NO oxidation on Fe- and Cu-zeolites mixed with BaO/Al2O3: Free oxidation regime and relevance for the NH3-SCR chemistry at low temperature

Abstract

Oxidative activation of NO is commonly regarded as a key step in the low-temperature mechanism of Standard SCR on metal-promoted zeolite catalysts for NH3-SCR of NOx. In the present work, we systematically investigate the dynamics of NO oxidation as well as the reactivity of NO+NO2 with adsorbed and gas-phase NH3 on Fe-ZSM-5 and Cu-CHA catalysts, using chemical trapping techniques. The approach is based on physically mixing the metal promoted zeolite catalyst with a NOx trap material, i.e. BaO/Al2O3. We show that in these combined systems, as long as NOx storage sites are available, the initial rate of NO oxidative activation is substantially higher than over the metal promoted zeolite alone, as measured in conventional steady-state activity tests for NO oxidation to NO2. Similar dynamics with enhanced initial NO conversions are observed also in the case of metal promoted zeolite catalysts with preadsorbed NH3, suggesting a close analogy between the role of BaO in the chemical trapping runs and the role of adsorbed NH3 in Standard SCR conditions, both species removing effectively the NO oxidation products from the gas phase and therefore preventing their inhibitory action. These novel findings are relevant to estimate correctly the intrinsic kinetics of the NO oxidative activation over metal-promoted zeolite SCR catalysts, and may help to explain the so far unresolved divergence between the rate of NO oxidation to NO2 and the rate of the Standard SCR reaction. We show, in fact, that the rate of uninhibited NO oxidation is able to sustain the Standard SCR activity over both Fe- and Cu-zeolite catalysts.