Catalytic Reforming of Oxygenates: State of the Art and Future

Review pubs.acs.org/CR

Catalytic Reforming of Oxygenates: State of the Art and Future Prospects Di Li, Xinyu Li, and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ABSTRACT: This Review describes recent advances in the design, synthesis, reactivity, selectivity, structural, and electronic properties of the catalysts for reforming of a variety of oxygenates (e.g., from simple monoalcohols to higher polyols, then to sugars, phenols, and finally complicated mixtures like bio-oil). A comprehensive exploration of the structure−activity relationship in catalytic reforming of oxygenates is carried out, assisted by state-of-the-art characterization techniques and computational tools. Critical emphasis has been given on the mechanisms of these heterogeneous-catalyzed reactions and especially on the nature of the active catalytic sites and reaction pathways. Similarities and differences (reaction mechanisms, design and synthesis of catalysts, as well as catalytic systems) in the reforming process of these oxygenates will also be discussed. A critical overview is then provided regarding the challenges and opportunities for research in this area with a focus on the roles that systems of heterogeneous catalysis, reaction engineering, and materials science can play in the near future. This Review aims to present insights into the intrinsic mechanism involved in catalytic reforming and provides guidance to the development of novel catalysts and processes for the efficient utilization of oxygenates for energy and environmental purposes.

CONTENTS 1. Introduction 2. Overview 2.1. Types and Sources of Oxygenates 2.2. Types of Reforming 2.2.1. Vapor-Phase Reforming 2.2.2. Aqueous-Phase Reforming 2.2.3. Supercritical Water Reforming 2.2.4. Plasma Reforming 2.2.5. Photocatalytic Reforming 2.3. Operational Conditions 3. Methanol Reforming 3.1. Steam Reforming 3.1.1. Reforming Mechanism 3.1.2. Catalyst Development 3.1.3. Cu/ZnO/Al2O3 Catalysts 3.1.4. Group VIII Metal-Based Catalysts 3.1.5. Oxidative Steam Reforming 3.2. Aqueous-Phase Reforming 3.3. Photocatalytic Reforming 3.3.1. TiO2-Based Photocatalysts 4. Ethanol Reforming 4.1. Steam Reforming 4.1.1. Reaction Pathways 4.1.2. Deactivation Mechanism 4.1.3. Catalyst Optimization 4.2. Dry Reforming 5. Polyols Reforming 5.1. Steam Reforming 5.1.1. Reaction Mechanism © XXXX American Chemical Society

5.1.2. Noble Metal-Based Catalysts 5.1.3. Non-Noble Metal-Based Catalysts 5.2. Aqueous-Phase Reforming 5.2.1. Reaction Mechanism 5.2.2. Catalyst Development 5.2.3. Catalyst Deactivation 5.3. Supercritical Water Reforming 5.3.1. Catalyst Selection 5.3.2. Process Optimization 5.4. Photocatalytic Reforming 5.4.1. Noble Metal Cocatalysts 5.4.2. Non-Noble Metal Cocatalysts 6. Dimethyl Ether Reforming 6.1. Reforming Mechanism 6.2. Catalysts for DME Steam Reforming 6.2.1. DME Hydrolysis Catalysts 6.2.2. Supported Cu-Based Catalysts 6.2.3. Spinel-Typed Cu Catalysts 6.2.4. Noble Metal-Based Catalysts 6.2.5. Oxide and Carbide Catalysts 6.3. Autothermal Reforming 6.4. Dry Reforming 6.5. Plasma Reforming 7. Bio-Oil Reforming 7.1. Overview 7.1.1. Bio-Oil Production and Conversion Technologies 7.1.2. Selection of Bio-Oil Model Compounds

B C C C C D D E E F F G G I I N V X X Z AA AB AB AF AJ AU AV AV AW

AW AY BA BA BB BI BJ BK BL BM BN BO BO BP BQ BQ BQ BS BU BV BV BW BW BW BW BW BX

Received: February 9, 2016

A

DOI: 10.1021/acs.chemrev.6b00099 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 7.2. Acetic Acid Reforming 7.2.1. Reaction Pathways 7.2.2. Catalyst Development 7.2.3. Production of High-Purity Hydrogen 7.3. Phenols Reforming 7.3.1. Supported Metal Catalysts 7.3.2. Catalysts Derived from Mineral Materials 7.4. Sugars and Sugar-Alcohols Reforming 7.4.1. Steam Reforming 7.4.2. Aqueous-Phase Reforming 7.5. Crude Bio-Oil and Its Aqueous Fraction Reforming 7.5.1. Steam Reforming 7.5.2. Reactor Design 8. Summaries and Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

Review

Approximately 95% of the hydrogen production in the U.S. is generated via steam methane reforming (SMR).8 It must be noted that currently industrial-scale hydrogen generation still relies on the conversion of easily available natural gas or shale gas, and in the next few years hydrogen production from renewable oxygenated hydrocarbons may not be costcompetitive.9,10 Nonetheless, as natural gas/shale gas serves as a cheap hydrogen source for now, sustainable feedstocks are most likely to be the source of choice for future hydrogen generation, and hence developing catalytic reforming technology of biomass-derived oxygenates is necessary. While considerable amounts of CO2, a major greenhouse gas (GHG), are released during the reforming reaction, using renewable oxygenated hydrocarbons instead of fossil feedstocks is advantageous because it is conceptually seen as a CO2-neutral precursor; that is, the CO2 emitted can be recycled back into the environment via photosynthesis during biomass growth.1,11 Moreover, it is more appropriate to regard SMR and oxygenated hydrocarbons reforming as complementary technologies rather than competitors, as the diverse feedstocks and operation conditions of the latter grant the process good adaptability in varied cases where conventional SMR is not suitable. Specifically, as compared to SRM where large-scale centralized units and high operation temperatures (e.g., 1073 K) are often required, reforming simple oxygenated hydrocarbons such as methanol and dimethyl ether is more flexible and initiated at low temperatures, which is quite suitable for small-scale decentralized H2 production and mobile fuel cell applications.12 In fact, vehicles powered by hydrogen fuel cells and hydrogen-fueled engines have been designed by manufacturers.2 Another example is bioglycerol, a low commercial valued byproduct of the fast expanding biodiesel industry, and its disposal often brings about detrimental impacts on the environment. Converting the bioglycerol via catalytic reforming is a preferable route to produce value-added secondary products such as hydrogen or syngas, allowing them to be finally used for energy generation in fuel cells or chemical production through methanol synthesis and F−T synthesis. The latter process, which ultimately transforms byproduct glycerol to value-added chemicals or fuels, is important because the need for sustainable C-containing sources is urgent.6 At the very center of catalytic reforming lies the surface reaction mechanism, which is determined by the structural and electronic properties of catalysts, and therefore catalyst is placed on the focus point.13 Progress has been made on understanding of the surface chemistry during catalytic reforming reactions thanks to the established and emerging experimental, characterization, and analysis technologies. Particularly, ultrahigh vacuum (UHV) experiments with surface science techniques have been very useful tools for observing important catalytic phenomena and elucidating the fundamental mechanism involved, while theoretical modeling such as density functional theory (DFT) calculations serves to predict and interpret experimental results.14,15 Moreover, model catalysis offers opportunities to study the catalytic influence of a single factor (size, support, interface, crystalline effects, etc.) on well-defined surfaces independently without interference of other factors.16 Guided by the fundamental studies, many researchers are dedicated to rationally designing active, selective, and stable catalyst using materials including metals, metal oxides, carbides, etc., and more preferentially, bifunctional supported catalysts where small nanoparticles (NPs) locate on an active support, and the interface between them

BY BY BZ CB CB CB CC CD CD CD CH CH CI CK CN CN CN CN CN CN CO

1. INTRODUCTION Energy is an indispensable element in our everyday lives. Nowadays most of the energy we use comes from fossil fuels, a nonrenewable and unclean energy source. Therefore, it is urgent to explore sustainable and clean energy. Hydrogen is an alternative fuel to support energy development because of its cleanness and high combustion efficiency. Traditional industrial applications for hydrogen mainly include ammonia synthesis, Fischer−Tropsch (F−T) synthesis, and refinery processes. Recent advances in fuel cell technologies offer a low-carbon route for energy generation with high efficiency, and demand for both is expected to grow.1,2 Among numerous approaches for producing hydrogen, catalytic reforming is classical and efficient, which can be traced back to the early 19th century.3 The term of catalytic reforming should not be confused with the process used for converting paraffinic hydrocarbons to high octane products in refining and petrochemical plants.3 In this Review, catalytic reforming refers to the reaction between water (gas-phase or aqueous-phase) and hydrocarbons or oxygenated hydrocarbons to yield a mixture of mainly hydrogen, carbon oxides, and methane.4 Oxygen and CO2 could be added as a coreactant or used as the replacement for water (in dry reforming). As Rostrup-Nielsen suggested, the term “oxygenolysis” could be more appropriate because the process mainly involves the cleavage of C−C and C−H bonds by means of oxygen-containing species.5 Recently, the development of technologies for converting biomass has resuscitated the surge of reforming renewable oxygenated hydrocarbons for hydrogen production. The renaissance of catalytic reforming from sustainable biomass can be clearly reflected by the ever-increasing number of peerreviewed publications in the last two decades. The increase synchronizes with the rise in the global price of natural gas, which is now the predominant and most economical feedstock for commercial hydrogen generation.6 According to the BP statistical review of world energy released in June of 2015, the price for natural gas has tripled at least in the past two decades in most parts of the world, with the only exception being North America, where a shale gas revolution is happening.7 B


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2. OVERVIEW

acts as the catalytic active site, thus stimulating fast growth of the field of catalyst synthesis.17,18 High-throughput catalytic tests are employed to screen out promising catalyst composition and determine suitable reaction conditions, and meanwhile long-term and cycling tests are carried out to examine catalyst lifespan and regenerability.19 Until recently, catalyst deactivation, mainly through sintering, coke deposition, oxidation state change, and active component leaching, puts a severe restriction on the efficiency of reforming reactions, and the deactivation behavior often serves as a feedback for catalyst design and synthesis for further optimization. Meanwhile, fundamental studies of the deactivation mechanism are important for addressing the deactivation issue, especially when coke deposition is the main culprit for catalyst decay.13 Identification of reaction pathways and key intermediates leading to carbonaceous deposits allows for manipulation of their relative importance in the overall reforming reaction by modification of catalyst (e.g., type of active component and promoter) as well as adjustment of operation conditions (e.g., temperature and feed composition) accordingly. Additionally, strategies to stabilize small active metal particles through various methods, for example, engineering novel encapsulated structures and/or inducing a strong metal support interaction (SMSI), are also of great research interest.20,21 Furthermore, process intensification and optimization are performed to maximize system efficiency, promote the overall economics, as well as achieve waste management and pollution control. For example, in situ sequestration of CO2 using mineral adsorbents promotes hydrogen yield, lowers CO2 emission, and reduces purification costs simultaneously.22,23 Indeed, catalytic reforming of oxygenates has been intensively investigated due to fundamental and practical significance in the context of catalysis, surface science, energy, and environmental sciences. There are several excellent reviews relevant to the hydrogen production via simple, model oxygenated hydrocarbons; however, these reviews primarily focus on a single specific oxygenate10,13,24−28 (e.g., methanol, ethanol, and glycerol), a single aspect of the reforming process13,25,29−31 (e.g., reaction mechanism, deactivation mechanism, and catalyst preparation), a single form of reforming reaction32−35 (e.g., steam reforming, aqueous-phase reforming, and sorption-enhanced reforming), or a single class of catalytic material (e.g., Ni-based catalysts and ceria-zirconia supported catalysts).9,20,36 This Review attempts to provide current understanding of design, synthesis, reactivity, selectivity, structural, and electronic properties of the catalysts for reforming of a variety of oxygenates (e.g., from simple monoalcohols to higher polyols, then to sugars, phenols, and finally complicated mixtures like bio-oil). Critical emphasis will be given on the mechanisms of these catalytic reactions and especially on the nature of the active catalytic sites and reaction pathways. Similarities and differences (reaction mechanisms, design and synthesis of catalysts, as well as catalytic systems) in the reforming process of these oxygenates will also be discussed. Furthermore, we will offer a critical overview regarding the challenges and opportunities for research in this area with an emphasis on the roles that systems of heterogeneous catalysis, reaction engineering, and materials science can play in the near future.

2.1. Types and Sources of Oxygenates

Oxygenates (refers to oxygenated hydrocarbons in this Review) are a large group of chemical compounds with oxygen as part of their chemical structure, ranging from simple monoalcohols to carbohydrates. The oxygenates are produced from a variety of sources including raw biomass and platform chemicals. In Table 1, the types and sources of major oxygenates are provided. Table 1. Type and Sources of Oxygenates type monoalcohol aldehyde polyol

model compound methanol ethanol formaldehyde acetaldehyde ethylene glycol

glycerol ether carboxylic acid

dimethyl ether formic acid

ketone

acetic acid (AcOH) acetone

aromatics carbohydrate bio-oil

phenols sugars raw bio-oil aqueous phase of bio-oil (APB)

source/production synthesis from syngas ethylene dehydration; sugars fermentation methanol oxidation ethylene oxidation hydration of ethylene oxide; synthesis from syngas; synthesis from biomass-derived glycerol and cellulose byproduct of biodiesel production via transesterification methanol dehydration; synthesis from syngas naphtha partial oxidation; methanol carbonylation; biomass and CO2 hydrogenation methanol carbonylation; bacterial fermentation of sugars and alcohols cumene process (benzene is alkylated with propylene to produce cumene, followed by oxidation by air to produce phenol and acetone) cumene process hydrolysis of lignocellulosic biomass fast pyrolysis of lignocellulosic biomass fractionation of bio-oil

2.2. Types of Reforming

2.2.1. Vapor-Phase Reforming. Typically, hydrogen production from hydrocarbons and oxygenated hydrocarbons in the vapor phase proceeds through three processes, steam reforming, partial oxidation, and oxidative steam reforming, and the latter two involve oxygen addition.37 Among them, steam reforming exhibits the highest hydrogen yield, but its strong endothermic nature demands high operation temperature and large external heat supply. Partial oxidation (POx), on the other hand, has the lowest hydrogen yield, but it provides heat internally via partial combustion of fuels and requires no heat input when it reaches steady state.38 As compared to steam reforming, the POx process is initiated at lower temperatures and has the advantage of fast start-up, and the transient response could be achieved by varied oxygen-to-carbon (O/C) ratios, and therefore is more suitable for onboard hydrogen production.37,39−41 Moreover, facile CO oxidation reaction inhibits CO production via POx reactions, which is highly desired for fuel cell systems. Note that in the case of methanol, the theoretical maximum of H2 concentration in POx approaches 67%; nevertheless, it could drop to 41% because air is often used in the real scenario.42 The H2 concentration is important for the performance of fuel cell systems, and therefore water is added to facilitate H2 production reactions such as the water−gas shift (WGS) reaction. Meanwhile, the high exothermicity of the reaction leads to waste heat generation and temperature control issues.40 Consequently, C


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reactants and water, significantly lowering the energy barrier as well as extending the feed resources to nonvolatile oxygenates.55 (ii) Low operating temperatures are able to minimize the undesired side reactions (primarily thermal decomposition) at elevated temperatures. (iii) WGS reaction is thermodynamically favorable at the APR conditions, and thus producing H2 with low CO content in a single-step process is viable.56 It is noteworthy that a typical steam reforming process requires multistage or multiple reactors to achieve the same purpose. (iv) The hydrogen-rich and CO-deficient, even CO-free, gas effluent of APR can be utilized directly as an energy source for fuel cell applications, or used as important reactants in many chemical processes such as catalytic NOx removal or in situ hydrogenation in the liquid phase.57−59 Notably, methanation and F−T synthesis are also favored under the moderatetemperature and pressurized APR conditions. Although these side reactions facilitate the removal of CO, they result in the formation of alkanes simultaneously, which are the main byproducts in the reaction. 2.2.3. Supercritical Water Reforming. In the past decade, supercritical water reforming (SCWR) or gasification of various hydrocarbons and biomass-derivatives has been an emerging technology that receives considerable attention.60−67 The term “supercritical” refers to a state where the distinction between liquid and gaseous phase ceases to exist. Water becomes supercritical when it reaches a temperature and pressure above its critical point, which is at 647 K and 221 bar (Figure 1).68

oxidative steam reforming couples the former two reactions, and in a measured ratio becomes thermal neutral, which is then called autothermal reforming (ATR).12 The idea of converting methanol to H2-rich gas via OSR could be traced back to the 1980s, and has been attracting much research interest across industrial and academic society.43−45 The OSR process delivers high H2 concentration (up to 65% with air operation) with low CO concentrations while maintaining an efficient and dynamic system. To maximize the H2 production from oxygenated hydrocarbons, sorption-enhanced steam reforming (SESR) has become an emerging area, featured by simultaneous steam reforming, water−gas shift reaction, and in situ CO2 removal in a single-step reactor system.46−48 CO2 sorption takes place in the course of the reaction, and thus hydrogen evolution is favored by shifted equilibrium based on Le Chatelier’s principle.49 Moreover, the higher H2/CO2 ratio in the effluent stream of SESR alleviates the burden of hydrogen purification and separation in the downstream process, thus reducing the capital cost for subsequent processing steps.50,51 A CO2 acceptor is necessary for the SESR process. For example, CaO-based solid acceptors have been utilized as in eq 1. CaO + CO2 → CaCO3

° ΔH298 = −178 kJ mol−1

(1)

The adsorption of CO2 coupled with WGS is represented in eq 2. CaO + CO + H 2O → CaCO3 + H 2 ° ΔH298 = −219 kJ mol−1

(2)

Consequently, an overall reaction for SESR of alcohol (e.g., glycerol) can be summarized as follows: C3H8O3 + 3H 2O + 3CaO → 3CaCO3 + 7H 2 ° ΔH298 = −407 kJ mol−1

(3)

Dry reforming of methane (DRM, eq 4) has recently become a hot topic as it offers a single-step process to produce syngas from methane and CO2 with simultaneous decrease of GHGs.52 As compared to dry reforming of hydrocarbons, the dry reforming of oxygenates (e.g., eq 4) could further improve the sustainability of the process considering that biomass-derived ethanol is a renewable source of carbon.53 CH4 + CO2 → 2CO + 2H 2

Figure 1. Critical point in phase diagram. Reprinted with permission from ref 68. Copyright 2011 Elsevier.

Supercritical water possesses unique properties that significantly differ from those of steam or liquid water, granting it several advantages as a promising reaction medium for reforming process: (i) As compared to liquid water, supercritical water has a lower dielectric constant and less amount of hydrogen bonds with weaker strength; therefore, it has high, even complete, miscibility with many organic compounds and gases, allowing for reforming reactions in a single fluid phase instead of multiphase under conventional conditions.69 The absence of phase boundaries reduces mass-transfer limitations and therefore leads to fast and complete reactions.70,71 (ii) As compared to steam, the density of supercritical water is higher, leading to a higher space time yield. Moreover, the higher thermal conductivity and specific heat of supercritical water make it very suitable for conducting endothermic reforming reactions.68 (iii) Under supercritical conditions, free radical degradation and ionic reactions may occur. According to the literature, the ionic products in supercritical water can be some orders of magnitude higher those than in water at ambient pressure.69 Therefore, water may serve as an acid/base catalyst due to the

° ΔH298 = 247 kJ mol−1

(4)

C2H5OH + CO2 → 3H 2 + 3CO

° ΔH298 = 297 kJ mol−1

(5)

2.2.2. Aqueous-Phase Reforming. Aqueous-phase reforming, as the name suggests, is a liquid-phase process, distinctively different from the aforementioned reforming processes, which proceed in the vapor phase. In 2002, the Dumesic group first demonstrated the APR process, in which methanol and other biomass-derived feedstocks (ethylene glycol, glycerol, sorbitol, glucose, etc.) were efficiently converted with water in the aqueous phase over appropriate heterogeneous catalysts at substantially low temperatures (e.g., 498 K) and moderate pressures (typically 15−50 bar) to generate primarily H2 and CO2.54 As compared to hydrogen production via SR, generating hydrogen via APR possesses several advantages. (i) Unlike MSR where reactions occur in the vapor phase, APR eliminates the need to vaporize both the D


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high concentration of H3O+ and OH− ions. The intrinsic reaction rates for decomposition of biomass are rapid, and thus a high H2 yield can be achieved at relatively low temperatures.72 (iv) Most reaction intermediates including hydrocarbons are completely soluble in SCW due to the low dynamic viscosity and high diffusivity observed for supercritical water, which contributes to suppression of undesired char and tar.73 (v) The H2-rich product gas available at high pressures can be stored directly, eliminating the high costs associated with compression steps.13,74,75 (vi) The SCWR process does not require the expensive and energy-consuming pretreatment of drying, which is typically necessary for other thermochemical processes.66,73,74,76 2.2.4. Plasma Reforming. The reforming techniques discussed previously all proceed at elevated temperatures, and these processes require an induction period for hydrogen production until the temperature is adequately high. This may reduce the energy efficiency of the processes. However, plasma is able to propel reactions at ambient conditions, and its utilization in H2 production has been well reviewed by Du et al.77 As compared to the traditional reforming, plasma reforming has several advantages, including high energy efficiency, compatibility with a broad range of feestocks (oxygenates and hydrocarbons), compactness, and reasonable cost.78 Moreover, the fast response time of plasma reforming removes the limitation of the induction period, which is desired for practical applications such as onboard hydrogen production for automobiles.79 Plasmas are streams containing highly energetic species including electrons, ions, radicals, and neutral atoms.80 Different from conventional reforming where reactions proceed via initial adsorption followed by surface reactions, reactant molecules are directly activated and dissociated through collision with highly energetic electrons, atoms, and radicals in plasma reforming.81−83 2.2.5. Photocatalytic Reforming. By far, the most abundant resource vector human has ever discovered is solar energy: 1.2 × 105 terawatts strikes the earth continuously, remarkably exceeding the rate at which the whole human civilization consumes energy.84 As compared to thermochemical processes, photochemical processes principally have an almost limitless and cheap source of energy. Since the seminal finding by Fujishima and Honda on photocatalytic water splitting over a TiO2 electrode under ultraviolet (UV) radiation in 1972, enormous efforts have been made in solar hydrogen generation via photocatalytic (PC) and photoelectrochemical (PEC) water splitting.85 Upon irradiation, O2 formed on the TiO2 photoanode due to water oxidation while H2 evolved on the Pt black cathode as a result of water reduction with some external bias by a power supply or pH difference as depicted in Figure 2. Innovated by this concept, Bard et al. further designed the photocatalytic systems utilizing semiconductor powders and particles for various chemical processes including water splitting.86−89 From a thermodynamic point of view, the conversion of photon energy into chemical energy via water splitting is an uphill reaction accompanied by a highly positive change in Gibbs free energy as shown in eq 4a. H 2O → H 2 + 0.5O2

° ΔG298 = 237.2 kJ mol−1

Figure 2. Honda−Fujishima effect for water splitting. Reprinted with permission from ref 90. Copyright 1995 American Chemical Society.

Figure 3. Fundamental principle of semiconductor-catalyzed photocatalytic conversion for H2 production.

conduction band (CB) and the valence band (VB) are separated by band gap with a proper width.91 When a photocatalyst absorbs light with an energy equivalent to or greater than its band gap, the electrons (e−) in the valence band are excited to the CB, while the holes (h+) are left in the VB, generating the electron−hole pairs. The separated electrons and holes migrate to the surface of the photocatalyst without recombination and can act as reducing and oxidizing agents for water molecules to produce hydrogen and oxygen, respectively. To facilitate this redox reaction, the bottom level of CB must be more negative than the reduction potential of H+/H2 (0 V vs normal hydrogen electrode (NHE)), while the top level of VB must be more positive than the reduction potential of O2/H2O (1.23 eV). Consequently, a theoretical minimum band gap energy (Eg) of 1.23 eV (equal to ΔG° = 237 kJ mol−1) is required for photocatalytic water splitting, which corresponds to the energy of a photon with a maximum wavelength of around 1010 nm according to eq 5a. This indicates the possibility of using photocatalysts under visible light irradiation (390−700 nm). band gap (eV) = 1240/ λ (nm)

(5a)

H2 evolution is determined by the amount of excited electrons on the photocatalyst/H2O interface in reducing water, and therefore other electron-consuming processes have to be suppressed to promote the efficiency of H2 production.86 Surface/bulk recombination of photogenerated electrons and holes is one of the primary challenges to photocatalytic performance because it reduces the amount of excited charges by light emission and leads to activity loss. Additionally, other factors including overpotentials, charge separation, mobility, and lifetime of the excited electrons and holes also affect the H2 production significantly, presenting a series of challenges for efficient photocatalytic water splitting.92 Although numerous efforts have been devoted to direct water splitting, its efficiency remains low.93 Consequently, sacrificial reagents are introduced as electron donors or hole scavengers,

(4a)

A schematic representation of basic principle of the photocatalysis on a semiconductor catalyst is depicted in Figure 3. Semiconductors possess a band structure where the E


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Table 2. Typical Operational Conditions for Oxygenates Reforming10,105−115 reaction steam reforming

reprensentative feedstock

reaction temp (K)

eq

pressure (bar)

S/C ratio

CH3OH + H 2O → 3H 2 + CO2

423−623

1

1−3

C2H5OH + 3H 2O → 2CO2 + 6H 2

573−973

1

1.5−8

423−673

1

O2:methanol = 0−0.5

573−973

1

O2:ethanol = 0−1.5

ethanol

3 O2 → 3H 2 + 2CO2 2 C2H5OH + CO2 → 3H 2 + 3CO

873−1073

1

CO2:ethanol = 1

dimethyl ether

CH3OCH3 + CO2 → 3H 2 + 3CO

550−973

1

CO2:DME = 1

ethylene glycol

C2H6O2 + 2H 2O → 2CO2 + 5H 2

450−540

34−56

0−10 (wt %)

glycerol

C3H8O3 + 3H 2O → 7H 2 + 3CO2

647−1073

above 221

0−50 (wt %)

room temperature

ambient

methanol ethanol

(1)CH3OH + H 2O → 3H 2 + CO2 methanol oxidative steam reforming

aqueous-phase reforming supercritical water reforming photocatalytic reforming

1 O2 → 2H 2 + CO2 2

(1)C2H5OH + 3H 2O → 2CO2 + 6H 2 ethanol

dry reforming

(2)CH3OH +

various oxygenates

(2)C2H5OH +

hν

Cx HyOz + (2x − k)H 2O → (2x + y/2 − z)H 2 + xCO2

spectrum changes accordingly. Generally, thermochemical reforming reactions are favored at high temperatures, but one should note that they are sometimes accompanied by side reactions such as decomposition. In this subsection, we present a brief summary of typical operational conditions for catalytic reforming as shown in Table 2.

which are readily, irreversibly oxidized by photogenerated holes.87,91,94 This process inhibits undesired charge recombination and increases the availability of free electrons. Moreover, the sacrificial reagents act as an extra driving force for the surface chemical reaction by suppressing the formation of H2O via surface back reaction between photogenerated H2 and O2.86 Some sacrificial reagents such as Na2S, Na2SO3, and EDTA have been utilized; however, these materials are nonrenewable and raise the overall cost for the process.94,95 Recently, the utilization of renewable biomass and biomass-derivatives including methanol, ethanol, glycerol, glucose, sucrose, cellulose, starch, and wood as sacrificial reagents in photocatalytic water splitting process, which is referred to as the photocatalytic reforming or photoreforming (PR) process, has received an increasing amount of attention globally, since it was first proposed by Kawai et al. in the 1980s.96−100 The redox potentials of these biomass oxygenates are much more negative than that of water, suggesting that they are much more easily oxidized than water by photogenerated holes.101 As a result, photocatalytic H2 generation rates in alcohol−water mixture could be 1−2 orders of magnitude higher than that obtained in pure water.102 In a typical photoreforming reaction, oxygenate denoted as CnHmOk is oxidized by the holes excited in the VB of a photocatalyst to form H2 and CO2 as shown in eq 6.103

3. METHANOL REFORMING Methanol (CH3OH) is one of the most important raw materials with an annual production of around 65 million tons as of 2014, of which more than one-half is produced in China, and the demand is still on the increase.116 Methanol as a hydrogen carrier receives much attention due to its molecular simplicity (no C−C bond), high hydrogen to carbon (H/C) ratio, wide availability, and easy storage and transportation. These characteristics make methanol one of the most promising feedstocks for fuel cell systems. To date, it has been demonstrated that hydrogen production from methanol primarily includes (i) thermal reforming and (ii) photocatalytic reforming. Catalytic production of hydrogen by methanol decomposition (eq 7) has attracted attention since the 1980s. However, this process is unsuitable for fuel cell applications as one of the main products CO has detrimental effects on the fuel cell anodes.

