Journal of Chemistry and Applications
Photocatalytic and Photoelectrochemical Reduction of CO2 to Methanol in Aqueous Medium
Shahed Khan UM1*, Frites M1, Shaban YA2 and Mac Grey M3*
1Department of Chemistry and Biochemistry, Duquesne University, USA 2Department of Marine Chemistry, King Abdul Aziz University, Saudi Arabia 3National Energy Technology Laboratory, Pittsburgh, USA
*Address for Correspondence: Shahed Khan UM, Department of Chemistry and Biochemistry, Duquesne University, USA, Pittsburgh, PA 15282, USA, Email: Khan@duq.edu
Submission: 04 May, 2019; Accepted: 28 May, 2019; Published: 06 June, 2019
Copyright: © 2019 Shahed Khan UM This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this article, we summarized the recent progress made on photocatalytic and photoelectrochemical reduction of carbon dioxide (CO2) to methanol (CH3OH) in aqueous medium on various photocatalytic materials, photoactive and electrocatalytic electrodes. We have provided a critical analysis as to how the final products were found to depend on the type of photocatalyst and photo electrocatalyst used. We have delineated the synthesis of some novel photocatalysts that preferentially convert CO2 to methanol as a major or a sole product. We also discussed the reactions and parameters that are critical in enhancing the reduction yield of carbon dioxide to methanol which remains to be the major challenge. Most importantly, we explained as to why the photocatalytic reduction rates of CO2 to methanol on nanomaterials are lower compared to those on photoelectrodes and explained the ways these rates can be enhanced in future studies. Furthermore, we highlighted the intriguing challenges involved in efficient conversion of CO2 directly to methanol in aqueous medium and provided possible ways to address them.
Warming of planet earth and its consequence devastating climate change prompted researchers towards developing energy sources that are carbon footprint free using photosynthesis [1-6]. It is well known that carbon dioxide accounts for the upmost share of greenhouse gas emissions by its startling increase in the atmosphere [4,7-11]. Nearly 1.0x109 tons of CO2 gas is added to the atmosphere every year, and as a result, there will be a 50% increase to ≥ 600 ppm from its level in the year 2000 in next 50 years . Such huge CO2 emissions are believed to be largely responsible for current changes in the global climate orderliness. To resolve the CO2 problem forever would be to convert it to a valuable product, namely, methanol (CH3OH). The energy source for such a conversion needs must be CO2 emission free energy. For the emission of 5x1014 moles of CO2 in next fifty years, it will be possible to convert most of CO2 to methanol in aqueous medium using sunlight as an energy source in presence of suitable photocatalyst or photoelectrocatalyst by mimicking the natural photosynthesis process . Hence, the process of CO2 capture and its conversion to methanol will provide an ideal carbon neutral energy source to diminish the global warming and the reliance on the earth’s fast exhausting fossil fuels.
Carbon dioxide is an inert molecule thereby making its reduction to methanol relatively challenging due to the high thermodynamic barrier. The drawbacks of CO2 reduction that result from the restricted selection of semiconductors and the competitive H2 generation in aqueous medium instead of CO2 reduction were discussed in a review article . Electrochemical studies on the reduction of CO2 [16-25], described the conversion of CO2 mainly to CO and formic acid by a 2e-(two electrons) reduction process. Importantly, Hori et al. [16,18] observed the formation of CO, CH4, C2H4 and alcohols, EtOH and PrOH during the electrochemical reduction of CO2 in aqueous electrolyte solution at Cu electrodes. Formation of CO was found to occur at a higher negative potential than – 1.2 V vs NHE; hydrocarbons and alcohols were found to be favorably produced at a more negative potential than – 1.3 V vs NHE. However, the hydrocarbons and alcohols generated were of negligible amounts by electrochemical methods due to a competitive H2 evolution reaction at more negative potentials than – 1.3 V vs NHE. Kuhl et al.  carried out the electrochemical study at copper metal at high negative potentials and reported a total of 16 different CO2 reduction products including aldehydes, ketones, alcohols, and carboxylic acids. Electrocatalytic conversion of carbon dioxide to methanol with low selectivity at high negative voltage on transition metal surfaces were also reported . Importantly, the reduction of CO2 at silver cathode produced CO and H2 as major and the methanol as one of the minor products . These important results indicate that at a high negative applied potential on transition metal cathodes, the selectivity of formation of specific product is highly compromised producing multiple reduction products. This is because, at high negative potential, the cathodes become extremely electron rich and generate strong reducing atmosphere and thereby produced various CO2 reduction products having virtually no selectivity.
