Photocatalytic reduction of carbon dioxide issues and prospects bentham science (2016)

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2. 80 Current Catalysis, 2016, Vol. 5, No. 2 Narayanan et al. [17]. This implies that greater effort will be needed to overcome the energy barrier for converting carbon dioxide than the water splitting. In spite of this unfavourable condition, enormous efforts have been expanded around the globe to convert carbon dioxide into chemical and value added fuels. It has received considerable attention in recent times due to its similarity to photosynthesis. There are various other routes available for carbon dioxide conversion with specific advantages and disad- vantages over photo-catalytic route. The photocatalytic conversion rate so far reported is up to few tens of micromoles per gram per hour, which is far from the levels required for the industrialization of the process. One of the primary reasons is that the carbon dioxide in its ground state is a linear molecule which is stable and the activation of this linear molecule for chemical conversion requires high overpotential. The first electron transfer to the carbon dioxide to produce carbon dioxide anions radical which needs high reduction potential up to -1.9 V vs Normal Hydrogen Electrode (NHE). The conduction band bottom of currently known photocatalysts is more positive than the first electron reduction potential. So, urgent need is a photocatalyst with conduction band edge more negative potential with necessary band gap for visible light absorption. The visible light absorption of the photocatalyst can be achieved by means of sensitization and band gap engineering, etc. But these possibili- ties have not provided any desired results. Since the photocatalytic process is a multielectron transfer process, the reaction rate depends upon each of the electron transfer steps, namely photon absorption, charge separation, surface reaction, electron-hole recombination, transfer of electron to the adsorbed substrate and desorption of the substrates and each of them must be sequentially taking place for the formation of multielectron reaction products like methanol, ethanol, formaldehyde. To the best of the author’s knowledge, no photocatalyst reported so far does both activation and visible light absorption in an appropriate way. At least one wants to achieve a 10% quantum efficiency or comparable to photosynthetic rate, then only the dream can be realized. Another hurdle associated with the efficiency is the measures like Quantum Efficiency, Turnover Rate, Turnover Number, Turnover Frequency, which reflects reactivity of the system, used for comparison and appropriate selection of materials in photocatalysis. These measures are based on either in terms of catalyst systems or in terms of illumination employed. In a reaction like photocatalysis, the illumination based measures is not directly reflecting the activity of the catalyst since the activity not only depends upon the illumination but also on many other factors. The illuminated beam itself undergoes multiple events like scattering, reflection, etc., hence the cal- culation of the exact number of photons absorbed by the photocatalyst is difficult. Hence, one wants to develop a new efficiency scale for representing the activity of the photocatalyst and it must be based on photocatalyst as well as the illumination and inde- pendent of these individual parameters. The new scale must be based on the number of successful photons absorbed by the photocatalyst, which only truly gives the quantum efficiency. The reaction rate can be increased by catalyst modification or by adding sacrificial reagent to the system. In the case of CO2 reduction, none of the above methods has not been yielding the desired result. The role of sacrificial reagent on the PCR of carbon dioxide is limited for a few hours and it also absorbs the light from the illumination source. The degradation of sacrificial reagent may affect the reaction mechanism of carbon dioxide reduction itself because the proposed mechanism for carbon dioxide reduction involves radical species. If nanoarchitecture will be a solution, the large scale production of catalyst with well-defined size, shape, surface, porosity, defects, and assemblies are the challenges. The doping and aliovalent sub- stitution of nanomaterials may lead to photochem- ical deactivation of the catalyst. Major limitation associated with carbon dioxide reduction is the accuracy of the analytical measurements employed. The photocatalytic process is a multielectron transfer process, hence the reaction leads to the formation of a variety of products like carbon monoxide, methane, higher hydrocarbons, alcohol, aldehydes, carboxylic acid etc., with some intermediates. The identification and quantifica- tion of the products are needed for the best selection of photocatalyst, comparison and elucidation of reaction mechanisms. Currently there is no standard analysis method that has been developed for product analysis of carbon dioxide reduction. Hence the results of these measurements also in- clude the products derived from the carbon con- tamination invariably present in the reaction sys-

