Relevance of the Co_(1-x)Ni_xWO_4 wolframite-type mixed oxide compositions on the synthesis and catalytic properties of W-based carbides

摘要

A series of wolframite-type oxides (Co_(1-x)Ni_xWO_4) with various compositions was prepared by urea-matrix combustion method and subsequently carburized using a temperature-programmed reaction (1 deg C min~(-1)) under a mixture of 10 vol.% C_2H_6/H_2,from room temperature to 700 deg 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 C03W3C and P-W2C carbides is favoured,whereas at Ni compositions greater than those of Co the main phases were Ni and a-WC.At intermediate composition (Coo.5Nio.5WQ-) bimetallic and monometallic carbides were formed.The CoWC* bimetallic catalyst showed greater activity in the steady state than Ni-containing catalysts.The HDN active phase present in CoWCx 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 Ni-C bond,for the latter,play a very important role in the catalytic process.