Supported Metal Particles Research Articles

Preparation of the Pd/C catalysts: A molecular-level study of active site formation

This review summarizes the results of molecular-level studies on the mechanism of Pd/C catalyst formation from the PdCl2 precursor. Two processes occur in acidic media during the contact of H2PdCl4 with carbon: (a) adsorption of palladium chloride to form surface complexes and (b) redox interaction between PdCl2 and carbon with the formation of palladium metal particles. The ratio between these adsorbed palladium species depends on the conditions of adsorption and especially on the size of carbon support grains and the oxidative atmosphere. The observations are explained by the fact that carbon support exhibits electrochemical and ligand properties. X-ray diffraction, X-ray scattering, XPS, and high-resolution electron microscopy revealed that the nanostructure of carbon materials, in particular the extent of their three-dimensional ordering, is crucial for the ligand properties. The presence of two forms, metallic and ionic, of sorbed palladium determines the bimodal size distribution of the metal. After the reduction of ionic species, metal particles are “blocked” with support. The nature of the ionic forms of palladium (mostly (PdCl2)n) clusters chemically and epitaxially bound to the carbon surface suggests the mechanisms of the bimodal distribution of the supported metal particles on the surface and the methods for the control of the ratio between “blocked,” low-dispersed, and highly-dispersed particles in the catalyst. One of these methods is the use of palladium polynuclear hydroxo complexes (PHCs) with low oxidation potentials as starting compounds for catalysts preparation. The data on the PHC structure in a solution and its change upon the adsorption of PHC on the surface of the carbon material obtained by the17O,23Na,133Cs, and35Cl NMR techniques are discussed. PHCs are shown to be a clew of the [Pd(OH)2]n polymeric filament, whose fractions are bound with alkali metal ions. When PHC is adsorbed on the surface of the carbon support and then dried, palladium oxide is formed from which highly dispersed metal particles are formed during reduction. The nature of alkali metal ions in PHC affects the activity of the Pd/C catalyst. An important role of the ligand, electrochemical, and lyophilic properties of carbon material during the formation of the species of the active catalyst component is discussed.

Influence of the Support on the Structural Characteristics of Carbon Nanofibers Produced from the Metal-Catalyzed Decomposition of Ethylene

The notion of using support materials to achieve high dispersions of metal particles has been extended to the synthesis of carbon nanofibers from the catalyzed decomposition of ethylene. By using this approach it has been possible to generate nanofibers whose widths are dictated by the dimensions of the supported metal particles. In addition, the support may alter the state of the bulk and/or the surface of the catalyst particle through metal−support interactions, and the impact of this effect is manifested by modifications in the structural characteristics of the nanofiber deposits. In an attempt to gain a clearer insight into the influence of metal−support interaction on the growth characteristics of GNF, three metals, Fe, Ni, and Co that are known to be active catalysts for this process were impregnated onto silica, graphite, and well-characterized graphite nanofiber supports. Characterization of the solid carbon products was performed by a variety of approaches including high-resolution transmission electron microscopy (HRTEM), gas-phase analysis, and thermal-programmed oxidation (TPO). The goal of this study was to correlate each nanofiber product with the behavior of the specific metal/support precursor system. The advantages of using selected support materials to control the morphologies and sizes of nanofibers is presented.

Advanced experimental approach to model studies of supported metal particle systems

An ambitious experimental system is described which is characterized by total instrumental integration of surface, high‐resolution microscopy, and catalytic reaction measurements. The design of this system is based upon an effective combination of different methodic approaches to catalytic model studies that have been used successfully but usually independently in the past. The combination of individually sophisticated experimental techniques has been possible because of major recent advances in the transportation and manipulation of samples in UHV environments. The advantages gained by the application of this experimental approach to the controlled preparation and analysis of particulate metal deposits on a variety of flat and electron transparent refractory oxide supports are discussed. Also, the opportunities created by integrated measurements of gas exposures and catalytic reactions at low and high pressures are explored. As a result, catalytic model studies can be moved closer to their ultimate goal of assisting in the development of improved real world catalysts of technological importance.