Cx HyOz + (2x − k)H 2O hν ,catalyst

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (2x + y/2 − k)H 2 + xCO2

CH3OH → 2H 2 + CO

(6)

° ΔH298 = 128.1 kJ mol−1

(7)

Alternatively, catalytic reforming of methanol is suitable for onboard hydrogen production for fuel cells. Christiansen was among the first who examined the reaction between methanol and steam on reduced copper surface.117 Thermal catalytic reforming of methanol generally includes steam reforming (eq 8), partial oxidation (eq 9), oxidative steam reforming (eq 10), and aqueous-phase reforming (eq 11). Photocatalytic methanol reforming is another promising process that allows H2 production at ambient conditions using solar energy, a source that is often considered to be inexhaustible.92 However, for now its H2 evolution rates and system efficiency are far from the commercial demands, largely due to the limitation of catalytic materials. A comparison

From a thermodynamical point of view, photoreforming reaction is less endergonic and alleviates the constraints associated with direct water splitting, and therefore is considered to be more feasible.104 It is important to note that, although CO2 evolves alongside H2 in PR, it originates from renewable biomass instead of fossil fuels and is conceptually recycled back into biomass via photosynthesis, resulting in a carbon-neutral H2 production process. 2.3. Operational Conditions

As discussed earlier, catalytic reforming of oxygenates could proceed under various reaction conditions in the vapor or liquid phase, utilizing thermal or solar energy, and the product F


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3.1. Steam Reforming

between these reforming routes is briefly summarized in Table 3.12,26

3.1.1. Reforming Mechanism. MSR is a well-developed and thoroughly studied process.12,26,118,119 The reaction can yield a gas containing up to 75% H2 while maintaining high selectivity toward CO2. However, the reaction mechanism of MSR and its reverse reaction, methanol synthesis, are more complicated and are still under debate, primarily over Cu-based catalysts, even though they have been vastly utilized in industry for decades.12,120−122 Therefore, numerous attempts have been dedicated to figure out the mechanistic aspects of MSR. Schomäcker et al. presented an entire catalytic cycle of MSR in Scheme 1, compiling several proposed mechanisms found in the literature.123−127 In MSR reactions, the initial steps were dissociative adsorption of methanol and subsequent dehydrogenation of methoxyl (CH3O*), and the latter was generally acknowledged as the rate-determining step (RDS) of MSR reaction according to extensive investigations.123,124,127−129 Therefore, formaldehyde (CH2O*) was the key intermediate with which a complicated MSR reaction network started. In fact, Takahashi et al. detected identical products in formaldehyde and methanol steam reforming under the same conditions.130 On the basis of kinetic investigations, it was assumed that the active site A (SA) was responsible for hydrogen adsorption and the active site B (SB) was responsible for the adsorption of all other intermediates, and moreover hydrogen adsorption did not compete for the active sites that the oxygen-containing species adsorb on.127,128 The catalytic circle consists of three mechanisms that have been proposed thus far, which are described respectively as follows. (A) Methanol decomposition (eq 7) followed by WGS reaction (eq 12):

CH3OH + H 2O → 3H 2 + CO2 ° ΔH298 = 130.9 kJ mol−1

CH3OH +

(8)

1 O2 → 2H 2 + CO2 2

° ΔH298 = −154.9 kJ mol−1

(9)

CH3OH + (1 − 2x)H 2O + xO2 → (3 − 2x)H 2 + CO2 ° ΔH298 = −12 kJ mol−1 for x = 0.25

(10)

CH3OH(l) + H 2O(l) → 3H 2 + CO2 ° ΔH298 = 49.5 kJ mol−1

(11)

Table 3. Comparison of Reforming Technologies technology

energy source

steam reforming

heat

partial oxidation oxidative steam reforming aqueous-phase reforming

heat heat

photocatalytic reforming

solar energy

heat

advantages oxygen not required; best H2/CO ratio for H2 production exothermic; fast thermally neutral or moderately exothermic without vaporizing water; low levels of CO ambient reaction condition

disadvantages strongly endothermic; large heat duty hot-spots and hot-zones requires complex and iterative calculations pressurized operations; leaching of catalyst components low hydrogen production rates

Scheme 1. Catalysis Cycle of Methanol Steam Reforming on the Basis of the Investigations of Wainwright et al.,124,125 Peppley et al.,127 and Takezawa et al.,126 Including Different Kinds of Reactive Surface Sites SA and SBa

a

Reprinted with permission from ref 123. Copyright 2007 Elsevier. G


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Figure 4. Energetics of the formate (a) and methyl formate routes (b) for MSR. The “//” symbol denotes that an H* is species removed from the next step of the calculation, while the symbol “⧧” suggests the barrier heights in the forward direction. Reprinted with permission from ref 147. Copyright 2011 American Chemical Society.

CO + H 2O → H 2 + CO2

° ΔH298 = − 41.2 kJ mol−1

MSR conditions where an excess of water was typically present.123,143−145 To date, direct evidence for the methyl formate route is still insufficient due to its weak adsorption and fast transformation kinetics over copper catalysts, but this does not necessarily exclude the involvement of methyl formate.141,146 DFT calculations by Lin et al. examined the relevance of the methyl formate route.147 Theoretical results indicated that the intermediacy of methyl formate only made a minor contribution to the MSR reactions, and meanwhile suggested that the formate-intermediate route could be more reliable. Figure 4 shows the energetics of the formate (Figure 4a) and methyl formate routes (Figure 4b). It can be found that the reaction of CH2O* with OH* had a lower barrier than its reaction with CH3O*, which explained the absence of methyl formate when there were plenty of OH* groups on the surface. Furthermore, the subsequent reaction of methyl formate with OH* held a higher barrier than its desorption, indicating a high desorption tendency. The theoretical study was in good agreement with previous experimental investigations, therefore discounting the credibility of the methyl formate route as the primary pathway for MSR. The formate-intermediated pathway (mechanism C) was proposed by Takezawa et al. on the basis of the observation of formaldehyde and formic acid (CHOOH*) intermediates in the MSR reaction over Cu-based catalysts.126 Later, the mechanism was supported by studies of several other groups, as they experimentally confirmed that CH3OH was dehydrogenated to HCHO* followed by a nucleophilic attack of H2O to form HCOOH, HCOO*, and HCHOO** (dioxomethylene), which finally decomposed to H2 and CO2.148 The formation of CO was ascribed to the reverse-WGS reaction. As shown in Figure 4a, a very low barrier (0.11 eV) for formaldehyde reacting with OH*, together with a strong exothermicity (−0.64 eV), suggested the facile formation of CH2OOH* that eventually dehydrogenated to yield H2 and CO2.147,149−151 In addition, theoretical studies upon methanol synthesis, generally considered as the reverse of MSR,

(12)

(B) Reforming via methyl formate intermediate: 2CH3OH → HCOOCH3 + 2H 2

(13)

HCOOCH3 + H 2O → CH3OH + HCOOH

(14)

HCOOH → CO2 + H 2

(15)

CO2 + H 2 → CO + H 2O

(16)

(C) Reforming via formate intermediate: CH3OH → HCHO + H 2

(17)

HCHO + H 2O → HCOOH + H 2

(18)

HCOOH → CO2 + H 2

(19)

Santacesaria et al. first proposed that MSR proceeded in the sequence of methanol decomposition and then WGS reaction (mechanism A).131 Accordingly, as CO is produced first in the reaction system, some claim that its concentration should be equal to or greater than the one at WGS equilibrium, which has been not observed.132−134 Instead, CO was found to be the secondary product generated via reverse-WGS reaction (eq 16).135,136 Moreover, this mechanism was further discredited by evidence that WGS reaction is suppressed in the presence of methanol.130,137−140 Takahashi et al., however, suggested a mechanism involving a methyl formate (CHOOCH3*) intermediate (mechanism B), which was later supported by Wainwright et al.124,125,130,141 In this scheme, formaldehyde reacts with methoxyl to yield methyl formate, which is then hydrolyzed to form formic acid and methanol, followed by decomposition of formic acid to form carbon dioxide.142 CO, as discussed earlier, is the result of reverse-WGS reaction.132,133 However, although methyl formate was observed in dry methanol decomposition or MSR at low concentrations of water, it was rarely detected under real H


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suggest that the roles of Cu0 and Cu+ differ in their adsorption species: methoxy, formaldehyde, and formate adsorb on Cu0, and oxygenates are stabilized on Cu+ sites.177 The oxidation state of Cu has been found to be closely associated with Cu−ZnO. Ressler et al. studied the redox behavior of Cu/ZnO catalysts in MSR.178 An oxidative treatment was introduced to fully reduce Cu/ZnO catalyst, and the formation of Cu+ and Cu2+ led to decreased reforming activity. Rereduction of the catalyst restored the metallic Cu0; nevertheless, the obtained activity was even higher than that at the initial stage. Accordingly, it was deduced that a large Cu0 surface area was essential for catalytic activity, yet it could not be fully responsible for the observed changes in catalytic performance. Consequently, bulk structural changes were taken into account, an increased structural disorder and microstrain in Cu particles, as evidenced by X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) measurements. The authors proposed that the microstrain could be the consequence of an epitaxial orientation of Cu and ZnO and lattice imperfection caused by Zn incorporation in Cu bulk or an incomplete reduction of copper, indicating the importance of a strong Cu/ZnO interaction on the reactivity of the catalysts.179 Consistent results were obtained by Ressler et al. for ternary Cu/ZnO/Al2O3 systems with those previously reported for binary Cu/ZnO model systems.180 They found that the H2 production was not linearly correlated with a specific Cu surface area, but was strongly influenced by the lattice strain in Cu particles induced by Cu−ZnO interaction. The presence of lattice strain was associated with an advanced Cu−ZnO interface. Such strain−activity correlation has been established for the Cu/ZnO/Al2O3 catalyst in the methanol synthesis reaction.163 On the basis of the literature examined, it could be concluded that the Cu−ZnO interface played a crucial role in the reactivity of Cu/ZnO-based catalysts. Rameshan et al. demonstrated that a UHV-grown Cu−Zn near surface alloy was transformed in situ to a Cu/ZnO catalyst in MSR conditions that possessed a large bimetal−Cu(Zn)0/ Zn(oxidized, ox) interface, as shown in Figure 5.181 The redox-

suggested a major pathway involving formate intermediate, corroborating the findings on MSR from a different aspect.152,153 By far, the evidence of methyl formate or formate route is not conclusive, and it is likely that both pathways coexisted in MSR and their relative contribution varies with different operation conditions and catalysts selected.150,154 3.1.2. Catalyst Development. MSR is able to produce H2 selectively even at low temperatures (e.g., 423−573 K), which is highly desirable to increase the heat integration potentials between the endothermic steam reforming and exothermic fuel cell processes.12,155 Hence, engineering highly active catalysts that provide fast kinetics is pivotal to acquire a high, commercially acceptable system efficiency. A wide variety of catalysts have been reported active for MSR, among which Cubased catalysts, represented by binary Cu/ZnO and ternary Cu/ZnO/Al2O3, are by far the most successful in terms of activity, stability, suppression of undesired CO formation, and cost.156−158 However, the major inherent drawbacks of copper, including high sintering tendency, coke deactivation, and pyrophoricity issue, necessitate the modification of Cu-based catalysts and the development of effective alternatives from other materials, such as group VIII metals.159−161 In this subsection, studies in both fields are discussed in detail. 3.1.3. Cu/ZnO/Al2O3 Catalysts. Although Cu/ZnO/Al2O3 (CuZnAl) has been a benchmark catalyst in methanol synthesis and reforming for decades, its structure−activity relationship remains illusive. In principle, Cu is generally considered as the active component, and alumina serves as the support with large surface area to accommodate well-dispersed Cu species that provide favorable kinetics to suppress CO formation.12,134,162 ZnO, on the other hand, plays a more complicated role. It has been acknowledged that the presence of ZnO is indispensable, as empirical development of active Cu/ZnO/Al2O3 catalysts focuses on maximizing Cu−ZnO contact.163 ZnO has been identified as a textural promoter to segregate Cu, whereas its promotional effects have also been explicitly observed and reported, indicating a multifunctional effect of ZnO. Consequenctly, accumulating evidence has been indicating a more delicate, microscopic structure−activity relationship rather than the one apparently observed. More importantly, elucidation of the intrinsic structure−activity relationship of Cu/ZnO/Al2O3 is a prerequisite for the rational design and further optimization of the catalyst. Unfortunately, unambiguous conclusion has not yet been reached to define the exact role of each component in Cu/ ZnO/Al2O3. Nevertheless, incessant efforts to understand the involvements of Cu/ZnO/Al2O3 in methanol-related catalysis have been reported in recent years.121,164−167 3.1.3.1. Structure−Activity Relationship. The influence of the oxidation state of Cu on catalytic reactivity has always been a focus of research in many Cu-catalyzed reactions including CO oxidation, dehydrogenation (e.g., cyclohexanol), and hydrogenation (e.g., dimethyl oxalate) of various feedstocks, as well as MSR.152,168−173 Cu2+ is the predominant Cu species in fresh catalysts, and undergoes a two-step reduction process to Cu0 via the Cu+ intermediate state. Much evidence reveals that a fraction of Cu0 is reoxidized under MSR conditions and stabilized by catalyst support.137 In methanol synthesis, a synergy between Cu0 and Cu+ is required, and similarly, many researchers agree that there exists an optimum cooperation between Cu0 and Cu+ for maximum catalyst reactivity in MSR, which is determined by catalyst preparation and composition as well as feed and reaction conditions.12,174−176 Some studies

Figure 5. (a) “As-grown” bimetallic CuZn(Cu:Zn10:1) precatalyst. (b) “In situ” formed active Cu(Zn)0 catalyst wetted with interfacial Zn(ox). Adapted with permission from ref 181. Copyright 2012 Wiley.

active interfacial sites assisted in H2O activation and then the transfer of oxygen-containing groups to the Cu(Zn)0 surface, on which they reacted with formaldehyde to finally yield H2 and CO2. Note that a bimetallic Cu(Zn)0 surface was essential to dehydrogenate methanol, whereas a clean Cu0 surface barely converted methanol. This model study suggested a bifunctional I


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role of Zn in the working Cu(Zn)0/Zn(ox) catalyst, which was further validated by Behrens et al.121 Recently, for the first time, Willinger and co-workers provided clear evidence for the formation of metastable “graphite-like” ZnO layers induced by SMSI during the reduction process of a real industrial Cu/ZnO/Al2O3 as shown in Figure 6.165 The disordered ZnO overlayer, together with the defected Cu surface underneath, could exhibit a synergistic effect that finally led to superior reactivity.

a modified or novel preparation process and tuning catalyst composition using different support and promoter materials. Tables 4, 5, and 6 summarize recent investigations using each strategy, respectively. To facilitate the comparison between different catalysts, the commercially available CuO/ZnO/Al2O3 catalyst is frequently used as a reference.186−188 The preparation process has a large impact on the structure and consequent catalytic performance of the catalyst for H2 production via MSR, and modification of preparation process is probably the most straightforward way to optimize catalyst performance without significantly altering the recipe. Typically, the preparation of a catalyst requires multiple selections, including precursor, preparation technique, calcination condition, and in some cases special pretreatment before use, etc., and it has been shown that each of them may affect the important properties of the resulting catalyst, mainly metal dispersion and metal−support interaction (Table 4). A common approach is to change the precursors of the Cu/ ZnO or Cu/ZnO/Al2O3 catalyst and compare the resultant catalysts with the commercially available ones to achieve novel, structured catalysts with enhanced performances in MSR. Catillion et al. used a copper foam and a sequential deposition− precipitation method to prepare active Cu/ZnO/Al 2 O 3 catalysts.189 Jones et al. used nanoparticle alumina to accommodate copper and zinc, and found that the nanoparticle-supported catalyst had a stronger Cu−Al2O3 interaction and high copper dispersion.190 As a result, the less reducible catalyst exhibited similar activity but much lower CO selectivity to commercial Cu/ZnO/Al2O3, possibly due to a large Cu surface area and the presence of Cu2O. Chen and coworkers designed a core−shell catalyst composed of a ZnO nanorod (NR) core and a shell of Cu NPs as shown in Figure 7, denoted as [email protected] The NR@NPs catalyst showed activity and stability superior to those of the commercial catalyst (11.5% vs 33.8% activity loss after 36 h time-on-stream (TOS)), which were attributed to the combined effects of uniformly dispersed small Cu NPs, formation of microstrain, a large fraction of active Cu0/Cu+ species, and the existence of SMSI due to the nanostructure. Song et al. compared several preparation methods including impregnation, coprecipitation, and hydrothermal synthesis, and identified the coprecipitated Cu/ZnO/Al2O3 with the highest copper reducibility as the efficient catalyst in MSR.192 Generally, Cu/ZnO and Cu/ZnO/Al2O3 are prepared by coprecipitating the mixture of copper, zinc (and aluminum) nitrates, as well as sodium carbonates in their aqueous solutions.179,193,194 Kniep et al. thoroughly investigated the effect of precipitate aging on the microstructural characteristics of Cu/ZnO catalysts in the process.156,179 They observed that after an appropriate aging procedure (more than 30 min), the structure of the precipitate was significantly tailored with respect to unaged sample, demonstrating a uniform CuO−ZnO distribution, which resulted in small-sized and highly disordered Cu/ZnO catalysts upon calcination/reduction. An obvious increase in catalytic activity was found exclusively for samples aged for more than 30 min, strongly indicating an activity− structure correlation that involved the crystallinity, phase composition, and homogeneity of the precipitate catalyst precursor. A modified coprecipitation method using oxalate precursors (called gel-coprecipitation) was introduced by Cao’s group to synthesize a nanostructured Cu/ZnO/Al2O3 catalyst with high specific surface area and copper dispersion that exhibited better

Figure 6. (A) High-resolution transmission electron microscopy (HRTEM) image of reduced Cu/ZnO/Al2O3 catalyst. The inset denotes the corresponding line scans taken from the assigned regions of interest (ROI). (B) TEM image of the region where energy-filtered TEM (EFTEM) maps of (C) and (D) were recorded. (C) and (D) are oxygen K edge and copper L edge EFTEM maps of reduced Cu/ZnO/ Al2O3, respectively. The scale bars in (B), (C), and (D) are 20 nm. Adapted with permission from ref 165. Copyright 2015 Wiley.

Lately, the conventional understanding of Al2O3 as a support or structural promoter in Cu/ZnO/Al2O3 catalyst has been rescrutinized. Behrens and co-workers demonstrated that trace amounts of Al3+ altered the metal surface area-normalized activity of Cu/ZnO catalysts in methanol synthesis.164 It appeared that a major function of Al was to introduce more defect sites in ZnO by incorporating into its lattice and occupying tetrahedrally coordinated sites, which has been widely utilized in semiconductor fields.166,182−185 As a result, the electronic perturbation of ZnO affected the extent of SMSI with Cu, which in turn led to changes in catalyst reactivity. The effect of doping was confirmed by Ga3+ incorporation in ZnO. These findings suggest a more complicated interaction within the ternary Cu/ZnO/Al2O3 system than that within binary Cu/ ZnO, while meanwhile providing another route to tune the properties of Cu/ZnO-based catalysts. 3.1.3.2. Catalyst Optimization. 3.1.3.2.1. Tuning Preparation Methods. In principle, the performance of Cu-based catalysts is affected by various factors including copper dispersion, metal−support interaction, and Cu oxidation state, etc., and two main strategies have been adopted to optimize their performance: synthesizing structured catalyst by applying J


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Table 4. Effect of Preparation on Catalyst Properties and MSR Reactivitya catalyst Cu/ZnO156 Cu/ZnO156 Cu/ZnO156 Cu/ZnO156 Cu/ZrO2/CeO2157 Cu/ZrO2/CeO2157 Cu/ZrO2/CeO2157 Cu/ZrO2/CeO2157 CC-PCZA198 3MW-PCZA198 5MW-PCZA198 8MW-PCZA198 10MW-PCZA198 Cu/ZrO2158 Cu/ZrO2158 Cu/ZrO2158 Cu/ZrO2158 CZ-350203 CZ-450203 CZ-550203 CZ-650203 CZ-750203 GC-CZ-0.5197 GC-CZ-2197 GC-CZ-4197 GC-CZ-8197 GC-CZ-12197 GN-CZ-4197 OC-CZ197 CC-CZ197 CP-CZA200 HP-CZA200 commercial CZA (G66B)200

preparation method CP CP CP CP CP+T CP+T CP+T CP+T MW MW MW MW MW OGC OGC CP WI OGC OGC OGC OGC OGC GC GC GC GC GC GC CP CP CP HP

SBET (m2/g)

SCu (m2 g−1)

Cu size (nm)

microstrain (%)

11 9.5 7 6.5 9.4 9.9 9.8 10.0 5.4 5.8 6.6 6.9 7.5 21.9 33.2 35.9 48.4 12.1 13.2 14.7 21.3 24.0 28.4 21.7 17.6 12.6 14.9 16.1 24.3 19.1

0.1 0.25 0.55 0.6

96 102 94 83 58.1 59.7 60.6 61.5 57.5 71.5 36.2 64.2 13.1 63 58 43 21 8 28 39 50 56 48 45 39 34 47.3 78.5 70.1

13 16 21 22 0.9 1.8 1.8 1.5 47.3 39.7 36.4 33.1 30.2 18.4 5.0 3.5 1.0 4.0 5.9 8.7 4.8 2.4 3.6 7.4 11.5 19.8 12.9 14.8 7.8 10.4 19.6 24.3 21.0

activity (μmolH2 gcat−1 s−1)

X (%)

yCO (ppm)

35 36 60 57.5

2.12 3.23 4.47 5.57 4.63

900 500 300 200 1800 1500 1200 1000 1100

78.0 80.9 86.2 91.9 87.8 ∼2.8 ∼55.6 ∼69.4 ∼91.7 ∼66.7 ∼69.7 ∼73.1 ∼67.2 ∼50.8 28.6 35.3 43.9 66.1 48.3 59.4 40.8 36.1 145.3 198.9 157.4

1.12 1.35 2.19 2.41 2.25 2.22 1.69 1.58

100 80 70 30 ∼79 ∼82.5 ∼87.5 ∼79.5 ∼60.5 34.0 41.8 52.0 78.8 57.6 70.9 48.6 43.0 77.0 90.1 88.7

1000 1200 1600 2200 2000 2600 2200 1800

a

CP, coprecipitation; CP+T, coprecipitation and template; MW, microwave-assisted synthesis; OGC, oxalate gel coprecipitation; WI, wet impregnation; GC, soft reactive grinding; HP, homogeneous precipitation; ME, microemulsion.

Table 5. Effect of Support and Promoters on MSR Activity catalysts CuO/ZrO2208 Cu−Mn spinel250 Cu/ZnO-CNTs258 Cu/Zn/Al2O3187 (commercial) La2CuO4 nanofibers259 ZnO NR@Cu NP191 Cu/GDC251 Al−Cu−Fe quasicrystal260 rod-like CuO−CeO2253 40-Cu/SiO2261

preparation method CP UNC WI template in situ synthesis CP melting+leaching template sol−gel

T (K)

S/C

523 513 593 533 323 523 513 573 513 523

1.5 1.3 1.5 1.4 1.3 1 1 1.5 1.5 1.5

activity than the conventional coprecipitated catalyst.158,195 In the process, oxalic acid was rapidly added to an alcoholic solution of copper, zinc, and aluminum nitrates to form gel-like MC2O4 (M = Cu or Zn) precipitates, which allowed a facile isomorphous substitution between Cu and Zn in the precursor and thus a homogeneous Cu−Zn interaction. Lorenzut et al. also used an oxalate gel coprecipitation method and reported that some promoters (Ni and Co) could be easily incorporated using this procedure.196 Furthermore, CaO and co-workers simplified this process to develop a solid-phase synthesis of the

X (%) 100 60 100 93 29 40 100 60

SH2 (%)

activity (μmolH2 gcat−1 s−1)

Sco (%)

∼111

sorbitol > glucose (Figure 22).54 Furthermore, the gas streams from APR of the oxygenates were found to contain low levels of CO ( Pt/ZrO2 > Pt/TiO2 ≈ Pt/C.595 The authors found that the activity sequence was closely associated with the interaction between Pt and support as well as the amounts of oxygen vacancies in the supports, indicating that Pt and oxygen vacancies with strong interaction served as the active sites for efficient H2 production. This is in agreement with the results obtained on Pt supported on partially reduced ceria.457 Low-temperature ESR performance of Ir- and Ru-based catalysts has also been reported (Table 11). Siang et al. obtained the hydrogen yield of more than 5.0 mol/molEtOH with insignificant content of CO and CH4 (5.4 5.5

100 70 100 99 80 100 100 >95 31 100

SH2 (%)

24 89

5.5 4.1 4.1 4.3 3.63 4.90 5.4 >5.7 40 30.8 1.7 2.2 4.52 0.9

SCO (%)

67 87.8 42 52

5.3 65 0.4 73.1

50 25 0 11 22 30 ∼50 ∼70 35 6.2 10.1 0 5 9.4 9.0 11 ∼2 98 100 95 100 100 100

YH2 (mol/molethanol)

SH2 (%)

SCO (%)

502

skeletal Ni Ni/Al2O3664 Ni/Al2O3531 Ni-AZ-0.2665 Ni/A3L534 Ni/Ca−Al2O3476 Ni/La2O3487 Ni/MgO545 Ni/Y2O3501 Ni/CeO2666 flowerlike Ni/CeO2667 Ni/ZrO2661 Ni/ZnO668 Ni/CeO2−ZrO2557 Ni/CeO2−ZrO2669 Ni/CeO2−ZrO2560 NiZnAl−Ce670 LaNiOx671 U−LaNiOx nanotubes672 a

24 500 27 000 10 000 78 000 100 000 13.5 g·h/mol (5)

35 75 89

0 4.6 0

60 73

4 6.2

4.11 4.39 >5.4 ∼5.25 4.0 4.95

30

62 4.8 95 75

70.1 2.1 28 8.0

5.66 (6) 70 g·h/mol 22 000 22 000

>95 100 100 100

69.0 89.8 5.41 70.0

3.7 19.4 0 0

In parentheses, WHSV (h−1).

correspondingly inspires various studies dedicated to achieve and stabilize small Ni particles during reaction, and maintain a clean catalyst surface for the efficient conversion of reactants. The performance of various Ni-supported catalysts is summarized in Table 13. 4.1.3.2.2.1. Supports and Promoters. Zhang et al. found that skeletal Ni is highly active and selective for H2 production

useful for Co ensemble size control. For example, Natile et al. stabilized Co through incorporation into a La−Sr−O perovskite structure, while they also reported that substituting a fraction of Co with Fe reduced the Co particle size.641 4.1.3.2.2. Ni-Based Catalysts. Systematic experimental and computational investigations have been conducted to elucidate the reaction and deactivation mechanism of ESR, which AO


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Figure 34. Schematic illustration and TEM images of (a,c) conventional supported Ni/ZrO2 catalyst, and (b,d) nanocomposite Ni@ZrO2; the insets in (c) and (d) are STEM images of reduced catalysts. (e) ESR activity test at 723 K. Ni@ZrO2, solid symbols; Ni/ZrO2, hollow symbols. (f) ESR stability at 873 K; the insets are TEM images of used catalysts. Reprinted with permission from ref 20. Copyright 2014 The Royal Society of Chemistry.

mitigated the acidity of Al2O3, inhibiting ethylene formation and promoting water adsorption facilitating the WGS reaction, but meanwhile weakened the interaction between Ni and Al2O3, resulting in large Ni particles. MgO is also used to modify the Al2O3 support to obtain a MgAl2O4 spinel phase, which could tune the interaction between the active metal and the Al2O3 support, suppress the formation of an inactive NiAl2O4 phase, and stabilize Ni particles.635 However, a high MgO loading (30 wt %) induced the formation of MgO layer over the MgAl2O4 spinel phase and thus weakened the interaction between Ni and supports.654 Frei et al. found that using the MgO support allowed catalysts to work at a high reaction temperature (923 K) with modest coke formation.545

that produced CO-free H2 at 623 K with full ethanol conversion.502 In more cases, the Al2O3 support is used, and commercial Ni/Al2O3 catalysts have shown good performance in industrial SRM process.652 However, a common problem for the Ni/Al2O3 catalysts in ESR is that support acidity promotes ethanol dehydration to ethylene, lowering H2 selectivity and often leading to coke deposition. Some studies focus on modifying Al2O3 with basic promoters such as CaO and MgO.653 Choong et al. found that the catalytic performance of 3 wt % Ca modified-Ni/Al2O3 was much higher and more stable than Ni/Al2O3 at 673 K, whereas catalysts with 5, 7, and 10 wt % Ca addition exhibited activity comparable to that of unpromoted catalyst.476 The introduction of Ca significantly AP


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Figure 35. Enhancement of redox property of ceria by dopant introduction. Reprinted with permission from ref 20. Copyright 2014 The Royal Society of Chemistry.

Figure 36. (a) Production of H2, CO2, and CH4 during ESR over Ce0.8Ni0.2O2−y catalyst. (b) H2 production during ESR over Ni0.1Ce0.9O2−y, Ni0.2Ce0.8O2−y, and 1 wt % Rh/CeO2 catalysts. Adapted with permission from ref 564. Copyright 2010 Wiley.

