Characterization of LaMnAl11O19 by FT-IR Spectroscopy of Adsorbed NO and NO/O2,

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 two types 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 3500C directly to N2 and O2. No NO decomposition is observed in absence of oxygen. The adsorbed nitro-nitrato species are inert toward 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 3500C due to the liberation of the active sites as a result of decomposition of the surface nitro-nitrato species. A mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed.