Spectroscopic investigation of palladium catalysts during wet methane oxidation

Abatement of the strong greenhouse gas methane in vehicle applications by use of catalytic end-of-pipe technologies is challenging due to the prevailing reaction conditions with low temperatures and high water content. In this thesis, the mechanisms behind low-temperature water inhibition of lean methane oxidation over palladium-based catalysts have been studied using an integrated experimental approach including wet-chemical preparation of model catalysts with systematic variation of key performance parameters, transient measurements of reaction kinetics in chemical flow reactor, and operando infrared and X-ray absorption spectroscopic characterisation. For Pd/γ-Al2O3 catalysts, well developed PdO particles is the most active phase and the apparent activation energy for methane oxidation is significantly higher in presence of water.  It increases with decreasing palladium particle size in the presence of water as opposed to dry conditions where it decreases slightly. Linear and bridge-bonded hydroxyl surface species on alumina evolve during dry methane oxidation by spill-over of hydrogen species to the Pd/γ-Al2O3 rim, which correlates with a declining catalytic activity.  Addition of water causes severe hydroxylation of the Pd/γ-Al2O3 catalysts that significantly hamper the methane turnover frequency.  On the contrary, for Pd/ZSM-5 catalysts, hydroxyl formation on the Pd-ZSM-5 rim can not be detected in dry conditions, and is minor upon water addition. A high but not too high palladium dispersion, with palladium particle size not smaller than about 2 nm, is suggested for Pd/γ-Al2O3 catalysts as to balance water tolerance against palladium utilization. Lastly, ZSM-5 supported Pd-based catalysts show outstanding long-term performance for methane oxidation in the presence of water vapor compared to catalysts supported by γ-Al2O3. This finding stimulates the use of hydrophobic support materials given that palladium particles are sufficiently stabilized.