Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al2O3 catalyst on its activity and stability in the decomposition of N2O in the absence or presence of O2, SO2, H2O and NOX was studied in the present work. Decomposition of pure N2O was slightly enhanced by the H2-pretreated catalyst (metallic Ru) compared to the O2- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O2 or H2O addition was reversible compared to SO2 which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO2 surface, which could only be removed by regeneration under reducing (H2 in He) atmosphere at temperatures of ca. 500 °C. Oxidative or inert regeneration required very high temperatures (i.e. >700 °C) in order to decompose these sulphates. A method of retaining N2O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10 min) regenerations of the catalyst under reducing atmosphere (ca. 5% H2 in He). The effect of co-feeding various reducing agents, such as CO or C3H6, on the N2O conversion activity in the presence of O2, SO2, H2O and NOX is negligible, mainly because they are oxidized at relatively low temperatures in the O2-rich feeds used in this study.