Kinetic Monte Carlo Simulations of Low-Temperature NH-SCR over Cu-Exchanged Chabazite.

Cu-exchanged chabazite (Cu-CHA) is widely used for ammonia assisted selective catalytic reduction of nitrogen oxides (NH-SCR). The Cu ions are at low temperatures solvated by NH forming mobile [Cu(NH)] complexes. The dynamic behaviour of the complexes is critical as O adsorption requires a pair of complexes to form a [Cu(NH)O] peroxo-species over which NO couples with NH. Here we introduce a first principles-based kinetic Monte Carlo approach to explore the effect of the Al-distribution on the reaction kinetics of NH-SCR over Cu-CHA. The method allows us to scrutinize the interplay between the pairing of [Cu(NH)] complexes and the reaction landscape for the NH-SCR reaction over the peroxo-complex. The Al-distribution affects the stability of the [Cu(NH)] pairs as well as the kinetic parameters of the SCR-reaction. The turn-over frequency is determined by the stability of the [Cu(NH)] pairs and the relative strength of NO and NH adsorption once a pair is present. The results establish the hierarchy of effects that influences the performance of Cu-CHA over NH-SCR and provide a computational basis for further development of the Cu-CHA material.