MODIFICATION OF THE SURFACE REACTIVITY OF CU-MFI DURING CHEMISORPTIONAND TRANSFORMATION OF THE REAGENTS IN THE SELECTIVE REDUCTION OF NO WITH PROPANE AND O-2

The steady-and unsteady-state catalytic behaviour of Cu-MFI in the con version of propane and NO in the presence of O-2 is reported, showing how the chemisorption and transformation of reactants may influence th e surface reactivity. Various effects were observed: (i) a change in t he surface reactivity and kinetics in going from low tn high concentra tions nf NO nr propane (ii) the transformation of NO to N-2 and N2O pr omoted at low temperature (250 degrees C) by oxygen in the absence of hydrocarbon, (iii) the influence of NO over the surface reactivity of the catalyst in the conversion of propane and (iv) the influence of su rface precoverage with oxidized nitrogen oxides (NxOy) or carboxylate species on the catalyst transient reactivity in the reduction of NO to N-2. In particular, Cu-MFI is initially more active when oxidized nit rogen oxides are present, suggesting that the active intermediate in t he reduction of NO with propane is a complex formed by the reaction of nitrate with activated hydrocarbon. It is shown, however, that strong ly bound oxidized nitrogen oxides may have also additional effects on the surface reactivity: (i) can promote the conversion of NO to N-2 an d N2O in transient conditions and (ii) can give a partial inhibition o f the surface reactivity blocking copper ions due to their strong chem isorption. Furthermore, it is shown that NO reacts faster with oxygen than hydrocarbon forming NxOy species which are then the oxidizing age nt for the hydrocarbon. It is thus suggested that the surface reactivi ty of Cu-MFI in the reduction of NO with propane/oxygen depends on the surface population of nitrogen oxide adspecies which influence not on ly the surface reactivity, but also the pathway of hydrocarbon oxidati on.