Anomalous Structural and Catalytic Behavior of Strained Pd Nanoparticles.

Pd Nanoparticles (NPs) are commonly used catalytic systems for high-temperature oxidation processes such as methane oxidation reactions. The catalytic activity of Pd NPs in these processes depends heavily on the oxidation of the metallic Pd phase to PdO. While the effect of the size of Pd NPs on their thermal oxidation has been widely studied, the role of the morphology and atomic arrangement of Pd NPs is largely unknown. In this work, the thermal oxidation of γ-Al2O3-supported Pd NPs was monitored using X-ray spectroscopy and high-energy scattering methods, enabling correlations between structure and catalytic properties in methane oxidation reactions. The results from this work demonstrate that the thermal oxidation of Pd NPs is highly dependent on tensile strain in the metallic Pd fcc lattice. The presence of tensile strain in polycrystalline Pd NPs impedes the oxidation of the Pd fcc phase to the PdO phase, leading to delayed methane oxidation activity. Thermal treatment of Pd NPs in open air led to the relaxation of tensile strain, and the oxidation of Pd NPs to the PdO phase started only after the complete removal of the tensile strain from the Pd fcc lattice. Larger polycrystalline Pd NPs, in which tensile strain was not removed by thermal treatment, did not show complete oxidation and thus had a significantly retarded onset for methane oxidation. Overall, this work highlights the importance of structural defects in the structure of Pd NPs on their chemistry and resulting catalytic behavior.