Recent Advancements, Fundamental Challenges, and Opportunities

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Recent Advancements, Fundamental Challenges, and Opportunities in Catalytic Methanation of CO2 Muhammad Younas,† Leong Loong Kong,‡ Mohammed J. K. Bashir,† Humayun Nadeem,† Areeb Shehzad,† and Sumathi Sethupathi*,† †

Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia ‡ Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Sungai Long, Bandar Sungai Long, 43000 Kajang, Selangor, Malaysia ABSTRACT: Commercial and environmental benefits have made carbon dioxide (CO2) methanation one of the topmost research projects all over the world both at the pilot plant and commercial scale. Mitigation of CO2 via carbon capture and storage (CCS) routes have less motivation from a commercial point of view. Therefore, an integrated system is of paramount importance to convert CO2 into value-added products such as methane (CH4) using solar energy (photosynthesis) or surplus electrical energy in hydrolysis for production of reactant hydrogen to use in CO2 methanation. To date, great efforts have been made to investigate both the reaction mechanism and catalysts development for methanation. Here in this review, up to date references have been cited, which are aimed at giving researchers a comprehensive overview of CO2 methanation with respect to the recent advancements in reaction mechanism, catalytic materials, and the novel combination of metal active phase and its synergy. Both thermochemical and electrochemical routes of CO2 methanation have been discussed, mainly focusing on thermochemical routes. Among the two routes, the thermochemical route seems to be a promising technique for producing an energy carrier due to the high selectivity of CH4.

1. INTRODUCTION Global warming and depletion of conventional sources of energy are the two important phenomena which are putting our social and environmental sustainability at high risk. The main source of energy for the world’s industrial development is the conventional sources of energy. According to 2014 statistics, renewable energy contributed 22% to our global energy consumption and the remaining 78% was achieved from conventional sources. These conventional sources of energy have a larger carbon footprint. Based on the World Bank data, each year 4.9 t of CO2 per capita was added to the atmosphere. Therefore, the atmospheric concentration of CO2 has increased by 30% since industrialization from levels of 280 ppm to about 399 ppm today. The forecast of the International Panel on Climate Change (IPCC) indicated that, this level of carbon dioxide (CO2) will increase to 570 ppm by the year 2100.1−3 This rapid increase has threatened the earth in the form of global warming and many other related environmental issues, such as extreme weather events, acid rain, migration of animals, unequal distribution of food, and disturbed nutrition for plants and animals. To minimize the negative externalities of atmospheric CO2, searching for renewable sources of energy and CO2 capture and storage is of paramount importance.1,4 To date, two main solutions are proposed for CO 2 mitigation: (i) CO2 capture and storage (CCS) in the geological subsurface or beneath the ocean; (ii) transformation of CO2 into value-added products. However, CCS has some major drawbacks such as elevated cost and transportation of CO2.5 The control of CO2 emissions from power-consuming units is still a big challenge for industries and society. Several technologies for CO2 mitigation do exist, but nothing © XXXX American Chemical Society

promising seems to be developed. As a substitute of capturing and storage, transformation of CO2 into value-added products such as methane (CH4) was suggested to be an essential process in the near future.6 Additionally, the requirements of energy for world industrial and social development have increased enormously. According to the U.S. Energy Information Administration,7 the world energy demand is expected to grow up to 48% by the year 2040, from 549 to 815 quadrillions Btu. The majority of the world’s energy needs are met from fossil fuel, and unfortunately it is depleting as predicted by Hubbert’s curve. This curve predicted that in the near future the recoverable crude oil will become significantly low. Though the Hubbert’s curve is widely controversial, it is certainly worthwhile to develop alternative fuel sources from non-fossil-fuel sources.8,9 Hydrogenation of CO2 is used for the production of several chemicals and fuel as well as fuel derivatives.9 Figure 1 shows possible products that may be obtained from CO2 hydrogenation via thermochemical or electrochemical routes. The methanation of CO2 can be used for purification of synthesis gas for the production of ammonia and syngas.10 Extensive research work has been conducted on metal-based catalytic systems for CO2 methanation. Recently, various metal catalysts have been developed that demonstrated CO2 methanation at low temperatures and atmospheric pressure. Through an electrochemical route, it has been made possible to convert CO2 to CH4 at temperature as low as 150 °C and atmospheric Received: July 13, 2016 Revised: September 26, 2016

A

DOI: 10.1021/acs.energyfuels.6b01723 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

electrochemical reduction of CO2 also provides a valuable cost efficient and environmentally friendly route for industrial processes with minimum waste and toxic material production.10,14 2.1. Thermochemical Route (Sabatier Process). Thermochemical methanation is the conversion of H2 and CO2 at a temperature of about 150−500 °C and pressure range from atmospheric pressure to 100 bar usually with metal-based catalysts. A mixture of gases containing CO2, CO, H2, and N2 is used, where the first three gases are used as reactant gases while N2 is used as a carrier gas. This reaction is predominantly carried out in fixed-bed, fluidized-bed, and three phase reactors. Thermochemical reaction of CO2 methanation is described by the following exothermic reaction;15,16 Figure 1. Products from CO2 hydrogenation via thermochemical and electrochemical routes.

CO2 (g) + 4H 2(g) ↔ CH4(g) + 2H 2O(g) ΔH 298K = −252.9 kJ/mol

pressure. These experiments showed that the recycling of CO2 to produce CH4, under friendly conditions, could be possible in the future.6,11 H2 production is the main energy-consuming step in CO2 methanation, which requires intensive use of resources and, thus, principally is a less favorable route for the production of fuels/CH4. However, other motivations may justify this route; for instance, market needs, CO2 mitigation, and a better energy integrated system, etc. Therefore, instead of looking only for fuel, an integrated technoeconomic and life-cycle assessment is necessary to evaluate this process.12 The greater share of H2 is currently produced by steam reforming method, though a highly interconnected energy system might play an increasingly significant role. H2 could also be obtained from an environmentally friendly process of water electrolysis via renewable energy sources such as wind turbines, hydroelectric generators, photovoltaic cells, or solar panels.5 Through these methods, renewable energy can be transformed into storable chemical energy in the form of CH4 via methanation.13

(1)

