Abstract/Summary

The surface chemistry and activity of a range of supported-palladium catalysts, known for their high activity in methane oxidation reactions, were studied using near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) and in-situ mass spectrometry. Using NAP-XPS, the chemical state of the catalyst was monitored under 0.33 mbar of a methane and oxygen gas mixture and a temperature ramp up to 800 K. In-situ mass-spectrometry was able to quantify the partial pressures of reactants and products throughout the temperature ramp, and the chemical state of the catalyst as determined by NAP-XPS was correlated to product formation. As a result, the impact of Pd particle size and support material on catalytic behaviour was studied, in addition to finding the active state of the catalyst. For supported-Pd catalysts on Al2O3, SiO2 and SiO2-Al2O3, NAP-XPS showed PdO to be the dominant oxidation state at 500 – 600 K. Despite a gas ratio of [CH4]:[O2] = 2, the complete oxidation of methane is favoured at these temperatures where CO2 and H2O are produced. PdO, therefore, was found to be the active state for complete methane oxidation. As the O2 depletes and the temperature increases to >650 K, PdO reduces to PdOx, where 0 ≤ x < 1. The partial pressures of CO2 and H2O decrease and syngas formation (H2 and CO), the product of partial methane oxidation, is dominant, suggesting reduced Pd is the active state for partial methane oxidation. From NAP-XPS, it is possible to identify this change in oxidation state from a shift in binding energy. The temperature at which the shift from high to low binding energy of the Pd 3d peaks occurs corresponds to the onset temperature of syngas, as determined by mass spectrometry. A particle size effect was observed for Pd/Al2O3 and Pd/SiO2 catalysts with Pd particle size ranges of 4 – 10 nm and 2 – 6 nm respectively, whereby the onset temperature of syngas, and the temperature at which PdO reduced, decreased with increasing particle size. This effect suggests that a larger particle size is more active towards partial methane oxidation, due to a greater metal-support interaction, and hence decreased reducibility, of smaller sized particles. The reactivity of support materials increased in the order: SiO2 < SiO2-Al2O3 < Al2O3.