Reducible Inverse CeO x-Based Catalyst as a Potential Candidate for Electroreduction

The inverse metal oxide/metal catalyst is very suitable for electrochemical reaction due to unique catalytic properties of metal oxide with small size and good conductivity of metal. To clarify the potential applications of inverse catalyst in electrochemistry, especially for reducible oxides, an inverse CeOx/Ag(111) model electrocatalyst was constructed and investigated by Density Functional Theory (DFT) for CO2 electrochemical reduction. It is found that Ag atoms acting as an electron donor, can partially reduce Ce4+ to Ce3+ in the supported CeOx cluster leading to the formation of interfacial Ce3+ active sites, which could promote the adsorption and reduction of CO2. As expected, all elementary reaction involved in the CO2 electrochemical reduction are more facile on CeOx/Ag(111) than pure Ag catalyst. Besides, the generation of CH3OH and CH4 is favored on CeOx/Ag(111), whereas the formation of CO, CH2O and H2 is obviously suppressed. More importantly, the weak interaction between H2O and CeOx cluster is beneficial for the desorption of OH intermediate, which makes the regeneration of the catalyst become easier and result in a great recyclability. All those results demonstrate that CeOx/Ag(111) is a potential excellent electrochemical catalyst.


Introduction
Oxide supported metal nanoparticles catalysts are widely used in heterogeneous catalysis and play an important role in industrial production.In the study of the adsorption of H 2 and CO on the oxides supported noble metal catalysts, the different adsorption capacities of H 2 and CO on pre-reduced catalysts at different temperatures was observed, which was attributed to strong metal-support interactions (SMSI) [1].Long-term studies have shown that this kind of interaction exists only in reductive metal oxides carrier (CeO 2 [2], FeO x [3]), which is not only act as supports to prevent the sintering of metal nanoparticles but also have a great influence on the catalytic activity of the catalysts.Recently, it is found that the inverse catalysts also exhibit excellent catalytic performance towards some reactions.In all cases, inverse catalysts exhibit strong interaction between the metal and oxide (SMSI) through sharing part of oxygens [4,5].Density Functional Theory (DFT) research reveals that all Ce atoms are Ce 3+ in CeO x /Metal (Cu and Ag) catalyst due to the covalent bond between CeO x cluster and metal carrier, leading to the formation of mixed oxides in the interface of the catalyst [6].Thus, it can be speculated that the unique interface in inverse catalysts will play a significant role in the reaction.
At present, studies have shown that the inverse catalysts can catalyze many important reactions such as dissociation of O 2 [5], water-gas shift reaction [7] and CO 2 reduction to produce methanol [8].The hydrogenation of CO 2 on CeO x /Cu catalyst indicate that CO 2 molecule can be activated when it adsorbed on the interface of the catalyst [8].This inspires that the Ce-related inverse catalysts may be a potential candidate for electrochemical CO 2 reduction reaction (CO 2 RR) to produce useful products such as CH 4 , CH 3 OH, HCOOH or CH 2 O. Recently, the DFT investigation about CO 2 RR on (TiO 2 ) 3 /Ag(110) have also been carried out.It is demonstrated that the introduction of metal oxides to a metal electrode provides the interfacial active sites for adsorption and activation of CO 2 and facilitates further reduction process.The metal carrier can guarantee good electrical conductivity, meanwhile the oxide cluster especially those reductive metal oxides can provide an electron rich environment, which can promote the adsorption and activation of reactant.However, it is also pointed out that the strong binding between O-containing species and Ti makes the OH* removal remain challenging [9].Thus it is very important to design a novel catalyst for facile removal the OH*.
Based on the above analysis, the inverse CeO x -based catalysts were constructed.For CeO x /M (M = Cu, Ag, Au) catalyst, different amounts of electrons could be transferred from metal to oxide clusters, in the order of Cu > Ag > Au [6].Herein, the CeO x /Ag(111) was employed, since it is relatively stable and its 4f states of Ce 3+ has high activity [6].Besides, detailed reaction mechanism of CO 2 electrochemical reduction to CO, HCOOH, CH 4 and CH 3 OH has been systematically studied.The presence of Ce 3+ generated by the SMSI effect between CeO x and Ag reduces the free energy for the reduction of CO 2 .Especially, it is beneficial to generate CH 3 OH and CH 4 in thermodynamic.Different from other catalysts, the removal of OH is also facile, due to the weak interaction between H 2 O and CeO x cluster.Those results provide valuable reference to design more efficient catalysts for electrochemical reduction of CO 2 .

