Photochemical Reduction of CO2

CHAPTER

PHOTOCHEMICAL REDUCTION OF CO2

8

ABSTRACT Photo-catalytic reduction of carbon dioxide is an emerging area of research. Starting from the basis of photo-catalytic reduction, the studies reported on various semiconducting systems like oxides, sulfides, and phosphides are considered in this chapter. The mechanistic aspects of this reduction and the limitations of this process are pointed out while the ways of improving the yields are also indicated. Keywords: Photo-catalytic reduction of CO2; semiconductors; comparison of various catalytic systems; strategies for enhancement of reduction; prospects of photo-catalytic reduction of carbon dioxide CHAPTER OUTLINE 8.1 8.2 8.3 8.4 8.5

Introduction ............................................................................................................................. 373 Basics of CO2 Photo-Reduction Systems ..................................................................................... 376 Typical Mechanisms ................................................................................................................. 377 Limiting Steps and Strategies for Enhancement .......................................................................... 381 Comparison Between Different Systems ...................................................................................... 381 8.5.1 Biological Systems ................................................................................................ 381 8.5.2 Semiconductor Systems ......................................................................................... 384 8.6 Summary and Outlook ............................................................................................................... 416 References ...................................................................................................................................... 418 Further Reading ............................................................................................................................... 436

8.1 INTRODUCTION Global energy demand, raising environmental pollution levels, and global climate change are viewed as the biggest challenges this century that humanity faces. The challenges we must tackle require an efficient method to convert raw materials into useful fuels and chemicals, to make the flow of energy and matter bidirectional, and to maintain the balance between production and consumption. For the efficient and sustainable production of fuels and chemicals, the method to be adapted has to utilize energy and resources that are naturally abundant and easily renewable. The sun delivers more energy to the Earth in 1 h than we currently use from fossil fuels, nuclear power, Carbon Dioxide to Chemicals and Fuels. DOI: http://dx.doi.org/10.1016/B978-0-444-63996-7.00008-0 © 2018 Elsevier B.V. All rights reserved.

373

374

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

and all renewable energy sources combined in a year. It’s potential as a renewable energy source is vast. Harvesting solar energy and its subsequent storage in the form of fuels can address the current and future demand of energy supply. There are various ways in which people are harnessing solar energy, such as for electricity and heating, and through bio-fuels and food [1]. Currently, most fuels for transport, electricity generation and many raw materials for industry are produced from coal, oil, or natural gas. But an alternative route to produce liquid and gaseous fuels could use technologies that harness sunlight, as shown in Fig. 8.1 [2]. The rate of global energy consumption averaged 15
17 TW in 2010 [3] and is projected to increase to 25
27 TW by 2050. In 2015, 79% of the energy needs were met by fossil fuels, while renewable sources accounted for 17%. The share of the latter is set to increase, but it is estimated that only up to 20 TW can by extracted from wind, tides, biomass, and geothermal sources [4]. In contrast, solar radiation provides a seemingly infinite

Sunlight is used to split water into hydrogen and oxygen Sunlight is used to produce carbonbased fuels from carbon dioxide and water. A source of carbon dioxide could be a coal-burning power station fitted with carbon capture technology

Hydrogen is used as a transport fuel

Hydrogen and simple carbon-based fuels are used as raw materials (feedstocks) by many industries

New

tec

hno

log

ies

Solar fuels Hydrogen (H2)

• Fe r tili ser last i c s •P har ma •S ceu ynt tica h for e ls tran tic fu spo els rt •P

Exi

stin

Carbon-based fuels such as methane (CH4), carbon monoxide (CO) and methanol (CH3OH)

g te

chn

olo

gie

s

FIGURE 8.1 An overview of the production and use of solar fuels. From A.J. Heeger, Solar fuels and artificial photosynthesis. Available from www.rsc.org/solar-fuels, with permission.

8.1 INTRODUCTION

375

source of energy, as the solar-light energy reaching Earth in 1 h corresponds roughly to the annual global consumption of energy [3]. That is, converting around 10% of the solar energy on 0.3% of the land surface of the earth would suffice to exceed the projected energy needs by 2050. The heavy reliance on fossil fuels for the production of energy results in emissions of 30.4 Gt of carbon dioxide into the atmosphere [4]. All human activities currently generate about 37 Gt of CO2 emissions. This number will likely increase to 36
43 Gt by 2035, depending on the policies governing emissions and energy use, even if renewable sources increase their share [5,6]. Consider that the natural cycle of CO2 emission and uptake (fixation by terrestrial plants and microorganisms, and underground inorganication) involves about 90 Gt [7], and it is known that the anthropogenic emissions significantly disturb the balance. The atmospheric level of CO2 rose from δ 5 270 ppm in the preindustrial era to almost δ 5 395 ppm in 2015. The current CO2 concentration far exceeds its natural fluctuation (δ 5 180
300 ppm) over the past 800,000 years, and is probably the highest in about 15 million years based on boron isotope ratios in planktonic shells [8]. The greenhouse gas effect is considered to be a factor for anthropogenic climate change, and therefore the steady rise in the CO2 level is of great concern. Mitigating the effect of increasing CO2 emissions through its sequestration has been studied intensively. However, the process generally requires a substantial input of energy and careful prevention of leakage of CO2 back into the atmosphere [1]. In this context, the use of solar energy to chemically reduce CO2 to higher energy compounds such as methanol or methane offers a possibility to address this problem. Solar fuels, as these compounds are often referred to, can then be combusted or used in fuel cells, thus releasing the CO2. This closes the carbon cycle, since the external addition of CO2 from fossil fuels is no longer part of the cycle. The term “methanol economy” was suggested for such a cycle involving methanol obtained from CO2 [9,10]. An additional benefit of such an approach is that the existing energy production and transport infrastructure could be used with mild alteration. Another important aspect of the conversion of CO2 into liquid or gaseous fuels is the high-density storage of solar energy in the form of chemical bonds, mainly C
H. Renewable sources generally provide only an intermittent and unreliable supply of energy; therefore, the energy storage capacity of solar fuels is an attractive option. Apart from the applications as fuel, some of the products of CO2 conversion may also be utilized in chemical synthesis. Thus, instead of being an undesired waste, potentially damaging the planet, CO2 may become a valuable and cheap carbon feedstock that also replaces fossil fuels in this role. Approaches which combine the chemical conversion of CO2 (e.g., synthesis of urea) with a change in the reduction state of carbon have also been proposed. Several methods to reduce CO2 by using solar energy are being pursued. The form of converting solar energy into energy stored in chemical bonds is termed solar fuel. Solar energy can be captured and stored directly in the chemical bonds of a material, or “fuel,” and then used when needed. What is new is not the fuels themselves, but the way that we can use energy from the sun to produce them. The word “fuel” is used in a broad sense: it refers not only to fuel for transport and electricity generation, but also to feed stocks used in industry. This concept of making fuels using sunlight is the basis of photosynthesis, where sunlight is used to convert water and carbon dioxide into oxygen and sugars (or other materials) which can be thought of as fuels (Fig. 8.2). Attempts to generate fuel using sunlight as the energy source began with work aimed at generating hydrogen, typically from water. The analogy, based solely on products, between this process and natural photosynthesis by considering sugars as hydrogen carriers has led to the term “artificial photosynthesis.” More recently, with an interest in the expanding diversity of

376

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

1

Sunlight is absorbed by plants, algae, and certain bacteria

2

This solar energy drives a complex process in which water and carbon dioxide are converted to oxygen and carbohydrates or other ‘fuels’

Plants Sunlight Algae

Water + Carbon dioxide

Oxygen +

Fuel

Cyanobacteria (microscopic view)

FIGURE 8.2 Nature’s way of making solar fuel in photosynthesis. From A.J. Heeger, Solar fuels and artificial photosynthesis. Available from www.rsc.org/solar-fuels, with permission.

energy-storing products, the term “solar fuels” has been adopted. Although the area of solar fuels dealing with the reduction of aqueous carbon dioxide and oxidation of water to form organic products and O2 is a closer analogy, based on both reactants and products, as photosynthesis than water splitting, this term has already been used to describe the former processes. Thus, it has been suggested that the reduction of CO2 be referred to as “reverse combustion.” To enable a carbon neutral cycle in modern industrial society, CO2 conversion into fuels utilizing natural (sustainable) energy is one of the ideal methods. Carbon monoxide, methane, methanol, acetaldehyde, and ethane obtained from CO2 are attractive products because they can be easily integrated into the existing fuel and chemical technology. Since artificial photosynthesis is an integrated system consisting of a light harvesting part and a catalytic conversion part, it is essential to maximize the performance of each unit and to design a combined system with optimum efficiency, based on a thorough understanding of each component and the interactions between them. There has been much progress in its constituent parts such as light harvesting, charge transport, and catalytic conversion CO2, and its design as a whole. The status and challenges in the field of photo-conversion of CO2 will be discussed in detail.

8.2 BASICS OF CO2 PHOTO-REDUCTION SYSTEMS A homogeneous CO2 photo-reduction system consists of a molecular catalyst, light absorber, sacrificial electron donor, and/or electron relay. While dealing with these types of systems, there are

8.3 TYPICAL MECHANISMS

377

three terms that are commonly used to quantify the efficiency of catalytic CO2 reduction processes see Eqs. (8.1)
(8.3). Catalytic selectivity (CS) is defined as the molar ratio of the CO2 reduction products to that of hydrogen (Eq. (8.1)). ðCSÞ 5

½CO2 reduction products ½H2 

(8.1)

Any catalyst that reduces CO2 to carbon monoxide or formate is also thermodynamically capable of reducing proton to hydrogen. The CS value provides quantitative data on the efficiency of CO2 reduction relative to hydrogen formation at specific experimental conditions (i.e., pH, solvent). Synthetic modification of the transition-metal catalysts provides the opportunity to tune relative reactivity through inductive and steric effects. The main figure of merit is the photochemical quantum yield, a measure of the molar fraction of incident photons that result in CO2 reduction products, and defined as in Eq. (8.2).
Photochemical quantum yield ðΦÞ 5 moles products=absorbed photons 3 ðnumber of electrons needed for conversionÞ

(8.2)

In the absence of current doubling reactivity that can occur when sacrificial donors are oxidized, the maximum Φ for a single-electron reduction is 1, while for n electron reductions it is 1/n. Lastly, the turnover number (TON) is the number of reductions that occur per catalyst over the catalyst’s lifetime. This is often calculated as the molar ratio of CO2 reduction products to the catalyst initially present (Eq. (8.3)). Turnover number ðTNÞ 5

½CO2 reduction products ½catalyst

(8.3)

It has to be mentioned that TONs reported for photochemical reductions are not always measured by Eq. (8.3). A common alternate practice is to illuminate for a designated time interval rather than continuous illumination until catalysis ceases and, therefore, one needs to carefully examine the definition of TON used by different authors in the literature.

8.3 TYPICAL MECHANISMS In principle, the basic process of photo-catalytic water splitting and CO2 reduction can be summarized into three steps: (1) formation of photo-generated charge carriers (electron-hole pairs) by a band-gap excitation, (2) separation and transportation of charge carriers, and (3) reduction of water or carbon dioxide by the photo-generated electrons [11]. The possible reactions which can occur during the reduction of CO2 in an aqueous medium are shown in Fig. 8.3 [11
16]. When a semiconductor is excited by a photon with energy equivalent to (or higher than) the band gap, electrons in the conduction band and positive holes in the valence band are generated. Thus, the photogenerated electrons and holes may migrate to the surface of the semiconductor, and respectively reduce or oxidize the reactants adsorbed on the semiconductor surface. In this case, such reduction reactions in the photo-catalytic system can catalyze the production of H2 from water splitting and induce the reduction of CO2 to hydrocarbon fuel. However, for the production of solar fuel in such a photo-catalytic system, the positions of the valence band and conduction band of semiconductor

378

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

FIGURE 8.3 Schematic illustration of a basic mechanism of hydrogen production and CO2 reduction with a semiconductor photo-catalyst. From Q. Xiang, B. Cheng, J. Yu, Angew. Chem. Int. Ed. 54 (2015) 2
19 with permission.

photo-catalyst should both match the oxidation and reduction potentials of water splitting and CO2 reduction. The catalyst is also required to have its suitable electron reduction potential. In an ideal system, the CB potential (also known as band edge) of the catalyst should be well above (more negative than) the reduction potential for a certain product at a predetermined pH. Meanwhile, the holes should be able to oxidize water to O2 and produce protons (H1), i.e., the VB edge should be well below (more positive than) the water oxidation level for an efficient photolysis. In fact, only a few semiconductors satisfy the above-mentioned requirements for the splitting of water and/or the reduction of CO2 under solar-light irradiation [1]. In Fig. 8.4, the energy levels of various semiconductors and the corresponding redox potentials in the CO2 photo-reduction system are presented [12]. In an ideal system, the CB potential (the band edge) of the catalyst should be well above (more negative than) the reduction potential for a certain product at a predetermined pH. Meanwhile, the holes should be able to oxidize water to O2 and produce protons (H1), i.e., the VB edge should be well below (more positive than) the water oxidation level for an efficient photolysis. It is apparent from Fig. 8.3, that CO2 photo-reduction involves multiple steps (Eqs. (8.4) and (8.15)). Photo-catalyst 1 hν-e
1 h1


1

e 1 h -heatðrecombinationÞ

(8.4) (8.5)

Photo-oxidation reactions: Water oxidation/decomposition, E0redox/V vs NHE H2 O 1 2h1 -1/2 O2 1 2H1 5 0:82

(8.6)

O2 1 2H1 1 2e2 -H2 O2

(8.7)

Hydrogen peroxide formation Photoredution reactions: Hydrogen formation, E0redox/V vs NHE

8.3 TYPICAL MECHANISMS

–2.0

379

SiC GaP CB

CdS

–1.0 Energy (V)

CB

0.0

HCOOH/H2CO3

CB

ZnO TiO 2

3.0 eV

2.3 eV

CB

2.4 eV

1.0 VB

WO3 CB

3.2 eV

3.0

2.8

eV

eV

VB

2.0

HCHO/H2CO3 H2/H2O CH3OH/H2CO3 CH4/CO2

SnO2 CB

O2/H2O

eV

VB

pH 5.0

3.0

VB

VB

VB

Redox systems

Semiconductors VB

FIGURE 8.4 Schematic diagram of the energy correlation between semiconductor catalysts and redox couples in water at pH 5. From T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aquesous suspensions of semiconductor powders. Nature 277 (1979) 637
638 with permission; Copyright 1979 Nature Publishing Group.

