Most of the world's hydrogen supply is currently obtained by reforming hydrocarbons. 'Reformate' hydrogen contains significant quantities of CO that poison current hydrogen fuel-cell devices. Catalysts are needed to remove CO from hydrogen through selective oxidation. Here, we report first-principles-guided synthesis of a nanoparticle catalyst comprising a Ru core covered with an approximately 1-2-monolayer-thick shell of Pt atoms. The distinct catalytic properties of these well-characterized core-shell nanoparticles were demonstrated for preferential CO oxidation in hydrogen feeds and subsequent hydrogen light-off. For H2 streams containing 1,000 p.p.m. CO, H2 light-off is complete by 30 (composite function)C, which is significantly better than for traditional PtRu nano-alloys (85 (composite function)C), monometallic mixtures of nanoparticles (93 (composite function)C) and pure Pt particles (170 ( composite function)C). Density functional theory studies suggest that the enhanced catalytic activity for the core-shell nanoparticle originates from a combination of an increased availability of CO-free Pt surface sites on the Ru@Pt nanoparticles and a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.

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Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

The use of sulfur in the next generation Li-ion batteries is currently precluded by its poor cycling stability caused by irreversible Li 2 S formation and the dissolution of soluble polysulfi des in organic electrolytes that leads to parasitic cell reactions. Here, a new C/S cathode material comprising short-chain sulfur species (predominately S 2 ) confi ned in carbonaceous subnanometer and the unique charge mechanism for the subnano-entrapped S 2 cathodes are reported. The fi rst charge-discharge cycle of the C/S cathode in the carbonate electrolyte forms a new type of thiocarbonate-like solid electrolyte interphase (SEI). The SEI coated C/S cathode stably delivers ≈600 mAh g −1 capacity over 4020 cycles (0.0014% loss cycle −1 ) at ≈100% Coulombic effi ciency. Extensive X-ray photoelectron spectroscopy analysis of the discharged cathodes shows a new type of S 2 species and a new carbide-like species simultaneously, and both peaks disappear upon charging. These data suggest a new sulfur redox mechanism involving a separated Li + /S 2− ion couple that precludes Li 2 S compound formation and prevents the dissolution of soluble sulfur anions. This new charge/discharge process leads to remarkable cycling stability and reversibility.

Rh@Pt core-shell, RhPt (1:1) alloy, and Rh + Pt monometallic nanoparticles (NPs) were prepared using standard polyol reduction chemistry in ethylene glycol (EG) with standard inorganic salts and polyvinylpyrrolidine (PVP(55000)) stabilizers. PVP-free colloids were also prepared but less stable than the PVP-protected NPs. Rh@Pt core-shell particles were prepared from 2.7, 3.3, and 3.9 nm Rh cores with varying shell thicknesses (approximately 1 and approximately 2 ML). The particles were characterized by a combination of TEM, single-particle EDS, EDS line scans, XRD analysis, Debye Function simulations, FT-IR, and micro-Raman CO-probe experiments. The three different architectures were evaluated for preferential oxidation of CO in hydrogen (PROX) using 1.0 wt % Pt loadings in Al(2)O(3) supports. For hydrogen feeds with 0.2% CO and 0.5% O(2) the Rh@Pt NP catalyst has the best activity with complete CO oxidation at 70 degrees C and very high PROX selectivity at 40 degrees C with 50% CO conversion.

We report on the first-principles-guided design, synthesis, and characterization of core-shell nanoparticle (NP) catalysts made of a transition metal core (M = Ru, Rh, Ir, Pd, or Au) covered with a approximately 1-2 monolayer thick shell of Pt atoms (i.e., a M@Pt core-shell NP). An array of experimental techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and temperature-programmed reaction, are employed to establish the composition of the synthesized NPs. Subsequent studies of these NPs' catalytic properties for preferential CO oxidation in hydrogen-rich environments (PROX), combined with Density Functional Theory (DFT)-based mechanistic studies, elucidate important trends and provide fundamental understanding of the reactivity of Pt shells as a function of the core metal. Both the PROX activity and selectivity of several of these M@Pt core-shell NPs are significantly improved compared to monometallic and bulk nonsegregated bimetallic nanoalloys. Among the systems studied, Ru@Pt core-shell NPs exhibit the highest PROX activity, where the CO oxidation is complete by 30 degrees C (1000 ppm CO in H(2)). Therefore, despite their reduced Pt content, M@Pt core-shell NPs afford the design of more active PROX catalysts. DFT studies suggest that the relative differences in the catalytic activities for the various core-shell NPs originate from a combination of (i) the relative availability of CO-free Pt surface sites on the M@Pt NPs, which are necessary for O(2) activation, and (ii) a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.

A comprehensive structural/architectural evaluation of the PtRu (1:1) alloy and Ru@Pt core-shell nanoparticles (NPs) provides spatially resolved structural information on sub-5 nm NPs. A combination of extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), pair distribution function (PDF) analyses, Debye function simulations of X-ray diffraction (XRD), and field emission transmission electron microscopy/energy dispersive spectroscopy (FE-TEM/EDS) analyses provides complementary information used to construct a detailed picture of the core/shell and alloy nanostructures. The 4.4 nm PtRu (1:1) alloys are crystalline homogeneous random alloys with little twinning in a typical face-centered cubic (fcc) cell. The Pt atoms are predominantly metallic, whereas the Ru atoms are partially oxidized and are presumably located on the NP surface. The 4.0 nm Ru@Pt NPs have highly distorted hcp Ru cores that are primarily in the metallic state but show little order beyond 8 A. In contrast, the 1-2 monolayer thick Pt shells are relatively crystalline but are slightly distorted (compressed) relative to bulk fcc Pt. The homo- and heterometallic coordination numbers and bond lengths are equal to those predicted by the model cluster structure, showing that the Ru and Pt metals remain phase-separated in the core and shell components and that the interface between the core and shell is quite normal.

Au-Pt heteroaggregate nanostructures were prepared by sequential reduction methods. The structures have approximately 11 nm Au cores with Pt "tendrils" attached to the Au surface. The heteroaggregates are active H2 oxidation catalysts and show high activity at 90 degrees C in the presence of 1000 ppm CO. The surprising CO-tolerant behavior arises from the composition and unusual architecture of the particles.