Surface characterization and catalytic properties of supported tungsten and platinum-tungsten carbide and oxycarbide

Abstract

Bulk tungsten carbide has been prepared from ammonium paratungstate. Oxidic oxygen in relatively small amount is always present on the WC surface regardless of the method of carburization. Tungsten(VI) oxide in Al2(WO4)3 can be reduced directly and in apparently one single step to the elemental W(0) state by Ar+ bombardment. Under H2 the reduction starts at 720 K. In 12 and 28% W03 supported on γ-Al2O3 and in Al2(WO4)3, almost 90% of the tungsten could be converted to the carbide form. Extensive hydrogenolysis products for n-hexane reactions on freshly prepared WC were observed while selectivity to isomerization associated with decrease in activity occurs upon exposure of the WC to oxygen at 620 K. Supported tungsten carbide(s) and oxycarbide(s) on γ-Al2O3 have comparable catalytic behaviour to those obtained on the bulk systems. The presence of Pt in these supported systems did not improve the catalytic performances and even did not show the catalytic properties of Pt. This was attributed to the severe conditions of catalyst preparations. Although the catalytic activity of tungsten carbide(s) obtained from the carburization of Al2(WO4)3 is very low, the selectivity suggests the presence of tungsten carbide plus some WO, species.