Jean-Marie Herrmann

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.

Active Agents in Heterogeneous Photocatalysis: Atomic Oxygen Species vs. OH^· Radicals: Related Quantum Yields

After a general survey of the fundamental characteristics of heterogeneous photocatalysis, the present article classifies the ensemble of reactions into three major categories: i) mild oxidations, ii) total oxidations, and iii) reactions involving hydrogen. Depending on the presence or absence of H2O, the active oxidizing species will be either a dissociated neutral oxygen species, denoted as O*, which is present in anhydrous systems and responsible for selective mild oxidation reactions, or an OH. radical, formed in the presence of H2O and responsible for totally degradative oxidation reactions. The existence of O* species is substantiated by photoconductivity measurements, oxygen-isotope exchange, and reactions in which oxygen-free NO is the oxidizing agent. The influence of the five main parameters that govern kinetics experiments i) the mass of the catalysts, ii) the wavelength, iii) the concentrations or partial pressures of reactants, iv) the temperature, and v) the radiant flux is examined to determine the best conditions for obtaining the optimum photocatalytic quantum yield (PQY), the definition of which is based on the quantum yield given in [1] for photochemistry.

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.

Photocatalytic Oxidations At Room Temperature in Various Media

Various inorganic compounds, such as CO, SO 2- 3 , l − , Br − and NH 3 , are photocatalytically oxidized at room temperature. The photocatalytic oxidation of hydrocarbons yields aldehydes and ketones, either in the gas phase or in the neat-liquid phase with a selectivity that depends on the molecular structure. For instance, cyclohexane is mainly transformed into cyclohexanone, and alkyltoluenes are selectively converted into the corresponding alkyltolualdehydes. Among various semiconductor oxides (TiO 2 , ZrO 2 , ZnO, CeO 2 , SnO 2 , Sb 2 O 4 , V 2 O 5 ), titania was generally the most active, whereas V 2 O 5 was totally inactive. As expected, for a given oxide, the activity depends on the particular specimen. From photoconductivity and oxygen isotope exchange measurements, it was inferred that the photoactivated oxygen species is atomic in gas phase reactions. In aqueous medium, the photoproduced holes react with OH − groups thus forming strongly oxidizing OH − radicals as shown by the primary products of the oxidation of monochlorophenols. These oxidations could be of interest for the synthesis of fine chemicals or for the removal of pollutants.