Titania based catalysts for photoreduction of carbon dioxide - NOPR

Indian Journal of Chemistry Vol. 51A, Sept-Oct 2012, pp. 1263-1283

Titania based catalysts for photoreduction of carbon dioxide: Role of modifiers V Jeyalakshmia, b, R Mahalakshmya, K R Krishnamurthyb & B Viswanathanb, * a

Department of Chemistry, Thiagarajar College, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India

b

National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India Email: [email protected] Received 24 May 2012; revised and accepted 22 June 2012

Photocatalytic conversions on titania utilizing sunlight as the energy source have been studied extensively for a variety of processes/ synthesis, like removal of pollutants in air and liquid streams, self-cleaning, anti-fogging and anti-bacterial applications, splitting of water into hydrogen and oxygen and photoreduction of CO2 by water to yield hydrocarbons. These processes are receiving global attention as an off-shoot of the frantic search for alternative energy sources. Though titania continues to be the preferred catalyst in view of its low toxicity, ability to resist photo-corrosion, versatility, and abundant availability at low cost, critical limitations do exist in terms of its inability to get activated with visible light and in achieving high conversion efficiency and quantum yield. Several techniques of modifying titania to improve its performance have evolved over the years resulting in correlations and concepts on structure-property-activity and the role of preparation methods. Such modifications have lead to changes in light absorption efficiency, electronic structure, energy levels, morphology, phase composition and other photophysical properties with moderate improvements in the performance. Efforts to understand the mode of action of the modifiers in terms of the first principles, i.e., rationalization of the activity in terms of electronic and structural properties and establishing theoretical basis for the photocatalytic action, have met with only partial success, due to conflicting observations/results. The objectives towards modifications, namely, extending the light absorption range, retarding charge carrier re-combination, facilitating their fast transport to the active sites on titania surface and incorporation of active elements suitable for redox reactions, have been achieved to a reasonable level. However, commensurate improvement in activity/CO2 conversion has not been observed. Maximization of selectivity (to methane or methanol) and arresting catalyst deactivation are the two major issues yet to be understood in clear terms. An in-depth study to understand the surface transformations at molecular level under activation by light energy, is needed to achieve further improvements in the activity of the catalysts and the process. This review brings forth an account of the investigations on modified titania, capturing some significant and selected contributions out of the vast literature available, with an emphasis on application for photocatalytic reduction of CO2 with water. Keywords: Photocatalysts, Photoreduction, Carbon dioxide photoreduction, Photophysical properties, Titania, Solar energy

Energy and environment are the two most critical issues that the modern society is seriously concerned about. Photocatalysis is emerging as one of the possible means that can provide viable solutions to the challenges from these two areas. Solutions in the form of different photocatalytic conversions/processes are at various stages of development. The key applications on the energy front utilizing the abundantly available sunlight are production of hydrogen by splitting of water using a photocatalyst, generation of electricity using solar cells (photovoltaic cells) and photocatalytic production of methane and other hydrocarbons (solar fuels) from CO2 and water (artificial photosynthesis). Removal of pollutants from the industrial effluents and drinking water (Advanced Oxidation Processes),

purification of air, self-cleaning, anti-fogging and anti-bacterial applications on different materials are the typical applications of photocatalysis towards environmental protection. Some like self-cleaning, anti-fogging and anti-bacterial applications are already being practiced on large scale.1-4 Historically, these processes center around titania and titania-based catalysts. Efforts to develop alternative catalysts, based on various metal oxides/sulphides/nitrides/phosphides, layered titanates, binary and ternary oxides of Nb, Ta, Ga and In in conjunction with alkaline, alkaline earth and rare earth oxides and with a host of co-catalysts and sensitizers, constitute the central theme of current research in this area.5-6

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Titania continues to be the preferred and hence extensively studied catalyst due to its low toxicity, ability to resist photo-corrosion, versatility in terms of wide spread applications and above all, abundant availability at low cost. In spite of these virtues, titania fails to fulfil some of the essential criteria for an ideal photocatalyst,7 like, stable and sustained photocatalytic activity, good overlap of absorption cross-section with solar spectrum, high conversion efficiency and quantum yield. Two major drawbacks with titania, that affect its overall efficiency as photocatalyst are: (i) Absorption of only a small fraction (< 5 %) in the UV region of the solar spectrum, in accordance with its band-gap of 3.0 – 3.2 eV. No absorption is observed in the visible region (λ > 400 nm) which constitutes the major part of the solar spectrum, and, (ii) Low interfacial charge-transfer rates of photo-generated charge carriers and consequently high recombination rate.8 Hence, intensive research efforts9-17 are being pursued to suitably modify the photophysical properties of titania and circumvent these shortcomings. The Need for Modifications The photocatalytic activity of a typical semiconductor like titania is initiated by the absorption of light energy corresponding to/or higher than the band gap energy, resulting in the generation of electrons and holes. The electron-hole pair, on migration to the semiconductor surface, interacts with the adsorbed reactants to facilitate reduction and oxidation process respectively. In the absence of such an energy transfer, the pairs recombine with the loss of energy. Electron-hole recombination process, which is two to three orders of magnitude faster, competes with the desirable redox processes. Therefore, the recombination process needs to be minimized by suitable modifications to increase the efficiency. Hence, modifications in TiO2 are aimed at dealing with the two major issues, namely, extending light absorption range beyond UV region, and, arresting the recombination of charge carriers. Several other modifications as detailed in the review have been attempted to improve the activity further. By suitable modification of the band gap, i.e., reduction in band gap by creation of additional/impurity energy levels, light absorption

range/wavelength could be increased. In the case of titania, light absorption range is increased to cover a part of the visible region.8-9 Arresting the recombination rate increases the life time of photo-electrons and holes, thus leading to a corresponding increase in photocatalytic activity. Highly conducting materials like carbon nanotubes (CNT) and graphene, when added to titania facilitate faster transport of the photo-generated electrons, minimizing further the charge carrier recombination (vide infra). It is observed that similar approaches for modification of titania have been adopted in all the three major applications, namely, removal of pollutants, splitting of water and reduction of CO2 with water. The reaction pathways and hence the required photophysical characteristics being different for each process, the types of modifications required could as well be different. Amongst the three major processes, photocatalytic reduction of CO2 involving multi-electron transfer steps, is highly complex and difficult and its reaction mechanism is yet to be established.5b,16b The objective of this review is to critically analyze the general approaches adopted for the modifications of titania for all the processes and then examine the type of modifications and characteristics that are essential for photocatalytic reduction of CO2 with water. Modifications in Titania – General Approaches Implications of modifications on the electronic structure of titania

A number of strategies, as listed below have been devised to bring about the modifications in the electronic structure of titania, with a view to overcome the drawbacks indicated earlier. These include: (i) Doping with metals, cations and anions (ii) Coupling with other semiconducting oxides (iii) Sensitization using light harvesting compounds/ dye molecules (iv) Plasmon resonance induced by specific metals Excellent and exhaustive reviews have been compiled describing the changes brought about by the above strategies in the electronic and photophysical properties of titania and the consequent improvements observed in the photocatalytic activity for various reactions.9-17 The main features of these modifications and the implications in the properties of titania are summarized in the Table 1. Schematic representation

JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS

of these modifications and the implications in each case are illustrated in the Fig. 1. Doped metals act as electron traps, facilitating charge separation and preventing recombination. Experimental evidence towards this effect was presented by Anpo et al.13 in their studies on Pt loaded on TiO2. As shown in Fig. 2, when irradiated with UV light, pure titania exhibits ESR signals attributed to Ti3+ formed by the photo-generated electrons. On loading the TiO2 with Pt, the intensity of the ESR signal due to Ti3+ falls sharply due to