Catalytic methanation of CO2 is widely studied in fixed-bed reactor (shown in Figure 2) in the past using powder catalyst due to its advantages such as high surface to volume ratio, low pressure drop in the column, a better controllability of the reaction parameters, and intensification of mass as well as heat transfer.17 It is known that severe methanation conditions are needed in the fixed-bed reactor; however, recently studies have reported that methanation process can be done at moderate temperatures and pressures. Beuls et al.11 demonstrated CO2 methanation reaction at low temperature (50−150 °C) and pressure (2 bar). Generally, increasing the temperature in the fixed-bed reactor improves the CO2 conversion and selectivity of CH4 up to a certain limit. However, temperature above 550 °C should be avoided to prevent catalyst deactivation by sintering. Moreover, the CH4 conversion is restricted by the thermodynamic equilibrium at temperatures above 300 °C (depending on the pressure).17 Therefore, a series of heat sink system is used with several methanation reactors. Liu et al.18 formed two same catalyst samples (Ni(15%)/TiO2-IMP and Ni(15%)/TiO2-DP) with two different preparation methods, namely, conventional impregnation (IMP) and deposition− precipitation (DP) method. The experiments conducted in fixed-bed reactor demonstrated that, beyond 200 °C, the CH4 selectivity decreased gradually for Ni(15%)/TiO2-IMP, and for Ni(15%)/TiO2-DP the selectivity slightly increased. The

2. METHANATION TECHNIQUES Currently, the main research focus in the field of CO2 methanation is the optimization of the processes developed in the past. Most of the research work has been done using the Sabatier thermochemical route for catalytic methanation of CO2 in fixed-bed and moving-bed reactors. Nevertheless,

Figure 2. Schematic diagram of fixed-bed reactor system for methanation of CO2. B


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today, the electrochemical reduction of CO2 into CH4 appears to be better than those of thermochemical processes where terms of mild reaction conditions are concerned. There are many advantages for electrochemical reduction of CO2 to CH4 such as versatility, energy efficiency, and cost effectiveness.14,23 Modern research has shown that the electrochemical reduction of CO2 can produce various types of organic compounds containing carbon monoxide (CO), formic acid, CH4, and ethylene with high current efficiency. Furthermore, these products can be utilized as feedstock in chemical synthesis or transformed into hydrocarbon fuels. The electrochemical reduction gives better yield of CH4 compared to byproducts. The Faradaic efficiency depends on the suitability of the electrodes and catalysts; employing appropriate electrode and catalytic systems gives high Faradaic efficiency, which means a lower energy requirement to complete the electrochemical reaction.7 To date, the highest Faradaic efficiency for CH4 is reported to be 64% on a copper single crystal. CH4 is formed through the following half electrochemical reaction:14,24

enhanced reactivity of Ni(15%)/TiO2-DP is because of high dispersion of Ni particles on the TiO2 support. On the other hand, decreasing CH4 selectivity with increasing temperature for Ni(15%)/TiO2-IMP is because of the exothermic nature of CO2 methanation reaction. Alternatively to a fixed-bed reactor, fluidized-bed reactors were used as a potential technology for CO2 methanation for large scale operations. In this technology, the fine catalyst particles were fluidized by the flow of gaseous reactants.17 Due to the uniform mixing of the reactant gases and the catalyst particles in a fluidized-bed reactor, the heat removal is more effective, creating nearly isothermal conditions inside the reactor. This is advantageous as it enables the use of a single reactor with a relatively simple design. Negative aspects of this process include incomplete conversion due to bubbling and the attrition and breakage by abrasion in heterogeneous catalysis.19,20 The catalyst sintering problem in traditional adiabatic fixedbed reactors and attrition in fluidized-bed reactors lead to catalytic deactivation in these two technologies. To cope with these problems, three phase technology is a far better option for methanation, where a solid catalyst (size < 100 μm) is suspended in a temperature stable inert liquid such as dibenzyltoluene.17 Figure 3 displays the three phase fluidized-

CO2 + 8H+ + 8e− ↔ CH4 + 2H 2O

(2)

Many studies have been carried out on the electrochemical reduction of CO2 using various metal electrodes in organic solvents. The reason for adoption of organic electrolytes is its higher potential of dissolving CO2. For instance, methanol dissolves approximately five times more CO2 than water at ambient temperature. Kaneco et al.24 tried electrochemical reduction of CO2 on copper electrode using a variety of organic solvents, such as acetonitrile, dimethyl sulfoxide, ethanol, methanol, N,N-dimethylformamide, and propylene carbonate at 273 K. Hydrocarbons were formed with relatively high Faradaic efficiencies when methanol was used as an electrolyte, and their Faradaic efficiencies increased with reducing temperature. In the electrochemical cell, the rate of CH4 formation and CO2 conversion enhanced with positive potentials (electrophobic behavior) and negative potentials (electrophilic behavior).23,24 Solid electrolytes have been also used for the methanation of CO2. Y2O3-stabilized ZrO2 (YSZ) as a solid electrolyte and Rh as electrodes had been investigated in a single chamber reactor, and this system yielded CH4 selectivity of 35% at 346 °C. Moreover, the same electrode and electrolyte system (Rh/ YSZ/Pt) was used in a monolithic electropromoted reactor (MEPR) with up to 22 thin plate cells to study the methanation of CO2 at atmospheric pressure and temperatures of 220−380 °C.11 The Rh/YSZ/Pt cells catalyzed CH4 formation, and the open-circuit selectivity of CH4 was less than 5%. Both electrophobic and electrophilic behavior positively changed the total hydrogenation rate but did not show significant changes in the selectivity of CH4. Moreover, the Cu/TiO2/ YSZ/Au system cells generated CH4 and C2H4 at temperatures of 220−380 °C with selectivity up to 80% and 2%, respectively.25 One of the latest studies by Gao et al.26 demonstrated that atomically thin layers of cobalt have higher intrinsic activity and selectivity toward formate (HCOO−) production during CO2 electroreduction. During this process, hydrogen atom is attached to carbon atom in CO2, stimulating an extra electron to be propelled into one of its oxygen atoms, which turns CO2 into formate. Furthermore, hydrogen atoms hydrogenate these species to produce CH4. Various routes have been developed for reducing CO2 into CH4 including thermochemical,

Figure 3. Conceptual model of the three phase methanation reactor.

