堿性介質(zhì)中CO對(duì)甲醇在PtAu(111)和Pt(111)表面氧化生成甲酸的影響的第一性原理研究

劉建紅1　呂存琴1,2　晉　春1,*　王貴昌3,*(1山西大同大學(xué)化學(xué)與環(huán)境工程學(xué)院，山西大同037009；2先進(jìn)能源材料化學(xué)教育部重點(diǎn)實(shí)驗(yàn)室(南開大學(xué))，天津300071；3南開大學(xué)化學(xué)學(xué)院，天津市金屬與分子基材料化學(xué)重點(diǎn)實(shí)驗(yàn)室(南開大學(xué))，天津300071)

堿性介質(zhì)中CO對(duì)甲醇在PtAu(111)和Pt(111)表面氧化生成甲酸的影響的第一性原理研究

劉建紅1呂存琴1,2晉春1,*王貴昌3,*
(1山西大同大學(xué)化學(xué)與環(huán)境工程學(xué)院，山西大同037009；2先進(jìn)能源材料化學(xué)教育部重點(diǎn)實(shí)驗(yàn)室(南開大學(xué))，天津300071；3南開大學(xué)化學(xué)學(xué)院，天津市金屬與分子基材料化學(xué)重點(diǎn)實(shí)驗(yàn)室(南開大學(xué))，天津300071)

采用密度泛函理論計(jì)算研究了堿性介質(zhì)中甲醇在清潔的PtAu(111)和Pt(111)表面、及有CO存在的PtAu(111)和Pt(111)表面的氧化。計(jì)算結(jié)果表明，在堿性介質(zhì)中，預(yù)吸附的CO促進(jìn)了甲醇在PtAu(111)和Pt(111)表面氧化的每一步反應(yīng)，這與其在Au(111)表面的作用相似。究其原因，是由于CO的吸附增強(qiáng)了OH的穩(wěn)定性和堿性，從而增強(qiáng)了OH奪取氫原子的能力。

甲醇氧化；M(111)(M=PtAu,Pt)；堿性介質(zhì)；CO促進(jìn)作用；密度泛函理論計(jì)算

[Article]

www.whxb.pku.edu.cn

In recent years,there are several successful attempts to prove the ability of theoretical modeling based on density functional theory(DFT)calculations in providing detailed mechanism and kinetics for complex surface reactions.However,although some theoretical studies about methanol oxidation have been performed14,15,the physical origin is unclear.The minimal energy pathways for COOH formation from CO and OH co-adsorbed on Au(111)and Au(110)surfaces have been investigated.The calculation results show that the ability of CO to enhance the adsorption of OH is related to the surface structure of gold and changes in the work function as induced by the adsorption of CO14. Wang et al.15investigated all elementary steps of methanol dehydrogenation and oxidation on atomic-oxygen-covered or OH-covered Au(111)surfaces,and they found that the reaction pathway is related to the oxygen coverage.On low oxygen coverage,methanol decomposition starts from α-H elimination and β-H elimination to produce CH2O and CO.On high oxygen coverage,the CH2O formed through the elimination of α-H,and one β-H is suitable for the cooperative interaction of two nearby atomic oxygen.Recently Lv et al.16investigated the kinetics of methanol oxidation into formic acid catalyzed by Au(111)surface with and without CO in alkaline media.The calculation results show that the pre-adsorbed CO promotes almost every step of methanol oxidation both thermochemically and kinetically.Yuan et al.17have studied the methanol oxidation on PtAu(111)surface via CO pathway,and proposed that proper arrangement of Au and Pt sites is suitable for non-CO pathway.Inspired by Yuan et al., methanol oxidation on the PtAu(111)surface along the non-CO pathway has been investigated,and a comparison has been made with that along the CO pathway by Zhong et al.18They found that the non-CO pathway of methanol oxidation on PtAu(111)is energetically more favorable than the CO pathway.

The catalytic properties of thin metal overlayers deposited on a foreign substrate could be enhanced with respect to those of the parent metals19－23.PtAu bimetallic surfaces show unique catalytic properties for methanol oxidation23,which could be attributed to the synergistic activity of Au and Pt.Analogous to the promoting effect of adsorbed carbon monoxide on the oxidation of methanol on a gold catalyst verified by experiments6and theoretical calculation16,is there the same effect for methanol oxidation to formic acid on platinum and PtAu alloy?To the best of our knowledge, there are no relevant reports about the effect of CO on the electrocatalytic activity of metal platinum and PtAu alloy in alkaline media obtained for methanol oxidation to formic acid.Therefore, we replaced the top slab of Au atoms on Au(111)surface with Pt atoms,referred as PtAu(111)surface hereafter,and investigated the methanol oxidation to formic acid on this surface and Pt(111) surface with and without CO by using density functional theory methods.Detailed comparisons for methanol oxidation to formic acid in alkaline media have been made among PtAu(111),Pt(111), and Au(111)surfaces,and similar promotion effect of CO on methanol oxidation has been found.

