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

CoY, with its low activity in selective catalytic reduction (SCR) of nitrog en 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 s imilar to the stability of the dinitrosyls formed on Co-ZSM-5 (1894 and 181 2 cm(-1)). This suggests that the Co2+ (NO)2 species are not involved in th e SCR. The stable species produced upon NO + O-2 co-adsorption on the two s amples are very different. The principal compounds formed on Co-ZSM-5 are s urface monodentate nitrates characterised by an IR band at approximate to 1 540 cm(-1). These nitrates easily interact with hydrocarbons, which confirm s that they are key species in SCR. No monodentate nitrates are formed on C oY. Stable symmetric nitrates (1488 and 1473 cm(-1)) and less stable specie s, probably bidentate nitrates (1620 and 1320 cm(-1)) appear instead. The s ymmetric nitrates are converted, during evacuation, into nitro-compounds (1 563 and 1383 cm(-1)) that are not removed even by evacuation at 743 K. Inte raction of methane with the nitrates on CoY only leads to their partial red uction to nitro-compounds. These results account for the low SCR activity o f CoY. (C) 2001 Elsevier Science B.V. All rights reserved.