ZrO2 stabilized by SiO2 exhibited highest H2 production without ethylene production.660 Li et al. synthesized a series of Ni catalysts supported on ZrO2 with different crystalline phase and particle size, and examined their ESR performances at 673 K.661 It was found that tetragonal ZrO2 exhibited better WGS activity but lower activity in methane reforming than monoclinic ZrO2. Additionally, the interaction between Ni and ZrO2 was greatly strengthened by a decrease in ZrO2 particle size, which accounted for higher catalytic activity. In this context, a nanocomposite Ni@ZrO2 catalyst was prepared by a modified hydrothermal method, in which Ni NPs were surrounded by tetragonal ZrO2 support NPs with comparable particle size (Figure 34b and d).662 As compared to conventional Ni/ZrO2 catalyst (Figure 34a and c), the Ni@ ZrO2 nanocomposite possessed smaller Ni particles with stronger metal−support interaction and more abundant surface active oxygen, exhibiting better ESR activity (Figure 34e) and stability during a long-term stability test (Figure 34f). TEM of used catalysts indicated that coke deposition was largely suppressed on Ni@ZrO2 catalyst (Figure 34f, inset), consistent with results from thermogravimetric analysis (TGA, 16.9% and 33.7% weight loss for used Ni@ZrO2 and Ni/ZrO 2 , respectively). Supports with large specific surface area were recently applied in ESR to disperse Ni particles. Rossetti et al. used the flame pyrolysis method to prepare Ni/SiO2 catalyst with a high specific surface area of 211 m2/g, and Costa-Serra et al. applied Mordenite with a specific surface area of 433 m2/g in ESR, and both contributed good activity.499,663 4.1.3.2.2.2. Ni/CeO2 Catalysts. Ni/CeO2 is a highly representative catalyst in ESR reaction, which generally exhibits excellent performance in terms of activity, selectivity, and stability.34 Rodriguez and co-workers focused on understanding

The advantages were attributed to the inhibition of the ethanol dehydration as well as electron enrichment of the supported metal, which contributed to depress the Boudouard reaction. Sánchez-Sánchez et al. reported that incorporation of La to Al2O3 obviously enhanced the stability of Ni catalyst.534 In addition to neutralization of surface acidity, La modification improved the dispersion of Ni and helped with gasification of coke precursors.655 This is supported by the study of Fatsikostas et al., in which H2 yield up to 5.4 mol/molEtOH could be obtained over Ni/La2O3 catalysts, and the catalysts exhibited good resistance to carbon deposition via lanthanum oxycarbonates.487 Note that Cerritos et al. reported that the La2O3 addition to Ni/Al2O3 could not promote the WGS reaction; moreover, some La species migrated and partially covered the Ni surfaces resulting in catalyst deactivation.656 Similar to Ni/Al2O3, Ni/ZrO2 catalysts are also active in steam reforming due to large surface area, good thermal and chemical stability, strong interaction with active phase, and the ability to adsorb and dissociate water, although the acid sites on the amphoteric ZrO2 may lead to dehydration.499,657 Nichele et al. showed that CaO addition effectively reduced the Lewis acidity of ZrO2 and inhibited coke deposition, and was also responsible for the formation of oxygen vacancies.657 Similar results were reported by Bellido et al., and they found that Ni/ ZrO2 modified by Y2 O3 or CaO exhibited higher CO2 selectivity, attributed to an enhanced support oxygen mobility.658 The different crystalline phase of ZrO2 has influence on the catalyst performance. Hou et al. found that Yb incorporation stabilized the cubic ZrO2, inhibiting the transformation of ZrO2 to thermodynamically more stable monoclinic ZrO2, which resulted in catalyst sintering.659 Benito et al. loaded Ni, Co, and Cu on ZrO2 with monoclinic and tetragonal structure, and found that Ni supported on tetragonal AQ


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Figure 37. Rietveld refinement of the ceria lattice strain for a series of fresh W-doped samples with and without Ni loading and (b) sequential Rieveld refinement of the variation in ceria lattice strain under ESR conditions. Reprinted with permission from ref 555. Copyright 2015 Elsevier.

the reaction mechanism over the Ni/CeO2 system and optimizing catalyst performance.524,555,564 The group developed Ce1−xNixO2−y nanocomposite catalysts by a reversemicroemulsion method, which could produce well-dispersed Ni NPs on partially reduced Ni-doped ceria during the ESR reaction.564 The electronic perturbation of ceria on Ni led to suppressed methanation activity, while Ni embedded in the ceria lattice induced the reduction of Ce4+ to Ce3+, generating oxygen vacancies that facilitated activation of water and ethanol. DFT calculations predicted that transition and noble metal ion substitution in CeO2 significantly enhanced the reducibility of Ce1−xMxO2 (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, and Ru).673 The introduction of dopant cations modifies the OSC and OM as well as the redox property (Ce4+ → Ce3+ → Ce4+) by lowering the barrier for oxygen migration and decreasing the activation energy for the reduction of ceria species as depicted in Figure 35.674 Moreover, fundamental studies on Pt/CeO2(111) and Ni/ CeO2(111) model systems revealed that significant electronic metal support interactions between small Pt or Ni particles and CeO2 tranformed the metals into excellent catalysts for WGS and ESR reactions.524 Consequently, the Ni0.2Ce0.8O2−y catalyst showed high H2 yield with minimal CH4 formation at temperatures above 673 K (Figure 36a), and was much more active than the Rh/CeO2 catalyst (Figure 36b). The sharp increase in hydrogen yield over Ni0.2Ce0.8O2−y at 673 K was attributed to the reduction of NiO to Ni, whereas insufficient amounts of Ni over Ni0.1Ce0.9O2−y accounted for the mild increase in activity, as evidenced by time-resolved XRD under reaction conditions. Furthermore, the group observed that codoping Ni and W into the ceria lattice resulted in larger lattice strain than for Ni− Ce and W−Ce (Figure 37).555 A synergistic effect between Ni and W generated a substantial amount of defects and oxygen vacancies, and the obtained catalysts with extremely small Ni NPs (2 nm) on Ni−W−Ce mixed oxides exhibited higher catalytic activity than Ni-doped ceria catalyst. The defects in NixWyCe1−x−y facilitated the dispersion and stabilization of extremely small Ni NPs (∼2 nm) through SMSI, and thus guaranteed a large Ni−ceria interface allowing efficient supply of OH groups to oxidize CHx species and carbon deposits on Ni surface. Studies on reaction pathways showed that the ESR reaction occurred on Ni−CeO2 interfaces, where the CeO2

support facilitated the oxidation of ethoxy to acetate, which was subsequently broken by Ni via the C−C bond cleavage to produce surface-bound carbonate and methyl.454 The interplay between Ni and Ce3+ sites was crucial to catalyst stability because Ni decomposed ethanol but also deactivated through the Ni3C formation and carbon deposition, while OH groups from water dissociation on Ce3+ sites essentially assisted in removal of surface carbon species (Figure 38a).675 The C 1s

Figure 38. (a) ESR reaction over a Ni-CeOx catalyst and (b) C 1s spectra of a 2 L ethanol dose on Ni-CeO1.8(111) surface at 300 K followed by heating to 700 K under a background of 2 × 10−8 Torr pressure of water. Reprinted with permission from ref 675. Copyright 2015 American Chemical Society.

spectrum (Figure 38b) indicates that two C peaks associated with ethoxy species appear at 300 K and transform to Ni3C or coke as the temperature increased, which can be subsequently removed at higher temperatures (700 K) in the presence of water, implying the importance of OH groups. In addition, post reaction studies showed that efficient cleaning of the metal surface in the water-rich environment during ESR favored the formation of filamentous carbon, which led to milder deactivation as compared to encapsulating carbon.454 The enhancement in catalytic performance of CeO2-based catalyst by lattice substitution has been reported in many studies. Incorporation of Zr is commonly used; for example, Biswas et al. doped Zr into CeO2, which greatly enhanced the oxygen mobility, and thus H2 yield as high as 5.66 mol/molEtOH AR


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could be achieved at 873 K.557,669 Zhang et al. showed that insertion of 7 mol % Mg into ceria lattice facilitated the reduction of Ce4+, leading to a maximum oxygen mobility of 4.8 mmol g−1 among Mg-doped ceria with 3−50 mol % Mg.553 The Ni/7MgCe exhibited higher H2 yield and better coke resistance as compared to Ni/CeO2. In addition, enhanced catalytic performances have been achieved by improving the redox properties of CeO2 through the formation of Ce−O−Ti and Ce−O−La solid solutions.556,676 Ribeiro et al. reported that doping CeO2−SiO2 with Mn not only facilitated the formation of mobile oxygen to promote the oxidative dehydrogenation of ethoxide to acetate, but also lowered surface basicity to decompose carbonate more readily.677 4.1.3.2.2.3. Catalysts with Encapsulated Structures. Confining/embedding metal NPs in well-defined cavities or channels, such as core−shell and core−sheath structures, is an alternative strategy to conventional supported catalysts.21 The confinement environment not only provides a high surface area for fine metal dispersion, but also exerts a spatial restriction on metal NP against sintering. In addition, the enhanced metal− support interaction, especially induced by the use of particular metal−support pairs, further contributes to stabilize metal clusters. Constructing such structured catalysts with the confinement effect via a proper synthetic method attracts much research interest, and relevant development has been nicely presented in several reviews.20,678−680 In the case of base metals, primarily Ni and Co, synthetic mineral-derived catalysts with a short-range ordered structure represent a typical class of catalytic materials that effectively protect metal particles from sintering via confinement effect.20 The synthetic mineral compound precursor includes LDH, phyllosilicates (PS), perovskites, and spinels.480,681−683 Synthesis of such compounds generally involved a solid-phase crystallization process, during which metal ions are homogeneously incorporated within the bulk structure. These precursors then were calcined and reduced upon use to finally achieve the structured Ni catalysts. Note that elevated reduction temperatures are often required for catalyst activation due to the presence of SMSI widely found in these materials.20 LDH-derived Ni catalysts, as discussed above in MSR, retain their lamellar structure with thermally stable metal NPs embedded in the metal oxide matrix. The LDH-derived Ni/ MgO/Al2O3 catalysts featured large surface area (e.g., 100−300 m2/g), high metal dispersion, and basic characters, therefore resulting in good catalytic performances in ESR.532,684−686 Moreover, the ion-exchangeable characteristic of HTLc allows tuning the catalyst composition for performance optimization: (i) addition of a second metal component such as Co, Cu, and Fe to form bimetallic catalysts;687−689 and (ii) substituting the ions that transform to the metal oxide component in the resultant catalyst to obtain a multifunctional catalyst. For example, Assaf et al. doped La and Ce into the Ni−MgO− Al2O3 LDH structure by anion exchange to enhance water adsorption and promote H2 production.690 Wu et al. synthesized the Ni−CaO−Al2O3 catalysts derived from LDH to achieve excellent in situ CO2-sorption ability in ESR, resulting in high-purity H2 production.691 Nickel phyllosilicate (Ni-PS) is a novel class of clay minerals with highly dispersed exchangeable Ni cations, large surface area, and neutral character.20 Zhang et al. reported that lamellar structure Ni-PS prepared by ammonia evaporation (Figure 39a and b) possessed small Ni particles with narrow size distribution and SMSI, thus exhibiting significantly higher

Figure 39. TEM images of (a) lamellar Ni-PS, (b) tubular Ni-PSn, (c) reduced lamellar Ni-PS, and (d) reduced tubular Ni-PSn; (e) ESR activity of Ni-catalysts at 673 K; and (f) ESR stability test of Ni-PSn at 773 K (inset: TEM image of used Ni-PSn and TGA analysis). Adapted with permission from ref 20. Copyright 2014 The Royal Society of Chemistry.

ethanol conversion and H2 production than conventional supported Ni/SiO2 (Figure 39e).480 Moreover, through a hydrothermal synthesis process, the lamellar PS could be crisped into a tubular PS nanotube (PSn) structure, which further stabilized the lamellar structure and offered a welldefined tubular structure for Ni confinement (Figure 39b and d).692−694 The enhanced SMSI and abundant surface hydroxyls in the Ni-PSn catalyst led to high catalytic performance (Figure 39e), and, more importantly, stable ESR reaction for as long as 100 h (Figure 39f). Recently, Kawi and co-workers successfully employed yolk−shell structured Ni-PS catalysts in the dry reforming of methane due to its superior thermal stability and antisintering ability, indicating that PS-based catalysts are promising for high-temperature reforming reactions.695−697 Perovskites, represented by a general formula ABO3, are a series of mixed oxides with the same typical crystal structure as calcium titanium oxide (CaTiO3).679 In the ABO3 structure, A is an alkali earth or rare earth element and B is typically a transitional metal, both of which could be partially substituted by cations of similar oxidation state and ionic radius.698 LaNiO3 is a well-studied perovskite-typed oxide, and Ni/La2O3 catalysts derived from it showed stable H2 production in ESR, attributed to its strong sintering resistance and coke gasification ability via a La2O3/La2O2CO3 transformation cycle.683,699 Wang and coworkers reported that LaNiO3 perovskite catalysts were very robust for low-temperature ESR, exhibiting complete ethanol conversion and 70% H2 selectivity at 623 K for 80 h without any sign of deactivation.671,683 Nevertheless, a challenge to the use of perovskite-derived catalysts is their small specific surface area, which is generally an order of magnitude lower than those of the LDH- and PSderived counterparts.700 For the same reason, spinel-derived catalysts are less studied for ESR. Consequently, many AS


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nanotubes (CNTs) was found in the reduced and used samples, indicating severe Ni agglomeration. It has been reported that Ni particles could diffuse out of the mesoporous channels to the outer surface of the silica support at elevated temperatures during reduction and reaction, due to the inherently weak interaction between nickel NPs and the silica support, resulting in the loss of the confinement effect and the subsequent sintering.711,717 In addition, Nichele et al. demonstrated that poor hydrothermal stability of SBA-15 was responsible for this phenomenon because of the gradual partial collapse of the mesoporous structure during the reforming process, which would hinder the exposure of active Ni surface and lose its confinement ability against Ni sintering.718 To address these issues, Gong and co-workers used a ceria promoter to anchor small nickel clusters in the channel of SBA15 (Figure 41a and b).719 The spatial restriction of mesoporous

researchers focus on the preparation of large surface perovskite materials, such as dispersing perovskite onto other support materials. Li and co-workers coated LaNiO3 onto LDH-derived Mg−Al mixed oxide to achieve a shell-like structure with high BET surface area (132 cm2 g−1).701 An effective synthesis process was presented by Nair et al., as they nanocasted the LaNiO3 onto SBA-15 as a hard template, and eventually obtained a mesoporous LaNiO3 with large surface area (150 m2/g) after removing the SBA-15 by NaOH treatment.702 The Ni/La2O3 catalyst derived from nanocasted perovskite stayed active in DRM after 50 h TOS at 973 K with negligible coke formation. Since their discovery in the early 1990s, ordered mesoporous materials (OMMs) have been a very successful class of materials in heterogeneous catalysis because they offer many excellent properties that could lead to exceptional catalytic performances in many reactions, as listed in Figure 40.703−706

Figure 40. Properties that OMMs offer to achieve excellent catalytic performances in various reactions. Reprinted with permission from ref 706. Copyright 2012 Elsevier.

Figure 41. (a) Reduced and (b) spent NiCe/SBA-15 catalysts; (c) reduced and (d) spent NiCe/SBA-16 catalysts. Adapted with permission from refs 717 and 719, respectively. Copyright 2015 Elsevier and 2013 American Chemical Society.

Particularly, these OMMs provide confinement environment for metal NPs, which is highly desired for preparing sinterresistant catalysts with high and uniform metal dispersions in reforming.680,707 Their large surface area and multiple, tunable pores enable a facile incorporation of metal NPs and promoters by simple impregnation or grafting processes.708−710 Ordered mesoporous silica, most representatively the famous MCM41/-48 and SBA-15/-16, possesses a series of unique properties such as high specific surface area, neutral character, long-range ordering of mesopores, and thick pore walls with high hydrothermal stability, and is therefore considered as an excellent catalyst host in the harsh steam reforming ́ et al. showed that reactions.706,711,712 Studies of Vizcaino MCM-41- and SBA-15-supported bimetallic CuNi catalysts exhibited better H2 yield and lower coke deposition than commerical Al2 O 3 -supported sample, with the best H 2 selectivity obtained on CuNi/SBA-15 due to the highest metal dispersion.713,714 Kang’s group reported that NiZr/ MCM-48 delivered stable H2 production close to 80% at a low S/C ratio (0.5) and a high temperature (1023 K) for more than 25 h, and catalyst was deactivated at low rates to keep a final production of 70% after 60 h TOS.715 The group also reported that NiW/SBA-15 maintained a H2 yield of 80% after 80 h in the long-term stability test.716 However, for both catalysts, gigantic Ni bulk accompanied by large amounts of carbon

silica support, together with the strong Ni−Ce interaction, effectively stabilized the nickel NPs (5.2 nm after 50 h TOS) during the long-term stability test. Meanwhile, CeO2 addition effectively assisted in water activation and suppression of coke deposits. Alternatively, Zhang et al. used a three-dimensional cubic SBA-16 with cage-like mesopores to achieve additional immobilization for Ni and ceria nanoclusters in DRM (Figure 41c and d).717 Lindo et al. reported that incorporation of Al into the structure of SBA-15 led to stronger MSI and stable ethanol conversion, but the generated acid sites resulted in high dehydration activity, lower H2 selectivity, and more coke formation as compared to bare Ni/SBA-15.720 Vizcaino et al. showed that CaO and MgO promoters stabilized small Cu−Ni crystallites in SBA-15, and their basic character was helpful for coke formation control.721,722 Interestingly, the carbon deposits on modified Ni/SBA-15 turned out to be nonencapsulating carbon nanofibers and multiwalled nanotubes (hollow fibers), whereas carbon deposits on bare Ni/SBA-15 had higher graphitization degrees and encapsulated a large proportion of ́ et al. demonstrated that smaller Ni Ni particles.723 Vizcaino particles and a stronger metal−support interaction impeded the carbon nucleation of tip-growth fiber and favored the baseAT


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Figure 42. (a−c) ESR performances for supported 0LaNiAl, ordered mesostructured meso-0LaNiAl, and La2O3-modified meso-3LaNiAl catalyst, respectively; reaction conditions: 873 K, 1 atm, W/F ratio = 2.5 g h mol−1, S/C = 4, 4 vol % ethanol in gas; (d) TG and DTG (inset) profiles for spent catalysts. Reproduced with permission from ref 727. Copyright 2016 Elsevier.

growth pathway, accounting for the improved catalytic performance.724 As compared to silica-based OMMs, ordered mesoporous alumina is comparatively less explored due to its surface acidity. Song’s group developed a series of active mesoporous Ni− Al2O3−ZrO2 xerogel catalysts for ESR, in which ZrO2 served to suppress surface acidity.725,726 Very recently, we prepared ordered mesoporous alumina with Ni embedded in the framework via a one-pot evaporation induced self-assembly method, which upon reduction generated smaller Ni particles (5.0 nm) as compared to conventional supported counterpart (9.6 nm).727 As a result, the mesostructured Ni exhibited higher H2 production for 50 h TOS (Figure 42). Modification by 3 wt % La2O3 addition further promoted the reduction of Ni from the alumina skeleton, mitigated the acidity of Al2O3, and stabilized the mesostructure, therefore demonstrating the highest activity, stability, and minimum coke deposition. Similarly, zeolites have been used to accommodate Ni and Co NPs, but passivation of zeolite acidity through ion exchange of alkali metal ions (Na+, K+, and Cs+) is often necessary.663,728

Figure 43. Carbon-formed and carbon-free regions at different pressues in EDR. Reprinted with permission from ref 112. Copyright 2009 Elsevier.

conditions, the reverse Boudouard reaction is thermodynamically favorable, while the excess of CO2 facilitates the fast removal of CHx species. The main coke precursor resulted from ethanol decomposition, via DRM reaction, which is also favored at elevated temperatures. In addition to thermodynamic limitations, suppressing carbon formation via kinetic control by the design of appropriate catalysts is important. Investigations upon the reaction methanism of ethanol dry reforming by far are limited; however, many studies of DRM suggest that dry reforming reaction proceeds dependently on the nature of catalyst support: a monofunctional mechanism occurs when inert materials (e.g., SiO2) are used where both methane and CO2 are dissociated solely on metal, while a bifunctional mechanism takes place when metal is loaded on basic (e.g., MgAl2O4, La2O3, CeO2) and acidic materials (e.g., Al2O3), in which CH4

4.2. Dry Reforming

The addition of CO2 to the ethanol-steam feed has been studied in low-temperature ESR to facilitate the reverse of the Boudouard reaction and thus suppress carbon formation.729 When CO2 replaces H2O to be cofed with ethanol, dry reforming occurs. However, thermodynamic analyses (Figure 43) showed that carbon formation was highly favorable even at high temperatures with stoichiometric feed (CO2/ethanol molar ratio = 1).112 Hence, some researchers have been focused on sequestering atmospheric carbon in the form of carbon nanofilaments in the presence of carbon steel catalyst through this process.730,731 For syngas production, high CO2/ethanol ratios and sufficiently high temperatures are required to diminish carbon deposition and thus alleviate catalyst deactivation.13 At such AU


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and CO2 are activated on metal and support, respectively.733 A possible mechanism was proposed by Noronha and co-workers based on DRIFTS results over Rh/CeO2 catalysts.732 The mechanism is similar to that of ESR with CO2 replacing H2O as the source of oxygen. Considering that reactants competing for adsorption sites could decrease reaction rates, together with undesired side reactions on acidic supports (e.g., dehydration), bifunctional catalysts involving active support materials with basic character are preferred.729,734 Selection of active metals mainly includes noble metals Rh, Pt, Ir as well as base metals Ni and Co.733,735,736 Noronha and co-workers reported that Rh/ CeO2 catalysts severely deactivated at 773 K and a CO2/ ethanol molar ratio of 1, but exhibited very stable performance for over 25 h TOS when the temperature was increased to 1073 K.732 Water was the only byproduct, and its formation led to CO/H2 ratios below unity due to reverse-WGS reaction. As shown in Scheme 14, Rh facilitates ethanol dehydrogenation

5. POLYOLS REFORMING A polyol is an alcohol with multiple hydroxyl functional groups. Among them, ethylene glycol (EG, C2H6O2) and glycerol (propane-1,2,3-triol, C3H8O3, also known as glycerin or glycerine) are two of the simplest polyols and important model compounds for polyols as they share the same functionalities including C−C, C−O, C−H, and O−H bonds as well as OH groups on adjacent carbon atoms. Recently, ethylene glycol and glycerol have been studied as promising feedstocks for hydrogen production via reforming due to their weak toxicity, nonvolatility, nonflammability, and compatibility with current infrastructure for transportation and storage.15,741,742 Moreover, such simple polyols are widely available: ethylene glycol can be obtained from biomass, sugars, and sugar alcohols and is the most abundant derivative from the direct catalytic conversion of cellulose, which makes it a sufficient and sustainable feedstock for industrial processes.56,743,744 In terms of the source, glycerol can be classified into two categories. Synthetic glycerol is mainly produced from propylene via the epichlorohydrin process, which is no longer in operation. Currently, the most important source of glycerol is as the byproduct of biodiesel production, which is manufactured via transesterification of triglycerides, the main constituents of vegetable oil and animal fats. The byproduct glycerol (crude glycerol) represents 10 wt % of biodiesel production. Recent annual biodiesel production in the U.S. has reached 1160 million gallons, according to the U.S. Energy Information Administration.745 As the biodiesel industry is rapidly expanding, an oversupply of byproduct glycerol is emerging. It is predicted that by 2020, the crude glycerol production would be 6 times more than its demand.746 The crude glycerol contains several other constituents including glycerides, methanol, moisture, fatty acids, and inorganic salts, which make it expensive for refining. Moreover, the impurities in crude glycerol limit its direct application such as combustion.747 Consequently, the storage and disposal of the glut of glycerol have become a threat to the economical and environmental feasibility of the biodiesel industry. The utilization of glycerol to produce value-added secondary products would be beneficial for the development of the biodiesel industry. To date, intensive research efforts have been devoted to finding new outlets for the oversupplied glycerol, among which conversion of glycerol via catalytic reforming processes to produce renewable hydrogen or syngas is a practical and promising pathway.24,27,747−751 This pathway not only assists in consuming the superfluous glycerol but also lowering the cost for H2 production. In this section, common catalytic reforming techniques will be introduced and assessed for H2 production using EG and glycerol as model compounds for polyols.

Scheme 14. Proposed Mechanism for EDR over Rh/CeO2 Catalystsa

a

Reprinted with permission from ref 732. Copyright 2011 Elsevier.

and the C−C bond cleavage, while efficient activation of CO2 on CeO2 at vacancy sites supplies abundant oxygen species. Moreover, Rh could offer additional O adatoms to oxidize surface carbon due to its ability to dissociate CO2, accelerating the removal of carbon species from surface.737 Kawi and coworkers also identified a Rh/Ce-SBA-15 catalyst as a promising candidate for syngas production via EDR.738 Supported Ni/ Y2O3−ZrO2 catalysts exhibited good performance in EDR, calling for more investigations on Ni-based catalysts.739 Recently, a bimetallic Ni−Fe/MgAl2O4 catalyst was reported to be highly coke-resistant for DRM.740 The Mars−Van Krevelen mechanism prevailed over the catalyst, where CO2 induced partial segregation of Ni−Fe alloy to form FeOx, which then oxidized the surface carbonaceous species derived from dehydrogenation of absorbed CH3 on Ni to produce CO, and finally the Ni−Fe alloy was restored to complete the redox cycle.740 As a result, a dramatically reduced amount of carbon deposits was found on Fe−Ni catalysts as compared to the pure Ni catalyst (5.6 molC/molmetal for Fe/Ni molar ratio of 0.7 vs 244.6 molC/molmetal, respectively). A similar strategy could also be applied for ethanol conversion to develop robust Ni catalysts for EDR reaction.

5.1. Steam Reforming

Steam reforming is an effective way to convert ethylene glycol and glycerol into hydrogen-rich gas products as shown in eq 57 and eq 58, respectively:11,752,753 C2H6O2 + 2H 2O → 2CO2 + 5H 2 ° ΔH298 = 85.9 kJ mol−1

(57)

C3H8O3 + 3H 2O → 3CO2 + 7H 2 ° ΔH298 = 127.7 kJ mol−1

AV

(58) DOI: 10.1021/acs.chemrev.6b00099 Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 15. Probable Reaction Pathways for H2 Production via SR of Glycerola

a


for SR of EG and glycerol are summarized in Table 14 and Table 15, respectively.

Generally, the reactions are considered to proceed via two stages: initial C−C bonds cleavage of the polyols’ backbone to yield a combination of H2 and CO followed by WGS reaction to produce H2 and CO2.754,755 These reactions are strongly endothermic and produce only H2 and CO2 ideally. In real scenarios, SR of polyols has a complex series of steps that involve multiple reactions and is often accompanied by side reactions. Consequently, byproducts such as CO, CH4, and coke are formed and end up in the outflow stream. 5.1.1. Reaction Mechanism. On supported catalysts, a bifunctional mechanism similar to those in MSR and ESR has been proposed.756,757 Scheme 15 illustrates major reactions that occur in the glycerol SR process. The first step of reforming would involve an initial dehydrogenation of glycerol via the dissociation of the O−H bond or C−H bond, which leads to 1,3-dihydroxy-2-propanone (dihydroxyacetone) or 2,3-dihydroxy-propanal (glyceraldehyde) absorbed on metal surface as key intermediates, respectively, together with molecular H2. Starting from this point, two different pathways may take place. Pathway I involves the dehydration of 1,3-dihydroxy-2-propanone followed by dehydrogenation to propanal-2-oxo, which undergoes C−C bond cleavage to yield ethanal (acetaldehyde). Further hydration and subsequent dehydrogenation of ethanal produce AcOH. Finally, AcOH decomposes to yield a mixture of H2, CO, CO2, and CH4. In pathway II, glyceraldehyde undergoes decarbonylation leading to CO species absorbed onto metal surfaces and 1,2-ethanediol (ethylene glycol), which subsequently dehydrogenates to H2 and OCCO intermediate. The latter then dissociates into absorbed CO species via the C−C bond scission. Finally, adsorbed CO species reacts with H2O to produce H2 and CO2, while CO may also desorb directly into the gas phase. The mechanism of glycerol SR is determined by the combination of active metals, catalyst supports, reaction conditions, etc. 5.1.1.1. Catalyst Development. Much emphasis has been placed on developing catalysts that exhibit high activities toward SR and WGS reactions and inhibit side-reactions such as methanation simultaneously. Noble metals including Pt, Pd, Ir, Rh, and Ru and non-noble metals (Ni and Co) catalysts have been investigated for polyol SR.758−761 Representative catalysts

Table 14. Representative Catalysts for SR of EG sample Ni/ Al2O3792 Ni/γAl2O3753 Ni/ MgO753 Ni/ CeO2753 Ni/ ZrO2753 Pt/γAl2O3775 Pt/C775 Pt/ TiO2775 NiPt/γAl2O3775 NiPt/C775 NiPt/ TiO2775

metal loading (wt %)

T (K)

P (bar)

S/ C

feed flow

X (%)

YH2 (%)

30

673

1

9

10.1a

96.7

89.6

15

673

1

3

10b

96.5

47.1

b

15

673

1

3

10

92.9

64.2

15

673

1

3

10b

96.0

47.5

15

673

1

3

10b

93.9

50.3

1.7

503

1

0.028c

∼5

∼60d

1.7 1.7

503 503

1 1

0.028c 0.028c

∼10 ∼5

∼70d ∼78d

Ni: 1.5, Pt: 1.7 Ni: 1.5, Pt: 1.7 Ni: 1.5, Pt: 1.7

503

1

0.028c

∼10

∼85d

503

1

0.028c

∼10

∼75d

503

1

0.028c

∼10

∼10d

Liquid hourly space velocity (LHSV, h−1). bW/F (gcat h mol−1). Flow rate (mL/min of 40 vol % EG in water). dH2 selectivity.

a c

5.1.2. Noble Metal-Based Catalysts. Rh has been widely examined as the active metal for SR due to its superior activity toward C−H and C−C bond cleavage.541,762−764 Suzuki and co-workers performed SR of glycerol over various supported metal catalysts and found the order of activity was as follows: Rh ≈ Ru > Ni > Ir > Pt ≈ Pd > Co > Fe.758 Chiodo et al. examined the catalytic activity of Al2O3-supported Rh and Ni catalysts to produce syngas for fuel cell systems. Rh/Al2O3 was more robust than Ni-supported catalysts but still suffered from coke formation.765 Moreover, catalyst screening by Adhikari et al. displayed that Rh/CeO2−Al2O3 performed best as compared to Pt, Rh, Ru, and Ni.759 AW


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Table 15. Representative Catalysts for SR of Glycerol catalyst Rh/Al2O3765 Rh/CeO2/Al2O3759 CZCoRh793 (Ce2Zr1.5Co0.47Rh0.07O8−δ) Ru/Y2O3758 Ru/Y2O3766 Ru/Ce0.5Zr0.5O2766 Ru/Al2O3766 Ru/MgAlO794 Pt/Al2O3768 Pt/Al2O3756 Pt/SiO2756 Pt/SiO2757 Pt/ZrO2756 Pt/CeO2/Al2O3795 Pt/La2O3/Al2O3795 C11-NK781 (commercial catalyst)759 Ni/MgO796 Ni/TiO2796 Ni/CeO2796 Ni/SiC784 Ni/MgO/Al2O3654 (MgO = 3 wt %) Ni−Mg−Al797 (NiO wt % 24.1, MgO wt % 26.1) Ni/CeO2798 La0.3Ce0.7NiO3786 La0.5Ca0.5NiO3700 Ni/SBA-15799 Ni/Mg/SBA-15723 Ni/Ca/SBA-15723 Ni−Cu−Al789 (NiO 29.2 wt %, CuO wt % 31.1) Ni−Pt/CeO2−Al2O3791 Co/Al2O3782 Co−Ni/Al2O3800 a

metal loading (wt %) 5 2.5 3 1 1 1 0.6 0.5 1 1 2 1 3 3 2.5 15 15 15 10 10 11.6

10 6.7 6.5 Ni, 5; Pt, 0.3 15 Co, 5; Ni, 10

T (K)

S/C ratio

X (%)

SH2 (%) or YH2 (%)

923 1173 923 873 873 873 873 823 1123 623 623 723 623 773 773 1123 1173 923 923 923 673 873 923 873 873 823 923 873 873 923 973 823 798

3 3 3 3.3 4 4 4 15.3 2.5 15.3 15.3 15.3 15.3 4 4 2.1 2 2 2 2 3 3 3 4 8 3 15.3 2 2 3 2

∼90

(71)a 71 (83) ∼90 68.3 71.8 62.1 (96) ∼70 61.1 69 70 62.2 60 60 (74) ∼80 65.6 62.2 66.7 H2/CO = 1.8 (77) 78.5 74.7 85 (∼85) (87) 53.2 53.0 92.9 (68.4) 65 65

4

100 ∼100 20b 27b 8b 96 ∼100 7.5 100 100 15.5 100 100 80 100 98 72.3 100 79 88 99 100 100 100 98.7 98.5 90.9 98 60

In parentheses: H2 yield (%). bTOFH2.