Electronic Characteristics for the CeO x /Ag(111) Catalyst
A Bader analysis of the charge was performed firstly for CeO x /Ag(111) catalyst, in order to reveal the electronic structure and interactions between ceria and Ag at the interface of this catalyst.As reported, the Bader charge of Ce in the particle is +2.00 e [5].While the calculated Bader charge for Ce in the CeO 2 /Ag(111) are ~+2.1 e and ~+2.2 e, respectively (Table 1).Thus it can be inferred that the formal oxidation states of Ce atoms in the ceria cluster are +4 and +3, respectively [5].Spin Density Difference (SDD) versus Z direction of the CeO x /Ag(111) catalyst is plotted in Fig. 1a, the isosurface of SDD is plotted in Figure 1b.From Figure 1a,b, obvious electron accumulation is founded on Ce atoms at the edge of a cluster.Therefore it can be concluded that the electrons are mainly located on the edge of the CeO x cluster.In other words, single electrons are concentrated in certain areas of ceria cluster, the computational results show that only parts of Ce atoms which located on the edges of ceria cluster have been reduced (Figure 1b).All those Ce atoms can be considered as Ce 3+ .In addition, the Electron Density Difference (EDD) versus Z direction of the CeO x /Ag(111) catalyst is plotted in Figure 2a, the isosurface of EDD is plotted in Figure 2b.From Figure 2, it was found that EDD indeed increased on CeO x cluster and decreased on surface Ag atoms, which indicates that there is a charge transfer from the Ag slab to the ceria cluster.And the electrons are mainly coming from the surface Ag layer, which located under the CeO x cluster, resulting in an oxidation of the Ag atoms and a reduction of the CeO x cluster.

CO 2 Adsorption and Activation
The adsorption and activation of CO 2 is considered as the first step for CO 2 reduction.In the initial CO 2 adsorption configurations, no matter where CO 2 was placed (on the interface of CeO x /Ag(111), Ag and CeO x , linear or bent structure).Only two stable structure of CO 2 adsorption (denoted as CO 2 *, * = CeO x /Ag(111)) were reached in the final (Figures 3 and 4c).CO 2 is linearly adsorbed on the edge (Ce 3+ ) or corner (Ce 4+ ) of the CeO x cluster using its O atom.The adsorption energy of CO 2 for those two structures is 0.42 eV and 0.40 eV, respectively.The shortest distance between O atom of CO 2 and Ce atom of CeO x cluster is 3.03 Å and 3.04 Å. Gradient isosurfaces of reduced density gradient (RDG) versus sign(λ 2 )/ (Figure 3a) [10] plot shows that there are Van Der Waals interactions between O-Ce and C-Ag for CO 2 /*(Ce 3+ ) simultaneously.Whereas, in CO 2 /*(Ce 4+ ), only O-Ce interaction has been found (Figure 3c).Spikes near zero in the corresponding scatter plot (Figure 3b,d) also reflect there exist weak interactions in CO 2 /*(Ce 3+ ) than CO 2 /*(Ce 4+ ).The synergism by Ag and CeO x in CO 2 /*(Ce 3+ ) configuration may be beneficial to the subsequent hydrogenating of CO 2 .Therefore CO 2 /*(Ce 3+ ) is chosen as the initial structure in this work.