2　Calculation methods and models

All DFT calculations were implemented by using the Vienna ab initio simulation package(VASP)24,25.The exchange-correlation energy and electron－ion interaction were described at the generalized gradient approximation level(Perdew-Wang 9126)and the projector-augment wave(PAW)scheme27,28.The energy cutoff applied to the plane wave basis has been set as 400 eV.The calculations were done on a(3×3)four-layer slab separated by a vacuum region around 1.5 nm.A 3×3×1 Monkhorst-Pack29kpoint sampling was implemented in all calculations.The geometry optimizations were performed by using the conjugated gradient method,and the top two metal layers as well as the adsorbed molecule were allowed to relax until the maximal forces on each relaxed atom were less than 0.5 eV?nm－1.The molecules in the gas phase have been calculated using a box with the size of 1.7 nm× 1.8 nm×1.9 nm,and only a Г-point was used during the calculation.Spin-polarized calculations were performed when needed. To find the saddle points along the adiabatic minimal energy path (MEP),which connected each initial state(IS)and final state(FS) of a given elementary step,the nudged elastic band(NEB)with quasi-Newton optimizer30implemented in the VASP has been systematically utilized.Eventually,the transition states were identified by the existence of only one normal vibrational mode associated with a pure imaginary frequency.The optimized lattice constants of Au and Pt are 0.418 and 0.399 nm,which agrees well with the experimentally measured values of 0.408 and 0.392 nm31for Au and Pt,respectively.Lattice mismatch resulted in the contraction or expansion of lattice of the bimetal surface compared with that of bulk pure metal32.For the PtAu(111)system,the calculations were performed with the DFT optimized clean Au lattice constant of 0.418 nm.In addition,we have also performed test calculations for the geometry convergence criterion(0.5 eV?nm－1)used in the present work.When the force tolerance is improved to 0.3 eV?nm－1,the calculated adsorption energy of OH is only changed by 0.004 eV,confirming the enough accuracy of the conclusion obtained using the more loose convergence criterion of 0.5 eV?nm－1.

There are four kinds of adsorption sites on the surface:top, bridge,hcp,and fcc sites.The adsorption energy(Eads)is defined as follows:Eads=Eadsorbate/substrate－Eadsorbate－Esubstrate,where Eadsorbate/substrateis the total energy of the adsorbate-substrate system,Eadsorbateexpresses the energy of the adsorbate in the gas phase,and Esubstrateexpresses the total energy of the substrate.In this work,for a reaction like AB=A+B,the calculated total energy change or reaction enthalpy(?H)is defined by the formula of?H=E(A+ B)/M－E(AB)/M,where E(A+B)/M and E(AB)/M are the total energies for the coadsorption system of the product and of the reactant,respectively.Activation energy,Ea,is calculated based on the equation of Ea=ETS－E(AB)/M,where ETSis the total energy of the transition state(TS).

Owing to the complexity of real-world direct methanol fuel cells systems,gas phase methanol oxidation investigations are often been used as prototype reactions to probe the mechanism of low-temperature methanol fuel cells18.Therefore,our present calculations were performed with gas phase models to investigate the effect of CO on the methanol oxidation to formic acid in alkaline media.

3　Results and discussion

Our previous study show that the CO adsorbed at the top site on Au(111)surface can promote the oxidation of methanol to formic acid16.We consider whether CO adsorbed on the other kinds of metals can have the same or the opposite effect on methanol oxidation to formic acid.For analyzing the effect of CO on methanol oxidation,we calculated the transition states for relevant reactions involved in methanol oxidation to formic acid on PtAu (111)and Pt(111)surfaces with and without CO.In order to compare the calculated results with that onAu(111)surface better, the following possible reaction mechanism were considered in our present work.

3.1Effect of CO to oxidize the methanol to formic acid on PtAu(111)surface

To make the comparison more reliable,we give the same initial state on PtAu(111)surface with that on Au(111)surface,i.e.,the molecular CO adsorbs on the top site of Pt for all the initial states, and the optimized configurations are shown in Figs.1 and 2.The corresponding energetic data are reported in Table 1 and Fig.3.

3.1.1Reaction of CH3OH and OH(CH3OH+OH→CH3O+H2O)

For the initial oxidative dehydrogenation step of methanol, methanol is adsorbed on the top site via oxygen atom on PtAu(111)and CO/PtAu(111)surfaces.The distance between the O atom of O―H and the H atom of the CH3O―H is 0.157 nm on CO/PtAu(111)surface,which is 0.007 nm smaller than that on PtAu(111)surface.In fact,on the PtAu(111)surface,the reaction of dehydrogenation for methanol is almost a neutral reaction with a ΔH equal to－0.01 eV,while on the CO/PtAu(111)surface,it becomes an exothermic reaction releasing only 0.11 eV in energy. Moreover,the energy barrier only increased by 0.10 eV with the presence of adsorbed CO.The slight difference of reaction energy and activation barrier on PtAu(111)surface with and without CO are the reasons that we donot take this step as the example to talk about the effect of CO.

