Surface density of particles in the design of nanostructured catalysts

Abstract

A discussion is given for theoretical and experimental data confirming the correlation between the catalytic properties of supported catalysts based on metal nanoparticles, the existence of contacts between the particles, and the possibility of charge transfer between them. Laser electrodispersion was used to prepare model systems. Efforts to increase the efficiency of supported metal catalysts require detailed knowledge of the processes occurring on the catalyst surface. Nanoparticle size is a very important factor determining the efficiency of catalysis featuring such particles [1-3]. However, the efficiency of nanocatalysis is not simply a function of optimal particle size. Such factors as the physicochemical environment of the particles on the catalyst surface and proper selection of support and promoter, providing for the electronic state of the metal necessary for maximum efficiency of the entire catalytic systems as a whole, play no less important roles. These questions have been analyzed in detail in a review by Siepua [4]. However, still another aspect of the dimensional properties of nanoparticles has not been studied extensively. The essence of these properties is that particles with diameter of a few nanometers are extremely sensitive not only to chemical interactions but also charge interactions between such particles. Thus, the properties of nanoparticles may depend not only on their own size but also the mean distance between particles or the particle surface density. The present study treats the theoretical and experimental development of a new approach to the creation of highly efficient catalysts by taking account of collective charge interactions in supported nanoparticle systems. The idea about a role of charge interactions in catalysis by nanoparticles arose from the discovery of unusual relationships between the catalytic properties of polymeric nanocomposites and the metal content [5]. For a series of transformations of chlorohydrocarbons, a sharp increase in the catalytic capacity of nanocomposites is observed precisely when conducting properties arise in the polymer film. This finding led to the hypothesis of a correlation between catalytic properties and the existence of contacts between particles and the possibility of charge transfer among them. More rigorous experimental proof for the role of charge effects in catalysis was found using catalytic systems having homogeneous size distribution of the supported metal particles. Such systems were obtained by a recently developed method