Search Results (6)

Gas turbines operate at exhaust gas temperatures exceeding 500 °C. Vanadium-based catalysts encounter challenges in NH₃-SCR denitrification due to vanadium volatilization and titanium dioxide support phase transition at high temperatures. This restricts the effective denitrification temperature range to 300~400 °C, falling short of gas turbine denitrification requirements. Zeolite-supported catalysts, known for their high specific surface area, abundant acid sites, and stable framework structure, demonstrate superior catalytic activity and hydrothermal stability at high temperatures. This review synthesizes recent advancements in high-temperature catalysts utilizing ZSM-5, Beta, SSZ-13, and SAPO-34 zeolites as supports. It elucidates the interaction mechanisms between active components (e.g., transition metals Fe, Cu, W, rare earth elements) and zeolite supports. Furthermore, it examines variations in denitrification performance through the lens of the high-temperature NH₃-SCR reaction mechanism, offering valuable insights for high-temperature denitrification catalyst development. Full article

Cu-exchanged small-pore zeolites have been extensively studied in the past decade as state-of-the-art selective catalytic reduction (SCR) catalysts for diesel engine exhaust NOx abatement for the transportation industry. During this time, Fe-exchanged small-pore zeolites, e.g., Fe/SSZ-13, Fe/SAPO-34, Fe/SSZ-39 and high-silica Fe/LTA, have also been investigated but much less extensively. In comparison to their Cu-exchanged counterparts, such Fe/zeolite catalysts display inferior low-temperature activities, but improved stability and high-temperature SCR selectivities. Such characteristics entitle these catalysts to be considered as key components of highly efficient emission control systems to improve the overall catalyst performance. In this short review, recent studies on Fe-exchanged small-pore zeolite SCR catalysts are summarized, including (1) the synthesis of small-pore Fe/zeolites; (2) nature of the SCR active Fe species in these catalysts as determined by experimental and theoretical approaches, including Fe species transformation during hydrothermal aging; (3) SCR reactions and structure-function correlations; and (4) a few aspects on industrial applications. Full article

Highly active metal chlorides grafted on silicoaluminophosphate number 5, MCl_x/SAPO-5 (M = Cu, Co, Sn, Fe and Zn) catalysts via simple grafting of respective metal chlorides (MCl_x) onto SAPO-5 are reported. The study shows that thermochemical treatment after grafting is essential to ensure the formation of chemical bondings between MCl_x and SAPO-5. In addition, the microscopy, XRD and nitrogen adsorption analyses reveal the homogeneous distribution of MCl_x species on the SAPO-5 surface. Furthermore, the elemental microanalysis confirms the formation of Si–O–M covalent bonds in ZnCl_x/SAPO-5, SnCl_x/SAPO-5 and FeCl_x/SAPO-5 whereas only dative bondings are formed in CoCl_x/SAPO-5 and CuCl_x/SAPO-5. The acidity of MCl_x/SAPO-5 is also affected by the type of metal chloride grafted. Thus, their catalytic behavior is evaluated in the acid-catalyzed acylation of 2-methylfuran under novel non-microwave instant heating conditions (90–110 °C, 0–20 min). ZnCl_x/SAPO-5, which has the largest amount of acidity (mainly Lewis acid sites), exhibits the best catalytic performance (94.5% conversion, 100% selective to 2-acetyl-5-methylfuran) among the MCl_x/SAPO-5 solids. Furthermore, the MCl_x/SAPO-5 solids, particularly SnCl_x/SAPO-5, FeCl_x/SAPO-5 and ZnCl_x/SAPO-5, also show more superior catalytic performance than common homogeneous acid catalysts (H₂SO₄, HNO₃, CH₃COOH, FeCl₃, ZnCl₂) with higher reactant conversion and catalyst reusability, thus offering a promising alternative for the replacement of hazardous homogeneous catalysts in Friedel–Crafts reactions. Full article

