OP-II-3

PP-I-5behavior was accompanied by a considerable decrease by 10-20 kJ mol –1 in the E app .The kinetic models, which provided the best fit for CO oxidation were the same forboth catalysts. More specifically, these were:(i) the standard redox model, incorporating a reaction order < 0.1 on P O2R =k P k PnCO CO O2 O2CO CO+nO2 O+2 CO2 CO20.5 k P k P (1 K P )(1)(ii) the modified redox model, which assumes a first order dependence on P O2(for surface oxidation) and includes the CO adsorption term:kKr COPCOkO P2 O2R =0.5 kK P + k P (1 + K P + K P )r CO CO O2 O2 CO CO CO2 CO2The same kinetic models were applied in the case of H 2 oxidation. All theequations incorporate the inhibiting effect of CO 2 . The heat of adsorption of CO onCuO-CeO 2 was estimated to be 25 kJ mol –1 . This value corresponds to CO adsorbedon oxidized sites, since CO adsorption on reduced (Cu + ) sites may also take place,but it does not influence the steady-state reaction rate. The activation energies forthe reduction and oxidation steps were found to be higher for the oxidation step(69.6 kJ mol –1 ) compared to the reduction step (40.6 kJ mol –1 ). The kineticparameters of CO oxidation over CuO took lower values than those of the CuO-CeO 2catalysts, reflecting the lower activity of this catalyst. The most pronounced differenceis in the rate constant of the reduction step, k CO , which in the temperature range of50-100°C is 25 times (at 50°C) or 6.5 times (at 100°C) smaller for the CuO catalyst.The rate constant of the oxidation step, k O2 , in CuO, on the other hand, is 3-4 timessmaller in the same temperature range. This implies that CeO 2 promotes to a greaterextent the reduction of copper oxide species than their reoxidation. The promotingeffect of ceria was also revealed in the case of CO 2 inhibition, since the calculatedheat of adsorption of CO 2 was twice as large on CuO compared to CuO-CeO 2 .ConclusionsRate expressions derived from redox (Mars-van Krevelen) mechanisms describesatisfactorily the observed kinetic behavior in PROX reaction. The presence of ceriaenhanced the reduction step and improved the tolerance towards CO 2 .References[1] G. Avgouropoulos and T. Ioannides, Appl. Catal. B 67 (2006) 1.[2] G. Sedmak, S. Hocevar, J. Levec, J. Catal. 213 (2003) 135.[3] W. Liu, M. Flytzani-Stephanopoulos, J. Catal. 153 (1995) 317.[4] G. Sedmak, S. Hocevar, J. Levec, J. Catal. 222 (2004) 87.(2)228