bed reactor (slurry bubble column). This concept is based on a three phase fluidized-bed system. In the three phase methanation slurry bubble column reactor, a decent heat dissipation of the reaction is a significant advantage of this method, as it allows good heat control of the reaction. Furthermore, the high heat capacity of the liquid phase used in this reactor makes it easier for the methanation process to handle the fluctuations in reactivity.21,22 However, a major drawback of three phase methanation is the liquid side, which provides less interaction between the gas and the metal catalyst particles. 2.2. Electrochemical Route. Applied electrochemistry provides valuable cost efficient and environmentally friendly contributions to development of many industrial processes with a minimum of waste production and usage of toxic material. Although the Sabatier thermochemical route is still practiced C


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Energy & Fuels electrochemical,27 photoelectrochemical,28 and photocatalytic29 routes. However, we are focusing here on the thermochemical route and recent advancements of this method.

of the same or less volume (incipient wetness amount) was added on the support containing pore volume.40 One modest technique to verify the pore volume of the support experimentally was by adding deionized water dropwise to the support until they had a glistening appearance. The appearance indicates that the pores are fully filled and the measuring was done through the total volume of water used.41 Solution added in excess of the support pore volume causes the solution transport to change from a capillary action process to a diffusion process, which is much slower. The capillary action is much faster than liquid diffusion, which draws the solution into the pores. The catalysts are then dried and calcined to deposit the metal on the catalyst surface. The concentration profile of the impregnated compound depends on the solubility of the precursor in the solution and mass transfer conditions within the pores during the impregnation and drying process. Several results have been reported in the literature for the synthesis of heterogeneous metal catalysts using this technique. IWI is the simplest preparation method for methanation catalyst, allowing for easier control of metal loading and no liquid waste generation.15,42 3.4. Double Impregnation Method. The activity and selectivity of metal loaded catalysts are greatly influenced by the amount of metal loaded and the size of the metal particles dispersed on the support. Double impregnation method (DIM) was used for well-dispersed and relatively high metal loading catalysts. DIM is completed in two stages. In the first stage, the inorganic support is preliminarily impregnated by organic reagents, such as EDTA, citric acid, salicylic acid, tartaric acid, resorcinol, and urea.43 In the second stage, after drying the impregnated support, the support was further impregnated using simple impregnation method (SIM) by solution containing active metal ions. Modification of the inorganic supports using impregnation by the nonmetal reagents in the first stage has influenced the way the metals are bonded. Application of nonmetal reagents favors high active metal phase dispersion and indirectly changes the concentration of metal in the catalyst and decreases average metal crystalline size.44 All these preparation methods are studied to obtained suitable characteristics for CO2 methanation, which include metal dispersion, crystalline size, and catalytic cycles. All the aforementioned techniques are currently used for the preparation of the catalysts. However, IWI was vastly studied by different investigators.15 Due to the capillary action, the active metal phase evenly diffused into the porous structure of the support, which gives a large active surface of the metal catalyst. Furthermore, IWI method involves a simple and sustainable chemistry tool for preparation of methanation catalyst with no liquid waste generation.41,45,46

3. PREPARATION TECHNIQUES FOR THE CATALYSTS The preparation method is an important consideration for designing catalysts in the thermochemical route. The techniques used for the combination of metal and its support can affect the crystal structure, catalytic activity, metal dispersion, and allowable metal loading. Different techniques have been used for the preparation of catalysts for CO2 methanation. A few of them are discussed as follows. 3.1. Sol−Gel Method. The sol−gel tool is a versatile chemistry method for preparation and understanding of catalytic materials. This method is an important tool in catalysis: e.g., the synthesis of heterogeneous catalysts through the entrapment of metal homogeneous precursors into titanium matrix or silica by a sol−gel method. In this method of catalyst preparation, a porous solid material is produced from small molecules of metal alkoxides, nitrides, or sulfides by colloidal route. Generally, for the CO2 methanation this solid catalyst is made via combining specific metal salts with its respective base metal.30 Various procedures such as aqueous and nonaqueous routes were used for the preparation with the following basisc order: (i) conversion of molecular precursor’s solution to the reactive state, (ii) polycondensation of activated molecular precursors into nanoclusters (formation of sol), (iii) gelation (formation of gel that encapsulates the solvent), (iv) aging (normally, 50−100 °C and 1−5 days), (v) calcination, (vi) washing, and (vii) drying/stabilization. The catalysts synthesized in such a way are stable and can be applied in CO and CO2 methanation processes at higher temperatures (up to 500 °C).31,32 3.2. Microemulsion Synthesis. Catalysts are used at severe conditions in industrial processes, but their usage at high temperatures is at times problematic. The microemulsion method has been considered an ideal route to synthesize organic and inorganic nanomaterial catalysts that possess high thermal stability. Additionally, the catalyst made via this method contributes high surface area and ultrahigh dispersion of the metal phase that enhances CO2 methanation.33 In this method, isotropic liquid mixtures of oil, water, and the combination of surfactant and cosurfactants are used. Essentially, the oil is a complex mixture of hydrocarbons and olefins, while the aqueous phase contains metal salt and other ingredients. Microemulsion can be categorized in two types: water-in-oil and oil-in-water, where the first compound is considered as solute and the second one is considered as solvent or dispersion medium.34,35 Park and McFarland36 prepared various monometallic and bimetallic catalysts, formed as an aggregate of highly dispersed palladium, magnesium, nickel, and lithium in silica that were prepared using a reverse microemulsion synthesis. In the formation of microemulsion the surfactant may be ionic or nonionic, which determines the stabilizing interactions of the hydrophilic end of the surfactant with the aqueous phase.37−39 This alternative approach potentially minimizes the CO byproduct using metal oxides that inhibit CO desorption and deactivation of the catalysts. 3.3. Incipient Wetness Impregnation. Incipient wetness impregnation (IWI) is also named as dry impregnation or capillary impregnation. IWI is generally used to synthesize heterogeneous catalysts. The active metal precursor is dissolved in aqueous or organic solution. The metal-containing solution

