Combinatorial High-Energy X-Ray Microbeam Study Of The Size-Dependent Oxidation Of Pd Nanoparticles On Mgo(100)

We have studied the oxidation of MgO(100) supported Pd nanoparticles in a size range from 4 to 24 nm at 570 K and near atmospheric pressures. Our experimental setup using high-energy x rays allows us to study in situ the oxidation behavior for different particle sizes under identical experimental conditions. We can identify three size regimes with a distinct oxidation mechanism. For particles smaller than 5 nm in diameter, kinetic barriers for bulk oxide formation are strongly reduced and complete transformation to PdO bulk oxide takes place, epitaxial to the MgO substrate. Particles with a diameter 5 nm 9 nm, we observe a complete passivation of the particle surface by a thin polycrystalline PdO layer, which prevents further oxidation. Our experiments demonstrate that particles with a few nanometers in size exhibit a much higher reactivity with oxygen, which is important for the performance of Pd nanoparticles as a CO oxidation catalyst.