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the transfer of electrons from titania to Pt, while the holes remain in TiO2, resulting in charge separation. Introduction of anions like N, S in TiO2 results in narrowing of band gap due to mixing of p states of dopants (N,S) with oxygen 2p states in the valance band of TiO2 and creation of impurity states above the valence band of titania.8, 9, 12 Such changes in the electronic structure of TiO2 facilitate visible light absorption. Another strategy to extend the light absorption range by TiO2 is to couple it with a narrow

Table 1 – Implications of different modifications in titania Modifications

Implications/Mode of action

Doping of metals/metal ions

• • •

Acts as electron traps and facilitate charge carrier separation Introduces impurity states and induce visible light absorption Absorbs visible light via surface plasmon resonance

Doping of anions

•

Narrowing of band gap due to mixing of p states of dopants (N,S) with O 2p states in the valance band of TiO2 Introduces impurity states above the valence band of titania Both states induce visible light absorption

• • •

Coupling with semi-conductors

• Sensitization with light harvesting components/dyes

• •

A narrow band gap semiconductor, with appropriate energy levels, absorbs visible light and transfers excited electrons into the conduction band of titania. UV light source not needed Besides visible light activity, effective separation of charge carriers is achieved Light absorbing components can absorb visible light and inject photo-excited electrons into the conduction band of titania Besides visible light activity, effective separation of charge carriers is achieved

Fig. 1 – Implications of modifiers on the electronic structure of titania.

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• possess higher electronegativity than partially filled electronic configuration, and, • capacity to trap and de-trap both e- /h+.

Fig. 2 – Growth of the ESR signal intensity of the photoformed Ti3+ active site on Pt-loaded and unloaded on TiO2 catalysts (record at 77 K). [Reproduced from Ref. 13 with permission from Elsevier, Amsterdam, The Netherlands].

band semiconductor (for example CuO with TiO2) which can be activated by visible light and injecting the photo-generated electron directly into the conduction band of titania.8, 9 Sensitization by light harvesting dyes/macrocyclic ligands (phthalocyanins, porphyrins) that transfer photo-generated electrons directly into the conduction band of TiO2, is another means of using the abundantly available visible light region in sunlight.8,9 Both strategies have inherent limitations. Coupling is restricted to those narrow band semiconductors with band energy levels appropriate for electron transfer to conduction band of TiO2. In the case of dye sensitization, stability of dyes and ease of regenerability are the concerns. In spite of such limitations, both strategies, especially dye sensitization, are utilized for several photocatalysts, including TiO2. Addition of methanol or SO32- as hole scavengers or Ag+, Fe3+ as electron scavengers is another means of restricting charge recombination and improving the quantum efficiency. Examples that illustrate the effect of the modifications in titania are presented in the following sections. Doping of metals

Choice of metals/metal ions for doping depends on several criteria like17: • ionic radii to be closer to that of Ti4+, • propensity to exhibit multiple oxidation states, • Mn+ /M(n+1) energy levels should lie closer to Ti3+ /Ti4+,

Ti,

Effect of metal ion doping on the characteristics of titania and the improvements observed in its photocatalytic activity for a variety of reactions has been studied extensively.3,9,11-14,18-22 A comprehensive study on the effects of the doping of 21 metal ions on titania has been reported by Choi et al.17, who observed that doping significantly influences photoreactivity for CHCl3 degradation, charge carrier recombination rates and interfacial electron-transfer rates. Doping titania with Fe3+, Mo5+, Ru3+,Os3+, Re5+, V4+ and Rh3+ at 0.1–0.5 at.% significantly increases the photocatalytic activity for both oxidation and reduction reactions, while Co3+ and Al3+ doping decreases the activity. Photoreactivity increases with the relative concentration of trapped charge carriers and appears to be a complex function of the dopant concentration, the energy level of dopants within the TiO2 lattice, their d electronic configuration, the distribution of dopants, the electron donor concentration and the light intensity. Doping of anions

Though a number of anions from C, N, F, P and S have been used for doping titania, studies on doping with nitrogen are predominant.9-16 According to Asahi et al.22,23 substitutional type of doping is possible by mixing of the 2p states of oxygen from titania with the 2p states of N resulting in intra-band states, which in turn can effectively narrow down the band gap, thereby facilitating visible light absorption. Interstitial type doping is found to be ineffective. In contrast, Valentin et al.24,25 have proposed that absorption of nitrogen in substitutional state is more effective in the formation of localized levels within the band gap and concluded that both substitutional and interstitial nitrogen may exist with energy levels as shown in Fig. 3, depending on the preparation conditions. Two N1s XPS peaks observed for N-doped titania, one at 396 eV for substitutional and the other at 400 eV for interstitial nitrogen species, lend credence to this theory.13 While the assignment of the XPS peak at 396 eV for susbstitutional N species has been corroborated by other researchers,26,27 there is a raging controversy on the proposed peak due to interstitial N species at 400 eV and has been attributed to different species like N-O-Ti-O or O-N-Ti-O.28-29


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Fig. 3 – Electronic band structure for (a) substitutional and (b) interstitial N doped anatase TiO2. [Reproduced from Ref. 24 with permission from American Chemical Society, Washington, USA].

It is well-known that the preparation history of doped titania (type of precursors, level of doping, method of preparation and pre-treatment) profoundly influences the type of nitrogen species, characteristics and activity in visible light and these observations hold good for titania doped with S,30,31 C,32,33 F34,35 and B36,37 as well. Conflicting views emerge from different studies on the mode of action, enhancements in visible light absorption and photocatalytic activity. Co-doping

An interesting and useful outcome of the extensive studies on the effects of doping on titania is the process of co-doping and the synergistic effects observed therein. Existence of synergistic effects in metal-non-metal co-doped systems has been established with sound theoretical basis.38 Long and English38 have investigated the co-doping effect of TM metal (TM = W, Re, Os) and X non-metal (X = N, C) on titania from first principles calculations. They found that co-doping indeed narrows the band gap of titania and that (N+ W) co-doping is highly effective amongst the other co-doping combinations studied. This theoretical deduction is in line with the experimental observations on W doping on N-doped titania.39,40 Co-doping is possible with (metal-metal), (metalnon-metal) and (non-metal-non-metal) pairs. In particular, metal and non-metal co-doping could lead to two distinct effects; first, narrowing of band gap by forming acceptor and donor levels, and, second, emergence of two different hetero structures with respect to titania. While the non-metal could enhance visible light absorption, the metal can facilitate the transfer of charge carriers thus retarding

Fig. 4 – Photogeneration and separation of charge carriers in Cu2O-TiO2 hetero-structure in UV and visible regions. [Reproduced from Ref. 49 with permission from Elsevier, Amsterdam, The Netherlands].

recombination. The critical factors are the choice of the pair for co-doping, level of doping and effective method of introducing dopants. Several pairs of co-dopants, metal-metal,41-43 metal-non-metal44 as well as non-metal- non-metal45-47combinations have been reported to be highly effective due to the synergistic effects. Driven by the enormous application potential of titania based photocatalytic processes for hydrogen generation by splitting of water and removal of pollutants in air and water, there has been an explosive growth of publications dealing with various strategies for modification, covering comprehensively the experimental as well as theoretical approaches. A special issue48 on “Doping and functionalization of photoactive semiconducting metal oxides” focusing on major scientific pursuits on modifications in titania has been published. Coupling with semiconductors

One of the elegant examples of coupled semiconductors is the heterostructured Cu2O-TiO2 system illustrated (Fig. 4) by Huang et al.49 Cu2O with a band gap of 2.0 eV is a typical narrow band semiconductor with both conduction and valence bands located above those of TiO2, a state which thermodynamically favours the transfer of excited electrons and holes between them. This state of

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Fig. 5 – Photocatalytic activity of Au/ TiO2 under (a) UV radiation, and, (b) in visible region due to excitation by surface plasmon band. [Reproduced from Ref. 59 with permission from American Chemical Society, Washington, USA].

