Spectroscopic characterisation of surface hydroxyls during catalytic methane oxidation
Implementation of methane rich fuels as to develop sustainable vehicle transports requires improved catalytic exhaust aftertreatment technologies to convert engine-out methane emissions to carbon dioxide and water by total oxidation. A main challenge is that water formed during combustion inhibits the methane oxidation reaction even over the most active palladium-based catalysts presently known. This work aims at elucidating the active state of palladium and main deactivating mechanism(s) caused by water for alumina supported palladium catalysts to build fundamental knowledge about how catalysts with improved water tolerance can be designed. A bottom-up research approach including synthesis, characterisation and evaluation of single catalyst components and model catalysts has been employed. In specific, operando X-ray absorption spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy have been used to monitor the oxidation state of palladium and adsorbed hydroxyls. A portable water generator device was constructed to ensure accurate feed concentrations of pure water vapor for spectroscopic characterisation. For Pd/Al2O3, oxygen pulse-response methane oxidation experiments reveal high catalytic activity for both reduced Pd and well-developed PdO. The transition between these two states depends on oxygen concentration and temperature and is associated with lower catalytic activity. During dry methane oxidation, terminal and bridge-bonded hydroxyl species evolve and upon water addition the hydroxyl coverage increases further, which correlates well with a decreased catalytic activity both for short (1 h) and long (24 h) periods. The availability of alumina Lewis acid sites dictates the formation of hydroxyl species such that when modified with Pd, fewer hydroxyl groups are detected for catalysts with high Pd dispersion. Surface hydroxyl formation routes and kinetics role of boundary sites for high catalytic activity are discussed.