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129. Photocatalytic Reduction of Carbon Dioxide: Issues and Prospects Current Catalysis, 2016, Vol. 5, No. 2 93 tion. The yield of product depends on the extent of adsorption of carbon dioxide on the photocatalyst. Anpo et al also studied the photoactivity of single crystal surface of Ti terminating rutile (100) and oxygen terminating rutile (110) and found after careful kinetic measurements that the TiO2 (100) facet was 7.4 times more active than TiO2 (110) in reducing CO2 [25]. This is due to the fact that the transfer of d electrons to the LUMO of CO2 is more favourable in Ti terminating surface. It is already known that carbon dioxide adsorbs strong- ly in steps sites, hence the creation of proper step active sites would lead to better carbon dioxide adsorption on the surface which facilitate the electron transfer from the photocatalyst to the CO2 and thereby increases the product yield. Fig. (15). Production of CO and CH4 on the untreated and the treated titania polymorphs Reproduced with permission from ref [123]. ©2012 American Chemical Society. Titanium oxide nanomaterials also have been studied extensively for CO2 reduction in the last decade [125-128]. Most of the studies conducted due to the common belief that the semiconductor nanomaterials increase the visible light activity. In a real way, the belief is no longer true, it’s one of the misconceptions in photocatalysis. Decreasing particle size from bulk to nano level increases the bandgap and their by decreases the visible light activity. Most of the semiconductor nanoparticles and clusters active only under UV irradiation be- low 320 nm. The Ti-oxide highly dispersed TiO4 single unit site absorbs only UV light under the wavelength 230-280 nm (Fig. 16) [27]. The absorption of that particular wavelength creates an electron-hole pair within the single site unit con- tributed to the high and unique photocatalytic activity. Incorporation of titanium dioxide single site photocatalyst into the zeolite matrix, thus a promising route towards the carbon dioxide conversion. However, it suffers by the lower product yield. Fig. (16). Bandgap diagram of different titania structures. Reproduced with permission from Ref. [27]. ©2012 Royal Society of Chemistry. By comparing the yield of the product by the different titanium dioxide structures for the conversion of CO2 (see Table 5), one can find that the product yield is too low as compared to the other material structures. Table 5 shows the product and product yield irrespective of their reaction conditions. TiO2 Degussa P25 (presently Evonik) is the most used material for CO2 reduction, normally used as a standard reference material in thermody- namically “downhill” photocatalytic processes. It is a mixture of two polymorphs of titanium dioxide, namely anatase (80%) and rutile (20%) and the particle size up to some nanometers. Recently, Ohtani et al found that Degussa P25 also contains trace amount of amorphous titanium dioxide [165]. The superior photocatalytic activity of P25 makes it as a standard reference in photocatalytic oxidation processes. The mechanism of superior activity of P25 is still a controversy among the photocatalytic community [166]. When scientists realize that CO2 can also reduce by the photocatalytic phenomenon, the followers used P25 as the reference material, even though it’s not a champion photocatalyst for CO2 reduction. In a process like CO2 reduction, the product yield depends on many factors interde- pendently as compared to the normal photocatalytic oxidation processes (See Table 6) [167]. The electron as well as proton transfer plays a crucial role in the thermodynamic uphill processes such as water or carbon dioxide splitting into valuable fuels that means the reaction led not alone by light irradiation. 21 18 15 12 9 6 3 0 CO CH4 TiA(UP) TiA(He) TiB(UP) TiB(He) TiR(UP) TiR(He) Productionofproduct(mmolg.1 ) TiO2 phase (A = anatase; B = brookite; R = rutile; UP = unpretreated; He = helium treated) Conduction band (cb) Valence band (vb) 400 nm (hv) ~350 nm Semiconducting bulk TiO2 Nanocluster of Ti-oxides Isolated Ti-oxide Charge transfer excited state ~320 nm 300 nm 250~ h+ e- O2- O2- O2- O2- O2- O2- O2- Ti4+ Ti4+ O2+ O2+ O2+ O2+ Ti4+ Ti3+ O2- O2- O2- hv O2-

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131. LUUHVSHFWLYH RI WKH UH DFWLRQ FRQGLWLRQV Photocatalyst Product (yield, μl h-1 gcat -1 ) References Year TiO2 Anatase Film 2

