Gas Phase and Surface Infrared Studies of Plasma-catalysis

Low-temperature plasma produces reactive species that can lead to new reaction pathways on a catalyst surface and lead to product enhancements. We will review two such examples: One, plasma-catalytic decomposition of CH₄ by O₂ over a Ni-SiO₂/Al₂O₃ catalyst and two, plasma-enhanced N₂ oxidation over a Pt-Al₂O₃ catalyst. For both, an atmospheric pressure plasma jet (APPJ) fed with the gasses of interest was interacted with the catalyst located 5 mm from the APPJ at variable catalyst temperatures. Fourier-transform infrared spectroscopy was used for measurement of downstream species, and diffuse reflectance infrared Fourier transform spectroscopy for analysis of surface species.



For the CH₄/O₂ system we find that O atoms are key for CH₄ oxidation when only Ar/O₂ passes through the plasma at near 25°C. Thermal oxidation of CH₄ becomes important as the catalyst is heated up to 500°C. On the other hand, CH₄ oxidation to CO and CO₂ is greatly enhanced if Ar/CH₄/O₂ plasma is used. In this case, the catalyst can be activated by large power plasma at 500°C and continues to produce CO after the APPJ is extinguished and not seen prior to plasma application.



For the N₂/O₂ system, N₂ oxidation occurs within the APPJ and shows a dependance on ambient temperature for the NO to NO₂ reaction, with NO becoming dominant at 350°C. In the presence of a Pt catalyst at 350°C, Pt promotes further oxidation and NO₂ becomes dominant. At temperatures below 270°, plasma produced nitrogen-oxygen species are stored on the catalyst surface, including nitrites and nitrates, and released at high temperatures. The reasons for these observations will be discussed, along with correlations of gas phase and surface IR data aimed at gaining more insights into the plasma catalysis reactions.