Direct 12-Electron Oxidation of Ethanol on a Ternary Au(core)-PtIr(Shell) Electrocatalyst.

Understanding the roles of metals and atomic structures in activating various elementary steps of electrocatalytic reactions can help rational design of binary or ternary catalysts for promoting activity toward desirable products via favorable pathways. Here we report on a newly developed ternary Au@PtIr core-shell catalyst for ethanol oxidation reaction (EOR) in alkaline solutions, which exhibits an activity enhancement of 6 orders of magnitude compared to AuPtIr alloy catalysts. Analysis of in situ infrared reflection absorption spectra for Au@PtIr and its bimetallic subsets, Au@Pt and PtIr alloy, found that monatomic steps and Au-induced tensile strain on PtIr facilitate C-C bond splitting via ethanol dissociative adsorption and Ir promotes dehydrogenation at low potentials. As evidenced by the CO band being observed only for the PtIr alloy that is rather inactive for ethanol dissociative adsorption, we propose that splitting the C-C bond at the earliest stage of EOR activates a direct 12-electron full oxidation pathway because hydrogen-rich fragments can be fully oxidized without CO as a poisoning intermediate. The resulting synergy of complementary effects of Au core and surface Ir leads to an outstanding performance of Au@PtIr for EOR as characterized by a low onset potential of 0.3 V and 8.3 A mg-1all-metals peak current with 57% currents generated via full ethanol oxidation.

Favorable Core/Shell Interface within Co2P/Pt Nanorods for Oxygen Reduction Electrocatalysis.

Nanostructures with nonprecious metal cores and Pt ultrathin shells are recognized as promising catalysts for oxygen reduction reaction (ORR) to enhance Pt efficiency through core/shell interfacial strain and ligand effects. However, core/shell interaction within a real catalyst is complex and due to the presence of various interfaces in all three dimensions is often oversimply interpreted. Using Co2P/Pt core/shell structure as a model catalyst, we demonstrate, through density functional theory (DFT) calculations that forming Co2P(001)/Pt(111) interface can greatly improve Pt energetics for ORR, while Co2P(010)/Pt(111) is highly detrimental to ORR catalysis. We develop a seed-mediated approach to core/shell Co2P/Pt nanorods (NRs) within which Co2P(001)/Pt(111) interface is selectively expressed over the side facets and the undesired Co2P(010)/Pt(111) interface is minimized. The resultant Co2P/Pt NRs are highly efficient in catalyzing ORR in acid, superior to benchmark CoPt alloy and Pt nanoparticle catalyst. As the first example of one-dimensional (1D) core/shell nanostructure with an ultrathin Pt shell and a nonprecious element core, this strategy could be generalized to develop ultralow-loading precious-metal catalysts with favorable core/shell interactions for ORR and beyond.

Surface Proton Transfer Promotes Four-Electron Oxygen Reduction on Gold Nanocrystal Surfaces in Alkaline Solution.

Four-electron oxygen reduction reaction (4e-ORR), a key pathway in energy conversion, is preferred over the two-electron reduction pathway that falls short in dissociating dioxygen molecules. Gold surfaces exhibit high sensitivity of the ORR pathway to its atomic structures. A long-standing puzzle remains unsolved: why the Au surfaces with {100} sub-facets were exceptionally capable to catalyze the 4e-ORR in alkaline solution, though limited within a narrow potential window. Herein we report the discovery of a dominant 4e-ORR over the whole potential range on {310} surface of Au nanocrystal shaped as truncated ditetragonal prism (TDP). In contrast, ORR pathways on single-crystalline facets of shaped nanoparticles, including {111} on nano-octahedra and {100} on nanocubes, are similar to their single-crystal counterparts. Combining our experimental results with density functional theory calculations, we elucidate the key role of surface proton transfers from co-adsorbed H2O molecules in activating the facet- and potential-dependent 4e-ORR on Au in alkaline solutions. These results elucidate how surface atomic structures determine the reaction pathways via bond scission and formation among weakly adsorbed water and reaction intermediates. The new insight helps in developing facet-specific nanocatalysts for various reactions.

Ultralow charge-transfer resistance with ultralow Pt loading for hydrogen evolution and oxidation using Ru@Pt core-shell nanocatalysts.

We evaluated the activities of well-defined Ru@Pt core-shell nanocatalysts for hydrogen evolution and oxidation reactions (HER-HOR) using hanging strips of gas diffusion electrode (GDE) in solution cells. With gas transport limitation alleviated by micro-porous channels in the GDEs, the charge transfer resistances (CTRs) at the hydrogen reversible potential were conveniently determined from linear fit of ohmic-loss-corrected polarization curves. In 1 M HClO4 at 23 °C, a CTR as low as 0.04 Ω cm(-2) was obtained with only 20 µg cm(-2) Pt and 11 µg cm(-2) Ru using the carbon-supported Ru@Pt with 1:1 Ru:Pt atomic ratio. Derived from temperature-dependent CTRs, the activation barrier of the Ru@Pt catalyst for the HER-HOR in acids is 0.2 eV or 19 kJ mol(-1). Using the Ru@Pt catalyst with total metal loadings <50 µg cm(-2) for the HER in proton-exchange-membrane water electrolyzers, we recorded uncompromised activity and durability compared to the baseline established with 3 mg cm(-2) Pt black.

