Protecting noble metal-sensitive

Improvement in the photocatalytic activity of these materials has been achieved in recent years by different methods including sensitization of the catalyst using dye molecules and doping the catalyst with norrmetals such as nitrogen, carbon, fluoride and iodide, and transition metals (Au, Pt, Ag, and Pd). Hahnemann and Ismail [110] have recently reviewed the recent developments in the syntheses of mesoporous TiOj as active photocatalysts by the surfactant assembly as well as the preparation and characterization of doped mesoporous Ti02 networks with transition metals ions, noble metals, and nonmetal species. Mesoporous titania nanoparticles will play an important role in the environmental protection and the search for renewable and clean technologies. 

By providing a barrier between the substfate and the environment, or by cathodically protecting the substrate, metallic coatings protect the substrate from corrosion. Coatings of chromium, copper, and nickel provide increased wear resistance and good corrosion resistance. However, these noble metals make the combination of the substrate (mostly steel or an aluminum alloy) with the protective layer sensitive to galvanically induced local corrosion. Nonnoble metallic layers such as zinc or cadmium provide good cathodic protection but show poor wear resistance. 

Clusters of noble and easily reducible metals (eg, Ag, Au, Pd) can be obtained by alcoholic reduction of metal ions in presence of polymeric stabilizers. Poly(vinylpyrrolidinone), poly(vinyl alcohol), poly(methylvinyl ether), etc are usually utilized. High quality polymer-protected metal clusters can be obtained at the end of reaction simply by addition of a nonsolvent liquid which causes the nanocomposite separation from the reactive mixture (coprecipitation) (77-79). However, the use of such nanocomposites is limited because of their excessive moisture sensitivity. 

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