Mechanistic View of the Main Current Issues in Photocatalytic CO2

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Perspective

A Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction Fernando Fresno, Ignacio Jose Villar-Garcia, Laura Collado, Elena AlfonsoGonzález, Patricia Reñones, Mariam Barawi, and Víctor A. de la Peña O'Shea J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02336 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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The Journal of Physical Chemistry Letters

A Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction Fernando Fresno, Ignacio J. Villar-García, Laura Collado, Elena Alfonso-González, Patricia Reñones, Mariam Barawi, Víctor A. de la Peña O’Shea* Photoactivated Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra 3, Parque Tecnológico de Móstoles, 28935 Móstoles, Madrid, Spain. Corresponding Author * [email protected]

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Abstract

After 40 years of research on photocatalytic CO2 reduction, there are still many unknowns about its mechanistic aspects even for the most common TiO2-based photocatalytic systems. These uncertainties include the pathways inducing visible-light activity in wide-band-gap semiconductors; the charge transfer between semiconductors and plasmonic metal nanoparticles; the unambiguous determination of the origin of C-bearing products; the very first step in the activation of the CO2 molecule; the factors determining the selectivity; the reasons of photocatalyst deactivation; the closure of the catalytic cycle by the hole-scavenging reagent; and the detailed reaction pathways and the most suitable techniques for their determination. This perspective discusses these controversial issues based on the most relevant investigations reported so far. For that purpose, we have tried to view the complex CO2 reduction in a holistic manner, considering today’s state-of-the-art approaches, strategies and techniques for the study of one of the hottest topics in energy research.

TOC IMAGE

KEYWORDS. Photocatalysis, CO2 reduction, Artificial photosynthesis, Charge transfer, Reaction mechanism, Advanced characterization techniques


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Controlling climate change and developing an efficient and sustainable global energy system are two of the main current scientific challenges. According to the directives of the Paris Agreement, adopted in 2015 at the UN Climate Change Conference and corroborated at the last two COPs in Marrakech and Bonn, greenhouse gas emissions should be reduced in order to hold the increase in the average worldwide temperature below 1.5 °C with respect to pre-industrial levels. At the same time, energy demand should minimise its dependence on non-renewable fuels. Thus, the development of a carbon-neutral energy supply system to ensure a sustainable future is urgently required. One of the potential strategies in order to achieve this goal is the use of CO2 mimicking Nature’s carbon-neutral cycle but in a manageable and commercial time scale.1 The main challenge for CO2 valorisation is the high stability of this molecule that requires a large amount of energy to be activated. Therefore, a key aspect for the proliferation of CO2 transformation technologies is their integration with sustainable energy sources.2 One of the most promising approaches for this strategy is photocatalytic reduction, one of the so-called artificial photosynthetic processes aiming at producing fuels and chemicals from carbon dioxide and water using sunlight as the only energy input.3 However, despite the potential of this technology to aid both climate change and sustainable energy goals and the significant efforts devoted to its development over the last few years, only modest yields have been achieved to date.4,5 The process of CO2 photoreduction involves a series of complex and consecutive multielectronic and chemical processes occurring at different timescales (from fs to s) in the presence of CO2 atmospheres and an electron donor (mainly H2O) and under illumination. The main sequential processes are: (1) light absorption and subsequent excitation of an electron through the semiconductor band gap and consequent generation of conduction band (CB) electrons and valence band (VB) holes; (2) separation and migration of these electrons and holes to reduction


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and oxidation sites at the surface of the semiconductor; and (3) surface reactivity, involving the adsorption of CO2 and H2O molecules and their respective reduction and oxidation via electron interchange with the excited semiconductor surface. For hybrid multifunctional photocatalytic systems composed of more than one phase, an additional and essential charge transfer step at the interface of the constituting materials eventually takes place and must be taken into account.5 Therefore, in all these processes, the photocatalyst plays a crucial role and their band structure determines its light absorption ability (band gap) and the capacity of the photogenerated electrons and holes to promote reduction and oxidation reactions (band-edge positions). Moreover, the charge carrier density and the presence of recombination centres determine the ability of the photogenerated holes and electrons to separate and migrate within the semiconductor. In order to improve the overall performance of photocatalytic materials, different synthetic strategies have been developed with the aim of modifying the textural, structural and optoelectronic properties of the target photocatalysts. These approaches include band structure engineering, doping, sensitisation, deposition of metal nanoparticles and development of heterojunctions. The principles underlying photocatalytic CO2 reduction, the reactivity of different photoactive materials and their compositional and structural modification have been the subject of several excellent reviews.4-12 However, despite nearly 40 years of research on CO2 photocatalytic reduction and the increasing number of works dealing with this application, there are still many unknowns on the mechanistic aspect of the reaction. These uncertainties even hold for the most commonly studied photocatalytic systems based on TiO2, which is still preponderant and used in around of 50% of the literature works on CO2 photoreduction.13 This perspective discusses these controversial issues, focusing on the most significant mechanistic, kinetic and characterisation studies reported over the last few years.


