SCR Reaction Research Articles

Selective catalytic reduction of NO x with NH3: opportunities and challenges of Cu-based small-pore zeolites.

Zeolites, as efficient and stable catalysts, are widely used in the environmental catalysis field. Typically, Cu-SSZ-13 with small-pore structure shows excellent catalytic activity for selective catalytic reduction of NOx with ammonia (NH3-SCR) as well as high hydrothermal stability. This review summarizes major advances in Cu-SSZ-13 applied to the NH3-SCR reaction, including the state of copper species, standard and fast SCR reaction mechanism, hydrothermal deactivation mechanism, poisoning resistance and synthetic methodology. The review gives a valuable summary of new insights into the matching between SCR catalyst design principles and the characteristics of Cu2+-exchanged zeolitic catalysts, highlighting the significant opportunity presented by zeolite-based catalysts. Principles for designing zeolites with excellent NH3-SCR performance and hydrothermal stability are proposed. On the basis of these principles, more hydrothermally stable Cu-AEI and Cu-LTA zeolites are elaborated as well as other alternative zeolites applied to NH3-SCR. Finally, we call attention to the challenges facing Cu-based small-pore zeolites that still need to be addressed.

Organotemplate-free synthesis of MOR zeolite from coal fly ash through simultaneously effective extraction of Si and Al

In this study, high-purity mordenite (MOR) zeolite was successfully synthesized from coal fly ash (CFA) using a simple and green method without organotemplate. The Al and Si used for MOR synthesis were simultaneously extracted from CFA, which were transformed to aluminosilicate gel by co-precipitation and then coupled with MOR seed for MOR synthesis instead of using organotemplate in common method. The results showed that the Al and Si extraction efficiencies were high, reaching up to 82.18% and 93.15%, respectively. The synthesized CFA-based MOR zeolite had a low Si/Al ratio of 3.92, and micropores is the dominating pore structure in the synthesized zeolite, accounts for up to 63.16% of its total pore volume. The possible mechanisms of MOR zeolite from CFA by gel coupling seed assisted synthesis were discussed. Furthermore, the CFA-based MOR was applied in CO 2 adsorption, heavy metal ions adsorption in aqueous solution and catalyst carrier for NH 3 -SCR reaction. The results presented that the CO 2 adsorption capacity of the CFA-based MOR was 1.92 mmol/g with high cycle stability. The removal rate of Pb 2+ was above 99.99% with Pb 2+ initial concentration of 51.30 mg/L. Mn-MOR catalyst was obtained through loading 3.36 wt% Mn by ion-exchange, which had a NO conversion rate of 93.58% at 200 °C. This study demonstrates that the gel coupling seed assisted method without organotemplate is a potential way to synthesize zeolite materials using the Si and Al simultaneously extracted from CFA. • CFA-based MOR zeolite was synthesized by an organotemplate-free method. • Al and Si were extracted from CFA with efficiencies of 82.18% and 93.15%, respectively. • Aluminosilicate gel coupling seed assisted method was used to synthesize MOR. • CFA-based MOR showed good performance in CO 2 capture, metal adsorption and NH 3 -SCR.

Mechanism of the hydrocarbon resistance of selective catalytic reduction catalysts supported on different zeolites

Cu–Mn/SAPO-34 demonstrated excellent HC resistance, NOx complexes and NH3 species could be formed and SCR reaction proceed. While on Cu–Mn/ZSM-5, the active site was occupied by acrylate leading to SCR reaction blocked.

One-pot synthesis of Na+-free Cu-SSZ-13 and its application in the NH3-SCR reaction.

Na+-free Cu-SSZ-13 zeolites have been rationally synthesized via a cooperative strategy, which has the advantages of rapid crystallization (9-48 h), high yield (86-94%) and adjustable Cu content. The NH3-SCR catalytic performance and hydrothermal stability of the calcined materials were studied and correlated with their physicochemical properties.

Enhancing catalytic performance of Cu-SSZ-13 for the NH3-SCR reaction via in situ introduction of Fe3+ with diatomite

In situ introduction of Fe3+ by diatomite greatly enhanced the hydrothermal stability and activity of Cu-SSZ-13 for the NH3-SCR reaction.

Porous Zeolitic Imidazolate Framework Loaded Mn as an Efficient Catalyst for the Selective Catalytic Reduction of NOx with NH3

ABSTRACT Zinc 2-methylimidazolate (Zn(Me-Im)2, ZIF-8) was synthesized through solvothermal method, and the Mn@ZIF-8 was synthesized by loading the Mn species onto the prepared ZIF-8 through impregnation. The effect of the addition amount of Mn species on the activity in selective catalytic reduction of NOx with NH3 (NH3-SCR), the stability of catalysts at ideal temperatures and the resistance to H2O and SO2 were investigated. Moreover, the characterizations of the catalysts including XRD, SEM, EDS, XPS, BET, TGA and H2-TPR were carried out. The results indicated that SCR activity of the catalysts was related to the addition amount of Mn species, the Mn@ZIF-8(0.8) showed the best NO conversion of over 90% ranging from 225°C–400°C with good stability in a long-time test. Besides, the Mn@ZIF-8 showed a high resistance to SO2. The ZIF-8 has a great potential to be an excellent support, and Mn@ZIF-8 is promising catalyst in SCR reaction.

Synthesis of Pd0.01FexCe(1−x)/2Zr(1−x)/2Oy catalysts and their catalytic performance for ammonia production by passive SCR reaction

The passive ammonia SCR system has been considered as a novel after-treatment method to reduce the emission of NOx and produce ammonia for downstream SCR.

