Hydrogen evolution catalysis by a cobalt porphyrin peptide: A proposed role for porphyrin propionic acid groups.

Cobalt microperoxidase-11 (CoMP11-Ac) is a cobalt porphyrin-peptide catalyst for hydrogen (H2) evolution from water. Herein, we assess electrocatalytic activity of CoMP11-Ac from pH 1.0-10.0. This catalyst remains intact and active under highly acidic conditions (pH 1.0) that are desirable for maximizing H2 evolution activity. Analysis of electrochemical data indicate that H2 evolution takes place by two pH-dependent mechanisms. At pH < 4.3, a proton transfer mechanism involving the propionic acid groups of the porphyrin is proposed, decreasing the catalytic overpotential by 280 mV.

Buffer pKa Impacts the Mechanism of Hydrogen Evolution Catalyzed by a Cobalt Porphyrin-Peptide.

The effect of buffer pKa on the mechanism of electrochemical hydrogen evolution catalyzed by a cobalt porphyrin peptide (CoMP11-Ac) at constant pH is presented. The addition of buffer to CoMP11-Ac in water and KCl leads to an enhancement of the catalytic current of up to 200-fold relative to its value in the absence of a buffer. Two distinct catalytic regimes are identified as a function of the buffer pKa. In the presence of buffers with pKa ≤ 7.4, a fast catalysis regime limited by diffusion 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, proposed to reflect the Co(II/I) couple, is maintained independent of the buffer pKa, consistent with rate-limiting Co(III)-H formation under these conditions. We conclude that the buffer species pKa impacts catalytic current and potential and the rate-determining step of the reaction.