Formation of Formic Acid and Methanol
The CO2 adsorption configuration shown in Figure 3a is set to be the starting point for further reduction.The first hydrogenation of CO2* may occur at either the O or C atom, forming carboxyl (HOCO*) or formate (HCO2*) species, respectively.Herein, the routes through both HOCO* and HCO2* intermediates were investigated.The Gibbs free energy diagrams for all possible products involved in CO2 reduction process on CeOx/Ag(111) were also examined.As mentioned above, hydrogenation of CO2 could occur either on oxygen or carbon atom of CO2.HOCO* or HCO2* intermediates are eventually formed at the interface of CeOx and Ag.The HCO2* (Figure 4d) intermediate is formed with a free energy release of −1.14 eV (Figure 4a).While the HCO2* intermediate can change to bidentate form using its O atoms with CeOx cluster, which is endothermic with free energy barrier of 0.73 eV.When the hydrogenation of CO2 occurs on the O atom (Figure 4b), forming HOCO* (Figure 4f) with a free energy barrier of 0.67 eV.This intermediate is bind to the

Formation of Formic Acid and Methanol
The CO 2 adsorption configuration shown in Figure 3a is set to be the starting point for further reduction.The first hydrogenation of CO 2 * may occur at either the O or C atom, forming carboxyl (HOCO*) or formate (HCO 2 *) species, respectively.Herein, the routes through both HOCO* and HCO 2 * intermediates were investigated.The Gibbs free energy diagrams for all possible products involved in CO 2 reduction process on CeO x /Ag(111) were also examined.
As mentioned above, hydrogenation of CO 2 could occur either on oxygen or carbon atom of CO 2 .HOCO* or HCO 2 * intermediates are eventually formed at the interface of CeO x and Ag.The HCO 2 * (Figure 4d) intermediate is formed with a free energy release of −1.14 eV (Figure 4a).While the HCO 2 * intermediate can change to bidentate form using its O atoms with CeO x cluster, which is endothermic with free energy barrier of 0.73 eV.When the hydrogenation of CO 2 occurs on the O atom (Figure 4b), forming HOCO* (Figure 4f) with a free energy barrier of 0.67 eV.This intermediate is bind to the catalyst using its C (C-Ag = 2.25 Å) and O (O-Ce = 2.56 Å) atom simultaneously.The Ag atom which bonds to C atom obviously migrates up from Ag surface.Obviously, the HCO 2 * formation is thermodynamically preferable over HOCO*.
When HCO 2 * or HOCO* intermediate is formed on the catalyst.Formic acid as an important product for CO 2 reduction can be generated firstly by hydrogenation of HCO 2 * or HOCO* intermediate.For the hydrogenation of HOCO*, this is thermodynamically preferable.The formation of cis-HOHCO* and trans-HOHCO* (Figure 4h,i) is exothermic by 0.72 and 0.82 eV, respectively.For the hydrogenation of bidentate HCO 2 *, subsequent reaction process can be achieved through two different ways.(a) The second hydrogen is bonded with carbon of HCO 2 *, a H 2 CO 2 * intermediate is formed.The free energy change for this process is 0.42 eV endothermic.(b) The hydrogenation occurs on oxygen atom of HCO 2 *, HOHCO* intermediate can be generated.There are also two possibilities here, (1) formation of cis-HOHCO* (0.47 eV, endothermic), (2) formation of trans-HOHCO* (0.37 eV, endothermic).The free energy change for the rate-limiting step of formic acid formation from HOCO* and HCO 2 * intermediate are 0.67 and 0.73 eV, respectively.Obviously, HOCO* reaction path is slightly more facile for the formation of formic acid.
As shown in Figure 4, trans-HOHCO* can further hydrogenate by (H + + e − ) to generate HOH 2 CO* intermediate.CH 3 OH and O atom co-adsorbed on the catalyst can be generated through the further hydrogenation of HOH 2 CO.Next, the formed CH 3 OH can be desorbed from the surface of the catalyst, leaving only OH*.Thermodynamically, the formation of CH 3 OH through HOH 2 CO* is uphill in free energy by only 0.05 eV.In general, once HOHCO formed, it is very easy to convert into CH 3 OH, although more hydrogenation steps are needed for this process.

