Insights Into the Temperature‐Dependent Oxidation Mechanism of Metallic Cerium in H ₂ O‐N ₂ Atmospheres

Metallic cerium (Ce), a widely used surrogate for actinide metals, enables mechanistic studies of reactions in complex gas environments under controlled and safer laboratory conditions. Here, the oxidation behavior of Ce in H 2 O‐N 2 mixed atmospheres was investigated from 200°C to 350°C using in situ thermogravimetry–mass spectrometry, in situ X‐ray diffraction, intermediate capture, and first‐principles calculations. A clear temperature‐dependent switching of oxidation pathways was identified. At low temperatures (235°C), Ce undergoes direct steam oxidation to CeO 2 , while N 2 remains essentially inert. At high temperatures (300°C), the reaction proceeds mainly through a coupled nitridation–hydrolysis/oxidation route: N 2 participates, a transient CeN intermediate forms, and subsequent reaction with H 2 O yields CeO 2 accompanied by NH 3 evolution. The pathway switch is controlled by the structure and chemical state of cerium hydrides (CeH x ) generated at an early stage. Low‐hydrogen, vacancy‐rich CeH x (typified by CeH 2.29 /CeH 2 ‐like structures) exhibits enhanced surface reactivity that promotes N 2 adsorption, activation, and dissociation, thereby facilitating Ce nitridation to CeN. By contrast, hydrogen‐rich CeH 2.97 /CeH 3 ‐like phases are limited by slow dehydrogenation and are ineffective in mediating N 2 activation. Although CeO 2 is the final product in both regimes, systematic differences in defect concentration and morphology indicate distinct controls on oxygen vacancy formation and particle growth kinetics.