Pierre Pichat

Oxidation of oxalic acid in aqueous suspensions of semiconductors illuminated with UV or visible light

The behaviours of various semiconductor powders (TiO2, Fe2O3, ZnO, ZrO2, Sb2O4, CeO2 and WO3) dispersed in oxalic acid solutions were examined either in the dark or under illumination at wavelengths shorter or longer than 400 nm in the presence of oxygen. All the specimens except Sb2O4 and WO3 were degraded in the dark, ZnO being the most unstable. UV illumination increased the oxidation of oxalic acid in the presence of TiO2, ZrO2 and CeO2, and the quantum yields were determined. Visible light enhanced the conversion in the presence of TiO2, Fe2O3 and WO3. Despite its weak absorption, TiO2 was the most reactive, and the two Fe2O3 samples exhibited markedly different activities. Doping of TiO2 with Cr3+ ions (0.86 at.%) substantially decreased its activity in both spectral regions, probably because of increased electron—hole recombination. It is concluded from these results that the surface properties and the presence of recombination centres, which depend on the preparation method, are very important factors in determining the activity of a given oxide. The results of a more detailed study of the effects of oxygen, oxalic acid concentration, pH and temperature performed using the TiO2 sample provided evidence for the photocatalytic character of the oxidation induced by UV light, and a mechanism involving an attack on the CC bond of adsorbed HC2O4− ions by atomic oxygen species activated by photoproduced holes is tentatively suggested.

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.

Photochemical reduction of gold(III) on semiconductor dispersions of TiO2 in the presence of CN− ions: disposal of CN− by treatment with hydrogen peroxide

Abstract The reduction and recovery of gold from samples containing CN− ions have been investigated employing TiO2 semiconductor dispersions irradiated with either UV light (λ ⩾ 210 nm) or AM1 simulated sunlight (λ ⩾ 310 nm). The photoreduction is very efficient at low pH; disposal of cyanide, which acts as a buffer to a pH of about 9.3, was imperative. The degradation of CN− was investigated employing two peroxides, H2O2 and S2O82−, in the presence and absence of light. Treatment of cyanide solutions with H2O2 and UV light irradiation leads to efficient conversion of CN− to NH3 and CO2 (the main products detected). The H2O2 decomposition of CN− also occurs in the dark, but to a lesser extent. The kinetics of the conversion process(es) have been studied; under irradiation, kobs is about ten times the rate of the dark process. A mechanism involving OH· radicals is proposed for the cyanide decomposition by UV light. Cyanate ion, OCN−, is the intermediate in both the dark and the illuminated pathways. Recovery of gold on TiO2 powder, following cyanide oxidation and removal of unreacted H2O2, is favored in acidic aqueous media. The implications of our findings towards practical disposal of cyanide and recovery of gold from actual indusrial waste samples are discussed.

Photocatalytic isotopic exchange between cyclopentane and deuterium over a bifunctional PtTiO2 catalyst

Cyclopentane-deuterium isotopic exchange (CDIE) was performed at −10 °C with a C5H10D2 ratio of ∼-6.4% over an illuminated (300–410 nm) 0.5 wt% Pt/anatase catalyst. A selectivity in C5H9D of ∼-90% was found with a total quantum yield of ∼-1.8%. The photocatalytic nature of CDIE under these conditions was established from these results: (i) no CDIE in the absence of photons of wavelengths shorter than the absorption edge of anatase or when TiO2 was replaced by SiO2, whereas the CDIE observed at higher temperatures (from 0 °C) without uv light mainly produced multiply exchanged cyclopentane, (ii) a linear CDIE rate-illumination time relationship was found, and (iii) an experiment carried out with the same catalyst sample showed that it was capable of exchanging, in successive runs, without decrease in activity, an amount of C5H10 78 times greater than that corresponding to a photoinduced stoichiometric surface reaction. By contrast, CDIE over pure TiO2 was limited and was only observed for hydroxylated specimens pretreated in D2 at 300 °C. It is concluded that (i) CDIE occurred on anatase between weakly adsorbed C5H10 and OD− ions activated by the trapping of photoproduced positive holes, and (ii) Pt, besides attracting the support-free electrons which decreased the recombination of electron-hole pairs, regenerated the titania OD− ions after their isotopic exchange, which involved D2 dissociative adsorption on the metal and subsequent migration of D atoms to the oxide. This constitutes an example of bifunctional heterogeneous photocatalysis.

Spectroscopic study of the cyclotrimerization of acetylene on NiY zeolites. Influence of the location of the Ni2+ ions

Abstract The cyclotrimerization of acetylene on NiY zeolites, containing 10, 14 and 19 Ni 2+ ions/unit cell treated at 200 and 600 °C, has been investigated by means of X-ray diffraction analysis, ir and EPR spectroscopies. It has been shown that this reaction does not alter the populations of Ni 2+ ions in the different zeolitic cages. Comparison of the various samples has shown that their activity depends on the number of dehydrated or partly dehydrated Ni 2+ ions inside the supercages. Cyclotrimerization of propyne can also occur. The effect of some ligands (NO, C 5 H 5 N, ND 3 ) on the cyclotrimerization of acetylene has also been examined. It has been established that the benzene formed does not remain bonded to the Ni 2+ ions. Finally, the reduction of a small amount of Ni 2+ ions into Ni + and Ni 0 species has been shown by EPR and one EPR signal was tentatively assigned to a Ni(C 2 H 2 ) n + species.

