Small metal particles structure

Unfortunately, until very recently experimental studies of small metal particle structure usually lacked the resolution necessary to observe differences between various geometries. In most cases only average particle sizes or size distributions are obtained and these usually assume simple cubic or spherical geometries. Technology was not available (and is still not very accessible) to study local surface disorders on an ordered core of a small particle. Some structural differences between small metal particles produced in different ways have been observed, however. These may be compared to large cluster compounds whose detailed structure can be determined by X-ray crystallographic methods (see Section III). 

Diffraction patterns having relatively well-defined sharp spots can be obtained from small unit-cell crystals with an incident beam of diameter 10-158. Such patterns have been used in the study of the structures of small metal particles (22). For particles 10-20A diameter the electron beam can illuminate the whole of the particle... 

The shape of the nanoparticles depends on numerous parameters such as the nature of the metal and the support, the metal loading. Of the various models of polyhedral metal particles [106], the cubooctaedral structure can be used to represent small metallic particles (Scheme 31). Note that these idealized structures can vary with the nature of chemisorbed species (vide infra) and very subtle atomic rearrangements probably occur during catalytic events. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

In the present paper non-conventional TEM methods to characterize small metallic particles are presented. The topographic information on the particles shape can be combined with micro-diffraction (using STEM) data to obtain a full characterization of the particle. The case of gold particles evaporated on a NaCl substrate is used as example. The particle shapes observed are discussed. It is shown that many particles have a crystal structure which is different from the bulk (Fee). 

The study of shape and crystal structure of small metallic particles is of prime importance in modern catalysis science. The relation between reactivity and structure is still not well known. The main problem in studying small metallic particles is that conventional techniques fail in the manometer diameter range. However it is possible to overcome these difficulties by the application of non-conventional methods. It is the purpose of this paper to review some of these methods and to present some results on the characterization of gold and platinum particles. 

Many investigations of small particles or of other materials may involve the collection and analysis of diffraction patterns from very large numbers of individual specimen regions. For small metal particles, for example, it may not be sufficient to obtain diffraction patterns from just a few particles unless there is reason to believe that all particles are of the same composition, structure, orientation and size or unless these parameters are not of interest. More commonly, it is of interest to obtain statistics on the variability of these parameters. The collection of such... 

The single crystal results are compared in Fig. 2 with three sets of data taken from Ref. 13 for nickel supported on alumina, a high surface area catalyst. This comparison shows extraordinary similarities in kinetic data taken under nearly identical conditions. Thus, for the Hj-CO reaction over nickel, there is no significant variation in the specific reaction rates or the activation energy as the catalyst changes from small metal particles to bulk single crystals. These data provide convincing evidence that the methanation reaction rate is indeed structure insensitive on nickel catalysts. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Chemical and Structural Analogy between Molecular Clusters and Small Metallic Particles 5... 

Decompose in a mild way the cluster after its adsorption on the surface of a metal oxide, to remove the ligands, and try to keep the size and possibly the structure of its metallic core as if the support by itself could stabilize a small metal particle with the same nuclearity of the metallic core of the original molecular cluster. 

The catalytic behavior of small metal particles in heterogeneous catalysts varies with metallic particle size and shape a phenomenon referred as a structure-sensitivity. Simple alkanes such as ethane, propane, n-butane and isobutane can be used as archetype molecules for studying hydrogenolysis reactions as they... 

We first review the factors affecting catalyst structures, sintering of small metal particles and ceramic substrates and describe the unique contributions of electron microscopy. 

We first describe the structural proprties of small metal particles. 

Small metal particles have a compressed crystallographic structure. This effect is very well documented in the literature (103-107). 

To answer these questions requires some understanding of the properties of small metal particles, both structural and electronic. In this review we shall examine first the evidence relating to metal particles prepared by direct methods, e.g., vapour deposition or condensation in the gas phase. Then we shall consider whether this information can be applied to the case of supported metals where both precursor decomposition and support effects may add to the complexity of the total system. We shall then consider whether further changes in catalytic properties occur after preparation, i.e., during the catalytic reaction. Finally, we shall summarize some of the more recent evidence concerning the nature of structure sensitivity. 

The Structure of Small Metal Particles (a) Theoretical Considerations. -... 

Baetzold and Hamilton31 recently have reviewed the quantum chemical theoretical models used to compare the stability of different structures of small metal particles. Only a brief summary is given here. 

Electronic Properties of Small Metal Particles (a) Theoretical Considerations. — Catalytic processes involve chemisorption at surfaces. The strength of the chemisorption bond will affect the catalytic activity, and is itself expected to be very sensitive to the electronic properties of the surface metal atoms. (The wide variation in catalytic activity among metals having the same structure is evidence for the paramount importance of electronic properties.) Within the particle size range typically encountered with supported catalysts (see Table 1) it is important to establish whether there will be variations in electronic properties with number of metal atoms. We examine first the theoretical evidence relating to this point. This work has been reviewed frequently31 152-155 so only a few brief comments will be made here. 

The foregoing sections have been concerned with the effect of particle size on the structure and properties of small metal particles. Several general comments can be made concerning the influence of particle size on catalytic properties. 

A final interesting observation both for Ni311 312 and Ru310 catalysts, is that small metal particles disappear during the course of the CO/H2 reaction. This effect has been attributed to the exothermicity of the reaction. The release of heat into a small particle could raise its temperature well above the reaction temperature. This in turn could result in major reconstruction of the particle to give a surface structure which best suited the adsorption of these particular reactants. How this would be reflected in terms of structure sensitivity is an interesting question. 

One further complication concerning the origin of selectivity changes during structural isomerization reactions has been indicated by the work of Kramer and Zuegg.352 They observed that the percentage of n-hexane produced from methyl cyclopentane increases with the amount of interface between Pt and the support. They propose that isomerization occurs by two parallel routes, one on the metal the other at the metal/oxide interface. Such effects, if confirmed, should be more important for small metal particles, and may have influenced the selectivity observed in the other work quoted above. 

There is a consensus from both theoretical and experimental studies that small particles may have unusual physical, chemical, and catalytic properties. Both in terms of numbers of sites of different co-ordination and with regard to electronic effects small means particles having diameters less than about 2 nm. For very small particles, sites having a particular co-ordination may be important, but the calculation of the number and distribution of such sites is subject to serious errors and requires assumptions about particle shapes, etc., which are difficult to confirm, and which may vary from one system to another. Although particles having unusual five-fold symmetry have been detected in certain circumstances, the large majority of small metal particles have conventional cubic symmetry. However, the difference in energy between two alternative structures is small - much smaller than typical heats of... 

Gas-induced morphological changes have been reported, and there is growing evidence that this may be a common occurrence with supported metal catalysts. There is further evidence that a small metal particle may consists of a solid core having a fluid-like surface layer of metal atoms. This raises the possibility that in addition to having catalytic reactions which are structure sensitive it may be necessary to allow that structures are sensitive to catalytic reactions, i.e., reaction-sensitive structures. It is possible that during the initial adsorption of the reactants a small particle will change its surface structure into one which best suits those particular reactants. 

---