Mechanism of NO reduction by CH4 in the presence of O2 over Pd-H-mordenite

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(O-Z)(n)(2+) 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(O-Z)(n)(2+) complex interacting with NO is present as the main Pd species, together with the bare Pd(O-Z)(n)(2+) and Pb(H2O)(n)(2+) complexes as the minor Pd species. The reaction rate for CH4-SCR correlated well with the amount of Pd(O-Z)(n)(2+) sites before the reaction, indicating that Pd(O-Z)(n)(2+) 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 NH4+ (3250 and 1430 cm(-1)) adsorbed on the zeolite acid site. NH4+ on Pd-H-MOR rapidly reacts in a how of NO or NO + O-2, which can result in N-2 formation. The results indicate that a Pd2+-NO complex and NH4+ play an important role as possible intermediates, and thus both Pd(O-Z)(n)(2+) sites and Bronsted 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. (C) 2000 Academic Press.