Understanding the Role of SnO2 Support in Water-Wolerant Methane Combustion: In situ Observation of Pd(OH)2 and Comparison with Pd/Al2O3

Quantitative analysis of poisonous Pd hydroxide phase formation during methane combustion in the presence of water is performed via in situ X-ray absorption spectroscopy from 200 to 500 deg C for palladium catalysts supported on tin and aluminum oxides. Water presence inhibits oxidation of the metallic Pd; for the Pd/Al2O3 catalyst the major oxidation product is Pd(OH)(2), with PdO being the dominant phase on the Pd/SnO2 system. Tin dioxide is reduced during methane oxidation in the presence of water, regardless of the excess oxygen in the feed. Temperature-programmed surface reactions at anaerobic and low-oxygen conditions revealed that the Pd/SnO2 catalyst has an inability to catalyze methane steam reforming, as opposed to Pd/Al2O3. We suggest that tin oxide extracts hydroxyls from the PdO surface, which makes the latter more active in methane combustion in the presence of water but inactive in methane steam reforming.