Time-resolved DXAFS study on the reduction processes of Cu cations in ZSM-5

The time-resolved reduction process of copper cations in/on ZSM-5 during te mperature-programmed reduction (300-700 K) was studied by energy-dispersive X-ray absorption fine structure (DXAFS) as well as transmission electron m icroscopy (TEM). Two Cu-ZSM-5 samples with different Cu loadings were prepa red by an ion-exchange method. The Cu K-edge DXAFS spectra for isolated Cu2 + species in the channels of ZSM-5 and CuO particles on the outer surfaces of ZSM-5 were recorded at an interval of 1 s during the reduction. The curv e fitting analysis of the EXAFS data and the XANES analysis revealed that t he isolated Cu2+ species in the channels were reduced stepwise. They were r educed to isolated Cu+ species at 400-450 K and the Cu+ species to Cu-0 met allic clusters at 550-650 K. Small clusters like Cu-4 were initially formed , followed by particle growth. A small part of them went out to the outer s urfaces of ZSM-5 during the reduction. In contrast, CuO particles on the ou ter surfaces were reduced directly to Cu-0 metallic particles around 450 K.