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

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...