13C NMR shift and spin-lattice relaxation measurements have been used to investigate 13CO (ex MeOH) on fuel cell grade Pt electrodes (having average particle diameters of 2, 2.5, and 8.8 nm) in an electrochemical environment from 80 to 293 K at 8.47 and 14.1 T. The temperature dependence of the 13C spin-lattice relaxation rate, 1/T1, shows a Korringa relationship which is independent of magnetic field, for all three samples. However, the peak positions and the corresponding T1T 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 13C T1 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 temperatures, 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(f)-LDOS) between the CO 5σ and 2π* orbitals and shows that the 2π*-like E(f)-LDOS at 13C is about 10 times larger than the 5σ-like E(f)-LDOS. Smaller Pt particles have shorter 13CO T1 values and more downfield shifts, due to the increase in the 2π*-like E(f)-LDOS. There is also a linear correlation between the value of the 2π*-like E(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 2π*-like E(f)-LDOS driven by the electrode potential, rather than a classical electrostatic effect. The two-band model developed here for ligand 13C atoms complements that described previously for 195Pt 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry