Structural Evolution Of Palladium Clusters And Heterogeneous Catalysis Of Hydrogen Molecules: A Density Functional Theory Study

A number of intense studies have been performed on clusters of precious and early transition metals at the nano/subnanoscales in recent years, one reason is their potential applications as building blocks for functional nanostructure materials, electronic devices, and nanocatalysts, the other is clusters not only can serve as a tractable model for the nanoparticles but can also provide an excellent platform for systematic understanding of structure/property evolution as they grow from a small size to a large size and ultimately into the bulk. The high ratio of surface area to volume is of paramount importance for reactions catalyzed by d-metal particles. Palladium clusters are promising catalysts in various applications. Ultra-dispersed palladium clusters supported on alumina, for example, were found to be more active than Pd(111) single crystals in CO oxidation by oxygen. The catalytic reduction of nitrogen monoxide with propane on zeolite-supported palladium clusters, as well as the extremely active NO reduction by CO on highly dispersed palladium clusters supported onγ-alumina, are the key reactions in exhaust gas treatment. Furthermore, ultra-dispersed supported palladium clusters of up to 2nm (150 atoms) in size are highly active catalysts in hydrogenation processes, having a much higher selectivity with respect to conversion of triple to double bonds than that of bulk palladium. In studies of this commercially important reaction, the hydrogen adsorPdion strength was shown not to depend on the Pd particle size, whereas the amounts of sorbed hydrogen and hydrogen-to-metal interaction strongly depend on the surface structure and the cluster size. In this thesis, structures of palladium clusters up to 15 were calculated using density functional theory under the generalized gradients approximation. The lowest structures of palladium clusters up to 15 were obtained according to lowest energy principle, and hydrogen molecules adsorPdion on Pdn(n=2-11, 13-b and 13-c) clusters were also calculated. The results indicate the lowest energy structures of palladium clusters up to 15 are close-packed, and the the growth pathway assumes the close-packed way. The energetic and topological similarity of the close-packed, icosahedral and fcc-like structures observed in small palladium clusters suggests that structural transitions from the close-packed configurations to icosahedral and, ultimately, to the bulk-like fcc configurations could occur at a relatively small cluster size. At around n=13, the average binding energies of both icosahedral and fcc-like structures are comparable to that of the close packed "irregular" clusters, indicating that the transition from the "irregular" structure to the icosahedral and fcc-like structures could occur at a small cluster size. In the approximate range of 20≤n≤50, the icosahedral and fcc-like, structures coexist but after that the icosahedral structures appear to dominate the cluster growth. The average binding energy and electron affinity increase as clusters size increases, ionization potential decreases as well. So palladium clusters are easy to lose electrons and oxidate, but difficult to receive and deoxidate. The magnetic moent of palladium clusters are surging up and down as increase of clusters size, and all are no bigger than 3_μB.The dissociation chemisorPdion of H₂ on palladium cluters are assumed for 3 steps: firstly, H₂ phsisorbed on palladium clusters, then it is dissociated on a palladium atom of the cluster, and finally one of the dissociated hydrogen atom moves from one atom to another atom. The barriers of dissociation and movement processes are quite small, so it is easy for H₂ to dissociate and dissociated hydrogen atoms to move around. When more H₂ are dissociated, chemsorPdion energy of H₂ decreases, and the ability to chemsorb H₂ becomes weak; chemdesorPdion energy of hydrogen molecules decreases, and dissociated hydrogen atoms are easier to desorb. Upon palladium dusters are saturated by H₂, chemdesorPdion energy around 0.85eV, in agreement with that on a single-crystal palladium surface. If the energy acquired by dissociated H atom is smaller than this value, it can not desorb from palladium clusters; if the energy acquired by dissociated H atom is a little bigger than this value, a part of dissociated H atoms will desorb from palladium clusters, the rest still chemsorb on palladium clusters. Then it results in a number of opening points for successive H₂ to dissociate. This process cycles, and then catalytic reaction goes on. It offered a good explaination to the ydrogen "spillover" phenomenon and also lay a foundation for the better choose of the temperature of catalytic reaction and offered a standard for the design of new and effective catalysts in future. Upon palladium clusters are saturated by H₂, charge transfer is from palladium clusters to hydrogen atoms and Pd/H ratio is 1: 2.