4. ACTIVE METALS FOR CATALYTIC METHANATION CO2 methanation started with the discovery of Sabatier and Senderens about 100 years ago. They discovered that CO2 and CO can be reacted with hydrogen to form CH4 and water using nickel as a catalyst. To date, several metals mainly in the group of 8−11 have been investigated and found to be successful in CO2 methanation (see Figure 4). Catalysts’ activity and CH4 selectivity are two different phenomena in methanation. Based on the metal catalysts’ activities and selectivities, different orders have been published by different investigators.47−49 Analyzing these orders and the previous literature, the following order of activity and selectivity for various metals in CO2 methanation is revealed: D


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Energy & Fuels

mostly used in other Fischer−Tropsch processes and ammonia synthesis.42

5. EFFECTS OF METAL COMBINATION ON METHANATION Methanation catalysts can be used as a single metal as well as a combination of different active metal phase in one catalyst. The effect of metals combined in a systematic way in one catalyst have been recently reported by different investigators.6,30,36,50−56 The results revealed that synergetic effects of these active metals further promote the adsorption and dissociation of H2 and CO2, which enhance the activity and selectivity of the catalysts in CO2 methanation. 5.1. CO2 Methanation over Monometallic Catalyst. Most of the CO2 methanation studies are carried upon multimetallic catalysts. However, some catalysts perform better alone than in combination with another metal catalyst. Table 1 shows monometallic catalysts and conditions employed in the catalytic methanation of CO2. Beuls et al.11 achieved 100% selectivity for CO2 using Rh/γ-Al2O3 catalyst at 150 °C and 2 bar pressure in a pulse reactor. CH4 is the distinctive hydrocarbon molecule formed with almost 100% selectivity. CO2 is considered as inert gas at low temperature ( Rh > Ni > Fe > Co > Os > Pt > Ir > Mo > Pd > Ag > Au

selectivity: Pd > Pt > Ir > Ni > Rh > Co > Fe > Ru > Mo > Ag > Au

Nickel-based catalysts are the most commonly used catalyst for CO2 methanation because of they are found to have high activity and CH4 selectivity. Some of the Ni catalysts could maintain a very good activity over a reaction time of nearly 100 h with high CH4 selectivity (about 100%) as well. However, because of the high activation energy required, the CO2 conversion was difficult to achieve at low temperatures on Ni catalysts.47 Rhodium- and ruthenium-based catalysts on the other hand are more active metals in CO2 methanation and produce almost exclusively CH4, but the high price of these metals make them less attractive for commercial uses. Other metals such as Pd, Pt, Mo, Ag, and Au catalyze CH4 product, simultaneously with CH3OH and CO as byproduct in a side reaction (reverse water gas shift reaction). Cobalt and iron catalysts exhibit almost the same methanation activity as that of Ni catalyst, but due to the high cost of cobalt, it is not widely used for industrial applications compared to Ni catalysts. Iron catalysts are reported to have a decent reactivity as well; however, their selectivity for CH4 was very low. Hence, iron is

ΔH ° = − 165 kJ/mol

CO2 + 4H 2 ↔ CH4 + 2H 2O

(3)

Table 1. Monometallic Catalysts Employed and Conditions Used in the Catalytic Methanation of C02

a

pressure (bar)

% CO2 conversion

% CH4 selectivity

11 15

1 15 20

35 70 96 67.3 80

100 30 90 99 87 99.5

350 350

15 1

97.1 85 82

45 46

fixed-bed reactor

350

1

10

fixed-bed microreactor fixed-bed reactor continuous fixed-bed reactor

280−320 400 300 300

5 1 1 1

99 81 16.8 88

100 99 98.4 60 87 92 98 96.1 99

catalysts

preparation method

reactor used

temp (°C)

1 wt % Rh/γ-A12O3 Ni/SiO2−Al2O3 Ni/RHA-Al2O3 15 wt %Ni/TiO2 25 wt % Ni/Al2O3 12 wt % Ni/γ-Al2O3

wet impregnation incipient wetness impregnation deposition−precipitation impregnation−co-precipitation incipient wetness, impregnation

fixed-bed reactor (pulse reactor) fixed-bed microreactor

50−150 500 500 260 325 210

2 1

10 wt %Ni/La2O3 Ni/MgAl2O4 DBDa Ni/MgAl2O4C 0.5 wt % Ni/SiO2 10 wt %Ni/SiO2 Ni/ZrO2 23 wt % Ni/Al2O3 3 wt % Ni-MCM-41 Ni/CeAl-p

incipient wetness, impregnation incipient wetness impregnation impregnation

fixed-bed reactor three phase reactor spinning-basket reactor (batch mode) fixed-bed reactor fixed-bed reactor

hydrogel

ref

18 22 41

62 63 64 65 66

DBD;, dielectric barrier discharge plasma decomposed catalyst. E


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Energy & Fuels CO2 + H 2 ↔ CO + H 2O

