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Role of Reduced CeO(110) Surface for CO Reduction to CO and Methanol Neetu Kumari, M. Ali Haider, Manish Agarwal, Nishant Sinha, and Suddhasatwa Basu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02860 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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The Journal of Physical Chemistry

Role of Reduced CeO2(110) Surface for CO2 Reduction to CO and Methanol Neetu Kumari†, M. Ali Haider†, Manish Agarwal‡, Nishant Sinha §and Suddhasatwa Basu†* †

Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016

‡

Computer Service Centre, Indian Institute of Technology Delhi, New Delhi 110016

§

Dassault Systemes, Galleria Commercial Tower, 23 Old Airport Road, Bangalore 560008

*

Corresponding author. Tel.: +91 11 2591035; fax: +91 11 26581120. E-mail address: [email protected]; [email protected]

1 Environment ACS Paragon Plus


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ABSTRACT

Density functional theory (DFT) calculations were performed to study the mechanism of carbon dioxide (CO2) reduction to carbon monoxide (CO) and methanol (CH3OH) on CeO2(110) surface. CO2 dissociates to CO on interacting with the oxygen vacancy on reduced ceria surface. The oxygen atom heals the vacancy site and regenerates the stoichiometric surface via a redox mechanism with intrinsic activation and reaction energies of 259.2 kJ/mole and 238.6 kJ/mole respectively. Lateral interaction of oxygen vacancies were studied by the generation of two oxygen vacancies per unit of CeO2 surface. Compared to a single isolated vacancy, the activation and reaction energies of CO2 dissociation on a di-vacancy were approximately reduced to half of its value. Hydrogen atom co-adsorbed on the surface was observed to assist CO2 dissociation by forming a carboxyl intermediate, CO2+H→COOH (∆Eact = 39.0 kJ/mole, ∆H = -69.2 kJ/mole) which on subsequent dissociation produces CO via the redox mechanism. On hydrogenation, CO is likely to produce methanol. The energetics of CO hydrogenation to produce methanol showed exothermic steps with activation barriers comparable to the DFT calculations reported for Cu (111) and CeO2-x/Cu(111) interface. While on the stoichiometric surface, COOH dissociation COOH→CO+OH (∆Eact = 55.6 kJ/mole, ∆H =5.7 kJ/mole) is likely to be difficult as compared to rest of the elementary steps, whereas on the reduced surface the energetics of the same step were significantly lowered (∆Eact = 47.4 kJ/mole, ∆H = -80.4 kJ/mole). In comparison, hydrogenation of methoxy, H3CO+H→H3COH, appears to be relatively difficult (∆Eact = 58.7 kJ/mole) on the reduced surface.


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INTRODUCTION

Rising levels of CO2 concentration in the environment and associated global warming concerns have necessitated prompt actions for CO2 mitigation. An effective strategy, therefore, is suggested to utilize CO2 as a feedstock to produce fuels and chemicals1,2,3. Since CO2 is a stable molecule (∆G = -396 kJ/mole), its utilization is limited by high energy requirements4. For example, thermal dissociation of CO2 to CO requires 523 kJ/mole at standard conditions4. Alternatively, CO2 reduction could be assisted by reactions with other molecules such as hydrogen or steam to achieve favourable thermodynamic conditions4. Copper is suggested as the most efficient catalyst for heterogeneous catalytic reduction of CO2 to CO and further to hydrocarbon fuels in presence of hydrogen5,6. Recent works of Graciani and co-workers have measured a 200 fold increase in the turn-over frequency of Cu catalyst by adding ceria, where CeO2-x/Cu(111) interface was attributed to the improved catalytic activity, resulting in the formation of methanol7. Interestingly, synergistic interaction between Cu nanoparticles and ceria has shown significant improvement in other similar reactions such as the one recently reported by Tamura et al. on the hydrogenation of dimethyl carbonate8. DFT calculations performed by Yang et al. recently, have identified the growth of small Cu clusters favoring a strong copperoxygen interaction which was ascribed to the high activity of Cu-ceria interface for H2O dissociation9. Further tuning of the Gibbs free energy change of the reaction can be obtained by applying an electric potential to run an electrochemical reaction10. Cu, Fe, Sn and Ag based catalysts have been suggested to be active for CO2 reduction through electrocatalytic reactions11,12,13,14. However at low operating temperatures, coke disposition and CO poisoning, resulting into catalyst deactivation remained serious concerns to be resolved15. Therefore, high temperature