2H1 1 2e2 -H2 5 2 0:41

(8.8)

CO2 1 e2 -CO2 2 5 2 1:90

(8.9)

CO2 1 2H1 1 2e2 -HCO2 H 5 2 0:61

(8.10)

CO2 1 2H1 1 2e2 -CO 1 H2 O 5 2 0:53

(8.11)

CO2 1 4H1 1 4e2 -HCHO 1 H2 O 5 2 0:48

(8.12)

CO2 1 6H1 1 6e2 -CH3 OH 1 H2 O 5 2 0:38

(8.13)

CO2 1 8H1 1 8e2 -CH4 1 2H2 O 5 2 0:24

(8.14)

CO2 1 12H1 1 12e2 -C2 H5 OH 1 3H2 O 5 2 0:16

(8.15)

CO2 radical formation Formic acid formation Carbon monoxide formation Formaldehye formation Methanol formation Methane formation Ethanol formation

380

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

As shown above, the initial step in the photo-catalytic reduction of CO2 is the generation of e2-h1 pairs (Eq. (8.4)). After photo-excitation, the e2-h1 pairs should be separated spatially, and be transferred on to redox active species across the interface and minimize electron-hole recombination. The lifetime of the e2-h1 pairs is only a few nanoseconds, but it is adequate for promoting redox reactions. However, the time scale of e2-h1 recombination (Eq. (8.5)) is two or three orders of magnitude faster than other electron transfer processes [16]. It has thus been considered as one of the major limiting steps in photo-catalysis. Therefore, any process which inhibits e2-h1 recombination would greatly increase the efficiency and improve the rates of CO2 photo-reduction. Ideally, after photo-excitation, water (or other hole scavengers in nonaqueous systems) is oxidized by holes to form oxygen and protons (H1) (Eq. (8.6)) which are essential for photo-reduction. In addition to the desired reactions, there are also several competing reactions, such as the formation of H2 and H2O2, which consume H1 and e2, respectively (Eqs. (8.7) and (8.8)). Research efforts to minimize the competing reactions are still going on. For example, some researchers have attempted to replace water with other reductants, such as methanol and ethanol [17]. This provides a high reaction yield and high selectivity to desired products by changing the mechanistic route. However, water (in practice) remains as the primary reactant since it is readily available and inexpensive. For the photo-reduction reaction pathways, some researchers proposed that CO2 activation involves the formation of a negatively charged CO2•2 species, which are 23 electron radical anions [18] and can be detected via infrared (IR) spectroscopy [19] and electron paramagnetic resonance spectroscopy [20]. However, the single-electron transfer to CO2 occurs at potentials as negative as
1.9 V versus NHE (Eq. (8.9)), which is highly endothermic because of the negative adiabatic electron affinity of CO2 [16,21,22]. The activation of CO2 molecules in both gas and liquid phases and the formation of CO2•2 still remain unresolved issues. Many researchers now agree that this pathway is a multielectron transfer (MET) process rather than a single-electron transfer process; for the later, the electrochemical potential of 21.9 V precludes the likelihood of a single electron transport mechanism [18,21]. Various products could be formed based on MET photo-reduction reactions (Eqs. (8.10)
(8.15)). Carbon monoxide (CO) is the most common product, since the reaction needs only two protons and two electrons. Formic acid (HCOOH) is also a common product found in the CO2 photo-reduction system, which consumes the same amount of protons and electrons, but requires slightly higher reduction potential than that for CO formation. The other products are generally difficult to form in gas
solid systems, since they require more electrons and protons even though the reactions are thermodynamically favorable. It should be noted that the MET reactions mentioned are just the simplest examples. Some products may be formed through multiple reactions. For example, methane (CH4) could be formed through Eq. (8.14) or through the following reactions (Eqs. (8.16) and (8.17)) as well as [23,24]. CO 1 6H1 1 6e
-CH4 1 H2 O 1




CH3 OH 1 2H 1 2e -CH4 1 H2 O

(8.16) (8.17)

In addition to these desired redox reactions, some backward reactions are also possible, making the CO2 photo-reduction system even more complex. For example, the strong oxidation power of the photo-excited holes, protons, OH radicals, or O2 could oxidize the intermediates and products to form CO2.

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

381

8.4 LIMITING STEPS AND STRATEGIES FOR ENHANCEMENT The kinetics of CO2 photo-reduction are dependent on many factors, such as reaction temperature, reactant adsorption, product desorption, CO2 activation, incident light intensity, fraction of the incident light absorbed by the photo-catalyst, the specific area of the photo-catalyst absorbing the light, and surface as well as crystalline properties of the photo-catalysts. For example, CO2 adsorption on the photo-catalyst surface is always considered as a very critical step for CO2 photo-reduction, which could be a pseudo-first-order reaction since the CO2 photo-reduction rate was found to increase with initial CO2 concentration [25]. The activation of CO2 or the electron transfer from the activated photo-catalyst to CO2 is also a major limiting step, where the mechanisms are rather complex and still debatable [16]. In addition, the e2-h1 recombination is also widely considered as a major limiting step, which draws attention. Slowing down the e2-h1 recombination rates has become a key area of research to develop efficient photo-catalyst systems. To address the above issues, in particular to slow down the e2-h1 recombination rates, several strategies have been developed to enhance the photo-reduction efficiency, which are presented below. The first is choosing semiconductors with appropriate band gap energies/edges. The proper structure of the photocatalyst can improve the product selectivity and yield and increase the rate of reaction. Therefore, photo-catalyst preparation is one of the most important steps that can enhance the capacity of a photo-reduction system. Wide band gap semiconductors are the most suitable photo-catalysts for CO2 photo-reduction, because they provide sufficient negative and positive redox potentials in conduction bands and valence bands, respectively. The disadvantage of using wide band gap semiconductors is the requirement for high energy input. Semiconductors with narrow band gaps, like cadmium sulfide (CdS, 2.4 eV) work in the visible range, but it is not sufficiently positive to act as an acceptor. This causes the photo-catalyst to decompose with the hole formation. In Table 8.1, several semiconductors with detailed information on their band gap and band edges are listed. The second common method is surface modifications of the photo-catalysts. For example, one way is to coat metal particles/islands on the semiconductor surface (Fig. 8.5, left). These metal islands serve as electron sinks to separate e2-h1 pairs and increase their lifetime [33]. Formation of composites of two or more semiconductors is also an interesting pathway. For instance, in a coupled CdS-TiO2 system, as shown in Fig. 8.5(right), due to the different band edges of two components, electrons generated from the CdS conduction band can transfer to the TiO2 conduction band, which increases the charge separation and efficiency of the photo-catalytic process [33]. Doping with nonmetals, such as nitrogen, is also a promising way to extend the absorption range of the wide band semiconductors from UV to visible range [34]. Lately, dye-sensitized and/or enzyme-modified semiconductor systems have also been developed to mimic nature and enhance the photo-reduction efficiency. Details of these modifications are provided in the following sections.

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS 8.5.1 BIOLOGICAL SYSTEMS Several algae species have been extensively investigated for their use in production of bio-fuels. This motivation stems from two premises: firstly, algae require CO2 to grow, thus removing

382

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

Table 8.1 List of Semiconductors With Detailed Band Gap Information [26
32] CB Edge (eV) Range

λ (nm)

Semiconductor

Eg (eV)

IR

1125 1032 1032 620 620 560 517 517 495
442 477 458 428 415 410 384 387 364 354 345 335 288 175 159

Si WSe2 CuO Cu2O CdSe QDs α-Fe2O3 BiVO4 CdS WO3 InTaO4 V2O5 In2O3 SiC TiO2 TiO2 ZnO SrTiO3 SnO2 MnO ZnS NiO Al2O3 MgO

1.1 1.4 1.35 1.9 200 2.2 2.4 2.4 2.5
2.8 2.6 2.7 2.9 3.0 3.02 3.23 3.2 3.4 3.5 3.6 3.7 4.3 7.1 7.3

VIS

UV

VB Edge (eV) at pH 7

2 0.6 2 0.25 4.07 2 1.30 1.2 0.13 2 0.30 2 0.6 0 2 0.75
4.7 (pH 6.54)
3.88 (pH 8.64) 2 1.5 2 0.52 2 0.29 2 0.31
3.24 (pH 8.6)
4.5 (pH 4.3)
3.49 (pH 8.61)
3.46 (pH 1.7) 2 0.5 2 3.6 2 3.0

0.5 1.15 5.42 0.60 3.2 2.33 2.1 1.8 2.5
2.8 1.85 2 2.0 2 0.98 1.5 2.5 2.94 2.91 0.16 2 1.0 0.11 0.24 4.8 3.5 4.3

CO2 from the atmosphere; secondly, while the algae grow, many species produce oils and other refinable byproducts that can be used as potential bio-fuel sources [35,36]. Therefore, by researching the possible ways to maximize algae growth using anthropomorphic CO2 sources, effective enhancement of a natural biological system that uses solar energy to convert CO2 into potential fuel sources can be achieved. Fig. 8.6 shows how photosynthesis occurs in algae. In algae, the fixation of CO2 occurs through two separate reaction steps. In the light-dependent set of reactions, photo-excited electrons are used to reduce the coenzyme NADP1 to NADPH as well as to create the high energy molecule ATP. In the second set of reactions, which are lightindependent reactions, these reduced molecules are used to convert CO2 to organic compounds that can then be used by algae as a source of energy. The oils produced by algae which are used for bio-fuel production contain the carbon that was fixed in the Calvin cycle. It can be seen that this biological system is similar in function to synthetic systems, with the added step of electron transfer to ADP and NADP1 to form high energy molecules for the occurrence of subsequent reduction in the Calvin cycle [37]. In a synthetic system, light energy is used to excite electrons in the

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

hν

383

Metal – – hν

Eg = 2.5 eV Eg = 3.2 eV

Electron trap

Schottky barrier Semiconductor

CdS

TiO2

FIGURE 8.5 Schematic diagram of metal-doped semiconductors for electron-hole separation (left) and composite semiconductor systems (right). From A.L. Linsebigler, G.Q. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces—principles, mechanisms, and selected results. Chem. Rev. 95 (1995) 735
758 with permission; Copyright 1995 American Chemical Society.

photo-catalyst which are then utilized to reduce CO2. Thus, light energy is stored in the bonds of the reduced CO2 products. It is apparent that the steps involved in these synthetic systems resemble that which are accomplished with algae. Tables 8.2 and 8.3 summarize different species of algae and synthetic systems with their CO2 conversion rates, respectively. In order to make a comparison between these systems it is important to use one consistent unit of measurement for CO2 conversion, such as μmol CO2 converted per gram catalyst per hour (μmol/g h). Among the several representations used in the literature to express the CO2 conversion efficiency for synthetic systems, this unit appears to the best one to demonstrate the performance of the system, as it typically reflects the potential of conversion in terms of amount of the catalyst used and the experimental time. For algae systems, a commonly reported unit in the literature is μg of CO2 consumed per liter of culture fluid per day (μg/L d). For instance, Ho et al. [36] reported the CO2 consumption rate of the algae species in Table 8.2 in the unit mg/L d. This unit has been converted to μmol/g h for comparison with synthetic systems. This unit conversion was accomplished by assuming 1 g of dry algae in the culture fluid, which is a typical range of dry algae reported for 1 L of culture fluid. As can be noted in the two tables, in general, synthetic systems conversion rates fall below the algal systems. However, it has to be emphasized that to produce well performing algae systems, many resources must be consumed. For instance, the tanks for the algae must be cleaned frequently in order to prevent water and nutrient build-up that blocks light penetration. As mentioned earlier, for just one gram of algae, nearly one liter of water is required. Therefore, a lot of space and water resources are required to get the performance outlined in the table. The CO2 uptake occurs in the algal biomass that requires cumbersome separation steps for the extraction of the oil product. Furthermore, algae systems are, in general, less robust in stability compared to synthetic systems. Algae are

384

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

Sunlight

Light-independent reactions

Light-independent reactions

CO2

O2 NADP+

Thylakoid

Cytosol

ADP+Pi Triacylglycerol Photosystem II Electron transport chain

3-Phosphoglycerate RuBP Calvin Cycle

Photosystem I

Stroma

Glycerol-3phosphate

Fatty acid

ATP NADPH H2O

G3P

NADPH,H+

Starch Acetyl CoA

Acetyl CoA

Mitochondria

FIGURE 8.6 Schematics of photosynthesis—CO2 fixation and carbon accumulation in microalgae cells. From X.H. Zeng, M.K. Danquah, X.D. Chen, Y.H. Lu, Microalgae bioengineering: from CO2 fixation to biofuel production. Renew. Sustain. Energy Rev. 15 (2011) 3252
3260 with permission; Copyright 2011 Elsevier.

living organisms that require a narrow range of living conditions, thus small changes in their environment can impact biomass growth as well as the amount of byproducts produced, for which a lot of time and energy is required in process control to guarantee optimal performance. Very few, if any, synthesis have demonstrated uptake of CO2 from concentrated systems exceeding a few percent. Synthetic systems however, do not have these growth-associated problems of a biological system. While it is true that current synthetic systems have decreased activity after a certain period resulting in decreased conversion rates, there is a lot of room for improvement and modification of these systems; whereas most of the issues associated with algae systems are due to algae being a living organism, complicating the improvement. Thus it is important to improve the conversion rates of synthetic systems and also the lifetime of these systems for practical and scalable use in the future.

8.5.2 SEMICONDUCTOR SYSTEMS The efficient utilization of solar energy requires the use of materials with a small band gap so that a large part of the visible light spectrum can be absorbed. Unfortunately, many promising semiconductors, especially oxides, have a wide band gap, which results in an onset of absorption below 400 nm. A red-shift of this onset to the visible range is achieved by either sensitizing the semiconductor or altering its band gap. In the former technique, the absorption region of the photo-catalyst is enhanced by the introduction of a secondary material which absorbs visible light. Typically,

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

385

Table 8.2 Summary of Algae Systems for CO2 Fixation and Conversion [38
58] Algae Species

Conversion (µmol/g h)

References

Nannochloris sp. Nannochloropsis sp. Phaeodactylum tricornutum Chlorella sp. Chlorococcum littorale Synechocystis Botryococcus braunii Chlorella sp. Chlorella vulgaris Chlorella emersonii Scenedesmus sp. Chlorella vulgaris Microcystis aeruginosa Microcystis ichthyoblabe Chlorella vulgaris Euglena gracilis Chlorella kessleri Scenedesmus obliquus Spirulina sp. Scenedesmus obliquus Spirulina sp. Scenedesmus obliquus Chlorella kessleri Chlorella vulgaris Chlorella sp. Chlorella sp. Chlorella sp. Chlorella sp. Aphanothece microscopica Nageli Aphanothece microscopica Nageli Anabaena sp. Scenedesmus sp. Scenedesmus obliquus

57
569 48
481 27
267 125
1246 85
852 142
1420 95
947 167
1673 7
71 7
73 44
436 58
580 49
493 46
464 591
5909 36
362 16
155 15
152 36
358 25
259 37
373 19
188 12
116 335
3352 81
812 68
680 109
1086 66
663 136
1364 515
5147 137
1373 39
387 52
521

[38] [38] [38] [39] [40] [41] [41] [42] [43] [43] [44] [44] [44] [44] [45] [46] [47] [47] [48] [48] [49] [49] [49] [50] [51] [52] [52] [53] [54] [55] [56] [57] [58]

nanoparticulate narrow-band semiconductors (quantum dots, QDs) or visible-light-active molecules (dyes) are used as sensitizing agents (Fig. 8.7). In this direction, for instance, coupled semiconductors such as CdS and TiO2, or CdSe and ZnO, have been exhibiting enhancement in the efficiency through the extension of the absorption range and improvement in the charge separation. When the band gap edges match, an electron is

386

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

Table 8.3 Literature Summary of CO2 Photo-Reduction by Semiconductors, Metal
Organic, and Hybrid Systems [55,59
89] Catalysts

Product(s)

Conversion (µmol/g h)

Reference

CH4 CH3OH CH3OH CO CO CO CO/CH4 CH4 CH4/CO/H2 CO CH4 CH3OH CH4, CH3OH CO CO CH4/CH3OH CH4/CH3OH CH4 CH4/C2H6 CH4/HCO2H HCOOH/CH4 CH3OH CH3OH CH4/CH3OH HCOOH CO/CH4 CH3OH CH4 Formate, CO CH4

1361 224.6 159.5 45 20 1.71 10.0 4.86 8.4 2.1 2.4 4.12 11 4 5 10.5 93.8 0.3 3.3 73.33 33.5 20 12.5 9.1 1.8 5.2 13 0.43 96.1 13.3

[59] [60] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [55] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86]

CO CO Malic acid HCOOH HCOOH

250 60 1107 0.01 110,000

[87] [88] [89] [48] [90]

Semiconductor Systems Pt NP/TiO2 film TNTs
Bi2S3 TNTs
CdS Cu/TiO2/SiO2 (wet method) Cu/TiO2/SiO2 (aerosol method) PbS QD/Cu
TiO2 TiO2/GO layered sheets Pt loaded c-NaNbO3 CdS-MMT Anatase (75%) Brookite (25%) TiO2-RMA Cu/Fe
TiO2
SiO2 Ti
Silica film MgO ZrO2 Ti
MCM-41 and 48 TiO2 (Degussa P-25) Cu
TiO2 dye sensitized Nafion layer on Pd
TiO2 MWCNT
TiO2 (anatase, rutile) CoPc
TiO2 Cu
TiO2 Cu
TiO2 Ti
Silica porous thin film TiO2 powder Rh
TiO2 TiO2/zeolite TiO2 (P-25) CdS coated by thiol TiO2 (P-25) TiO2/zeolite Bio-Hybrid Systems TiO2/CODH I/Ru dye CdS QD/CODH I TiO2/malic enzyme TiO2/Formate dehydrogenase Enzyme/graphene complex

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

387

Table 8.3 Literature Summary of CO2 Photo-Reduction by Semiconductors, Metal
Organic, and Hybrid Systems [55,59
89] Continued Catalysts

Product(s)

Conversion (µmol/g h)

Reference

CO CO CO CO/CH4 CO CO H2/CO CO/H2

30 TONa 90 TON 30 TON 23.6 TON 3 TON 10.9 TON 8.3 TON 15.6 TON

[91] [92] [93] [94] [95] [96] [97] [98]

