Molecular Origins of Selectivity in the Reduction of NO_x by NH₃

The fundamental principle underlying the selective catalytic reduction (SCR) of NO_x to N₂ is the promotion of reactions of reductant with NO_x over competing, and thermodynamically preferred, reactions with a large excess of O₂. A similar competition between NO_x and O₂ exists in the noncatalytic, thermal reduction of NO_x with NH₃. In this work, density functional theory calculations are used to elucidate the origins of the remarkable selectivity in thermal deNO_x. Thermal deNO_x is initiated by the conversion of NH₃ into the active reductant, NH₂ radical. NH₂ radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H₂NNO adduct that readily rearranges and decomposes to N₂ and H₂O. In contrast, NH₂ radical reacts exceedingly slowly with O₂:  the H₂N−OO adduct is weakly bound and more readily falls apart than reacts to products. The pronounced discrimination of NH₂ against reaction with O₂ is unusual behavior for a radical but can be understood through comparison of the electronic structures of the H₂NNO and H₂NOO radical adducts. These two key elements of thermal deNO_xreductant activation and kinetic inhibition of reactions with O₂are similarly essential to successful catalytic lean NO_x reduction, and are important to consider in evaluating and modeling NO_x SCR.