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[Display omitted] • Microfibrous-structured Cu-ZSM-5/Al-fiber catalysts with different Si/Al molar ratios were synthesized for SCR reaction. • Adsorbed bridged nitrates species is in accordance with the enhanced NO degradation activity. • Microstructured Cu-ZSM-5 catalysts are more active at high-throughout conditions due to the enhanced mass transfer. Selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) is one of the most efficient technologies to remove NO x emissions. The copper ion-exchanged zeolites with broad temperature windows and high N 2 selectivity are very promising for this process. In this work, we synthesized a series of microfibrous-structured Cu-ZSM-5/Al-fiber catalysts for SCR reaction by changing the Si/Al molar ratios. The in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NH 3 adsorption results show that the Cu-ZSM-5/Al-fiber catalysts with higher Si/Al molar ratios have higher NH 3 adsorption capacity. The subsequent NO adsorption results show that the amount of adsorbed bridged nitrates species is in accordance with the enhanced NO x degradation activity. At moderate temperature, Cu-ZSM-5/Al-fiber catalyst shows better NH 3 -SCR performance under higher gas hourly space velocity (GHSV), indicating microstructured Cu-ZSM-5 catalysts are more active at high-throughout conditions due to the enhanced mass transfer. [ABSTRACT FROM AUTHOR]

• Intracrystalline and effective diffusivity of NO and NH 3 within Cu-ZSM-5. • Validation of equation for estimation of NO diffusivity within Cu-ZSM-5. • Kinetic study for NH 3 -SCR of NO by Cu-ZSM-5 with different crystal size. • NO diffusion rate control overall reaction rate of NO reduction with NH 3. Micropore diffusivities of NO and NH 3 were experimentally measured for Cu-ZSM-5 using constant volumetric method at a temperature range from 373 K to 423 K. Intracrystalline diffusivities and effective diffusivities of NO were lower than those of NH 3 , and the activation energy for intracrystalline diffusivity of NO (21 kJ mol−1) was higher than that of NH 3 (6.3 kJ mol−1). The intracrystalline diffusivity of NO in Cu-ZSM-5 catalysts at 373 K was independent of Cu content and Si/Al ratio of Cu-ZSM-5. We proposed an equation for calculation of effective diffusivity of NO in Cu-ZSM-5 which was validated by the experimental data. Kinetic studies for NH 3 -SCR and NH 3 oxidation were carried out using Cu-ZSM-5 with different crystal thickness. The apparent reaction rate of NH 3 -SCR at 523 K depended on the zeolite crystal thickness; the rates by a small Cu-ZSM-5 (0.088 μm) was 2.75 times higher than that by a large crystal catalyst (2.7 μm), corresponding to the effectiveness factor of 0.36. The apparent reaction rate of NH 3 oxidation did not depend on the zeolite crystal size. It is concluded that NH 3 -SCR is controlled by intracrystalline diffusion of NO, or in other words, the active sites in zeolite crystal are not fully utilized due to the NO diffusion limitation. The effective diffusivity of NO in Cu-ZSM-5 estimated by the catalytic study was close to that calculated using the NO diffusivity results, which supported our conclusion. [ABSTRACT FROM AUTHOR]

• A model for NH 3 -SCR based on kinetics and computational fluid dynamics (CFD). • Model validation with SCR by monolithic Cu-ZSM-5. • Inhibition of effect of hydrocarbons in engine emission tests. A simple kinetic model for the standard NH 3 -SCR reaction by Cu-ZSM-5 catalysts has been developed by assuming three reaction steps: NH 3 adsorption (desorption), reactions of adsorbed NH 3 with O 2 (NH 3 oxidation) and NO (NH 3 -SCR). The model is based on Arrhenius parameters for NH 3 -SCR and NH 3 oxidation by a powder catalyst, combined with models for heat and mass transport and parameters of monolithic catalysts. The model is validated with the experimental results, not included in the estimation of the model, for NH 3 -SCR by monolithic catalysts with different parameters (catalyst loading and cell density). NOx removal from diesel exhaust, generated by engine bench, was also carried out by using monolithic Cu-ZSM-5 and Cu-AFX catalysts. The results suggest the poisoning effect of hydrocarbons in the exhaust emissions on NOx conversion is more significant for Cu-ZSM-5 than Cu-AFX. [ABSTRACT FROM AUTHOR]

