New operando method for quantifying the relative half-cycle rates of the NO SCR redox cycle over Cu-exchanged zeolites

An operando experimental method for monitoring relative variations in half-cycle rates of the Cu redox-cycle model of selective-catalytic-reduction (SCR) is demonstrated, along with a kinetic model for predicting the same. Conversion inflections (CI) can occur at SCR onset within Cu/SCR catalysts; transient CI involves initial conversion growth to an intermediate value greater than exists at steady state (SS) before degrading to the SS value. While similar CI has been reported for Fe/SCR catalysts as being due to NH3 inhibition, Cu/SCR CI is inconsistent with a similar origin. Rather, Cu/SCR CI is due to imbalances in the redox-half-cycle rates, and will occur when reduction is faster than oxidation. The temporal CI shape varies with the redox-cycle kinetic parameters and model formulation, and can be used to study the same. We demonstrate the first measurements of dynamic half cycle rate balancing at SCR onset, use CI and onset transients to study variations in half-cycle rates under Standard, Fast and NO2 SCR, and present the first redox-cycle model showing CI. The new operando method and model provides a basis for better understanding the SCR redox process, and determining the individual half cycles kinetic model formulation and parameters.