In situ time-resolved energy-dispersive XAFS study on the reduction processes of Cu-ZSM-5 catalysts

The reduction process of Cu-ZSM-5 was investigated by a time-resolved energ y-dispersive XAFS (DXAFS) technique. The spectra were recorded every second and the entire spectra were analyzed to elucidate the dynamic change of th e local structure around the Cu species. Two Cu-ZSM-5 samples with differen t Cu loadings were prepared by an ion-exchange method followed by calcinati on. In the low-loading sample (84% ion-exchanged), Cu2+ cations dispersed i solatedly in the channels of ZSM-5, while in the high-loading sample (104% ion-exchanged) CuO particles were formed. The isolated Cu2+ cations were re duced to isolated Cu2+ cations at 400-450 K and the Cu+ ions were reduced t o CuO at 550-650 K. The curve fitting analysis of the DXAFS data revealed t hat Cu4-6 clusters were initially formed in the reduction process. The clus ters went out to the outer surface and grew into big particles. On the cont rary, the CuO particles on the outer surfaces were reduced directly to Cu-0 metallic particles at ca. 450 K. The dynamic structure change around the C u species in the zeolite during the reduction with H-2 was monitored in sit u by DXAFS technique for the first time.