ELECTROCATALYTIC ACTIVITY OF AN IMMOBILIZED COFACIAL DIPORPHYRIN DEPENDS ON THE ELECTRODE MATERIAL

A thiolated bis(cobalt) cofacial diporphyrin chemisorbed on an edge pl ane pyrolytic graphite electrode has the electrocatalytic activity of a four-electron (n greater than or equal to 3.6) dioxygen reduction ca talyst. When chemisorbed on a gold electrode surface, the same catalys t exhibits the activity of only a two-electron catalyst, producing hyd rogen peroxide (n = 2.1). The edge plane graphite surface thus plays a crucial, but not understood, role in designed dioxygen reduction cata lysis. Analysis of X-ray photoelectron spectroscopy and UV-vis results is consistent with the rings of the thiolated porphyrins being coplan ar to the Au electrode plane. A structurally modified catalyst exhibit s greater coplanarity and a slight increase in activity (n approximate to 2.4). The present results set the stage for a strategy of cochemis orbing functionalities onto the thiolated diporphyrin-coated Au surfac e, seeking those functionalities which will chemically mimic the graph ite surface and elevate the catalytic reactivity to a four-electron di oxygen reduction. Such functionalities could include host-guest cochem isorption of putative carbon surface ligands within the porphyrin elec trode cavity.