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.

We report the results of a carbon-13 nuclear magnetic resonance spectroscopic investigation of the structure of carbon nanohorn aggregates (CNHs). The results show that CNHs consist of two components, characterized by different chemical shifts and spin lattice relaxation (T-1) behavior. The first component has a chemical shift of 124 ppm and displays rapid spin-lattice relaxation behavior and is assigned to the nanotubelike horns on the particles' surfaces. The second component has a chemical shift of 116 ppm and much slower spin-lattice relaxation behavior and is assigned to the graphitelike part of the CNH aggregrate. The results of integrated peak area measurements indicate a 1:2 ratio of nanohorns to the graphitelike substrate. The absence of a clear Korringa behavior for the temperature dependence of T-1 and the lack of a Knight shift ruled out any metallic behavior and indicated instead behavior characteristic of semiconductor materials with paramagnetic centers due to structural defects providing an effective relaxation mechanism in the nanohorn domains. We also observed an anomalous change in T-1 near 17 K in the nanohorn domains suggesting the development of an antiferromagnetic correlation between localized electron spins.

The NMR data obtained after adsorbing (CO)-C-13 on Pt by partial oxidation of (CH3OH)-C-13 showed a nonlinear variation of the chemical shift and the linewidth with decreasing particle size, more change for average sizes less than 5 nm, which can be explained in terms of the variation of relative populations of different CO adsorption sites. There was also a noticeable increase in the chemical shift of CO adsorbed on carbon-supported Pt catalysts as compared to Pt black samples of similar size, which we attribute to a metal-support interaction in the supported catalysts. (c) 2005 The Electrochemical Society. All rights reserved.

Ruthenium is the most active and stable promoter of platinum known for low-temperature fuel cell anode reactions such as the oxidation of methanol and CO. The mechanism of promotion by Ru, including bifunctional and ligand effects, is discussed in this selective review. Also examined are effects of surface structure, Ru distribution, and Ru oxidation state. For this purpose, some review of the mechanism of CO and methanol oxidation on Pt/Ru surfaces is provided. The review concentrates on pure Pt nanoparticles and bulk electrodes modified by deposition of Ru, although some discussion of Pt/Ru alloys and Pt-modified Ru electrodes is included in order to address and discuss all aspects of promotion by Ru.

(195)Pt NMR spectroscopic and electrochemical measurements were carried out on commercial Pt-Ru alloy nanoparticle samples to investigate the effect of high-temperature annealing in different vacuum/gas-phase environments. Samples annealed at 220 degrees C in Ar gas, or in a vacuum, did not show any demonstrable change in catalytic activity vs electrochemically reduced, room-temperature samples. In contrast, annealing at 220 degrees C in H(2) gas led to a 3-fold increase in reactivity toward methanol oxidation (per surface site). NMR experiments show that annealing at 220 degrees C (in both Ar and H(2)) leads to a slight reduction in the Fermi level local density of states (E(F)-LDOS) at the Pt sites, which we attribute to surface enrichment of Ru. This electronic effect alone, however, appears to be too small to account for the increase in the catalytic activity observed for the H-treated catalyst. By comparing the electrochemical and NMR data of the H- and Ar-treated samples, we conclude that annealing at 220 degrees C in the hydrogen atmosphere reduces surface Ru oxides into metallic Ru, and consequently, the presence of metallic Ru and its enrichment on the surface are essential for the enhanced catalytic activity. In contrast, heat treatment at 600 degrees C in both vacuum and argon atmosphere increases the particle size and reduces the amount of platinum on the nanoparticle surface, thereby increasing the surface Ru content beyond the optimum surface composition values. This causes a large reduction in catalytic activity. Our results suggest that optimizing the amount of surface Ru by heat treatment at temperatures near 200 degrees C, in a hydrogen atmosphere, can be utilized to produce Pt-Ru alloy nanoparticles with high methanol oxidation activity. Finally, our NMR and electrochemical data, taken together with the lattice parameter measurements, lead to a novel model of Pt-Ru alloy nanoparticles having a Ru-rich core and a Pt-Ru alloy overlayer.

