Abstract
In this work, a kinetic model is developed for the reduction of CuII sites by NO + NH3 and the reoxidation of NH3-solvated CuI sites by O2 and NO in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy and spatially resolved capillary inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parameters associated with NO + NH3 RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH3 and gas-phase NH3. The resulting NO + NH3 RHC model is validated using spatiotemporal N2 measurements covering a wide range of temperatures (200–450 °C) and space velocities (53 000–640 000 h–1). N2O formation is observed and modeled during NO + NH3 RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measured enthalpic and entropic changes associated with O2 adsorption on NH3-solvated CuI (ZCu(NH3)2) complexes [Kamasamudram, K. Catal. Today 2010, 151(3–4), 212−222], along with activation energies estimated computationally for the intercage diffusion of ZCu(NH3)2 complexes [Paolucci, C. Science 2017, 357(6 354), 898−903], are incorporated into a mean field kinetic model for the low-temperature oxidation half-cycle (OHC). Significant NH3 release is observed during the isothermal oxidation of CuI sites, attributed to desorption of NH3 ligands from NH3-solvated CuII dimers (Z2Cu2(NH3)4O2). Reduction of these dimeric complexes leads to the consumption of one NO/CuII, contradicting the expected reduction stoichiometry. Inclusion of a global Arrhenius rate for the NO titration of Z2Cu2(NH3)4O2 complexes provides accurate representations of standard SCR on reduced and oxidized catalysts, predicting transient NO and NH3 consumption between 150 and 250 °C as a function of hydrothermal aging. Deactivation of low-temperature standard SCR by NH3 is observed at high NH3 pressures, modeled via the formation of superoxo amino (ZCu(NH3)3OO*) complexes during NH3 titration of Z2Cu2(NH3)4O2 complexes [Negri, C. J. Am. Chem. Soc. 2020, 142(37), 15884−15896]. The redox kinetic model presented here provides a foundational description of active site redox during low-temperature standard SCR, combining the recent kinetic, spectroscopic, and computational findings on the mechanism of standard SCR over Cu-SSZ-13.
