Cation migration in siliceous CHA probed with DFT simulation and its catalytic implication

Abstract

The migration or diffusion of cations in siliceous CHA with an approximate Si/Al ratio of 11 has been investigated to explore the impact of their location on catalytic performance, using DFT simulation. The migration of protons within the CHA cage and the resulting total energy were calculated to determine the activation energy for this migration. As the distance between the Al site and the nearby framework oxygen decreased, the activation energy for site-to-site migration decreased as well. Eventually, the proton was stabilized near the Al site, where the activation energy for reverse migration reached its maximum at 1.60 eV. The migration of Na cations and Cu cations was also examined. The activation energy for Na cations was only 0.49 eV, which can be attributed to multiple coordination and explains the facile migration. On the other hand, Cu+2 migration had an activation energy of 1.56 eV, while Cu+1 migration had an activation energy of 1.06 eV. Consequently, under the given reaction conditions, the facile migration of the catalytic component Cu+1 can be expected. Thus, CHA containing Na cations exhibits improved low-temperature activity in NH3 SCR, despite the potential for thermal degradation at high temperatures.