CATALYTIC ACTIVITY OF COPPER-ZINC MANGANITES FOR THE REDUCTION OF NO AND N2O BY HYDROCARBONS

The catalytic behaviour of copper-zinc manganites with general formula CuxZn1-xMn2O4 (x = 0, 0.01, 0.05, 0.10) was investigated for the reduction of NO by hydrocarbons. Also the N2O decomposition and its reduction by propane and propene were tested on representative samples. The catalysts were obtained by thermal decomposition at 973 K of carbonate precursors. Regardless the composition, all the precursors are monophasic and made by a rhodochrosite-like phase, CuxZnyMn(1 x-y)CO3, in which Cu2+ and Zn2+ ions entered in solid solution. The metal dispersion in the precursors is preserved by the final catalysts that, in spite of the presence of copper, are monophasic and made by a CuxZn1-xMn2O4 spinel like phase. The reduction of NO was studied by using CH4, C3H8 and C3H6 as reducing agents and the last two hydrocarbons were used for testing the N2O reduction. Propene is the most effective hydrocarbon followed by propane whereas methane is efficient only at higher reaction temperatures. Pure zinc manganite is an active catalyst but the presence of copper in the spinel enhanced the catalytic activity of NO reduction by propane and slightly improved that of N2O reduction. Moreover, regardless the presence of copper, a high conversion (70-100%) of NO to N2 was attained at 873 K and the selectivity to N2 and CO2 was always close to 1. The catalysts are stable in the CH4 and C3H8 containing streams within the whole range of temperature explored. By contrast, when C3H6 is used as reducant the spinel structure is preserved only below 773 K because at higher temperatures it collapses by reduction of Mn3+ to Mn2+ ions forming a mixture of ZnO and MnO oxides. At this stage the catalysts are still active and an attempt to explain this evidence is made.