Kinetic Model for the Reduction of CuIISites by NO + NH3 and Reoxidation of NH3-Solvated CuISites by O2and NO in Cu-SSZ-13

In this work, a kinetic model is developed for the reduction of CuIIsites by NO + NH3and the reoxidation of NH3-solvated CuIsites by O2and NO in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy and spatially resolved capillary inletmass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parametersassociated with NO + NH3RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH3andgas-phase NH3. The resulting NO + NH3RHC model is validated using spatiotemporal N2measurements covering a wide range oftemperatures (200-450 degrees C) and space velocities (53 000-640 000 h-1). N2O formation is observed and modeled during NO +NH3RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measuredenthalpic and entropic changes associated with O2adsorption on NH3-solvated CuI(ZCu(NH3)2) complexes [Kamasamudram,K.et al.Catal. Today2010,151(3-4), 212-222], along with activation energies estimated computationally for the intercage diffusionof ZCu(NH3)2complexes [Paolucci, C. et al.Science2017,357(6 354), 898-903], are incorporated into a meanfield kinetic modelfor the low-temperature oxidation half-cycle (OHC). Significant NH3release is observed during the isothermal oxidation of CuIsites, attributed to desorption of NH3ligands from NH3-solvated CuIIdimers (Z2Cu2(NH3)4O2). Reduction of these dimericcomplexes leads to the consumption of one NO/CuII, contradicting the expected reduction stoichiometry. Inclusion of a globalArrhenius rate for the NO titration of Z2Cu2(NH3)4O2complexes provides accurate representations of standard SCR on reducedand oxidized catalysts, predicting transient NO and NH3consumption between 150 and 250 degrees C as a function of hydrothermal aging.Deactivation of low-temperature standard SCR by NH3is observed at high NH3pressures, modeled via the formation of superoxoamino (ZCu(NH3)3OO*) complexes during NH3titration of Z2Cu2(NH3)4O2complexes [Negri, C. et al.J. Am. Chem. Soc.2020,142(37), 15884-15896]. The redox kinetic model presented here provides a foundational description of active site redox duringlow-temperature standard SCR, combining the recent kinetic, spectroscopic, and computationalfindings on the mechanism ofstandard SCR over Cu-SSZ-13.

Automated summary

In this study, a kinetic model is developed for the reduction and oxidation reactions of Cu sites in Cu-SSZ-13. The rate parameters of these reactions are investigated using Fourier transform infrared spectroscopy and spatially resolved mass spectrometry. The model is validated with experimental measurements and successfully predicts the formation and consumption of nitrogen oxides. NH3 release is observed during the oxidation reaction at low temperatures, and the presence of NH3 negatively affects the catalyst's activity. This study provides important insights into the reduction and oxidation mechanisms of Cu-SSZ-13 catalysts.