Formation of Methane
With the aim of comparison, both HCO 2 * and HOCH* reaction path have been plotted in Figure 5.When the hydrogenation of HOH 2 CO* firstly occurs on the carbon atom, methoxy (CH 3 O*) and hydroxyl (OH*) intermediates are formed and co-adsorbed on the catalyst.This process is endothermic with free energy change of 0.71 eV.Further hydrogenation for the formation of H 2 O and CH 4 are all uphill with free energy barrier of 0.54 eV and 0.20 eV, respectively.Obviously, the total free energy change from CO 2 * to CH 3 OH and CH 4 are almost the same.Thus it is speculated that both CH 3 OH and CH 4 are easy to form on the interface of this catalyst, as both of process are free energy downhill from the CO 2 *.However, it is noteworthy that the formation of CH 4 needs more steps than that of CH 3 OH.Thus, CH 3 OH formation is probably facile than the formation of CH 4 .The most endothermic steps for the formation of HOCO* or trans-HOHCO* are observed, which will be the potential-limiting steps for the formation of CH 3 OH and CH 4 on the CeO x /Ag(111) catalyst.
free energy change from CO2* to CH3OH and CH4 are almost the same.Thus it is speculated that both CH3OH and CH4 are easy to form on the interface of this catalyst, as both of process are free energy downhill from the CO2*.However, it is noteworthy that the formation of CH4 needs more steps than that of CH3OH.Thus, CH3OH formation is probably facile than the formation of CH4.The most endothermic steps for the formation of HOCO* or trans-HOHCO* are observed, which will be the potential-limiting steps for the formation of CH3OH and CH4 on the CeOx/Ag(111) catalyst.

Formation of Carbon Monoxide
Although CO2 only weakly binds to CeOx/Ag(111), less energy is required for the formation of HOCO* from CO2* than that on pure Ag [9], on which more than 1.2 eV is needed for the hydrogenating of CO2 to form HOCO*.The formation of CO* (Figure 6b) from HOCO* is endothermic on CeOx/Ag(111) with a free energy change of 0.69 eV.However, further hydrogenating of HOCO* is exothermic with a free energy change of −0.82 eV.Clearly, further hydrogenating of HOCO* to form HOHCO* is more facile on CeOx/Ag(111) than the formation of CO (0.18 eV, CO*).Simultaneously, CO adsorption is quite stable on CeOx/Ag(111).As CO desorption is endothermic with a free energy change of 0.83 eV.Consequently, CO* may be further reduced on CeOx/Ag(111) to other products rather than desorbed from the catalyst.The reduction pathway and the free energy profile of the corresponding intermediates from CO* have been mapped out (Figure 6).As shown in Figure 6, apart from the formation of HCO* (0.25 eV), all other processes after HCO* formation are downhill in free energy.

