Low-temperature oxidation of carbon monoxide and methane over alumina and ceria supported platinum catalysts

Abstract

The ignition and extinction processes for total oxidation of CO, CH4 and CO+CH4 mixture in oxygen excess over Pt/Al2O3 and Pt/CeO2 catalysts with the platinum phase distributed either homogeneously or heterogeneously (i.e., locally high platinum concentration) in the support have been studied by temperature programmed oxidation experiments. Following the preparation methods by Arnby et al. [42], the samples have the same Pt load and dispersion. Generally the catalytic activity follows the order: Pt/CeO2 (heterogeneous)>Pt/CeO2 (homogeneous)>Pt/Al2O3 (heterogeneous)>Pt/Al2O3 (homogeneous) as indicated by lower ignition and/or extinction temperatures. For Pt/Al2O3, the addition of NO2 to the reactant stream increases the rate of oxidation of CO in the pre-ignition regime although the light-off temperature T50 is shifted towards higher temperatures (except for low CO concentrations). In the case of the Pt/CeO2, the CO conversion generally decreases. For CH4 oxidation in the presence of NO2, the conversion increases for Pt/Al2O3 and decreases for Pt/CeO2. The addition of CO2 in the reactant stream has minor influence on CO oxidation over Pt/Al2O3 while for Pt/CeO2, T50 is shifted towards higher temperatures. For the simultaneous oxidation of CO and CH4, a reverse hysteresis for methane oxidation is observed, i.e., the extinction process occurs at higher temperature than the corresponding ignition process. The improved activity for CO oxidation over samples with heterogeneous Pt distribution is likely due to less tendency towards CO self-poisoning through the development of steeper concentration gradients in the Pt containing regions in the porous support material. The significant increase of activity for both reactions over ceria-supported Pt is here assigned to highly active sites at the platinum–ceria boundary but also, to some extent, the oxygen storage and release function and dynamics of the transport of oxygen in the Pt/CeO2 system.