IR STUDY OF CO AND NOX SORPTION ON AG-ZSM-5

The properties of ion-exchanged Ag-ZSM-5 have been studied by means of IR spectroscopy. CO sorption on the sample leads to formation of Ag+- CO species (band at approximate to 2190 cm(-1)) which, at low temperat ures and in the presence of CO in the gas phase, are converted into Ag +(CO)(2) dicarbonyls (bands at 2195 and 2189 cm(-1)). Introduction of water vapour to Ag-ZSM-5 with pre-adsorbed CO results in a shift of th e Ag+-CO band to 2181 cm(-1) and a drop of its intensity. The 2181 cm( -1) band disappears after evacuation and is assigned to Ag+(H2O)CO com plexes. It is demonstrated that H2O and CO replace each other from the surface, which allows testing of silver-containing catalysts by CO wi thout their preliminary activation. Due to the weak electrophilicity o f the univalent Ag+ ions and the lack of a pi-backdonation, NO sorptio n on Ag-ZSM-5 does not lead to formation of nitrosyl complexes. Co-sor ption of NO and O-2 (in deficiency) results in the appearance of a ser ies of species: NO+ (band at 2140 cm(-1)), N2O3 (1880 and 1590 cm(-1)) , N2O4 (1748 cm(-1)), chemisorbed NO2 (1676 cm(-1)), physisorbed NO2 ( 1607 cm(-1)), bridged and bidentate nitrates (1630 and 1576 cm(-1), re spectively) and nitrites (1440 cm(-1)). When the oxygen introduced is in excess, NO+ and N2O3 are converted into N2O4 and NO2. Only the ioni c compounds are stable upon evacuation, i.e. the surface nitrates and nitrites. The latter species are bound to Ag+ ions and affect their pr operties: CO is strongly bound to some Ag+-NOx sites and is characteri zed by a band at 2182 cm(-1). The possible role of the sorbed nitrogen -ore species in the selective catalytic reduction of NOx is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.