Alumina-Supported Palladium Oxide Clusters Catalyze the Nitric Oxide Reduction by Hydrogen under Oxygen-Rich...

Hydrogen-assisted selective catalytic reduction (H₂-SCR) of NO under O₂-rich conditions is crucial for lean exhaust aftertreatment. Experimentally, Pd/Al₂O₃ catalysts can exhibit ∼100% NO conversion and up to ∼70% N₂ selectivity at temperatures as low as 150 °C. Given Pd's excellent combustion properties, H₂ combustion and NO oxidation are instead expected. Establishing a mechanistic understanding and determining the structure of the active site are crucial for reconciling experimental data and for designing next-generation noble-metal-free catalysts. Here, we combine density functional theory (DFT) calculations with state-based microkinetic modeling (sMKM) to interrogate Pd clusters on γ-Al₂O₃(110). DFT-calculated activation energies suggest the following trends in intrinsic NO dissociation rates: Pd₂O₂ > Pd₂ > Pd₃ > Pd₁ > Pd₁O₂ > Pd₄ > Pd₅ > Pd₃O₂. Electron-density difference isosurfaces reveal that strong NO → Pd σ-donation and Pd → NO π* backdonation are responsible for the low NO dissociation barrier on Pd₂. Contrary to popular belief, O₂-rich conditions promote H₂-SCR at intermediate temperatures due to the formation of electron-rich 2O* from O₂ dissociation, which further facilitates N-O activation due to an unexplored electron donation from coadsorbed 2O*. sMKM qualitatively reproduces experimental product distribution, identifying Pd₂O₂ with Pd in the +2 oxidation state as the eminent active motif. Our work exposes a reversal in selectivity with varying cluster size, a profound coadsorbate-induced promotion of chemistry, metal oxide clusters as prominent catalytic centers, and spatially evolving catalyst states. The model also reveals distinct oxidation-induced deactivation at high temperatures and NO- or N₂O-induced poisoning at low temperatures.