3.1.2Reaction of CH3O and OH(CH3O+OH→CH2O+ H2O)

In relation to the second step,from Table 1,we can find that the barrier of this step on PtAu(111)surface is higher than that of 0.56 eV on Au(111)surface16,and the reason why this step has so high barrier to happen is that Pt has stronger adsorption ability thanAu, which makes CH3O more stable on the PtAu(111)surface and harder to dissociate from the surface to react with OH.In the presence of CO on PtAu(111)surface,the energy barrier is reduced to 0.64 eV from 0.99 eV on PtAu(111)surface.As we cannot find any obvious differences between the initial states on PtAu(111)surface with and without CO,we try to find where the differences come from with the analysis of the reaction steps. Through the transition states analysis,shown in Figs.1 and 2,we can find that this reaction goes along similar paths on the different surfaces.On PtAu(111)and CO/PtAu(111)surfaces,the CH3O first dissociates from the surface and the abstracted H atom which will react with OH tries to approach the OH.Then after the C―H bond cleavage and O―H formation,the C―O bond rotates until the C,O,and H atoms of CH2O on a same plane.This induces the formation of the hydrogen bond between the CH2O and H2O to lower the energy of this system.From Figs.1 and 2,it can be seen that one H bonding of C―H…O is formed between C―H bond of CH3O and O of OH without and with co-adsorbed CO.However,besides the C―H…O bond,it seems another H bonding is formed with the presence of CO,i.e.,the O―H…O bond between H in OH and O in CO,which may lead to the increased stability in the transition state and final state and therefore the change in reaction heat and activation barrier.

3.1.3Reaction of CH2O and OH(CH2O+OH→CHO+ H2O)

The third step of methanol oxidation to formic acid has a little difference on the two different surfaces in the enthalpy by 0.05 eV, which can be explained by the data listed in Figs.1 and 2.We can point out that there is no big difference between the final stateson the two different surfaces,while the initial state on the CO/ PtAu(111)surface is less stable than that on the PtAu(111)surface, with a longer distance of 0.013 nm between the H and O which combines to react.However,with the existence of CO,it is obviously advantageous for the initial state to overcome the energy barrier of 0.38 eV compared with that of 0.85 eV on PtAu(111) surface.From the configurations shown in Figs.1 and 2,we can find that there are many differences between the two TSs.First, the C―H bond,which will break in this step,is elongated to 0.130 nm from 0.113 nm on the CO/PtAu(111)surface.In comparison, it is elongated to 0.136 nm from 0.112 nm on the PtAu(111) surface.Therefore,it needs more energy for the surface with CO to catalyze the reaction.Besides,there exists a big difference of almost 0.07 nm for the distance between C and Pt on the surface, which lowers the energy of the transition state on the surface with CO.Through the figures of the transition states,we can figure out that this step also goes in different paths on the two different surfaces.On the surface with CO existence,CH2O comes to the surface firstly and then the C―H bond breaks to react with OH. And finally,the O―H bond forms.However,on the surface without CO,the C―H bond breaks down first and at the same time,CHO goes down to the surface to form a C―Pt bond.Therefore,the energy barrier on the PtAu(111)surface is much higher than that on the surface with the existence of CO by 0.47 eV.

Fig.1　Optimized adsorption configurations of the initial states,transition states,and final states involved in the methanol oxidation to formic acid on the PtAu(111)surface

Fig.2　Optimized adsorption configurations of the initial states,transition states,and final states involved in the methanol oxidation to formic acid on the CO/PtAu(111)surface

3.1.4Reaction of CHO and OH(CHO+OH→HCOOH)

For the fourth step,from Table 1,we can find that the reaction on the surface with CO existence releases more energy than that on the clean PtAu(111)surface by 0.29 eV,which means that it is more beneficial for this step on the PtAu(111)surface with the existence of CO.From Figs.1 and 2,it can be found that the distance of OH and CHO is 0.011 nm closer with the existence of CO,which may explain the difference of the enthalpy for this reaction.

From the computed energetic data on PtAu(111)surface with and without CO shown above,we find that the existence of CO is not beneficial for the first step,but it is beneficial for the other three steps.From Figs.1 and 2,we can find that there is a big difference between the adsorption sites on the surface for CO in the four steps.In the first and second steps,CO adsorbs on the fcc site and its C―O bond is about 0.120 nm.While in the third and fourth steps,CO sits on the top site with a bond length of 0.116nm.When CO is adsorbed on the fcc site,it has a larger adsorption energy which makes the PtAu(111)surface poisonous and it is hard to catalyze the reaction.However,when CO adsorbs on the top site,it has the same effect as that on the Au(111)surface,that is,CO is beneficial for OH to oxidize the methanol to HCOOH16. To check out the reason why CO adsorbed on the fcc site will hinder the reaction,we calculated the third step on the PtAu(111) surface with CO on the fcc site.According to the results,we found that the activation barrier is 0.48 eV,which is 0.10 eV higher than that when CO adsorbs on the top site,but it is still lower than that on PtAu(111)surface without CO.This result can prove that CO adsorbed on the fcc site will also promote the reaction,and the promotion effect is smaller than that on the top site.Through the analysis of the adsorption energy of OH,we found that the adsorption energy of OH with the presence of CO at the fcc site is also smaller than that on the clean surface by 0.16 eV,which may be the reason why it is beneficial for the reaction.And,CH2O adsorbs on the top site of one Pt atom,which is not affected by CO.So the reason why CO is not beneficial for the oxidation of methanol to HCOOH on the PtAu(111)surface is that CO adsorbed on the fcc site will prevent the second step,which makes it impossible to react to become CH2O and thus impossible to produce HCOOH.

