Chapter 15 Charge Effects in Catalysis by Nanostructured Metals

Abstract

The catalyst structures, which are thin granulated films consisting of Cu, Ni, or Pd nanoparticles, were fabricated by means of laser electrodispersion technique. This technique allows producing nearly monodispersive and amorphous metal nanoparticles (the particle sizes are 5.0 nm for Cu, 2.5 nm for Ni, and 2.0 nm for Pd; the size dispersion is less than 10%). These particles were deposited on dielectric (thermally oxidized silicon) or semiconductor (naturally oxidized Si) supports and the resulting particle surface densities were closely controlled by the time of deposition. The most important common feature of the fabricated catalysts is their unusually high (up to 10 5 product mole per metal mole per hour) specific catalytic activity measured in several chlorohydrocarbon conversions (Cu, Ni) and hydrogenation (Ni, Pd) reactions. In all the reactions, strong dependencies of the specific catalytic activity on the particle surface density and solution polarity have been observed. The nature of the support affected the activity as well, for instance, different activities were measured when using p- or n-doped Si supports. These experimental facts are explained assuming that, along with the small size and amorphous state of the particles, particles charge fluctuations (resulting from inter-particle or particle–support tunnel electron transitions) determine the catalytic activity of these structures. A theoretical model is developed providing means for calculating the number of the charged particles in case when the structure is deposited on a dielectric or on a conducting support. The speculations on the mechanism of tunnel electron transfer from the charged nanoparticle to the chemisorbed reagent molecule show that, for the reactions proceeding with the electron transfer, nanoparticle charging may result in substantial reduction of the reaction activation energy. Combining these two models allows quantitative estimation of the effect of the particle charge on the catalytic activity. Estimations made on this basis are in good agreement with the experimental results. Utilization of the described phenomenon of particle interaction (related with their charging) opens up a new way for managing the catalytic properties of immobilized metallic nanoparticles.