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Light absorption and charge separation/transport. The absorption of light by the photocatalytic system constitutes the very first step needed to initiate the CO2 photoreduction, taking place in timescales in the range of fs, followed by charge carrier formation and subsequent transport to the surface. According to thermodynamics, in order for a single semiconductor material to drive the photocatalytic process, the energy separation between its conduction and valence bands needs to be at least larger than the difference in the related CO2 reduction and water oxidation potentials. On top, there is an activation barrier in the charge transfer process, which requires an overpotential to drive the reaction. This implies the use of semiconductors with absorption onsets typically in the UV region that only cover 4-6% of the solar energy reaching the Earth crust. To overcome this limitation, several strategies have been developed to reduce the energy necessary for driving the photocatalytic process (Figure 1), such as: (1) coupling semiconductors or mixed phases with different band-edge positions forming a heterojunction; (2) doping the semiconductor with metal (e.g. Fe, Ag, Cu) and non-metal (e.g. N, S) elements to narrow the bang gap; (3) deposition of noble metal nanoparticles with localized surface plasmon resonance (LSPR) effect; (4) sensitisation with light harvesting dyes or macrocyclic ligands (Ru-bipyridine, phthalocyanins, porphyrins, among others) that transfer photo-generated electrons directly into the conduction band of the semiconductor.5,10 Nevertheless, despite the significant advances in synthetic routes, most strategies present inherent limitations that still need to be solved. The stability of dyes and their regenerability are the most important concerns for sensitisation. In the case of semiconductor heterojunctions, appropriate band energy levels are required to an efficient coupling that improves the charge transfer processes. On the other hand, dopants can also act as recombination centres partially supressing the benefits of increased absorption.5


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Figure 1. Strategies to modify semiconductors to extend their optical absorption range by (1) semiconductor heterojunctions, (2) doping to create intra-band gap states (dashed line), (3) deposition of metal nanoparticles with local surface plasmon resonance (LSPR) and (4) sensitisation. Reproduced from ref. 14. Light absorption and photogeneration of charge carriers is followed by their transfer to the surface where the redox reaction takes place. These processes take place in the range form picosecond to millisecond and usually result in electron-hole recombination before they can participate in their respective reactions. The efficiency of charge separation and multi-electron transfer is strongly determined by the intrinsic physico-chemical properties of the semiconductor. Surface and bulk defect sites can also act as recombination centres of photogenerated charges, inevitably reducing the overall quantum yield. Therefore, in order to maximise the charge


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separation and transfer efficiencies, it is essential to understand how the materials manage these photogenerated charges and the intermediate steps implied in these processes. Time-resolved spectroscopic techniques can supply meaningful information about the nature of the defect sites and the efficiencies of charge carrier trapping, migration and transfer. All of them are key aspects for understanding the CO2 photoreduction mechanism and designing new photocatalysts with improved efficiencies and controlled selectivity. Previous studies have provided significant insights into the photosynthetic water splitting mechanism by tracking the charge carrier dynamics in the photocatalytic systems. By using transient absorption spectroscopy (TAS), Durrant and co-workers15,16 studied the hole lifetimes of TiO2 over a range of excitation intensities, and proposed a mechanism for oxygen production in which four holes/photons are required to form each molecule of oxygen. In addition, electron/hole recombination occurs in the range of picoseconds to milliseconds depending on the materials nature and light intensity excitation,15,17 being a critical factor that limits the photoactivity. On the other hand, the appearance of trap states within the semiconductor band gap are also the responsible for the recombination of these charge carriers.18,19 By using photoluminescence (PL) measurements under weak excitation conditions, Wang et al. showed that the distribution of trap states produces visible emission in anatase TiO2 and NIR emission in rutile with lifetimes up to the millisecond range.18 Specifically, trapped electrons and holes in anatase recombine with oxygen vacancies and hydroxyl groups, while in rutile trapped states are very deep and electrons recombine with free holes. These deep energy levels may not participate in photocatalytic reactions, which would at least partially explain the generally lower photocatalytic activity of rutile in comparison to anatase.20 Jiang et al.21 employed ultrafast TAS and time-resolved photoluminescence spectroscopy to study the CO2 photoreduction process and found a long-lived


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electron trap state on a porphyrin-based MOF photocatalyst that substantially suppressed the detrimental electron−hole recombination, thereby boosting the efficiency of the visible light driven-CO2 photoreduction. Regarding organic sensitisation, several studies have tried to track spectroscopically the electron injection and the recombination dynamics in different photocatalytic systems. By using femtosecond TAS, for instance, Hilgendorff et al.22 reported electron injections from a dye molecule (fluorescein 27) to TiO2 with a characteristic time constant of 300 fs, and a slowdown in the recombination rates ranging from ∼10 ps up to nanoor even microseconds timescales. Schnadt et. al. used the core hole clock method, that allows the measuring of charge transfer phenomena down to the attosecond scale, to monitor electron transfer times from an organic adsorbate to a TiO2 semiconductor of less than 3 fs.23 As mentioned before, the deposition over semiconductors of nanoparticles (NPs) that possess localized surface plasmon resonance (LSPR) improve the photocatalytic activity mainly in the range of visibly light.24-26 Although the physics of LSPR in NPs is well understood thanks to the intensive research over the last years,25,26 the charge transfer mechanism in plasmonic photocatalysis is still a matter of debate. Understanding this charge transfer mechanism is not an easy task and only a few studies have provided spectroscopic evidence to explain the photocatalytic behaviour. Cushing and co-workers26 proposed a resonant energy transfer mechanism in an Au@SiO2@Cu2O sandwich nanostructure using transient absorption and photocatalysis action spectrum measurements. They found that the gold core converted the energy of incident photons into localized surface plasmon resonance oscillations, and this energy was transferred to the Cu2O semiconductor generating electron-hole pairs, which greatly enhanced the visible-light photocatalytic activity as compared to the semiconductor alone.


