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.

Oxygen reduction reaction (ORR) measurements and (195)Pt electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy were combined to study a series of carbon-supported platinum nanoparticle electrocatalysts (Pt/CB) with average diameters in the range of roughly 1-5 nm. ORR rate constants and H(2)O(2) yields evaluated from hydrodynamic voltammograms did not show any particle size dependency. The apparent activation energy of 37 kJ mol(-1), obtained for the ORR rate constant, was identical to that obtained for bulk platinum electrodes. Pt/CB catalysts on Nafion produced only 0.7-1% of H(2)O(2), confirming that the direct four-electron reduction of O(2) to H(2)O is the predominant reaction. NMR spectral features showed characteristic size dependence, and the line shapes were reproduced by using the layer-deconvolution model. Namely, the variations in the NMR spectra with particle size can be explained as due to the combined effect of the layer-by-layer variation of the s-type and d-type local density of states. However, the surface peak position of (195)Pt NMR spectra and the spin-lattice relaxation time of surface platinum atoms showed practically no change with the particle size variation. We conclude that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.

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.

No abstract

We have carried out a series of 195 Pt and 13 C 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 195 Pt NMR results strongly suggest that there is a surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles. The CO-stripping cyclic voltammogram (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 13 C 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 13 C 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 2π* 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".

No abstract

Study of the diffusion of small molecules on catalyst surfaces is of broad general interest, and there have been numerous investigations of surface CO diffusion on Pt under ultrahigh vacuum (UHV) or gas phase conditions. 1-7 Both diffusion coefficients (D CO ) as well as activation energies (E d ) for diffusion have been measured and are of importance in the context of, among other topics, CO hydrogenation in fuel synthesis 8 and CO oxidation in heterogeneous catalysis. 9 The latter topic is also of interest in the context of fuel cell catalysis, but there has been no direct experimental determination of D CO in an electrochemical environment due to problems associated with the presence of the electrolyte. 10 Fortunately, however, NMR methods are not plagued by these problems, [11][12][13] and in this paper, we report the first direct determination of the diffusion constants of CO on Pt in a liquid electrochemical environment, together with the activation energy for diffusion, using the techniques of electrochemical NMR (EC-NMR) 11-14 and selective spin inversion NMR. 7 To determine diffusion constants, we used the "S-shape" pulse sequence developed by Becerra et al. 7 The S-shape pulse sequence (Figure 1) exploits the fact that CO molecules adsorbed on a Pt nanoparticle can have different 13 C resonance frequencies, depending on the angle of CO's unique molecular axis with respect to the external magnetic field. A part of the magnetization is selectively inverted by the first two pulses, and the 13 C spins are then allowed to diffuse to different regions of the nanoparticle during the evolution period T ev . Motion of a CO molecule due to surface diffusion alters the 13 C spin's Larmor frequency (ω), and experimentally, the amplitude M + (T ev ) of the noninverted part of the spectrum is measured for various values of T ev . If only T 1 processes are involved, M + (T ev ) grows back to its equilibrium value independent of T ev , but when CO molecules diffuse, a mixing of inverted and noninverted parts of the nuclear magnetization occurs, leading to an initial decrease in M + (T ev ), which then grows back to its equilibrium value with increasing T ev .To calculate the diffusion rate, we follow the time evolution of a normalized signal amplitude, A + , defined by the following equation, at various T ev :where ∆ is the line width and ω 0 is the frequency where M + (T ev ) has its maximum; λ n and A n are the coefficients from a Fourier series solution that are determined by boundary and initial conditions. 7 D ω (the diffusion coefficient in the frequency domain) is obtained as the sole fitting parameter to eq 2, and D ω can then be converted to D CO , the diffusion constant, using the relation, D CO ) (π 2 d 2 /32Ω 2 )D ω , where d is the average particle diameter, and Ω is the upper bound for diffusion in the frequency domain, as reported elsewhere. 7 NMR measurements were carried out on 13 CO adsorbed onto electrochemically cleaned platinum black in 0.5 M D 2 SO 4 using a home-built 8.47 T NMR spectrometer. 14...

Broadband and fluorescence line narrowing optical spectroscopic studies have been used to investigate the local environments of Eu 3ϩ ions in lithium fluoroborate glasses. From the vibronic spectra, different borate groups coupled with the Eu 3ϩ ions have been identified. A pulsed tunable dye laser has been used to selectively excite the 5 D 0 level of the Eu 3ϩ ion and the subsequent 5 D 0 → 7 F 1 fluorescence spectra have been monitored as a function of the exciting wavelength. From these FLN studies, three 7 F 1 Stark levels have been identified and a C 2v orthorhombic symmetry has been assumed in the subsequent calculation of the crystal-field parameters for the different environments occupied by the Eu 3ϩ ions in the glass. The second rank crystal-field parameters have been systematically analyzed for the Eu 3ϩ :lithium fluoroborate glass from the site dependent behavior of the 7 F 1 level splitting. The importance of the J-mixing in the crystal-field analysis has been emphasized. An appropriate method for comparing the crystal-field interactions in different glasses has been proposed by analyzing the 7 F 1 level. Thus, results obtained for the Eu 3ϩ :fluoroborate have been compared with recalculated results in other Eu 3ϩ doped fluoride, borate, silicate, and borosilicate glasses. An intermediate behavior between Eu 3ϩ :oxide and Eu 3ϩ :fluoride glasses is observed for the local structure of the Eu 3ϩ ions in the fluoroborate glass, indicating the active participation of fluorine ions in the immediate environments of the lanthanide ion in this glass.