F.J. Cadete Santos Aires

Diffusion and aggregation of large antimony and gold clusters deposited on graphite

Abstract We present a study of the first stages of growth of thin films produced by low-energy cluster beam deposition (LECBD) on graphite. Our experiments are analyzed in the framework of new models including three physical ingredients, which are the deposition, the diffusion and the aggregation of the clusters. The comparison between computer simulations of the model and the experimental structures reveals that only the incident clusters diffuse on the graphite, the clusters stick irreversibly upon contact, and allow us to quantify the diffusion of clusters on graphite. Two kinds of metallic cluster films are studied: thin films produced by deposition of antimony clusters (containing 2300 and 250 atoms) and others by deposition of gold clusters (containing about 250 atoms). In both cases, we find that the clusters, in spite of their large size, diffuse very rapidly on the surface. The different microscopic diffusion mechanisms proposed in the literature are investigated, but none is compatible with our experimental results. Finally, we suggest a collective mechanism where the cluster rotates on the surface as a rigid entity to explain our results.

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. ...

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.

Plasma synthesis of catalytic thin films

Plasma sputter deposition is introduced in the field of catalyst preparation. It is shown that growth kinetics and morphologies are determined by ion to neutral flux ratio and kinetic energies of sputtered atoms. Catalytic activity of such catalysts compares very well with classical catalysts.

Reaction mechanism in combustion synthesis of α-Si3N4 powder using NaN3

A combustion synthesis method for the synthesis of α-Si 3 N 4 from a reactant compact composed of Si, NaN 3 , and NH 4 X and wrapped up with an igniting agent was investigated. Wrapping the reactant compact with the igniting agent (i.e., a mixture of Ti and C powders) was found necessary for the synthesis of Si 3 N 4 . In addition to NH 4 Cl, which was considered previously to function as a catalytic agent, other ammonium halides (i.e., NH 4 F, NH 4 Br, and NH 4 I) were found to be capable of catalyzing the synthesis reaction with NH 4 Cl being the most effective. Si 3 N 4 could not be produced when NaN 3 was replaced by C 3 H 6 N 6 . NaN 3 was thus considered to exert an essential effect on the combustion synthesis reaction other than functioning as a solid-state nitrogen source as considered previously. It was proposed that Na vapor produced by decomposition of NaN 3 reduces SiX x (formed by reaction of Si and NH 4 X), promoting the nitridation reaction to form Si 3 N 4 . NaN 3 thus plays a role as a reducing agent in the synthesis reaction.

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.

Palladium Catalysts Deposited on Silicon Nitride in the Deep Oxidation of Methane

The physicochemical and catalytic properties of palladium catalysts were studied in the deep oxidation of methane. The catalysts were deposited on silicon nitride from aqueous (Pd/Si3N4-a) and toluene (Pd/Si3N4-t) solutions of palladium acetate. The use of aqueous and organic solutions of palladium acetate, all other preparation conditions being equal, resulted in the formation of palladium systems with different catalytic properties. The sample from Pd/Si3N4-t was characterized by high activity and stability. The systems studied had different structures and adsorption properties of palladium nanoparticles, which influenced the form of reagent adsorption, catalytic properties, and mechanism of surface reactions. The suggestion was made that the solvent played a key role in the formation of the active surface of Pd-containing catalytic systems.

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.