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By using surface photovoltage spectroscopy (SPS), Xie et al. investigated charge transfer, catalytic activity and selectivity in the photoreduction of CO2 to formate using Na2SO3 as electron donor and Pd/RuO2/TiO2, Pd/TiO2, a-TiO2, and b-TiO2 as photocatalysts.27 The surface photovoltage response increased after the deposition of the co-catalysts, leading to a new response band in the SPS curve of Pd/RuO2/TiO2 or Pd/TiO2 appearing in the near IR. Moreover, the catalytic activity of CO2 photoreduction improved. These authors found that the electron transfer from TiO2 to Pd surface sites promotes the reduction of CO2 to formate, while the holes are injected into RuO2 surface sites promoting the oxidation of SO32− to SO42−. Studies of gas-phase photocatalytic CO2 reduction over Au/TiO2 catalysts show that an increasing lifetime of charge separated states through interfacial electron transfer between the TiO2 and Au NPs, followed by TAS,28 which led to a remarkable improvement of the production of high electron-demanding products, especially CH4. The activation of the CO2 molecule. A list of common CO2 reduction potentials to different products compared to the band edge energies of some common photocatalytic materials is shown in Figure 2. In most reviews and articles the Eº for CO2/CO2- pair (-1.9 V at pH=0) is included, pointing out that it is thermodynamically unfeasible for most semiconductors. However, in order for this reduction to occur, carbon dioxide needs first to adsorb onto the surface leading to the molecule activation and changing its structure from linear to bent (CO2*).29,30 This molecular bending is ascribed to a charge transfer from the material (even at dark conditions) leading to a partially charged CO2-δ (activated),31-34 which has been differentiated from the CO2- ion by others,35,36 and is thermodynamically more favourably for the electron injection into the CO2 molecule. STM studies have shown that the adsorption of CO2 on oxygen vacancies leads to more stable conformations where the charge transfer can occur37 and Indrakanti et al. have


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studied the adsorption of CO2 on small clusters using both post-Hartree–Fock and DFT methods, and showed that transferring an excited electron is favourable at oxygen deficient surfaces and unfavourable on stoichiometric TiO2.38 Once it has been adsorbed/activated, the photoinduced charge transfer occurs, however it is not still clear how photoelectrons are implied in these processes. Reaction pathways based on twoelectron processes, that would avoid the energetic barrier, have indeed been calculated to be more energetically favoured than one-electron processes both for the photoreduction of CO2 and for some of the potential intermediates.39-41 However, there is very little experimental evidence for such multi-electron processes. The production of CO as a consequence of a single-photon, two-electron transfer has only been supported experimentally for a TiMCM-41 sieve system.41-42

Figure 2. Band gaps of some photocatalytic materials and their valence and conduction band positions compared to the potentials of redox pairs relevant to CO2 reduction at pH=7. Based on ref. 8. The CO2 activation and subsequent charge transfer over catalytic surfaces have also been demonstrated by different STM studies, in which the transfer of an electron to a CO2 molecule adsorbed on an oxygen vacancy site is forced by applying a negative potential. (Figure 3 a-d) In


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turn, this adsorption and consecutive charge transfer led to the dissociation of the CO2 molecule by filling the oxygen vacancy and liberating a CO molecule43 which was found to fully or mainly (90% of observed molecules) desorb from the surface after dissociation in these studies (Figure 3e). The calculated dissociation potential, around 1.4 eV, is higher in energy than the top of the valence band of TiO2 (Figure 3f).29 This suggests that the actual dissociation of the CO2 molecule in working systems might be related to other experimental factors that cannot be reproduced at STM analysis conditions.

Figure 3. (a) STM Images of bare TiO2 (110)-1×1 surface, (b) after CO2 adsorption in situ at 80 K, and (c) after tip-induced CO2 dissociation. (d) Typical I-t curve during the voltage pulse. (e) Schematic drawing of the tip-induced CO2 dissociation, leading to the healing of an oxygen vacancy and CO, either desorbed or adsorbed at a Ti4+ site. (f) Plot of CO2 dissociation as a function of the tunnelling current measured at different bias voltages. Reproduced with permission from ref. 44. Copyright 2011 American Physical Society.


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It was then suggested that electron injection could be correlated to the presence of other coadsorbed species such as water in order to be thermodynamically feasible. In fact, the surface of TiO2 at pressures higher than 2 mbar is mainly hydroxylated and covered by H2O multilayers as shown in XPS experiments.45 At the same time, molecular dynamics calculations have suggested that under ambient conditions adsorbed water does not cover surfaces uniformly even at 100% relative humidity, thus allowing for the gaseous molecules to bind directly to the surfaces sites.32 The consequence of the presence of H2O and OH groups is that CO2 is mainly chemisorbed as different carbonate and bicarbonate species. In addition, the presence of carboxylated species such as activated CO2* has also been identified in several FTIR studies46,32,35,47 and DFT calculations.31 The quantity of CO2* has also been shown to increase under illumination conditions and the activation barrier was found by Dimitrijevic et al. calculations to be lower in an aqueous dispersion of the photocatalyst due to the stabilization of the surface charge of the anion radical by dipolar interactions with water molecules.39 However, on working systems, CO2* dissociation to form CO has only been observed for TiO2 with metal nanoparticle systems (as no dissociation was observed for only TiO2).48-50 In conclusion, it seems like the activation of CO2 molecules to CO2* requires the confluence of several factors such as adsorption conformation and the presence of co-adsorbed reactants and co-catalysts in order to occur. For example, the CO2 chemisorption capacity of TiO2 has been improved by the addition of a second oxide like MgO or La2O3.42-51 Moreover, CO2 reduction products have been observed under photocatalytic conditions using bare TiO2 systems and further investigations are needed in order to fully identify the factors behind CO2 activation or alternative multielectronic processes behind these observed reactions.


