Elucidation of Oxygen Chemisorption Sites on Activated Carbons by H-1 DNP for Insight into Oxygen Reduction Reactions

Activated carbons (ACs) are widely used in many industrial and medical adsorbent applications because of their distinct ability to adsorb numerous gaseous and/or liquid analytes. More recently, ACs have been actively explored as an inexpensive alternative to metal catalysts (Pt) for numerous oxygen reduction reactions including microbial fuel cells (MFCs) for wastewater treatment. Although it is well established that O-2 is chemisorbed on ACs, the actual chemical site has not been elucidated. In this study, we characterize adsorption of benzene on the surface of ACs in the presence and absence of O-2. The AC samples have been heat treated and cover the range of 350-600 degrees C. The flowing benzene is monitored by solid/liquid intermolecular transfer (SLIT) H-1 dynamic nuclear polarization (DNP). We find that the introduction of benzene (N-2 atmosphere) flowing over an AC interface leads to a scalar (positive) H-1 Overhauser effect in high temperature activated carbons (550-600 degrees C), whereas this nanoscale close-in Fermi interaction is completely suppressed upon introduction of oxygen (air) to the flowing benzene/activated carbon interface. We propose these results are consistent with a benzene/delocalized singlet-triplet radical carbene or diradical interaction at the zigzag sites edges of disordered graphene motifs. These unique radical sites chemically react with O-2 to form quenched diamagnetic sites. In contrast, a solid-state H-1 DNP effect is observed at lower heat treatment temperatures representing different radical sites (e.g., aromatic heteroatom radical sites) in ACs.