Formation of Carbon Monoxide
Although CO 2 only weakly binds to CeO x /Ag(111), less energy is required for the formation of HOCO* from CO 2 * than that on pure Ag [9], on which more than 1.2 eV is needed for the hydrogenating of CO 2 to form HOCO*.The formation of CO* (Figure 6b) from HOCO* is endothermic on CeO x /Ag(111) with a free energy change of 0.69 eV.However, further hydrogenating of HOCO* is exothermic with a free energy change of −0.82 eV.Clearly, further hydrogenating of HOCO* to form HOHCO* is more facile on CeO x /Ag(111) than the formation of CO (0.18 eV, CO*).Simultaneously, CO adsorption is quite stable on CeO x /Ag(111).As CO desorption is endothermic with a free energy change of 0.83 eV.Consequently, CO* may be further reduced on CeO x /Ag(111) to other products rather than desorbed from the catalyst.The reduction pathway and the free energy profile of the corresponding intermediates from CO* have been mapped out (Figure 6).As shown in Figure 6, apart from the formation of HCO* (0.25 eV), all other processes after HCO* formation are downhill in free energy.As shown in Figure 6b, CO* is bonded to Ag only through C atom.Bond length of C-O and C-Ag are 1.15 and 2.15 Å, respectively.This indicates that CO is chemically adsorbed on the Ag surface and has been activated.With the formation of CHO*, O atom is weakly bonded to the Ce atom of the CeO x cluster, distance of C-Ag and O-Ce are 2.20 and 2.60 Å.There is an obvious deformation of Ag surface.The Ag atom, which interacts with C atom, is moved upward from the Ag surface.Gradient isosurfaces of RDG and sign(λ 2 )/ [10] plot also shows clearly a strong attractive interaction between carbon and Ag (Figure S1).With further hydrogenating of CHO*, the formed CH  To sustain the catalytic cycle, no matter in what kind of reaction path, the O* species has to be cleared through consecutive hydrogenating to OH* and further to H2O.The hydrogenating of OH* to H2O is endothermic, with a free energy barrier of 0.70 eV, which indicate that OH is the most stable intermediate species at U = 0 V.This makes OH removal become the free energy limiting step.However, the free energy change is still lower than the desorption energy (1 eV) needed on TiO2/Ag system [9] or pure metals (such as Pt (0.75 eV)) [11].The free energy change needed for the formation and removal of H2O is only slightly higher than the energy required for the generation of HOCO* intermediate (0.67 eV) on CeOx/Ag(111) catalyst.The weak interaction between H2O and CeOx cluster is beneficial for the formation of H2O (the distance of O-Ce has enlarged from 2.15 Å to 2.72 Å, for OH* and H2O* intermediate), which makes the regeneration of the catalyst become easier, resulting To sustain the catalytic cycle, no matter in what kind of reaction path, the O* species has to be cleared through consecutive hydrogenating to OH* and further to H 2 O.The hydrogenating of OH* to H 2 O is endothermic, with a free energy barrier of 0.70 eV, which indicate that OH is the most stable intermediate species at U = 0 V.This makes OH removal become the free energy limiting step.However, the free energy change is still lower than the desorption energy (1 eV) needed on TiO 2 /Ag system [9] or pure metals (such as Pt (0.75 eV)) [11].The free energy change needed for the formation and removal of H 2 O is only slightly higher than the energy required for the generation of HOCO* intermediate (0.67 eV) on CeO x /Ag(111) catalyst.The weak interaction between H 2 O and CeO x cluster is beneficial for the formation of H 2 O (the distance of O-Ce has enlarged from 2.15 Å to 2.72 Å, for OH* and H 2 O* intermediate), which makes the regeneration of the catalyst become easier, resulting in a great recyclability.
Finally, the hydrogen evolution reaction (HER), which is the main competitive reaction of CO 2 RR, has been calculated.The computational result shows that the free energy change for the formation of H 2 is 1.22 eV, which is far higher than the rate-limiting step of CO 2 RR on CeO x /Ag(111) (Figure 7e).Based on all those advantages mentioned above, it is reasonable to believe that the CeO x /Ag(111)is a potential excellent catalyst for CO 2 electroreduction.
The above analysis shows that CH 3 OH and CH 4 are the main products on the surface of CeO x /Ag(111) catalyst.In order to change into exothermic and spontaneous process for all the reaction steps, the free energy diagram with an external potential of −0.67 V (RHE) was depicted and shown in Figure 8.The potential is selected based on the potential-limiting step, that is the formation of HOCO*.Under this applied potential (U = −0.67V vs RHE), the free energy change of all steps involving the proton-electron pairs will be corrected according to Equation (2).Under this potential, all steps following HOCO* formation become downhill on the free energy diagram.

