Michael E. Bothwell

Electrochemical regeneration of clean and well-ordered Pd(111) surfaces

In this paper, we suggest a new procedure for the electrochemical or in situ regeneration of clean and well-ordered electrode surfaces. All the steps in this method, which is demonstrated here for a Pd(111) single-crystal electrode, are performed inside the electrochemical cell

Surface coordination chemistry of noble-metal electrocatalysts: oxidative addition and reductive elimination of iodide at iridium, platinum and gold in aqueous solutions

Abstract The oxidative addition and reductive elimination of the iodo ligand has been compared at smooth polycrystalline gold, platinum and iridium surfaces in aqueous solutions. On these three metals, the iodo species undergoes spontaneous oxidative chemisorption to form a close-packed monolayer of zero-valent iodine, the saturation coverage of which is limited by the van der Waals radius of the iodine atom; this oxidative addition process is further manifested by evolution of hydrogen gas from proton reduction. Elimination of iodine from these surfaces can be achieved by its reduction back to the anion either by application of sufficiently negative potentials or by exposure to ample amounts of hydrogen gas. On Pt and Ir, the reductive desorption of iodine is coupled with reductive chemisorption of hydrogen; consequently, the reaction is a two-electron, pH-dependent process. A plot of E 1/2 , the potential at which the iodine coverage is decreased to half its maximum value, against pH yields information concerning the redox potential of the I (ads) /I − (ads) couple in the surface-coordinated state. On Au, where dissociative chemisorption of hydrogen does not occur, the iodine-stripping process is a pH-independent, one-electron reaction. The difference in the redox potentials [ E o I(ads) - E o I(aq ] for the I (ads) and I 2(aq) /I − (aq) redox couples was found to be −0.90 V on Au, − 0.76 V on Pt, and −0.72 V on Ir. These values imply that the ratio of the formation constants for surface coordination of the iodine and iodide species ( K f,I / K f,I− ) is 2 × 10 28 on Au, 1 × 10 26 on Pt, and 2 × 10 25 on Ir.

Structure, composition, thermal stability and electrochemical reactivity of HS−(aq)-derived species chemisorbed at Pd(III) electrode surfaces

Abstract The structure, composition, thermal stability, and electrochemical reactivity of the chemisorbed layer formed when a clean and ordered Pd(111) electrode is exposed to a dilute solution of Na 2 S buffered at pH 10 have been studied. Under these adsorption conditions, the predominant solution species is the bisulfide ion, HS (aq) − . Experimental measurements were based upon low-energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry in conjunction with classical electrochemical methods. The bisulfide ion is oxidatively chemisorbed onto Pd(111) as zerovalent sulfur which forms a highly ordered structure upon slight heating, Pd(111) and coverage Γ = 1.07 ± 0.06 nmol cm 2 . This particular adiattice is thermally stable up to 1100 K, the highest temperature investigated, or up to 900 K in the presence of 10 −6 Torr of O 2(g) . On the other hand, it is easily oxidized electrochemically to aqueous SO 4 2− ions. Application of negative potentials has no effect either on the structure or the composition of this chemisorbed layer. However, if copious amounts of electrogenerated H 2(g) are present at the applied negative potentials, all the sulfur is hydrogenatively desorbed from the surface, most likely back to HS − (ag) species.

Reversible redox chemistry, hydrodesulfurization, and anodic oxidation of thiophenols chemisorbed at smooth polycrystalline iridium electrodes

Abstract The reversible redox chemistry, electrocatalytic hydrodesulfurization, and anodic oxidation of thiophenols chemisorbed at smooth polycrystalline iridium electrodes have been investigated. The experimental measurements were based upon thin-layer electrochemical methods. 2,5-Dihydroxy-thiophenol (DHT), 2,5-dihydroxy-4-methylbenzylmercaptan (DHMBM), and pentafluorothiophenol (PFT) were used as the model compounds. The principal findings of this study are as follows: (i) The surface coverages of DHT, DHMBM, and PFT are consistently lower on Ir than on Pt or Au. (ii) Whereas surface-chelated chemisorption occurs for DHT on Ir which explains its lower coverage and electrochemical inactivity, no such mode of binding is evident for DHMBM and PFT: Pentafluorobenzene itself is inert towards Ir and a sharp quinone/diphenol redox is displayed by DHMBM which indicates that its diphenolic moiety is pendant. The decreased surface packing densities suggest that the strength of the metal-thiolate interaction is weaker on Ir than on Pt, especially at higher coverages, (iii) All three thiophenols undergo hydrodesulfurization (HDS), the extent of which increases in the order: DHMBM (15%)

Surface chelation of 2,5-dihydroxythiophenol at polycrystalline iridium electrodes

