NO-assisted N2O decomposition over Fe-based catalysts:: Effects of gas-phase composition and catalyst constitution

The decomposition of N2O is strongly promoted by NO over Fe catalysts supported on zeolites (ex-framework FeMFI catalysts, sublimed Fe/ZSM-5, ion-exchanged Fe-ZSM-5, Fe-beta, and Fe-USY) and conventional supports (Fe/Al2O3 and Fe/SiO2) in a wide temperature range (550-900 K). Mixtures of NO and O-2 in the N2O-containing feed lead to the same promoting effect as NO only. The promoting effect of NO is catalytic, and in addition to NO2, O-2 is formed much more extensively at lower temperatures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with no significant improvements at molar NO/N2O feed ratios higher than 0.25, No inhibition by NO was identified even at a molar NO/N2O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition by N2O. The latter sites seem to be remote from each other. Multitrack experiments on ex-framework FeZSM-5 show that release of oxygen from the catalyst surface during direct N2O decomposition is a rate-determining step, due to the slow oxygen recombination, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption. Adsorbed NO accommodates oxygen from N2O and the formed adsorbed NO2 can react with a second oxygen from the neighboring site, thus accelerating the recombination of oxygen from N2O. Less than 0.9% of the Fe seems to participate in this promotion. Adsorbed NO may even facilitate the migration of atomic oxygen to enhance their recombination. A model is proposed to explain the phenomena observed in the NO-assisted N2O decomposition, including NO2 decomposition. (C) 2002 Elsevier Science (USA).