Study On H₂Adsorption And Dissociation On Pd_n（N=4,6,13,19,38）Clusters By Density Functional Theory

Clusters are the agglomerates of a few to a few thousand atoms with relative stability and with a radius smaller than50nm. Clusters are the borderland between the microscopic single atom and the macroscopic solid state. The palladium-ydrogen system has attracted intensive studies in physical chemistry as a model system in view of its high storage capacity, its high sensitivity and selectivity to H2gas, and its ability to easily release hydrogen at room temperature. In particular, palladium has been widely utilized in many chemical processes such as hydrogenation, oxidation and reduction process. Hydrogen dissociative chemisorption on Pd transition metal catalysts is of great industrial importance in various hydrogenation or dehydrogenation processes. In these reactions, hydrogen molecule is dissociated with active hydrogen atoms adsorbed on the catalyst surfaces. Understanding the elementary processes of the chemical reactions is significant to design novel high efficiency catalysts and provide important theoretical knowledge to solve practical problems of industrial catalytic.In this work, we use DFT calculations and transition state theory, and we optimize the different sizes’of Pdn(n=4,6,13,19,38) by GGA/PW91function in DMol3software. We investigate systematically a series of nature analysis on the base of optimized structures, including hydrogen molecular adsorption geometry and stability, mechanisms of dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38). We also examine electronic properties of palladium-hydrogen system by analyzing Hirshfeld charges, HOMO-LUMO gap and the density of states. Through the detailed system’s theoretical calculation, we discuss hydrogen dissociative chemisorption on palladium clusters and the electronic properties with the cluster sizes increase.In Chapter1, we summarized basic theory of the cluster and surface adsorption. In Chapter2, we introduced the theoretical basis of modern quantum chemistry. In Chapter3, we studied the mechanisms of hydrogen adsorption, dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38) and electronic and geometric properties of palladium-ydrogen system.The main conclusions of this work include:1. The average binding energies of the Pdn(n=4,6,13,19,38) clusters significantly increase from1.660to2.785eV with cluster size increased. Pd-Pd bond distances gradually change from2.661to2.756A. The stable adsorption sites for H2molecule are top sites of Pdn(n=4,6,13,19,38), with the respective adsorption energy of0.568,0.670,0.629,0.453and0.456eV. The stable adsorption sites of Pd19and Pd38are at top sites of five-coordinated Pd atoms.2. Hydrogen atom adsorbed on the Pd4edge site is more stable while the adsorption on face site is more stable for Pd6, Pd13, Pd19and Pd38. From Pd4to Pd38. the rate-determining step of dissociating H2molecule is overcoming the energy barrier of0.448,0.401,0.314,0.304and0.782eV, respectively. The results show that Pd19has much lower chemisorption energy and the lowest energy barrier. Adsorptive H atom shifts on the Pdn(n=4,6,13,19,38) cluster are testified to be quite flexible.3. In the charge transfer process, the Pd cluster behaves as a donor and H2behaves as an acceptor. There is charge being transferred from the Pd atoms to the antibonding orbital of H2, associated with the breaking of the H-H bond.