• Cu-exchanged zeolites achieving both high C 2 H 3 CN conversion and N 2 selectivity. • Oxidation/hydrolysis mechanism was proposed on experimental and theoretical approaches. • Change transfer during nitrile elimination was analyzed by Mulliken method. A series of Cu (5 wt%) modified microporous zeolites (ZSM-5, Beta, MCM-22, MCM-49, and Y) was investigated for selective catalytic combustion of acrylonitrile (C 2 H 3 CN-SCC), associated with N 2 yield order of Cu-ZSM-5 (SiO 2 /Al 2 O 3 = 26) > Cu-MCM-49 > Cu-MCM-22 > Cu-Beta > Cu-Y. Various characterizations of XRD, N 2 adsorption, H 2 -TPR, NH 3 -TPD and XPS were employed to reveal structure-activity relationship. It was suggested that (i) isolated Cu cation acting as active center played a major role in C 2 H 3 CN-SCC; (ii) lowering SiO 2 /Al 2 O 3 ratio could enhance ion-exchanging capacity and thereby improve the zeolitic catalytic behavior; (iii) Cu-ZSM-5 (SiO 2 /Al 2 O 3 = 26) possessed the largest amounts of Cu cations as well as the highest Cu2+ → Cu+ redox ability, being responsible for its excellent catalytic behavior. C 2 H 3 CN-SCC mechanism over Cu-ZSM-5 was further investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) and density functional theory (DFT) calculation. C 2 H 3 CN-SCC mainly followed an oxidation mechanism via intermediate of NCO in absence of H 2 O; however, after an introduction of H 2 O hydrolysis mechanism was dominated involving NH 3 intermediate. DFT simulation majorly focusing on NCO formation mechanism reveals that C 2 H 3 CN could be initially adsorbed on active center Cu of Cu-ZSM-5 through its N, associate with atomic C of CN being adsorbed with a dissociated atomic O. Such kind of adsorption mode greatly favored C 2 H 3 CN bond fracture to produce NCO, of which the energy barrier was calculated to be 17.0 kcal mol−1. Mulliken charge transfer (CT) analysis was further conducted, based on which the electronic properties of active center Cu, zeolite framework and dissociated atomic O were well illustrated. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • La doping effectively improves the dispersion of the Cu species. • Ni doping benefits the formation of the (Cu2+-O2−-Cu2+)2+. • Co-doping with La and Ni shows synergistic effects of the two promoters. • La-Ni-Cu-ZSM-5 shows a higher activity than that in the single doped sample. Abstract A number of M-Cu-ZSM-5 (M = La, Ni, La-Ni) samples were prepared via an aqueous solution ion-exchange method. The co-doping with Ni and La into a Cu-ZSM-5 catalyst enhanced the NO decomposition activity in a temperature range 350–550 °C. The characterization results indicate that doping with La effectively improved the dispersion of the Cu species forming more Cu2+ ions in the sample, while the effect of Ni is likely to facilitate the implantation of Cu2+ to form (Cu2+-O2–-Cu2+)2+ dimers. The synergistic effect of La and Ni contributes to the increased (Cu2+-O2−-Cu2+)2+ content in the co-doped sample, thus enhancing the NO decomposition activity. [ABSTRACT FROM AUTHOR]