An atomic X-ray absorption fine structure study of the influence of hydrogen chemisorption and support on the electronic structure of supported Pt particles

The physical principles of atomic XAFS (AXAFS) are presented along with important details on how to isolate the AXAFS contribution from the experimental XAFS data. Intuitive illustrations are given showing how various interactions of the absorber atom with its neighbours influence the AXAFS contribution. Hydrogen chemisorbed on the surface of the supported metal particles is shown to have a strong influence on the amplitude of the Fourier transform AXAFS peak. The effect of the support (with different amounts and types of charge compensating ions (H + ,K + ), different Si/Al ratio and extra-framework Al) on the experimental AXAFS spectra of Pt dispersed in zeolites (LTL,Y) and on flat supports (Al 2O3‐SiO2, MgO‐Al2O3) are summarised. It is shown that the essence of the metal‐support interaction, as revealed in the AXAFS, is a shift in the ionisation potential of the valence d-orbital electrons brought about by the polarisation induced primarily by the changing charge on the support oxygen atoms.

Anodic catalysts for polymer electrolyte fuel cells: the catalytic activity of Pt/C, Ru/C and Pt-Ru/C in oxidation of CO by O 2

The catalytic activity for oxidation of CO by O 2 was investigated on commercial Pt/C, Pt-Ru/C (Pt/Ru atomic ratio = 20, 3, 1, 1/3) and Ru/C. All samples contained 20 wt.% metal. Assuming equal surface and bulk composition, the number of surface Pt and Ru atoms was calculated from the average size of the supported metal particle as determined by TEM. On Pt-Ru/C alloys, the turnover frequency per Ru atom, N Ru/molecules s −1 Ru-atom −1, was independent of chemical composition. This finding suggests that the active site in these alloys is Ru. In the temperature range 300–400 K, the turnover frequency per active metal atom was 50–300 times higher on Pt-Ru/C than on Pt/C. The turnover frequency was 400 times higher on Ru/C than on Pt/C at 313 K and 90 times higher at 353 K. Addition of water vapor to the reactant mixture left the catalytic activity of Ru/C unchanged but slightly increased the activity of Pt/C. On both catalysts the activation energy and reaction orders were nearly the same as in dry atmosphere. Conversely, the addition of water markedly decreased the activation energy for Pt-Ru(1 : 1)/C alloy (from 19 to 11 kcal mol −1). These findings suggest that fuel cells equipped with Pt-Ru/C anodes perform better than cells with Pt/C anodes. They do so because Ru effectively oxidizes the carbon monoxide present as an impurity in the H 2-reformed fuel.

On Metal Blocking in Micropores of Carbon Supports

Platinum catalysts prepared on activated carbon supports of different origin are compared in the hydrogenation of cyclohexene and cyclooctene, which differ much in their reactivity. A decrease in the catalytic activity upon decreasing the dimensions of the micropores in the support and the size of the supported metal particles has been confirmed. The results give evidence against a trivial role of the diffusional factor in such an effect and indicate complete blocking of part of the metal surface in narrow pores.

A New Model Describing the Metal–Support Interaction in Noble Metal Catalysts

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO2). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt–H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal–support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline.

Adsorption of molecules on the surface of small metal particles studied by optical spectroscopy

The influence of adsorbate layers on surface plasmon excitation of small supported metal particles with mean sizes ranging from R=20 to 40 nm has been investigated. For this purpose, sodium and potassium clusters adsorbed on dielectric substrates served as model systems. Their optical transmission spectra were first recorded under ultra-high vacuum conditions directly after preparing the particles and, subsequently, after dosage of gases such as O 2, N 2O, CO 2, H 2 and N 2. The optical spectra are dominated by two maxima that originate from the excitation of surface plasmon resonances in the direction of the long and short axis of the oblate particles. Changes are found upon adsorption of as little as 0.04 monolayers on the cluster surface. The results of a series of measurements together with a theoretical treatment using the quasi-static approximation indicate that the variations of the spectra allow one to characterize the strength of the surface chemical bond. In addition, diffusion of the atoms into the bulk of the particles can be followed. Particularly interesting is the observation that the clusters can experience a change in shape if gases such as O 2 or CO 2 react with their surface.

