• Oxidation up to 1 monolayer on Mg(0001) and Mg(101 ̅0) surfaces is reported. • Evolving oxidation mechanisms strongly depend on surface orientation. • In Mg(0001), Mg-O unit structure transitions from wurtzite to hexagonal type. • In Mg(101 ̅0), wurtzite-type structure evolves to a more perfect Mg-O unit. Magnesium (Mg) is an abundant metal which has been used in aviation, medicine, hydrogen energy storage, etc. However, Mg can be rather reactive, and therefore an improved understanding of corrosion and oxidation mechanisms can enhance the efficiency of these processes to control and widen applications. The study presented here investigates the mechanisms of oxidation from the initial to full monolayer stages, on two low-index Mg surfaces, Mg(0001) and Mg ( 10 1 ¯ 0 ) . By analysing the valence electron changes during the oxidation process, we reveal a connection between oxidation and electron properties, suggesting that oxygen (O) atoms preferentially adsorb in the regions of charge accumulation on the surfaces. After the adsorption of a first O atom, the charge distribution on the surface changes, and following O atoms are attracted to neighbouring charge-rich regions. In addition, the oxidized Mg-O units form geometric structures initially different from the rocksalt structure commonly reported for a fully oxidized surface. In Mg(0001), the Mg-O unit structure transitions from a wurtzite type to hexagonal, while on Mg ( 10 1 ¯ 0 ) a more perfect Mg-O unit of wurtzite structure forms.

• Dopants prefer localize on the Fe2O3[001] surface rather than in the bulk. • Alkali dopants significantly lower surface oxygen vacancy formation energy (Evac). • In both pure and Na-doped Fe2O3, H2 oxidation prefers O top sites over Fe sites. • Surface O atoms thermodynamically and kinetically facilitate H2 oxidation. • H2 oxidation proceeds more favorably on surface O sites than Fe sites, with surface O atoms promoting H2O formation both thermodynamically and kinetically. The reactivity of Fe 2 O 3 oxygen carriers (OCs) in the presence of alkali and alkaline earth metal substitutions was investigated using density functional theory (DFT) to enhance their reduction behavior. Our calculations reveal that these substitutions preferentially occupy surface sites on Fe 2 O 3 [001], rather than the bulk. Compared to alkaline earth metals, the surface oxygen vacancy formation energy (E vac ), a measure of reducibility, is substantially lower near alkali substitutions, indicating more oxygen release. Additionally, we investigated H 2 oxidation and adsorption on pure and Na-substituted Fe 2 O 3 [001] surfaces that have an oxygen vacancy. Adsorption energies demonstrate that H 2 preferentially dissociates on O top and hollow sites rather than on Fe-related sites. The oxidation of H 2 is both thermodynamically and kinetically more advantageous on O sites, resulting in the production of H 2 O via either direct adsorption or H atom migration pathways. Conversely, Fe sites demonstrate elevated steric hindrances and reduced reactivity. Finally, oxygen migration from the bulk to the surface was identified as a mechanism driven by high temperatures, which may influence oxygen availability during cycling. These findings offer essential understanding of the impact of substitutions on the redox behavior of Fe 2 O 3 OCs, relevant to applications in chemical looping and sustainable hydrogen consumption.