DRIFT, XPS and XAS Investigation of Au–Ni/Al₂O₃ Synergetic Catalyst for Allylbenzene Isomerization

Abstract

Au/Al₂O₃ and Ni/Al₂O₃ prepared by deposition-precipitation and impregnation methods, respectively, and Au–Ni/Al₂O₃ prepared by impregnation of the Au/Al₂O₃ system with nickel have been studied by physicochemical methods. The catalysts were tested in allylbenzene isomerization at 443 K. The isomerization on Au/Al₂O₃ proceeds with a rather low rate, while monometallic Ni/Al₂O₃ is inactive. Unlike monometallic catalysts, Au–Ni/Al₂O₃ demonstrates a strong synergetic effect: the isomerization rate on the bimetallic sample exceeds the sum of the rates over the individual metals by an order of magnitude. The catalysts structure was estimated by XPS, DRIFT, and XAS techniques. It was revealed that nickel in Au–Ni/Al₂O₃ and Ni/Al₂O₃ exist as Ni²⁺ cations with the coordination state close to that in the precursor, gold in Au/Al₂O₃ exist as Au⁰ nanoclusters, whereas the Au⁰ nanoclusters co-exist with Au³⁺ cations in the bimetallic Au–Ni/Al₂O₃ sample. Reasons of the synergetic catalytic effect for Au–Ni/Al₂O₃ are discussed in terms of the formation of new positively charged Au catalytic centers.