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 m 2 /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 PtCl 4 . 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.

Stable gold clusters around 1 nm in size have been prepared by evaporation of the metal into an organosilicon polymer solution. A controlled particle growth of the entrapped gold particles was achieved by annealing in helium at ∼410 °C. Above 460 °C the growth rate is dramatically increased, concurrent with the onset of pyrolysis of polysilazane to form a porous solid. X-ray diffraction and simulation calculations using Debye functions of model clusters of different sizes and both crystallographic and noncrystallographic symmetry were used to characterize these particles. The exclusive presence of noncrystallographic (decahedral and icosahedral) multiply twinned particles (MTPs) in the as-prepared material is confirmed by EXAFS. The high precision of the XRD experiments and theoretical data fits gives detailed insight into the thermally induced growth process, during which the MTPs progressively disappear and are replaced by singly twinned fcc particles of larger size. This transition occurs in the range ∼2−3 nm. This analysis requires the introduction of a size-dependent contraction for the Au−Au distance in the order of 1−3% compared to bulk gold. This is discussed in the context of theoretical predictions.

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COMMUNICATIONS [ (Cp"*CoH)2-/t-{q2 :q2-&-/t-[ (CZH5)2Al-CH = CH -Al(C2H5),]}] and [{Cp*(q2-C2H,)CoAl(C zH,J] 2 ] :

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