Juan B. Abreu

Electrochemistry of the I-on-Pd single-crystal interface : studies by UHV-EC and in situ STM

Abstract A single chemisorbed layer of zerovalent I atoms has been found to enhance, at ambient temperatures, the reactivity of Pd electrode surfaces. Three unique reactions (anodic dissolution in non-corrosive electrolyte, regeneration of well-ordered single-crystal surfaces, and “electrochemical digital etching”) have been investigated at Pd(111) and Pd(100) single-crystal electrodes and are described in this paper. Experimental measurements were based upon a combination of electrochemistry (EC), low-energy electron diffraction (LEED), and in-situ scanning tunneling microscopy (STM).

Electrochemical regeneration of clean and ordered Pd(100) surfaces by iodine adsorption-desorption: evidence from low-energy electron diffraction

In atomic-level investigations of electrocatalytic processes, the preparation and regeneration of clean and well ordered electrode surfaces under electrochemical conditions constitute major concerns [1,2]. Standard single-crystal orientation procedures performed outside an inert environment do not guarantee the existence of clean and ordered surfaces [1,3]; even for well defined surfaces prepared in ultrahigh vacuum (UHV), interfacial disorder occurs at potentials where surface oxide layers are formed 141. Towards the solution of this problem, two general schemes have been adopted [l]. One employs high temperatures (thermal annealing), and the other applies electrode potentials (electrochemical annealing). Thermal annealing has been done in UHV (following which surface analysis is performed to ascertain the interfacial structure and composition) [1,51 and at ambient-pressure conditions (provided precautions are taken to prevent surface contamination from atmospheric impurities) [6-91. Electrochemical annealing is based on the likelihood that, at an appropriate potential, disordered interfacial atoms can either be activated to diffuse to ordered sites or be dissolved to expose ordered layers. Electrochemical annealing may be electrolyte unassisted (such as the ordering of Au(ll1) electrodes by sequential voltammetric scans between the oxygen and hydrogen evolution regions [lo]) or electrolyte assisted (such as the etching of

Anodic dissolution and reordering of Pd(110) induced by chemisorbed iodine

Abstract Adsorbate-induced reordering and anodic dissolution, reported previously for Pd(111) and Pd(100) surfaces that contained an ordered iodine adlattice, were examined at an I-pretreated Pd(110) electrode. Experimental measurements were based upon a combination of electrochemistry, electron spectroscopy, and scanning tunneling microscopy; remarkable consistency was observed between the ex situ and in situ results. Similarities and differences exist between Pd(110) and the two other low-index planes. The expected congruencies: (i) well-ordered iodine adlattices (Pd(110)-c(2 × 2)-I and Pd(100)-pseudohexagonal-I) are formed spontaneously upon exposure of a Pd(110) surface to an aqueous solution of iodide, even when the surface was previously disordered by oxidation-reduction cycles; (ii) anodic dissolution of the metal substrate occurs only in the presence of chemisorbed iodine; (iii) the I-catalyzed corrosion does not alter the coverage of the iodine adlayer. The notable disparities: (a) reductive desorption of the chemisorbed iodine does not yield an ordered (1 × 1) surface; neither does the removal of iodine by displacement with and subsequent oxidative desorption of CO; (b) anodic stripping of the metal surface disorders the structure of the iodine adlattice; no reordering takes place upon exposure of such disordered surface to aqueous iodide; (c) the iodine-catalyzed corrosion occurs selectively at stepedges along the {100} and {110} directions; dissolution at the {110}-directed step develops preferentially over that at the {100}-directed edge to form rectangular pits; (d) the propagation of new (smaller) pits at the bottom of the (enlarged) rectangular pits leads to progressive surface roughness.

Adsorbate-catalyzed layer-by-layer metal dissolution in inert electrolyte: Pd(100)-c(2 × 2)-I

Abstract Studies on the corrosion of Pd in inert ( halide-free ) H 2 SO 4 solution, catalyzed by a single adsorbed layer of iodine, have been extended to a Pd(100) single-crystal electrode that contained an ordered c(2 × 2) adlattice of iodine. Experimental measurements were based upon a combination of linear-sweep voltammetry, potential-step coulometry, low-energy electron diffraction, and Auger electron spectroscopy. As was earlier noted with polycrystalline electrodes, Pd dissolution occurred only when iodine was present on the surface. More significantly, neither the coverage nor the ordered structure of the iodine adlattice was affected by the dissolution process. These observations strongly suggest that the iodine-catalyzed corrosion occurs one layer at a time.

In situ reordering by iodine adsorption-desorption of extensively disordered (ion-bombarded) Pd(100) electrode surfaces

Abstract Previous studies have shown that the single-crystallinity of a Pd(100) electrode surface mildly disordered by electrochemical oxidation can be reestablished if the spent surface is immeresed at ambient temperatures in aqueous iodide followed by reductive desorption of the interfacial iodine. In this short note, we show that an extensively disordered (Ar+-ion-bombarded) Pd(100) surface can also be reordered by the iodine-chemisorption method. Multiple surface oxidation—reduction cycles on the ion-bombarded electrode did not regenerate an ordered surface, but the nature and/or degree of disorder was altered to resemble an anodically oxidized surface. Reordering was attained only when multiple sequences of iodine oxidative chemisorption (deposition) and reductive desorption (stripping), at potentials close to the hydrogen evolution region, were performed. Experiments were carried out in alkaline solutions to ensure that the reordering process is driven predominantly by the strong chemisorption of iodine and not by the dissolution of Pd. Electrode-surface characterization consisted of cyclic voltammetry, low-energy electron diffraction, and Auger electron spectroscopy.

The Interfacial Chemistry of Grignard Reagent Formation: Reactions of Clean Mg(0001) Surfaces

The Grignard reagent, RMgX, where R is a hydrocarbon group and X is a halogen, is one of the more important and versatile reagents for organic synthesis [1]. It is formed in a heterogeneous reaction between magnesium and an organic halide in an appropriate organic solvent [2, 3, 4, 5]

Absorbate-catalyzed dissolution in inert electrolyte: layer-by-layer corrosion of palladium(100)-c(2 .times. 2)-iodine

{\rm{RX + Mg}} \to {\rm{RMgX + RR + Mg}}{{\rm{X}}_{\rm{2}}}{\rm{ + other by - products}}

The structure, composition and reactivity of clean and ambient-exposed polycrystalline and monocrystalline Mg surfaces

(1)