Azuma et al.  reported the formation of trace amounts of methane and ethylene by the reduction of CO2 at various metal electrodes, except some reasonable amounts of these products were observed at Cu electrodes. The high selectivity of the copper electrode in forming hydrocarbons during CO2 reduction triggered many studies in finding the reaction mechanism on the Cu electrode surface [26-29]. Ohta et al.  found the main products as methanol and formic acid at copper tube electrode, under atmospheric pressure, between -1.4 V and -2.0 V vs NHE. Under the optimum experimental conditions, the Faradaic efficiencies for converting CO2 electrochemically to methanol and methane were found to be 18% and 20% at -1.7 V and -1.8 V respectively. Hence, the over potential need for these reactions on copper cathode are quite excessive and dwarfs the over potential required for the oxygen evolution reaction at the counter electrode. The electrochemical reduction of CO2 to methanol at such negative voltage is energy intensive and therefore becomes impractical for cost-effective production of methanol from CO2 and water. Hence, the studies on photocatalytic and photoelectrocatalytic reduction of CO2 specifically to methanol should be focused.
Various products of CO2 can be obtained when it is reduced in an aqueous medium either electrochemically, photocatalytically or photoelectrocatalytically. Two of these products can be directly used as fuels, one is methanol (CH3OH), a liquid and other is methane (CH4), a gas. The third product is carbon monoxide gas (CO), which must be used with H2 as syngas to produce methanol in a two-step process. The two-step method of fuel production from CO may not be desirable at this time in the absence of efficient carbon neutral hydrogen generation from water. Hence, the challenge at this time is to develop a single step process that can selectively produce methanol from carbon dioxide and water with high yields.
The reactions and thermodynamic potential (at pH 7) for methanol formation during CO2 reduction in aqueous medium by simultaneous electron (e-) and proton (H+) transfer reactions can be given as ,
Though the thermodynamic potential of this reduction reaction in water is low, the thermodynamic potential for its accompanying water oxidation reaction is quite high,
Hence, the overall reaction of CH3OH formation by CO2 reduction in aqueous medium is obtained by addition of Eqs. (1) and (2) as,
The limitations of the photocatalytic CO2 reduction arise from the high voltage needs (Equation 3) and the competitive hydrogen evolution reaction in the aqueous medium . In addition to the high potential needs, there is also the catalyst poisoning effect of CO2 reduction products or intermediates which ultimately may diminish the catalyst active sites [30,31]. Most importantly, CO2 may preferentially adsorb in its molecular form on some catalyst surfaces that facilitates its reduction by simultaneous electron and proton transfer processes [30,32]. The preferential adsorption of CO2 could be advantageous to limit the competitive H2 evolution reaction on these surfaces.
In this article, we concentrated on the photocatalytic and photoelectrochemical reduction of CO2 to solar fuel methanol in aqueous media. We have also discussed the present challenges faced by this process, how some of these were partially addressed and how to overcome them in future studies.
Photocatalytic reduction of CO2 to methanol:Several studies [1,33-48] were reported on CO2 reduction to methanol using photocatalysts in aqueous medium. Among various semiconducting photocatalyst materials, TiO2 was found the most feasible by various authors to use for the reduction of carbon dioxide in aqueous medium [1,35,38,39,41,42,44,47,49-52]. This is for the reason that high band gap semiconductors were needed to overcome the high thermodynamic barrier involving CO2 reduction coupled with water oxidation in an aqueous medium.
Photocatalytic reduction of carbon dioxide using Ru-doped Titanium oxide anatase mounted on silica was reported by Sasirekha et al. . In this study, Ru-TiO2/SiO2 showed practically four-fold enhanced activity for the methanol production compared to that on Ru-TiO2. Methane formation was favored instead of methanol when TiO2 photcatalyst was used in a suspension in aqueous media (0.1%, w/v) in presence of a depolarizer or hole scavenger. This indicates that Ru-decoration of TiO2 was essential for the selective reduction of CO2 to methanol.
Tseng et al.  studied the photoreduction of CO2 using solgel derived TiO2 and Cu loaded TiO2 catalysts and observed the formation of methanol. Figure 1 shows the process of O2 separation from water by photogenerated holes in TiO2 and reduction of CO2 to CH3OH on Cu surface by electrons photogenerated on TiO2.
However, the copper loaded titanium oxide-based photocatalyst was found active only under UV light though the Sunlight at Earth’s surface is about 3 to 5 percent ultraviolet.
Ohno et al.  photocatalytically reduced CO2 on exposed brookite phase TiO2. The methanol generation enhanced considerably by Photo-deposited Au or Ag nanoparticles on brookite (TiO2) nanorods. These metal nanoparticles may have acted as highly active sites for the reduction of CO2 which involves multi-electron and proton transfer processes. Using the results of an isotope labeling experiment using 13CO2, the generation of CH3OH was identified to be from the 13CO2 reduction.
