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Here, a comparative study is made on four pairs of Cu-LTA and Cu-SSZ-13 zeolites with the same bulk Si/Al ratio (16) but different Cu/Al ratios (0.14–0.65), in an attempt to understand the similarities and differences in the nature of their catalytically active sites for the NH3-SCR reaction. Two different types of copper ions, i.e., Cu2+ and [Cu(OH)]+, can exist in these two catalyst systems. However, their location and distribution were characterized to be quite different. Unlike the case in Cu-SSZ-13, both Cu2+ and [Cu(OH)]+ ions in Cu-LTA catalysts with Cu/Al ≤ 0.50 are predominantly located near the single 6-ring sites in the LTA structure, and copper species when over-exchanged reside within the 8-ring windows. It was also found that both Cu-SSZ-13 and Cu-LTA have nitrate and ammonium nitrate as reaction intermediates, but the relative amounts of these intermediates are always smaller in the latter catalyst system regardless of the Cu/Al ratio. This is primarily due to the preferential location of copper ions in single 6-rings that are spatially more confined than the 8-ring window sites. Consequently, Cu-LTA shows a better deNOx activity under the fast SCR conditions than Cu-SSZ-13.

Here we report an economical synthesis method for high-silica (Si/Al∼17) LTA zeolites using colloidal silica and ammonium fluoride as Si and F sources, respectively. When both 1,2-dimethyl-3-(4-methylbenzyl)imidazolium and tetramethylammonium ions are used as organic structure-directing agents, the LTA crystallization was found to strongly depend on the particle size of the colloidal silica employed. However, the type of Al and F sources had little effect on the formation of this small-pore zeolite. The catalytic properties of three high-silica Cu-LTA catalysts synthesized using the same Si source (Ludox HS-40 colloidal silica) but different Al sources (aluminum hydroxide, aluminum metal, and pseudoboehmite) for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are also presented. Despite the presence of a large amount of extraframework Al, all of the catalysts exhibited considerably higher activity maintenance than the current commercial Cu-SSZ-13 catalyst even after hydrothermal aging at 900 °C.

The catalytic properties of iron-exchanged LTA and SSZ-13 zeolites with different Si/Al (11–23) and Fe/Al (0.18–0.37) ratios hydrothermally aged at 750–900 °C have been investigated in the selective catalytic reduction of NOx by NH3 (NH3-SCR). When the zeolite support possesses the same bulk ratio (16), each fresh Fe-LTA catalyst always exhibits a higher NO conversion than the corresponding Fe-SSZ-13 catalyst with a similar Fe/Al ratio in the reaction temperature range studied. Hydrothermal aging at 900 °C led to a noticeable decrease in NH3-SCR performance of both series of catalysts. However, the extent of the decrease was found to be considerably smaller in the Fe-LTA series. This allows each of the Fe-LTA series to outperform even Fe-beta, which is known as the most hydrothermally stable among the iron-exchanged zeolite catalysts tested so far. The overall characterization results of this study reveal that the active iron species (e.g., isolated Fe3+ ions and small oligomeric FexOy species) in Fe-LTA are more stable than those in Fe-SSZ-13, mainly due to notable differences in their iron ion sites.

We have recently demonstrated that divalent copper ions exchanged into the large-pore zeolite UZM-35 with MSE topology (Cu-UZM-35) when hydrothermally aged at 750 °C exhibit a considerably wider operating temperature window for NH3-SCR than those into the small-pore zeolite SSZ-13 with CHA topology (Cu-SSZ-13), the current commercial catalyst for this reaction. Here we report the physical states and locations of copper ions in the fresh and 750 °C aged forms of Cu-UZM-35 determined using quantitative EPR measurements and synchrotron powder X-ray diffraction and Rietveld analyses. Among the four different cation sites (Cu1–Cu4) identified in fresh Cu-UZM-35, sites Cu3 and Cu4 are easily accessible via 12-ring and 10-ring channels, respectively, allowing their copper ions to serve as the main active sites for NH3-SCR. It was also found that while the Cu2+ ions at site Cu3 remain unchanged even after hydrothermal aging at 750 °C, the [Cu(OH)]+ ions at site Cu4 are transformed into CuOx and/or CuAl2O4 species...

We have recently reported an increase in low-temperature NH3–SCR activity of the copper-exchanged (Cu/Al = 0.50), high-silica (Si/Al = 23) LTA catalyst when it is hydrothermally aged at 1023–1173 K. Here we demonstrate that this unexpected phenomenon originates from the migration of Cu+ ions present inside the sod cages of Cu-LTA to the vacant single 6-rings which accompanies their oxidation to Cu2+ ions during (hydro)thermal aging. Hence, the sod cages, which are inaccessible to almost all reactant species so as to be regarded as useless for zeolite catalysis, were found to serve as a “catalyst reservoir” during the course of nitrogen oxides (NOx) reduction with NH3.

