Reduction of benzoic acid on CeO2 and, the effect of additives Original Research Article

Catalytic behaviour of CeO2 in the reduction of benzoic acid has been studied in the temperature range 523–723 K. Two types of catalytic behaviour are observed in the whole temperature range. One type is observed from 523 up to 723 K. In this range, the selectivity to benzaldehyde can be higher than 95% and the reaction proceeds by the redox (Mars and van Krevelen) mechanism, with the oxygen vacancy as the active site. The activity is controlled by the steady-state concentration of oxygen vacancies under reaction conditions. The second type behaviour is observed above 648 K. Under these conditions the conversion of benzoic acid achieves nearly 100%, but the selectivity to benzaldehyde decreases, while that to toluene and benzene increases. In this range of temperatures, the reaction proceeds too far, due to a very high concentration of vacancies. Besides the redox mechanism decar☐ylation (hydrogenolysis) of the relative stable adsorbed benzoic acid takes place. The results observed with the reaction of adsorbed benzoic acid, under a flow of H2 reflects the two types of reaction. The potential promotion effects by Al, Cr, Mn, Fe, Ga, Zr, In and Pb oxides added to CeO2 were investigated. The catalytic behaviour of CeO2 changes by the addition of the metal oxides, positive effects are observed after adding Mn, Zr, In and Pb oxides.