Acid Strength of Support Materials as a Factor Controlling Oxidation State of Palladium Catalyst for Propane Combustion

The support effect on the low temperature catalytic combustion of propane over palladium catalyst was studied by using a series of metal oxides as support materials: MgO, ZrO2, Al2O3, SiO2, SiO2–ZrO2, SiO2–Al2O3, and SO42−–ZrO2. The catalytic activity varied with the kind of the support materials; a support material with moderate acid strength gave maximum conversion. In order to discuss more details, 5 wt% Pd/ZrO2, Pd/Al2O3, and Pd/SiO2–Al2O3 were subjected to catalytic testing under the various conditions, and the catalysts after the test were characterized by XRD and XPS. The activity changed with the oxygen concentration, and the concentration providing the maximum activity depended on the support material. The maximum activity was obtained at high concentration on Pd/SiO2–Al2O3, at moderate concentration on Pd/Al2O3, and at low concentration on Pd/ZrO2. This sequence corresponded to that of acid strength of the support materials measured by Hammett indicators. The relation between the oxidation state of palladium and its catalytic activity under the various mixtures indicated that the catalytic activity on each catalyst depends on the oxidation state of palladium, and that partially oxidized palladium is effective for propane combustion. Moreover, palladium on acidic support kept the metallic state even under high oxygen concentration, indicating that acidic support gives palladium the resistibility against oxidation. These results suggest one guideline for the design of active palladium catalyst; an acidic support must be chosen under the mixture of the higher oxygen concentration and vice versa.