In the past Jean-Marie Herrmann has collaborated on articles with Henri Courbon and Mikhail Kerzhentsev. One of their most recent publications is Effect of chromium doping on the electrical and catalytic properties of powder titania under UV and visible illumination. Which was published in journal Chemical Physics Letters.

More information about Jean-Marie Herrmann research including statistics on their citations can be found on their Copernicus Academic profile page.

Jean-Marie Herrmann's Articles: (17)

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

AbstractAlthough 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.

Metal-support interactions: An in situ electrical conductivity study of Pt/TiO2 catalysts

AbstractAn in situ investigation of the electrical conductivity of Pt/iO2 catalysts (0.5 and 5 wt% Pt) has been carried out under various conditions corresponding to two pretreatments (473 and 773 K in H2), to H2 or O2 adsorptions and to H2O2 titrations at 295 K. To explain the results the following hypotheses have been proposed: (i) in H2, H atoms migrated from Pt onto the O2− sites of TiO2 and released one electron per OH− group formed, and (ii) under vacuum at 295 K an “electron pumping effect” of the metal decreased the conductivity of the reduced oxide. When the catayst was under strong metal-support interaction (SMSI) conditions (viz. after reduction in H2 at 773 K), the metal was markedly enriched in electrons. Accordingly, the particular chemisorption and catalytic properties of Pt under these conditions have been interpreted by the filling of its d orbitals which tend to the d10 configuration. The disappearance of the SMSI state after exposure to O2 has been attributed to the back donation of electrons to the titania by the platinum which thus recovered its normal electronic configuration.

Metal content and temperature effects on the photocatalytic isotopic exchange cyclopentane-deuterium over Pt or NiTiO2 catalysts in the “normal” or “strong metal-support” “interaction” state

AbstractA cyclopentane-deuterium isotopic exchange (CDIE) was carried out in gas phase, in the temperature range 228–273 K, using a static reactor with a fixed-bed layer of illuminated (300–410 nm) catalyst. Over PtTiO2, an optimal temperature of about 260 K was found; at lower temperatures CDIE was limited by the hydrogen desorption from the metal crystallites; above 260 K the rate became dependent on the cyclopentane adsorption on TiO2. At 260 K, an optimal content of about 0.5 wt% Pt, corresponding to ca. 1 Pt particle (homodispersed; ~2 nm diam, regardless of the content) per TiO2 grain, was found for catalysts pretreated at 473 K in D2 and containing 0.1 to 10 wt% Pt; beyond 0.5 wt% Pt, the detrimental recombination of the photoproduced charges at the Pt particles became apparent. PtTiO2 samples pretreated at 773 K in D2 showed the inhibition of CDIE by the so-called “strong metal-support interaction” (SMSI) effect as expected since reversible hydrogen chemisorption is required. Finally, CDIE also took place over NiTiO2 samples where the metal was deposited as much larger particles (~15–18 nm diam); however, its rate was markedly reduced most likely because of the poorer interaction between the two catalyst components. These results are interpreted in terms of electronic exchanges and hydrogen spillover and substantiate the mechanism we proposed previously.

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

AbstractThe 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).

Termodynamic considerations of strong metal-support interaction in a real PtTiO2 catalyst

AbstractThe nature of lattice defects induced by strong metal-support interaction (SMSI) treatment in titania supporting platinum has been studied by in situ electrical conductivity measurements as a function of oxygen pressure. The variations between 425 and 500 °C of the electrical conductivity σ vs PO2 (σ = k · PO2−14) are indicative of the presence of singly ionized vacancies. Their heats of formation are equal to twice the eneration determined by microcalorimetry corresponds, in absolute value, to the heat of filling two anionic vacanciactivation energy of conduction (ΔH = +81 kcal mol-1). The heat of PtTiO2 reges by one molecule of O2. It is substantially lower than the heats of titanium suboxide reoxidation, which rules out the presence of such species on PtTiO2 real catalysts in the SMSI state. Titania has a stoichiometry of TiO1.996, i.e., still close to that of anatase. These anionic vacancies can be filled by the oxygen of water molecules arising from the methanation reaction (ΔH = −23 kcal mol−1), thus explaining the suppression of the SMSI state under working conditions. To each singly ionized anionic vacancy corresponds one free electron of conduction, which increases the Fermi level of titania and makes possible an electron transfer to the metal. The electron transfer can inhibit H2 or CO chemisorption, if these molecules chemisorb as dipoles with a donor character to the metal as evidenced by conductivity.

