Influence of the Preparation Method, Outgassing Treatment, and Adsorption of NO and/or O2 on the Cu2+ Species in Cu-ZSM-5: An EPR Study

Two Cu2+-loaded ZSM-5 zeolites prepared by ion exchange from liquid solution (LS) and solid-state reaction (SR) are studied by EPR after vacuum treatments at several temperatures, Tv, up to 413 K and subsequent NO and/or O2 adsorption. In addition to the changes in Cu2+ coordination upon dehydration, 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 Tv=383 K. This is ascribed to fast spin relaxation, due either to a ligand geometry effect or to the establishment of copper–copper interactions. Subsequent NO adsorption at 298 K generated (Cu–NO)+ complexes, apparently by reaction of square planar Cu2+ (possibly assisted by water or OH− still present), while NO and O2 coadsorption led to a large change, all copper now being EPR-visible either as a sharp signal for a square planar species or as a broad band due to spin-interacting Cu2+. For sample SR, the resolved Cu2+ EPR signals, representing ca. 25% of the total copper, behave similarly to those found in sample LS (except that their overall intensity decreases little upon outgassing), but most copper exists as Cu-oxide clusters giving a broad EPR signal. The latter, resulting from the solid-state preparation method, seem scarcely reactive on adsorption of NO or NO+O2; they may be connected with the lower catalytic activity in NO decomposition previously observed for this latter sample.