70% of that of pure glycerol due to the difficulties in reforming long-chain fatty acid impurities.768 To further reduce the energy consumption of steam reforming of glycerol, Dumesic and co-workers have made great efforts to develop Pt-based catalysts that are able to convert glycerol into synthetic gas at temperatures from 498 to 620 K, which are significantly lower than that for conventional gasification of biomass (e.g., 800−1000 K).769 Catalytic tests showed that Pt supported on Al2O3, ZrO2, CeO2/ZrO2, and MgO/ZrO2 all suffered from deactivation at 623 K during the reactions, whereas Pt/C catalyst showed stable conversion of glycerol into syngas (H2/CO = 1.3) for more than 30 h on stream. Moreover, they reported that upon the addition of an equimolar amount of Re to Pt/C (i.e., Pt−Re/C), the resulting catalyst was an order of magnitude more active than the Pt/C catalyst.770 The promotional effect of Re has been extensively investigated and discussed on the basis of both experimental and theoretical studies in the APR of oxygenates.771−774 In brief, Re addition favors the formation of Pt−Re alloy and weakens the binding energy of CO to neighboring Pt sites, increasing the rates of WGS and thus allowing the catalyst to operate at high rates.773 SR of EG was also performed to compare NiPt bimetallic catalysts with monometallic nanoclusters (i.e., Ni and Pt) on different supports (i.e., γ-Al2O3, TiO2, and carbon) by Chen

Suzuki et al. initiated the study on Ru-based catalysts, as they prepared a series of supported Ru catalysts using Y2O3, ZrO2, CeO2, La2O3, SiO2, MgO, and Al2O3 as supports, among which Ru/Y2O3 exhibited the best performance but still underwent sintering.758,766 The same group also investigated the promotional effects of second metal dopants, but limited improvements in TOFH2 were observed over bimetallic Ru−Me (Me = Ni, Co, Mo) catalysts. Notably, the formation of surface MoO3 species hindered the sintering of Ru clusters. Combining the antisintering effect of the Mo dopant and the anticoking ability of the Y2O3 support, Ru−Mo/Y2O3 was claimed as the best performing catalyst among the samples examined. DFT calculations of glycerol decomposition on Pt(111) revealed that there existed a competition between C−C bond breaking and C−O bond breaking, where the selectivity of C− C bond scission was relatively higher.767 Thus, the preference in the C−C bond cleavage over the C−O bond cleavage disfavored the formation of undesired hydrocarbons and therefore promoted H2 production. Slinn et al. performed SR of glycerol over Pt/Al2O3 to investigate the feasibility of utilizing byproduct glycerol stream of a biodiesel plant via the SR process, and found that at high temperature (1123 K), SR of pure glycerol reached almost 100% gas yield and 70% selectivity, whereas the yield of byproduct glycerol was only AX


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and Vlachos.775 Bimetallic NiPt catalysts were more active than their parent metal catalysts over each support, consistent with the predictions from surface science studies.55,776,777 NiPt/γAl2O3 suffered from some activity loss probably due to surface structural changes, which was also observed under the APR conditions employing in situ EXAFS.778 TiO2 and carbon exhibited excellent stabilities as supports for bimetallic catalysts; nevertheless, TiO2 increased the formation of C2 oxygenates such as ethanol and acetic acid. The overall yield for H2 generation identified NiPt/C as a promising catalyst. The robust gas-phase conversion of polyols into syngas is quite promising at low temperatures. Note that F−T synthesis is typically operated in the temperature range of 470−550 K. Therefore, the endothermic conversion of glycerol into synthesis gas can be combined with exothermic F−T synthesis to provide efficient and low energy-intensive routes for the production of liquid fuels.769 Dumesic and co-workers designed a simple two-bed reactor system consisting a Pt−Re/C reforming catalyst followed by a Ru/TiO2 Fischer−Tropsch catalyst to couple the steam reforming of glycerol with F−T synthesis for liquid fuels production.779 The integrated catalytic system was carried out efficiently, perhaps synergetically, over a wide range of glycerol feed concentrations (25%−85%) and pressures up to 17 bar. This process represents a viable, energyefficient alternative for producing liquid fuels from petroleum. Similarly, by substituting the F−T synthesis catalyst with an active WGS catalyst such as Pt/CeO2/ZrO2, the two-bed reactor system can be tuned for H2 production.780 It was reported that the modified integrated system displayed almost complete carbon conversion of concentrated glycerol feed (30− 80 wt %) with a H2 yield up to 80% of the calculated equilibrium at 573 K after 40 h on stream. 5.1.3. Non-Noble Metal-Based Catalysts. The pioneering work by Czernik et al. converted crude glycerol to hydrogen-rich gas using a commercial nickel-based naphtha reforming catalyst (denoted as C11-NK).781 Catalyst screenings by Fernando and co-workers showed that Ni/Al2O3 catalysts possessed the highest activities for H2 generation as compared to other Al2O3-supported platinum group metal catalysts at all temperatures investigated in the study (873−1173 K) and the highest glycerol conversion at 1173 K.759 Adesina’s group developed active and selective Co-based catalysts toward H2 production via SR of glycerol. Over Co/Al2O3, they acquired near-stoichiometric values of H2:CO2 (2−2.30) with relatively high H2:CO ratios (6−12), depending on the feed composition (30−60 wt %).782 Thermogravimetric analysis suggested the existence of two types of carbonaceous species.783 A small portion of residue was reactive toward H2 and O2, whereas a larger reservoir could be only removed by O2. According to TGA-MS and FTIR spectra results, the former could be attributed to atomic carbonaceous species, while the latter was primarily polymeric species derived from dehydro-polycondensation of CxHy fragments. Kim et al. utilized a neutral SiC support because it inhibited the coke deposition caused by the C−C bond scission of the intermediates via dehydration over acidic sites and condensation over basic sites.784 Moreover, due to the inactivity toward the WGS reaction of SiC, the produced syngas with a H2/CO ratio up to 1.9 was more appropriate for F−T synthesis. Fernando et al. reported in their catalyst screening studies that all catalysts including Ni, Ir, Pd, Pt, and Ru showed better performances on CeO2-modified Al2O3 support than on unmodified Al2O3.759 The promotional effects of ceria have

been extensively explored, increasing the reducibility of Ni and avoiding the phase transition of γ-Al2O3 to α-Al2O3, which was thermodynamically more stable at high temperatures. A similar promotional effect was reported for CeO2-modified Ni/Al2O3 in methane dry reforming and La2O3-modified Ni/Al2O3 in ESR.450,547,785 Furthermore, perovskite-typed catalysts have also been applied in polyol reforming reactions. Rihko-Struckmann et al. synthesized partially Ce-substituted La1−xCexNiO3 by the coprecipitation method, and showed that reduced La1−xCexNiO3 catalysts exhibited activity comparable to that of Pt catalysts.786 Fraga et al. correlated the degrees of La substitution by Ce with the amount of carbonaceous deposits on the used catalysts.787 As shown in Figure 44, the sample

Figure 44. Correlation between the amount of coke deposition on used catalysts and degree of La substitution by Ce in the perovskitetype mixed oxides. Reprinted with permission from ref 787. Copyright 2014 Elsevier.

with a 50% La substitution was more coke-resistant as compared to other substitution degrees.788 It was proposed that this behavior was associated with the formation of CeO2− La2O3 solid solutions serving as an oxygen buffer and therefore facilitated coke gasification, while the composition of the solidsolution was critical to its OSC. The use of metal promoters has also been reported. For example, Cu is used to eliminate large ensembles of Ni atoms that are favored for coke deposition and promote WGS ́ et al. reaction.789 Similar results were also observed by Vizcaino in ESR.790 Assaf and co-workers used Ir, Pt, Pd, Ru additives to Ni/CeO2−Al2O3. All modified samples boosted H2 yield, particularly over Ni−Pd and Ni−Pt bimetallic catalysts.791 TPR and XANES analyses indicated that noble metals modified the reducibility of surface Ni species and stabilized Ni during SR by H2 spillover. Additionally, the neighboring noble metal atoms could alter the electronic properties of the Ni surfaces by shifting the d-band center closer to the Fermi level, leading to changes in the adsorption energies of reaction intermediates. This is consistent with the surface science studies on bimetallic Pt-based surfaces in EG reforming by the Chen’s group, in which they correlated the reforming activity with the d-band center of the surfaces and observed a linear trend that activity AY


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SESR of glycerol.804,812,813 However, better multicycle performance was observed for dolomite.814−816 5.1.3.2. Oxidative Steam Reforming. Cofeeding oxygen has also been examined by Dauenhauer and co-workers to couple partial oxidation to provide heat for reforming reactions internally. In this case, the need for external heat input was eliminated (eq 60).817

increased as surface d-band center moved closer to the Fermi level.55,776,777 5.1.3.1. Sorption-Enhanced Steam Reforming. Researchers have been dedicated to lowering the heat requirement for the energy-intensive SR process through CO2 capture processes.752,801 The exothermic carbonation generates extra heat to propel the reforming reactions and shifts WGS equilibrium toward H2 evolution. The SESR could provide high purity of H2 (above 99%), while meanwhile eliminating the formation of CO. For example, the SESR of glycerol (eq 59) can be summarized as follows:

C3H8O3 + x H 2O + yO2 → 3CO2 + z H 2 ° ΔH298 = 0 kJ mol−1

It is reasonable to assume that the active catalysts for conventional SR can be applied in OSR as well as ATR. Schimdt and co-workers examined the ATR of several oxygenates including glycerol and EG over Pt- and Rh-based catalysts supported on alumina foams at millisecond contact times.818 Higher H2 selectivity was observed over Rh-based catalysts as compared to Pt-based counterparts, which was attributed to the stronger surface oxygen bond to Rh than Pt.747,819 The strong binding of oxygen on Rh suppressed glycerol decomposition, whereas decomposition occurred on Pt surfaces leading to nonequilibrium products such as acrolein and hydroxyacetone. A long-term stability test of 450 h was performed over the Rh−Ce catalyst by Schimdt’s group, where high-yield syngas production was maintained for 75 h followed by slow deactivation.820 Deactivation analysis revealed that poisoning and thermal sintering were the main reasons for the activity loss. Poisoning by impurities could be resolved by purification of the feedstock. However, deactivation via thermal sintering could be a significant impediment to the commercial implementation of Rh−Ce catalyst due to the high expense of Rh. However, several alternatives can possibly tackle this problem: (i) incorporation of Rh into support that is capable of stabilizing it against deactivation;821 (ii) alloying Rh with cheaper metals such as transitional metals to minimize Rh content;822 and (iii) replacing Rh-based catalysts by other catalytic materials including Ni-based catalysts and perovskite.823−825 Liu et al. utilized BASF dual layer monolith catalysts in ATR of glycerol, in which the catalytic partial oxidation Pt catalyst layer was in direct contact with the steam reforming Rh/Pt catalyst layer next to the monolith wall, allowing the two coupled reactions to occur simultaneously in a single reactor (Figure 45).826 The honeycomb structure of the monolith catalyst possessed thousands of gas flow passages with numerous microchannels, which enhanced the mass and heat transfer dramatically, thus enabling the use of high gas hourly

C3H8O3 + 3H 2O + 3CaO → 3CaCO3 + 7H 2 ΔH ° = −407 kJ mol−1

(60)

(59)

It is noteworthy that the overall reaction of SESR is strongly exothermic and thermodynamically favorable at low temperatures, which significantly differs from the conventional endothermic SR reaction. Thus, it is possible to reduce heat usage and improve the energy efficiency. To evaluate the feasibility of this integrated process, systematic thermodynamic analyses have been conducted by several research groups.22,46,802−804 He et al. pointed out that in situ CO2 capture during the SR process should be operated at temperatures below 923 K due to the limitation of the equilibrium of the carbonation reaction between CO2 and CaO.22,803 In fact, no substantial difference was observed between conventional SR and SESR at temperatures exceeding 1023 K.22 On the basis of thermodynamic calculations, Chen et al. suggested that the most favorable temperature range for SESR is 800−850 K. They predicted that a maximum number of 7 mol of H2 could be obtained from 1 mol of glycerol due to CO2 adsorption, which equals 100% H2 concentration.802 Dupont and co-workers reported a highest H2 purity of 97% at 773 K in the presence of calcined dolomite as a CO2 sorbent.805 Ca-based adsorbents are presently the dominating species investigated in the in situ CO2 capture in SR of polyols. Dolomite, typically consisting of CaO and MgO, is widely utilized due to its high capacity for CO2 adsorption and low costs with little poisonous impurities such as sulfur.22,806 Previous investigations in SESR of CH4 have evidenced the advantages of dolomite practically.801,806,807 Kinetics studies based on various models including shrinking core and 1Ddiffusion suggested that the carbonation reaction undergoes a two-phase regime: a fast initial reaction phase leading to CaCO3 followed by a relatively slow reaction phase controlled by CO2 diffusion through the carbonate product layer formed on CaO.22,805,808 The active zone for efficient CO2 removal from the SESR system is restricted to the layer on the outer surface of the dolomite particle, whereas the sorbent is considered to be saturated and inactive under the second reaction phase as indicated by CO2 breakthrough. The saturated CO2-acceptor then would be reactivated by calcination.809 Therefore, the capture capacity as well as the stability of the sorbent to undergo many carbonation−regeneration cycles are important issues to be addressed. According to the literature, the capture capacity of dolomite is 2 orders of magnitude higher than that of K2CO3-promoted hydrotalcite materials, which have been used in the SESR of methane.804,810,811 The CaO adsorbent derived from natural limestone, which was reported to possess greater stoichiometric capacity than dolomite (0.79 and 0.46 gCO2/gadsorbent, respectively), was used by Illiuta et al. in the

Figure 45. Dual layer monolith reactor for ATR. Reprinted with permission from ref 826. Copyright 2013 Elsevier. AZ


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space velocity (GHSV). Activity tests showed that the duallayer catalyst exhibited nearly 100% conversion of glycerol yielding H2, CO2, and CO close to equilibrium at GHSV as high as ∼104 h−1. Furthermore, no significant deactivation was observed for 30 h stability test. The study provides a possible pathway for developing efficient catalysts for industrial utilization.

(iii) The WGS reaction takes place, converting CO into CO2 while meanwhile producing H2. CO + H 2O → CO2 + H 2

(iv) Further hydrogenations of CO and/or CO2 on metals lead to alkanes and water via methanation and F−T synthesis. This pathway places a challenge toward the selectivity of hydrogen production.

5.2. Aqueous-Phase Reforming

xCO + (2x + 1)H 2 → Cx H 2x + 2 + x H 2O

Preliminary investigations have shown that it is possible to achieve high conversions as well as high selectivities for H2 production using a maximum feed concentration of 10 wt % glycerol and EG.54 Although processing such diluted solutions is less economically favorable, APR of polyols remains an efficient and energy-saving process for clean hydrogen production due to its mild operating conditions. 5.2.1. Reaction Mechanism. As shown in Figure 22, EG and glycerol produce hydrogen via APR with very high selectivity, only lower than that of methanol. Moreover, the simplicity and integrity (C−C, C−O, and C−H bonds) of these simple polyols make them very good biomass model compounds, and therefore much emphasis has been placed upon APR process to obtain a fundamental understanding of the reaction, particularly on a metal-based catalyst, over the past decade.55,56,115,386,752,754,827−829 In the case of EG, multiple conversion pathways exist simultaneously depending on the operating conditions, reactants, and catalysts involved. These pathways control the catalytic activity and selectivity for reforming reactions. Possible pathways (I−IV) are shown schematically in Scheme 16.

(63)

(v) Dehydration of the adsorbed C2H6−xO2 species followed by hydrogenation (pathway II) results in the formation of ethanol on the metal catalyst, which may be further converted to alkanes, H2O, and H2 on the metal surface.386 This pathway is another challenge toward the selectivity of hydrogen production. C2H6O2 + H 2 → C2H5OH + H 2O

(64)

(vi) The adsorbed C2H6−xO2 species can react via another pathway (pathway III) involving desorption from metal surface followed by rearrangement to form organic acids (that generally occur on the catalyst support and/or in the aqueous phase), which may be further converted to alkanes, H2O, and H2 via surface reactions.386 This pathway becomes an additional challenge toward the selectivity of hydrogen evolution. C2H6O2 → CH3COOH + H 2

(65)

(vii) One should note that in addition to metal-catalyzed reactions, undesirable bifunctional catalysis takes place by the combination of the active metal and the support. 828 Dehydration of ethylene glycol on an acidic support (e.g., Al2O3) and subsequent hydrogenation on a metal surface (pathway IV) lead to alcohols, which may in turn be converted to alkanes as shown in pathway II. This bifunctional route increases alkane production at the expense of desirable hydrogen. In the case of APR of glycerol, Trans et al. summarized a mechanism similar to that of EG based on the literature. 24,386,830−835 Upon probing surface reactions and intermediates using ATR-IR spectroscopy, Lercher and coworkers demonstrated that the catalytic reactions of glycerol in the aqueous phase proceeded as bifunctional reactions over catalysts involving metal and support acid−base functions.832 In terms of the type of the initial reactions of glycerol, two major competing reaction pathways were proposed as shown in Scheme 17: dehydrogenation/decarbonylation route (route I) involving C−C bond cleavage and dehydration/hydrogenation route (route II) involving C−O bond cleavage. In route I, the initial step is identified as the irreversible dehydrogenation of hydroxyl groups at primary carbon atoms of glycerol to form adsorbed glyceraldehyde intermediates, which suffer from rapid C−C bond cleavage to yield CO and EG.832 CO then undergoes the WGS reaction in the aqueous solution to produce CO2 and H2, or it may undergo methanation or F−T synthesis to produce alkanes. Further decarbonylation of EG produces methanol, whereas dehydration and subsequent hydrogenation of ethylene glycol lead to ethanol. Moreover, acetic acid is present from dehydrogenation followed by disproportionation. Route II begins with dehydration (C−O bond cleavage) of glycerol followed by hydrogenation to produce propylene glycol (1,2-PDO). This dehydration/hydrogenation sequence is repeated until propane is obtained. However, if this sequence is accompanied by the

Scheme 16. Reaction Pathways for Hydrogen Production via Reforming of Ethylene Glycol with Watera

“*” represents a surface metal site. Reprinted with permission from ref 386. Copyright 2005 Elsevier.

a

(i) First, irreversible dehydrogenation of ethylene glycol leads to surface-adsorbed intermediates such as glycolaldehyde and the C2H6−xO2 species. x C2H6O2 → C2H6 − xO2 + H 2 (61) 2 (ii) Adsorbed intermediates undergo selective C−C bond cleavage to form the CHxO species (pathway I), and then subsequent dehydrogenation leads to CO2 and H2. C2H6O2 → CO2 + H 2

(12b)

(62) BA


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Scheme 17. Major Reaction Pathways for APR of Glycerola

a

Reprinted with permission from ref 24. Copyright 2013 The Royal Society of Chemistry.

C−C bond rupture, ethanol will appear and in turn result in alkane formation (e.g., methane and ethane). For the latter route, it is noteworthy that 1 mol of H2 will be consumed per mole of C−O cleavage, which disfavors hydrogen production.771 From the above mechanisms, it is apparent that economically feasible H2 production via APR must overcome significant selectivity challenges because several parallel and competing pathways including methanation, F−T synthesis, and dehydration exist, some of which eventually lead to undesired alkanes, deteriorating the productivity of H2. Moreover, the conversion of alkanes to hydrogen is thermodynamically unfavorable under low temperatures. Therefore, the identification of suitable catalysts is brought up as a key issue in polyol APR catalysis. Note that the chemical versatility of polyols enables the APR process to be operated for synthesizing other value-added chemicals with tuned reaction conditions. For example, Chaudhari et al. reported that 1,2-PDO, a common humectant or solvent in pharmaceutical, food, and tobacco industries, could be efficiently produced by aqueous phase hydrogenolysis of glycerol using the H2 generated in situ by glycerol reforming over a mixture of Ru/Al2O3 and Pt/Al2O3 catalysts.748,836 As compared to the conventional hydrogenolysis process, this integrated process required no external H2 supply. In addition, Pt catalysts supported on solid acid supports such as H-ZSM-5 were utilized by Murata et al. for propane production via APR of glycerol.837 The Pt/H-ZSM-5 catalysts were extremely active for the dehydration/hydrogenation route, therefore resulting in efficient propane evolution. 5.2.2. Catalyst Development. Controlling the selectivity of C−C, C−O, O−H, and C−H bond cleavages is the key to acquiring high-purity H2 streams. To attain good catalytic performance in the APR process, catalysts must have high rates of C−C bond scission and promote removal of adsorbed CO species by WGS, while it must not facilitate C−O cleavage and hydrogenation of CO and CO2.19 By comparing the catalytic performances of Pt, Pd, Ru, Rh, Ir, and Ni, Dumesic and coworkers demonstrated that Pt and Ni are the most promising for APR of oxygenates.56,386,838 Specifically, they compared various silica-supported monometallic catalysts including Ni, Pd, Pt, Ir, Rh, and Ru. Figure 46 summarizes the catalytic

Figure 46. Catalytic performances of metals for aqueous-phase reforming of ethylene glycol. Reaction conditions: 483 K and 22 bar. Reprinted with permission from ref 56. Copyright 2003 Elsevier.

performances of APR of EG over several silica-supported metal catalysts at 483 K. Experimental results suggested that Pt, Ni, and Ru exhibited considerable reforming activity, while high selectivities toward H2 production were observed exclusively on Pt and Pd. Ni-based catalysts, without modification, were susceptible to methane formation. The difference in selectivity among these catalysts could be ascribed to their different abilities to catalyze the WGS reaction and methanation. Indeed, Pt and Ni showed considerable WGS activities.839 Vannice et al. demonstrated that Pt and Pd exhibited low rates of methanation, whereas Ni showed much higher activity toward methane formation.840 Other catalysts examined including Rh failed to meet the criteria for suitable APR catalysts due to low catalytic activity as well as poor selectivity toward H 2 production. It can be concluded that Pt-based catalysts are by far the most attractive systems in consideration of H2 yield, whereas Ni-based catalysts also receive much attention despite their tendency to produce undesired alkanes.56,386 A substantial amount of work has aimed at catalyst optimization to acquire commercially viable reaction rates while maintaining high selectivity toward H2. Relevant APR catalysts for EG and glycerol are summarized in Table 16 and Table 17, respectively. BB


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Table 16. List of Catalysts for APR of EG catalysts Pt/SiO2

56

Pd/SiO256 Ni/SiO256 Ru/SiO256 Rh/SiO256 Pt-black741 Pt/SiO2741 Pt/ZrO2741 Pt/C741 Pt/TiO2741 Pt/SiO2−Al2O3741 Pt/CeO2741 Pt/γ-Al2O3741 Pt/ZnO741 Pt/CNF851 Pt/MWNT848 Pt/CMK-3850 Pt/AC850 Pt/γ-Al2O3850 Pt/CMK-8849 Pt1Ni1/Al2O319 Pt1Co5/Al2O319 Pt1Fe9/Al2O319 Pt−Co-ySWNT-EG867 Pt−Co/ySWNT867

Raney-NiSn874 NP-Ni875 Co/ZnO878 Co/CNF851 Pd1Fe9/Al2O319 Pd/Fe2O319 Pd/Fe3O4879

metal loading (wt %) 6 6 5 5 19 19 6 6 9 9 99.9 0.75 0.69 0.89 0.85 0.79 0.59 0.59 0.88 4.8 8 7 7 7 7 3 3 3 3 7.2c 7.2d

0.075e 0.67f 0.3g 14.6 3 3 6 6 4.7

T (K)

P (bar)

EG (wt %)

483 498 483 498 483 498 483 498 483 498 498 498 498 498 498 498 498 498 498 503 498 538 538 538 538 483 498 483 498 453 498 498 498 538 498 538 498 498 503 453 483 453 483 498

22 22 22 22 22 22 22 22 22 22 29.4 29.4 29.4 29.4 29.4 29.4 29.4 29.4 29.4 6.0 27.0 45.6 45.6 45.6 45.0 25.4 29.3 25.4 29.3 10.0 26.2 26.2 25.8 51.4 25.8 51.4 25.8 25.8 6.0 10.0 19.6 10.0 19.6 25.8

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 10 10 5 5 5 5 5 5 10 5 5 5 5 5

WHSVa (h−1)

2 71.8 2 2 2 2 (70.2) (70.2) (57.4) (57.4) (3.8) 72.8 72.8 (4.13) (8.26) (2.57) (10.3) (3.61) 0.59 (3.8) (14.4) (3.9) (14.5) 3.6

Xb (%)

TOFH2 (min−1)

SH2 (%)

Salkane (%)

8.6 21.0 1.47 3.06 11.5 2.3 26.0 42.0 7.0 15.0 8.34 1.29 9.34 16.9 16.3 9.63 1.88 12.4 0.55

0.075 0.275 0.015 0.030 0.054 0.001 0.005 0.020 2 × 10−4 2 × 10−4 8.54 0.70 4.87 7.52 11.1 4.60 1.20 7.04 1.55

52.5 77.9 98 98.5 42 57 3 7 0.34 0.17

26.6 13.0 0.0 0.0 31 13 66 58 66 60

75.8 38.1 38.6 85.8 3.9 5.9 3.8 8.4 32.5

9.48 1.48 1.38 0.58 35.6 5.18 8.06 5.06 10.39 1.12

97 103 93 97 8.2 5.2 8.5 8.7 7.6 35.1 27.0 14.4

1.1 1.4 0.22 1.69 0.22 1.38 4.30 14.9 39.1 109

52.5 84.7 74.8 70.8 55.7 98.7 88.3 91.0 93.9 88.2 98.8 ∼100 ∼100 35 28 90 96 63 89 21.4 108.5 103.5 105.1 95.3 133

5.6 6.8 5.8 10.8 4.7 0.0 0.0 0.0 0.5 0.0 19.7 7.4 44 47 9 7 31 29 0.0 0.0 0.0 0.0 0.03

In parentheses, the liquid hourly space velocity (LHSV), when WHSV is unavailable. LHSV (h−1) is defined as (volumetric feed flow rate)/(catalyst volume). bConversion (%) = (C atoms in gas-phase products)/(total C atoms in the feedstock) × 100. cCo wt % = 0.7. dCo wt % = 7.2. eNi/Sn atomic ratio. fNi/Al atomic ratio. gCo/Zn atomic ratio. a

5.2.2.1. Pt-Based Catalysts. 5.2.2.1.1. Pt-Based Monometallic Catalysts. By far, supported Pt-based catalysts are the most systematically examined materials in APR of EG. Typically, theses catalysts display high surface-specific activity and hydrogen selectivity. Therefore, revealing the structure− activity relationship is vital to the rational design of highperformance APR catalysts. The support effect of Pt catalysts has been examined. Shabaker et al. acquired turnover frequencies for hydrogen production over Pt-black and Pt NPs supported on TiO2, Al2O3, carbon, SiO2, SiO2−Al2O3, ZrO2, CeO2, and ZnO.741 Pt supported on Al2O3, TiO2, and carbon exhibited higher

catalytic activities, while moderate TOFs were observed over Pt supported on SiO2−Al2O3 and ZrO2. Pt supported on CeO2, ZnO, and SiO2 did not show much appreciable catalytic activities. The acidity of a catalyst is pivotal to control the product distribution in APR process, as Tian et al. found that the activities of several supported Pt catalysts decreased in the following order: Al2O3 > MgO > SiO2 > HUSY zeolite > active carbon > SAPO-11.841 By correlating WGS activity with APR performance, Guo et al. showed that basic supports were preferred for the WGS reaction (Figure 47a), which in turn enhanced the catalyst activities in the APR process (Figure 47b). BC


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Table 17. List of Catalysts for APR of Glycerol samples Pt/Al2O3

54

Pt/Al2O3841 Pt/SiO2841 Pt/MgO841 Pt/AC841 Pt/HUSY841 Pt/SAPO-11841 Pt/MgO882 Pt/ZrO2882 Pt/MgO842 Pt/Al2O3842 Pt/CeO2842 Pt/TiO2842 Pt/SiO2842 Pt/Ce4Zr1α883,b Pt/CeO2−Al2O3884 (CeO2 = 3 wt %) Pt−Re/C771 Pt−Mo/C865 (Pt/Mo atomic ratio = 1) Pt−Cu/Mg(Al)O863 (Mg:Al = 2.95) Pt−Ni/CeO2−Al2O3864 (CeO2 = 3 wt %) Raney-NiSn874

T (K)

metal loading (wt %) 3

498 538 503 503 503 503 503 503 498 498 498 498 498 498 498 523 513 498 503 473 513 498 538 498 543 543

4.4 4.3 9.8 4.2 5.1 5.3 1.4 1.1 0.79 0.74 0.75 0.72 0.76 1 3 Pt: 3, Re: 3 5 Pt: 0.9, Cu: 0.4 Pt: 1, Ni: 6 0.075c

AP-Ni876 (Ni73.4−B26.6 alloy) Ni−5Cu−Mg−Al885,d Ni−10Cu−Mg−Al885,e

P (bar) 29 56 32 32 32 32 32 32

27.6 27.6 27.6 27.6 27.6 44 40 29 30 29 40 25.8 51.4 27.6 50 50

glycerol (wt %)

X (%)

SH2a (%)

10 10 10 10 10 10 10 10 1 1 5 5 5 5 5 10 1 10 30 10 1 5 5 5 10 10

83 99 18.9 10.8 13.8 17.2 22.0 13.3

75 51 (69.7) (71.8) (79.9) (69.6) (71.8) (72.8) (71.9) (62.7) (58.1) (60.7) (58.9) 56.7 62 81 80 24.5 55 55.3 84 (64) (62) 91 43 40

48 38 35 27 20 29 85 58.5 26 98 96 81 99 18.4 80 65

a In parentheses, the H2 content in gas-phase product (%). bCe4Zr1α represents α-Al2O3 modified with 4 wt % CeO2 and 1 wt % ZrO2. cNi/Sn atomic ratio. dHTLc precursor consists of 19 wt % NiO, 5 wt % CuO, and 48 wt % MgO. eHTLc precursor consists of 21.3 wt % NiO, 11.7 wt % CuO, and 43.4 wt % MgO.

