Abstract
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 inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parameters associated with NO + NH3RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH3and gas-phase NH3. The resulting NO + NH3RHC model is validated using spatiotemporal N2measurements 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 + NH3RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measured enthalpic and entropic changes associated with O2adsorption on NH3-solvated CuI(ZCu(NH3)2) complexes [ Kamasamudram, K.et al. Catal. Today 2010, 151(3-4), 212-222 ], along with activation energies estimated computationally for the intercage diffusion of ZCu(NH3)2complexes [ Paolucci, C. et al. Science 2017, 357(6 354), 898-903 ], are incorporated into a mean field kinetic model for 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 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)4O2complexes provides accurate representations of standard SCR on reduced and oxidized catalysts, predicting transient NO and NH3consumption between 150 and 250 °C as a function of hydrothermal aging. Deactivation of low-temperature standard SCR by NH3is observed at high NH3pressures, modeled via the formation of superoxo amino (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 during low-temperature standard SCR, combining the recent kinetic, spectroscopic, and computational findings on the mechanism of standard SCR over Cu-SSZ-13.
| Original language | English |
|---|---|
| Pages (from-to) | 6418-6433 |
| Number of pages | 16 |
| Journal | ACS Catalysis |
| Volume | 12 |
| Issue number | 11 |
| DOIs | |
| State | Published - Jun 3 2022 |
Funding
This research was supported, in part, by the DOE Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) and used resources at the National Transportation Research Center, a DOE-EERE User Facility at Oak Ridge National Laboratory (ORNL). The authors thank Neal Currier, Michael Cunningham, Krishna Kamasamudram, and Aleksey Yezerets from Cummins Inc. for their valuable support and promotion of the ORNL-Cummins Catalyst CRADA (cooperative research and development agreement) partnership (No. 97-0489), within which a portion of this work was performed, and their SCR-catalyst insights. The authors thank Josh Pihl, group leader of the ORNL Applied Catalysis and Emissions Research Group, for his critical review and suggestions regarding catalysis methods and experiments and help with experimental system automation. The authors thank DOE VTO Program and Technology Managers, Gurpeet Singh, Siddiq Khan, and Ken Howden, for supporting the CRADA project. This manuscript has been co-authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The publisher acknowledges the US government license to provide public access under the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).
Keywords
- Cu-zeolites
- OHC
- RHC
- SCR mechanism
- hydrothermal aging
- microkinetic model
- redox
- selective catalytic reduction