Formaldehyde emissions in addition to the methane-slip have recently been addressed for natural gas fueled combustion engines and thus require an aftertreatment catalyst system. In this work, two Pt-based catalysts supported on Al2O3 and TiO2–SiO2 were investigated by considering their activity, inhibition and stability towards SO2-poisoning. The catalysts were analyzed by X-ray diffraction, electron microscopy, BET surface area, ex situ and operando X-ray absorption spectroscopy, which revealed differences in the oxidation state of Pt depending on the support material and that slightly reduced Pt sites seem to be the active species for this reaction. Systematic activity tests showed that NOx presence leads to a significant inhibition, possibly due to oxidation of the active species, whereas CO addition promotes formaldehyde oxidation above the onset temperature of CO oxidation. These results were supported by spatially resolved concentration profiles along the channel of a catalyst coated monolith. Addition of small amounts of SO2 to the gas mixture resulted in a complete loss of the low-temperature activity, slightly more pronounced for the TiO2–SiO2 supported catalyst.
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We thank the Research Association for Combustion Engines e.V. (FVV) for financial support, and the Karlsruher synchrotron facility (CAT-ACT beamline) and DESY (P65 beamline, PETRA III) for providing beam time. Dr. A. Zimina (KIT), Dr. T. Prüßmann (KIT) and Edmund Welter (PETRA III) are gratefully acknowledged for their help and technical support during the XAS experiments.