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Interactions of N2 and H2O at transition metal oxide surfaces are of fundamental interest for gaining insight into electrocatalytic nitrogen reduction reaction (NRR) mechanisms. N2/H2O interactions at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and N2 partial pressures between 10-9 Torr and 10-1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with significant V(III) and V(V) components. Such films have been previously demonstrated to be NRR active at pH 7. There is little understanding, however, of the detailed nature of N2-surface interactions. XPS measurements were acquired at room temperature in environments of both pure N2 and equal pressures of N2 and H2O vapor, up to a N2 partial pressure of 10-1 Torr. In the absence of H2O, broad N 1s features were observed at binding energies of 401 eV and 398.7 eV with relative intensity ratios of ~ 3:1, respectively. These features remained upon subsequent pumpdown to 10-9 Torr, indicating that adsorbed nitrogen is stable at room temperature in the absence of equilibrium with gas phase N2. In the presence of equal pressures of N2 and H2O vapor, the 401 eV N 1s feature was reduced in intensity by ~ 50% at 10-1 Torr N2 partial pressure, with the feature at 398.7 eV binding energy barely observable. DFT calculations show that the above NAP-XPS data demonstrating stable N2-surface binding in the absence of N2 overpressure are consistent with N2 binding at V(IV) or V(III) sites, but not at V(V) sites, and further show that N2/H2O binding is competitive. SCF-HF calculations suggest that the two N 1s XPS features correspond to "shake" and normal transitions at 401 eV and 398.7 eV, respectively, for N2 bonded end-on to the surface. The shake feature involves a charge transfer from V 3d to N2 pi* in addition to N 1s ionization. The difference in binding energies of the two features, ~ 2.3 eV, strongly suggests N2 -V(III) binding. The data presented demonstrate the ability of NAP-XPS, in concert with theory, to provide atomic-level insight concerning interfacial reactions relevant to electrocatalysis.
Acknowledgement:
This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility, under Contract No. DE-AC02-05CH11231. Work at UNT was supported in part by the NSF through grants DMR-2112864 (JAK, TRC), and via NSF support for the UNT CASCaM HPC cluster via Grant CHE-1531468. PSB was supported by the Geosciences Research Program, Office of Basic Energy Sciences, U.S. DOE through its Geosciences program at PNNL.
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