Enhancing Atomic Layer Deposition Efficiency of the Classical Medium-Pore ZSM-5 Zeolite by an Artful Alkali-Etching Technique for Boosting NH ₃ -SCR Behaviors

Atomic layer deposition (ALD), as a novel metal-loading approach, allows the customization of interesting metal active sites or precise control of the surface chemistry. The diffusion limitation of pore structures prevents the efficient deposition of relatively large organometallic precursors on medium/small-pore zeolites. A rich variety of etching strategies are proposed to facilitate deep deposition of reactive precursors. The Cu-ZSM-5(Na₂CO₃) catalyst showed a 2.3-fold enhancement in Cu loading compared with parent zeolite and more copper active sites at the same Cu-ALD cycles. The Cu-ZSM-5(Na₂CO₃) achieved the most excellent low-temperature <i>de</i>NO_<i>x</i> performance and was accompanied by a wide temperature window, announcing the compatibility of medium-pore zeolite with the ALD technique. It was found that the diffusion of pore structures is the key to the coupling of ALD with zeolites when Si/Al is lower than 20, and the insufficient framework Al (FAL) is fatal for copper(II) hexafluoroacetylacetonate [Cu(hfac)₂] deposition when Si/Al is higher than 20. Density functional theory (DFT) calculations verified that Cu(hfac)₂ prefers to be deposited on FAL sites at the intersectional channels and the extra-framework Al is detrimental to copper deposition. DFT simulation also confirmed that the [Cu(OH)]⁺ ions have the lowest energy barrier in the rate-controlling step of redox pathways.