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 unsupported and silica-supported Cu/ZnO catalysts. It was shown that, by applying isotopic enrichment (63Cu and 68Zn), the copper and zinc signals can be separated. LEIS measurements showed a considerable amount of lead on the surface of the 63Cu/68ZnO catalyst. It was concluded that lead originated from the 68ZnO raw material and segregated during catalyst reduction to the surface. In contrast, no impurities were detected on the surface of the 63Cu/68ZnO/SiO2 catalyst. This seemingly contradictory observation was attributed to the higher degree of copper and zinc dispersion in the latter catalyst or interaction of lead with the support. It was shown previously, that catalytic activity of Cu/ZnO/SiO2 catalysts in methanol synthesis and ester hydrogenolysis is proportional with the reduction temperature applied (600−750 K). A depth profile of copper and zinc obtained for the high-temperature reduced silica-supported catalyst revealed that the surface was enriched in zinc, whereas copper was more abundant in layers beneath the surface. In the low-temperature reduced catalyst, the zinc enrichment was less prominent. These results suggest that the reduction at high-temperature results in segregation of zinc oxide to the surface.