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Distinct photocatalytic performance was observed when Ta 3 N 5 was synthesized from commercially available Ta 2 O 5 or from Ta 2 O 5 prepared from TaCl 5 via the sol−gel route. With respect to photocatalytic O 2 evolution with Ag + as a sacrificial reagent, the Ta 3 N 5 produced from commercial Ta 2 O 5 exhibited higher activity than the Ta 3 N 5 produced via the sol−gel route. When the Ta 3 N 5 photocatalysts were decorated with Pt nanoparticles in a similar manner, the Ta 3 N 5 from the sol−gel route exhibited higher photocatalytic hydrogen evolution activity from a 10% aqueous methanol solution than Ta 3 N 5 prepared from commercial Ta 2 O 5 where no hydrogen can be detected. Detailed surface and bulk characterizations were conducted to obtain fundamental insight into the resulting photocatalytic activities. The characterization techniques, including XRD, elemental analysis, Raman spectroscopy, UV−vis spectroscopy, and surface-area measurements, revealed only negligible differences between these two photocatalysts. Our thorough characterization of the surface properties demonstrated that the very thin outermost layer of Ta 3 N 5 , with a thickness of a few nanometers, consists of either the reduced state of tantalum (TaN) or an amorphous phase. The extent of this surface layer was likely dependent on the nature of precursor oxide surfaces. DFT calculations based on partially oxidized Ta 3 N 4.83 O 0.17 and N deficient Ta 3 N 4.83 consisting of reduced Ta species well described the optoelectrochemical properties obtained from the experiments. Electrochemical and Mott−Schottky analyses demonstrated that the surface layer drastically affects the energetic picture at the semiconductor−electrolyte interface, which can consequently affect the photocatalytic performance. Chemical etching of the surface of Ta 3 N 5 particles to remove this surface layer unites the photocatalytic properties with the photocatalytic performance of these two materials. Mott−Schottky plots of these chemically etched Ta 3 N 5 materials exhibited similar characteristics. This result suggests that the surface layer (1−2 nm) determines the electrochemical interface, which explains the different photocatalytic performances of these two materials.

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Positioning of absolute energy levels and the quantitative description of occupied levels obtained for TiO2 nanopowders, combining UPS and UV-Vis spectroscopies.

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Experimental. Solids, characterizations and pretreatmentsIn the present study, the sulfate free and sulfated TiO 2 solids are P25 from Degussa (55 m 2 /g) and DT51 (80 m 2 /g) from Millenium Inorganic Chemical respectively which have been (a) used as supports of NH 3 -SCR catalysts and (b) characterized considering their a = 18 torr) on TiO 2 -P25 and Al 2 O 3 have been ascribed by Kakeuchi et al. 15 to d-H and Hbond IR bands respectively of polymeric H 2 O chains. It must be noted that the positions of the d-H IR bands, 36-37 are in the same wavenumber range than the OH groups of the metal oxides and the ν 1 and ν 3 IR bands of isolated H 2 O species, 11-17 leading to difficulties in the interpretation of the IR spectra of adsorbed H 2 O species.

Methylene Blue (MB) has been chosen as a model molecule to evaluate the impact of inorganic salts, present in textile waste waters, on the adsorption properties and on the photocatalytic efficiency ofTiO2. NoOH∘radical scavenging by anions such asNO3−,Cl−,SO42−,PO43−, andCO32−was observed at neutral and basic pH, while this phenomenon can be suggested at acidic pH for some anions except carbonate anions which are totally neutralized and/or eliminated asCO2in these conditions. The decrease in the rate MB photocatalytic degradation in the presence of inorganic salts was shown to be due to the formation of an inorganic salt layer at the surface ofTiO2, which inhibits the approach of MB molecules. The correlation between the amount of MB adsorbed and the rate of its photocatalytic degradation, whatever the nature of the salt, its concentration and the pH of the solution, indicates (i) that photocatalysis occurs at the surface and not in the solution and (ii) thatOH−ions added at basic pH do not participate to the increase in the photocatalytic efficiency by inducing an increase inOH∘formation. They increase the surface density in adsorption sitesTiO−. The effect of various salts is similar on various titania samples of industrial origin (MillenniumTiO2PC 500, PC 50, and Degussa P 25). It is however more important on Millennium PC 10 probably because of its smaller surface area.

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