The effect of platinum distribution in monolithic catalysts on the oxidation of CO and hydrocarbons

Monolithic Pt/γ-Al2O3 catalysts were prepared by depositing of the platinum phase, either homogeneously or heterogeneously (i.e., local high platinum concentration), in the washcoat. The influence of the platinum distribution on the ignition and extinction processes for oxidation of CO, propene, and propane, respectively, was investigated by both temperature-programmed and oxygen step-response flow-reactor experiments. In addition, in situ XANES spectroscopy was used to follow changes in the chemical state of platinum during propane oxidation. For samples with heterogeneous platinum distribution, the results show an improved low-temperature activity for CO oxidation, whereas no clear improvement is observed for oxidation of propene or propane. Comparison of the results for CO and hydrocarbon oxidation shows that the improved activity cannot be explained by thermal effects. Moreover, calculations indicate that the enhanced activity for CO oxidation can be due to mass-transfer phenomena. This could not be confirmed by our experiments, however. Instead, the improved activity for CO oxidation can be due to varying platinum particle size between the samples. Furthermore, the oxidation of propane is shown to be highly influenced by the oxygen concentration, showing an activity maximum for gas compositions close to the stoichiometric. High oxygen levels result most probably in a predominantly oxygen-covered surface, which inhibits the reaction and, as supported by the in situ XANES experiments, passivates the surface by forming platinum oxide.