Abstract The surface coordination and reversible redox chemistry of 2,5-dihydroxythiophenol (DHT) have been studied at smooth polycrystalline iridium electrodes in 1 M H 2 SO 4 in order to examine the aromatic-Ir interaction in comparison to that for Pt and Au. Experiments were based upon thin-layer electrochemistry. The three main findings of this investigation are: (i) The maximum obtainable coverage of DHT at Ir is 0.40(3) nmol cm −2 , a value considerably lower than that obtained [0.55(4) nmol cm −2 ] at Pt and Au electrode surfaces, (ii) As on Pt and Au, the diphenolic moiety remains intact when DHT is chemisorbed on Ir. (iii) In contrast to results for Pt and Au, DHT bound to Ir exhibited no reversible quinone/diphenol reactivity. These findings have been used to suggest that on Ir, DHT is chemisorbed as a bidentate surface ligand in which surface coordination is through the sulfur and aromatic moieties. This surface chelation of Ir-bound DHT occurs evidently because Ir has stronger affinity towards the organic moiety in DHT than Pt and Au. That is, the aromatic-Pt interaction is sufficiently strong to enforce surface chelation of DHT on a loosely packed layer, but not strong enough to block formation of S-η 1 DHT. In comparison, the strength of the aromatic-Ir interaction is of such magnitude that formation of a fully vertical DHT layer is actually inhibited.

The influence of coadsorbed iodine on the surface chelation of 2,5-dihydroxythiophenol at indium electrodes

Abstract The influence of coadsorbed iodine on the surface chelation and reversible redox activity of 2,5-dihydroxythiophenol (DHT) has been studied at a smooth polycrystalline indium electrode as a model system to examine how the structure, bonding and electrochemical reactivity of chemisorbed redox-active intermediates are influenced by strongly surface-active electrolytes. When DHT is chemisorbed at full coverages on a clean Ir surface, it is bound (chelated) through both the sulfur and diphenol moieties and no reversible quinone/diphenol redox activity is exhibited. If DHT is adsorbed onto an iodine-pretreated surface, partial displacement of iodine occurs and a mixed I/DHT coadsorbed layer is formed; however, about 40% of the total DHT in this mixed layer remains chelated. No changes were noted when an Ir electrode coated with surface-chelated DHT at full coverage was exposed to aqueous iodide. The present results are different from those observed previously on Pt and Au; for example, coadsorbed iodine completely prevents the diphenolic ring in DHT from interacting directly with the Pt surface. These findings indicate that: (i) the interaction between Ir and the diphenol ring in DHT is stronger than that between Ir and iodine; and (ii) the surface binding strength of the aromatic group in DHT depends upon the electrode material and decreases in the order Ir > Pt > Au.

Probing the surface electrochemical properties of bimetallic alloys by chemisorption of redox-active species: Iodine at smooth Au90Pt10

Abstract The interfacial electrochemical properties of a smooth polycrystalline Au + Pt alloy consisting of 90% Au and 10% Pt (Au 90 Pt 10 ) were investigated by comparison of the redox-activated adsorption-desorption behavior of iodine at the bimetallic surface with that at pure Au and Pt. This approach is possible since the electrochemical properties of iodine absorbed on pure Au are different from those of iodine chemisorbed on pure Pt. Experimental measurements were based upon thin-layer voltammetry and coulometry. The primary findings are: (i) In terms of its influence on the electrochemical properties of chemisorbed iodine, the Au 90 Pt 10 electrode behaves virtually identically to pure Au. (ii) The hydrogen evolution reaction, which is retarded at Au, is enhanced at the bimetallic electrode whether iodine is chemisorbed or not. (iii) Oxidized Pt atoms at the alloy surface facilitate the reduction of unadsorbed IO − 3 to I 2 relative to that at Au. (iv) The surface of the Au 90 Pt 10 electrode is homogeneous; that is, domains of aggregated Pt and Au atoms do not exist.

Surface Coordination/Organometallic Chemistry of Monometal and Bimetallic Electrocatalysts

The interaction of selected organic and inorganic functional groups, which are reversibly electroactive and strongly surface-active, with monometal (Rh, Pd, Ir, Pt, Au) and mixed-metal (Au-Pt, Ag-Pt) electrocatalysts has been studied to help establish the interfacial organometallic/coordination chemistry of these metals in aqueous solutions. Experimental measurements were based upon thin-layer electrochemical and ultra-high vacuum surface spectroscopic methods; the latter included low-energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and thermal desorption mass spectrometry. The results to date indicate the following trends: (i) Electrode surface phenomena can be modeled in terms of monometal and cluster coordination/organometallic chemistry. (ii) Chemisorption is analogous to oxidative addition; desorption is similar to reductive elimination. (iii) Chemisorption of an electroactive center favors its oxidized state over the reduced form. (iv) Chemisorption of electroinactive anionic reagents forms polyprotic surface acids. (v) Substrate-mediated interactions between pendant electroactive centers may arise if the redox group itself is surface-active. (vi) Substrate-mediated adsorbate-adsorbate interactions can be viewed similarly to mixed-valence complexes.