Structure-function relationships of tungsten carbide catalysts.

The relationships between the catalytic and structural properties of tungsten carbides have been elucidated. Tungsten carbides with different bulk phases (i.e. WC, W$\sb2$C and WC$\sb{\rm 1-x}$) and surface areas were prepared via the temperature programmed carburization of tungsten oxides and nitrides with CH$\sb4$-H$\sb2$ mixtures. The bulk phase and surface area were strongly influenced by the final synthesis temperature and solid reactant. The orientation of the carbide crystallites in the aggregate particles resulted in the formation of three distinct mesopore size regions. The grouping of the pores into distinct size regions suggested that the development of the carbides pore network was related. The tungsten carbides were exceptional butane hydrogenolysis and dehydrogenation catalysts despite the presence of graphitc carbon after H$\sb2$ treatment. The reactions were structure-sensitive over the carbides because the catalytic properties differed for the different crystallographic phases. The carbide activities were superior to those of a Pt-Sn/$\rm Al\sb2O\sb3$ catalyst, but their selectivities were markedly different. The carbides and the Pt-Sn/$\rm Al\sb2O\sb3$ catalyst were selective to hydrogenolysis and dehydrogenation products, respectively. Variations in the catalytic properties were not completely accounted for by changes in the bulk phase suggesting that the surface properties also varied. High resolution electron microscopy and x-ray photoelectron spectroscopy were employed to determine the surface structure and composition. The carbide surface structures were similar to those in the bulk. The surface lattice parameters for the high surface area WC and W$\sb2$C catalysts were greater than those in the bulk because of the oxygen in the near-surface region of the catalysts. The surface parameters for the low surface area WC and W$\sb2$C catalysts and the WC$\sb{\rm 1-x}$ materials were similar to those in the bulk. The near-surface region of the carbides contained oxygen after reduction in H$\sb2$. The excess oxygen was most likely in the form of acidic WO$\sb{\rm x}$ and $\rm WC\sb{x}O\sb{1-x}$ species. The acidity of the carbides was confirmed by NH$\sb3$ temperature programmed desorption. Since hydrogenolysis is acid-catalyzed and the WO$\sb{\rm x}$ and $\rm WC\sb{x}O\sb{1-x}$ are acidic, it was concluded that hydrogenolysis occurred over the oxide and/or oxycarbide sites.PhDApplied SciencesChemical engineeringInorganic chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129595/2/9542820.pd