Metal clusters on ultrathin oxide films: model catalysts for surface science studies

Characterization and reaction kinetics studies have been performed over a variety of planar model supported catalysts prepared by the vapor deposition of catalytically interesting metals onto ultrathin oxide films on refractory metal single crystal substrates in ultrahigh vacuum. These unique systems feature many of the advantages for fundamental study associated with single crystals, while addressing important issues for supported catalysts, such as the intrinsic effects of particle size and the role of the support. The oxide thin films have been shown to roughly mimic the chemical and physical properties of the bulk analogs, and yet they are electrically conductive via defects and tunnelling to the single crystal substrate. This renders them amenable to the various charged particle spectroscopies that comprise the core of modern surface science; also, because they are flat as well conductive, they are suitable for scanning–tunnelling and atomic force microscopy. Characterization studies carried out over these models focusing on structural, electronic, and chemical properties as a function of particle size have been related to parallel studies of relevant catalytic reactions, providing fundamental insight into these processes at the atomic level. The select group of experiments presented here provides a broad illustration of the versatility and utility of these materials for elucidating the properties of small, supported metal particles and for simulating catalysis over `real world' high surface area supported catalysts.

Chemical reactions on the surface of metal nanoparticles studied by optical spectroscopy

The influence of molecular adsorbate layers on surface plasmon excitation in small supported metal particles has been investigated and exploited to study adsorption reactions on their surfaces. For this purpose sodium nanoclusters on quartz and LiF substrates served as model systems. Their optical transmission spectra are dominated by two maxima which are due to the excitation of surface plasmon resonances in the direction of the long and short axes of the oblate particles. By recording the spectra under ultrahigh-vacuum conditions and, subsequently, after exposure to gases such as O2, N2O, CO2, H2 and N2, changes in the optical spectra can be identified if the clusters are covered by as little as half a molecular monolayer. Depending on the adsorbed molecules, different modifications of the maximum position, the width and the amplitude of the surface plasmon resonances are observed. The results of a series of measurements together with calculations using the quasi-static approximation indicate that the variations in the spectra allow one to distinguish between physisorption and chemisorption, i.e. to characterize the strength of the chemical bond. In addition, diffusion of the molecules into the bulk of the particles can be detected. Particularly interesting is the observation that the clusters can experience a change in their shape if gases such as O2 or CO2 react with their surface. © 1998 John Wiley & Sons, Ltd.

Chemical reactions on the surface of metal nanoparticles studied by optical spectroscopy

The influence of molecular adsorbate layers on surface plasmon excitation in small supported metal particles has been investigated and exploited to study adsorption reactions on their surfaces. For this purpose sodium nanoclusters on quartz and LiF substrates served as model systems. Their optical transmission spectra are dominated by two maxima which are due to the excitation of surface plasmon resonances in the direction of the long and short axes of the oblate particles. By recording the spectra under ultrahigh-vacuum conditions and, subsequently, after exposure to gases such as O2, N2O, CO2, H2 and N2, changes in the optical spectra can be identified if the clusters are covered by as little as half a molecular monolayer. Depending on the adsorbed molecules, different modifications of the maximum position, the width and the amplitude of the surface plasmon resonances are observed. The results of a series of measurements together with calculations using the quasi-static approximation indicate that the variations in the spectra allow one to distinguish between physisorption and chemisorption, i.e. to characterize the strength of the chemical bond. In addition, diffusion of the molecules into the bulk of the particles can be detected. Particularly interesting is the observation that the clusters can experience a change in their shape if gases such as O2 or CO2 react with their surface. © 1998 John Wiley & Sons, Ltd.

Supported metals derived from organometallics

This article reviews recent progress in the application of organometallic compounds for the preparation of supported metal catalysts. The approach is a very general one in that the organometallics considered are the main group element complexes, the mononuclear complexes and clusters of transition metals (in different oxidation states). The surfaces can be that of metal oxides, the internal or external surface of zeolites or the surface of supported metal particles in a zero-valent state. Volatile complexes with the right size and reactivity have been used to prepare zeolite-entrapped metals and supported catalysts via chemical vapour deposition technique.

