Résumé
4-benzenethiol-functionalized High Surface Area Graphite powder was prepared and decorated with bimetallic Cu 100-x Rh x nanoparticles (NPs) to serve as electrocatalysts for the reduction of nitrates. In the first step, the HSAG powder was grafted with in situ generated diazonium compounds from 4-aminothiophenol (ATP), in acidic medium using NaNO 2 for the diazotization process. The surface composition was tuned using different initial quantities of ATP. The surface XPS-determined S/C atomic ratio was found to increase stepwise with the initial amine quantity. In a second step, the grafted and untreated HSAG powders were decorated with Cu 100-x Rh x NPs by a wet chemical method and the elemental composition of the end composites assessed by EDS-SEM and ICP, whereas TEM and EDS-TEM served to characterize the NPs morphology and their composition at the nanometer scale. In all cases, the NP size was invariably found to be ~1.7 nm but with a size distribution becoming narrower under increasing grafting rate and the global composition enriched in copper. Voltammetry was performed with a cavity microelectrode to evaluate the electrocatalytic performances of the composites for nitrate reduction. Increasing diazonium grafting led to a progressive reduction of the peak current intensity and a shift of the peak potentials towards cathodic values. Maximum intensity was obtained for 0.005 µmol of diazonium salt per mg of HSAG, 2 with a gain of 40 % in comparison to the best untreated sample. This improvement and a change of the voltammogram characteristics after grafting seem to result from modifications of the local composition at the level of NPs that differs from the global composition. This work conclusively shows that diazonium surface modification is important not only to attach electrocatalytic NPs to carbon supports but also to provide narrower size distribution of the electrocatalysts together with finely tuned catalytic properties.