X-ray photoelectron spectroscopy characterization of the reduction and oxidation behaviour of Ni-containing HZSM-5 zeolites

Abstract

The surface characteristics of NiHZSM-5 zeolites prepared by ion-exchange, impregnation and mechanical mixing methods have been studied by XPS. The results show that because of the interaction between Ni^II species and the zeolite, nickel can exist in many forms on the surface of the zeolite, e.g. NiO, Ni(OH)₂ and NiAI₂O₄. The degree of reduction of Ni^II species depends mainly on its form, and decreases in the order NiO > Ni(OH)₂ > NiAI₂O₄. The metallic nickel particles formed by reduction of Ni^II species by hydrogen were unstable, and the redox cycle was not completely reversible either. The analysis of surface composition indicates that HZSM-5 zeolite does not have a uniform composition. Surface enrichment of aluminium and nickel was observed and can be explained by several factors, including the sample preparation method, the stage of the redox cycle and the treatment conditions. Toluene conversion on this kind of catalyst has also been investigated. The surface NiO species provide weak catalytic sites for the dealkylation of toluene whilst the surface Ni(OH)₂ species act as relatively weak catalytic sites for the disproportionation of toluene. However, the Ni^o species exhibit strong catalytic activity towards the cracking and deakylation of toluene.