Effects of Temperature and Atmosphere on Carbon-Supported Platinum Fuel Cell Catalysts

Pt/Vulcan XC 72 powders were prepared via a colloidal method. TEM and XRD studies were carried out from 273 to 573 K under oxidizing, inert, and reducing atmospheres. Under air flow, a small aggregation of platinum particles is observed, but neither fusion of particles nor increase of the Sherrer's length L-v This is explained by the presence of oxygen species on the platinum surface. Under reducing atmosphere (3%H-2/He), aggregation of platinum particles and increase of the crystallinity and/or apparent mean crystallite size are observed. L-v increases from ca. 3 nm for the fresh Pt(40 wt %)/C catalyst to ca. 7 nm after thermal treatment at 573 K. Under inert atmosphere, TEM and XRD measurements did not show any reconstruction process or L-v increase for temperatures lower than 473 K. But from 523 K, some platinum entity reconstruction was evidenced, Under reducing atmosphere, two kinetics of grain growth process seem to exist. The induction time tau before the second growth process is dependent on the temperature, but independent of the platinum loading. For T < 523 K, the formation of a surface Pt-H quasiliquid phase leading to crystalline reconstruction and an agglomeration process are proposed to occur. This step is then followed by grain boundary formation and grain boundary diffusion leading further to the increase of L-v (controlled by the second growth kinetics). For T >= 523 K, reconstruction by size effect is invoked for the first growth process.