Pollution abatement in waterPhotocatalytic pollutant removal in water at room temperature: case study of the total degradation of the insecticide fenitrothion (phosphorothioic acid O,O-dimethyl-O-(3-methyl-4-nitro-phenyl) ester)

AbstractThe photocatalytic purification of water containing fenitrothion has been performed in TiO2 aqueous suspensions. The mineralization into CO2, H2PO4−, SO42−, NO3− was achieved. Several intermediates have been identified by HPLC and GC/MS. They illustrate the transformation of PS into PO, the splitting between the phosphorous part and the aromatic moiety and the transfer of methyl or methoxy groups. Formate ions, principally produced from the CH3O and CH3, groups and acetate ions were also detected as intermediate products.

Regular ArticlePhotocatalytic Degradation of Dyes in Water: Case Study of Indigo and of Indigo Carmine

AbstractThe TiO2/UV photocatalytic degradations of indigo and of indigo carmine have been investigated both in aqueous heterogeneous suspensions and in the solid state. In addition to prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able to oxidize the dye, with almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO2, NH+4, NO−3, and SO2−4, respectively. A detailed degradation pathway has been determined by careful identification of intermediate products, in particular, carboxylic acids, whose decarboxylation by photo-Kolbe reactions constitutes the main source of CO2 evolution. The only persistent organic compound was acetic acid, whose degradation required a longer period of time. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted wastewaters in textile industries. The irradiation of titania with visible light did produce a photoinduced decolorization of the dye, probably induced by the breaking of the double-bond conjugation system of the chromophoric group. However, this decolorization was not accompanied by any degradation of the molecule since no loss of total organic carbon (TOC) nor release of inorganic ions were observed. This corresponded to a stoichiometric reaction of an electron transfer from the dye molecule excited in visible irradiation to titania. Because indigo is very poorly soluble (≈2 ppm), it was tentatively degraded in its solid state, mixed with titania in a photocatalytic solid-solid-type reaction. Observation of the decolorization and of the degradation of solid indigo constitutes a surprising and encouraging result for the development of self-cleaning titania-coated objects (glasses, steel, aluminium, metals, walls, etc.) fouled by solid dirt particles.

Effects of methanol, formamide, acetone and acetate ions on phenol disappearance rate and aromatic products in UV-irradiated TIO2 aqueous suspensions

AbstractEffects of methanol, formamide, acetone and acetate ions on the TiO2 photocatalytic degradation of phenol in water at equal initial concentrations (∼1 mmol L−1) have been studied. The amount of phenol adsorbed in the dark and the apparent first-order rate constant of phenol disappearance remained the same within experimental accuracy. The temporal variations in the concentrations of hydroquinone and catechol, the two main primary intermediate products, were not significantly affected either, except by acetate ions which increased these concentrations, particularly that of catechol. In the presence of each of the aliphatic additives, the phenol-phenol coupling products were less numerous and their lifetime was decreased. Traces of products showing reactions between phenol and acetate were found. Interpretations are tentatively presented for these observations. Overall, the absence of marked effects of these aliphatic pollutants upon phenol degradation are of great interest for the photocatalytic treatment of water, since these aliphatics represent intermediate products in the mineralization of organic matter.

The electronic factor and related redox processes in oxidation catalysis

AbstractCatalysis by oxides concern mild selective oxidation as well as total oxidation reactions. Oxides are all semiconductors, either pure, mixed, doped or supported and the redox reactions they catalyse are all connected with their electronic properties. n-Type semiconductors possess anionic vacancies which, when once or twice ionized, are the real oxidative agent of the oxide catalyst. This is illustrated by the Sn–Sb–O system. p-Type oxide semiconductors have positive holes h+ as charge carriers, associated with an excess of anionic oxygen. They are illustrated by the case study of vanadyl-pyrophosphate (VO)2P2O7 able to selectively oxidize butane into maleic anhydride in a single pass. In situ measurements of the electrical conductivity of titania during the catalytic oxidation of CO clearly indicated that the oxidizing agent was O− species. All what is described in thermal catalysis can be transposed to photocatalysis at room temperature, with the simultaneous formation of photoelectrons and holes. In dry medium (gas or liquid phase), selective mild oxidation occur involving a neutral activated species O*, illustrated by the direct oxidation of 4-tert-butyl-toluene into 4-tert-butyl-benzaldehyde.