We report the first direct measurement of CO diffusion on nanoparticle Pt electrocatalysts at the solid/liquid interface, carried out using 13C nuclear magnetic resonance (NMR) with a spin-labeling pulse sequence. Diffusion parameters were measured in the temperature range of 253-293 K for CO adsorbed on commercial Pt-black under saturation coverage. 2H NMR of the same system indicates that the electrolyte remains in the liquid state at temperatures where the CO diffusion experiments were performed. The CO diffusion parameters follow typical Arrhenius behavior with an activation energy of 6.0 +/- 0.4 kcal/mol and a pre-exponential factor of (1.1 +/- 0.6) x 10-8 cm2/s. Exchange between different CO populations, driven by a chemical potential gradient, is suggested to be the main mechanism for CO diffusion. The presence of the electrolyte medium considerably slows down the diffusion of CO as compared to that seen on surfaces of bulk metals under UHV conditions. This work opens up a new approach to the study of surface diffusion of adsorbed molecules on nanoparticle electrode catalysts, including the possibility of correlating diffusion parameters to catalytic activity in real world applications of broad general interest.

Studies of rf-SQUID effect in ternary (YRuB2, LuRuB2, LaRu3Si2) and in binary V3Si intermetallic bulk superconductors have been carried out for investigating the nature of grain boundaries in these superconductors. YRuB2 (Tc∼7.5 K), LuRuB2 (Tc∼10 K) and LaRu3Si2 (Tc∼7 K) ternary superconductors are found to show rf-SQUID voltage–flux modulations from 4.2 K to close to Tc. This indicates that the natural grain boundaries in the ternary intermetallic superconductors behave as Josephson weak links. Rf-SQUID effect is not observed in V3Si binary superconductors, which indicates that the grain boundaries in V3Si are not weak link type. STM/STS studies of the YRuB2 and V3Si grain boundaries (GB) are also reported. The interior of the GB in YRuB2 is found to be of quasi-insulating type and thus provides a weak link effect. On the contrary the interior regions of the GB of V3Si is found to be of metallic type and thus provides proximity coupling between the grains of V3Si.

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 behaviour of adsorbed CO. The C-13 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 (E-a) 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 5sigma 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 (E-f-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.

Spectra obtained by electrochemical infrared reflection absorption spectroscopy (EC-IRAS) for carbon monoxide (CO) adlayers formed by partial CO dosing on various ruthenium-decorated platinum nanoparticle films are reported. The need to achieve a well distributed rather than aggregated metal nanoparticle array is demonstrated, given that such nanoparticle aggregates induce complex dielectric behavior. The strategy here is to use an "organic glue matrix" (short chain SAMs) between the nanoparticles and the gold substrates. The observed promotion in CO electrooxidation by the existence of a Ru island on Pt nanoparticles, of interest to fuel-cell catalysis, showed a strong relationship with Ru surface concentrations, consistent with previous studies on single crystal or polycrystalline bimetallic surfaces. Two distinctive CO infrared bands, one for the Pt-CO and one for Ru-CO domain were found after the dipole coupling of CO within the two CO domains was minimized. Interestingly, those two CO bands showed independent electrooxidation behavior with electrode potential changes. Also, it is shown that the electrooxidation of CO on large Ru islands is less facile than on small Ru islands. In addition, the activity of commercial Pt/Ru alloy nanoparticles to CO stripping was tested and IRAS spectra were reported as a comparison to our Ru-decorated Pt nanoparticles.

