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

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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.