Adsorbed cobalt porphyrins act like metal surfaces in electrocatalysis

Electrodes chemically modified with molecular active sites are potent catalysts for energy conversion reactions. Such electrodes are typically presumed to operate by the same redox mediation mechanisms as the analogous soluble molecules, with electron transfer and substrate activation in separate elementary steps. Here we uncover solvent-dependent concerted reaction mechanisms for cobalt porphyrins attached to glassy carbon electrodes by flexible aliphatic linkages. In acetonitrile, outer-sphere CoII/I reduction mediates H2 evolution in a stepwise sequence. However, in aqueous media, outer-sphere reduction is not observed and H2 evolution proceeds instead by concerted proton–electron transfer pathways typical of metal surfaces. Consequently, catalysis is not defined by the reduction potential of the parent molecule, but rather by the free energy of hydrogen binding. We attribute these mechanistic changes to electrostatic coupling between the molecule and the surface arising from adsorption. Our results motivate a re-examination of the reaction mechanisms of and design principles for molecularly modified electrodes. Heterogenized molecular catalysts are often assumed to operate via analogous mechanisms to their homogeneous counterparts. Here, the authors demonstrate that a tethered cobalt porphyrin exhibits either molecule-like or metal-like behaviour depending on the strength of adsorption between the molecule and the electrode surface.