François Quentel

Electrochemical hydrogen production in aqueous micellar solution by a diiron benzenedithiolate complex relevant to [FeFe] hydrogenases

The diiron hydrogenase model Fe(2)(bdt)(CO)(6) (1, bdt = benzenedithiolate) was dispersed in aqueous micellar solution prepared from sodium dodecyl sulfate (SDS). Aqueous solution of 1 showed no sign of decomposition when left in contact with air over a period of several days. Current-potential responses recorded at a dropping mercury electrode over pH 7-3 were consistent with reduction of freely diffusing species. Catalysis of proton reduction was observed at pH < 6 with current densities exceeding 0.5 mA cm(-2) at an acid-to-catalyst ratio of 17. Bulk electrolysis at -0.66 V vs. SHE of solution of 1 at pH 3 confirmed the production of hydrogen with a Faradaic efficiency close to 100%. A mechanism involving initial reduction of 1 and subsequent proton-coupled electron transfer is proposed.

Determination of trace amounts of chromium(VI) in water by electrochemical methods

Abstract A sensitive procedure for the determination of chromium(VI) is described. A chelate of chromium with an organic ligand is adsorbed on a hanging mercury drop electrode and the reduction current of the accumulated chelate is measured by differential-pulse voltammetry. The effects of various parameters (pH, ligand concentration, potential and collection time) on the response are discussed. Possible interferences by trace metals and organic matter are considered.

Electrocatalytic hydrogen evolution from neutral water by molecular cobalt tripyridine–diamine complexes

A cobalt complex with a tripyridine-diamine pentadentate ligand was found to be a highly active catalyst for electrochemical H2 production from neutral water, with an activity of 860 mol H2 (mol cat)(-1) h(-1) (cm(2) Hg)(-1) over 60 h CPE experiment at -1.25 V in a pH 7 phosphate buffer solution, without considerable deactivation.

A silver-based metal–organic framework material as a ‘reservoir’ of bactericidal metal ions

The silver-based MOF material Ag3(3-phosphonobenzoate) was evaluated as a bactericidal material. A sustainable release of Ag+, which was quantified by cathodic stripping voltammetry, was responsible for bactericidal activity against the 6 bacterial strains tested.

Natural attenuation processes applying to antimony: A study in the abandoned antimony mine in Goesdorf, Luxembourg

The processes leading to the attenuation of the antimony concentration in the water draining from the abandoned antimony mine in Goesdorf, Luxembourg, have been studied. Antimony has been mined in Goesdorf since Roman times from a stibnite-rich mesothermal vein system hosted in metasedimentary schist. The draining waters have pH values between 7 and 8 because the mineralization itself contains calcite and dolomite. This study combines the identification of minerals in the supergene zone with the application of bulk techniques (e.g., measurement of antimony in the waters of the adit and the creek draining the mine, sediment sequential extractions) over a period of five years. Antimony concentrations in the water that leaves the supergene zone are controlled by the dissolution of stibnite and the subsequent formation of Sb(III) oxides and sulphates. The relative proportions of the main secondary minerals can be qualitatively estimated as follows: 70% valentinite, 15% senarmontite and 12% sulphates (coquandite, klebelsbergite and peretaite). Further antimony attenuation along the adit and the creek that drain the mine waters is due partly to dilution, through mixing with waters that have not been in contact with the ore, and partly to sorption onto amorphous iron and manganese oxides present in the colluvial sediments.

Square-Wave Voltammetry of Molybdenum-Fulvic Acid Complex

The reduction process of molybdenum in the presence of fulvic acids was investigated by square-wave voltammetry (SWV). The influence of the parameters (frequency, amplitude) of the SW on the reduction signal is analyzed according to the theoretical model. The molybdenum-fulvic acid complex is reduced reversibly with adsorption of the reactant.

Photocatalytic Hydrogen Production Using Models of the Iron–Iron Hydrogenase Active Site Dispersed in Micellar Solution

