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Stabilization of Platinum Nanoparticles by Substitutional Boron Dopants in Carbon Supports

Chethan Acharya and Heath Turner. Chemical and Biological Engineering, University of Alabama, Box 870203, Tuscaloosa, AL 35487-0203

A common anode catalyst used in a proton exchange membrane (PEM) fuel cell is platinum on a carbon support. The performance of the fuel cell can be enhanced by well-dispersed, stable platinum nanoparticles. One of the causes for the loss of activity in a fuel cell (as well as in other catalytic processes) is the growth, or sintering, of the nanoparticles. We suggest that the structural stability of nm-sized platinum catalyst particles can be enhanced by increasing the interaction of the nanoparticles with the carbon support, relative to the Pt-Pt interactions. In an effort to increase the interaction between the metal and the support, carbon substituted Group III (B and Al), IV (Si), and V (N and P) dopants were introduced in graphite, and the influence of the dopants on the Pt-carbon interaction was investigated using first-principles density functional theory (DFT) calculations. Among the dopants considered, boron significantly increased the interaction of the Pt1 atom with the graphite support when compared to the pristine graphite sheet without affecting the structural integrity of the support. To see if the boron dopant increased the binding energy of larger Pt clusters, Pt2 to Pt6 atoms were adsorbed on various boron-doped graphite models and a similar phenomenon was observed. To avoid the possible effects of the edges on the calculations involving the graphite models, a fullerene structure was also chosen as a model for the carbon support. Pt1 to Pt5 clusters were adsorbed on pristine and boron-doped fullerene models and the results obtained with the fullerene supports were consistent with the results obtained from the graphite model, which was a higher binding energy of the Pt atoms in the presence of the boron dopant. To see if the boron doping increased the binding energy of other metals, Ru and Au atoms were adsorbed on the fullerene models. The effect of boron doping was more predominant with the Ru and Au atoms, with an increase in their binding energies of ~40 kcal/mol, relative to their adsorption on the pristine fullerene model. The adsorption enhancements that we predict may potentially be used to stabilize catalyst nanoparticles on carbon supports, and experiments are currently underway to test this possibility. We are currently expanding our computational work to incorporate periodic graphite models, and we are also exploring the impact of the boron doping procedure on the catalytic properties of our systems. We plan to present the stability enhancement phenomenon that we have predicted with our calculations, as well as the rationale for this unusual behavior.