On the Character of Active Sites in Copper Exchanged Zeolites during NH3-SCR
Nitrogen oxides (NOx) are formed during combustion in diesel engines and one current approach to reduce NOx to N2 is selective catalytic reduction with NH3 as reducing agent (NH3-SCR). To acquire both high activity and selectivity, the active site is often atomically dispersed metal atom, so-called single-site catalysts. In the case of NH3-SCR, one example is Cu-exchanged chabazite (Cu-CHA). This is a kind of ion-exchanged zeolites that over the past decade has emerged as a promising candidate for NH3-SCR thanks to its good performance over a wide temperature window (473-773 K) and a high hydrothermal stability. In this thesis, the mechanisms for solid-state ion exchange of copper into CHA and activation of oxygen during NH3-SCR in Cu-CHA have been studied through density functional theory (DFT) in combination with ab-initio thermodynamics and molecular dynamics. Firstly, the energetic conditions for functionalization of CHA with copper ions from Cu2O(111) via the formation of Cu(NH3)2+ are explored. It is found that the diamine complexes form easily on Cu2O(111) and diffuse with low barriers over the surface and inside the CHA framework. The charge neutrality of the systems is maintained via counterdiffusion of H+ in the form of NH4+ from the zeolite to the Cu2O surface where water can be formed. The efficient solvation of Cu+ and H+ by ammonia renders the ion-exchange process exothermic. The dynamic character of Cu ion sites in CHA in the presence of ammonia is also highlighted.
Secondly, the activation of oxygen, one crucial step during NH3-SCR reactions, is investigated. The character of ligand-solvated Cu ions under SCR conditions is explored and Cu(NH3)2+ is found to be the preferable species under low-temperature operation conditions (below 523 K). Direct dissociation of O2 is found to be facile over a pair of Cu(NH3)2+, whereas dissociation on a single Cu(NH3)2+ species is unlikely due to a high activation energy. Theffect of Al distribution on the stability of the pair formation in CHA is also investigated. The Al distribution is found to strongly affect the propensity to form the pairs in CHA. At temperatures above 623 K, copper sites are preferably framework-coordinated without any ligands. In this case, the NH3-SCR catalytic cycle is completed over framework-coordinated Cu sites. The character of active sites during NH3-SCR is consequently condition-dependent, where the low temperature reaction occurs on Cu-solvated species. The NH3-SCR process in Cu-CHA is consequently just at the border between homogeneous and heterogeneous catalysis.