Table 1　Energetic data(in eV)for the reactions involved in methanol oxidation to formic acid on OH/PtAu(111)and CO/OH/PtAu(111)surfaces

Fig.3　Schematic diagram of the production of HCOOH by methanol oxidation on OH/PtAu(111)and CO/OH/PtAu(111)surfaces

Besides,we calculated the binding energy of CO/OH and coadsorption on PtAu(111)surface(Table 2),and the optimized adsorption configurations of CO and OH co-adsorbed on the PtAu(111)surface with CO at the top site,bridge site and hollow site(fcc site)are shown in Fig.4.Then,it can be found that CO prefers to adsorb on the fcc site,which will obviously reduce the adsorption energy of OH at fcc site.This is disadvantageous for the production of HCOOH by methanol oxidation,especially for the second step.However,when CO adsorbs at the top site,it will not affect the adsorption energy of OH obviously.In fact,it may conclude that the promotion effect may come from the weaken binding energy of OH due to the presence of CO species.This can be explained as following:as the R―H+OH→R+H2O belongs to the association reaction type,and the weaken binding energy of these two species,or at least one of the species,would lead to the high activity for the respective reaction.In our present study,the effect of CO on the binding energy of OH depends on its adsorption sites,either top site,bridge site,or the three-hollow site. From Table 2 we can find that the most significance case occurred when CO adsorbed at the fcc site,followed by bridge site,and the least one is the top site.So structure dependence of CO promotion is available.Moreover,to provide insight into the Pt―CO interaction with OH at the fcc site and CO at the top site,and OH at fcc site and CO at the fcc site,we first plot the projected density of states on d orbitals(d-PDOS)of Pt atoms(before and after CO adsorption on top and fcc site)in PtAu(111)configurations(shown in Fig.5),and then the d band center has been calculated.The dband centers are－1.70,－1.96,and－3.86 eV for the PtAu(111), CO(top)/PtAu(111)andCO(fcc)/PtAu(111)surfaces,respectively.It is consistent well with the order of d band center for the Au(111),CO(top)/Au(111),and CO(fcc)/Au(111)surfaces (shown in Fig.6).In general,the closer the d-band center to the fermi level,the more reactive the metal is.Therefore,the calculated d-band centers suggests that the adsorption energy of OH on PtAu(111)surface is slightly decreased with the presence of CO at the top site,whereas the OH adsorption is relatively large decreased with CO on the fcc site,compared with that on the clean PtAu(111)surface.This is consistent well with the adsorption energy of OH at fcc site(OH moves from initial top site to fcc site) with and without CO at the top and fcc sites on the PtAu(111) surface listed in Table 2.From the Bader charge analysis shown is Table 2,we find there is a repel interaction between CO and OH species owing to the same signs of charges for CO and OH in all investigated co-adsorption cases.

Table 2　DFT results of binding energy of CO,OH and CO/OH co-adsorption on PtAu(111)surface

3.2Effect of CO to oxidize the methanol to formic acid on Pt(111)surface

Fig.4　Optimized adsorption configurations of CO and OH co-adsorbed on the PtAu(111)surface with CO at the top site(a), bridge site(b),and hollow site(fcc site)(c)

In order to investigate the effect of CO,we also calculated methanol oxidation to formic acid on Pt(111)surface.The calculated binding energies of CO/OH and co-adsorption on PtAu (111)surface were calculated first,and the corresponding energetic data are listed in Table 3.From the calculation results,we found that OH is adsorbed stably at the fcc site with the adsorption energy of－2.46 eV and CO also prefers the fcc site with the adsorption energy being－1.76 eV,which agrees well with that on PtAu(111)surface.To investigate the methanol oxidation with production of formic acid better,the co-adsorption of OH and CO at different adsorption sites were investigated.The calculation results show that the adsorption energies of OH at the fcc site all decreased,with CO coadsorbed at the top,bridge,and fcc sites. Whereas the adsorption energy of OH in the initial top adsorption is not affected with the CO coadsorbed at the fcc site.It is valuable to point out that the OH moves to the fcc site from the initial top site in the co-adsorption case of OH(top)+CO(fcc).To provide insight into the Pt―CO interaction,the d-PDOS of Pt atoms (before and after CO adsorption on fcc and top site)was plotted (shown in Fig.7)and the d band center has been calculated.The d-band center is－2.19 eV for Pt(111),and it is－2.33 and－3.64 eV for Pt(111)surface with CO on the fcc and top sites.The calculated d-band centers indicate that the adsorption energy of OH at the fcc site of Pt(111)surface is slightly decreased with CO at fcc site,whereas it is relatively large decreased with CO on the top site,compared with that on the clean Pt(111)surface.This is agreement well with the adsorption energy of OH at the fcc site with and without CO at the fcc and top site on the Pt(111)surface shown in Table 3.As shown in Table 3,the Bader charge analysis indicates there is a repel interaction between CO and OH species because of the same signs of charges for CO and OH in all investigated co-adsorption cases.

Fig.5　d-PDOS of the Pt atom in clean PtAu(111),CO(top)/ PtAu(111),and CO(fcc)/PtAu(111)configurations

After determining the most stable co-adsorption of OH and CO, the relevant reactions involved in methanol oxidation to formic acid on Pt(111)surface were calculated.The optimized configurations are shown in Figs.8 and 9,and the corresponding energetic data are listed in Table 4 and Fig.10.

