Kinetics of CH4, H2S and SO2 oxidation on precious metal catalysts under stagnation point flow conditions

A new methodology which combines reactor experiments and numerical modelling to derive kinetic rates of solid-gas heterogeneous reactions in a stagnation point flow reactor (SPFR) is developed and used to investigate the effects of small concentrations of H2S and SO2 on the lean catalytic combustion of methane on precious metal catalysts. The activity of polycrystalline Pt foil, then Pt-, Rh- and Pd-containing washcoats supported on stainless-steel foils are investigated in the SPFR, where the washcoat consisted of a thin layer of γ-alumina or ceria/γ-alumina. The porous washcoats supported on steel foil were more active than the equivalent flat surface of pure precious metal (Pt) with pre-exponential factors of 7.4 × 105 cm s-1 and 4.9 × 104 cm s-1 respectively. Repeated use of the washcoated catalyst and the effect of pore opening gives rise to a change in the pre-exponential factor from 7.4 × 105 cm s-1 to 1.1 × 109 cm s-1 for Pt. The high-temperature activity behaviour of Pt in presence of sulphur species in the feed was, however, superior to that of Pd, due to the changing nature of the active site in CH4 oxidation (PdO/Pd), as is consistent with the literature based on other type of reactor studies. Methane combustion was enhanced by both H2S and SO2 on the Pt catalysts as opposed to the Pd and Rh catalysts, which saw a decrease in the methane conversion.