Li et al.  reported photocatalytic activities of CdS (or Bi2S3)/ TiO2 nano tube for the reduction of CO2 to CH3OH under visible light illumination. The photocatalytic activity for the reduction of CO2 to methanol on Bi2S3/TiO2 under visible light illumination was found to be much higher than that on CdS/TiO2 nanotubes (TNTs). Furthermore, the yield of methanol on Bi2S3/TNTs photocatalyst under visible light irradiation was found to be ~ 45.0 micromoles per gram of catalyst per hour (μmol.g-1cat.h-1), which was ~ 2-fold higher than that on undecorated TNTs.
The reduction of carbon dioxide was investigated on photocatalyst, AgBr/TiO2 under exposure of visible light of wavelength, λ > 420 nm . The AgBr/TiO2 exhibited a high activity for the reduction of CO2 to methanol in aqueous media. This was ascribed to its strong visiblelight activity. Furthermore, this photocatalyst was found stable on multiple uses. This may be due to transfer of photogererated electrons from the conduction band of AgBr to that of TiO2.
The role of copper species (e.g. Cu0, CuI , or CuII ) on photoinduced reduction of CO2 to methanol was reported . Among the Cu loaded TiO2 species the CuIIO/TiO2 enhanced methanol generation compared to other Cu species. The activation energy (Ea) of CO2 reduction on TiO2 Degussa-P25 was found 26 kJ. mol-1 and on 3% CuIIO/TiO2 it was found 12 kJ. mol-1. The diminished activation energy on 3% CuIIO/TiO2 implies a catalytic role of copper in boosting the methanol production rate. Enhanced yield indicated that the copper (II)-oxide species acted as an electron trap and thereby reduced electron-hole recombination. The visible light absorption by p-type CuIIO and generation of p/n junction between p-CuIIO/n-TiO2 also contributed in diminishing the recombination of photogenerated electrons and holes and consequently improved the yield.
Fe2O3-TiO2 nanoparticles synthesized by a sol-gel method were utilized for CO2 reduction to methanol . In addition to UV light absorption, the presence of Fe2O3 helped to broaden the absorbance of Fe2O3-TiO2 composite material to visible region. The yield of methanol formation was found to be as high as 45.6 μ mol. g-1cat.h-1. The mechanism of the photocatalytic reduction of CO2 on Fe2O3-TiO2 was also explored in terms of electron-hole generation, transition and separation. Nitrogen doped n-TiO2 was synthesized using amorphous TiO2 anatase and ammonium hydroxide at 100 °C . The reduction of CO2 to methanol in aqueous solution on this catalyst yielded methanol of 23.0 μmol. g-1 cat. h-1.
The photocatalytic activity of Ag-loaded TiO2 was found to be about 9 times higher than that of bare TiO2 . The optimum amount of Ag loading on TiO2 was found to be 2.5 % which produced an energy efficiency of 0.5 % and methanol yield of 30.0 μmol. g-1cat.h-1. Furthermore, a synergetic mechanism between UV light excitation and surface plasmon resonance enhancement was proposed to elucidate CO2 reduction under various reaction conditions.
Cu2O/TiO2 nanotube (TNTs) arrays showed an enhancement in the photocatalytic activity during the reduction of CO2 to methanol . The surface morphology showed that the Cu2O nanoparticles decorated the TNTs. The increased absorption in the visible region by Cu2O/TiO2 compared to the plain TNTs was determined by UVVis spectral analysis. This could be due to lower band gap energy of Cu2O of ~ 2.2 eV compared to 3.2 eV for TiO2. The Cu2O/TiO2 composite material facilitated the reaction process where the CO2 photoreduction occurred on p-Cu2O sites and water photooxidation on n-TiO2 sites. The p/n junction between p-Cu2O and n-TiO2 was also responsible for high rate of CO2 photoreduction due to voltage drop across the junction that helped to minimize the rate of recombination of photogenerated carriers.
Pure TiO2 and silver-enriched TiO2 powders were tested for the photocatalytic reduction of CO2 to methanol . Ag particles improved the photocatalytic activity of Ag/n-TiO2 compared to pure n-TiO2. When the Ag loading in TiO2 was ≤ 5%, the impurity band due to presence of Ag was produced in the TiO2 band gap that helped to lower the absorption edge and thereby enhanced the photogeneration of electron-hole pairs. However, metallic clusters of Ag were formed on TiO2 crystals when the Ag loading was > 5%.This produced Schottky junction at the metal-semiconductor interface that helped to enhance electron-hole separation and thereby minimized their recombination rate.
TiO2-passivated p-GaP was utilized to reduce CO2 to methanol . The TiO2 layer was used to prevent the photocorrosion of the GaP. In addition to increased stability by TiO2 the photoconversion efficiency did enhance due to formation of a p/n junction which causes to better separation of photogenerated carriers and hence minimized their recombination. This also affected a shift in the energy need by about 0.5 eV. For the TiO2 thicknesses above 10 nm no enhancement in the photoactivity was observed due to insulating nature of the TiO2, thus, outweighing the benefits of passivation. The process of reduction of CO2 in aqueous media is shown in Figure 2.
