Molecular origins of selectivity in the reduction of NO _x by NH ₃

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 ₂NNO adduct that readily rearranges and decomposes to N ₂ and H ₂O. In contrast, NH ₂ radical reacts exceedingly slowly with O ₂: the H ₂N-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 ₂NNO and H ₂NOO 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.