Mechanistic Insights into Heterogeneous Mechanisms for Water Formation on O ₂ -Precovered Pd(111): A Combined DFT and AIMD Study

Metal surface-catalyzed reactions can proceed through distinct mechanisms depending on the reaction conditions. Here, we investigated the water formation reaction from H2 and O2 on Pd(111) with a focus on the role of O2 coverage. Static density functional theory (DFT) calculations identified stable O2 adsorption configurations and corresponding reaction pathways, showing that the Langmuir–Hinshelwood (LH) mechanism dominates on most surfaces. However, on certain surfaces where all six Pd atoms surrounding the H2 molecule adsorption site are bonded with O2, steric hindrance favors alternative channels such as the hot–atom (HA) mechanism. Building on these insights, ab initio molecular dynamics (AIMD) simulations were conducted to investigate the interaction between the dissociated gas H atom and the O2 precovered Pd(111) surfaces (the coverage reaching 0.33 monolayers (ML)). Among 100 MD trajectories, 22% led to the formation of the key intermediate OH* via an HA mechanism, in which OOH* formed first and subsequently dissociated. The present finding demonstrates a shift from LH to gas H collision HA mechanisms at a higher O2 coverage. The work provides detailed atomic-level insights into coverage-dependent catalytic behavior.