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

Abstract

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.

Insights into the Mechanism of Ammonia Decomposition on Molybdenum Nitrides Based on DFT Studies

“…Among these transition states, the major difference is the H location. These results are similar to the previously determined transition states of NH x dissociation on Ru(0001) and Rh(111). − Following the first pathway, the dissociating N–H bond length is stretched to 1.58 Å (Figure a) with a Mo III –N distance of 2.12 Å. The resulting NH 2 is stabilized on Mo III , whereas H diffuses to the hollow site in the final state.…”

Ab initiodensity-functional theory study ofNHxdehydrogenation and reverse reactions on the Rh(111) surface

“…We conclude that this point is not a transition state as is described in the harmonic transition state theory. The existence of the secondary stationary point for NH over the bridge agrees with the conclusion of Crawford et al 13 that the H atom determines the minimum reaction path for NH dissociation.…”

“…The hydrogenation of NH 2 has even a higher barrier, 1.24 eV ͑TS1͒ and 1.32 eV ͑TS2͒. In the following paper from the same group 13 Crawford et al analyzed the factors which govern the minimum energy reaction pathways for NH and NH 2 hydrogenations on close-packed surfaces of the 4d transition-metals from Zr-Pd. They found that the potential energy surface of the H adsorbed on a metallic surface determines the activation barrier and the minimum reaction path, over the top or over the bridge.…”

“…They concluded that for the NH 3 adsorbed molecule the top site is the most stable position. A first estimate of the energetics for the ammonia synthesis was presented by Liu et al and Crawford et al 7,13 They searched the transition states by constraining the H-NH x distance as the reaction path. In fact there are many questions to be further addressed concerning the stability of the NH x species and transition states on the potential energy surface and kinetic and thermodynamic characteristics of catalytic phenomena derived from ab initio data.…”

Sequential Dihydrogen Desorption from Hydride-Protected Atomically Precise Silver Clusters and the Formation of Naked Clusters in the Gas Phase

“…Adsorption of hydrogen on transition metal surfaces is a popular area of research because of its application in heterogeneous catalysis and fuel cells. − A number of reports are available in the literature on the understanding of associative desorption of dihydrogen from transition metal surfaces . In many cases, hydrogen-adsorbed solid surfaces act as hydrogen sources. − Effects of adsorbed hydrogen on the catalytic activity of transition metals have also been studied in detail. , Here, we isolated a gaseous species, Ag 17 H 14 + , which also undergoes associative desorption of seven hydrogen molecules (7H 2 ), leading to the formation of naked Ag 17 + ion.…”