energy levels facilitates all the four charge transfer processes indicated in Fig. 4, in UV as well as in visible regions. While processes 1 and 2 can proceed with UV as well as visible radiation, processes 3 and 4 require UV radiation. All four processes together constitute a highly effective coupled semiconductor system, with very high efficiency. Heterostructures with 30 % and 70 % Cu2O, both under UV and vis radiation display 6 times and 27 times higher activity respectively, compared to the base material, P-25.49 Examples wherein narrow band semiconductors like PbS, CdS and CdSe are used to form effective heterostructures with TiO2 are known.50-52 Inherent drawbacks with such systems are photo-corrosion and charge carrier recombination. In such cases electron donors like sulfide or sulfite are added as hole scavengers. Surface sensitization with dyes

Certain organic dyes can harvest visible light and inject photo-generated electrons into the conduction band of titania. Various dyes such as erythrosine B, eosin, rose bengal, rhodamine B, crestalviolet, thionine, chlorophyllin, porphyrins, phthalocyanines, and carbocyanines have been used as sensitizers.53-55 Since the dyes undergo oxidative degradation during the process, redox couples are often employed to regenerate the sensitizer. Principles of dye sensitization have been extensively applied in the development of dye sensitized solar cells (DSSC).56,57 Plasmonic photocatalysis

Increase in photocatalytic activity for decomposition of methyl blue under UV radiation by silver nanoparticles embedded in titania was first noticed by Awazu et al.58 It was attributed to the Surface Plasmon Resonance (SPR) displayed by the Ag nano particles and hence, was termed as plasmonic catalysis. Besides UV radiation, SPR provides additional means of excitation in the visible region. Resonance wavelength depends on the nanoparticle size and shape.

Fig. 6 – Photocatalytic splitting of water by Au/TiO2 enhanced by SPR effect. [Reproduced from Ref. 60 with permission from American Chemical Society, Washington, USA].

Nanoparticles of gold also display SPR. Silva et al.59 applied this concept in their studies on photocatalytic splitting of water using Au nanoparticles supported on P-25 titania in UV as well as visible light regions (> 400 nm). It was observed that Au nanoparticles play a dual role (Fig. 5). With UV light, Au nanoparticles act as electron traps and facilitate H2 gas evolution. In the visible region, plasmonic photocatalysis takes place by injection of electrons into the conduction band of titania. It was observed that the gold loading, catalyst preparation and pre-treatment conditions influence the Au nanoparticle size and activity. Maximum activity was displayed by 0.2 wt% Au (Au crystallite size-1.87 nm) loaded on titania. The catalyst was prepared by precipitation of Au as hydroxide from HAuCl4 solution using 0.2 M NaOH at pH 9, followed by calcination at 200 °C. Chen et al.60 elucidated the effect of SPR (Fig. 6) by carrying out similar studies on Au/TiO2 using radiation of different wavelengths. Maximum


hydrogen and oxygen evolution rates (53.75 and 25.87 µM/g of catalyst respectively) were observed while using UV and visible radiations together whereas the corresponding yields were less (35.04 and 17.52 µM/g of catalyst) when only UV light was used. No activity was observed with only visible light. These observations clearly bring out the role of SPR in utilization of the full spectrum of sunlight. Au nanoparticles act as electron traps in UV region and promote excitation through SPR with visible light. Hu et al.61 have added another dimension to the effect of SPR. With Au nano particles deposited on TiO2, plasmonic enhancement of the photocatalytic reduction of CO2 with H2O is observed under visible light (λ-532 nm). With the same catalyst, when irradiated with high energy UV light (λ-254 nm), inter-band (d-band) transitions occur within Au, resulting in excitation to a level higher than the conduction band of TiO2. This set of excitons provide additional means of photocatalytic conversions and products, thus maximizing overall activity. Modifications in Physicochemical Properties Besides the electronic properties, a number of other characteristics of titania like particle size, bulk and surface crystal structure, morphology, textural properties and nano size characteristics affect its performance. Many of these characteristics are acquired during the preparation and pre-treatment stages and this aspect explains the importance of preparation conditions. Lattice defects and impurities

Lattice defects and trace impurities can provide intermediate energy levels within the band gap, thus facilitating excitations from valence band to intermediate levels and then from that level to conduction band, besides direct valence band to conduction band transitions.62 Pre-treatment processes adopted for titania e.g., oxidation/reduction, may induce lattice defects like oxygen vacancies, electron deficient oxygen, Ti vacancies and interstitial Ti.63-65 Oxygen vacancies and Ti interstitials are known to generate donor defect levels at 0.75–1.18 and 1.23–1.56 eV below the conduction band of TiO2, respectively, while Ti vacancies lead to the acceptor defect levels located above the valence band of titania.66,67 Oxygen vacancies due to doping can effectively capture electrons.63-65

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Quantum size effects

Photocatalytic reactions include the following major steps: • Photo-generation of charge carriers by excitation, • Bulk diffusion and interfacial transfer of charge carriers to surface species, and, • Reduction/oxidation and further conversion of surface species Effective charge transfer to the surface species with minimum loss due to recombination (life-time of charge carriers) is an important parameter. While metal ion doping could reduce the degree of recombination by electron trapping, titania particle size could play a different role. Quantum size effect, applicable to particles < 100 nm, leads to an increase in the band-gap (resulting in blue-shift), thus improving the photocatalytic activity. In addition, this effect induces specific electronic modifications within TiO2 nanoparticles that ensure close existence of charge carriers and effective transfer to surface species.14,68 However, according to Serpone et al.69 size quantization effect may not be observed below a threshold value. Surface structure

Observations on quantum size effect on nano sized titania opened up several approaches towards design of highly active and dispersed titania catalysts.14 Anchoring of highly dispersed titania on various inert supports, incorporation within zeolite networks, supporting on high surface area binary oxides and incorporation of titania in the zeolite framework provided the means of exposing titania in different structural environments on the surface (isolated titania clusters in tetrahedral/octahedral environments). Anpo et al.70,71a observed that at lower Ti content, small/fine crystallites of TiO2 displayed high activity for photocatalytic hydrogenolysis of unsaturated hydrocarbons with water, decomposition of NO into N2 and O2 and reduction of CO2 with water. Highly dispersed and isolated TiO2 within the zeolite framework, exposed in tetrahedral coordination, were found to be selective for methanol formation during reduction of CO2 with water. Correlation established between co-ordination number of titania in zeolites, (determined by EXAFS analysis) and the selectivity for methanol formation is shown in Fig. 7. Influence of surface structure of titania on activity and selectivity during CO2 photoreduction was

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Fig. 7 – Correlation between selectivity for methanol formation and coordination number of titania. [Reproduced from Ref. 13 with permission from Elsevier, Amsterdam, The Netherlands].

observed earlier by Anpo et al.70,71a in their studies on single crystal surfaces (100) and (110) of TiO2. While the formation of both methanol and methane could be observed on TiO2 (100), only methanol could be observed on TiO2 (110). Surface density and coordination environment of Ti ions strongly influence the activity/selectivity. Besides, carbon dioxide is known to get adsorbed preferentially on four and five coordinated surface sites.71a Our recent investigations on CO2 photoreduction on different titania catalysts have shown that methane and methanol are formed on five co-ordinated surface Ti4+ while on four co-ordinated surface Ti4+ ethanol is formed by dimerization of methyl and methylene species.71b It is pertinent to mention that the kind of structural features discussed above are acquired during the preparation of titania,72-75 and hence, preparation methods/techniques play a vital role in evolving the surface structures that direct the transformation of the transient species on the surface to various products. Photocatalytic Reduction of CO2 by Water Process steps

