Studies of nickel-tungsten-alumina catalysts by X-ray photoelectron spectroscopy

Abstract

Nickel-tungsten catalysts supported on γ-alumina have been studied by ESCA. Various treatments such as reduction and sulfiding were carried out on the catalyst as a function of time and temperature. Chemical species present on the catalyst surface were identified and measured semiquantitatively. For the oxidic nickel-tungsten-alumina catalyst, nickel sesquioxide and nickel aluminate were detected on the catalyst surface. Tungsten was found to form a stable interaction complex with the γ-alumina support. When the catalyst was reduced at 500°C in hydrogen, tungsten remained in the +6 oxidation state and was not reduced even after prolonged treatment. On the other hand, nickel was readily reduced to nickel metal. Sulfiding of the catalyst was carried out at different temperatures using different sulfiding mixtures. The sulfiding rate was found to increase with increasing temperature or hydrogen sulfide concentration. The oxidic catalyst was found to be very active in hydrodesulfurizing thiophene. When the catalyst was sulfided with a 9 vol % H2S/H2 mixture at 350°C, nickel sulfide, nickel subsulfide, and tungsten disulfide were formed. Both carbon and oxygen signal intensities were reduced. Diffusion of nickel species to the surface and an increase in surface conductivity were also observed. Tungsten +3 species which are believed to be the active sites were not detected. The above results are explained in terms of structural models.