Effect of support and calcination on the properties of cobalt catalysts prepared by gas phase deposition

Abstract

The effect of support and dispersion on the hydrogen adsorption properties and the toluene hydrogenation activity of cobalt catalysts were studied. The reducibility of the catalysts was also studied. The metal loading and the treatment of the catalysts (calcination) were varied. The catalysts were prepared on silica and alumina supports by atomic layer epitaxy (ALE) using cobalt acetylacetonate precursors (Co(acac) 3, Co(acac) 2). The cobalt loading was increased by repeating the precursor addition and air calcination steps up to five times, which gave samples with 5–20 wt.% Co on silica and with 3–10 wt.% Co on alumina. Two sets of catalysts were prepared, one where air calcination was performed after each precursor addition step and another where calcination was omitted after the last precursor addition step. The reducibility properties were studied by temperature programmed reduction (TPR) and oxygen titration. Calcination was found to produce silicates and aluminates. The formation of aluminates was not as severe as the formation of silicates. Dispersion and hydrogen adsorption properties were studied by static chemisorption measurements and by temperature programmed desorption (TPD). The dispersion was higher on the alumina supported catalysts than on corresponding silica supported samples. Chemisorption of hydrogen on these catalysts was shown to be activated. Toluene hydrogenation was performed in a transient mode by increasing the temperature from 100 to 200°C. A reaction rate maximum was observed regardless of support or metal loading between 100 and 110°C. No obvious dependence of dispersion or support on the intrinsic activity was found. Therefore, the total surface area of metallic cobalt was the determining factor for the overall catalytic activity.