EAGER: Collaborative Research: Consequences of Co-Adsorbed Chlorine on Surface Dynamics and Selectivity in Ethylene Epoxidation on Silver Catalysts

环氧乙烷(EO)是一种主要的日用化学品，用于生产材料、工业溶液、表面活性剂和消费品。环氧乙烷是通过乙烯和氧气之间的催化反应制造的，使用高度复杂的催化剂，这些催化剂是多年来主要通过工业研究而发展起来的，涉及对大量催化、促进剂和稳定材料的实验筛选。虽然这些努力已经导致了含有五种或五种以上促进材料的高性能催化剂，但催化剂技术仍有很大的改进机会。最新的理论、机器学习、光谱和反应分析方法将结合在一起，以更好地了解氯的作用，这是关键的促进元素之一，并确定提高其有效性的机会。性能更好的EO催化剂将提高工艺能效，减少排放，并提升美国在2016年占450亿美元的化工市场的竞争力。在工业EO催化剂中报道的许多不同的助剂组合中，氯(Cl)是最普遍的助剂，它与未改性的银(Ag)催化剂的加入使EO的选择性得到了最大的提高。长期以来，反应物和启动子诱导的表面动力学和重构在许多催化反应中起着关键作用。该研究结合了催化剂合成、光谱表征、反应活性测试和机器学习增强的分子模拟，探索了在反应条件下氯促进的银表面的动态性质。自下而上的合成和表征方法产生了几个机会来调和文献中相互矛盾的光谱指定，对这个系统的机械建议，以及通过结合最先进的计算和实验方法对氯的作用方式的假设。具体地说，该研究将利用高效的计算方法来模拟具有高配置复杂性的系统的动态演化。从分子动力学模拟得到的时间平均模拟拉曼光谱将与实验拉曼光谱的多变量曲线分辨率相结合，对不同的表面氧物种以及氯对其结构的影响进行分子精确的指认。这些系统的瞬时行为也将与与工业系统相关的条件下的稳态操作的观察结果联系在一起。该项目的协作性质将为两个研究小组的研究生提供交叉接触理论和实验方法的机会，从而传授有效研究协作的技能。研究中开发的方法及其应用将被整合到合作研究人员教授的研究生课程中，以展示拥抱催化系统复杂性的重要性，即使在基础研究中也是如此。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Ethylene oxide (EO) is a major commodity chemical used in the production of materials, industrial solutions, surfactants and consumer goods. EO is manufactured via a catalytic reaction between ethylene and oxygen gas utilizing highly complex catalysts that have evolved over the years primarily through industrial research involving experimental screening of a large array of catalytic, promoter, and stabilizing materials. While those efforts have led to high-performing catalysts containing five or more promoting materials, there remains significant opportunity to improve the catalyst technology. State-of-the art theory, machine learning, spectroscopic and reaction analysis methods will be combined to better understand the role of chlorine, one of the key promoting elements, and identify opportunities for increasing its effectiveness. Higher-performing EO catalysts would improve process energy efficiency, reduce emissions, and promote U.S. competitiveness in a chemical market sector that accounted for $45B in 2016. Among the many different combinations of promoters reported in commercial EO catalysts, chlorine (Cl) is the most ubiquitous promoter, and its addition to an otherwise unmodified silver (Ag) catalyst leads to the greatest increase in selectivity to EO. Reactant and promoter-induced surface dynamics and reconstruction have long been known to play a critical role in many catalytic reactions. The study combines catalyst synthesis, spectroscopic characterization, reactivity testing, and machine-learning enhanced molecular simulations, to explore the dynamic nature of the Cl-promoted Ag surface under reaction conditions. The bottom-up approach to synthesis and characterization yields itself to several opportunities for reconciling conflicting spectroscopic assignments in the literature, mechanistic proposals for this system, and hypotheses for the mode of action of Cl through the combination of cutting-edge computational and experimental methods. Specifically, the study will utilize efficient computational approaches for modeling dynamic evolution of systems with high configurational complexity. Time-averaged simulated Raman spectra derived from molecular dynamics simulations will be used in conjunction with multi-variate curve resolution of experimental Raman spectra to make molecularly precise assignments for different surface oxygen species and the influence of Cl on their structure. The transient behavior of these systems will also be tied to observations at steady-state operation at conditions relevant for industrial systems. The collaborative nature of the project will provide opportunities for cross-exposure of graduate students from the two research groups to theoretical and experimental methods, thereby teaching skills for effective research collaboration. The methodologies developed in the research and their application will be integrated into graduate courses taught by the co-investigators to demonstrate the importance of embracing complexity of catalytic systems, even within fundamental studies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.