Level Local Density Of States Research Articles

Spatially Resolved Electronic Alterations As Seen byin Situ195Pt and13CO NMR in Ru@Pt and Au@Pt Core–Shell Nanoparticles

Pt-based core–shell (M@Pt where M stands for core element) nanoparticles (NPs) have recently been under increasing scrutiny in the fields of fuel cell and lithium air battery electrocatalysis due to their promising prospects in optimizing catalytic activity, reducing Pt loading and consequently lowering its cost. To achieve the latter, delineating spatially resolved local (surface) elemental distribution and associated variations in electronic properties under working condition (i.e., in situ) is arguably a prerequisite of fundamental importance in investigating electrocatalysis but unfortunately is still sorely missing. In this regard, in situ195Pt electrochemical NMR (EC-NMR) of Pt-based NPs is unique in terms of accessing such information, particularly the spatially resolved partition between the s- and d-like Fermi level local density of states (Ef-LDOS) modified by the core elements. In this paper, we report a comparative in situ195Pt EC-NMR investigation of Ru@Pt vs Au@Pt NPs which was complemented by in situ13C EC-NMR of the 13CO adsorbed on the respective NPs generated via dissociation of methanol and by ab initio DFT calculations. The obtained results showed opposing electronic effect between Ru vs Au cores: the former reduced substantially the s-like but not the d-like Ef-LDOS of the Pt shell while the latter did the opposite. According to recent quantum calculations, a reduction in d-like partition would weaken the Pt–O bond while a reduction in s-like partition would weaken the Pt–H bond, which is largely in agreement with experimental observations.

CO surface diffusion on platinum fuel cell catalysts by electrochemical NMR

We report on surface CO diffusion processes in relation to properties of nanoparticle Pt and Pt/Ru fuel cell catalysts. The CO ad 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 CO ad 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 ( E f-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.

Activation of Nanoparticle Pt−Ru Fuel Cell Catalysts by Heat Treatment: A 195Pt NMR and Electrochemical Study

(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.

NMR evidence of a spatially resolved oscillation in the Ef-LDOS in a nanoscale platinum electrocatalyst

Indirect nuclear spin–spin J-coupling measurements have been carried out on a conducting, carbon-supported 8.8 nm platinum electrocatalyst by using 195 Pt 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 f, 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 195 Pt NMR spectrum.

Nanostructured electrode surfaces studied by electrochemical NMR

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 ( E f-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 E f-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.

An NMR investigation of CO tolerance in a Pt/Ru fuel cell catalyst.

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.

Correlation between the Knight Shift of Chemisorbed CO and the Fermi Level Local Density of States at Clean Platinum Catalyst Surfaces

We have investigated the 195Pt and 13C nuclear magnetic resonance (NMR) spectroscopy of clean-surface platinum catalysts and of CO chemisorbed onto Pt catalyst surfaces. We use Knight shift, relaxation, and J-coupling data to deduce information about the Fermi level local density of states (Ef-LDOS) at catalyst surfaces. There is a linear correlation between the Knight shifts of chemisorbed CO and the clean surface Ef-LDOS of platinum onto which the CO is bound, as determined by 13C and 195Pt NMR. The correlation amounts to ∼12 ppm/Ry-1·atom-1, the same as that which can be deduced for CO on palladium, as well as from the electrode potential dependence of 13C Knight shifts and infrared vibrational frequencies, νCO, and the relationship between νCO and the Ef-LDOS at clean platinum surfaces. The ability to now directly relate metal and adsorbate electronic properties opens up new avenues for investigating metal−ligand interactions in heterogeneous catalysis and electrocatalysis.

Correlation between the Stretching Frequency of Carbon Monoxide Adsorbed and the Fermi Level Local Density of States at Surfaces of Platinum Catalysts

We have studied the stretching frequency (extrapolated to zero coverage) of adsorbed CO and the 195Pt NMR (before adsorption of CO) of Pt catalysts on zeolite-NaY (three dispersions), on zeolite-NaHY (two dispersions), and on zeolite-NaX (one dispersion); we have also measured the hydrogen and oxygen chemisorption capacity of these samples. For comparison, we studied three samples on an oxidic carrier (unpromoted and promoted by Li or K salts, all from the same initial batch) by infrared and by NMR. In the zeolite case, we find a direct correlation between the CO stretching frequency and the density of states at the Fermi energy on surface sites (surface Ef-LDOS) derived from the clean-surface NMR data, but the chemisorption results seem to be determined by steric as well as electronic effects, and show no simple relation to the two other quantities. The values for the oxidic samples appear to form a distinct set, with probably some promotor-specific effects. The zeolite data support an interpretation of ...

ChemInform Abstract: Electron Availability and the Surface Fermi Level Local Density of States: An Alternative Way to See Catalytic Activity of Metals

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Modification of transition metal electronic structure by P, S, Cl, and Li adatoms

We have evaluated the electronic perturbations induced on a thin Rh(001) film by the adsorption of 1/4 monolayer coverages of P, S, Cl or Li atoms. The self-consistent Surface Linearized Augmented Plane Wave (SLAPW) calculations indicate that the P, S, and Cl adlayers cause only very slight work function changes, and generally give rise to perturbations in the valence charge that are small beyond the nearest neighbor Rh atoms. We attribute the latter result to the effectiveness with which the charge density is screened. In contrast, reductions induced in the Fermi level Local Density of States (Ef-LDOS), whose magnitude reflects the ability of the surface to respond to the presence of reactants, are substantial even above next nearest neighbor Rh atoms, which are 8 a.u. away. The effect is strongest for Cl and weakest for P. When Li is adsorbed, the work function is reduced by almost 2 eV. This decrease is associated with an increase in electronic charge density over most of the surface, and at the same time, an increase in the Ef-LDOS. Experimental observations of CO adsorption and dissociation on transition metal surfaces indicate a “poisoning” effect by coadsorbed P, S, and Cl atoms and a “promotion” effect by coadsorbed alkali atoms. The SLAPW results for the Ef-LDOS parallel these observations.