The importance of hydrogen's potential-energy surface and the strength of the forming R-H bond in surface hydrogenation reactions

An understanding of surface hydrogenation reactivity is a prevailing issue in chemistry and vital to the rational design of future catalysts. In this density-functional theory study, we address hydrogenation reactivity by examining the reaction pathways for N+H-->NH and NH+H-->NH(2) over the close-packed surfaces of the 4d transition metals from Zr-Pd. It is found that the minimum-energy reaction pathway is dictated by the ease with which H can relocate between hollow-site and top-site adsorption geometries. A transition state where H is close to a top site reduces the instability associated with bond sharing of metal atoms by H and N (NH) (bonding competition). However, if the energy difference between hollow-site and top-site adsorption energies (DeltaE(H)) is large this type of transition state is unfavorable. Thus we have determined that hydrogenation reactivity is primarily controlled by the potential-energy surface of H on the metal, which is approximated by DeltaE(H), and that the strength of N (NH) chemisorption energy is of less importance. DeltaE(H) has also enabled us to make predictions regarding the structure sensitivity of these reactions. Furthermore, we have found that the degree of bonding competition at the transition state is responsible for the trend in reaction barriers (E(a)) across the transition series. When this effect is quantified a very good linear correlation is found with E(a). In addition, we find that when considering a particular type of reaction pathway, a good linear correlation is found between the destabilizing effects of bonding competition at the transition state and the strength of the forming N-H (HN-H) bond.