Buffer Pka Impacts the Mechanism of Hydrogen Evolution in Water Catalyzed By a Cobalt Porphyrin-Peptide

Abstract

A detailed study of the impact of buffer pKa (pKa 5.6 to 10.5) on electrochemical hydrogen evolution at constant pH catalyzed by CoMP11-Ac, a cobalt porphyrin peptide, is presented. The addition of buffer leads to a significant enhancement of the catalytic current of up to 20-fold relative to its value in absence of buffer. Two distinct catalytic regimes are identified based on the buffer pKa . In the presence of buffers with pKa ≤ 7.4 a fast catalysis regime limited by diffussion of buffer is reached. The catalytic half-wave potential (Eh ) shifts anodically (from -1.42 to -1.26 V vs Ag/AgCl/KCl(1M)) as the buffer pKa decreases from 7.4 to 5.6, proposed to result from fast Co(III)-H formation following the catalysis-initiating Co(II/I) reduction. With higher-pKa buffers (pKa > 7.7), an Eh = -1.42 V reflecting the potential of the Co(II/I) couple is maintained independent of the buffer pKa , consistent with a rate-limiting Co(III)-H formation under these conditions. We conclude that the buffer species pK a, even at constant pH, can have a large effect on both catalytic current and potential, as well as the rate-determining step of the reaction. Thus, buffer is a factor to consider in optimizing reaction conditions for hydrogen-evolving catalysis and for other proton-requiring reactions in water. Finally, buffer acids are shown to be a probe for studying hydrogen evolution mechanism in water.