132. Au (0.25%) /Pt (0.75%)/TiO2 nanofiber +

133. 2

134. Cu/Pt2/TiO2 nanotube +

135. Pt (0.21%)/TiO2 +

136. Pt (0.94%)/TiO2 +

137. Pt (1.3%)/TiO2 single crystal +

138. Pt (2.5%)/TiO2 +

139. Au (1.5%)/TiO2 +

140. Au (0.5%)/Cu (1%)/P25 +

141. Cu (1.5%)/P25 +

142. Pt (0.5%)-red P-g-C3N3 +

143. Ni/SiO2-Al2O3 (AM 1.5 filter) +

144. NiO (AM 1.5 filter) +

145. Fe2O3 (AM 1.5 filter) 2

146. CoO (AM 1.5 filter) 2

147. +

148. Ni/SiO2-Al2O3 ( Ȝ 420nm) +

149. NiO ( Ȝ 420nm) +

150. 2

151. Au3Cu/SrTiO3/TiO2 2

152. Hex-ZnIn2S4 0)

153. Cub- ZnIn2S4 0)

154. MIL-101 (Fe) +2

155. NH2- MIL-101 (Fe) +2

156. NH2- MIL-53 (Fe) +2

157. CdS-Co-ZIF-920,1 2

158. CeO2-TiO2 composites +

159. 2

160. Au/Pt-TiO2 nanotube +

161. 2

162. Ag/Zn-Ga2O3 2

163. +

164. 2

165. In2O3/g-C3N4 +

166. ZLWK 3W Cu based MOF +2+ SSP J K

167. C3N4/Ru complex +22+

168. Porphyrin/Graphene + + —PRO P K

169. Ru/RuX2-TiO2 +

170. 23.3 wt % AgBr-TiO2 +

171. +2+

172. Photocatalytic Reduction of Carbon Dioxide: Issues and Prospects Current Catalysis, 2016, Vol. 5, No. 2 95 Table 5: contd…. C2H5OH (13.28) CO (32.14) CdS/TiO2 nanotubes CH3OH (159.5) 148 2012 Bi2S3/TiO2 nanotubes CH3OH (224.6) 148 2012 GaP/TiO2 CH4 (118) 149 2014 Ru (0.5%)/ TiO2 CH4 (200) H2 (250) 150 2006 3 wt % CuO/TiO2 CH3OH (2655) 151,152 2005,2009 2 wt % CuO/TiO2 CH3OH (1000) O2 (20) 153,154 2002,2004 2 wt% Cu/TiO2/SBA-15 CH4 (627) 155 2009 0.1% Y-TiO2 HCHO (382.62) 156 2011 3 wt % Ag TiO2 H2 (100) CH4 (6) C3H6 (14) 157 CeF3/TiO2 CH3OH (162) 158 2014 Pt/TiO2 CH4 (277.2) C2H6 (12.4) H2 (389.2) O2 (785.3) 159 2014 Pt/MgO/TiO2 nanotubes CH4 (100.22 ppm/hr cm2 ) CO (10.4 ppm/hr cm2 ) 160 2014 In/TiO2 CH4 (244) CO (81) 161 2015 N-TiO2/Ni CH3OH (482) 162 2010 N-TiO2/Pt-Cu CO (111 ppm/h cm2 ) 32 2009 N-TiO2 nanotubes CH3OH (1132.6) HCHO (921.6) HCOOH (12475.8) 163 2012 C/TiO2 HCOOH (2610.98) 164 2011 The CO2 adsorption and the concurrent reaction with the electrons highly depends on the nature of the material surfaces. Not all the semiconductors have the similar surface arrangement, therefore, the reaction pathway and the interaction of CO2 moiety with surfaces varies according to the materials used. The product and yield vary within the titanium dioxide compounds itself [124]. Since the CO2 reduction doesn't only depend on the visible light activity of the photocatalytic material, but also many other factors described in Table 6, so the benchmarking of titanium dioxide P25 for CO2 reduction seems to be irrelevant and any comparison of reactions is only possible after the implementation of a standard procedure. A new benchmark system has to be evolved instead of TiO2 P25 because it is not a pure single component system. Recently, scientific efforts are reported in the literature for benchmarking the photocatalytic material for electrochemical water splitting reactions [168-170]. It would be a surprising

