Small metal particles chemisorption

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

The Characterization and Properties of Small Metal Particles. Y. Takasu and A. M. Bradshaw, Surf. Defect. Prop. Solids p. 401 1978). 2. Cluster Model Theory. R. P. Messmer, in "The Nature of the Chemisorption Bond G. Ertl and T. Rhodin, eds. North-Holland Publ., Amsterdam, 1978. 3. Clusters and Surfaces. E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Cornell National Science Center, Ithaca, New York, 1978. 4. Determination of the Properties of Single Atom and Multiple Atom Clusters. J. F. Hamilton, in "Chemical Experimentation Under Extreme Conditions (B. W. Rossiter, ed.) (Series, "Physical Methods of Organic Chemistry ), Wiley (Interscience), New York (1978). 

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 reveal a not fully developed valence band (they have a system of discrete levels rather than a quasi-continuous metallic-like band), which effect influences the binding energy as determined by XPS and might be, in principle, important also for chemisorption and catalysis (99, 100). 

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. 

Henglein, A. Physicochemical properties of small metal particles in solution Microelectrode reactions, chemisorption, composite metal particles, and the atom-to-metal transition, J. Phys. Chem. 1993, 97, 5457. 

It has been a major point of interest in the study of small metal particles to determine the precise point (if indeed there is a precise point) at which metallic character is lost. The broader context of this problem as it relates to other metals such as mercury and sodium has been discussed in a series of important papers by Peter Edwards and his associates.135,136 The difficulty seems to be that there is no agreed criterion by which membership of the metallic state can be judged, and various physical techniques give somewhat different answers because they sense slightly different aspects of electron behaviour. The question has been earnestly addressed in the case of gold, partly because of the familiar sensitivity of catalytic activity to particle size as noted above (Section 2.1) the way in which electrons are used to form metallic bonds determines the character of the free valences at the surface, and hence the kind of chemisorption bond that is formed with the reactants. 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

Chemisorptive properties of small metal particles depend upon the size of the particle. We find that the orbital spectrum of halogen (Cl) is dependent upon the size of the silver particle to... 

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. 

Immediately evident from the data in Table 3 is the small metal particle diameters and high metal dispersion. Chemisorption data for a literature catalyst prepared with activated carbon as a support is also given in Table 3 [3]. It follows that the metal dispersion is lower and the metal diameter is larger for the carbon nanofiber supported catalysts most probably due to the micropores of the activated carbon. 

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