Cobalt oxide/γ-alumina catalysts prepared by equilibrium deposition filtration: The influence of the initial cobalt concentration on the structure of the oxide phase and the activity for complete benzene oxidation

Abstract

In the present work we studied the influence of the initial concentration of the impregnating solution used for mounting Co(II) species on the γ-alumina surface by equilibrium deposition filtration method (edf) on the physicochemical properties and the catalytic activity of the “cobalt oxide”/γ-alumina catalysts. The complete oxidation of benzene has been used as a model reaction. Two series of catalysts (edf-X-A and edf-X-B) of varying Co content (X: up to 21 wt.% Co) were prepared using the above-mentioned method and tested at various temperatures in the range 200–300 °C using a fixed-bed reactor. In the first series (A) various Co loadings were obtained by varying the initial Co(II) concentration of the impregnating solution. In the second series (B) the corresponding Co loadings were obtained by using the impregnating solution used for the preparation of the catalyst of A series with the maximum Co(II) content and varying the impregnation time. All catalysts were characterized using various techniques, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), X-ray powder analysis (XRD), nitrogen adsorption (BET) and temperature-programmed reduction (TPR). It was found that the initial Co(II) concentration of the impregnating solution used for depositing the corresponding species on the γ-alumina surface by edf influences the catalytic activity of the “cobalt oxide”/γ-alumina catalysts with respect to the complete oxidation of benzene. The increase of the initial Co(II) concentration of the impregnating solution brings about a change in the composition of the deposited phase formed in the impregnation step by decreasing the ratio “Co(II) surface inner-sphere complexes/surface Co(II) precipitates”. Upon calcination, the Co(II) surface inner-sphere complexes are transformed to well-dispersed “cobalt oxide” phase strongly interacting with the support surface while the surface Co(II) precipitates are transformed to Co 3O 4 crystallites loosely interacting with the support surface. The former phase is responsible for the relatively high dispersion of Co observed in the A series of catalysts but it is rather inactive, while the latter exhibits lower dispersion but higher activity. Thus, the edf catalysts prepared using high initial Co(II) concentration exhibited higher catalytic activity than the corresponding ones prepared using low initial Co(II) concentration.