Effect of Dissolved Carbon on Activation of Methane on Palladium: Experimental and Theoretical Study

Using experimental and theoretical methods, we studied the dissolution of carbon in near-surface layers of palladium, the formation of palladium carbide and its reactivity toward oxygen. By studying self-sustained oscillations during the oxidation of methane over Pd foil by X-ray diffraction and mass spectrometry, we found that the formation of palladium carbide proceeds simultaneously with a sharp increase in catalytic activity. However, ab initio calculations indicated that the dissolution of carbon in near-surface layers of palladium leads to increasing activation barriers of methane decomposition steps. For example, for the hydrogen abstraction from CH adsorbed species, the activation barrier increases from 54 to 80 kJ/mol. Since the C–H bond breaking activity is the limiting factor in the hydrocarbon oxidation, such increase means that the formation of palladium carbide leads to a decrease in the catalytic activity. Therefore, the formation of palladium carbide is a result of high activity of metallic palladium in the methane decomposition; the carbon formed in this case can not only accumulate on the surface, but also dissolve in the near-surface layers of palladium; both processes reduce the activity of palladium.

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