J.M. Herrmann

Preparation and characterization of TiO2 photocatalysts supported on various rigid supports (glass, quartz and stainless steel). Comparative studies of photocatalytic activity in water purification

Abstract In the present study TiO 2 has been supported on several rigid substrates. Deposition on glass and quartz was carried out by a dip coating procedure and the deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. The materials were then tested for the photocatalytic degradation of malic acid. For this reaction, the sample supported on quartz showed the highest catalytic activity. The photocatalytic activity pattern as a function of the nature of the support (fused silica, glass, stainless steel) followed the decreasing order: TiO 2 /quartz > TiO 2 /steel ≈ TiO 2 /glassaphotolysis. This decline in activity has been correlated with the presence of cationic impurities (Si 4+ , Na + , Cr 3+ , Fe 3+ ) in the layer as a consequence of the necessary thermal treatments to improve the cohesion of the titania layer and its adhesion onto the support.

Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz

Abstract In the present study, TiO2 and Ag-TiO2 catalysts have been supported in the form of thin layers by a dip-coating procedure on quartz substrate. The resulting materials have been characterized by SEM/EDX, XRD, XPS and UV-vis absorption spectroscopy. The immobilized catalysts were tested in the photocatalytic degradation of malic acid. For this reaction, the presence of metallic silver does not produce an intrinsic increase in photocatalytic activity in comparison with pure titania. The apparent increase observed in activity is principally due to the increase in the exposed surface due to the textural characteristics of the Ag-TiO2 layer in comparison with TiO2. In addition, the presence of metallic silver always produces an increase in activity in comparison with oxidized Ag+ ions. This can be explained by the increase in the electron-hole pair-separation efficiency induced by trapping of electrons by metallic silver.

Heterogeneous photocatalysis: In situ photoconductivity study of TiO2 during oxidation of isobutane into acetone

Using a special flow-reactor, the electrical photoconductivity σ of TiO2 and the photocatalytic activity for isobutane oxidation at room temperature over this catalyst have been simultaneously measured. The formation rates in acetone (partial oxidation) and CO2 (partial and total oxidation) as well as σ are proportional to light intensity, provided the photon energy is greater than the band gap of anatase. Kinetic orders have been determined. A reaction mechanism is suggested. In the pressure range from 100 to 500 Torr for O2 and i-C4H10 and under steady-state conditions, the electron-hole recombination is considered negligible; the dependence of σ upon PO2−1 is explained by the capture of photoproduced electrons by O2ads, species in equilibrium with gaseous O2, whereas the independence of the formation rate in acetone upon PO2 is interpreted by the neutralization and activation of Oads− species (adsorbed at saturation) by photoproduced holes; isobutane reacts in a neutral adsorbed state with the activated oxygen species thus obtained.

Semiconductivity study of ceria-supported nickel related to its methanation catalytic activity

The electrical conductivity study of a ceria-supported nickel catalyst (d≈2.1 nm) showed the presence of anionic vacancies created in ceria by reduction in hydrogen. At reaction temperature, this gas is able to spill over the surface of the oxide. It was also inferred that electrons can migrate from the support to the metal. The catalytic behaviour of Ni/CeO2 was compared with that of a Ni/SiO2 homologue. Ni/CeO2 has a higher methanation activity but the selectivity with respect to methane depends strongly on the reaction temperature and the hydrogen pressure. At low temperature (533 K) and under low PH2, a substantial production of carbon dioxide is observed. If the temperature is increased to 573 K and if hydrogen is in large excess (H2/CO=17), Ni/CeO2 becomes highly active and selective with respect to methane with good stability of its activity. From the results of electrical conductivity and the variations in the selectivity, it was inferred that the active sites are formed by a surface nickel atom at the metal—support interface in close vicinity to an anionic vacancy on the support. Carbon monoxide chemisorbs on the nickel atom, where it dissociates, the oxygen atom being trapped by the anionic vacancy. For a low H2/CO ratio, this oxygen atom is removed from the surface by reaction with a carbon monoxide molecule, thus explaining the high amount of carbon dioxide produced. For a high H2/CO ratio, anionic vacancies are preferentially regenerated by hydrogen, which forms water and prevents the formation of carbon dioxide at the metal—support interface.

Nature Of Surface Sites In The Selective Oxide Hydrogenation Of Propane Over V-Mg-O Catalysts

Abstract In order to explain the difference in catalytic behavior between the three VMgO reference phases, (orthovanadate Mg 3 V 2 O 8 , pyrovanadate α–Mg 2 V 2 O 7 , metavanadate β–MgV 2 O 6 ), in propane oxidehydrogenation, temperature-programmed desorption (TPD) of the probe NO, X-ray photoelectron spectroscopy (XPS) of the catalysts pretreated “in situ”, Electron Spin Resonnance and Electrical Conductivity measurements, under static conditions, have been performed. All these techniques can explain the highest selectivity into propene for the magnesium pyrovanadate by the stabilization of V 4+ ions observed for this phase which was associated to the formation of oxygen vacancies.

