Influence of the preparation method, outgassing treatment, and adsorption of NO and/or O-2 on the Cu2+ species in Cu-ZSM-5: An EPR study

Two Cu2+-loaded ZSM-5 zeolites prepared by ion exchange from liquid solutio n (LS) and solid-state reaction (SR) are studied by EPR after vacuum treatm ents at several temperatures, T-v, up to 413 K and subsequent NO and/or O-2 adsorption. In addition to the changes in CU2+ coordination upon dehydrati on, from octahedral to square pyramidal and planar, outgassing produced in sample LS a decrease of the CU2+ EPR signal, initiated by brief outgassing at 298 K and reaching a maximum (ca. 75% of the total copper not detected) for T-v = 383 K. This is ascribed to fast spin relaxation, due either to a ligand geometry effect or to the establishment of copper-copper interaction s. Subsequent NO adsorption at 298 K generated (Cu-NO)(+) complexes, appare ntly by reaction of square planar CU2+ (possibly assisted by water or OH- s till present), while NO and O-2 coadsorption led to a large change, all cop per now being EPR-visible either as a sharp signal for a square planar spec ies or as a broad band due to spin-interacting Cu2+. For sample SR, the res olved CU2+ EPR signals, representing ca. 25 % of the total copper, behave s imilarly to those found in sample LS (except that their overall intensity d ecreases little upon outgassing), but most copper exists as Cu-oxide cluste rs giving a broad EPR signal. The latter, resulting from the solid-state pr eparation method, seem scarcely reactive on adsorption of NO or NO + O-2; t hey may be connected with the lower catalytic activity in NO decomposition previously observed for this latter sample. (C) 2000 Academic Press.