Morphologie de cristallites de palladium sur support d'alumine Morphology of Palladium Crystallites Supported on Alumina

Nous avons étudié, par microscopie électronique à transmission, la structure tridimensionnelle des particules métalliques de catalyseurs Pd/Al2O3. Un modèle de cuboctaèdre tronqué est proposé pour des particules de taille de l'ordre de 30 nm. Pour des particules de taille inférieure (~ 5 nm) l'observation des profils et des orientations cristallographiques vis-à-vis du support sont compatibles avec un modèle cuboctaédrique. <br> Transmission electron microscopy has been used to study the three dimensional structure of metal particles in Pd/Al2O3 catalysts. A model in the form of a truncated cuboctahedron is proposed for particles of size about 30 nm. For smaller (~ 5 nm) particles observations of profiles and crystallographic orientations with respect to the support are compatible with a cuboctahedron model

In Situ Electrochemical, Electrochemical Quartz Crystal Microbalance, Scanning Tunneling Microscopy, and Surface X-ray Scattering Studies on Ag/AgCl Reaction at the Underpotentially Deposited Ag Bilayer on the Au(111) Electrode Surface

Ag/AgCl reaction at the Ag bilayer, which was underpotentially prepared on a Au(111) surface, was investigated using electrochemical quartz crystal microbalance (EQCM), scanning tunneling microscopy (STM), surface X-ray scattering (SXS), and electrochemical techniques. When the potential was scanned positively from -200 mV, the Cl^[-] ion was adsorbed on the Au(111) electrode surface around 0 mV, and then the phase transition of the adsorbed Cl^[-] ion layer from random orientation to (√3 x √3) structure took place at around +130 mV. The Ag bilayer and Cl^[-] ions were oxidatively reacted to form the AgCl monolayer with (√13 x √13)R13.9° structure around +200 mV, accompanied with the formation of AgCl monocrystalline clusters on the AgCl monolayer surface. The structure of the AgCl monolayer on the Au(111) surface was changed from (√13 x √13)R13.9° structure to (4 x 4) structure around +500 mV. When the potential was scanned back negatively, the AgCl monolayer was electrochemically reduced, and a Ag monolayer, not a bilayer, was formed on the Au(111) surface. In the subsequent potential cycles, the structural change between the Ag monolayer and the AgCl monolayer was reversibly observed. All oxidative structural changes were much slower than the reductive ones