Photocatalytic Gas Phase Reactions - Chemistry of Materials (ACS

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Photocatalytic Gas Phase Reactions Murielle Schreck, and Markus Niederberger Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04444 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Chemistry of Materials

Photocatalytic Gas Phase Reactions Murielle Schreck and Markus Niederberger*

Laboratory for Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland

* E-mail: [email protected]

Abstract Photocatalytic gas phase reactions represent a less explored, yet promising direction within the highly active research area of photocatalysis. While photocatalytic liquid phase reactions are typically performed in water, photocatalytic gas phase reactions focus on gaseous and vaporized compounds. With this change in reaction environment, it is possible to address completely new questions and topics. Examples lie in the field of solar fuels, air pollution, global warming, or green chemistry. For this review, we selected five photocatalytic gas phase reactions, namely the reduction of CO2, water splitting, the oxidation of volatile organic compounds, the degradation of nitrogen oxides, and the synthesis of ammonia, which we discuss in the context of why they are important from a scientific and technological, but also from a societal point of view. We present the chemical mechanisms behind these photocatalytic processes and we propose ideas and strategies, how these processes can be made more efficient. Our literature survey results in a list of eleven points, how the selectivity and the yield of photocatalytic gas phase reactions can be increased by optimizing the composition of the photocatalysts, their surface chemistry, and experimental parameters such as temperature, gas flow, or gas composition.

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1

Introduction

The term ‘photocatalysis’ arose the first time at the beginning of the 19th century in scientific publications. However, without any practical application, not much effort was put into this research field. Only when human kind realized that fossil fuels are finite at the beginning of the 1970s, things changed. In parallel to the drastically increased crude prices due to an oil crisis triggered by an embargo of the OAPE Countries (Organization of Arab Petroleum Exporting Countries), also the number of publications dealing with alternative energy sources increased. A turning point in photocatalysis research was reached in 1972,1-3 when Honda and Fujishima conducted photoelectrochemical water splitting with TiO2 as the semiconductor electrode material in a photoelectrochemical cell.4 From this moment on, the practical aspects of photocatalysis motivated many groups to do research in this direction. The opportunity to perform complex chemistry with light as the single energy source without high temperatures and pressures makes photocatalysis also a hot topic of modern research.2 Research fields, where photocatalytic reactions are particularly attractive, include environmental remediation (e.g., wastewater treatment,5 air purification6), generation of energy and alternative fuels (e.g., artificial photosynthesis7 and hydrogen generation8), applications in the fields of medicine and biomedicine,9 food packaging10 and green chemistry11 (Figure 1).3

Figure 1 Selected application areas of photocatalysis.

Photocatalysis is the “change in the rate of a chemical reaction or its initiation under the action of ultraviolet (UV), visible, or infrared radiation in the presence of a substance – the photocatalyst – ACS Paragon Plus 2 Environment

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that absorbs light and is involved in the chemical transformation of the reaction partners”.12 It is important to mention the difference between photocatalysis and classical catalysis. In catalysis, chemical reactions are accelerated, i.e., the kinetics is changed, while the overall thermodynamics and the products of the reactions remain the same. Accordingly, only spontaneous reactions with a negative Gibbs free energy can be catalyzed. A photocatalyst, on the other hand, is not activated by heat, but by light or, more precisely, by photons with a specific energy.1 As a result, photocatalysts can also accelerate non-spontaneous reactions like e.g. water splitting or artificial photosynthesis and the degradation of contaminants in water or air.13 In photocatalysis, high temperatures can be disadvantageous, because thermal excitation can lead to desorption of the reagent from the surface of the photocatalyst and to increased charge carrier recombination.2 Similar to classical catalysis, a distinction is made between homogeneous (reactant and photocatalyst are co-existing in the same phase) and heterogeneous photocatalysis (reactant and photocatalyst are in different phases).1 Depending on the phase of the targeted species to be reduced or oxidized and depending on the reactor, the photocatalytic reaction happens in the liquid or in the gas phase. Most research effort is put in semiconducting photocatalysts, because the band gaps lie in the UV and visible light energy range. Therefore, semiconductors are ideal for harvesting photons with the appropriate energies. During the photoexcitation process, a photon with an energy equal or higher than that of the gap between valence band (VB) and conduction band (CB) is absorbed and an electron is excited from the VB to the CB (Figure 2). A hole is left behind in the VB. This process is referred to as charge carrier separation. Afterwards, the charge carriers migrate to the surface of the photocatalyst and, in the ideal case, they are transferred to the surface-adsorbed reactant, participating in the redox reaction. In the non-ideal case, recombination of electrons and holes within the bulk or at the surface of the semiconductor occurs, preventing any redox processes (Figure 2). This is a major problem during photocatalytic processes using semiconductors, because it decreases their efficiency.2, 14