ΔH ° = 41 kJ/mol

another metal changes the catalytic performance of single-metal catalysts, such as selectivity toward CO2 methanation and lowtemperature and -pressure activity. Ren et al.54 impregnated monometallic catalyst of Ni/ZrO2 with Fe, Co, and Cu to make it a bimetallic catalyst. It was demonstrated that the 70% CO2 conversion associated with monometallic catalyst named Ni30 increased to 93% for bimetallic catalyst named Co3Ni30 (3 wt % Co, 30 wt % Ni) at the reaction temperature of 240 °C. The introduction of Cu and La paves the way for both structural and electronic promotion effects, resulting in a smaller particle size of the active phase and weaker interaction between active phases and the support, higher dispersions, and reducibility of active phases.68 The addition of Fe as a second metal to monometallic catalyst not only improves the dispersion and degree of reduction of Ni but also can boost the partial reduction of support ZrO2. The reason for this is the strong electrondonating ability of Fe that forms Fe2+ upon reduction at 400 °C. These effects further enhance adsorption and dissociation of CO2 into CO and subsequent reaction of CO with H2 to generate CH4, thus promoting the activity of the catalyst in CO2 methanation.53 Nickel- and rhodium-based methanation catalysts are easily deactivated by CO2 dissociation into CO that is considered as a poisoning effect on hydrogen adsorption.6 Thus, to maintain the catalytic activity of bimetallic catalysts (nickel/praseodymium), Buang et al.30 doped the catalyst with Mg, Zr, Mo, Mn, Co, Fe, and Cu. The results showed that Mg-, Co-, and Fedoped catalysts could achieve CO2 conversion from 70 to 94% at temperature above 350 °C. However, CO2 conversion for Zr-, Mo-, Mn-, and Cu-doped catalysts is less than 50% at any temperature. The addition of praseodymium (Pr) changes the physical properties of the catalyst by increasing the surface area of the multimetallic catalysts. The increase of activity for the multimetallic catalyst was associated with the enhancement of the H2 adsorption and by the weakening of the bond between the metal active phase and CO.69 It has been reported that the surface carbonate and formate species formed by the reaction of CO2 on a metal-containing oxide support play a critical role in CO2 methanation. The spilled-over hydrogen atoms hydrogenate these species on the metal catalyst to produce CH4. Therefore, the existence of a large number of strong basic sites is favorable for CO2 methanation at lower temperatures.10 Moreover, to investigate the effect of strong basic sites, He et al.51 used KOH as an additive to Ni−Al2O3−HT to form K−Ni/Al2O3−HT. As expected, both the CO2 conversion and CH4 selectivity increased after introducing a small amount of potassium. The addition of potassium on the catalyst provided extra strong basic sites, which was suggested to be the reason for the enhanced catalytic performance for CO2 methanation. The monometallic catalyst of Pd/SiO2 has activity only for CO2 reduction to CO. However, when this catalyst was used as a bimetallic catalyst with magnesium (Pd−Mg/SiO2) a greater selectivity of CH4 (95%) was achieved at CO2 conversion of 59%.36,42 The bifunctional efficiency of the Pd−Mg/SiO2 catalyst is motivated by the properties of Pd atoms to dissociate molecular hydrogen into atoms and make available for the subsequent reaction with activated surface carbonate species formed by the reaction of CO2 on Mg-containing oxide.10,70 The same way, Wan Abu Bakar et al.55 recently investigated monometallic, bimetallic, and trimetallic alumina supported metal oxide catalyst. The results showed that as the number of

(4)

The positive effects of water’s presence in CO2 hydrogenation are reported by Jimenez et al.59 for Ru-based catalysts, where the vapor presence in the feed is suggested to inhibit the reverse water gas shift reaction or to increase the amount of surface carbon for chain growth, which enhanced CO2 methanation.60 Although noble metals such as Pd, Pt, Ru, and Rh are known to give better activity and selectivity as a catalyst, but the scarce availability and high price of these metals have restricted their uses.55 Nickel-based catalysts are the most commonly studied for CO2 methanation because of their high activity and low price, but the sintering effect at reaction conditions diminishes their commercial practicability. Generally, the catalysts affect the chemical reaction mainly by its exposed active sites or surface properties. The number of active sites increases, while dispersion of metal decreases with increasing metal loading. Qin et al.61 applied the microwave calcination method at 450 °C for the higher degree of Ni and Ce particle dispersion on the alumina support. H2-TPR, H2 pulse chemisorption, TPSR, TEM, and XRD characterization showed that microwave calcination can enhance the formation of amorphous NiO with weak interaction with the alumina support, promote the degree of Ni and Ce dispersion, and increase the active surface area. Furthermore, these results improved catalytic performance of Ni- and Ce-based catalysts in CO2 methanation. Additionally, at low metal loading the CO selectivity is much higher, while better catalytic activity has been noticed for CH4 selectivity at higher metal loading.62 The reason for higher CO selectivity compared to CH4 at low metal loaded catalyst is the low H2 coverage on small active metal particles, which leads to the quick formation of CO from the m-HCOO intermediate. A larger number of active nickel sites implies a higher CH4 yield. However, Chang et al.15 reported that CO2 conversion is almost independent of nickel loading at temperature 500 °C. The reason is the bulk formation of NiO, which decreases the active sites gradually. 5.2. CO2 Methanation over Multimetallic Catalyst. The introduction of a second metal on the surface of monometallic catalyst completely changes the electronic and geometric structures of the catalysts. The interaction of metals in multimetallic catalysts drastically changes the physical and chemical properties of the metals; those properties are different from the individual metals.54,67 In most of the cases, loading more than one metal makes a significant synergic effect for CO2 methanation. Rh and Ni catalysts give low selectivity for CO2 methanation when used separately. However, Swalus et al.6 used a mechanical mixture of two catalysts (1 wt % Ni/AC + 1 wt % Rh/γ-Al2O3) at a temperature as low as 125 °C and 2 bar pressure. It was noticed that using the monometallic catalyst (1 wt % Ni/AC, 1 wt % Rh/γ-Al2O3) individually, no activity was observed at this reaction condition. While synergy was observed for the mechanical mixture of the two catalysts in the form of 4 times higher production of CH4 and 8% higher CO2 conversion compared to pure Rh catalyst (1 wt % Rh/γAl2O3) and Ni catalyst (1 wt % Ni/AC). The same results have been reported by Ocampo et al.52 for bimetallic systems of Ni− Rh and Ni−Ru catalysts. This synergy effect is because of the supportive properties of each catalyst: Rh catalyst is a storage source for CO2 adsorption and also helps in dissociation while Ni catalyst helps in H2 adsorption and activation via spillover mechanism.67 Thus, the deposition of a small amount of F


G

1

1

1

1 1 1

5

1 10

350

450

400

380 400 250

250

400 320

co-precipitation method sol−gel method incipient wetness impregnation impregnation method

fixed-bed reactor (continuous) fixed-bed down-flow reactor down-flow tubular reactor

microreactor fixed-bed reactor

impregnation method PEG-free method

two-nozzle flame spray pyrolysis

fixed-bed reactor

high-pressure fixed-bed reactor

reverse microemulsion synthesis

fixed-bed reactor

catalyst preparation method incipient wetness impregnation sol−gel method

reactor used

fixed-bed tubular flow microreactor flow-bed reactor

CZ = CeO2/ZrO2,

2

125

a

P (bar)

T (°C)

Ni/Al2O3

Ni/ZrO2

17.8

90

82 71.5 11.4

36.8 0.8 55

Ni/SiO2 Mg/SiO2 Rh/Al2O3

Ni/Al2O3-HT 5Ni/CZa Fe/Al2O3

40.8

0

% CO2 conversion

Pd/SiO2

1 wt % Ni/AC

monometallic catalysts

90

99.2 98.5 96.5

81.8 10.3 94

10.4

% CH4 selectivity

93 96 50 99.74 10

43 86 77.8 22.1

Rh−Ba/Al2O3 K−Ni/Al2O3-HT 5Ni−0.5Rh/CZ Fe−Ni/Al2O3 Co−Ni/ZrO2 Fe−Ni/ZrO2 Cu−Ni/ZrO2 Ru−Mn−Ni/Al2O3 NiRu/SiO2-P

