Evolution of Pd catalyst structure and activity during catalytic oxidation of methane and ethane

• Rate of alkane oxidation over Pd catalyst becomes oscillating.
• Oscillations include periodic accumulation/removal of C and O.
• Pd surface evolves dramatically during oscillations.
• Pd activity depends on C and O content in Pd.
• Metallic Pd can be highly active, if carbonized, or low active, if blocked by O2.

Methane and ethane oxidation in alkane-rich mixtures over Pd powder in temperature range of 300–450 °C has been studied using combined mass-spectrometry and thermogravimetry. Fresh Pd powder was found to have very low stationary initial catalytic activity. The activity gradually rose with time on stream reaching a certain value where oscillations of the reaction rate began. The oscillation phases with relatively high catalytic activity were accompanied by a periodic accumulation/removal of carbon in Pd bulk, whereas during the low activity phases a periodic accumulation/removal of oxygen in Pd surface layer took place. The correlation between Pd oxidation state and catalytic activity was drawn. The metallic Pd had the lowest catalytic activity in alkane oxidation at high partial pressure of O2 and while the highest catalytic activity was observed at low partial pressure and high conversion of O2. Microscopic study of Pd samples revealed that the catalyst evolution caused changes in Pd surface morphology resulting in formation of sponge-like structure. In turn, such morphological changes were found to increase duration and intensity of the high activity phase.

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