Fig.6　d-PDOS of theAu atom in cleanAu(111),CO(top)/Au(111), and CO(fcc)/Au(111)configurations

Table 3　DFT results of binding energy of CO,OH and CO/OH co-adsorption on Pt(111)surface

For the first dehydrogenation step(CH3OH+OH→CH3O+H2O),on the surface of CO/Pt(111),it only releases 0.02 eV energy more than that on the Pt(111)surface.And from the activation energies shown in Table 3,we can see that this reaction is nonactivated with and without the presence of adsorbed CO.Therefore,this step is not chosen as an example to talk about the effect of CO.

Fig.7　d-PDOS of the Pt atom in clean Pt(111),CO(fcc)/Pt(111), and CO(top)/Pt(111)configurations

For methoxy dehydrogenation into CH2O and H,we find that this reaction is likely to occur on Pt(111),Au(111),and PtAu(111) surfaces with the barriers of 0.53,0.5616,and 0.99 eV,respectively. On Pt(111)surface,the barrier reduced to 0.15 eV from 0.53 eV and the reaction energy is more exothermic by 0.07 eV in the presence of CO,which indicates that the oxidative dehydrogenation of CH3O into CH2O by OH can be promoted by the preadsorbed CO both thermochemically and kinetically.On Pt(111) and CO/Pt(111)surfaces,there is not any obvious difference between the initial states and final states.We try to analyze the transition states and find the differences that bring the different barriers.At the TS,the breaking C―H bond is elongated to 0.129 and 0.127 nm from 0.110 nm on Pt(111)and CO/Pt(111)surfaces, respectively.The distances between the abstracted hydrogen and the O of OH are 0.130 nm on Pt(111)surface and 0.134 nm on CO/ Pt(111)surface.These both lower the barrier of methoxy dehydrogenation on Pt(111)surface in the presence of CO.

Fig.8　Optimized adsorption configurations of the initial states,transition states,and final states involved in the methanol oxidation on the Pt(111)surface

After CH2O is formed,it can be further oxidized by OH to CHO.As one part of initial state,CH2O is adsorbed on the surface with the C―O bond tilting at 59.4°and 59.6°from the surface normal on Pt(111)and CO/Pt(111)surfaces.We also computed the geometry reported by Greeley and Mavrikakis33that CH2O is adsorbed on Pt(111)surface in a“top-bridge-top”geometry through oxygen and carbon with the C―O bond almost paralleling to the surface,and found that it is only 0.09 eV more stable than it is adsorbed via oxygen atom at the top site of Pt.Herein we describe the CH2O dehydrogenation reaction with CH2O adsorbed at the top site of Pt through O atom as the initial state.Similar to CH3O dehydrogenation,there is also not any obvious difference between the initial states and final states on Pt(111)and CO/Pt (111)surfaces.The different barriers come from the configurationsof TSs.On Pt(111)surface,CHO adsorbs at the top site by carbon atom with the distance between the C atom and the abstracted H atom being 0.129 nm,which is longer than that of 0.114 nm on CO/Pt(111)surface.This needs more energy to catalyze the dehydrogenation of CH2O.Besides,the distance between the abstracted H and the O of OH is 0.173 nm on the CO/Pt(111)surface,which becomes longer than that of 0.138 nm on the Pt(111) surface.This also decreases the barrier of CH2O dehydrogenation on Pt(111)surface with the presence of CO.From the data listed in Table 3,we can see that the pre-adsorbed CO promotes the CH2O oxidation both thermochemically and kinetically:the barrier for CH2O dehydrogenation to CHO is 0.1 eV on CO/Pt(111) surface and it is 0.41 eV on the Pt(111)surface,and the reaction energy changes from－1.70 eV on Pt(111)to－1.76 eV on CO/ Pt(111)surface.

For the fourth step,the barriers are both 0.2 eV on Pt(111)and CO/Pt(111)surfaces,which indicates that the CO has little effect on the reaction of CHO and OH.The energy of this reaction is－0.66 eVon Pt(111)surface and－0.70 eVon CO/Pt(111)surface, which means that it is also beneficial for this step with the existence of CO.

Based on the computed energetic data listed above,we find that the existence of CO has little effect on the first and fourth steps, but it is beneficial for the second and the third steps.In the second and third steps,CO adsorbs on the fcc site,and it has the same effect as that on PtAu(111)and Au(111)surfaces,which is beneficial for OH to oxidize the methanol producing HCOOH. However,when CO adsorbs on the top site,it has not obvious promotion effect on methanol oxidation to formic acid.We calculated the second step on the Pt(111)surface with CO on the top site,and found that the barrier is 0.52 eV,which is 0.37 eV higher than that when CO adsorbs on the fcc site,but it is almost the same as that on the surface without the presence of CO.This result indicates that CO adsorbed on the fcc site will promote the reaction more than it on the top site.On this Pt(111)surface,the ratedetermining step is the second step,which is the same as that on PtAu(111)surface and different from that onAu(111)surface.This indicates that it is hard for the methanol to be oxidized to CH2O, which can deter the reaction to continue to form CO2on Pt(111) surface.