Copper or Cobalt loaded TiO2/ZSM-5 catalysts were used for the reduction of CO2 to methanol .The conversion efficiency of CO2 to methanol was found to enhance under low energy irradiation compared to bare TiO2. The highest rates of CH3OH formation were found to be 50.05 and 35.12 μ mol. g-1 cat. h-1 for Cu and Co loaded TiO2/ZSM-5, respectively. This indicates a better selectivity of Cu compared to Co in reducing CO2 specifically to methanol in aqueous medium. Plasmonic Au decorated TiO2 photocatalyst produced CH4, CH3OH and HCHO as main products by CO2 reduction.
Graphene-TiO2 (Gn-TiO2) photocatalyst synthesized by reducing graphite oxide using hydrothermal method, reduced CO2 to methanol and formic acid . With 8.5% graphene loading the yield of methanol and formic acid, under light illumination of 365 nm wavelength, reached 160 and 150 μmol g−1 cat. h-1 respectively. However, it was found that an increase in graphene loading beyond the optimum 8.5% decreased the efficiency of CO2 reduction by shielding the light from reaching the surface of the photocatalyst.
Copper nanoparticle (Cu-NP) -covered graphene oxide (GO) was used to reduce CO2 under visible light illumination . Photocatalytic reduction of CO2 was found to enhance by 60-fold or more on Cu-NP/GO (10 wt. % Cu load) compared to that on pristine GO. A schematic illustration showing the CO2 reduction process to methanol and acetaldehyde on these nanoparticles is shown in Figure 3.
Ruthenium trinuclear polyazine complex grafted on graphene oxide support containing phenanthroline ligands (GO-phen) was used for the production of methanol from CO2 by its photocatalytic reduction . After 48 h illumination the yield of methanol was found to be 82.0 μmol. g-1cat.h-1. Ruthenium trinuclear polyazine complex grafted on graphene oxide support exhibited a higher photocatalytic activity compared to those on bare graphene oxide itself.
Carbon dioxide reduction products such as CH4, CH3OH and HCHO were observed when Pt deposited nanocomposite, g-C3N4- Pt photocatalyst was used for photocatalytic reduction of CO2 under simulated solar irradiation . Platinum deposited co-catalyst helped to enhance both selectivity of the products and as well as its reaction rates. The Pt nanoparticle (NP) co-catalyst facilitated the electron transfer from g-C3N4 to its surface for CO2 reduction. This study shows the effect of tiny amounts of Pt to increase the photoactivity and selectivity of g-C3N4 for the generation of CH3OH, CH4 and HCHO as the reduction products of carbon dioxide (Figure 4). However, the yield was quite low.
Li et al.  investigated the photocatalytic performance of visible light active Cu2O modified SiC nanoparticles (Cu2O/SiC NPs) for the CO2 reduction mainly to methanol. Under visible light irradiation, the yields of methanol generated using SiC (band gap of 2.23 eV), Cu2O (band gap of 1.95 eV) and Cu2O/SiC photocatalysts were 153, 104 and 191 μ mol. g-1 cat. respectively in 5-hour reaction time. It should be noted that the p/n junction between p-Cu2O and n-SiC in Cu2O/ SiC NPs helped to enhance the CO2 reduction rate by enhancing the separation of photogenerated carriers and thereby minimizing their recombination.
Li et al.  also explored the photocatalytic activities of Bi2S3, CdS and Bi2S3/CdS for carbon dioxide reduction to methanol under the exposure to visible light. Bi2S3/CdS hetero-junction photocatalyst exhibited a superior photocatalytic activity during the CO2 reduction compared to individual Bi2S3 and CdS photocatalysts. The highest yield of methanol was 122.6 μmol. g-1Cat. h-1 when the optimum weight ratio of Bi2S3 to CdS was 15%.
Aluminum or gallium decorated ordered layered double hydroxides (LDHs) of zinc and/or copper was found to efficiently reduce CO2 to methanol under illumination of light . Producing a yield of 170 nano mol. g-1 cat. h-1. Additionally, the methanol selectivity was found to improve by inclusion of Cu from 5.9 to 26 mol% in Zn–Al LDH photocatalyst. Also, methanol selectivity was found to enhance by addition of Cu from 5.9 to 26.0 mol% in Zn-Ga LDH photocatalysts.
Lamellar BiVO4 reduced CO2 selectively to methanol under exposure to visible light . This lamellar BiVO4 was prepared using the surfactant-assisted hydrothermal process. Addition of NaOH solution to the reaction mixture with BiVO4 suspension was found to significantly enhance methanol yield. A mechanism for the methanol generation using BiVO4 photocatalyst by CO2 reduction was also discussed. The possible photocatalytic mechanism was illustrated in Figure 5. The dependences of CH3OH production by photocatalytic CO2 reduction on photocatalyst, BiVO4 on NaOH concentration are shown in Figure 6. This figure shows enhanced amounts of methanol formation with increase in NaOH concentration. At a higher concentration of 1.0 M NaOH the yield of methanol was found to be 27.5 μ mol. g-1cat. h-1.
