Ion beam assisted deposition and carbon-nitrogen bond activation character of molybdenum nitride films.

Abstract

The purpose of research described in this dissertation was to synthesize molybdenum nitride films with well-defined structures and stoichiometries, to evaluate their C-N bond activation characters, and to correlate them with those of molybdenum nitride powder catalysts. The molybdenum nitride films were synthesized using ion beam assisted deposition (IBAD). The effects of ion-to-atom arrival rate ratio, ion angle of incidence, and ion energy on the film composition and phase constituents were examined. The film nitrogen to molybdenum stoichiometry increased linearly with increasing arrival rate ratio irrespective of the ion energy and varied significantly with changes in the ion angle of incidence. The phase constituents were functions of all of the deposition parameters investigated. It is postulated that a single parameter, the effective energy density per deposited atom (E$\sb{\rm s}$), accounts for the effects of ion energy, mass and angle of incidence on film structure. For low energy ions E$\sb{\rm s}$ is proportional to E$\sp{1/2},$ a dependence that it shares with other models for phase formation during IBAD. The advantage of the energy density treatment is that it has a more obvious influence on the temperature in the growth zone, a factor controlling phase formation. The C-N bond activation character of films with different phases and textures was evaluated using CH$\sb3$NH$\sb2$ thermal desorption spectroscopy. Decomposition of CH$\sb3$NH$\sb2$ was also studied over a series of Mo nitride powders. It was found that all of the films were active for CH$\sb3$NH$\sb2$ decomposition, and the product formation depended significantly on the phase and texture. Characteristics of the $\delta$-MoN(200) and $\gamma$-Mo$\sb2$N(200) films resembled those of the low surface area Mo nitride powders. Characteristics of the $\beta$-Mo$\sb{16}$N$\sb7$(400) and $\beta$-Mo$\sb{16}$N$\sb7$(203) films resembled those of the high surface area molybdenum nitride powders. Given the results it was concluded that methylamine C-N bond activation was structure-sensitive over molybdenum nitrides. Structure-function relationships described in this dissertation could lead to more active and selective industrial Mo nitride hydrodenitrogenation catalysts.