A series of metal modified zeolite including Cu-(Beta, FER, MCM-22, MCM-49, MOR) and M-(M = Cu, Co, Fe, Mn, Ni)-ZSM-5 was prepared by impregnation method and characterized by BET, XRD, and H 2 -TPR. HCN-SCC (selective catalytic combustion) activity measurement results suggest that Cu-ZSM-5 exhibiting excellent HCN conversion activity (T 90 = 350 °C) and N 2 yield (>95% at T > 350 °C) constitutes one kind of promising candidate for HCN catalytic abatement. The HCN-SCC mechanism was further investigated by density functional theory (DFT) over constructed 20T-Cu-ZSM-5 models, which suggests that: (i) the 20T-Cu-ZSM-5 containing double active [Cu] + site was much more favorable than that of 20T-Cu-ZSM-5 with single active [Cu] + site during NCO generation step, being mainly related to the cooperation effect of double active [Cu] + site; (ii) NCO + NO → N 2 + CO 2 was confirmed to the one kind of probable reaction pathway for N 2 and CO 2 generations, wherein the NO could be produced through oxidation of NCO; (iii) the DFT energy calculation results as well as the microkinetic analyses suggested that the NCO formation step possessing the highest energy barrier (10.4 kcal mol −1 ) was proposed to be the rate determining step of the whole catalytic cycle. [ABSTRACT FROM AUTHOR]

Highlights: [•] Metal foams of enhanced mass and heat transfer properties. [•] Metal foams generates low pressure drop and offer considerable reduction of a reactor size. [•] Cu-ZSM-5 directly synthesized on metal foams active in NH3-SCR reaction. [•] Short channel structures as an alternative for honeycomb monoliths and packed bed of spheres. [ABSTRACT FROM AUTHOR]

Abstract: The present work studies RedOx properties and reactivity in N2O decomposition and selective catalytic reduction of NO (NO-SCR) by propane of Cu-modified ZSM-5 catalysts prepared by hydrolytic polycondensation (HPC). Their characteristics are compared with properties of Cu-ZSM-5 samples having the same copper content (1wt.%) and produced by ion exchange (IEX) with water–ammonia solutions of copper chloride at NH4OH/Cu2+ equal to 6–30. The HPC method consists in the treatment of the copper-impregnated zeolite by water–ammonia solution for polycondensation of hydrolyzed copper ions in the zeolite pores. It is shown that varying the NH4OH/Cu2+ ratio allows a regulation of the size and structure of copper active complexes. The copper-impregnated zeolite contains copper as CuO-like particles after calcination. At NH4OH/Cu2+ equal to 1–3, active sites in dried samples are disordered polynuclear copper-hydroxo complexes, which are characterized by symmetric ESR signal with g 0 =2.18. Increase of NH4OH/Cu2+ to 6–30 lead to the orbital ordering of Cu2+ ions in the copper–ammoniate complexes, observed as the rhombic ESR signals (g z ex,1 =2.18, g y ex,1 =2.08, g x ex,1 =2.03 and g ∥ ex,2 =2.24, g ⊥ ex,2 =2.03), and to dispersion of CuO-like particles during following thermal treatment at 500°C. The dried 1%Cu(30)ZSM-5-HPC sample prepared at NH4OH/Cu2+ =30 has a higher amount of isolated tetraammine–copper complex and a lower number of weak associates of Cu2+ ions among samples. The addition of ammonia solution to the copper-impregnated zeolite for hydrolytic polycondensation has a favorable impact on the Cu-ZSM-5 activity in DeNOx processes. In N2O decomposition, the ion-exchanged CuZSM-5 catalysts were more reactive in the low-temperature region, while the ex-polycondensation catalysts provided a high N2O conversion at high temperatures. In NO-SCR by propane, the more reactive were the ion-exchanged catalysts, and NO reduction rate increased with NH4OH/Cu2+ grew for 10. It is suggested that the ordered associated Cu2+ ions of ex-polycondensation and ion-exchanged catalysts have higher reactivity and selectivity in both DeNOx processes as compared to the isolated copper ions. [Copyright &y& Elsevier]

Abstract: The adsorption of NO has been investigated in the temperature range 50–150°C on Cu-ZSM-5 and La-Cu-ZSM-5 also in the presence of water and O2 in a continuous flow reactor. TPD experiments have been performed after each adsorption test monitoring all desorbed NO x species. Water has been found to displace NO from copper sites strongly reducing its adsorption except for nitrate-like species which are partially preserved also in the presence of water. O2 addition, although decreasing NO adsorption, limits the negative effect of water. Lanthanum co-exchange results in improving the NO adsorption capacity of the zeolite both in the absence and in the presence of water. [Copyright &y& Elsevier]

