Supported metals, small particles electronic properties

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 ... 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

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]. 

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. 

However, all this work has been performed on flat (111) surfaces. Small supported metal particles contain mostly edges and comers, which adsorb H stronger than atoms in a (111) flat surface, and may result in a different preferred adsorption site. Moreover, there are strong indications that the support properties influence the electronic structure of the supported Pt... 

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. 

This chapter focuses on metallic NMR behavior, i.e., on properties that are governed by electrons and holes in extended states with small energy differences, such as typically found in metals. Such NMR properties obviously allow us to determine whether the supported metal particles are indeed metallic and not simply small molecules built from atoms that would form a metal in the bulk. In addition, from NMR of adsorbed molecules, some adsorbates become a piece of the metal, (which tells us something about the nature of the chemisorption bond), as frequently happens with chemisorbed carbon monoxide and sometimes with hydrogen. This aspect of the NMR of these adsorbates is discussed later, but work related to their dynamics and reactions is only partially covered other adsorbates are not treated at all. 

It appeared finally that the support can no longer be considered as a pure inert stabilizing partner. Actually, the support acts as a supramolecular ligand and has been claimed to promote specific electronic properties and/or geometrical features of the nano-sized supported metal particles. Any metal-support interaction (MSI) does occur in any case when small particles are deposited on a carrier. However, the extent of their interaction depends on the nature of the metal, but much more on the size of the particles and the nature of the support. 

The results that have been obtained with the catalysts after reduction and passivation are the same as those after calcination, i.e. the textural and structural properties of the support material have completely been retained after the treatments (as determined with nitrogen physisorption. X-ray diflfiaction and transmission electron microscopy). Information concerning the metallic nickel particles has been obtained with X-ray diffraction and transmission electron microscopy. Diflractograms of the catalysts after passivation are shown in Fig. 8. The observed features are exactly the same as for the oxidic systems (Fig. 4) only very broad and low diffractions are visible for the catalyst ex citrate, whereas sharp, intense peaks with a broad onset are observed for the catalyst ex nitrate. Consequently the nickel particles of the catalyst ex citrate have resisted sintering during the reduction treatment, thereby conserving the high dispersion of the catalyst. These results were confirmed by transmission electron microscopy measurements (not shown) only very small nickel nanoparticles situated inside the mesopores were found for the catalyst ex citrate. 

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. 

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