Computational Details
Spin-polarized calculations are conducted with the Quickstep [12] module of the CP2K program package [12,13], which based on the Gaussian and plane waves formalism [14].The exchangecorrelation energy was described by the generalized-gradient approximation (GGA) with spinpolarized revised PBE functional (revPBE).In order to investigate the influence of dispersion, the structures and energies for all the stationary points included in our work are evaluated at the DFT-D3 level with the approximation suggested by Grimme [15] added to the revPBE calculated energy.Fronzi et.al have carried out studies on water adsorption on the stoichiometric and reduced CeO2(111) surface [16].They have pointed out that the DFT+U method has some inherent defects, such as no unique way to decide on what value to use for U; weather DFT+U method indeed provides a systematic improvement of the energetics of a system compared to plain DFT and so forth.Moreover, theoretical studies for the CO oxidation on AuxCe1-xO2 catalyst also pointed out there is no hard evidence that GGA gives unusually large errors when used to calculate the energy differences [17].In short, it is impossible to find a universal method which gives a satisfactory description of both the electronic structure and the energetics.Because this article focuses on the energy changes of the CO2RR reaction process, Therefore DFT+U method is not considered in this paper.The wavefunctions were expanded in an optimized double-ζ Gaussian basis sets [16,18] with a cutoff energy of 500 Rydberg [14].Core electrons have been modelled by scalar relativistic norm-conserving pseudo potentials [19] with 12, 6, 1, 4 and 11 valence electrons for Ce, O, H, C and Ag, respectively.Brillouin

Computational Details
Spin-polarized calculations are conducted with the Quickstep [12] module of the CP2K program package [12,13], which based on the Gaussian and plane waves formalism [14].The exchange-correlation energy was described by the generalized-gradient approximation (GGA) with spin-polarized revised PBE functional (revPBE).In order to investigate the influence of dispersion, the structures and energies for all the stationary points included in our work are evaluated at the DFT-D3 level with the approximation suggested by Grimme [15] added to the revPBE calculated energy.Fronzi et.al have carried out studies on water adsorption on the stoichiometric and reduced CeO 2 (111) surface [16].They have pointed out that the DFT+U method has some inherent defects, such as no unique way to decide on what value to use for U; weather DFT+U method indeed provides a systematic improvement of the energetics of a system compared to plain DFT and so forth.Moreover, theoretical studies for the CO oxidation on Au x Ce 1−x O 2 catalyst also pointed out there is no hard evidence that GGA gives unusually large errors when used to calculate the energy differences [17].In short, it is impossible to find a universal method which gives a satisfactory description of both the electronic structure and the energetics.Because this article focuses on the energy changes of the CO 2 RR reaction process, Therefore DFT+U method is not considered in this paper.The wavefunctions were expanded in an optimized double-ζ Gaussian basis sets [16,18] with a cutoff energy of 500 Rydberg [14].Core electrons have been modelled by scalar relativistic norm-conserving pseudo potentials [19] with 12, 6, 1, 4 and 11 valence electrons for Ce, O, H, C and Ag, respectively.Brillouin zone integration is performed with a reciprocal space mesh consisting of only the gamma point.Thermochemistry calculation is implemented by TAMkin [20], a post-processing toolkit for normal mode analysis.All other electronic property analyses deal with Multiwfn software, a program for realizing electronic wavefunction analysis [21].
The following equation was used to calculate the adsorption energy of an adsorbate where SD(ED) (CeOx/Ag(111)) is the Spin Density (Electron Density) of the catalyst, SD(ED) (CeOx) and SD(ED) (Ag(111)) are Spin Density (Electron Density) of CeO x and Ag(111) respectively, the cartesian coordinates used for the computation of CeO x and Ag(111) are consistent with that in the CeO x /Ag(111).
The Gibbs free energy diagrams for CO 2 reduction along different pathways were calculated with reference to the computational hydrogen electrode (CHE) proposed by Nørskov et al. [22].The free energy of each species is obtained using TAMKin (Table S1) [20].The free energy change of each elementary step at external potential U will be shifted by eU as ∆G(U) = ∆G(U = 0) + eU (2) where e is the electronic charge.According to Nørskov et al., the thermodynamic activation barrier equals the largest of the free energy differences [22].A Ce 6 O 13 cluster, which is the most stable structure found by other theoretical work [6], is selected and supported on an Ag(111) metal surface described by means of 7 × 7, five atomic layers thick of Ag slab (245 Ag atoms) and a vacuum region of 20 Å between repeated slabs.In our calculations,

