CATALYTIC REACTIONS OF NORMAL-ALKANES ON BETA-W2C AND WC - THE EFFECT OF SURFACE OXYGEN ON REACTION PATHWAYS

Tungsten carbide powders with WC and β-W2C structures catalyze alkane hydrogenolysis reactions. Alkanes adsorb on fresh carbide surfaces strongly, leading to rapid deactivation by carbon fragments and to high selectivity to hydrogenolysis products. n-Hexane isomerization or dehydrocyclization products were not observed on fresh carbides. Chemisorbed oxygen lowers the binding energy of adsorbed intermediates and decreases both hydrogenolysis and deactivation rates. Oxygen-exposed carbides catalyze n-hexane and n-heptane isomerization with high selectivity (74-99%). Isomerization involves methyl-shift pathways that require alkene intermediates. These results and the well-known acid properties of WOx, surface species suggest that oxygen-exposed tungsten carbides catalyze bifunctional (dehydrogenation/carbenium-ion) alkane isomerization pathways. © 1991.