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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry