Synthesis, characterization, and catalytic properties of clean and oxygen-modified tungsten carbides

Abstract

High surface area WC and β-W 2C powders (30–100 m 2g −1) were prepared by direct isothermal carburization of WO 3 and W 2N in CH 4-H 2 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.