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The origin of carbon products. In order to study a chemical process, it seems obvious that it is essential first to ascertain the nature of the starting reactants and thus the origin of the products. This is not a trivial task in CO2 photocatalytic reduction though, since desorption or decomposition of organic surface contaminants can be falsely interpreted as the formation of products.52-55 Indeed, experiments in the absence of CO2 often lead to the evolution of carboncontaining products that in some cases are of the same order of magnitude as those obtained in the actual reaction.55-58 Several strategies have been followed so far to rule out or at least minimize the effect of adsorbed organics on the surface of photocatalysts. Calcination in an air atmosphere can drastically reduce the organic content on the surface of the photocatalyst,54 and this is a procedure followed in some works, often as part of the catalyst synthesis6,59 or, less frequently, specifically to eliminate these impurities when using commercial catalysts.28,58,60 However, Ishitani and co-workers showed that, despite calcination at 350 ºC eliminates most of the organic impurities in a commercial TiO2 catalyst like P25, after this treatment some organics could still be extracted from the catalyst surface as revealed by gas and ion exclusion cromatographies.54 These remaining impurities, at concentration levels of 10-2 µmol per gram of catalyst, could be removed to virtually zero by further washing the powder with ultrapure water, although carbonate and bicarbonate species could still be detected. The reduction in the organic content of the catalyst from untreated to calcined and to further rinsed P25 was accompanied by a decrease in methane production during photocatalytic experiments. In any case, the produced CH4 amount was larger than the remaining impurities in treated samples. In order to rule out the effect of impurities, a photocatalytic pre-cleaning step has been also proposed by some authors, consisting of the irradiation of the catalyst sample in a humid inert gas stream for several hours before introducing CO2 into the reactor.55,58 Both cited works have shown that organic impurities


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on the surface of TiO2 can be either hydrogenated to methane or oxidized to CO and CO2 during this cleaning step, and that the products observed during cleaning arise in similar concentrations or even higher than those quantified during CO2 reduction. In any case, the evolution of carboncontaining products after removing surface organics by photocatalytic pre-cleaning apparently discards that those arise only from impurities and not from the conversion of CO2. In this regard, the fact that, in some experiments, the products are formed at a constant rate after pre-cleaning points in this same direction.58,61,63 Nevertheless, a steady state is not always observed,54,55,57,64 as discussed below regarding catalyst deactivation. Given the low CO2 conversion levels that are being reported even in the best case scenarios, pre-treatment protocols like the ones described above, and the use of high purity conditions, seem advisable.52,65 In addition, true CO2 reduction can in principle be elegantly confirmed or discarded as the origin of the carbon-containing products against impurity desorption or decomposition by isotopic tracing experiments, using

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C-enriched CO2 as feedstock and then

differentiating the product carbon isotope(s) in the products by mass spectrometry or infrared spectroscopy (Figure 4). However, even this apparently final proof has given controversial results in the literature. Most of the works using 13

CO,61,63,64,66-70

13

CH4,62,71-73 or

13

13

CO2 have reported the formation of

C-labelled carboxylates,34 depending on the catalyst and the

reaction conditions, leading the authors to claim, with more or less certainty, that these products actually arise from the conversion of CO2. However, the assignments are not always unequivocal. On the other hand, works in which the detected products are categorically stated, after isotope tracing experiments, to form from organic impurities rather than from CO2 are extremely rare.57 In addition, some authors have reported, after reaction of 13CO2 with H2O, the formation of products containing both 13C and 12C, which in the case of CO is proposed to result


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either from a photocatalytic reverse Bouduard reaction (eq. 1) or from photocatalytic gasification of carbon-containing impurities (eq. 2).2 Another possibility for the presence of

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C products is

the conversion of carbonate species that remain on the catalyst surface after calcination and rinsing, or are re-adsorbed upon contact with the atmosphere.3 These species, frequently detected by infrared spectroscopy, are known to form on the surface of oxides with higher or lower stability depending on the nature of the oxide,46 even with the CO2 concentrations present in the atmosphere,74 and their conversion into carbon products would in any case be electronically equivalent to CO2 reduction. With these data, it is not possible to discard the photocatalytic CO2 conversion into reduced products on irradiated semiconductor surfaces, but it is clear that adventitious carbon on the surface of catalysts exerts an influence on the obtained products. Probably, the most conclusive evidence of CO formation in the absence of carbon residues was reported by Frei and co-workers,65 who observed only 13CO production in a high vacuum IR cell upon CO2 reduction under

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CO2 and H2O over isolated Ti sites in mesoporous materials after

residual carbon removal by ozonation. 13

CO2 + “C” →13CO + 12CO

(1)

H2O + 12C → 12CO + H2

(2)


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Figure 4. In-situ DRIFTS spectra for

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CO2 and

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CO2 interaction with copper-modified TiO2

under irradiation for 20 min. Reproduced with permission from ref. 66 Copyright 2013 Elsevier B.V. Currently proposed reaction pathways. The full reaction mechanism of CO2 photoreduction involves a series of sequential steps, depending on the final products, that combine proton transfer, breaking of oxygen bonds and electron transfer. Moreover, the reaction of different intermediates involves, as suggested by theoretical calculations, several changes in adsorption modes.36 From the plethora of potential reaction pathways,75 three main different mechanisms have been proposed in the literature for the photoreduction of CO2 into its different observed reaction products: (A) the formaldehyde, (B) the carbene and (C) the glyoxal pathways (Figure 5).8,52 The first two (A-B) contain only reduction steps that involve the transfer of electrons and hydrogen atoms to produce different radical and non-radical intermediates. Meanwhile, the glyoxal pathway involves both reduction and oxidation steps.