Abstract: NO oxidation is considered as the rate determining step of the standard selective catalytic reduction (SCR) reaction (NO+NH3 +O2) on metal-exchanged zeolite catalysts. A comprehensive experimental and kinetic modeling study of NO oxidation reaction was carried out and a comparison made between Fe-ZSM-5, Cu-ZSM-5 and recently developed small pore Cu-chabazite catalysts. Steady state integral measurements of NO oxidation show Fe-zeolite to be a more active NO oxidation catalyst than Cu-zeolite. Both NO2 temperature programmed desorption and decomposition experiments show that the NO2 is more strongly bound on the Cu-chabazite compared to Fe-ZSM-5. Increasing amounts of NO2 in the NO x feed decrease the NO oxidation activity on both the catalysts. Differential kinetic studies show a positive order rate dependence of NO oxidation reaction with respect to both NO (∼1) and O2 (∼0.5) while showing a negative order dependence with respect to NO2 (−0.5 to −1), which confirms that the NO2 inhibits the NO oxidation reaction. NO2 inhibition on NO oxidation was found to be more severe on Cu-chabazite (∼ −1 order) compared to Fe-ZSM-5 (∼ −0.5 order). The overall activation energy of NO oxidation was found to be 39kJ/mol on Fe-ZSM-5 compared to 56kJ/mol on Cu-chabazite. The presence of feed water strongly inhibits the NO oxidation reaction. Transient NO2 uptake experiments reveal nitrate formation on both catalysts. Catalysts pretreatments by different oxidizing/reducing agents (O2, NO2, H2 and NH3) did not affect steady state NO oxidation results. Experiments with monolith catalysts having the same washcoat volume but different washcoat thicknesses rule out the presence of washcoat diffusion limitations for NO oxidation. A global kinetic model is developed which accurately predicts the experimentally observed reaction orders, NO conversions and NO2 and water inhibition effects for a wide temperature range and different feed conditions. The kinetic model accounts for the experimentally observed instability of nitrates in the presence of feed NO and shows that the steady state NO oxidation data can be predicted with a global kinetic model assuming the surface reaction between adsorbed O atoms and gaseous NO as the rate determining step. [Copyright &y& Elsevier]

Abstract: The decomposition activities of nitrogen monoxide (NO) were investigated for the copper ion-exchanged ZSM-5 zeolites (Cu-MFI) containing the rare-earth elements: La, Ce, Pr, Nd, Sm, and Gd. It was found that Sm and Gd acted as promoter for NO decomposition over Cu-MFI with 60–70% of the exchange level of copper ion, while the addition of the other rare-earth elements to Cu-MFI showed no positive effect on NO decomposition activity. The introduction of Sm and Gd to Cu-MFI had little influence on the effectiveness of each copper ion for NO adsorption in the MFI zeolite. The presence of Sm and Gd stabilized Cu+ as an active center. [Copyright &y& Elsevier]

Abstract: The effect of ion exchange conditions, such as Si/Al ratio, precursor copper salt, pH and concentration of the solution, on the catalytic activity in SCR of NO by propane and on the electronic state of copper ions in Cu-ZSM-5 has been studied. The NO conversion in NO SCR by C3H8 has been found to reach a maximum value at Cu/Al ratio about 0.37–0.4 and remain constant at higher Cu/Al. ESR and UV–vis DR spectroscopy have been used to elucidate stabilization conditions of copper ions in Cu-ZSM-5 zeolites as isolated Cu2+ ions, chain copper oxide structures and square-plain oxide clusters. The ability of copper ions for reduction and reoxidation in the chain structures may be responsible for the catalytic activity of Cu-ZSM-5. These transformations of copper ions are accompanied by the observation of intervalence transitions Cu2+–Cu+ and CTLM of the chain structures in the UV–vis spectra. [Copyright &y& Elsevier]

