A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn2O4, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N2O decomposition as well as the reduction of N2O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973K for 24h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N2 and CO2 was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773K and disaggregate, by reduction of the spinel framework Mn3+ cations to Mn2+, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.