High-Temperature Reaction Mechanism of NH<sub>3</sub>-SCR over Cu-CHA: One or Two Copper Ions?
Yingxin Feng, Ton V. W. Janssens, Peter N. R. Vennestrøm, Jonas Jansson, Magnus Skoglundh, Henrik Grönbeck
Abstract
High Resolution Image Download MS PowerPoint Slide Cu-exchanged chabazite (Cu-CHA) shows good performance for selective catalytic reduction of nitrogen oxides using NH 3 as a reducing agent (NH 3 -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 3 ) 2 ] + complexes, whereas the high-temperature mechanism should proceed over framework-bound Cu ions. To explore the NH 3 -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 2 coadsorption. The coadsorption competes with NH 3 adsorption, and the NH 3 -SCR rate is largely determined by the adsorption energy of NH 3 . Combining the high-temperature kinetic model with our previous low-temperature model for NH 3 -SCR over pairs of mobile [Cu(NH 3 ) 2 ] + 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.