The electrocatalytic hydrogenation (ECH) of phenol has been studied using palladium supported on gamma-alumina (10% Pd-Al2O3) catalysts. The catalyst powders were suspended in aqueous supporting electrolyte solutions containing methanol and short-chain aliphatic acids (acetic acid, propionic acid, or butyric acid) and were dynamically circulated through a reticulated vitreous carbon cathode. The efficiency of the hydrogenation process was measured as a function of the total electrolytic charge and was compared for different types of supporting electrolyte and for various solvent compositions. Our results show that these experimental parameters strongly affect the overall ECH efficiency of phenol. The ECH efficiency and yields vary inversely with the quantity of methanol present in the electrolytic solutions, whereas the presence of aliphatic carboxylic acids increased the ECH efficiency in proportion to the chain length of the specific acids employed. In all cases, ECH efficiency was directly correlated with the adsorption properties of phenol onto the Pd-alumina catalyst in the studied electrolyte solution, as measured independently using dynamic adsorption isotherms. It is shown that the alumina surface binds the aliphatic acids via the carboxylate terminations and transforms the catalyst into an organically functionalized material. Temperature-programmed mass spectrometry analysis and diffuse-reflectance infrared spectroscopy measurements confirm that the organic acids are stably bound to the alumina surface below 200 degrees C, with coverages that are independent of the acid chain length. These reproducibly functionalized alumina surfaces control the adsorption/desorption equilibrium of the target phenol molecules and allow us to prepare new electrocatalytic materials to enhance the efficiency of the ECH process. The in situ grafting of specific aliphatic acids on general purpose Pd-alumina catalysts offers a new and flexible mechanism to control the ECH process to enhance the selectivity, efficiency, and yields according to the properties of the specific target molecule.

Original and versatile new materials for the electrocatalytic hydrogenation of organic compounds were designed. The materials consist of reticulated glassy carbon cathode electrodes in which the modified silica particles (average diameter 40-63 microm) were dynamically circulated. The modification of the silica surface is 2-fold. First, the silica is surface-modified using organic functions such as -OSi(CH3)2(CH2)3OCH2CH-(OH)(CH)2OH (SiO2-Diol), -OSi(CH3)2(CH2)7CH3 (SiO2-C8), and -OSi(CH3)2C6H5 (SiO2-Phenyl). Second, these silica particles were further modified by vapor phase deposition of nickel nanoaggregates (used as sites for hydrogen atoms and electric contacts with the electrode material), which does not destroy or alter the organic functionalization as demonstrated by thermogravimetric analysis-mass spectrometry and Raman, diffuse reflectance IR Fourier transform, and Auger electron spectroscopies. The new concept stems from relative adsorption and desorption properties of the organic molecules and their corresponding reduced products into the organic functionalization of the surface-modified silica. In this work, the electrocatalytic hydrogenation cyclohexanone was used to test the concept. The performances (amount of cyclohexanol vs time of generated electrolysis at constant current) are measured and compared for the various bonded organic functions of the silica surface listed above, along with the unmodified silica particles (but still containing nickel nanoaggregates) and the presence or absence of methanol in solution. The measurements of the adsorption isotherms of cyclohexanone, and the calculations of the interaction energies (MM3 force field) between the chemisorbed organic functions and the substrates, corroborate perfectly the electrocatalysis results.

Henry's law constants were determined for cyclohexanone adsorption onto aluminium oxide – hydroxide powders by dynamic isotherm measurements using the HPLC method. The key parameters that control cyclohexanone adsorption were established. Further, it is suggested that the amount of cyclohexanone adsorbed onto aluminium oxide – hydroxide compounds (AOHC) is of paramount importance for its electrocatalytic hydrogenation in the presence of composite nickel–AOCH powders.Key words: adsorption isotherm, electrocatalytic hydrogenation (ECH), cyclohexanone, aluminum oxide – hydroxide powders.

The electrocatalytic hydrogenation of catechol was carried out in aqueous media in different pH ranges with Rh-Al 2 O 3 powder catalyst. The reactions were conducted in an H-cell used as a dynamic cell, with a reticulated vitreous carbon electrode in contact with the catalyst powder as the working electrode. It was shown that the final product is 1,2-cyclohexanediol (cis and trans isomers) and that several intermediates are detected depending on the pH of the solution. Different media, from pH 5 to 13, were studied. One of the intermediates is 1,2-cyclohexanedione, detected at all pH values. The other is 2-hydroxycyclohexan-1-one, only observed at pH ≤ 7. To determine the mechanism of the reactions involved, the electrocatalytic hydrogenation of these intermediates to form the final 1,2-cyclohexanediol product was also conducted, and their UV spectroscopy and cyclic voltammetry data recorded. The influence of the nature of the solution was screened by measuring the Henry constant of each molecule.Résumé : L'hydrogénation électrocatalytique du catéchol a été menée en milieu aqueux à différents pH sur des poudres de Rh-Al 2 O 3 comme catalyseur. Les réactions ont été réalisées dans une cellule en H utilisée en mode dynamique avec des électrodes de carbone vitreux réticulé en contact avec le catalyseur comme électrodes de travail. Il a été montré que le produit final est le 1,2-cyclohexanediol (isomers cis et trans) et que plusieurs intermédiaires sont détectés dépendam-ment du pH de la solution. Divers milieux de pH 5 à 13 ont été étudiés. Un des intermédiaires est la 1,2-cyclohexanedione, formée quel que soit le pH. L'autre est la 2-hydroxycyclohexan-1-one, uniquement observée à pH ≤ 7. Afin de détermi-ner le mécanisme des réactions impliquées, l'hydrogénation électrocatalytique de ces intermédiaires en 1,2-cyclohexanediol a été réalisée ainsi que la spectroscopie UV-visible et la voltampérométrie cyclique. L'influence de la nature du milieu a été évaluée en mesurant les constantes de Henry pour chacune de ces molécules.Mots clés : hydrogénation électrocatalytique du catéchol, mécanisme de réduction dépendant du pH, spectroscopie UVvis, voltampérométrie cyclique, constantes de Henry.Brisach-Wittmeyer et al. 1647