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Platinum supported on carbon is widely used as a catalyst for polymer electrolyte membrane fuel cells. The efficiency of the catalyst is affected by the size distribution and morphologies of the platinum particles. The degradation of the catalyst activity arising from the coarsening of the primary platinum particle microstructure is regarded as a serious problem. Therefore, the formation process of platinum particles is of great interest. We think that the phase-field simulation [1] performed on the nano-second time scale can provide useful information about nano-scale phenomena. The objective of this study is to extend the phase-field approach to describe the formation process of platinum particles on graphite. The bulk chemical free energy was based on the regular solution approximation to the metal-vacancy complexes. The microstructure evolution of a nanoparticle was described by the temporal evolution of the field variables related to the concentration of vacancy, long-range crystallographic ordering, and phase transition. The interaction energy between a platinum cluster and graphite was evaluated from the electronic-state calculation by applying the density-functional theory [2]. The interaction energy became weak as the coordination number of the adsorbed platinum atoms increased. Supersaturated platinum vapor was initially supplied above the graphite surface. In the case of a platinum loading of 1.9 mg/m2 at a temperature of 293 K, platinum atoms were gradually concentrated on the graphite surface, and this density profile led to the formation of the platinum particle with the mean particle diameter of about 2 nm. We have also investigated the influence of the platinum-graphite energy site distribution supposing various carbon blacks. The simulation clarified that the platinum particle size distributions were sensitive to the carbon crystallite size distributions. This study was supported by a grant from Core Research for Evolutional Science and Technology (CREST) by the Japan Science and Technology Agency (JST), Japan. [1] L. Q. Chen, Ann. Rev. Mater. Res. 32, 113(2002). [2] K. Okazaki-Maeda, Y. Morikawa, S. Tanaka, and M. Kohyama, Mater. Res. Soc. Symp. Proc. 900E, O06-35(2005). |