Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds

Abstract

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH 4 traces under lean-burn conditions was studied in the presence or the absence of water or H 2S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H 2S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H 2S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H 2S-poisoned Pd/Al 2O 3 catalyst was studied by TPD in O 2/He. Poisoning species decompose above 873 K as SO 2 and O 2 in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O 2/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H 2S-poisoned Pd/Al 2O 3. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.