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Figure 8. NH3-SCR over Cu-Exchanged CHA-Type Zeolite NH3-SCR over Cu-exchanged CHA-type zeolite involves a reduction half-cycle and an oxidation half-cycle. studies utilized the state-of-the-art operando characterization and computational techniques to investigate the NH3-SCR process. For instance, the temperature- dependence of NH3-SCR reactions and the atomic-scale behavior of Cu active sites in the 150C–400C temperature range were studied via operando XAS/XES measurement;57 [CuI(NH3)2]+O2[CuI(NH3)2]+ intermediate was identified to play an important role in the reaction kinetics of low-temperature NH3-SCR;58 mobilized Cu ions were found to travel through CHA windows and form dynamic ion pairs that participated in a CuI / CuII redox step.56 These theoretical and experimental studies have provided important clues for the discovery of new NH3-SCR catalysts with improved performance. Recently, Cu-exchanged high-silica LTA zeolites were reported to be a promising alternative to commercialized Cu-SSZ-13.59,60 Cu-LTA exhibited excellent NH3-SCR activity under simulated vehicle conditions even after hydrothermal aging at 900C, a critical temperature that current commercial Cu-SSZ-13 catalyst could not overcome. Such superior performance was attributed to the Cu2+ ions bound at the center of 6-rings in Cu-LTA, which acted not only as a catalytically active center but also as a dealumination suppressor. Another example is the MSE-type large-pore zeolite Cu-UZM-35, which was reported to possess a considerably wider operating temperature window after aging at 750C than Cu-SSZ-13.61 Its high-temperature stability was attributed to the formation of CuOx and CuAl2O4 phases during NH3-SCR. NH3 is another N-containing air pollutant mainly present in coking wastewater streams. Metal-exchanged zeolite catalysts can be used for NH3 elimination via selective catalytic oxidation of NH3 into N2 (NH3-SCO: NH3 + O2 / N2 + H2O).53 For instance, many transition-metal (Cr, Mn, Fe, Co, Ni, Cu) exchanged zeolites ZSM-5 were found to be active for NH3-SCO.62 Among them, Fe-ZSM-5 exhibited the best performance, achieving 99% NH3 conversion and 100% N2 selectivity at 450C. More importantly, Fe-ZSM-5 maintained a high NH3 conversion of 94% at 500C in the presence of water vapor and a small amount of SO2, which were often found in waste streams. The high activity of Fe-ZSM-5 was attributed to the variable valences of iron (FeIII and FeII), favoring oxygen adsorption and activation during NH3-SCO. These results indicated that Fe-ZSM-5 might be a practical catalyst for NH3-SCO in a wet stream. Noble-metal-modified zeolites are another important group of NH3-SCO catalysts. For instance, zeolites H-Y loaded with 0.05–2.5 wt % Pd were reported to exhibit high N2 selectivity (>90%) at 100% NH3 conversion in the temperature range of 250C–450C.63 In particular, H-Y loaded with 2.5 wt % 942 Chem 3, 928–949, December 14, 2017PDF Image | Applications of Zeolites in Sustainable Chemistry
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