Catalytic site requirements for N₂O decomposition on Cu-, Co-, and Fe-SSZ-13 zeolites

N₂O decomposition is investigated on Cu, Co and Fe-exchanged SSZ-13 zeolite catalysts at relatively low metal loadings. The catalysts are synthesized by solution ion exchange, and subjected to X-ray diffraction (XRD), temperature-programed-reduction by H₂ (H₂-TPR), temperature-programed-reaction of N₂O (N₂O-TPR) coupled with in-situ transmission FTIR, and finally steady-state flow reaction tests. At low N₂O pressures (<0.05 kPa), all catalysts display pseudo first-order kinetics. From Arrhenius analysis, Cu and Fe-SSZ-13 display very different apparent activation energies but similar pre-exponential factors, suggesting their similar reaction mechanisms. N₂O decomposition follows a dual-site mechanism, occurring on dimeric M-O-M sites in these catalysts, and O₂ is formed by the combination of two O ad-atoms from two vicinal metal sites. Under low N₂O pressure (0.05 kPa) and first-order kinetic regime, the reaction is limited by N-O cleavage on bare metal active sites. In comparison to Cu-SSZ-13, the much higher N₂O decomposition rate over Fe-SSZ-13 is attributed to the much lower activation barriers for the N-O cleavage step. N₂O decomposition occurs on isolated Co²⁺ ions in Co-SSZ-13. The rate-limiting step is N-O cleavage on an O-occupied Co site in the low-pressure first order kinetic regime. This single-site mechanism leads to much higher pre-exponential factors as compared to the dual-site mechanism. This beneficial factor for reaction rate enhancement, however, is compromised by the much higher activation barriers over this catalyst.