Figure 2 Scheme of the photoexcitation process in a semiconductor particle. At the surface of the particle, the photogenerated electrons can reduce and the holes can oxidize the electron acceptor A and the electron donor D, respectively. As a side reaction, electron and hole recombination can happen at the surface (a) or in the bulk (b) of the semiconductor particle.

In this review article, we exclusively focus on photocatalytic reactions occurring in the gas phase. Such reactions play an important role in diverse technological applications. TiO2-coated glass with self-cleaning properties has been commercialized more than 15 years ago.15 The self-cleaning effect is also used in house and street construction, where cements, paintings, paving stones and tiles with a photocatalytic component (mostly TiO2) provide a surface that is able to degrade organic pollutants or gases like NOx or SOx. Here, we selected the reduction of CO2, water splitting, the oxidation of volatile organic compounds, the degradation of NOx, and the synthesis of ammonia as technologically and socially relevant model reactions. Every of these five reactions is discussed under the following three aspects: why is it important? What are the chemical mechanisms? What are the challenges and possible improvements? More specifically, we present facts and numbers, why these reactions are important for our society. We then cover the chemical mechanisms of the photocatalytic processes. At the end of every reaction-specific section, we briefly present, which problems and limitations are responsible for the low photocatalytic efficiency of these processes and we mention some general ideas, how these issues can be overcome. In section 3, the core of the review, we converge the state-of-the art in the literature into 11 bullet points, which summarize the main findings how the experimental conditions of the photocatalytic processes and the photocatalysts themselves can be changed to improve the efficiency of the overall photocatalytic reactions. Every point is illustrated by concrete examples from literature. ACS Paragon Plus 4 Environment

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It is clear that not all aspects can be covered in a review article on such a broad topic as photocatalysis. Two important areas are not addressed: homogeneous photocatalytic systems and the efficiency of photocatalytic gas phase reactions. Although our review strongly focuses on conventional heterogeneous systems, homogeneous molecular systems are also promising for photocatalysis, providing advantages such as high quantum efficiencies, high selectivity for the target molecule and the possibility to control the reactivity at the molecular level.16-17 Metal-complex catalysts with the metal component being ruthenium or rhenium are commonly used for such homogeneous processes. The disadvantage of using these metals is that they are rare and expensive. 14

Additionally, the turnover numbers for metal-complex catalysts in general are low due to the fast

degradation of the complexes. A deeper discussion of homogeneous molecular photocatalytic systems can be found somewhere else.18 Throughout the review, we almost completely refrained from disclosing any efficiency values for the materials and systems described. The reasons for this are missing standards, different efficiency definitions and the immense variety of experimental setups and conditions used by the different research groups, making a meaningful comparison impossible. In contrast to conventional thermal heterogeneous catalysis, the reaction rates in photocatalysis are not proportional to the number of active sites, and thus not to the weight or surface area of the photocatalyst.19 As a matter of fact, photocatalytic rate constants depend on quantum yield, incident light intensity, light path, extinction coefficient and concentration of photocatalyst, and therefore only (apparent) quantum yields can and should be compared.20 In any case, there is a great need to improve the reporting of photocatalytic results to ensure reproducibility and effective benchmarking.19

2

Different photocatalytic gas phase reactions

2.1 2.1.1

Photocatalytic CO2 reduction to solar fuels Why is it important?