Figure 47. (a) The activity of WGS reaction over different supported-Pt catalysts. (b) H2 production rates over different supported-Pt catalysts in APR of 5 wt % glycerol at 498 K, 27.6 bar, and a flow rate of 0.06 mL min−1. Adapted with permission from ref 842. Copyright 2012 Elsevier.

another relevant study, Ciftci et al. observed the formation of boehmite on amorphous SiO2−Al2O3 (ASA)-supported Pt catalysts during the reaction, and its presence increased surface acidity, which in turn led to increased 1,2-PDO selectivity via glycerol dehydration and hydroxyacetone hydrogenation.847 Carbonaceous materials exhibited several unique properties as catalyst supports including resistance to dissolution in hightemperature aqueous solutions and stability in acidic or basic media.848 Therefore, several types of carbon including CNF, CNT, activated carbon (AC), and mesoporous carbon have been investigated in the APR process.848−851 Bitter et al. demonstrated that Pt/CNF showed the highest H2 selectivity as compared to Ni/CNF and Co/CNF.851 Haller and co-workers

Moreover, the physical stability of the supports should be a concern if the catalysts are to be utilized in long-term APR process. Catalyst supports could suffer from degradation through sintering (e.g., ZrO2), surface area loss through phase transformation (e.g., γ-Al2O3 to α-Al2O3), or dissolution (e.g., SiO2) by acids in the hydrothermal environment.741,843,844 de Vlieger et al. reported that hydroxylation of Al2O3 occurred in the presence of acidic intermediates in the APR process, and subsequent redeposition of the dissolved Al2O3 blocked the active Pt sites, thus resulting in deactivation of Pt/Al2O3 catalysts.845 The transformation of Al2O3 to hydrated boehmite was also observed by Jongerius et al., and then Pt sintering took place due to the weakened metal−support interaction.846 In BD


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Figure 48. Schematic representation of the structure of CMK-3 and Pt/CMK-3 in the APR reaction. Reprinted with permission from ref 850. Copyright 2011 The Royal Society of Chemistry.

reported that both single-walled and multiwalled CNT supported Pt catalysts exhibited higher APR activity than oxide supported catalysts.848,852 Kim et al. showed that an ordered mesoporous carbon CMK-3 supported Pt catalyst (Figure 48) produced a more than 2-fold higher amount of hydrogen than Pt/AC and Pt/γ-Al2O3.850 The enhanced activity was attributed to the outstanding hydrothermal stability, large surface area (typically above 800 m2 g−1), and open mesoporous structure of CMK-3, resulting in higher Pt dispersion as well as better mass transportation and diffusion. Furthermore, they found that a 3D-OMC supported Pt catalyst (Pt/CMK-9) had several advantages over a 2D-OMC supported Pt catalyst (Pt/CMK-3) due to the synergetic effect of the stronger antisintering ability and more favorable mass transfer promoted by the hollow-type framework configuration.744 Mesoporous carbon-supported Pt catalysts showed promising activity and long-term stability due to the high surface area and the hydrophobic nature of the support. The H2 production rate has been found to be closely dependent on the Pt particle size. Yamaguchi and co-workers examined the TOFH2 in the APR of glycerol as a function of Pt NP size over Pt/Al2O3 catalysts, consistent with the findings by Lehnert and Claus.853,854 A rapid rise in TOFH2 was observed as the particle diameter decreased below 1.6 nm, because smaller Pt particles favor C−C bond cleavage and promote reforming reactions, whereas larger Pt particles limit the relative availability of the metal function.832 In fact, the findings of Xiao et al. revealed that Pt tended to aggregate upon reduction when high Pt loadings were employed, resulting in activity decay.855 5.2.2.1.2. Pt-Based Bimetallic Catalysts. Bimetallic catalysts have been widely utilized in heterogeneous catalysis due to their unique properties that differ from those of the corresponding monometallic catalysts distinctly.24,856 To rapidly screen catalysts for the APR process, Huber et al. developed a high-throughput reactor (HTR, shown in Figure 49) with 48 reaction wells, in which multiple catalysts were tested in parallel, enabling a much faster screening rate in the HTR than in the traditional fixed bed reactor.19 More than 130 bimetallic Pt and Pd catalysts were systematically screened for H2 production. Potential catalysts were further tested in a fixed bed reactor. Although monometallic Ni- and Co-based catalysts were less active than their Pt-based counterparts, bimetallic PtNi and PtCo exhibited TOFs for hydrogen production 1.5− 2.8 times greater than those of monometallic Pt catalysts at 483 K. DFT calculations and experimental studies revealed that alloying Pt with Co or Ni caused a shift of the d-band center of Pt, lowering the adsorption heats of H2 and CO as compared to

Figure 49. Photos and schematic of a high-throughput reactor: (A) reactor with common headspace top plate for catalyst reduction. (B) Reactor with isolated headspace place for reaction and GC analysis. (C) Schematic of cross section of reactor with isolated head space. Reprinted with permission from ref 19. Copyright 2006 Elsevier.

pure Pt, leading to a decrease in the surface coverage of absorbed H2 and CO, therefore allowing more active sites accessible for EG.829,857−860 It is noteworthy that the addition of Ni or Co must be controlled precisely to obtain bulk or surface alloys with sufficient active sites and high activity per active site. Excessive amounts of Ni or Co may not alloy with Pt and even result in catalyst deactivation due to the extensive sintering of Ni and Co under APR conditions.838 Moreover, bimetallic PtFe catalysts also possessed a higher TOFH2 for H2 production than Pt/Al2O3 especially at lower temperature (e.g., 453 K). DFT calculations have also shown a decrease in the Pt d-band for PtFe alloys, similar to those observed for PtNi and PtCo, weakening the strength of H2 and CO adsorption.857,858 The most active Al2O3-supported PtFe catalysts are Pt1Fe9/Al2O3 (Pt/Fe atomic ratio of 1:9) with a TOF value 3 times higher than that of Pt/Al2O3. Decreased catalytic activity was observed as the amount of Fe increased. It was suggested by Huber et al. that alloying Pt with Ni, Co, or Fe enhanced the H2 production via APR by lowering the dband center, which induced a decrease in the adsorption heats of CO and H2, therefore increasing the fraction of surface available for conversion of EG.19 BE


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The catalytic activity and structural properties of various Pt− Me (Co, Ni, and Re) bimetallic catalysts in APR of glycerol have been investigated and discussed by several groups.771,833,861−866 By addition of Re to Pt/C, Wang and King et al. improved the conversion of glycerol by at least 1 order of magnitude, even though it was accompanied by a reduction in H2 selectivity.771 Different hypotheses have been put forward to explain the promotional effect of Re on Pt-based catalysts. First, alloying Re to Pt increased the rate of the metalcatalyzed C−C bond scission, which was considered as the key step to generate H2 and CO from glyceraldehyde. Second, the WGS reaction is boosted. Two explanations were provided for the enhanced activity: (i) the formation of Pt−Re alloy lowered the binding energy of CO and thus CO adsorption strength was weakened,770 and consequently the absorbed CO on Pt−Re was more likely to undergo WGS as compared to pure Pt; and (ii) the presence of oxidized ReOx species provided a redox route for WGS reaction wherein ReOx was reduced by CO, generating CO2, and reoxidized by H2O to form H2.774,833 Hensen et al. showed that PtRe/C catalysts remained in the reduced state, as evidenced by H2-TPR and EXAFS. Moreover, they suggested the role of Pt in PtRe bimetallic catalysts was to facilitate the reduction in Re and provided sites with appropriate bond energies for adsorption of reactants such as CO for WGS and glycerol for dehydrogenation and the C−C bond cleavage.834 Re sites were also responsible for water activation and covered by oxygen-containing species such as hydroxyl groups. NH3-TPD and pyridine-IR spectroscopic studies confirmed the generation of acidity in the PtRe alloy, and they were correlated to the higher C−O bond cleavage rates in PtRe/C. Experimental results and DFT calculations by Dumesic et al. also supported that the presence of hydroxyl groups associated with Re atoms was the cause for the catalytic properties of PtRe-based catalysts as depicted in Figure 50.773

Figure 51. One possible structure of oxidized Pt−Re in the presence of water. Reprinted with permission from ref 833. Copyright 2012 Elsevier.

Dietrich et al. prepared a carbon-supported PtMo bimetallic catalyst and observed a turnover rate 4 times higher than that of Pt/C at comparable conversions.865 DFT calculations indicated that metallic Mo altered the electronic properties of Pt, lowering the binding energy of CO and decreasing the activation energies for dehydrogenation and C−O bond rupture.866 Because the promotional effect of Mo was more pronounced on the breaking of the C−O bond, the H2 yield was ultimately lowered as conversion approached 100% due to reduced H2 selectivity. Dietrich et al. also examined the effect of cobalt as a promoter for Pt-based APR catalysts. Unlike the PtMo bimetallic catalyst, PtCo/MWNT (multiwalled carbon nanotube) not only exhibited a 4.6 times higher glycerol consumption rate than Pt but also multiplied the H2 formation rate by a factor of 4.862 The improved catalytic performance was attributed to multiple mono- and bimetallic structures (Pt shell/Co core, well-mixed PtCo alloy, pure Pt), which maintained surface Pt sites that were selective for H2 evolution while the cobalt promoter increased reaction rates.862 These mixed structures are further confirmed by STEM-EELS line scans (Figure 52).

Figure 52. HAADF-STEM micrographs and STEM-EELS line scans at Pt M4,5 edges (dashed red) and Co L1,2 edges (solid blue) for NPs with (a) Pt only, (b) Pt shell/Co core, and (c) mixed PtCo alloy configurations. Reprinted with permission from ref 861. Copyright 2014 American Chemical Society.

It is found that a higher Co loading led to a higher fraction of alloyed PtCo particles, which possessed Pt functionality over WGS or less CO binding energy, and thus the APR reaction site time yields (STYs, normalized to CO chemisorption sites) increased as well. For PtCo/MWNT with a Pt:Co ratio of 1:5, the APR STYs increased by a factor of 4 while maintaining high selectivity to CO2 and H2 (>45% and >85%, respectively) at conversions up to 60%.861 Unlike in MSR, Cu-based catalysts do not present excellent reforming activities toward hydrogen in APR of glycerol because it requires capability for the C−C bond rupture prior to the C−O bond rupture.386 Despite the incapability for APR of glycerol, the high WGS activity makes Cu a potential candidate for Pt-based bimetallic (i.e., Pt−Cu) catalysts. Alloying Cu with Pt supported on LDH-derived Mg(Al)O, methane formation in APR was suppressed, and ultimately high hydrogen selectivity was observed.863

Figure 50. Schematic representation for the roles of Re−OH species on the catalytic behavior of Pt−Re/C catalyst in steam reforming of oxygenated hydrocarbons. Reprinted with permission from ref 773. Copyright 2008 Elsevier.

However, in situ XPS measurements by Wang and coworkers indicated that under hydrothermal reaction conditions, Pt in reduced PtRe/C became electron deficient and a fraction of Re was oxidized due to its oxophilicity.833 The Pt−Re interaction increased local surface acidity as depicted in Figure 51, which promoted the dehydration pathway leading to the reduction in H2 selectivity. BF


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A majority of investigations on the support effect of Pt-based bimetallic catalysts focus on Al2O3, whereas studies on other supports are relatively limited. Notably, Minett et al. demonstrated that when nickel is introduced to Pt on CeO2doped Al2O3, the loading of Pt could be reduced from 3% to 1% with the same level of H2 production. In addition, Haller et al. prepared a series of PtCo bimetallic catalysts supported on their lab-synthesized single-walled carbon nanotubes (denoted as ySWNTs) using different preparation methods.867 It is interesting that the synthesized Pt−Co-ySWNT-EG (prepared by ethylene glycol solution reduction) and Pt−Co/ySWNT (prepared by sequential impregnation) catalysts possessed different structures and active phases in EG APR reactions. Structural characterizations using XANES and EXAFS showed that in Pt−Co-ySWNT-EG, individual Pt and Co particles were both active phases, and the interactions between them maintained the metallic state of Co particles, whereas Pt− Co/ySWNT had a core−shell structure with a Co core and Pt− Co alloy shell, and the PtCo alloy acted as the active phase. Catalytic tests showed that Pt−Co-ySWNT-EG achieved the highest hydrogen production of 4.6 mmolH2 g cat−1 min−1 and the H2 yield from APR decreased in the order of Pt−CoySWNT-EG > Pt−Co/ySWNT > Pt/Al2O3. Other supports for Pt-based bimetallic catalysts are yet to be studied. The synergistic effects obtained from Pt-based bimetallic catalysts have made them promising alternatives for monometallic catalysts in the APR process. 5.2.2.1.3. Mechanistic Understanding on Bimetallic Surfaces. Previous mechanistic studies primarily focused on the reforming pathways on bimetallic metal surfaces. The spectroscopic identification of surface reaction intermediates was accomplished by using state-of-the-art UHV surface science techniques including TPD, high-resolution electron energy loss spectra (HREELS), AES, XPS, low-energy ion scattering (LEIS), LEED, and in situ EXAFS. Moreover, DFT calculations were also applied in an effort to confirm these experimental observations for ethylene glycol conversion over different metals. Chen and co-workers extensively studied the dehydrogenation and decarbonylation of EG and ethanol on 3d/Pt bimetallic surfaces including Ni/Pt bimetallic surfaces using TPD, which were considered as probe reactions for the reforming of oxygenates to produce H2. By varying the deposition temperature and time during the physical vapor deposition (PVD) process, they successfully prepared a series of surfaces including surface monolayer Ni−Pt−Pt(111), subsurface monolayer Pt−Ni−Pt(111), Pt(111) films, and Ni(111) films.776 The Ni−Pt−Pt(111) represented a monolayer of Ni on Pt at 300 K, while Pt−Ni−Pt(111) stood for a surface with nickel atoms diffusing into the subsurface region, leaving the top layer enriched with platinum (Figure 53). TPD results showed that the reforming activity of ethylene glycol followed the trend: Ni−Pt−Pt(111) > Ni(111) film > Pt(111) > Pt−Ni−Pt(111,) where Ni/Pt bimetallic surfaces displayed better catalytic performance than either of the parent metal surfaces.776 On the basis of TPD results and DFT calculations, a linear correlation between reforming activity of EG and the d-band center of these surfaces was established, with activity increasing as the surface d-band center moved closer to the Fermi level as shown in Figure 54. The presence of surface monolayer 3d metals on Pt(111) shifted the surface d-band center toward the Fermi level, strengthening the interactions with absorbates such as hydrogen. However,

Figure 53. Idealized bimetallic surface structures with one monolayer of 3d metals on a Pt(111) substrate. Reproduced with permission from ref 15. Copyright 2012 American Chemical Society.

Figure 54. Correlation of ethylene glycol and ethanol reforming activity with the d-band center on Pt(111) and bimetallic Ni/Pt(111) surfaces. Reprinted with permission from ref 776. Copyright 2006 American Chemical Society.

substituting a 3d metal in the subsurface of Pt(111) decreased the d-band center, resulting in a weaker interaction with adsorbed species.868 Therefore, it can be assumed that the reforming activity on metal surfaces was affected by the adsorption of surface species significantly. On Ni/Pt(111) bimetallic surfaces, Chen’s group nicely drew a linear correlation between the binding energy of species derived from ethylene glycol and the surface d-band center, with increasing BE as the d-band center shifted closer to the Fermi level, consistent with the theory of chemisorption proposed by Hammer et al.777,869 Combined with the previous results, a correlation between reforming activity and BE of surface adsorbates was built. Moreover, HREELS measurements were applied to identify the bond cleavage sequence of ethylene glycol on metal surfaces. The HREELS results revealed that ethylene glycol molecularly adsorbed and reversibly desorbed with the O−H bond intact on Pt(111) and Pt−Ni−Pt(111), whereas ethylene glycol underwent initial decomposition through O−H bond scission to form an ethylenedioxy (−OCH2CH2O−) intermediate on Ni−Pt−Pt(111) and Ni(111) film surfaces. This intermediate further reacted via dehydrogenation and the C−C bond cleavage to form adsorbed CO, which subsequently BG


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used catalyst reversed the Ni-segregated surface back to a Ptrich surface (Figure 55c). The reversibility of the PtNi bimetallic catalyst demonstrated its versatility to be used for various reactions; yet its long-term stability is concerning due to surface reconstruction during the APR process. 5.2.2.2. Ni-Based Catalysts. It has been discussed above that Ni-based catalysts are highly active but show poor H2 selectivity. Among supported Ni catalysts, Ni/Al2O3 has shown good performance toward H2 production.741,838 Studies of Ni/Al2O3 catalysts showed that the Ni−Sn/Al2O3 catalyst was the most active and selective among the Sn, Au, and Zn doped Ni catalysts.838 However, Ni−Sn/Al2O3 was not durable and suffered from considerable deactivation by sintering during APR. Dumesic’s group reported the superior capability of Snpromoted Raney-Ni catalysts in APR of biomass-derived oxygenated hydrocarbons such as EG, glycerol, and sorbitol.874 The Sn-modified Raney-Ni catalysts exhibited higher selectivity for hydrogen production and lower selectivity for alkanes production than Pt/Al2O3 while maintaining catalytic stability for more than 340 h on stream.874 Kinetic measurements indicated that Raney-NiSn exhibited rates of H2 production comparable to those of Pt/Al2O3, even though TOFs for H2 formation over Raney NiSn were several times lower than that over Pt/Al2O3.838 The addition of Sn significantly improved the H2 selectivity. At a Ni:Sn ratio of 14:1, the H2 selectivity increased to 90%, while alkane production was nearly eliminated. XRD, SEM, and in situ 119Sn Mössbauer spectroscopy (Figure 56a) results

underwent WGSR to produce H2. Combined with the results from DFT calculations, the initial O−H bond cleavage was identified as the kinetically controlling reaction for reforming of EG on metal catalysts instead of C−C bond breaking, which is consistent with the recent findings by the Vlachos and Dumesic groups.870,871 A catalytic kinetic model parametrized based on atom binding energy descriptors showed that an increase in atomic oxygen BE from 89 to 119 kcal/mol led to increased initial dehydrogenation reaction rates and in turn to enhanced catalytic activity, therefore rationalizing that the Ni−Pt− Pt(111) catalyst was more active than the Pt catalyst toward EG reforming reaction.871 Chen and co-workers further systematically examined the reforming activity of other 3d/Pt bimetallic surfaces using Ti and Fe.872 Similar to Ni/Pt(111) bimetallic surfaces, Fe or Ti surface monolayer on Pt(111) displayed higher reforming activity for EG than their corresponding subsurface counterparts. However, experimental results showed that highest reforming yield was observed on Ni−Pt−Pt(111) instead of Ti−Pt−Pt(111) and Fe−Pt−Pt(111), which exhibited slightly decreased selectivity via total decomposition of oxygenates to adsorbed carbon and oxygen instead of carbon monoxide over these surfaces. This experimentally confirmed the assumption that the adsorption of absorbates greatly affected the reforming activity. In this case, ethylene glycol tended to undergo complete decomposition instead of reforming at sufficiently high binding energies on Ti−Pt−Pt(111) and Fe−Pt−Pt(111). The decrease in selectivity as the d-band center shifted to the Fermi level was much more pronounced for ethanol reforming, and this might be attributed to the larger BE of ethanol, leading to a total decomposition. It is important to note that the trends observed on Pt single crystals can be extended to NiPt surfaces prepared on polycrystalline Pt substrates.873 In situ characterization of NiPt catalysts under APR conditions was carried out to draw connections between the catalyst structure and activity at practical operating conditions. Supported NiPt bimetallic catalysts have been synthesized using various synthesis methods and displayed as effective APR catalysts.15 In situ EXAFS studies verified that the surface of NiPt catalyst was mainly Ptterminated (Figure 55a) under APR conditions, similar to the Pt−Ni−Pt subsurface structure, in a reductive environment as reported previously.55,778 However, Ni segregated to form a Ni-terminated surface (Figure 55b) under APR conditions, which accounts for the enhanced activity of NiPt bimetallic catalysts by both theoretical and experimental studies on the Ni−Pt−Pt(111) surfaces under the UHV condition.55,778,873 Rereduction of the

Figure 56. In situ 119Sn Mössbauer spectra of Raney-NiSn catalyst (A) after reduction at 533 K and (B) after reaction at 533 K and 51.4 bar with 5 wt % ethylene glycol feed for 60 h. Reprinted with permission from ref 874. Copyright 2003 Science/AAAS.

suggested that the addition of Sn to the Raney-Ni catalyst facilitated the formation of a core−shell structure, in which the Ni core is surrounded by an oxidation-resistant Ni−Sn (Ni3Sn, Ni3Sn4) alloy shell after thermal treatment above 533 K.743,838,874 Upon exposure to reaction conditions, Ni3Sn became predominant, and some Sn(IV) species appeared (Figure 56b). The addition of Sn significantly decreased the rate of methanation from the C−O bond cleavage without inhibiting the rate of the C−C bond scission necessary for H 2 production.838 The origin of the promoting effect of Sn can be attributed to the following reasons: (i) Sn may selectively decorate Ni defect or edge sites that are crucial for CO

Figure 55. Structural modeling of NiPt/C catalyst based on the fitting results of the Pt L3-edge spectra. (a) The catalyst was reduced in 10% H2/He at 623 K for 2 h. (b) 1 h after exposure to APR reaction conditions (10 v/v % ethylene glycol in H2O at 498 K and 450 psig). (c) The catalyst was reduced in 10% H2/He at 623 K for 1 h after exposure to the APR conditions. Adapted with permission from ref 778. Copyright 2012 American Chemical Society. BH


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coprecipitation method exhibited a H2 selectivity of 89% and a TOFH2 of 101.4 h−1, which were higher than those of the Raney-Ni catalyst.878 Additionally, a negligible amount of CO was observed. However, metallic Co was vulnerable to oxidation and subsequent leaching by acids formed in the aqueous solution and thus deactivated rapidly.851 As discussed earlier, although Pd-based catalysts were selective toward hydrogen production, the conversion rate was relatively low. For example, the TOFH2 of Pd/SiO2 catalyst was only 1/9 of that of Pt/Al2O3 under identical APR conditions, although its H2 selectivity was much higher than that of Pt/Al2O3 (98.5% vs 77.9%).56 It was deduced that the WGS reaction could be the RDS for APR of EG because the rates of C−C scission were similar over Pd and Pt catalysts, whereas the rates of the WGS reaction over Pd catalysts were lower than those over Pt catalysts.19 As effective industrial catalysts for WGS, iron oxides have been added into Pd-based catalysts as supports.19,879−881 By loading Pd on high surface area Fe2O3, Huber et al. found that the rates of H2 production approached an order of magnitude higher than those over the Pt/Al2O3 catalyst.19 Nevertheless, Pd/Fe2O3 lost 50% of its initial activity upon heating to 483 K under the APR conditions. Recently, a high TOF for H2 (109 min−1) was reported by Liu et al. over a Pd/Fe3O4 catalyst, which was attributed to promoted WGS activity due to the synergy between highly dispersed Pd particles ( 8), the growth of nickel NPs was almost completely suppressed. Stable H2 production Table 18. Representative Catalysts for SCWR of Glycerol catalyst K2CO3892 893

KOH K2CO3894 KOH894 Na2CO3894 NaOH894 Ru/Al2O375 Ru/ZrO2896 Pt/CeZrO2897 (Ce0.08Zr0.92O2) Ni/ZrO2897 Ni/CaO−6Al2O3897 NiCu/CeZrO2897 (Ce0.46Zr0.54O2) CuZn alloy897 Pt−Ni/Al2O3891 Raney-Ni898 Co/YSZ899 Fe2O3−Cr2O3900

metal loading (%) b

(0.5) (2.4)c (0.5) (0.5) (0.5) (0.5) 5 1 1.8 15 10.5 Ni: 25.5, Cu: 8.5 Cu:Zn = 4 Pt: 1.2, Ni: 0.4 10 Fe2O3: 91, Cr2O3: 9

T (K)

P (bar)

glycerol (wt %)

873 773 799 799 799 799 1073 823 932 912 931 933 937 723 653 773 673−823

250 450 250 250 250 250 241 350 255−270 255−270 255−270 255−270 255−270 250 230 250 170−250

10 7 10 10 10 10 5 5 10 10 10 10 10 15 3 5 2−30

X (%) 100 ∼100 ∼100 ∼100 ∼100 ∼100 100 ∼100 ∼100 ∼100 ∼100 ∼100 100 85 94 ∼100

SH2a (%) 55 28 65.2 67.4 67.0 68.9 70 3.5d 51 41.9 51.1 53.4 44.8 41.0 27e 3.7d ∼62

H2 concentration in gas-phase product (%). bIn parentheses, the figure represents the concentration (wt %) of a homogeneous catalyst in the reaction solution. cUnit: mol/L. dH2 yield (mol/mol glycerol). eH2 yield (mmol/g feed). a

BJ


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Scheme 19. Possible Reaction Network for SCWR of Glycerol in the Presence of a Pt−Ni Bimetallic Catalysta

a

Reprinted with permission from ref 891. Copyright 2013 American Chemical Society.

catalyst in the SCWR of crude glycerol.893 The promotional effect of KOH was attributed to in situ CO2 removal (eq 66), which shifted the equilibrium of the WGS reaction toward H2 evolution.

To overcome these dilemmas, de Vlieger et al. introduced the SCWR process, in which the APR was performed at supercritical conditions (e.g., 723 K and 250 bar) over alumina-supported monometallic Pt, Ir, Ni, and Pt−Ni catalysts with a feed concentration range of 5−15% in a continuous flow reformer.886 The unique properties of supercritical water grant the process several advantages over other reforming processes such as higher reaction rates, lower interface mass transfer resistance, better heat transfer, and lower amount of undesired carbon deposition, as stated before in section 2.2.3.111,742,750 SCWR of simple polyols, especially glycerol, is a versatile process as it can be tuned for producing liquid components with low molecular weight (mainly acrolein, an important intermediate for the chemical industry) or permanent gases (mainly H2-rich gases) with or without a catalyst.750,887 In this section, we focus on H2 production via SCWR. 5.3.1. Catalyst Selection. Catalysts for SCWR process can be classified into two categories: homogeneous and heterogeneous catalysts. Representative catalysts for SCWR are summarized in Table 18. Homogeneous catalysts mainly consisted of acidic catalysts and alkali catalysts. The acidic catalysts applied in glycerol SCWR are typically H2SO4, NaHSO4, and ZnSO4, which facilitate the production of liquid products (mainly acrolein) via glycerol dehydration.887−890 To convert glycerol into H2-rich gases, alkali compounds including NaOH, Na 2 CO 3 , KOH, and K 2 CO 3 were utilized as homogeneous catalysts. The alkali catalysts enhance the WGS reaction by steering the product composition toward H2 and accelerating the glycerol gasification.891 Chakinala et al. reported that the addition of K2CO3 (0.5 wt %) into 10 wt % glycerol feed resulted in a high H2 production of 2.69 molH2/ molglycerol at 873 K and 250 bar.892 Yang et al. employed KOH

CO2 + 2KOH → K 2CO3 + H 2O

(66)

A quantitative kinetics study by Guo’s group indicated that the addition of alkali catalyst greatly increased the rate constants of WGS reaction.894 Evaluation of the performance of four alkali catalysts showed that their hydrogen yields decreased in the following order: NaOH > Na2CO3 > KOH > K2CO3. Because of the unique properties of supercritical water, the solubility of alkali and metal salts is low. Therefore, the concentration of homogeneous catalyst in the system should not be too high; otherwise, it may lead to precipitation and plugging of the reactor.750,894 Supported metal catalysts are by far the most widely used. Gupta’s group reported that Ru/Al2O3 was an effective catalyst for SCWR of ethanol and glucose and extended its use to glycerol.64,895 Near-theoretical H2 yield (6.5 mol/mol glycerol) was achieved over Ru/Al2O3 at 1073 K.75 May et al. found that ZrO2-supported Ru enhanced glycerol conversions and presented good stability, but the reported carbon balance was below 80%, indicating the presence of liquid byproducts such as acetaldehyde in the effluent.896 Catalyst screening by van Bennekom et al. investigated the performance of Pt/CeZrO2, Ni/ZrO2, Ni/CaO−6Al2O3, NiCu/CeZrO2, and a CuZn alloy catalyst.897 All samples facilitated the decomposition of glycerol resulting in complete conversions, while only approximately 40% glycerol conversion was achieved in the absence of catalysts. The WGS reaction was promoted, whereas the effect was much less pronounced over BK


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Figure 58. Effect of operation parameters on product gas yield: (a) Temperature. P = 241 bar, residence time = 1 s, feed concentration = 5 wt % glycerol. (b) Feed concentration. T = 1073 K, P = 241 bar, residence time = 1 s. (c) Residence time. T = 1073 K, P = 241 bar, feed concentration = 5 wt % glycerol. Adapted with permission from ref 75. Copyright 2008 Elsevier.

for SCWR of several biomass-derived feedstocks including methanol, ethanol, and glycerol using Gibbs free energy minimization as well as gas fugacities.111 They indicated that high temperatures were slightly favorable for H2 yield from glycerol, whereas the increase in glycerol feed concentration had a negative effect on hydrogen production. These results were in good agreement with the experimental data acquired by Byrd et al., in which H2 yield decreased accompanied by increased CH4 formation as the glycerol feed concentration rose from 5 to 40 wt % at 1073 K and 241 bar (Figure 58a and b).75 Furthermore, Ortiz et al. suggested that the optimal conditions for H2 production are 1173 K and 1 wt % glycerol in the feed because a H2 yield close to 100% was observed under such conditions.901 However, this is not viable considering the intensive energy consumption and enlarged unit size for such conditions. They recommended to operate the APR process in temperature range of 1023−1073 K because glycerol conversions were complete for all feed concentrations investigated. H2 selectivity suffered a sharp decline below 1023 K, while the durability of facility materials was challenged at a temperature above 1073 K. Moreover, Fiori and co-workers pointed out that the char formation occurred only when processing very concentrated streams (above 60 wt %) and was inhibited by high temperatures.902 Residence time is another important operation parameter. Both Byrd et al. and Liu et al. investigated the effect of residence time between 1 and 4 s. They showed that the shortest residence time exhibited the highest H2 yield; nevertheless, the H2 yield decreased sharply with a slight decline in the CO2 yield (Figure 58c). Moreover, the increased amount of CH4 at longer residence times indicated secondary reaction of methane formation, which has been reported for

CuZn. Moreover, Ni catalysts promoted methanation. In fact, the strong promotional effect of Ni/CaO−Al2O3 made it potential for methane production from glycerol in supercritical water. Cu addition into the NiCu/CeZrO2 catalysts suppressed the methanation activity, but its long-term stability was questionable because it disintegrated in the supercritical water. Chakinala et al. studied SCWR of glycerol over Pt− Ni/Al2O3 catalyst at near critical conditions (e.g., 723 K and 250 bar).891 Complete conversion of glycerol was achieved at relatively low temperature (723 K), but H2 selectivity was below 50% due to a considerable amount of alkanes. On the other hand, Pt−Ni/Al2O3 exhibited stable performances during a run up to 85 h, and such long-term stability has also been observed for Pt−Ni catalysts in the SCWR of EG. Moreover, Chakinala et al. deduced a possible reaction network scheme for glycerol reforming in supercritical water as shown in Scheme 19.891 The primary route for H2 production proceeds through glyceraldehyde intermediate, suggesting some similarities in the mechanistic nature between APR and SCWR of glycerol. Farnood and co-workers used Raney-Ni for SCWR of glycerol as a model compound for activated sludge.898 Their results showed that the Raney-Ni catalyst was even more active than Ru-based catalysts under the conditions investigated. Pairojpiriyakul et al. developed a series of Co-based catalysts supported on La2O3, ZrO2, α-Al2O3, γ-Al2O3, and YSZ, among which Co/YSZ produced the highest H2 yields in the temperature range of 723−848 K.899 5.3.2. Process Optimization. Thermodynamic analyses serve as useful tools for predicting reaction behaviors under certain conditions and thus identifying suitable operation parameters. Voll et al. calculated the equilibrium compositions BL


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Scheme 20. Block Flow Diagram for Glycerol Autothermal SCWR System Integrated with Methanol Synthesis Processa

a


methanol and glucose reforming in supercritical water.60,895 Low residence time prohibited H2-consuming methanation from approaching equilibrium, and the residence time should be limited to the order of seconds.64 According to the literature, the effect of operation pressure seemed to be irrelevant in the supercritical region.64,900 However, processing at high pressure above the critical point of water not only raises the investment and operation costs but also challenges the design and maintenance of the system. As a consequence, low pressures sufficient to keep the system in the supercritical state seem advisible. Ortiz’s group conducted a series of process and energy integration studies aiming to further reduce the energy input for glycerol SCWR process. Autothermal reforming in supercritical water was investigated by introducing oxygen into the reaction system.903 On the basis of process simulation and calculations using Aspen Plus, they proposed an energysufficient autothermal SCWR system for maximum electrical power generation by using a turbine expander and a PEMFC unit.904−906 The power generation mainly comes in two pathways: (i) the expansion of product gas in the turbine at the outlet of the reformer released huge pressure energy; and (ii) purified hydrogen (up to 99.999 mol % H2) was converted into electrical energy in the PEMFC after being upgraded using serially two WGS reactors and a pressure swing adsorption (PSA) unit. In addition, PSA off gas, which mainly consisted of CH4 and unrecovered H2, was fed into a furnace to provide a proportion of thermal energy for the reforming process. As a result, the oxygen needed within the autothermal reformer was reduced. A total net power of 1600.73 KW was achieved by feeding 1000 kg/h of pure glycerol in aqueous solution with an oxygen-to-glycerol ratio of 0.17. Finally, the self-sustaining system was integrated with methanol synthesis process, in which the PSA unit was adjusted to produce syngas (Scheme 20).907 Ortiz et al. reported that a specific methanol production of 0.270 kg

methanol/kg glycerol as well as an overall energy efficiency of 38% were obtained. Moreover, CO2 sequestration in PSA unit yielded 0.38 kg CO2/kg glycerol, providing added value for the proposed process. 5.4. Photocatalytic Reforming

In addition to methanol, various biomass-derived feedstocks have been studied as hole scavengers. Among them, low-cost glycerol is an economically favorable choice in great abundance.908 More imporantly, Bowker et al. evaluated the H2 production rates for the PR of several biomass-derived oxygenates in the literature (Figure 59), and glycerol was found to exhibit the highest H2 evolution rates.909 The trend was

Figure 59. Relative rates of hydrogen production from photoreforming of several oxygenates. Reprinted with permission from ref 909. Copyright 2015 Springer. BM


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further substantiated by recent studies of Waterhouse and coworkers.413,908 The group examined the PR activity of a series of M/TiO2 (M = Pd, Pt, Au) catalysts for H2 production in various alcohol−water mixtures. Interestingly, they found that for each M/TiO2 catalyst, the hydrogen production rates decreased in the order glycerol > 1,2-ethanediol (ethylene glycol) > 1,2-propanediol > methanol > ethanol > 2-propanol > tert-butanol > water. The H2 production rates were positively correlated with the free energy change of the alcohol oxidation reaction, which is the potential separation between the alcohol oxidation potential and the valence band of TiO2 (Figure 60).908

Figure 61. Rates of H2 and CO2 production versus irradiation time obtained from Pt/TiO2 catalyst in a 0.368 mM glycerol aqueous solution. The solid line represents data acquired from pure water. The highlighted region is the additional H2 evolution due to the presence of glycerol in solution. Incident light intensity: 3.79 × 10−7 einstein s−1. Reprinted with permission from ref 97. Copyright 2008 Springer.

by photogenerated species, which eventually yielded H2, CO, and CO2. It is noteworthy that CO2 is formed via the WGS reaction of CO, which is known to take place rapidly in photocatalyst suspension under irradiation.912 Palladium has also been utilized to decorate the TiO2 surface for enhancing the PR activity. According to Bowker and coworkers, Pd NPs served as active sites for decarbonylation and dehydrogenation of adsorbed glycerol to yield H2 and strongly bounded CO species as shown in Figure 62.913−915 On the

Figure 60. Oxidation potentials (vs NHE) of different alcohols shown with respect to the CB/VB of TiO2. Reprinted with permission from ref 908. Copyright 2015 Elsevier.

