Model Supported Metal Catalysts Research Articles

In Situ Synthesis and Nanoscale Evolution of Model Supported Metal Catalysts: Ni on Silica

The ability to disperse metal nanoparticles over a high surface area support is critical for catalyst preparation. For impregnation techniques, there are many fundamental questions about the spreading, diffusion, and coarsening behavior of metal intermediates during the drying and thermal treatments. We employ in situ environmental transmission electron microscopy to monitor the nanoscale processes taking place during the preparation of Ni on silica-supported metal model catalysts. Conventional salt impregnation techniques were employed, and the electron microscope was used to follow the spatial evolution of the material from salt impregnation through calcination and reduction. The best precursor dispersion was obtained with a nickel nitrate solution prepared in an ethanol solvent, followed by performing impregnation in an atmosphere saturated with ethanol vapor to avoid premature drying or deliquescence during mixing. In situ observations showed that no significant wetting or dispersion of the nickel pre...

Aliphatic Nitrile Adsorption on Al2O3and ZrO2As Studied by Total Internal Reflection Sum-Frequency Spectroscopy

Total internal reflection sum-frequency spectroscopy has been used as an in situ vibrational spectroscopic probe of the interface between neat liquid aliphatic nitriles (acetonitrile, butyronitrile) and flat oxide surfaces (ZrO2, Al2O3). Such oxide surfaces serve as models for high surface area supports used for supported metal catalysts that are effective for nitrile hydrogenation reactions. Two types of surface nitrile species were observed to form on the oxide surfaces when exposed to neat nitriles. A weakly bound species coordinates to the surface through the nitrogen of the CN group and exhibits a νCN stretching frequency very similar to that of the bulk liquid nitriles. The lower νCN frequencies of this species on Al2O3 as compared to ZrO2 suggest a stronger interaction with the former surface. It is proposed that this stronger interaction arises from increased hydrogen bonding that is caused by a larger amount of native OH on the Al2O3. A second type of species exhibits a further downshifted νCN stretch and is assigned to intermolecular polar interactions and nitrile clusters at the interface. Cluster formation is diminished in the case of butyronitrile, likely due to increased steric hindrance. Such steric hindrance is suggested by gauche defects detected by sum-frequency measurements in the C−H stretching region. Finally, using polarization-dependent measurements of the CH3 stretching mode, the average orientation of acetonitrile on these surfaces was found to be around 20−25° from the surface normal. The prospects for using this approach for examining model-supported metal catalysts are discussed in light of these findings.

HREELS study of the interaction of ethylene with Pt films supported on ZrO 2(100) and ZnO(0001)

The adsorption and reaction of ethylene on epitaxial Pt films supported on ZrO 2(100) and ZnO(0001) was studied using high-resolution electron energy-loss spectroscopy (HREELS). On the (111) oriented surfaces of the supported Pt films, ethylene was found to adsorb in both di-σ and π-bonded configurations at temperatures near 100 K. This result is in contrast to the interaction of ethylene with Pt(111) where only the di-σ species is stable at 100 K and suggests that the surfaces of the films contained a high concentration of steps and other defects. Both di-σ and π-bonded ethylene were also detected on Pt particles supported on ZrO 2(100) at 130 K. On the supported Pt films and particles, adsorbed ethylene underwent dehydrogenation and rearrangement to form ethylidyne upon heating to 300 K. The results of this study serve to further demonstrate the utility of HREELS in characterizing adsorbates on model supported metal catalysts.

The Impact of Aqueous Impregnation on the Properties of Prereduced vs Precalcined Co/SiO 2

One of the methods of preparing model supported metal catalysts for fundamental studies of promoter effects is by sequential impregnation of reduced catalysts. In general, such promotion has been found not to alter the metal particle size distribution. However, the promotion process itself may possibly result in chemical and/or structural modifications of certain catalysts. In order to better understand how the process of promotion may itself modify a Co catalyst, the effect of water impregnation and drying at various temperatures (25-110°C) of reduced or calcined 20% Co/SiO 2 was investigated. The results show that water impregnation and subsequent drying had a significant effect on the structure of the reduced and passivated Co catalysts. The Co oxide phase declined in favor of a Co silicate/hydrosilicate phase which reduced completely only above 800°C. Steady-state isotopic transient kinetic studies of ethane hydrogenolysis revealed that the surface concentration of reaction intermediates declined following water impregnation, which suggests that the Co silicate/hydrosilicate formed was inactive. Aqueous impregnation and drying of calcined Co/SiO 2, on the other hand, did not appear to modify the catalyst in any significant way.