Oxygen gas sensing behavior of nanocrystalline tin oxide prepared by the gas phase condensation method

Nanocrystalline SnO2 powders have been synthesized by the gas phase condensation method. The synthesis consists of the evaporation of commercial SnO powders in a He atmosphere with calcination at 573 K of the collected ultrafine powder. The material obtained shows a fractal texture and contains small amounts of SnO in addition to the SnO2 cassiterite phase. At a temperature up to 573 K the surface interacts with gaseous oxygen and the electrical conductivity (σ) follows a law of the type: σ = Po2.−14. The response of σ varies reversibly with the oxygen pressure and rather rapidly for a powder sample. This has been explained by a combination of a high surface area with very good electrical interconnections between grains. This last effect has been attributed to the nanocrystalline character of the particles, their high degree of coalescence, and to the presence of small amounts of quasi-metallic SnO in the present material. The exponential variation of σ with temperature is evidence for the semiconductor character of the SnO2 nanocrystalline powder and allowed the determination of the enthalphy of formation of anionic vacancies. The electrical behavior of the stannic oxide sample has been compared with a commercial (Aldrich) SnO2 powder and the differences observed have been discussed.

Dynamics of the oxidative dehydrogenation of propane over VMgO catalysts studied by in situ electrical conductivity and step transients

The oxidative dehydrogenation of propane (ODHP) over redox VMgO catalysts was studied under non-steady state conditions using in-situ electrical conductivity and step-transient kinetic experiments. It was found that higher yields to propene can be obtained compared to steady state operation. The reaction of propane over these catalysts occurs by a fast reduction of the surface VO34- polymers and slow reduction of the bulk Mg3V2O8. Important kinetic parameters for the bulk diffusion of oxygen necessary for a possible scale up of transient operation can be obtained from the modelling of the conductivity and kinetic transient experiments.

Photoassisted deposition of rhodium on platinum/titania samples as a method of preparing bimetallic catalysts

Abstract Rh-Pt/TiO2 samples obtained by UV illumination at room temperature in the absence of oxygen of suspensions in aqueous Rh3+ solutions of particulate 2.5 wt.-% Pt/TiO2 (previously prepared by impregnation and reduction in hydrogen), were examined by X-ray photoelectron spectroscopy and transmission and scanning transmission electron microscopy. Depending on the medium (water, 2-propanol-water, ammonia solution buffered at pH 11 with ammonium chloride or nitrate), rhodium particles (ca. 1 wt.-% loading) of various mean size and degree of reduction were found. Interestingly, rhodium was deposited on titania in neutral medium and on platinum in basic medium. These results, which demonstrate that different types of bimetallic catalysts can be prepared by photoassisted deposition, are discussed in terms of the separation of photoproduced charges and surface charge.

Mild Oxidation of Cyclic C6-C10 Hydrocarbons in Liquid Phase at Room Temperature by Heterogeneous Photocatalysis

Abstract The mild oxidation of various liquid hydrocarbons (cyclohexane, methyl- and 1,4-dimethylcyclohexane, decalin, cyclohexene and tetralin) by O2 (or air) over UV-illuminated TiO2 at room temperature has been studied in a static slurry reactor using wavelengths λ > 300 nm. Some photochemical transformations (i.e. in the absence of titania) are only observed with the unsaturated hydrocarbons, but remains limited compared with the photocatalytic transformation. The main photocatalytic products are the corresponding ketones : cyclohexanone and its methylated derivatives, 2-decalone, and 1-tetralone. Cyclohexene is also oxidized into cyclohexenone mainly, but some cyclohexene oxide is also formed. The influence of various parameters (mass of catalyst, initial concentration in acetonitrile used as an inert solvent, temperature, radiant flux, wavelength) has been studied, as well as the modification of titania by a metal deposit (0.5

Heterogeneous Photocatalysis as a Method of Water Decontamination: Degradation of 2-, 3- and 4-Chlorobenzoic Acids Over Illuminated Tio2 at Room Temperature

Abstract Chlorobenzoic acids(CBA) are completely mineralized into CO 2 and Cl-over UV-illuminated TiO 2 in water according to the following reactivity order : 4-CBA〈2-CBA〈3-CBA. The identification of various intermediates of 2-CBA shows the decarboxylation and the progressive hydroxylation of the aromatic ring. This study confirms the potentialities of photocatalysis for water detoxification.