Physicochemical characterization of ZnO/Al2O3 and ZnO–MoO3/Al2O3 catalysts

Abstract

The interaction modes of ZnO with Al₂O₃ and MoO₃/Al₂O₃ catalysts have been investigated as a function of ZnO content (1–30 wt% with respect to Al₂O₂) and calcination temperature (400–800 °C) by using X.p.s., A.e.s., and other physicochemical techniques. Particular attention was paid to the depth distribution of zinc ions in the surface layer of Al₂ O₃ by using the Zn LMM/Zn 2p_3/2 intensity ratio coupled with Ar⁺-sputtering techniques. Zinc ions are highly dispersed, forming a ‘surface spinel’ irrespective of the calcination temperature in ZnO/Al₂O₃ catalysts having a low ZnO content,(<10–15 wt%). With the ZnO/Al₂O₃O₃ catalysts containing 1–2 wt% ZnO, it has been found that zinc ions are retained in the surface layer of the Al₂O₃, even when calcined at 800 °C. In the case of ZnO/MoO₃/Al₂O₃ catalysts an enhanced segregation of zinc ions in the surface layer of Al₂O₃, was deteced, suggesting the occurrence of the interaction with pre-existing molybdates on Al₂O₃ while no appreciable effects of ZnO on the distribution and reduction behaviour of the molybdenum species were observed.