Abstract
In this work, we employed density functional theory to elucidate the energetics associated with elementary steps along a Langmuir-Hinshelwood mechanism for the Haber-Bosch syn-thesis of ammonia from N-2 and H-2 on a hexagonal, Mo-terminated molybdenum nitride surface. Using nudged elastic band calcu-lations, we determined the energy barriers involved in the reaction processes. An active site consisting of four nearest-neighbor Mo atoms, previously identified as an active site on similar surfaces, was chosen to investigate the reaction processes. Using this approach, we calculate a barrier of -0.5 eV for the dissociation of N-2. The superior activity of the dissociation of the strong N-2 bonds is rationalized based on the unique geometric and electronic configurations present at these active sites. Despite the favorable energetics for nitrogen dissociation, the energy cost for hydrogenation of NHx (0 <= x <= 2) species is shown to be energetically limiting for the formation of ammonia through the Langmuir-Hinshelwood mechanism at these sites, with elementary step activation barriers calculated to be as large as -2 eV. A comparison to Haber-Bosch results derived from a similar gamma-Mo2N model system suggests the relative independence of surface and bulk for rhombic Mo-4 active sites on nitrides.