Metal
Organic Complex Systems [Re(bpy)(CO)3Cl] Ru(II)/Re(I) dimers Rhenium complex Metal
organic
polyoxometalate hybrid complex Ru-substituted polyoxometalate [Re(I)(CO)3(dcbpy)Cl](H2L4) derivatized MOF4 Co(bpy)321 with Ru(bpy)321 [fac-Re(bpy)-(CO)3P(OiPr)3]1

a (TON 5 turnover number) results for organic complex systems are reported in turnover numbers because of lack of information for conversion to the metric μmol/g h. The turnover number is the maximum number of substrate molecules that the molecular catalyst can convert to product, per site available on the catalyst.

preferentially transferred to the semiconductor with the lower CB edge (TiO2), while a hole moves in the opposite direction to a higher VB level (CdS), thereby contributing to the decrease in the recombination rate. Sensitization with dyes is a commonly used technique in solar cells, where a large number of inorganic (especially ruthenium-based) or organic dyes have been investigated [99]. The LUMO of the dye has to be higher in energy than the conduction band edge of the semiconductor for the electron transfer to occur, but its rate also depends on a number of parameters such as the orientation and distance of the light absorbing unit with respect to the surface or size and flexibility of the anchoring unit [100,101]. Similar to sensitization with QDs, the added benefit is the improved charge separation, because the opposite charge is associated with the oxidized dye. The weakly reductive conduction band of many semiconductors, for example, TiO2, does not offer much space to minimize the band gap by lowering its energy. In contrast, there is significant difference between the energy levels in the valence band (often constituted by 2p orbitals of the O atoms) and the water oxidation potential [1.23 V at pH 0]. Therefore, apart from sensitization, two main strategies have been employed to shift the onset of absorption toward longer wavelengths: Doping to create intraband states above the valence band (Fig. 8.8B) and raising the valence band by forming a solid solution of two or more semiconductors (Fig. 8.8C). Another possibility is to create intraband impurity states, for example, oxygen vacancy states, below the Fermi level (Fig. 8.8D) [102] or additional states arising from the aggregation of semiconductor nanoparticles [103]. Examples of each strategy are given later in this chapter. At first in the following section, the photo-catalytic properties of TiO2 are discussed in detail followed by an extensive overview of experimental approaches for the photo-reduction of CO2 on TiO2, other oxides, and various sulfides and phosphides. Apart from the strategies to improve the absorption properties presented above, special emphasis is placed on the effects of employing a cocatalyst as well as the role of the nanostructure and crystallographic properties of the photo-catalyst.

388

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

FIGURE 8.7 Sensitization of a wide band gap semiconductor with (A) a narrow band gap with aligned conduction band minima or (B) a dye that absorbs in the visible region.

8.5.2.1 TiO2-based systems 8.5.2.1.1 Photo-catalytic properties of TiO2 The prerequisite for photo-catalysts to have a sufficiently wide band gap is met by titanium dioxide, which has a band gap exceeding 3.0 eV. TiO2 is a most common semiconductor in photo-catalysis, and is employed in pollutant degradation, dye bleaching, and water purification. Early studies by Fujishima and Honda [104] have demonstrated its applicability to the photo-catalytic splitting of water and reduction of CO2 [12] upon irradiation with UV light. Apart from its applications as a photo-anode in inorganic and hybrid solar cells [99,105
107] as well as in photo- and electrochromic devices [108], TiO2 has attracted enormous interest owing to its unique crystal lattice, surface,

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

389

FIGURE 8.8 (A) Semiconductor with a wide band gap and approaches to decrease its width, (B) creation of dopant states above the VB level, (C) hybridization with other orbitals to raise the VB, and (D) creation of impurity states, for example, from oxygen vacancies, below the fermi level.

charge transport, and other interesting properties [16,109
111]. The dominance of TiO2 as a semiconductor of choice can be attributed to a variety of reasons. It is cheap, nontoxic, made up of earth-abundant elements, and has resistance to photo-corrosion [110]. Its band structure (Fig. 8.4) allows for simultaneous occurrence of water reduction and oxidation [112]. The CB edge is located at around 20.16 V/NHE at pH 0 [113,114], so that the reduction of protons (ENHE(H1/H2) 5 0.0 V at pH 0) is feasible. The low position of the valence band means that the oxidation of water (ENHE(O2/H2O) 5 1.23 V at pH 0) is also thermodynamically favorable. Herein lays the abovementioned resistance to photo-corrosion, which is induced by photo-generated charge carriers initiating redox reactions which eventually lead to a disintegration of the material [112]. In an aqueous

390

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

environment, however, TiO2 is not prone to this damaging process because the oxidation of water is thermodynamically more favorable than the formation of oxygen molecules from oxide anions. Since this stability extends over a wide range of pH values, TiO2 is a favorable semiconductor material for use in suspension-based photo-catalytic systems. TiO2 exists mainly in four different crystallographic phases (polymorphs): Rutile (tetragonal), anatase (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic) [110]. For photo-catalytic applications, the first two phases have been investigated extensively. The structures of these two polymorphs have chains of elongated TiO6 octahedra [33,111] with each Ti41 ion surrounded by six O22 ions. In the rutile configuration, this octahedron exhibits a slight orthorhombic distortion, while the distortion of the anatase octahedron is much more significant, thus exhibiting a lower symmetry. This has been reflected by different distances between the titanium and oxygen atoms in the two phases. While the Ti
Ti distances are larger in anatase than in rutile, the Ti
O distances are smaller. The anatase unit cell can be regarded as octahedra connected by their vertices. In contrast, the unit cell of rutile is built up of octahedral connected on their edges, and thereby each octahedron is in contact with ten neighboring octahedra. Generally, TiO2 is an n-type semiconductor, a property which is attributed to a small number of oxygen vacancies which are then compensated by the presence of Ti31 centers. The valence band is formed by the overlap of 2p orbitals of the oxygen atoms. The lower part of the conduction band is formed by 3d states of titanium Ti41 ions with t2g symmetry. The differences in the crystal structures are responsible for the different band gaps of anatase (3.2 eV) and rutile (3.0 eV). The minimum energy of photons required to initiate an interband transition lies in the UV range, with 389 nm for anatase and 413 nm for rutile. It has been shown that the difference between anatase and rutile is mainly due to the position of the CB edge, which lays 0.2 eV higher for anatase [113]. The crystal symmetry of TiO2 in its natural configuration allows only indirect interband transitions [110]. Therefore, photo-exciting an electron from the valence band into the conduction band requires the simultaneous presence of a photon to provide the energy and a phonon to provide a change in momentum. This decreases the photonic efficiency, as the rate of absorption events that produce available charge carriers is limited. Further, the rate of recombination of the photogenerated charge-carrier pairs can reach up to 90% within a period of 10 ns after their generation [110,115], thereby significantly limiting the number of charge carriers available for surface reactions. According to thermodynamic calculations based on calorimetric data, rutile is the most stable polymorph at all temperatures and pressures up to 6 kbar. The Gibbs free energy of formation of anatase is 15 kJ/mol lower than that of rutile, but it can be considered stable at ambient conditions. Annealing anatase at temperatures above 400 C promotes the transformation to rutile. It has been demonstrated that the surface crystallinity of TiO2 plays a key role in determining the efficiency of the photo-catalyst, with anatase found to possess notable adsorption affinity for organic compounds. Anatase has received much attention by researchers because it also exhibits recombination rates nearly ten times lower than that observed with rutile. Grela and Colussi [116] observed nearly 10 times increase in the quantum yield for the photo-degradation of nitrophenol with highly crystalline TiO2 compared to poorly crystalline TiO2. This may be attributed to the carrier lifetime enhancement with improved crystallinity. High crystallinity is also assumed to lead to a greater efficiency in the electronic coupling between orbitals of adsorbed dyes and CB states in TiO2. For example, it has been shown that the observed rates of electron transfer between anthracene carboxylic acid dyes and TiO2 nanoparticles are faster in crystalline than in amorphous TiO2 [117]. The correlation of higher crystallinity with improved photo-catalytic performance is attributed to fewer

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

391

dangling bonds and lattice distortions, which can constitute both trapping and recombination sites [117]. There has been extensive effort to determine which of the two phases of TiO2 is more beneficial for photo-catalytic applications, and has been reviewed by Henderson [117]. It has been found that mixed-phase TiO2 shows particularly interesting photo-catalytic properties, such as enhanced photo-catalytic efficiencies, that are not found in either of the phases separately [118
120]. The standard material in the field of photo-catalysis is P-25 from Evonik (formerly Degussa), a TiO2 powder, which is a composite made of about 80% anatase and 20% rutile. The particle size usually ranges from 20 to 80 nm in diameter (with an average of about 25 nm in diameter). Its effective surface area (BET, Brunauer
Emmett
Teller) is considered to be 49
56 m2/g. Numerous studies have been carried out on P-25 powder [121
127] and it serves as a benchmark system in experiments focused on new materials and systems. The high activity of P-25 is attributed to complementary effects of the two constituting TiO2 phases [128,129]. There is still much debate about the exact nature of those effects, especially because it is challenging to clearly determine the distribution of rutile and anatase in the mixed-phase powder. There is, however, a general agreement that the interfaces between the phases could play a key role [130,131]. The first explanation is based on the different positions of their electronic bands (the CB of anatase is about 0.2 V higher), which leads to the formation of a heterojunction. In this configuration, it is energetically preferential for photo-generated electrons to transfer from anatase to rutile, while the concomitant holes stay in the anatase VB, thereby improving the charge-carrier separation (Fig. 8.9). This model, however, is challenged by various research groups, stating that the data used to support this explanation is too ambiguous [123]. An alternative explanation is based on the presence of trapping sites in anatase which are up to 0.8 eV lower in energy than the CB edge [132]. This would suggest that electrons may actually transfer in the opposite direction than previously proposed, with the anatase trap states acting as a sink for electrons generated in the rutile phase. These trap states are considered to be more stable in anatase than in rutile, which results in a stabilization of the charge separation, thus allowing both charge carriers to contribute to the photo-catalytic reactions on the surface. Finally, the interface region between anatase and rutile itself may act as a site for charge-carrier trapping, thus facilitating charge separation [133]. In terms of the generation of solar fuel, the wide band gap of TiO2 has a considerable drawback, because it limits the absorption of light to the UV range. Only about 10% of the power of the incident solar photon flux can thus be absorbed [33]. The two types of modifications mentioned earlier are commonly used to overcome this limitation and shift the onset of the absorption from the UV to the visible region: sensitization and doping [110]. Examples of both approaches are given below and the results are summarized in Table 8.4.

8.5.2.1.2 Reduction of CO2 on TiO2 The TiO2 band structure has direct implication on the photo-catalytic reduction of CO2. Most importantly, the single-electron reduction potential of CO2 to the radical anion CO2•2 at 21.9 V/NHE is much more negative than the conduction band of TiO2, and hence a direct electron transfer from TiO2 to CO2 is thermodynamically hampered. The reduction process may proceed only through adsorbed CO2 species on TiO2 [168
170] or on selected metal catalysts [171,172], and even then the over potential is very low [173]. Since, proton reduction is more favorable and competes with CO2 for electrons, to avoid a decrease in the efficiency, the catalyst would have to suppress the H2 generation process. It is known that the multielectron reduction of CO2 to formic acid, methanol, or methane is less negative than the conduction band of TiO2, although the

392

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

FIGURE 8.9 Two possible mechanisms responsible for the high performance of mixed-phase TiO2: (A) the offset of the two conduction band edges of 0.2 eV is the cause for the improved separation of the charge carriers through electron transfer from anatase to rutile; and (b) trap states located 0.8 eV below the CB edge of anatase are populated by electrons from rutile, thus facilitating charge separation.

mechanisms of such reactions remain to be elucidated and proven [174]. Furthermore, the hole in the valence band has a highly positive potential of about 12.94 V/NHE, thereby creating a strong oxidizing environment through surface trapped holes and OH• radicals (E0(OH•/H2O) 5 2.27 V/NHE). This is why most photo-catalytic applications of TiO2 are oxidation reactions of various organic substances [175,176]. There is a possibility that the products of CO2 reduction, such as formates or methanol, could be easily oxidized back to CO2, especially in the presence of a cocatalyst [177,178]. Sacrificial hole scavengers are employed to prevent the reoxidation. Despite these constraints, TiO2 remains as an attractive semiconductor for photo-catalytically converting CO2 into valuable hydrocarbons at ambient temperatures and pressures [179], even with a simple system composed of a photo-catalyst (TiO2), water, and a UV light source. A simple and straightforward way to implement such a system is to disperse TiO2 nanoparticles in a CO2-saturated aqueous solution [132]. Both liquid- (formic acid, formaldehyde, and methanol) and gas-phase (methane) products were detected under ambient conditions. Various approaches

Table 8.4 TiO2-Based Photo-Catalytic Systems for CO2 Reduction Category TiO2 (P-25) in suspension TiO2 anatase particles (crystallite diameter 4.5
29 nm) TiO2 powder (P-25) in 2-propanol TiO2 (anatase) powders in liquid CO2 TiO2 powder in supercritical fluid CO2 TiO2 powder (P-25) coated glass pellets TiO2 (anatase, 325 mesh) in aqueous suspension with propan-2-ol Sputtered TiO2 films TiO2 coated onto sapphire disks Immobilized TiO2 particles Immobilized TiO2 particles Sensitized TiO2 particles in suspension Sensitized TiO2 particles in suspension Sensitized TiO2 particles in suspension Sensitized TiO2 particles in suspension Sensitized TiO2 particles in suspension Sensitized TiO2 particles in suspension Sensitized TiO2/SBA- 15 composite in suspension Sensitized TiO2 particles in suspension

Best System

Cocatalyst

Major Product

Rmaxa

Minor Products

Traces

Ref.

CH4 CH3OH


H2, CO

[73] [134]

TiO2 (P-25) particles 14 nm anatase particles P-25 particles Anatase particles









CH3OH CH4

3.4 0.4









HCOOH HCOOH

1.2 1.2









[84] [135]

Anatase particles







CH4

1.8







[81]

P-25 particles







CH4

4.11

CO, C2H6




[136]

Anatase particles







CO

15.3c

CH4, H2




[137]

Mixed-phase TiO2 Anatase particles JRC-TiO-4 particles JRC-TiO-4 particles Ag/TiO2 particles





Ag





7.0 wt%

CH4 CH4 CH4 CH4 CH4

0.032 7 93 0.020 9

CH3OH C2H6 C2H6, C2H4 C2H4, C2H6, O2


[138] [139] [140] [141] [142]

Ag/TiO2 particles

Ag

5.2 wt%

CH4 1 CH3OH

10.5


H2

H2, CH3OH,CO





[143]

Cu/TiO2 particles

Cu

2.0 wt%

CH3OH

19.8







[79]

Cu/TiO2 particles

Cu

2.0 wt%

CH3OH

17







[144]

Cu/TiO2 particles

Cu

2.0 wt%

CH3OH

20







[78]

Cu/TiO2 (P-25) particles (Cu/TiO2) 45 wt %/SBA-15 Cu
Ce/TiO2 particles

Cu

3.0 wt%

CH3OH

443







[145]

Cu

2.0 wt%

CH3OH

627







[146]

Cu/Ce

2.0 wt%

CH3OH

11.3







[147]

b

(Continued)

Table 8.4 TiO2-Based Photo-Catalytic Systems for CO2 Reduction Continued Category Sensitized TiO2 suspension Sensitized TiO2 suspension Sensitized TiO2 zeolite Sensitized TiO2 suspension Sensitized TiO2 suspension Sensitized TiO2 suspension

Best System

Cocatalyst

particles in

CuO/TiO2 particles

CuO

particles in

Enzyme/TiO2 (P-25)

Enzyme

supported on

Fe
TiO2/HZSM-5

Fe

particles in

1.0 wt%

10.0 wt%

Pd

Major Product

Rmaxa

Minor Products

Traces

Ref.

