Abstract
The performance of Cu-exchanged chabazite (Cu-CHA) for the ammonia-assisted selective catalytic reduction of NO(x) (NH(3)-SCR) depends critically on the presence of paired [Cu(NH3)2]+ complexes. Here, a machine-learning force field augmented with long-range Coulomb interactions is developed to investigate the effect of Al-distribution and Cu-loading on the mobility and pairing of [Cu(NH3)2]+ complexes. Performing unbiased and constrained molecular dynamics simulations, we obtain unique information inaccessible to first-principle calculations and experiments. The free energy barrier for [Cu(NH3)2]+ diffusion between CHA-cages depends sensitively on both the local and distant Al-distribution. Importantly, certain Al-distributions and arrangements of neighboring [Cu(NH3)2]+ and NH4+ cations make paired [Cu(NH3)2]+ complexes exothermic with respect to separated configurations. Our results suggest that the NH(3)-SCR activity can be enhanced by increasing the Cu-loading and Al-content. The dynamic interplay between [Cu(NH3)2]+ and NH4+ diffusion is crucial for the [Cu(NH3)2]+ mobility and stresses the need to explore large systems including long-range Coulomb interactions when studying diffusion of charged species in zeolites.