High-Temperature Reaction Mechanism of NH-SCR over Cu-CHA: One or Two Copper Ions?

Cu-exchanged chabazite (Cu-CHA) shows good performance for selective catalytic reduction of nitrogen oxides using NH as a reducing agent (NH-SCR). The temperature dependence of the activity has a characteristic nonmonotonic behavior with a minimum in the range 300-350 °C. The minimum signals that different reaction mechanisms or active sites dominate at low and high temperatures. The low-temperature mechanism is believed to occur over a pair of mobile [Cu(NH)] complexes, whereas the high-temperature mechanism should proceed over framework-bound Cu ions. To explore the NH-SCR reaction over framework-bound Cu ions, we use first-principles calculations combined with mean-field microkinetic simulations. We find that the reaction proceeds over a single framework-bound Cu ion and that the first step is NO and O coadsorption. The coadsorption competes with NH adsorption, and the NH-SCR rate is largely determined by the adsorption energy of NH. Combining the high-temperature kinetic model with our previous low-temperature model for NH-SCR over pairs of mobile [Cu(NH)] complexes makes it possible to describe the nonmonotonic behavior of the reaction rate. The work provides a detailed mechanistic understanding of the role and transformation of different forms of Cu ions during low- and high-temperature standard SCR in Cu-CHA.