Photocatalytic reduction of carbon dioxide with water on semiconductor oxide catalyst surfaces using solar energy to yield fuels/chemicals (methane, methanol, etc.,) has the potential to become a viable and sustainable alternative energy source to fossil fuels.76-79 The process involves two major steps, (i) splitting of water to yield hydrogen, which in turn helps in the (ii) reduction of carbon dioxide to

Fig. 8 – Conduction band and valence band potentials for photocatalysts versus energy levels of redox couples in water. [Reproduced from Ref. 79 from Nature Publishing Group, London, UK].

different hydrocarbon products in the second step. Design of effective catalysts for such a complex process involving multi-electron transfer steps, holds the key for the viability of the process. Ideal catalysts are expected to display maximum efficiency towards solar energy absorption and possess requisite band energy levels to drive the redox reactions on the semi-conducting catalyst surface (Fig. 8). Functionally, the catalysts should have valence band top energy level suitable for splitting of water to generate hydrogen, which is the primary step. The second and the most crucial step is the reduction of CO2 to hydrocarbons, which requires the bottom energy level of the conduction band to be more negative with respect to reduction potential for CO2. It is essential that the photo-generated electrons and holes are spatially separated so that they have a sufficient life time to initiate redox reactions on titania surface. In this respect, life time of the photo-generated electrons is a crucial factor for the photoreduction of water as well as CO2. A complex sequence of process steps involving two, four, six or eight electrons for reduction, lead to the formation of formic acid/CO, formaldehyde, methanol and methane respectively,80-81 depending on the type of the catalyst and reaction conditions. Thermodynamics and kinetics

Reduction potentials for water splitting and reduction of CO2 to various reduced products with


reference to NHE at pH 7 are given below. Conduction band electrons of titania at Ecb = – 0.5 V have sufficient energy to reduce CO2 to CH4 (E0 – (CO2/CH4) = –0.24 V). In fact, based on the reduction potentials, methane formation is a thermodynamically facile process vis-a-vis hydrogen evolution E0–(H+/H2) = –0.41 V. However, hydrogen evolution is kinetically faster than methane formation, since eight electrons are involved in the latter process. Hence, facile generation of photoelectrons with longer life cycles is the primary criterion. 2H+ + 2e-

H2

CO2 + 1eCO2 + 2H+ + 2e-

CO2.CO + H2O

CO2 + 4H+ + 4eCO2 + 2H+ + 2eCO2 + 4H+ + 4e-

C + 2H2O HCOOH HCHO + H2O

CO2 + 6H+ + 6eCO2 + 8H+ + 8e-

CH3OH + H2O CH4 + 2H2O

Eo = -0.41V Eo = -2 V Eo = -0.52 V Eo = -0.20 V Eo = -0.61 V Eo = -0.48 V Eo = -0.38 V Eo = -0.24 V

Unique features of CO2 photoreduction

Activity for hydrogen generation from water is a necessary but is not sufficient condition for a catalyst to be active for CO2 reduction. Conduction band bottom energy level has to be more negative vis-à-vis CO2 reduction potential. Besides, metal, metal oxide, and anionic dopants, structural and textural features, morphology and addition of sensitizers have different roles to play in the modification of photophysical properties and photocatalytic activity. Several examples to this effect are enumerated in the following sections. Role of metal dopants in TiO2 for CO2 photoreduction

Doping of titania with different metals enhances the activity by minimization of electron-hole recombination since they act as electron traps and facilitate CO2 reduction process by photo-generated electrons. Besides improvement in activity, distinct selectivity patterns with respect to different metals have been observed. While the incorporation of metals like Pd, Pt on TiO2 results in preferential formation of methane, methanol is the preferred product on titania doped with Cu oxide. Among the series of metal doped titania (metal loading of 2 % w/w on P-25), Ishitani et al.82a have observed that the rate of methane formation (values within brackets, expressed as µM/g cat./h) during CO2 photoreduction in water decreases in the following order: Pd (32.9) >

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Rh (13.3) > Pt (6.7) >Au (4.4) > Cu (2.5) > Ru (0.8). However, the exact mode of action of these metals/ co-catalysts in terms of their electronic, structural and surface reactivity characteristics in promoting methane or methanol formation is yet to be understood. Observations by Dzhabiev et al.82b on CO2 photoreduction (Table 2) on different metals loaded on TiO2 and SrTiO3 supports prove this point further. Significant variations in products selectivity is observed, thereby unravelling the crucial role played by metals in directing the surface reaction pathways. Considering the fact that CO2 photoreduction involves two processes, splitting of water to yield hydrogen and reduction of CO2 to hydrocarbons, it is seen from the rate data in Table 2, the rate for H2 formation is 6-7 times faster on Pt/ TiO2 vis-à-vis Pt/ SrTiO3. Rh/SrTiO3 is more selective towards H2 formation than Pt/SrTiO3. Other metals like Ag and Pt-Ru supported on SrTiO3 form CO indicating that reduction of CO2 is partial, up to CO only. Possibly the differences in the mode of activation of CO2 on these metals, modulated with the nature of the supports, could explain the rate data for various products. Studies in this direction could lead to catalysts with higher selectivity for methane. A series of publications by Solymosi et al.83-85 on the photo-assisted conversion of CO2 + H2O brings out the effect of modification of TiO2 with WO3, loading of Rh and of oxidation state of Rh on product formation. Pure TiO2 at 333 K yielded 0.283 µmoles/h of HCOOH and 0.594 µmoles/h of HCHO with total conversion of 0.877 µmoles/h. Doping with 0.1 wt% WO3 brings down the activity (0.404 µmoles/h) with respect to pure TiO2 (0.877 µmoles/h), but selectivity shifts totally to formaldehyde (0.404 µmoles/h). Increasing WO3 loading to 2 % does not change the activity or selectivity. Introduction of 1 % Rh on TiO2 lowers total activity (0.479 µmoles/h), but methanol Table 2 − Product patterns for photocatalytic reduction of CO2 by water on metal doped semiconductor oxidesa Catalyst Pt/TiO2 Pt/SrTiO3 Rh/SrTiO3 Ag/SrTiO3 Pt-Ru/SrTiO3 a

Ref. 82b.

Ratio of initial rates of product formation CH4:H2:CO 1.2 : 8.0 : 0.0 1.0 :1.0: 0.0 1.0 : 250.0 : 0.0 1.0 : 7.0:15.0 1.0 : 6.0 : 1.2

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appears as additional product. On TiO2+2WO3 modified support, overall conversion increases to 0.669 µmoles/h. On reduction of Rh to metallic state, both formaldehyde and formic acid are converted to methanol, with an associated increase in total conversion to 0.7µmoles/h. However, when external hydrogen is used for photo-assisted conversion of CO2 on Rh/TiO2, CO is formed as the main product in the oxidized state and methane in the reduced state86. Thus the nature of the support and the oxidation state of metal affect overall conversion and selectivity. Rh in metallic state facilitates the supply of photoelectrons towards deeper reduction products. Methane and methanol are formed when Rh is in the metallic state and HCOOH and HCHO in the oxide form. Ru and Os colloids, in homogeneous phase, with Ru (bpy)32+ as photo-sensitizer, tri-ethanolamine as electron donor, and N,N'-dimethyl 2,2'-bipyridinium as charge relay, lead to the conversion of CO2 to methane, ethylene and hydrogen. However, changing over to Ru(II) tris(bipyrazine) as sensitizer results in methane, ethylene and ethane but with no evolution of hydrogen.87 Ligand modifications direct the selectivity in homogeneous phase. In the heterogeneous phase, Ru/TiO2 follows a different pathway. Formaldehyde, formic acid and methanol were observed for Ru loaded on TiO2.88 Ru supported on titania displays higher conversion of

Fig. 9 – Products distribution of the photocatalytic reduction of CO2 with H2O on the (a) anatase TiO2 powder, (b) the imp-Tioxide/Y-zeolite (10.0 wt% as TiO2), (c) the imp-Ti-oxide/ Y-zeolite (1.0wt % as TiO2), (d) the ex-Ti-oxide/Y-zeolite, (e) the Pt-loaded ex-Ti-oxide/Y zeolite catalysts. [Reproduced from Ref. 92 with permission from American Chemical Society, Washington, USA].

