Silica pillared metal(IV) phosphate materials as supports for nickel catalysts

Abstract

Two silica pillared phosphate materials containing different amounts of silica have been used as supports to prepare, by impregnation, two series of metallic nickel catalysts (8, 10 and 15% loading). Thermal programmed reduction curves of nickel show two peaks at ca. 690 and 838 K denoting two different sites for Ni ²⁺ ions assigned to the external and inner surface, respectively. These high reduction temperatures indicate high dispersion and strong interactions with the supports. XPS analyses show that the reduction degree at 873 K (2 h) ranges between 22.5 and 30%. The metal dispersion and particle size were calculated from hydrogen adsorption isotherms at 298 K. These nickel catalysts are active in the hydrogenation of benzene (at 443 K). In both series of catalysts no correlation between activities and metal dispersion was found, and turnover frequencies indicate an apparent structure sensitivity of the reaction, likely to be due to the presence of superficial unreduced nickel ions. TEM micrographs reveal that deactivation of catalyst occurs by formation of coke and filaments of carbon separating the nickel particles from the pillared materials.