The effect of CeO2 additions into a PdO catalyst supported on χ-Al2O3 has been investigated. Changes in crystal structure have been examined in a methane atmosphere, in relation to catalytic activity for methane oxidation. The mass ratio Al2O3/PdO of the catalysts was kept constant at a value of 9, whereas the atomic ratio R=Pd/Ce was set to 0, 1, 2 and 5. The PdO-CeO2/χ-Al2O3 catalyst with R=1 showed the best catalytic activity in methane oxidation. When the catalyst was heated at 300°C, Pd metal appeared under the O2-deficient atmosphere, whereas PdO was reformed when the reduced catalyst was heated at 300°C in an atmosphere with an O2-excess. Line broadening of X-ray diffraction patterns was detected for both the (110) plane of PdO and the (200) plane of the Pd formed after heating, suggesting that stacking faults were introduced into these planes by the loss of oxygens in the PdO/Pd lattice during the reaction with methane. As the (110) planes in PdO correspond to the (200) planes of the fcc Pd metal formed during reaction, in terms of the array of the constituent metal only, methane oxidation would easily proceed by way of these oxygen-deficient lattice planes.
