Effect of method of preparation and calcination temperature on the catalytic activity of NiO/Al2O3 solids

ABSTRACT

Two series of NiO/AL[2]O[3] solids of different composition were prepared by both coprecipitation and impregnation of - delta -AL[2]O[3]. The catalytic activity of various solids, heated in air at 250-300C were determined under the oxidation of CO by O[2] at 250-300C under static conditions and the magnitudes of the catalytic reaction rate constant perunit surface area [k] were calculated for the different catalysts. Both bilk and surface concentration of nickel ions for each solid was determined using atomic absorption spectrometry and ESCA techniques, respectively. The obtained results revealed that, surface concentration of Ni[2+] ions [sni] was much greater than that in the bulk and decreased regularly by increasing the calcination temperature through an inward diffusion of nickel ions into the interior of the grains of the solid. Such activated diffusion was, however, less pronounced for the solids rich in NiO content. The catalysts prepared by coprecipitation, although, measured higher specific surface areas than those obtained by impregnation, yet they exhibited lower specific catalytic activities. This discrepency was explained in terms of actual surface concentration of nickel ions. This was much larger [3-fold] for the solids obtained by impregnation than those prepared by copreciptation and having the same composition. On this basis, the surface concentration of nickel ions played the most important role in determining the catalytic activity of NiO/AL[2]O[3] solids