Co₃O₄ and Pd/Co₃O₄ catalysts were analyzed for Pd and Co speciation during wet lean methane combustion in temperature ranges of 150–450 °C by means of in situ X-ray absorption spectroscopy. As water inhibits Pd reoxidation during methane combustion, the catalysts were reduced prior to the in situ XAS analyses to follow the evolution of the oxidized species. The contributions from metallic Pd, PdO and Pd(OH)₂ and metallic Co, Co(II), and Co(III) oxides were quantified as a function of temperature. The Pd-free sample showed remarkable stability of its species regardless of temperature in the dry feed but the presence of water inhibited its reoxidation. In a partially pre-reduced Pd/CoO_x catalyst in the wet feed, cobalt species progressively oxidized to the level of the Pd-free sample. Pd remained largely unoxidized below the surface, with the Pd(0) fraction remaining stable in the studied 150–450 °C region. This was ascribed to the effect of the cobalt oxide support, as Pd(0) is known to oxidize progressively for alumina and tin dioxide supports in the wet feeds. We considered bulk thermodynamics of the involved processes, as well as the reported activation energies and kinetic data from a previous study on methane combustion. In combination with the results from the in situ XAS, we suggest that cobalt oxide, being a preferential oxygen but unlikely hydroxyl acceptor, inhibits Pd oxidation, and does not provide benefits in terms of inactive Pd(OH)₂ formation.

中文翻译：

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Pd / Co3O4催化剂在湿甲烷燃烧中的强金属-载体相互作用：原位X射线吸收研究

利用原位X射线吸收光谱法分析了在150-450°C的温度范围内，湿式贫甲烷燃烧中Co ₃ O ₄和Pd / Co ₃ O ₄催化剂的Pd和Co形态。由于水在甲烷燃烧过程中会抑制Pd的再氧化，因此在进行原位XAS分析之前，应先还原催化剂，以追踪氧化物种的演变。金属Pd，PdO和Pd（OH）₂以及金属Co，Co（II）和Co（III氧化物是温度的函数。不论干饲料中的温度如何，无钯样品均显示出其种类的显着稳定性，但是水的存在抑制了其再氧化。在部分还原的Pd / CoO _x中在湿进料中使用催化剂，钴逐渐被氧化到无钯样品的水平。在表面以下，Pd基本上保持未氧化状态，在研究的150–450°C范围内，Pd（0）组分保持稳定。这归因于氧化钴载体的作用，因为已知Pd（0）在湿进料中逐渐被氧化铝和二氧化锡载体氧化。我们考虑了所涉及过程的整体热力学，以及先前的甲烷燃烧研究报告的活化能和动力学数据。结合原位XAS的结果，我们建议氧化钴是一种优先级的氧气，但不太可能是羟基受体，它抑制了Pd的氧化，并且在无活性的Pd（OH）₂形成方面没有任何益处。