Changing the Selectivity of O2 Reduction Catalysis with One Ligand Heteroatom

The development of catalytic systems that selectively reduce O-2 to water is needed to continue the advancement of fuel cell technologies. As an alternative to platinum catalysts, derivatives of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide one benchmark for catalyst design, but incorporation of these catalysts into heterogeneous platforms remains a challenge. Co-porphyrins can be heterogeneous O-2 reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired four-electron reduction of O-2 to H2O is often poor. Herein, we demonstrate substantial improvements in the O-2 reduction selectivity using a Co-porphyrin that incorporates a 2-pyridyl group at one of the mesa-positions of a Co-tetraarylporphyrin (cobalt(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin, CoTPPy). The properties of CoTPPy immobilized on EPG were investigated using cyclic voltammetry, rotating disk, and rotating ring-disk electrochemistry. The presence of a single 2-pyridyl group in the CoTPPy gives rise to the four-electron reduction of O-2, as opposed to the two-electron reduction commonly associated with cobalt porphyrins. Detailed electrochemical studies of CoTPPy and related Co and Fe improves porphyrins are described. Use of Co instead of Fe overpotentials by over 200 mV with a factor of 2 increase in maximum turnover frequency (TOFmax). This work demonstrates that a simple change in catalyst structure can dramatically change the selectivity for O-2 reduction.