In Situ X-ray Probes of Surface-Mediated Pollutant Formation

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This project aims to show how an X-ray spectroscopy apparatus can be used to probe detailed microscopic aspects of pollutant formation that occur at interfaces in combustion sources. Many pollutants are introduced into the environment as a result of combustion processes, and there is a growing realization that important pathways related to the production of the pollutants involve heterogeneous processes, i.e., surface reactions, for which, there is no clear mechanistic picture of the reaction pathways or kinetics. This project carries to develop instrumentation that generates essential information regarding how pollutants with significant health effects are formed, and thereby provide the knowledge required for reducing their production. An end-station for synchrotron-based x-ray spectroscopy experiments will be developed for monitoring microscopic aspects of combustion processes. The end-station will be a vacuum system with a temperature-regulated sample holder that can be dosed with reactants of interest. The system will be designed so that fluorescence yield XAFS(s-ray absorption fine structure) spectroscopy can be performed in a variety of ways. Different types of x-ray spectra will be generated. Extended x-ray absorption fine structure (EXAFS) spectroscopy will be used for determining structural information, and x-ray absorption near edge structure (XANES) spectroscopy will be used for chemical fingerprinting applications. Specifically, this end-station will be optimized for probing how chlorine-containing aromatic species (e.g., chlorophenols) are adsorbed on metal oxide surfaces, and how they react to form polychlorinated dibenzo-p-dioxins and dibensofurans (PCDD/Fs). EXAFS spectra will be used to determine how aromatic compounds (such as 2-chlorophenol) are adsorbed on oxidized metal surfaces. Also, EXAFS spectra will be used to determine structural information on reaction intermediates after the reaction has been allowed to progress. It will also be possible to generate kinetic data on combustion processes by probing a more limited spectral range, the XANES region.%%%The key point unifying these ideas is that by combining techniques from environmental science and x-ray spectroscopy, wholly new types of data can generated to elucidate chemical pathways that generate pollutants in combustion processes. Students trained in areas of instrumentation development and applications to environmental sciences, which are also priorities for U.S. industry, will be highly competitive in this job market.