We report the first combined application of solid-state electrochemical NMR (EC NMR), cyclic voltammetry (CV), and potentiostatic current generation to investigate the topic of the ruthenium promotion of MeOH electro-oxidation over nanoscale platinum catalysts. The CV and EC NMR results give evidence for two types of CO: CO on essentially pure Pt and CO on Pt/Ru islands. There is no NMR evidence for rapid exchange between the two CO populations. CO molecules on the primarily Pt domains behave much like CO on pure Pt, with there being little effect of Ru on the Knight shift or on Korringa relaxation. In sharp contrast, COs on Pt/Ru have highly shifted (13)C NMR resonances, much weaker Korringa relaxation, and, at higher temperatures, they undergo thermally activated surface diffusion. For CO on Pt, the correlation observed between the 2pi* Fermi level local density of states and the steady-state current suggests a role for Ru in weakening the Pt-CO bond, thereby increasing the CO oxidation rate (current). The combined EC NMR/electrochemistry approach thus provides new insights into the promotion of CO tolerance in Pt/Ru fuel cell catalysts, in addition to providing a novel route to investigating promotion in heterogeneous catalysis in general.

New insights into the formation chemistry of chalcogenate-protected metal nanoparticles (NPs) synthesized via the well-known Brust-Schiffrin two-phase method are presented here. On the basis of Raman, NMR, and surface plasmon resonance characterizations, it is concluded that, before the formation of any metal-chalcogen bonds, metal nucleation centers/NPs are first formed inside the inverse micelles of the tetrabutylammonium bromide in the organic solvent, where the metal ions are reduced by NaBH(4). The ensuing formation of the metal-chalcogen bonds between the naked metal NPs inside the micelles and the organo-chalcogen ligands in the organic solvent is the mechanism by which the further growth of the metal core can be controlled. This proposed mechanism is further examined in the formation of Ag and Cu NPs.

No abstract

Direct experimental evidence that can be unambiguously attributed to the need of an ensemble of a minimum number of neighboring Pt atoms for methanol electro-oxidation has been observed for the first time. This was realized by a Pt coverage-dependent investigation of methanol and CO electro-oxidation on Pt sites generated via spontaneous deposition onto both Au and Ru surfaces. CO stripping voltammograms also show clear evidence of a substantially strengthened CO-Pt bonding for submonolayer Pt deposited on the Au substrate over a range of ca. 0.22 to 0.77, which is in qualitative agreement with the theoretical prediction based on the Hammer-Nørskov d-band center model. However, the degree of the bond strengthening depends on the Pt coverage, being stronger for lower coverage. Additionally, evidence of an Ostwald ripening process for Pt islands formation has also been observed.

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/T 1, shows a Korringa relationship which is independent of magnetic field, for all three samples. However, the peak positions and the corresponding T 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 13C T 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 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 T 1 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.