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Figure 5. Three proposed mechanisms for the reduction of CO2 to methane. Adapted from ref. 8. The formaldehyde pathway should be favoured by fast hydrogenation conditions while the carbene pathway is proposed to be favoured by fast deoxygenation conditions Meanwhile, the glyoxal pathway involves both reduction and oxidation steps, including the dimerization of HCO radicals, and can explain the observation of C2 and C3 products for some reaction systems. This pathway, proposed by Handoko et al.,76 is based on a work by Shkrob and co-workers concerning CO2 fixation on metal oxides.77 More recently, Li et al proposed a modification of the carbene pathway, based mainly on theoretical calculations, that include the hydrogenation of


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CO to produce formaldehyde as an intermediate that is then transformed into CH3• and finally CH4 or CH3OH.37 The formaldehyde pathway could be favoured by fast hydrogenation conditions and by the adsorption of CO2 in a bidentate mode in a way that the carbon atom is available for protonation and formation of the formate intermediate.78 In this sense, a study combining theoretical calculations and EPR has shown that if the CO2- radical is doubly bounded through its oxygen atoms to the metal ions at the surface, this radical can be further reduced to formate.79 Several intermediate species characteristic of the formaldehyde mechanism have been detected (HCOOH, H2CO, CH3OH, CH4),80 and it has been recently found to be the most thermodynamically favoured mechanism from the suggested ones.81 However, this mechanism cannot explain the formation of CO, which is one of the most frequently observed products. CO could also be formed, however, from the partial oxidation of some of the reaction products or intermediates. The carbene pathway is favoured by the already described formation of a CO molecule after the dissociation of CO2 in an oxygen vacancy described above. Moreover, the presence of oxygen vacancies also favours the dissociation of water into OH groups and protons, which are available for the protonation steps in the mechanism. Several of its radical and neutral intermediates have also been observed.82-84 Kinetic modelling experiments also support this mechanism where CH3OH is a product (and not an intermediate as in the formaldehyde pathway).82 One of the shortcomings of this mechanism is that it does not include the commonly observed HCOOH molecule as one of the reaction intermediates or products (although again it could be formed by partial oxidation of the products/intermediates).


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The glyoxal pathway is the only mechanism that explains the dimerization of HCO radicals and the observation of C2 and C3 molecules as reaction products. This pathway, proposed by Handoko et al.,76 is based on a work by Shkrob and co-workers concerning CO2 fixation on metal oxides.77 The key step is the formation of glyoxal after the combination of two formyl radicals (HCO•) after CO2 adsorption onto neighbouring Ti atoms. EPR studies have demonstrated the tendency of proposed glyoxal and glycolaldehyde intermediates to be reduced to HOC•HCHO and vinoxyl radicals respectively (rather than oxidised).77 However, the same study could not confirm the sequential formation of the hydrogenated neutral products and neither glyoxal nor glycolaldehyde have been observed to give rise to reduction products under photocatalytic reaction conditions in a recent and comprehensive study on potential reaction intermediates.54 On the positive side, acetic acid, one of the main intermediates in this reaction pathway, was the only studied compound found to reduce rather than oxidise under reaction conditions in the same study. More recently, Ji et al proposed a modification of the carbene pathway, based mainly on theoretical calculations, that include the hydrogenation of CO to produce formaldehyde as an intermediate that is then transformed into CH3• and finally CH4 or CH3OH.41 In fact, most commonly proposed intermediates have been shown to rather oxidise than reduce at photocatalytic conditions by theoretical and experimental studies.

8,52,78,85,86

In a recent

theoretical study, the energy barrier for the photo-oxidation of the proposed intermediates (namely: HCOOH, H2CO, CH3OH, CO) molecules was shown to be lower than those of their photo-reduction reactions.59 Moreover, experimental studies by Dilla and co. have subjected the most probable C1 and C2 reaction intermediates to photocatalytic conditions and found that only acetic acid and acetaldehyde gave similar product distributions as when using CO2.58 Other


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proposed C2 intermediates such as glyoxal and glyoxylic acid tended to decompose to produce CO and CO2. From the C1 intermediates tested, CO showed no sign of reaction while methanol, formaldehyde and formic acid were oxidised to CO2. Dimitrijevic et al. had also arrived at similar conclusions in their studies on formic acid, formaldehyde and methanol.85 The observed behaviours cannot completely rule out all these compounds as intermediates of the CO2 photoreduction reaction as certain adsorption conformations and co-adsorption might actually lead to their reaction towards CH4. However, they do suggest that there has to be a strong competition between photooxidation and photoreduction reaction that might actually be one of the reasons for the low rates observed. Competition of H2 evolution. The role of water in CO2 photoreduction is an important aspect to be considered. Water molecules can play a dual role in the process since, on the one hand, they act as the necessary electron donors as just discussed, and, on the other hand, they compete with CO2 for conduction band electrons to reduce protons to hydrogen (hydrogen evolution reaction, HER) which is thermodynamically more favourable than CO2 reduction.5,11 In addition and prior to this, another competition is established between CO2 and water molecules for adsorption sites on the photocatalyst surface. Indeed, when water is initially adsorbed, it can block or alter the active sites of the catalyst, which is translated into lower CO2 adsorption, as already shown by Herderson some time ago.47 Similarly, if CO2 is adsorbed first, water causes a displacement towards lower bonding energies (lower desorption temperature in TPD experiments), meaning weaker CO2 adsorption. However, H2O adsorption is not affected by the presence of CO2. In the same line, Smith and coworkers87 revealed that the adsorption energies over TiO2 (110) are larger for H2O than for CO2, and that, at low temperatures, CO2 can be trapped by a water layer. At least in gas-solid reactions, the enhancement of CO2 adsorption is


