Reactions of nitrogen oxides and oxygen with hydrocarbons over manganese and related oxides

Abstract

The catalytic reactions of nitric oxide, nitrous oxide or oxygen with either ethane or 1-butene were studied. Manganese (III) oxide was evaluated as a catalyst for these reactions in a differential-reactor flow system operated at atmospheric pressure. This system allowed the kinetics of reaction to be calculated and kinetic modelling studies were used as an aid in determining reaction mechanisms. Nitric oxide reacted in a complex sequence of steps beginning with an irreversible bimolecular reaction between adsorbed species on the catalyst surface. In the case of the reactions with ethane, each reactant was adsorbed on one surface site, while in reactions with 1-butene each was adsorbed on two sites. Nitrous oxide was one of the products of re¬ action when nitric oxide was a reactant. In reactions with nitrous oxide or oxygen, similar mechanisms operated and only the products of complete hydrocarbon oxidation were observed. In all reactions, the manganese (III) oxide went through a phase change from the alpha to the gamma phase. A nitro or nitrate species appeared on the catalyst surface after reactions involving nitric oxide. The effects of a series of oxide catalysts were observed on the nitrous oxide / 1-butene reaction. Reactivity decreased down the series: CuO, Fe₂O₃, NiO Mn₂O₃, Cr₂O₃, TiO₂, Sc₂O₃, SnO, ZnO, V₂O₅, Al₂O₃. The use of carbon fibres as catalyst supports or cocatalysts was assessed, and the kinetics of the oxygen / ethane and oxygen / 1-butene reactions over manganese oxide / carbon fibre were determined.