Synergistic Effects of Keggin-Type Phosphotungstic Acid-Supported Single-Atom Catalysts in a Fast NH₃‑SCR Reaction

Fast selective catalytic reduction of nitrogen oxide with ammonia (NH₃-SCR) (2NH₃ + NO₂ + NO → 2N₂ + 3H₂O) has aroused great interest in recent years because it is inherently faster than the standard NH₃-SCR reaction (4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O). In the present paper, the mechanism of the fast NH₃-SCR reaction catalyzed by a series of single-atom catalysts (SACs), M₁/PTA SACs (PTA = Keggin-type phosphotungstic acid, M = Mn, Fe, Co, Ni, Ru, Rh, Pd, Ir, and Pt), has been systematically studied by means of density functional theory (DFT) calculations. Molecular geometry and electronic structural analysis show that Jahn–Teller distortion effects promote an electron transfer process from N–H bonding orbitals of the NH₃ molecule to the symmetry-allowed d orbitals (d_xy and d_x2–y2) of the single metal atom, which effectively weakens the N–H bond of the adsorbed NH₃ molecule. The calculated free energy profiles along the favorable catalytic path show that decomposition of NH₃ to *NH₂ and *H species and decomposition of *NHNOH into N₂ and H₂O have high free energy barriers in the whole fast NH₃-SCR path. A good synergistic effect between the Brønsted acid site (surface oxygen atom in the PTA support) and the Lewis acid site (single metal atom) effectively enhances the decomposition of NH₃ to *NH₂ and *H species. M₁/PTA SACs (M = Ru, Rh, Pd, and Pt) were found to have potential for fast NH₃-SCR reaction because of the relatively small free energy barrier and strong thermodynamic driving forces. We hope our computational results could provide some new ideas for designing and fabricating fast NH₃-SCR catalysts with high activity.