Acid strength of support materials as a factor controlling oxidation stateof palladium catalyst for propane combustion

The support effect on the low temperature catalytic combustion of propane o ver palladium catalyst was studied by using a series of metal oxides as sup port materials: MgO, ZrO2, Al2O3, SiO2, SiO2-ZrO2, SiO2-Al2O3, and SO42--Zr O2. The catalytic activity varied with the kind of the support materials; a support material with moderate acid strength gave maximum conversion. In o rder to discuss more details, 5 wt% Pd/ZrO2, Pd/Al2O3, and Pd/SiO2-Al2O3 we re subjected to catalytic testing under the various conditions, and the cat alysts after the test were characterized by XRD and XPS. The activity chang ed with the oxygen concentration, and the concentration providing the maxim um activity depended on the support material. The maximum activity was obta ined at high concentration on Pd/SiO2-Al2O3, at moderate concentration on P d/Al2O3, and at low concentration on Pd/ZrO2. This sequence corresponded to that of acid strength of the support materials measured by Hammett indicat ors. The relation between the oxidation state of palladium and its catalyti c activity under the various mixtures indicated that the catalytic activity on each catalyst depends on the oxidation state of palladium, and that par tially oxidized palladium is effective for propane combustion. Moreover, pa lladium on acidic support kept the metallic state even under high oxygen co ncentration, indicating that acidic support gives palladium the resistibili ty against oxidation. These results suggest one guideline for the design of active palladium catalyst; an acidic support must be chosen under the mixt ure of the higher oxygen concentration and vice versa. (C) 1999 Academic Pr ess.