First-Principles Investigation on the Effect of Al and Cu Density on the Mobility of CuI Ions in Cu-CHA Zeolites for NH3-SCR

Selective Catalytic Reduction of NO_x by NH₃, also known as NH₃-SCR, is a critical technology for mitigating NO_x emissions from heavy-duty diesel engines. Cu-CHA zeolites demonstrate superior performance in NH₃-SCR, catalyzing the reaction through a redox mechanism involving both reduction and oxidation half cycles. The oxidation half cycle entails the pairing of two Cu^I(NH₃)₂ ions, activating an oxygen molecule to form Cu dimer species—the key active sites for low-temperature NH₃-SCR. The kinetics of the oxidation half cycle are governed by the mobility and pairability of Cu^I(NH₃)₂ ions through 8R windows of CHA zeolites, influenced by the aluminum distribution. In this study, we utilize ab-initio molecular dynamics and metadynamics sampling to compare the mobility of local and remote diffusion of Cu^I(NH₃)₂ ions from their respective charge-compensating aluminum atoms. Recent experimental observations have revealed that higher aluminum and copper densities lead to higher rate constants for the oxidation half cycle. To rationalize these experimental findings, we also model the local diffusion of Cu^I(NH₃)₂ ions in the presence of additional NH4⁺ and Cu^I(NH₃)₂ ions. Our investigation shows that the presence of ammonium or copper cations does not significantly affect the local diffusion barriers. This lack of effect is attributed to the choice of the Al T-site within the chabazite cage, which governs the distance between the diffusing copper ion and the co-occupying cation. Additionally, we observe that local diffusion exhibits significantly lower energy barriers than remote diffusion, consistent with prior studies. These calculations provide evidence that the T-site location for the co-occupying ion in the CHA cage plays a pivotal role in the diffusion process, as outlying ions do not exhibit meaningful electrostatic interactions with the copper ion. Future work will investigate other T-sites closer to the T-site of the diffusing Cu^I(NH₃)₂ ion.