Fig.9　Optimized adsorption configurations of the initial states,transition states,and final states involved in the methanol oxidation on the CO/Pt(111)surface

3.3Comparison about effect of CO to oxidize the methanol to formic acid on Au(111)16,PtAu(111), and Pt(111)surfaces

On Au(111)surface16,the activation energy barriers for the first three steps considered in this work generally decrease in the presence of CO adsorbed on the top site,particularly for the second and third steps.The third step or the conversion of CH2O into CHO species is the rate-controlling step owing to its relatively high energy barriers of 0.72 and 0.61 eV without and with the presence of CO.Therefore,methanol can just be oxidized to CH2O without the product of HCOOH on Au(111)surface at the low potential.On PtAu(111)surface,the energy barriers are decreased for the second and third steps with the CO adsorbed on the top site,which is the same as that on Au(111)surface.On this sur-face,the methoxy dehydrogenation is the rate-determining step, which means that the oxidation of methoxy to CH2O is difficult. On Pt(111)surface,the presence of CO is beneficial for the four steps,especially for the second and third steps,which is similar to that onAu(111)surface and PtAu(111)surface.Whereas the adsorption site of CO is fcc site on Pt(111)surface,which is different from the top adsorption of CO onAu(111)and PtAu(111)surfaces. Moreover,the rate-determining step is the oxidation of methoxy, which is the same as that on PtAu(111)surface and different from that onAu(111)surface.

Based on the analysis above,we find that the adsorbed CO species can promote methanol oxidation to formic acid in aqueous alkaline media.Moreover,the promotion effect of CO is related to the adsorption site on different surfaces.It is the top adsorption on Au(111)and PtAu(111)surfaces,and the fcc adsorption on Pt(111)surface,which is owing to the higher stability of OH with CO adsorbed on the top sites of Au(111)and PtAu(111) surfaces,and on the fcc site of Pt(111)surface.For the PtAu(111) surface,the adsorption energy of OH at the fcc is not obviously decreased with the CO at the top site compared with other coadsorption cases.Whereas on the Pt(111)surface,OH adsorbed at the top site with the CO co-adsorbed at fcc site moves to the fcc site,and the corresponding adsorption energy is not decreased obviously in all the investigated co-adsorption cases.

3.4Physical original of CO promotion

Our present work indicates that CO can promotes the OH oxidative dehydrogenation of methanol on Au(111),PtAu(111),and Pt(111)surfaces,and the reaction mechanism should be determined.The ability of striping hydrogen atoms for OH species is related to its stability and basicity character34.In general,the more negative adsorption energy,the more stable adsorption.For Au (111)and PtAu(111)surfaces,the OH adsorption at the fcc site with the CO co-adsorbed at the top site is most stable,whereas OH adsorbed at the fcc site stably with CO at fcc site is most stable on Pt(111)surface,which promotes the methanol oxidation to formic acid.Moreover,the stronger basicity of OH species,the stronger promotion of X―H(X=CH3O,CH2O,CHO)bond breaking.CO2/ SO2is a typical electrophilic agent,and it is usually used to measure the basicity of surface O/OH species35,36.For methanol oxidation,hydrogen atom is one of the products.So,hydrogen atom is used as the probe atom to measure the OH basicity directly.And the basicity of OH species can be predicted by the reaction energy of X―H+OH→X+H2O,in which a more negative reaction energy indicates stronger basicity of OH species, and a stronger basicity means higher activity for the X―H bond breaking.More negative reaction energy of reaction,the stronger basicity of OH species.The reaction energy of each dehydrogenation step with the CO is generally more exothermic than that without CO,which indicates that the basicity of OH species is stronger with CO than that without CO.

Table 4　Energetic data(in eV)for the reactions involved in methanol oxidation to formic acid on OH/Pt(111)and CO/OH/Pt(111)surfaces

Fig.10　Schematic diagram of the production of HCOOH by methanol oxidation on OH/Pt(111)and CO/OH/Pt(111)surfaces

4　Conclusions

Our present DFT calculations demonstrate that the promotion effect of CO was acted on the reactions involved in methanol oxidation to formic acid on the PtAu(111)and Pt(111)surfaces, which is similar to that on Au(111)surface.The ability that CO could promote the methanol oxidation to formic acid is owing to the higher stability and stronger basicity of OH species induced by the co-adsorption of CO.The stability of OH and the basicity of OH species can be predicted by the adsorption energy and by the reaction energy of X―H+OH→X+H2O,respectively.More negative adsorption energy of OH,higher stability of OH.More negative reaction energy of reaction,the stronger basicity of OH species,and the higher activity for the X―H bond breaking. Considering the effect of CO on the different adsorption sites, such as top,bridge,and hollow,we found that the effect of CO on the methanol oxidation producing formic acid is different.On PtAu(111)surface,the CO adsorbed on the top site can promote the oxidation of methanol,which is the same as that on Au(111) surface,whereas the promotion effect only occurred with the fcc adsorption for CO on Pt(111)surface.