Figure 62. Probable reaction mechanism for PR of glycerol over Pd− TiO2 catalysts. Reprinted with permission from ref 24. Copyright 2013 The Royal Society of Chemistry.

5.4.1. Noble Metal Cocatalysts. Over a 0.5 wt % Pt/TiO2 catalyst, a significant enhancement in initial H2 production rates was observed in glycerol aqueous solution, which was 1 order of magnitude greater than that in pure water (Figure 61).97 Moreover, the reaction rate of PR was very low in the absence of Pt, confirming the promotional effect of Pt cocatalysts. A series of mechanistic studies have been conducted over Pt−TiO2 catalyst by Kondarides and co-workers.97,910,911 Specifically, the PR reaction is initiated by excitation of TiO2 with light absorption to produce electrons and holes. The photogenerated electrons migrate to the Pt−TiO2 interface to reduce the protons into H2. Meanwhile, the glycerol molecules dissociatively adsorb on the TiO2 surface, possibly through OH groups, and were readily oxidized and decomposed by photogenerated holes. In addition, hydroxyl radicals generated from the oxidation of water by holes acted as oxidant for glycerol as well. A series of hydrogenolysis and dehydrogenation reactions were triggered by the attack of adsorbed glycerol

other hand, the excitation of TiO2 by light absorption led to charge separation and the formation of active oxygen species such as O−, which subsequently removed the adsorbed CO from metal surface to gas-phase CO2, leaving an oxygen vacancy (V−) behind. The oxygen vacancy V− was finally refilled by the reduction of H2O with more H2 production to complete the catalytic cycle. To some extent, this process can be considered as a photoinduced WGS reaction, which was also reported for the Pt−TiO2 system previously.24,423,916 As another noble metal, Au cocatalyst has been studied in various photocatalytic reactions including the biomass PR.98,917,918 Unlike Pd−TiO2, mechanistic studies showed that photoinduced WGS reaction would not take place for Auloaded TiO2 under ambient conditions, possibly due to the relatively weaker CO adsorption on Au.913,919 The reduction of H+ by electrons and the total oxidation of glycerol by the holes BN


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seem to be responsible for H2 production via PR of glycerol over Au-modified TiO2 photocatalysts, but this mechanism is not univocal and remains to be explored.914,916 Recently, enhanced photocatalytic activities toward PR of glycerol were reported over TiO2 decorated with Au-based bimetallic nanoclusters. Under both UV and simulated sunlight irradiation, the hydrogen production via PR of glycerol was higher over Pt−Au bimetallic catalysts than those obtained over monometallic catalysts. 920 This enhancement could be attributed to the nature of Pt−Au alloy, which facilitated the desorption of hydrogen. Stable H2 production was observed for at least 65 h under simulated sunlight irradiation. By loading Pd−Au cocatalysts onto commercial P25 titania (80% anatase and 20% rutile), Bowker et al. found that alloying Au with Pd resulted in higher hydrogen yield than either of the parent metals at the same weight loading.913 A synergy was proposed that the formation of the Pd−Au alloy enhanced the storage of the photogenerated electrons as well as the rates of trapping them. Detailed studies into the intrinsic nature of this promotional effect are yet to be conducted. Several noble metal oxides such as RuO2 and IrO2 have been used as efficient cocatalysts in water-splitting.921,922 Such a promotional effect is also found in PR. Gu et al. enhanced the H2 production in PR of biomass feedstocks (methanol, ethanol, glycerol, and glucose) by depositing highly dispersed RuO2 particles on the surface of TiO2.923 Moreover, when SnOx was further grafted on RuO2/TiO2, they noted a dramatic enhancement in catalytic activity. As shown in Figure 63, it

Figure 64. HRTEM micrograph of embedded CuOx@TiO2 catalyst. The digital diffraction pattern (DDP) of the particle in the black square can be assigned to metallic Cu. The thin amorphous layer covering the Cu nanoparticle in the black square may consist of either titania or oxidized copper phase as a result of the preparation process. Reprinted with permission from ref 927. Copyright 2010 American Chemical Society.

to Cu−TiO2 by the conventional impregnation method. The large contact area between CuOx and TiO2, combined with the possible dissolution of Cu ions into the TiO2 lattice modifying the band gap, could account for the enhanced reactivity of this system.927,928 Moreover, Arai et al. reported an active NiOx-loaded TiO2 catalyst for photocatalytic H2 production from aqueous glycerol solution.929 They demonstrated that the formation of n-type (NiOx) and p-type (TiO2) heterojunctions decreased the band gap energy by 0.58 eV as compared to the TiO2 substrate, extending the energy range of photoexcitation for the system and improving the efficiency of PR process. Apart from TiO2-based systems, identification of alternative metal oxides has proceeded for H2 production via PR process. Recently, a series of iron(III) oxide polymorphs prepared by chemical vapor deposition (CVD) technique were investigated in the PR of ethanol, glycerol, and glucose, among which two scarcely studied β-Fe2O3 and ε-Fe2O3 were used in the PR process for the first time.103 All samples exhibited stable PR performances. As compared to common α-Fe2O3 (hematite), βFe2O3 and ε-Fe2O3 possessed superior catalytic activities toward hydrogen production. This study indicated the potential application of an iron oxides-based system in the PR of biomass-derived oxygenates. Additionally, the control of crystal phase could also be a key issue for the fabrication of robust photocatalysts.

Figure 63. Schematic representation of the inner electric mechanism inside the comodified RuO2/SnOx/TiO2 photocatalyst. Reprinted with permission from ref 923. Copyright 2013 Elsevier.

was proposed that an inner electric field formed between RuO2 and SnOx, which served, respectively, as a cathode and an anode. As a consequence, this electric field directed the electron flow to the SnOx species, while the holes were trapped by RuO2, resulting in remarkable accelerated charge separation and enhanced photocatalytic performance. A similar mechanism was also reported for Pt/RuO2/TiO2 catalyst in watersplitting by Borgarello and co-workers.924,925 5.4.2. Non-Noble Metal Cocatalysts. Exploration of cheap and largely available cocatalysts has been conducted. Cobalt loaded TiO2 was reported to be effective in PR of glycerol by Kumari and co-workers.926 They impregnated copper ions onto TiO2 and calcined the samples at 623 and 723 K to achieve CuO/TiO2 and Cu2O/TiO2, respectively. The Cu2O/TiO2 catalyst exhibited continuous H2 production for glycerol−water mixtures, suggesting a positive role of Cu ions in the +1 oxidation state. Gombac et al. prepared an embedded CuOx@TiO2 catalyst through a microemulsion method to achieve highly dispersed CuO species on TiO2 (Figure 64).927 Superior performance in glycerol PR was observed with respect

6. DIMETHYL ETHER REFORMING Dimethyl ether (DME), also known as methoxymethane, is the simplest ether with a formula of CH3OCH3. DME is a nontoxic, noncarcinogenic, nonmutagenic, and noncorrosive compound.930 Under standard atmosphere conditions (1 bar and 298 K), DME is in a gaseous state, but it can be easily liquefied when pressurized above 5 bar or cooled at 258 K. Conventionally, DME is produced via a two-step process, methanol synthesis from syngas followed by dehydration of methanol. The direct synthesis of DME from syngas is a more thermodynamically and economically favorable alternative.931,932 In principle, DME can be produced from a variety of feedstock such as natural gas, coal, residual oil, crude oil, and biomass. Recently, the Chemrec Co. built and operated the world’s first bioderived DME plant in Pitea, northern Sweden, as a part of the European project BioDME. The synthetic process of DME is based on the gasification of black liquor, a BO


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Table 19. Representative Catalysts for DME SR sample Cu/SiO2+HPA/Al2O3942 CuZnAl945 CuZnAl+ZSM-5-90 (Si/Al = 90)946 CuZnAl+desilicated HZSM-5949 CuZnAl+HZSM-5953 CuZnAl+MgO-HZSM-5950 Cu/ZnO/carbon+Al2O3955 CuZnAl0.8Zr0.2O956 CuZnAlCe0.1957 CuZn/ZrO2-monolith961 Cu−CeO2/Al2O3959 Cu−Ce−Co−O960 Cu−Ce−Mn−O960 Cu/CeO2+H-mordenite940 Cu/CeO2+WO3/ZrO2966 CuFe2O4+Al2O3241 CuMn2O4+Al2O3241 CuCr2O4+Al2O3241 CuGa2O4+Al2O3241 CuAl2O4+Al2O3241 CuFe0.75Mn0.25O4+Al2O3241 CuZnAl-γ-Al2O3941 CuFe2O4-γ-Al2O3941 CuCr2O4-γ-Al2O3941 CuMn2O4-γ-Al2O3941 CuNiFe2O4−Al2O3981 CuFe2O4−Al2O3984 (heat treatment at 973 K) CuFe2O4−Al2O3984 (heat treatment at 1073 K) Rh/Al2O3990 Pd/CeO2+H-mordenite940 Pd/ZrO2991 Ru/Al2O3993 Pt/Al2O3993 Au/CeO2−Al2O3997 K-promoted Au/CeO2−Al2O3997 Ga2O3/Al2O31000 Ga2O3/TiO21000 ZnO−Al2O31002 ZnCr−TiAl1003 ZnAlCe0.21005 Mo2C/norit + Al2O31013

T (K)

S/C ratio

GHSVa (h−1)

X (%)

523 673 573 573 573 563 573 673 673 753 623 673 673 523 548 623

3.2 1.5

1200

36.6

3 2 2 2.5 2 2.5 1.5 1.5 1.5 1.5 1.7 1.8 2.5

0.3d 0.6d 4000 2000 (15 000) (12 000) (11 300) 10 000 30 000 30 000 4500 4500 2000

623 623 623 623 623 623 623 623 648 648

2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

2000 2000 2000 2000 2000 2000 2000 2000 9100 9100

93 70 79 88 90 90 ∼30 ∼90 ∼65 ∼75 76 86.6

648

2.5

9100

84.3

923 523 753 673 673 773 773 673 673 733 703 693 673

3.0 1.7 1.5 2.5 2.5 0.5 0.5 1.5 1.5 2.4 2.4 2.5 1.5

100 000 4500 1500 5400 5400 8000 8000 (20 000) (20 000) 7900 7900

∼100 75 ∼80 ∼100 ∼95

(8000)

90.0 75 85 95 87 95 56 100 85 68 86

∼100 100 98 ∼100 ∼90 ∼100 ∼80

H2 production rate (mol kgcat−1 h−1)

SH2/YH2b (%)

SCO (%)

SCO2 (%)

2.9c 87b

9.5 12

90.5

70b 81b 93b

5 8

90

2

92b 85b 70.4

17 13 3.7

83 80 25.4

0.4

70 70

20 10

7 12

8 13 5 10 4 7 88 86 91 80 19.3

89 83 93 86 94 90 2 10 4 15 79.6

3 4 2 4 2 3 10 4 5 5 1.1

5.9

10.6

1.7

3

22 20

60 17

SCH4 (%) 1

144 180

610

71 ∼50 35e 24e 27e 30e 32e 31e ∼20 ∼70 ∼45 ∼50 186

42 53 70 18 60 73b 87b 67b 55b 315 345 ∼100b 65b

∼30 30 59 59 ∼6 ∼7

∼4 65 38 38 ∼95

a In parentheses, space velocity (mL gcat−1 h−1), when GHSV is unavailable. bH2 yield (%). cH2/(CO+CO2) molar ratio. dUnit: gcat h gDME−1. eUnit: mL min−1.

due to the absence of C−C bonds. Additionally, the nontoxicity of DME offers an advantage over methanol. Furthermore, the physical properties of DME are very similar to those of liquefied petroleum gas (LPG), which enables the utilization of well-established LPG infrastructures for DME transportation and storage.930

residual product from the Kraft process in pulp and paper industries. This project could be a herald of large-scale DME production from renewable sources. Since the mid-1990s, DME has been widely exploited as an excellent diesel alternative with high cetane number (i.e., 55− 60). As compared to conventional petroleum fuels, DME is considered to be greener as it burns with no sulfur, SOx, soot, and low NOx emission.933 DME is atmospherically benign with a very small global warming potential (0.3 over a 100-year integration).934 DME is also an important hydrogen carrier with high hydrogen-to-carbon ratio (H/C = 3). In recent years, DME has been gathering interest as a potential feedstock for hydrogen production. Similar to methanol, DME can be converted to H2-rich gas at low temperatures of 473−673 K

6.1. Reforming Mechanism

Generally, the SR of DME (eq 67) proceeds via a two-step pathway involving two consecutive reactions: DME first undergoes hydrolysis (eq 68) to methanol and then MSR takes place to produce H2-rich gas. The RDS is typically considered as DME hydrolysis, which is known to be an equilibrium-limited reaction.935,936 Additionally, several side reactions are involved in the process affecting the product BP


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temperature exceeds 573 K, therefore demanding exploration of thermally stable MSR catalysts.939 Obviously, the development of acidic catalysts is a key issue to achieve satisfactory performance in DME hydrolysis and overall SR reactions. As discussed earlier, a suitable DME SR catalyst requires intimate cooperation of acidic and metallic functions. The role played by metallic catalysts as well as the interaction between the two functions is by no means less important. Specifically, the catalyst development for DME SR is categorized according to the classification of MSR catalysts used, while relevant investigations upon acidic catalysts and their interactions will be introduced. 6.2.2. Supported Cu-Based Catalysts. Because the hydrolysis reaction is equilibrium-controlled and limited at a low conversion (ca. 20% at 523 K), it becomes the RDS for the overall DME SR reaction.940 Sufficiently high MSR activity is essential to eliminate methanol instantaneously to keep DME hydrolysis away from equilibrium, and thus particular emphasis has been placed upon developing robust MSR catalysts.935,941 Cu-based catalysts, the most widely used system in MSR due to their low cost and high activity, has been introduced to the DME SR process. However, the inherent tendency for copper crystallites to rapidly sinter at temperatures above 573 K is the primary challenge toward stable catalytic performance over Cubased catalysts.939 In addition, the blockage of surface active sites by coke deposition often leads to catalyst deactivation. Many efforts have been dedicated to address these issues by altering catalyst composition, modifying catalyst surface properties, and tuning the acidic function. Sobyanin and coworkers were among the first to study hydrogen production via DME SR using a mechanical mixture of a Cu/SiO2 catalyst and a heteropoly acid-containing catalyst H4SiW12O40/Al2O3.942 Complete DME conversion with a H2 outlet concentration of 71 vol % was obtained at 563 K, showing the feasibility of Cubased composite catalysts for DME reforming. As elaborated in section 3.1.3, CuO/ZnO/Al2O3 is an efficient versatile catalyst for methanol synthesis and conversion.943,944 In the case of DME reforming, in addition to dispersing Cu and ZnO, Al2O3 also acts as the catalyst for hydrolysis reaction, and meanwhile CuZnAl could flexibly cooperate with various solid acids to modify its acidic properties. Semelsberger et al. observed that the H2 yield reached 89% of the predicted equilibrium over a CuZnAl catalyst, but the high temperature (673 K) required for DME hydrolysis on γ-Al 2 O 3 raised concerns about catalyst durability.945 Kawabata et al. combined CuZnAl with several solid acid catalysts, among which CuZnAl/ZSM-5 composite exhibited the highest DME SR activity.946 Stable performances were observed up to 573 K; however, gradual deactivation occurred as temperatures further increased, mainly due to the formation of carbonaceous species on Cu catalyst. Erena and co-workers confirmed that the reforming temperature could be lower for the CuZnAl catalyst when using H-ZSM-5 zeolite instead of Al2O3 as the acidic function.947 They reported that the acidic properties of zeolite were crucial to the product distribution, because they observed substantial formation of C2−C4 hydrocarbons due to the high acidity and acidic strength of HZSM-5. Therefore, alkaline treatment was adopted to moderate the zeolites’ acidity.948 The treatment led to slight activity loss in DME hydrolysis, and nevertheless effectively suppressed undesired hydrocarbon formation. Consequently, the composite of CuZnAl and alkaline-modified HZSM-5 with 1:1 weight ratio struck an optimized balance between metallic

distribution, including reverse-WGS reaction, methanol decomposition, and DME decomposition. CH3OCH3 + 3H 2O → 6H 2 + 2CO2 ° ΔH298 = 135 kJ mol−1

CH3OCH3 + H 2O → 2CH3OH

(67) ° ΔH298 = 37 kJ mol−1

(68)

6.2. Catalysts for DME Steam Reforming

For DME SR, dual active sites are essential to catalyze the two main reactions efficiently. The hydrolysis of DME requires acidic sites, whereas metallic sites are necessary for MSR. By far, solid acids (e.g., H-ZSM-5 zeolite) and acidic metal oxides (e.g., Al2O3) have been developed for DME hydrolysis. On the other hand, Cu-based catalysts are most commonly used in MSR, and meanwhile noble metals such as Pd and Pt have also been identified as effective alternatives. It is noteworthy that the combination of both acid catalyst and metal catalyst is indispensable for H2 production via DME SR, because neither of them is able to facilitate the overall reaction alone. Consequently, bifunctional catalysts containing both acidic and metallic sites are desired for the DME SR process. Two approaches are feasible: (i) mechanical mixing of a DME hydrolysis catalyst and a MSR catalyst; and (ii) synthesizing a single bifunctional catalyst possessing actives sites for both reactions. Relevant studies on catalyst development in DME SR are summarized in Table 19. 6.2.1. DME Hydrolysis Catalysts. DME hydrolysis on an acidic catalyst has been considered to be the rate-determining step in DME SR as its reaction rate is much slower than that of MSR.936 Catalytic tests reveal that the DME SR activity is dependent on the DME hydrolysis activity, with higher hydrolysis activity leading to better SR performance.937 It is reasonable that the crucial role played by acidic catalysts in DME SR stimulates many research activities. Currently, the acidic catalysts employed in the DME hydrolysis and SR can be classified into two main categories, zeolites and metal oxides. H-mordenite and ZSM-5 are the representative zeolites used for DME hydrolysis, while various types of alumina are reported to be active was well. Eguchi’s research group systematically examined the effect of acidic catalyst on DME hydrolysis and SR.938 They demonstrated that the acid strength, the type of acid site, as well as the acid amount determined the performance of an acidic catalyst in DME hydrolysis and SR reactions.935 It was proposed that stronger acid strength and higher acid amount led to enhanced activity in DME hydrolysis, which in turn contributed to higher H2 yield via overall DME SR. Zeolites possess mainly strong Brønsted acid sites and exhibit high hydrolysis activity in the low temperature range of 473− 573 K.937 Nevertheless, rapid catalyst deactivation was observed due to severe coke deposition, which originated from the decomposition reactions favored on the strong acid sites. On the other hand, alumina catalysts with weak Lewis acid sites require higher reaction temperatures over 573 K to effectively hydrolyze DME. Alumina catalysts, despite being less active than zeolites, showed less carbon formation and superior durability. It should be noted that higher temperatures also facilitated decomposition reactions, resulting in lower reformate quality due to increased CO and CH4 formation. Moreover, copper is known to be highly susceptible to sintering when BQ


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Scheme 21. Schematic Representation of Carbonization Process for Ion Exchange Resins Loaded with Cu or Cu/Zna

a


Snytnikov et al. reported that Cu-CeO2/γ-Al2O3 was highly selective toward H2 and CO2.958,959 The obtained product stream with low CO content (2 vol %) was viable to be directly introduced to high-temperature PEMFC, which could operate with a CO tolerance up to 5 vol %. Co and Mn were applied to modify Cu−CeO2/Al2O3, considering their oxidation ability in the presence of steam to reduce CO selectivity.960 The promotional effect was more pronounced for the Mn-promoted sample, resulting in a much higher H2 yield and CO2 selectivity as compared to Cu−CeO2/Al2O3. Ledesma et al. prepared a series of catalytic monoliths coated with CuZn supported on CeO2, ZrO2, and CeO2−ZrO2 for the DME SR process.961 The CuZn/ZrO2 catalyst possessed the highest H2 yield and traceable CO and CH4. They also investigated the feasibility of a Pd promoter for the catalytic monoliths; although the CuPd/ZrO2 catalyst exhibited higher initial activity, its stability was relatively inferior to CuZn/ZrO2 as shown in Figure 65.962 In situ XPS and IR studies revealed

and acidic functions, selectively producing hydrogen at 573 K in the absence of hydrocarbons.949 Similar results have been reported by Long et al., as they managed to acquire a H2 yield of 93% by minimizing C2−C4 hydrocarbons formation over a physical mixture of MgO-modified HZSM-5 and commercial CuZnAl.950 Erena’s group further investigated the deactivation behavior of the DME SR catalysts. It was reported that Cu NPs would be stabilized below 598 K in the CuZnAl/H-ZSM-5 composite catalysts, whereas coke deposition, primarily on metallic active sites, was the main culprit for catalyst deactivation.951 The origin of coke was tentatively attributed to the degradation of methoxy ion intermediates as proposed Agarwal et al. in MSR, which could be readily decreased by tuning the operational parameters.952,953 Regenerability is a vital criterion for catalyst selection from an industrial standpoint. Arteta et al. found that postcombustion in air at 588 K effectively removed the coke deposited on the metallic sites of CuZnAl/H-ZSM-5 composite catalyst, whereas 773 K was necessary for complete removal of coke deposited on the zeolite.954 At such high temperature, copper underwent sintering irreversibly, and thus the recyclability of the catalyst was limited. Various modifications have been made. Kudo et al. prepared a novel Cu/Zn/carbon catalyst by carbonization of an ion-exchange resin containing both Cu and Zn cations.955 The carbonized resin had a microporous structure with Cu and ZnO particles, and ZnO content contributed to disperse Cu as shown in Scheme 21. As compared to the monoloaded Cu/carbon, the stable bond of the Zn cation to the carboxyl group and less migration ability of ZnO confined the growth of Cu NPs during the carbonization process. When combined with Al2O3, the composite catalyst showed enhanced activity in DME SR with respect to the conventional CuZnAl catalyst. Meng’s research group reported the partial incorporation of CeO2 or ZrO2 attenuated the Cu−Al interaction to enhance the reducibility of copper, while the SMSI between Cu and CeO2/ ZrO2 not only promoted the Cu dispersion but also stabilized Cu species against thermal sintering, even at temperatures above 673 K.956,957 Moreover, XANES spectra revealed that the presence of CeO2/ZrO2 contributed to maintain high amounts of Cu+ species, leading to better catalytic performances.

Figure 65. Presence of Zn or Pd affects the DME SR performance by altering the predominated reaction intermediates on catalyst surfaces. Reprinted with permission from ref 962. Copyright 2011 American Chemical Society.

that the presence of Zn or Pd affected the distribution of surface copper species, which, in turn, determined the evolution of reaction intermediates. Specifically, low valence copper species (Cu0 and Cu+) prevailed over CuZn/ZrO2 surface and favored the formate intermediates under reaction conditions, whereas oxidized copper species (Cu2+) were predominant over CuPd/ZrO2 surface and stabilized the less reactive methoxy intermediates, leading to the formation of carbonaceous residues. BR


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in the catalysts. Specifically, upon reduction, all Cu spinel samples possessed greater Cu+ species on the surface than CuZnAl. Moreover, the reduced CuFe2O4 and CuMn2O4 possessed higher content of Cu+ species, whereas reduced CuCr2O4 provided lower proportion of Cu+. In situ XPS analysis revealed that reduced Cu spinel catalysts maintained Cu+-rich surfaces during reforming reaction, while the change of oxidation state Cu was blamed for the fast degradation of CuZnAl catalyst.974,975 It has also been reported that CH3OH was activated via the O−H bond scission to CH3O on a Cu0-rich surface, whereas the formed methoxy group was subsequently oxidized to formate ion on a Cu+-rich surface.941,976 DFT calculations were performed to investigate methoxy dehydrogenation, which was widely assumed as the RDS of MSR over several Cu/B-metal oxide (Fe3O4, MnO, and Cr2O3) surfaces.977−980 Cu/Fe3O4 displayed the smallest reaction energy and the lowest energy barrier for methoxy dehydrogenation (Figure 66) with the highest reforming activity, consistent with experimental results.

The reforming process is more economically favorable to proceed at lower operation temperatures in consideration of energy-saving, and copper is less prone to agglomeration under such conditions. Imamura and co-workers studied the combination of solid acids and a Cu/CeO2 catalyst in the DME SR process.940 Indeed, hydrogen production was obtained over H-modenite-Cu/CeO2 at a temperature down to 523 K. Additionally, solid superacids (i.e., WO3/ZrO2) exhibited good low-temperature performance when combined with CuO/CeO2.963−965 It was found that the loading of WO3 had a great influence on the reforming activity due to different surface configurations.966 A monolayer of WO3 was formed with high density of active W(V) species on the ZrO2 surface up to WO3 loading of 10+ wt %, while excessive WO3 formed less reactive bulk WO3. It is noteworthy that coke deposition is a grand challenge upon the solid acid-CuO/CeO2 system as coke precursors migrated from solid acids to the metallic copper sites and deactivated the catalyst. The large amount of coke precursors was presumably related to the excessive WO3 species, which could be passivated by vigorous calcination. Upon calcination at 1073 K, the 10 wt % WO3/ZrO2−CuO/ CeO2 composite gave considerable durability at 523 K for 100 h on stream. 6.2.3. Spinel-Typed Cu Catalysts. To allieviate deactivation of copper-based catalysts by sintering, synthetic mineraltype precursors, mainly spinel-typed catalysts, have attracted a growing research interest.20,967−970 The resultant metal catalysts typically possess high metal dispersion and a strong metal−support interaction, therefore exhibiting superior activity and sintering-resistance even under harsh reaction conditions.480,700 Particularly, spinel-typed Cu-based spinel oxides with a general formula of CuB2O4 (B = Fe, Mn, Cr, etc.) have been applied in many chemical processes including DME SR.242,971,972 Eguchi’s group has studied extensively the utilization of Cubased spinel oxides in DME reforming. A series of CuB2O4 spinel oxides (B = Fe, Mn, Cr, Ga, Al, or FeMn) were prepared via a sol−gel method followed by a subsequent solid-state reaction.241,973 The spinel oxides were mechanically mixed with Al2O3, and the composite catalysts were evaluated in SR of DME. The descending order of activity was as follows: CuFe 2 O 4 , CuFe 1.5 Mn 0.5 O 4 > CuAl 2 O 4 > CuCr 2 O 4 > CuMn 2 O 4 > CuGa 2 O 4 , whereas only CuFe 2 O 4 , CuFe1.5Mn0.5O4+, CuCr2O4, and CuMn2O4 remained stable during the reforming reaction at 623 K. The stability of the B metal oxides (e.g., Fe2O3, MnO, MnFe2O4, and Cr2O3) and the interaction between Cu species and B metal oxides significantly contributed to the durability of the catalysts. As compared to commercial CuZnAl catalysts, the CuB2O4 spinel-Al2O3 (B = Fe, Mn, Cr) composite catalysts exhibited superior activity, selectivity, and stability in DME SR.941 The order of overall DME SR activities over the composite catalysts, CuFe2O4-γ-Al2O3 > CuMn2O4-γ-Al2O3 > CuCr2O4-γ-Al2O3 > CuZnAl-γ-Al2O3, was identical to that of MSR activities observed over corresponding Cu catalysts, suggesting the pivotal role of Cu catalysts in the overall reforming reaction. The authors proposed that the improvement in reactivity was associated with the oxidation state of Cu species after H2 reduction. In MSR, it is generally accepted that there exists an optimum balance between metallic Cu0 and oxidized Cu+ for optimal performance in a particular catalyst under certain reaction conditions.12 In this study, the activity sequence observed was in good agreement with the surface Cu+ content

Figure 66. Diagram for CH3O dehydrogenation barrier energies (Ea) and reaction energies (ΔE) on Cu4−B−metal oxide surfaces and the structures of adsorbed CH3O* (left), coadsorbed CH2O*+H* (right), and corresponding transition states (middle). Reprinted with permission from ref 941. Copyright 2013 American Chemical Society.