HCOOCH3

1602







[148]

CO

300







[149]

CH4N2O

18d







[150]

HCOO2

2.22e







[151]

particles in

Pd/TiO2 particles

Pd

2.0 wt%

CH4

0.6

CO

C2H6

[152]

particles in

Pd/TiO2 particles

Pd

2.0 wt%

CH4

0.3

C2H6

[153]

TiO2 anchored on Vycor glass

TiO2 particles







CH4

0.11

CH3OH

TiO2 powder

JRC-TIO-4 (anatase) particles TiO2 (P-25) particles







CH4

0.02







CH4

0.1

CH3OH, CO CO, H2

CH3OH, HCOOH, CH3CO2H C2H4, C2H6, CO, O2 C2H4, C2H6, O2


[155]

TiO2 pellets TiO2 (P-25) particles Ti-β(OH)











CH4 CH4 CH4

0.22e 135f 5.8

H2
CH3OH

CO
CO, C2H4, O2

[156] [157] [158]

Ti-PS (hexagonal, Si/ Ti 5 50) Ti
SBA-15







CH4

7.1

CH3OH




[23]







CH4

106

CH3OH

[159]

Sensitized TiO2 on glass fiber Anatase films

Ag

1.0 wt%

CH3OH

4.1




C2H4, C2H6, CO, O2


CH4

15g

C2H6; HCHO; CH3OH




[161]

Quartz wool immersed in P-25 suspension TiO2 pellets (made of P-25) Immobilized TiO2 (P-25) powder Two types of Ti-b-zeolites: Tiβ(OH) and Ti-β(F) Ti-containing porous silica (PS) films Ti
MCM-41; Ti
MCM-48; Ti
SBA-15 Optical fiber coated with sensitized TiO2 Sensitized TiO2 films

Au

[154] [141]

[160]

Optical fiber coated with sensitized TiO2 Glass wool supported sensitized TiO2
SiO2 composite Glass wool supported sensitized TiO2
SiO2 composite Optical fiber coated with sensitized TiO2 on SiO2 support Optical fiber coated with sensitized TiO2 Optical fiber coated with sensitized TiO2 Sensitized TiO2 films Sensitized anatase-type TiO2 nanorods Titanium oxide species anchored within the Y-zeolite cavities Mesoporous titania zeolites (Ti
MCM-41, Ti
MCM-48, TS1) Immobilized TiO2 particles a

Sensitized TiO2 on glass fiber Porous TiO2
SiO2

Cu

1.2 wt%

CH3OH

0.5







[162]

Cu

0.5 wt%

CO

60

CH4




[61]

TiO2 (2 mol%)
SiO2 composite particles Sensitized TiO2 on glass fiber Sensitized TiO2 on glass fiber Sensitized TiO2 on glass fiber TiO2 on glass beads TiO2/Pt nanorods

Cu

0.01 mol%

CO

17.5







[62]

Cu
Fe

CH4; C2H4

CH3OH, C2H6

[163]




CH3OH, C2H6

[164]







[165]

CH4 CH4

1.86; 0.56 0.91; 0.58 0.85; 0.56 0.3 6




Cu
Fe, N3 Pt Pt

0.5
0.5 wt % 0.5
0.5 wt % 0.5
0.5 wt % 0.25 wt% 1.0 wt%









[166] [167]

C2H4, C2H6, CO, O2 C2H4, C2H6, CO, O2

[86]




[82]

Cu
Fe

CH4; C2H4

Pt/ex-Ti-oxide/ Y-zeolite Pt/Ti
MCM-48

Pt

1.0 wt%

CH4

6.7

CH3OH

Pt

1.0 wt%

CH4

12.3

CH3OH

JRC-TiO-4 particles

Rh

4.0 wt%

CO

5.1

CH4

Maximum formation rate reported for the major product(s) in micromole per gram per hour In mgL-1h-1. c In mL. d In ppmg-1h-1. e μmolh-1. f In ppm. g In μmolm-2h-1. b

CH4; C2H4

[72]

396

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

have been investigated to improve the efficiency of bare TiO2 in liquid suspensions. For instance, an increase in the CO2 pressure brought an increase in the yield of the liquid-phase products and of gaseous products such as methane and ethane, and ethane [180]. The addition of electrolytes improved the overall yield under ambient as well as high pressure conditions, and also resulted in the formation of C2 products such as ethanol and acetaldehyde. The lack of electron donors is considered to be one of the rate-limiting factors. Addition of isopropanol function as a hole scavenger enhances the rate of formation of methane under higher pressure conditions [84], yet none of the previously observed C2 compounds were reported in the presence of isopropanol. A similar trend was also reported under ambient pressure [137,181], but additionally, the formation of CO was observed. When carbonate anions rather than gaseous CO2 were used as the carbon source in an aqueous suspension, the formation of methanol and methane was improved at lower pH values [73], and the optimal concentration of P-25 nanoparticles was found to be 1.0 g/L. The overall decrease in the efficiency at higher concentrations was attributed to the limited penetration by the UV light, although it should be noted that this optimal concentration relates to a specific intensity of 12.64 W/m. It was also shown that, compared to other sizes, anatase nanoparticles with 14 nm diameter gave the highest yields of methanol and methane [134]. A decreased performance with larger particles may be due to smaller surface areas and longer electron migration paths. The decrease in activity with smaller particles was associated with changes in the optical and electronic properties, especially with a widening of the band gap and associated blue shift in the onset of absorption [182]. Since, water reduction occurs competitively with CO2 reduction, attempts to replace water by supercritical liquid CO2 were made [81,135]. Washing the suspended TiO2 powder with water after exposure to light yielded formic acid, indicating that the illumination of TiO2 in liquid CO2 resulted in the formation of CO2•2 radicals, which were subsequently protonated. It has to be mentioned that washing TiO2 with acidic solutions instead of pure water gave higher yields of formic acid. TiO2 microcrystals embedded in SiO2 matrices (Q-TiO2/SiO2) exhibited improved photo-catalytic properties compared to bulk TiO2/SiO2 powders [183]. These composites were dispersed in water mixed with isopropanol as a hole scavenger. The added CO2 was photocatalytically converted into formate, methane, and ethene. The efficiency of the product formation was increased with smaller Ti/Si ratios. The difference between the observed product spectrum and the previously reported one was ascribed to the role of isopropanol as an electron donor. In order to explore the role of solvents, TiO2 nanoparticles embedded in SiO2 were spin-coated onto a quartz plate, and illuminated upon immersion in a suspension containing isopropanol and solvents with different dielectric constants [184]. The detected CO2 reduction products were formate and CO. The ratio of formate to CO increased with an increase in the dielectric constant of the solvent. Studies with isotopically labeled 13CO2 revealed that both were products of the CO2 reduction process and did not originate from the decomposition of either the solvent or the hole scavenger. A comparison of the Q-TiO2/SiO2 with bulk P-25 TiO2 powder illustrated the superior performance of the composite. The formation of methanol was observed when Q-TiO2 was immobilized in a poly(vinylpyrrolidone) film. This finding suggests that the matrix used for the immobilization of TiO2 also influences the reduction pathway of CO2. Ammonia and urea were found in the products together with formate and CO when lithium nitrate was present in the solution [185]. In this case, an increase in the dielectric constant led to larger amounts of ammonia and urea, while the amounts of both formate and CO decreased. The improved efficiency of the TiO2/SiO2 systems was attributed to enhanced charge separation through the presence of bridging Ti
O
Si bonds. This

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

397

proposition was verified by doping the composite with ruthenium by a solid
state dispersion method [186]. Loading pure anatase nanoparticles with 0.5 wt% Ru by wet impregnation had been previously shown to improve their photo-catalytic activity because of the formation of a Schottky barrier and improved charge separation. In contrast, doping the composite with Ru led to a decrease in activity. This detrimental effect was associated to the inhibition encountered in the formation of the Ti
O
Si bonds. A composite system of TiO2 and kaolinite was examined for the photoreduction of CO2 by Koci et al. [187], and compared with pure that of P-25 TiO2 particles in an aqueous solution of 0.2 M NaOH under UV illumination. An increase in the rates of formation of methane and methanol was observed, but the rate of formation of CO, although generally small, was smaller for the TiO2/kaolinite system than for P-25. Various factors were hypothesized to explain this enhanced performance of the composite photo-catalyst. These include a larger effective surface area emerging from a weaker agglomeration of the particles, reduced recombination rate of the photo-generated charges, modified acidity of the surface of the composite system, and a smaller size of anatase crystallites. With a focus to develop photo-catalysts to form C2, and possibly C3 compounds from CO2, TiO2 catalysts were supported on amphiphilic materials [188]. It has been revealed that the nature of catalytic support influences the selectivity for specific photo-catalytic products. The highest yields and best selectivity was obtained for a 10 wt% TiO2/Pd/Al2O3 system, which produced acetone and ethanol as major products, along with ethane and methane as minor products from a CO2-saturated aqueous solution. In analogous to electrochemical approaches which employ various metals such as copper, platinum, and palladium to catalyze CO2 reduction [172,173], metal doping was used in photo-catalytic systems. In early studies, Hirano et al. [189] added Cu nanoparticles to TiO2 slurry and this system produced mainly methanol and showed no activity in the absence of copper. It was assumed in this case that the copper not only acted as a catalyst, but also as a hole scavenger. Copper-loaded TiO2 particles prepared via sol-gel method tested in an alkaline suspension for CO2 reduction yielded the largest methanol with a Cu loading of 2 wt% [78,79], compared to nondoped TiO2. The active state of the Cu for the photo-reduction was assumed to be highly dispersed CuI on the basis of a decrease in the photo-activity on reduction of CuI to Cu0 or aggregation upon reduction with H2 [144]. The promoting effect of copper loading on the photo-catalytic reaction is ascribed to the ability of the metal to trap electrons and thereby reducing the recombination losses of the photo-generated charges without affecting the mobility of the electrons. In another study, a loading of 3 wt% CuO on P-25 was suggested to be optimal for the formation of methanol in an aqueous suspension [145]. A higher surface loading was associated with shading effects, which reduced the overall absorption capacity of the TiO2 particles. It was also claimed that CuO is the most active copper species compared to Cu and Cu2O. In another approach, TiO2
CuO composites were synthesized by using cetyl trimethylammonium bromide as a dispersant [148] and the highest photo-catalytic activity for CO2 reduction was observed with 1.0 wt% CuO. Annealing the catalyst at 450 C also showed an improved performance, possibly because of the high crystallinity which enabled rapid electron transfer. In this study, the reaction was carried out in methanol to enhance the solubility of CO2 and to provide an additional hole scavenger. The main product was methyl formate, resulting from the dimerization of formaldehyde. Simultaneous doping of TiO2 with Cu and Ce was beneficial for the formation of methanol from CO2 in an alkaline suspension [147], with an optimal Cu loading of 2 wt%. According to calculations, the cerium atoms were inserted into the TiO2 crystal lattice, thereby creating distortions and facilitating the formation of catalytically active adsorbed CO2 and H2O species. The effect of

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Ce- appeared to be stronger than that of Cu-doping. A two-way approach was chosen to improve the photo-catalytic activity of TiO2 by dispersing TiO2 nanocrystallites in the channels of a mesoporous silicate (SBA-15) and loading this composite system with copper [146]. Irradiation of the photo-catalyst in an aqueous 0.1 N NaOH solution saturated with CO2 led to methanol formation. The highest photo-catalytic activity was observed for a highly dispersed anatase nanoparticles loaded with 2 wt% Cu embedded in SBA-15. The superior performance was attributed to the highly dispersed state of TiO2 because of the mesoporous structure of SBA-15, on the one hand, and to the catalytic properties of copper, on the other hand. The molecular sieve 5 A has been tested as another mesoporous support to disperse Cu-loaded TiO2 nanoparticles [150]. The photo-catalytic reduction of CO2 bubbled through an aqueous 0.2 M NaOH solution mainly yielded oxalic acid through dimerization of •COOH radicals, along with smaller amounts of acetic acid and methanol. The highest formation rate of oxalic acid in alkaline conditions (65.6 μg/g h) was obtained for a sieve with 10 wt% P-25 TiO2 loaded with 2 wt% Cu. Under neutral conditions, however, P-25 particles without Cu outperformed the other composite structures. Copper is particularly appealing compared to other metals such as platinum, rhodium, palladium, gold, and ruthenium because it is inexpensive and abundant. However, all of these metals are known for their outstanding catalytic properties. In an early study by Ishitani et al. [153], palladium led to the largest increase in the rate of formation of methane when P-25 particles were loaded with 2 wt% of the metals. Other reported products included ethene, formic acid, acetic acid, and traces of methanol. Isotope labeling with 13 CO2 proved that CO2 indeed served as the carbon source for the formation of methane. Additionally, a much smaller amount of methane was detected and production ceased after about 4
5 h under N2. It was concluded that, in this case, methane formed from the carbonate dissolved in the water. XPS measurements showed the presence of partly oxidized Pd species after the illumination. Oxidation and deactivation of the catalyst explains the decrease in the formation rate after 8 h and eventual termination of the process. This proposal was confirmed by the fact that methane was formed at a lower rate after degassing the reactor and reintroduction of CO2 after 24 h. Palladium loading on TiO2 colloids in aqueous suspension had previously been reported to lead to the photo-reduction of carbonates to formates [151]. This finding was corroborated by another study in which TiO2 particles were doped with palladium and ruthenium oxide (RuO2) [190]. The deposited Pd was assumed to catalyze the conversion of CO2 into formate, while RuO2 catalyze the oxidation reaction. Surface photo-voltage spectrum measurements showed a strong response by the metal-doped particles in the near IR region, which could be correlated with an increased photocatalytic activity. Other metals such as gold and silver can, in addition to their catalytic function and acting as an electron sink, also extend the light-absorption range of the photo-catalyst through plasmonic excitations. These excitations in metal nanoparticles are known to enhance photocatalytic oxidation processes on TiO2. Accordingly, Koci et al. [142] reported that loading TiO2 nanoparticles with 1
7 wt% Ag indeed improved the rate of formation of methane and methanol. The increase in the rate was correlated with the amount of silver. Although the effective surface appeared to increase due to silver doping, it was not the decisive effect in explaining the increase in product yields. Instead two different causes were proposed: (1) doping with up to 5 wt% Ag led to a noticeable shift of the absorption toward visible wavelengths, and (2) the silver atoms formed metal clusters, which led to the formation of Schottky barriers at the metal-semiconductor interface. This phenomenon is known to enhance the charge-carrier separation [174]. In a separate study, an improvement in the product yield with metal loading was observed for illumination at 254 nm, but

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not at 365 nm [143]. P-25 TiO2 nanoparticles coated with Nafion (a perfluorinated polymer with sulfonate groups) membranes exhibited improved photo-conversion of CO2 into methane and ethane, and this enhancement was attributed to the excellent proton-conducting ability of the membrane, as it stabilizes the reaction intermediates, in addition to its inertness toward photo-induced redox processes. The combination of a Nafion layer with silver loading further improved the photoconversion. In this case, however, the main products were methanol and formaldehyde, whose formation rates increased with the amount of silver. In the most common approach to sensitization, dye molecules are chemisorbed on the TiO2 surface. If the LUMO of the dye lies above the conduction band edge of TiO2 then, after the absorption of a photon, the excited electron can be injected into the semiconductor. In this context, TiO2 nanoparticles with 20 nm diameters synthesized by a sol-gel method via homogeneous hydrolysis technique have been employed with cobalt phthalocyanine dye (TiO2
CoPc) as a sensitizer [191]. The electron transfer to TiO2 separates the electron from the hole, and thus improved the overall photo-catalytic activity. This was demonstrated by a larger total yield of the photo-reduction products of CO2 bubbled through an aqueous 0.15 N NaOH solution containing Na2SO3 as a hole scavenger when a TiO2
CoPc system was used compared to pure TiO2. The formation rates of formic acid and formaldehyde strongly increased, but at the same time a small decrease in the formation rates of CO and methane was noted. The presence of sacrificial electron donors (Na2SO3 and OH2 groups) improved the photocatalytic performance of the system. Woolerton et al. [87,149] adapted an alternate approach to the suspension-based system by sensitizing P-25 TiO2 particles with a ruthenium dye ([RuII(bpy)2(4,40 -(PO3H2)2-bpy)]Br2) (bpy 5 2,2-bipyridine) (referred to as RuP), to increase the visible light absorption. Instead of using a metal catalyst for the catalytic reduction of CO2, an enzyme ChCODH I (carboxydothermus hydrogenoformans) from the anaerobic microbe was attached to the TiO2 particles, and this enzyme is known to catalyze a two-electron reduction of CO2 to yield CO [192]. In such a system, the RuP sensitizer absorbs visible light and subsequently injects an excited electron into the conduction band of the TiO2 particles. After migrating through the conduction band of TiO2, the electron is transferred into the enzyme and its active sites, and CO2 is reduced to CO. The addition of EDTA as an additional electron donor increased the rate of formation of CO by up to 40%, which led to the conclusion that the limiting factor was the regeneration of the ruthenium sensitizer. Silver bromide (AgBr) is known to be highly sensitive to light, and it was also used as a sensitizing agent [193]. It has been found that the system consist of 23.2 wt% AgBr coupled with P-25 TiO2 nanoparticles performed best at pH 8.5 (aqueous suspension with 0.2 N KHCO3) giving the maximum yield, under visible light irradiation. Methane, methanol, CO, and ethanol were detected as reduction products. The system exhibited a high stability and almost no deterioration in performance over repeated reduction cycles under visible light. As photo-catalysts, chemically bonded nanocomposites of TiO2 (P-25) and graphene exhibited good adsorptivity of dyes, extended light-absorption range, and efficient charge separation through the graphene sheet [194,195]. Two types of graphene with different defect densities were synthesized and coupled to P-25 TiO2 particles to form a composite thin film, and photo-reduction of CO2 in the presence of water was attempted under UV-A (365 nm) and visible light and compared with the results obtained for a pure TiO2 film. Both hybrid systems resulted in an enhanced photo-catalytic rate of formation of methane under visible light. However, the catalyst with fewer defects (SEG
TiO2) performed three times better than the graphene/TiO2 (SRGO-TiO2) structure containing more defects. The same trend was observed under UV illumination, where only SEG
TiO2

