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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2
Direct Donation of Proton from HO to CO in Artificial Photosynthesis on the Anatase TiO(101) Surface 2
Yongfei Ji, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11936 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019
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Direct Donation of Proton from H2O to CO2 in Artificial Photosynthesis on the Anatase TiO2(101) Surface Yongfei Ji† and Yi Luo∗,‡,¶ †Department of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, Guangdong, China. ‡Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China. ¶KTH, the Royal Institute of Technology, Department of Theoretical Chemistry and Biology, S-106 91 Stockholm, Sweden. E-mail: [email protected]
Abstract Conversion of CO2 and H2 O into value-added organic molecules via artificial photosynthesis is a promising solution to current energy and environment problems. In the reaction, it is generally believed that CO2 is converted into organic molecules by photogenerated electrons and protons that result from photooxidation of H2 O. In this work, we investigate the possibility that H2 O, without being oxidized, directly donates protons to CO2 and other intermediates adsorbed at the oxygen vacancy on the anatase TiO2 (101) surface. We found that this can greatly lower the barriers (by about 0.3 eV) for the hydrogenation of CO2 , CO, H2 CO, and CH3 O because less energy is required to displace these adsorbates to accept the proton (in H2 O). OH – group produced in these reactions can recombine with a surface-adsorbed proton to form a new H2 O molecule, making H2 O a shuttling center of the adsorbed protons, or it can take part in the oxygen evolution reaction with a lower barrier. The results suggest that H2 O can play multiple roles in the artificial photosynthesis, and the reduction and oxidation part of the reaction may have synergistic effects.
Introduction Converting solar energy into chemical energies via artificial photosynthesis is a promising solution for current energy shortage and global warming. 1,2 Take conversion of CO2 and H2 O into CH4 and O2 as an example, the overall reaction (reaction (1)) has been generally split into two parts: oxidation of H2 O to O2 and protons by photogenerated holes (reaction (2)), and reduction of CO2 and the protons to CH4 by photogenerated electrons (reaction (3)). Photooxidation of H2 O was achieved in 1972 and photoreduction of CO2 in 1979 on TiO2 by Fujishima et.al. 3,4 However, the efficiencies of these reactions are far below sufficient. Numerous studies on TiO2 have been conducted to reveal the mechanisms of the reactions and to improve the performance of the materials. 5–8 For the oxidation of H2 O, the molecular mechanism of the reaction has been well explored by Selloni et al. 8,9 The first electron-coupled 2
proton transfer was suggested to be the rate-limiting step. For CO2 reduction, two pathways, namely the fast hydrogenation path and the fast deoxygenation path, have been proposed 6,10 to explain various experimental results. However, it is difficult to rationalize why and predict when the reaction follows one of the specific pathways. These pathways on both the surface Ti site 11,12 and the oxygen vacancy (Ov ) site 7 (reaction (4) or (5) in which Ob2 – is lattice bridging oxygen) on the anatase TiO2 (101) surface have been investigated systematically by first-principles calculations. Based on the results, a new mechanism which is compatible with most of the experiments was proposed. In this new mechanism, Ov , which can be regenerated in the oxygen evolution process(reaction (6)), 9 was proposed to be the real active site; it can either dissociate a H2 O molecule to cure the vacancy and produce two surface-adsorbed protons (reaction (7)), 9 or more importantly adsorb and activate CO2 for further reduction. 7 However, the calculated barriers of several key steps for the photoreduction of CO2 to CH4 are still very high (1.0 ∼ 1.2 eV).
As shown in Scheme1 (a), constrained by the structure of the anatase TiO2 (101) surface, the distance between the CO2 molecule adsorbed at the Ov and the proton adsorbed at the adjacent bridge oxygen is large (about 3.6 ˚ A). CO2 molecule (and other intermediates) binds so strongly at the vacancy that it may cost large energy to move and reach the proton, which results in the high barriers for its reduction. On the other hand, the distance between the intermediates and the proton in an adsorbed H2 O molecule is much smaller(1.6 ˚ A). In addition, H2 O molecule adsorbed on the surface is more mobile 13,14 than CO2 adsorbed at the vacancy. It should be easy for the H2 O molecule to approach the CO2 molecule. Although it requires additional energy to dissociate H2 O molecule, the effective barrier to donate proton directly from H2 O to CO2 (reaction (8)) can be lower than that to transfer the surface-adsorbed proton to CO2 (and other intermediates). If all the resulting OH – groups from reaction (8) recombine with surface-adsorbed protons from reaction (6-7) to form new H2 O molecules (reaction (9)), then the net role of the H2 O molecule in the CO2 reduction reaction is a shuttling center of the surface-adsorbed proton (Scheme1 (b)). In fact, it is generally believed that H2 O directly donates its proton to CO2 in electrochemical reduction of CO2 at the neutral and alkaline conditions because the concentration of proton is too low. 15 Furthermore, theoretical calculations showed that the shuttling process can significantly lower the barriers for the electrochemical reduction of CO2 on Cu surface. 16 These pathways may also play important roles in photocatalytic CO2 reduction, but they have not been investigated theoretically.
