CAS: Mechanistic Study of Reaction Intermediates in Nanoparticle-Enhanced Plasma-Assisted Catalysis

燃烧化石燃料产生的氮氧化物(NOx)和硫氧化物(SOx)等有毒气体会产生烟雾和酸雨。将这些有害污染物转化为更安全的副产品是一项重大而重要的挑战。在这个项目中，在化学系化学催化计划的资助下，南加州大学的斯蒂芬·克罗宁博士正在研究等离子体驱动的催化作为消除这些污染物的一种方法。催化剂是加速化学反应的物质，本身不会被消耗。催化剂需要能量才能发挥作用，研究小组正在研究等离子体(荧光灯中的低温发光气体是等离子体的一个例子)来传递这种能量。在这项工作中获得的基本知识使补救这些有害污染物的系统得以改进。用于研究等离子体驱动的化学反应的专门技术也可以应用于广泛的其他催化反应。克罗宁博士在其研究的基础上积极参与推广活动，以促进学生对科学、技术、工程和数学(STEM)学科的参与。这些活动包括为高中科学教师举办讲习班，旨在改善对有前途的高中生的教育，鼓励他们对STEM职业的兴趣。在化学系化学催化项目的资助下，南加州大学的克罗宁博士正在使用原位衰减全反射(ATR)-FTIR光谱技术研究关键反应中间体的振动特征。在这里，使用包含纳米颗粒(例如，铂、银、金)的衬底上的纳秒高压脉冲产生热电子、低温瞬变脉冲等离子体。这些纳米粒子在血浆的生成方面提供了高达1000倍的增强，这些血浆定位于纳米粒子的表面，在那里它对催化最有用。这种瞬变等离子体的低温性质对于保持这些微妙的纳米颗粒的结构完整性至关重要，这在传统的射频(RF)等离子体中是不可能的。克罗宁博士和他的团队探索了几种测试反应系统，包括NO、NO2和SO2的修复。虽然这些反应都是使用基于等离子体的过程来演示的，但这些等离子体辅助反应的详细化学路径还没有被很好地理解。虽然其他形式的光谱已经在等离子体上得到了广泛的应用(例如，激光诱导荧光光谱)，但ATR-FTIR是一种固有的表面敏感光谱，它通过阐明关键中间物种来提供对等离子体驱动的催化反应机理的新见解。通过使用几种不同类型的金属纳米颗粒表面(例如，铜、镍、铂)来识别表面中间体，Cronin博士和他的团队在气相和液态中测试了几种假设的化学路径。例如，一种假设是SO2的修复是由OH自由基(即SO2→HSO3→H2SO4)驱动的，这些自由基可以由等离子体产生。克罗宁博士积极参与STEM外展计划，重点是招募女性学生进入STEM领域和高中学生研究实习，以支持该项目的更广泛影响。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。

Toxic gases such as nitrogen oxides (NOx) and sulfur oxides (SOx), produced by the burning of fossil fuels, give rise to smog and acid rain. Converting these harmful pollutants to safer by-products is a significant and important challenge. In this project, with funding from the Chemical Catalysis Program of the Division of Chemistry, Dr. Stephen Cronin of the University of Southern California is investigating plasma-driven catalysis as a way to eliminate these pollutants. Catalysts are substances that accelerate chemical reactions without themselves being consumed. Catalysts require energy to function, and the research team is investigating plasmas (the low temperature, glowing gas in fluorescent lamps is an example of plasma) to deliver that energy. The fundamental knowledge gained in this work enables improved systems for remediating these harmful pollutants. The specialized techniques used to investigate plasma-driven chemical reactions can also be applied to a wide range of other catalytic reactions. Dr. Cronin is actively engaged in outreach activities that build upon his research to promote engagement of students in science, technology, engineering and mathematics (STEM) disciplines. These activities, which include a workshop for high school science teachers, are directed at improving the education of promising high school students and encouraging their interest in STEM careers. With funding from the Chemical Catalysis Program of the Division of Chemistry, Dr. Cronin of the University of Southern California is studying the vibrational signatures of key reaction intermediates using in situ attenuated total reflection (ATR)-FTIR spectroscopy. Here, a hot-electron, low-temperature transient pulsed plasma is generated using nanosecond high voltage pulses across a substrate containing nanoparticles (e.g., Pt, Ag, Au). These nanoparticles provide up to 1000X enhancement in the generation of the plasma, which is localized to the surface of the nanoparticles where it is most useful for catalysis. The low-temperature nature of this transient plasma is crucial to maintaining the structural integrity of these delicate nanoparticles and would not be possible with a conventional radio frequency (RF) plasma. Dr. Cronin and his group explore several test reaction systems, including NO, NO2, and SO2 remediation. While these reactions have all been demonstrated using plasma-based processes, the detailed chemical pathways of these plasma-assisted reactions are not well understood. While other forms of spectroscopy have been performed extensively on plasmas (e.g., LIF spectroscopy), ATR-FTIR is an inherently surface-sensitive spectroscopy that provides new insights into the plasma-driven catalytic reaction mechanisms through the elucidation of key intermediate species. By identifying surface intermediates using several different types of metal nanoparticle surfaces (e.g., Cu, Ni, Pt), Dr. Cronin and his group test several hypothetical chemical pathways in both the gas and liquid phases. For example, one hypothesis is that SO2 remediation is driven by OH radicals (i.e., SO2 → HSO3 → H2SO4), which can be produced by the plasma. Dr. Cronin is actively engaged in STEM outreach programs focused on female student recruitment into the STEM fields and in high school student research internships, in support of the broader impacts of the 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.