Changing the Selectivity of O₂ Reduction Catalysis with One Ligand Heteroatom

The development of catalytic systems that selectively reduce O₂ 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 O₂ reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired four-electron reduction of O₂ to H₂O is often poor. Herein, we demonstrate substantial improvements in the O₂ reduction selectivity using a Co-porphyrin that incorporates a 2-pyridyl group at one of the meso-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 O₂, as opposed to the two-electron reduction commonly associated with cobalt porphyrins. Detailed electrochemical studies of CoTPPy and related Co and Fe porphyrins are described. Use of Co instead of Fe improves overpotentials by over 200 mV with a factor of 2 increase in maximum turnover frequency (TOF_max). This work demonstrates that a simple change in catalyst structure can dramatically change the selectivity for O₂ reduction.