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enhanced the photo-catalytic yield. Though defect sites are usually considered to be highly reactive, they appear to be rather detrimental for the photo-reduction of CO2 and this has been related to the decreased electron mobility because of charge-carrier scattering events. In contrast, the better performance of the composite with fewer defects was linked to the high electron mobility, which translates into a longer mean-free diffusion path, ensuring rapid separation of photo-generated charges, thus reducing the recombination rate and increasing the chances of an electron reaching and interacting with an adsorbed reactant. Moreover, the electronic coupling between SEG and TiO2 appears to be stronger, facilitating electron injections from one structure to the other. Another approach that combines nonstoichiometric Ti0.91O2 and graphene to a composite photo-catalyst was to stack nanosheets of each material to form robust hollow spheres [64]. The increased yield of CO and CH4 from CO2 compared to with P-25 TiO2 was attributed to the thin form of the nanosheets, which enabled a very rapid transport of the photo-generated electrons to the surface, where the reaction could occur. In addition, the electron transfer from Ti0.91O2 to graphene was presumed to enhance the lifetime of the charge carriers. It was also speculated that the hollow spheres might be able to enhance the absorption of light through multi-scattering of incident photons. The photo-catalytic reduction of CO2 at the liquid
solid interface has been generally carried out in aqueous dispersion of a nanoparticulate photo-catalyst, while for reactions at the gas
solid interface the photo-catalyst has to be immobilized. Although in the later case, the accessible area for photons and reactants would decrease, this approach significantly facilitates the separation of products, reactants, and photo-catalytic material [179]. In this case, both the structure of the photocatalyst and of its substrate affects the overall efficiency of the process. Another drawback of aqueous suspensions is the low solubility of CO2, which can be circumvented by using gaseous mixtures of CO2 and H2O. The ratio of H2O to CO2 is tunable in the gas phase, and it impacts the reaction rate [154]. For example, in a system of TiO2 nanoparticles immobilized on Vycor glass, the rate of formation of methane increased as the ratio of the H2O vapor to CO2 increased. The other major product, methanol, was formed at the highest rate with a H2O/CO2 ratio of 5:1. In contrast, methane and CO were the only detected CO2 reduction products while P-25 TiO2 nanoparticles were immobilized on glass wool [155]. The same compounds were obtained for P-25 TiO2 pellets illuminated with UV light in the presence of water vapor and CO2. These pellets were more effective than photo-catalysts immobilized by thin-film or anchoring techniques. A UV light source of 253.7 nm gave a significantly higher yield than illumination with UV light of 365 nm, as expected. With P-25 and anatase nanoparticles coated onto sapphire glass for photo-reduction of CO2, ethane was detected among the products of CO2 reduction [139]. In this study, a steady increase in the cumulative yield was observed on increasing the temperature up to 200 C. Saturation limits have been attained in the product formation after 4 h (at all temperatures) and such behavior can be attributed to the strong adsorption of O2 produced in the CO2 photolysis, blocking the reaction sites. This conjecture was supported by two observations: At first, only about 25% of the stoichiometrically produced O2 was detected and secondly, the temperature-dependent measurements indicated that desorption of O2 and intermediate products might be the rate-limiting step. An increased product yield at elevated temperature was also observed in an investigation on various TiO2 thin films. It was found that the largest methane yield was obtained at 80 C and 45 vol% CO2 [138]. The employed photo-catalytic structure was a sputtered mixed-phase film (70% anatase, 30% rutile) deposited at low angle. This film proved to be superior to pure-phase or high-angle sputtered films, as well as a P-25 film prepared by dip coating. This finding was attributed to the high photo-catalytic

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activity of the numerous rutile-anatase phase interfaces and to the enhanced absorption of visible light (up to 550 nm) associated with nonstoichiometric oxygen vacancies. A photo-catalytically active zeolite structure was synthesized by replacing silicon atoms in the zeolite framework with titanium [196]. For CO2 reduction experiments carried out in an H2 atmosphere under UV illumination, methane has been formed, while silicate without titanium did not exhibit photo-catalytic activity. Upon exposure to UV illumination, the amount of adsorbed carbon species on the catalyst increased by two orders of magnitude, and this must be due to hydrogenized CO2 molecules, such as •CH3 radicals. The Ti-oxide/Y-zeolite catalysts in the presence of CO2 and gaseous H2O significantly improved the yield of CO2 reduction products (mostly methane, methanol, and some ethene, ethane, as well as CO) under UV illumination compared to anatase TiO2 powder as photo-catalyst [83,86]. The largest yield of methanol was obtained with Ti-oxide/Y-zeolite (1.1 wt% TiO2) catalyst prepared by an ion-exchange method. The absorption spectra indicate, however, that the absorption of such a catalyst was blue-shifted compared to that of Ti-oxide/Y-zeolite structures prepared by impregnation techniques. This shift is attributed to a higher dispersion of Ti oxide species, which is in turn considered to be linked to the enhanced reactivity of the catalyst to form methanol. Additionally, the Ti-oxide/Y-zeolite obtained through ion-exchange technique exhibited a strong photoluminescence originating from the radiative decay of a long-lived (54 μs) charge transfer excited state to the ground state of the tetrahedrally coordinated Ti atoms. These are considered to be highly efficient photo-catalytic reaction sites, which is supported by the observation that the photoluminescence is quenched by both water vapor and CO2. The effect of loading the catalyst with Pt was also investigated and found to increase the methane yield as well as strongly decrease the methanol yield. The observed quenching of the photoluminescence was attributed to the transfer of photo-excited electrons to the metal, thus effectively separating these electrons from their concomitantly generated holes [197]. The redox half-reactions are thus spatially separated so that carbon and •OH radicals form at different reaction sites, thus preventing them from reacting with each other to form methanol. The microporous TS-1 zeolite was compared with titanium-containing mesoporous silicate zeolites (Ti
MCM-41, Ti
MCM-48); the product distribution was the same as found previously and all the zeolites outperformed bulk TiO2 powder, with Ti
MCM-48 resulting in the highest rate of formation of both major products [72,83]. This was again attributed to a higher dispersion of the reactive TiO2 species. Additionally, the comparatively larger pore size ( . 20 &) within the 3D channel structure may have contributed to this result. Accordingly, the yield was further improved up to 20 times by increasing the pore size to 70 & (Ti
SBA-15) [159]. Photoluminescence measurements of two types of Ti-β zeolites Ti-β(OH) and Ti-β(F) showed that the photoluminescence of the OH system was much stronger [83,198,199]. While the addition of CO2 led to a similar quenching in both zeolites, the addition of water had a much stronger effect on Ti-β(OH). This was attributed to its hydrophilic character and translated directly into its photocatalytic performance: the yield of methane and methanol was much larger for Ti-β(OH). A remarkable feature of fluorine-containing, hydrophobic, Ti-β(F), however, was a selectivity of 41% for methanol, which was unmatched by previously investigated zeolitic systems. These results were attributed to the low concentration of water molecules compared to CO2 molecules, emphasizing the importance of the adsorption affinity of H2O on highly dispersed TiO2 species incorporated in zeolite structures. Analogous to photo-catalysts of highly dispersed titanium species in zeolite structures, an amine-functionalized titanium metal
organic framework (NH2-MIL-125(Ti)) was demonstrated to be able to photo-reduce CO2 to formate under irradiation with visible light,

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although at comparably low conversion rates [200]. Thin, porous silica films containing titanium species increased the rate of CO2 photo-reduction twofold compared with the best zeolite powders [80,201,202]. The highest quantum efficiency (0.3%) was obtained with the Ti-PS(50) catalyst, which has a hexagonally shaped pore structure. Interestingly, a thin-film photo-catalyst with a cubic pore structure exhibited a high selectivity for methanol (nearly 60%). This was explained by this material having a less hydrophilic surface so that fewer water molecules were in the vicinity of the reaction sites, thus leading to preferential formation of methanol. Conversely, an anatase TiO2 powder catalyst (JRC-TiO-4, 49 m2/g) with a large band gap and numerous surface OH2 groups deposited on the bottom of a quartz cell was shown to reduce CO2 into methane most efficiently in the presence of water vapor. The selectivity could be tuned by loading the catalyst with Cu1 ions, which increased the yield of methanol [140]. The effect of copper loading on methanol generation was also investigated in a custom-built reactor made of optical fibers coated with Cu
TiO2. Methanol was the only reported reduction product from CO2 and water vapor [162], and its yield was better than with pure TiO2. Such selectivity in the gas phase is possibly due to a preference of the hydrogen atoms adsorbed on copper to react with oxygen atoms of the coadsorbed methoxy intermediate, thereby forming methanol. On the other hand, in the liquid phase, protons originate from the solution and react with the carbon atom of the methoxy group, to form methane [172]. The optimum copper loading was found to be 1.2 wt%; the lower formation rates at larger concentrations were assumed to be caused by shading the TiO2 and thus reduced light absorption. Copper (I) was identified to be the primary state of the copper, in agreement with earlier findings [79,203]. Increased adsorption of hydroxyl radicals on the Cu surface was considered as another factor that enhanced the reactivity of the system. A similar improvement in the yield of methanol was also achieved by loading with silver [160]. The addition of copper and iron to the TiO2 fiber coating led to the photo-catalytic formation of ethene and methane and trace amounts of ethane and methanol [164]. The yield of ethene was lower when either of the metals was added individually, compared to the addition of both metals together. Additionally, in the latter case, the rate of ethene formation was stable over longer periods of time, whereas the systems loaded with a single metal dopant worked well for only a limited time. It has been explained that the function of the iron was to improve the charge separation and the transfer of photo-generated holes from TiO2 to H2O via the valence band of iron oxide, to enhance the absorption of light, and to provide additional specific catalytic centers. At the same time, adding the iron apparently slowed down the rate of formation of methane. However, a mixture of TiO2 and SiO2 instead of pure TiO2 as a photo-catalytic substrate was more selective for methane than ethane formation [163]. A TiO2 (0.5 wt% Cu, 0.5 wt% Fe) system sensitized with a ruthenium dye (N3) to further extend the absorption range, improved the yield of methane under concentrated sunlight [165]. Overall, the decoration of TiO2 particles with copper and the use of highly dispersed TiO2 species inside mesoporous silica structures have both proven to be successful strategies for improving the photo-catalytic efficiency of the CO2 reduction process. Therefore, the two approaches were combined in a one-pot sol-gel synthesis of a Cu/TiO2
SiO2 catalyst, which was then coated onto glass wool [61]. The optimal loading concentration of copper was found to be 0.5 wt%. In the course of the illumination of the photo-catalyst, a color change from greenish white to dark gray was observed. This color change was associated with the reduction of CuI to Cu0 by photo-generated electrons, which was accompanied by a decrease in the photo-catalytic reactivity. The catalyst was regenerated upon exposure to air, whereby the gray color faded to white again, likely due to reoxidation of Cu0 to CuI. Mesoporous

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Cu
TiO2
SiO2 composite particles synthesized by self-assembly presents highly homogeneous phase [62]. These particles supported on glass wool were demonstrated to photo-reduce CO2 to CO. The highest yields were obtained with 2 mol% TiO2 and 0.01 mol% Cu. Hollow nanocubes of CuO
TiO2-xNx were shown to improve the rate of formation of methane compared to pure TiO2 P-25 particles [204]. The improved photo-catalytic activity was linked to various properties of these hybrid structures. In particular, the heterojunction formed between CuO and TiO2 was presumed to allow rapid charge separation and improve catalytic performance, while the presence of nitrogen in the hybrid structure improved the absorption in the visible range (Fig. 8.8B). Arrays of TiO2 nanotubes (NTs) were investigated by Varghese et al. [23,24] as a means for improving the generation of charge carriers and transport properties of the system. They possess a large surface area, but their wall thickness is in the range of the diffusion lengths of charge carriers, thus allowing the charges to migrate to the surface where they participate in the catalytic reactions. The columnar shape of the NTs was assumed to additionally facilitate directed charge transfer. The absorption spectrum of these NTs was extended into the visible range by doping with nitrogen, while copper and platinum were sputtered onto the surfaces to provide catalytic centers. The products reported to have formed photo-catalytically under natural sunlight included CO, H2, olefins, branched paraffins, and methane. Experiments with NTs of different lengths showed that the performance of the structure increased up to a length of 25 μm, remained uncorrelated between 25 and 70 μm, and decreased for longer NTs. The latter observation was ascribed to an inhomogeneous distribution of the metal NPs. The catalytic effects of the NPs were not the same: while platinum facilitated the formation of H2, Cu increased the formation of CO fivefold. The synergistic surface loading with Cu (52%) and Pt (48%) by sputtering resulted in an increase in the amount of produced hydrocarbons compared to individual loading. Moreover, the uniform deposition of copper nanoparticles by a wet chemistry route led to a further increase in the hydrocarbon yield. A similar increase was observed for uniformly distributed platinum nanoparticles obtained through microwave-assisted solvothermal method [205]. Anatase TiO2 nanorods with dominant reactive (010) facets and decorated with 1 wt% Pt were used to photo-catalytically convert CO2 into methane at a higher rate than P-25 nanoparticles decorated with the same amount of Pt [167], emphasizing the influence of the shape of the catalyst, as P-25 is generally considered to be a more photo-catalytically active material than pure anatase. Nanostructured platinized TiO2 films (Pt-TiO2) exhibited high efficiency for photo-converting CO2 into methane [59], with maximum rate of 1361 μmol/g h, and the corresponding quantum efficiency was 2.41%. Dye sensitization has also been employed for photo-reduction at a solid
gas interface. In this approach, TiO2 films prepared by dip coating glass beads were functionalized with ruthenium and perylene dyes [166,206]. The major product of the CO2 photo-reduction was methane; traces of CO and H2 were also formed. These products were obtained through illumination with visible light only when the sensitizer was used. Further, an improvement in the methane yield was only found for films decorated with platinum nanoparticles, but not for impregnated ones. In fact, platinum nanoparticles immersed in TiO2 might be detrimental to the efficiency of the photo-catalysis because of their function as electron traps [207]. In a similar approach to utilize visible light, a TiO2-based composite was sensitized with QDs, which led to an alignment of the energy band for a fast injection of electrons into TiO2 and better charge separation [208]. The QDs of 2.5 nm CdSe [209] and 3
5 nm PbS [63] were used in conjunction with metal cocatalyst nanoparticles, such as Cu or Pt. The CdSe/Pt/TiO2 composite yielded mostly methane and small amounts of methanol,

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CO, and H2; however, no products were observed under illumination with visible light without the CdSe QDs. PbS/Cu/TiO2 composites produced CO, methane, ethane, and H2. A fivefold increase in the yield of products was noted when 4 nm PbS QDs were used under illumination with white light, compared with the Cu/TiO2 composite alone. After prolonged illumination, the PbS QDs become deactivated through photo-oxidation. In another study, the sensitization of TiO2 NTs with Bi2S3, a semiconductor with a narrow band gap (Eg 5 1.28 eV), increased the rate of formation of methanol by a factor of 2.2
43.6 μmol/g h [210]. Sensitization with rhodium was also demonstrated to have a positive effect on the photo-catalytic reduction of CO2 on TiO2. In the liquid phase, Rh/TiO2 increased the yield of methanol, formaldehyde, and formic acid from a CO2-saturated aqueous suspension [211]. In the gas phase, functionalization of the TiO2 with rhodium improved the rate of formation of CO from CO2 and hydrogen [82,212]. The reduction of rhodium to its metallic state was accompanied by a decrease in activity and a shift in the product selectivity from CO to methane. The CO2 photo-reduction was also investigated on plasmonic gold nanoparticles deposited on anatase films [161]. Their illumination with a monochromatic light in the visible region of the spectrum (532 nm) afforded methane with a rate 24 times higher than that with bare TiO2. This effect was attributed to the excitation of the plasmon resonance of the nanoparticles leading to an enhancement of the local electromagnetic field, which in turn generates electron-hole pairs in the adjacent TiO2 at a much larger rate than the incident light [213]. In contrast, plasmon excitations on Au nanoparticles directly transfer electrons to the TiO2 conduction band only at an excitation wavelength of 254 nm, which matches the excitation energy of d-band electrons in gold. These electrons can transfer to the lower-lying conduction band states of TiO2. At this excitation wavelength, additional photo-catalytic products were detected including methanol, ethane, and formaldehyde. Methane as the main product was obtained while the TiO2 loaded Ag/Au bimetallic nanoparticles prepared by a microemulsion method was employed as photo-catalyst [157]. Charge recombination is often one of the limiting factors of the photo-catalytic performance of TiO2 materials. Multiwalled carbon nanotubes (MWCNTs), used as a support for TiO2 nanoparticles, provide a means to effectively separate the charges and reduce their recombination [214,215], in a similar manner to TiO2-graphene composites. However, most applications operate in an oxidative environment, for example, in the photo-catalytic degradation of dyes [216], wherein the hole stays on the TiO2 and participates in the oxidation reaction while the electron is transported away. Nevertheless, MWCNT
TiO2 composites have been shown to increase the efficiency of the CO2 reduction to ethanol, methane, and formic acid [76]. Low concentrations of MWCNTs gave the best results; it has been suggested that higher concentrations resulted in a shading effect of the NTs, thereby limiting the absorption of light by TiO2. It was also shown that the activity of the composite structures in repeated cycles was much higher than of pure TiO2 structures.

