In molecular catalysis, precise active site control is enabled by ligand design which controls catalytic metal centers. In contrast, heterogeneous catalysts contain a distribution of active site structures which hinders the rational design of active sites. Metal, nitrogen-doped carbon materials (M-N-C, with M = 1st row transition metals), have emerged as a heterogeneous system with hypothesized, nitrogen-coordinated MNx sites that mimic the active sites in metal complexes. In this work, we study NiPACN, a Ni-N-C material derived from pyrolysis of polyacrylonitrile, which is a highly active and selective catalyst for CO2 electroreduction to CO. We show evidence that dispersed Ni sites are potential active sites for this reaction and use a suite of advanced characterization techniques to determine the physiochemical structure of these sites. We use high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) to show the existence of Ni single atoms [Fig 1A]. Then, atomic resolution electron energy loss spectroscopy (EELS) showed both Ni and N EELS signal at the Ni atoms which suggests the existence of NiNx moieties [Fig 1B]. This analysis was supplemented with time of flight secondary ion mass spectrometry (ToF-SIMS) that revealed NiNxCy fragments originating from NiPACN which are similar to fragments from Ni Phthalocyanine (NiPc), which gives strong evidence that molecular-like active sites are present in M-N-C materials [Fig 1C]. Finally, synchrotron-based 61Ni Mossbauer spectroscopy was conducted to gain atomic insight into these NiNx sites [Fig 1D].

Advanced characterization of atomically dispersed, nitrogen-coordinated Ni catalytic active sites for conversion of CO2 to CO

ACS Fall 2020 Virtual Meeting

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