Surface spectroscopic studies of model supported-metal catalysts

A new surface science approach to the study of supported-metal catalysts will be described. Thin oxide films (~100 Å) of SiO2, Al2O3, or MgO supported on a refractory metal substrate (e.g., Mo or W) have been prepared by depositing the oxide metal precursor in a background of oxygen (ca. l×l0-5 Torr) [1]. The thin-film catalysts facilitate investigation by an array of surface techniques, many of which are precluded when applied to the corresponding bulk oxide [1,2]. In particular, the oxide films have been characterized by AES, ELS, HREELS, XPS, UPS, ISS, IRAS, and TD spectroscopies and shown to have essentially identical electronic and vibrational properties of the corresponding bulk oxides. These studies indicate then that these films can serve as convenient models for oxide catalysts or metal supports. Metal thin films (e.g., Cu, Pd, Ni) have subsequently been deposited onto the oxide films and the properties of the metal/oxide system then studied with the above array of surface techniques [3]. By properly defining the metal thin film thickness, metal particles of varying sizes can be synthesized with dispersions from a few nanometers to tens of nanometers.

Construction of model supported metal catalysts for scanning tunneling microscopy examinations: Platinum ultrafine particles on silica and on alumina

Scanning tunneling microscopy (STM) images of Pt ultrafine particles dispersed on silicon oxide and on aluminum oxide surfaces were obtained. The silicon oxide sample employed consists of a thin (ca. 0.6 nm thick) native silica film present on a Si(100) surface. The aluminum oxide sample was prepared by vacuum-depositing aluminum metal on a native-oxide-covered single crystal silicon surface, followed by its air oxidation. Model Pt/SiO2 and Pt/Al2O3 catalysts were prepared by vacuum-depositing Pt metal on those oxide surfaces. The STM observations of these sample surfaces were critically dependent on the tunneling conditions, and the optimum STM observation conditions for those supported metal catalyst systems were identified.

Structural and Catalytic Properties of Model Supported Nickel Catalysts

The surface structure and catalytic properties of model silica-supported nickel catalysts have been investigated with infrared reflection-absorption spectroscopy (IRAS) and reaction kinetics of ethane hydrogenolysis and carbon monoxide methanation. Nickel particles in the range 15-80 %L were vacuum deposited onto silica thin films using established preparation procedures. Specific rates and activation energies for ethane hydrogenolysis and carbon monoxide methanation over the model catalysts are remarkbly similar to the corresponding values for both traditional silica-supported nickel catalysts and nickel single crystals. The turnover frequency for CO methanation is independent of particle size for the dimensions studied. The ethane hydrolysis rate, however, increases with particle size to a maximum at -25 A and thendecreases. This reactivity trend is found to parallel a change in the percentage of bridging CO on the nickel particles as determined by IRAS. An understanding of the relationship among surface structure, particle size, and chemical properties is of primary importance in the design of a cata1yst.l Typically, oriented single crystals have been used as model catalysts to investigate the influence of morphology on reactivity/selectivity. These surface science studies, under well-defined conditions, have provided insights into the catalytic mechanisms, active sites, and reacting species of heterogeneous catalysts at the molecular level.2 There are, however, distinct differences between the well-defined single crystals used in surface science studies and metal catalysts supported on high surface area oxides. It has been shown that the interaction between a dispersed modelcatalyst and its support can modify the activity of the catalyst depending on the reaction and the reaction condition^.^ These metal-support interactions have been investigated by depositing an oxide onto the surface of a single-crystal metal surface and monitoring the modifications to the reactivity of the metal? There are inherent problems with this approach to studying supported metal catalysts in that the metal maintains its bulklike characteristics. This is contrary to the effects observed for dispersed metal catalysts, where activity and selectivity often depend upon the size of the metal particle.5 A more favorable approach to the synthesis of a realistic model of a dispersed supported metal catalyst is to deposit a metal onto an oxide support material. The incorporation of surface science techniques in the study of these model supported metal catalysts has been limited due to the experimental difficulties related to sample mounting, sample heating/cooling, and surface charging of insulating oxides. Recently, we have circumvented these problems with an approach utilizing metal deposition onto a silicon dioxide thin film ( N 100 A) which, in turn, is supported on a molybdenum metal substrate.Gs These planar Sios thin film models have been shown to be good models for high surface area ~ilica.~ The Mo(ll0) substrate does not alter the chemical properties of the thin film, yet improves the thermal stability of the Si02 relative to Si02 grown on silic~n.~J~ The specific activity of supported nickel catalysts toward alkane hydrogenolysis generally depends markedly upon the metal particle size.sJ1 Specific activity is defined as the catalytic rate normalized to the number of exposed metal atoms and is therefore representative of the molecules produced per surface atom. Although the relationship between particle size and activity has been well documented, there is not a generally accepted expla- Abstract published in Aduonce ACS Abstracts, January 1, 1994.

