Creation of surface catalytic active sites on tungsten oxide(s) and a study of the catalytic behavior in isopropanol and 1-hexene reactions

Abstract

Catalytically active sites formed after the partial reduction of tungsten trioxide using hydrogen gas at different reduction temperatures were studied by measuring the catalytic activity of the system in reactions with isopropanol and 1-hexene. These reactions were activated by either metal, acid, or acid-metal (bifunctional) functionalities. The identification of the chemical composition of the specific catalytic active sites was achieved by in situ characterization of the system using the XPS-UPS surface technique carried out at the same time as the experimental catalytic measurements. Tungsten trioxide species possess acidic character before applying the reduction treatment. However, when WO3 is reduced to WO2 a metallic character is obtained, imparting the delocalization of π electrons over the tungsten atoms located along the C-axis of the deformed rutile structure of WO2. Changes in the spin–orbit coupling of W(4f) and the structure of the density of states (DOS) at the Fermi level confirmed the development of metallic properties. The bonding of active H, obtained from the dissociation of molecular hydrogen during the reduction procedure, to the oxygen surface sites results in a build-up of Bronsted W-OH acid functionalities. Therefore, a metal–acid bifunctional catalyst is obtained on the surface of the sample in the form of WO2−x(OH)y/TiO2. Titanium dioxide is utilized as a support to mechanically strengthen the catalyst and enhance the surface area.