Development of Energy-Dispersed Surface XAFS Method

本研究项目的目的是开发一种新的表面XAFS技术，使用能量分散的软X射线，并将其应用于表面反应的研究作为一种工具，用于实时监测的反应。在本项目的第一年，我们在东京大学光谱化学研究中心的KEK-PF中的7A光束线上构建了色散表面XAFS的实验装置。与传统的方法相比，这种技术，典型的数据采集时间减少了十倍，这使我们能够进行现场跟踪的表面动态过程。第二年，对测量系统作了进一步改进，用柱面镜代替平面镜，使光子更好地聚焦在样品上，并改进了空间分辨电子能量分析器的电子透镜系统，使电子更好地聚焦在探测器上，从而使信号强度和能量分辨率都有了显著的提高。此外，该技术被应用于表面反应。所研究的反应是CO在Pt（111）上的氧化反应。虽然该反应是研究最多的催化反应之一，但只有最近的STM研究提供了关于反应下表面物种的原子水平方面的信息。STM研究似乎揭示了反应机理，但不能保证所有的表面物种都清晰成像，并且观察到的局部区域代表整个表面。我们已经研究了这种反应与色散表面XAFS技术在相同的条件下使用的STM研究，发现两个反应过程具有不同的反应级数，其中之一是不能观察到的STM由于快速扩散的反应物种。总之，我们已经成功地开发了分散表面XAFS技术作为一个新的强大的工具，表面动力学过程的研究。

The purpose of this research project is to develop a novel surface XAFS technique using energy-dispersed soft x rays and to apply it to studies of surface reactions as a tool for real-time monitoring of the reactions. In the first year of this project, we constructed the experimental setup for the dispersed-surface-XAFS at beamline 7A in KEK-PF that belongs to Research Center for Spectrochemistry of the University of Tokyo. With this technique, the typical data acquisition time is reduced by a factor of ten compared to the conventional method, which enables us to conduct in-situ tracing of surface dynamic processes. In the second year, this measurement system has been further improved by replacing a plane mirror with a cylindrical mirror to obtain better photon focusing on the sample and by modification of the electron lens system of the spatially-resolved electron energy analyzer to get better electron focusing on the detector, which resulted in significant increases both hi signal intensity and energy resolution. Furthermore, this technique was applied to a surface reaction. The reaction studied here was CO oxidation reaction on Pt(111). Although this reaction is one of the most studied catalytic reactions, only recent STM study has provided information about the atomic-level aspect of the surface species under the reaction. The STM study seems to reveal the reaction mechanism but does not guarantee that all the surface species are clearly imaged and that the observed local area is representative of the whole surface. We have investigated this reaction with the dispersive-surface-XAFS technique in the same condition as used in the STM study and found two reaction processes with different reaction orders, one of which was not observable by STM due to fast diffusion of the reaction species. To summarize, we have successfully developed the dispersed-surface-XAFS technique as a new powerful tool for studies of surface dynamic processes.