Unveiling Water-Vapor-Promoted Oxidation of Palladium Nanoparticles via Atomic-Scale Transmission Electron Microscopy at Atmospheric Pressure

Water vapor plays a critical role in metal oxidation, profoundly impacting structural integrity and functional performance. However, its effects on metal oxidation dynamics under realistic temperatures and pressures remain poorly understood. In this study, we utilized Cs-corrected transmission electron microscopy (TEM) in combination with atmospheric-pressure techniques to investigate the oxidation dynamics of Pd nanoparticles in water vapor, and systematically compared their behavior to that in oxygen, both under atmospheric conditions. Atomic-resolution TEM images reveal that protruding PdO nucleates on the Pd{110} surface in a water vapor atmosphere, with PdO(1̅1̅2) aligned parallel to Pd(22̅0), or rotated within 7.4° around the consistently parallel directions of PdO[111] and Pd[001]. In contrast, PdO nucleates and grows on the Pd{100} surface in oxygen to form continuous films with a fixed epitaxial relationship, i.e., PdO(001)//Pd(100), PdO[1̅10]//Pd[001]. Density functional theory (DFT) calculations combined with crystallographic analysis demonstrate that facet-dependent adsorption energy and interfacial mismatch govern the distinct nucleation and growth behaviors. Particularly, the self-enhanced adsorption capability for water vapor during the oxidation, along with the presence of interstitial hydrogen in the oxide, facilitates accelerated overall oxidation in water vapor. This work bridges the pressure gap in study of metal oxidation dynamics in the presence of water vapor and offers mechanistic insights into oxidation processes under practical environmental conditions.