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

Hydrogen-assisted selective catalytic reduction (H 2 –SCR) of NO under O 2 -rich conditions is crucial for lean exhaust aftertreatment. Experimentally, Pd/Al 2 O 3 catalysts can exhibit ∼100% NO conversion and up to ∼70% N 2 selectivity at temperatures as low as 150 °C. Given Pd’s excellent combustion properties, H 2 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 ( s MKM) to interrogate Pd clusters on γ-Al 2 O 3 (110). DFT-calculated activation energies suggest the following trends in intrinsic NO dissociation rates: Pd 2 O 2 > Pd 2 > Pd 3 > Pd 1 > Pd 1 O 2 > Pd 4 > Pd 5 > Pd 3 O 2 . 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 Pd 2 . Contrary to popular belief, O 2 -rich conditions promote H 2 –SCR at intermediate temperatures due to the formation of electron-rich 2O* from O 2 dissociation, which further facilitates N–O activation due to an unexplored electron donation from coadsorbed 2O*. s MKM qualitatively reproduces experimental product distribution, identifying Pd 2 O 2 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 N 2 O-induced poisoning at low temperatures.