Indeed, CuFe2O4 spinel-based catalysts exhibited excellent activity and stability in DME SR, making them very attractive candidates for further investigation. For instance, the promotional effect of doping a small amount of Ni to the Cu−Fe spinel was reported by Eguchi and co-workers.981 As shown in Figure 67, the Cu0.95Ni0.05Fe2O4−Al2O3 catalyst maintained complete DME conversion for ca. 700 h and gave a final DME BS


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requirement for higher operation temperatures, because CuFe2O4 spinel catalyst had good sintering-resistance. The mixing ratio of acidic catalyst and Cu catalyst is crucial to balance the two functions to obtain maximum H2 production via DME SR. High Cu spinel/Al2O3 ratios lead to low DME conversion due to insufficient acidic sites, while the ratios not only result in low methanol SR activity but also facilitate formation of undesired CH4 and CO via DME decomposition.938 According to Eguchi and co-workers, the optimum ratios of Cu spinel/Al2O3 are in the range of 1:1−2:1 by weight.937,973 Faungnawakij et al. reported that thermal pretreatment affected the crystallinity, reducibility, and surface area of the composite catalysts, which in turn influenced their catalytic performances.254 It turned out that rigorous heat pretreatment at 973−1073 K was highly effective in ameliorating the activity and stability of the composite catalyst, attributed to the formation of a new active CuFe2−xAlxO4 phase via the solidstate reaction at the CuFe2O4 spinel−Al2O3 interface as shown in Scheme 22a.983,984 Stable DME conversion (∼85%) was observed over the pretreated samples at 648 K during 55 h on stream. In addition, thermal treatment could also be applied to the prereduced samples. The CuFe2O4 spinel structure was first reconstructed, and then aluminum atoms migrated the spinel structure via solid-state reactions to yield the new CuFe2−xAlxO4 phase (Scheme 22b). As indicated in Scheme 22, the pretreatment temperature should be controlled cautiously. The solid reaction would not proceed at temperatures lower than 773 K, while it was expedited at temperatures above 1073 K leading to significant shrinkage of surface area due to the phase transformation of alumina (α-Al2O3 to δ-Al2O3).938,983,984 Moreover, heat treatment at 1173 K totally degraded the catalyst, possibly assigned to the deterioration of the acid sites on Al2O3. It should be noted that the thermal pretreatment hampered the

Figure 67. DME SR over CuFe 2O 4−Al2O3 (rectangle) and Cu0.95Ni0.05Fe2O4−Al2O3 (sphere). Reaction conditions: GHSV = 500 h−1, S/C = 2.5, T = 598 K. Reprinted with permission from ref 981. Copyright 2009 American Chemical Society.

conversion of ca. 95% at a TOS of 1000 h, whereas CuFe2O4− Al2O3 composite began to deactivate at TOS of 300 h. The outstanding stability of Ni-doped catalyst was ascribed to the formation of CuNi alloy, which obviously contributed to suppress the sintering rate. Shimoda et al. combined CuFe2O4 spinel with various zeolites including mordenite, β-zeolite, Y-zeolite, and ZSM-5 in search for high-performance formulas.982 All CuFe2O4−zeolite composites showed comparable activities in a low temperature range of 473−548 K. However, excessive coke deposition due to the strong acidic sites on the zeolites led to severe deactivation. Consequently, the stability of CuFe2O4−zeolite composites was inferior to that of CuFe2O4−Al2O3. It appears that the less acidic Al2O3 is more appropriate despite

Scheme 22. Proposed Mechanism of the Formation of New CuFe2−xAlxO4 Phase by Thermal Pretreatment of Fresh (a) and Prereduced (b) CuFe2O4−Al2O3 Composite Catalysta

a

Adapted with permission from ref 984. Copyright 2009 Elsevier. BT


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reducibility of CuFe2O4−Al2O3 composite, and thus an extended activation was essential. The promotional effect of thermal pretreatment was observed exclusively for Cu spinelAl2O3 among -ZSM-5, -mordenite, and -TiO2 systems, indicating that alumina was necessary for the formation of a new interfacial active phase via solid-state reaction.983 In addition, thermal reduction in H2 atmosphere is typically necessary to activate metal catalysts prior to the reaction. According to Faungnawakij et al., the CuFe2O4−Al2O3 catalyst underwent a two-step reduction process at 623 K or lower temperatures.985 CuFe2O4 was initially transformed to metallic Cu and Fe2O3, followed by Fe2O3 to Fe3O4. Upon reduction, Cu clusters were well confined within the matrix of iron oxides with strong chemical interaction as shown in Scheme 23. High

973 K. During the regeneration process, the construction of the Cu−Fe spinel phase led to redispersion of copper species, and meanwhile the coke deposits were easily removed due to its nongraphitic nature.255,975 Restoration of reforming activity was also observed for CuFe2O4−zeolite catalysts.988 Interestingly, the heat treatment in air at 773−973 K even improved the stability of the regenerated catalysts with respect to the fresh catalysts. Moderation of zeolite acidity during the regeneration process was attributed to the enhancement in catalyst durability, because coke deposition favored on strong acidic sites mainly accounted for the severe deactivation of CuFe2O4− zeolite composite catalysts. Therefore, the CuFe2O4−Al2O3 composite has been deemed as a very promising catalytic system for DME SR with excellent activity, stability, and regeneration capability. 6.2.4. Noble Metal-Based Catalysts. Noble metal-based catalysts, such as Rh, Ru, Pd, and Pt, exhibited better thermal stability with respect to Cu.989 Gucciardi et al. demonstrated that DME was efficiently reformed over Rh/Al2O3 catalysts at temperatures of 823−923 K.990 It is worth noting that using DME as the starting fuel gave higher H2 yield and less coke formation than ethanol, due to dehydration reactions on Al2O3. However, noble metals are not selective for reforming reaction because decomposition reactions are highly favored over metal surfaces, generating substantial amounts of CO. WGS converts a fraction of CO to CO2, but the reaction is kinetically slower.12 Imamura and co-workers carried out DME SR over a series of solid acid/Pd-based catalysts. Although Pd-based composite catalysts exhibited much higher DME SR activity than their Cubased counterparts, significant CO was produced irrespective of the supports.940 Llorca and co-workers loaded palladium on monoliths coated with various metal oxides including CeO2, ZrO2, Ce0.5Zr0.5O2, MnO2, SnO2, Al2O3, WO3, and WO3− ZrO2, among which Pd/ZrO2 showed the highest H2 yield in DME SR.991 DRIFTS experiments, combined with TPD studies, suggested that Pd facilitated the formation of methoxy species, which were subsequently converted to formate and carbonate intermediates on ZrO2. Finally, the formate and carbonate species decomposed to yield H2 and CO2 upon heating. The proposed reaction pathway (Scheme 24) is similar to that over PdZn alloys as reported in the MSR section.992 Additionally, methane formation is undesired. High throughput catalyst screening upon 60 precious metal-based catalysts (Pt, Pd, Ir, Rh, Ru) conducted by Yamada et al. identified Pt/Al2O3 to be highly active in terms of DME conversion; however, substantial formation of methane was observed.989 Fukunaga et al. confirmed that H2 yield over Pt/

Scheme 23. Schematic Diagram for the Reduction Process of CuFe2O4 Spinel Catalysta

a


reduction temperatures (e.g., 723 K) were not preferred because Fe3O4 would be further reduced to metallic Fe, and Cu particles would sinter under vigorous heat due to the loss of confinement by iron oxides.257 Catalytic tests showed that composite catalysts reduced at 623 K or below maintained high H2 production after 25 h on stream, whereas catalysts reduced at 723 K or above deactivated rapidly. The agglomeration of copper occurs during the prereduction process, which is detrimental for the performance of the catalysts in SR of DME. However, prereduction is not necessarily indispensable. Faungnawakij et al. reported that unreduced CuFe2O4−Al2O3 composite exhibited lower initial activity than the reduced catalyst, but showed comparable activity upon a longer activation period.254 Recently, Gao et al. have demonstrated that the omission of prereduction process prevented the sintering of Cu prior to the reaction, and the CuAl2O4 spinel catalyst gradually released active Cu species during reaction, resulting in a low copper-sintering rate.986 The deactivation and regeneration behaviors of CuFe2O4− Al2O3 composite catalysts have also been investigated. Eguchi and co-workers performed a 1100 h-stability test over CuFe2O4−Al2O3 composite catalyst and achieved complete DME conversion with a high H2 concentration of around 80% at 453 K for 800 h.255 The composite catalyst suffered partial deactivation after 800 h due to the concomitant effect of copper sintering and coke deposition. Other possible routes for the catalyst deactivation such as metal oxidation and poisoning were considered but excluded. It was deduced that the deactivation of the composite catalyst followed a first-order kinetics.134,987 The deactivation kinetic model offered a useful tool for catalyst lifetime prediction. The spent CuFe2O4−Al2O3 catalyst could be fully regenerated by calcination in air in the temperature range of 773−

Scheme 24. Schematic Reaction Process of DME and Methanol over Pd/ZrO2 Catalysta

a

BU

Reprinted with permission from ref 991. Copyright 2011 Elsevier. DOI: 10.1021/acs.chemrev.6b00099 Chem. Rev. XXXX, XXX, XXX−XXX

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a ZnO/(ZnO+Al2O3) ratio of 0.4−0.5. Aiming to accelerate the rate-determining hydrolysis reaction, TiO2 was deposited on Al2O3 to strengthen its acidity via the formation of Ti−O−Al bonds. 1003,1004 The resultant ZnO−Cr 2 O 3 −TiO 2 −Al 2 O 3 (ZnCr−TiAl) catalyst maintained a high H2 production in the absence of obvious activity decay for 150 h on stream. Recently, a Ce-substituted ZnAlCex ternary oxide has been applied in DME SR, and its TOF was 3 times that of ZnAlO.1005 Zn K-edge XANES and EXAFS spectra revealed that the incorporation of Ce hindered the transformation of ZnO to the less reactive ZnAl2O4 spinel. It was assumed that the introduction of Ce induced the formation of an interstitial ZnxCe4+1−2xCe3+2xO2 solid solution at the oxide interfaces, which contained abundant Ce4+−O−Zn2+ linkages. The Ce4+− O−Zn2+ linkages were more active than the Zn2+−O−Zn2+ linkages for methanol dehydrogenation, the RDS for ZnObased catalyst in the MSR reaction.310 Apart from metal oxides, metal carbide is another group of materials widely used in heterogeneous catalysis including WGS reaction, F−T synthesis, and partial oxidation of methane.1006−1009 Previously, Solymosi and co-workers used a Mo2C catalyst to produce H2 from methanol and ethanol via reforming and/or decomposition reactions, and extended its application to DME reforming.1010−1012 MoC2 prepared by MoO3 reaction with carbon Norit solely exhibited decomposition activity, but H2 production was greatly promoted when mixed with alumina, which facilitated the hydrolysis of DME into methanol.1013 Recent studies on model catalysis by Chen et al. revealed that depositing a metal monolayer (e.g., Ni, Pt) on metal carbides (e.g., WC, MoC2) significantly altered the property of substrate, and the resultant bimetallic surfaces showed superior performances in various reactions including reforming, suggesting that metal carbide is a prospective catalyst for the reforming process.1014−1018

Al2O3 was severely compromised by high levels of CH4, which was related to the high methanation activity of platinum-group metals.993−996 Au NPs supported on CeO2−Al2O3 were reported by Solymosi et al. for DME reforming. DME hydrolysis took place on Al2O3, while fast decomposition of methanol occurred at the Au/CeO2 interface.997 Further incorporation of K promoted WGSR, and the H2 yield finally approached ∼87%. However, the durability of Au-based catalyst should be a concern because Au is precarious under reaction conditions.12 6.2.5. Oxide and Carbide Catalysts. Metal oxides are promising candidates for reforming due to their stability, wide availability, and no need for prereduction. Yamada and coworkers first studied the possibility of a metal-free Ga2O3 catalyst for DME reforming.998,999 Together with Al2O3 as a hydrolysis catalyst, the Ga2O3−Al2O3 mixed oxides prepared by the sol−gel method efficiently produced H2 via DME SR, although elevated temperatures (623−673 K) were required as compared to the Cu-loaded samples. The support effect of Ga2O3 was examined on various oxides, and corresponding DME reforming performances in terms of H2 yield are shown in Figure 68.1000 Ga2O3 supported on TiO2 exhibited the

6.3. Autothermal Reforming

The feasibility and efficiency of DME ATR have been evaluated by several groups. Thermodynamic analysis by Faungnawakij et al. demonstrated that thermo-neutral conditions could be achieved by varying the O/C and S/C ratios.

Figure 68. H2 yield on Ga2O3 on various oxides at different temperatures. Reaction conditions: GHSV = 20 000 h−1; feed composition of 1% DME and 3% H2O diluted with N2. Reprinted with permission from ref 1000. Copyright 2006 Elsevier.

CH3OCH3 + 0.5O2 → 2CO + 3H 2 ° ΔH298 = −38 kJ mol−1

highest H2 yield, followed by Ga2O3/Al2O3 and Ga2O3/SiO2 with comparable activity. XRD and XPS results revealed that the dispersions of Ga2O3 on Al2O3 and SiO2 were lower due to the formation of poorly dispersed Ga2O3 clusters or to the burial of Ga2O3 by the support. Furthermore, the existence of an electronic interaction between Ti and Ga probably contributed to the enhanced reactivity. Nevertheless, further investigations are necessary to identify the active centers of Ga2O3 catalysts in DME SR. Previous studies in MSR have demonstrated that metal-free ZnO-based catalysts are active and selective toward H2 and CO2 production.310,1001 Considering the important role of MSR, ZnO-based catalysts combined with proper acidic catalysts seem very promising in DME reforming. Chen’s research group synthesized a series of ZnO−Al2O3 catalysts with various ZnO/(ZnO+Al2O3) molar ratios, in which Al2O3 hydrolyzed DME and ZnO subsequently converted the methoxy intermediates via the formate species into H2 and CO2.1002 The composition of the catalyst determined its reactivity, and optimal results were obtained over samples with

(69)

One should note that a catalyst active for both SR and POx reactions is essential for the ATR process. As stated before, Cucontaining catalysts are not suitable for ATR processes due to hot spot formation and pyrophoricity issues.376 Noble metals are well-known to be active for oxidation reactions, among which Pd and Pt are the most commonly used. Moreover, Pd-based catalysts are also efficient for DME reforming when combined with an acidic function. Nilsson et al. reported PdZn/Al2O3 catalysts were effective for ATR of DME.1019a The Pd−Zn interaction favored reforming reactions over decomposition reactions over Pd metal, thus generating less CO in the product gas. By thermal treatment of the ZnO− Al2O3 support, Pd supported on ZnO/ZnAl2O4/Al2O3 mixtures showed better reforming activity than PdZn/Al2O3.1019b Asami and co-workers used Pt/Al2O3 and a Ni−MgO solid solution as the oxidation and reforming catalysts, respectively.1019 They achieved a H2 yield higher than 90% with complete DME conversion at 973 K via the coupled process over a dual catalyst bed. It was found that the manner of BV


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Scheme 25. Strategies for Fuel Production from Lignocellulosic Biomassa

a

Adapted with permission from refs 1033, 387, and 35. Copyright 2006 American Chemical Society, 2010 The Royal Society of Chemistry, and 2011 Wiley.

were inhibited under reaction conditions, demonstrating that cold plasma was promising for H2 production from DME.1024

catalyst loading was important as a hybrid of oxidation (Pt/ Al2O3) and reforming (Ni−MgO) catalysts presented better performance than the separated dual-bed configuration.1020 Further studies showed that Rh/Al2O3 was more robust than Pt/Al2O3 as a POx catalyst.1021 Alkaline metals (e.g., Li, Na, and K) were introduced to modify Rh/Al2O3; however, only Na addition led to elevated reactivity. The promotional effects of Li+, Na+, and K+ were tentatively explained as their different electronic effects on the adsorption behaviors of reacting species on Rh/Al2O3.

7. BIO-OIL REFORMING 7.1. Overview

7.1.1. Bio-Oil Production and Conversion Technologies. The diminishing availability of fossil fuels, together with the associated environmental and geopolitical benefits, has rekindled the interest in biomass-derived renewable transportation fuels. Numerous scientific research has been dedicated to biomass conversion to transportation fuels, as summarized by several well-organized reviews.35,387,1025−1029 On the basis of the chemical structural features, Dumesic et al. divided biomass feedstocks suitable for fuel production into three classes: starches (including sugars), triglycerides, and lignocellulose.387 Among them, lignocellulosic biomass, mainly comprised of cellulose (40−50%), hemicellulose (25−35%), and lignin (15−20%), is the most abundant class. Production of first generation biofuels, mainly bioethanol and biodiesel, utilizes starches and triglycerides that are generally obtained from food crops (e.g., corn, sugar cane, soybean, etc.). The technologies are well-established; however, their feedstocks only represent the minority of biomass, and are restricted to a certain fraction of specific edible crops.1030 Comparatively, lignocellulosic biomass has a wider variety of sources including most energy crops, virgin and waste biomass, and therefore its conversion to transportation fuels does not interfere with current food supply. As a result, next generation biofuels based on lignocellulosic biomass receive increasing interest due to high sustainability and compatibility with current society, considering the fact that food shortage remains a serious challenge in China and over the globe.1031,1032 As shown in Scheme 25, three processes, hydrolysis and two thermochemical processes, gasification and pyrolysis, are currently the primary competing technologies in lignocellulosic biomass transportation fuel production.1026,1027 In the hydrolysis process, the most attracting characteristic is that it allows selective production of particular target chemicals from sugar monomers via either biological or chemical routes. However, a

6.4. Dry Reforming

Dry reforming (eq 70) of DME is regarded as the reverse reaction of the direct synthesis of DME from syngas. Typically, the exhaust of DME engine contains considerable amounts of DME and CO2; therefore, onboard CO2 reforming of DME could be a practical technology to maximize DME utilization, reduce CO2 emission, and recover heat simultaneously.1022 CH3OCH3 + CO2 → 3H 2 + 3CO ° ΔH298 = 246.0 kJ mol−1

(70)

By far, investigations of CO2 reforming of DME are limited. Ma et al. first studied CO2 reforming of DME over Ni/γ-Al2O3 catalyst as a novel route for syngas production.1023 Complete DME conversion with high selectivity of syngas was obtained at temperatures above 923 K. Asami et al. reported that the Cu− Zn−Al2O3 composite exhibited good reforming activity toward syngas production, but the catalyst was only stable at low temperatures (40 wt %).1140 As discussed before, recent investigations in single-atom catalysis may lead to superior catalysts with only a trace of noble metals, which is of great research and economical interest. 7.3.2. Catalysts Derived from Mineral Materials. In recent years, the utilization of natural mineral materials as catalysts (or at least catalyst precusors) has attracted much attention, because they are cheap, widely available in nature, and thermally stable at high operation temperatures (>1073 K).1141 The mineral-based materials, with a definite, but generally not fixed, composition and an ordered atomic arrangement, can be directly or after physical treatment (e.g., calcination) used as catalysts.1142 Calcined dolomites (CaCO3· MgCO3) and olivines ((Fe,Mg)2SiO4) are most commonly used for tar elimination in biomass gasification processes.1143,1144 Moreover, Keller et al. found that natural ores such as bauxite and ilmenite (FeTiO3) were promising for chemical looping reforming applications.1145 Nonetheless, the use of natural material catalysts is limited in the SR of phenol. Constantinou et al. studied the SR of phenol for H2 production in the range of 923−1073 K over calcined calcite (CaCO3) materials of different geological origins.1146 CC


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Figure 73. Schematic model of Co/TiO2 catalyst with and without ALD TiO2 layer decoration. (a) Bare Co/TiO2, (b) TiO2/Co/TiO2, and (c) calcined TiO2/Co/TiO2. Reprinted with permission from ref 1167. Copyright 2014 The Royal Society of Chemistry.

Scheme 29. (a) Reaction Pathways Involved in the Reforming of Glucose and Sorbitol; and (b) Schematic Representation of a Two-Step Conversion of Glucose for Hydrogen Productiona

a


tube with sufficient length before the catalyst bed.1155 Marquevich et al. used a nozzle to spray sugar solutions directly on top of the catalyst bed to minimize coke formation in the freeboard of the fixed bed reactor.1047 He et al. suggested that utilization of fluidized bed reactors could be a solution to the decomposition issues.23 In addition to optimizing reactor design, modifications of operation conditions have also been conducted, such as the introduction of air to initiate char combustion reactions.1155 However, technical challenges including high operation temperatures (∼973 K) and rapid catalyst deactivation demand more investigations in the SR of sugars process. 7.4.2. Aqueous-Phase Reforming. 7.4.2.1. Hydrogen Production. APR of glucose could be conducted in a single flow reactor at temperatures near 500 K, much lower than those required for conventional thermal pyrolysis and steam reforming.1156,1157 The low temperature favors WGS reaction and thus results in low levels of CO. The reactor is operated above the saturation pressure of water to prevent the vaporization and decomposition of glucose.386,1158,1159 Direct conversion of cellulose via APR has been reported, but relevant studies are limited due to its complexity.1160,1161 Catalyst screening revealed that Pt/Al2O3 was an efficient catalyst for APR of glucose in terms of reforming activity and H2 selectivity. Tanksale et al. reported that H2 could be effectively produced via APR of sugar solutions (e.g., glucose, fructose, sucrose, and a physical mixture of glucose and fructose) over a commercial Pt/Al2O3 catalyst.1162 Irmak and co-workers developed an AC-supported Pt catalyst using supercritical fluid deposition technique, which exhibited better performance than Pt/Al2O3 due to higher metal dispersions.1163−1165 Although Sn-modified Raney Ni catalysts showed performance comparable to those of Pt/Al 2O 3 catalyst for H 2 production from small oxygenated hydrocarbons such as EG and glycerol, its utilization in APR of glucose has been scarcely reported.828,874,1166 One possible reason is that deactivation of Raney NiSn catalysts by Ni sintering, oxidation, and leaching is strongly associated with their interactions with water, which is

The authors found that increased temperature and water concentration in the feed were beneficial for hydrogen production, whereas cofeeding CO2 and H2 significantly lowered the rates of reforming reaction.1147 In situ DRIFTSCO2 adsorption showed that catalytic activity of calcined calcite was compromised due to the occupation of active sites by CO2 chemisorbed in the unidentate form. Meanwhile, water and hydrogen competed for dissociation adsorption on surface lattice oxygen anions, and thus a high partial pressure of H2 had an inhibiting effect on the reforming activity. Thus, reforming products, mainly H2 and CO2, should be readily removed from the catalytic system. Additionally, olivines and calcined dolomites exhibited good reforming activity at low (e.g., 923 K) and high reaction (e.g., 1073 K) temperatures, respectively, indicating that natural materials are feasible for SR of phenol.1148 7.4. Sugars and Sugar-Alcohols Reforming

Sugars and their corresponding polyols (sugar-alcohols) make up a large fraction of the heavy constituents in bio-oil. Particularly, sugar monomers such as glucose and xylose are the building blocks of cellulose and hemicellulose, and are readily obtained via catalytic or enzymatic hydrolysis of cellulose and hemicellulose.1149−1151 Therefore, the sugar monomer, mainly glucose, is often chosen as the model compound for sugars, and its corresponding reduction product sorbitol represents for sugar-alcohols or polyols.1152 Although sugar monomers have characterstics such as high water solubility, low volatility, and high oxygen content (C:O ratio = 1:1) that present serious challenges upon bio-oil upgrading via C−C coupling reactions, they are suitable for reforming reactions, especially APR.466,1153 7.4.1. Steam Reforming. Research on steam reforming of sugars (glucose, xylose, and sucrose) and sugar-alcohols is rather limited, possibly due to their exceedingly high coke formation tendency via decomposition and polymerization reactions.1047,1154 Hu et al. reported that glucose decomposed to a significant extent even before approaching the catalyst bed.792 To avoid the undesired decomposition, Sharma et al. made glucose bypass a preheater and vaporize in the reactor CD


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supposed to be mitigated by the use of concentrated feeds.831 However, previous experimental results revealed that low H2 yields were obtained when solutions of high glucose concentrations were processed.54 As a result, APR of glucose over Raney NiSn catalysts is less likely to guarantee efficient hydrogen production and stable long-term performance simultaneously. Even so, lowering the expense of catalysts is practically attractive, and thus the development of non-noble metal-based catalysts is of significant research and industrial interest. Recently, a thin atomic layer deposition (ALD) TiO2 (1.2 nm thickness) was proven to be effective for the stabilization of cobalt NPs against sintering and leaching (Figure 73) during aqueous-phase hydrogenation of furfural alcohol, furfural, and xylose, etc.1167 Very recently, it was found that a similar structure of TiO2-covered Co NPs could be obtained through high-temperature calcination and reduction treatments of Co/TiO2 catalyst due to the SMSI effects, presenting a less expensive and more industrially relevant method to prepare stable non-noble metal-based catalysts in aqueous-phase reactions.1168 A major drawback of reforming glucose (pathway G1, Scheme 29a) in the aqueous phase is that H2 selectivity decreases correspondingly with the increase of feed concentration from 1 to 10 wt %, partially attributed to heterogeneous dehydration reactions (pathway G2) that consume hydrogen, as well as homogeneous decomposition reactions (pathway G3) that possess faster kinetics than the heterogeneous reforming reactions (pathway G1).1158,1169 The low selectivity for hydrogen presents a serious issue upon the economics of direct APR of sugars, because processing such diluted aqueous solutions would obviously raise the total cost of the process.56,874 Therefore, Dumesic and co-workers suggested optimization of reactor designs to maximize surface active sites to promote reforming reactions and meanwhile minimize reactor void volume to suppress homogeneous reactions.54 Alternatively, it was found that H2 selectivity in reforming of sorbitol (pathway S1), the product of glucose hydrogenation and a common sugar alcohol, was insensitive to liquid concentrations.54,1149 Moreover, sorbitol was more reactive than glucose for the APR process and gave higher reaction rates under similar conditions. Accordingly, a two-step conversion scheme has been proposed by Davda et al. that glucose hydrogenation to sorbitol (pathway G-S) is followed by reforming of sorbitol to yield H2 and CO2 (pathway S1).1153,1170 To achieve this scheme, careful manipulation of reaction conditions is critical, because hydrogenation is wellknown to be favored at low temperatures and high pressures, whereas elevated temperatures and moderate pressures are typically used for reforming reactions.828,1171,1172 Therefore, Davda et al. developed a two-reactor system, comprised of a hydrogenation reactor at 393 K and a subsequent reforming reactor at 538 K.1153 As shown in Scheme 29b, an aqueous solution containing 10 wt % glucose was cofed with H2 into the hydrogenation reactor and then flowed downstream to the reforming reactor. Experimental results showed that the addition of a hydrogenation reactor increased the net moles of H2 produced per mole of glucose by 290%, from 1.5 to ca. 6 with a theoretical maximum of 12. After being cooled, the effluent of the reformer was sent to a gas−liquid separator operated at low temperature. Finally, the H2-rich reformate was purified by PSA, and then directed to fuel cells for power generation.

From an economical point of view, it has been considered that reforming of sorbitol was less practical than that of glucose, because glucose is less reduced and more immediately available.386,1173 Nevertheless, in recent years, growing focus has been placed on direct conversion of cellulose to sorbitol, which, if well established, grants a virtually unlimited source of sorbitol and does not interrupt with food supply.1149,1174−1176 The Dumesic group has demonstrated that APR of sorbitol successfully produced H2 in the presence of Pt-based and Nibased catalysts.874,1153 Tanksale et al. reported that the metal component of supported catalysts played a significant role in the rate of H2 formation in sugars reforming, decreasing in the order Pt > Pd > Ni.1162 Moreover, the group demonstrated that addition of a small fraction of Pt or Pd to Ni supported on alumina nanofiber (Alnf) multiplied its TOF of H2 production by 3−5 times as shown in Figure 74.1177 The improvement in

Figure 74. TOFH2 number (min−1, black column) and sorbitol conversion (%, gray column) acquired over alumina nanofibers supported mono- and bimetallic Ni catalysts. Adapted with permission from ref 1177. Copyright 2008 Elsevier.

activity was attributed to two factors: more active sites resulted from the enhanced reducibility, and weakened CO binding strength due to the formation of Ni−Pt and Ni−Pd alloy that favored WGS reaction to produce more H2.1178,1179 Moreover, replacing an alumina nanofiber support by ZrO2 and ceriazirconia-silica (denoted as CZxS) resulted in decreased activities, indicating the important involvement of the support in the APR of sorbitol.1180 D’Angelo et al. found that a bimetallic Pt−Ru catalyst exhibited increased activity and decreased H2 selectivity than the monometallic Pt catalyst, and therefore they used a microchannel reactor with continuous H2 stripping to propel the reforming reaction toward H 2 formation.1181 The in situ H2 removal has also been achieved by the utilization of a carbon-coated ceramic membrane reactor packed with the commercial Pt/C catalyst.1182 APR operation in the membrane reactor showed an increase in the H2 yield by a factor of 2.5 to a reference reactor without membrane. To further investigate the reforming process and develop more effective catalytic systems, kinetic understanding of reactions pathways involved in the process is necessary. Murzin and co-workers studied the reaction intermediates and product distribution from APR of sorbitol over a Pt/Al2O3 catalyst.1183 Indeed, the reaction network was complicated, as more than 260 compounds were detected and 50 major products were identified in the process. Aiouache et al. first established a path CE


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lumping kinetic model for APR of sorbitol that simplified the complex mechanism of the process by a generic intermediate that generates gaseous products via reforming route and liquid oxygenate route including HDO, decarbonylation, and dehydrogenation, and described the reforming results over several supported mono- and bimetallic Ni catalysts.1184 However, this study focused on the formation of gaseous products in a batch reactor, while liquid products were also important and the APR process was more commonly performed in continuous reactors. Consequently, Kirilin et al. developed a mechanism-based kinetic model that was able to describe both gas-phase and liquid-phase products involved in the APR of sorbitol over a Pt/Al2O3 catalyst in a continuous fixed bed reactor.1185 On the basis of the kinetic models and experimental data, reaction pathways that are responsible for the formation of gaseous products such as reforming and methanation are favored at low conversions, whereas those that lead to liquid products including hydrodeoxygenation become more important at high conversions (Figure 75).1183,1185,1186

Scheme 30. Bifunctional Reaction Pathway for the Production of Alkanes via Aqueous Phase Reforming of Sorbitola

a

Reprinted with permission from ref 1187. Copyright 2004 Wiley.

