RESUMEN

Ethanol, with its high energy density, likely production from renewable sources and ease of storage and transportation, is almost the ideal combustible for fuel cells wherein its chemical energy can be converted directly into electrical energy. However, commercialization of direct ethanol fuel cells has been impeded by ethanol's slow, inefficient oxidation even at the best electrocatalysts. We synthesized a ternary PtRhSnO(2)/C electrocatalyst by depositing platinum and rhodium atoms on carbon-supported tin dioxide nanoparticles that is capable of oxidizing ethanol with high efficiency and holds great promise for resolving the impediments to developing practical direct ethanol fuel cells. This electrocatalyst effectively splits the C-C bond in ethanol at room temperature in acid solutions, facilitating its oxidation at low potentials to CO(2), which has not been achieved with existing catalysts. Our experiments and density functional theory calculations indicate that the electrocatalyst's activity is due to the specific property of each of its constituents, induced by their interactions. These findings help explain the high activity of Pt-Ru for methanol oxidation and the lack of it for ethanol oxidation, and point to the way to accomplishing the C-C bond splitting in other catalytic processes.

RESUMEN

We demonstrate a new approach to synthesizing high-activity electrocatalysts for the O(2) reduction reaction with ultra low Pt content. The synthesis involves placing a small amount of Pt, the equivalent of a monolayer, on carbon-supported niobium oxide nanoparticles (NbO(2) or Nb(2)O(5)). Rotating disk electrode measurements show that the Pt/NbO(2)/C electrocatalyst has three times higher Pt mass activity for the O(2) reduction reaction than a commercial Pt/C electrocatalyst. The observed high activity of the Pt deposit is attributed to the reduced OH adsorption caused by lateral repulsion between PtOH and oxide surface species. The new electrocatalyst also exhibits improved stability against Pt dissolution under a potential cycling regime (30,000 cycles from 0.6 V to 1.1 V). These findings demonstrate that niobium-oxide (NbO(2)) nanoparticles can be adequate supports for Pt and facilitate further reducing the noble metal content in electrocatalysts for the oxygen reduction reaction.

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RESUMEN

We demonstrated that platinum (Pt) oxygen-reduction fuel-cell electrocatalysts can be stabilized against dissolution under potential cycling regimes (a continuing problem in vehicle applications) by modifying Pt nanoparticles with gold (Au) clusters. This behavior was observed under the oxidizing conditions of the O2 reduction reaction and potential cycling between 0.6 and 1.1 volts in over 30,000 cycles. There were insignificant changes in the activity and surface area of Au-modified Pt over the course of cycling, in contrast to sizable losses observed with the pure Pt catalyst under the same conditions. In situ x-ray absorption near-edge spectroscopy and voltammetry data suggest that the Au clusters confer stability by raising the Pt oxidation potential.

RESUMEN

We investigated the oxygen-reduction reaction (ORR) on Pd monolayers on various surfaces and on Pd alloys to obtain a substitute for Pt and to elucidate the origin of their activity. The activity of Pd monolayers supported on Ru(0001), Rh(111), Ir(111), Pt(111), and Au(111) increased in the following order: Pd/Ru(0001) < Pd/Ir(111) < Pd/Rh(111) < Pd/Au(111) < Pd/Pt(111). Their activity was correlated with their d-band centers, which were calculated using density functional theory (DFT). We found a volcano-type dependence of activity on the energy of the d-band center of Pd monolayers, with Pd/Pt(111) at the top of the curve. The activity of the non-Pt Pd2Co/C alloy electrocatalyst nanoparticles that we synthesized was comparable to that of commercial Pt-containing catalysts. The kinetics of the ORR on this electrocatalyst predominantly involves a four-electron step reduction with the first electron transfer being the rate-determining step. The downshift of the d-band center of the Pd "skin", which constitutes the alloy surface due to the strong surface segregation of Pd at elevated temperatures, determined its high ORR activity. Additionally, it showed very high methanol tolerance, retaining very high catalytic activity for the ORR at high concentrations of methanol. Provided its stability is satisfactory, this catalyst might possibly replace Pt in fuel-cell cathodes, especially those of direct methanol oxidation fuel cells (DMFCs).

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RESUMEN

We synthesized a new class of O2 electrocatalysts with a high activity and very low noble metal content. They consist of Pt monolayers deposited on the surfaces of carbon-supported nonnoble metal-noble metal core-shell nanoparticles. These core-shell nanoparticles were formed by segregating the atoms of the noble metal on to the nanoparticles' surfaces at elevated temperatures. A Pt monolayer was deposited by galvanic displacement of a Cu monolayer deposited at underpotentials. The mass activity of all the three Pt monolayer electrocatalysts investigated, viz., Pt/Au/Ni, Pt/Pd/Co, and Pt/Pt/Co, is more than order of magnitude higher than that of a state-of-the-art commercial Pt/C electrocatalyst. Geometric effects in the Pt monolayer and the effects of PtOH coverage, revealed by electrochemical data, X-ray diffraction, and X-ray absorption spectroscopy data, appear to be the source of the enhanced catalytic activity. Our results demonstrated that high-activity electrocatalysts can be devised that contain only a fractional amount of Pt and a very small amount of another noble metal.

RESUMEN

A dramatic multilayer substrate relaxation is observed for the (square root 19 x square root 19)-13CO adlayer phase on a Pt(111) electrode by surface X-ray scattering. Within the (square root 19 x square root 19) unit cell, a vertical expansion of 0.28 A was determined for the Pt atoms under near-top-site CO molecules, whereas only 0.04 A was found under near-bridge-site CO molecules. The lateral displacements involve small rotations toward more symmetric bonding. Both the expansions and rotations extend into the bulk with a decay length of 1.8 Pt layers. This nonuniform layer expansion, hitherto unseen, appears to be a manifestation of the differential stress induced by CO adsorption at different sites.

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RESUMEN

Surface enhanced infrared reflection-absorption spectroscopy with an attentuated total reflection configuration (ATR-SEIRAS) was used for the first time to identify the intermediates of the oxygen reduction reaction (ORR) on gold electrodes. Our study employed a Au thin-film electrode in acidic and alkaline solutions. In alkaline solutions, a potential dependent band at 1268 cm(-1), which we assigned to the antisymmetric bending mode of OOH of adsorbed HO2-, was observed between 0.1 and -0.6 V versus Ag|AgCl, Cl-, exactly in the potential range where the ORR occurred. The assignment was supported by our isotope exchange experiment. The adsorbed HO2- is a reaction intermediate in the 4e- serial mechanism. In acidic solutions, there was only a very weak band at the same position, reflecting the fast protonation of HO2-. This finding may imply that the interaction between HO2- and Au surfaces is very weak in acidic solutions, in agreement with the observed 2e- reduction mechanism.

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X-ray reflectivity, cyclic voltammetry, and scanning tunneling microscopy (STM) are used to examine the structure of alpha-SiW12O4(4-) or silicotungstic acid (STA) adsorbed on Ag(100) in acid solution. The voltammetry shows that STA passivates the Ag surface relative to electron transfer to a solution redox species. STM images reveal the formation of a series of lattice structures, one of which can be associated with a commensurate ( radical13x radical13)R33.69 degrees structural model. X-ray reflectivity measurements show uniquely that STA orients with its four-fold axis perpendicular to the Ag(100) surface and that the center of the STA molecule is 4.90 A above the top layer of the Ag substrate. Analysis of bond lengths leads to a footprint of STA on Ag(100), in which the four terminal O atoms are located near the hollow sites and have a Ag-O bond length of 2.06 A. This bond length is consistent with a strong covalent interaction between STA and the Ag surface.