Influence of the preparation method on the activity of ceria zirconia mixed oxides for naphthalene total oxidation

Abstract

Cerium/zirconium mixed oxides, with different Ce/Zr ratios, have been synthesised by a co-precipitation method using two different precipitating agents (sodium carbonate and urea) and tested for the total oxidation of naphthalene. Catalysts were characterised by N2 adsorption, XRD, Raman, TPR, XPS and DRIFTS. Ceria prepared by carbonate precipitation had low activity and this is likely to be related to the high concentration of residual surface carbonate that covers catalytic sites and inhibits reaction. For carbonate precipitation, increasing the Zr content to 1% resulted in a significant increase of activity, which is related to the decrease of surface carbonate. Increasing the Zr content up to 50% resulted in catalysts more active than ceria, but activity decreased as Zr content increased. This was in spite of increasing the number of oxygen vacancies, and this effect has been related to the decrease in the number of surface hydroxyl groups, which favours naphthalene adsorption. Ceria prepared by urea precipitation was markedly more active than that produced by carbonate precipitation. The urea-derived catalyst with 1% Zr is marginally more active than pure ceria, whilst for higher Zr contents activity was marginally lower. Two factors can account for these observations; they are the increase of oxygen vacancies contributing positively to activity and the opposing negative effect of decreasing the number of surface hydroxyl groups when the zirconium content increases.