NO reduction by CH4 in the presence of O2 over Pd-H-ZSM-5

An investigation of the interaction of NO and NO2 with Pd-H-ZSM-5, as well as the reduction of NO by CH4, has been conducted using mass spectrometry and in situ infrared spectroscopy. Prior to reaction most of the Pd in Pd-H-ZSM-5 (Pd/Al = 0.048) is present as Pd2+ cations. NO reduction by CH4 in the absence of O-2 results in the progressive reduction of Pd2+ cations above 610 K and the formation of small Pd particles. Reduction of Pd2+ cations is significantly suppressed when O-2 is added to the feed of NO and CH4. In situ infrared spectroscopy reveals the presence of NO+ and NO as the principal adsorbed species. NO+ is present as a charge-compensating cation (e.g., Z(-)NO(+)) and is believed to be formed via the reaction 2 Z(-)H(+) + 2 NO + 1/2 O-2 = 2 Z(-)NO(+) + H2O. NO+ does not react with CH4 at temperatures up to 773 K. Adsorbed NO reacts with CH4 above 650 K and CN species are observed as intermediates. The latter species react with both NO, O-2, and presumably NO2. Based on the accumulated data, a mechanism is proposed to explain the reduction of NO by CH4 both in the presence and absence of O-2. (C) 1999 Academic Press.