Thermal Hydroquinone Oxidation on Co/N-doped Carbon Proceeds by a Band-Mediated Electrochemical Mechanism.

ABSTRACT

Molecular metal complexes catalyze aerobic oxidation reactions via redox cycling at the metal center to effect sequential activation of O2 and the substrate. Metal surfaces can catalyze the same transformations by coupling independent half-reactions for oxygen reduction and substrate oxidation mediated via the exchange of band-electrons. Metal- and nitrogen-doped carbons (MNCs) are promising catalysts for aerobic oxidation that consist of molecule-like active sites embedded in conductive carbon hosts. Owing to their combined molecular and metallic features, it remains unclear whether they catalyze aerobic oxidation via the sequential redox cycling pathways of molecules or band-mediated pathways of metals. Herein, we simultaneously track the potential of the catalyst and the rate of turnover of aerobic hydroquinone oxidation on a cobalt-based MNC catalyst in contact with a carbon electrode. By comparing operando measurements of rate and potential with the current-voltage behavior of each constituent half-reaction under identical conditions, we show that these molecular materials can display the band-mediated reaction mechanisms of extended metallic solids. We show that the action of these band-mediated mechanisms explains the fractional reaction orders in both oxygen and hydroquinone, the time evolution of catalyst potential and rate, and the dependence of rate on the overall reaction free energy. Selective poisoning experiments suggest that oxygen reduction proceeds at cobalt sites, whereas hydroquinone oxidation proceeds at native carbon-oxide defects on the MNC catalyst. These findings highlight that molecule-like active sites can take advantage of band-mediated mechanisms when coupled to conductive hosts.