Molecular Origins of Selectivity in the Reduction of NOx by NH₃

The fundamental principle underlying the selective catalytic reduction (SCR) of NO x to N 2 is the promotion of reactions of reductant with NO x over competing, and thermodynamically preferred, reactions with a large excess of O 2 . A similar competition between NO x and O 2 exists in the noncatalytic, thermal reduction of NO x with NH 3 . 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 NH 3 into the active reductant, NH 2 radical. NH 2 radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H 2 NNO adduct that readily rearranges and decomposes to N 2 and H 2 O. In contrast, NH 2 radical reacts exceedingly slowly with O 2: the H 2 N−OO adduct is weakly bound and more readily falls apart than reacts to products. The pronounced discrimination of NH 2 against reaction with O 2 is unusual behavior for a radical but can be understood through comparison of the electronic structures of the H 2 NNO and H 2 NOO radical adducts. These two key elements of thermal deNO x reductant activation and kinetic inhibition of reactions with O 2 are similarly essential to successful catalytic lean NO x reduction, and are important to consider in evaluating and modeling NO x SCR.