Temperature-programmed desorption and decomposition of NH3 over molybdenum nitride films

The surfaces of β-Mo16N7, γ-Mo2N, and δ-MoN films were characterized using NH3 temperature-programmed desorption (TPD). Ammonia adsorption at ∼ 280 K and TPD using a heating rate of 6 K/s produced NH3 peaks at ∼ 360 K. The desorption kinetics depended on the structure and composition of the film. Ammonia desorption from the β-Mo16N7 and γ-Mo2N films was first-order; however, desorption from the δ-MoN film appeared to be second-order. Assuming a pre-exponential factor of 1013 s−1, the desorption energy for the β-Mo16N7 and γ-Mo2N films was 22 kcal/mol. The NH3 saturation capacity increased in the following order: δ-MoN < β-Mo16N7 < γ-Mo2N. This order is similar to that expected for the Mo surface atom density. Some of the NH3 decomposed into H2 and N2. Two H2 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 N2 desorption spectrum consisted of a peak at ∼ 340 K and several peaks in the range 500–900 K. 15NH3 TPD experiments indicated that the low-temperature N2 desorption peak was due to NH3 decomposition while the origin of the high-temperature peaks was the nitride itself. The amount of N2 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 γ-Mo2N powders. Characteristics of the γ-Mo2N film resembled those of the low-surface-area powder (< 20 m2/g), while the behavior of the β-Mo16N7 and δ-MoN films was similar to that for the higher-surface-area powders.