SAPO-34 was prepared with a mixture of three templates containing triethylamine, tetraethylammonium hydroxide, and morpholine, which leads to unique properties for support and production cost reduction. Meanwhile, Cu/SAPO-34, Fe/SAPO-34, and Cu-Fe/SAPO-34 were prepared through the ion-exchanged method in aqueous solution and used for selective catalytic reduction (SCR) of NO_x with NH₃. The physical structure and original crystal of SAPO-34 are maintained in the catalysts. Cu-Fe/SAPO-34 catalysts exhibit high NO_x conversion in a broad temperature window, even in the presence of H₂O. The physicochemical properties of synthesized samples were further characterized by various methods, including XRD, FE-SEM, EDS, N₂ adsorption-desorption isotherms, UV-Vis-DRS spectroscopy, NH₃-TPD, H₂-TPR, and EPR. The best catalyst, 3Cu-1Fe/SAPO-34 exhibited high NO_x conversion (> 90%) in a wide temperature window of 250–600 °C, even in the presence of H₂O. In comparison with mono-metallic samples, the 3Cu-1Fe/SAPO-34 catalyst had more isolated Cu²⁺ ions and additional oligomeric Fe³⁺ active sites, which mainly contributed to the higher capacity of NH₃ and NO_x adsorption by the enhancement of the number of acid sites as well as its greater reducibility. Therefore, this synergistic effect between iron and copper in the 3Cu-1Fe/SAPO-34 catalyst prompted higher catalytic performance in more extensive temperature as well as hydrothermal stability after iron incorporation. Full article

Cu-containing CHA type (Cu-CHA) zeolites have been widely investigated owing to their excellent low-temperature activity and high hydrothermal stability in selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Herein, a series of Cu-SAPO-44 zeolites were prepared by one-pot method with dual-amine templates and the subsequent ion exchange (IE) with NH₄NO₃. The effect of NH₄NO₃ treatment on Cu species was investigated by X-ray powder diffraction (XRD), N₂ adsorption-desorption isotherm, inductively coupled plasma (ICP); field-emission scanning electron microscope (FE-SEM), high-resolution transmission electron microscope (HRTEM), X-ray absorption fine structure (XAFS), and H₂-temperature-programmed reduction (H₂-TPR). The results indicated that—besides the main SAPO-44 structure—the CuO phase was detected by XRD in original samples. After IE with NH₄NO₃, the Cu contents decreased greatly from ICP analysis. The removal of CuO agglomerations and the presence of highly dispersed CuO nanoparticles (~2.36 nm) were confirmed by SEM, TEM and H₂-TPR. Furthermore, a significant increase in the proportion of isolated Cu²⁺ was derived from XAFS. As a result, the activity at higher temperature (≥350 °C) was improved a lot. Full article

The adsorption of NO, NH₃, H₂O, and SO₂ gaseous molecules on different transition metal oxides was studied based on density function theory (DFT), and three better-performing transition metal elements (Fe, Co, and Ce) were selected. Cu–Mn/SAPO-34 catalysts were prepared by impregnation method and then modified by the selected transition metals (Fe, Co, and Ce); the SO₂ resistance experiments and characterizations including Brunner−Emmet−Teller (BET), X-ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), and thermal gravity analysis (TG)-differential thermal gravity (DTG) before and after SO₂ poisoning were conducted. The results showed that the deactivation of the Cu–Mn/SAPO-34 catalyst is ascribed to the deposition of lots of ammonium sulfates on the surface, depositing on the active sites and inhibiting the adsorption of NH₃. After the modification of Fe, Co, and Ce oxides, the SO₂ resistance of the modified Cu–Mn/SAPO-34 catalyst was significantly enhanced due to the less formation of ammonium sulfates. Among all these modified Cu–Mn/SAPO-34 catalysts, the Cu–Mn–Ce/SAPO-34 exhibited the highest SO₂ resistance owing to the decreased decomposition temperature and the trapper of ceria for capturing SO₂ to form Ce(SO₄)₂, further inhibiting the deposition of ammonium sulfates. Full article