The energy demand of our society is constantly increasing and at the same time the energy reserves in the form of fossil fuels like oil, gas, and coal are running out.21-22 Solar energy is an abundantly available source of energy. It is safe and available all over the world free of charge.3 However, its direct usage in photovoltaics to produce e.g. electrical energy reveals also its biggest disadvantage: it is subject to daily and seasonal fluctuations.1 Storing the converted energy in batteries is one possibility to solve this problem. Another attractive solution from an environmental and economical point of view is the conversion of solar energy into chemical energy, i.e., storage of the energy of the sun in chemical bonds of solar fuels like methanol, hydrogen, and others.3, 23 ACS Paragon Plus 5 Environment


In addition to the problem that fossil fuels are limited in supply, their consumption releases a lot of carbon dioxide, which is a greenhouse gas and as such contributes to the climate change.24 Therefore, immense efforts are undertaken to solve the energy and the global warming problem. One possibility is the photocatalytic hydrogen generation via water splitting (section 2.2). Hydrogen can replace fossil fuels and its combustion does not release any CO2 into the atmosphere.25 A second and quite elegant solution is called artificial photosynthesis, i.e., the photocatalytic reduction of CO2 to methanol, methane, and other solar fuels. Although combusting those solar fuels will release CO2, it can be used again for the synthesis of solar fuels. In this way, the carbon cycle is closed and neither is CO2 released from the cycle nor will external addition of CO2 to the cycle happen (Figure 3). The ultimate goal is then to replace fossil fuels by CO2 as carbon feedstock.14

Figure 3 The closed carbon cycle: CO2 and H2O are converted into solar fuels via artificial photosynthesis. The combustion of these solar fuels leads to the emission of CO2, which is converted into solar fuels again.

The increasing CO2 level in air is a life-threatening problem. The concentration of CO2 in the atmosphere was 280 ppm before the industrial revolution. In 1958, when the monitoring of this gas started on the volcano Mauna Loa in Hawaii, the value was 316 ppm. In April 2013, the hourly average of CO2 in the atmosphere exceeded 400 ppm for the first time since the start of recording, and in 2016 the monthly average values were permanently higher.26 Nowadays, 35 gigatons of CO2 are emitted per year.27 There is definitely a huge need in doing research on decreasing the CO2 level in our atmosphere.


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Several strategies are currently in practice, how the CO2 concentration can be reduced with the help of solar energy. The first two approaches address indirect reduction pathways. In photovoltaics, solar energy is converted into electricity, which is then applied for the electrochemical reduction of CO2.28 The second indirect approach uses hydrogen obtained from photocatalytic water splitting to hydrogenate CO2 with the help of a catalyst.29 For large-scale production of chemicals like methanol and methane, on the other hand, direct mechanisms represent the state-of-the-art: CO2 is photocatalytically reduced on semiconductors in the liquid or in the gas phase.14, 30

2.1.2

What are the chemical mechanisms?

The reduction of CO2 to hydrocarbons like formic acid, formaldehyde, methanol, and methane is an endothermic process and thus requires energy input (solar energy). Under light illumination, electron-hole pairs are generated (2.1). While the photogenerated electrons are excited over the band gap of the semiconductor photocatalyst (Pc) into the CB and transferred onto the acceptor molecule CO2, which is adsorbed on the surface of the photocatalyst, the holes are left behind and migrate to the surface of the photocatalyst to oxidize adsorbed H2O molecules (2.2-2.3). Suitable semiconductor photocatalysts need to exhibit a potential level of the VB edge (green lines in Figure 4) that is more positive than the oxidation potential of the H2O molecule (O2/H2O 0.82 V, i.e., the electrochemical oxidation potential of H2O to O2 versus the NHE at pH = 7, blue-dotted line in Figure 4).14, 31 Regarding the reduction potential of the CO2 molecule no semiconductor exhibits a potential level of the CB edge (red lines in Figure 4) negative enough to transfer a photogenerated electron (2.4) to a free CO2 molecule (CO2-/CO2 -1.9 V, i.e., the electrochemical reduction potential of CO2 to CO2- versus the NHE at pH = 7, yellow-dotted line in Figure 4).1-2, 14 The CO2 molecule is chemically inert and very stable.14, 32 C-O bonds have to be cleaved and C-H bonds formed. If multiple electrons together with protons are transferred to CO2 (2.5-2.9) to form e.g. formic acid (HCOOH/CO2 -0.61 V – 2 (HCHO/CO2 -0.48 V – 4 (CH4/CO2 -0.24 V – 8