8.5.2.2 Reduction of CO2 on other semiconductors Presently, the photo-catalysts employed for the reduction of CO2 are not just limited to TiO2, but several other semiconductor-based systems can be found in the literature. This is mainly because the efficiency of the process remains low for photo-catalytic reduction of CO2 with TiO2 and derivative materials. This is due to the band structure of TiO2, which is acceptable, but certainly not optimal for the reduction of CO2 driven by solar energy. Firstly, the potential of its conduction band electrons is only slightly more negative than the multielectron reduction potentials of CO2, thus providing very small over potentials; whereas the potential of the valence band holes is much

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more positive than the water oxidation potential, which is thermodynamically not useful. Secondly, the wide band gap (3.2 eV) limits the absorption of light to the UV range. The two most-common methods adapted for extending the absorption range to visible light are sensitization or doping, but neither of these could fully address the issue. Sensitizing agents (dyes or QDs) often degrade when exposed to UV light and photo-generate oxidizing holes in TiO2, while the dopant atoms become centers of charge recombination. Further, the additional energy states associated with foreign atoms are highly localized, in effect suppressing charge mobility. Therefore, while TiO2 remains as a benchmark photo-catalyst, there is a lot of interest in alternative semiconductors, such as transition or main group metal oxides, chalcogenides, nitrides, and phosphides for the photo-reduction of CO2. These highly diverse groups of materials contain both narrow and wide band gap semiconductors, yet many of them offer a favorable band structure than TiO2. One of the possible advantages of using non-TiO2 semiconductors is a narrower band gap that results in a wider range of wavelengths to induce excitation of electrons. This has implications in visible light range sensitivity without the use of sensitizers that are subject to photo-degradation, thus allowing for a possibly higher stable system under visible light conditions. It is noteworthy that many recent developments on relevant semiconductors originated from research on the photo-catalytic splitting of water, as both processes share similar constraints on the energy bands [217
219]. Specifically, the search for new semiconductor materials is focused on the following points: 1. 2. 3. 4.

To raise the valence band energy to decrease the band gap. To move the conduction band to more reductive potentials. To improve the quantum efficiency of exciton formation and reducing charge recombination. To exploit novel nanoscale morphologies to provide a large surface area with multiple photo-catalytically active sites [220].

Examples for the non-TiO2 photo-catalytic systems are presented in Table 8.5 and detailed discussions on sulfide, oxide, and phosphide semiconductors can be found in the following sections.

8.5.2.2.1 Sulfides From a historical perspective, sulfide semiconductors initially received intense attention. Their valence band, constituted by 3p orbitals of the sulfur atoms is shifted upwards compared with those of the oxide analogues, while the conduction band electrons are also more reductive [219]. Many sulfides have a narrow band gap (e.g., PbS, Bi2S3), with the onset of absorption in the visible or even infrared region. Therefore, they have been employed as photosensitizers for TiO2. CdSe has also been used as a photosensitizer for ZnO nanorods [264]. The major disadvantage is that sulfides are generally not stable under illumination in an aqueous dispersion because of the oxidation of lattice S22 ions to elemental sulfur and eventually to sulfates [265]. To prevent the photo-corrosion, additional reducing agents such as sulfite (SO322), thiosulfate (S2O322), or hypophosphite (H2PO22) anions, tertiary amines (e.g., triethylamine, TEA), or alcohols (e.g., isopropanol) were used to scavenge the photo-generated holes. ZnS and CdS were the most studied sulfides for CO2 reduction. The former is a direct wide band gap semiconductor (Eg 5 3.66 eV in the bulk), hence it absorbs only in the UV range, but it possesses a strongly reducing conduction band (ECB 5 21.85 V/NHE at pH 7) [266]. In several reports, ZnS has been shown to preferentially reduce CO2 to formates with high quantum yields [221
223]. Henglein et al. [267] and Yoneyama [268] speculated that smaller ZnS nanoparticles are more efficient due to their larger band gap,

Table 8.5 Non-TiO2 Semiconductor-Based Systems for CO2 Reduction Semiconductor

Structure

Ega

Cocatalyst 21

Major Product

Rmaxb d

PEc

Other Products

Ref.

H2

[221]

H2, CO

H2

[222] [223] [224] [225] [223] [17] [226] [227]

ZnS

Nanoparticles

3.66

Cd

HCOOH

10.5

ZnS ZnS ZnS CdS

Nanoparticles Nanoparticles Nanoparticles in silica Nanoparticles

3.66 3.66 3.66 2.4


Zn21



HCOOH HCOOH HCOOH CO

75.1d 15.4d 7000 8.37d

CdS CdS CdS CdS

Nanoparticles Nanoparticles Nanoparticles Nanoparticles in silica

2.4 2.4 2.4 2.4

Cd21
TMA1e


CO CO Oxalate HCOOH

3.0d 0.13d 58.3d 81.5

32.5% (280 nm) 72% (313 nm)

9.8% (400 nm)





CdS

2.63




CH4

1.0




MnS Bi2S3/CdS CuxAgyInzZnkSm Cu2ZnSnS4 WO3 W18O49 ZrO2 SrTiO3 BaTiO3

Nanoparticles in MMT[k] Particles Particles Nanoparticles Photocathode Monoclinic crystals Nanowires Particles Particles Particles


H2, HCOOH Glyoxalate HCHO, oxalic acid H2, CO

3.0 1.28 1.4 1.5 2.79 2.7 5.0 3.2 3.2



RuO2 Ru complex

1 wt% Cu Pt



88 34.3 .5f 0.34 666g 2.5d



4.5% (UV)





0.01% 0.0021%

CO




H2



[228] [229] [230] [231] [232] [233] [234] [235] [226,236]

K2Ti6O13

Particles

.3.0

0.3 wt% Pt

HCOOH CH3OH CH3OH HCOOH CH4 CH4 CO CH4 HCOOH, HCHO CH4

0.2




[237]

BaLa4Ti4O15 KNbO3 NaNbO3

Particles SSR particles[g] Cubic crystals nanoparticles Nanowires Nanobelts

3.8 3.1 3.29

Ag Pt 0.5 wt% Pt

CO CH4 CH4

22d 70g 4.86






H2, HCOOH, HCHO H2, HCOOH



3.4 3.66

Pt


CH4 CH4

653g 3.58









[240] [241]

NaNbO3 HNb3O8

[66]

[238] [239] [65]

LiNbO3 N
Ta2O5

Particles Nanoparticles

3.6 2.4


Ru-dcbpye

HCOOH HCOOH

7700d 70

N-Ta2O5 BiVO4 Bi2WO6 InTaO4 InTaO4 N-InTaO4

Nanoparticles Monoclinic crystals Nanoparticles SSR particlesi Particles SSR particlesi

2.4 2.24 2.75 2.6 2.6 2.28

HCOOH C2H5OH CH4 CH3OH CH3OH CH3OH

60f 406.6d 1.1 1.39 11.1 165

InTaO4 Zn2GeO4

SSR particlesi Mesoporous

2.5 4.5

Ru-dcbpye

1 wt% NiO 1 wt% NiO 3.2 wt% Ni in NiO 1 wt% NiO 1 wt% Pt

CH3OH CH4

1.58 28.9g

Zn2GeO4 Zn2GeO4 ZnxGeyOzNk Zn1.7GeN1.8O

Nanobelts Nanorods Mesoporous Sheaflike

4.5 4.65 2.70 2.6

Pt,RuO2 3 wt% RuO2 Pt Pt, RuO2

CH4 CO CH4 CH4

25 17.9k 2.7g 11.5

In2Ge2O7(En) Zn2SnO4 β-Ga2O3 ZnGa2O4 CuGaO2 p-GaP

Nanowires Nanoplates Mesoporous Mesoporous SSR particlesi Photocathode

3.98 3.87 4.9 4.4 2.6 2.3

Pt Pt,RuO2
1 wt% RuO2
Pyridine

CO CH4 CO CH4 CO CH3OH

0.9 86.7g 1.46 50.4k 9g


p-InP p-InP

Photocathode Photocathode

1.35 1.35

Ru complex Ru complex

HCOOH HCOOH

140d


a

2.0%h 1.9% (405 nm)


2.45%j 0.063%


HCHO H2, CO

[242] [243]

CO CH3OH





[244] [245] [246] [247] [248] [249]


0.2% (251 nm)


0.024% (420 nm)

3.9%

2.6% (465 nm)
0.04%h





[250] [251]


CH4 CO


[252] [253] [254] [255]



CH4




[256] [257] [258] [259] [260] [261]





[262] [263]

The values in eV of the band gap for the specific structure. Maximum rate of formation of the main product(s) in micromolesper gram per hour. c Photonic efficiency, unless stated otherwise. d In μmolh-1. e Turnover number. f In ppmg-1h-1. g Sintered particles prepared by solid-state reaction. h Power conversion efficiency. i (Internal) Quantum efficiency. j In ppmh-1. k dcbpy 5 4,4’-dicarboxy-2,2’-bipyridine, dpbpy 5 4,4’-diphosphonate-2,2’-bipyridine, MMT 5 montmorillonite, TMA 1 5 tetramethylammonium cation. b

408

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

higher surface area, and greater affinity for CO2. Loading of 3.9 nm ZnS nanoparticles with 0.035 mol% Cd21 in an aqueous dispersion at pH 5.5 in the presence of isopropanol increased the quantum yield to twofold B32.5% [221]. A further increase in the cadmium content resulted in the formation of CO. Kanemoto et al. [222] obtained a cumulative quantum yield of 72% with irradiation at 313 nm (75.1 μmol/h HCOOH, 86 μmol/h H2, and 1.7 μmol/h CO) at pH 7 in the presence of NaH2PO2 and Na2S [222]. Sulfide anions are protonated to HS2 at pH 7. The role hydrogen sulfide anions were to eliminate surface sulfur vacancies which trap CB electrons, rather than to be sacrificial electron donors. They reportedly also shift the conduction band edge of sulfide semiconductors to more negative potentials, because of modified surface charging by the absorbed anions [269]. Fujiwara [223] observed the selective production of formate by ZnS nanoparticles with a diameter of 2 nm in DMF when zinc acetate was used as a precursor of the nanocrystals; however, a mixture of HCOOH and CO was obtained with a zinc perchlorate precursor. This was explained by the strong interaction of acetate anions with Zn cations, which inhibits the formation of sulfur vacancies that act as catalytic sites for the formation of CO. On the other hand, perchlorate interacts weakly with Zn21 species, which remain coordinated by the solvent. Excess amounts of Zn(OAc)2 further enhanced the rate of formation of HCOOH. ZnS nanoparticles were also loaded on a silica matrix with a large surface area (340 m2/g) to improve the stability of the semiconductor and adsorption of CO2 [224]. The best results were obtained for silica loaded with 13% ZnS (7 μmol/g h HCOOH). Interestingly, the addition of a platinum cocatalyst resulted in further reduction steps to yield HCHO and methanol at the expense of formate. As a consequence of the narrow band gap of 2.4 eV, the onset of absorption with CdS is at 520 nm [208], although its conduction band electrons are less reductive (ECB 5 20.9 V at pH 7/NHE) [265] than those in ZnS. Accordingly, 3
5 nm diameter CdS nanoparticles were shown to reduce CO2 to CO in DMF in the presence of TEA and small amounts of water (1%) under irradiation with visible light (l . 400 nm) [225]. A quantum yield of 9.8% was reported at 405 nm (8.37 μmol/h of CO). The selective formation of CO originated from the bidentate adsorption of a CO2 molecule at Cd atoms near a sulfur vacancy and the subsequent transfer of one electron to form a bound anion radical CO2 •2. This adsorbate reacted with a second CO2 molecule, followed by another electron transfer to form CO through an elimination reaction, by leaving behind a carbonate anion [223]. The cation
dipole interaction of the lattice Cd21 with DMF led to a cationic surface by preventing the adsorption of H1, consequently generating H2 or formate. However, the use of solvents with a higher dielectric constant than DMF (isopropanol, CH3CN, etc.) increased the ratio of formate to CO. Only formate was produced in water, because in polar solvents the radical anion is weakly bound to the surface and a proton can interact with its C atom [17]. Tetramethylammonium cations form an aprotic layer at the surface, and hence in the absence of H1, the dimerization of CO2•2 radicals occurs to form oxalate anions [226]. Further reduction products of oxalic acid, such as glyoxylic acid and glycolic acid, were also observed, thus drawing analogies to natural photosynthetic processes. A similar dimerization process was observed by Kisch and Lutz [227] for CdS particles embedded in a silica matrix, which yielded formate, HCHO, and oxalate as products. The reaction of the radicals was assumed to proceed in solution or on silica rather than on the CdS surface. CdS NPs were also anchored to a phyllosilicate clay mineral (montmorillonite) to protect the material against photooxidation in an aqueous dispersion [66]. In this setup, hydrogen, methane and CO were reported with an overall efficiency 4
8 times larger than the analogous system containing P-25 particles. Methane was considered to be a product of the hydrogenation of CO2 by evolving H2, instead of

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

409

the direct photo-reduction of CO2. Other sulfides were used less often. Manganese sulfide (Eg 5 3.0 eV) reduced bicarbonate dissolved in water (pH 7.5) to formate with a photonic efficiency of 4.2% under UV irradiation, although the semiconductor was degraded by oxidation of S22 to sulfate [228]. Bi2S3, with a narrow band gap of 1.28 eV, absorbs in the visible range and has been shown to reduce CO2 to methanol, especially when used in conjunction with CdS particles [229]. The highest formation rate (B88 μmol/g h) was obtained for 15 wt% Bi2S3, which was nearly two to three times higher than for either of the semiconductors while used alone. Methanol was also obtained with the complex sulfide CuxAgyInzZnkSm in the presence of sodium nitrite [230]. This sulfide is a solid solution of wide band gap ZnS with narrow band gap Cu2S and AgInS2, and has an intermediate band gap of about 1.4 eV, depending on the proportions. The highest formation rate, 34.3 μmol/g h was observed for Cu0.12Ag0.30In0.38Zn1.22S2. Recently Arai et al. [231] employed Cu2ZnSnS4, a quaternary sulfide with a narrow band gap (Eg 5 1.5 eV) and reductive conduction band (ECB 5 21.3 V), to photo-electrochemically reduce CO2 to formate in the presence of a metal complex electro-catalyst [231]. This semiconductor operated effectively under illumination with visible light, and its p-type character facilitated electron transfer to the catalyst, where CO2 reduction proceeded preferentially to proton reduction. Nevertheless, it required an external bias of 20.4 V, in place of a hole scavenger, to prevent photo-oxidation.

