Toward an Atomic-Level Understanding of the Catalytic Mechanism of Selective Catalytic Reduction of NOx with NH3

As primary air pollutants, nitrogen oxides (NOx) result in a series of environmental issues, such as photochemical smog, acid rain, ozone depletion, and fine particle pollution, which threaten the health of all human beings. Selective catalytic reduction with NH3 (NH3-SCR) is the most powerful technique for the abatement of NOx. The development of NH3-SCR catalysts is the key to the normal operation of NOx removal systems. The currently used V2O5-WO3(MoO3)/TiO2 catalysts are widely applicable for medium-/high-temperature conditions but are not suitable for NH3-SCR operated at low temperatures. The development of high-efficiency low-temperature SCR catalysts is the central issue at present. The rational design of efficient NH3-SCR catalysts requires an atomic-level understanding of their mode of operation, including the identification and characterization of the active sites, reaction paths, and rate-determining steps of NH3-SCR. Herein, we summarize the recent advances in catalysts for low-temperature NH3-SCR. The active sites, intermediates, and reaction pathways of oxide catalysts and molecular sieve catalysts mainly studied by researchers will be reviewed, with a particular focus on an atomic-level understanding of the mechanism. We provide an atomic-level understanding of the current general NH3-SCR catalytic mechanism and further combine the atomic-level understanding of the mechanism to provide design guidelines for low-temperature SCR catalysts. We hope that our account will trigger research activities and discussions in NH3-SCR catalysis and bridge the material gap between idealized and real catalytic systems.