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CO2 electrolysis combined with steam (as a source of hydrogen) is explored16,14. A reversible solid oxide fuel cell (SOFC) running in the electrolysis mode provides a potential platform to develop this technology17. For this reaction, electrode materials based on doped ceria are being explored to assess the viability of desired product formation. For example, Kleiminger and group have studied CO2 reduction to CO on Cu/gadolinium doped ceria (GDC) composite electrodes in a solid oxide electrolysis cell (SOEC)18. At 1023 K current efficiency, up to 100%, was measured towards CO formation18. Doping of an aliovalent cation in place of Ce leads to the reduction and formation of oxygen vacancies19,20,21. On the introduction of a dopant, oxygen vacancies are formed by both structural and electronic modifications in ceria22,23. An increase in the dopant concentration results into a greater likelihood of vacancy clustering24. Size of the vacancy cluster, in general, depends upon the dopant type24. Yang and co-workers suggested by performing DFT calculations that Cu is stable as both surface- adsorbed (Cu+1) and surfacedoped (Cu+2) atoms on ceria25. While surface-doped Cu was attributed to the creation of oxygen vacancies, surface-adsorbed Cu was responsible for the suppression of oxygen vacancies25. On Cu doped CeO2(111) surface the oxygen vacancy formation energies, at different positions, were calculated to lie within the range of 3.8 to 86.8 kJ/mole which was observed to be lower than on stoichiometric CeO2(111) surface (282.7 kJ/mole)25. All of the aforementioned experimental observations suggest a synergistic effect of Cu and ceria responsible for high activity of CO2 reduction. The role of Cu has been established and Cu is suggested to be active in the production of hydrocarbons7. The role of the extended surface of ceria remains to be explored. In this regard, in the previous work a detailed DFT study was reported to explore all suggested routes of CO2 hydrogenation to produce methanol on the extended CeO2(110) surface26. CO2 conversion to CO via reverse water gas shift (RWGS)


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mechanism and its further hydrogenation was inferred to be the likely route for methanol synthesis26. Alternatively, a redox mechanism is proposed by Cheng et al. where surface oxygen vacancies were suggested to be actively participating27. For the reverse reaction, involving CO oxidation to CO2 on ceria, the redox mechanism is an established route where surface oxygen vacancy concentration is suggested to be influential in determining the catalytic activity28,29. The hypothesis, however; needs to be studied for CO2 reduction reaction on ceria surface. Recent work by Cynthia and co-workers have highlighted the important role of CeO2(110) surface in CO2 hydrogenation to produce methanol by utilizing a microkinetic analysis, performed at the experimental temperature of 500 K for the heterogeneously catalyzed reaction30 .The present study aims to explore the reaction pathways for methanol synthesis on the ceria surface under both catalytic and electrocatalytic reaction conditions by studying the energetics of elementary reactions. The research work presented in this study is an attempt to study the effect of lateral vacancy interactions in the form of di-vacancies on the catalytic reduction of CO2 via the redox mechanism. Utilizing DFT calculations, redox mechanism was further explored to study the energetics of CO2 conversion to CO in presence of hydrogen. At the end, a full reaction energy diagram for CO2 reduction to methanol on reduced ceria is presented, wherein the CO hydrogenation steps to produce methanol are similar to the RWGS mechanism. Calculations on reduced ceria surface were compared to the calculations on stoichiometric ceria surface, to understand the role of surface vacancies in activating different elementary reaction steps, involved in the synthesis of methanol.



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COMPUTATIONAL METHOD DFT calculations were performed by using double-numerical basis sets which are implemented in DMol3 module31,32 of Materials Studio 8.0 (Accelrys Inc., San Diego, USA). Ion-electron interactions were represented by semi-core pseudopotential. The generalized gradient approximation (GGA)33 of PBE33 was used as exchange-correlation functional. The relative energy of low index ceria surface are in the order of CeO2(100) > CeO2(110) > CeO2(111), which indicates that CeO2(100) is the least stable and CeO2(111) is the most stable surface of ceria34. CeO2(110) is the metastable surface35. The energy required for the oxygen vacancy formation on CeO2(111) and CeO2(100) surfaces are 248.3 and 219.0 kJ/mole34, which are much higher than on CeO2(110) surface (∆Evac = 152.8 kJ/mole)26. Therefore, the reduced CeO2(110) surface is catalytically more active than CeO2(111) and CeO2(100). CeO2(110) surface was modelled by three-atomic layer slab with a p(2x2) supercell and each slab was separated from periodic image in z-direction, by 20 Å thick vacuum-layer. Bottom two atomic layers of the slab model were fixed at the optimized bulk coordinates of atoms corresponding to the lattice constant a = 5.411 Å and the top layer of the surface was allowed to relax with adsorbed molecules. A grid of 3x3x1 Monkhorst-Pack k-points was used to perform integration in the first Brillouin-zone27. All the reported calculations were carried out in spin-unrestricted conditions. Convergence criteria for total energy, maximum force, maximum displacement, and selfconsistent field (SCF) density were set to 0.026 kJ/mole, 5.251 kJ/(mole.Å), 0.005 Å and 10-6 respectively. The method thus adopted is similar to our previous work on CeO2(110) surface calculations26. Removal of one neutral oxygen atom from the ceria surface leads to the formation of oxygen vacancy with two unpaired electrons. The electrons are suggested to localize on forbital of two neighboring Ce atoms which leads to the change in the formal valence from +4 to