CO2 to methane and methanol compared to bare titania support. However, when Ru is supported on same titania (10 % w/w) in the dispersed state on high surface area silica, CO2 conversion decreases, possibly due to the inhibition of Ti-O-Si bonds formation by Ru.89 Earlier studies on Pt or Pd supported titania were carried out with aqueous carbonate/bicarbonate media which resulted in formaldehyde and formic acid respectively.90, 91 Anpo and co-workers92 established the crucial role of Pt in increasing CO2 conversion and selectivity towards methane formation (Fig. 9). Dispersion of titania on zeolite-Y resulted in the formation of isolated TiO2 with Ti in tetrahedral coordination which facilitates selective formation of methanol, while Ti in octahedral coordination leads to methane formation. Doping with Pt invariably results in methane formation. Zhang et al.93 also observed that Pt loaded on to nano sized titania powder and titania nanotubes yielded exclusively methane (Fig. 10). Pd loading (0.5 wt%) on titania, which has been made free of any adsorbed hydrocarbon impurities, leads to mainly methane, with a little CO. Prolonged irradiation caused deactivation of the catalyst due to oxidation of Pd to PdO.94 Nature of the support influences surface reaction pathways of Pd catalysts. Pd when supported on titania, whose acid-base properties are modified by addition of Al2O3, SiO2 and MgO, yields different products.95 Acidic supports yield C1 products, (CH4 and CH3OH), while basic supports yield C2 hydrocarbons (C2H6, C2H5OH, CH3COCH3). Sasikala et al.96 have also reported that titania when dispersed on zeolites (NaY) and zirconia displays better photocatalytic activity (for hydrogen generation from water) as

Fig. 10 – Effect of Pt metal content in Pt/TO-NP on methane yield for photocatalytic reduction of CO2 after 7 h irradiation at 323 K. (H2O/CO2= 0.02). [Reproduced from Ref. 93 with permission from Elsevier, Amsterdam, The Netherlands].


compared to dispersion on ceria or alumina. The acidity of the support also plays a key role. Promotion with metals like Pt,93 Ru,89 Rh,27 and Ag97 vastly enhances the rate in several ways, e.g., charge separation, retarding re-combination and trapping of charge carriers, besides activation of CO2 and water reduction, thus facilitating further surface transformations leading to hydrocarbon products. Doping of titania (prepared by sol-gel inverse micelle method) with silver increases CO2 photoreduction (Fig. 11), leading to increase in the formation of methane and methanol.97 Upto Ag loading of 5 %, impurity levels formed within the band gap of titania extend the light absorption range8, and beyond this level, Ag forms metal clusters that retard electron-hole recombination via electron trapping. Electron transfer from conduction band of TiO2 to the metallic silver particles could take place at the interface since the Fermi level of TiO2 is higher than that of silver metal.98 Such a phenomenon results in the formation of Schottky barrier at the Ag-TiO2 contact region, which increases the life of charge carriers and hence improvement in the photocatalytic activity.99 Similarly, AgBr/TiO2 nanocomposites also display very high activity for CO2 photoreduction under visible light.100 A unique feature with AgBr promoted titania catalyst is that besides methane and

Fig. 11 – Dependence of product yields on Ag content in Ag/TiO2 (after 24 hrs). [Reproduced from Ref. 97 with permission from Elsevier, Amsterdam].

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methanol, ethanol is also observed in measurable quantities. While ethanol is one of the minor products with the titania support, loading with Ag significantly increases the formation ethanol. Formation of ethanol is expected to proceed by dimerization of transient surface C1 species, but mechanistic pathways to this end is yet to emerge. Using optical fibre photo-reactor, Wu101 could give a methanol yield of 4.12 µmoles/g cat./h, with 1 % Ag/TiO2 catalyst under UV light intensity of 10 W/cm2. Modifications with metal oxides

As discussed vide supra, coupling of metal oxides that possess electronic band energy levels suitable for charge transfer, with TiO2 is one of the options available for increasing its photocatalytic activity. Copper oxide for coupling with TiO2 has been investigated by several authors. Use of Cu (as oxides) as a co-catalyst was reported by Adachi et al.102 wherein Cu (≤ 5 wt%) loaded TiO2 powder was suspended in a CO2 pressurized (to 50 kg/cm2 initially) aqueous solution at ambient temperature. Under Xe lamp illumination, methane, ethylene and ethane are formed with attendant decrease in pressure to 28 kg/cm2. Dimerization of surface ·CH2 species was proposed as the route for the formation of ethylene. However other investigators observed methanol as the major product when the reaction was carried out at atmospheric pressure. Tseng et al.103-105 studied the effect of copper loading on titania prepared by sol-gel technique. With an optimum Cu loading of 2 wt%, methanol yield of 118 µmoles/g cat. was observed after 6 h of UV illumination. It was proposed that the redistribution of the electric charge and the Schotky barrier of Cu and TiO2 facilitates electron trapping via supported Cu. Lowering of the combination probability for holeelectron pairs helped in increasing the activity of Cu/TiO2 (quantum efficiency of 10 % and energy efficiency of 2.5 %) versus bare TiO2. Based on XPS analysis, Cu2O was identified as the active phase and reduction of the oxide to metallic Cu affected the activity adversely. Slamet et al.106 in their studies of Cu/TiO2(P-25) observed 3 wt% Cu loading to be optimum and could achieve very high activity, with 2655 µ mol/g of methanol after 6 h of irradiation compared to 809 µmol/g of methanol on bare P-25 TiO2. While Tseng et al.103-105 used TiO2 (anatase) prepared by sol-gel route and 0.2 N aqueous NaOH as the reaction medium, Slamet et al.106 employed P-25 titania (mixture of anatase and rutile)


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and 1 M KHCO3 as the medium. To what extent these factors affect the CO2 photoreduction is not clear. Besides, Slamet et al.103-106 suggest that CuO may be the more active dopant as compared to Cu2O, based on the redox values for Cu2+ and Cu+ and the interaction of Cu species with TiO2 lattice. Cu2+ +2eCu2+ + eCu+ + e-

Cu0 Cu+ Cu0

E0 = 0.34 V E0 = 0.17 V E0 = 0.52 V

With the highest value of redox potential, Cu+ should be more effective for trapping of electrons so as to avoid recombination. For the same reason, interaction of Cu+ with TiO2 lattice would be stronger and, hence, electrons are not easily transferred to the conduction band of TiO2. Therefore, Cu2O in TiO2 could become a centre for recombination and CuO with lower redox potential could become a more effective dopant. According to Roy et al.76 location of the band edge levels of the oxides TiO2, Cu2O and CuO, could promote highly facile charge transfer between the oxide phases. Figure 12 shows the band edge positions of Cu oxides and that of TiO2 in contact with water and CO2. Both CuO (1.6 eV) and Cu2O (2.4 eV) could couple with TiO2 and transfer photo-electrons with UV as well as visible radiations, in line with coupling of the electronic energy levels envisaged by Huang et al.49 as shown in Fig. 4. During CO2 photoreduction, due to the presence of H2 it is possible that CuO gets reduced, though the

Fig. 12 – Band-edge positions of CuO, Cu2O and anatase TiO2 in contact with water vapor and CO2. [Reproduced from Ref. 76 with permission from American Chemical Society, Washington, USA].

