Unusual Facet-Dependent Sintering in Pd-TiO₂ Catalysts Revealed by Theory and Experiment

Engineering of support facet and metal valence states is recognized as an effective strategy for manipulating the sintering and dispersion of metal nanoparticles (NPs). Here, we report an unusual valence state-independent but facet-dependent effect of sintering/dispersion behaviors of Pd NPs on anatase TiO2 surfaces, which is confirmed by density functional theory (DFT) calculation and in situ environmental scanning transmission electron microscopy (ESTEM). All the Pd species, including Pd atoms and Pd2+ ions, sintered on the TiO2 (100) surface through both the particle migration coalescence (PMC) and Ostwald ripening (OR), while dispersed into single atoms/ions rapidly on the TiO2 anatase (001) surface. Further dynamical simulations revealed the specific interactions between Pd, surface lattice O, and the surface oxygen vacancies in the supports, responsible for the mobility of Pd NPs and diffusion barriers for Pd adatoms/ions. Our results contrast with the conventional understanding of relationships between the valence state of noble metal and the intrinsic facet-dependent effects and suggest an avenue for the rational design of targeted catalysts under different reaction conditions.