Redox behavior of iron at the surface of an O(100) single crystal studied by ambient-pressure photoelectron spectroscopy

Abstract

We have studied the oxidation and reduction of iron in an Fe-doped MgO single crystal by O 2 , H 2 and CO 2 using ambient-pressure XPS and NEXAFS. Surface charging of the crystal was rendered manageable by the elevated temperatures and the gas atmospheres. The oxidation state of iron was found to shift reversibly between the Fe 2+ and Fe 3+ states, with a strong asymmetry in the rates; while oxidation by O 2 or CO 2 was nearly complete at 100 • C, reduction by H 2 began at ∼ 300 • C, and was still incomplete at 600 • C. Grazing-incidence XRD characterization of the crystal indicated the presence of octahedral, nanoscale inclusions assigned to the magnesioferrite spinel (MgFe 2 O 4 ). It is proposed that the redox behavior observed involves interconversion between the rock-salt (Fe x Mg 1−x O) and spinel phases, with the more open lattice containing Fe 3+ enabling more rapid ion diffusion and thus more facile oxidation compared to reduction. ARTICLE HISTORY

A Co‐ and Ni‐Free P2/O3 Biphasic Lithium Stabilized Layered Oxide for Sodium‐Ion Batteries and its Cycling Behavior

“…The spectrum shows satellite peaks and reveals an oxidation state of +3 for Fe. [ 17,25 ] Overall, the XPS results confirm that the oxidation states of Mn and Fe for the pristine Na 0.8 Li 0.2 Fe 0.2 Mn 0.6 O 2 material are +4 and +3, respectively. These values are expected for achieving a complete charge balance in the compound assuming O (−2) and Na/Li (+1).…”

Bimetallic Nanoparticles as a Model System for an Industrial NiMo Catalyst

“…Despite the fact that a well-ordered alloy structure is not formed, the interaction between Ni and Mo is important for the activity of the catalyst, as can be shown using a combination of XPS and H 2 -TPR to probe the chemical information of the catalyst (Figure 6). The XPS study of the insulating industrial samples was conducted under 1 mbar of air, showing that the charging effect that occurs can be effectively reduced by the presence of a gas [39]. Two peaks can be resolved in the Mo 3d XPS spectrum at binding energies of 233.1 eV and 236.2 eV (Figure 6b).…”

Surface oxide development on aluminum alloy 6063 during heat treatment

“…Secondly, the first moment of the O 1s peak is observed to decrease in binding energy with increasing temperature, as presented in Table . Previous measurements assigned Al 2 O 3 to binding energies ranging from 532.6 to 531.4 eV, while MgAl 2 O 4 (spinel) and MgO have been assigned binding energies() of 531.5 and 530.1 eV, respectively. Hence, the shift toward lower binding energy of the O 1s peak is consistent with a change in oxide composition, as suggested by the Al 2p and Mg 2p spectra.…”