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+3. Few studies on ceria have incorporated the Hubbard (U) parameter in the conventional DFT schemes such as GGA+U and LDA+U to account for the localized f-electron of reduced Ce atoms27,36,28. A value of U = 5 eV was suggested to be appropriate for modeling on stoichiometric and reduced ceria surface27. To ascertain the importance of the U parameter, two calculations were performed, one with U = 5 eV and second without incorporating U to calculate the oxygen vacancy formation energy on the ceria surface. In both cases, the obtained results(153.2 and 152.8 kJ/mole) were similar to the value of ~153 kJ/mole which is consistent and comparable to the

value reported27. Therefore, in this study U parameter was not

incorporated in DFT calculations which is consistent to our earlier work on ceria26. Gas phase optimizations were performed in a periodic cubic unit cell of length 20 Å. The optimized gas phase species were allowed to adsorb on the stoichiometric and reduced CeO2(110) surface. Binding energy (∆Ebinding) of the reactant and product state of the intermediate species were calculated by using Eq. 1. ΔE = E () − E () − E (1) whereE () is the total energy of the 2x2 supercell, E is the total energy of ‘x’ species, and E () is the total energy of the composite system. Negative adsorption energies imply exothermic adsorption and stronger surface–adsorbate bonding. Long range dispersion-type interactions could play an important role in calculated energetics of bigger molecules such as; CH3OH, H3CO and H2CO. In order to ascertain the effect of dispersive interactions on the binding energies of big and small molecules, DFT-D (Grimme model) implemented in DMol3 was utilized. Binding energies for CH3OH, H3CO, H2CO and CO2 were calculated to be -77.5, 235.7, -52.3 and -19.0 kJ/mole respectively. The numbers are similar to the calculated adsorption energies of these molecules in the absence of any dispersive interactions (-68.8, -228.6, -48.7 and



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-18.6 kJ/mole for CH3OH, H3CO, H2CO and CO2 respectively). The comparison of adsorption energies with and without dispersion is presented in Table S1. On a close observation, the dispersive interactions tend to increase the binding energy of a relatively bigger molecule, for example by 8.7 kJ/mole for methanol. However, calculations of reaction energies of intrinsic reaction steps are expected to cancel out any dispersive interactions in the adsorbed state of reactants and products. Similar observations were made by Cheng et al.30 in ignoring the effect of dispersive interactions. Reaction energy on surface is determined by Born-Haber cycle as follows37: %&' !"#$

∆E = 0& ∆E +,-. = ∆/)

1&'(

%&'

− (&#$&)$ (2) ,.3+

.33+ + ∆ E − ∆ E (3) %&' !"#$

where ∆E is the reaction energy in vapour phase,

%&'

and (&#$&)$ are the total

energy of the reactant and the product respectively in gas phase (calculated from DFT ,.3

.33+ calculations), ∆E +,-. is the reaction energy on the surface, ∆E and ∆E are

the binding energies of the reactant and the product. Linear synchronous transit and quadratic synchronous transit (LST/QST) calculation with conjugate gradient minimization within the transition state search tool in DMol3 module were used to isolate the transition states and estimate the corresponding activation barriers for all the elementary steps considered. The root mean square (RMS) force on the transition state was optimized to less than 4.82 kJ/(mole.Å).This value is similar to the previously reported calculations26,27. The transition state structures were confirmed using vibrational frequency analysis, wherein a single imaginary frequency corresponding to the reaction mode was obtained, Table S2-S6. Activation barriers were calculated from the following equation:


