Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS

Interactions of N-2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N-2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10(-1) Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N-2 and equal pressures of N-2 and H2O vapor. In pure N-2, broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7 eV, with a relative intensity of similar to 3:1, respectively. These features remained upon subsequent pumpdown to 10(-9) Torr. The observed maximum N surface coverage was similar to 1.5 x 10(13) cm(-2)-a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10(-1) Torr is similar to 25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N-2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree-Fock-based cluster calculations for N-2-V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N-2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions.