A comprehensive kinetic model is developed to predict the consumption of NOx and NH3 along with the change in oxidation state of active Cu sites in Cu-SSZ-13 upon NH3 solvation, reduction in NO and/or NH3 in addition to re-oxidation in NO and/or O2. A kinetic formalism that considers several types of experimentally quantified framework aluminum sites (Z2Cu, ZCuOH, ZH and ZCu) is utilized for the SCR redox model. Transient kinetic experiments are used to identify the rate constants of reduction and oxidation half cycle (RHC and OHC) reactions. The influence of catalyst composition is studied through hydrothermally aging the commercial Cu-SSZ-13 catalyst. Experimental and modeling studies provide new insights into the nature and reactivity of active sites in Cu-SSZ-13 under operando conditions. Reduction rates of Z2Cu and nominally ZCuOH sites in NO + NH3 are shown to be identical, and second order in CuII site number densities, consistent with recent reports. An alternate pathway exists for the reduction of NH3-free CuII sites with gas-phase NH3. Enthalpic and entropic changes associated with activation of O2 on pairs of ZCu(NH3)2 complexes are consistent with microcalorimetry data and first-principles calculations. Oxidation of partially solvated and NH3-free ZCu sites under exposure to O2 or NO + O2 involves Brønsted NH3. NO oxidation is a redox reaction with NO2 production from reduction of nominally ZCuOH sites, and direct oxidation of ZCu sites. NH3 oxidation is a redox reaction with second order NH3-only RHC kinetics over nominally ZCuOH sites, and direct oxidation of ZCu sites. N2O formation under standard SCR is proposed to occur during NO + NH3 RHC. The resulting redox mechanism is validated extensively as a function of hydrothermal aging, feed concentration and temperature. Furthermore, spatially resolved N2 measurements are utilized to consolidate the proposed kinetic model. This model successfully predicts the rates and conversions associated standard SCR, NO oxidation, NH3 oxidation and N2O formation. Future work will include the role of sulfur in modifying the RHC and OHC rates under SCR conditions.

Redox model to predict standard SCR, N2O formation, NO oxidation and NH3 oxidation in Cu-SSZ-13

ACS Spring 2022

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