A detailed NMR-based model for CO on Pt catalysts in an electrochemical environment: Shifts, relaxation, back-bonding, and the Fermi-level local density of states
{sup 13}C NMR shift and spin-lattice relaxation measurements have been used to investigate {sup 13}CO (ex MeOH) on fuel cell grade Pt electrodes (having average particle diameters of 2, 2.5, and 9.9 nm) in an electrochemical environment from 80 to 293 K at 8.47 and 14.1 T. The temperature dependence of the {sup 13}C spin-lattice relaxation rate, 1/T{sub 1}, shows a Korringa relationship which is independent of magnetic field, for all three samples. However, the peak positions and the corresponding T{sub 1}T values depend on particle size, with those of the 8.8 nm sample approaching values found for unsupported polycrystalline platinum black in an electrochemical environment (J.B. Day et al., J.Am.Chem.Soc. 1996, 118, 13046--13050). The {sup 13}C T{sub 1} is single exponential, independent of particle size and temperature, in contrast to previous results obtained on oxide-supported Pt-CO systems in a ``dry'' environment, in which relaxation was nonexponential at low temperatures, but exponential at high temperature, suggesting strongly a quantum size effect in the dry systems at low T. A detailed two-band model is developed to analyze the partitioning of the Fermi level local density of states (E{sub f}-LDOS) between the CO 5{sigma} and 2{pi}* orbitals and shows that the 2{pi}*-like E{sub f}-LDOS at {sup 13}C is about 10 times larger than the 5{sigma}-like E{sub f}-LDOS. Smaller Pt particles have shorter {sup 13}CO T{sub 1} values and more downfield shifts, due to the increase in the 2{pi}*-like E{sub f}-LDOS. There is also a linear correlation between the value of the 2{pi}*-like E{sub f}-LDOS and the corresponding infrared stretching frequency, due to back-bonding. This indicates that the ``Stark tuning'' effect (the response of the vibrational stretch frequency to an applied field) is dominated by variations in the 2{pi}*-like E{sub f}-LDOS driven by the electrode potential, rather than a classical electrostatic effect. The two-band model developed here for ligand {sup 13}C atoms complements that described previously for {sup 195}Pt atoms in the metal electrode, and should be applicable to other nuclei and adsorbates as well, enabling Fermi level densities of states information to be obtained from both sides of the electrochemical interface, which can then be correlated with other spectroscopies (e.g., infrared) and chemical (e.g., catalytic activity) properties.
- Research Organization:
- Univ. of Illinois, Urbana, IL (US)
- Sponsoring Organization:
- National Science Foundation; Defense Advanced Research Project Agency; US Department of Energy
- DOE Contract Number:
- FG02-91ER45439
- OSTI ID:
- 20017360
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 6 Vol. 122; ISSN JACSAT; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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