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∆&#$ = 56 − (&#$&)$ (4) where∆&#$ is the activation energy,56 is the total energy of transition state and (&#$&)$ is the energy of reactant state. The reaction and activation energies of the hydrogenation reactions were calculated using a single hydrogen atom adsorbed on the surface, reacting with the molecule. The lateral interaction of hydrogen atoms adsorbed on ceria surface were considered to be negligible. RESULTS AND DISCUSSION Oxygen vacancies in ceria are formed due to the mixed oxidation state of cerium (Ce) in reducing environments38. One vacancy can be generated in p(2x2) supercell system of CeO2(110) surface. The vacancy formation energy on CeO2(110) surface was calculated to be 152.8 kJ/mole26. Similar value of oxygen vacancy formation energy for CeO2(110) surface was calculated by Cheng et al.27. Adsorbed CO2, with ∆Ebinding = -109.8 kJ/mole, on the neighboring site of oxygen vacancy is activated into a bent configuration with ∠OCO = 128.2° (Figure 1). In general, the C-O bond in a carbonate like structure are of partially double bond character measuring a length of 1.28 Å39. While the two symmetrical C-O bond in the bent structure of adsorbed CO2 are of length 1.27 Å, the C-O bond in which the C atom of CO2 molecule is interacting with the surface O atom of ceria was measured to be 1.37 Å. Therefore the bent structure is an activated form of CO2 adsorbed on the ceria surface, which is not representing a carbonate structure, despite having a CO3 configuration. Electrons in the activated bent structure are calculated to be transferred from the reduced ceria surface to the adsorbed CO2 molecule to dissociate the C-O bond27. The activated CO2molecule could therefore be directly reduced to CO on the ceria surface via the incorporation of an oxygen atom directly into the surface vacancy40,41. Alternate possibility could be the dissociation of CO2 to CO and adsorption of atomic oxygen on the surface. However, CO2 dissociation leading to surface adsorbed atomic



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oxygen was calculated to be highly endothermic (362.3 kJ/mole, Figure S1) and is unlikely to proceed. The redox mechanism could be thought of to proceed in two steps. Adsorbed CO2 in the vicinity of an oxygen vacancy, dissociates into CO and atomic oxygen. Oxygen atom on the surface heals the vacant site, leading to re-oxidation of the reduced surface into stoichiometric or partially reduced form of cerium oxide. Figure 1 shows the calculated intrinsic activation barrier and reaction energy of this step on CeO2(110) surface, which are of the value 259.2 and 238.6 kJ/mole respectively. Similar calculations were performed by Cheng et al. on CeO2(110) surface27. The intrinsic activation barrier (∆Eact = 264 kJ/mole) and reaction energy (∆H = 225.3 kJ/mole) for CO2 reduction to CO via the redox mechanism is comparable to the values reported in this work. The results thus establish the consistency in the calculations presented in this study. The reverse reaction, involving CO oxidation to CO2, has been studied in detail and a Marsvan Krevelen (MVK) mechanism is suggested in which the lattice oxygen of the oxide catalyst participates in the oxidation of surface adsorbed CO. On a thin-film FeO2/Pt(111) catalyst, CO reacts with the lattice oxygen of FeO2 to form CO2, leaving an oxygen vacancy on the surface. Experiments utilizing X-ray photoelectron spectroscopy to study CO2 adsorption on Pt/CeO2catalyst have shown the importance of oxygen vacancies in the decomposition of CO2 to CO42. On Pt surface, the activation and reaction energies for CO oxidation reaction are calculated to be 92.6 and -69.2 kJ/mole respectively43. On the interface of Au/CeO2(111)44 and on a substituted Au/(Ce-X)O2catalyst (X=Au, Pt, Pd, Zr, Ru and Ce)45, it is suggested that the vacancy formation energy could be a descriptor to the reactivity44. Therefore, charge carriers or vacancies on the surface can be thought of playing a significant role in these redox reactions. For CO2 reduction, introduction of extra charge carriers or surface vacancies could thus pave the way


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for achieving efficient catalytic conversion by reducing the activation barrier. Henkelman and co-workers have studied the kinetics of CO oxidation reaction on Au13/CeO2-x catalyst, in presence of molecular oxygen adsorbed on stoichiometric and reduced ceria surfaces29. Reduced ceria surface was simulated with three oxygen vacancies. It was suggested that the ceria surface with increased number of oxygen vacancies is likely to be more active towards the reduction of CO2 to CO. Ramani and co-workers have reported an order of magnitude enhancement in the surface reactivity of the reduced ceria nanoparticles on increasing the surface vacancy concentration by Zr doping46. All of these observations indicate a favorable pathway for efficient catalytic reduction on ceria having higher concentration of oxygen vacancies in the form of pairs or clusters. In order to evaluate the influence of vacancy interactions on the energetics of CO2 reduction, two oxygen vacancies on CeO2(110) surface were created side by side or at the diagonal positions as shown in Figure 2. The energy required for the generation of second vacancy (∆E2vac) was calculated. E2vac for the lattice oxygen sitting at the side by side position (Figures 2a, 2c and 2e) with respect to the first vacancy are 357.4, 357.3 and 347.4 kJ/mole respectively. Compared to this, the vacancy formation energy for diagonally positioned oxygen vacancy (Figures 2b and 2d), were calculated to be relatively lower i.e., ∆E2vac = 309.1 and 308.8 kJ/mole respectively. Thus, oxygen di-vacancy formed at the diagonal positions are likely to be more stable, which is consistent with the structure reported by Yang et al. on CeO2(110) surface47. CO2 molecule was adsorbed, having binding energy of -98.6 kJ/mole, on the di-vacancy site with a bent configuration (with ∠OCO = 128.2° and C-O bond length of 1.27 Å) which are similar to the structure on an isolated vacancy site. The adsorbed structure of CO2 was therefore unaffected by the vacancy interaction. In order to determine the effect of vacancy interactions on the



