Growth of palladium nanoparticles on nanostructured highly ordered pyrolytic graphite

We report on the growth of palladium nanoparticles on the basal plane of as-cleaved highly oriented pyrolytic graphite (HOPG) samples, and on CO2 ion sputtered nanostructured HOPG surfaces. The morphology of Pd nanostructures grown at room temperature is investigated by scanning tunneling microscopy (STM). The STM observations indicate that the morphology of the Pd films is strongly dependent on the HOPG surface. Stabilized Pd particles only form on the sputtered surface, while ramified Pd particles decorate the clean HOPG terraces. The prestructuring of HOPG surface leads to a selective location of particles at the rim of the nanopits generated by the CO2 ion sputtering and annealing of the surface. The correlation between size, form, density, spatial distribution of the Pd nanoparticles and the quantity of metal added on surface is discussed. We also describe trench channeling of graphite or graphene basal planes by means of Pd nanoparticles in an ambient environment. Copyright (c) 2014 John Wiley & Sons, Ltd.

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010

Etude des propriétés morphologiques et catalytiques d'agrégats d'or triés en taille et déposés sur le TiO2(110)

Raphaël Vallotton

This thesis aims to find, for the first time, a direct relation between the size and morphology of small metallic nanostructures (gold in this case) supported on a metal-oxide surface to their catalytic activity. In this perspective, three main topics have been treated during this thesis. The first part concerns the design and realization of a Scanning Tunneling Microscope (STM) supposed to work over a wide temperature range (4K < T ≤ 300K). This new device replaces an existing solution. The coarse approach of the scanning tip towards the sample is realized by an axial motor based on a sapphire prism gliding on shear piezos in the stick&slip mode. The new STM is very rigid moving the resonance frequencies to higher values with respect to the existing solution. Operation of the motor down to T = 8K has been proven, however topographic imaging has been performed only in the temperature range 77K < T ≤ 300K. The second part of this work focuses on a study of the evolution of the morphology of gold nanoparticles on a TiO2(110) surface. Size-selected clusters Aun+ (n = 5, 7) are deposited at a well defined kinetic energy on the surface held at room temperature. Subsequent annealing of the sample has been performed stepwise. After each temperature increase, the morphology has been determined by STM. The evolution as a function of surface temperature has been studied for two different surface reconstructions, TiO2(110)-(1×1) and TiO2(110)-(2×1). The deposition process leads only to small fragmentation and the morphology is stable up to T = 400K. Further increase of the surface temperature leads to sintering of the particles by Ostwald ripening as shown by an exponential decrease of the island density with temperature. The main topic of this thesis, the correlation between morphology and catalytic activity, is described in the last part of this manuscript. For the first time we are able to relate the onset of CO2 production from CO and O2 to a clear change in the morphology. The catalytic activity of the particles strongly depends on their size and dimensionality. The relative activity per particle has been determined and we find a clear maximum for clusters containing 60 atoms and are 3 to 4 monolayers high. These results are discussed in contrast of literature data on the same but also on different metal-oxide surfaces.

EPFL2009

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

EPFL2006