Graphene as a Platform in Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) for Amino-TEMPO Electro-Grafting

Abstract

2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) has emerged as an environmentally benign catalyst for the electrochemical oxidation of primary and secondary alcohols to aldehydes and ketones (1). This simple organic free radical reagent possesses high activity, and versatility, and is one of the most promising alternatives to noble metal catalysts that were previously studied. The catalytic activity and reactivity of TEMPO is mostly studied in bulk solutions, for fuel cells, aqueous redox flow batteries and sensors (2, 3). For more practicality, various approaches were developed to immobilize TEMPO on active supports, such as anionic polyelectrolytes, and the electropolymerization of TEMPO-substituted pyrrole or thiophene (4, 5). However, these substrates suffer from instability, loss of catalytic activity, degradation of the polymer film in alkaline environment mostly used for TEMPO regeneration. One other way, is to use carbon-based substrates that are stable in aqueous acid and base environment, as well as non-aqueous solutions and have shown enhanced electrocatalytic behavior (6, 7).In this work, we studied the electro-grafting of amino-TEMPO on graphene using in-situ attenuated total reflection infrared absorption spectroscopy (ATR-IRAS). We observed that TEMPO is attached on the surface through covalent bonding between the amine and the graphene lattice, as previously reported (6, 7). While grafting was successfully demonstrated via the oxidation of the primary amine to a radical cation which reacted with the graphene surface, ATR-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) provided additional spectral feature confirming TEMPO grafting through plasmon induced signal enhancement. Hickey, D. P.; Milton, R. D.; Chen, D.; Sigman, M. S.; Minteer, S. D., TEMPO-Modified Linear Poly(ethylenimine) for Immobilization-Enhanced Electrocatalytic Oxidation of Alcohols. ACS Catalysis 2015, 5 (9), 5519-5524. Mishra, A.; Kim, J.; Zorigt, M.; Romo, A. I. B.; Gaddam, R.; Braun, J. E.; Ziviani, D.; Rodríguez-López, J., Highly Selective TEMPO Catalyzed Bulk Electrooxidation of Isopropanol to Acetone for Application in Electrochemical Heat Pumping. ACS Sustainable Chemistry & Engineering 2023, 11 (16), 6241-6249. Elouarzaki, K.; Mandoc, L.-R. P.; Gorgy, K.; Holzinger, M.; Amarandei, C.-A.; Ungureanu, E.-M.; Cosnier, S., Synthesis and electrochemical characterization of original “TEMPO” functionalized multiwall carbon nanotube materials: Application to iron (II) detection. Electrochemistry Communications 2015, 60, 131-134. Belgsir, E. M.; Schäfer, H. J., Selective oxidation of carbohydrates on Nafion®–TEMPO-modified graphite felt electrodes. Electrochemistry Communications 2001, 3 (1), 32-35. Grimshaw, J.; Perera, S. D., Poly(pyrrole-pyromellitimide) modified electrodes. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1990, 278 (1), 279-286. Barman, K.; Askarova, G.; Jia, R.; Hu, G.; Mirkin, M. V., Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode. Journal of the American Chemical Society 2023, 145 (10), 5786-5794. Yuan, B.; Xu, C.; Zhang, D.; Zhang, R.; Su, H.; Guan, P.; Nie, J.; Fernandez, C., Electrografting of amino-TEMPO on graphene oxide and electrochemically reduced graphene oxide for electrocatalytic applications. Electrochemistry Communications 2017, 81, 18-23.