Ordered bilayer ruthenium-platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts.

Fabricating subnanometre-thick core-shell nanocatalysts is effective for obtaining high surface area of an active metal with tunable properties. The key to fully realize the potential of this approach is a reliable synthesis method to produce atomically ordered core-shell nanoparticles. Here we report new insights on eliminating lattice defects in core-shell syntheses and opportunities opened for achieving superior catalytic performance. Ordered structural transition from ruthenium hcp to platinum fcc stacking sequence at the core-shell interface is achieved via a green synthesis method, and is verified by X-ray diffraction and electron microscopic techniques coupled with density functional theory calculations. The single crystalline Ru cores with well-defined Pt bilayer shells resolve the dilemma in using a dissolution-prone metal, such as ruthenium, for alleviating the deactivating effect of carbon monoxide, opening the door for commercialization of low-temperature fuel cells that can use inexpensive reformates (H2 with CO impurity) as the fuel.

Truncated ditetragonal gold prisms as nanofacet activators of catalytic platinum.

We report a facile, seed-mediated method to synthesize nanoscale gold truncated ditetragonal nanoprisms (TDPs) enclosed by 12 high-index {310} facets. The method leads to the formation of nanoparticles with high size and shape monodispersity and allows for easy surfactant removal. The dependence of particle shape on the synergetic contribution of metallic ions, halide ions, and surfactant adsorbates during synthesis is described. The resulting high-index nanoparticle facets were demonstrated as efficient activators of a supported catalytic material (platinum). A Pt monolayer deposited onto the Au TDP nanofacets with sharp electrochemical signatures exhibits an enhanced catalytic activity.

Kirkendall effect and lattice contraction in nanocatalysts: a new strategy to enhance sustainable activity.

Core-shell nanoparticles increasingly are found to be effective in enhancing catalytic performance through the favorable influence of the core materials on the active components at the surface. Yet, sustaining high activities under operating conditions often has proven challenging. Here we explain how differences in the components' diffusivity affect the formation and stability of the core-shell and hollow nanostructures, which we ascribe to the Kirkendall effect. Using Ni nanoparticles as the templates, we fabricated compact and smooth Pt hollow nanocrystals that exhibit a sustained enhancement in Pt mass activity for oxygen reduction in acid fuel cells. This is achieved by the hollow-induced lattice contraction, high surface area per mass, and oxidation-resistant surface morphology--a new route for enhancing both the catalysts' activity and durability. The results indicate challenges and opportunities brought by the nanoscale Kirkendall effect for designing, at the atomic level, nanostructures with a wide range of novel properties.

Low-coordination sites in oxygen-reduction electrocatalysis: their roles and methods for removal.

Low-coordination sites, including edges, kinks, and defects, play an important role in oxygen-reduction electrocatalysis. Their role was studied experimentally and theoretically for various Pt surfaces. However, the roughness effect on similar-sized nanoparticles that could elucidate the role of low-coordination sites has attracted much less attention, with no studies on Pd nanoparticles. Here, using Br- adsorption/desorption, we introduce an effective approach to reduce surface roughness, yielding Pd nanoparticles with smoother surfaces and an increased number of (111)-oriented facets. The resulting nanoparticles have a slightly contracted structure and narrow size distribution. Pt monolayer catalysts that contain such nanoparticles as the cores showed a 1.5-fold enhancement in specific and Pt mass activities for the oxygen reduction reaction compared with untreated ones. Furthermore, a dramatic increase in durability was observed with bromide-treated Pd(3)Co cores. These results demonstrate a simple approach to preparing nanoparticles with smooth surfaces and confirm the adverse effect of low-coordination sites on the kinetics of the oxygen-reduction reaction.

Melamine related bilateral renal calculi in 50 children: single center experience in clinical diagnosis and treatment.