Photocatalytic degradation of aqueous hydroxy-butandioic acid (malic acid) in contact with powdered and supported titania in water

AbstractMalic acid, a model molecule for partially oxidized products found either in biomass or in various degradative processes, could totally be mineralized by photocatalytic degradation. Laboratory experiments enabled one to determine the identity of 15 intermediates, logically presented in four parallel degradation pathways. The major one is initiated by a decarboxylation (‘photo-Kolbe’) reaction in the α-position with respect to the OH group. Parallel experiments have been performed in a slurry pilot solar photoreactor at PSA (Almeria, Spain), on a large scale (250 l). The same main intermediate products and the same kinetic laws were observed. TiO2 has been used either in slurries or supported on several rigid inert plates. Deposition of titania 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 exhaustively characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. Their photocatalytic activity pattern as a function of the nature of the support followed the decreasing order: TiO2/quartz > TiO2/steel = TiO2/glass > > photolysis. This decline in activity has been correlated with the presence of cationic impurities (Si4+, Na+ for glass and Cr3+, Fe3+ for stainless steel) in the titania layer as a consequence of the thermal treatment necessary to improve the cohesion of the titania layer and its adhesion onto the support.

Photocatalytic degradation pathway of methylene blue in water

AbstractThe TiO2/UV photocatalytic degradation of methylene blue (MB) has been investigated in aqueous heterogeneous suspensions. In addition to a prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able to oxidize the dye, with an almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO2, NH4+, NO3− and SO42−, respectively. A detailed degradation pathway has been determined by a careful identification of intermediate products, in particular aromatics, whose successive hydroxylations lead to the aromatic ring opening. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted waste waters in textile industries.

Solar efficiency of a new deposited titania photocatalyst: chlorophenol, pesticide and dye removal applications

AbstractA specially designed titania photocatalyst was prepared by coating Ahlstrom non-woven paper, used as a flexible photocatalytic support, with Millennium PC500 anatase. At the same time, a new solar photoreactor (STEP) was designed based on the multi-step cascade falling-film principle to ensure good exposure to sunlight and good oxygenation of the effluent to be treated. Several types of reactants were treated: 4-chlorophenol as a model organic pollutant; formetanate, a widely used pesticide in horticulture; a mixture of pesticides used in vineyards; and indigo carmine (IC) and Congo red (CR), which are complex multifunctional dye molecules. Each reaction was performed simultaneously in a solar CPC slurry photoreactor and in the STEP photoreactor under identical solar exposure to better evaluate and validate the results obtained. The STEP solar reactor was found to be as efficient as the CPC for 4-chlorophenol and formetanate total degradation. In contrast, both dyes required longer treatment in STEP experiments. This new system, in which the final tedious filtration can actually be avoided, constitutes a good alternative to slurries.

Kinetics and reactional pathway of Imazapyr photocatalytic degradation Influence of pH and metallic ions

AbstractSome of advanced oxidation processes (AOP) are characterised by a special chemical feature: the ability to use the high reactivity of OH radicals in driving oxidation processes. These radicals are suitable for achieving the complete abatement, even including the mineralization of less reactive pollutants.In this study, a photocatalytic process is used to degrade one herbicide of the imidazolinone family, Imazapyr. It was shown to be photodegraded rapidly and extensively in an aqueous solution. The decline of Imazapyr concentration in the solution followed a first-order kinetics. The apparent first order rate constant was found equal to 0.19 min−1 in distilled water at natural pH 3.8. The smaller activities found at acidic and basic pH were explained by considering the ionisation state of Imazapyr and the charge density of TiO2.The present work dealt with the influence of metal ions like Ni2+ and Cu2+ which are frequently present in agricultural wastewater on the photocatalytic efficiency of TiO2 in the elimination of Imazapyr. A detrimental effect of the presence of metallic species was observed only with samples containing amount of copper and nickel in the presence of TiO2. Several hypotheses were proposed to explain this phenomenon, passivation of TiO2 surface by adsorption of Cu2O and/or Cu0, formation of a complex or recombinaison of the e−/h+ pairs. At higher concentrations of metallic species like Cu2+ and Ni2+, a plateau was reached which could be explained by the photo-Fenton like reaction.In an attempt to understand the basic mechanisms of the degradation of Imazapyr in water by TiO2 photocatalysis, we discussed the primary degradation mechanism on the basis of the experimental results together with molecular orbital calculation of frontier electron density and partial charge.

Characterization of Mo–Sn–O system by means of Raman spectroscopy and electrical conductivity measurements

AbstractMo–Sn–O systems were characterized by Raman spectroscopy and electrical conductivity measurements. The catalysts were obtained from precipitation of SnCl4 by ammonia in the presence of (NH4)2Mo7O24 using four different levels of Mo concentration. The electrical conductivity measurements showed that particles are formed by agglomeration of SnO2 crystals aggregated by polymolybdate. Raman spectroscopy suggested that four-coordinated species are dispersed at the external surface while six-coordinated species are inside the particles. For high Mo concentration (Mo >10%), octahedral coordinated species are also on the surface. Bulk MoO3 oxide was not observed. These results confirm the model previously proposed.