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therefore a key step to suppress the competing HER and improve the overall efficiency of CO2 photoreduction. The main strategies followed for such enhancement have been thoroughly reviewed recently.7 In aqueous media, CO2 photoreduction is hampered by the low solubility of CO2 in water and the adsorption and activation of CO2 are generally more difficult than those of H2O.88 This is usually overcome by using basic solutions, in which, on the one hand, CO2 turns easily into CO32- or HCO3- and, on the other hand, the concentration of “free” protons is minimised. Modification of the surface of the photocatalyst by doping has also been claimed to alter the selectivity of the reduction half-reaction and drive it towards the conversion of CO2. Thus, Gadoping of ZnO has been reported to exhibit a high selectivity towards CO against H2 in aqueousphase photocatalytic NaHCO3 under UV irradiation.63 Similar results have been reported using Co-doped TiO2.89 A more critical aspect determining the shift of this competition to one side or the other is the presence of a co-catalyst and its nature.4 For example, Pt, which is a well-known H2 evolution co-catalyst, can lead to a high selectivity to CH4 against CO in CO2 photoreduction, but producing at the same time large amounts of hydrogen.90 However, covering Pt nanoparticles with Cu2O in a core-shell structure (Figure 6) has been shown to supress significantly the reduction of H2O to H2, resulting in a high selectivity for CO2 reduction.91 This system was proposed to provide sites for the preferential activation and conversion of CO2 at the Cu2O shell while the Pt core acts as a sink for the electrons photogenerated in TiO2.


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Figure 6. HRTEM micrographs of different TiO2/Pt/Cu2O systems obtained by photodeposition of Cu on Pt/TiO2 for increasing times from a) to d): incomplete covering of Pt by Cu2O after 1 h (a) and 2 h (b) and core-shell structures formed after 5 h (c) and 10 h (d). Reproduced with permission from ref. 91 . Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Some works have reported the preferential CO2 reduction vs. proton reduction in Ag/semiconductor systems. For instance, Ag-loaded BaLa4Ti4O15 resulted in CO2 reduction to CO and HCOOH being more efficient than H2 evolution in a liquid-phase reaction, while other co-catalysts like NiO, Ru and Au promoted mainly water reduction and Cu increased the activity for both processes.92 In similar working conditions, decoration of ZnO/ZnGa2O4 photocatalysts with silver nanoparticles resulted in decreased H2 evolution rates while keeping the CO production constant with respect to the undecorated coupled semiconductors.93 In general, metals


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with a low overpotential for CO2 reduction and a high one for HER should favour the first of these two processes and the selectivity is strongly related to the binding energy of the reduction intermediates. Indeed, in CO2 electroreduction, metals like Pb, Hg, Sn, Bi, among others, have shown high selectivity towards formate, while on Au, Ag, Zn, and Pd CO is the preferential product. On the contrary, Ni, Fe, Pt, and Ti mainly lead to the formation of H2.94 However, some of the CO2 reduction selective metals are hardly applicable in the photocatalytic reaction conditions for stability reasons. An interesting case is that of Cu, which has shown good selectivity both in electrocatalytic94 and photocatalytic5,91 conditions. The fate of photo-produced holes. Although it is not the most common case, some works report photocatalytic CO2 reduction using sacrificial electron donors other than water.10 However, even if this practice may be useful to test the CO2 reduction ability of new materials, the consumption of these sacrificial donors significantly penalizes the environmental benefits of the process and, furthermore, sometimes they are the same desired products of the CO2 reduction reaction. Thus, the full potential of artificial photosynthesis can only be reached if CO2 reduction is combined with the oxidation of water. In any case, in order to establish electro-neutrality, there has to be a parallel and isoelectronic oxidation reaction. The full water oxidation product should therefore be molecular oxygen. Stoichiometric detection of O2 would then assure complete artificial photosynthesis and exclude other possible processes mentioned above like the Bouduard reaction and the decomposition of organic impurities.53 However, as in other aspects of this reaction, reported results vary considerably, both qualitatively and quantitatively. Many works using water as electron donor do not mention the evolution of oxygen or any other oxidation product, or assume that O2 is forming but do not present experimental data. It has to be noted here that evaluating the oxygen balance is not an experimentally easy task, not only


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because of possible air leaks or residues into the reaction system, but also because of the complexity of the process, which may lead to different reaction products, some of which may remain undetected. Apparently, whether O2 evolution is reported or not does not depend on the used catalyst nor the reaction phase (gas or liquid) or mode (batch or continuous). Among the works that report oxygen quantification, there are cases in which the holes consumed by the 4-electron O2 production perfectly or nearly match the electrons used in reduction reactions. Some of these works use TiO2, either bare or modified,95-97 doped metal decorated NaTaO3,61 or other modified layered perovskites.92 In the case of NaTaO3,61 Nakanishi and co-workers carried out a systematic study of the influence of different dopants and co-catalysts on photocatalytic activity. The reported product amounts reveal that the hole/electron ratio somewhat varies with the guest cation and the co-catalyst metal and, for a fixed catalyst composition, with the pH of the aqueous reaction medium. Indeed, at basic pH, in which CO2 reduction is favoured against water splitting, O2 evolution rates were slightly higher than the stoichiometric values. This deviation from stoichiometry is found quite often in the literature, and cases with higher98-100 or lower100-103 oxygen production than expected from the reduction products have been reported. As mentioned above, the former case can be explained in terms of undetected reduction products, while the latter may arise from photoadsorption or back reaction. In contrast with the results just mentioned, there are cases with no detectable oxygen evolution during photocatalytic CO2 reduction both in liquid and in gas phase. He et al. reported no obvious increase in the gas chromatograph O2 signal, even after multiple replications, during photocatalytic experiments carried out by bubbling CO2 into an aqueous basic dispersion of surface-fluorinated anatase TiO2 nanosheets.104 Similarly, Dilla et al. revealed the absence of O2 detection in their gas-phase


