Synthesis of Ni/SiO2 catalysts through precipitation of silica-sols: effect of aging and Ca(Ba) additives

The preparation of Ni/SiO2 catalysts by precipitation of silica sols with N H4OH is described. Nickel introduction during precipitation of silica sols did not affect significantly the silica surface area, yielding amorphous Ni /SiO2 catalysts in both calcined and reduced forms. This method of preparat ion results in Ni/SiO2 catalysts showing homogenous distribution of compone nts, surface areas ca. 255 m(2) g(1), and mean size of nickel particles bet ween 4 and 6 nm. The effects of gel aging and the addition of Ca or Ba cati ons on the textural, structural and catalytic properties of Ni/SiO2 catalys ts were studied. Results are interpretated according to reported processes of dissolution-precipitation of silica occurring under basic conditions [R. K. ner, The Chemistry of Silica, Chapters 3 and 4, Wiley, New York, 1979]. Alkaline earth addition to the nickel/silica catalysts decreased the surfac e area to 100-155 m(2) g(-1). For reduced Ni/SiO2-Ba and Ni/SiO2-Ca catalys ts, the X-ray diffraction (XRD) peak of (111) planes of metal nickel crysta ls was identified. Several XRD peaks not corresponding to any Ni, Si and Ba /Ca containing crystalline phase reported in the ICDD database were reveale d. They were assigned to new crystalline phases including Ba and Ca in the second coordination sphere of nickel. Hydrogen adsorption on these new phas es was stronger than on metal nickel. Variation of hydrogen chemisorption p roperties and reaction rate for benzene hydrogenation were observed with al kaline earth addition. (C) 1998 Elsevier Science B.V. All rights reserved.