Small metal particles electronic properties

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]. 

Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

These questions lead on to further fundamental questions concerning the shapes and properties of small metal particles. For example, what is the stable shape for a small metal particle How is this affected by size, method of preparation, temperature, gaseous environment, precursor compound, support morphology, etc. Do small metal particles have different electronic properties from bulk metal Do surface electronic properties depend on particle size, and if so, do they vary in the same way as bulk electronic properties When, indeed, is a particle small enough to have unusual properties ... 

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. 

All the experimental results summarized in this section have been interpreted in the original papers as evidence that there is a change in electronic properties of small metal particles due to their size. Most researchers agree that upwards of 150 atoms is required to attain bulk-like properties. The narrowing of the electron bands is attributed to the fact that in a small crystal there are fewer molecular orbitals which go to make up the electron band. This particular point is not in dispute. However, what has been challenged is the interpretation of shifts in the position of the centroid of the d-band, or... 

The conclusion is that particle size effects on catalytic activity or selectivity due to variations in the inherent properties of small metal particles (geometric or electronic) are unlikely to be important for particles larger than about 1.5-2.0 nm. If size effects are observed for larger particles it is necessary to consider the nature and origin of such effects. 

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... 

Specific catalytic properties of synthesized Pd-PPX nanocomposites have been explained by the tunnel charge transfer between nanoparticles. As mentioned in Section 2, the energy of Fermi level of small metal particle depends on its size [14], At the same time, M nanoparticles immobilized in PPX matrix have rather wide size distribution in the range 2-8 nm (Section 3). Electron transfer between particles of different size results in their mutual charging that leads to equalization of their electrochemical potentials [15],... 

Chuistov K.V., Shpak A.P., Perekos A.E., Rud A.D., Uvarov V.N. (2003) The small metallic particles fabrication methods, atomic and electronic structure, magnetic properties and practical applications, Uspekhi Fiziki Metallov 4, 235-270 (in Russian). 

There are many ways in which small metal particles can be created and examined (Section 3.2). When the gold particles are supported, the first step is to determine their mean size and size distribution for this there is no real substitute for transmission electron microscopy (TEM). The various energetic and electronic properties then need to be examined, and the bases of the available experimental techniques will be briefly rehearsed in Section 3.3. Of particular interest is the point at which the change from metallic to nonmetallic behaviour occurs as size is decreased, because this corresponds very roughly to the point at which catalytic activity (at least for oxidation of carbon monoxide) starts to rise dramatically. Relevant experimental results and theoretical speculations are reviewed in Section 3.4. 

Small metal particles on the nanometer dimension are of particular interest regarding their solid-state properties as well as their important application as catalysts. Such particles offer a useful model allowing the study of structural effects with relevance for a variety of applications in chemistry and physics. The key point of interest lies in the fact that such particles often possess very different and sometimes novel properties compared to those of bulk materials, i.e., their physical properties (spectroscopic, electronic, magnetic) differ from those of the bulk phase and are particle size-dependent [68]. One such property concerns the variation in the electrochemical redox potential between metals in a dispersed and bulk state, as was shown by Plieth [69], when he demonstrated that the redox potential depended on the radius (r) of a metal particle on nanometer dimension (the radius was assumed to vary from 1 to lOnm) according to the following equation ... 

Murray has demonstrated that soluble metallic clusters exhibit coulomb staircase-type behaviour [102]. The ionic space charge formed around the dissolved MFCs is reported to contribute to its capacitance, upon charging of the metal core. It is well known that small metal particles exhibit double layer charging (capacitive charging) properties in liquid electrolytes [104]. The sub-attofarad capacitance associated with the MFCs leads to charging of the tiny capacitor by single electron processes in potential intervals of A V that surpass ke T where is the Boltzmann constant and T is the temperature [102, 105]. 

It is not surprising therefore that the optical properties of small metal particles have received a considerable interest worldwide. Their large range of applications goes from surface sensitive spectroscopic analysis to catalysis and even photonics with microwave polarizers [9-15]. These developments have sparked a renewed interest in the optical characterization of metallic particle suspensions, often routinely carried out by transmission electron microscopy (TEM) and UV-visible photo-absorption spectroscopy. The recent observation of large SP enhancements of the non linear optical response from these particles, initially for third order processes and more recently for second order processes has also initiated a particular attention for non linear optical phenomena [16-18]. Furthermore, the paradox that second order processes should vanish at first order for perfectly spherical particles whereas experimentally large intensities were collected for supposedly near-spherical particle suspensions had to be resolved. It is the purpose of tire present review to describe the current picture on the problem. 

Some attempts have been made to measure the electronic properties of small particles by X-ray photoelectron spectroscopy (XPS). The preparation of samples of isolated small metal particles is not easy. The most successful methods are either vapor deposition of noble metals (Pt or Pd) on carbon or silica, or ion exchange used to prepare metals in Y zeolite. For the noble metals and inert supports used, it is assumed that the metal particles are isolated from each other. 

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

Proportions of atoms having unusually low CN will change relatively little as size is increased beyond about 2 nm (ca. 60 % dispersion). (2) Bulk electronic properties are not likely to be shown by particles having less than about 150 atoms (1.7 nm, 70 % dispersion) (3) Typical surface electronic properties are however probably shown by particles with only 25-30 atoms (90 % dispersion). (4) Unusual crystallographic structures are rare. (5) Since the heat released by chemisorption is enough to convert one structure into another, metastable stmctures are unlikely to survive under reaction conditions, and surface reconstruction may occur frequently. (6) A small metal particle may comprise a solid core and a semi-fluid surface layer. 

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