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In the field of energy technology, proton exchange membrane fuel cell has been the front runner for the environmentally friendly power source. However, the widespread use of fuel cells has been hampered by the high cost of metal catalysis, such as platinum (Pt). To develop cost-effective fuel cell systems, it is highly desirable to maximize the efficiency of Pt catalyst or replace it with cheaper solutions. Given recent suggestions from numerous experiments studies that Pt-doped carbon nanotubes (Pt-CNT) have better performance over the conventional carbon black based systems, we carried out electronic structure calculations to understand the electronic properties and catalytic effects of Pt-CNT.  During the past grant year, we have completed the electronic structure calculations of some representative Pt clusters adsorbed on (5,5) single-walled nanotube (SWNT), up to four platinum atoms, and started investigating oxygen dissociation reactions on the Pt-CNT system.

     Pt-monomer/CNT: There are three distinct adsorption sites for Pt on CNT: on-top site (above carbon atom), hole site (above C₆ honeycomb), and bridge site (above C-C bond). Calculations showed that the bridge sites have much higher binding energies than others.  Bridge sites can be further classified into "orthogonal" (B1, Fig. 1(left)) and "skewed" (B2, Fig.1(right)) sites. The Pt-C bond lengths are 2.06Å for B1 site and 2.08Å for B2 site. These bond lengths are more compatible with van der Waals interaction than covalent interaction.

Fig.1 Single platinum adsorbed on "orthogonal" site (B1, left) and "skewed" site (B2, right) of (5,5)-SWNT

However the binding energies of  Pt adsorbed on (5,5) CNT are quite large, 2.03 eV for B1 and 2.18 eV for B2, which indicates that Pt-C bonds have some covalent characters. Bader charge analysis showed that the extent of charge transfer from Pt to CNT is not negligible, 0.077e for B2 and 0.025e for B1 site, but these are not substantial numbers either. It is also intriguing to observe that the B2 site has a larger binding energy than the B1 site even though its Pt-C bond length for B2 site is longer. Further analyses of electronic structures of bridges sites revealed that the band structures of B1 and B2 sites are significantly different. The projected density of state of Pt atom showed that the occupied states of valence electrons for B2 sites are located much further below the Fermi level than those for B1 site, which is consistent with the fact that the binding energy of B2 site is higher. In addition, occupied states of Pt-CNT around HOMO showed significant mixing between the d-states of Pt on the B2 site and the HOMO of pristine CNT, but those of B1 site are mostly pure d-states of Pt. This indicates that detailed analysis of electronic structure is necessary to understand the bonding characters of metal-doped nanotubes, not just the amount of charge transferred.

      Linear Pt Chain/CNT :  We investigated the geometries and electronic structures of a wide variety of Pt-clusters absorbed on (5,5)-SWNT to understand the nature of Pt-C interaction as well as the role of Pt-Pt interaction in the formation of Pt-CNT.  For example, we studied the linear chains of Pt clusters adsorbed on CNT, starting from Pt dimer to Pt tetramer. Due to the size of our system (four CNT unit cell), a linear Pt tetramer on (5,5)-SWNT forms a infinite Pt chain. We included both linear Pt-chains adsorbed on the B1 and B2 sites in our calculations (see Fig. 2).

Fig. 2 Linear platinum chains on (5,5)-SWNT : (a) B1 site, and (b) B2 site.  

For the Pt dimer and trimer on B2 site, it was found that Pt chains are not parallel to the CNT axis, but slightly rotated. This increases the Pt-Pt bond length of dimer on B2 site to 2.63Å. As the number of Pt increases, Pt-Pt distance decreases and reaches 2.5 Å when the chain becomes infinitely long and parallel to the CNT axis. Interestingly, the binding energy of Pt-chain on B1 site increases faster as the number of Pt increases and eventually becomes larger than that of Pt-chain on B2 site. On the other hand, Pt-C distance increases as the chain length increases. These results indicate that Pt-Pt interaction becomes stronger, whereas the overall strength of Pt-CNT interaction diminishes as the length of chain increases. This is also reflected in the fact that the binding energy per Pt atom steadily decreases as the chain length increases.

      Oxygen dissociation on Pt-CNT :  To understand the catalytic role of platinum on the surface of nanotube, we began investigating the oxygen dissociation reaction on the surface of (5,5)-SWNT with and without platinum clusters. Since the catalytic activity of CNT doped with Pt cluster is expected to change drastically depending on its geometry and electronic properties, a variety of Pt clusters adsorbed on (5,5)-SWNT are currently under investigation. As an example, we are reporting oxygen dissociation on the B1 site of (5,5)-CNT doped with a single Pt atom. The adsorption of O₂ on a pristine SWNT is quite weak and the oxygen has to change its electronic structure from triplet to singlet state upon dissociation. The activation barrier of this reaction on the B1 site of pristine (5,5)-SWNT (Fig. 3a) was found to be 3.0 eV. The triplet-singlet transition occurs right before the bond dissociation. On the other hand, the barrier height for the same reaction on the B1 site of Pt-doped (5,5)-SWNT is only 1.3 eV, which is still high, but much lower than on the pristine (5,5)-CNT. The dissociation actually occurs on the single Pt atom and the oxygen atom migrates to a nearby B1 site (Fig. 3b). The activation barrier is expected to decrease as the size of Pt cluster increases.   

            Fig. 3   Reactant (right) and product structure (left) for the oxygen dissociation on a (a) pristin (5,5)-SWNT and (b) Pt-doped (5,5)-SWNT. The energy level diagram is include in the middle. Oxygen  atoms are in red and platinum atoms are in cyan color.