Jean-Luc Rousset

Stereoselective stilbene epoxidation over supported gold-based catalysts

The gold reference catalyst Au/TiO(2) exhibits high activity in the stereoselective epoxidation of trans-stilbene in methylcyclohexane in the presence of 5 mol% TBHP, by taking part in a chain reaction involving the activation of molecular oxygen by a radical produced from methylcyclohexane.

Study of bimetallic Pd-Pt clusters in both free and supported phases

We study PdPt bimetallic clusters in both free and supported phases. These clusters have been produced with a laser vaporization source. Free clusters directly produced by the source are studied by time of flight mass spectrometry and photofragmentation technique. We observed a sequential evaporation of Pd atoms in the mixed clusters consistent with a palladium segregation process. This tendency has been also observed on supported particles from which the structure and the composition are determined by high resolution transmission electron microscopy and energy dispersive x‐ray analysis. A main result is that each particle has the composition of the massic rod vaporized in the source. The supported particles are well crystallized and exhibit truncated octahedron shapes. Experimental observations are well explained using a modified tight binding model. Indeed, within this model, we found that the equilibrium shape is strongly related to the variation of the cohesive energy with atomic coordination number. ...

Functional nanostructures from clusters

Functional cluster-assembled nanostructures with original structures and properties are prepared using the Low Energy Cluster Beam Deposition method (LECBD). This technique consists in depositing supersonic clusters produced in the gas phase using a combined laser vapourisation-inert gas condensation source. Low energy clusters with typical sizes ranging from ∼1 to a few nm are not fragmented upon impact on the substrate (soft landing regime) leading to the formation of cluster-assembled nanostructures which retain the original structures and properties of the incident free clusters. Model nanostructured systems of any kind of materials (metallic, covalent, oxides) well suited for fundamental studies in various fields (nanoelectronics, nanomagnetism, nanophotonics, catalysis or nanobiology) and for applications to very high integration-density devices (∼Tbits/in/²) are prepared using this method. After a brief review of techniques to produce, analyse, mass select, and deposit clusters in the LECBD-regime, the specific aspects of the nucleation and growth process which govern the formation of cluster-assembled nanostructures on the substrate are presented, especially the preparation of 2D-organised arrays of cluster-assembled dots by depositing low energy clusters on FIB-functionalised substrates. Characteristic examples of cluster systems prepared by LECBD are also described: i) metallic (Au, Ag, Au-Ag, Ag-Ni, Ag-Pt)) and oxide (Gd2O3, ZnO) cluster-assembled nanostructures for applications to linear and non linear nano-optics; ii) magnetic nanostructures from Co-based nanoclusters (i.e., Co-Pt) exhibiting a high magnetic anisotropy which is well suited for applications to high density data storage devices; iii) gold or Pd-Pt or Au-Ti clusters for chemical reactivity and catalysis applications. In some specific cases, we were able to perform studies from an isolated individual nanocluster up to 2D or 3D-collections of non-interacting or interacting particles leading to a rather good understanding of the intrinsic as well as the collective properties at nanoscale.

STRUCTURAL, VIBRATIONAL, AND OPTICAL PROPERTIES OF SILICON CLUSTER ASSEMBLED FILMS

Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2–3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination. As for the oxygen con...

On the reactivity of 2D Pd surface alloys obtained by surface segregation or deposition technique

Abstract Two-dimensional Pd surface alloys can be obtained by two different methods: either by surface segregation or by deposition followed by controlled annealing. For a similar Pd surface concentration, Pd-Ni samples, prepared by these two ways, exhibit similar activity towards the hydrogenation of 1,3-butadiene and are more effective than pure Pd. Different UHV techniques allow us to think that the same catalytic behaviour is due to the same chemical composition and atomic arrangement at the very surface. Moreover, XPS measurements show similar electronic properties of the active site which might be a Pd pair modified by surrounding Ni atoms.

Structures and associated catalytic properties of well-defined nanoparticles produced by laser vaporisation of alloy rods

Bimetallic clusters, all containing gold, have been produced by laser vaporisation of bulk alloys followed by deposition of the formed clusters onto Al2O3 and TiO2 powders or flat silica supports. This technique allows a narrow size distribution of highly dispersed gold-based nanoparticles on powders and nanocrystalline structured thin films on 2D supports to be obtained. The catalytic performances of the as-obtained AuFe, AuNi, AuTi powdery catalysts have been studied in the PROX reaction and compared with those obtained in the oxidation of CO in the temperature range 25-300 degrees C. By comparing the activities of the different catalysts, it is concluded that the nature of the gold partner directly affects the activity of gold. The following tendency is observed: AuFe and AuNi have rather similar activities, significantly lower than that of AuTi. In this paper, we also present a first attempt to study reactivity of original self-supported systems. We show that significant CO oxidation reactivity can be obtained over unsupported nanoporous AuTi and PdAu thin films. By completely excluding the support effect, unsupported catalysts could provide a way of understanding the relevant catalytic mechanisms more easily.

Characterization and reactivity of Pd–Pt bimetallic supported catalysts obtained by laser vaporization of bulk alloy

Abstract Bimetallic Pd–Pt clusters produced by laser vaporization of bulk alloy have been deposited on high surface alumina. Energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM) show that they have a perfectly well-defined stoichiometry and a narrow range of size. Therefore, they constitute ideal systems to investigate alloying effects towards reactivity. Pd–Pt alloys are already known for their applications in the hydrogenation of unsaturated hydrocarbons, especially aromatics, because this system is highly resistant to sulfur and nitrogen poisoning. In this context, the catalytic properties of this system have been investigated in the hydrogenation of tetralin in the presence of hydrogen sulfide. Preliminary results show that this model catalyst is more sulfur-resistant than each of the pure supported metals prepared by chemical methods.

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.

Sizeable deactivation effect for the 1,3-butadiene hydrogenation on vapor-deposited Pd aggregates on graphite

The catalytic activity of Pd aggregates vapor-deposited on graphite has been measured in the 1,3-butadiene hydrogenation. It is generally decreasing with particle size. But, such a decrease is largely depending upon the hydrogen and the hydrocarbon pressure. Indeed, with low hydrocarbon pressure (about 1 Torr) and in presence of large excess of hydrogen (Ph2/Pc4h6=100) the activity of the smaller aggregates is comparable to that of larger particles. The different behaviour is consequently attributed to a size dependent deactivation. Such a deactivation could be due to carbon deposits generated by dehydrogenation of the diene admolecules. This behaviour is tentatively related with marked differences observed by XPS on the smaller aggregates, both with respect to the core level energies and to the density of states into the valence band.

Supported Pd-Cu catalysts prepared from bimetallic organo-metallic complexes: relation between surface composition measured by Ion Scattering and reactivity

The direct synthesis on a silica support of mixed Pd-Cu acetylacetonates clusters is achieved by crystallization from solutions of the pure compounds. The stochiometric clusters are identified by X-ray diffraction. After elimination of the organic ligands under argon followed by in situ XRD, alloys are formed for all the studied compositions. Low energy ion scattering shows that the surface of the particles has a higher Cu concentration than the bulk. As expected, these alloys have a lower activity than pure palladium for the hydrogenation of butadiene, but only butenes are formed even at 100 % conversion of the reactant. This selectivity is explained by the preferential migration of Cu atoms on given sites of the surface as demonstrated by Monte Carlo simulations.