Influence of metal-support interactions on the stability of Ni/SiO₂ catalysts during cyclic oxidation-reduction treatments

Ni/SiO₂ catalysts prepared by impregnation and deposition-precipitation have been subjected to cyclic treatments in hydrogen at 500 °C and oxygen at 450 °C to test their resistance to sintering. The activity of the catalysts in benzene hydrogenation was tested after each cycle. The resulting behaviour was found to depend upon the method of preparation and whether the initial treatment of the precursor was reducing or oxidizing. For a catalyst prepared by impregnation and reduced initially, a strong reduction in catalytic activity was recorded as the number of cycles was increased; this behaviour is ascribed to facile sintering which occurs in the absence of an interaction between metal particles and the support. When the same precursor was calcined initially large NiO crystallites were formed on the external surface of the support; subsequent reduction-oxidation cycles acted to change their texture, making them more irregular or porous and resulting in an increased metallic surface area and catalytic activity. Catalysts prepared by deposition-precipitation were very resistant to sintering owing to strong interactions between the metal and support; at the precursor stage, a nickel hydrosilicate was formed which had a turbostratic morphology. Direct reduction of this compound gave rise to finely divided particles of metallic nickel trapped between the SiO₂ support which retained its turbostratic morphology. Calcination of this precursor caused some nickel to enter the support, further anchoring the active phase and rendering it very resistant to sintering.