An in situ IR 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.