Supported Palladium Particles Research Articles

Reaction Kinetics on Complex Model Catalysts under Single Scattering Conditions

To approach a microscopic understanding of the reaction kinetics on complex surfaces of heterogeneous catalysts, we combine multimolecular beam techniques and a supported model catalyst approach. The model systems are prepared under ultrahigh vacuum (UHV) conditions and have been characterized in detail with respect to their geometric and electronic structure. To probe the kinetics of catalytic reactions on these systems, we have developed a molecular beam instrument, which allows us to cross up to three beams on the sample surface. The simultaneous detection of reaction products and surface species is established by a combination of angle- and time-resolved gas-phase detection and in situ time-resolved IR reflection absorption spectroscopy. In this paper, we review a variety of representative experiments, illustrating the experimental possibilities of the molecular beam approach. As a model surface, we focus on alumina supported palladium particles. We cover the adsorption and desorption kinetics of small molecules and the kinetics of simple surface reactions on these systems. The reaction kinetics is probed via systematic steady state measurements, transient experiments, time-resolved in situ IR spectroscopy and measurements of the angular distribution of products. A central topic is the CO oxidation, a model reaction, which has been thoroughly investigated on a variety of single crystal surfaces. For the supported model catalysts, it is shown how structure and size dependencies can be identified by performing systematic kinetic measurements. These effects can be linked to the inherent heterogeneity of the model surfaces via microkinetic mean-field and Monte Carlo simulations. It is shown that the particular kinetic effects on the model catalyst surfaces can be understood by explicitly accounting for their inherent complexity. Finally, we outline possible future directions of the molecular beam approach applied to complex model surfaces.

Characterization and Activity of Supported Palladium Combustion Catalysts

The catalytic activity of Pd supported on γ-Al2O3, Ba-Al2O3, and La–Al2O3 has been examined in complete oxidation of methane when operating in excess of oxygen. Two different sizes of Pd particles have been considered. Foreign ions have a strong influence with respect to the stabilization of alumina when submitted to a temperature as high as 1000°C. In contrast, no specific effect can be detected when the samples are calcined at 500°C. Interaction with the supported palladium particles, observed during the combustion reaction, has been investigated by X-ray photoelectron spectroscopy and temperature-programmed oxidation. The difference in combustion activity is attributed to the difference in surface oxidation states of the Pd particles. The presence of foreign ions in the alumina structure results in surface PdO only. When supported on γ-Al2O3, small amounts of metallic Pd can be detected, resulting in a lower ignition temperature.

Study of CO adsorption on crystalline-silica-supported palladium particles

Supported palladium particles have been studied using a recently developed crystalline silica film. The SiO 2 film, as seen for the first time by scanning tunnelling microscopy (STM), is sufficiently flat to be used for model catalyst studies. Palladium particles grown on silica at room temperature are round and somewhat disordered as seen by STM, suggesting a strong interaction between the Pd particles and the silica support. The structural properties of the palladium particles were probed by CO adsorption at 90 K, and monitored by IRAS. The same general adsorption sites are observed for the Pd particles grown on silica as have previously been observed for crystalline Pd particles on alumina, however a larger fraction of on-top sites was observed on silica, indicative of the smaller and more disordered particles. These first results obtained for the Pd/SiO 2/Mo(1 1 2) system are encouraging for the use of the crystalline SiO 2 substrate as a model-catalyst support.

Size-effects in the acetylene cyclotrimerization on supported size-selected Pd n clusters (1≤ n≤30)

Motivated by the suggestion that mainly geometric arguments can explain the observed structure sensitivity in the cyclotrimerization of acetylene to benzene on palladium single crystals and on supported palladium particles, we studied this reaction on size-selected Pd n clusters (1≤ n≤30) supported on thin MgO(100) films by thermal desorption spectroscopy. Already on small Pd n (1≤ n≤6), benzene is readily produced, mainly at low temperature (300 K). For larger clusters (7≤ n≤30) benzene also desorbs at a temperature of 430 K, indicating that a critical ensemble of seven atoms is necessary for high-temperature cyclotrimerization. Using density functional theory, we attribute this high-temperature desorption of benzene to the presence of three-dimensional clusters.

