Catalytic properties of chromium–palladium loaded alumina in the combustion of methane in the presence of hydrogen sulfide

Abstract

Impregnation of γ-Al₂O₃ by chromium(III) ions leads to surface chromate species after calcination under oxygen at 773 K. This solid is very active in methane combustion, however selectivity towards CO₂ is not complete because some CO is also produced even in the presence of a large excess of oxygen. To prevent the formation of CO, subsequent ultra-low palladium deposition (0.05–0.2 wt% Pd) has been performed on the chromate supported alumina. In this case the activity is unchanged but full selectivity towards CO₂ is now observed. For this bimetallic catalyst, palladium is present in the form of large PdO particles, which do not participate in the conversion of methane to CO₂, mainly due to their size, however, they promote the oxidation of CO to CO₂. The presence of H₂S in the feed leads to a severe deactivation of the Pd/Al₂O₃ catalyst owing to the formation of sulfate groups at the surface of PdO particles. On the contrary, the activity of the Cr/Al₂O₃ catalyst was not dramatically affected, but selectivity towards CO was strongly improved. In the presence of H₂S the chromate groups of the Cr/Al₂O₃ are reduced to Cr³⁺ ions probably in the form of Cr₂O₃. For the mixed catalyst containing both palladium and chromium, H₂S addition has a limited effect on the global catalytic activity, whereas the formation of CO is fully suppressed. An electronic interaction between chromium oxide and palladium oxide is postulated leading to a weakening of the Pd–S bond of the adsorbed sulfate.