Monte Carlo Simulation Of Hydrogen Adsorption Kinetics On Ni And Pd Surfaces

The adsorption of gas on transitional metal surfaces is an elementary step in the process of heterogeneous catalysis. Nowadays it has been one of the most important subjects in the field of surface science and numerous studies have been carried out in experiments as well as in theories. Theoretical calculation has been playing an essential role in the aspects of helping us to understand the adsorption mechanisms and explain the experimental phenomenon. Adsorption kinetics can be used to study the effect on adsorption caused by chemisorption, desorption rate and other factors.The research of adsorption kinetics facilitates us to find out the factors which can affect the rate of adsorption and desorption, educe the equation of reaction rate. Also it is helpful for us to have a further understanding of adsorption isotherm formula, based on the elementary function of adsorption rate and desorption rate. Furthermore, with the characters of the molecules and some fundamental constants, it can help us to well understand the essence of the mechanism of adsorption kinetics by applying the transition state theory to the research of the adsorption rate and desorption rate. According to the thermodynamic properties which given by experiments and theories, the Grand canonical Monte Carlo method is applied to simulate the adsorption kinetics of hydrogen molecule on the surface of nickel and palladium in this thesis. For the different properties of adsorption when hydrogen molecules are adsorbed on the surfaces of Pd, Ni (111) and (100), different models are presented to simulate the process of adsorption. Three processes are considered when hydrogen molecules adsorb on the surfaces of metal: dissociative adsorption of hydrogen molecules, the diffusion of hydrogen atoms, the desorption of hydrogen molecules. Surface structure of hydrogen/metal systems, equilibrium adsorption isotherms, sticking coefficient and the rates of adsorption and desorption are calculated with our models.The present calculations suggest that hydrogen atoms are mainly adsorbed at Fcc hollow sites on the surfaces of Ni (111) and Pd (111). However, on the surfaces of (100), hydrogen atoms are mainly adsorbed at 4-fold hollow site. Hydrogen atoms have an ordered structure when they are adsorbed on surface of Pd (111); while on other three kinds of surface, hydrogen atoms form a disordered structure. The maximum coverage of hydrogen given by our model are H/Ni (111) and H/Ni (111) 0.93ML, H/Pd (111) 0.92ML, H/Pd (111) 1.4ML, respectively. All these results agree well with experimental results, which prove that the models we propose are reasonable. The results of equilibrium adsorption isotherms suggest that the higher the temperature, the lower the coverage will be; and the higher the pressure, the higher the coverage will be. The sticking coefficient will reduce when coverage increases, reducing the adsorption rate. Statistics show that H-Metal system reaches equilibrium when the adsorption rate equals to the desorption rate.