Infrared study of CO adsorption on alumina supported palladium particles

We have studied the adsorption of CO on small palladium particles deposited on a well-ordered thin Al 2O 3 film on NiAl(110) using infrared reflection absorption spectroscopy (IRAS). Recent studies in our group with STM and SPA-LEED have shown that the deposited particles are predominantly terminated by (111)-type surfaces. The IR spectra exhibit several absorption bands, which are associated with linearly bound CO (2090–2120 cm −1) and bridge bonded CO (1930–1970 cm −1). On the larger, better ordered palladium aggregates another CO-induced band appears in the region of 1970–2000 cm −1, which we assign to CO molecules bound on the edges of the particles. Evaporation of Pd in presence of a CO atmosphere leads to the formation of a palladium carbonyl-like compound which is stable only at low temperatures. The IR spectra of this species show apparent similarities with the spectra of CO on the small particles.

Adsorption of Carbon Monoxide on Pd(111) and Palladium Model Catalysts

The adsorption of carbon monoxide on Pd(111) and SiO2/Si(100)-supported palladium particles, ranging in size from 1.5 to 9.5 nm, was investigated with ellipsometry. By calibrating the sensitivity of the ellipsometric parameter δΔ for the carbon monoxide coverage on Pd(111) with LEED, ellipsometry could be used for a quantitative analysis of the adsorption isotherms. The saturation coverage of carbon monoxide appeared to be ≈0.5 ML on all samples, with respect to the number of surface atoms and within the temperature (400–540 K) and pressure range (up to 1 Pa) studied. The initial isosteric heat of adsorption was 148±5 kJ/mol on all samples. The heat of adsorption decreased with an increasing CO coverage. The decrease of the heat of adsorption with an increasing CO coverage occurred in a similar way on all samples. From these results we conclude that there are no particle size effects with the adsorption of CO on SiO2/Si(100)-supported palladium particles. Since other studies on the adsorption of CO on palladium model catalysts do find a particle size effect, it is concluded that the particle size effects are dependent on the support or that the observed particle size effect is in fact a support effect.

Support of Ultrafine Palladium Particles upon a Glassy Carbon Substrate by a Chemical Method

Abstract Isolated ultrafine palladium particles prepared by a gas-evaporation method have been chemically supported on a flat glassy carbon substrate using a silane coupling agent. The modification of the ultrafine palladium particles with the silane coupling agent was confirmed by FT-IR spectroscopy and transmission electron microscopy (TEM). The supported palladium particles did electrochemical sorption–desorption of hydrogen in an acidic solution.

Surface restructuring of palladium particles induced by CO adsorption

In order to characterize chemisorption induced reconstruction the surface of supported palladium particles (model catalyst) has been investigated during CO adsorption by SSIMS (static secondary ion mass spectrometry). The SSIMS signal ratio ∑n PdnCO+/Pdn+ has been used to monitor the CO adsorption kinetics. The relative occupancy of linear and bridged CO sites has been determined by the value of PdCO+/(∑n PdnCO+) and the variation of the Pd-Pd next neighbour distance by plotting Pd2+/Pd+ during CO exposure. It has been shown that CO chemisorption induces a surface restructuring by increasing the Pd-Pd distance in the particle surface. This phenomenon has the features of a cooperative phenomenon. On the reconstructed particle one bonding state has been identified which appeared to be a precursor of the CO dissociation.

EXAFS and XPS studies of the influence of metal particle size, nature of support and H2 and CO adsorption on the structure and electronic properties of palladium

Abstract For the gas-phase hydrogenation of vinylacetylene the highly dispersed supported palladium particles show a low turnover number, regardless of the nature of the support (silica, γ-alumina, carbon). The peculiarities of their structural and electronic properties were studied by EXAFS and XPS. Regardless of the nature of the support (SiO 2 , C), and the size of the Pd metal particles (19–108 A), the Pd-Pd interatomic distances are close to those for the bulk metal. The quantitative correlation between turnover number and the binding energy, E b , of the Pd core level was found.

