Precise design of Cu-exchanged zeolites with diverse topologies via atomic layer deposition for boosting low-temperature NH3-SCR

Abstract

Atomic layer deposition (ALD), as an advanced grafting technique, enables precise tailoring of the active sites using self-limiting growth at atomic level. The primary difficulties faced by this metal post-synthesis method are the low metal loading and expensive precursors due to the highly developed internal micropores of zeolite as a support. Cu(hfac)2 was deposited over zeolites with different topologies in order to clarify the matching principles as well as key factors between zeolites and ALD, and the method was applied for the first time to selective catalytic reduction (SCR) of NO by NH3. It was shown that Cu loading on EMC-2 zeolite can be reached to 1.58wt% after only twice cycles, and the Cu-EMC-2 exhibits excellent performance, achieving more than 90% NO conversion at 200-550 °C. The temperature-programmed reduction by hydrogen (H2-TPR) and electron paramagnetic resonance (EPR) demonstrated that Cu-based zeolites prepared by ALD were dominated by isolated Cu+ or [Cu(OH)]+ ions, which are combined with single Al sites. Kinetic diameter computation, molecular dynamic simulation, and X-ray photoelectron spectroscopy (XPS) together indicate that the copper precursors could successfully enter the zeolites with 12-member rings (MR), while having difficulty entering the zeolites with 10-MR or 8-MR because of pore diffusion limitation, making poorly dispersed copper ions on the zeolitic surface. This study guides how to improve metal precursor utilization and clarifies that the essentials in coupling ALD with zeolites is the pore structures or membered rings.