Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions #DNPNMR
Liu, Xiaoyang, Juan Gu, James Wightman, and Harry C. Dorn. “Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions.” ACS Applied Nano Materials, November 25, 2019, acsanm.9b01308.
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 O2 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 O2. The AC samples have been heat treated and cover the range of 350−600 °C. The ﬂowing benzene is monitored by solid/liquid intermolecular transfer (SLIT) 1H dynamic nuclear polarization (DNP). We ﬁnd that the introduction of benzene (N2 atmosphere) ﬂowing over an AC interface leads to a scalar (positive) 1H Overhauser eﬀect in high temperature activated carbons (550−600 °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 O2 to form quenched diamagnetic sites. In contrast, a solid-state 1H DNP eﬀect is observed at lower heat treatment temperatures representing diﬀerent radical sites (e.g., aromatic heteroatom radical sites) in ACs.