In Situ X-ray Absorption Spectroscopy Studies on Chemical Oscillations in the CO/O2 System on Supported Pd Catalysts

The combustion of carbon monoxide on transition metals is known to exhibit an intriguing oscillatory behavior in reaction rate as well as reaction temperature under certain conditions. Chemical oscillations during the oxidation of CO at atmospheric pressure on supported Pd catalysts were studied in situ by employing X-ray absorption spectroscopy (XAS) in an energy-dispersive mode. Self-sustained thermokinetic oscillations with a period of about 12 min were found in both reaction temperature and carbon dioxide partial pressure. Energy-dispersive XAS experiments were carried out in situ at the Pd K-edge (24.35 keV) during the occurrence of chemical oscillations with a time resolution of several seconds. An elliptically bent, rectangular Si(400) crystal in transmission geometry was used. From the Pd absorption spectra, a constant phase correlation of edge position (threshold energy) with respect to the observed temperature oscillations could be revealed. This points toward a periodic oxidation/reduction proc...