Molecular adsorption at well-defined electrode surfaces: hydroquinone on Pd(111) studied by EC-STM

The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.     

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.     

Reaction mechanism of the benzoquinone/hydroquinone couple at platinum electrodes in aqueous solutions: Retardation and enhancement of electrode kinetics by single chemisorbed layers

The electrochemical kinetics of the benzoquinone (Q)/hydroquinone (H₂Q) redox couple at platinum electrodes in aqueous solutions has been found to be extremely sensitive to the nature of species adsorbed on the electrode surface at monolayer coverages. Experimental measurements were based on thin-layer cyclic voltammetry; the use of thin-layer electrodes was dictated by the need to minimize surface contamination. Bulky neutral or anionic aromatic adsorbates led to the familiar U-shaped rate-vs.-pH curves; the rate minimum occurred near pH 4. Kinetic effects due to oriental changes of chemisorbed species were noted only when the rate was low. Adsorbed 1 atoms led to comparatively rapid reactivity (rate constant k° > 10^?3 cm s^?1) and virtual independence of pH. Profound retardation resulted from pretreatment ofthe surface with CN^? and SCN^?; total irreversibility (k° < 10^?6 cm s^?1) was observed at pH 4, with a further decrease in rate at pH 7. In contrast, when the surface contained n layer of chemisorbed phenyltriethylammonium cations, the electrode rate increased with increasing pH. The results indicate that different reaction pathways predominate when different absorbates are present.