extent of reduction is not known. In fact, Li et al.107 in their investigations on Cu/TiO2 dispersed on mesoporous silica have observed that during CO2 photoreduction the colour of the catalyst changes from very light green to dark grey, possibly due to reduction which also signified the onset of deactivation. On exposure to atmospheric oxygen the catalyst readily undergoes re-oxidation. Thus, the exact nature Cu oxide phase during reaction needs to be identified by suitable experimental methods. Synergistic effects due to co-doping

Doping of two metals on titania results in higher hydrocarbon yield as well as changes in product patterns. Varghese et al.108 observed that on nitrogen doped titania nanotube arrays, introduction of Cu yields C1-C4 hydrocarbons up to 104 ppm/cm2/h, and addition of Pt increases the hydrocarbon yields to 116 ppm/cm2/h. However, the product patterns for individual doping were different. Hydrocarbons and CO were predominant with Cu, while with Pt, hydrogen yields were higher. With Cu and Fe (at −0.5 wt% each) dispersed on TiO2-SiO2, CO2 photoreduction with UV radiation yields methane and ethylene, while the bare support TiO2-SiO2 yields only methane.101 This could be due to the fact that the reduction potential for methane is lower than that of ethylene. Synergistic effect of Cu and Fe brings out the formation of methane and ethylene. Wavelength of the radiation is an additional factor that affects the product pattern. Only methane is formed with the same Cu-Fe bimetallic system under natural sunlight (with negligible UV light). Thus the effect of metal doping on titania for CO2 photoreduction appears to be complex, with no clear correlations established between the properties of the dopants and activity/selectivity/product patterns. Variations in metal dopant compositions, (mono, bi-metallic, metal-non-metal) type of radiations used and other experimental conditions adopted make it difficult to arrive at meaningful correlations. Detailed understanding of the activation of CO2 and reaction pathways with different metals could help in arriving at the most efficient metal/pair of metals or metalnon-metal combinations. Modes of activation of CO2 by different metals could play a key role in activity/selectivity improvements. Anions as dopants on TiO2 for CO2 photoreduction

Nitrogen and carbon constitute the most effective anion dopants on titania for CO2 photoreduction.


Varghese et al.108 introduced nitrogen into titania nanotube arrays during its preparation by anodic oxidation process and subsequent annealing at 600 °C. Nitrogen doping with N2p states located above the valence band of titania shifted light adsorption end into the visible range, to 540 nm. Fan et al.109 introduced nitrogen at various levels of doping using urea as the nitrogen precursor by sol-gel route, during controlled hydrolysis of titanium tetrabutoxide in n-butanol medium. Ni2+, with nickel nitrate as precursor, was incorporated as co-dopant during the sol-gel process itself. Synergistic effect of Ni and N co-doping shifted the light absorption edge to 430 nm. With a light source at 365 nm, doping with 4 wt% N and 6 % Ni was found to be optimum as revealed by the methanol yield. Methanol yield of 3 µl/g with bare titania increased to 167.6 µl/g with 4 wt% N doping and further to 253.5 µl/g with co-doping of 6 wt% Ni. While it is clear that N doping does help in extending the light absorption edge, it is also expected that in the process, trap states, i.e., Ti3+, could be created leading to charge recombination.110,111 Sulfur (4−5 wt%) doped titania samples with particle size in the range 8−12 nm have been reported to be very active for the formation of methanol, ethanol and propanol.112 Increase in S doping from 4 to 5.2 % decreases the band gap from 3.31 to 3.25 eV. XPS peak at 160−161 eV indicates the presence of S2−. It is suggested that S-doping and the optimum particle size enhance the photocatalytic activity. Enhancement of photocatalytic activity of titania by carbon based nanomaterials has been studied extensively.113,114 The survey by Leary and Westwood115 covers comprehensively the details and recent progress in the application of TiO2-carbon nanocomposites, using carbon in different forms, e.g., activated carbon, carbon doping, carbon nanotubes, fullerenes, graphene, thin layer carbon coating, nanometric carbon black and carbon in different morphologies. Of the many non-metal dopants, carbon, especially in the nanosize, has the unique ability to circumvent the two major drawbacks with titania, by extending the light absorption edge to visible range and facilitating charge carrier separation. Several other benefits could be realized115, e.g., synthesis of carbon based nanomaterials with well-defined and controllable structure and electrical properties for facile charge transport. Their high surface area ensures availability of a large number of

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active sites for adsorption of reactants and promotion of forward transformation of surface species. It is possibile to incorporate different functional groups on carbon, adding a new dimension to its application. An intimate contact between titania and carbon nanomaterials leads to creation of a number of hetero junctions, ensuring efficient charge transfer to the surface species. While the band gap tuning by carbon doping in titania has been established by several experimental observations as well as theoretical calculations (Table 2 in Ref. 115), it is not clear whether it exists in the lattice as substitutional anion or interstitial cation116. DFT calculations by Valentin et al.117 have shown that at low oxygen concentration, carbon substitutes for oxygen in TiO2 lattice with the formation of oxygen vacancies, while under oxygen rich conditions C atoms occupy the interstitial sites. Along with variations in band gap, several intermediate states are induced. Most of the applications of C-doped titania pertain to removal of pollutants, as exemplified by its activity for degradation of model pollutant compounds/dyes/indicators.115 Results on the effects of co-doping of other anions (N and S) with carbon are controversial. Increase in methylene blue degradation has been observed for C and N co-doped titania, which is attributed to a synergistic effect resulting from band-gap narrowing caused by N substituting for oxygen and surface carbon species acting as a photo-sensitizer.118 With C-S co-doping, no improvement in activity vis-à-vis TiO2 for methanol degradation could be observed by Sun et al.119 though earlier investigations by Tachikawa et al.120 showed a negative effect. Carbon nanotubes (CNTs) possess several characteristics essential for photocatalytic activity, like high-surface area, high concentration of active sites, facile transport of photo-generated electrons leading to minimization of electron-hole recombination and simultaneously facilitating visible light catalysis by modification of band-gap and/or sensitization.121,122 These functions arise through the effective synergistic interactions between the electronic levels of titania and CNT. CNT can (a) act as a sink for electrons (storage capacity of one electron for every 32 C atoms) from the conduction band of titania and inhibit recombination, (b) function as a photo-sensitizer by injecting electrons/holes into conduction/valence band of titania, depending upon the respective energy

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levels, and, (c) create impurity levels thus promoting activation by visible light. Morphology of CNT, especially the aspect ratio, loading of CNT and the degree of dispersion of TiO2 on CNT, as governed by the method of preparation of TiO2-CNT composites, are the factors that define the mode and extent of activity enhancement. Combined effect of carbon doping and its morphology in TiO2-xCx carbon nanotube array in improving the activity for photochemical splitting of water was established by Park et al.123 Similar improvements in activity of nanocomposites of multi-walled carbon nanotubes (MWCNT) supported on titania have been observed for water splitting as well as CO2 photoreduction with water.124-126 [60]-Fullerenes are well-known for their unique electronic properties, which make them interesting materials for applications in photocatalysis.127-130 Fullerenes absorb light in the visible as well as UV regions. In their excited state, they are good electron donors as well as good electron acceptors, capable of accepting as many as six electrons.131-132 Since the excited state of fullerene lies at −0.2 eV (vs NHE) and the conduction band of titania at −0.5 eV, electrons may be transferred from titania to fullerene, leading to the formation of radical [60]-fullerene anion C60−. This anion may then react with species adsorbed at an interface between [60]-fullerene and TiO2, resulting in reduced products. Under certain conditions, fullerene can also act as acceptor as observed by Kamat et al.133,134 and Krishna et al.135, 136 Water soluble polyhydoxy fullerenes (PHF) adsorbed on titania was found to be very effective by Krishna et al. in terms of electron transfer between PHF and TiO2 and consequent enhancement of activity for degradation of procion red dye and degradation of E. coli. Several other examples of enhancement of activity by TiO2-fullerene[60] composites have been cited by Leary and Westwood115 in their review, which pertain to degradation of dyes/pollutants. Some of the unique characteristics of TiO2-fullerene [60]composites (fullerene acting as donor or acceptor of electrons) are yet to be explored for hydrogen generation from water and CO2 photoreduction. Graphene is emerging as a material with vast application potential due to its strikingly different properties like very high surface area (~ 2600 m2/g for exfoliated graphene) on which the electrons can travel without scattering at mobilities exceeding ~15,000 m2 V-1 s-1 at room temperature. In this sense

