Electronic Properties of Small Metal Particles

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... 

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

See also a review on metal colloids in ionic crystals [8]. — See also a report on electronic properties of small metallic particles [9]. — According to a review on quantum size effects [10]. d = 22.4 + 3.2 A in original paper [4]. ... 

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. 

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. 

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. 

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). 

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],... 

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. 

The techniques that have been most employed for investigating the electronic properties of small particles are photoemission (UPS, XPS), soft X-ray spectroscopy, EXAFS, photoionization mass spectrometry, and AES (23, 111, 240, 257d,e). While there is some controversy from theoretical work about the minimum particle size required to give bulk properties—from 10 (258) to several hundred atoms (259)—there seems to be a consensus that a cluster of about ISO atoms or more is required to observe a photoemission spectrum similar to that of the corresponding bulk metal (23, 260). When other properties are considered (ionization potential, density of states, valence bandwidth, etc.), the agreement is less satisfactory between the results obtained with different techniques (23). 

Supported iridium clusters on oxides were intensively investigated [267] because they form rather stable metal frames such as Ir4. Despite the substantial work on such samples, questions remained due to limitations of EXAFS spectroscopy widely used for the characterization and the lack of other experimental techniques suitable for investigating metal species dispersed in porous solids. To clarify how the interaction with a zeolite framework can modify the structural and electronic properties of small supported metal particles, we carried out a scalar relativistic DKH DF study of the interaction between fr4 clusters and a zeolite fragment, using a cluster model of a faujasite six-ring [268]. 

Ideally, the substrate should be sufficiently inert to guarantee that the metal-substrate interaction will not affect the properties of the metal particle. Often, however, a relatively strong interaction can occur between substrate and metal species, in particular for small clusters. In a sense, the oxide surface can be considered to be a very special case of a ligand environment stabilizing the metal cluster. The metal electronic states are, however, always perturbed by interaction with the substrate, just as for proper ligands. 

Also transport properties, which are strictly related to the electronic structure of a metal particle, critically depend on size (37). For small particles, the electronic states are not continuous, but discrete, because of the confinement of electron wave function. Consequently, also properties like electrical and thermal conductivity may exhibit quantum-size effects. 

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