The origin and assignment of the complex main and satellite X-ray photoelectron spectroscopy (XPS) features of the cations in ionic compounds have been the subject of extensive theoretical studies using different methods. There is agreement that within a molecular orbital model, one needs to take into account different types of configurations. Specifically, those where a core electron is removed, but no other configuration changes are made and those where in addition to ionization, there are also shake or charge-transfer changes to the ionic configuration. However, there are strong disagreements about the assignment of XPS features to these configurations. The present work is directed toward resolving the origin of main and satellite features for the Ni 2p XPS of NiO based on ab initio molecular orbital wave functions (WFs) for a cluster model of NiO. A major problem in earlier ab initio XPS studies of ionic compounds has been the use of a common set of orbitals that was not able to properly describe all the ionic configurations that contribute to the full XPS spectra. This is resolved in the present work by using orbitals that are optimized for averages of the occupations of the different configurations that contribute to the XPS. The approach of using state-averaged (SA) orbitals is validated through comparisons between different averages and through use of higher order excitations in the WFs for the ionic states. It represents a major extension of our earlier work on the main and satellite features of the Fe 2p XPS of Fe2O3 and proves the reliability and the generality of the assignments of the character and origin of the different features of the XPS obtained with orbitals optimized for SAs. These molecular orbital methods permit the characterization of the ionic states in terms of the importance of shake excitations and of the coupling of ionization of 2p1/2 and 2p3/2 spin–orbit split sub shells. The work lays the foundation for definitive assignments of the character of main and satellite XPS features and points to their origin in the electronic structure of the material.

Characterizing the chemical state and physical disposition of uranium that has persisted over geologic time scales is key for modeling the long-term geologic sequestration of nuclear waste, accurate uranium–lead dating, and the use of uranium isotopes as paleo redox proxies. X-ray absorption spectroscopy coupled with molecular dynamics modeling demonstrated that pentavalent uranium is incorporated in the structure of 1.6 billion year old hematite (α-Fe2O3), attesting to the robustness of Fe oxides as waste forms and revealing the reason for the great success in using hematite for petrogenic dating. The extreme antiquity of this specimen suggests that the pentavalent state of uranium, considered a transient, is stable when incorporated into hematite, a ubiquitous phase that spans the crustal continuum. Thus, it would appear overly simplistic to assume that only the tetravalent and hexavalent states are relevant when interpreting the uranium isotopic record from ancient crust and contained ore systems.

Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction mechanisms. Interactions of N2 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 N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at 401.0 eV and 398.7 eV binding energy, with relative intensities of ~ 3:1, respectively. These features remained upon subsequent pump down to 10-9 Torr. Observed maximum N surface coverage was ~ 1.5 x 1013 cm-2-a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10-1 Torr is ~ 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 N2 bonding to both V(IV) and V(III) cation sites, but less so for V(V) sites. Hartree-Fock-based cluster calculations for N2-V end-on adsorption show that experimental XPS doublet features are consistent with calculated shake-up and normal, final ionic configurations, for N2 end-on bonding to V(III) sites, but not V(IV) sites. 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 electrolyte under NRR-relevant conditions.

Passivation of silicate surfaces by accumulated reaction products is an obstacle to efficient CO2 mineralization. In this study, we investigate a unique passivation effect during the carbonation of the basalt mineral forsterite (Mg2SiO4) in humid supercritical CO2 (50 °C, 90 bar). Using in situ high-pressure infrared spectroscopy, we demonstrate that dissolution of forsterite into a thin water film slows significantly after reaction for ≈24 h, even under far-from-equilibrium conditions. 29Si magic angle spinning nuclear magnetic resonance spectroscopy detects a highly polymerized amorphous silica at this stage. On the basis of transmission electron microscopy and energy dispersive X-ray spectroscopy, we show that the silica is present as a Mg-depleted layer that is just 2–3 nm thick on the reacted forsterite particles. The decrease in the level of forsterite dissolution in the presence of an extraordinarily thin Mg-depleted layer can be strongly linked to properties of the thin fluid film at the surface, highlighting the importance of water during mineral carbonation. This study furthers our understanding of silicate mineral carbonation under select low-water, humidified fluid conditions relevant to basaltic geologic reservoirs, recovery of critical elements by carbonation of mafic ores, and sequestration of atmospheric CO2 by enhanced rock weathering.