In Situ XPS and MS Study of Methane Oxidation on the Pd–Pt/Al₂O₃ Catalysts

The bimetallic Pd–Pt/Al₂O₃ supported catalysts with different Pd/Pt ratios have been prepared and studied in methane oxidation reaction. Comparison of their catalytic properties with those of monometallic Pd/Al₂O₃ and Pt/Al₂O₃ samples has shown that the bimetallic catalysts with a relatively low Pt loading (less than 0.4 mol%) are more active than the monometallic ones indicating the synergy effect. An increase in Pt loading above 0.5 mol% decreases the catalytic activity, so that these catalysts become less active than the monometallic Pd/Al₂O₃ catalysts, but still keep an activity higher than that of the least active monometallic Pt/Al₂O₃ sample. The chemical states of the palladium and platinum for all the catalysts produced depending on the reaction conditions have been investigated with in situ XPS. It has been shown that even at room temperature the reaction mixture affects the surface composition of the catalysts, which does not change much at higher reaction temperatures. The data have led us to suggestion that activity in the total methane oxidation is promoted by formation of Pd²⁺, surface fraction of which depends on Pt content in the bimetallic catalysts.