Nitrogen doping was recently shown to extend the absorptivity of TiO 2 photocatalysts into the visible. We find that N-doped TiO 2 materials fail, however, to catalyze the oxidation of HCOOinto CO 2 •-, or of NH 3 -OH + into NO 3 -, under visible illumination. By N-doping anatase at ambient or high temperature according to the literature we obtained yellow powders A and H, respectively, that absorb up to ∼520 nm. Aqueous H suspensions (pH ∼ 6, 1 atm O 2 ) photocatalyze the oxidation of HCOOinto CO 2 •radicals at λ ∼ 330 nm, but the quantum yield of CO 2 •formation at λ > 400 nm remains below ∼2 × 10 -5 and is probably zero. A is similarly inert toward HCOOin the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO 2 by visible photons are unable to oxidize HCOOeither by direct means or via intermediate species produced in the oxidation of water or the catalyst. Reports of the bleaching of methylene blue (MB) on N-doped TiO 2 , which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by MB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.

Microporous and mesoporous silicas are combined with nanoparticulate CdS particles to form hybrid photocatalysts that produce H2 from water/ethanol solutions under visible light irradiation. Catalyst structures are characterized by XRD and SEM. All hybrid materials are active photocatalysts for water splitting, and the order of photoactivity is found to be zeolite-Y > SBA-15 > zeolite-L. Silica cavity size, which determines, in part, CdS particle size, and photocatalytic activity are found to be correlated. Photocatalytic activity is seen to decrease under acidic or basic conditions with associated negative ionic strength effects. In addition, XPS analysis indicates loss of ion-exchanged and Cd2+ ion from the silicate supports occurs during the course of the photochemical reaction in solution with the complete retention of preformed and surface-bound CdS.

The photocatalytic production of H2 in water with visible light using nanocomposite catalysts, which include quantum-sized (Q-sized) CdS, CdS nanoparticles embedded in zeolite cavities (CdS/zeolite), and CdS quantum dots (Q-CdS) deposited on KNbO3 (CdS/KNbO3 and Ni/NiO/KNbO3/CdS), is investigated. The rate of H2 production in alcohol/water mixtures and other electron donors at λ ≥ 400 nm is the highest with the hybrid catalyst, Ni/NiO/KNbO3/CdS with a measured quantum yield, φ, of 8.8%. The relative order of reactivity as a function of catalyst is Ni(0)/NiO/KNbO3/CdS > Ni(0)/KNbO3/CdS > KNbO3/CdS > CdS/NaY-zeolite > CdS/TiY-zeolite > CdS, while the reactivity order with respect to the array of electron donors is 2-propanol > ethanol > methanol > sulfite > sulfide. In addition, the rates of H2 production from water and water−alcohol mixtures are correlated with fluorescent emission spectra and fluorescence lifetimes. Irradiation of Ni/NiO/KNbO3/CdS proceeds via the partial reduction of Cd(II) to Cd(0) on the surface of CdS. The coupling of Ni(0)/NiO and Cd(0) on the surface of KNbO3 appears to have some of the chemical principles of a Ni/Cd battery at high overvoltages. Evidence for the formation of nickel hydride as an important intermediate has been obtained.

3. Enhanced activity is most likely due to effective charge separation of photogenerated electrons and holes in CdS that is achieved by electron injection into the conduction band of KNbO 3 and the reduced states of niobium (e.g., Nb(IV) and Nb(III)) are shown to contribute to enhanced reactivity in the KNbO 3 composites by mediating effective electron transfer to bound protons. We also observed that the efficient attachment of Q-size CdS and the deposition of nickel on the KNbO 3 surface increases H 2 production rates. Other factors that influence the rate of H 2 production including the nature of the electron donors and the solution pH were also determined. The Ni/NiO/KNbO 3 /CdS nanocomposite system appears to be a promising candidate for possible practical applications including the production of H 2 under visible light.

Nitrogen-doped TiO 2 was synthesized by high-temperature exposure of TiO 2 to ammonia. The catalytic efficiency was tested by monitoring the photocatalytic degradation of formate (HCO 2 -) to CO 2 and H 2 O under visible-light irradiation. The N-doped TiO 2 powders were found to be active for the degradation of formic acid under visible light. However, the catalytic efficiency of the N-doped TiO 2 under UV light alone is less than that of the pure TiO 2 starting material. FTIR evidence indicates that the visible-light-active N-doped TiO 2 has defect sites in the form of Ti-N triple bonds and that the increase of these sites leads to a loss of crystallinity that accounts for the reduced photocatalytic activity under UV irradiation. An optimal synthesis temperature of 550°C was determined as a balance point between catalyst crystallinity and the presence of defect sites that absorb visible-light photons.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminatedTiO2surfaces under both vacuum conditions, and in the presence of organic molecules (decane and methanol). In the presence of hole scavengers, electrons are trapped at Ti(III)–OH sites, and free electrons are generated. These free electrons are seen to decay by exposure either to oxygen or to heat; in the case of heating, reinjection of holes into the lattice by loss of sorbed hole scavenger leads to a decrease in Ti(III)–OH centers. Decane adsorption experiments lend support to the theory that removal of surficial hydrocarbon contaminants is responsible for superhydrophilicTiO2surfaces. Oxidation of decane led to a mixture of surface-bound organics, while oxidation of methanol leads to the formation of surface-bound formic acid.

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