Control of the catalytic activity of tungsten carbides. I. Preparation of highly dispersed tungsten carbides

Abstract

A study of the preparation of carbides using hydrous oxide precursors is presented. Two different precursors (WO3 · H2O and hexagonal WO3 · xH2O), obtained by homogeneous precipitation from ammonium tungstate solutions, have been converted into carbides after preliminary dehydration and reduction steps. The carburization step was performed either in pure CO or in COCO2 mixture ( CO CO2 ratio = 10:1). Carburization in pure CO always resulted in the deposition of free carbon on the surface of the carbides, as was shown by XPS analysis. The use of a CO CO2 mixture made it possible to minimize the coverage of the carbide particles by free carbon, probably as a result of a slackening of the CO dismutation reaction. Hence, the amount of free carbon in excess over the quantity necessary to convert W2C to WC was maintained at the lowest possible level. The so obtained carbides proved to be extremely well dispersed (30 m2/g). They appeared to consist of particles which were pseudomorphic with the starting oxides. A passivation treatment, consisting of contacting the samples with diluted oxygen at 21 °C, was found necessary in order to prevent their spontaneous ignition in air. © 1986.