Structural changes of noble metal catalysts during ignition and extinction of the partial oxidation of methane studied by advanced QEXAFS techniques

Abstract

The dynamics of the ignition and extinction of the catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide over Pt–Rh/Al₂O₃ and Pt/Al₂O₃ were studied in the subsecond timescale using quick-EXAFS with a novel cam-driven X-ray monochromator employing Si(111) and Si(311) crystals. The experiments were performed under reaction conditions in a small fixed-bed capillary reactor. For the first time XAS data were taken with this QEXAFS technique with a Si(311) crystal that opens the energy range up to 35 keV. In addition, both XANES and EXAFS data are shown at the Pt L₃-edge, allowing to discuss the potential and limitation of this technique in catalysis and related areas. With respect to the noble metal catalysed partial oxidation of methane, several interesting observations were made: structural changes during ignition were—independent of the chosen reaction conditions—significantly faster than during the extinction of the reaction. The dynamic behavior of the catalysts was dependent on the flow conditions and the respective noble metal component(s). Higher reaction gas flow led to a faster ignition process. While the ignition over Pt–Rh/Al₂O₃ occurred at lower temperature than over Pt/Al₂O₃, the structural changes during ignition were significantly faster in the latter case. The rate of reduction of the catalyst during ignition was also dependent on the axial position in the fixed-bed. The spectroscopic results provide important insight into the ignition and extinction behavior of the CPO of methane and are complementing results from time-resolved infrared thermography and full field X-ray microscopy studies.