173. 96 Current Catalysis, 2016, Vol. 5, No. 2 Narayanan et al. fact that such kinds of effort are not seen in the CO2 reduction field especially in the electrocatalytic CO2 reduction. Table 6. Factors that determine the photoactivity of a semiconductor surface. Reproduced with permission from Ref. [167] ©2015 American Chemical Society. Properties Factors Impacts Optical Band gap Photonic Efficien- cy Electronic Carrier Mobility Quantum Effi- ciency Structural and Morphological Surface Area Crystallinity/ Crystal Faces Particle Size Defects Crystal Phase Multiple Ways Multiple Ways Multiple Ways Multiple Ways Multiple Ways Surface Chemical Acid-Base character Surface Groups Multiple Ways The innovation of new materials, alternative to TiO2 has been in progress. Many materials like carbon nitride, BiVO4 offer a better activity than that of TiO2 but it lacks the in detail in situ and op- erando studies. The interaction of CO2 with these two materials has not been revealed in full swing. New studies on these materials for CO2 reduction do not consider the nature of interaction with CO2 (adsorption mechanism, since it varies with materials) and the followed reaction pathways. The ra- tional modelling of these materials under real conditions is still missing, limiting the generation of alternative or next generation photocatalytic materials. Since the photocatalytic reaction is too complex, the use of state-of-the-art techniques and methods are needed to overcome the barriers associated with process and to find an alternative photocatalytic material for solar fuel generation. The combination of theoretical modelling, time re- solved spectroscopies, surface science methodolo- gies and radical chemistry is needed for the successful understanding of the mechanism of solar fuel generation both in terms of thermodynamics and kinetics [171]. 6. PERSPECTIVES Photocatalytic reduction of carbon dioxide is an immediate need in the energy as well as environmental points of view. The amount of product formed during the reaction still suffered by lower yield say up to some micromoles. A complete revisit to each individual process associated with the carbon dioxide reduction is required for elucidating the reaction pathways and preparing a suitable mechanism. At this point, the scientist should deviate from the conventional TiO2 route [167,172]. The mecha- nistic studies using TiO2 are highly encouraged due Fig. (17). Best Solar Cell Efficiencies. This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO. 50 48 44 40 36 32 28 24 20 16 12 8 4 0 1975 1980 1985 1990 1995 2000 2005 2010 2015 Best Research-Cell Efficiencies Efficiency(%) Multijunction Cells (2-terminal, monoltic) LM - Latice matched MM - meternophic Three-junction concentrator Three-junction (non-concentrator) Two-junction concentrator Two-junction (non-concentrator) Four-junction of more (concentrator) Four-junction of more (non-concentrator) Sigle crystal Concontrator Thin-flm crystal Sigle-junction GaAs Crystaline Si Cells Single crystal (concentrator) Single crystal (non-concentrator) Multisoysiline Slioon hebterestructures (HiT) Thin-frm crystal IMM- irverled, metanorphic Quantum dot cells Inorganic cells (CZTSSe) Organic Landen cells Organic cells (various typed) Poroside cells (non stabiled) Dye-sensitized cells Emerging PV IBM (TJ. Watson Research Center) Mobel Solar Macsushita Macsushita U of Marie U of Marie RCA RCA RCA RCA RCA RCA RCABoeing Boeing Boeing Solarex Kodsk Boeing Kodsk Kodsk Kodsk Solarex Solarex Boeing ARCO No, Canolina State U. RCA ARCO ARCO Boeing AVETEK Spine Varian Flonda U, So Photon Energy United Solar EPFL EPFL EPFL EPFL United Solar NREL Matsushita Euro-CIS NREL NREL NREL NREL NREL NREL NREL NREL U Sungart United Solar NREL UNSW UNSW Georgia Tech Georgia Tech Georgia Tech UNSW UNSW UNSW Spire UNSW UNSW ARCO Sandia Westing- house Stanford NREL Kepin Vanian Varisn Varisn Varisn Stanford (140x) (205x) (216x) NREL NREL SunPower (B6x) UNSW UNSW UNSW UNSW Japan Energy NREL/ Spectrpiab Spectrpiab Spectrpiab Spectrpiab Spectrpiab Spectrpiab SpectrpiabSpectrpiab Spectrpiab Spectrpiab Boeing NREL Spectrpiab Boeing NREL (MM 179x) (MM 299x) Boeing (MM 240x) (MM 454x) Boeing (MM) Fraunholar ieSE (MM 364x) Boeing Eurosolare NREL (14x) Sanyo Sanyo Sanyo Sanyo Sanyo NREL U. Stuttgart FiG-ISE Radood U Radood U FiG-ISE FiG-ISE Amonts (92x) (232x) IES-UPM (1026x) NREL Boeing (MM) Sharp (MM) Sharp (MM) FiG-ISE (117x) NREL Ati Devious Panasonic Alta Devioes NREL (457x) NREL (MM, 325 .7x) Solar Junction (MM, 416x) Sotec (4J, 319x) NREL (4J, 327x) NREL Boieng- Spectrotab (5-J) Sharp (MM) Sharp (MM, 302x) Solar Soitec Spine Semoonductor (MM, 406x) Junction (LM, 942x) (4-J, 297x) Fraunholar ISE/ Soitec LG Electronics SunPoewr (Nurgo Anda)Panasonic Ati Devious Panasonic NREL (154x) NREL 2SW NREL (14.7x) Solend ZSW Solar Frontier First Solar EPFL Trine Solar Souro KRACT Solar Fronder GE Globel Research KRICT AIST Hong Kong UST IBM IBM First SolarGE Gobal Research LG Electronics FrX Sorc ISFH Fraunhclor ISE Misubshi ChomUnited Solar (aSadusSuscus) Sharp IBM IBM NIMS EPEL Sharp UCLA Sumi- lomo Konana Solamer KonatoNREL / Konanta U- LineGroningen U. Line U. Line Semeng Pleelectronis U. Dresden Helacek NREL (ZnOPbS-CO) U. Tanonda (PbS-CO) MiT Heliees U. Tanonda U. Tanonda UCLA Sumdomo Chew ZSW-EMPA (Flex poly) Per.12.1670) 10.4% 10.4% 11.4% 11.4% 12.4% 13.6% 20.4% 21.0% 21.2% 21.5% 22.5% 23.3% 25.0% 25.6% 27.5% 27.4% 28.3% 29.1% 31.1% 34.1% 37.9% 38.8% 44.4% 46.0% Thin-Film Technologies CIGS (concentrator) CIGS CdTe Amonphous ScH (scebTzed)

