Jean Disdier

Effect of chromium doping on the electrical and catalytic properties of powder titania under UV and visible illumination

Abstract Although Cr-doped (0.85 at%) TiO2 absorbs in the visible region, it becomes a photoconductor only through band-gap illumination and the doping causes a considerable decrease in photoconductance. Its activity for oxidations (oxalic acid, propene, 2-propanol) and for oxygen isotope exchange is nil under visible illumination and is 25–1000 times diminished under UV light. This is attributed to an increase in electron—hole recombination at the Cr3+ ion sites. Conversely, similar doping might be envisaged to enhance the light stability of TiO2-containing materials if the colorimetric properties were maintained.

Photocatalytic deposition of silver on powder titania: Consequences for the recovery of silver

The effects of various parameters on the photoassisted reduction of silver ions to metallic deposits on powder titanium dioxide has been studied. The initial deposition rate (i) varied with the starting concentration of Ag+ ions according to a Langmuir-Hinshelwood mechanism, (ii) was almost independent of the temperature around 300 K, and (iii) was proportional to the radiant flux (300 nm < λ < 400 nm) of photons absorbable by TiO2, at least up to a value of 7.6 × 1015 photons s−1 cm−2. For the same illumination conditions, an apparent initial quantum yield of 0.16 was calculated at room temperature in the case of complete coverage of the surface by Ag+ ions. Transmission electron microscopy revealed that silver initially formed particles between 3 and 8 nm in diameter. For larger amounts of silver deposited and longer illumination times, large crystallites (up to 400 nm) were also observed. This prevented the photosensitive surface from being substantially covered and, for instance, a silver-to-titania mass ratio of ca. 2.4 was reached without a decrease in the deposition rate. This phenomenon, as well as the removal of silver ions down to the detection limit, supports the possible use of this method for the recovery of silver from dilute aqueous solutions. In this connection, preliminary experiments showed that silver was selectively extracted from equimolar solutions of Ag+ and Cu2+ ions (cf. electrolytic baths) and that it was also recovered in the presence of thiosulfate ions (cf. photographic fixing baths).

Comparison of various titania samples of industrial origin in the solar photocatalytic detoxification of water containing 4-chlorophenol

The degradation of 4-chlorophenol (4-CP) was used as a model reaction to investigate the photocatalytic properties of different industrial TiO2 catalysts and to compare their efficiencies in the treatment of contaminated waters. Parallel experiments have been performed using either artificial UV-light in a batch photoreactor in laboratory experiments or solar energy in the flow reactor of the pilot plant at Plataforma Solar de Almeria (PSA) in Spain. Depending on the kinetic criteria chosen for comparison ((i) initial rate of pollutant disappearance; (ii) amounts of intermediate products present in solution at a given time; (iii) time necessary to obtain total mineralization), different classifications of photocatalyst activities were observed. Among various physical characteristics such as particle size, structure or active site density, which may intervene on the photocatalytic activity, the influence of the surface area appeared of prime importance. For lower surface area catalysts, there is a decrease in the readsorption rate of intermediate products and consequently in the overall photodegradation rate. By contrast, for higher surface area catalysts, it was observed a lower final rate of total organic carbon (TOC) disappearance because of a very low coverage in pollutant which favors electron–hole recombination. The comparison of the kinetic results for decontaminating water at PSA and in laboratory experiments indicated the same kinetic order (apparent first order) for 4-CP disappearance and the same apparent quantum yield. However, fewer intermediate products and a faster TOC disappearance were observed in the solar pilot reactor at PSA, which, in addition, worked with a smaller optimum concentration of suspended titania. This was ascribed to the design of the photoreactor.

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.

Platinum/titanium dioxide catalysts. A photoconductivity study of electron transfer from the ultraviolet-illuminated support to the metal and of the influence of hydrogen

Photoconductivity measurements have been carried out for a series of powder Pt/TiO2 catalysts containing from 0.05 to 10 wt% Pt as well homodispersed particles (mean diameter ca. 2 nm). The presence of platinum, at least for contents ca. 1 wt%, caused a decrease in the anatase photoconductance σeq at equilibrium under vacuum and this decrease was more pronounced for the highest loadings (5 and 10 wt%). In addition, differences in the times required to reach the steady state were observed, and an attenuation of the u.v. light flux lowered σeq of the Pt-loaded samples, whereas it did not affect that of TiO2 alone. These phenomena can be rationalized in terms of electron transfer from the titania to the platinum. Accordingly, the existence of such a withdrawal by small metal crystallites is substantiated. Conversely, in H2 the Pt deposits decreased the resistance of TiO2, illuminated or not, and this is tentatively attributed to a migration of adsorbed hydrogen atoms from Pt to TiO2 where they form OH– ions and release electrons.

Titania-supported bimetallic catalyst synthesis by photocatalytic codeposition at ambient temperature : preparation and characterization of Pt-Rh, Ag-Rh, and Pt-Pd couples

