Mechanism of NO Reduction by CH4 in the Presence of O2 over Pd–H–Mordenite

Abstract

Palladium-exchanged H–mordenite catalysts (Pd–H–MOR) were studied for the selective catalytic reduction of NO by CH4 (CH4-SCR). In situ UV–Vis results showed that the isolated Pd2+ ions coordinated to cation exchange sites, Pd(OZ)2+n, are present on the dehydrated samples, and their amount increases linearly with increasing Pd loading to 1.5 wt%. During the CH4-SCR reaction, a Pd(OZ)2+n complex interacting with NO is present as the main Pd species, together with the bare Pd(OZ)2+n and Pb(H2O)2+n complexes as the minor Pd species. The reaction rate for CH4-SCR correlated well with the amount of Pd(OZ)2+n sites before the reaction, indicating that Pd(OZ)2+n is involved in the crucial steps for NO reduction. The kinetic results suggest that strongly adsorbed NO-derived species and weakly held CH4-derived species are involved in the crucial steps for NO reduction. In situ IR spectra showed that a Pd2+–NO complex (1860 cm−1) is a dominant adspecies during CH4-SCR on Pd–H–MOR over a wide range of temperature. The adsorbed NO species on Pd2+ are reduced in a flow of CH4 to form NH+4 (3250 and 1430 cm−1) adsorbed on the zeolite acid site. NH+4 on Pd–H–MOR rapidly reacts in a flow of NO or NO+O2, which can result in N2 formation. The results indicate that a Pd2+–NO complex and NH+4 play an important role as possible intermediates, and thus both Pd(OZ)2+n sites and Brønsted acid sites of a zeolite are required for this bifunctional catalysis. A proposed mechanism is consistent with the kinetic results, indicating that it can be the dominant NO reduction pathway during the SCR reaction in a steady state.