-Large-scale Quantum Chemical Molecular Dynamics Study on CO Oxidation Reaction on Precious Metal Surface (cid:3)

To study atomistic behavior of CO oxidation reaction on Pt(332) surface, we used ultra accelerated quantum chemical molecular dynamics method. First, we estimated the CO oxidation reactivity on step and terrace site of Pt(332) surface. Then, we conﬁrmed that the eﬀect of PtO 2 stripe on step site on reactivity of the CO oxidation. It is shown that ultra accelerated quantum chemical molecular dynamics method is available for the analysis of reaction dynamics on metal surface. [DOI: 10.1380/ejssnt.2010.272]


I. INTRODUCTION
The oxidation reaction of carbon monoxide (CO) is one of the key processes in automotive exhaust gas treatment [1,2].This reaction is promoted by the precious metal catalyst.Especially, extensive studies have focused on the reaction on Pt(332) surface.It is because the step edges of Pt(332) lead to high activity [3].In recent years, computational approaches have become powerful research tool for understanding that kind of chemical reaction in atomic scale.Although there is a great interest, first principle molecular dynamics method demands high computational cost for treatment a metal surface structure.In this regard, we recently developed ultra accelerated molecular dynamics method (UAQCMD).This tool is over 10 million times faster than conventional first-principles molecular dynamics method.Hence, the chemical reaction dynamics on a large catalyst model can be simulated.In the present study, we applied above mentioned UAQCMD method to the oxidation reaction of CO over Pt(332) surface using a large-scale model.The dynamics behavior of CO and oxygen is also analyzed and discussed detail.The aim of this study is to verify the validity of this new approach for the analysis of metal surface reaction.

II. METHOD
We used DMol3 program to compare the adsorption energy by density functional theory (DFT) and UAQCMD method.UAQCMD simulator consists of two parts.The first part is our tight-binding quantum chemistry (TBQC) simulator [4].In this simulator, an electronic structure calculation is performed by solving the Schrödinger equation (HC = εSC; H, C, ε, and S refer to the Hamiltonian matrix, eigenvector, eigenvalue, and overlap integral matrix, respectively) with the diagonalization condition (C T SC = I; I refers to the unit matrix).In order to determine the off-diagonal elements of H, H rs , the corrected distance-dependent Wolfberg-Helmholz formula [5] was used.In the TBQC method, various parameters, such as those for Slater-type basis sets and orthogonal Hamiltonian term, are used for accelerating computations.To determine the accurate parameters, we determined them on the basis of first-principles calculations.The second part is our classical molecular dynamics simulator [6].At first steps of MD, UAQCMD is carried out to renew the Morse type function.The dynamics calculations of CO oxidation on Pt(332) were performed at 300K.Details of above UAQCMD method is described elsewhere [7].

III. RESULTS AND DISCUSSIONS
In UAQCMD method, various parameters, such as those for Slater-type basis sets and orthogonal Hamiltonian term are used for accelerating computations.To determine the reliable parameters, we determined them on the basis of first-principles density functional theory tively.We could see a good agreement between results by UAQCMD and DFT calculations.We verified the validity of the parameters for CO oxidation dynamics calculation.
We then performed the analysis of CO oxidation dynamics on Pt(332) at 300 K.
Figure 1 shows the initial structure of CO oxidation dynamics on the Pt(332).Pt(332) is composed of (111) terraces, six atomic rows wide, and a (111) type step.It is known that the CO oxidation reactivity of step is higher than that of terrace.In this study, we attempted to theoretically evaluate the reactivity.Then, we verify the validity of UAQCMD method.
First we studied CO oxidation reactivity dependent on the site difference on the surface.The oxygen atoms are separately shown as the oxygen atoms of CO (O A ) and the oxygen atoms on the Pt surface (O B ). Earlier studies reported that the rate determining step of CO oxidation on Pt surface is the bond formation of C-O B [8].To compare the reactivity of step and terrace, we analyzed the bond energy and bond distance between O B -CO A . Figure 2(a) shows the time profile of the bond energy between O B -CO A .We observed the significant increase of the O B -CO A bond energy on step rather than that on terrace.This indicates that the rate of O B -CO A bond formation on step is comparatively higher than that on terrace.Furthermore, we confirmed that the bond distance between O B -CO A on step is shorter than that on terrace as shown in Fig. 2(b).This suggests that O B -CO A bond formation is accelerated on the step.Li et al. obtained the activation energy of CO oxidation reaction.The activation energy on step and terrace were 0.60 eV (13.84 kcal/mol) and 0.71 eV (16.37 kcal/mol), respectively [9].This result is corresponding to the result of this study.DFT result also shows that the reactivity of CO oxidation on step is higher than that on terrace.
Then, we studied the effect of PtO 2 stripe on step arise from oxidation condition.It is known that the Pt atoms of step site of Pt(332) become PtO 2 state in oxidation condition [9]. Figure 1(b) shows the PtO 2 stripe on Pt(332) in oxidation condition.The oxygen atoms of CO and PtO 2 stripe were shown as O A and O B , respectively.To study the CO oxidation reactivity of PtO 2 stripe, we com-puted the time profile of the distance between O B -CO A , as shown in Fig. 3.Then, we compared CO oxidation reactivity of PtO 2 stripe to that of a single oxygen atom.We supposed that the closer bond, the higher reactivity.During 2 ps, the shortest C-O B distance of PtO 2 stripe and the single oxygen atom were about 2.5 Å and 2.6 Å, respectively.This indicates that the O B -CO A bond of PtO 2 stripe is easier to form compared with that of single oxygen atom.To study the nature of this tendency, we evaluated the bond energy of Pt-O B with single oxygen atom and PtO 2 .The bond energy of Pt-O B with single oxygen atom and PtO 2 stripe are about −12 and −25 kcal/mol, respectively.This suggests that the weaker Pt-O B bond around the PtO 2 stripe promotes the O B atoms to an active state.

IV. CONCLUSIONS
In this work, we evaluated the CO oxidation reaction reactivity on Pt(332) by using ultra accelerated quantum chemical molecular dynamics method.The adsorption energy of CO and O on Pt(332) were well compared with the results by DFT.We then observed that the CO oxidation is promoted on step O compared with that on terrace O.It is also shown that the CO oxidation is accelerated by oxygen atoms of PtO 2 stripe.We successfully simulated the previous knowledge about the CO oxidation reaction under temperature condition.The large scale simulation in this study also enables to consider the reasonable cutoff distance and the variable concentration of adsorbate.

FIG. 1 :FIG. 2 :FIG. 3 :
FIG. 1: The initial configuration of CO oxidation dynamics calculation on Pt(332) for study of (a) reactivity of step and terrace sites, (b) effect of the PtO2 stripe.