it is potentially an ideal electron sink and acts as electron transfer-bridge. Graphene has a two dimensional one-atom-thick structure and its surface properties can be tuned via chemical modification. Graphene oxides (GO) are derived by partial chemical oxidation of graphene which expose carboxylic groups on the surface. Titania nanoparticles can be readily anchored to GO by the carboxylic groups. GO containing anchored TiO2 can be dispersed in ethanol. Upon UV irradiation, a part of the photoelectrons generated reduce GO to some extent to yield reduced graphene oxide (RGO) and the remaining electrons are retained by GO matrix. Removal of solvent yields photoreduced grapheneTiO2 composite, which is useful for photocatalytic processes. Kamat et al.137-138 have demonstrated anchoring of TiO2 and Pt separately on RGO nanosheet which acts as a conduit for shuttling of electrons from titania to Pt site where gas evolution could take place. Such a scheme (Fig. 13) would be ideal for solar water splitting process. Many examples of graphene-TiO2 composites displaying better activity as compared to bare TiO2 for photocatalytic degradation of organic pollutants are known115. Graphene obtained by solvent exfoliation (SEG) is expected to be relatively free from structural as well as chemical defects compared to RGO.139-141 Liang et al.142 have prepared two different TiO2-graphene nanocomposites, one by solvent exfoliation in DMF (SEG) and the other by solvent reduced graphene oxide (SRGO) by thermal reduction process. Both nanocomposites have been characterized by Raman spectroscopy, four point probe sheet resistance measurement and UV-vis-NIR optical absorbance spectroscopy. Sheet resistance of SRGO films was ~ 2.4 times higher than that of SEG films, indicating higher electrical mobility in SEG which is expected to facilitate the diffusion of photoelectrons. Both composites were stabilized by ethyl cellulose and then studied for photocatalytic

Fig. 13 – Shuttling of electrons from TiO2 site to Pt site via reduced grapheme nanomat. [Reproduced from Ref. 137 with permission from American Chemical Society, Washington, USA].


activity for oxidation of acetaldehyde and reduction of CO2 with water. SEG composite films exhibited around 2-fold and 7-fold increase in activity for photo-oxidation of acetaldehyde and photoreduction of CO2 respectively as compared to TiO2 alone under visible radiation. No significant activity enhancement was observed with SRGO-TiO2 nanocomposite. Higher activity with SEG nanocomposite is attributed to the higher electrical mobility and facile photoelectron transport on less defective SEG surface. Applications of graphene based composites as photoactive supports are worthy of further investigation. Functionalized carbon nanoparticles could be used for harvesting photons from visible light. Cao et al.143 have observed that a two-step surface functionalization of carbon nanoparticles (< 10 nm) by first refluxing with aqueous nitric acid (2.6 M) followed by treatment with oligomeric poly (ethylene glycol) diamine yields finely dispersed carbon nanoparticles in water. When Au is deposited on suspended nanoparticles by photolysis under UV radiation, UV absorption maxima around 550 nm is observed. Similarly, Pt could be deposited by photolysis. Under UV radiation carbon nanoparticles act as electron donors facilitating the reduction of Au/Pt salts. Functionalized carbon nanoparticles with Pt/Au could then be used as photocatalysts for reduction of CO2 in the most desired visible region (425-720 nm) yielding formic acid as the reduction product. Besides, these catalysts were also active for generation of hydrogen from water. Thus, titania-carbon composites in several configurations continue to receive attention as catalysts for photoreduction of CO2. Modifications in physical properties of titania

The methods/approaches described above aim to improve photocatalytic activity of titania through chemical methods. Besides the control of particle size, several other physical properties like morphology (nanosheets, nanobelts, foams) and pore structure (ordered mesoporous sturcture) and spatial structure (nano tubes) offer several advantages in terms of high surface area, easy transport of charge carriers and effective charge separation which ultimately contribute towards improving photocatalytic activity. Aprile et al.144 have reviewed the effect of “spatial structuring” or controlling nano particles on length scale and encapsulation in nano size cavities of zeolites on photo catalytic activity.

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Encapsulation of titania nano clusters within the zeolite cavities with precise size control leads to significant improvements in photocatalytic activity and these aspects have been extensively covered by Anpo and co-workers145-153 for NOx and CO2 fixation. Further improvements in catalyst design has been reported by Bossmann and co-workers,154 who have encapsulated light harvesting component, Ru(bipy)22+, along with titania so that photoelectrons from the complex could be transferred to the conduction band of TiO2. In this case TiO2 acts as relay, transferring electron to the external Co macrocyclic Co (dphen)33+ unit. Titania based catalysts systems with both chemical as well as physical modifications, i.e., encapsulation and doping of anions/metals have been designed and explored for different photocatalytic applications.155-157 Ordered meso-porous titania with a large surface area and pore volume and nano-sized titania dispersed on mesoporous silica matrix have been studied extensively as catalysts for photocatalytic reduction of CO2 with H2O.89,107,158 The mesoporous structure is expected to facilitate fast electron transfer within the matrix and retard charge recombination, both of which lead to increased photocatalytic activity.145,147,159-161 One-pot sol-gel method107 and evaporation driven self-assembly route in a furnace aerosol reactor (FuAR)158 have been adopted to prepare Cu-TiO2-SiO2 catalysts for photoreduction of CO2 with methane and CO as products. Catalyst preparation technique adopted has resulted in very high dispersion as well as synergy between the active phases, TiO2 and Cu oxide on mesoporous silica matrix, thereby increasing the rate for photocatalytic reduction of CO2 to CO and methane. As shown in Table 3, Li et al.107 observed a distinct synergy effect between the components when the Cu-TiO2-SiO2 prepared by the one-pot sol-gel method is used for CO2 photoreduction. While the nominal effects of dispersion of TiO2 on SiO2 (increase in CO formed from 8.1 to 22.7 µmol/g TiO2/h) and Cu loading on TiO2 Table 3 − CO and CH4 formation on Cu/TiO2 supported on mesoprous silica catalysts – Synergy effecta Catalyst TiO2 TiO2-SiO2 4% Cu/TiO2 0.5% Cu/TiO2-SiO2 a

Ref. 107.

