Amorphous Microporous Titania Modified with Platinum(IV) ChlorideA New Type of Hybrid Photocatalyst for Visible Light Detoxification

Abstract

Amorphous microporous metal oxides of titanium (AMM-Ti) modified with a few percent of a platinum(IV) halide have been prepared by a sol−gel procedure and characterized by various surface analytical methods. By these methods the metal salt is homogeneously distributed in an almost exclusively amorphous powder of high specific surface area (160−200 m2/g) and a pore size of 0.8 nm. From the coordination number of 4.3 and a Pt−Cl distance of 2.28 Å, as calculated from extended X-ray absorption fine structure results, we conclude that the platinum salt is present as PtCl4. The hybrid sample (Pt(IV)/AMM-Ti) catalyzes the photodegradation of 4-chlorophenol with visible light. Apparent disappearance quantum yields decrease from 8.6 × 10-3 at 335 nm to 4.5 × 10-3 at 366 nm and 2.8 × 10-3 at 400 nm; lower values of 1.6 × 10-3 and 1.3 × 10-3 are found at 436 and 546 nm, respectively. The quantum yield at both 366 and 436 nm decreases linearly with the square root of the incident photon flux, suggesting increasing recombination of the photogenerated charge carriers. We postulate that the photocatalytic activity of Pt(IV)/AMM-Ti originates from local excitation of the platinum(IV) chloride chromophore, affording a halogen atom and Pt(III) as intermediate species. Charge-trapping by the titania matrix generates reducing and oxidizing surface sites from which subsequent photodegradation proceeds. Good photostability of Pt(IV)/AMM-Ti is demonstrated by repeated use of the photocatalyst.