50.5 59.2 25

93.3 91.5 78.5 44.7

8

% CO2 conversion

Pd−Ni/SiO2 Pd−Mg/SiO2 Rh−K/Al2O3

Ni−Mg−Pr Ni−Fe−Pr Ni−Co−Pr Pd−Fe/SiO2

1 wt % Ni/AC + 1 wt % Rh/Al2O3

multimetallic catalysts

Table 2. Comparison of the Monometallic and Multimetallic Catalysts Employed and Conditions Used in the Catalytic Methanation of CO2

90 92 70 72.36 80

95 99.8 99.2 99.5

89 95 0

2.8

100

% CH4 selectivity

55 56

54

51 52 53

50

36

30

6

ref

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Energy & Fuels Table 3. Performance of Unsupported Catalysts in Methanation catalyst Raney Ni catalystsa Ni 42 Ni 50 Ni 60 nickel nanoparticles (Ni NPs) MoS2 a

preparation method

reduction in aqueous solution thermal decomposition method

T (°C)

% CO2 conversion

% CH4 selectivity

ref

300 300 300 400

60 73 80 42 82

90 98 100 33 60

71

72 73

The number in the catalyst represents the percentage of Ni content in the Ni−Al alloy.

catalytic methanation and are termed as “real nanoeffects”. Thus, nanoparticles of active metals are mostly used in the CO2 methanation as unsupported catalysts.74

incorporated metals increased, the CO2 conversion and CH4 selectivity also increased. Thus, for trimetallic catalyst Ru− Mn−Ni/Al 2O3 the values of CO2 conversion and CH4 selectivity were noted as 99.7% and 72.4%, respectively. However, the Pd−Mn−Ni/Al2O3 incorporation unfavorably affect the catalytic activity and selectivity, which might be due to the least activity of Pd for CO2 hydrogenation, and Mn and Pd were not a good metal oxide combination in methanation process. It was also found that Pd and Pt promote the undesired RWGS reaction.59 Table 2 gives comparative study based on CO2 conversion and CH4 selectivity on the monometallic and multimetallic catalyst systems used by different researchers. It has been summarized that the synergic effect in multimetallic catalysts plays an important role in enhancing the CO2 methanation process. Therefore, in the future, the research should be focused on the multimetallic system. However, in some occasions, turning monometallic catalyst to bimetallic catalyst adversely affects the CO2 methanation. For instance, pure Rh/ Al2O3 catalyst showed the best selectivity to CH4 while making Rh−K/Al2O3 by introducing potassium produces selectively CO below 450 °C. The K-containing catalyst directly transformed CO2 to CO.50

7. REACTION MECHANISM OF CO2 METHANATION 7.1. Chemical Aspect. The CO2 methanation reaction seems to be a simple reaction. However, the nature of the intermediate compounds and steps involved make it a sophisticated chemical reaction. Two different paths are proposed for CO2 methanation. The first path involves the dissociation of CO2 to CO prior to methanation, and in the subsequent reaction, CO converts to CH4 by reacting with H2. The second path does not involve the formation of CO as intermediate steps and direct hydrogenation of CO2 to CH4 take place.10,75 The proposed first path is further suggested to be completed by two different mechanisms. The first mechanism advocates the formation of a formiate (CHOO−) species as intermediate, where these species are decomposed in CO. The second mechanism suggests the dissociation of CO2 into CO and O adsorbed species without formation of formiate. In both mechanisms, CO would be the intermediate that can be desorbed or react with H2 to form CH4, after dissociation into C and O monatomic species. In both mechanisms, the intermediate formed is the adsorbed CO. The only difference is the way in which this intermediate is formed.5,11,41 Most of the researchers believe that CO2 methanation follows the model involving a CO intermediate without the formation of formiate. The reactions below summarize the reduction process:

6. UNSUPPORTED CATALYSTS Mostly, the methanation catalysts are supported on the porous network to achieve a larger surface area, nevertheless besides supported catalysts, unsupported methanation catalysts are also used. Unsupported catalysts include nickel nanoparticles, Raney nickel and MoS2 shown in Table 3, but these catalysts also require a large surface area to achieve sufficient methanation activity. Several methods had been used to prepare unsupported Mo-based catalysts including solution reactions, thermal decomposition of thiosalts, and hydrothermal and solvothermal processes.71,72 The unsupported MoS2 catalyst was prepared by Liu and his co-workers73 via thermal decomposition of ammonium heptamolybdate, (NH4)6Mo7O24·4H2O, or other precursors with sulfur powder in a quartz tube reactor. They concluded that unsupported MoS2 prepared by thermal decomposition of thiosalts demonstrated higher activity for methanation than those catalysts prepared by other methods. Molybdenum-based unsupported catalysts synthesized by sulfiding process are also potential catalysts for methanation. Usually, the activation of molybdenum through sulfiding process is achieved by exposing its precursor to a gaseous mixture of hydrogen sulfide, resulting in highly active MoS2 catalysts.73 The particle size of unsupported active metal catalysts plays a vital role in the methanation process. Furthermore, a few new phenomena arise in the methanation process through the transition of solid state behavior of metal catalysts to atomic/molecular properties due to the nanoparticle size. These new effects positively change the

CO2 + H 2 ↔ CO + H 2O

(5)

CO + 3H 2 ↔ CH4 + H 2O

(6)

The equilibrium for the first reaction between CO2 and H2 is rather unfavorable, which is due to the faster generation of CO in the first reaction than its consumption in the second reaction. The excess amount of CO generated in the first reaction deposits on the catalyst, which produces sintering and coking effects. To avoid the problem, methanation of CO must proceed much faster than CO production. The methanation of CO2 has been reported at temperatures as low as 50−150 °C.11,76 CH4 formation at such a low temperature could be explained through the dissociation mechanism of CO2 and H2 on the surface of the catalyst. New insights on CO 2 methanation gave a three steps mechanism: (i) chemisorption of CO2 on the catalysts, (ii) dissociation of CO2 into CO and O as well as H2 into atomic hydrogen on the catalyst surface, and (iii) the reaction of dissociated species with spilled-over H2 (see Figure 5). These steps were evidenced by in situ DRIFT experiments and could be represented as given in Schemes 1−3.5 H


