Crystallite growth of platinum dispersed on graphitized carbon black II. Effect of liquid environment

The effect of various liquids on the growth of platinum crystallites dispersed on graphitized carbon has been determined from electrochemical surface area measurements. Although significant crystallite growth was not observed on exposure to hydrogen gas at temperatures less than 600 °C, rapid growth occurred below 200 °C in the presence of a liquid environment. Crystallite growth rates were independent of the platinum content on the catalyst and of the electrode potential. Furthermore, the nature of the liquid environment did not appear to have an influence on the rate of Pt crystallite surface area loss. When fitted to a simple power law, the order of the rate process lay between 11 and 13 and the activation energy was 21 kcal/mole. The implications of these experimental results are discussed with reference to recent theoretical models for the mechanism of crystallite growth. It is concluded that the evidence strongly suggests that two-dimensional Ostwald ripening (the migration of platinum atoms on the carbon surface) is the process whereby platinum is transferred from one crystallite to another, producing a decrease in the Pt crystallite density and surface area.