SYNTHESIS, CHARACTERIZATION, AND CATALYTIC PROPERTIES OF CLEAN AND OXYGEN-MODIFIED TUNGSTEN CARBIDES

High surface area WC and beta-W2C powders (30-100 m2g-1) were prepared by direct isothermal carburization of WO3 and W2N in CH4-H2 mixtures. After surface cleaning with H-2, their surfaces are equilibrated with bulk stoichiometric carbides and free of polymeric carbon; they chemisorb 0.2-0.4 monolayers of CO and H. These carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on tungsten carbides and introduces surface sites that lead to high isomerization selectivity (70-99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC(x)) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC(x) sites and introduces Bronsted acid surface sites (WO(x)). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.