Evolution of bimodal distributions in the sintering of model supported metal catalysts

Model Pt/γ-Al 2O 3 catalysts were prepared and characterized in our laboratories. They were sintered in flowing oxygen at 600 °C for up to 24 h. Particle sizes, shapes, and relative positions were determined from bright field images, and crystal structures from diffraction in the transmission electron microscope. Ion scattering spectroscopy gave surface compositions. The platinum particles in the unsintered samples were uniformly sized and evenly distributed upon the support. The sintered samples contained some larger particles, along with smaller (2-nm) particles that uniformly covered the support surface, giving rise to a bimodal size distribution. The average size of the particles in the larger mode increased with time, while both the size and the number of the particles in the smaller mode remained substantially constant. The average size of the larger particles after 24 h sintering was compared to a theoretical model that considers Ostwald ripening and particle migration. Agreement between the calculated and the observed distributions requires a lower than expected value for the metal-support interfacial tension. The value is consistent with the presence of metal oxide rather than metal in contact with the support. Surface analysis showed that at least the free surface of the metal particles is oxidized, while electron microscopy indicates that the interior remains as metal. This structure is also expected to affect the particle size distribution by promoting splitting until the entire particle is converted to oxide.

Simulation of sintering of model supported metal catalysts

AbstractDuring the past decade, the sintering of model supported metal catalysts, i.e. catalysts consisting of metal deposited on very thin and flat oxide films, has been intensively investigated by transmission electron microscopy. In the current paper, a mathematical model, based on an atomic migration mechanism, for the sintering of these catalysts is presented. The predictions of the model, based on Monte Carlo simulations, are in very good agreement with the experimental observations, i.e. phenomena such as ‘apparent’ particle migration, splitting and neck formation between particles are predicated by the model.

High resolution EELS measurements of CO adsorption on Pt/Al 2O 3 model catalysts via impact scattering

The use of high resolution electron energy loss spectroscopy (EELS) to study adsorption on model supported metal catalysts is investigated, using CO adsorption on Pt/Al 2O 3 as a simple test case. The model catalysts are formed by evaporating Pt onto an oxidized, polycrystalline, aluminum foil. Transmission electron microscope pictures show that the model catalyst consists of metal particles of average diameter 1.1–1.3 nm covering 18%–25% of the surface area of the support. EELS spectra recorded for specular and off-specular electron scattering reveal that the strong loss peaks due to vibrations of the smooth substrate occur via inelastic dipole scattering, but that the loss peaks due to CO adsorbed on the rough particles occur via the inelastic impact scattering mechanism. This has important implications for the use of EELS to study model supported metal catalysts.

Specimen preparation technique for high resolution transmission electron microscopy studies on model supported metal catalysts

SUMMARYA new technique is described which can be used for preparing transmission electron microscopy (TEM) specimens suitable for high resolution studies on supported metal catalysts. By conventional silicon processing techniques 200 × 200 μm2 Si3N4 membranes on Si wafers are produced. These membranes are extremely flat and have a uniform thickness of 13 nm. They can be used as a support in various kinds of thin film deposition. A TiO2 film, optimally structured with respect to the requirements for high resolution TEM work in TiO2–metal cluster systems, is deposited on the Si3N4 layer. It consists of one monolayer of 10–25 nm TiO2 crystallites. TiO2 lattice images show that a line resolution down to 0.19 nm is possible. Examples of TiO2–Pd and TiO2–Rh are given using respectively photodeposition and impregnation reduction to produce l.5–4 nm metal clusters.

