Manuel P. Soriaga

Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction An In Situ XANES and EXAFS Investigation

The electrocatalysis of the oxygen reduction reaction (ORR) on five binary Pi alloys (PtCr/C, PtMn/C, PtFe/C, PtCo/C, and PtNi/C) supported on high surface area carbon in a proton exchange membrane fuel cell was investigated. All the alloy electrocatalysts exhibited a high degree of crystallinity with the primary phase of the type Pt3M (LI2 structure with fcc type lattice) and a secondary phase (only minor contribution from this phase) being of the type PtM (LIo structure with tetragonal lattice) as evidenced from x-ray powder diffraction (XRD) analysis. The electrode kinetic studies on the Pt alloys at 95~ and 5 atm pressure showed a two- to threefold increase in the exchange current densities and the current density at 900 mV as well as a decrease in the overvoltage at i0 mA em -2 relative to Pt/C eleetrocatalyst. The PtCr/C alloy exhibited the best performance. In situ EXAFS and XANES analysis at potentials in the double-layer region [0.54 V vs. reversible hydrogen electrode (RHE)] revealed (i) all the alloys possess higher Pt d-band vacancies per atom (with the exception of PtMn/C alloy) relative to Pt/C electrocatalyst and (it) contractions in the Pt-Pt bond distances which confirmed the results from ex situ XRD analysis. A potential excursion to 0.84 V vs. RHE showed that, in contrast to the Pt alloys, the Pt/C electrocatalyst exhibits a significant increase in the Pt d-band vacancies per atom. This increase, in Pt/C has been rationalized as being due to adsorption of OH species from the electrolyte following a Temkin isotherm behavior, which does not occur on the Pt alloys. Correlation of the electronic (Pt d-band vacancies) and geometric (Pt-Pt bond distance) with the electrochemical performance characteristics exhibits a volcano type behavior with the PtCr/C alloy being at the top of the curve. The enhanced electrocatalysis by the alloys therefore can be rationalized on the basis of the interplay between the electronic and geometric factors on one hand and their effect on the chemisorption behavior of OH species from the electrolyte. The role of Pt/C and Pt alloys on the mechanism of the oxygen reduction reaction (ORR) has been investigated previously, 1-4 however the mechanism still remains elusive. One of the first investigations I of the ORR on Pt alloy electrocatalysts was in phosphoric acid; the effect of changes in the Pt-Pt interatomic distances, caused by alloying, was examined. The strength of the [M-HO2]aas bond, the intermediate formed in the rate-determining step of the molecular dioxygen reduction, was shown to depend on the Pt-Pt bond distance in the alloys. A plot of the electrocatalytic activity vs. adsorbate bond strength exhibited a volcano type behavior. 5 It was shown that the lattice contractions due to alloying resulted in a more favorable Pt-Pt distance (while maintaining the favorable Pt electronic properties) for dissociative adsorption of 02. This view was disputed by Glass et al. ~ in their investigation on bulk alloys of PtCr (the binary alloy at the top of the volcano plot) of different compositions. The latter investigation showed no activity enhancement for the ORR in phosphoric acid. This study therefore suggested the possibility of differences in electrochemical properties of bulk vs. supported alloy electrocatalysts (small particles of 35-85 A). A recent study on supported PtCo electrocatalysts ~ revealed the possibility that particle termination, primarily at the vicinal planes in the supported alloy electrocatalyst, is the reason for the enhanced ORR electrocatalysis (i.e., vicinal planes are more active than ). Paffett et al., 3 attributed higher activities for the ORR on bulk PtCr alloys in phosphoric acid to surface roughening, and hence increased Pt surface area, caused by the dissolution of the more oxidizable alloying component Cr. In contrast to these findings on bulk alloys, the supported alloy electrocatalysts have been reported to retain their nonnoble alloying element in the electrode during long periods (6000-9000 h) of operation in phosphoric acid fuel cells (PAFCs) 6 and proton exchange membrane fuel ceils (PEMFCs). 7 Based on these previous investigations and in the context of the ORR mechanisms, the principle explanations for the

Electrocatalysis of hydrogen production by active site analogues of the iron hydrogenase enzyme: structure/function relationships

A series of binuclear FeIFeI complexes, (μ-SEt)2[Fe(CO)2L]2 (L = CO (1), PMe3 (1-P)), (μ-SRS)[Fe(CO)2L]2 (R = CH2CH2 (μ-edt): L = CO (2), PMe3 (2-P); R = CH2CH2CH2(μ-pdt): L = CO (3), PMe3 (3-P); and R = o-CH2C6H4CH2 (μ-o-xyldt): L = CO (4), PMe3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The redox levels for H2 production were established by spectroelectrochemistry to be Fe0Fe0 for the all-CO complexes and FeIFe0 for the PMe3-substituted derivatives. As electrocatalysts, the PMe3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H2. An (η2-H2)FeII–Fe0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