References

(1)Zhang,K.;Yang,W.;Ma,C.;Wang,Y.;Sun,C.W.;Chen,Y. J.;Duchesne,P.;Zhou,J.G.;Wang,J.;Hu,Y.F.;Banis,M.N.; Zhang,P.;Li,F.;Li,J.Q.;Chen,L.Q.NPG Asia Mater.2015, 7,e153.doi:10.1038/am.2014.122

(2)Sun,S.;Zhang,G.;Gauquelin,N.;Chen,N.;Zhou,J.;Yang, S.;Chen,W.;Meng,X.;Geng,D.;Banis,M.N.;Li,R.;Ye,S.; Knights,S.;Botton,G.A.;Sham,T.;Sun,X.Sci.Rep.2013,3, 1775.doi:10.1038/srep01775

(3)Ganesan,R.;Lee,J.S.Angew.Chem.Int.Edit.2005,44,6557. doi:10.1002/anie.200501272

(4)Wasmus,S.;Küver,A.J.Electroanal.Chem.1999,461,14. doi:10.1016/S0022-0728(98)00197-1

(5)Roberts,J.L.,Jr.;Sawyer,D.T.Electrochim.Acta 1965,10, 989.doi:10.1016/0013-4686(65)80011-1

(6)Rodriguez,P.;Kwon,Y.;Koper,M.T.M.Nat.Chem.2012,4, 177.doi:10.1038/nchem.1221

(7)Zope,B.N.;Hibbits,D.D.;Neurock,M.;Davis,R.J.Science 2010,330,74.doi:10.1126/science.1195055

(8)Ketchie,W.C.;Fang,Y.L.;Wong,M.S.;Murayama,M.; Davis,R.J.J.Catal.2007,250,94.doi:10.1016/j. jcat.2007.06.001

(9)Burke,L.D.;Nugent,P.F.Gold Bull.1998,31,39.doi: 10.1007/BF03214760

(10)Kita,H.;Nakajima,H.;Hayashi,K.J.Electroanal.Chem. 1985,190,141.doi:10.1016/0022-0728(85)80083-8

(11)Zhang,T.F.;Liu,Z.P.;Driver,S.M.;Pratt,S.J.;Jenkins,S. J.;King,D.A.Phys.Rev.Lett.2005,95,266102.doi:10.1103/ PhysRevLett.95.266102

(12)Gan,L.Y.;Zhao,Y.J.J.Chem.Phys.2010,133,094703.doi: 10.1063/1.3483235

(13)Liu,Y.H.;Wei,L.;Hu,Y.Z.;Huang,X.Y.;Wang,J.Q.;Li,J. Q.;Hu,X.L.;Zhuang,N.F.J.Alloy.Compd.2016,656,452. doi:10.1016/j.jallcom.2015.10.004

(14)Shubina,T.E.;Hartnig,C.;Koper,M.T.M.Phys.Chem. Chem.Phys.2004,6,4215.doi:10.1039/B407669A

(15)Wang,L.;He,C.Z.;Zhang,W.H.;Li,Z.Y.;Yang,J.L. J.Phys.Chem.C 2014,118,17511.doi:10.1021/jp501620h

(16)Lv,C.Q.;Liu,J.H.;Wang,H.;Wang,G.C.Catal.Commun. 2015,60,60.doi:10.1016/j.catcom.2014.11.013

(17)Yuan,D.W.;Gong,X.G.;Wu,R.Q.J.Chem.Phys.2008, 128,064706.doi:10.1063/1.2835545

(18)Zhong,W.H.;Liu,Y.X.;Zhang,D.J.J.Phys.Chem.C 2012, 116,2994.doi:10.1021/jp210304z

(19)Montero,M.A.;Gennero de Chialvo,M.R.;Chialvo,A.C. Int.J.Hydrog.Energy 2011,36,3811.doi:10.1016/j. ijhydene.2010.12.115

(20)Ren,H.;Humbert,M.P.;Menning,C.A.;Chen,J.G.;Shu,Y. Y.;Singh,U.G.;Cheng,W.C.Appl.Catal.A 2010,375,303. doi:10.1016/j.apcata.2010.01.018

(21)Qiu,C.C.;Guo,Y.G.;Zhang,J.T.;Ma,H.Y.;Cai,Y.Q. Mater.Chem.Phys.2011,127,484.doi:10.1016/j. matchemphys.2011.02.041

(22)Xu,J.B.;Zhao,T.S.;Yang,W.W.;Shen,S.Y.Int.J.Hydrog. Energy 2010,35,8699.doi:10.1016/j.ijhydene.2010.05.008

(23)Yang,L.;Chen,J.H.;Zhong,X.X.;Cui,K.Z.;Xu,Y.;Kuang, Y.F.Colloids Surf.A 2007,295,21.doi:10.1016/j. colsurfa.2006.08.023

(24)Kresse,G.;Hafner,J.Phys.Rev.B 1994,49,14251.doi: 10.1103/PhysRevB.49.14251

(25)Kresse,G.;Furthmüller,J.Comp.Mater.Sci.1996,6,15.doi: 10.1016/0927-0256(96)00008-0

(26)Perdew,J.P.;Chevary,J.A.;Vosko,S.H.;Jackson,K.A.; Pederson,M.R.;Singh,D.J.;Fiolhais,C.Phys.Rev.B 1992, 46,6671.doi:10.1103/PhysRevB.46.6671

(27)Kresse,G.;Joubert,D.Phys.Rev.B 1999,59,1758.doi: 10.1103/PhysRevB.59.1758

(28)Bl?hl,P.E.Phys.Rev.B 1994,50,17953.doi:10.1103/ PhysRevB.50.17953

(29)Monkhorst,H.J.;Pack,J.D.Phys.Rev.B 1976,13,5188.doi: 10.1103/PhysRevB.13.5188

