Relationship between metal ion adsorption and catalytic properties of carbon-supported nickel catalysts

Abstract

During the preparation of supported metal catalysts, the adsorption of aqueous metal ions is influenced by the intrinsic properties of the support. Any attempt to model this adsorption phenomenon requires at least three elements: the pH-dependent surface charge speciation of the support, the pH-dependent aqueous phase speciation of the precursor ion, and a suitable model to describe the equilibrium exchange reaction. A series of activated carbons derived from a low ash content carbon source was prepared by controlled oxidation using nitric acid. The surface ionization constants for the protonation/de proton ation equilibria of the hydroxyl groups on each carbon were determined assuming a classical Gouy-Chapman diffuse layer. They were found to vary systematically with extent of oxidation. Nickel adsorption data were obtained over a wide range of pH and Ni concentration. Under the conditions of this study, Ni 2+ was the dominant ion. A surface complexation model was employed to test the hypothesis that the model's parameters could be correlated to the results of characterization studies of these Ni/carbon catalysts. Specifically, it was found that the surface stability constant (≈Δ G ads) of the precursor and the point of zero charge of the carbon varied in accordance with results from temperature-programmed reduction and temperature-programmed oxidation. It is proposed that the point of zero charge of the carbon support can serve as a convenient index for the design of carbon-supported metal catalysts with desired catalytic properties.