Mechanistic aspects of NO+CO reaction on La2-xSrxNiO4-δ (x=0.00-1.50) perovskite-type oxides

The mechanistic aspects of NO+CO reaction on perovskite-type series La2-xSrxNiO4-delta(x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) prepared by the nitrate route have been studied. The catalysts were active between 380 and 580 degrees 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 N2O production. The reactants NO and CO react in a 2:1 ratio at low reaction temperatures (T less than or similar to 480 degrees C, 2NO+CO-->products) but they convert in a 1:1 ratio at high temperatures (T greater than or similar to 480 degrees 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 delta and Delta(X-NO/X-CO)/Delta T. 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 delta pass from negative to positive ones or from samples rich in oxygen to samples poor in oxygen. (C) 1997 Elsevier Science B.V.