Combustion of Methane over Palladium-Based Catalysts: Support Interactions

Palladium-based catalysts are attractive for methane combustion at low temperatures, although deactivation is an issue, making it important to understand the mechanism for the catalytic combustion reaction. Support effects on the catalytic combustion reaction were investigated at temperatures below 450 degrees C using continuous flow of CH4 plus O-16(2) or O-18(2) over various combinations of isotopically labeled PdO/metal oxide catalysts. Our experimental results are consistent with a PdO/Pd redox combustion mechanism. However, the oxide supports are also shown to provide oxygen for the catalytic combustion reaction. Pd/PdO serves as a porthole for gas phase oxygen to dissociate, migrate to the oxide support, and exchange with oxygen from the oxide support. The migration and exchange process allows oxygen that was originally bonded to the support to participate in the catalytic combustion reaction. Water, a reaction product, inhibits the exchange of oxygen between the support and PdO. The rate of oxygen exchange is temperature and support dependent. Mechanisms for the low temperature combustion of methane over PdO-based catalysts need to include steps for oxygen and hydroxyl exchange between the Pd phase and the support.