Wet Oxidation of Phenol on Ce1−xCuxO2−δCatalyst

Abstract

Ce1−xCuxO2−δcatalysts with 0.05<x<0.20 for catalytic wet oxidation of phenol in aqueous solutions have been synthesised using the coprecipitation method. The three most important synthesis parameters, the concentration of the mixed metal salt solution, the rate of coprecipitant addition and the stirrer speed during coprecipitation, were optimised with central composite design using the catalytic activity as a response function. The catalytic activity strongly depends on stirrer speed during coprecipitation. A high mutual dispersion of copper oxide and ceria, having the average crystallite size of about 9 nm, enhances solid solution formation. The unit cell parameter of ceria decreases when the overall concentration of copper in the catalyst increases, most probably obeying Vegard's law. The catalysts proved to be very stable in hydrothermal reaction conditions at low pH values. After 5 h of reaction in the semibatch CST reactor less than 100 ppm of Cu was leached out of catalyst samples that were calcined in a flow of air for 2 h above 1033 K, and only a very low quantity of carbonaceous deposits were formed on the surface of the catalysts (0.6 wt%). The kinetics of phenol degradation could be interpreted by an equation valid for homogeneous autocatalytic reactions, in which the rate constant depends linearly on the heterogeneous catalyst (Cu) concentration. This demonstrates that the reaction proceeds through a heterogeneous–homogeneous radical-branched chain mechanism.