Study of the scale formation mechanism on gold modified with an alkanethiol monolayer.

Scaling is a problem in many industrial processes. To control and minimize it, it is important to understand the dynamics of the scale formation. In this paper, the scale formation was examined on two kinds of gold surfaces. One was a pure metallic gold surface, and the other was a gold surface modified with an alkanethiol self-assembled monolayer. A series of surface characterization experiments were performed to ensure a good understanding of the gold-thiol bond stability in a caustic solution.

Adsorption enthalpy determination on silica-supported metallic nanoparticles.

A new specific method to measure adsorption enthalpy on supported metallic nanoparticles has been developed. This method is based on gas chromatography measurements, and it allows the calculation of adsorption enthalpies on metallic nanoparticles while neglecting the effect of the supporting particle. In this paper, we discuss the specific case of the adsorption of benzene on the surface of silica-supported gold nanoparticles. The results show a good correlation with similar values found in the literature.

In situ control of phenol adsorption on conductive Pd-fluorine-doped tin dioxide-supported and Pd-alumina-supported catalysts in electrocatalytic hydrogenation.

In the context of the electrocatalytic hydrogenation (ECH) process of unsaturated organic molecules, we have shown using infrared spectroscopy and water contact angle measurements that catalysts powders made of palladium on conductive tin dioxide (10% Pd/SnO2:F) and on alumina (10% Pd/Al2O3) are functionalized with organic chains when they were dipped in supporting electrolyte aqueous solutions containing different carboxylic acids. The carboxylic acids are bound to the supports (SnO2:F and Al2O3) through either the carboxyl or carboxylate groups. The measurement of contact angles confirmed that the support surface is functionalized by the carboxylic acids but also indicated the hydrophobic or hydrophilic character of the resultant surface. With these functionalized catalysts, the effectiveness of electrocatalytic hydrogenation of phenol could be modulated by controlling the adsorption of phenol. The adsorption depends mainly on the functionalization agent (carboxylic acid) and to a lesser extent on the identity of the support material (SnO2:F or Al2O3). Because adsorption is the step that induces the selectivity of the ECH process, controlling this phenomenon by functionalizing the catalyst support in situ is promising for obtaining molecules of choice.

Determination of the surface area of Pd and nanometric Au aggregates supported on a micrometric solid support by thiol adsorption and GC-MS.

A new sensitive and specific method to measure gold and palladium surface areas using alkanethiol adsorption coupled with analysis by gas chromatography with mass spectrometry detection has been developed. The effectiveness of the method was tested with metallic samples having a known surface area. The results have also been compared with BET specific surface area measurements. The results obtained with both methods show a good correlation.

Modification of the surface adsorption properties of alumina-supported Pd catalysts for the electrocatalytic hydrogenation of phenol.

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.

Electrocrystallization of rhodium clusters on thiolate-covered polycrystalline gold.

This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.

Rational design of original materials for the electrocatalytic hydrogenation reactions: concept, preparation, characterization, and theoretical analysis.

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.