8.5.2.2.2 Oxides Many oxides do not suffer from photo-oxidation under irradiation unlike sulfides and hence can be conveniently employed in photo-catalyzed reduction and oxidation reactions. Indeed, two groups of metal oxides with closed-shell electronic configurations of the metal have been the center of interest for the photo-catalytic reduction of CO2. The first group contains octahedrally coordinated d0 transition-metal ions—Ti41, Zr41, Nb51, Ta51, V51, and W61—which have an empty orbital. TiO2 is the most salient member of the group, which includes binary oxides (e.g., ZrO2, Nb2O5, Ta2O5) and a number of several complex oxides referred to as titanates, niobates, tantalates, etc. [270,271]. These often occur in a perovskite, AMO3 (e.g., SrTiO3, NaNbO3), or in perovskiterelated structures. The second group contains main group metal oxides in a d10 configuration (i.e., with a fully occupied d orbital) with a general formula MyOz or AxMyOz, where M stands for Ga, Ge, In, Sn, or Sb, while A is an electropositive cation such as an alkali, alkaline earth, or rare earth metal ion [219,272]. Many of these photo-catalytically active binary and ternary oxides initially found application in the photo-catalytic splitting of water, but later tested for the photo-reduction of CO2. The top of the valence band in binary d0 and d10 metal oxides is primarily made of oxygen 2p orbitals. This means that a photo-generated hole in the VB has a strongly oxidizing character, which is thermodynamically capable of oxidizing water. Scaife [273] noted that their approximate potential is 2.94 V/NHE, while Matsumoto [27] estimated as 1.23 V 1 Eg/2. In any case, a wide band gap is needed for the conduction band to be sufficiently reductive. The theoretical limit for the reduction of CO2 is 20.24 V versus the NHE at pH 7, but in practice some over potential is necessary and the band gap has to be larger than 3.0 eV. The conduction band of d0 oxides has a strong d-orbital character, which originates from the antibonding interaction between transitionmetal t2g (i.e., d) orbitals and oxygen [110,271]. In simplified terms, the excitation of the electron from the VB to CB can be seen as charge transfer from the oxygen atom to the metal, for instance, leading to the excited state (Ti31-O2) , postulated for TiO2. In these oxides, the minimum of the CB and hence the band gap increases as the effective electronegativity of the transition-metal ion

410

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

decreases, which can be ordered as W61 . Nb51BTi41 . Ta51 . Zr41 [271]. Accordingly, WO3 has the smallest band gap of 2.7 eV, and with the edge of its conduction band located at 0.0 V/NHE at pH 7, it cannot reduce CO2 [12,274]. However, a rectangular sheet like monoclinic crystal of WO3 with dominant {002} facets has been shown to possess a slightly larger band gap (2.79 eV) and the conduction band elevation by 0.3 V [232]. This was sufficient to enable formation of small amounts of CH4 from CO2 (0.34 μmol/g h; see Table 8.5). Competition from proton reduction was negligible because the ECB potential was still too positive for this reaction to proceed (ENHE (H1/H2) 5 20.41 V at pH 7). In another investigation, 0.9 nm thin and several micrometer long W18O49 nanowires were shown to reduce CO2 under the illumination of visible light [233]. This was attributed to the presence of numerous oxygen vacancies or defects in the monoclinic structure of the nanowires providing reduction sites and improving the adsorption of CO2. The photo-catalytic activity was suppressed after exposure to H2O2, presumably by filling the vacancies. This was accompanied by a change in the color of the catalyst from deep blue to the standard yellow of WO3. Other oxides in the series have wider band gaps, up to 5.0 eV for ZrO2. With its CB minimum at a strongly negative potential (ECB 5 21.4 V vs the NHE at pH 7), ZrO2 is capable of reducing CO2 to CO in an aqueous dispersion without a cocatalyst or a sacrificial electron donor, although the rate of concurrent water reduction was nearly 100 times higher [234]. At the same time, the width of its band gap severely restricts the absorption of light. Despite its weakly reductive CB, TiO2 is the binary oxide used most extensively for CO2 reduction. Similar results as for TiO2 were obtained with alkaline earth metal titanates, which have band gaps of comparable energy (ca. 3.2 eV). SrTiO3 was shown to photo-reduce CO2 to methane in the presence of Pt [235], while the use of BaTiO3 for the reduction of CO2 resulted in the formation of formate and HCHO [226,236]. The rates of the reactions were low; for example, in the latter case, the photonic efficiency was 0.0021% for illumination at 320 nm , l , 580 nm. Interestingly, a different system of barium titanate, BaTi4O9, with an orthorhombic crystal structure was considered to be more photoactive because of distortions in the TiO6 octahedral structure (Ti41 ions displaced from the center of gravity of six oxygen ions). This local anisotropy is responsible for the generation of a dipole moment, which is thought to promote charge separation through an internal polarization field, a typical effect in ferroelectric materials [272]. BaTi4O9 was, however, not tested for the reduction of CO2, but only for the splitting of water. Potassium hexatitanate, K2Ti6O13, loaded with Pt was also able to reduce CO2 to CH4, HCOOH, and HCHO, but their combined rate of formation was still 240 times lower than that of H2 [237]. These results highlight the importance of choosing an appropriate cocatalyst. For example, silver is a known electro-catalyst for the reduction of CO2 to CO. With this background, Iizuka et al. [238] have produced CO almost selectively by using a wide band gap (3.8 eV) quaternary titanate, BaLa4Ti4O15, decorated with 10 nm Ag nanoparticles formed by the reduction of Ag1 in solution with hypophosphite. Apart from traces of HCOOH, hydrogen was generated at a lower rate than CO, while O2 was formed at a stoichiometrically matching rate. In this case, no hole scavenger was required because the water functioned as an electron donor. It was claimed that the crystal structure of the material facilitated the separation of the reduction and oxidation sites, thereby preventing reoxidation of the products. The reduction sites were located at the edges of the layers of the perovskite structure, while the oxidation proceeded at the basal plane. Niobates with a perovskite structure share many characteristics (nontoxicity, stability, indirect wide band gap) with titanates, although their conduction band is slightly more reductive. For example, the minimum of the CB of NaNbO3 is located at 20.71 V/NHE at pH 7 [275].

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

411

NaNbO3 and KNbO3 decorated with platinum nanoparticles by photo-deposition were both shown to reduce CO2 to CH4 without sacrificial electron donors [239]. Potassium niobate performed nearly 2.5 times better, which was attributed to a smaller band gap (3.1 eV for KNbO3 vs 3.4 eV for NaNbO3) and higher mobilities of the charge carriers, which were deduced from DFT calculations of the energy bands. Similar explanation on more dispersive conduction band were used to explain the twofold higher rate of formation of CH4 with the cubic crystal structure of NaNbO3 compared to the more common orthorhombic one (4.86 vs 2.45 μmol/g h) [65]. It was also demonstrated that NaNbO3 nanowires with a diameter of about 100 nm and a length of about 50 mm produced methane at a significantly higher rate than micrometer-sized particles [240]. The nanostructured material possessed a much larger surface area and its crystal lattice had fewer defects. Moreover, the high aspect ratio provided short pathways for electron and hole transport. These results confirm the trend that the photo-catalytic activity of nanostructured niobates is at least tenfold higher that of their bulk counterparts [276]. Lamellar perovskite niobates and tantalates hold great promise as active photo-catalysts [277,278]. They are composed of thin (ca. 1 nm) sheets of edge- and/or cornershared NbO6 or TaO6 octahedra, with the other cation intercalated between the layers [270]. The structure is favorable for the separation and transfer of photo-excited charges, deposition of cocatalysts, and selective intercalation of reactants [279,280]. In this context, layered calcium niobate HCa2Nb3O10 [281] or strontium niobate Sr2Nb2O7 and tantalate Sr2Ta2O7 [282] or niobic acid HNb3O8 [283] have proven effective for the generation of H2. The acid in the form of 50 nm thick, 300
500 nm wide, and 5 mm long nanobelts prepared by a hydrothermal method was also employed successfully to selectively photo-reduce CO2 to CH4 [241]. The rate of formation with the acid was twice as high as that with KNb3O8 because of its hydrophilic nature and Brønsted acidity. It was speculated that, because of the high concentration of CO2 at the reaction sites, the photo-generated HC radicals were trapped by CO2 molecules instead of combining to form H2. Moreover, the nanobelts led to a 20 times higher yield than the same compounds prepared by a solid
state reaction and nearly 10 times higher yield than P-25 particles. A sustained dipole in ferroelectric materials causes significant band bending at the interfaces and thus effectively separates the photo-induced charges [284,285]. This means that the reduction and oxidation sites can be spatially separated, which slows down back reactions. Ferroelectric material LiNbO3, with remnant polarization of 65 μC/cm2 and a low electron affinity (i.e., highly reducing), photo-reduced CO2 to formic acid and HCHO with a power conversion efficiency of 2%, which is 11 times more efficient than P-25 particles [242]. In addition, the ferroelectric surfaces might facilitate CO2 adsorption by lowering its reduction potential. Nitrogen doping is a typical example of band gap engineering designed to improve the absorption of light in the visible range (see Fig. 8.8B). This approach relies on additional intraband states originating from nitrogen 2p orbitals which have higher energies than oxygen 2p orbitals and, therefore, lie above the valence band of the oxides. In the case of niobic acid, nitrogen doping reduced the band gap by 0.8 eV and red-shifted the onset of absorption by 80 nm (Fig. 8.10). Importantly, the lamellar structure of the acid was preserved in the doping process, thereby the resulting material remained highly photo-catalytically active [283]. Tantalum pentoxide Ta2O5 has a wide band gap of 4.0 eV, but the following treatment with gaseous NH3 aided the gap to shrink to 2.4
2.5 eV, leading to a shift in the adsorption edge from 320 to 500 nm [286]. It lost the n-type character in the process through the elimination of oxygen vacancies and became a p-type semiconductor. This is a favorable feature for the injection of a photo-excited electron into a

412

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

CB

HNb3O8

N-doped HNb3O8

Nb 4d

Nb 4d

Eg= 3.5 ev

Eg= 2.7 ev N 2p

VB

O 2p

O 2p

FIGURE 8.10 Schematic band structures of HNb3O8 and N-doped HNb3O8. From H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012) 229
251 with permission.

cocatalyst. Sato et al. [243] employed N
Ta2O5 anchored with a ruthenium-based electro-catalyst [Ru(dcbpy)2(CO)2]21 to reduce CO2 to formic acid and CO under visible light illumination. The quantum yield at 405 nm was estimated to be 1.9% and 1.0% at 435 nm. The maximum of the valence band was shifted upward in N
Ta2O5 to about 1.1 V, and hence the photo-catalyst was unable to use water as an electron donor (i.e., oxidize it). TEOA fulfilled the role of a hole scavenger and partially of a proton source. Chemisorption of the metal complex through phosphonic groups rather than carboxylic groups was found to increase the efficiency almost fivefold because of stronger electronic coupling between the semiconductor and the electro-catalyst [244]. The band gap can be narrowed by further replacement of oxygen with nitrogen atoms [219]. Oxy(nitride) TaON has a band gap of 2.4 eV, while that of the nitride Ta2N3 is even smaller at 2.1 eV. The position of the conduction band, composed in each case of Ta 5d orbitals was not affected, thus offering a way to improve the absorption of light without lowering the reduction potential. The disadvantages of doping through an increased recombination rate and decreased charge mobility can be suppressed by the formation of multimetal oxides. The introduction of cations, such as Ag1, Pb21, and Bi31, with fully occupied low energy d or s orbitals into the VB, contributes to the upshift of the band. In the case of AgTaO3 and AgNbO3, the new valence band formed by hybridization of oxygen 2p and silver 4d orbitals effectively decreases the band gap by 0.6 eV without imparting notable changes in the perovskite structure [287]. As a consequence of the higher VB, AgNbO3 becomes active in the visible range ( . 420 nm) and results in the generation of H2. The findings regarding CuNbO3 are even more promising [288]. This p-type semiconductor has a narrow band of 1.95 eV originating from the Cu 3d character of the valence band and a conduction band minimum appropriate for CO2 reduction. Furthermore, its electronic structure permits nearly direct band gap transitions because of Cu to Nb d
d excitations facilitating light harvesting [289]. In copper(I) tantalites Cu5Ta11O30 and Cu3Ta7O19 with band gap widths 2.59 and 2.47 eV, respectively, the conduction band composed of Ta 5d orbitals is even more reductive at 21.53 and 21.28 V [290]. The crystal structures of these oxides consist of pentagonal TaO7 bipyramids instead of TaO6 octahedra. Such a structure exhibits a stronger delocalization of the conduction band states within layers of the bipyramids, which results in a higher electron mobility. It appears that the copper (I) compounds are suitable candidates for the photo-catalytic reduction of CO2 and have been discussed in such a context, but this presumption has to be verified experimentally

8.5 COMPARISON BETWEEN DIFFERENT SYSTEMS

413

[288]. Such verification has already been performed for several d0 metal oxides containing Bi31 ions, where the valence band is shifted upward by hybridization of occupied oxygen 2p and bismuth 6s orbitals [291,292]. For example, bismuth vanadate BiVO4 with a monoclinic scheelite structure has a band gap of only 2.24 eV, showing selectivity to photo-reduce CO2 to ethanol, although under weaker illumination some methanol was also formed [245]. It has been presumed that the dimerization of intermediate C1 species occurred as a result of their high concentration at the surface and the intensity of the irradiation led to the production of a large number of photogenerated electrons. The rate of formation of ethanol obtained with monoclinic BiVO4 was nearly 17 times higher than with tetragonal BiVO4, possibly because of the easier adsorption of CO2 on the former and the larger (2.56 eV) band gap of the latter. Square nanoplates of Bi2WO6 with 10 nm thickness prepared via hydrothermal route photo-catalytically reduced CO2 to methane when illuminated with visible light [247]. Their band gap of 2.75 eV was a little larger than that in the bulk material (2.69 eV), but the size and shape allowed for quick electron transfers to the surface and for provision of a highly active surface with a {001} facet for CO2 adsorption and subsequent dissociation. The photo-catalytic splitting of water by indium tantalum oxide doped with nickel, In12xNixTaO4 (x 5 0
0.2), under irradiation with visible light was reported by Zou et al. [293] generated much interest in this group of semiconductors. It was the first observation of the concurrent photocatalytic generation of both H2 and O2 without an applied bias and a UV light source; however, the measured optical band gaps for InTaO4 (Eg 5 2.6 eV) and InNbO4 (Eg 5 2.5 eV) caused some controversy [294]. Both ternary oxides have a wolframite structure which contains corner-shared InO6 and TaO6 (NbO6) octahedra arranged in a zigzag pattern. DFT calculations have shown that both systems have wide band gaps (3.70
3.96 eV for InTaO4 [295] and 3.43 eV for InNbO4) [102] with an indirect transition between the VB dominated by the oxygen 2p orbital and the CB dominated by the Ta 5d (Nb 4d) orbital. Clearly, the calculated band gaps were significantly larger than the experimental ones. The actual discrepancy was even larger because DFT methods tend to underestimate rather to overestimate the Eg value. Various hypotheses have been put forward to account for the activity of these oxides in visible light: (1) oxygen vacancies that create inter band (gap) impurity states below the Fermi level [102,296] (Fig. 8.8D), (2) absorption by Frenkel excitons [102], or (3) In2O3 impurities which exhibit a matching (2.62 eV) band gap [295]. None of these hypotheses has as yet been sufficiently corroborated with evidence, and the explanation remains elusive. This unclear picture is exacerbated by the fact that nickel plays two different roles. Firstly, it serves as a dopant further decreasing the band gap of In12xNixTaO4 due to the distorted NiO6 octahedral structure [295] or due to the introduction of dopant states above the valence band [296]. It can also weakly contribute to the absorption of light nearly at 2.7 eV, which originates from internal d
d transitions; however, this excitation is photo-catalytically inactive. Secondly, nickel is also a cocatalyst, often in a Ni in NiO configuration, referred to as NiOy. Despite this lack of understanding, InTaO4 and InNbO4 prepared by either a high-temperature solid
state reaction or by a sol-gel synthesis [297] have been employed in studies of CO2 photo-reduction. Pan and Chen [247] suspended 1
2 mm InTaO4 particles in an aqueous solution of KHCO3 and observed that methanol was produced at a rate of 1.39 μmol/g h under illumination from a 500 W halogen lamp. According to the authors, this rate corresponded to an (internal) quantum efficiency of 2.45%. Wang et al. [248] showed that the addition of a NiO cocatalyst acts as an electron trap and facilitates the formation of HC radicals that react with CO2 increasing the rate of formation of methanol in the aqueous

