COLL 605 |
| The C-O stretching frequency (w∞) of CO adsorbed on Pt in PtRu alloys is used as a probe for analyzing the electronic effects alloying. The Blyholder-Bagus mechanism (BBM) that refers to increased back-donation from the metal d-band to the hybridized 2p* CO molecular orbitals is the dominant mechanism for explaining the electron transfer process during CO chemisorption over the anode surface. However application of BBM has been paradoxical due to 1) Pt-C bond contraction, expected with increased back-donation as the Pt mole percent is reduced, and (2) the lowering d-band center and increased d-band vacancies upon alloying must be reconciled with the alloy-induced red shift of the w∞ . Using Density Functional Theory electronic and phonon property calculations on a library of spin optimized CO/Pt, and CO/PtRu clusters coupled with Mulliken/FEFF8 population analysis and FEFF8 local density of states it is revealed that when Pt is allowed with Ru atoms and although the center of the Pt d-band is shifted towards more negative energies an asymmetric increase in the dispersion of the d-band is accompanied by a broadening of the 5£m/2p* manifold with additional lower energy states closer to the HOMO of the alloy cluster. This effect enhances the 2p* backdonation and thus lowering the w∞ . The localized electron density is manifested as an electrostatic wall effect, preventing the Pt-C bond contractions expected with increased back-donation to the 2p* CO MOs. We have extended our calculations to examine the CO coverage effect over the Pt surface. We observed that DFT calculated w∞ increases proportional to CO mole percent as long as CO remains atop over the Pt surface (no CO tilt). |
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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 |