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.
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The authors thank A. Alabugin, O. Jung, A. Chu, T. Wesley, N. Lewis, M. Jackson, M. Pegis, P. Smith and C. Costentin for helpful discussions. This research was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0020973. C.J.K. and S.W. are supported by the National Science Foundation Graduate Research Fellowship under grant no. 1122374. This research used the Inner Shell Spectroscopy 8-ID beamline of the National Synchrotron Light Source II, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. This work made use of Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award no. DMR-1419807. Support for ICP-MS instrumentation was also provided by a core center grant P30-ES002109 from the National Institute of Environmental Health Sciences, National Institutes of Health. Y.S. acknowledges the Sloan Foundation, Research Corporation for Science Advancement (Cottrell Scholar) and the Canadian Institute for Advanced Research (CIFAR Azrieli Global Scholar).
The authors declare no competing interests.
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Kaminsky, C.J., Weng, S., Wright, J. et al. Adsorbed cobalt porphyrins act like metal surfaces in electrocatalysis. Nat Catal 5, 430–442 (2022). https://doi.org/10.1038/s41929-022-00791-6