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activation barrier of CO2 reduction to CO, a transition state search was performed on the minimum energy path connecting CO2 to CO via a redox mechanism. Interestingly, the activation barrier for CO2 dissociation to CO on ceria surface having two vacancies, was reduced to 134.3 kJ/mole (Figure 3), as compared to the calculations on the surface having an isolated vacancy (∆Eact =259.2 kJ/mole, Figure 1). Furthermore, the CO2 reduction step on a di-vacancy is calculated to be less endothermic (∆H = 127.3 kJ/mole) as compared to the one on an isolated vacancy (∆H = 238.6 kJ/mole, Figure 1). Therefore, incorporation of an additional vacancy to form a di-vacancy on the surface could be linked to the improvement of the surface activity due to increased vacancy interactions, which subsequently reduces the activation barrier48,27. In the presence of hydrogen, CO2 reduction to CO is likely to be assisted by the atomic hydrogen adsorbed on reduced ceria surface. Structural configuration of adsorbed reactants and products of the elementary steps and corresponding transition states are illustrated in Figure 4. The reaction energy diagram with intrinsic activation barriers and reaction energies is shown in Figure 5. CO2 was shown to be adsorbed in a linear configuration on reduced ceria, which is likely to participate in hydrogenation26. Molecular adsorption of CO2 was estimated to be atop of surface oxygen with a bond length, C-Osurface measured to be 2.90 Å. The reactant state shows that the CO2 molecule is interacting with the hydrogen atom sitting in the vicinity of the four fold hollow site with a H-Osurface bond distance of 2.34 Å (Figure 4, a). Adsorbed CO2 could hydrogenate to formate (HCOO) or the carboxyl (COOH) species49,50. Both intermediates may lead to the formation of CO. Li et al. have suggested the reduction of CO2 to CO via the formation of HCOO on Fe(111) and W(111) surfaces, with an estimate of CO2 hydrogenation barrier to be 35.7 and 52.1 kJ/mole respectively for the two surfaces51. The mechanistic route involving the formate species is widely debated52,53. This is primarily due to the highly stable


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structure of the formate (e.g. ∆Ebinding= -263.3 kJ/mole on Cu(111) surface) leading to its inactivity for further reactions7. In our previous study, formate intermediates were observed to be tightly bound on the ceria surface (∆Ebinding= -222.9 kJ/mole) which may act as a spectator. Furthermore, the dissociation of H2COOH, which is produced from the hydrogenation formate, was calculated to be significantly endothermic (∆H = 146.2 kJ/mole)26. Therefore, formate species are unlikely to produce methanol, unless and until favorable kinetic conditions are available30. Numerous studies have therefore suggested the formation of a carboxyl species for direct CO2 hydrogenation54,55. Carboxyl intermediate (CO2+H→COOH) is generated with an exothermic reaction energy and activation barrier of -69.2 kJ/mole and 39.0 kJ/mole respectively (Figure 5). Zhao et al. have performed DFT studies for CO2 hydrogenation to methanol on Cu(111) surface and have calculated an activation barrier for carboxyl formation (CO2+H→COOH) to be 33.7 kJ/mole, which is comparable to the barrier on reduced ceria surface. At the transition state, CO2 molecule was observed to be activated in a bent configuration with a 1.46 Å distance of O-H bond, (Figure 4, TS1). The carboxyl species thus produced was adsorbed on top of the oxygen vacancy site of reduced ceria surface with a 2.78 Å distance from the surface (Figure 4, b). Lateral interaction between adsorbed hydrogen and COOH species was studied by analysing the binding energies of co-adsorbed and isolated species on the reduced ceria surface. The coadsorption energy of COOH and H was calculated to be 223.1 kJ/mole, which was observed to be similar to the sum of the adsorption energies of individual COOH (-151.2 kJ/mole) and H (77.6 kJ/mole) species on the reduced ceria surface. Therefore, lateral interaction of co-adsorbed species were considered to be negligible in all the calculations. This assumption is consistent to the earlier study on CO2 hydrogenation7.The carboxyl is likely to dissociate into CO and OH by