Room temperature hydrogen production from aliphatic alcohols over uv-illuminated powder Ni/TiO2 catalysts

Abstract The dependence of the room temperature H 2 production from liquid methanol or propan-1-ol on the degree of reduction of Ni and on the Ni content (0.03–13.8 wt%) of UV ( λ ⩾ 297 nm) illuminated Ni/TiO 2 catalysts has been studied. A complete reduction of Ni was found beneficial and an optimum rate of H 2 production from methanol (quantum yield ∼0.05) was observed for ∼5 wt% Ni. Magnetic measurements indicated that the mean Ni crystallite size varied with the content and was in the 6.5–18 nm range, as compared with ∼2 nm for Pt particles on the same support. At least the highest Ni percentages induced a decrease in the anatase photoconductance under vacuum, which can be attributed to an electron transfer from the semiconductor grains (∼25 nm dia.) to the Ni particles. The optimum in the photocatalytic H 2 production was considered as resulting from a compromise between the Ni catalytic properties and this electron transfer either because of a decrease in the number or strength of the TiO 2 basic sites or an increase in the electron-hole recombination at the Ni particles. Finally, the difference in the optimum metal content for Ni and Pt(0.1–1 wt%) was attributed mainly to the distinct particle sizes.

Modification of the TiO2 Electron Density by Ion Doping or Metal Deposit and Consequences for Photoassisted Reactions

Effects of Pt deposit (as particles 2 nm dia.) or Cr 3+ doping (0.85 at %) on the free electron density ns of anatase powder has been studied. The decreased photoconductance and conductivity (after H 2 -pretreatement in this latter case) under vacuum at 295 K of the Pt/TiO 2 samples compared with naked TiO 2 are attributed to electron transfer from TiO 2 to Pt and the resulting average electron enrichment of the Pt atoms are estimated from photoadsorbed oxygen amounts. The diminished dark conductivity of the Cr-doped sample relative to that of pure TiO 2 is ascribed to the acceptor centres associated with the doping and the decreased ns under illumination is considered to result from electron-hole recombination at these centres. This increased recombination rate caused substantial decreases in the rates of various oxidations photocatalyzed by TiO 2 and in the rate of oxygen isotope exchange (OIE). The rates of alcohol dehydrogenation and cyclopentane-deuterium isotopic exchange over illuminated Pt/TiO 2 were maxima for a Pt content of ca.0.5 wt% (% 1 Pt particle per TiO 2 grain). A similar optimum content was also found for OIE, a reaction for which Pt does not play a catalytic role, provided the samples were not preoxidised. It is inferred that, for low Pt contents, decreased ns reduces the charge recombination in TiO 2 , whereas, for high Pt contents, the recombination at the metal particles progressively cancels this beneficial effect. These results illustrate the impediments encountered when attempting to improve the surface and/or photosensitive properties of photocatalysts by doping or metal deposition.

Hydrogen production by water photoelectrolysis with a powder semiconductor anode

Abstract It has been shown that a photoelectrolysis H cell with a powder semiconductor anode, a platinum collector electrode and a counter electrode, can operate. Hydrogen can be produced from water dissociation by this cell which combines the advantages of a large photosensitive surface as in photocatalysis and a separation of the products as in classical photoelectrolysis.

Phototransformation of O-Xylene over Atmospheric Solid Aerosols in the Presence of O2 and H2O

The photodegradation (300 < λ/nm < 400) of o-xylene over four oxides which are present as aerosols in the atmosphere has been studied. 0rtho-xylene in the gas phase is stable over the SiO2 and Fe2O3 samples used and is oxidised to o-tolualdehyde over Ti02 and, to a much lesser extent, over AI2O3. Poisoning of Ti02 is observed. No reaction occurs when o-tolualdehyde is used as a reactant instead of o-xylene over Ti02. In the presence of liquid water, 2-methyl-benzylalcohol, 2,3-dimethylphenol and 3,4-dimethylphenol are produced (probably because of the formation of OH- radicals on Ti02) in addition to o-tolualdehyde, and all the aromatic compounds progressively disappear, which suggests the mineralization of o-xylene. No product of o-xylene is found when AI203 is used instead of Ti02 in liquid water.

Heterogeneous Photocatalysis: NO Decomposition and Oxidation of Butanols by no Over TiO2 at Room Temperature

Over UV irradiated anatase NO decomposes, producing N 2 and N 2 O. In the presence of an alcohol, such as a butanol, the decomposition is increased and the corresponding aldehyde or ketone is formed. Besides the possibility of using NO as an alternative oxidant, this brings information on the mechanism of photo-catalytic oxidations.