414

CHAPTER 8 PHOTOCHEMICAL REDUCTION OF CO2

phase to almost tenfold [248]. They also achieved a rate of formation of 11.1 μmol/g h in the vapor phase in an optical fiber reactor under intense irradiation of 327 mW/cm2. The (external) quantum efficiency in the aqueous- and vapor-phase experiments was 0.0045% and 0.063%, respectively. Tsai et al. [249] decreased the band gap of InTaO4 to 2.28 eV by doping with nitrogen. They inferred that the conduction band of InTaO4 is not reductive enough (ECB 5 20.8 V/NHE at pH 0) for transferring electrons to NiO (ECB 5 20.96 V/NHE). Hence, they used a Ni in NiO structure to trap electrons in the nickel and to provide catalytic centers on the NiO surface. The reported rate of formation of methanol was 65 μmol/g h upon illumination with visible light (390
770 nm). It was increased twofold by nitrogen doping and threefold by the addition of the cocatalyst. In another study, Ni in NiO was found to be a more effective cocatalyst of InNbO4 than Co3O4 nanoparticles, although the difference was only about 5% in favor of nickel [250]. The presence of small amounts of Nb2O5 during preparation helped to maintain the small size of the NiO particles needed for their photo-catalytic activity. Metal oxides in which the core ion is in a d10 electronic configuration have attracted considerable attention in recent years because of the characteristics of their conduction band. In contrast to d0 oxides, where the conduction band is primarily composed of d orbitals, in d10 oxides hybridized s and p orbitals form the lower end of the band. Such mixing of orbitals confers a large dispersion in k-space on the band and thereby a high mobility of the photo-generated electrons [270,272]. Many of the ternary d10 oxides also contain Zn21 ions because the p-d repulsion of the occupied oxygen 2p and zinc 3d orbitals raises the valence band without affecting the level of the conduction band, and thus decreasing the gap [298]. The respective oxides are also in general stable in water under UV irradiation. Zinc germanate, Zn2GeO4, has been the subject of several studies on the photo-catalytic reduction of CO2 in the last few years. As a consequence of the wide band gap of 4.5 eV, it was earlier proven to be capable of splitting water, especially when decorated with RuO2 nanoparticles as oxidation catalysts [299]. Its willemite crystal structure consists of GeO4 and ZnO4 tetrahedra. Interestingly, the GeO4 tetrahedra are heavily distorted so that a dipole moment of 1.6 D is generated inside. The associated field is thought to promote the electron-hole separation, as was previously discussed for BaTi4O9. Later, computational studies have shown that CO2 preferentially adsorbs on the Zn2GeO4 surface near or at an oxygen vacancy site [300]. In the latter case it can dissociate to CO, thereby filling the vacancy with its oxygen atom. Without a vacancy, a CO2 molecule adsorbs through either bridged carbonate like species or bidentate carbonates, depending on the crystal shows the effect of decorating the structures with RuO2 and platinum nanoparticles on the efficiency of the photo-reduction, with the best results obtained when both types of nanoparticles were employed together. While methane was the major product of the CO2 reduction on zinc germanate, carbon monoxide was produced almost selectively with the indium germanate In2Ge2O7(En) (En 5 ethylenediamine) [256]. Ultrathin nanowires of the material prepared in an En/water solvent and loaded with platinum nanoparticles were active in the UV range (Eg 5 3.98 eV) and formed CO in the presence of water vapor at a rate of about 0.9 μmol/g h when illuminated with a Xe arc lamp. The benefits of nanostructures are further exemplified by hexagonal nanoplates of zinc stannate, Zn2SnO4, arranged to form micrometer-sized octahedron [257]. As a consequence of the conduction band minimum located at 21.17 V/NHE, this d10 ternary oxide is a suitable material for the photo-reduction of CO2, albeit only under UV illumination (Eg 5 3.6 eV in the bulk and 3.87 eV for the nanoplates). The rate of formation of methane with the nanostructured sample was 20.1 ppm/g, compared to the value of 1.5 ppm/g with the bulk sample. The rate of increase was attributed to the high surface area, high rate of electron

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transfer along the nanoplates, facile electrolyte diffusion within the mesh-like structure of the octahedron, and the significant light scattering by the micrometer-sized objects, which is beneficial for the absorption of light. The addition of cocatalysts, RuO2, and Pt nanoparticles substantially improved the results, and led to the highest CH4 formation rate of 86.7 ppm/g under identical experimental conditions. Gallium oxide, especially the polymorph α-Ga2O3 with high surface area has a good affinity toward CO2 through surface OH groups and the formation of various (bi) carbonate species [301]. This property has been used in studies of the photo-catalytic hydrogenation of CO2 on the surface of Ga2O3 that is reduction with H2 [302]. The same reductant was also used in the reduction of CO2 to methanol on gallium oxide based zinc-copper double layered hydroxides [303]. However, Ga2O3 has a high reducing potential (21.45 V/NHE at pH 7) and is able to photoreduce CO2 without the use of additional reductants and cocatalysts. Park et al. [258] employed mesoporous β-Ga2O3 with a pore size of 3.9 nm and microporous channels to convert CO2 into CO and CH4 with formation rates of 1.46 and 0.21 μmol/g h, respectively. The rates were nearly four times higher than for the bulky β-Ga2O3 analogue and this improvement is attributed to the higher active surface area as well as to efficient electron transfer properties, good penetration of light, and gas diffusion in the porous structures. Mesoporous zinc gallate, ZnGa2O4 with a spinel crystal structure and a wide band gap of 4.4 eV was shown to photo-reduce CO2 to methane [259]. In contrast, the same compound prepared by a solid
state reaction produced only traces of methane. Similar to other d10 oxides, the improved separation of the electrons and holes by decoration of the semiconductor with RuO2 nanoparticles led to a significant (almost tenfold) increase in the rate of formation of CH4. Gallium zinc (oxy) nitride, (Ga12xZnx)(N12xOx), could alleviate the major deficiency of zinc gallate, that is the width of its band gap. This semiconductor can be construed as a solid solution of GaN and ZnO, but its activity to visible light is a little surprising because both components have wide band gaps [260]. This activity is thought to originate from the interaction of oxygen 2p with nitrogen 2p and zinc 3d orbitals which shift up the valence band [304]. Lately it was shown that the band gap can be modulated between 2.2 and 2.7 eV by varying the content of GaN and ZnO [305]. With a band gap of 2.2 eV, which corresponds to an onset of absorption at 565 nm, a significant part of the solar spectrum can be covered. This semiconductor has been applied successfully to the splitting of water under illumination with visible light [217], but to our knowledge has not yet been used for the photo-reduction of CO2, unlike its Ge-containing counterpart [254]. All of the methods presented here for narrowing the band gap to harvest visible light have relied on raising the energy of the valence band. The futility of modifying the conduction band level is illustrated by employing delafossite Fe-substituted CuGaO2 [260]. Delafossites are materials with the general formula ABO2 containing layers of octahedrally coordinated B31 ions interlaced with layers of A1 ions. They usually absorb only UV light; however, CuGaO2 has a weak absorption feature at 2.6 eV, which is associated with an indirect gap transition. This is presumably due to a similar effect as discussed earlier for CuNbO3, in which copper 3d orbitals raising the valence band. While some of the Ga31 ions were substituted with Fe31, the resulting CuGa12xFexO2 (x 5 0.05
0.20) system absorbed strongly in the visible region because of a narrow band gap of 1.5 eV. CuGaO2 was shown to photocatalytically reduce CO2 to CO at a relatively low rate of about 9 ppm/g h, but this rate was not increased with CuGa12xFexO2, irrespective of the iron concentration. The authors explain that the introduction of Fe31 ions lowered the conduction band because of distortions in the crystal lattice

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so that its level was below the reduction potential of CO2 to CO. Therefore, despite enhancing the absorption of light, the photo-catalytic activity did not improve but worsened.

8.5.2.2.3 Phosphides Several main group metal phosphides, especially GaP and InP have been investigated for their ability to reduce CO2 in photochemical or photo-electrochemical systems. GaP is a p-type semiconductor with a reasonably narrow band gap (2.3 eV) along with a highly favorable reducing position of the conduction band. It was shown in the notable work by Inoue et al. [12] to photo-reduce CO2 to methanol and smaller amounts of HCHO much more efficiently than, for example, TiO2 or ZnO [12]. Barton et al. [261] employed p-GaP as a photocathode in a photo-electrochemical cell, in which a photonic (external quantum) yield of 2.6% at 465 nm was obtained for the conversion of CO2 into methanol while the electrode was held at 20.5 V/SCE (saturated calomel electrode). Pyridine was added to the electrolyte to function as a cocatalyst and thought to be responsible for the selectivity of the process toward methanol through carbamate intermediate species. On the other hand, p-InP semiconductors tend to photo-reduce CO2 to formate, although the minimum of its conduction band is located about 0.85 V lower. Arai et al. [262] built a photo-electrochemical cell with a zinc-doped InP photocathode (Eg 5 1.35 eV) modified with a ruthenium coordination polymer [Ru(L-L)(CO)2]n, in which L-L is a diimine ligand. The system exhibited good selectivity for the formation of formate in an aqueous environment under irradiation with visible light (l . 400 nm) and an applied potential of 20.6 V. Importantly, isotope analysis using 13CO2 and D2O has proven that the CO2 and water acted as sources of carbon and protons, respectively. The system was further combined with a second photo-catalyst based on TiO2-Pt, which was designed to oxidize water (i.e., extract electrons) in a separate compartment [263]. The constructed system has been referred as a Z-scheme and is composed of two photo-catalysts which have similar band-gap widths but shifted band edges. It operates on the basis of a two-step excitation, where the less reductive electron recombines with the less oxidative hole, leaving the more active species to drive redox reactions. In such a scheme, InP anchored with a ruthenium electro-catalyst photo-reduced CO2 to formate without any external bias, with a power conversion efficiency of 0.03%, demonstrating the viability of the two-compartment photo-electrochemical system.

8.6 SUMMARY AND OUTLOOK Photo-reduction of CO2 is a promising approach to reduce carbon emission while simultaneously recycling it as a fuel feedstock by harnessing readily available solar energy. A wide variety of semiconductor systems have proven capable of harnessing near-ultraviolet and visible light and using that energy to drive the reduction of carbon dioxide through photo-catalysis. The improved production of solar fuels is an essential component of the renewable energy portfolio of the future, enabling intermittent power sources to become more prevalent by storing excess energy with fewer losses and thus greater recoverability. Early research in this field employed suspensions of simple semiconductor materials in aqueous solution, while later developments have focused on various doping and nanostructuring techniques, combinations of multiple semiconductors, and the addition

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of heterogeneous or homogeneous catalysts and sensitizers. The use of nanomaterials in particular has led to improved charge separation and to enhanced catalysis. The reduction mechanisms, however, are complex, involving various limiting steps. Overcoming the limiting steps is the major task for developing efficient photo-catalyst systems. Several strategies have been attempted for this purpose, including selection of semiconductors with suitable band edges, coating electron sinks, forming heterogeneous junctions, designing novel morphologies, and developing hybrid structures. Comparison of different state-of-the-art photocatalytic systems for CO2 reduction was made in this chapter aiming to demonstrate the advances in this area and provide an overview of the research trend for future development of photo-catalysts for CO2 photo-reduction in a large scale. It can be noted that at present, the formation rates of products of the photo-catalytic reduction of CO2 on semiconductors rarely exceed tens of μmol/g h and therefore the efficiency of the process is generally lower than in natural photosynthesis or in the photo-catalytic generation of H2. However there has been a surge of interest in the field over the last few years. Recent developments are concentrated on new photo-catalytic materials (e.g., novel oxynitrides of metals with d0 electronic configurations or oxides of d10 metals) and on new nanoscale structures that offer a large surface area, improved charge separation, and directional electron transfers. The mechanism of the process is also the subject of many studies, usually based on a combination of experimental and computational methods. These studies attempt to answer the unresolved issues concerning the chemical pathways of CO2 reduction which determine the selectivity of the entire reaction. Of particular interest are the approaches to overcome the barrier associated with the activation of a CO2 molecule toward the first one-electron reduction. This step appears to be a rate-limiting one, because of the highly negative reduction potential of CO2 to the anion radical CO2•2 compared to the conduction band levels of commonly employed semiconductors. Other important aspects of the mechanism which are relevant for a deeper understanding of the process include: (1) charge-carrier dynamics within the semiconductors and at their interfaces with the metal cocatalysts; (2) the effect of the nanostructure of the photo-catalyst itself and the employed cocatalyst; and (3) the impact of the choice of the catalytic metal on the outcome of the process. As a result of the extensive body of research already devoted to TiO2, it is considered a good model system on which these fundamental mechanistic questions can be investigated. The lack of a single measure of efficiency which would allow for an unequivocal comparison of heterogeneous photo-catalytic systems has to be considered as a serious impediment to advancing the field. The situation could be partially ameliorated by standardizing the experimental conditions as well as by specifying at least two of the popular measures, one related to the amount of the catalyst (e.g., formation rate) and the second one related to the strength of the illumination (e.g., photonic efficiency). An alternative would be to devise a new measure that encompasses both relationships. Both approaches would be valuable steps forward for a consistent assessment of the efficiency of any system with respect to others and thereby enable identification of the best performing ones and paths for further improvement. Photo-catalytic reactions taking place in the presence of sacrificial electron donors are inherently short-term, as they only work until the donor is used up. So far, virtually all the studied systems focused on the half-reactions of the CO2 reduction. Full cycle systems, with a firm emphasis on the oxidation half-reaction (e.g., generation of oxygen or hydrogen peroxide), will be essential to allow for long-term studies. This could give insight into whether CO2 photo-reduction is a commercially viable approach for the production of solar fuel. A reliance

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solely on cheap and abundant materials will be an important aspect for the future adoption of this technology, even if small sacrifices in efficiency have to be conceded. In this context, semiconductors such as TiO2 might have an advantage over some novel materials in terms of production costs. Some of the innovations in the later days in photo-catalysis, such as photosensitization, were first studied for use in photo-voltaics. Another such innovation, the use of organic semiconductors has, to our knowledge, not yet been exploited for CO2 reduction catalysis. Although organic photovoltaics are still less efficient than silicon [306], the use of these semiconductors in catalysis may prove to be particularly interesting due to their mutability. The vast scope of synthetic methods available permits tuning of the electronics [307], but functional groups could also be appended to bind and reduce CO2 more effectively. The vast majority of CO2-reduced products are singlecarbon species such as methane, methanol, carbon monoxide, and formic acid. Some multicarbon products, primarily light hydrocarbons, have been reported to form but typically in low yields. Whereas the established Fischer-Tropsch process is capable of transforming solar-generated CO and H2 to liquids, an important advance in catalysis would be the development of materials that can make carbon
carbon bonds and, thus, higher order hydrocarbons and alcohols at ambient conditions directly from CO2, water, and light. In terms of reactor design aspects, one significant problem in the photo-catalytic redox reactions is the separation of the two half-reactions to avoid reverse processes. The photo-electrochemical cells—hybrid systems of photovoltaic and photo-catalytic devices—offer a possibility to effectively address this issue and utilize the advantages of both types of solar devices, and more attention has to be paid in future studies. The heterogeneous photo-catalytic reduction of CO2 on semiconductors is not yet ready for implementation in real-life solar fuel applications. However, with the current pace of developments in the field, the emerging understanding of the mechanism, as well as novel materials should bring the promise held by this process much closer to fulfillment in the near future.

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FURTHER READING Y.T. Liang, B.K. Vijayan, K.A. Gray, M.C. Hersam, Nano Lett. 11 (2011) 2865
2870.