CO formation (µmol/g TiO2/hr) 8.1 22.7 11.8 60.0

CH4 formation (µmol/g TiO2/hr) 1.8 10

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(increase in CO formed from 8.1 to 11.8 µmol/g TiO2/h and additional methane formation of 1.8µmol/g TiO2/h) could be observed separately, synergistic effect in the composite catalyst is observed by significant increase in CO (60 µmol/gTiO2/h) and methane formation (10 µmol/gTiO2/h). Specific catalyst preparation techniques thus facilitate the positive interaction between the components of the composite leading to improved performance. Titania nanotubes possess several features that make them interesting materials for photocatalysis.162 Compared to spherical particles, one-dimensional nanotubes provide a high surface area and a high interfacial charge transfer rate. Free movement of charge carriers across the length of the nanotube is expected to reduce the e−/h+ recombination probability. Time-resolved diffuse-reflectance spectroscopy163 has shown that charge recombination is disfavored by the tubular morphology of the titania. It is observed that the half-life of photo-generated hole, τ1/2, as measured by laser flash photolysis is the longest with titania nanotube, at 3.5 ± 0.4 µs, as compared to 0.6−1.0 µs observed for spherical nanoparticles of TiO2. Nanotubes with a higher aspect ratio facilitate longer diffusion length of charge carriers and better charge separation. According to Roy et al.76 in vertically oriented TiO2 nanotubes, both inner and outer walls consist of anatase crystallites that are made available to the reactants which results in enhanced activity. The hole diffusion length in titania164 is reported to be ~10 nm, while electron diffusion length165 is ~100 µm and half of nanotube wall thickness is ~10 nm. Such dimensional features ensure facile contact between the surface species and charge carriers. Since half of nanotube wall thickness is less than or comparable to minority charge carrier diffusion length, both electrons and holes are generated very near to the interface between tube surface-reactant-gas. Thus, all charge carriers formed deep inside the nanotube walls reach the interface by diffusion and those created near the interface are readily available for the reactants. Such dimensional advantages at nanoscale could accelerate CO2 conversion. Similar advantages for Pt supported on titania nanotubes has been reported by Zhang et al.93 and by Xia et al.125 for titania-MWCNT composites for CO2 photoreduction with water. Nano sized hollow titania spheres with large surface area and porosity could be another potentially useful material for different photocatalytic conversions.166 Methods for synthesis of nano sized titania in various

morphological modifications have been covered extensively by Chen and Mao.167 Sensitization of titania by macrocyclic ligands

Macrocyclic ligands like phthalocyanins (Pc) and porphyrins (Pr) are good candidates for sensitization of titania because of their high absorption coefficient in the solar spectrum, especially in visible region and good chemical stability. (Pr)-titania composites display excellent activity for photocatalytic degradation of Rhodamin-B under visible light, while titania incorporated with Fe-porphyrin are active in the UV range.168 Since photoelectrons are directly transferred to the conduction band of titania, charge carrier separation is very effective. Zhihuan et al.169 and Shaohua et al.170 observed that phthalocyanins containing Zn and Co prepared by sol-gel technique were active for photocatalytic reduction of CO2, but the yields were low.169,170 Zhao et al.171 reported that in situ synthesis of CoPc/TiO2 (Fig. 14) results in highly efficient photocatalysts for CO2 reduction yielding formic acid, methanol and formaldehyde in an aqueous solution using visible light irradiation. In situ synthesis route results in the formation of isolated CoPc species (within the confined space of mesoporous titania), which effectively absorbs visible light enabling ultrafast injection of electrons from the excited state into the conduction band of the titania support. This helps charge carrier separation and increase in photo-conversion efficiency.171 Since the reported band gap for TiO2 is 3.22 eV versus 2.14 eV for CoPc, such a sensitization process is highly feasible. CO2 photoreduction data presented in Table 4 show that in situ CoPc/TiO2 catalyst is more

Fig. 14 – Flow diagram for the preparation of in situ CoPc/ TiO2. [Reproduced from Ref. 171 with permission from Elsevier, Amsterdam, The Netherlands].


Table 4 − Product profile for photoreduction of CO2 on TiO2-CoPc systema Catalyst

Product yield (µmoles/g catalyst) HCOOH CH3OH

TiO2 1wt%CoPc/TiO2 0.7% in situ CoPc/TiO2

221 450.6 1487

12.1 93

HCHO Total organic carbon 38.5 134.3

221 501.2 1714

a

Data taken from Ref. 171 with permission from Elsevier, Amsterdam, The Netherlands.

active than a simple mechanical mixture of CoPc and TiO2, indicating a cooperative effect between dispersed CoPc and titania surface for the effective transfer of photogenerated electrons. Compositing titania with macrocyclic ligands has emerged as one of the effective means of increasing photocatalytic reduction of CO2 on titania surface. Future Trends Titania and modified titania formulations continue to be the preferred catalysts for photoreduction of CO2 and other photocatalytic applications. Though the improvements achieved by various modifications are moderate, several useful concepts and correlations have emerged. Photophysical properties of titania originate during the preparation/pre-treatment conditions and hence preparation methods have attained tremendous importance.71b When used as support, properties of titania affect the reducibility and dispersion of the active phase like CuO, induce electronic level changes and affect the catalytic performance.172 Limitations in light absorption efficiency of titania could be overcome by the choice of visible light active mixed oxides of In, Nb and Ta,173,174 besides the modifications described in this review. Alkali metal tantalates like NaTaO3 promoted by La with NiO as co-catalysts, display significant activity for CO2 photoreduction with water, forming methanol and ethanol as major products.175 Detailed investigations in the following aspects would be helpful in arriving at catalysts with improved performance: 1 Modes of adsorption and activation of CO2 on metals, metal oxides, which act as co-catalysts– experimental as well as theoretical approaches; 2 Use of different co-catalysts, single as well as bi-component systems; 3 Exploring the application of TiO2-nanocarbon composites with graphene, fullerene and carbon nanotubes, etc.;

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4 Elucidation of the mechanistic pathways for the transformation of transient surface species and formation of C2 and higher carbon number products; 5 Modes of deactivation of catalysts and possibilities for regeneration. It is expected that the use of photoelectrochemical cells (PEC) for conversion of CO2 to methanol176, 177 would add another dimension to the process to get improved yields of hydrocarbons. Recent advances in these directions on photocatalytic conversion of CO2 to fuels have been compiled by Izumi178, wherein the necessity of ensuring that the source of carbon for the hydrocarbons formed originates from the CO2 used and not from the carbon containing impurities gathered on the catalyst surface by various means like residual organic templates, and solvents used during preparation is highlighted and strategies for recycling of sacrificial donor and direct supply of protons electrons released from water oxidation photocatalysts have been demonstrated for the photoreduction of CO2. Application of 13CO2 isotopic labelling and in situ FT-IR spectroscopic studies on surface transformations, backed by mass spectroscopic analysis of the intermediates and products to this end, have helped to unravel the reaction path for CO2 photoreduction. This has established unequivocally that the hydrocarbons do originate from the reactant CO2. However, according to Izumi178 the mechanism for the preferred formation of methane on TiO2 from CO2 + H2O is yet unclear, though pathways based on surface carbon species179, viz., CO and formyl group species, have been proposed101,180,181 backed by several in situ surface spectroscopic techniques. Summary and Conclusions As enumerated above, titania, subjected to a wide range of modifications, in the form of titania based composites and in different morphological forms has been explored to enhance the activity for photocatalytic reduction of CO2 with water with respect to pristine titania. Primary objectives behind the modifications, namely, promoting visible light activity, retarding the recombination of charge carriers by effective physical separation (by doping with metals, anions and cations), facilitating their transport through titania surface, isolation of titania sites by dispersion on high surface area supports

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and incorporation of suitable active elements to bring about the required redox reactions, have been realized to a significant extent. However, the expected improvement in activity has been moderate (an increase by 2-4 times), which is too low for any possible practical applications. Multiple and complex surface reaction pathways that involve several carbon containing ion radicals, render the selective formation of methane or methanol a difficult task. Further, decomposition of the products and promotion of backward reactions also contribute towards lower yields. Catalyst deactivation proceeds through the formation of carbonaceous species on the surface. This implies that the metal function responsible for the hydrogenation of carbonaceous species needs to be improved though availability of hydrogen via water splitting may not be the issue. Indepth investigations on the surface reaction pathways by in situ spectroscopic methods, supported by sound theoretical studies on the activation and surface transformations of CO2 and other aspects for investigation, as detailed in the earlier section, will be helpful in controlling deactivation and achieving higher conversions. Acknowledgement The authors gratefully acknowledge the Department of Science and Technology, New Delhi, Govt. of India, for the grant towards establishing NCCR at IIT Madras, Chennai, India and M/s Hindustan Petroleum Corporation Limited, Mumbai, India, for funding the project on photocatalytic conversion of CO2. References 1

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