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An extensive infrared investigation of the CO/Rhl AI,OJ system has revealed the presence of eight different CO/Rh species including two which have not been observed previously. It has been shown that Rh loading on an alumina support is more critical than reduction temperature in effecting infrared spectral changes in the 1800-2200 cm -\ region. The oxidation state of Rh for the various CO/Rh species has been discussed; it has been postulated that for several of the species the oxidation state of Rh is greater than 0. Furthermore, this work indicates that there is significant atomic dispersion on Rhl AI,OJ catalysts prepared from RhCI J ·3H,OI AI,OJ by a procedure described originally by Yang and Garland. These catalysts retain appreciable amounts of chlorine even following hydrogenation at 673 K.

It is well-known that platinum/ruthenium fuel cell catalysts show enhanced CO tolerance compared to pure platinum electrodes, but the reasons are still being debated. We have combined cyclic voltammetry (CV), temperature programmed desorption (TPD), electrochemical nuclear magnetic resonance, and radio active labeling to probe the origin of the ruthenium enhancement in Pt electrodes modified through Ru deposition. The results prove that the addition of ruthenium not only modifies the electronic structure of all the platinum atoms but also leads to the creation of a new form of adsorbed CO. This new form of CO may be ascribed to CO chemisorbed onto the "Ru" region of the electrode surface. TPD and CV results show that the binding of hydrogen is substantially modified due to the presence of Ru. Surprisingly though, TPD indicates that the binding energy of CO on platinum is only weakly affected. Therefore, the changes in the bond energy of CO due to the ligand effect only play a small role in enhancing CO tolerance. Instead, we find that the main effect of ruthenium is to activate water to form OH. Quantitative estimates based on the TPD data indicate that the bifunctional mechanism is about four times larger than the ligand effect.

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.