Adsorbed Cobalt Porphyrins Act like Metal Surfaces in Electrocatalysis

Abstract

Carbon electrodes chemically modified with molecular active sites are potent catalysts for key energy conver-sion reactions. Generally, it is assumed that these molecularly modified electrodes operate by the same redox mediation mechanisms observed for soluble molecules, in which electron transfer and substrate activation occur in separate elementary steps. Here, we uncover that, depending on the solvent, carbon-bound cobalt porphyrin can carry out electrolysis by the non-mediated mechanisms of metal surfaces in which electron transfer and substrate activation are concerted. We chemically modify glassy carbon electrodes with cobalt tetraphenylpor-phyrin units that are anchored by flexible aliphatic linkages to form CH-CoTPP. In acetonitrile, CH-CoTPP dis-plays a clear outer-sphere Co(II/I) process which catalyzes the H2 evolution reaction by a step-wise, redox-mediated reaction sequence. In contrast, clear surface redox waves are not observed for CH-CoTPP in aqueous media and H2 evolution proceeds via a non-mediated, concerted proton-electron transfer reaction sequence over a wide pH range. The data suggest that, in aqueous electrolyte, the CoTPP fragments reside inside the electro-chemical double layer and are electrostatically coupled to the surface. This coupling allows CH-CoTPP to carry out catalysis without being pinned to the redox potential of the molecular fragment. These studies highlight that the simple adsorption of molecules can lead to reaction mechanisms typically reserved for metal surfaces, ex-posing new principles for the design of molecularly-modified electrodes.

Small molecule binding to surface-supported single-site transition-metal reaction centres

“…Indeed, the emerging field of single-site heterogeneous catalysis takes advantage of molecular design concepts from homogeneous catalysts to prepare relatively well-isolated and controlled metal-organic sites for reactivity that are anchored to surfaces, typically through solution-based chemistry on oxide surfaces [38][39][40][41] . This control borrowed from homogeneous catalysis is appealing, however, even with a relatively benign surface, changes in reactivity brought about by the surface environment 41,42 or participation of surface atoms 41,43 offer up complexity alongside opportunities for reaction design if the interaction with the surface is well understood.…”

Tuning Electrode Reactivity through Organometallic Complexes

“…119 Cyclic voltammetry is a powerful tool to characterize the electrode surface modified with molecular functionalities. 21,39,40,42,114 Unlike their solubilized counterparts that are free to move around in an electrolyte solution, surface-anchored compounds have a limited diffusion range depending on the linkage length and solvent, and usually do not diffuse through the electrical double layer. They gain or lose electrons through electron transfer due to the directional potential difference with the electrode.…”

“…Appended amines on the ligand of an organometallic complex may be subject to covalent linkage via amide bond formation and diamine condensation. 20,21,[92][93][94][95][96]114 The presence of carboxylic acids and o-quinones on the edge plane of graphitic electrodes provides available sites for forming amide bonds and fully conjugated pyrazine linkages. Re(5,6-diamino-1,10phenanthroline)(CO) 3 Cl was fully conjugated to graphite and achieved superior activity in CO 2 reduction (Figure 9a).…”

“…To summarize, a typical design of a molecularly modified electrode generally encompasses a molecular catalyst; in this case, an organometallic compound, with optimal performance for a specific type of reaction, and an electrode support as the electron source. 114 However, such a design assumes that the activity of the molecular catalyst translates invariably after heterogenization. In reality, ligand structure modifications can change the catalytic behavior of the selected organometallic compound dramatically.…”

Electrocatalytic CO2Reduction: from Discrete Molecular Catalysts to Their Integrated Catalytic Materials

“…Considering the negligible contribution of the non-conjugated linkages to electron coupling, the strong electron coupling between surface-tethered CoPc fragments and electrode is posited to be origin from the π-π interactions and/or axis coordination of the Co center to oxygen-containing groups on graphite. [100] These molecular catalysts covalently-modified to Reproduced with permission. [92] Copyright 2019, American Chemical Society.…”