174. Photocatalytic Reduction of Carbon Dioxide: Issues and Prospects Current Catalysis, 2016, Vol. 5, No. 2 97 to its appreciable stability in most of the vigorous medium compared to other photocatalysts. Most of the studies have not explained the source of product formed during the reduction reaction, whether it is from the artifact or not. Iso- topic study or blank run is recommended to all CO2 reduction processes in order to reveal the exact carbon source thereby avoiding the bias in CO2 reduction. A standard experimental procedure is an immediate need in the carbon dioxide reduction process. Only with the standard methods one can compare the efficiency of material innovated from the different research groups, and also to choose the prominent candidate as the champion photocatalyst. Now different groups normalize the reaction efficiency in different ways, no standard method is also available for the normalization of efficiency of the CO2 reduction process. Standard method and normalization of efficiency is needed for developing an efficiency chart for selection of best photocatalysts as in the photovoltaics. In photovoltaics field, the National Renewable Energy Laboratory (NERL) maintains a plot that combines all research cell efficiencies reported so far since 1976 (Fig. 17) [173]. The plot includes the current state of the art efficiencies that are reported on a standardized basis based on Standard Test or Re- porting Conditions defined by IEC 60904-3 edi- tion 2 or ASTM G173 at 250 C. The reaction rate and the product yields are low compared to the natural photosynthesis. The catalyst is deactivated in the operation stage itself. Hence the quantum efficiencies are often very low. For the real life implementation of the photocatalytic reduction of CO2 or so called artificial photosynthesis, one has to prepare catalyst systems which will be more effective in the visible region and also give more than 10% quantum efficiency [174]. Osterloh in 2008, has excellently depicted a pictorial representation on the selection of materials for water splitting reaction (Fig. 18) [174, 175]. The materials represented in the Fig. 20 are also relevant to the CO2 reduction process. The figure clearly depicts that if a photocatalyst is said to be economic, it has to give at least 10% quantum efficiency in the visible region of the electromagnetic spectrum. To the best of the author’s knowledge, no material which has been identified to satisfy both criteria as shown in the shaded part of the figure. Photo-fuel cell is an attractive route to reach the solar energy conversion in an effective way. It offers a spatial separation of redox reactions, the anode catalyst oxidise the water and the cathode catalyst reduces the H+ ions to H2 using holes and electrons respectively. The theoretical efficiency is found to be 32% when the device is based on a single bandgap absorber that constitute a semiconductor [176]. The same principle is also applied for CO2 reduction by Hurst et al. in 2010. In that, water oxidation catalyst ejects electrons and holes when irradiated by light and the concurrently formed electron undergoes a series of reduction reactions to produce H2, which will further react with the CO2 to yield fuels (Fig. 19) [177]. The reaction is suffered by the rapid deactivation of the water oxidation ligands. The discovery of visible light active, durable, cost effective water oxidation molecular photo-catalyst and stable ligands accel- erate the yield of fuels. The present scenario does Fig. (18). Quantum efficiencies of various materials used for photocatalytic water splitting reaction. Reproduced with permission from [174]. ©2010 The Royal Society. 100 10 1 0.1 100 200 300 400 500 600 700 wavelength (nm) UV Ni: lnTaO4 Cr/Ta: SrTiO3 +LaTiO2N + + WO3 visible K4Sr1.5Ta3O10 (M = K, Rb, Cs) M2La2Ti3O10 Zn :La2O3/Ga2O3 HCa2Nb3O10 La2Ti2O3 M4Nb6O17 (M = K, Rb) TiO2 + Sr2Nb2O7 CdS ZHS La :NaTaO3 Nb2O3 :TaO2 +La4CaTi3O12 ZnNaTaO3 Ge3N4 KBa2Ta3O10 MTa2O6 TiO2 (anatase) (M = Sr, Ba, Sn) + GaN :ZnO + MZnS (M = Ni, Pb, Cu) +PbBi2Nb2O9 +Zn :[In(OH)ySz]+BiVO4 + + + + +++ ++ ++ ++ + + quantumefficiency(%) ?