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experiments in high-purity conditions over P25 TiO2.58 Apart from the variation of results from one work to another, this raises the question of the fate of photogenerated holes if they are not consumed by water to release oxygen. Several explanations have been invoked for this. A possible explanation, as mentioned above, is that released oxygen reacts to a certain extent with the reduction products resulting in the back formation of CO2 in a classical photocatalytic oxidation reaction, which is thermodynamically more favourable than CO2 reduction. This would lower the product formation rate and eventually lead to total reaction inhibition, as it has been observed when introducing oxygen in the reaction medium.58 Other postulated possibilities involve the formation of species other than dioxygen as the final product of water oxidation. Thus, H2O2 has been suggested to be this final product in view of the absence of oxygen.104,105 Disproportionation of hydrogen peroxide should result in oxygen formation too, although in the presence of an irradiated photocatalyst and electron scavengers such as the CO2 reduction products, H2O2 could lead to a back-reaction preventing O2 from being detected.106 Another possible pathway is that H2O2 forms surface peroxo complexes with Ti64,107 or mediates in the formation of peroxocarbonates, which may also remain on the surface of the catalyst.64 The formation of peroxocarbonates or related species by direct trapping of holes by surface carbonate species instead of water has been also proposed by several authors.52,78,104 The competition of carbonate and bicarbonate with water for valence band holes was proposed by Dimitrijevic et al. on the basis of EPR spectra, both by direct observation of CO3- radical anions and by spin trapping experiments with DMPO (Figure 7).39 In any case, further decomposition of these species would finally result in the formation of oxygen,78 so in order for the oxidation reaction to be stopped at that stage, stabilization by the formation of surface complexes should take place. Such surface peroxo-related species have been suggested to account for the lack of oxygen and


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the concomitant catalyst deactivation by the authors of this Perspective on the basis of Raman spectra.64

Figure 7. EPR spectra recorded under illumination at 4.5 K of 10 mg of TiO2 in different environments. The spectrum in aqueous Na2CO3 shows a signal with principal g-tensor values in agreement with orthorhombic CO3- radical anions and suggests binding or adsorption of CO3- on Ti sites. The spectrum in CO2/H2O can be simulated with contributions from holes on titania and CO3- radical anions. Reproduced from ref. 39. Copyright American Chemical Society. Photocatalyst deactivation. The deactivation of catalysts with time-on-stream is a crucial factor that limits the industrial development of both the material and the catalytic process. In CO2 reduction, several photocatalytic systems have been shown to be active only for a few hours.82,108 Therefore, this is one of the major issues yet to be understood in clear terms. The primary causes of deactivation proposed in CO2 photoreduction processes are (1) catalyst poisoning, blockage of


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the active sites by reaction intermediates that chemisorb or further react forming surface complexes (e.g. carbonaceous species); (2) slow desorption kinetics of products and/or reaction intermediates; (3) sintering, by agglomeration of small metal crystallites below the melting point; and (4) change in the nature of active sites, by depletion of reaction sites (e.g. oxidation of Ti3+ species, depletion of oxygen vacancies, consumption of surface hydroxyl groups) or increase in recombination sites.66,109,111 Of special relevance is the deposition of carbon species on the catalyst surface. This species, mainly bi-and monodentate carbonates, bicarbonates and carboxylates, are formed from the interaction of CO2 with surface OH groups.111,112 Some studies have addressed a poisoning effect of surface-accumulated bidentate carbonates, while bicarbonate species may be considered as a more active intermediate for CO formation in CO2 photoreduction.32,111,113 On the other hand, Li et al.114 found by in situ DRIFTS spectroscopy that either CO or CH4 were selectively generated on Ag-loaded brookite TiO2 if either carbonates or bicarbonates were more efficiently adsorbed respectively, depending on the Ag loading. The desorption of carbonaceous species and reaction intermediates can be obviously favoured by increasing the reaction temperature. However, this may also lead to a weaker CO2 adsorption that would be detrimental for the reaction, thus being necessary to balance the reaction kinetics in order to achieve an enhanced CO2 photoreduction performance. Moreover, in situ DRIFTS measurements by Zhao et al. led to a possible deactivation mechanism of TiO2 due to the increased adsorption of H2O compared to that of CO2 under illumination.108 A combination of in situ DRIFTS and XANES/EXAFS experiments allowed Liu and co-workers to determine the deactivation mechanism of Cu/Ti(H2), which was caused by the consumption of Ti3+-OH and Cu+ sites by the photogenerated holes.115


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In addition, the deactivation and regeneration of the photocatalysts has been also correlated to the creation and replenishment of oxygen vacancies, respectively. The loss of surface lattice oxygen causes the deactivation of the photocatalyst, while the exposure to gas-phase O2 in the dark is able to replenish the lattice oxygen vacancies.116-118 Despite these findings, there is not a clear experimental evidence of the reaction mechanism that governs the deactivation of CO2 photoreduction catalysts. In summary, we have exposed here the, in our opinion, main controversial aspects of photocatalytic CO2 reduction. For that purpose, we have tried to view this complex process in a holistic manner, considering the different approaches, strategies and experimental and theoretical techniques that constitute today’s state of the art of one of the hottest research topics in energy research. Taking into account the high dispersion in the photocatalytic productions (several orders of magnitude even at similar conditions) is imperative that the scientific community agrees on a series of operational criteria that allow a better sharing of results and facilitate future decision making. In addition, from the authors’ point of view, the major breakthroughs will come in the hands of integrated in-situ and operando time resolved spectroscopies in combination with theoretical studies that are necessary to identify the main bottlenecks underlying the main processes behind the photocatalytic reduction of CO2 that are currently hindering its development and commercial application. Understanding these key steps that occurs at different timescales will make possible to comprehend the global reaction mechanism, enabling the development of novel strategies to design a wide range of emergent more active, selective photocatalysts with improved long-term stability, not only for light-controlled reactions but also for other sustainable energy production technologies.


