Electrocatalytic activity of an immobilized cofacial diporphyrin depends on the electrode material

A thiolated bis(cobalt) cofacial diporphyrin chemisorbed on an edge plane pyrolytic graphite electrode has the electrocatalytic activity of a four-electron (n greater than or equal to 3.6) dioxygen reduction catalyst. When chemisorbed on a gold electrode surface, the same catalyst exhibits the activity of only a two-electron catalyst, producing hydrogen peroxide (n = 2.1). The edge plane graphite surface thus plays a crucial, but not understood, role in designed dioxygen reduction catalysis. Analysis of X-ray photoelectron spectroscopy and UV-vis results is consistent with the rings of the thiolated porphyrins being coplanar to the Au electrode plane. A structurally modified catalyst exhibits greater coplanarity and a slight increase in activity (n approximate to 2.4). The present results set the stage for a strategy of cochemisorbing functionalities onto the thiolated diporphyrin-coated Au surface, seeking those functionalities which will chemically mimic the graphite surface and elevate the catalytic reactivity to a four-electron dioxygen reduction. Such functionalities could include host-guest cochemisorption of putative carbon surface ligands within the porphyrin electrode cavity.