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Table 4. Activation Energies (Ea) for CO2 Methanation on Different Catalysts

Figure 5. Conceptual model of hydrogen spillover mechanism on catalyst supported adsorbent. HC is hydrogen atom adsorbed on catalyst; HA is hydrogen atom diffuse in adsorbent porous network.

catalyst

T (°C)

Ea (kJ/mol)

ref

1% Rh/TiO2 5% Rh/SiO2 0.5% Rh/TiO2 Ru/γ-Al2O3 Rh/Al2O3 0.5 wt % Ni/SiO2 10 wt % Ni/SiO2 5% Rh/Al2O3 Rh (sheet) Ni(100)

200−275 200−275 200−275 160 50−150 400

81 72 68 83 10.7 43 69 103 67 89

9 9 9 9 11 62

200−300 250−400 177−477

69 77 78

Scheme 1 al.11 in their work noticed that the values of activation energy are lower than those reported in the previous studies. Different experimental conditions might be the reason for variances in the results, especially the lower temperature used in their work. It has been reported in the literature that the mechanism in which formiate produces as an intermediate step, required higher temperature and higher activation energy. 7.2. Physical Aspect. The literature revealed that not only chemical characteristics of methanation reaction but also the physical aspect of reaction influence the methanation process.47 The idea of simultaneous use of solid as a methanation catalyst and gas sorbent aims to overcome the complex problems related with the handling of two different solids as a catalyst and gas sorbent. Supports are often considered as inert media: catalysis occurs at the catalyst field, and the existence of support is viewed to provide high surface area only. Various experiments indicate that this interpretation of the hybrid material (support and metal active phase) is oversimplified or even wrong. The gases adsorption and dissociation are the first steps in CO2 methanation; therefore catalysts having the ability to adsorb the reactant gases are more important for this mechanism. These materials are required for CO2 sorption enhanced methanation. Furthermore, it is also decreasing the cost of the total solid material used for methanation.80 The catalysts investigated for CO2 methanation are mostly made using group viii, ix, x, and xi transition metals.42 The loading of active metallic particles containing Ce, Co, Ni, Rh, and Ru acts synergistically with sorbents in the process of CO2 methanation. These hybrid materials can be developed by loading noble and transition metals on different sorbent matrices such as SiO, Al2O3, Ca12Al14O33 (mayenite), activated carbon, and bentonite, etc. The use of porous sorbent matrices as support for the methanation catalyst offers three interesting characteristics: (i) the high surface area of these sorbents allowing one to obtain highly dispersed active catalytic phases, (ii) weak metal−carbon or promoter−carbon interactions that can favor a better metal−promoter interaction, and (iii) the porous network of the sorbent trap H2 and CO2 that helps in methanation.80,81 To understand the role of the supports in CO2 adsorption and methanation, Pandey and Deo53 studied the CO2-TPD profile for different supports impregnated with the same ratio of metal catalyst. These studies determined that different amounts of CO2 were adsorbed on each support which trailed the following trend:

Scheme 2

Scheme 3

Scheme 1 depicts the hydrogen spillover mechanism on the catalyst, where HC was hydrogen atom adsorbed on the metal active phase. Scheme 2 shows the CO2 dissociation to CO and C, from where it has been learned that CO needs more activation energy (Ea = 286.56 kJ mol−1) than CO2 (Ea = 122.54 kJ mol−1) in the dissociation process. Furthermore, the CO dissociation has two pathways; one is CO(ads) → C(ads) + O(ads) and the other is 2CO(ads) → C(ads) + CO2(ads). The calculated activation energy of these two steps were 286.56 and 186.22 kJ mol−1, respectively, while Scheme 3 shows the formation of CH4 from the derived species of the previous two schemes.75 At low temperature the extent of this reaction was slow but thermodynamically was favorable. In a subsequent step, these dissociated species react to produce CH4. The reaction of CO2 and H2 takes place at the interface between the metal active phase and support forming CO that further react with hydrogen atoms to produce CH4.9 It has been suggested that CO production was an intermediate during CO 2 methanation. The stability of the CO deposits on the catalyst determine whether the CO will desorb and progress for further reduction or not. The CO dissociation was thought to be the rate-determining step for the remaining reduction stages.36 Table 4 gives the activation energy (Ea) values for CO2 methanation reactions as reported in the literature. The great loss to CO2 methanation was the high stability and activation energy required for CO2, which imposes scientific challenges in its activation. New concepts and fundamental techniques are highly requisite to develop and promote emergent technologies to activate CO2 at affordable cost for methanation.79 Beuls et

Al 2O3(838) > TiO2 (263) > Nb2 O5(239) > ZrO2 (205) > SiO2 (34) I


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Figure 6. Effect of temperature and pressure on CO2 conversion and CH4 selectivity during methanation, a comman trend.

and CO formed via reverse water gas shift reaction (RWGS), which are shown in the following chemical reactions:17

