Mechanism of WO3 reduction and carburization in CH4/H-2 mixtures leading to bulk tungsten carbide powder catalysts

The mechanism of bulk tungsten carbide catalysts synthesis from WO3 in CH4/ H-2 mixtures has been studied using temperature programmed reactions associ ated with CH4/D-2 exchange reaction and in situ X ray diffraction, Various experimental parameters have been studied such as partial pressures of reac tants, heating rate, mass of precursor, or flow rate in order to determine the most important steps occurring during the transformation of WO3 to WC. It is shown that at temperatures below 900-923 K the diffusion within the s olid particles is slow with respect to the rate of reduction of the surface , allowing the carburization of the surface in the presence of a core still partially oxidized. At higher temperatures, the diffusion is rapid, leadin g to a uniform reduction within the solid. In this case the surface is cont inuously replenished in oxygen thus inhibiting the activation of methane an d allowing the carburization to proceed only when the solid is deeply reduc ed. An inhibiting effect of hydrogen pressure on the interaction of methane with the surface has also been evidenced, an effect which excludes the pos sibility of an independent control of the reduction process from that of ca rburization, Finally the role of space velocity has also been elucidated. ( C) 2000 Academic Press.