Catalysis

10.2.1. Nature of Catalysis

Catalysis involves the modification of the rate of a chemical reaction, usually a speeding up or acceleration of the reaction rate, by the addition of a substance, called a catalyst, that is not consumed during the reaction Ordinarily the catalyst participates in the reaction by combining with one or more of the reactants, and at the end of the process it is regenerated without change. In other words, the catalyst is being constantly recycled as the reaction progresses. When two or more chemical reactions are proceeding in sequence or in parallel, a catalyst can play the role of selectively accelerating one reaction relative to the others.

There are two main types of catalysts. Homogeneous catalysts are dispersed in the same phase as the reactants, the dispersal ordinarily being in a gas or a liquid solution. Heterogeneous catalysts are in a different phase than the reactants, separated from them by a phase boundary. Heterogeneous catalytic reactions usually take place on the surface of a solid catalyst, such as silica or alumina, which has a very high surface area that typically arises from their porous or spongelike structure. The surfaces of these catalysts are impregnated with acid sites or coated with a catalytically active material such as platinum, and the rate of the reaction tends to be proportional to the accessible area of a platinum-coated surface. Many reactions in biology are catalyzed by biological catalysts called enzymes. For example, particular enzymes can decompose large molecules into a groups of smaller ones, add functional groups to molecules, or bring about oxidation-reduction reactions. Enzymes are ordinarily specific for particular' reactions.

Catalysis can play two principal roles in nanoscience: (1) catalysts can be involved in some methods for the preparation of quantum dots, nanotubes, and a variety of other nanostructures; (2) some nanostructures themselves can serve as catalysts for additional chemical reactions. See Moser (1996) for a discussion of the role of nanostructured materials in catalysis.

10.2.2. Surface Area of Nanoparticles

Nanoparticles have an appreciable fraction of their atoms at the surface, as the data in Tables 2.1, 9.1 demonstrate. A number of properties of materials composed of micrometer-sized grains, as well as those composed of nanometer-sized particles, depend strongly on the surface area. For example, the electrical resistivity of a granular material is expected to scale with the total area of the grain boundaries. The chemical activity of a conventional heterogeneous catalyst is proportional to the overall specific surface area per unit volume, so the high areas of nanoparticles provide them with the possibüity of functioning as efficient catalysts. It does not follow, however, that catalytic activity will necessarily scale with the surface area in the nanoparticle range of sizes. Figure 4.14, which is a plot of the reaction rate of H2 with Fe particles as a function of the particle size, does not show any trend in this direction, and neither does the dissociation rate plotted in Fig. 10.5 for atomic carbon formed on rhodium aggregates deposited on an alumina film. Figure 10.6 shows that the activity or turnover frequency (TOF) of the cyclohexene hydrogénation reaction (frequency of converting cyclohexene (^H, to cyclohexane CsHf,) normalized to the concentration of surface Rh metal atoms decreases with increasing particle size from 1.5 to 3.5 run, ami then begins to level off. The Rh particle size had been established by the particular alcohol C„H2„+ )OH (inset erf Fig. 10.6) used in the catalyst preparation, where n — 1 for methanol, 2 for ethanol, 3 for 1-propanol, and 4 for 1-butanol.

The specific surface area of a catalyst is customarily reported in the units of square meters per gram, denoted by the symbol S, with typical values for commercial catalysts in the range from 100 to 400 m2/g. The general expression for this specific surface area per gram S is

where p is the density, which is expressed in the units g/cm3. A sphere of diameter d has the area/I = red2 and the volume V — ltd3¡6, to give A/ V — 6/d. A cylinder of

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