Preparation of nickel-tungsten bimetallic carbide catalysts

A series of nickel and tungsten bimetallic oxides have been prepared using the solid state reaction of nickel nitrate and tungsten trioxide, which were carburised under temperature-programmed conditions with a mixture of 20% CH4/H-2 and 10% C2H6/H-2 to prepare the corresponding carbides. A GC-MS was employed to monitor the carburisation process, and the resulting oxides and carbides have been characterised with X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is shown that a greater proportion of nickel tungstate is formed at a high nickel-to-tungsten ratio during the solid state reaction. At a Ni/(Ni + W) (molar) ratio of 0.4, the surface is completely covered with NiWO4, although the bulk phase is completely converted to NiWO4 when the Ni/W ratio approaches unity. Carburising the oxide with a mixture of 20% CH4/H-2 or 10% C2H6/H-2 leads to beta-W2C. However, the carbide is formed at a lower carburisation temperature when using 10% C2H6/H-2 instead of 20% CH4/H-2. The nickel metal is either highly dispersed in or occupies the tungsten carbide lattice. After passivation, the surface of the Ni and W carbide is converted into the oxide of NiWO4, although the bulk structure is still that of beta-W2C carbide. The carbon content in the bimetallic carbide decreases and the carbide particle size becomes larger with increasing nickel content. (C) 2002 Elsevier Science (USA).