An in situIR spectroscopic investigation of adsorption of hexa- and penta-cyanoferrates to metal oxides from aqueous solution

Abstract

The adsorption of some hexa- and penta-cyanoferrate ions to several metal oxides from aqueous solutions has been investigated by in situ IR spectroscopy. Thin TiO₂, ZrO₂, Al₂O₃ and Ta₂O₅ particle films were prepared on ZnSe and Ge 45° attenuated total internal reflection prisms and CaF₂ transmission IR windows by drying of aqueous sols. The Fe(II) adsorbates, [Fe(CN)₆]⁴⁻ and [Fe(CN)₅NH₃]³⁻, were both found to coordinate to the metal oxides. [Fe(CN)₆]⁴⁻ binds ia an ambidentate –CN– ligand, while [Fe(CN)₅NH₃]³⁻ binds in a bidentate fashion with displacement of the –NH₃ ligand from the Fe(II) coordination sphere. Adsorption of the Fe(III) adsorbates: [Fe(CN)₆]³⁻, [Fe(CN)₅NH₃]²⁻ and [Fe(CN)₅H₂O]²⁻, to metal oxides prepared on ZnSe was found to involve: (i) electrostatically adsorbed species, (ii) the formation of Zn²⁺-adsorbate complexes on the prism surface and (iii) reduction of the Fe(III) adsorbates at photo-generated surface reduction centres forming adsorbed Fe(II) products. Adsorption of the Fe(III) adsorbates to metal oxides prepared on Ge prisms was also found to involve only surface complexation of an Fe(II) product following reduction ia Ge, while adsorption to films prepared on CaF₂ showed only electrostatically adsorbed species.