New findings on CO electrooxidation at platinum nanoparticle surfaces

Different unsupported platinum nanoparticles are synthesized via water-in-oil microemulsion and colloidal methods (PAA). TEM, adatom adsorption and cyclic voltammetry measurements give very coherent results concerning the surface structure of the crystallites. The combination of these results with CO stripping experiments leads us to assign the oxidation peak multiplicity to surface structure rather than to pure size effect. Keywords: Oriented platinum nanoparticles, CO electrooxidation, Cyclic voltammetr

Colloidal Syntheses of Shape- and Size-Controlled Pt Nanoparticles for Electrocatalysis

International audienceDifferent colloidal synthesis methods of platinum nanoparticles with controlled sizes and shapes that are relevant for electrocatalysis studies are reviewed. Four main methods, i.e., water in oil microemulsion (w/o) method, polyacrylate (PA) method, tetradecyltrimethylammonium bromide (TTAB) method, and ethylene glycol method, are used to synthesize platinum nanoparticles. The PA method allowed us to synthesize reproducibly nanocubes, nanooctahedrons, or nanocuboctahedrons/truncated nanooctahedrons with size between 8 and 10 nm, the TTAB method led to the synthesis of nanocubes of about 10 nm, and the w/o method allowed the synthesis of spherical particles of about 3 nm. All these samples could be cleaned for further electrochemical characterization of their surface structure by hydrogen underpotential deposition and by spontaneous deposition and oxidation of bismuth and germanium, leading to a quantitative determination of the (100) and (111) surface domains. The samples prepared by the ethylene glycol method, in the presence or not of polyvinylpyrrolydone as surfactant, were size or shape controlled, but did not allow the electrochemical characterization of their surface due to remaining of strongly adsorbed organic species even after cleaning steps

Formation de coke à partir de propène sur silicalite

Sur silicalite et zéolithe MFI acide, l'adsorption du propène a été suivie en microbalance sur une gamme de température variant de 25 à 450 °C et pour une pression d'alcène de 97 kPa. L'analyse de la phase irréversiblement adsorbée (coke) montre que, malgré la faible acidité de la silicalite, dès 25 °C le propène subit des réactions d'oligomérisation. La quantité maximale de coke est obtenue entre 100 et 150 °C. Pour ces températures, des oligomères avec un nombre de carbone non multiple de trois sont présents dans les pores de la zéolithe, des réactions d'oligomérisation et de craquage sont responsables de la formation de ces composés. Dans ces conditions, on peut estimer qu'environ 10 % de coke entraîne le complet blockage des pores de la silicalite. À plus haute température, des composés aromatiques et polyaromatiques sont formés par réactions successives (oligomérisation, craquage, cyclisation, transfert d'hydrogène). La présence des composés polyaromatiques est responsable de la forte désactivation de l'adsorbant par un rapide bloquage de pores. Sur zéolithe acide (Si/Al = 40), les réactions de formation de coke sont plus rapides mais les molécules formées sont similaires à celles identifiées dans les pores de la silicalite

Electrochemical Characterization and Stability of Ag_<i>x</i>Pt_1–<i>x</i>/Pt(111) Surface Alloys

As part of an extensive study of the performance of structurally well-defined Ag_<i>x</i>Pt_1–<i>x</i>/Pt­(111) monolayer surface alloys in the O₂ electroreduction we have systematically investigated the electrochemical properties and the stability of these electrodes in 0.5 M H₂SO₄ under mass controlled conditions in a flow cell setup, varying the surface Ag content over a wide range. The surface alloys were prepared and structurally characterized on an atomic scale by scanning tunneling microscopy (STM) under UHV conditions, and a transfer system allowed electrochemical measurements without intermediate contact to air. Cyclic voltammetry (CV) measurements showed distinct changes in the hydrogen and (bi)­sulfate sorption behavior of the surface alloys with increasing Ag surface content. The electrochemical stability of the electrode surfaces was tested by CV in 0.5 M sulfuric acid supporting electrolyte with a stepwise increase of the potential limit up to 0.95 V (vs RHE). CVs as well as STM measurements revealed that surface alloys with Ag contents up to 50% are stable in the electrochemical environment under these conditions, while for higher surface Ag contents dissolution of Ag sets in. The modifications in sorption behavior and stability are compared with recent findings for adsorption on similar surfaces under UHV conditions and related calculations and discussed in a coherent picture

Oxygen reduction on structurally well defined, bimetallic PtRu surfaces:monolayer PtxRu1-x/Ru(0001) surface alloys versus Pt film covered Ru(0001)

The electrocatalytic activity of different, structurally well defined bimetallic PtRu surfaces in the oxygen reduction reaction was investigated by a combination of scanning tunnelling microscopy and electrochemical measurements performed under controlled mass transport conditions in a flow cell. We compare the effect of pseudomorphic Pt cover layers, mimicking the situation in a core-shell Pt/Ru nanoparticle, and of mixed PtxRu1-x monolayer surface alloys, reflecting the situation in an alloyed nanoparticle. The results unambiguously demonstrate that these bimetallic surfaces can reach activities well in excess of that of Pt(111), both for the film surfaces and the surface alloys, by optimizing the Pt surface content (surface alloys) or the Pt film thickness (film surfaces). The results are compared with simulated kinetic current-potential profiles based on existent density functional theory calculations (Greeley and Norskov, J Phys Chem C 113:4932, 2009; Lischka et al., Electrochim Acta 52:2219, 2007) revealing very good agreement in trends. Potential and limits of this approach are discussed

Improvement of toluene catalytic combustion by addition of cesium in copper exchanged zeolites

HY and HMFI zeolites exchanged with copper and cesium have been studied for the catalytic combustion of toluene (800 ppm) with air. The catalysts activity has been analyzed by comparison of light-off curves and in both Cu zeolites, the addition of Cs leads to a decrease of the light-off temperature by 50 °C. Temperature-programmed reduction (H2-TPR) and EPR studies have evidenced clear differences in the characteristics of the copper species both in the presence and absence of Cs co-cations. These differences account for the nature of the active centers in the Cu zeolites for the toluene oxidation. The position and geometry of the copper ions in the zeolite matrix are of great significance for the redox behavior and activity for toluene oxidation. In both MFI and FAU structures, the bulky Cs co-cations are located in the more accessible main zeolite pores, forcing the copper ions to occupy the most stable, but less accessible positions within each structure. In the case of the MFI zeolite, the EPR study revealed that the Cs exchange resulted in an increased abundance in the number of square pyramidal Cu2+ ions relative to the other Cu environments. Cs co-cations also lead to an increase in the reducibility of the copper ions mainly due the reduction of protons in Cu, Cs-containing samples