The absence of H2 gas in the product gases confirmed that the competitive hydrogen evolution reaction (HER) did not occur. This result is consistent with the previous report that BiVO4 cannot produce H2 due to its unmatched energy band structure with the water reduction potential . Importantly, Figure 6 shows the enhanced CH3OH production with the increase of NaOH concentration. This observation was attributed to increased dissolution of CO2 in higher concentration of NaOH, higher reduction rate of CO2 by minimizing the proton reduction rate and as well as enhanced coupled O2 evolution reaction in the alkaline medium.
The influence of loading of various metal oxides such as Fe2O3, CuO and NiO on the photocatalytic activity of InVO4 was reported . The Fe2O3-loaded InVO4 (Fe2O3/InVO4) markedly enhanced the methanol yield by minimizing the recombination of the photogenerated carriers due to their effective separation on it. The yield of methanol was found to be 35.6 μ mol. g-1 cat. h-1.
Martin et al.  investigated the effect of the reaction media such as NaOH, NaOH + Na2SO3 (1:1), NH4OH, NH4OH+Na2SO3 (1:1), on photocatalytic reduction of CO2 on ZnS deposited montmorillonite (ZnS-MMT). The NaOH solution was found to be most favorable reaction medium among these for the reduction of CO2 to methanol. It should be noted that the addition of Na2SO3 which acted as a reducing agent resulted in improving methanol yields by minimizing its back oxidation reaction.
The photocatalytic reduction of CO2 to methanol was carried out on the reduced graphene (rG) modified Ta2O5 photocatalyst (Ta2O5- rG) loaded with Ni/NiOx in aqueous solution . The composite photocatalyst was prepared using a one-step hydrothermal method. Ni/NiOx was used as a co-catalyst and its load was limited to 3% per weight of the Ta2O5-rG. Different weight percent ratios of rG to Ta2O5 were investigated. In this study, methanol generation was found to depend on the amount of reduced graphene in the Ta2O5-rG photocatalyst. The highest activity of the composite photocatalyst was observed when 1% of rG was used. However, if the percentage of rG was increased beyond 1% then the yield of methanol decreased due to negative effect of rG on absorption of light.
Detailed studies were performed by Kavil et al. [77,78] for the reduction of CO2 dissolved in sea water to methanol using their hydrothermally synthesized copper loaded carbon modifiedn-TiO2 (Cu/CM-n-TiO2) under both UV-light and actual natural sunlight illumination. Generation of methanol of maximum 582 μ mol. g-1 cat. h-1 under UV light illumination and 182 μ mol g-1 cat. h-1 under actual natural sunlight were observed. These are the highest yield of methanol reported so far under UV and natural sunlight illumination on copper loaded carbon modified n-TiO2 (Cu/CM-n-TiO2). It is important to note that such a high yield on this photocatalyst nanoparticles can be attributed to mainly three factors such as: (1) greater visible light absorption by carbon modified n-TiO2 [79,80] (2) enhanced catalytic effect of Cu for the conversion of CO2 specifically to methanol in aqueous medium  and (3) increased separation of photogenerated carriers by Schottky junction at the Cu/CM-n-TiO2 interface that minimized their recombination.
Mechanism of reduction of CO2 to methanol on photocatalyst (e.g., n-TiO2) surface:The mechanism of reduction of CO2 in aqueous medium on the widely used TiO2photocatalyst surface can be depicted in terms of the following equations which involve six electrons and six protons transfer processes. equation (4) below shows the photogeneration of electrons ecb- (cb conduction band) and holes hvb+ (vb valence band). The photogenerated electrons in the conduction band (ecb-) react with H+ ions to generate hydrogen radical (H•) according to equation (5). The highly reactive H• reacts with CO2 molecule as in equation (6) to produce methanol and water. As a counter reaction, the photogenerated holes (hvb+) react with hydroxide ions OH- to generate O2 and H2O as given in equation (7).
The overall reaction is given by addition of (Equations 4-7) as,
Photoelectrochemical reduction of CO2 to methanol:The semiconductor photocatalysts [8,35,38,39,41,49], have been used for photoreduction of carbon dioxide in an aqueous electrolyte to produce various products such as methane, methanol, ethylene, ethanol, carbon monoxide and formic acid etc. However, the limitations of the photocatalytic method are the low yield and hence the low photoconversion efficiency. This is because under sunlight illumination the photocatalysts are unable to generate high enough photovoltage to enhance the yields of this energetically challenging reaction of CO2 reduction to methanol in aqueous medium. On the other hand, in photoelectrochemical method, in addition to the solar energy, a minimal amount of energy would be possible to be supplied from an external power source or from solar panels to enhance the rate of the reduction of CO2 to methanol. In this section, we will discuss various studies conducted on the photoelectrochemical reduction of CO2 to methanol.
