In a proton-exchange membrane fuel cell, nano-particles of Pt supported on carbon materials are used as electrode catalysts. In order to attain excellent catalytic activity, it is necessary to understand and control the microstructures of Pt/C electrodes, as recently observed by high-resolution electron microscopy [1]. On the theoretical side, it is essential to understand the interactions between Pt nano-particles and carbon materials for this purpose. This should lead to the understanding of the formation or evolution processes of microstructures, and also lead to the understanding of the effects of carbon materials or interfaces on the catalytic activity of Pt particles. In this paper, we concentrate on the initial growth process of Pt small particles on various graphite surfaces, through first-principles calculations using the STATE code.
First, we have examined the stable configurations and adsorption sites for a Pt atom, and Pt2, Pt3, and Pt4 clusters on the graphite surface without defects. It has been shown that a Pt atom is stably adsorbed on the bridge site between two carbon atoms with the adsorption energy of about 2eV. For the case of Pt2, both of the Pt atoms are adsorbed on the bridge site. For the case of Pt3, we have examined linear and triangle clusters, and found that the latter is more stable. For the case of Pt4, we have examined planar and three-dimensional clusters, and found that the latter is more stable. The adsorption energies on the surface without defects are 0.55 eV/adatom for Pt2 and 0.05 eV/adatom for Pt3. The interaction energy between the Pt cluster and the graphite surface per Pt atom becomes weaker as increasing the number of Pt atoms. Second, we have also examined the effect of a surface vacancy of graphite. The adsorption energy for a Pt atom on the vacant site is 8.00 eV/adatom, which is stronger than the formation energy of a Pt-Pt bond (about 2 eV/bond) for small clusters. Therefore, we propose that the growth of Pt clusters with smaller contact angles may occur at defective sites.
[1] T. Akita, et al., J. Power Sources 159 (2006) 461. |