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ACKNOWLEDGMENTS The authors thank financial support from the Spanish Ministry of Science, Innovation and Universities through the projects Ra-Phuel (ENE2016-79608-C2-1-R), SOLPAC (ENE201789170-R) and FOTOFUEL (ENE2016-82025-REDT). M. B. thanks the award of a Juan de la Cierva grant (FJCI-2016-30567) from the same Ministry. Support from Comunidad de Madrid through the program MAD2D (S2013/MIT-3007) is gratefully acknowledged.


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Author Biographies

Fernando Fresno is a Senior Assistant Researcher at the IMDEA Energy Institute. He obtained his PhD in Chemistry from Universidad Autónoma de Madrid in 2006 with a thesis carried out at the Institute of Catalysis and Petrochemistry of CSIC. He has worked as a postdoctoral researcher at CIEMAT and ICP-CSIC, and as Research Associate at the University of Nova Gorica, Slovenia. He has spent visiting periods in IRCELYON (France), and in the Universities of Aberdeen (UK) and Niigata (Japan). His scientific activity focuses on the development


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photoactive materials and processes for an efficient use of solar light in energy and environmental applications.

Nacho Villar is postdoctoral researcher in the photoactivated unit at IMDEA Energy Instituto. He received his Chemical Engineering Degree and Masters from the University of Oviedo, Spain, and obtained his PhD: "XPS of ionic liquids" from the University of Nottingham (2009). Then, he moved to Ethiopia as an Associate Professor in the Department of Chemistry of Addis Ababa University. In 2011 he moved back to UK, Imperial College London, where he worked initially in ionic liquids at the chemistry department in the group of Prof. Tom Welton and from 2013, in the materials department where he managed the high pressure XPS service.

Elena Alfonso González is a predoctoral researcher in the Photoactivated Processes Unit of IMDEA Energy Institute. She is graduated in Chemistry from the Complutense University of Madrid in 2014. She got a Master of Science degree in Advanced Spectroscopy in Chemistry from Lille and Leipzig Universities in 2016. Her research interests focus on the obtaining of solar fuels by water splitting and CO2 photoreduction through the use of a photoelectrochemical tandem cell.

Patricia Reñones Brasa is graduated in Chemical Engineering by the Rey Juan Carlos University of Madrid in 2013. Currently, she is doing her PhD in the development of hybrid semiconductor for the production of fuels by artificial photosynthesis at the IMDEA Energy Institute. Her research interests include synthesis of materials (photocatalysts & redox materials), characterization of them also with in situ and operando techniques and their evaluation in photocatalytic reactions for energy and environmental applications both in lab and pilot plant solar photoreactors.


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Laura Collado is a former member of the Photoactivated Processes Unit at IMDEA Energy Institute. She received her PhD in Environmental and Chemical Engineering in 2015, and followed with a postdoctoral stage at Heriot-Watt University (UK). Her main research lines involve artificial photosynthesis for CO2 valorization and solar fuels production, development of multifunctional materials, and advanced ex-situ and in-situ characterisation studies.

Mariam Barawi is a Juan de la Cierva postdoctoral researcher at IMDEA Energy institute. She received her Ph.D from Universidad Autonoma de Madrid in 2015, it was based on the investigation of photoelectrochemical cells for energy conversion. After that, she carried out a postdoctoral in Lecce (Italy) at the Italian Institute of Technology (IIT) where she carried out research activities to develop Smart Windows devices by using electrochromic materials and localised surface plasmon resonance phenomena. Currently she is working on the development of efficient photoelectrochemical systems for solar fuels production by artificial photosynthesis under solar irradiation.

Victor A. de la Peña O’Shea is the Head of Photoactivated Process Unit at IMDEA Energy Institute. He obtained his PhD in Chemistry from Universidad Autónoma de Madrid in 2003 with a thesis carried out at the Institute of Catalysis and Petrochemistry of CSIC and worked as a postdoctoral researcher at Universidad de Barcelona. Currently, he is the chair of the CO2 uses Group in the Spanish technological platform of CO2 (PTECO2) and coordinator of the Spanish excellence network of solar-fuels (FOTOFUEL). His research interest is focused on heterogeneous catalysis, theoretical chemistry, in-situ characterization and reactor design and built-up applied to energy and environmental technologies such as CO2 uses and artificial photosynthesis, sustainable fuels production and energy storage.


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Pull Quotes (Proposed) “despite nearly 40 years of research on CO2 photocatalytic reduction and the increasing number of works dealing with this application, there are still many unknowns on the mechanistic aspect of the reaction” “Time-resolved spectroscopic techniques can supply meaningful information about the nature of the defect sites and the efficiencies of charge carrier trapping, migration and transfer” “It is imperative that the scientific community agrees on a series of operational criteria that allow a better sharing of results and facilitate future decision making”


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Mechanistic View of the Main Current Issues in Photocatalytic CO2

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