Catalytic oxidation of carbon black and propylene over Ru/CoxMgyAl2 and Cu/CoxMgyAl2 catalysts

Abstract

CoxMgyAl2 oxides are prepared using the hydrotalcite route. The obtained solids are thermally stabilized at 500°C and then impregnated with ruthenium or copper nitrate solution. X-ray diffraction results show that the calcination of the impregnated solids led to the formation of various mixed oxides (CuO, RuO2, Co3O4, Co2AlO4, CoAl2O4, CoxMgyO4, MgO …). The different impregnated and non impregnated solids are then tested in the total oxidation of carbon black (CB) and propylene. It was shown that the Co3O4 is the reactive species in absence of ruthenium or copper and that the catalytic activity depends on the quantity of this oxide. Copper does not significantly affect the supports reactivity in the studied reactions. However, ruthenium enhances the reactivity in propylene oxidation but does not affect it in carbon black oxidation, only in loose contact conditions. The good reactivity of ruthenium impregnated catalysts is attributed to the formation of easily reducible ruthenium and cobalt oxide species at the surface of the support. The addition of ruthenium made the reduction of surface and bulk cobalt oxides possible at lower temperatures. For a given series of catalysts and whatever the reaction studied, solids based on Co6Al2 support are the most reactive and the ones based on Mg6Al2 support are the less reactive. A kinetic study was done for the reaction of CB oxidation. The study showed that this combustion followed a phase boundary controlled reaction (contracting linear) P1 and that the activation energy was decreased from 151 kJ/mol to 111 kJ/mol with Ru/Co6Al2 in tight contact conditions.