N–H bond activation in ammonia by TM-SSZ-13 (Fe, Co, Ni and Cu) zeolites: a first-principles calculation

Abstract

N–H activation is crucial for NH₃-assisted selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) of NH₃ in exhaust emission treatment of the diesel engine. Copper-exchanged CHA-type zeolites show high performance for N–H activation, however, the atomic-level understanding of the activation mechanism is inaccessible. In this work, we use density functional theory (DFT) calculations to investigate the origin of high activity through electronic structures, magnetic properties and the N–H bond of NH₃ activation on the naked Cu²⁺ as the active site in Cu-SSZ-13. Fe, Co and Ni-SSZ-13 were also systematically studied for comparison. The incorporation of transition metal ions leads to high-spin d-states around the Fermi level. The magnetic moment of 3.858, 2.889, 1.920 and 0.946 μ_B is observed for Fe, Co, Ni, and Cu-SSZ-13, respectively. A distorted square planar crystal field is found. For Cu-SSZ-13, the electronic configuration of Cu²⁺ ensures the lowest unoccupancy of b_1g (d_x²−y²), enabling to receive electrons from base NH₃ and thus activate the N–H bond with a barrier of 1.26 eV significantly lower than that in Fe, Co and Ni-SSZ-13. These findings may provide fundamental insights into the high activity and selectivity of Cu-SSZ-13 catalysts for NO_x removal and NH₃ oxidation at high temperature.