Temperature-programmed desorption of H2 from molybdenum nitride thin films

Abstract

The thermal desorption of hydrogen from [beta]-Mo16N7, [gamma]-Mo2N and [delta]-MoN thin films was investigated. Hydrogen adsorption at [approximate]270 K produced two hydrogen desorption peaks, one at [approximate]370 K (designated as the [beta]1 site) and the other at [approximate]800 K (designated as the [beta]2 site). We concluded that the [beta]1 peak was due to a low-energy surface site and the [beta]2 peak to a subsurface site. Desorption from the [beta]1 state obeyed first-order kinetics. The hydrogen saturation capacity increased in the following order: [beta]-Mo16N72N. This variation in saturation coverage tracked with the nominal molybdenum atom surface density. Nitrogen also desorbed from the Mo nitride surfaces during the H2 temperature-programmed desorption experiments producing a low-temperature peak at [approximate]370 K and several high-temperature peaks in the range 500-900 K. The amount of nitrogen that desorbed increased with increasing H2 dose, suggesting a hydrogen-induced nitrogen desorption process. Hydrogen may have weakened the Mo-N bond thereby facilitating the desorption of nitrogen. We believe that the low-temperature peak was due to nitrogen bound to Mo atoms at the surface. The high-temperature peaks were probably a consequence of hydrogen that diffused into the surface altering the Mo-N bonding in the subsurface region. Thermal desorption characteristics of the Mo nitride films were similar to those of a series of bulk [gamma]-Mo2N powders that we previously investigated. Apparent energies for hydrogen desorption from the low-temperature sites were near 25 kcal/mol, and both the thin films and powders possessed high-temperature subsurface binding sites. The most striking observation was the similarity between the characters of the [delta]-MoN film and the Mo nitride powder with the highest hydrodenitrogenation activity. This similarity suggested that the high activity was due to the presence of [delta]-MoN-like structures at or near the bulk powder surface.