The reaction pathways for CO oxidation over gold pentamer as a model catalyst were studied by UB3LYP/Lan2dz-ECP/ccpvtz-ECP)for Au,6-311+G* for C and O.The reaction mechanisms for Au5CO(O2)initial structures(R1),in which CO or O2 was adsorbed on the trapezia Au5 cluster top apex site,while the other was on the nearest top site,were analyzed.The reaction proceeds through a typical L-H mechanism in R1 as initial structures.First,the reaction via a prereactive complex IM(CO-OO)where the O(9)-C(6)bond was shorten to 1.314(A)from 2.164(A)in R1 with the relative free energy(RFE)of 12.65 kcal·mol-1.IM resulted in the TS geometry(TS2)with a 21.13 kcalmol-1 higher energy relative to that of reactant R1.Then,TS2 lead to forming IMF(O-OCO)with RFE-47.46 kcal·mol-1.Whereafter,IMF dissociates into the first CO2 and Au5O radical(R2)with RFE-54.95 kcal·mol-1.RRKM calculations show that the rate is increasing with temperature increase.Two paths of CO attack O(refer to side 1)and Au(refer to side 2)in Au5O radical also were analyzed.The results show that the two ways are also follow L-H mechanism,the side 1 has higher CO oxidation rate than the side 2,these rate are also increasing with temperature increase.However,the possibilities of CO oxidize over Au5 and Au5O radical via an E-R mechanism without any success.