Computational and experimental insights into reactive forms of oxygen species on dynamic Ag surfaces under ethylene epoxidation conditions
Computational and experimental insights into reactive forms of oxygen species on dynamic Ag surfaces under ethylene epoxidation conditions
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DOI：
10.1016/j.jcat.2021.11.031
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发表时间：
2022-01-13
影响因子：
7.3
通讯作者：
Paolucci, Christopher
中科院分区：
文献类型：
--
作者：
Liu, Changming;Wijewardena, Devinda P.;Paolucci, Christopher
乙烯部分氧化为环氧乙烷（EO）进行负载银粒子。分子氧在Ag上的吸附形成了负责环氧化和燃烧反应的活性氧物质，然而，尽管过去使用了适当形式的原位光谱，但催化剂表面结构和存在的活性氧物质的分子结构和身份仍然存在争议。具体而言，没有共识的分子起源的广泛报道的拉曼功能，出现在接触只有O-2或在C2 H4和O-2的反应物混合物中的压力和温度相关的工业过程中，部分原因是由于缺乏一致的从头计算研究，计算含氧的银表面的振动频率。在这里，我们阐明了催化剂表面和活性氧物种的分子结构，通过协调光谱解卷积的瞬态和稳态表面增强拉曼光谱的银催化剂暴露于氧气（2 - 101 kPa O-2，523 - 673 K）和氧乙烯混合物（2 - 101 kPa O-2，0.5 - 9.8 kPa C_2H_4，523 K）的热力学模型和振动频率计算。这些比较表明，在EO催化重建的表面氧化物的形式和部分或完全包住金属银颗粒。拉曼光谱在600 cm（-1）附近的O2-或C2 H4中的特征代表了仅在表面氧化物和高氧覆盖率的AgxOy表面上形成的O-Ag-O结构基序。我们分配的功能集中在810 - 840 cm（-1）和更高的频率，这是普遍存在于整个文献中，部分嵌入在氧化物的覆盖层，在暴露过程中形成的氧-2或在稳态epoxidations的双氧复合物。两者合计，我们的研究结果暗示存在的单原子和双原子表面氧物种的组合，出现了可观数量的表面下的氧在温度和O-2压力代表EO催化。(C)2021爱思唯尔公司All rights reserved.
The partial oxidation of ethylene to ethylene oxide (EO) proceeds over supported Ag particles. Adsorption of molecular oxygen upon Ag forms the reactive oxygen species responsible for both epoxidation and combustion reactions, however, the catalyst surface structure and the molecular structure and identity of the reactive oxygen species present remain debated despite past use of appropriate forms of in situ spectroscopy. Specifically, there is no consensus for the molecular origin of widely reported Raman features that appear on contact with only O-2 or in reactant mixtures of C2H4 and O-2 at pressures and temperatures relevant for the industrial process, in part, due to the lack of concerted ab initio studies that compute vibrational frequencies for oxygen-containing Ag surfaces. Here, we elucidate the molecular structure of the catalyst surface and reactive oxygen species by coordinating spectral deconvolution of transient and steady-state surface-enhanced Raman spectroscopy of Ag catalysts exposed to oxygen (2 - 101 kPa O-2, 523 - 673 K) and mixtures of oxygen and ethylene (2 - 101 kPa O-2, 0.5 - 9.8 kPa C2H4, 523 K) with ab initio thermodynamic modeling and vibrational frequency calculations. These comparisons suggest that during EO catalysis reconstructed surface oxides form and partially or completely encase metallic Ag particles. Raman features near 600 cm(-1) that persist in O-2 or cofed C2H4 represent O-Ag-O structural motifs that form only on surface oxides and high oxygen-coverage reconstructed AgxOy surfaces. We assign features centered near 810 - 840 cm(-1) and at higher frequencies, which are ubiquitous throughout the literature, to dioxygen complexes partially embedded within an oxide-like overlayer that forms during exposure either to O-2 or during steady-state epoxidations. Taken together our results implicate the presence of a combination of monatomic and diatomic surface oxygen species, which emerge with appreciable quantities of subsurface oxygen at temperatures and O-2 pressures representative of EO catalysis. (C) 2021 Elsevier Inc. All rights reserved.