Abstract We have examined the possibilities of preparing some bimetallic catalysts supported on TiO2 (Degussa P-25) by UV illumination (λ > 300 run, radiant flux, 60 MW cm−2), at room temperature for 24 h, of aqueous suspensions containing 500 mg of this oxide, and cations of the desired metals in amounts corresponding to metal loadings between 0.5 and 2.5 wt% if complete reduction were achieved. In the case of the H2PtC16RhCl3 couple, Rh deposition almost did not take place. For the mixture AgNO3RhC13 at pH 2.6, Rh deposition was substantial but still incomplete. In contrast, Rh deposition was complete in aqueous ammonia (pH 11). TEM and STEM-EDX analysis of the solid recovered in this latter case showed that Rh formed 1- to 3-nn particles and was also found in 8- to 30-nm particles containing ca. 80 wt% Ag (with respect to Rh) presumably as a result of its deposition on more rapidly crystallized Ag. Solids prepared from H2PtC16 and Pd(NO3)2 were studied in more detail because both metals were completely deposited, at least at levels ≤1 wt%. They were heterogeneous, since Pd and Pt formed ca. 1- or 3-nm crystallites, respectively; however, most of the crystallites were found to contain both metals with a high and varied proportion of Pd. An XPS Ar+-sputtering depth profile of the I wt% Pt-1 wt% Pd/TiO2 sample showed a similar behavior for Pt and Pd, which might indicate the formation of a solid solution in some particles. As this formation is attributed to similar reduction rates for both metallic salts, it is inferred that alloys are obtained only in cases where the salts and conditions can be chosen to fulfill this requirement. This limits the capabilities of the photoassisted codeposition method for preparing effective bimetallic supported catalysts.

Oxygen species ionosorbed on powder photocatalyst oxides from room-temperature photoconductivity as a function of oxygen pressure

Using a cell designed to study the electronic interactions of gases with illuminated powders, the photoconductance, σ, of a series of oxides (TiO2, ZrO2, V2O5, ZnO, SnO2, Sb2O4, CeO2 and WO3) has been measured at equilibrium under various oxygen pressures PO2(from 7 × 102 down to 0.7 Pa) at room temperature in an attempt to obtain information on the nature of negative oxygen species adsorbed on these illuminated surfaces. For the majority of the samples, the conductance of the non-illuminated part was negligible compared with that of the illuminated layer. In a log–log plot, the slopes of σ against PO2 were very close to –1, –½ or 0. A slope of –1 (found only for TiO2 on the higher-pressure side) and a slope of –½(observed for ZrO2, ZnO and, for part of the pressure range examined, for TiO2, Sb2O4 and CeO2) were interpreted as indicating that O–2 and O– species, respectively, controlled the equilibrium between the free electrons and the gas. SnO2 and WO3 were characterized by a poor relative response to illumination, so that σ variations with PO2 were masked; consequently the measurements failed in determining with certainty the type of adsorbed oxygen species, although the σ sensitivity to initial O2 admission showed the occurrence of ionosorption. The V2O5 sample did not behave as a photosemiconductor. This is consistent with the photocatalytic activities of these samples in propene oxidation (very small for SnO2 and WO3, nil for V2O5). However, no simple correlation exists between the photoconductance variations thus measured and the photocatalytic properties, which shows that the surface densities of electrons and of ionosorbed oxygen species are not the only factors to consider for oxidation reactions.

Measuring the effect of photocatalytic purifiers on indoor air hydrocarbons and carbonyl pollutants.

Abstract Laboratory tests of photocatalytic air purifiers are usually performed with a single pollutant, in the parts per million by volume domain and at airflow rates [H11349]0.1 m3/hr. Clearly, it is necessary to probe photocatalytic materials and apparatuses under real conditions or conditions closely mimicking reality. Photocatalytic prototypes were placed in an ordinary room. To collect hydrocarbons over a shorter period (15 min) than with adsorbent-containing cartridges, solid-phase microextraction (SPME) was used. Typically, concentrations in substituted benzene hydro-carbons and tetrachloroethene were decreased to 20–35% of initial values; toluene and m- [H11001] p-xylene concentrations dropped to 2–6 parts per billion by volume, and o-xylene and benzene concentrations were still lower. In the absence of appropriate, commercialized SPME fibers, carbonyl compounds (both formed and destroyed by photocatalysis) were extracted using cartridges containing 2,4- dinitrophenylhydrazine-coated silica. The concentration ranges (in parts per billion by volume) were shifted to higher values in treated air: from 9–15.5 to 12.5–18 for methanal, from 1.5–3 to 8–11.5 for ethanal, and from 4.5–19 to 8–26.5 for propanone with the prototype used; these unprecedented results do not exclude using photo-catalysis to treat air, but they illustrate that improvement is needed. Because these tests are time-consuming, preliminary tests are useful; results obtained with a 225-L closed-loop, airtight, photocatalytic reactor with an external turbine enabling the ambient air inside the reactor to be circulated through the purifier device at 15–450 m3/hr flow rates are reported.

Semiconductive and redox properties ofV2O5/TiO2catalysts

The dc electrical conductivities of a series of V 2 O 5 /TiO 2 samples were measured at 250°C and observed to be sensitive to the V 2 O 5 loadings: 1, 2, 3, 4 and 6 V 2 O 5 wt.% corresponding to 0.9, 1.8, 2.65, 3.5 and 5.3 V atom%. Up to 3% V 2 O 5 loading (half monolayer), the electrical conductivity, slightly but constantly, increased whereas, for 4% vanadium oxide loading, the electrical conductivity increased dramatically by over one order of magnitude. It was concluded that, for low vanadia loadings, well dispersed fractions of a monolayer could be obtained with a better dispersion than that of Eurocat ones, previously studied with the same technique. It appears that the electrical conductivity provides a sensitive method to determine the total amount of V 5+ ions incorporated in titania during the preparative calcination. In situ measurements of the electrical conductivity during oxygen and methanol cycles suggest that methanol reduces both vanadia and titania. Reduction by the introduction of methanol strongly increases the electrical conductivity of the samples, whereas oxygen decreases the conductivity to the original value. The reproducible conductivity levels indicate the reversible redox process occurring during the exposure of the samples to methanol (reduction) and to oxygen (oxidation). No quantitative relationship exists, between the amounts of dissolved V 5+ ions detected by the variations in electrical conductivity and the methanol oxidation activity, for this series of V 2 O 5 /TiO 2 samples. TiO 2 appears as a reducible, non-inert support in electronic interaction with supported V 2 O 5 .

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.