In SituX-ray Absorption Spectroscopy Studies on Chemical Oscillations in the CO/O₂System on Supported Pd Catalysts

Abstract

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 process accompanying the deactivation/activation cycles of the catalyst. Furthermore, an oscillatory behavior in height and position of the first Pd peak was found in the radial distribution function FT(χ(k)). From a comparison with theoretical cluster calculations, it can be deduced that the evaluated oscillations in Pd coordination number and Pd−Pd distance indicate an oscillatory change of the oxygen surface coverage as well as the ratio of linearly and bridged bonded carbon monoxide. Taking these surface-coverage oscillations and the periodic oxidation/reduction process into account, a surface activation/deactivation mechanism for chemical oscillations during CO oxidation on supported Pd catalyst can be confirmed. The activity of the catalyst appears to be decreased by a surface oxidation leading to an increasing concentration of linearly bonded CO molecules. Thereafter, an increasing amount of bridging CO and a decreasing amount of linearly bonded CO accompany the reduction of the catalyst and subsequently the return to the high activity branch of the reaction.

Solid-state kinetics from time-resolved in situ XAFS investigations: reduction and oxidation of molybdenum oxides

“…energy calibration, etc.) can be found in references (Ressler et al 1997). X-ray absorption fine structure (XAFS) analysis was carried out using the software package WinXAS v2.0 (Ressler, 1998) following recommended procedures from the literature (Koningsberger und Prins, 1988).…”

Probing Active Sites During Palladium-Catalyzed Alcohol Oxidation in “Supercritical” Carbon Dioxide

“…references [16,17]), X-ray absorption spectroscopy (XAS) is a valuable technique for operando spectroscopy because it can give direct information on the oxidation state of noble metal atoms [18]. Though X-ray absorption spectroscopy has not been used for unraveling the catalyst structure in SCFs, the technique was widely used for identification of the structure at the gas-solid interface [18][19][20][21][22][23][24][25][26] under reaction conditions, leading in ideal cases to structureactivity relationships [24,[26][27][28][29][30]. For liquid-solid reactions, in situ EXAFS studies are rarer, but are equally powerful [31,32].…”

Thermal decomposition of (NH4)2[PtCl6] – an in situ X-ray absorption spectroscopy study

“…In (Meyer 1991). Therefore, a further study of these reactions with techniques that allow in situ observations on a much shorter time scale than the above mentioned techniques is of interest (Epple 1996, TrOger 1997, Ressler 1997. Time-resolved XAS experiments, especially those using the energy dispersive mode, enable the acquisition of spectra in less…”