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the incorporation of the hydroxyl oxygen into the surface vacancy as shown in Figure 4, c-1 by the redox mechanism, with an exothermic reaction energy of -80.4 kJ/mole and activation barrier of 47.4 kJ/mole (Figure 5). An alternate possibility could be the surface adsorption of reaction generated hydroxyl on ceria, instead of the incorporation of the hydroxyl oxygen in the vacancy site (Figure S2). However, the calculated activation energy (67.2 kJ/mole) of the later mechanism was higher than the former. In addition, the reaction energy for COOH dissociation to form a surface adsorbed hydroxyl species was endothermic, ∆H = 24.6 kJ/mole (Figure S2). Therefore, the redox mechanism for the dissociation of the carboxyl is considered to be the preferred route. On surface incorporation of oxygen, the hydrogen atom of hydroxyl species appears as the chemisorbed hydrogen on the ceria surface. The dissociating bond distance (C-O) in the transition state is 1.58 Å (Figure 4, TS-2). In the product state, the bond distance of adsorbed hydrogen atom with the surface oxygen is 0.99 Å (Figure 4, c-1). Once CO is formed on the surface, it can possibly hydrogenate to produce methanol. The proposed sequence of elementary steps for CO2 reduction to methanol via the formation of carboxyl intermediate on active CeO2-x/Cu(111) interface is: CO2 → COOH → CO → HCO → H2CO → H3CO → H3COH7. The same route has also been suggested by DFT calculations on the extended ceria (110) surface26. The adsorption energy of the intermediate species associated with COOH mediated route are discussed in the previous study26. Adsorption of relatively larger intermediate species (H2CO, H3CO and H3COH), in different orientation, are illustrated in Figure S3-S5 respectively. It is evident from these calculations that the change in the orientation of the adsorbed species does not lead to any difference in their final adsorbed geometry. CO on the surface, interacts with the adsorbed hydrogen atom (Figure 4, c-2) to produce a formyl (HCO) species. The intrinsic activation barrier and reaction energy of this step are 19.4


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and -79.9 kJ/mole respectively. At the CeO2-x/Cu(111) interface the calculated barrier was higher (∆Eact = 63.7 kJ/mole)7 than on the CeO2-x(110) surface. HCO species were adsorbed on top of the oxygen vacancy site. On subsequent hydrogenation, HCO produces formaldehyde (H2CO) on interacting with another surface hydrogen adsorbed on top of the surface Ce atom (Figure 4, d2). The step is calculated to be highly exothermic (∆H = -288.8 kJ/mole). The activation energy is slightly higher than the previous step of CO hydrogenation (∆Eact= 19.4 kJ/mole). This is likely due to the difference in the relative binding energies of the reactants CO (∆Ebinding = -104.9 kJ/mole) and HCO (∆Ebinding= -148.3 kJ/mole), indicating favorable interaction of HCO species with the oxygen vacancy present on the ceria surface. Similar values of activation energies were reported by Zhao et al. for calculations on Cu(111) surface (∆Eact=44.4 kJ/mole)49. In contrast, Graciani and co-workers have obtained a relatively lower (∆Eact =22.2 kJ/mole) activation barrier of HCO hydrogenation on CeO2-x/Cu(111)7. H2CO is adsorbed on top of the oxygen vacancy site (Figure 4, e-1) at 2.47 Å distance from the surface. On subsequent hydrogenation, H2CO produces a methoxy (H3CO) species with an activation barrier of 21.9 kJ/mole (Figure 5). Oxygen atom of H3CO was observed to interact with the vacancy site, by partially substituting the oxygen vacancy on the surface at a distance of 0.44 Å from the vacancy site (Figure 4, f2).This is in contrast to the adsorbed structures of H2CO and HCO species where the oxygen was interacting at a relatively larger distance of 3.01 Å (Figure 4, e-2) and 1.54 Å (Figure 4, d-2) respectively from the vacancy site. The binding energy of the adsorbed H3CO structure (∆Ebinding = -329.6 kJ/mole) was therefore calculated to be higher than the H2CO (∆Ebinding= -126.4 kJ/mole) and HCO (∆Ebinding= -148.4 kJ/mole) species. Thus subsequent hydrogenation of the H3CO species is likely to be difficult and a higher activation energy (∆Eact =58.7 kJ/mole) was estimated for H3CO as compared to the HCO and H2CO hydrogenation. Similar trend in the