PURPOSE: We investigated the clinical diagnosis and treatment features of bilateral renal calculi in young children who ingested melamine tainted infant milk formula. MATERIALS AND METHODS: We retrospectively analyzed clinical data on 50 children (mean +/- SE age 23.4 +/- 3.1 months) with a history of ingesting melamine tainted infant milk formula and suffering from bilateral renal calculi. All patients underwent ultrasound and renal function evaluation. Treatment included cessation of melamine tainted formula consumption, hydration, urine basification and hemodialysis if necessary. RESULTS: Bilateral renal calculi peaked in 6 to 18-month-olds (58% of cases). The male-to-female ratio was 3.1:1.0. Calculi ranged in diameter from 4 to 10 mm in 33 patients (66%) and 2.5 to 4 mm in 17 (34%). Of the 11 patients with renal failure 8 underwent 1 to 4 sessions of hemodialysis. Of the 9 bilateral obstruction cases with renal failure 8 did not require surgical intervention but 1 required ureteral catheterization to drain the renal pelvis. All children experienced a good outcome and were discharged home after a mean +/- SE hospitalization of 8.1 +/- 0.7 days. CONCLUSIONS: Melamine related urinary calculi were most often seen in patients 6 to 18 months old. Conservative management has been sufficient in most cases. However, these children need to be monitored for long-term effects of melamine tainted milk formula consumption.

Oxygen reduction on well-defined core-shell nanocatalysts: particle size, facet, and Pt shell thickness effects.

We examined the effects of the thickness of the Pt shell, lattice mismatch, and particle size on specific and mass activities from the changes in effective surface area and activity for oxygen reduction induced by stepwise Pt-monolayer depositions on Pd and Pd(3)Co nanoparticles. The core-shell structure was characterized at the atomic level using Z-contrast scanning transmission electron microscopy coupled with element-sensitive electron energy loss spectroscopy. The enhancements in specific activity are largely attributed to the compressive strain effect based on the density functional theory calculations using a nanoparticle model, revealing the effect of nanosize-induced surface contraction on facet-dependent oxygen binding energy. The results suggest that moderately compressed (111) facets are most conducive to oxygen reduction reaction on small nanoparticles and indicate the importance of concerted structure and component optimization for enhancing core-shell nanocatalysts' activity and durability.

Intrinsic kinetic equation for oxygen reduction reaction in acidic media: the double Tafel slope and fuel cell applications.

According to Sergio Trasatti, "A true theory of electrocatalysis will not be available until activity can be calculated a priori from some known properties of the materials." Toward this goal, we developed intrinsic kinetic equations for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) using as the kinetic parameters the free energies of adsorption and activation for elementary reactions. Rigorous derivation retained the intrinsic connection between the intermediates' adsorption isotherms and the kinetic equations, affording us an integrated approach for establishing the reaction mechanisms based upon various experimental and theoretical results. Using experimentally deduced free energy diagrams and activity-and-barriers plot for the ORR on Pt(111), we explained why the Tafel slope in the large overpotential region is double that in the small overpotential region. For carbon-supported Pt nanoparticles (Pt/C), the polarization curves measured with thin-film rotating disk electrodes also exhibit the double Tafel slope, albeit Pt(111) is several times more active than the Pt nanoparticles when the current is normalized by real surface area. An analytic method was presented for the polarization curves measured with H2 in proton exchange membrane fuel cells (PEMFCs). The fit to a typical iR-free polarization curve at 80 degrees C revealed that the change of the Tafel slope occurs at about 0.77 V that is the reversible potential for the transition between adsorbed O and OH on Pt/C. This is significant because it predicts that the Butler-Volmer equation can only fit the data above this potential, regardless the current density. We also predicted a decrease of the Tafel slope from 70 to 65 mV dec(-1) at 80 degrees C with increasing oxygen partial pressure, which is consistent with the observation reported in literature.

Double-trap kinetic equation for the oxygen reduction reaction on Pt(111) in acidic media.

We derived an intrinsic kinetic equation for the four-electron oxygen reduction reaction (ORR) in acidic media using free energies of activation and adsorption as the kinetic parameters. Our kinetic model consists of four essential elementary reactions: a dissociative adsorption (DA) and a reductive adsorption (RA), which yield two reaction intermediates, O and OH; a reductive transition (RT) from O to OH; and a reductive desorption (RD) of OH. Analytic expressions were found for the O and OH adsorption isotherms by solving the steady-state rate equations. For the ORR on Pt(111) in 0.1 M HClO4 solution, we analyzed the measured polarization curves, thereby deducing activation free energies that are consistent with the values from theoretical calculations. The reductive adsorption (DeltaG*0RA=0.46 eV) is not the rate-determining step (RDS) for the ORR on Pt because dissociative adsorption (DeltaG*0DA=0.26 eV) offers a more favorable pathway at high potentials. It, however, generates strongly adsorbed O. The high activation barriers for the O to OH transition (DeltaG*0RT=0.50 eV) and OH desorption (DeltaG*0RD=0.45 eV) cause a large potential loss for the desorption-limited ORR. As the OH coverage increases to a constant value with decreasing potential, the Tafel slope increases to the value determined by a symmetric electron-transfer coefficient. We discuss the role of adsorption isotherm in kinetic analysis and, via activity-and-barrier plots, illustrate why the RDS may vary with reaction conditions or may not exist. Recognizing such features of electrocatalytic reactions can facilitate reaching the long-standing goal of quantitative descriptions and predictions of electrocatalysts' activities.