Proton-Coupled Electron Transfer Catalyst: Homogeneous Catalysis. Application to the Catalysis of Electrochemical Alcohol Oxidation in Water
Cyrille Costentin
Abstract
Proton-coupled electron transfer (PCET) catalysts are investigated in the framework of cyclic voltammetry (CV). We analyze homogeneous catalysts and provide a detailed formal kinetic analysis of the various responses expected in the case of a PCET catalyst following either stepwise or concerted pathways. Both buffered solution and nonbuffered aqueous media are considered. In the first case we show that, in addition to possible limitation by buffer diffusion, the PCET sequence may be rate limiting. The CV wave shape, position, and intensity are discussed in terms of thermodynamic and kinetic penalties associated with each mechanism. In the second case, we show that a nonclassical pseudo-canonical cyclic voltammogram with hysteresis due to hydronium production is obtained for a Nernstian PCET catalyst with water as the proton acceptor. Thermodynamic and kinetic penalties resulting from water-based PCET specific pathways are analyzed. Finally, we use the present analysis to investigate the electrochemical catalysis of alcohol oxidation in water with a PCET catalyst, N-hydroxyphthalimide. We have found that the rate constant for benzyl alcohol oxidation is independent of pH, strongly suggesting a true hydrogen atom transfer (HAT) as the rate-determining step. When water is the proton acceptor to generate the active form of the catalyst, i.e. N-oxyl PINO radical, the catalytic process follows a stepwise PTET pathway whose kinetics partially limits the catalytic current.