Isokinetic consecutive reactions in heterogeneous catalysis

AbstractThe formal kinetics of a system of irreversible “isokinetic” consecutive reactions (i.e. reactions having the same rate and the same rate constants) has been established, considering a common (pseudo-) first order rate constant k. The characteristics of the kinetics of all intermediates have been determined. The nth intermediate goes through a maximum located at time tnmax=nk and is equal to (an!) (ne)n, a being the initial number of moles of the reactant. For large values of n (n>4), this maximum tends to a/2φn. The selectivity in the nth intermediate has been found equal to ((kt)n−1n!)(n − kt). Other relationships independent of time with dimensionless parameters correlating the partial conversion τn of each intermediate product with the total conversion τ of the initial reactant have also been determined. τn has been found to vary as a function of τ as: τn=(1−τ)n![ln (1(1−τ))]n. The maximum of τn for the first intermediate tends to 1e, whereas for higher values of n, this maximum tends to 1/2φn. A new concept of “molecular exposure”, expressed in moles×second (or in molecules×second), has been defined. It corresponds to the surface area comprised between each curve and the x-axis. It has been demonstrated that it remains constant, as well for the reactant as for all the intermediates formed in isokinetic reactions. It is equal to ak. Some examples from the literature on the catalytic conversion of hydrocarbons such as mono- and dihydrogenations of diolefins and deuterium-alkane isotopic exchange, illustrate and substantiate this kinetic model.

Naphthalene degradation in water by heterogeneous photocatalysis: An investigation of the influence of inorganic anions

AbstractIn a pollution control context, the degradation of naphthalene in water was performed by photocatalysis in UV-irradiated TiO2 suspensions. The influence of physicochemical parameters such as concentration, photonic flux, temperature, pH and mass of catalyst has been investigated. An optimum titania concentration was found equal to 2.5 g L−1. This value is identical to that observed in other liquid phase reactions, either in water or in liquid organic phases, confirming that this value depends on the design of the photoreactor (geometry, texture of the catalyst, optical pathway). The almost nil effect of the pH upon the kinetics indicates that protons do not intervene at the limiting step level. It was also confirmed that the kinetics were slightly accelerated by a limited increase in temperature with a small apparent activation energy of reaction equal to 22 kJ mol−1. The presence of common salts generally found in natural waters was followed versus kinetics, including NaCl to simulate the treatment of seawater. It has been found that small amounts of carbonates strongly inhibit naphthalene adsorption and degradation. Hydrogenocarbonates were also found to inhibit naphthalene adsorption at low concentration, but no inhibition was observed at concentrations below 0.3 mol HCO3− L−1. Surprisingly, addition of sodium chloride makes the initial reaction faster and more selective. It was interpreted as an enhancement of naphthalene adsorption by sodium chloride. The main intermediates of naphthalene photodegradation have been identified by HPLC–DAD and GC–MS. They result from naphthalene hydroxylation and ring-cleavage by action of oxygenated radicals.

Photocatalytic degradation of chlorobenzoic isomers in aqueous suspensions of neat and modified titania

AbstractThe three isomers of chlorobenzoic acid (CBA) have been degraded using photocatalysis. The three pollutants disappeared from water in the following order: 3-CBA < 2-CBA < 4-CBA, the para position being the most reactive. The zeroth kinetic order was interpreted by a Langmuir-Hinshelwood mechanism involving a saturation of the adsorption sites. The chemisorption of CBA molecules involves the carboxylic group linked to the surface with the aromatic ring possibly oriented perpendicularly to the surface. The initial step of CBA disappearance was the decarboxylation of the molecule (photo-Kolbe reaction) with an initial rate of CO2 formation equal to that of the disappearance of CBA. The mass balance could be established for all the final products. All the intermediate products found correspond to successive hydroxylations of the corresponding chlorophenol obtained after the initial photo-Kolbe reaction. They all disappeared within less than 2 h of UV irradiation. In 2-CBA degradation, a transient condensation product (4′-chloro,3,4-dihydroxy-(1,1′)biphenyl) could be identified before its degradation. Modification of titania by doping with Cr3+ ions or by depositing 1 wt.% Pt was detrimental for the activity, the best catalyst remaining neat titania. The present study illustrates the difference in the kinetics of photodegradation of various isomers for one compound.

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