Relevance of the Co_1-xNi_xWO₄ wolframite-type mixed oxide compositions on the synthesis and catalytic properties of W-based carbides

A series of wolframite-type oxides (Co_1-xNi_xWO ₄) with various compositions was prepared by urea-matrix combustion method and subsequently carburized using a temperature-programmed reaction (1 °C min^-1) under a mixture of 10 vol.% C₂H ₆/H₂, from room temperature to 700 °C, to obtain a mixed Co, Ni and W carbide catalysts. The catalytic performance was evaluated in a continuous flow reactor using hydrodenitrogenation of pyridine as model reaction. The wolframite-type oxides and the carbide catalysts pre- and post-HDN reaction were characterized using elemental analysis, X-ray diffraction (XRD), laser Raman spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetric (DSC), transmission electron microscopy (TEM) and BET surface area measurements. Urea-matrix combustion method is a convenient tool to prepare highly pure wolframite-type oxides, whose composition affects strongly the W-based carbide phase distribution and the HDN catalytic behaviour. At Ni compositions lower than Co contents the formation of Co₃W ₃C and β-W₂C carbides is favoured, whereas at Ni compositions greater than those of Co the main phases were Ni and α-WC. At intermediate composition (Co_0.5Ni_0.5WC_x) bimetallic and monometallic carbides were formed. The CoWC_x bimetallic catalyst showed greater activity in the steady state than Ni-containing catalysts. The HDN active phase present in CoWC_x is different than that present in the Ni-containing catalysts, that is, carbon-metal bond strength of the bimetallic carbide, for the former, and metal nickel or weak NiC bond, for the latter, play a very important role in the catalytic process.