Improvement of the Platinum Nanoparticles-Carbon Substrate Interaction by Insertion of a Thiophenol Molecular Bridge

International audienceThe effect of thiophenol layer grafted on carbon for platinum catalyst stabilization was studied. The grafted layer was prepared by reduction of 4-thiophenoldiazonium ions in the presence of Vulcan XC72 substrate. The grafted layer was characterized by elemental analysis, thermogravimetric analysis coupled with mass spectrometry, and X-ray photoelectron spectroscopy. Platinum nanoparticles prepared by the "water in oil" microemulsion method were then deposited on modified substrates and bare Vulcan XC72. The platinum stability improvement was characterized by in situ X-ray diffraction and electrochemical aging. These experiments enabled to evidence a lower crystallite growth during heat treatment under hydrogen atmosphere and a lower active surface area loss for platinum particles deposited on modified substrates compared to those deposited on bare Vulcan XC72. This stability improvement can be attributed to a better interaction between platinum particles and carbon substrate due to the thiophenol molecular bridge

Préparation et caractérisations physicochimiques de nanoparticules de platine à distributions de taille et de forme contrôlées

Des nanoparticules de platine non supportées ont été préparées par différentes voies de synthèses colloïdales. Les méthodes mises en œuvre ont permis d obtenir une collection d objets de forme variable dans une gamme de taille nanométrique. Différentes techniques de caractérisations ont été utilisées pour la détermination de paramètres comme la taille et la forme (microscopie) ou encore la structure superficielle (électrochimie) des nanocristaux. Ainsi, il a été démontré que les nanoparticules préparées exposent des fractions variables de surface orientées (111) et (100) et de défauts en accord avec leurs formes. L étude de nanoparticules de platine de formes contrôlées apparait aujourd hui comme le lien manquant entre les catalyseurs industriels et les surfaces monocristallines. La réaction d électro-oxydation du monoxyde de carbone a été étudiée afin d apporter des informations sur les mécanismes mis en jeu à la surface des nanoparticules. Les résultats obtenus mettent clairement en évidence une forte dépendance du signal électrochimique et plus précisément du potentiel d oxydation à la structure superficielle exposée par les nanoparticules. Enfin, la possibilité d un nouveau mécanisme à plus bas potentiel sans adsorption dissociative de l eau a aussi été mise en avant.Unsupported platinum nanoparticles were prepared by using some different colloidal synthesis procedures. These methods lead to the preparation of a collection of object including a wide range of shape in nanometric scale. Size and shape of nanocrystals were determined by microscopy while superficial structure was obtained by electrochemical methods. It was evidenced that platinum nanoparticles expose variable fractions of surface oriented domains (111) or (100) and defect sites in agreement with the observed shapes. Study of well-defined shaped nanoparticles now appears as the missing link between carbon supported platinum nanoparticles and platinum single crystal surfaces. Carbon monoxide electro-oxidation was studied on these samples in order to give some information about the mechanisms occurring at nanoparticles surface. Results point out a strong effect of the superficial structure of platinum particles on the voltammetric signal and particularly the oxidation potential. Finally, another oxidation mechanism not involving water dissociative adsorption is supposed to occur at lower potential.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles.

International audienceThe oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle.

International audienceAn understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously

Selective Syntheses and Electrochemical Characterization of Platinum Nanocubes and Nanotetrahedrons/Octahedrons

International audiencePlatinum nanocubes and tetrahedrons/octahedrons were synthesized by different methods using different surfactants (tetradecyltrimethylammonium bromide (TTAB) and sodium polyacrylate). Transmission electron microscopy (TEM) investigations show high selectivity in shape and low size dispersion. However, the small nuclei observed at the surface of some nanocubes synthesized using the TTAB method is not yet clarified. The presence of such small particles could be explained by a double nucleation or different growth processes which might occur during synthesis. Electrochemical characterizations indicated that particles exhibited ca. 60% of oriented surface domains. Cyclic voltammetry and adatom adsorption displayed a very good agreement with TEM results

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation

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Conductive Polymer Grafting Platinum Nanoparticles as Efficient Catalysts for the Oxygen Reduction Reaction: Influence of the Polymer Structure

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Electro-oxidation of CO_chem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle

An understanding of the oxidation of chemisorbed CO (CO_chem) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO_chem oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO_chem preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO_chem oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO_chem oxidative stripping unambiguously