175. 98 Current Catalysis, 2016, Vol. 5, No. 2 Narayanan et al. not offer such a kind of materials and stable and self-reparable ligands. More research effort is needed. [178]. Swiegers and coworkers put forward an idea towards the efficient production of hydrogen from solar energy based on a bio-inspired photosynthetic mechanism [178, 179]. The set-up consists of two electrons separated by a plastic membrane which allows the one directional transfer of protons. In this, the manganese impregnated membrane is attached to the ruthenium based DSSC (Dye Sensitised Solar Cell) which absorbs visible light irradiation and transfer an electron into the TiO2 nanoparticles. The water oxidation catalyst (Mn4O4) also absorbs light and grab electrons from water. The electrons are transmitted to the cathode by means of an external circuit and the protons passed through the H+ conducting nafion membrane further reduced to H2 at the cathode by the electrons (See Fig. 20). This cell is too inefficient to produce hydrogen economically. If succeeded, one can make 15 times more hydrogen compared to the conventional hydrogen evolution pathways. The realization of this idea and incorporating this to CO2 reduction process offers a significant research avenue. In a similar way, Seger et al investigated methanol as a source of hydrogen in a PEFC with TiO2 photoanode and Pt cathode with no applied bias [180]. The acidified methanol oxidized under UV irradiation on TiO2 and the H+ formed is transport- ed to the Pt cathode through the nafion membrane. Fig. (19). Solar fuel cells based on the concept of a Polymer Electrolyte Fuel Cell (PEFC). Reproduced with permission from ref [176, 177]. ©2006 National Academy of Science (left) ©2010 American Association for the Ad- vancement of Science (right). Fig. (20). Photoanode consisting of titania sensitized Ru dye coated over a nafion layer doped with manganese based water oxidation catalyst along with the cross sectional arrangement of photoelectrochemical cell. Reproduced with permission from Ref. [179] ©2010 American Chemical Society. Fuel Cell Solar Fuel Cell anode membrane cathode 4H+ e- e- H2 O2 H2O O2 cat anode O2 A Solar PV assembly 4H+ H2 cat cathode 2H2 02 2H2 O 4H++ WOC Light energy 4e- Photoactive element 4H+ (+ CO2) 2H2 (or CH2O + H2H) Dye Nafion Electrolyte Cathode Conductive glass TiO2 TiO2 Nafion 4x1e-4e- SO3 SO3 SO 3 SO3 SO 3 O 2 O O O OO H+ HH H MnMn Mn Mn 2H2 SO 3 SO 3 SO3 4x1e- 4e-hv4hv diffusion Electrolyte FTOCoatedGlass O O O O C C N N N NRu N N 2H2 O 2H2 O2 hv hv Ru* e- e- e- e- 4H+ Ru*/Ru* [Mn4 O4 L6 ]4

176. Photocatalytic Reduction of Carbon Dioxide: Issues and Prospects Current Catalysis, 2016, Vol. 5, No. 2 99 The hydrogen ion is reduced to H2 with a rate of 69 μL h−1 cm−2 . Kim et al also studied the nafion layer-enhanced photocatalytic conversion of CO2 [181]. The main role of nafion membrane is to en- hance the proton activity. It also prevents the re- oxidation of the CO2 reduction products. The current efficiency of solar fuel cells restricts this process from real-life implementation. Since water oxidation is one of the key steps in the photocatalytic reduction of carbon dioxide, any breakthrough in water splitting research also con- tributes to the CO2 reduction process. Materials used for water splitting reactions and its pros and cons were excellently reviewed elsewhere [175, 182-186]. Recently more research has been concentrated on hydrogen evolution reaction [187- 191] and the evolved hydrogen is channeled for the CO2 reduction [192-205]. Some reports also used CH4 as a reductant, instead of water and ex- ternally channeled hydrogen [199, 201]. Such kind of CO2 reduction systems is innovative but one cannot term it as photocatalytic even though the hydrogen produced ex situ by photocatalytic water splitting. Photocatalytic process involves generation and combined utilization of charge carriers within the reaction system and the in-situ generation of hydrogen ions from water oxidation and consecutive reduction to hydrogen, which is directly used for CO2 reduction on the surface of a photocatalyst. The artificial photosynthetic systems should make use of the state-of-the-art microscopic and spectroscopic techniques for the in-situ experimental studies. Chromatography based analytical measurements do not give any insight into the mechanism of photocatalytic reaction. It only helps to identify the products formed during the process or the reaction pathways. Most of the product formed are also under the detectable limit of chromatographic measurements or the product formed during the reaction will interface with each other over the chromatographic column. The unre- acted carbon dioxide should be vented out proper- ly, otherwise it will adversely affect the chromatographic column during the continuous operation. Incorporation of XANES, XES like spectroscopic techniques give a detailed information on the reaction mechanism of the surface photocatalytic reactions. It reveals the chemical environment, charge transfer and charge symmetry with respect to the particle size and impurity level, which will throw light on the surface reaction mechanism of the photocatalytic reaction. X-ray photoelectron technique is another important tool in photocatalytic research while the exact potential of that technique is not fully explored. Casalongue et al directly probed the relationship between the adsorbed chemical species on the surface and the electrochemical potential in the case of polymer electrolyte membrane fuel cells using ambient pressure X-ray photoelectron spectroscopy (See Fig. 21) [206]. It is found that non-hydrated hy- droxyl species were the protuberant surface species and the exclusion of hydration increases the reactivity of oxygen species on the surface of the electrode. This type of in situ experiments can also be useful in solar energy conversion processes, which may throw light on the dark shaded CO2 reduction mechanism and many other processes. Fig. (21). Schematic representation of a PEM fuel cell set-up for in situ APXPS investigations Reproduced with permission from Ref. [206]. ©2013 Macmillan Publishers Limited. For in-situ experiments most commonly used microscopic and spectroscopic techniques are environmental transmission electron microscopy (E- TEM) [207-209], In-situ vibrational spectroscopic [210-212], ambient pressure X-ray photoelectron spectroscopy [206,210,213], X-ray absorption spectroscopy [213,214-217], and Raman spectroscopy [218]. Making use of this in-situ experiments, the solar fuel generation processes will get a new dimension to the state-of-the-art beliefs. Moreover, the catalysts structure, coverage and composition also change with time, the combination of ultrafast of in-situ spectroscopic techniques reveal the structure and catalytic activity relationships (See Table 7) [217]. x-ray e- e- e- e- e- e- e- e- e- e-e-e- e- e- Cathode Anode Nafion H+ H+ H+ H+ O2 H2, H2O e- e- A V