Iron-thiolate complexes of the type [Fe2 (μ-bdt)(CO)6-x P(OMe3 )x ] (bdt=S2 C6 H4 =benzenedithiolate, x≤2) are simplified models of iron-iron hydrogenase enzymes. Recently, we have shown that these water-insoluble organometallic complexes, when included into micelles formed by sodium dodecyl sulfate (SDS), are good catalysts for the electrochemical production of hydrogen in aqueous solutions at pH<6. We herein report that the all-CO derivative [Fe2 (μ-bdt)(CO)6 ] (1), owing to its comparatively low reduction potential, is also a robust molecular catalyst for visible-light-driven production of H2 in aqueous SDS solutions at pH 10.5. Irradiation at λ=455 nm of a system consisting of complex 1, Eosin Y as a sensitizer, and triethylamine as an electron donor produced up to 0.86 mL of H2 in 4.5 h, corresponding to a turnover number of 117 mol of H2 per mol of catalyst. In the presence of a large excess of sensitizer, the production of H2 lasted for more than 30 h, stressing the relative stability of complex 1 under the photocatalytic conditions used herein. Thermodynamic considerations and UV/Vis spectroscopy experiments suggest that the catalytic cycle begins with the photo-driven reduction of complex 1. The reduced intermediate reacts with a proton source to yield iron hydride. Subsequent reduction and protonation steps produce H2 , regenerating the starting complex. As a result, the iron-thiolate complex 1 is a versatile proton reduction catalyst that can utilize either solar or electrical energy inputs, providing a starting point for the construction of noble metal-free molecular systems for renewable H2 production.

Silver-Based Hybrid Materials from meta- or para-Phosphonobenzoic Acid: Influence of the Topology on Silver Release in Water

Three novel silver-based metal-organic frameworks materials, which were synthesized from either 3-phosphono or 4-phosphonobenzoic acid and silver nitrate, are reported. The novel hybrids were synthesized under hydrothermal conditions; the pH of the reaction media was controlled by adding different quantities of urea thereby producing different topologies. Compound 1 (Ag3(4-PO3-C6H4-COO)), synthesized in the presence of urea, exhibits a compact 3D structure in which both phosphonic acid and carboxylic acid functional groups are linked to the silver-based inorganic network. Compound 2 (Ag(4-PO3H-C6H4-COOH)), which was synthesized at lower pH (without urea), has a layered structure in which only the phosphonic acid functional groups from 4-phosphonobenzoic acid moieties are linked to the silver inorganic network; the carboxylic acid groups being engaged in hydrogen bonds. Finally, material 3 (Ag(3-PO3H-C6H4-COOH)) was synthesized from 3-phosphonobenzoic acid and silver nitrate without urea. This material 3 features a layered structure exhibiting carboxylic acid functional groups linked via hydrogen bonds in the interlayer space. After the full characterization of these materials (single X-ray structure, IR, TGA), their ability to release silver salts in aqueous environment was measured. Silver release, determined in aqueous solution by cathodic stripping voltammetry, shows that the silver release capacity of these materials is dependent on the topology of the hybrids. The more compact structure 1 is extremely stable in water with only trace levels of silver ions being detected. On the other hand, compounds 2 and 3, in which only the phosphonic acid functional groups were bonded to the inorganic network, released larger quantities of silver ions into aqueous solution. These results which were compared with the silver release of the previously described compound 4 (Ag6(3-PO3-C6H4-COO)2). The results clearly show that the release capacity of silver-based metal organic framework can be tuned by modifying their topology which, in the present study, is governed by the regio-isomer of the organic precursor and the synthetic conditions under which the hybrids are prepared.

Determination of inorganic and organic selenium species in natural waters by cathodic stripping voltammetry

The determination of the total concentration of a trace element gives only few informations about its toxicity and its bioavailability which depend mainly on the different chemical forms under which it is present. The studies of the chemical speciation is thus particularly important. We propose, in this paper, a selenium speciation model for the studies of natural waters based on Se(IV) voltammetric determination. The separation of the organic and inorganic forms was realized by a rapid ion exchange method and total selenium was determined after a u.v. irradiation.

A Binuclear Iron–Thiolate Catalyst for Electrochemical Hydrogen Production in Aqueous Micellar Solution

The substituted iron-thiolate complex [Fe(2)(μ-bdt)(CO)(4){P(OMe)(3)}(2)] (bdt=benzenedithiolate) is an active catalyst for electrochemical hydrogen production in aqueous sodium dodecyl sulfate solution, with a high apparent rate constant of 4×10(6) M(-1) s(-1). The half-peak potential for catalysis of proton reduction is less negative than -0.6 V versus the standard hydrogen electrode at pH 3. Voltammetric data are consistent with the rate of electrode reaction controlled by diffusion. A mechanism that begins with the rapid protonation of the iron-thiolate catalyst is proposed. The Faradaic efficiency in diluted HCl solutions is close to 100%, but the catalytic activity decayed after about twelve turnovers when electrolysis was carried out in the presence of acetic acid.