AmericanChemicalSociety.com

Despite significant improvement over the years, high cost of metal catalyst remains one of the bottle necks to be relieved for wide-spread use of fuel cell technology. One possible alternative is to use platinum-doped carbon nanotube (CNT/Pt) as catalyst, which has been shown to have superior performance than the traditional approach based on carbon black containing platinum particles. To gain fundamental understanding of the catalytic effect of CNT/Pt, computational investigations of their roles in the electrode reactions are in progress. Last year, the first part of this project was carried out and the electronic structures of CNT/Pt were investigated. More specifically, a series of density functional theory (DFT) calculations were performed on a (5,5) carbon nanotube (4 unit cell, 80 carbon atoms) doped with platinum using ultrasoft pseudopotential, PW91 exchange-correlation functional and plane wave basis set.

Overall, there are three different types of adsorption sites for Pt on CNT: on-atom site (above carbon atom), hole site (above C₆ honeycomb structure) and bridge site (above C-C bond). Bridge sites can be classified into two different kinds, which is denoted as B1 (C-C bond is perpendicular to the tube axis) and B2 (other bridge sites). Our calculations showed that bridge sites have much larger binding energies than others and, within bridge sites, B2 sites are more stable with the binding energy of E_b^B2 = 2.30 eV, whereas B1 site has the binding energy of E_b^B1 = 2.12 eV.

For certain numbers of platinum atoms, platinum clusters have non-zero magnetic moment and open-shell ground state. In fact, it has been shown in the literature that platinum dimer has magnetic moment of 1 µ_B per atom, which was confirmed in the present work as well. However, our calculations showed that CNT doped with a platinum dimer exhibits zero magnetization. The minimum energy structure of CNT-Pt₂ has two Pt atoms on B2 sites along the tube axis with the binding energy E_b = 2.08 eV [E_b = E(CNT-Pt_n)-E(CNT)-E(Pt_n)] and the Pt-Pt distance of 2.6 Å. The fact that the binding energy of CNT-Pt₂ is much smaller than that of CNT-Pt implies weak Pt-C interaction. On the other hand, a calculation showed that the CNT-Pt₂ structure with two platinum atoms (B2 sites) separated far apart has a 1.4 eV higher energy than that with a platinum dimer. Therefore, platinum tends to form a cluster on the surface of CNT, rather than being dispersed. When there are more than two Pt atoms in the system, various geometries are possible for platinum clusters and exhaustive search for the minimum energy structure was carried out. So far, we have identified the minimum energy CNT/Pt structures with three and four platinum atoms. The binding energy of platinum cluster on (5,5) CNT was found to gradually increase as the cluster grows. Platinum cluster in the CNT-Pt₃ structure forms a triangle with two platinum atoms located on the B2 sites along the tube axis and the third platinum atom located above the Pt-Pt bond, which, again, reflects the weakness of Pt-C bond. However, with four platinum atoms on CNT, the minimum energy structure has linear Pt₄ along the tube axis, although the one with tetrahedral Pt₄ is located nearby with the binding energy difference of only 0.07 eV. Currently, we are investigating the band structures of a few representative CNT/Pt structures. This systematic study will reveal the effect of platinum clusters on the electronic structure and the metallic nature of (5,5) carbon nanotube.