Elucidation of structure–activity relationships of model three way catalysts for the combustion of methane

The combustion of methane on model three way catalysts (TWC) Pd/CeZrO2 and Pd/LaFeO3 (2 wt.% Pd) was studied under lean conditions in the temperature range between ca. 200 and 800 °C to investigate the catalytic performance at and after high temperatures. The catalysts were prepared by incipient wetness impregnation and the nature and distribution of palladium species was characterized by N2-physisorption, X-ray diffraction (XRD), visible Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), H2-temperature programmed reduction (H2-TPR), CO adsorption followed by diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) and CO chemisorption. The behaviour of Pd during the combustion was monitored by in situ XANES spectroscopy. The characterization results indicate that the dispersion of Pd is higher in Pd/CeZrO2 than in Pd/LaFeO3. The catalytic data reveal that Pd/CeZrO2 is more active and stable compared to Pd/LaFeO3 in the whole studied temperature range. This is also true even after subjecting the catalysts to reaction at ≥800 °C as evidenced by the extent of hysteresis observed in the conversion profiles. In situ XANES studies indicate that palladium had been in the oxidized state when the catalysts showed the best combustion performance. The studies also indicate that a fraction of PdOx had been reduced when the hysteresis was observed in the conversion profiles (while cooling between ca. 680 and 560 °C) implying that the PdOx species are the active sites, at least at T ≤ 700 °C.