TEMPERATURE-PROGRAMMED DESORPTION AND DECOMPOSITION OF NH3 OVER MOLYBDENUM NITRIDE FILMS

The surfaces of beta-Mo16N7, gamma-Mo2N, and delta-MoN films were characterized using NH3 temperature-programmed desorption (TPD). Ammonia adsorption at similar to 280 K and TPD using a heating rate of 6 K/s produced NH3 peaks at similar to 360 K. The desorption kinetics depended on the structure and composition of the film. Ammonia desorption from the beta-Mo16N7 and gamma-Mo2N films was first-order; however, desorption from the delta-MoN film appeared to be second-order. Assuming a pre-exponential factor of 10(13) s(-1), the desorption energy for the beta-Mo16N7 and gamma-Mo2N films was 22 kcal/mol. The NH3 saturation capacity increased in the following order: delta-MoN < beta-Mo16N7 < gamma-Mo2N. This order is similar to that expected for the Mo surface atom density. Some of the NH3 decomposed into H-2 and N-2. Two H-2 desorption peaks were produced: a low-temperature peak due to recombination of surface hydrogen and a high-temperature peak due to hydrogen that emerged from the nitride subsurface. The N-2 desorption spectrum consisted of a peak at similar to 340 K and several peaks in the range 500-900 K. (NH3)-N-15 TPD experiments indicated that the low-temperature N-2 desorption peak was due to NH3 decomposition while the origin of the high-temperature peaks was the nitride itself. The amount of N-2 that desorbed in this high-temperature envelope increased with increasing NH3 dose. We believe that nitrogen desorption from the nitride was induced by the presence of hydrogen which altered the Mo-N bonding. Ammonia desorption and decomposition spectra for the films were similar to those for a series of bulk gamma-Mo2N powders. Characteristics of the gamma-Mo2N film resembled those of the low-surface-area powder (< 20 m(2)/g), while the behavior of the beta-Mo16N7 and delta-MoN films was similar to that for the higher-surface-area powders.