Metal carbonyl clusters as precursors of heterogeneous catalysts: Hydrogenation and disproportionation of monoenes, dienes, and arenes in the presence of H2Ru4(CO)13, H2FeRu3(CO)13, and HRuCo3(CO)12 supported and activated on chromosorb. Characterization of the metal particles derived from H2Ru4(CO)13 by means of IR spectroscopy and TEM microscopy

The tetrahedral hydridic clusters H2Ru4(CO)13 (1), H2FeRu3(CO)13 (2), and HRuCo3(CO)12 (3) were supported on Chromosorb P and activated under dihydrogen flow. The resulting metal particles are active in the hydrogenation of pentenes, cyclic monoenenes and dienes, benzene, and toluene; these catalysts are effective under mild conditions and with a low metal loading. Experiments under dinitrogen showed that complex hydrogenation-dehydrogenation processes occur, as already observed for the same clusters during the homogeneous hydrogenation of cyclohexadienes. After the gas-chromatographic catalytic runs with cluster 1 as precursor, TEM microscopy showed the presence of very small supported metal particles (mean size 7.5 nm). The decomposition of cluster 1 to metal particles upon thermal treatment on Aerosil under vacuum or under dihydrogen was followed by means of IR spectroscopy; this catalyst hydrogenates benzene at room temperature with 100% conversion in a very short time (calculated activity was about 3200 TOFs).

Environmental high resolution electron microscopy and applications to chemical science

An environmental cell high resolution electron microscope (EHREM) has been developed for in situ studies of dynamic chemical reactions on the atomic scale. It allows access to metastable intermediate phases of catalysts and to sequences of reversible microstructural and chemical development associated with the activation, deactivation and poisoning of a catalyst. Materials transported through air can be restored or recreated and samples damaged, e.g. by dehydration, by the usual vacuum environment in a conventional electron microscope can be preserved. A Philips CM30 HRTEM/STEM system has been extensively modified in our laboratory to add facilities for in situ gas-solid reaction studies in controlled atmospheres of gas or vapor at pressures of 0–50 mbar, instead of the regular TEM high vacuum environment. The integrated new environmental cell capability is combined with the original 0.23 nm TEM resolution, STEM imaging (bright field/annular dark field) and chemical and crystallographic microanalyses. Regular sample holders are used and include hot stages to > 1000°C. Examples of applications include direct studies of dynamic reactions with supported metal particle catalysts, the generation of defects and structural changes in practical complex oxide catalyst systems under operating conditions and carbon microstructures.

Surface study of supported metal particles by 129Xe NMR.

The surfaces of platinum clusters in the supercages of NaY zeolite have been studied by ${}^{129}\mathrm{Xe}$ NMR over the temperature range 80--293 K. The sensitivity of this weakly perturbing technique makes it possible to reveal the heterogeneity of the electronic structure of the particle surface. The temperature dependence of the ${}^{129}\mathrm{Xe}$ spin-lattice relaxation indicates that the surface sites with the highest adsorption energy possess metallic character.

Size-dependent catalytic activity of supported metal clusters

BECAUSE catalysis by metals is a surface phenomenon, many technological catalysts contain small (typically nanometre-sized) supported metal particles with a large fraction of the atoms exposed1. Many reactions, such as hydrocarbon hydrogenations, are structure-insensitive, proceeding at approximately the same rate on metal particles of various sizes provided that they are larger than about 1 nm and show bulk-like metallic behaviour1. But it is not known whether the catalytic properties of metal particles become size-dependent as the particles become so small that they are no longer metallic in character. Here we investigate the catalytic behaviour of precisely defined clusters of just four and six iridium atoms on solid supports. We find that the Ir4 and Ir6 clusters differ in catalytic activity both from each other and from metallic Ir particles. This raises the possibility of tailoring the catalytic behaviour of metal clusters by controlling the cluster size.