177. 100 Current Catalysis, 2016, Vol. 5, No. 2 Narayanan et al. The potential of electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy is also not fully explored in photocatalytic studies which is primarily used for the rapid screening and high throughput evaluation of photocatalysts. The use of in-situ analytical techniques used for heterogeneous catalysis is extensively reviewed elsewhere [213-218]. The main advantage of in-situ experiments is; one can avoid the artifacts or interference from sample transfer or ion beam cleaning when using surface sensitive techniques. The applicability of this online techniques to the artificial photosynthesis is highly recommended. It will help to map the entire photocatalytic reaction leading to better understanding of the overall photocatalytic processes. Commonly used microscopic techniques ana- lyze the photocatalytic material prior to the experiments while the properties of photocatalyst vary in solution or restructured with respect to the sol- vent medium. Most of the present literature does not consider this type of changes or postulates that the reaction rate decreases without any scientific evidences for the catalyst deactivation. This kind of changes can be monitored using in-situ experiments by mapping the entire life span of the catalyst during the photocatalytic process. It also reveals the deactivation mechanism of photocatalysts which will help us to modify the catalyst surface. Most of the laboratories suffered by the in- situ setup due to its high cost. One can at least do the characterization of photocatalyst prior and after the reaction or one can design the experiment in such a way that conducting the same experiment in different times and characterize the catalyst. This kind crude methods will impose impurities to the samples while transferring. Separation of products formed during the reaction imposes an additional challenge to the photocatalytic conversion process. Recently, Alexis Bell and Meenesh Singh proposed an effective method for synthesizing pure liquid ethanol from carbon dioxide, which uses a saturated salt electrolyte [219, 220]. According to the theoretical calcula- tions, the system would be capable of producing 15.27 million gallons of ethanol per year per square kilometer. The suggestion is based on an effect known as salting out. They used an electrolyte supersaturated with caesium carbonate, the attraction between the salt and water weakens the water-ethanol interaction which leads to the formation of a micro-emulsion that can be separated by means of a liquid-liquid separator (Fig. 22). The main challenge in front of the proposers is to make this idea in a cost effective manner. If it is experimentally proven, then it will be a break- Table 7. Some of In situ used techniques and applications in the solar fuel generation. Extracted from Ref [211,216, 218]. Method Applications AP-XPS Mainly used for studying the elemental compositions, the redox state of the surface and the subsurface region etc. HP-STM Elucidating the surface structure. SFG-FTIR Identifying the reaction intermediates and molecular nature of active sites. EPR Detection and identification of paramagnetic centers and free radicals TA It will give a reaction up to femto to the microsecond. Effective tool for unravelling at the kinetics of a single step from the subsequent reaction steps under relevant reaction conditions and probing the charge carrier dynamics and lifetimes. Ultrafast TA was used for probing the hole transfer kinetics. XAS,EXAFS, NEXAFS Extracts the evidence about the chemical state environment like the oxidation state, symmetry or the local charge distribution. It also gives the local environment of atoms (coordination number, distance between the neighboring atom etc.) ab- sorbing the incident radiation. KPFM Used for the investigation of surface potential of materials Electrochemical Imped- ance Spectroscopic Methods Reveals the chemistry at electrolyte / semiconductor interfaces AFM, SECM, STM Visualization of catalyst structure in oper- ando conditions. Provide finest details of the surface atom by atom. TEM In situ TEM reveals atomic level structure activity relationships of photocatalysts under reaction conditions. EELS Used in gas composition studies and sim- ultaneous atomic structural characterization of the particles during the reaction. Raman Spectroscopy Used to study the spectral features of solar fuel materials under reaction conditions.

