Transition metal-nitrogen-carbon (TM-N-C) materials are now regarded as the most promising class of platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells catalysts [1]. An Fe catalyst with atomically-dispersed FeNx sites imbedded in a graphene matrix (Fe-N-C), derived from an iron-doped zeolitic imidazolate framework (ZIF), has recently been shown to have outstanding ORR activity and to be a relatively durable PGM-free catalyst [2,3]. This presentation will describe methods to quantify the amount and relative adsorption energies (i.e., desorption temperatures) of nitric oxide (NO), a molecular surface probe, adsorbed on different surface sites on Fe-N-C catalysts. Catalyst powder samples are electrochemically reduced and oxidized using an electrochemical cell specially designed for easy retrieval of the powder after electrochemical treatment. Using a commercial temperature-programmed reactor (TPR), as-prepared and electrochemically-reduced catalyst samples are exposed to gas-phase NO followed by temperature-programmed desorption (TPD) with mass spectrometric detection of NO and thermal conductivity detection of all desorbed species. The TPD results quantify the amount and relative adsorption energies (i.e., desorption temperatures) of NO adsorbed on different surface sites on the Fe-N C catalyst. In addition, following NO exposure, the powders are also transferred to the above-mentioned electrochemical cell or incorporated into a catalyst-ionomer ink and deposited onto a rotating disk electrode (RDE) tip for electrochemical stripping of the adsorbed NO and for ORR activity and selectivity measurements. The potentials and stripping charges and desorption peak temperatures and areas for both the as-prepared and electrochemically-reduced forms of the catalyst are compared and used to provide insight into the active site identity and density of the Fe-N-C catalysts. References S.T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics, 319 (2018) 68-76.H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, Gang Wu, J. Am. Chem. Soc., 139 (2017) 14143-14149.H. T. Chung, D. A. Cullen, X. Yin, S. Komini Babu, K. L. More, D. J. Myers, P. Zelenay, 234th Electrochemical Society Meeting 1540, Oct 4 2018, Cancun, Mexico. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357.