Influence of aluminium distribution on the diffusion mechanisms and pairing of [Cu(NH₃)₂]⁺ complexes in Cu-CHA

The performance of Cu-exchanged chabazite (Cu-CHA) for the ammonia-assisted selective catalytic reduction of NO_x (NH₃-SCR) depends critically on the presence of paired ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ 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 ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ complexes. Performing unbiased and constrained molecular dynamics simulations, we obtain unique information inaccessible to first-principle calculations and experiments. The free energy barrier for ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ diffusion between CHA-cages depends sensitively on both the local and distant Al-distribution. Importantly, certain Al-distributions and arrangements of neighboring ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ and ${{\mathrm{NH}}_4}^{+}$ cations make paired ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ complexes exothermic with respect to separated configurations. Our results suggest that the NH₃-SCR activity can be enhanced by increasing the Cu-loading and Al-content. The dynamic interplay between ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ and ${{\mathrm{NH}}_4}^{+}$ diffusion is crucial for the ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$ mobility and stresses the need to explore large systems including long-range Coulomb interactions when studying diffusion of charged species in zeolites.