Reaction Pathways in H 2 –Assisted NO Reduction on Alumina-Supported Platinum Clusters and Extended Surfaces

Hydrogen internal combustion engines necessitate the implementation of selective catalytic reduction of nitrogen oxides under O 2 -rich environments in exhaust. This is challenging as noble metals favor combustion, selectively burning H 2 instead of reducing NO to N 2 . Yet, high NO conversion and selectivity to N 2 can be achieved on Pt/Al 2 O 3 catalysts at low temperatures. To rationalize this paradox, we construct a microkinetic model on Pt(111) using electronic-structure calculations and find that the model fails to capture experiments. We conjecture that high-performing reduction catalysts consist of small metal clusters. Density functional theory calculations on Pt dimers supported on γ-Al 2 O 3 (110) (Pt 2 /Al 2 O 3 ) reveal lower activation energies for NO dissociation and N–N recombination than O 2 dissociation and N–NO recombination, respectively. Electron density difference isosurfaces suggest that lower-lying and partially occupied π* orbitals of NO require significantly less electron back-donation from Pt 2 for bond activation than those of O 2, rationalizing the lower NO dissociation barrier compared to O 2 . The higher barrier of N–NO recombination is likely due to the NO binding in a stable O δ− –N δ+ state. State-based microkinetic model accurately describes experimental observations and reveals a spatially varying Pt oxidized state, where NO reduction occurs on metallic states. Interestingly, the oxidation–reduction selectivity reverses at the extremes of metal size.