Methane oxidation over palladium-washcoated foils in the presence of sulphur dioxide

The catalytic oxidation of methane in the presence and absence of SO2 was investigated on palladium catalysts supported on Al2O3 and CeO2/Al2O3 washcoats. The washcoats adhered onto thin stainless steel foils and were investigated under fuel-lean conditions in a stagnation point flow reactor (SPFR). Both palladium-loaded washcoats exhibited hysteresis in the conversion versus temperature profile during heating and cooling. This was attributed to the change in oxidation state of palladium between PdO and Pd. The presence of CeO2 stabilised PdO, reducing the extent of the conversion discontinuities caused by the PdO ↔ Pd transitions during heating and cooling. However, the Pd/CeO2/Al2O3 catalyst had a lower activity for methane oxidation. The presence of SO2 in the feed deactivated both catalysts, but unexpected higher conversions following exposure to SO2 reflected a promotional effect in the thermal decomposition region of PdO. Exposure to 30 ppm SO2 in the lean methane feed stream eliminated the stabilising effects that CeO2 had on conversion during the PdO–Pd transition. Frequency factors and activation energies were calculated for a single step reaction mechanism using a power law kinetic model. The numerical code SPIN successfully modelled the conversion curves using the derived kinetic data, providing a means of kinetic validation. The activation energies in absence of SO2 were 44 ± 1 kJ mol−1 and 58 ± 4 kJ mol−1 for the 3 wt.% Pd/Al2O3 and the 3 wt.% Pd/12 wt.% CeO2/γ–Al2O3 catalyst respectively, one of the lowest reported in the literature. The aged catalysts also exhibited relatively low activation energies (49–84 kJ mol−1).