Insights Into the Temperature-Dependent Oxidation Mechanism of Metallic Cerium in H2O-N2 Atmospheres

en This link goes to a English section zh This link goes to a Chinese section 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. 摘要 en This link goes to a English section zh This link goes to a Chinese section 作为锕系金属的常用模拟金属,金属铈(Ce)可在受控且更安全的实验条件下开展复杂气体环境中的反应机理研究。本文利用原位热重-质谱联用、原位X射线衍射、反应中间体捕获以及第一性原理计算,研究了Ce在H 2 O-N 2 混合气氛和200°C–350°C范围内的氧化行为。结果表明氧化途径存在显著的温度依赖性切换:在低温区(235°C),Ce直接与水蒸气反应生成CeO 2 ,而N 2 基本呈惰性。在高温区(300°C),反应主要通过耦合的氮化-水解/氧化路径进行,金属Ce与N 2 反应形成CeN中间体,随后与H 2 O反应生成CeO 2 并释放NH 3 。氧化反应途径切换受早期阶段生成的氢化铈(CeH x )的结构与化学状态控制。低氢、富空位缺陷的CeH x (以CeH 2.29 /CeH 2 为典型)显示出更高的表面反应活性,可促进N 2 的吸附、活化与解离,从而有利于Ce氮化生成CeN。相比之下,富氢的CeH 2.97 /CeH 3 因脱氢速率较慢,难以有效活化N 2 。尽管两种反应路径的最终产物均为CeO 2 ,但缺陷浓度和形貌的系统性差异表明氧空位形成和CeO 2 颗粒生长动力学受不同氧化机制的控制。