Gas-Phase Photodegradation of Decane and Methanol on TiO_2: Dynamic Surface Chemistry Characterized by Diffuse Reflectance FTIR

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminated TiO2 surfaces 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 superhydrophilic TiO2 surfaces. Oxidation of decane led to a mixture of surface-bound organics, while oxidation of methanol leads to the formation of surface-bound formic acid

Gas-Phase Photodegradation of Decane and Methanol on TiO

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminated TiO2 surfaces 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 superhydrophilic TiO2 surfaces. Oxidation of decane led to a mixture of surface-bound organics, while oxidation of methanol leads to the formation of surface-bound formic acid

Photocatalytic production of hydrogen on Ni/NiO/KNbO_3/CdS nanocomposites using visible light

The photocatalytic production of H2 from water splitting was demonstrated on Ni/NiO/KNbO3/CdS nanocomposites using visible light irradiation at wavelengths >400 nm in the presence of isopropanol. The inherent photocatalytic activity of bulk-phase CdS was enhanced by combining Q-sized CdS with KNbO3 and Ni deposited on KNbO3. 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 KNbO3 and the reduced states of niobium (e.g., Nb(IV) and Nb(III)) are shown to contribute to enhanced reactivity in the KNbO3 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 KNbO3 surface increases H2 production rates. Other factors that influence the rate of H2 production including the nature of the electron donors and the solution pH were also determined. The Ni/NiO/KNbO3/CdS nanocomposite system appears to be a promising candidate for possible practical applications including the production of H2 under visible light

Applications of Semiconductor Photocatalysis for Both Degradation of Organics and Hydrogen Production

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study illuminated TiO2 surfaces under both vacuum conditions and in the presence of organic molecules (decane and methanol). In the presence of a hole scavenger, electrons are trapped at Ti(III)-OH sites, and a free electrons are generated. These free electrons are seen to decay by either exposure 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 hydrocarbon contaminants is responsible for superhydrophilic TiO2 surfaces. Oxidiation of methanol led to formation of surface bound formic acid. Titanium dioxide was then doped with nitrogen atoms via high temperature treatment with ammonia, toward the goal of developing a catalyst capable of using visible light to degrade organic substrates. Catalyst efficiency was tested by monitoring formate degradation to CO₂ and H₂O under visible light using ion chromatography. However, reduced photocatalytic activity in the UV region, as well as a strong synthesis temperature dependence on catalytic efficiency, was observed. The N-doped TiO₂ surface was probed with diffuse infrared Fourier transform spectroscopy (DRIFTS), leading us to conclude that Ti-N triple bond defect sites control visible light activity and lead to an apparent reduction in overall crystallinity. Visible light photocatalytic H₂ production was then studied. Microporous and mesoporous silicas (Zeolite-Y, Zeolite-L, SBA-15) and niobium oxides (KNbO₃, K₄Nb₆O₁₇) were combined with nanoparticulate CdS particles and Ni to form hybrid photocatalysts that produced H2 from water/ethanol solutions under visible light irradiation. Silica cavity size, which determines CdS particle size, and photocatalytic activity were found to be correlated. Photocatalytic activity was seen to decrease under acidic or basic conditions with an associated negative ionic strength effect. In the niobate catalysts, Ni doping was shown to lead to higher-energy Nb-O bonding states and to compete with Cd for ion exchange sites. XPS analysis indicated loss of Cd²⁺ ion from the metal oxide supports occured during the course of the photochemical reaction, with apparent retention of bound CdS for most catalysts.</p

Visible-Light Photoactivity of Nitrogen-Doped TiO_2: Photo-oxidation of HCO_2H to CO_2 and H_2O

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_2O 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

Oxidative Power of Nitrogen-Doped TiO_2 Photocatalysts under Visible Illumination

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 HCOO^- into CO_2^(•-), or of NH_3OH^+ 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 HCOO^- into 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 HCOO^- in 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 HCOO^- either 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

Photocatalytic hydrogen production with visible light using nanocomposites of CdS and Ni on niobium oxide

Niobium oxides (KNbO_3, K_4Nb_6O_(17)) were combined with nano-particulate CdS and Ni to form composite photocatalysts that are able to produce H_2 gas from water/ethanol solutions under visible light illumination. Ni doping was shown to affect the Nb–O bonding states, and to compete with Cd for ion exchange sites, although the Ni/CdS-doped catalysts still were found to have higher photo-catalytic activity than composite potassium niobates with nanoparticle CdS deposits alone. XPS studies showed the loss of excess Cd^(2+), but not sulfur, during the course of the photoreaction for the KNbO_3 support matrix, but significant loss of both Cd and S for the K_4Nb_6O_(17) support

Photocatalytic Production of Hydrogen from Water with Visible Light Using Hybrid Catalysts of CdS Attached to Microporous and Mesoporous Silicas

Microporous and mesoporous silicas are combined with nanoparticulate CdS particles to form hybrid photocatalysts that produce H_2 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 Cd^(2+) 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

Photocatalytic Production of H_2 on Nanocomposite Catalysts

The photocatalytic production of H_2 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 KNbO_3 (CdS/KNbO_3 and Ni/NiO/KNbO_3/CdS), is investigated. The rate of H_2 production in alcohol/water mixtures and other electron donors at λ ≥ 400 nm is the highest with the hybrid catalyst, Ni/NiO/KNbO_3/CdS with a measured quantum yield, φ, of 8.8%. The relative order of reactivity as a function of catalyst is Ni(0)/NiO/KNbO_3/CdS > Ni(0)/KNbO_3/CdS > KNbO_3/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 H_2 production from water and water−alcohol mixtures are correlated with fluorescent emission spectra and fluorescence lifetimes. Irradiation of Ni/NiO/KNbO_3/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 KNbO_3 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