Density Functional Theory Study On AuPd Cluster Structures And Small Molecules Adsorption And Dissociation Mechanism

Bimetallic and monometallic nanoclusters as catalysts have attracted great interest in various fields of scientific research and industrial applications. They possess extremely large surface-to-volume ratios, small size effect and tunable structure and catalytic properties, etc. In recent years, bimetallic AuPd nanoclusters present more broad application prospect, which are often superior to catalytic performance and stability of monometallic nanoclusters. Experimental researchers currently devote their efforts to synthesis of AuPd nanoclusters with controllable size, structure and properties. Theoretical study on AuPd nanocluster structures will contribute to synthesis of novel AuPd nanoclusters. Pd nanocatalyst has been widely used in hydrogen storage, hydrogenation and dehydrogenation, and automobile tail gas treatment, etc. At the microscopic level researchers have explored the relationship between Pd nanocluster structures and catalytic activity for small molecule dissociation reaction. This is of great significance for searching efficient Pd nanocluster catalysts and understanding complex chemical reaction on their surfaces.In this dissertation, we investigated these effects of size, composition (Au/Pd) and distribution of active centers Pd on structure stability, electronic properties and H2 adsorption activity for bimetallic AuPd clusters using density functional theory (DFT). We explored the adsorption and dissociation mechanism of H2 and NO small molecules on different palladium cluster structures. We studied a resistance to CO poisoning for different chemical composition sites in the AuPd(111) surfaces with Au monomers, Au dimers and Au trimers. The main conclusions are as follows:(1) The stability of AumPdn clusters oscillates with increasing size. The order is Au24Pd14?Au52Pd27?Au36Pd19 (m/n?2:1) and Au32Pd6> Au66Pd13> Au46Pd9 (m/n?5:1). With increasing size, the binding energy per atom increases, the adiabatic ionization potential decreases and the adiabatic electron affinity oscillates for the AumPdn clusters with the same composition. This trend agrees with that of pure Pdv clusters. (2) The stability order of Pd substituents in Au79-nPdn (n=1-55) clusters is face> mid-edge> corner> edge. The H2 adsorption activity is corner-edge> mid-edge> face. For the Au36Pd43(3) cluster with Pd trimers, direct dissociation of H2 at Pd-atop sites occurs. The locations and numbers of Pd substituents significantly affect the stability and H2 adsorption activity of Au79-nPdn clusters. (3) Among the Pdn(n=4,6,13,19,55) clusters, direct dissociation of H2 at Pd-atop sites in icosahedron Pdn occurs. The H2 dissociation at Pd-atop sites in octahedron Pd19 and truncated octahedron Pd55 have lower barriers and are thermodynamically favorable. Thus the Pdn(n=13,19,55) structures have higher catalytic activity for H2 dissociation. Hirshfeld charge transfers from Pdn cluster to H2 in adsorption processes. The amount of the charge transfer increases up H2 dissociation. (4) The NO adsorption at atop, bridge and hollow sites in the Pdn(n=8,13,19,25) clusters were calculated. The results showed that the stretching frequencies of the adsorbed NO have significant redshifts. The redshifts scales are 85-148 cm-1 (top sites),265-360 cm-1 (bridge sites) and 385-416 cm-1 (hollow sites), In adsorption processes, the adsorption of NO at bridge and hollow sites is easier in the icosahedron Pd13 and icosahedron-based Pd25. The NO adsorption on octahedron Pd19 has minimum stability and is favorable at its bridge sites. On the stable ortho-bicapped octahedron Pd8a, NO competitive adsorption may be observed. In NO dissociation reaction, the dissociation barriers for NO on hollow site of the ortho-bicapped octahedron Pd8a and bridge site of the octahedron Pd19 are lower than those for NO on hollow sites of the icosahedron Pd13 and icosahedron-based Pd25. The adsorption and dissociation properties of NO on the Pdn(n=8,13,19,25) clusters exhibit significant structure sensitivity. (5) The adsorption and co-adsorption of H atom and CO on active sites with different chemical composition for AuPd(111) surfaces were studied. The results showed that the AuPd(111) surface with Au monomers have a significant selectivity for H atom and CO adsorption. The hollow sites with Au dimers significantly reduce CO adsorption energy with about 1.10 eV. On hollow sites with Au trimers, CO direct dissociation occurs. It may be due to Au trimers with a large number of negative charges. On CO-preadsorbed AuPd(111) surface with Au monomers, the H atom co-adsorption changes CO adsorption sites and further lowers CO stability, helping to CO desorption. The AuPd(111) surfaces exhibit a strong ability of CO poisoning resistance.