In a study by De Brito et al. , the photoelectrochemical reduction of CO2 was carried out using a Cu/Cu2O electrode under UV-visible radiation in which the reaction products were monitored overtime. The formation of methanol, ethanol, formaldehyde, acetaldehyde, and acetone was reported. The photoelectrons in the p-Cu2O reduced CO2 to methanol for period of < 30 min, but produced acetaldehyde, and acetone after a longer period > 120 min. The pH of the electrolyte was found to act as a key factor for the selective generation of the methanol. It was an interesting observation that the types of products were found time dependent. However, no explanations were provided for such a finding. This result indicates that the initially formed methanol was converted to other products after 2 hours. Hence, to collect methanol as the main product, it must be removed within 30 min from the reaction mixture. Also, the use of the Cu-oxide as photocathode may not be realistic since it may be ultimately reduced to metallic Cu under cathodic polarization and thereby will lose its photoactivity.
Morikawa et al.  used reverse photo-fuel cells for the photoelectrochemical oxidation of water and reduction of CO2 using and WO3 and a layered double hydroxide (LDH), separated by a polymer electrolyte (PE) film. WO3 was used for the photooxidation of water, whereas LDH, comprised of Zn, Cu, and Ga, was utilized for the reduction of CO2. The 68% - 100% of observed photocurrents were found to be due to reduction of CO2 to methanol.
In an important communication, Barton at al.  investigated the reduction of CO2 to methanol on pyridine-catalyzed p-GaP photoelectrode in a photoelectrochemical cell (PEC). The band gap energy of this p-GaP photoelctrode is 1.6 eV which allowed it to absorb most of the visible light in solar spectrum. In the presence of pyridinium, methanol formation was observed at - 0.4 V vs SCE with faradaic efficiencies extending from 88 to 100%. However, no methanol production was observed in the absence of pyridinium. The mechanism is depicted in Figure 7.
The quantum efficiency for the photoelectrochemical conversion of CO2 to methanol was found to be as high as 10.9 % at – 0.50 V/ SCE under UV light illumination of wavelength 365 nm (3.39 eV). However, at the same voltage condition of – 0.5 V/SCE, the quantum efficiency was found to be 1.05 % under visible light illumination of wavelength 465 nm (2.66 eV) at which the intensity of solar light of AM 1.5 (1 sun) is maximum. These results indicate that the photogenerated carriers under lower energy photons recombined faster than those generated under high-energy photons and thereby tenfold lower quantum efficiency was observed. Reduction of CO2 selectively to methanol is challenging due to closeness of reduction potentials for methanol, formic acid, and formaldehyde. However, reduction of CO2 in presence of pyridine may have overcome this limitation. A probable mechanism of reduction of CO2 in presence of homogeneous catalyst pyridine was put forward by Barton et al. .
A cluster model was used for the theoretical prediction of structures and binding energies for charged and neutral adsorbates on the GaP(110) surface . The model calculations were made both with the use and without the use of van der Waals interactions and solvation. The binding energy contributions for various adsorbates were found relevant in the CO2 reduction process.
The composition, the crystal size, pyridine coverage, and the applied bias on p-CuInS2 photocathode were found to influence the methanol yield during the CO2 reduction . The mass transfer across the adsorbed pyridine layer was proposed to be the ratedetermining step.
Deposition of the transition metal islets (i.e. Ag, Au, Cd, Cu, Pb, and Sn) on CuO/Cu2O films increased CO2 reduction to fuel as was identified from faradaic efficiency . For example, Pb on CuO/ Cu2O showed outstanding results among the transition metals. For instance, formation of 0.524 μ mol. cm-2. h-1 formic acid and 0.102 μ mol. cm-2. h-1 MeOH were observed with 40.45% faradaic efficiency at -0.16 V/ SHE. However, the CuO layer was found from XPS results to be readily reduced to Cu under cathodic polarization that degraded the performance of the photoelectrode. This indicates that even the metal islets deposition on CuO/Cu2O could not protect the copper oxide layer from reduction when used as a photocathode even under the low cathodic polarization of - 0.16 V/SHE.
Carbon-modified titanium oxide (CM-n-TiO2) was used as a photoanode and Cu metal gauze as a dark cathode to reduce CO2 to methanol in a two-compartment PEC in an aqueous electrolyte of 5.0 M NaOH [81,95]. The main product was found to be methanol. The highest amount of methanol formation was observed at low negative voltage when the pressurized carbonated water mixed with 5.0 M NaOH solution was used as the electrolyte. The rate of methanol formation decreased after 5 min which may be due to back reaction at the photoanode and/or consumption of all the dissolved carbon dioxide (aqueous CO2) in the electrolyte. A two-fold increase in methanol formation was observed when 0.5 mM methanol was initially added. This enhancement may be due to added methanol acting as a depolarizer or a sacrificing agent at the photoanode and due to higher solubility of CO2 in initially added methanol.
