Cobalt–molybdenum–alumina hydrodesulphurisation catalysts. Part II. Incorporation of cobalt(II) and molybdenum(VI) into γ-alumina

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Abstract
The incorporation of cobalt(II) and molybdenum(VI) into γ-alumina from aqueous solutions and the magnetic and spectroscopic properties of γ-alumina containing cobalt(II) and molybdenum(VI) have been studied. At low concentrations cobalt(II) is adsorbed as an octahedral aquo-species which on dehydration at 130° enters tetrahedral sites of the alumina. At higher concentrations cobalt(II) is incorporated mainly by occlusion of the aqueous phase in the alumina pores. Occluded cobalt(II) remains octahedral after dehydration. On calcining cobalt–alumina samples in air, octahedral cobalt(II) is partially oxidised to cobalt(III), but tetrahedral cobalt(II) is not oxidised. On oxidation the magnetic moments of the samples decrease and a new u.v. band at 20,000–30,000 cm.–1 appears. Molybdenum(VI) is adsorbed from solutions of ammonium paramolybdate mainly as the tetrahedral MoO4 2– ion. In the presence of molybdenum(VI), the incorporation of cobalt(II) and the proportion of octahedral cobalt(II) both increase. In the presence of cobalt(II), the incorporation of molybdenum(VI) increases. In the presence of molybdenum(VI), aerial oxidation of octahedral cobalt(II) does not occur. There is an interaction between cobalt(II) in the solution and/or at the alumina surface. The relevance of the results to the preparation of the catalyst is discussed.

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