Influence of nanostructure and composition on the catalytic properties of mono and bimetallic nitrides.

Abstract

There were two primary goals to this work. The first objective was to determine the influence of elemental composition on the performance of phase-pure high surface area early transition metal nitride catalysts for n-alkane and ethanol conversion. Both the effect of oxygen and the consequence of metal atom type and constitution were investigated. The second objective was to ascertain the influence of nanostructure on nitride catalytic properties. Several important conclusions were drawn regarding the effect of solid-state oxygen. Thermogravimetric analysis and x-ray diffraction revealed that bulk oxygen contents for the VN and NbN catalysts were a function of the temperature-programmed reaction synthesis temperature. According to results from H₂ temperature-programmed reduction, 1--4 monolayers of oxygen were incorporated preferentially at the surface during passivation of the nitrides. Hydrogen reduction temperatures necessary for passivation layer removal correlated well with the metal-oxygen bond strength. The presence of oxygen, as a function of content, negatively influenced the performance of mono and bimetallic nitride catalysts for all reactions studied. Metal atom type and composition also influenced the properties of bimetallic nitride catalysts. Bimetallic nitrides composed of Group 5 and 8 metal combinations were not phase-pure and the segregation was attributed to lattice parameter disparity. X-ray diffraction, sorption, and catalytic results were all consistent with the nitrides comprised of Group 5 and 6 metals being solid-solutions. These solid-solutions exhibited areal reaction rates and product distributions that were functions of metal composition and distinct from their monometallic counterparts. The catalytic properties for the bimetallic nitrides were less desirable than those for the best monometallic nitrides for a given reaction. The nanostructure analysis focused on the VN catalysts due to their similarity to Pt for n-alkane reforming reactions. Initial n-butane conversion rates increased proportionately with H₂ pretreatment temperature. The areal reaction rates converged as an exponential function of temperature to similar steady-state values. Chemisorption with CO revealed that VN possessed a single type of catalytic site. This site only activated C-H bonds. Site densities appeared to track with the metal:nitrogen stoichiometry. These and other results suggested that non-metal vacancy defects were the active site for n-butane and n-hexane dehydrogenation.