[show abstract][hide abstract] ABSTRACT: We report on surface CO diffusion processes in relation to properties of nanoparticle Pt and Pt/Ru fuel cell catalysts. The COad diffusion was studied by the use of 13C electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy. Measurements were carried out in the temperature range 253–293 K, where the solution side of the nanoparticle–electrolyte interface is liquid, in contrast to previous measurements, in ice. We offer a concerted view of the effect of particle size and surface coverage on COad diffusion, and find that both are important. We also found that the diffusion parameters were influenced by the variations in the distribution of chemisorption energies on particles of different sizes, and by the CO–CO lateral interactions. On all Pt nanoparticle surfaces investigated, we conclude that CO surface diffusion is too fast to be considered as the rate-limiting factor in methanol reactivity. The addition of Ru to Pt increases the surface diffusion rates of CO, and there is a direct correlation between the Fermi level local density of states (Ef-LDOS) of the 2π* molecular orbital of adsorbed CO and the activation energy for surface diffusion. These results are of interest since they improve our knowledge of surface dynamics of molecules at electrochemical interfaces, and may help to formulate better models for the electrooxidation of adsorbed CO on nanoparticle surfaces.
[show abstract][hide abstract] ABSTRACT: We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed.
[show abstract][hide abstract] ABSTRACT: Spontaneous deposition of Pd onto catalytic grade Pt nanoparticles has been shown to yield Pt/Pd catalysts having enhanced catalytic activity toward formic acid oxidation, when compared to pure Pt- and Pd-black. Here, we report the results of electrochemical nuclear magnetic resonance (EC NMR) and electrochemical measurements of CO chemisorbed onto these Pt/Pd catalysts, to probe the nature of the CO chemisorption bond, as well as the motional behavior of adsorbed CO. The 13C NMR spectra are broad and can be deconvoluted into two peaks, assigned to CO adsorbed on Pt and Pd sites. From the temperature dependence of the spin−lattice relaxation rates, we conclude that CO chemisorbed on Pd undergoes fast diffusion. The activation energy (Ea) obtained from these results for CO on Pd is smaller than that found for CO adsorbed onto Pd nanoparticles supported on alumina. A two-band model analysis of the NMR data shows that the 5σ orbital of CO makes a significant contribution to the chemisorption bond of CO on Pd, which agrees well with theoretical predictions. The interaction of Pd with Pt leads to a reduction in the Fermi level local density of states (Ef-LDOS) at the Pd sites, which reduces the strength of CO and, most likely, OH adsorption. This electronic modification is proposed to be responsible for the improved catalytic performance of Pt/Pd in formic acid oxidation.
Journal of Physical Chemistry B - J PHYS CHEM B. 11/2004; 108(52).