We acknowledge financial support from the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea and from Hyundai Motor Group. We also thank J. Y. Koo (POSTECH) for assistance with ESR measurements, K.-S. Lee and Y. H. Jung (8C and 9B, PAL, respectively) for help in X-ray absorption and diffraction data, S. P. Thompson (I11,DLS) for help in obtaining powder X-ray diffraction data, and PAL and DLS for beam time. PAL is supported by MSIP and POSTECH. We acknowledge financial support from the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea and from Hyundai Motor Group. We also thank J. Y. Koo (POSTECH) for assistance with ESR measurements, K.-S. Lee and Y. H. Jung (8C and 9B, PAL, respectively) for help in X-ray absorption and diffraction data, S. P. Thompson (I11,DLS) for help in obtaining powder X-ray diffraction data, and PAL and DLS for beam time. PAL is supported by MSIP and POSTECH. Diesel engine technology is still the most effective solution to meet tighter CO2 regulations in the mobility and transport sector. In implementation of fuel-efficient diesel engines, the poor thermal durability of lean nitrogen oxides (NOx) aftertreatment systems remains as one major technical hurdle. Divalent copper ions when fully exchanged into high-silica LTA zeolites are demonstrated to exhibit excellent activity maintenance for NOx reduction with NH3 under vehicle simulated conditions even after hydrothermal aging at 900 °C, a critical temperature that the current commercial Cu-SSZ-13 catalyst cannot overcome owing to thermal deactivation. Detailed structural characterizations confirm the presence of Cu2+ ions only at the center of single 6-rings that act not only as a catalytically active center, but also as a dealumination suppressor. The overall results render the copper-exchanged LTA zeolite attractive as a viable substitute for Cu-SSZ-13. Postprint Postprint

The synthesis of zeolite UZM-35 with the MSE topology and the catalytic properties of copper-exchanged UZM-35 (Cu-UZM-35) for the selective catalytic reduction of NOx with NH3 (NH3-SCR) are presented. When the simple dimethyldipropylammonium cation is used as an organic structure-directing agent together with Na+ and K+, crystallization of pure UZM-35 is very sensitive not only to the types of Al and Si sources employed, but also to the SiO2/Al2O3 and K2O/(Na2O + K2O) ratios in the synthesis mixture. In the temperature range studied, fresh Cu-UZM-35 shows comparable deNOx performance to fresh Cu-SSZ-13, the best NH3-SCR catalyst known to date. When hydrothermally aged at 750 °C, although the latter catalyst outperforms the former one, the operating temperature window of Cu-UZM-35 is considerably wider than that of Cu-SSZ-13. Like Cu-beta, Cu-UZM-35 also produces a higher amount of N2O during SCR than Cu-SSZ-13 and Cu-ZSM-5. However, it shows a significantly lower NH3 oxidation activity than Cu-SSZ-13 and Cu-ZSM-5. The overall characterization results of this study demonstrate the highly stable nature of framework Al atoms in the large-pore zeolite UZM-35 and their strong interactions with Cu2+ ions. This may lead to a moderate alteration of exchanged Cu2+ ions to CuOx and CuAl2O4 phases at high temperatures, thus rendering Cu-UZM-35 hydrothermally stable during NH3-SCR.

A series of LTA zeolites with Si/Al = 8.3-∞ and a UFI zeolite with Si/Al = 11 are synthesized using benzylimidazolium-based cations as organic structure-directing agents in fluoride media, if required, together with the tetramethylammonium ion. Two LTA zeolites with Si/Al = 11 and 16 showed an enhanced operating temperature window with excellent thermal durability for selective catalytic reduction of NOx with NH3, compared to Cu-SSZ-13 commercially being used for automotive applications.

The catalytic properties of iron-exchanged UZM-35, beta, LTA, and SSZ-13 zeolites with similar Si/Al and Fe/Al ratios (8.9–12 and 0.35-0.37, respectively) and their hydrothermally aged forms at 1023−1173 K are compared in NH3-SCR. When fresh, the low-temperature (

Ag-doped perovskite catalysts (La1−xAgxMnO3) prepared by the citric acid method were investigated for the catalytic oxidation of NO. Compared to LaMnO3, La1−xAgxMnO3 revealed a superior NO oxidation activity with its maximum activity at x = 0.2, confirming the potential of La0.8Ag0.2MnO3 as one of the most promising NO oxidation catalysts. The high reactivity of La0.8Ag0.2MnO3 is mainly due to the high oxygen vacancy concentration on its surface produced by the partial substitution of La3+ with Ag+ in LaMnO3, as determined by the XRD, SEM, XPS, O2-TPD, H2-TPR, (NO + O2)-TPRD and DRIFT studies. The mono- and bi-dentate nitrates formed on the oxygen vacancies of perovskite appear to be the primary reaction intermediates for the NO oxidation reaction over the Ag-doped perovskite catalyst. A plausible reaction pathway of the NO oxidation over La1−xAgxMnO3 has been postulated on the basis of the redox process involving the oxygen vacancy, oxygen, NO, NO2 and NO3.

This article is part of the themed collection: 2015 Journal of Materials Chemistry A Hot Papers, We have synthesized an aluminosilicate RTH-type zeolite with Si/Al = 10 using 1,2,3-trimethylimidazolium (123TMI+) as an organic structure-directing agent (OSDA) together with Na+ or K+ in hydroxide media and without the use of seed crystals. The zeolite obtained is characterized by a cuboid morphology made of very small ill-defined crystallites, largely different from the plank-like morphology typically observed for RTH-type zeolite crystals thus far. More interestingly, we show experimental evidence demonstrating that two 123TMI+ ions are located within each [46586484] cavity of the RTH framework, forming antiparallel dimers, as found by Rietveld refinement. When hydrothermally aged at 1023 K, Cu-RTH is much less active for NO reduction with NH3 than Cu-SSZ-13, the best catalyst known for this reaction to date. However, while the CO2 uptake (3.2 mmol g−1) on Na-RTH at 298 K and 1.0 bar is lower than that (4.5 mmol g−1) on zeolite Na-Rho, a well-studied small-pore zeolite that selectively adsorbs CO2, it exhibits much faster CO2 sorption kinetics. This renders our RTH zeolite potentially useful as a selective CO2 adsorbent., This work was supported by the National Creative Research Initiative Program (2012R1A3A2048833) and the BK 21-plus Program through the National Research Foundation of Korea. PAL is supported by MSIP and POSTECH.