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activation barriers for HCO (∆Eact = 79.1 kJ/mole), H2CO (∆Eact = 39.5 kJ/mole) and H3CO (∆Eact = 128.3 kJ/mole) hydrogenation were observed for CO2 reduction to produce methanol on Pd/In2O356. Nevertheless, the hydrogenation of methoxy species is an exothermic step, with ∆H = -199.2 kJ/mole, and is likely to proceed with a slightly higher activation barrier to produce methanol. On reduced surface, OH species adsorbed on the oxygen vacancy site (Figure 4, g-2) may possibly combine with H atom, adsorbed on a top of the neighbour Ce atom (Figure 4, g-2), to form the H2O molecule which could be disrobed from the surface. The intrinsic activation barrier and reaction energy of the formation of H2O were calculated to be 56.4 and -140.5 kJ/mole respectively. CH3OH and H2O were desorbed from the reduced surface with an overall desorption energy of 69.1 kJ/mole as shown in Figure 5. In order to understand the catalytic activity of reduced CeO2 (110) surface, similar calculations of the transition states of the intrinsic steps involved in CO2 hydrogenation via the carboxyl mediated route were performed on stoichiometric ceria. Binding energy of the intermediate species at the most favorable site on stoichiometric surface are discussed in the earlier work26. Adsorption of H2CO, H3CO and H3COH in different orientation are calculated to study the interaction of the molecules with the surface Ce and O atoms respectively. Flipping of these intermediates as shown in Figure S6 to S8 does not lead to a difference in the final adsorbed state. The adsorbed configuration of reactant, products and the transition states are shown in Figure 6. Corresponding reaction energy diagram showing intrinsic activation and reaction energies is illustrated in Figure 7. CO2 molecule was adsorbed in a linear configuration, as was observed on the reduced ceria surface. The bond distance, d(O-Ce), of the adsorbed CO2 structure is 3.04 Å (Figure 6, a). Adsorbed CO2is hydrogenated to COOH with a reaction and activation energies of -22.2 and 49.5 kJ/mole (Figure 7) respectively. Carboxyl is adsorbed on


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the surface at a distance of 2.86 Å from surface oxygen atom (Figure 6, b). As observed on the reduced ceria surface, adsorbed COOH species dissociates into CO and OH, however; the resultant hydroxyl is surface adsorbed, since no vacant site for surface adsorption is available. Thus, the activation barrier for COOH dissociation (∆Eact =55.6 kJ/mole) is estimated to be higher. Surface adsorbed CO participates in hydrogenation (Figure 6, c-2) to form HCO with an exothermic reaction energy of -102.3 kJ/mole (Figure 7). The calculated activation barrier for CO hydrogenation (∆Eact = 26.3 kJ/mole) on the ceria surface was observed to be lower than on Cu(111) surface (∆Eact = 41.5 kJ/mole)49 and slightly higher than on CeO2-x/Cu(111) interface (∆Eact = 22.2 kJ/mole)7. The resultant HCO interacts with the neighbouring hydrogen atom (Figure 5, d-2), to form H2CO. The activation energy of this hydrogenation step is 42.9 kJ/mole, which is significantly higher than on pure Cu(111) surface (13.5 kJ/mole)49 and on CeO2x/Cu(111)

interface (14.5 kJ/mole)7. Methoxy is produced by the hydrogenation of H2CO with

∆H = -195 kJ/mole and ∆Eact = 42.9 kJ/mole (Figure 7). It is adsorbed on four fold hollow site with Ce-O distance of 2.02 Å from the surface (Figure 6, f-1). Subsequent hydrogenation of H3CO lead to the formation of methanol, with an activation energy of 24.7 kJ/mole which is of similar value as calculated for CeO2-x/Cu(111) interface (∆Eact =21.3 kJ/mole)7. Recombination of OH and H on stoichiometric surface is highly exothermic (∆H = - 441.1 kJ/mole) as compared to that on reduced surface. Activation energy of water formation is slightly lower (∆Eact = 39.0 kJ/mole, Figure 7) on stoichiometric surface as compared to the reduced surface (56.4 kJ/mole). The product CH3OH and H2O were desorbed from the surface with an overall desorption energy of 95.3 kJ/mole as shown in Figure 7. A comparison of the intrinsic reaction and activation energies of the corresponding elementary steps of CO2 hydrogenation to methanol on reduced and stoichiometric ceria surface is presented



