NSF-BSF: Surface-Sensitive Spectroscopy and Microscopy on Metal/Oxide Interfaces at Atmospheric Pressures

表面科学是阐明发生在材料表面的化学和物理的基本方面的领域，目的是为催化、电化学、腐蚀和润滑等工业重要领域提供基本信息。经典的表面科学是在真空压力和极低温度的精细条件下进行的；几乎和外太空一样空旷寒冷的环境。表面科学，作为实践，直到20世纪末，提供了我们目前对固体表面的理解，由于广泛的表面敏感显微镜和光谱学技术，揭示了许多晶体表面的原子，电子和化学结构。然而，真实的表面总是与气体或液体接触，我们对这种现实条件下表面的知识远远不够广泛，因为只有少数实验技术可以探测现实条件下的表面。这种知识的缺乏被称为表面科学和化学技术之间的压力-温度差距。本课题主要研究氧化铝负载银和铜催化剂用于乙烯氧化反应。这是一个商业上重要的反应，因为全球每年对环氧乙烷的需求超过2500万吨。本项目的优点不仅限于提供了对乙烯氧化反应的原子和分子水平的理解，而且还将为其他基础反应的研究提供基准。金属/氧化物界面是一个高度复杂的系统（多组分，多界面边界），并且由于环境试剂气体的存在而增加了复杂性。该项目旨在通过同时弥合与压力和温度差距平行的复杂性差距来改变当前的范式。因此，该项目包括开发适用于其他金属/氧化物系统和其他反应物气体外加剂的新技术和方法。工业过程中使用的90%以上的催化剂是氧化物支撑的金属，这一事实表明了它的重要性。与传统表面科学研究中使用的稀薄超高真空（UHV）压力和低温相比，在大多数工业过程中，催化反应通常发生在高压（1bar）和高温（295k）下。为了弥合科学技术之间的压力差距和温度差距，拟议的研究将在295k及以上的温度下进行，并且在Torr-bar压力范围内存在气体，从而接近与工业生产相关的范围，同时避免在测量分辨率和精度方面做出重大牺牲。这将涉及利用显微镜和光谱学技术，即x射线光电子能谱（XPS），红外反射吸收能谱（IRRAS）和扫描隧道显微镜（STM），这些技术在过去几十年中已经过特别调整，以便在环境压力下进行。研究了乙烯环氧化反应条件下Ag/Al2O3和Cu/Al2O3金属/氧化物表面的化学、原子和电子结构。这一具有重要商业意义的反应在2013年的全球产量为250亿公斤环氧乙烷。由于Al2O3是一种强电介质，除了上面提到的方法之外，还引入了新的工具和方法：独立超薄膜方法，原子力显微镜（AFM）成像和光谱。将这四种技术结合起来，将为在模型非均相催化剂上发生的催化反应提供无与伦比的机理见解，特别是本项目中选择的乙烯环氧化反应。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical SummarySurface science is the field of elucidating the fundamental aspects of chemistry and physics occurring on the surface of materials with the goal of providing fundamental information to the industrially important fields of catalysis, electrochemistry, corrosion, and lubrication. Classical surface science is carried out in refined conditions of vacuum pressures and very low temperatures; conditions which are almost as empty and cold as outer space. Surface science, as practiced until the end of the 20th century, has provided much of our present understanding of solid surfaces, thanks to an extensive array of surface-sensitive microscopy and spectroscopy techniques, which have revealed the atomic, electronic, and chemical structure of many crystal surfaces. However, real surfaces are always in contact with gases or liquids, and our knowledge of surfaces under such realistic conditions is far less extensive, because only a few experimental techniques can probe surfaces in realistic conditions. This lack of knowledge is referred to as the pressure-temperature gap between surface science and chemical technologies. This project focuses on silver and copper catalysts supported on alumina for ethylene oxidation reaction. This is a commercially important reaction because the annual global demand for ethylene oxide is over twenty five million tons. The merit of this project is not limited to providing an atomic and molecular level understanding of ethylene oxidation reaction, but will also serve as a benchmark, for other fundamental reaction studies. The metal/oxide interface is a highly complex system (multicomponent, multiple interfacial boundaries) and poorly understood, and adds complexity due to presence of environmental reagent gases. The project aims to shift the current paradigm by simultaneously bridging the complexity gap in parallel to the pressure and temperature gaps. Therefore, the project includes developing novel techniques and methods that will be applicable to other metal/oxide systems and other reactant gas admixtures. A measure of its importance is seen in the fact that more than 90% of the catalysts used in the industrial processes are metals supported on oxides. Technical SummaryCatalytic reactions occur typically at high pressures (1 bar) and temperatures (295 K) in most industrial processes compared to the rarefied ultra-high vacuum (UHV) pressures and cryogenic temperatures used in traditional surface science studies. To bridge this pressure gap and temperature gap between science and technology the proposed studies will be performed at 295 K and above, and in the presence of gases in the Torr-bar pressure range, thus approaching the range relevant to industrial production, while avoiding significant sacrifices in terms of measurement resolution and accuracy. This will involve the utilization of microscopy and spectroscopy techniques, namely x-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and scanning tunneling microscopy (STM), that have been specially adapted in the last decades to be performed at ambient pressures. Chemical, atomic, and electronic structure of the Ag/Al2O3 and Cu/Al2O3 metal/oxide surfaces under ethylene epoxidation reaction conditions will be investigated. This commercially important reaction has an annual global volume of 25 billion kg ethylene oxide produced in 2013. Because Al2O3 is a strong dielectric, new tools and methods are introduced in addition to those mentioned above: Freestanding ultra-thin membrane approach, and atomic force microscopy (AFM) imaging and spectroscopy. Combining these four techniques will provide unparalleled mechanistic insights into catalytic reactions occurring on model heterogeneous catalysts, particularly to ethylene epoxidation reaction chosen in this project.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.