Operando infrared (IR) spectroscopy and density functional theory (DFT) calculations were combined to investigate the selective catalytic reduction (SCR) of NOx by NH3 over H-AFX zeolites. The steady-state kinetics shows that SCR reactions involving NO2 proceed much more rapidly than those of NO. Data from in situ IR combined with on-line mass spectrometry under transient conditions demonstrate that Brønsted acid sites (BASs) promote the reaction of NO2 with NH3 to form N2, H2O and NH4NO3 at low temperatures (50-150 °C). Combined with DFT results, these data suggest that NO promotes the reduction of NH4NO3 to NH4NO2, which then decomposes into N2 and H2O. Therefore, the accumulation of NH4NO3 in the zeolite is inhibited by NO. Furthermore, when NO is absent, NH4NO3 decomposition into N2O and H2O occurs only at high temperatures (> 200 °C). A comparison of H-AFX and Cu-AFX implies that Cu sites are not active for the reduction of NO2 by NH3, and that BASs are responsible for the NH3-SCR reactions involving NO2.

Synthesis of new zeolites with controlled pore architectures is important in the field of catalysis and separation related to chemical transformation, environmental protection, and energy-saving. Zeolites containing channels of different sizes in the same framework have been desirable. We report here the synthesis and structure of a novel aluminosilicate zeolite (designated as YNU-5), the first zeolite containing interconnected 12-, 12-, and 8-ring pores, as well as independent straight 8-ring channels. The synthesis procedure is quite simple and consists of conventional hydrothermal conditions as well as readily available starting materials. The framework structure is stable enough and Si/Al ratio is controllable between 9 and 350. Determination of the crystal structure is performed by utilizing X-ray diffraction-based techniques, revealing 9 independent tetrahedrally coordinated atoms. This robust structure is expected to be industrially valuable and several unusual combinations of composite building units are of considerable interest in an academic sense. The new zeolite YNU-5 is promising catalyst for the production of useful light olefins such as propylene and butylenes in the dimethyl ether-to-olefin reaction, when the Si/Al ratio is properly tuned by dealumination through simple acid treatments.

Reduction/oxidation half-cycles of the selective catalytic reduction of NO with NH 3 (NH 3 -SCR) at 200 °C were investigated using in situ and operando spectroscopies to propose a general mechanism for four different catalysts (TiO 2 -supported and bulk vanadium oxides and Cu-AFX and Cu-CHA zeolites). The reduction half-cycle is initiated by the reaction of NH 3 on Lewis acid sites [V(V) or Cu(II); L-NH 3 ] and NO, followed by the gradual reaction of NH 3 on Brønsted acid sites (B-NH 3 ) and NO; this results in the formation of V(IV) or Cu(I) and protons (H + ) on the surface, along with N 2 and H 2 O. The UV−vis measurements for the reduction half-cycle indicate that N 2 is continuously generated until the surface V(V) or Cu(II) species is depleted. The subsequent reoxidation of the reduced catalysts under O 2 leads to the regeneration of V(V) or Cu(II) and the reaction of surface H + , yielding H 2 O (oxidation half-cycle). The higher consumption rates of B-NH 3 and L-NH 3 under NO + O 2 than those under NO, which has been previously reported in the literature, were explained based on the continuous reduction/oxidation of V(V)/ V(IV) or Cu(II)/Cu(I) where the regenerated V(V) or Cu(II) site is reused in the subsequent (second) reduction half-cycle. Namely, upon the recovery of V(V) or Cu(II) via reoxidation, the leftover B-NH 3 species react with the supplied NO to yield N 2 ; this suggests that B-NH 3 is not a spectator but a reservoir of NH 3 to participate in the second reduction half-cycle possibly via the migration of NH 3 or HONO species. These results provide comprehensive evidence of the general mechanism of NH 3 -SCR, which was found to be applicable to both V and Cu catalysts.

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