Electrochemical study of ultrathin silica films supported on a platinum substrate

The aqueous electrochemistry of ultrathin silica films supported on metal substrates has been investigated in an apparatus that combines an ultrahigh vacuum analysis chamber with an electrochemical cell. The silica films were prepared in vacuum by evaporating silicon in oxygen ambient onto a platinum substrate. It is established that the silica films completely cover the substrate and isolates it from the solution. The silica films on platinum are stable under electrochemical conditions of cycling. It is shown that the thin silica films (15–100 Å) are electrically conductive, with a conductivity several orders of magnitude larger than bulk silica. The silica/Pt electrodes behave over a wide range of potentials as ideally polarizable, which facilitate electrochemical studies of supported metal particles of Pd and Pt. The under-potential deposition of copper onto silica-supported palladium particles and methanol oxidation on platinum particles have also been demonstrated.

Carbon Fibrils Mechanism of Growth and Utilization as a Catalyst Support

ABSTRACTEspecially with the production of fine chemicals catalytic reactions are being performed in which solid catalysts suspended in liquids are used. Carbon is stable in both acid and alkaline liquids and is not preferentially wetted by water. Carbon is therefore an attractive support for catalysts to be used in liquid-phase reactions. The relatively low diffusion coefficients in liquids call for catalyst bodies having wide pores and dimensions between 1 and 10 μm. Since particles smaller than 1 μm cannot be readily separated from liquids, attrition of carbon support bodies must be avoided. However, it is difficult to produce active carbon bodies of 1 to 10 μm of a high mechanical strength. Also with gas-phase reactions, the low mechanical strength of active carbon bodies limits the applicability of active carbon.From supported metal particles carbon filaments of a diameter between 10 and 300 nm can be grown rapidly. The diameter is generally of the same order of magnitude as the size of the metal particles. The graphite layers within the fibrils, that have a circular cross-section, are oriented either parallel to the fibril axis or at an angle, which leads to cone-shaped graphite layers. The mechanism of growth of carbon fibrils from supported metal particles will be dealt with. The parallel orientation of the graphite layers leads to a very high mechanical strength, while the porous structure is extremely open. A representative value for the surface area is 225 m2/g and for the pore volume 1.6 ml/g, which leads to an average pore dimension of 28 nm.To apply active precursors, such as, palladium salts, the surface of the carbon fibrils must be oxidized, e.g., by treatment with nitric acid. A very high dispersion of palladium can thus be achieved. The dispersion of components active in hydrogenation can be assessed by the ability to react the support with hydrogen to methane. The activity of catalyst on carbon fibrils in the hydrogenation of nitrobenzene as well as the filterability of carbon fibril catalysts will be compared with that of commercial catalysts.

Monitoring of the Early Stages of Thin Film Growth by the Generation From Second Harmonic Radiation of Supported Metal Particles

AbstractOptical second harmonic generation (SHG) of metal clusters supported on a dielectric substrate has been investigated. The SHG intensity was detected in situ as a function of cluster size during the nucleation of the particles at constant number density. SH radiation is detectable for particles as small as approximately 1 nm. In further experiments the chemisorption of oxygen on the surface of the metal particles was studied. The SHG signal strongly decreases as a function of oxygen coverage and disappears almost completely upon chemisorption of only one monolayer.

Desorption of metal atoms with laser light: Mechanistic studies.

Results on laser-induced desorption of metal atoms from small metal particles are presented. Experiments have been performed on sodium, potassium, and silver particles supported on a LiF(100) single-crystal surface under ultrahigh vacuum conditions. Measurements include the determination of the desorption rate as a function of laser wavelength, laser intensity, average particle size, and substrate temperature, the determination of the kinetic energy of the desorbed atoms, the investigation of the optical spectra of the supported metal particles, and the study of the influence of adsorbate molecules on the desorption rate. Furthermore, theoretical extinction and absorption spectra of the metal particles have been calculated with the classical electrodynamical Mie theory as a function of average particle size and excitation wavelength. Also, the radial electric field at the particle surface was computed. The results of the experiments and theoretical calculations are combined to give a consistent picture of the mechanism of metal-atom desorption by electronic excitation with laser light. A realistic surface potential from which the atoms escape and nonlocal optical effects are taken into account. The latter introduce additional absorption channels by the formation of electron-hole pairs in the surface layer of the particle which relax into antibonding states before desorption occurs. Finally, the mechanism is discussed in the light of similar phenomena observed for thin metal films. Possibilities for future work are outlined.