pubs.acs.org/Langmuir © 2009 American Chemical Society
In Situ Control of Phenol Adsorption on Conductive Pd-Fluorine-Doped Tin Dioxide-Supported and Pd-Alumina-Supported Catalysts in Electrocatalytic Hydrogenation Dihourahouni Tountian,† Anne Brisach-Wittmeyer,*,‡ Paul Nkeng,† Gerard Poillerat,† and Hugues Menard‡ † Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie - UMR 7177 CNRS - Universit e de Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg, France, and ‡Laboratoire Sciences de Mat eriaux d’Electrodes, D epartement de Chimie, Universit e de Sherbrooke, 2500 boul. de l’Universit e, Sherbrooke J1K2R1, Qu ebec, Canada
Received April 15, 2009. Revised Manuscript Received July 14, 2009 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.
Introduction Electrocatalytic hydrogenation (ECH) is a heterogeneous catalytic process in which chemisorbed hydrogen, produced in situ by the electroreduction of water, is used to hydrogenate an unsaturated organic molecule. This process is accomplished in four principal steps: production of chemisorbed hydrogen (reaction 1), adsorption of an unsaturated organic molecule (reaction 2), its hydrogenation (reaction 3), and desorption (reaction 4) of the hydrogenated product compound. The hydrogenation step is competing kinetically with the hydrogen evolution reaction (reactions 5 and 6), and the efficiency of the ECH process is determined by this competition. H3 Oþ þ e - þ M a MHads þ H2 O Volmer reaction K ads
YdZ þ A sr fs ðYdZÞads A adsorption
ð1Þ ð2Þ
K des
ðYdZÞads A þ 2MHads a ðYH-ZHÞads A þ 2M ð3Þ
hydrogenation reaction K des
ðYH - ZHÞads A sr fs YH - ZH þ A desorption K ads
ð4Þ
H3 Oþ þ MHads þ e - a M þ H2 þ H2 O ð5Þ
Heyrovsky reaction 2MHads a 2M þ H2 Tafel reaction
ð6Þ
*Corresponding author. E-mail:
[email protected].
Langmuir 2009, 25(18), 11105–11111
Different studies have shown that among these steps the adsorption of the target molecule is the rate-determining step.1-6 They also showed that when unsupported metal catalysts are used the ECH process is very inefficient. Furthermore, two different adsorption sites were identified on the supported catalysts: adsorption sites for chemisorbed hydrogen were found to be located on the metal catalyst whereas the adsorption of unsaturated organic molecules occurred on the support material. The efficiency and the selectivity of the hydrogenation process is dependent on the nature of the supporting matrix.2,8,9 For example, Laplante et al. observed that when Pd-supported catalysts were used in the ECH of phenol the efficiency increased in the following order: Pd/BaCO3