), carbon monoxide (CO/CO2 -0.53 V – 2

), methanol (CH3OH/CO2 -0.38 V – 6

), formaldehyde

) or methane

), the reduction potentials are less negative (blue bar in Figure 4), lying

in the range of the CB edge potentials of many semiconductors.14 However, one has to keep in mind that the proton reduction (H2/H -0.41 V) of the water splitting reaction,2, 33-34 which has a similar reduction potential as the multi-electron transfer reactions above, can compete with the photoreduction of CO2. *

Pc +

 Pc (

)


(2.1)


4 Pc (

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) + 2 H2O  O2 + 4 H

(2.2)

Pc (

) + H2O  OH + H

(2.3)

Pc (

) + CO2  CO2 

(2.4)

2 Pc (

) + CO2 + 2 H  HCOOH

(2.5)

2 Pc (

) + CO2 + 2 H  CO + H2O

(2.6)

4 Pc (

) + CO2 + 4 H  HCHO + H2O

(2.7)

6 Pc (

) + CO2 + 6 H  CH3OH + H2O

(2.8)

8 Pc (

) + CO2 + 8 H  CH4 + 2 H2O

(2.9) *

Pc - photocatalyst

Because such multi-electron processes are rather unlikely, difficult to achieve, and there is no evidence in literature that they indeed occur, single-electron reactions remain a more probable option. Yet, the linear symmetry of the free CO2 molecule makes it very stable and the barrier for accepting the first electron is very high. A slight bending of the molecule due to a partially charged molecule, e.g. upon adsorption on the surface of a photocatalyst or upon obtaining an electron from a metallic co-catalyst, lowers this barrier, because the potential of the CB edge becomes more positive. Such activated CO2 molecules can be reduced following a series of single-electron reactions.14

Figure 4 Potentials of VB and CB versus the NHE at pH = 7 and band gap energies for different semiconductors, which are potentially useful for CO2 reduction.


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2.1.3

Challenges and possible improvements

Although a lot of research is conducted on photocatalysis and especially on photocatalytic CO2 reduction, the conversion efficiencies and production rates are far from what we would need for commercial applications. Literature rarely reports production rates, which exceed tens of μmol of solar fuels per gram of photocatalyst per hour of illumination (μmol g-1 h-1). The main issues are the low solar conversion efficiency of the whole process due to fast recombination rates of photogenerated electrons and holes and the bad selectivity, because the reduction of CO2 is subject to a complex mechanism and leads to several different products.2, 14 Further problems are the slow charge consumption during the redox reactions and the poor light utilization.35 Most of the photocatalysts predominantly absorb UV light. However, the main part (around 44%) of the solar radiation spectrum reaching the Earth surface consists of visible light, and only 4-5% are composed of UV light (Figure 5).3 The prerequisites for a good photocatalyst for the CO2 reduction reaction to solar fuels are the following. First, its band gap should be big enough to include the potentials of both redox reactions, which are participating in the artificial photosynthesis,3, 14 but at the same time, it should be small enough to be activated by visible light photons. Therefore, band gap narrowing is a big topic in photocatalysis. In this context it has to be mentioned that the term “band gap narrowing” is not entirely correct. In the most cases, experimental strategies are targeted at introducing energy levels into the band gap rather than at narrowing the band gap itself. Nevertheless, as band gap narrowing is widely used in the community, we will also do that in this review. Second, the recombination of electrons and holes must be prevented or at least heavily suppressed.



