Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes of ultrathin oxide films formed on Ni(111) surfaces by thermal oxidation under vacuum and a hydrogen atmosphere were investigated by in situ time-resolved photoelectron spectroscopy. On the basis of these results, we propose a reaction model for the reduction of Ni oxide films. Our results show that the reduction of Ni oxide films on heating under vacuum does not yield a clean Ni(111) surface owing to formation of a residual stable suboxide structure on the Ni(111) surface. Conversely, in a hydrogen atmosphere of 1 × 10⁻⁵ Pa, the Ni oxide was completely reduced and a clean Ni(111) surface was obtained, even when heating below 300 °C. The reduction in a hydrogen atmosphere was best described by a two-step reaction model. The rate of the first step depends on the reduction temperature, and the rate of the second step depends on the H₂ pressure. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H₂ molecules.

中文翻译：

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氢还原超薄氧化镍反应的两步模型

镍 (Ni) 用作一氧化氮分解和氨生成的催化剂，但它很容易氧化和失活。澄清氧化镍的还原过程对于促进更有效地使用镍催化剂至关重要。在这项研究中，通过原位时间分辨光电子能谱研究了在真空和氢气氛下通过热氧化在 Ni(111) 表面上形成的超薄氧化膜的还原过程。在这些结果的基础上，我们提出了一个还原 Ni 氧化膜的反应模型。我们的结果表明，由于在 Ni(111) 表面上形成残留的稳定低价氧化物结构，在真空下加热时 Ni 氧化膜的还原不会产生干净的 Ni(111) 表面。相反，在 1 × 10 的氢气气氛中^-5  Pa，Ni 氧化物被完全还原并获得干净的 Ni(111) 表面，即使加热低于 300 °C。两步反应模型最好地描述了氢气气氛中的还原。第一步的速率取决于还原温度，而第二步的速率取决于H ₂压力。第一步的限速过程是O原子的表面沉淀，第二步的限速过程是H ₂分子的解离。