Structure of small metal particles

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 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. 

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. 

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... 

In Sect. 4, some aspects of the electronic structure of small gold particles in general, and of AU55 in particular, are considered. The question of metallic bonding of the gold atoms in the cluster is discussed, utilizing the reported results of a number of different measurement techniques. 

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. 

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. 

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. 

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... 

It seems plausible that the catalytic activity of small metal particles would be influenced by the crystal structure. Yacaman et al. (118) have studied pentane hydrogenolysis over Rh -A C, Rh/Si02, Rh/C, Rh/Ti02, and Rh/MgO. The support and preparation method, all for particles of d < 5 nm, determined whether cubooctahedrons or icosahedrons were formed, but the catalytic properties depended more on d than on crystal type. 

E. Structure and Electronic Properties of Small Metal Particles... 

Important questions of metal adsorption at ionic metal oxides are (i) preferred adsorption sites (ii) strength and nature of metal-support interactions on regular and defect-rich surfaces (iii) charge redistribution between deposits and supports (iv) geometric and electronic structure as well as magnetism of small metal particles and deposition-induced alteration of these features (v) implications for the reactivity. We will also discuss accuracy improvements due to more precise xc functionals as well as more realistic cluster models of oxide supports. 

Related to the Surface Science approach to catalysis is the use of molecular beam techniques . With Mass Spectroscopy and other detection techniques the reactivity of small metal particles effusing into a vacuum is studied. Again reaction conditions are well defined, but the temperature is close to zero Kelvin. This approach enables the study of chemical reactivity 2ls a function of particle size. Knowledge on the reactivity of isolated particles is relevant especially for theoretical studies since these data refer to well-defined particles without interaction with a support. Theoretical studies can be conveniently done with isolated small clusters. As with surface science studies, molecular beam experiments are fundamental to the study of elementary reactions as a function of surface or particle structure. We will frequently refer to data collected by these approaches. 

Returning for a moment to the description of bonding inside the crystal, " those d-orbitals whose interactions are responsible for bonding nearest neighbours (viz. the t2g family) will form a band which is broader than that formed by the 6g family, since interactions between next-nearest neighbours are less strong. Extending this concept to surface atoms, we see on the (KX)) surface for example that the absence of atoms above the plane means that the overlap of dxz and d y orbitals has decreased and their band is narrowed, while the dyz orbitals in the surface plane are unaffected, and their band remains broader. Similar but smaller effects will occur with the Cg and 5-orbitals. The modification of electronic structure of atoms at steps and kinks is then easily rationalised, and the story will be resumed in Chapter 2, where other concepts developed in the context of small metal particles will be considered. 

This brief survey must suffice to introduce a pervasive phenomenon which must often affect the structure and composition of small metal particles, and hence their catalytic behaviour this will be a recurring item in later chapters. Nunc est bibendum. 

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