It should be noted that no methanol formation was observed when CO2 was dissolved directly in 5.0 M NaOH solution [81,95]. This observation may be attributed to formation of carbonate ion in solution when CO2 is dissolved directly in 5.0 MNaOH, instead of having molecular form of CO2 present in pressurized carbonated water. It was possible to reduce the linear molecular form of CO2 to methanol at low negative voltage but not the non-linear CO3 22 anion at the same low voltage.
Using platinum-deposited reduced graphene oxide (Pt-r-GO) as a cathode and Pt-decorated TiO2 nanotubes (Pt-TNT) as a photoanode in a PEC, CO2 was converted to methanol, ethanol etc. . Total combined rate of production of CH3OH, C2H5OH, HCOOH, and CH3COOH) was found to be 0.6 μmol. cm-2. h-1. It should be noted that when Pt-modified carbon nanotubes or platinized carbon was used as a cathode instead of Pt-r-GO the conversion rate of CO2 was much lower. This observation indicates that the reduced graphene has unique property in reducing CO2 in a PEC though it is not specific to methanol formation and the total yield was quite low.
One dimensional wedged or compact nitrogen-doped CuO was prepared on Cu substrate by its anodization. The resulting CuO semiconductor exhibited an energy band gap of 1.34 eV . The photoelectrocatalytic reduction of CO2 produced predominantly methanol with a current efficiency of 84.4% which was about 15 times higher compared to 5.84 % on bare CuO film. However, the methanol output (600 μmol. L-1. cm-2. h-1) was 139 times higher than that on the regular CuO film (4.33 μmol. L-1.cm−2.h-1). This highly enhanced photo response of wedged nitrogen doped CuO compared to regular undoped CuO film can be attributed to the fact that nitrogen sites acted as the active center for CO2 adsorption and as well as enhanced surface area of wedged sample.
Hydrothermally synthesized highly ordered TiO2 nanotube arrays (TiO2NTs) were decorated by MoS2-rods to produce MoS2-rods/TiO2 NTs heterojunction , the band gap of which was found to be 1.55 eV. The rate of photoelectrochemical reduction of CO2 using MoS2- rods/TiO2 NTs / Pt PEC enhanced more than two folds compared to that on the regular TiO2 NTs /Pt PEC. Also, the faradaic efficiency or current efficiency increased by 2.65 times and the methanol yield increased by 2.29 times from 263.25 μmol. L-1. cm-2. h-1 to 603.75 μ mol. L-1. cm-2. h-1. This marked enhancement of methanol yield on MoS2-rods/TiO2 NTs heterojunction can be attributed to low band gap visible light active Mo-S2-rods and lowering the recombination rate of the photogenerated carriers due to p/n junction produced at the p-type MoS2 and n-type TiO2 interface. This advanced photoelectrochemical cell (PEC) to produce methanol isV shown in Figure 8.
Electrochemical reduction of CO2 to alcohol was achieved using a self-organized TiO2 nanotube arrays (TNAs) photoanode . Electrochemical anodization method was used (applying 20 V for 2 h at 30 °C) to fabricate the self-organized TiO2 nano tube arrays (TNAs) with Ti foils as anode in 1 M (NH4)2SO4 electrolyte solution containing 0.5 wt% NH4F. The photocatalytic conversion of CO2 and H2O to alcohols predominately methanol and ethanol was carried out using the annealed TNAs under Xenon lamp illumination. The generation rate of methanol and ethanol were found to be 10 and 9 nano mol. cm-2 h-1, respectively. Such low rates are due to absorption of only UV light by the regular n-TiO2 synthesized by anodization of Ti metal foil. As well as due to the sluggish O2 evolution on Pt catalyst used as anode.
Layered CuO/Cu2O semiconductor nanorods was prepared by thermal oxidation of Cu-foil to CuO and followed by electrodeposition of p-Cu2O on it .These nanorods photoelectrochemically reduced CO2 to methanol with 95% faradaic efficiencies. This high percentage of Faradic efficiency indicates high selectivity of CuO/Cu2O for photoelectrochemical reduction of CO2 to methanol. However, such p-type copper oxide semiconductors will be eventually transformed to their corresponding metals under the cathodic polarization condition and hence their photocatalytic activity will be completely diminished as was observed by other authors .
p-CuInS2 thin film photocathode with co-catalyst pyridinium ion was found to reduce CO2 to methanol with 97 % the faradaic efficiency . Yield of methanol was reported to be as high as ~ 200 μmol.L-1.cm-2. h-1.
