Thèse de doctorat en Chimie
Sous la direction de Jacqueline Belloni.
Soutenue en 1989
à Paris 11 .
Radiolytic synthesis of metal nanoaggregates in fluoropolymer ion exchange membranes : aggregation dynamics : electrocatalytic activity
This work describes a new radiolytic process for the synthesis at room temperature of metal aggregates within the bulk volume (and/or at the surface) of a perfluorosulfonated ion exchange membrane such as NAFION (French Patent Nr 87-13295). The membrane is first impregnated with a solution of one (or more) metallic salt(s) in a polar solvent enabling the metal salt(s) to dissolve and the membrane to swell, and then subjected to an ionizing radiation (X rays, 60Co y rays, accelerated electrons), so as to induce a radiolytic reduction of the metal ions down to nascent metal atoms. The latter diffuse in the hydrophilic phase of the membrane and aggregate in the form of nanometer size metal particles (nanoaggregates). The electrocatalytic efficiency of bimetallic nanoparticles (such as Pt-Ru and Ni-Ru), once grafted onto bulk metal electrodes (Ti or Ni), has been investigated by respectively measuring the overpotential for chlorine or hydrogen evolution. Experimental conditions are similar to those of the so-called chlorine-soda industrial process. A synergistic effect is shown when Pt and Ru are alloyed with a 2:1 atomic ratio. Then the chlorine overpotential is minimum. Conversely, there is no synergy between Ni and Ru, but a minimum amount of Ru in Ru-Ni (40% atomic) ensures a very low hydrogen overpotential. Time-resolved studies of aggregation dynamics in a membrane medium have been carried out for the first time by a nanosecond pulse radiolytic reduction of silver ions in NAFION 117 swollen with 2-propanol/water mixtures. Kinetic measurements show that the earliest stages of silver aggregation are quite fast, while the following are far slower. This growth inhibition is interpreted as resulting from a slow diffusion of the transient species of silver aggregation through the hydrophilic phase of the membrane. A computer simulation of silver aggregation, occurring in competition with the corrosion of the earliest species by H30+ ions, has given estimated values of the corresponding rate constants, and an estimation of the size beyond which the diffusion of silver particles is slowed down to zero. Such a model well accounts for the stabilization in the membrane medium of silver species made up of very few atoms.