Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 21

Another size‐dependent property of nanoparticles is their chemical reactivity. This is demonstrated most dramatically by gold, which in the bulk is the archetypal inert material. This is one of the reasons it is so highly valued since it does not corrode or tarnish and so has a timeless quality. The only acid that is known to attack it is a hellish brew of concentrated nitric and hydrochloric acids mixed to form what has been poetically named aqua regia (royal water). Gold would therefore seem to be useless as a catalyst to speed up chemical reactions, but this is not so for gold nanoparticles. Catalysts provide a surface whose chemical properties reduce the energy required for a specific reaction between two other species to occur and are vital to the chemical industry. Because it is only the surface that is active, catalysts are in the form of nanoparticles to maximize the surface area per gram of material (see Problem 1) and so their chemical properties are different from the bulk form, which is also true for gold. When gold is in the form of nanoparticles with diameters less than about 5 nm it becomes a powerful catalyst, especially for the oxidation of carbon monoxide (CO). The full story is quite complicated because the reactivity of gold nanoparticles appears to depend not only on their size but also on the material on which they are supported.

An assessment of a number of research papers on the effectiveness of gold in catalyzing the above reaction, however [16], has concluded that the dominant effect is that of the gold nanoparticle size, with the nature of the support playing a secondary role. Figure 1.15 shows a compilation of data on the oxidation of carbon monoxide (CO) by gold nanoparticles on various supports as a function of their size and shows the impressive performance of the gold, which is completely inert in macroscopic‐sized pieces. Since catalysis can only occur at the surface layer of atoms, the dominant size effect is the proportion of gold atoms that are at the surface. In fact, the most important atoms for catalysis are those at the corners between different facets. These low co‐ordinated atoms are where the reacting carbon monoxide (CO) molecules preferentially bond during the reaction. The fraction of this type of atom (highlighted in red in Figure 1.15) is proportional to 1/d³, where d is the particle diameter, and the black line in Figure 1.15 is a fit to the data using this law demonstrating that the dominant size effect is indeed the proportion of corner atoms at the surface. The focus here has been on gold because of its extreme demonstration of a size‐dependence, that is, from a completely inert material to a powerful catalyst but as a general rule, the performance of all catalysts depends on the particle size. Because of the importance of catalysts to the chemical industry, the effect is the focus of a significant research activity.

Figure 1.15 Reactivity of gold nanoparticles. Measured activities of gold nanoparticles on various supports (box) for carbon monoxide oxidation as a function of particle size. The black line is a fit using a 1/d³ law and is seen to broadly represent the variation indicating that the dominant size effect is the proportion of gold atoms that are at a corner between facets at the surface (see text). Such atoms are highlighted in red on the nanoparticle shown.

Source: Reproduced with the permission of Elsevier Science from N. Lopez et al. [16].