Figure 5 Solar radiation spectrum: comparison of AM0 standard extraterrestrial radiation spectrum (yellow), the theoretical emission of a blackbody at 5250 °C (black line), and the actual radiation reaching the surface of Earth at sea level (red). Reproduced with permission from Ref. 36 under the Creative Commons Attribution-Share Alike 3.0 Unported license (CC BY-SA 3.0).

In addition to the optoelectronic properties of the photocatalysts, also structural and morphological aspects play a role. The photocatalyst should have a high surface, providing a large active area and a large number of sites, where CO2 can adsorb. A high porosity is important for an optimized gas flow. Furthermore, the photocatalyst should exhibit high crystallinity for fast mobility of the charge carriers,37 provide high mechanical stability, be ecologically harmless, and it should be synthetically accessible in an easy and cheap way.1-2, 38

2.2 2.2.1

Photocatalytic hydrogen generation via gas phase water splitting Why is it important?

World’s population is growing steadily and the standard of living is rising. Therefore, the global energy consumption is predicted to increase drastically until 2050.1 In 2001, the world consumed 4.25∙1011 GJ energy, which is equivalent to the energy consumption rate of 13.5 TW.23 In 2016, the worldwide energy consumption was approximately 5.6∙1011 GJ (18.26 TW).39 For 2050, 25 TW are predicted.40 Non-renewable energy sources like fossil fuels will not be able to keep up with the ACS Paragon Plus 10 Environment

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global energy demand due to their decreasing availability. The world needs an energy source, which is reliable, abundant, and renewable, but additionally also clean and safe. Although hydrogen is not an energy source, it attracted great interest as an energy carrier. It is the most common element in the Universe. However, on Earth, although it is the third-most abundant atom in the Earth’s crust, it does not exist in a sufficient amount in the molecular form of H2. It is bound as part of hydrocarbons (oil and natural gas), biomass, and water. Therefore, H2 must first be generated from these compounds before it can be combusted with O2 in a strongly exothermic (change in Gibbs free energy ΔG0 = -237 kJ mol-1) reaction (2.10). Additionally, apart of the release of a big amount of energy (the energy content of H2 is very high with 120 MJ kg-1 compared to e.g. petroleum with 46 kJ kg-1 41), no harmful greenhouse gases are produced. The only product is H2O.1-3, 8, 23 ½ O2 + H2  H2O

(2.10)

96% of H2 is still produced from fossil fuels via reforming processes from primary sources like methanol, methane, heavy oil and coal.42 Accordingly, the process of combusting H2 is clean and green, but the production of this energy carrier from fossil fuels is not. Because we are looking for abundant and renewable energy sources and most of the H2 originates from dirty processes, it is not surprising that only 2% of the currently manufactured H2 is used to produce energy. The majority of the rest is utilized as raw material in the chemical and petrochemical industry, e.g. for the production of ammonia.42-43 H2 can be produced from fossil fuels via several different routes. The most common one (48%) is the steam reforming process of natural gas. During the reforming reaction (2.11), water steam reacts with methane to produce H2 and carbon monoxide. This process happens at high temperatures (625-925 °C) and high pressures (5-25 bar). The reaction is endothermic and needs energy input. The heated gas is passed over a nickel catalyst together with the steam. Afterwards, the water gas shift reaction (2.12) produces more H2 in a slightly exothermic reaction at lower temperatures (125-400 °C). CH4 + H2O  CO + 3 H2

(2.11)

CO + H2O  CO2 + H2

(2.12)

The carbon containing primary sources to produce H2 release a lot of CO2 and tens of ppm of CO upon combustion. High temperatures and pressures are needed. Consequently, it is important to ACS Paragon Plus 11 Environment


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find COx-free hydrogen production routes, which can be performed under economically more viable conditions. A promising solution to avoid the use of fossil fuels and to harness a renewable, abundant, clean, and safe energy source, is the already industrially implemented electrolysis of water into H2 and O2 (

Photocatalytic Gas Phase Reactions - Chemistry of Materials (ACS

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