Cu/ZnO and Cu/ZnO/SiO2 catalysts studied by Low-Energy Ion Scattering

Low-Energy Ion Scattering (LEIS) was employed to study the surface of unsup ported and silica-supported Cu/ZnO catalysts. It was shown that, by applyin g isotopic enrichment (Cu-63 and Zn-68), the copper and zinc signals can be separated. LEIS measurements showed a considerable amount of lead on the s urface of the Cu-63/(ZnO)-Zn-68 catalyst. It was concluded that lead origin ated from the (ZnO)-Zn-68 raw material and segregated during catalyst reduc tion to the surface. In contrast, no impurities were detected on the surfac e of the Cu-63/(ZnO)-Zn-68/SiO2 catalyst. This seemingly contradictory obse rvation was attributed to the higher degree of copper and zinc dispersion i n the latter catalyst or interaction of lead with the support. It was shown previously, that catalytic activity of Cu/ZnO/SiO2 catalysts in methanol s ynthesis and ester hydrogenolysis is proportional with the reduction temper ature applied (600-750 K). A depth profile of copper and zinc obtained for the high-temperature reduced silica-supported catalyst revealed that the su rface was enriched in zinc, whereas copper was more abundant in layers bene ath the surface. In the low-temperature reduced catalyst, the zinc enrichme nt was less prominent. These results suggest that the reduction at high-tem perature results in segregation of zinc oxide to the surface.