On the stability of vapor grown palladium particles supported on amorphous alumina

Transmission electron microscopy studies of small supported metal particles grown by vapor deposition under ultrahigh vacuum (UHV) conditions often require stabilizing overcoats (also called “fixing layers”) in order to preserve the initial structure of the deposit. Whether such stabilizing overcoats must be used depends on the specific combination of support and deposit materials and on the stage of deposition to be studied. In this study, the influence of a SiO x overlayer on nanometer-size palladium particles deposited on an in situ cleaned amorphous alumina support film is investigated. For supported palladium particles at the very early stages of metal film growth, deposits coated in situ with SiO x show higher number densities and smaller mean sizes than uncoated ones. The comparison between uncoated and coated palladium deposits shows that the SiO x substantially retards sintering of palladium cluster deposits which had to be exposed to laboratory air during transfer from the UHV deposition environment to a standard transmission electron microscope for size and number density analysis. SiO x also retards but does not prevent electron beam effects during high-resolution transmission electron microscopy.

Three-dimensional transmission electron microscopy observations of supported palladium particles

Three-dimensional transmission electron microscopy observations of supported palladium particles

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd n CO+ (n=1, 2) for molecular adsorption and Pd n C+ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO+/σ n Pd n CO+ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O2 (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO+ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.

Crystallographic structure and chemisorption activity of palladium/mica model catalysts: III. Static secondary ion mass spectrometry study of CO chemisorption on small palladium particles

The application of static secondary ion mass spectrometry (SSIMS) to the surface investigation of small supported palladium particles is described. The method is applied within the frame of a catalytic reaction study (CO oxidation). The secondary ion yields are measured during CO adsorption and desorption. It is shown that it is possible to identify two states of CO bonding and to make quantitative measurements of adsorbed amounts in relation to previous thermal desorption results. The effects of heating treatments in CO + O 2 mixtures are investigated for various particle sizes.

In-situ TEM evidence of lattice expansion of very small supported palladium particles

A direct, in-situ TEM technique was employed to demonstrate that under otherwise identical conditions the lattice parameter of palladium particles in the 1–2 nm size range (supported in random orientation on ex-situ prepared muscovite mica) is expanded by (2.9 + 0.2)% when compared to particles of 5 nm mean size in which case the lattice parameter is believed to have reached the bulk value. The expansion is not due to pseudomorphism and is considered to only secondarily be a size effect. It is instead believed to be primarily due to a transformation of the 1.5 nm as-deposited Pd particles with predominantly composite crystal structures into larger particles with true fcc structure and with inherently smaller lattice parameter.

In-situ tem evidence of lattice expansion of very small supported palladium particles

In-situ tem evidence of lattice expansion of very small supported palladium particles

A TEM study of the electron-beam interaction with small supported palladium particles and alumina and titania substrates

The interaction of 100 and 200 keV electron beams with amorphous alumina and titania substrates and nanometer-size palladium particulate deposits was investigated for the two extreme cases of (a) large-area electron-beam flash-heating and (b) small-area high-intensity electron-beam irradiation. The former simulates a short-term heating effect with minimum electron irradiation exposure; the latter simulates high-dosage irradiation with minimum heating effect. All alumina and titania samples responded to the flash-heating treatment with significant recrystallization. However, the size, crystal structure, shape, and orientation of the grains depended on the type and thickness of the films and the thickness of the Pd deposit. High-dosage electron irradiation also readily crystallized the alumina substrate films but did not affect the titania films. The alumina recrystallization products were usually either all micron-size grains in the alpha phase, or they were a mixture of much smaller grains in a number of low temperature forms including γ-, δ-, κ-, β-, and θ-aluminas. Palladium deposits reacted heavily with the alumina substrates during either treatment, but they were very little affected when supported on titania. The study suggests that under illumination conditions typical for very high resolution, very high magnification microscopy with modern high-performance electron microscopes, electron beam effects on oxide-supported metal particle specimens may be severe.

Growth characteristics of supported palladium particles in gaseous environments

Growth characteristics of supported palladium particles in gaseous environments

The Physical Structure and the Effect of Silica Xerogel Support on Metal Catalyst

Silica xerogel is used as a support of metal catalysts. This paper deals with the relation between the physical structure and the supporting effect of the silica xerogel on the dispersibility and the heat resistivity of the supported palladium particles. It was found that specific area of metallic palladium (a measure of dispersibility) was a function of the micropores and the rate of sintering of palladium particles (a measure of heat resistivity) was proportional to the mean radius of the micropores. Consequently, it may be concluded that the mean radius of the support may play an important role in the dispersibility and heat resistivity of palladium particles.