Scheme 1: Three pathways for hydrogenation of CO2 adsorbed at the Ov on the anatase TiO2 (101) surface. (a) Green: transfer of H from an H2 O molecule adsorbed on the surface Ti site to the CO2 molecule adsorbed at the Ov , red: transfer of H adsorbed on the surface oxygen to the CO2 adsorbed at the Ov ;(b) shuttling of H adsorbed on the surface oxygen to CO2 via an H2 O molecule adsorbed on the surface Ti site. In this work, we used the first-principles calculations to investigate the direct donation of protons from H2 O to CO2 and other intermediates adsorbed at the oxygen vacancy on the anatase TiO2 (101) surface. We found that the barriers of several key elementary steps in the photocatalytic reduction of CO2 can be greatly lowered in this pathway, which suggests that it may play important roles in artificial photosynthesis. It can be a very general mechanism for the hydrogenation of strongly adsorbed species on the surface.
Computational Methods We performed spin-polarized calculations with density functional theory implanted in Vienna Ab-initio Simulation Package (VASP). 17–21 PAW pseudopotential 22 was applied to describe the ion-electron interaction. An energy cutoff of 460 eV was used for the plane-wave basis set. The anatase TiO2 (101) surface was presented by a five-layer slab with a vacuum layer of 13 ˚ A. A 3 × 1 supercell along the  and [10¯1] direction was used. The system contains 180 atoms with a size of 10.2 ˚ A ×11.3 ˚ A, therefore, only the Γ point was included for the Brillouin zone sampling. As in previous works, 7,11 all the structures were relaxed with the atoms in the center layer fixed to their bulk positions until the maximal force on the atoms was smaller than 0.02 eV/ ˚ A. Such kind of slab model has been applied successfully in the study of other photocatalytic reactions. 23,24 The nudged elastic band method with climbing images was used to search the transition states. 25 To describe the d electrons of Ti, PBE+U 26,27 method with 5
a typical U = 4.0 eV was used to for the structure optimization and transition state searching. The HSE06 28 functional was subsequently applied for the single-point calculations on the PBE+U optimized structure for the energies of the systems. Note that the reaction can take please at both gas/solid 29 and liquid/solid interface. To account for the solvent correction, we adopted the implicit solvent model 30 for the hydrogenation steps (shown later in Table 1).
Results and Discussion For convenience, we name the hydrogenation by H2 O the direct path, and by surfaceadsorbed protons (comes from the H2 O photooxidation reaction, therefore indirectly from H2 O) the indirect path. We have carried out the calculations at both the PBE+U level and the PBE+U/HSE level (single-point calculations with HSE hybrid functional on the PBE+U optimized structure). The PBE+U/HSE results are shown and discussed first because they are closer to those at the full HSE level. 11 All the full PBE+U and PBE+U/HSE barriers will be summarized later in Table 1, and they agree well with each other qualitatively. In previous work, 7 CO2 was proposed to be reduced following the path: CO2 → CO → HCO → H2 CO → CH3 O → ·CH3 → CH3 OH → CH4 . It is worth to mention that the reactions look like thermal reactions instead of photocatalytic ones. But from reaction (4-5, 8), one can see that excess electrons are consumed with the oxygen vacancies which will be regenerated in the oxygen evolution reaction (reaction (6)). 9 We first investigate the direct donation of protons from H2 O to a CO2 molecule adsorbed at the Ov . The structures and the potential energy surface of this step are shown in Figure 1. At the initial state, a Hydrogen-bond is formed between the H2 O molecule and the CO2 molecule; at the transition state, the H2 O molecule dissociates and transfer one proton to the CO2 molecule; and at the final state, a COOH group is formed at the Ov with a OH – group adsorbed at the adjacent Ti (state A). The barrier of this step is calculated to be 0.74