The values shown in parentheses are the amount of CO2 adsorbed in millimoles of CO2 per gram of support. CH4 yield also followed the same trend for different supports. In the case of Rh/Al2O3 catalyst, more than half of the CO2 adsorption takes place on alumina support, while rhodium deposition increases CO2 adsorption.36 Beuls et al.11 highlighted that 3−4 times more physisorption of CO2 on Rh deposited alumina (Rh(1%)/γ-Al2O3) compared to the pure alumina support, which further improved the CO2 methanation reaction. In order to get a high yield of CH4, it was necessary to make sure the catalyst was able to store and activate a high amount of hydrogen. Via spillover mechanism shown in Figure 5, the metal catalyst helps hydrogen to migrate from the metal surface onto the nonmetal surface and finally diffuse to specific sites where they are strongly adsorbed on the hybrid material.6 The loose bonding between the spilled-over hydrogen and the adsorbent surface allows the hydrogen atom to desorb through the reverse spillover effect, which is the migration of hydrogen atoms from adsorbent onto the metal phase where they recombine to form molecular hydrogen.82 Several factors influencing the hydrogen storage at room temperature on hybrid materials have been examined. These factors include the amount of metal load, the nature of the metal precursor, hydrogen pressure, catalyst preparation method, metal particle dispersion, mechanical mixing of metal precursor, and sorbent support.83 de la Casa-Lillo et al.84 demonstrated that activated carbon alone physisorbed a small amount of hydrogen: whereby at high pressure of 30 bar about 0.15 wt % was adsorbed. In contrast, when the metal phase is introduced to activated carbon, the hydrogen uptake increased to 0.53 wt % at the same condition that is almost four times higher than pure activated carbon. Extrapolating these results of 0.53 wt %, hydrogen physisorption showed that pure activated carbon required higher pressure (100 bar) to achieve these results at room temperature.82 Except the adsorption of gases, the supports in methanation catalysts also tend to act as a stabilizer to avoid Oswald ripening or sintering of the catalytically active particles. Alumina achieved good results, followed by silicas and titania when employed as supports in the CO2 methanation process. The performance was reported better for alumina as it hinders sintering due to strong metal− support interactions that lead to a well-dispersed catalyst and high methanation activity.85

CO2 + 4H 2 ↔ CH4 + 2H 2O

(7)

nCO2 + (3n + 1)H 2 ↔ CnH 2n + 2 + 2nH 2O

(8)

nCO2 + (3n)H 2 ↔ CnH 2n + 2nH 2O

(9)

CO2 + H 2 ↔ CO + 2H 2O

(10)

The ultimate product of the CO2 hydrogenation was decided by the process conditions. Selectivity of CH4 depends on different parameters, which include the type of catalysts, reaction conditions such as temperature, pressure, and mass and heat transfer. The equilibrium of the CO2 methanation reactions was influenced by pressure and temperature. In the CO2 methanation, the general trend of CO2 conversion and CH4 selectivity over most of the catalysts are shown in Figure 6. In thermodynamic equilibrium, high pressures increase the conversion of CO2 and selectivity of CH4. On the other hand, the CO2 conversion increases rapidly as the reaction temperature rises, while the selectivity of CH4 increases as the reaction temperature rises to a certain limit and then starts decreasing at elevated temperature.18,45−47 This can be further explained thermodynamically based on the exothermic nature and reversibility of the CO2 methanation process that proceeds at a suitable rate at moderate temperatures required for high CH4 selectivity only when a suitable catalyst is used. CH4 selectivity was suggested to be more sensitive to temperature for Rh, Ru, and Co catalysts due to their good hydrogenation properties.51,60 Furthermore, the elevated temperature resulted in the inactivation of the catalyst and affected the thermodynamic equilibrium. A higher reaction temperature promotes side reactions such as endothermic reverse water gas shift reaction, which was favored at higher temperatures and implies a lower CH4 selectivity. Therefore, reaction proceeding under suitable temperatures can promote the selectivity of CH4.15,51,64 CO2 conversion and CH4 selectivity are affected not only by the process condition but also by the type of catalyst and support. Ruthenium is apparently the most active catalyst. It was reported that Ru/SiO2 catalyst has the highest selectivity for CH4 (99.8% CH4 selectivity and 5.7% CO2 conversion) at 229 °C. Nearly 100% selectivity toward CH4 over Ru/TiO2 catalyst at low temperature and atmospheric pressure was also reported.65 However, in most of the studies, nickel-based catalysts were chosen because of its higher CO2 methanation in terms of activity and selectivity, and cheaper compared to ruthenium- and rhodium-based catalysts.17 Using ceria−

8. CO2 CONVERSION AND CH4 SELECTIVITY During the hydrogenation process of CO2, not only CH4 was produced but also other side products such as alkanes, alkenes, J


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dissociation of CO2 into CO and O2. The addition of the higher Gibbs free energy H2 as a co-reactant in the methanation reaction makes the conversion of CO2 thermodynamically easier with a ΔH of −167 kJ/mol.42 9.2. Deactivation of the Catalysts. Deactivation of the metal catalysts in the methanation reaction is also a big challenge. Catalyst deactivation is a chemicophysical process that could be mainly classified into two types: (i) chemical deactivation and (ii) physical deactivation. Chemical Deactivation. Poisoning and vapor−solid reaction are considered as chemical deactivation. Methanation catalysts are sensitive to different gas impurities containing carbon deposition, chlorine compounds, tars, ammonia, sulfur compounds, or alkalis. Poisoning occurs because of chemisorption of impurities present in the feed stream on catalytically active sites, thereby blocking sites for catalytic reaction. These impurities can also modify the chemical nature of the catalysts and result in the formation of new compounds that definitively alter the catalyst performance.86 Limited presence of oxygen enhance the methanation rate, where improvement was attributed to the formation of more reactive species because of the oxygen oxidative property. However, when the amount of oxygen exceeded a certain limit, the deep oxidation deactivates the catalyst and negatively affects the rate of reaction.47,53,86 Vapor−solid reactions are the reaction of fluid, support, or promoter with catalytic phase, which also causes a form of chemical deactivation. Formation of nickel carbonyls in the carbonization reaction at temperatures below 230 °C in the presence of CO is an example of this type of deactivation.47 In the case of iron catalyst, there was an irreversible formation of iron carbide−Fe5C2−because of the iron−carbon reaction. This stable and inactive carbide is accountable for the catalyst deactivation in the CO2 hydrogenation process.12 During the methanation process, when niobium(V) oxide (Nb2O5) was used as a support for iron or nickel active phase, niobates were formed, which decreased catalytic active sites causing deactivation of the catalysts.53 Physical Deactivation. Thermal degradation, fouling, and attrition are considered as physical deactivation. The catalyst life not only depends on the catalyst composition but also by the reaction conditions. It was found that higher reaction temperatures and pressures could rapidly reduce the activity and catalyst lifetime by promoting agglomeration and thermal degradation or sintering effects. Sintering of the catalyst leads to a loss of surface area and hence, reducing the catalyst activity.8 Thus, the exothermicity of the methanation reaction is somewhat also responsible for the catalyst deactivation, which actually enhances sintering effect.17 However, due to thermal degradation, temperatures above 500 °C have to be avoided in some cases of catalytic activity. Though, catalysts for hightemperature (

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