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 temperatures 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 the 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 complexes. It is demonstrated that H2O and CO replace each other from the surface, which allows testing of silver-containing catalysts by CO without their preliminary activation. Due to the weak electrophilicity of the univalent Ag+ ions and the lack of a pi-backdonation, NO sorption on Ag-ZSM-5 does not lead to formation of nitrosyl complexes. Co-sorption of NO and O-2 (in deficiency) results in the appearance of a series 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), respectively) and nitrites (1440 cm(-1)). When the oxygen introduced is in excess, NO+ and N2O3 are converted into N2O4 and NO2. Only the ionic compounds are stable upon evacuation, i.e. the surface nitrates and nitrites. The latter species are bound to Ag+ ions and affect their properties: CO is strongly bound to some Ag+-NOx sites and is characterized 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.