Structure-function relationships of supported molybdenum nitride HDN catalysts.

A series of $\gamma$-$\rm Al\sb{2}O\sb{3}$ supported Mo nitrides with loadings of 4, 8, and 16 wt% Mo were prepared by the temperature programmed nitridation of supported molybdates with $\rm NH\sb3.$ Several heating rates and space velocities were employed in an attempt to vary the properties of the nitrides. The supported Mo nitrides were composed of $\gamma$-$\rm Mo\sb{2}N$ (fcc) domains. The sizes of the supported nitride particles ranged from less than 10 A for the low loaded materials to greater than 50 A for the high loaded materials. Oxygen chemisorption and electron microscopy suggested that they had a two-dimensional, plate-like morphology. The pyridine HDN activities (588-648 K, 1 atm) of the supported Mo nitrides were similar to the activities of unsupported Mo nitrides and better than those of a sulfided Ni-Mo/Al$\sb2$O$\sb3$ commercial catalyst. The specific HDN activity of the supported Mo nitrides was a strong inverse function of Mo loading. Changes in the space velocity and heating rates had only modest effects on the activity of the materials. Several features correlated with differences in the activities of the supported Mo nitrides: particle size, reducibility, and the amount of chemisorbed CO. The reduction temperature was a function of the Mo loading. The lowest loaded materials (small particles) were reduced at lower temperatures than the highest loaded materials (large particles). Distinct differences were observed between the adsorptive/desorptive chemistry of the supported Mo nitrides and bulk $\gamma$-$\rm Mo\sb{2}N$ powders. Carbon monoxide dissociatively adsorbed on the unsupported powders and to a lesser extent on the supported nitrides. Associative adsorption of CO was observed only for the supported materials. The amount of associatively adsorbed CO scaled linearly with the HDN activity. This CO apparently adsorbed on regions at the perimeter (particle/support interface) of the two-dimensional particles. Dissociative adsorption was linked with adsorption on-top of the plate-like domains. The results suggested that the highest HDN active sites were located along the perimeter of the nitride particles. The turnover frequency for these sites was 1.5 x 10$\rm\sp{-3}\ s\sp{-1}$ at 633 K, five-fold higher than that of sites on top of the particles.PhDApplied SciencesChemical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129680/2/9610100.pd