Abstract: This review summarises experimental and modelling studies concerning the development of kinetic oscillations during the catalytic decomposition of nitrous oxide. Oscillations over Cu-ZSM-5 catalysts have been investigated using several methods and it could be shown that the formation of a strongly adsorbed nitrate formed in side reactions and the interaction of this species with atomic oxygen is responsible for the development of kinetic oscillations. The reaction of nitrous oxide over Fe-ZSM-5 catalysts shows similarities to the observations made with Cu-ZSM-5, but kinetic oscillations occur only in the presence of water vapour. On the other hand, isothermal oscillations observed over precious metal catalysts can be explained by an oxidation/reduction mechanism including the restructuring of the active catalyst surface under reaction conditions. [Copyright &y& Elsevier]

Toluene and propene are used as model hydrocarbon gases for producing coke over Cu-ZSM-5. Temperature programmed oxidation technique is adapted to investigate roles of NO and O2 on coke deposition and removal over Cu-ZSM-5 catalyst. It is found that when a gas mixture of NO and O2 is added during hydrocarbon exposure, the carbonaceous structure is not changed but the deposition rate is dramatically increased. It is suggested that the presence of NO + O2 accelerates hydrocarbon deposition or that coke may be formed in another route involving SCR. In addition, N2 is detected during TPO by O2 of this exposed Cu-ZSM-5. This indicates that nitrogen compounds have to be a part of carbonaceous deposits or they may adsorb on the deposits on the zeolite. By using XRD and ex situ FT-IR, it is found that the structure of coke is polyaromatic, not graphite-like form. It is also found that gasification reactivity of the carbonaceous deposits depends on the oxidizing gas (O2, NO, NO + O2) with respect to both composition and concentration. However, the rate of the gasification at each maximum temperature is about similar, independent on the oxidizing agent. Furthermore, N2 peaks appear to coincide with CO2 peaks. Therefore, in the presence of NO, CO2 appears to be formed preferably by NO reduction rather than coke combustion. NO seems to be more reactive for oxidizing of carbonaceous deposits rather than O2. Certainly, NO plus O2 in the TPO oxidizing gas accelerates coke removal. [Copyright &y& Elsevier]

The effect of finely coating Cu-ZSM-5 with nanoparticles of ceria has been studied in situ by XANES, DRIFTS and TPR. The combination of these techniques reveals a lower temperature of reduction for both metals (Cu and Ce) in the bifunctional catalyst. Copper interacts with ceria nanocrystals at the oxide–zeolite interface and forms a localized phase comparable to doped ceria solid solutions commonly used as oxidation catalysts. As a result the catalyst is able to activate C3H6 and form NO2 at low temperatures. Analysis of the stability of adsorbed CO by DRIFTS suggests that the addition of water promotes the oxidation of Cu1+ on the coated catalyst. In addition, TPD and TGA results, in combination with supplementary DRIFTS experiments, indicate that the ceria coating retains water at the interface during reaction conditions. The combination of these events yields relatively high C3H6-SCR activity at temperatures as low as 250 °C. It is believed that the affinity towards water and extended coverage of the coating also protects the zeolite against degradation by restricting the diffusion of water molecules into the zeolite pores. [Copyright &y& Elsevier]

Bifunctional catalysts comprising metal-exchange zeolites coated by metal oxides have been prepared and shown to be active catalysts for the selective catalytic reduction of NOx with propylene. Characterization of the materials by various means shows that oxide nano-particles adhere to the external surface of the zeolite and create a fine permeable coating which does not block the zeolite pores. When ceria is used, the combination of phases lowers the maximum activity temperature of the deNOx reaction as much as 150 °C relative to the metal-zeolite only. In addition, a unique activity enhancement in the presence of water is observed. These effects are observed in different zeolite systems to various degrees indicating that the nature of the exchanged metal or zeolite framework is not critical. In particular, CeO2/Cu-ZSM-5 displays a maximum of activity at a temperature as low as 250 °C. The benefits of low temperature and water-enhanced activity obtained by the coating technique exceed those displayed by other bifunctional materials combining cerium and zeolites through different approaches. [Copyright &y& Elsevier]