Mechanistic aspects of NO+CO reaction on La 2− xSr xNiO 4−δ ( x=0.00–1.50) perovskite-type oxides

Abstract

The mechanistic aspects of NO+CO reaction on perovskite-type series La 2− x Sr x NiO 4−δ x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25and1.50) prepared by the nitrate route have been studied. The catalysts were active between 380 and 580°C. The kinetics of NO+CO reaction exhibits a rather complex behaviour which is differentiated according to low and high reaction temperatures as well as to small and large degree of substitution x of La by Sr. The conversion of CO runs parallel to NO conversion with a hysteresis which leads to N 2O production. The reactants NO and CO react in a 2 : 1 ratio at low reaction temperatures ( T≲480°C, 2NO+CO→products) but they convert in a 1 : 1 ratio at high temperatures ( T≳480°C, NO+CO→products). Plots of the ratio of the degrees of conversion X of NO and CO versus temperature, X NO/X CO=f(T) , show that the transition from one mechanism to the other is perfectly linear. An inverse relation is observed between the quantities of δ and Δ(X NO/X CO)/ΔT . From the kinetic analysis it is estimated that the difference of the apparent activation energy for samples with small x and large x, at low reaction temperatures corresponds to the enthalpy of adsorption of NO while at high reaction temperatures it corresponds to the enthalpy of adsorption of oxygen. The difference between the apparent activation energies corresponding to high and low temperature, for both reactants, passes from negative to positive values at the same point where the values of δ pass from negative to positive ones or from samples rich in oxygen to samples poor in oxygen.