A.J. Renouprez

Investigations on supported bimetallic PdPt nanostructures

Abstract PdPt bimetallic clusters have been produced by a laser vaporisation source and studied after deposition on a support. We have been able to obtain clusters with a well-defined composition, identical to that of the target rod as deduced from energy dispersive X-ray analysis measurements. An evaporation of Pd atoms upon the impact of the electron beam has been observed, suggesting that the surfaces of the clusters are Pd-enriched. The segregation behaviour using Monte Carlo simulations has been studied for the Pd 15 Pt 85 and Pd 50 Pt 50 cubo-octahedral clusters composed of 586 and 2406 atoms. The results indicate a large Pd segregation in all the systems, in qualitative agreement with the experimental observations, with preferential occupation of corner and edge sites.

Electrochemical preparation of nanometer sized noble metal particles into a polypyrrole functionalized by a molecular electrocatalyst precursor

Uniform spatial dispersion of a fine distribution of Pd, Ru and Rh nanoparticles has been achieved in a polypyrrole (ppyr) film functionalized by a cationic precursor of metal–metal bonded polymers [Ru(L)(CO)2]n (L = 2,2′-bipyridine (bpy) and derivatives) by using convenient electrochemical techniques. As evidenced by Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and X-ray Photoemission Spectroscopy (XPS), the metal particle sizes in the resulting composite material were found to be in the range from 1 to 6 nm, depending on the metal.

Electron microscopy and 195Pt nuclear magnetic resonance of platinum particles in a zeolite-Y matrix

Abstract We present the first 195 Pt NMR spectra of platinum particles embedded in a zeolite, and compare them with earlier results on oxide-supported Pt. Slight variations in the details of the preparation can make a bulk-like NMR signal appear, indicating the presence of particles larger than roughly 1.5 nm, that cannot fit into the zeolite supercage. TEM observations confirm that they are nevertheless inside the matrix. Although the particles have nominally clean surfaces, a fraction of Pt atoms is not in a metallic environment: this may be caused by interaction with the framework and its counterions, and/or by an intrinsic size effect. Hydrogen chemisorption seems to restore the metallic character at least partially. This may be related to the change of lattice constant between clean and hydrogen-covered particles, known to occur from EXAFS studies.

Sulphur resistant palladium-platinum catalysts prepared from mixed acetylacetonates

Well-defined mixed acetylacetonates precursors were synthesised on a silica support and identified by their X-ray differaction spectra. After a calcination treatment to decompose the ligands and a reduction in hydrogen, bimetallic particles of 3.5 to 5 nm have been obtained. Analytical microscopy shows that these catalysts have a perfectly homogeneous composition. Low energy ion scattering experiments show that the surface of the particles is palladium enriched, as expected from Monte Carlo simulations. However, in the simple hydrogenation of cyclooctadiene to cyclooctene, where Pd is two orders of magnitude more active than Pt, it is observed that the rate is not proportional to the Pd surface concentration. This is interpreted as an ensemble effect on the chemisorption of the reactant. A study of the hydrogenation of tetralin in the presence of H 2 S shows a maximum of sulphur resistance for the Pd 20 Pt 80 atomic composition.

Electron availability and the surface fermi level local density of states: an alternative way to see catalytic activity of metals

The vanishing tail of electronic density that enters the vacuum from a metal surface consists mainly of electrons at the Fermi level. The farther away from the surface, the higher is the fraction of Fermi-level electrons. To first order approximation, one expects that interactions between incident reactants and a metal surface will be dominated by these electrons. Therefore, they should play an important role in heterogeneous catalysis. The importance of this concept is illustrated by an analysis of data for the Fermi-level local density of states at the surface of Pt catalysts, obtained from 195Pt nuclear magnetic resonance.

Amorphous SiC films prepared by low-energy cluster beam deposition

Abstract Amorphous silicon carbide films have been prepared by low-energy cluster beam deposition. Transmission electron microscopy and tapping mode atomic force microscopy reveal the structure of the film which is formed by a random distribution of SiC nanoparticles. The size of the particles was about 8–10 nm and it is greater than the size of the incident free particles (less than 1 nm). However, the inner structure of the supported particles does not reveal long-range order. Both Raman spectroscopy and Fourier-transformed infrared spectroscopy show clearly limited chemical ordering and phase separation evidenced by the presence of silicon-rich, carbon-rich and SiC-rich areas.

PREPARATION OF BIMETALLIC PtnPdm SUPPORTED CLUSTERS WITH WELL-DEFINED STOICHIOMETRY

Supported bimetallic Pd-Pt clusters with a well-defined stoichiometry are produced using a laser-vaporization source. Free clusters are also studied by time-of-flight mass spectrometry and photofragmentation. The supported clusters are characterized by energy dispersive x-ray analysis. The low binding energy of palladium atoms compared to that of platinum is clearly demonstrated in both free and supported clusters. The reactivity is briefly discussed.

Hollow metallic particles obtained by oxidation/reduction treatment of organometallic precursors

Abstract Oxidation and rapid reduction of supported organometallic precursors on silica yield large metallic particles (5 to 20 nm) that exhibit in transmission electron microscopy (TEM) a typical difference of contrast between the center and the borders. Z-contrast high angle annular dark field (HAADF) imaging in the scanning transmission electron microscope (STEM) has been successfully used to show that these particles are hollow in the case of Pd. PdMn particles have also been prepared and it is shown that they exhibit surface cavities instead of internal cavities.