Beatriz G. Bravo

Reversible redox, hydrodesulfurization and anodic oxidation of chemisorbed thiophenols: A comparison at platinum and gold electrodes

Abstract The reversible redox, hydrodesulfurization, and anodic oxidation of chemisorbed 2,5-dihydroxythiophenol (DHTP) and pentafluorothiophenol (PFT) have been compared at smooth polycrystalline gold and platinum thin-layer electrodes. The packing density and the mode of binding (through the -SH moiety) of the subject compounds were found to be identical on both surfaces. The width of the quinone/diphenol redox peak of the pendant hydroquinone group in DHTP was twice as large on platinum as it was on gold, indicating substantial substrate-mediated adsorbate-adsorbate interactions on the former surface. The extent of DHTP hydrodesulfurization (cleavage of the C-S bond with or without impairment of the pendant aromatic ring) was an order of magnitude lower on Au than on Pt. but the extent of anodic oxidation was only slightly lower on gold. PFT was readily and quantitatively desulfurized on Pt but not on Au, even at potentials negative of the hydrogen evolution reaction; on the other hand, oxidative removal of PFT at potentials below that for oxygen evolution occurred on Au but not on Pt.

Chemisorption and electrocatalytic reactivity of 3,6-dihydroxypyridazine at Au and Pt electrodes: a comparison

Abstract The chemisorption and electrocatalytic reactivity of 3,6-dihydroxypyridazine (DHPz) has been compared at smooth polycrystalline platinum and gold electrodes in aqueous acid and neutral solutions. Analytical measurements of packing densities and electrochemical reactivities were based upon thin-layer electrochemical techniques. At ambient temperatures, DHPz irreversibly adsorbs on Pt via only one nitrogen, regardless of pH. This means that the driving force for coordination of the nitrogen heteroatom to the Pt surface is greater than its protonation, even in molar acid. The same mode of η 1 -N attachment also occurs on gold but saturation or space-limited coverages are not attained in very acidic media. Evidently, the driving force for protonation of the nitrogen heteroatom in molar acid is about the same magnitude as its coordination to the Au surface. The surface-coordination strength of DHPz is therefore greater on Pt than on Au. Chemisorbed DHPz can be hydrogenated and oxidized at both electrodes, although the extent of these reactions is lower on Au than on Pt. Coulometric measurements suggest that oxidation of surface-coordination DHPz is limited to cleavage of the NN bond and formation of terminal nitro groups.

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.

ELECTROCHEMISTRY OF MIXED-METAL INTERFACES : CHEMISORPTION AND ELECTROCHEMICAL REACTIVITY OF THIOPHENOLS AT AU90PT10 ELECTRODES

Abstract The surface electrochemical properties of thiophenolic compounds chemisorbed at a smooth polycrystalline Au 90 Pt 10 alloy electrode have been studied in comparison with those at the pure monometal electrodes. 2,5-Dihydroxythiophenol (DHT) and pentafluorothiophenol (PFT) were used as model compounds since these are strongly chemisorbed on Au and Pt, and their electrochemical reactivities [reversible redox activity, hydrodesulfurization (HDS), and anodic oxidation] in the chemisorbed state depend strongly upon the composition of the electrode surface. Experimental measurements were based on thin-layer electrochemical methods. The major findings are as follows: (i) The packing density and the mode of binding (through the -S group) of DHT and PFT at Au 90 Pt 10 are the same as at the pure Pt and Au electrodes, (ii) Substrate-mediated DHT-DHT interactions which occur on Pt but not on Au are promoted, albeit to a small extent, at the alloy interface, (iii) The extent of HDS is greater at Au 90 Pt 10 than at Au but much lower than at Pt. (iv) HDS of DHT at Au 90 Pt 10 unlike that at Pt, is not accompanied by hydrogenation and/or hydrogenolysis of the pendant aromatic ring; that is, HDS at the alloy surface is selective towards simple cleavage of the C-S bond. (v) Irreversible oxidation of DHT chemisorbed at Au 90 Pt 10 yields two distinct anodic peaks; the first appears to be associated with oxidation involving the Pt surface sites, whereas the second is suggestive of oxidation involving Au surface atoms, (vi) Chemisorbed PFT, which on pure Pt is inert towards anodic oxidation, undergoes irreversible oxidation at Au 90 Pt 10 similarly but not identically to that at pure Au. (vii) Based upon the separate Pt-oxide and Au-oxide reduction peaks, the surface composition of the Au 90 Pt 10 alloy is the same as that in the bulk. (viii) On the unoxidized alloy surface, the distribution of the Au and Pt sites is predominantly homogeneous, (ix) Phase separation occurs after the alloy surface is oxided; reduction of the surface oxide appears to restore the homogeneity of the bimetallic interface, (x) Under the conditions of the reactions studied, no surface roughening or segregation occurred at the alloy electrode. The present results reflect the complex interplay between the relative surface activities and catalytic reactivities of the Au and Pt sites at the Au 90 Pt 10 interface.