Orientational transitions of aromatic molecules adsorbed on platinum electrodes

Abstract A method to determine the orientation of molecules adsorbed at solid-liquid interfaces, based on thin-layer electrochemical techniques, is described. This method has been applied to determine orientational changes of molecules adsorbed from solution onto smooth platinum electrodes as a function of adsorbate concentration. Twenty-six diphenols and quinones, representing a variety of structures and chemical properties, were studied. In general, these compounds are adsorbed with the diphenol or quinonoid ring parallel to the surface at low concentrations and re-orient irreversibly to non-random edgewise-orientations as the concentration is increased. A possible explanation for these edgewise-orientations is discussed.

Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); Studies by leed, auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I2 vapor to form the Pt(111) (7 × 7)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.

Electrodeposition on a well-defined surface: Silver on Pt(111)(√7×√7)R19.1°−I

Abstract An exploration is reported of the structures formed by electrodeposition of silver on well-defined Pt(111) surfaces in various amounts up to a few monolayers. Prior to electrodeposition, the Pt(111) surface was treated with I2 vapor to form a Pt(111)(√7×√7)R19.1°−I superlattice which effectively protected the Pt and Ag surfaces from attack by the aqueous HClO4 electrolyte and residual gases. Silver electrodeposited in three widely separated underpotential deposition stages, forming distinct lattice structures having (3×3) or (√3×√3)R30° LEED patterns at all coverages studied. Formation of ordered Ag layers has therefore been demonstrated. Measurements of Auger electron spectroscopic current for Pt, Ag and I revealed that the silver was located underneath the iodine atomic layer, which remained attached during multiple cycles of electrodeposition and dissolution of silver from iodine-free solutions.

Ultra-high vacuum techniques in the study of single-crystal electrode surfaces

The development of powerful, surface-sensitive analytical methods has led to spectacular advances in the field of gas-solid interfacial science over the past two decades. Earlier research had been based upon thermodynamic and kinetic methods that portrayed only the macroscopic properties of the interfacial ensemble. The dearth of atomic-level information at that time is remarkably similar to what presently handicaps classical electrochemistry. In search of a more fundamental, microscopic view of electrode processes, research in modern electrochemistry has incorporated non-traditional approaches to the study of the electrode-solution interface. One approach, motivated by the overwhelming successes in vacuum-metal surface science, is the adaptation of ultra-high vacuum (UHV) surface spectroscopic techniques; such approach is the subject of the present review. This article describes the capabilities and limitations of coupled UHV-electrochemistry (UHV-EC) as a means to extract an atomic-level picture of the solid-electrolyte interface. After a brief introduction that outlines the experimental and theoretical obstacles in electrochemical surface science, this review presents a detailed discussion on experimental protocols (sample preparation, surface analytical techniques, instrument design) and critical processes (emersion, evacuation, surface characterization) inherent in the UHV-EC methodology. The final segment of this article summarizes selected studies with single-crystal electrode surfaces that showcase the power and elegance of the UHV-EC strategy; a more extensive bibliography of published investigations is provided in the Appendix. This review is concluded with a commentary on the future prospects of the UHV-EC approach.

Electrocatalysis of the hydrogen-evolution reaction by electrodeposited amorphous cobalt selenide films†

Using an electrochemical method under ambient conditions, crystallographically amorphous films of cobalt selenide have been deposited from aqueous solution onto planar Ti supports. These films have been evaluated as electrocatalysts for the hydrogen-evolution reaction. In 0.500 M H2SO4, the cobalt selenide films required an overpotential of ∼135 mV to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm−2. Galvanostatic measurements indicated stability of the electrocatalytic films for >16 h of continuous operation at −10 mA cm−2. The facile preparation method, and the activity of the cobalt selenide films, suggest that electrodeposited metal chalcogenides are potentially attractive earth-abundant electrocatalysts for the hydrogen-evolution reaction.