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in Table 1. On reduced ceria surface, all the intrinsic reaction energies of the elementary steps are exothermic. However, hydrogenation of the methoxy on reduced CeO2 appears to be relatively difficult as compared to the stoichiometric surface and hence the activation barrier for H3CO hydrogenation is higher than stoichiometric surface. On the stoichiometric surface, carboxyl dissociation (COOH→CO+OH) is endothermic (5.7 kJ/mole) with a significantly high activation energy (55.6 kJ/mole, Table 1). In contrast, on the reduced ceria surface, the COOH dissociation step is exothermic (-80.4 kJ/mole) and is of relatively lower activation energy (47.4 kJ/mole, Table 1). CONCLUSIONS DFT calculations were performed to construct a reaction energy diagram of mechanistic pathways for CO2 reduction to CO and methanol on CeO2(110) surface. CO2 is suggested to dissociates into CO on interacting with the oxygen vacancies on reduced ceria surface. On CO2 activation, oxygen atom is incorporated directly into the vacant site, via a redox mechanism. The energetics of CO2 reduction to CO were observed to be dependent on vacancy interactions and favoured on di-vacancies as compared to a mono vacancy. In presence of hydrogen, CO2 is activated to form a carboxyl intermediate which is dissociated to CO via the redox mechanism. On the stoichiometric ceria surface, the COOH dissociation step is of significantly higher activation barrier, however; on the reduced surface, interaction with vacancies reduces the activation barrier of this step. CO thus produced is subsequently hydrogenated to produce methanol in a series of exothermic steps. Compared to the overall energetics of CO2 hydrogenation on Cu(111) and CeO2-x/Cu(111) interface, the energetic on extended CeO2(110) surface shows an equally favoured path for the synthesis of methanol.


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Funding Sources Financial support from GAIL R&D, and Department of Science and Technology (DST), Government of India. ACKNOWLEDGMENT We acknowledge financial support of GAIL R&D, and Department of Science and Technology (DST), Government of India. We acknowledge the computer service center of Indian Institute of Technology, for providing the high performance computing systems. Supporting Information. Reaction diagram of CO2 dissociation to CO and atomic oxygen(O); COOH dissociation to CO and OH on reduced ceria surface are given as Figure S1 and S2 respectively. Figures S3-S8 illustrate the adsorption of H2CO, H3CO and H3COH in different orientations, on reduced and stoichiometric ceria surface. Table S1 shows that effect of dispersion parameter on binding energy of H3COH, H3CO, H2CO and CO2. Vibrational frequencies of the transition states are shown in Table S2-S3. Atomic co-ordinates of transition state structuresas calculated from DFT simulations, are given in Table S4-S6. Binding energy of H3CO and HCOO on stoichiometric, mono-vacancy and di-vacancy surface.

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Figure 1.Reaction diagram of CO2 dissociation to CO on CeO2(110) surface having one oxygen vacancy per unit cell. Distance are in Å.



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Figure 2.Formation of a di-vacancy in different orientation on CeO2(110) surface. The number, ∆Evac reports the calculated energy of formation of the di-vacancy.


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Figure 3.Reaction diagram of CO2 dissociation to CO on the CeO2(110) surface having a divacancy.Distance are in Å.



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Figure 4.Configuration of reactants, products and transition states of elementary steps for CO2 reduction to methanol on reduced ceria surface. Distance are in Å.


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Figure 5. Reaction diagram for CO2 reduction to methanol on reduced CeO2(110) surface.



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Figure 6. Adsorbed configuration of reactant, product and transition states of elementary steps of CO2 reduction to methanol on stoichiometric ceria surface. Distance are in Å.


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Figure 7.Reaction diagram of CO2 reduction to methanol on stoichiometric ceria surface.



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Table 1.Reaction and activation energies of intrinsic step of CO2 reduction to methnaol on reduced and stoichiometric CeO2(110) surface. Reduced surface S.N

Stoichiometric surface

Elementary Reactions ∆H(kJ/mole)

∆Eact (kJ/mole) ∆H(kJ/mole)

∆Eact (kJ/mole)

1

9:; ∗ + = ∗ → 9::= ∗

-69.2

39.0

-22.2

49.5

2

9::= ∗ → 9:∗ + := ∗

-80.4

47.4

5.7

55.6

3

9:∗ + H ∗ → H9O∗

-79.9

19.4

-102.3

26.3

4

H9O∗ + H ∗ → =; 9:∗

-288.8

42.3

-414.7

10.9

5

=; 9:∗ + H ∗ → =A 9:∗

-225.4

21.9

-195.2

42.9

6

=A 9:∗ + H ∗ → =A 9:= ∗

-199.2

58.7

-388.4

24.7


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CO2 Reduction to CO and Methanol on CeO2(110) Surface 298x173mm (300 x 300 DPI)

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