Calculation of cyclic voltammetric responses for the reductive formation of catalyst–substrate adducts on electrode surfaces

Abstract

Electrocatalysts based on monolayers of transition-metal complexes attached to electrode surfaces frequently follow mechanisms in which a chemical step is interposed between the first and subsequent electron-transfer steps. The cyclic voltammetric responses to be anticipated for such systems were calculated using finite difference procedures to solve the relevant differential equation. The calculated variation of the peak currents and peak potentials with the kinetic parameters governing the three steps in the mechanistic scheme are presented in graphical form. Application of the results to a specific experimental system, the catalysis of the electroreduction of 0, by a macrocyclic complex of Co"' adsorbed on graphite electrodes, produced reasonable agreement between calculated and observed cyclic voltammograms.The catalysis of electrode reactions that involve the transfer of multiple electrons, e.g. the reduction of 0, to H,O, or H,O, may proceed by a variety of conceivable mechanisms. One way to examine possible mechanistic schemes is to compare experimental current-potential responses with those calculated on the basis of feasible mechanisms. We have employed this approach in three recent studies in which the currentpotential responses to be expected were calculated for single monolayers of molecules adsorbed on electrode surfaces where they acted as catalysts in simple outer-sphere,' inner-sphere, or intramolecular3 electron-transfer mechanisms. In the present study we extend the approach to a somewhat more complex mechanism in which a chemical reaction is interposed between the first and subsequent electron-transfer steps and a separate response appears from an intermediate in the catalytic cycle. Such ECE4 mechanisms have been treated previously by Laviron5y6 but only for cases where the electrode reactions were assumed to be Nernstian and the chemical reaction was assumed to be totally irreversible. We were interested in cases where these restrictive assumptions were not made, in order to compare the calculated results with those obtained in an experimental system in which the catalysed reduction of 0, to H,O, by a complex of Co"' adsorbed on the surface of graphite electrodes appeared to proceed by such an ECE mechani~m.~ In our previous reports'-3 the calculational procedures employed were based primarily on the original treatments of Andrieux and Saveant' and Aoki et aL9 as elaborated and extended to encompass the mechanistic schemes of interest. In the present case it was possible to carry out all of the necessary calculations using finite difference procedures. '' Because of the availability of previous descriptions of the details of the calculations they are only outlined in the present report.The mechanism we wish to consider is depicted in Scheme I:The catalyst, consisting of the O/R couple, is confined in a monolayer on the electrode surface. The substrate, Z, can be reduced only after it is bound by the reduced form of the catalyst to form the adduct, (R-Z)ads. This reaction is governed by equil...

(5,10,15,20-Tetramethylporphyrinato)cobalt(II): A Remarkably Active Catalyst for the Electroreduction of O2 to H2O

“…In addition, the values of E 1/2 shift to more negative values as the rotation rate of the electrode is increased and the disk current increases. This is the behavior expected for an irreversible electrode process catalyzed by an adsorbed catalyst . The smaller catalytic current obtained under cyclic voltammetric conditions allows the reduction to begin at potentials that are somewhat more positive than the formal potentials of particularly active adsorbed catalysts because the small quantities of reduced catalyst generated at these potentials are nevertheless adequate to provide the cyclic voltammetric currents.…”

Transport in Polymer-Electrolyte Membranes

“…The consensus is that at lower hydration levels, those that correspond to a membrane equilibrated with vapor, the structure of the ionic phase is an inverted micelle. 37,39 The clusters are connected by short pathways of around 1 nm in diam, as calculated experimentally, 40 and were determined by Hsu and Gierke to be transient connections with a stability on the order of ambient thermal fluctuations. 37 In this model, the water is contained in a spherical domain of about a 4 nm diam into which the polymer side chains infiltrate.…”

Ion and Water Transport Characteristics in Membranes for Polymer Electrolyte Fuel Cells Containing H + and Ca2 + Cations

“…Manuscript submitted Dec. 13,1996; revised manuscript received March 17, 1997. MITI assisted in meeting the publication costs of this article.…”