Electrochemical oxidation of aromatic compounds adsorbed on platinum electrodes: The influence of molecular orientation*

Abstract Previous work has demonstrated that aromatic molecules adsorb on platinum electrodes in specific molecular orientations which change as the solution concentration is increased. The present article describes electrochemical oxidation of these adsorbed molecules as a function of orientation. The number of electrons, n ox , per molecular oxidized in aqueous 1 M HClO 4 was determined by thin-layer electrochemical methods. Twenty-nine compounds, representing a variety of structures and chemical properties, were studied: benzene; simple diphenols; alkyldiphenols; polyhydroxybenzenes; polyhydroxybiphenyls; tetrafluorohydroquinone; N -heteroaromatics; diphenols having surface-active side-chains; polycyclic phenols and quinones; hydroquinone mercaptans; and hydrogen sulfide. The magnitude of n ox is strongly dependent on initial molecular orientation, being smaller for edgewise than for flat orientations. These changes in n ox imply variations in oxidation product distribution with orientation, although direct identification of products was not made.

The Reaction Mechanism with Free Energy Barriers for Electrochemical Dihydrogen Evolution on MoS2

We report density functional theory (M06L) calculations including Poisson-Boltzmann solvation to determine the reaction pathways and barriers for the hydrogen evolution reaction (HER) on MoS2, using both a periodic two-dimensional slab and a Mo10S21 cluster model. We find that the HER mechanism involves protonation of the electron rich molybdenum hydride site (Volmer-Heyrovsky mechanism), leading to a calculated free energy barrier of 17.9 kcal/mol, in good agreement with the barrier of 19.9 kcal/mol estimated from the experimental turnover frequency. Hydronium protonation of the hydride on the Mo site is 21.3 kcal/mol more favorable than protonation of the hydrogen on the S site because the electrons localized on the Mo-H bond are readily transferred to form dihydrogen with hydronium. We predict the Volmer-Tafel mechanism in which hydrogen atoms bound to molybdenum and sulfur sites recombine to form H2 has a barrier of 22.6 kcal/mol. Starting with hydrogen atoms on adjacent sulfur atoms, the Volmer-Tafel mechanism goes instead through the M-H + S-H pathway. In discussions of metal chalcogenide HER catalysis, the S-H bond energy has been proposed as the critical parameter. However, we find that the sulfur-hydrogen species is not an important intermediate since the free energy of this species does not play a direct role in determining the effective activation barrier. Rather we suggest that the kinetic barrier should be used as a descriptor for reactivity, rather than the equilibrium thermodynamics. This is supported by the agreement between the calculated barrier and the experimental turnover frequency. These results suggest that to design a more reactive catalyst from edge exposed MoS2, one should focus on lowering the reaction barrier between the metal hydride and a proton from the hydronium in solution.

Electrode-surface coordination chemistry: ligand substitution and competitive coordination of halides at well-defined Pd(100) and Pd(111) single crystals

Abstract Ligand (adsorbate) substitution (displacement) and competitive-coordination (chemisorption) reactions between chloride, bromide and iodide anions have been studied at well-defined Pd(100) and Pd(111) single-crystal electrode surfaces in aqueous solutions. Experiments involved: (i) pretreatment of the Pd( hkl ) surfaces with a full monolayer of one halide followed by exposure to a dilute aqueous solution of another halide, and (ii) exposure of a clean Pd( hkl ) electrode to a solution that contained a binary or ternary mixture of the halides. The resulting monolayers were then characterized by low-energy electron diffraction. Auger electron spectroscopy and temperature-programmed desorption. The results were as follows: (i) the subject halides were oxidatively chemisorbed to produce well-defined halogen adlattices. (ii) In the absence of the heavier halides, chloride ions were adsorbed to form Pd(100)-(2×2)-Cl and Pd(111)-(√3×√3) R30°-Cl. (iii) These were displaced, spontaneously, irreversibly and quantitatively, by bromide ions to yield Pd(100)-(2×2)-Br and Pd(111)-(√3×√3) R30°-Br. respectively. (iv) The latter, in turn, were spontaneously, irreversibly and quantitatively displaced by iodide to produce Pd(100)-c(2×2)-I and Pd(111)-(√3×√3) R30°-I. Only Pd(100)-(2×2)-Br and Pd(111)-(√3×√3) R30°-Br were produced when the electrodes were exposed to a solution that contained a mixture of Cl − and Br − . (vi) Only Pd(100)-c(2×2)-I and Pd(111)-(√3×√3) R30°-I were produced when the electrodes were immersed in a solution that consisted of all three halides. These results, consistent with the thermal desorption data, demonstrate that the interaction of the subject halides with Pd electrode surfaces closely follows the homogeneous coordination chemistry of halo-Pd complexes: the strength of chemisorption or surface coordination decreases in the order I − > Br − > Cl − .