Carbon incorporation and deactivation of MgO(0 0 1) supported Pd nanoparticles during CO oxidation

We have investigated the structure and composition of the model catalyst system Pd/MgO(0 0 1) during oxidation, CO reduction and CO oxidation at near atmospheric pressures by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy and spectroscopy techniques. From the in situ X-ray experiments, we find: (a) the Pd nanoparticles with 9 nm in diameter transform into epitaxial PdO above 10−1 mbar O2 pressure at 570 K, (b) the oxidation process can be reverted by CO exposure, recovering Pd nanoparticles in their initial orientation, and (c) during CO oxidation in a mixture of 50 mbar O2 and 50 mbar CO a new phase is evolving with lattice constant close to the MgO substrate value, which we assign to expanded Pd nanoparticles forming upon carbon incorporation. Ex situ transmission electron microscopy and different spectroscopy techniques uncover the CO2 induced growth of a disordered overlayer containing C, Mg and O, which forms during CO oxidation and leads to an overgrowth of Pd nanoparticles thereby deactivating the catalyst.