Restructuring of Palladium Nanoparticles during Oxidation by Molecular Oxygen

An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time‐resolved X‐ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided. XAS analysis revealed the restructuring of the fcc‐like palladium surface into the 4‐coordinated structure of palladium oxide upon absorption of oxygen from the gas phase and formation of core@shell Pd@PdO structures. The reconstruction starts from the low‐coordinated sites at the edges of palladium nanoparticles. Formation of the PdO shell does not affect the average Pd‒Pd coordination numbers, since the decrease of the size of the metallic core is compensated by a more spherical shape of the oxidized nanoparticles due to a weaker interaction with the support. The metallic core is preserved below 200 °C even after continuous exposure to oxygen, with its size decreasing insignificantly upon increasing the temperature, while above 200 °C, bulk oxidation proceeds. The Pd‒Pd distances in the metallic phase progressively decrease upon increasing the fraction of the Pd oxide due to the alignment of the cell parameters of the two phases. Time‐resolved synchrotron‐based X‐ray spectroscopy complemented by theoretical calculations is used to film atomic‐level restructuring of supported palladium nanoparticles upon exposure to oxidizing conditions. Below 200 °C, a palladium oxide core is formed on top of the metallic shell, the particle's shape becomes more spherical and the Pd‐Pd distances decrease. Further heating leads to complete bulk oxidation.