178. Photocatalytic Reduction of Carbon Dioxide: Issues and Prospects Current Catalysis, 2016, Vol. 5, No. 2 101 through in artificial photosynthesis where the greatest challenge is to separate alcohols from water due to its high solubility and crossover [221]. Generally, the natural photosynthetic process is associated with the absorption of light in the near infrared region of the electromagnetic spectrum. Most of the semiconductors are not activated by the absorption of light near infrared region due to the bandgap values. One way to make the artificial photosynthesis efficient would be to increase the visible light absorption of the photocatalyst. Stud- ies are reported for activating the photocatalyst using light near infrared region [222, 223]. A recent study reports the strategy for converting the infrared light into visible light using quantum dots films [224, 225]. The film was made up of a lead sulphide semiconductor quantum dot glazed with a single layer of fatty acids that make the surface inactive. The top crystalline layer was made up of rubrene, an organic molecule. The top layer absorbs infrared light and transfers it in the bottom layers (See Fig. 23). The energy inside the system exists in the form of excitons which diffuse over the rubrene through a process called triplet-triplet annihilation. When it is irradiated with infrared radiation, glowing of the film occurred which is the direction of the production of visible during the process. It claims that the collision of two low energy excitons creates a high energy exciton or singlet, which can further emit visible light. If it is possible to create the materials in large scale, then it will be an asset to the artificial photosynthesis process if and only if the champion photocatalyst is discovered. 7. CONCLUSION Carbon dioxide reduction is a promising route to the sustainable solar energy conversion. The Fig. (22). Proposed system incorporated with the liquid-liquid extractor for the conversion of carbon dioxide into pure liquid ethanol fuel. Reproduced with permission from Ref. [219]. ©2016 Royal Society of Chemistry Fig. (23). Schematic representation of quantum dot sensitized upconversion of IR to Visible light via triplet-triplet annihilation. Reproduced with permission from [224] © 2015 Macmillan Publishers Limited. Saturated Salt Electrolyte Liquid Fuel Make-up Water ArtificialLeaf CO2Reservoir Carhode AEM CO2 Anode LightAbsorber O2 e + Pbs NC Transfer Rubrene TTA 1 nm Emitter;Annihilator; Rubrene DBPPbs NC Sensitizer; Excitation Emission G S1 S1TTA T1= 1.14 eV E1 q = 4.8nm Rubrene;0.5% DBP 80 nm NC submonolayer Glass

179. 102 Current Catalysis, 2016, Vol. 5, No. 2 Narayanan et al. existing level of knowledge does not permit the design of economically viable process for the conversion carbon dioxide into chemicals and value added fuels. It could happen only by the collabora- tive research effort from various groups, irrespective of the implicit bias among the scientific community. The material development and the lack of a standard procedure has been the bottleneck. The material should give at least 10 % quantum efficiency in the visible region of the solar spectrum. Currently, all such CO2-to-fuel research efforts have been scattered around the various parts of the globe. A global initiative is needed for the resur- rection of CO2-to-fuel technology in a real life process and it should be targeted on the policy makers as well as the general audience. The global initiative should be like a compound parabolic concentrator in which all the scattered research efforts concentrated into a central point. The current pace shows that the plants will face a tight competition from the CO2-to-fuel solar business in the near future! LIST OF ABBREVIATIONS AFM = Atomic Force Microscopy AP-XPS = Ambient Pressure X-Ray Photoe- lectron Spectroscopy CPC = Compound Parabolic Concentrator DMF = N, N-dimethylformamide EELS = Electron Energy Loss Spectrosco- py EPR = Electron Paramagnetic Resonance or electron spin resonance (ESR) EXAFS = Extended X-ray Absorption Fine Structure Fc+/0 = ferrocenium/ferrocene couple HOMO = Highest Occupied Molecular Or- bital HP-STM = High-Pressure Scanning Tunneling Microscopy IR = Infra Red KPFM = Kelvin Probe Force Microscopy LUMO = Lowest Occupied Molecular Or- bital MeCN = Acetonitrile MOT = Molecular Orbital Theory NEXAFS = Near Edge X-Ray Absorption Fine Structure NHE = Normal Hydrogen Electrode PCR = Photocatalytic Reaction PZC = Point of Zero Charge SECM = Scanning Electrochemical Micros- copy SFG-FTIR = Sum Frequency Generation Vibra- tional Spectroscopy SPR = Surface Plasmon Resonance STM = Scanning Tunneling Microscopy TA = Transient Absorption TEM = Transmission Electron Microscopy UV light = Ultraviolet light XANES = X-Ray Absorption Near Edge Structure XAS = X-Ray Absorption Spectroscopy XES = X-ray emission spectroscopy CONFLICT OF INTEREST The authors have no conflict of interest to dis- close. ACKNOWLEDGEMENTS Financial support from the UGC BSR to one of the authors (HN) by means of Junior Research Fel- lowship is gratefully acknowledged. We thank to Department of Science and Technology, Govt. of India for the prolonged support for establishing research facilities at NCCR. REFERENCES [1] Ciamician, G. The Photochemistry of the Future, Science, 1912, 36, 385-394. [2] Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 1972, 238, 37 – 38. [3] Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Photoelectrocata- lytic reduction of carbon dioxide in aqueous suspensions of semiconductor. Nature, 1979, 277, 637 – 638. [4] Hautala, R.R.; King, R.B.; Kutal C. Solar Energy: Chemical Con- version and Storage, Humana Press, 1979.

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