Amorphous microporous titania modified with platinum(IV) chloride - A new type of hybrid photocatalyst for visible light detoxification

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 amorph ous powder of high specific surface area (160-200 m(2)/g) and a pore size o f 0.8 nm. From the coordination number of 4.3 and a Pt-CI distance of 2.28 Angstrom, as calculated from extended X-ray absorption fine structure resul ts, we conclude that the platinum salt is present as PtCl4. The hybrid samp le (Pt(IV)/AMM-Ti) catalyzes the photodegradation of 4-chlorophenol with vi sible light. Apparent disappearance quantum yields decrease from 8.6 x 10(- 3) at 335 nm to 4.5 x 10(-3) at 366 nm and 2.8 x 10(-3) at 400 nm; lower va lues of 1.6 x 10(-3) and 1.3 x 10(-3) are found at 436 and 546 nm, respecti vely. The quantum yield at both 366 and 436 nm decreases linearly with the square root of the incident photon flux, suggesting increasing recombinatio n of the photogenerated charge carriers. We postulate that the photocatalyt ic activity of Pt(IV)/AMM-Ti originates from local excitation of the platin um(IV) chloride chromophore, affording a halogen atom and Pt(IU) as interme diate species. Charge-trapping by the titania matrix generates reducing and oxidizing surface sites from which subsequent photodegradation proceeds. G ood photostability of Pt(IV)/AMM-Ti is demonstrated by repeated use of the photocatalyst.