C–H Bond Activation by Pd-substituted CeO₂: Substituted Ions versus Reduced Species

Substituted metal oxides containing ionic species have been attracting a great deal of attention because of their potential ability to reduce the usage of precious metals in heterogeneous catalysts. We investigate Pd-substituted CeO 2 for C–H bond activation reactions including the partial oxidation and dry reforming of CH 4 . This catalyst has been previously studied for CO oxidation, NO x reduction, and the water-gas shift reaction. Pd-substituted CeO 2, Ce 1– x Pd x O 2−δ, was prepared as a powder with high surface area and a hollow sphere morphology using ultrasonic spray pyrolysis. The catalysts were extensively characterized using synchrotron X-ray diffraction and other techniques, confirming phase pure samples up to 10 mol % Pd substitution. Ce 0.95 Pd 0.05 O 2−δ was found to be active for partial oxidation of CH 4 around 500 °C and higher. Our studies, including postcatalytic synchrotron diffraction, suggest that the single-phase Ce 1– x Pd x O 2−δ material is not the active species and that catalysis occurs instead over the reduced two-phase Pd 0 /CeO 2 . This observation has been further confirmed by verifying the activity of the reduced Pd 0 /CeO 2 catalysts for ethylene hydrogenation, a reaction that is known to require Pd 0 .