Interactive metal ion–silicon oxidation/reduction processes on fumed silica

Abstract

Fumed silica is shown to represent an active oxidation/reduction site. Arresting results demonstrate that the average oxidation state of silicon, at least at the surface of fumed silica, is +1 in contrast to the assumed value of +4, thus allowing the preparation of silica with desired reduced silicon surface oxidation states without post-synthesis treatment. The nature of this silica interface is demonstrated by preparing Fe/silica and Cu/silica materials with unprecedented control of the transition metal oxidation state. Supported iron or copper catalysts were prepared by contacting anhydrous iron(III) chloride, iron(III) nitrate nonahydrate, or hydrated copper(II) chloride with fumed, amorphous silica (CAB-O-SIL®) in dry methanol at room temperature. Subsequently, the solids were vacuum-dried (∼10⁻³ Torr) at room temperature for two hours. These solids, and in particular the iron-silica interface, were examined by X-ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FTIR) spectroscopy, UV-vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), colour analysis, and water contact angle analysis. No discernible evidence was found that indicated the formation of large crystallites of the transition metals. The products of iron(III) interaction at the interface with amorphous silica were also investigated using phenanthroline complexation to confirm the presence of Fe(II) ions. This body of data showed compelling evidence that a portion of the transition metal ions in contact with the fumed silica were reduced to lower oxidation states while some of the silicon ions were observed to be “oxidized” to higher oxidation states. The ratio of Fe(II) over Fe(III) found by XPS deconvolution for the chloride spectra matches well with theoretical prediction based upon a simple surface reaction between the Fe(III) ions and the lower valent Si ions. The Fe doping was deduced to be more likely at the axial position of the Si–O bond rather than the equatorial. It is remarkable that these observed transitions in the metal ion oxidation states occurred at room temperature. The inherent simplicity of this technique is general to many reducible metal oxides, and thus, its use in preparations may provide a new way of controlling the ratio of various oxidation states of metal elements.