FTIR study of species arising after NO adsorption and NO+O-2 co-adsorption on CoY: comparison with Co-ZSM-5

CoY, with its low activity in selective catalytic reduction (SCR) of nitrogen oxides, differs from Co-exchanged pentasil zeolites (e.g. Co-ZSM-5). To obtain more information on the SCR mechanism, the NOx species formed after NO adsorption and NO + O-2 co-adsorption on CoY were studied by means of IR spectroscopy and the results were compared with those obtained for Co-ZSM-5. NO adsorption on CoY leads to the formation of Co2+(NO)(2) species (v(s) at 1900 and v(as) at 1819 cm(-1)) which are characterised by a stability similar to the stability of the dinitrosyls formed on Co-ZSM-5 (1894 and 1812 cm(-1)). This suggests that the Co2+ (NO)2 species are not involved in the SCR. The stable species produced upon NO + O-2 co-adsorption on the two samples are very different. The principal compounds formed on Co-ZSM-5 are surface monodentate nitrates characterised by an IR band at approximate to 1540 cm(-1). These nitrates easily interact with hydrocarbons, which confirms that they are key species in SCR. No monodentate nitrates are formed on CoY. Stable symmetric nitrates (1488 and 1473 cm(-1)) and less stable species, probably bidentate nitrates (1620 and 1320 cm(-1)) appear instead. The symmetric nitrates are converted, during evacuation, into nitro-compounds (1563 and 1383 cm(-1)) that are not removed even by evacuation at 743 K. Interaction of methane with the nitrates on CoY only leads to their partial reduction to nitro-compounds. These results account for the low SCR activity of CoY. (C) 2001 Elsevier Science B.V. All rights reserved.