CoZSM-5: Why this catalyst selectively reduces NOx with methane

Methane oxidation with NO2, NO, O-2 catalyzed by CoZSM-5 and CuZSM-5 was st udied, as well as NO oxidation with O-2 to form NO2. The efficacy of NO, NO 2, and O-2 for the oxidation of CH4 has been evaluated. CuZSM-5 is very act ive for combustion and it is thus the dominant pathway for NO, SCR with CH4 . CoZSM-5 is much less active than CuZSM-5 for the same reaction. Although CH4 conversion with NO2 is approximately the same for both catalysts, the p aths of NO2 conversion differ dramatically. Whereas with CoZSM-5, 50% of NO 2 converts to Nz in the presence of CH4 at temperatures below 450 degrees C , only 10% of NO2 is converted to N-2 with CuZSM-5. However, the selectivit y toward N-2 for CuZSM-5 increases with temperature and reaches approximate ly 50% at 600 degrees C. Despite the completely different behavior of both catalysts in the CH4 + NO2 reaction they are equally selective for the CH4 + NO reaction: only CO2 and N-2 form but CuZSM-5 was more active. The catalytic behavior of CuZSM-5 and CoZSM-5 under SCR conditions with CH4 is related to the unique chemistry of Cu and Co species in the host ZSM-5. In CuZSM-5, dispersed Cu oxide clusters are highly active in a direct (rea ction with O-2) and an indirect (reaction with NO2 to form NO followed by i ts oxidation with O-2) methane combustion. The different intrinsic ability of Cu2+ and Co2+ ions to activate NOx results in the formation of an adsorb ed species that is reactive toward gaseous CH4 and also has a significant i mpact on the origin of CoZSM-5's ability to selectively reduce NOx with met hane. (C) 1999 Academic Press.