Wang et al.  reported the photoelectrochemical reduction of CO2 on amine-functionalized TiO2 supported on Ni-Foam (aminefunctionalized TiO2/Ni-Foam) as a photocathode and BiVO4 as photoanode. Methanol was the main product and the rate of which was found to be ≤ 153 μ mol. h-1.cm-2. This value was found to be 15 times higher than that on bare TiO2/Ni-foam photocathode.
Mechanism of reduction of CO2 to Methanol in a PEC:One possible mechanism for the reduction of adsorbed (CO2) ad to methanol (CH3OH) in aqueous solution involves adsorbed hydrogenradicals, H• ads on the cathode surface generated by coupled electron and proton transfer reactions followed by saturation of double bonds in CO2(O=C=O) by the hydrogen radicals, formation of formaldehyde and H2O and then CH3OH formation by further saturation of the double bond in formaldehyde (CH2 = O) by hydrogen radicals (see equations 9 -13):
At the metal cathode (M):
Oxygen separation from H2O occurs at the counter photoelectrode (e.g., n-TiO2 photoanode) as:
Addition of (Equations 9-15) give the overall reaction,
Summary and Conclusion
Numerous studies were carried out on photocatalytic conversion of CO2 in aqueous medium to various organic products or fuel. Most of the studies generated limited yield of methanol on photocatalyst surface under light illumination. Thus, future challenges involve the identification of photocatalysts that can absorb both UV and visible lights of the solar spectrum, preferentially adsorb the dissolved CO2, limit the competitive hydrogen evolution reaction and as well as minimize the generation of non-fuel reduction products.
Nanoparticulate photocatalysts have the advantage that coupled CO2 reduction and water oxidation take place on the same particle. This aids in reducing the recombination of photogenerated carriers due to minimal transport distance prior to their reaction with the species at the particle-solution interface. Furthermore, nanoparticles with hetero-junction have the added advantage of generating the photo voltage that helps efficient separation of photoexcited electronhole pairs prior to their recombination. In addition, the high surface area of the nanoparticles enhances the yield. Conversely, the limitations of the photocatalysts are the low yield and consequently the low photoconversion efficiency. Under sunlight illumination, the photocatalysts are unable to generate high enough photo voltage to enhance the yield of CO2 reduction to methanol in aqueous medium (see Equation 3). This intriguing limitation dictates the use of only UV-light active photocatalysts of high band gap. Also, there is no simple way to minimize the reverse reaction that oxidizes the newly formed methanol on the same particle surface unless methods are developed to separate the product continuously from the reaction chamber.
On the other hand, the photoelectrochemical reduction of CO2 to methanol in aqueous medium has several advantages. This is because, in addition to solar energy, a minimal amount of energy from the external sources will be possible to utilize. The added advantage is that the external energy source could be from renewable solar or wind power. Alternatively, the use of appropriate combination of p-type and n-type semiconductor photoelectrodes will enable the CO2 reduction without the need of external bias [101,102]. In this case, both photoelectrodes will supply the needed photovoltage to enhance the reaction rate.
Moreover, for the photoelectrochemical method to be efficient, the appropriate cathode or photocathode materials on which the dissolved CO2 is preferentially adsorbed will need to be identified. Furthermore, photoanodes are to be selected such that they can efficiently separate oxygen from water equations 14,15 and can absorb most of the photons of the solar spectrum.
From the survey in this review it looks obvious (Table 1) that Cu or Mo containing materials such as p-CuInS2 , CuO  or Cu2O  or MoS2-rod/TiO2 NTs heterojunction  act as specific cathode or photocathodes for the reduction of CO2 mainly to methanol at low applied potential. Similarly in presence of homogeneous catalyst, pyridinium, the p-GaP photocathode efficiently reduces CO2 to methanol in aqueous medium . It is also observed in (Table 1) that Bi2S3/CdS , Mo-Clusters Cs2  [Mo6Br14] , Mo-Clusters TBA[Mo6Br14] ,Cu(II) imidazolate  are the high methanol yielding photocatalysts. However, among the photoelectrodes N-doped CuO , MoS2 rods/TiO2 NTs heterojunction  were found to have the high rates of methanol formation due to affinity of CO2 to preferentially adsorb on nitrogen sites.
Future studies may focus in identifying appropriate ratios of aqueous-non-aqueous mixed solvents where the dissolution of CO2 mainly in the molecular form is enhanced. The detailed studies should be made on the effect of the concentration of alkaline electrolyte (e.g. NaOH or KOH) on the yield of methanol. In depth study is also needed in finding the effect of the combination of various amounts of pressurized carbonated water (where CO2 dissolves mainly in the molecular form) and the concentrated NaOH solution on the yield of methanol.
In a photoelectrochemical cell appropriate membrane should be used to separate the anode and cathode compartments to stop the reverse reaction by blocking movement of methanol formed. Also, for photocatalytic reaction on photoactive nanoparticles the product methanol must be removed continuously by passing an inert gas and be collected by cold trap to avoid back reaction.
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