Figure 6 .
Figure 6.Free energy diagrams for the carbon monoxide.(a) reaction path for the formation of CO, (b) CO*, (c) HCO*.
2 O* bonds to CeO x /Ag(111) only through O-Ce interaction (C-Ag 4.28 Å, O-Ce 2.71 Å).The C-Ag in CHO* is completely broken upon formation of CH 2 O*.The same situation occurs in the formation of CH 3 O*, in which methoxyl bonds to the CeO x /Ag(111) only through O-Ce (C-Ag 6.14 Å, O-Ce 2.13 Å).The high oxygen affinity of CeO x /Ag(111) keeps the O atom of CH 3 O* bound to Ce strongly.And this, in turn, will cause the breaking of the C-O bond of CH 3 O*.Eventually, CH 4 can be formed in the subsequent hydrogenation step.2.6.Formation of Acetaldehyde The formation of formaldehyde can be achieved through (a) hydrogenation of CO* and HCO*, which is mentioned above in 2.5, or (b) dehydrogenation of methoxy in the (CH 3 O_OH)* intermediate.As plotted in Figure 7, Breaking C-O bond of HOH 2 CO* intermediate can form CH 2 O and OH co-adsorption intermediate (Figure 7c).With the formation H 2 O, CH 2 O* is ultimately formed.Free energy change for this process is 0.63 eV, which is slightly lower than the formation of HOCO*, thus the rate-limiting step for this reaction path is also the HOCO* generation.Besides, the dehydrogenation of CH 3 O* intermediate can also lead to the formation of CH 2 O* (0.88 eV).Obviously, this reaction path is relatively difficult to achieve.Compared with the formation of CH 3 OH and CH 4 , more energy is needed for the generation of CH 2 O. Thus CH 2 O formation is unfavorable on this catalyst.Catalysts 2018, 8, x FOR PEER REVIEW 8 of 11

Catalysts 2018, 8 ,
x FOR PEER REVIEW 9 of 11 stable (0.19 eV lower) than CH3OH + OH*.This result indicates that CH3OH formation is less favorite than CH4 thermodynamically.However, CH3OH formation requires fewer proton reduction and hydrogenation steps, therefore, both CH3OH and CH4 are likely to be generated.

Figure 8 .
Figure 8. Free energy diagram for CO 2 reduction on CeO x /Ag(111) along HOCO* pathway at −0.67 V (RHE).When hydrogenation of CO 2 occurs on the oxygen atom, HOCO* is the key intermediate and plays a central role in the entire mechanism.Further hydrogenating HOCO* leads to the formation of HOHCO* (trans-formic acid) intermediate.When hydrogenation of CO 2 occurs on the carbon atom, trans-HOHCO becomes the critical intermediate.A HOH 2 CO* intermediate can be formed through the hydrogenation of trans-HOHCO.Breaking different C-O bond of HOH 2 CO* will lead to the formation of CH 3 OH, CH 2 O or an adsorbed methoxy (CH 3 O*) species on the Ce site and an OH*.Subsequently, CH 3 O* can be hydrogenated to produce CH 4 or CH 2 O.And the catalyst will recover its original state with the desorption of H 2 O by the hydrogenation of OH*.Based on the free energy profile for the HOCO* path shown in Figure 8, CH 3 OH desorption from the catalyst release more energy (−0.69 eV) than the formation of CH 3 O* (−0.13 eV), whereas, the CH 3 O* intermediate is more stable (0.19 eV lower) than CH 3 OH + OH*.This result indicates that CH 3 OH formation is less favorite than CH 4 thermodynamically.However, CH 3 OH formation requires fewer proton reduction and hydrogenation steps, therefore, both CH 3 OH and CH 4 are likely to be generated.

Table 1 .
Bader charge of the Ce and Ag atoms under the CeO x cluster of CeO x /Ag(111).

Table 1 .
Bader charge of the Ce and Ag atoms under the CeOx cluster of CeOx/Ag(111).

Table 1 .
Bader charge of the Ce and Ag atoms under the CeOx cluster of CeOx/Ag(111).