Development of New Tools for the Chemical Analysis of the Internal Surfaces of Porous Materials using Annihilation Gamma Spectroscopy

在化学系化学测量和成像（CMI）项目的支持下，由德克萨斯大学阿灵顿分校（UTA）的亚历山大韦斯、瓦格斯·奇拉亚斯和阿里·科伊曼教授领导的研究小组正在开发一种新技术，用于探测多孔材料内部表面的化学成分。UTA的研究小组最近表明，正电子（反物质电子）与表面电子湮灭产生的伽马射线的能谱可用于确定外表面最顶层的元素组成。该团队目前正在努力证明正电子湮没伽马光谱法可用于揭示催化或反应性气体暴露所带来的纳米多孔金属内部隐藏表面的化学变化。该项目旨在开发一种迄今为止尚不具备的用于多孔材料内表面化学分析的工具。 这项研究的结果对许多研究领域都有潜在的影响，包括催化、储能、气体分离、碳捕获和稀合金设计。该项目为本科生和研究生提供高级表面改性和表征方法方面的培训。参与这个项目的学生来自UTA的高度多样化的学生团体，这是一个西班牙裔服务机构。该研究小组还与阿灵顿独立学区（AISD）的STEM学院合作，为选择高中生提供培训。学生参与该项目的前沿研究有助于塑造先进化学仪器的下一代领导者，并为国家急需的这一领域的劳动力发展做出独特贡献。多孔材料的催化活性内表面对其环境的反应强烈表明，表面组成控制催化功能，这表明需要能够在操作条件下以高表面选择性探测内表面的工具。有一个明显的缺乏实验方法，可以非破坏性地提供化学信息的顶部原子层的内表面，是免费的散装信号的干扰。在化学测量和成像（CMI）计划的支持下，UTA的正电子研究团队正在开发一种用于不可接近的内表面的操作表征工具，其元素敏感度低至最顶层原子层的1%。该团队正在将正电子光谱学的上级表面选择性与511 keV湮灭伽马辐射的能力相结合，以通过毫米的样品或反应单元而不丢失任何信息。该技术无与伦比的表面选择性源于湮灭前在样品表面真空侧的图像势阱中捕获注入的正电子。正如UTA团队最近所证明的那样，由表面捕获的正电子湮灭产生的伽马辐射的能谱通过湮灭伽马能量的元素特异性多普勒展宽反映了最顶层原子层的元素组成。使用从受控外表面获得的校准光谱来基准多普勒展宽湮灭光谱，该团队现在正在开发测量纳米多孔金（Au）和纳米多孔铜（Cu）内表面化学成分的技术，包括暴露于反应气体或催化过程中发生的变化。研究小组正在探索这些材料中催化活性位点的性质，目的是通过在真空和操作条件下的受控实验，在臭氧和一氧化碳等反应性刺激的作用下，提供纳米多孔Au和Cu的内表面构型的全局视图。除了工作的更广泛的科学影响外，该项目还为研究生和本科生提供高级培训机会，并支持外展活动，让当地高中生参与STEM研究。该奖项反映了NSF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。

With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, the research team led by Professors Alexander Weiss, Varghese Chirayath, and Ali Koymen from the University of Texas at Arlington (UTA) are developing a new technique to probe the chemical composition of otherwise inaccessible internal surfaces of porous materials. The research team at UTA has recently shown that the energy spectrum of gamma rays resulting from the annihilation of positrons (anti-matter electrons) with electrons at the surface can be used to determine the elemental composition of the topmost layer of external surfaces. The team is now working to demonstrate that positron-annihilation gamma spectroscopy can be used to reveal chemical changes to the hidden surfaces inside of nanoporous metals brought about by catalysis or reactive gas exposure. The project is aimed at the development of a hitherto unavailable tool for the chemical analysis of porous materials' internal surfaces. The results of this research have potential impact across many areas of research, including catalysis, energy storage, gas separation, carbon capture, and dilute alloy design. The project provides training to undergraduate and graduate students in advanced surface modification and characterization methods. The students involved in this project are from the highly diverse student body of UTA, which is a Hispanic Serving Institution. The research team also collaborates with the STEM academy of the Arlington Independent School District (AISD) to provide training to select high-school students. Participation by students in the cutting-edge research of this project is serving to mold the next generation of leaders in advanced chemistry instrumentation and making a unique contribution to workforce development in this area of critical national need.The response of catalytically active internal surfaces of porous materials to their environment strongly suggests that surface composition controls catalytic function, demonstrating the need for tools capable of probing internal surfaces with high surface selectivity under in operando conditions. There is a glaring absence of experimental methods that can non-destructively provide chemical information about the top atomic layer of internal surfaces that is free of interference from bulk signals. With the support of the Chemical Measurement and Imaging (CMI) program, the positron research team at UTA is developing an in operando characterization tool for the inaccessible internal surfaces with elemental sensitives as low as 1% of the topmost atomic layer. The team is combining the superior surface selectivity of positron spectroscopy with the ability of 511 keV annihilation gamma radiation to exit through millimeters of sample or reaction cells without any loss of information. The unparalleled surface selectivity of the technique stems from the trapping of the implanted positrons in an image-potential well on the vacuum side of the sample surface before annihilation. As recently demonstrated by the UTA team, the energy spectrum of the gamma radiation resulting from the annihilation of the surface-trapped positrons reflects the elemental composition of the topmost atomic layer through the element-specific Doppler broadening of the annihilation gamma energy. Using calibration spectra obtained from controlled external surfaces to benchmark the Doppler broadened annihilation spectrum, the team is now developing the technique to measure the chemical composition at the internal surfaces of nanoporous gold (Au) and nanoporous copper (Cu), including variations that occur upon exposure to reactive gases or during catalysis. The research team is exploring the nature of catalytically active sites in these materials with the goal of providing a global view of the internal surface configuration of nanoporous Au and Cu under the action of reactive stimuli like ozone and carbon monoxide through controlled experiments in ultrahigh vacuum and in operando conditions. In addition to the scientific broader impacts of the work, the project provides advanced training opportunities for graduate and undergraduate students, and supports outreach activities to engage local high school students in STEM research.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.