More importantly, Dumesic and co-workers reported that more than 90% of the energy content of the original carbohydrate feed and only 30% of the original mass were retained in the alkane products, resulting in a dramatically increased energy density.386 Mechanistic understanding of surface chemistry of aqueousphase hydrodeoxygenation of sorbitol revealed that retro-aldol condensation and decarbonylation that occurred on metallic sites were the major C−C bond scission reactions, while dehydration on acidic sites involved C−O bond scission reaction.1188 Therefore, the product selectivity could be adjusted by tuning the relative rates of C−C versus C−O bond cleavage through these key reactions. Huber et al. showed that SiO2−Al2O3 supported Pt and Pd were active catalysts for alkanes production via aqueous-phase reforming of sorbitol.1187 SiO2−Al2O3 provided the active sites for dehydration, while it could be replaced by a homogeneous mineral acid (such as HCl) cofed with aqueous feed. For the metallic function, Pt catalysts gave higher rates of C−C bond cleavage than Pd catalysts, whereas Pd favored hydrogenation reactions and presented higher hexane selectivities. The properties of acidic catalyst significantly affected the performance of aqueous-phase reforming of sugar alcohols for alkane production. Weingarden et al. found that zirconium phosphate (ZrP) with a high Brønsted to Lewis acid ratio possessed the optimal combination of activity, C 5 −C 6 selectivity, and stability among ZrP, SiO2−Al2O3, WOx/ZrO2, Al2O3, and HY zeolites for aqueous dehydration of xylose.1189 Vlachos et al. demonstrated that Lewis acid catalysts (e.g., SnBeta, Ti-Beta) facilitated the isomerization of aldose sugars (glucose and xylose) to ketose sugars (fructose and xylulose), whereas the Brønsted acid, in fact, catalyzed the dehydration of glucose, xylose, and fructose.1190,1191 It could be possible that a similar mechanism is involved in the dehydration of sugar alcohols. As a consequence, the Pt/ZrP catalyst exhibited stable catalytic performance for alkanes production from aqueous sorbitol solutions without significant deactivation after 200 h TOS.1192,1193 Recently, Kirilin et al. have drawn a correlation between the surface acidity of Pt/C catalysts and rates of alkane formation in APR of xylitol as shown in Figure 76, indicating that catalysts bearing high acidity are recommended for alkane production, yet not for hydrogen production.1194 Modifying Pt/C by addition of Re has resulted in a promising catalyst for APHDO of sorbitol.1152,1195 According to Zhang et al., a fraction of Re in PtRe/C catalyst was oxidized when exposed to hydrothermal APR reaction conditions.833 The oxidized ReOx particles, possessing both Brønsted and

Figure 75. Carbon distribution between gas and liquid products and total carbon balance versus sorbitol conversion in APR: (a) whole conversion range, and (b) carbon distribution at 62% conversion. Reaction condition: T = 598 K, P = 29.7 bar, WHSV = 0.22−2.16 h−1, catalyst = 1% Pt/Al2O3. Reprinted with permission from ref 1186. Copyright 2015 American Chemical Society.

Godina et al. also found that the reaction behaviors of polyols with different chiralities (sorbitol and galactitol) were almost the same, except for some minor discrepancies in liquid product distribution.1186 This result may be beneficial to the expansion of feedstocks that are viable for the APR process. 7.4.2.2. Alkanes Production. By facilitating C−O bond cleavage and suppressing C−H and C−C bond scissions, the APR process can be tailored to convert sorbitol, xylitol, and other sugar-alcohols into a clean stream of light alkanes including primarily butane, pentane, and hexane (Scheme 30).1187 In this scheme, sorbitol undergoes repeated sequential dehydration−hydrogenation reactions over the acidic and metallic sites, respectively, on a bifunctional catalyst (e.g., Pt/ SiO2−Al2O3). This process is also referred to as aqueous-phase hydrodeoxygenation (APDHO). H2 consumed in the hydrogenation step is provided in situ by H2 produced via reforming reactions over the metallic function, or supplied externally by cofed H2 with the aqueous solution. In principle, this process removes hydroxyl groups from carbohydrates, which increases the volatility of the feed and decreases its hydrophilicity, leading to spontaneous separation of products from aqueous solution and thus the omission of energy-intensive distillation steps.1043 CF


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promote hydrogenation reactions over the bimetallic catalysts.445,1199−1201 Huber and co-workers made a comparison between Pt/ZrP and PtRe/C catalysts in the APHDO of sorbitol.1202 The two catalysts had similar TOFs, but PtRe/C exhibited 34 times higher APHDO activity than Pt/ZrP on a Pt mass basis. Additionally, sorbitol was isomerized to mannitol over the PtRe/C catalyst, whereas no mannitol production was observed over Pt/ZrP. Mannitol, as compared to sorbitol, was more reactive and less prone to coke formation. As a result, the amount of coke deposited on PtRe/C (6.8 wt %) after 140 h TOS was significantly lower than that on Pt/ZrP (15.9 wt %) after 114 h TOS. However, Pt/ZrP exhibited a higher C6 product selectivity than PtRe/C (35.8% and 11.6%, respectively). On the basis of literature investigations, a detailed schematic network of the major pathways for APHDO of sorbitol is given in Scheme 31.1188,1202,1203 The differences in reactivity have been proposed to be associated with the different surface structure of two catalysts as shown in Figure 77. First, the PtRe/C catalyst has welldispersed Pt particles (1.2 nm), and Pt sites and acid sites were in intimate contact. In contrast, larger Pt particles (26 nm) on Pt/ZrP were separated from acid sites. The isomerization of sorbitol to mannitol took place over the well-mixed metal-acid sites. Hydroxylated Pt-ReOx species could be responsible for the acid-catalyzed isomerization reaction, because they had deprotonation energies similar to that for solid acids with strong Brønsted sites.1196 Furthermore, the increased H2 spillover on Pt-ReOx catalysts could contribute to suppress coke formation over acid sites.1204 Additionally, the Pt/ZrP catalyst possessed a higher density of surface acid sites and a higher ratio of surface acid to metal sites than PtRe/C catalyst,

Figure 76. Dependence of alkane formation rate and H2/CO2 ratio on the surface acidity of Pt/C catalysts. Reaction conditions: 498 K, 29.3 bar, xylitol conversion ∼10−12%. Reprinted with permission from ref 1194. Copyright 2014 The Royal Society of Chemistry.

Lewis acid sites, increased the surface acidity of catalyst proportionally to the amount of Re in PtRe catalyst. Such improvement in acidity has also been observed for the ReOxpromoted Rh/C catalyst in hydrolysis of polyols and cyclic ethers.1196 Consequently, the enhanced acidity facilitated C−O bond breaking and thus the dehydration reactions, which was considered as the rate-determining step during dehydration/ hydrogenation.1197 Moreover, Kirilin et al. reported that, as compared to monometallic Pt catalyst, bimetallic Pt−Re catalysts were more selective toward alkane production.1195,1198 Both DFT calculations and experimental results suggested that the role of Re was to favor C−O over C−C bond cleavage and

Scheme 31. Major Pathways and Reactions Involved in the APHDO of Sorbitola

a

Reprinted with permission from ref 1202. Copyright 2013 Elsevier. CG


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Figure 77. Possible surface structure of Pt/ZrP and PtRe/C catalysts under reaction conditions. Reprinted with permission from ref 1202. Copyright 2013 Elsevier.

which could be related to its faster relative rate of C−O bond scission and thus enhanced hexane selectivity. Moreover, Wang and Ma reported that high selectivity of isohexane could be obtained over a Ni/HZSM-5 zeolite catalyst.1205 For APHDO of xylitol, Ni/HZSM-5 showed much higher pentane selectivity of 98% than Pt/HZSM-5 at similar xylitol conversions, due to the stronger C−C bond cleavage ability of Pt to produce C1−C4 alkanes.1206 However, the recyclability of Ni/HZSM-5 was challenged by severe coke deposition after five recycle runs, while Ni/MCM-22 with larger surface area and better pore structures maintained high pentane yield (90−95%).1207 Nonetheless, the light alkanes (C4−C6) could not be used directly for fuel applications due to their low octane number (ca. 37) and high volatility. Hence, a strategy to produce liquid alkanes (C7−C15) has been outlined and practiced by Dumesic and co-workers, in which an upstream aldol-condensation step for carbon chain growth was added to the dehydration− hydrogenation process.1208,1209 Specifically, carbohydrates such as glucose and xylose first dehydrated over mineral or solid acid catalysts to furan derivatives hydroxymethylfurfural (HMF) and furfural, respectively, which then underwent aldo-condensation with ketones (e.g., acetone), and finally dehydration−hydrogenation to yield liquid alkanes.1195 It should be noted that catalysts were highly prone to deactivation by coke deposition in the dehydration−hydrogenation reactions of large organic compounds. Consequently, a four-phase reactor system (Scheme 32) was employed, in which a hexadecane alkane stream served to readily remove the hydrophobic species from the catalyst surface before coke formation took place.1209 An alternative strategy targeting at the utilization of monofunctional intermediates produced by the aqueous-phase reactions of sugars and sugar alcohols is shown in Scheme 33.1210 APHDO of sorbitol was first performed over a PtRe/C catalyst at 503 K. After gas-phase light alkanes (C1−C6) and aqueous-phase higher oxygenates (e.g., isosorbide) were removed, a liquid stream of monofunctional intermediates (comprised of carboxylic acids, ketones, alcohols, and heterocyclic compounds) was transferred to a dual-bed catalyst system. In the dual-bed reactor, carboxylic acids first underwent C−C coupling by ketonization on a CeZrO2 mixed oxide catalyst bed to form higher linear ketones, and subsequently aldo-condensation/hydrogenation reactions of ketones and alcohols on the downstream Pt/ZrO2 catalyst bed to yield

Scheme 32. Schematic Representative of a Four-Phase Dehydration-Hydrogenation Reactor, Consisting of (i) an Aqueous Feed Stream, (ii) a H2 Gas Stream, (iii) a Hexadecane Alkane Stream, and (iv) a Solid Bifunctional Catalysta

a

Adapted with permission from ref 1209. Copyright 2006 Elsevier.

branched ketones, which were finally converted to heavy alkanes via dehydration/hydrogenation over a Pt/SiO2−Al2O3 catalyst. The overall yield of C7+ products for the dual-bed system reached 42% of the inlet carbon in the sorbitol solution. Therefore, the process could effectively produce fuel-grade compounds from sorbitol, which is desirable for diesel fuel applications. 7.5. Crude Bio-Oil and Its Aqueous Fraction Reforming

7.5.1. Steam Reforming. Aqueous phase of bio-oil is acquired by phase separation of crude bio-oil, generally through water addition.1034 The APB mainly consists of simple carbohydrate-derived soluble organics, acids, ketones, alcohols, aldehydes, sugars, phenols, and a large amount of water including original free water and water added afterward, whereas lignin-derived oligomers predominate in the remaining hydrophobic phase.1211 Table 22 lists the composition of a crude bio-oil and its aqueous fraction (wt %, dry basis) obtained by flash pyrolysis of pine sawdust.1212 The water addition required for APB extraction varies with the origin and pyrolysis process of crude bio-oil, but typically water content is around 80 wt % in the aqueous phase. CH


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phenol-formaldehyde resins).1213 As a result, a high H2 yield up to 85% of the stoichiometric value could be obtained using commercial nickel-based catalysts. Furthermore, the preliminary economic analysis of the process suggested that it had the potential to be cost competitive with established H2 production processes such as methane reforming. As compared to steam reforming of model compounds discussed above, reforming APB, although more difficult, is more realistic and industrially relevant.1214 Furthermore, the direct conversion of crude or raw bio-oil is another leap forward for the utilization of entire bio-oil, and, if established, the efficiency of H2 production from bio-oil would be very much likely to be high enough for commercial-scale application. Nevertheless, the lignin-derived oligomers in bio-oil are facile to form coke deposits upon heating, and these oligomers could not be completely removed by the extraction of APB. Shanks and co-workers demonstrated that the low molecular weight species in APB, such as acetic acid and acetol, were converted more effectively to H2-rich gas, whereas the presence of heavier molecules (e.g., levoglucosan and furfural) reduced the H2 production due to the coke deposition.1215 The finding was substantiated by Wu et al., as they showed that heavy oxygenates in bio-oil demanded higher reaction temperatures (1073 K vs 923 K) and S/C ratios (10 vs 7) than light oxygenates, yet still suffered from more severe coke deposition.1042 Kechagiopoulos et al. carried out steam reforming of APB derived from beech wood in a pilot scale fixed bed reactor under conditions similar to those in SMR.1216 Much lower H2 yields around 60% were obtained for APB as compared to those for bio-oil model compounds (acetic acid, acetone, ethylene glycol). The coke deposits were most likely the consequence of excessive thermal decomposition and polymerization. Commercial Ni/Al2O3 has been utilized in steam reforming of crude bio-oil and its aqueous fraction.1211,1217−1219 Nevertheless, the harsh reaction conditions (≥973 K) and complex feedstocks often require modifications to achieve high reaction rates and minimal coke deposition. Similarly, three principles have been employed to inhibit coke deposition in steam reforming of APB, enhancing steam adsorption using basic materials, modifying surface properties of catalysts with oxides and metallic promoters, as well as introducing oxygen species via cofed oxygen or the use of redox supports.1211,1212,1220−1224,1225 7.5.2. Reactor Design. 7.5.2.1. Fixed Bed and Fluidized Bed Reactors. Currently, coke deposition is well acknowledged as the bottleneck in SR of bio-oil. Previous studies show that, apart from the catalysts used, the severity of coke deposition is also associated with the configuration of reactors. Traditional fixed bed reactors, generally used for reforming natural gas or naphtha, have been initially applied in the conversion of bio-oil. The utilization of fixed bed reactors is feasible; however, drawbacks have been identified for this system: vigorous thermal decomposition and polymerization of bio-oil feed led to layers of coke deposits on the freeboard and the upper part of the catalyst bed, resulting in partial and complete blockage of the reactor. The coke formation significantly limits the operation time and requires frequent regeneration processes. Therefore, fluidized bed reactors are preferential over their fixed bed counterparts, because fluidized beds allow a better contact between feedstock and catalysts, and the enhanced exposure of catalyst surface is advantageous for coke gasification at a higher rate.781,1212,1216,1226

Scheme 33. Schematic Representation of a Dual-Bed Catalyst System for Upgrading Monofunctional Intermediates Produced from APHDO of Sorbitol to FuelGrade Compounds, Carbon Distribution for the APHDO of 60 wt % Sorbitol Solution over PtRe/C Catalyst, and the Sequential Ketonization over Ce1Zr1Ox Catalyst and AldolCondensation/Hydrogenation over Pd/ZrO2 Catalyst of Monofunctional Intermediatesa

a


Table 22. Composition (wt %, dry basis) of Raw Bio-Oil and Its Aqueous Fractiona component/group

raw bio-oil

aqueous fraction

acetic acid acetone acetol glycoaldehyde levoglucosan phenols other acids other alcohols other ketones other aldehydes esters ethers others nonidentified

12.8 5.5 16.3 8.5 11.0 16.6 4.0 2.4 3.8 6.5 5.1 1.4 2.3 2.6

19.1 1.0 8.7 1.8 19.6 13.4 7.4 3.6 8.1 5.5 3.1 0.3 3.8 3.8

a

Water content: crude bio-oil, 35 wt %; aqueous fraction, 82 wt %. Adapted with permission from ref 1212. Copyright 2013 American Chemical Society.

Catalytic steam reforming of APB has been carried out to produce H2-rich gas.1046,1125 As shown in Scheme 34, the principle of the process is simple: crude bio-oil from fastpyrolysis undergoes fractionation, and then the APB is processed through a SR unit to yield hydrogen product, while the hydrophobic fraction is finally converted to phenolics substituted coproduct (e.g., used as a phenol replacement in CI


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Scheme 34. (a) Schematic Diagram and (b) Flow Diagram for the Integrated Process of Fast Pyrolysis and Steam Reforming of Biomassa

a

Adapted with permission from refs 1046 and 1125. Copyright 1997 and 1998 American Chemical Society.

Scheme 35. Schematic Diagram of a Sprouted Bed Reactora

a

Adapted with permission from refs 1229 and 1230. Copyright 2007 Elsevier and 2009 American Chemical Society.

Remon et al. made a comparison between a fixed bed reactor and a fluidized bed reactor in the steam reforming of APB (S/C = 7.6) derived from pine sawdust.1214 They confirmed that catalysts deactivated at higher rates in the fixed bed reactor, although higher carbon conversion and H2 yields were obtained at the initial stage. In contrast, the fluidized bed reactor gave stable hydrogen production over time. Yan and co-workers also

demonstrated that carbon deposition was less significant in the fluidized bed reactor than in the fixed bed reactor at the same temperature due to higher reaction efficiencies.1227 Meanwhile, the fluidized bed reactor demanded higher S/C ratios because steam served as reactant and fluidized agent simultaneously. High S/C ratios favored coke elimination reactions with CJ


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(mainly CH4) to yield H2-rich products. Pyrolytic lignin deposition mainly occurred over the expendable dolomite catalysts in the first reactor, and thereby catalysts in the primary reformer would suffer less from deactivation. Similarly, Rossum et al. studied the SR of bio-oil using a staged reactor system, consisting of an inert fluidized bed (packed with sand) reactor and a catalytic fixed bed (packed with commercial Ni/Al2O3 catalysts) reactor.1238 Valle and co-workers used a two-step continuous process with two reactors in line: an initial, controlled pyrolytic lignin deposition step via thermal treatment (473−773 K) of raw biooil in a U-shape steel tube reactor was followed by SR of the remaining volatiles over Ni/La2O3−Al2O3 catalysts in a fluidized bed reactor.1224,1239 Both raw bio-oil and APB (2:1 water addition) were tested in the two-step process. The H2 production obtained from raw bio-oil was higher than APB (58.6 vs 50.5 mmolH2/g bio-oil); nevertheless, SR of APB enabled a longer duration before catalyst regeneration due to less significant coke deposition. Additionally, the use of the fluidized bed reactor allowed coupling of in situ CO2 capture, and thereby resulted in high H2 yields of 80−82% with negligible CO at 873 K from raw bio-oil without water addition (S/C ratio = 1.1).1040 Moreover, the exothermic carbonation reaction provided additional heat supply for the endothermic reforming reactions, and this effect can be amplified by cofeeding a biogas (containing 30%−40% CO2) stream in the SESR of bio-oil.1240,1241 It is worth noting that aging is detrimental to bio-oil and its aqueous fraction because chemical changes including polymerization occur. Ortiz-Toral et al. reported that fresh APB turned from a translucent red, homogeneous material to a dark, suspended material after an aging process (30−90 days).1215 Catalytic tests showed that the aged samples were no longer appropriate for SR process. Although it has been reported that addition of alcohols (e.g., methanol, ethanol, and isopropanol) lowered the viscosity and molecular mass increase during the aging of bio-oil, these stabilizing additives may lead to a rise in total production costs. 1242 Consequently, a continuous operation, where the instable raw bio-oil and APB are processed instantly after its production, is more suitable for their conversion to value-added chemicals. Recently, Remiro et al. reported that cofeeding bioethanol (e.g., 50 wt %) with crude bio-oil and its aqueous fraction in the two-stage (thermal and catalytic) SR process was promising. The bioethanol not only served as stabilizing agent during the storage of bio-oil, but also increased H/C ratios in the feed, thereby contributing to suppress pyrolytic lignin deposition. One should note that the cost of bioethanol is declining in recent years due to advances of its production technologies, and meanwhile the coconversion of the bio-oil/bioethanol mixture improves the sustainability of the overall process.1243,1244

improved heat and mass transfers, but also diluted the feed and raised the heat duty of reactors.1228 A pilot scale spouted bed reactor was applied by Kechagiopoulos et al. as shown in Scheme 35.1229,1230 The spouted bed reactor is conceptually a modified fluidized bed reactor. The APB feed was kept cool until it reached the reactor, and was subsequently mixed in liquid form with the spouting jet, steam, and gas feed from preheating zone, just before the nozzle at the cone apex. The liquid APB then was injected and came into immediate contact with the catalytic particle bed. In this case, the decomposition of APB prior to approaching the catalytic bed was prevented, and the favorable hydrodynamics of the spouted bed drastically inhibited coke deposition in the reforming process. Consequently, the spouted bed reactor was successful for the SR of EG, AcOH, and APB. Ni/olivine was proven to possess sufficient mechanical strength for this fluidized operation, but the low surface area of natural olivine (3.02 m2/g) limited the loading of high amounts of Ni (5 wt % in this case), thus resulting in insufficient activity for steam reforming of APB. This is supported by the study of Bimbela et al., who reported that coprecipitated Ni/Al catalysts with high nickel loadings around 28% gave good performances in APB reforming.1231 As a result, appropriate fluidizable catalyst supports should possess both attrition-resistance and large surface areas to accommodate enough active sites. 7.5.2.2. Two-Stage Operation. In industrial utilization, SRM is often carried out in tubular reformers that are expensive, and therefore reducing the size of the reformer and the number of tubes is a concern.1232 Increasing the inlet temperature of reactors is one strategy, but it involves the risk of thermal cracking of long-chain hydrocarbons in the preheater and thus leads to severe coke formation. Consequently, a two-stage reformer system is utilized, a prereformer (preconverter) and a primary reformer (Scheme 36).1233 The installation of an Scheme 36. Flow Diagram of Steam Reforming Process with a Prereformer and a Tubular Primary Reformera

a

Reproduced with permission from ref 1233. Copyright 1988 Elsevier.

upstream adiabatic reformer is to convert all high hydrocarbon feedstocks to C1-components without intermediate products at low temperatures (e.g., 623−823 K).1234 CH4 is then preheated and reformed to syngas at high temperatures in the primary reformer.430 To minimize coke deposition, an analogous two-stage reactor system has also been adopted.1235−1237 Wu et al. set up a twostage fixed bed reactor system for hydrogen production via SR of bio-oil, as shown in Scheme 37.1235 Cheap dolomite catalysts were packed in the prereformer for the vaporization and decomposition of crude bio-oil, whereas a metal catalyst Ni/ MgO facilitated the conversion of gaseous intermediates

8. SUMMARIES AND PERSPECTIVES In this Review, we have summarized current understandings of catalytic reforming of several important oxygenated hydrocarbons, and special attention has been placed upon the progress in the design, synthesis, and structure−activity relationship of catalysts involved in these processes. In general, a bifunctional mechanism prevails in most (thermal-chemical) reforming reactions, where typically metal surface is active for the activation and transformation of oxygenated hydrocarbons, while metal oxide site serves for the activation of water (or CO2). Hence, one could deduce that the interface between the CK


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Scheme 37. Schematic Diagram of the Two-Stage Fixed Bed Reactor System for Steam Reforming of Fast Pyrolysis Bio-Oila

a


for protection against metal sintering and leaching.1167 Moreover, ALD offers a simple method to prepare model supported catalysts to develop fundamental knowledge upon reaction mechanism over a rational-designed catalyst.18 For instance, ALD-prepared metal NPs with controllable size as well as extremely narrow distribution enable studies on particle size effect, which is of great research interest in understanding the nature of catalytic active site in powder catalysts under reaction environments. With much progress in catalytic reforming achieved already, there still are several issues to address: (i) Fundamental studies on surface reaction mechanism and catalyst properties have been fruitful; however, relevant aspects, such as effects of electronic and structural properties on the activity and stability of reforming catalysts, remain elusive, demanding for further experimental and theoretical investigations. To collect first-hand experimental data under reaction conditions, development of in situ and/or operando characterization techniques is crucial.587,588 As accumulated evidence shows, the structure of a working catalyst can be dynamic, and thus in situ and time-resolved XAFS and XRD have provided important information on the structural changes in catalysts during reaction by determining the short-range bonding environment and long-range order of the examined catalyst.9,15,555,564 Meanwhile, combined in situ vibrational spectroscopic (e.g., DRIFTS and Raman) and TPD studies offer important information on surface species and their transformation pathways.1245 Correlating the results of catalysts structure with those of surface species would lead to a deeper understanding of the reaction mechanism and structure− activity relationship in catalytic reforming. Additionally, stateof-the-art characterization techniques such as environmental TEM (E-TEM) offer imaging of catalyst in near-reaction environment, therefore allowing in situ observation of catalyst structures and their changes during reaction.1246 The E-TEM could be a powerful tool to investigate the origin of catalyst deactivation, as major deactivation pathways including sintering, leaching, and coke deposition are expected to be monitored in real time using this operando technique. On the other hand, DFT calculations have been very useful for exploring elementary steps and mechanisms of reforming reactions at the atomic scale. Recent progress in computational architectures and processing speed allows theoretical calculations to

two sites is where the reforming reaction could occur. In this context, supported metal catalysts are by far the most investigated for reforming processes. Transitional metals are efficient active components, and they are selected according to the type and properties of oxygenate feedstock. For example, regarding simple oxygenates (e.g., methanol and DME) without C−C bonds in their molecular structure, copper catalysts exhibit excellent performances, yet they can not effectively convert higher oxygenates including ethanol or ethylene glycol involving C−C cleavage. When it comes to complicated feedstocks, precious metals such as Rh, Pt, and Pd are favored for their high bond breaking ability, good thermal stability, and low affinity to coke deposits. However, this does not mean that precious metal catalysts are a better choice in the reforming process due to their high costs. Hence, strategies are (i) lowering the loading of precious metals while improving the site-specific activity (e.g., ideally single-atom catalysis); and (ii) developing robust nonprecious alternatives, the most representative being Ni-based catalysts, which have been commercialized in SMR. Conventional support materials such as SiO2 and Al2O3 provide sufficiently large surface area for fine metal dispersion, but they either weakly interact with metal clusters or have strong surface acidity, resulting in rapid activity decay through metal particle agglomerization and/or coke deposition. Synthesizing catalysts with encapsulated structures using a well-define precursor or an ordered mesoporous oxide (mainly silica and alumina) has been employed to spatially confine metal clusters, and promoters with basic natures are commonly used to mitigate surface acidity. On the other hand, redox oxide supports, represented by ceria-based materials, allow facile water dissociation and transport of oxygen-containing speices to the metal surface, and meanwhile stabilize metal clusters via the SMSI effect, thus becoming very promising for achieving selective and stable H2 production. Nevertheless, considering that ceria-based oxides are relatively more expensive than Al2O3 or SiO2, the use of these materials as promoters seems more practical. Emerging synthesis techniques such as ALD and molecular layer deposition (MLD) could deposit a homogeneous, ultrathin oxide layer on the metal clusters, thus obtaining an enlarged metal−oxide interface, strengthening metal−support interaction, and maximizing the utilization efficiency of the material on a mass basis, and successes have been met with Al2O3 and TiO2 CL


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of hydrogen to mechanical energy through thermal energy; however, its efficiency is limited by the Carnot efficiency, whereas the other one allows “cold” combustion of the hydrogen−air mixture to occur electrochemically in fuel cells converting chemical energy directly to electrical energy to drive an electric engine, and in this process heat-power conversions are avoided, and hence an efficiency of 50−60% is attainable, twice as much as that of the former process.2,1249 To date, in addition to efficient generation and use of hydrogen, a serious challenge is the onboard storage of hydrogen, which is also the key to link the two ends of the hydrogen economy for vehicle application. Conventionally, hydrogen is stored (1) in gaseous state in high-pressure vessels, (2) in liquid phase in cryogenic tanks, or (3) chemically bound in metal hydrides and other solids, and the hydrogen density increases from the gas phase to the solid phase while its accessibility follows a reverse trend as illustrated in Figure 78.1250

describe complicated bifunctional supported catalysts, which is highly desired for reforming reactions, despite some limitations such as large amounts of possible positions of given sites due to the lattice mismatch between particle and support.446,1247 Note that advanced in situ characterizations also facilitate the identification of active sites in complex catalytic systems and offer more guidelines for theoretical studies to build more reliable models. (ii) As insights have been gained from classical surface science studies, one can notice that these studies are generally carried out under UHV conditions over model catalysts with well-defined single-crystal surfaces, which is far from the catalysts under the working environment. Hence, bridging the “material” and “pressure” gaps between model catalyst under UHV conditions and supported catalyst under realistic reaction condition is the key to translate the surface science outcome into practical catalysis. For example, Chen and co-workers have provided a successful example to build strong correlations between model surfaces and supported catalysts in the hydrogenation of CC and CO bonds over Pt-3d bimetallic materials with the aid of various characterization techniques (AES, XPS, LEIS, scanning tunneling microscope (STM), chemisorption, TPD, TEM, DRIFTS, XAS, etc.) and DFT calulations, exhibiting the potential of designing efficient catalyst from first principles.15 Furthermore, the group has extended their investigation to reforming of oxygenates including methanol, ethanol, and ethylene glycol, and observed similar trends in reforming activity between single-crystal and polycrystalline materials, although challenges were met when building correlations between model surfaces and supported catalysts, due to the structural transformations of catalyst surfaces during the commonly harsh calcination, reduction, and reaction processes in catalytic reforming.776,778,873,1246,1248 For further advancements, synthesis efforts should be made to prepare supported catalysts with well-defined strucutures and compositions analogous to those of corresponding model surfaces, and, more importantly, increase their thermodynamic and chemical stability. In addition, the relevant particle size, support, and interface effects should also be considered in supported catalyst systems. (iii) Next is turning catalytic reforming of oxygenates into practical technology. With many advances achieved in the catalytic materials and reactor designs, the key to build a hydrogen economy based on catalytic reforming of oxygenates is to turn current findings into commercialized technologies. Among them, hydrogen-powered vehicle is one of the most promising application. Most encouraging news is the successful commercialization of Toyota Mirai, the world’s first massproduced hydrogen fuel cell car released by the Japanese automotive enterprise in 2015. The hydrogen-driven vehicle demands on-board hydrogen generation, storage, and utilization. First, for onboard hydrogen generation, design of reforming reactors is required. Considering the heat duty and reactor volume, low-temperature autothermal reforming and aqueous-phase reforming seem more suitable than other reforming types. Meanwhile, safety issues should be taken for serious consideration. Moreover, oxygenates compatible with current infrastructure for transport and storage, such as methanol and glycerol, are favored. Second, efficient hydrogen usage is essential to driving the vehicles. Mainly two processes are present for hydrogenpowered vehicles: one is to burn hydrogen with oxygen from air in an internal combustion engine, converting chemical energy

Figure 78. Hydrogen density and accessibility of different storage materials. Reprinted with permission from ref 1250. Copyright 2011 American Chemical Society.

The former two forms are not viable for commercial mobile applications due to their low volumetric energy densities (typically

Catalytic Reforming of Oxygenates: State of the Art and Future

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