Characterization of LaMnAl11O19 by FT-IR spectroscopy of adsorbed NO and NO/O-2

Abstract

The nature of the NOx species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on LaMnAl11O19 sample with magnetoplumbite structure obtained by a sol–gel process has been investigated by means of in situ FT-IR spectroscopy. The adsorption of NO leads to formation of anionic nitrosyls and/or cis-hyponitrite ions and reveals the presence of coordinatively unsaturated Mn3+ ions. Upon NO/O2 adsorption at room temperature various nitro–nitrato structures are observed. The nitro–nitrato species produced with the participation of electrophilic oxygen species decompose at 350 8C directly to N2 and O2. No NO decomposition is observed in absence of molecular oxygen. The adsorbed nitro–nitrato species are inert towards the interaction with methane and block the active sites (Mn3+ ions) for its oxidation. Noticeable oxidation of the methane on the NOx -precovered sample is observed at temperatures higher than 350 8C due to the liberation of the active sites as a result of decomposition of the surface nitro–nitrato species. Mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed. (c)2005 Elsevier B.V. All rights reserved.