Adsorption of cobalt species on the interface, which is developed between aqueous solution and metal oxides used for the preparation of supported catalysts: a critical review

In the present review article, we present the efforts done so far for elucidating the mechanism of adsorption of the Co(II) species, mainly Co(H₂O)₆²⁺, on the interfacial region developed between metal oxide particles, used as catalytic supports, and aqueous electrolytic solutions. Specifically, we present: (i) the principal modes of deposition of the transition metal ionic species (TMIS) on the surface of oxidic supports related with the various methodologies used for the preparation of the supported catalysts; (ii) the state of the art concerning the general aspects of the adsorption mechanisms of the TMIS on the aforementioned interfacial region; and (iii) the works reported so far dealing with the adsorption of the Co(II) species on the surface of γ-Al₂O₃ (γ-alumina), -Al₂O₃ (-alumina), TiO₂ (rutile), and SiO₂ (silica).

It was concluded that the mechanism of adsorption depends on two main factors: on the Co(II) surface concentration and on the nature of the support surface. It seems that, generally, the mechanism changes progressively along the Co(II) surface concentration from the deposition of monodentate–mononuclear inner sphere complexes, weakly evidenced in too low values of the Co(II) surface concentration, to multidentate, multinuclear inner sphere surface complexes at relatively low Co(II) surface concentrations, and then into surface Co(OH)₂-like, eventually mixed precipitates, at relatively high Co(II) surface concentrations but at pH values lower than those required for bulk precipitation. In all cases, Co(II) forms surface species with Co(II) in octahedral symmetry.

However, the exact Co(II) surface concentration values, in which the abovementioned two transitions (concerning the deposited phase) take place, depends on the kind of the support. Thus, SiO₂ favors the formation of the Co(OH)₂-like precipitates even at relatively low Co(II) surface concentrations. In contrast, TiO₂ favors the formation of mononuclear or oligonuclear surface complexes. Finally, alumina, which exhibits the maximum adsorption capacity, favors the formation of highly defected Co(OH)₂-like precipitates, probably mixed Co–Al precipitates.

The exact local structure of the inner sphere Co(II) surface complexes, formed by exchanging the H₂O ligands with surface oxygens, has been already approached but only for the surface planes of the -Al₂O₃ and rutile monocrystals. This structure remains up to now rather unclear for the polycrystalline oxides used as catalytic supports.