eV, which is 0.45 eV lower than that in the indirect path. 7 We can see that the structure of the CO2 molecule does not change much between the initial and the transition states; whereas in the indirect path, CO2 molecule is greatly distorted to reach the proton adsorbed at the adjacent surface oxygen. 7 Although, some energy is cost to dissociate the H2 O molecule in the direct path, more energy is gained by distorting the CO2 molecule to a less extend, which greatly lowers the barrier. The resulting OH – can then recombine with a proton to form a new H2 O which makes water a shuttling center of protons. Note that, the proton from H2 O oxidation may be adsorbed far away from the Ov at which CO2 and other intermediates are adsorbed. Therefore, we calculate the shutting process of a proton from one surface oxygen to another by H2 O. As shown in Figure 2, the dissociation of H2 O is exothermic by 0.13 eV with a relatively low barrier of 0.42 eV. This is also the effective barrier for shuttling the proton. The low barrier suggests that H2 O plays an important role in proton diffusion on the surface. On the other hand, the association reaction of OH – with proton is exothermic with a small barrier of 0.29 eV. In the following hydrogenation of other intermediates, we will focus on the hydrogenation step without calculating the recombination barrier of OH – with proton again. The relative energy of the system before and after the recombination of OH – with proton (from state A to B in Figure 1) can be calculated to be 0.25 eV by assuming that there is another proton adsorbed far away from the Ov in state A (detail of the method is in ref. 7 ). The barrier for the dissociation of the COOH into a CO adsorbed at the oxygen vacancy and an adsorbed OH – group has been calculated to be 0.1 eV. 7 The OH – group can again react with a surface-adsorbed proton to form another H2 O molecule. Overall, the rate limiting step for CO2 reduction to CO the hydrogenation of CO2 to COOH with an effective barrier of 0.74 eV. We continue to investigate the direct hydrogenation of CO to methanol. The full potential energy surface is shown in Figure 3. CO can be hydrogenated to methanol by four protons
and four electrons. These four steps are named step H-I ∼ H-IV as shown in Figure 2. The transfer of electrons in each step has been discussed in our previous work. 7 For step H-I, the CO molecule is initially adsorbed at the vacancy nearly vertically. It transforms to a bridging geometry by overcoming a barrier of 0.70 eV, which is 0.05 eV lower than that in the indirect path. 7 Further hydrogenation of the bridging CO to HCO only needs to overcome a small barrier of 0.28 eV, which is 0.53 eV lower than that in the indirect path. 7 As the OH – adsorbed on the surface is hydrogenated to H2 O, the energy of the system decreases by 0.18 eV. In step H-II, hydrogenation of HCO to H2 CO, the barrier becomes 0.65 eV in the direct path, which is 0.32 eV higher than that in the indirect path. This is probably because the transition state in the indirect path does not require much structure distortion, but additional energy is needed in the direct path to dissociates H2 O which increases the total barrier. Further conversion of the adsorbed OH – to H2 O lowers the energy of the system greatly (by 0.86 eV), probably because the H2 CO molecule is also stabilized by forming a C-Ti bond with the surface. In step H-III, hydrogenation of H2 CO to CH3 O, the barrier in the direct path is calculated to be 0.38 eV lower than that in the indirect path. 7 The reason is that the H2 CO is greatly distorted in the indirect path with only its O bonded to surface Ti atom, whereas, in the direct path, H2 CO at the vacancy is greatly stabilized via bonding with both Ti atoms at the vacancy. If we continued to hydrogenate the CH3 O (step H-IV) at the vacancy with H2 O, we should obtain a CH3 OH at the vacancy and a OH – group on the Ti. But we found that the reverse reaction, CH3 OH reacts with OH – to form CH3 O and H2 O, is barrierless. This agrees with the result in the indirect path that the dissociation barrier of CH3 OH at the oxygen vacancy is very small. 7 We found that CH3 OH molecule can be greatly stabilized in the presence of one additional proton at the adjacent oxygen. However, the reverse reaction is still exothermic with a relatively low barrier. Therefore, CH3 O should be the relevant
species for further evolution of the reaction. It has been shown in previous work that CH3 O can dissociate by overcoming a barrier of 0.63 eV to form a methyl radical, which will further be converted to CH4 or CH3 OH, depending on the availability of proton/hydroxide and electron/hole. 7 The methyl radical has been detected by EPR experiment 31,32 which suggest it to be an intermediate in the reaction. Therefore, we are not studying the direct hydrogenation of CH3 O adsorbed at the oxygen vacancy to the CH4 by H2 O (reaction (10)).
