Insight into association reactions on metal surfaces: Density-functional theory studies of hydrogenation reactions on Rh(111)

[[abstract]]Hydrogenation reaction, as one of the simplest association reactions on surfaces, is of great importance both scientifically and technologically. They are essential steps in many industrial processes in heterogeneous catalysis, such as ammonia synthesis (N 2 3H 2 →2NH 3). Many issues in hydrogenation reactions remain largely elusive. In this work, the NH x (x0,1,2) hydrogenation reactions NH→NH, NHH→NH 2 and NH 2 H→NH 3) on Rh111 are used as a model system to study the hydrogenation reactions on metal surfaces in general using density-functional theory. In addition, C and O hydrogenation CH→CH and OH→OH and several oxygenation reactions, i.e., CO, NO, OO reactions, are also calculated in order to provide a further understanding of the barrier of association reactions. The reaction pathways and the barriers of all these reactions are determined and reported. For the C, N, NH, and O hydrogenation reactions, it is found that there is a linear relationship between the barrier and the valency of R RC, N, NH, and O. Detailed analyses are carried out to rationalize the barriers of the reactions, which shows that: i The interaction energy between two reactants in the transition state plays an important role in determining the trend in the barriers; ii there are two major components in the interaction energy: The bonding competition and the direct Pauli repulsion; and iii the Pauli repulsion effect is responsible for the linear valency-barrier trend in the C, N, NH, and O hydrogenation reactions. For the NH 2 H reaction, which is different from other hydrogenation reactions studied, the energy cost of the NH 2 activation from the IS to the TS is the main part of the barrier. The potential energy surface of the NH 2 on metal surfaces is thus crucial to the barrier of NH 2 H reaction. Three important factors that can affect the barrier of association reactions are generalized: i The bonding competition effect; ii the local charge densities of the reactants along the reaction direction; and iii the potential energy surface of the reactants on the surface. The lowest energy pathway for a surface association reaction should correspond to the one with the best compromise of these three factors.[[notice]]補正完畢[[journaltype]]國內[[incitationindex]]SCI[[booktype]]紙