(30)Henkelman,G.;Uberuaga,B.P.;Jónsson,H.J.Chem.Phys. 2000,113,9901.doi:10.1063/1.1329672

(31)Kittel,C.Introduction to Solid State Physics,8th ed.;Wiley: New York,2004

(32)Hammer,B.Surf.Sci.2000,459,323.doi:10.1016/S0039-6028(00)00467-2

(33)Greeley,J.;Mavrikakis,M.J.Am.Chem.Soc.2004,126, 3910.doi:10.1021/ja037700z

(34)Pacchioni,G.;Ricart,J.M.;Illas,F.J.Am.Chem.Soc.1994, 116,10152.doi:10.1021/ja00101a038

(35)Torres,D.;Lopez,N.;Illas,F.;Lambert,R.M.Angew.Chem. Int.Edit.2007,46,2055.doi:10.1002/ange.200603803

(36)Zhao,S.;Ma,X.D.;Pang,Q.;Sun,H.W.;Wang,G.C.Phys. Chem.Chem.Phys.2014,16,5553.doi:10.1039/C3CP55048F

First-Principles Study of Effect of CO to Oxidize Methanol to Formic Acid in Alkaline Media on PtAu(111)and Pt(111)Surfaces

LIU Jian-Hong1Lü Cun-Qin1,2JIN Chun1,*WANG Gui-Chang3,*
(1College of Chemistry and Enviromental Engineering,Shanxi Datong University,Datong 037009,Shanxi Province,P.R.China;2Key LaboratoryofAdvancedEnergyMaterialsChemistry(Ministry ofEducation),Nankai University,Tianjin300071,P.R.China;3Department of Chemistry and the Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry, Nankai University,Tianjin 300071,P.R.China)

Density functional theory calculations have been performed to investigate methanol oxidation to formic acid on PtAu(111)and Pt(111)surfaces with and without CO in alkaline media.The calculated results show that the pre-adsorbed CO species promotes almost every step involved in the oxidation of methanol on PtAu(111)and Pt(111)surfaces,which is similar to that observed on aAu(111)surface.These findings may be attributed to the relatively high stability and strong basicity of the OH species induced by the adsorption of CO, and the enhanced ability to strip the H atoms.

Methanol oxidation;M(111)(M=PtAu,Pt);Alkaline medium;CO promotion effect; Density functional theory calculation

1　Introduction

Given that methanol exhibits their distinctive advantages of high energy density and the ready availability,inexpensiveness, and the safe storage and transportation potential as a liquid fuel, direct methanol fuel cells have attracted much attention for portable power applications1－4.As an intermediate species producedin methanol oxidation,carbon monoxide has a strong tendency to adsorb on the catalyst surface to hinder the sites needed by reactants in electrocatalysis,which makes the adsorbed CO act evidently as a poison or poisoning intermediate in methanol oxidation.Gold is known to be an excellent electrocatalyst for carbon monoxide oxidation5,and alkaline conditions are necessary for efficient aqueous-phase electrocatalysis6－10.Rodriguez et al.6investigated the oxidation of several alcohols,including methanol, on cleanAu(111)and CO-modifiedAu(111)electrodes in aqueous alkaline media,and found that the CO can promote alcohol oxidation in a solution.This can be attributed to the strong bonding of OH to the surface induced by adsorbed CO.Moreover,the adsorbed OH may act as the oxidant for CO oxidation,leading to a significant rate enhancement.Besides the co-adsorption of CO and OH,the effect of other co-adsorption systems on the reaction has been reported.Zhang et al.11observed an interesting stabilizing co-adsorption system of CO and electronegative NO2on Au(111) surface in ultra-high vacuum(UHV);they proposed that such stabilizing system may be responsible for catalytic promotion, particularly for the association reaction in IB metals.Gan and Zhao12investigated the interaction of the co-adsorption of CO and different adatoms(such as Na,S,O,and Cl)on Cu(111),Ag(111), Au(111),and Pd(111)surfaces,and found that the enhancement of CO adsorption by S originates from S-induced positive polarization of Au and Ag surfaces.The recent experimental results show that the CO has an anti-poisoning effect on graphene supported PtPd alloy for the ethanol oxidation reaction13.

September 28,2015;Revised:January 18,2016;Published on Web:January 19,2016.*Corresponding authors.WANG Gui-Chang,Email:wangguichang@nankai.edu.cn;Tel:+86-22-23503824. JIN Chun,Email:jinchun0828@126.com;Tel:+86-352-7563093. The project was supported by the National Natural Science Foundation of China(21503122,21346002),Natural Science Foundation of Shanxi for Youths,China(2014021016-2),Scientific and Technological Programs in Shanxi Province,China(2015031017),and Foundation of Key Laboratory ofAdvanced Energy Materials Chemistry(Ministry of Education),China.

O641；O647

10.3866/PKU.WHXB201601191

國(guó)家自然科學(xué)基金(21503122,21346002),山西省青年科技研究基金(2014021016-2),山西省科技攻關(guān)項(xiàng)目(2015031017)及先進(jìn)能源材料化學(xué)教育部重點(diǎn)實(shí)驗(yàn)室開放基金資助