James H. White

Electrochemical processes at well-defined surfaces

Abstract The structures of layers of atoms and ions formed on well-characterized single-crystal electrode surfaces in vapor and in electrolytic solutions at atmospheric pressure have been investigated by means of LEED, Auger spectroscopy, cyclic voltammetry and related techniques. Electrodeposited layers of metals were generally found to be highly ordered when deposited onto well-defined substrates. Layer structure proved to be a sensitive function of the structure of pre-adsorbed adlattices. In related studies, organic compounds of various types were found to form a layer of oriented adsorbed molecules on atomically smooth substrates. Reactivity of these oriented adsorbed intermediates was sharply dependent upon orientation. Findings of this latter type involved accurate packing density measurements using thin-layer electrodes. Recent work will be reviewed and additional findings presented.

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

Abstract 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°

Effect of surface roughness on the adsorption, orientation and anodic oxidation of hydroquinone at platinum electrodes

Abstract Adsorption and anodic oxidation of hydroquinone (HQ) have been studied at platinum thin-layer electrodes purposely roughened by platinization or abrasion. The concentration-dependent packing density transition observed for HQ at smooth surfaces diminished as the surface roughness was increased. Beyond a roughness factor of about 5, the transition had essentially vanished. Taken together, the adsorption and oxidation data suggest that the suppression of the packing density transitions by surface roughness arises from inhibition of adsorption in the vertical orientation. The results help account for the fact that packing density (orientational) transitions were not reported in studies using extensively platinized Pt surfaces. Surface roughness effects are attributable to breakdown of cooperative adsorption/reorientation phenomena, and disruption of optimum conditions for stable bridge-bonding in the edgewise orientation. Mechanical polishing and electrochemical etching decreased surface roughness appreciably but did not produce smoothness comparable to high-temperature annealing.

The orientation and electrochemical oxidation of hydroquinone chemisorbed on platinum electrodes in various weakly surface-active supporting electrolytes

Abstract Studies on the adsorption, orientation and electrochemical oxidation of hydroquinone at smooth polycrystalline Pt electrodes in aqueous solutions of weakly surface-active electrolytes are reported. The electrolytes were compared with respect to how they influenced (i) the packing density vs. concentration curves, (ii) the oxidation n -values ( n ox ), and (iii) the packing density vs. electrode potential plots. Analytical measurements were based on thin-layer electrochemical methods. Six electrolytes were studied: HClO 4 (and NaClO 4 ), H 2 SO 4 , H 3 PO 4 , NaF, NaPF 6 , and CsClO 4 . No significant electrolyte- or pH-dependences were found for thevs. c adsorption profiles. The n ox data for η 6 - and η 2 -orientations likewise were unaffected by changes in supporting electrolyte, although n ox was slightly lower at pH=7 than in 1 M H + . Changes in packing density with electrode potential correlate closely with adsorption of hydrogen (at 9 E E >0.40V); weak but noticeable features at E >0.20 V are consistent with the expected variation in specific adsorption of anions of supporting electrolyte.

Kinetics of hydroquinone chemisorption at polycrystalline platinum electrodes: The effect of surface roughness

Abstract The rate of hydroquinone (HQ) chemisorption from aqueous H 2 SO 4 onto annealed (smooth) and platinized (rough) polycrystalline Pt has been studied. Previously, equilibrium adsorption measurements indicated that at smooth Pt surfaces, adsorption of HQ at c HQ ⩽ 0.1 m M yielded flat (η 6 )-oriented species while adsorption at c HQ ⩾ 1 m M resulted in edge (2,3-η 2 )-attached intermediates. Edge-attached species were not formed efficiently at roughened Pt surfaces. The present data show that the rate of η 6 -HQ chemisorption was significantly lower at roughened than at smooth surfaces. Analysis of the rate data gave the following enthalpies and entropies of activation at smooth (sm) and platinized (pt) surfaces: ΔH ≠ sm,0.1 m M = 12.5 kJ/mol; Δ H ≠ sm,2 m M = 8.3 kJ/mol; Δ S ≠ sm,0.1 m M = −83 J/mol K; Δ S ≠ sm,2 m M = −117 J/mol K; Δ H ≠ pt,0.4 m M = 12.5 kJ/mol; Δ H ≠ pt,2 m M = 12.5 kJ/mol; Δ S ≠ pt,0.4 m M = −92 J/mol K; Δ S ≠ pt,2 m M = −100 J/mol K. The similarity between Δ H ≠ pt,0.4 m M , Δ H ≠ pt,2 m M and Δ H ≠ sm,0.1 m M , and between Δ S ≠ pt,0.4 m M , Δ S ≠ pt,2 m M and Δ S ≠ sm,0.1 m M correlate with the earlier finding that adsorption of HQ onto roughened Pt surfaces occurred primarily in the flat orientation at all concentrations studied.