Synergistic Effects of Keggin-Type Phosphotungstic Acid-Supported Single-Atom Catalysts in a Fast NH 3 -SCR Reaction.

Fast selective catalytic reduction of nitrogen oxide with ammonia (NH 3 -SCR) (2NH 3 + NO 2 + NO → 2N 2 + 3H 2 O) has aroused great interest in recent years because it is inherently faster than the standard NH 3 -SCR reaction (4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O). In the present paper, the mechanism of the fast NH 3 -SCR reaction catalyzed by a series of single-atom catalysts (SACs), M 1 /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 NH 3 molecule to the symmetry-allowed d orbitals (d xy and d x 2 -y 2 ) of the single metal atom, which effectively weakens the N-H bond of the adsorbed NH 3 molecule. The calculated free energy profiles along the favorable catalytic path show that decomposition of NH 3 to *NH 2 and *H species and decomposition of *NHNOH into N 2 and H 2 O have high free energy barriers in the whole fast NH 3 -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 NH 3 to *NH 2 and *H species. M 1 /PTA SACs (M = Ru, Rh, Pd, and Pt) were found to have potential for fast NH 3 -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 NH 3 -SCR catalysts with high activity.