Cu/SSZ-13 zeolites prepared by in situ hydrothermal synthesis method as NH3-SCR catalysts: Influence of the Si/Al ratio on the activity and hydrothermal properties

Abstract

This study focuses on investigating the influence of Si/Al ratio on the performance of hydrothermally aged Cu/SSZ-13 catalysts prepared by in situ hydrothermal synthesis method. It was found that NH3-SCR performance and hydrothermal stability of Cu/SSZ-13 decreased with increasing Si/Al ratio. Fresh and hydrothermally aged Cu/SSZ-13 with Si/Al ratio of 6.5 showed the best SCR activity and N2 selectivity in a broad temperature of 200–500 °C, in which more than 90% NOx was reduced with superior N2 selectivity above 95%. The effect of Si/Al ratio on the properties of the physicochemical structure, the change of acidity intensity, and Cu species migration or transformation of the fresh and aged Cu/SSZ-13 catalysts were comprehensively characterized by NH3-TPO, NH3-TPD, H2-TPR, N2 physisorption, XRD, ICP-AES, and XPS. The increase of the surface CuO species, the decrease of the amount of Cu2+ in active sites, the chemisorption NH3 species, the specific surface area, the total pore volume, and the increase of average pore diameter, may be the main reasons for performance deterioration over high-Si/Al-ratio catalysts after hydrothermal aging. Furthermore, in situ DRIFTS indicate that, with the increasing Si/Al ratio, the zeolitic structure was easy to collapse, and that the surface acid sites, the active copper species, as well as the adsorbed surface nitrate, decreased after hydrothermally aged. After hydrothermal aging, the amount of isolated Cu2+ decreased, and more CuO species were aggregated, mainly coming from the conversion of the unstable isolated Cu2+ species in D6R or CHA cages. The decrease of isolated Cu2+ and CuO species aggregation were significant in Cu/SSZ-13 with high Si/Al ratios. The CuO leaded to the instability of the SSZ-13 skeleton structure and NH3 oxidation promotion, resulting in catalysts deactivation.