The effect of steam on supported metal catalysts

In order to examine the effect of steam on supported metal catalysts, model supported metal catalysts of Ni, Co, or Fe on alumina have been heated in steam at 700 °C. The transmission electron micrographs show that for all these metals, patches of film extend from the crystallites. Prolonged heating results in the disappearance of the patches which probably spread as a contiguous film over the entire surface of the substrate. The degree of spreading is in the order: Co > Ni > Fe. On subsequent heating in H 2, small crystallites were generated, probably via the rupture of the contiguous film. The contraction of the patches of film bridging two or several particles caused the coalescence of the latter. This subsequent heating in H 2 favors redispersion only when the heating time is sufficiently short. Prolonged heating in H 2 leads to the disappearance of the small particles.

Inelastic electron tunneling spectroscopic study of acetylene chemisorbed on alumina supported palladium particles

Inelastic electron tunneling spectroscopy was used to study the adsorption of acetylene on palladium particles supported by alumina. Low peak intensities in the spectra complicated the analysis of the multiple species present. Nevertheless, some species could be tentatively identified including weakly bonded acetylene, two types of strongly bonded acetylenic species and two other further rehybridized species. The prospect for the future application of tunneling spectroscopy to the study of other hydrocarbons adsorbed on model supported metal catalysts is discussed.

Regeneration of model supported metal catalysts

Regeneration of a model catalyst of Pt over γ-Al 2O 3 was studied as a function of time using a TEM. The decrease in particle diameter was accomplished by heating the sample in air at 500 °C for times up to 2 h. The results were analyzed using a model which is shown to be particularly valid for short times and large particle sizes. The model postulates a surface layer of oxide on the supported metal particle. The difference between PtPt distances in the surface layer of platinum oxide and in the unoxidized metal gives rise to forces which may crack and split the particle. The experimental results indicate that the initial regeneration rate is rapid, and that the rate falls off after about 2 h of heating. Applying the model to the experimental data yields values for an average rate of oxidation of 2.37 nm/h, and a minimum particle diameter for crack initiation of 8.6 nm. These values are consistent with numbers obtained for these parameters from completely different circumstances.

The application of high resolution electron energy loss spectroscopy to the study of model supported metal catalysts

Model supported metal catalysts were fabricated by evaporating a small quantity of rhodium onto an oxidized aluminum substrate. High resolution electron energy loss spectroscopy (ELS) was successfully used to obtain the vibrational spectra of carbon monoxide chemisorbed on these model catalysts. The ELS spectra are compared with vibrational spectra obtained by infrared spectroscopy and inelastic electron tunneling spectroscopy for CO chemisorbed on similar model catalysts.

Particle growth in model supported metal catalysts—I. Theory

A theoretical study of capillarity driven growth has been performed for several regimes of importance to the ripening of noble metal particles supported on flat, partially wetted, metal oxide substrates. Three related models are presented, and in each case two modes of interparticle transport are considered, namely: vapor phase transport and substrate surface diffusion. The first model is analogous to classical Ostwald ripening. The second is a simple extension of the first but allows for interparticle transport via a volatile oxide species. The third model is applicable to the growth of faceted particles and incorporates the concepts of “pill box” nucleation as a growth inhibiting factor. The growth kinetics predicted by the third model are quite different from those predicted by classical ripening models. In particular, changes in the initial particle size distribution, which cause only short-lived transients in the case of classical ripening, are found to have a profound effect on the growth kinetics as well as other characteristics of nucleation inhibited growth.

Particle growth in model supported metal catalysts—II. Comparison of experiment with theory

The growth of platinum particles supported on flat alumina substrates has been measured in the temperature range 600–800°C, in two different oxidizing atmospheres having oxygen partial pressures of 2 and 20 kPa respectively. Comparison of the measured growth kinetics with predictions of classical ripening models does not yield satisfactory agreement. However, good agreement is obtained when the data are compared with predictions of a model of nucleation inhibited growth.