COLL 611 |
| Pt is one of the best catalysts for the hydrogen oxidation reaction in fuel cells. However, Pt suffers from poisoning by CO. The CO poisoning is mediated by compositional-tuning to a Pt alloy. While this solution has advanced the prospects of low temperature fuel-cells, the underlying science behind alloy-tuning to lower CO adsorption, the alloy best tuned being Pt50Ru50, has not been explained without controversy. The controversy lies in the role of Ru in changing the electronic structure of CO on Pt. The accepted mechanism for CO adsorption on Pt proposes that the CO molecules adsorb on Pt surfaces at a normal angle, and the bonding consists of donation of charge from the CO 5s orbitals to Pt and a back-donation of charge from the dxy,yz orbitals to the CO 2p*. We probed the molecular orbitals (MOs) of CO with near-edge X-ray absorption fine structure (NEXAFS) post CO (sub)monolayer adsorption onto Pt and Pt-Ru. Individual MOs were accessed by aligning the near-linearly polarized electric field of the synchrotron beam. We have found that there is an increase in the 2p* resonance as Ru is added to Pt. Since the resonance intensity is proportional to the density of the molecular states, the observed increased intensity is a result of charge transfer from CO back to the metal catalyst. However, Ru also has the added role of adding electrons back into the 2p* MO of those CO molecules that are on Pt sites. This subtle effect has been predicted by DFT published calculations. |
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Physical and Computational Characterization of Nanostructured Electrocatalysts
2:00 PM-4:20 PM, Thursday, 14 September 2006 Sir Francis Drake -- Empire Room, Oral
Division of Colloid & Surface Chemistry |