Precise control of post-treatment significantly increases hydrothermal stability of in-situ synthesized cu-zeolites for NH3-SCR reaction

Abstract

The origin of the beneficial effects of post-treatment processing with HNO3 and NH4NO3 solutions on the catalytic performance of copper-amine templated zeolites used in the NH3-SCR reaction was investigated. By careful design and optimization of the dual-treatment procedure (HNO3-NH4NO3), Cu-SSZ-13 catalysts were obtained using Cu-TEPA as a template that exhibited NOx conversion above 90% in a wide temperature window from 250 to 450 ℃ with GHSV of ∼ 400,000 h−1, even after hydrothermal aging at 750 and 800 ℃. The results of XRD and H2-TPR indicated that the HNO3 post-treatment adjusts the zeolite crystallinity and optimizes the copper species distribution in Cu-SSZ-13 catalysts. Further treatment with NH4NO3 reduces the Cu/Al ratios effectively and avoids the accumulation of Cu2+ ions to form CuOx clusters during hydrothermal aging. We investigated the influence of Cu2+ on the hydrothermal stability of Cu-SSZ-13, and two opposing effects were found. Cu2+ ions inhibit dealumination of the SSZ-13 zeolite structure, while excessive quantities of them easily accumulate to form CuOx clusters, leading to collapse of long-range order in the zeolite structure during hydrothermal aging. Before the zeolite structure collapses, Cu-SSZ-13 catalysts with high Cu loading exhibit higher NH3-SCR activity due to the preservation of more active Cu2+. However, Cu-SSZ-13 catalysts totally lose deNOx activity in NH3-SCR once the zeolite structure collapses. To guarantee both the activity for NOx reduction and hydrothermal tolerance, the optimal Cu loading (Cu/Al = 0.22∼0.31, here) and structural stability of in-situ synthesized Cu-SSZ-13 should be carefully tuned by HNO3 and NH4NO3 treatment.