CH3 O− + H2 O + 2 e− −−→ CH4 + OH− + Ob2−
We summarize the barriers for the direct and indirect paths at both the PBE+U and the PBE+U/HSE levels in Table 1. We can see that the barriers of all the steps, except for the hydrogenation of HCO, are significantly lowered in the direct path by 0.24-0.45 eV at the PBE+U/HSE level. The barrier of the rate-limiting step, hydrogenation of formaldehyde, is lowered by 0.38 eV. The results at the PBE+U level agree well with those at the PBE+U/HSE level with the barriers change at the same order of magnitude, and the solvent corrections to the barrier are within 0.1 eV. These results strongly suggests that the direct path plays important role in the reaction in both the gas/solid and liquid/solid interface. Table 1: Summary of the Barriers in the Direct and Indirect Paths at the PEB+U and PBE+U/HSE Levels Direct Path Barriers/eV Reaction steps PBE+U PBE+U/HSE CO2 −−→ COOH 0.82 (0.94) 0.74 CO −−→ HCO 0.15(0.12) 0.28 HCO −−→ H2 CO 0.58(0.57) 0.65 H2 CO −−→ CH3 O 0.84 (0.92) 0.88 CH3 O −−→ CH3 OH 0.32 (0.36) 0.47 1 2
(reaction(11)). This means the rate limiting step for oxygen evolution is no longer the first proton coupled electron transfer which has a relatively high barrier. In addition, OH – group is a much stronger hole scavenger than H2 O 8,33 which suggests that the reaction can be greatly promoted. Interestingly, OH – has also been proposed to be the species that was oxidized to O2 for electrochemical oxygen evolution reaction in the alkaline conditions (reaction(12)). 34
OH− + Ob2− + 4 h+ −−→ O2 + H+ + Ov
4 OH− −−→ O2 + 2 H2 O + 4 e−
Thus, we can see that the oxidation part and the reduction part of artificial photosynthesis are more closely related than it was known: the oxidation part provides protons and oxygen vacancy as the active site for CO2 adsorption and reduction; the reduction part provides OH – groups which promotes oxygen evolution. In most studies, these two parts have been studied separately. Our calculations suggests that there can be synergetic effects owing to the multiple roles H2 O plays. And depending on the role H2 O plays, the artificial photosynthesis cycle can be completed in several different ways: if H2 O is simply oxidized to produce O2 and H+ , the cycle can be completed by coupling reaction (2) with (3); if Ov is produced during oxygen evolution, the cycle can be completed by coupling reaction (6) with (4), or by coupling reaction (6) with (5)and (7): (6) × 2 + (5) + (7) = (1); if H2 O also serves as the shuttling center of surface-adsorbed protons, the cycle can be completed by coupling reaction (6) with (7-9): (6) × 2 + (7) + (8) + (9)× 6 = (1); and if some of the OH – groups from reaction (8) take part in the oxygen evolution reaction directly, the cycle can be completed by coupling reaction (11) with (7-9): (11) × 2 + (7) + (8) + (9) × 4 = (1).
Conclusion In summary, we have carried out first-principles calculations on the direct hydrogenation of CO2 and other intermediates by H2 O on the anatase TiO2 (101) surface. The results demonstrate that direct donation of protons from H2 O can significantly lower the barriers of several key steps in the photocatalytic reduction of CO2 on the anatase TiO2 (101) surface. This is mainly determined by the surface structure of the catalysts and the mobility of the adsorbates. H2 O can serve as a shuttling center of adsorbed proton if the OH – group recombines with another proton. Alternatively, the OH – group can directly be oxidized by photogenerated holes, and thus promotes photooxidation of H2 O. These suggest that H2 O has multiple roles in the reaction, and most importantly, the oxidation part and the reduction part of the artificial photosynthesis may have synergistic effects that we must combine them together to comprehensively understand the mechanism of the overall reaction.
Acknowledgement This work is supported by the Ministry of Science and Technology of China (2017YFA0303500), the National Natural Science Foundation of China (21633007, 21790350), Anhui Initiative in Quantum Information Technologies (AHY090000), and the Swedish Research Council (VR). The Swedish National Infrastructure for Computing (SNIC) is acknowledged for computer time.
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