Catalytic Performance Of Au-Ag Nanoalloy On WGSR

With the development of science and technology,the understanding of physical world went from macro to micro,and into the level of molecules and atoms,nano-scale clusters have attracted a wide spread attention and as well applied in many reactions because of their unique catalytic properties.As to the water gas shift reaction?WGSR?,existing researches were more about the metal surface and pure clusters,this thesis proceeds from the unique nature of nano-gold clusters to study the catalytic capability of smallest gold cluster with a positive charge,i.e.Au₈⁺,and its corresponding nanoalloy Au_nAg_8-n⁺?n=7-4?for the WGSR in the hope to improve the adsorption intensity of one of the reactants H₂O.The global minimum energy structure of nanoalloy clusters with different composition is searched by genetic algorithm which based on the first principles theory.After reoptimization of these GM structures in Gaussain,their respective adsorption abilities of the reactants of WGSR,namely CO and H₂O,are calculated.The influence of composition and structure on the adsorption capability obeys the following pattern: the Au in cluster has stronger adsorption ability to CO than H₂O.The contrary is the case of Ag in the same cluster.But not all Ag has the higher adsorption energy to H₂O than Au,this conclusion is consistent with the previous research results.In addition,with reducing the ratio,Au: Ag,the cluster adsorption ability to CO is declined,and the adsorption ability of cluster to H₂O has no obvious change.Furthermore,in the same cluster,the adsorption capability to CO and H₂O in different positions is relevant with its charge distribution and orbital shape.In general,the smaller charges and better shape matching,the higher adsorption energy.When adding electron-withdrawing group like cyano into the structure of Au₆Ag₂⁺,the adsorption ability of CO and H₂O are both decreased.Instead when mixing an atom of N into the cluster,which has larger electron negativity than Au and Ag,the adsorption energy is increased obviously.But it can be also concluded that the corporation of Ag cannot improve the adsorption strength of H₂O molecular on Ag to implement the co-adsorption of CO and H₂O.Last,the reaction mechanism of WGSR catalyzed by Au₈⁺ and Au₆Ag₂N⁺ is calculated.Each step for the both structures coincide with those reported in literature.Computational results show that the cluster of Au₈⁺ has higher dissociation energy of H₂O,while for cluster Au₆Ag₂N⁺,the activation energy is decreased by 10 kcal/mol.