We have carried out a series of Pt-195 and C-13 NMR spectroscopic and electrochemical experiments on commercial Pt-Ru alloy nanoparticles and compared the results with those on Pt-black samples having similar particle sizes. The Pt NMR spectrum of the alloy nanoparticles consists of a single Gaussian peak, completely different from the broad "multi-Gaussian" NMR spectra, which are generally observed for carbon-supported Pt catalysts. Spin-echo decay measurements show that the intrinsic spin-spin relaxation time (T-2) is much larger in the alloy compared to Pt-black. A "slow-beat" is observed in the spin-echo decay curve of the alloy, implying that the NMR frequencies of spin-spin coupled Pt nuclei in the alloy nanoparticles are quite similar, unlike the situation found with Pt-black. These Pt-195 NMR results strongly suggest that there is a surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles. The CO-stripping cyclic voltammograrn (CV) of the Pt-Ru alloy nanoparticles is broader than that observed with platinum black and is shifted toward lower potential. The two-peak structure observed previously for the CO-stripping CV behavior of Pt-black containing spontaneously deposited Ru (Tong et al. J. Am. Chem. Soc. 2002, 124, 468-473) is absent in the alloy sample. The C-13 NMR spectrum of CO adsorbed on the Pt-Ru alloy consists of a single peak, exhibiting only a small Knight shift. An analysis of the C-13 spin-lattice relaxation results indicates that Ru addition causes a reduction in the Fermi level local density of states of the clean metal surface atoms and the 2pi* orbital of adsorbed CO. These NMR results suggest that alloying with Ru reduces the total density of states (DOS) at the Pt sites, in accord with conclusions drawn previously from synchrotron X-ray absorption studies of Pt-Ru electrocatalysts. This electronic alteration could be the basis for the ligand field contribution to the "Ru enhancement".

Indirect nuclear spin-spin J-coupling measurements have been carried out on a conducting, carbon-supported 8.8 nm platinum electrocatalyst by using Pt-195 NMR. The J-coupling values show a marked variation across the spectrum. These J-coupling values were then transformed into the corresponding s-like Fermi level local density of states D-s(E-r, x), where x is the distance from the particle surface, revealing a spatially resolved oscillatory decay in D-s(E-f) which is responsible for the line broadening of the bulk peak in the Pt-195 NMR spectrum. (C) 2002 Elsevier Science B.V. All rights reserved.

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.

Electrochemical nuclear magnetic resonance (EC-NMR) is a powerful local probe, which combines solid state NMR with electrochemistry. It is a unique technique that permits a unified, electronic-level study of the metal and adsorbate side of the electrochemical interface. Experiments can be performed either under direct potentiostatic control and in situ potential adjustment, or with samples prepared in a separate electrochemical cell and transferred to an NMR cell, where the potential is both known and constant. A phenomenological two-band model was applied to the NMR parameters to yield quantitative information about the Fermi level local density of states (Ef-LDOS) that are relevant to the type of chemisorption bond involved in the systems under investigations. A layer-model analysis was found to be effective in interpreting the 195Pt-NMR spectra of carbon-supported Pt nanoparticles. The surface peak of the 195Pt-NMR spectrum was found to be very sensitive to the chemical nature of the adsorbate present. The 195Pt Knight shifts show a direct correlation with the electronegativity of the adsorbate, and the 13C Knight shift of the CO adsorbate shows a correlation with the clean metal surface Ef-LDOS. The electrode potential dependence of 13C-NMR spectra of CO adsorbed on Pt and Pd black show evidence of the alterations to the electrochemical interface by the application of the electric field. EC-NMR of Pt electrode surfaces modified by spontaneous deposition of ruthenium has provided new insights into the enhancement in CO-tolerance of these catalysts for methanol oxidation.

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.

With our DC-magnetisation studies on single crystal CeRu2, Bi-2212, polycrystalline 1 at % Fe-doped Nb and MoxRe1-x alloy samples, we show that the recently reported "history effects" are not general to all superconductors exhibiting "peak effect". Our studies on MoxRe1-x showed that these "history effects" do not correlate with the currently held belief that "peak effect" needs to be at fields close to H-c2(T). (C) 2000 Elsevier Science Ltd. All rights reserved.

Abstract We have observed, in our dc magnetization studies, the peak effect (PE) in the superconducting mixed state of two compositions, Mo0.825Re0.175 and Mo0.8Re0.2, of the binary alloy system Mo1−xRex. The PE occurs close to the H c2(T) line at temperatures below T C, similar to the well studied compound CeRu2. A detailed investigation of the various characteristics of this peak effect and a comparison with other superconductors of current interest show that the observed behaviour can be explained within the realm of critical state models of hard type-II superconductors. Our results show that the anomalies reported in CeRu2 and NbSe2 are not universal to materials showing a PE close to the H c2(T) line and may, therefore, have an important bearing on the phenomenology of type-II superconductors.