A DFT-D study on the reaction mechanism of selective catalytic reduction of NO by NH3 over the Fe2O3/Ni(111) surface

The adsorption and SCR reaction mechanism of NH3, NO, and O2 molecules on the Fe2O3/Ni(111) catalyst surface was revealed.

Synergistic degradation mechanism of chlorobenzene and NOx over the multi-active center catalyst: The role of NO2, Brønsted acidic site, oxygen vacancy

Development of an efficient synergistic degradation technology to prevent the emission of chlorinated organics and NOx from industrial sources without forming harmful by–products, is a major challenge in environmental chemistry. The designed multi–active center catalyst (PdV/TiO2) showed excellent CB/NO synergistic removal conversion efficiency and target product selectivity. Experimental and DFT calculation results indicated that Pd was the common center for adsorption of CB, O2, NH3 and NOx molecules. The O vacancies (VOx–1 and TiOx–1) generated during the CB ring opening process was conducive to adsorption of reaction gas molecules, accelerating CBCO and NH3–SCR reaction. The adsorbed NH3 species would dissociate H protons to provide Brønsted acidic sites necessary for Cl conversion. O vacancies structure was easier to be restored by reoxidation with the participation of NO2. Certain synergistic effect in the removal of CB and NO on the multi–active center catalyst surface was found.

水熱劣化したCu-CHA 触媒におけるSCR 反応のモデル化

水熱劣化したCu-CHA 触媒におけるSCR 反応のモデル化

Catalytic Performance of MIL‐88B(V) and MIL‐101(V) MOFs for the Selective Catalytic Reduction of NO with NH3

AbstractIn this paper, the SCR properties of MIL‐88B(V) and MIL‐101(V) MOFs materials by hydrothermal synthesis were investigated, and the effect of the coordination and spatial topology of MOFs materials on the coordination state, chemical state and surface properties of V ions, as well as the relationship with SCR catalysis, were discussed. The results showed that the NO conversion of MIL‐88B(V) can be as high as 80 %, while those of MIL‐101(V) can only reach to 20 %, but the SCR reaction selectivity of both were excellent. The coordination structure and spatial topological structure of MOFs materials had a great influence on the coordination state, chemical state and surface properties of V ions, resulting in different SCR catalytic performance. The reaction performance of SCR was related to the structural properties of V3Ox units and followed the E‐R mechanism. The staggered and layer upon layer close‐packed structure of MIL‐88B(V) made the density of V3Ox unit cluster higher and the distance of adjacent V3Ox unit cluster smaller. More V5+ ions and V−O−V bonds can form more Brønsted acid sites, also was conducive to the progress of the SCR reaction. Although MIL‐88B(V) has insufficient high temperature stability, but its good SCR reaction selectivity and SCR catalytic performance at medium and low temperature can still have the application prospect of medium and low temperature industrial denitration in non‐electric industries.

The promoting mechanism of in situ Zr doping on the hydrothermal stability of Fe-SSZ-13 catalyst for NH3-SCR reaction

Fe-SSZ-13 has been proven to be a highly efficient catalyst for the ammonia selective catalytic reduction (NH3-SCR) of NOx, especially at medium and high reaction temperatures. Nevertheless, the hydrothermal stability of Fe-SSZ-13 catalyst is inferior, limiting its application in vehicle exhaust purification. Impressively, it is found that the hydrothermal stability of Fe-SSZ-13 can be remarkably enhanced (as high as 30 % improvement at 300 °C) by in situ introduction of Zr and Fe into initial gels via one-pot synthesis in this work. A small amount of Zr addition (0.31 wt.%) is beneficial for improving the relative crystallinity of Fe-SSZ-13. The characterization results (obtained by 27Al NMR, NH3-TPD, UV–vis, XPS and in situ DRIFTS) revealed that Zr doping by a one-pot method would not only enhance the dispersion of active isolated Fe ions but also inhibit the migration and agglomeration of isolated Fe ions during hydrothermal aging, improving the stability of the catalyst. Additionally, the introduction of Zr generates new hydroxy groups and Lewis acid sites, facilitating the formation of reaction intermediates, such as -NH2, -HNO2, -NO2, etc., and promoting the SCR reaction especially over the aged catalyst.

A basic comprehensive study on synergetic effects among the metal oxides in CeO2-WO3/TiO2 NH3-SCR catalyst

• The impregnated CeW/Ti-I exhibits over 80% NO x conversion at 200–550 °C for NH 3 -SCR. • Substitution of Ce 4+ /Ti 4+ by W 6+ at the interface depends on dispersion of metal oxides. • Formation of solid solutions produces more lattice defects and M O M’ bridges. • Ce 3+ and oxygen vacancies facilitate the low-temperature activity via fast SCR route. • Unsaturated cations and Ce(Ti) O W bonds serve as Lewis and Brønsted acid sites. Two Ce-W-Ti mixed oxide catalysts were prepared by wet impregnation (CeW/Ti-I) and mechanical mixing (Ce/W/Ti-M) methods. CeW/Ti-I catalyst exhibits over 80% NO x conversion and nearly 100% N 2 selectivity at 200–550 °C. The superior catalytic performance is associated with the high dispersion of active CeO 2 and acidic WO 3 on TiO 2 and the synergistic effects among different components. The X-ray Diffraction, Raman spectroscopy, Nitrogen physisorption, X-ray photoelectron spectroscopy, H 2 temperature-programmed reduction, temperature-programmed desorption of ammonia and infrared spectroscopy were performed. It is found that the formation of solid solutions, as well as the high specific surface area, would increase the amounts of Ce 3+ , oxygen vacancies and acid sites in the catalyst. These broaden the operating temperature window of NH 3 -SCR reaction and enhance the N 2 selectivity by facilitating “fast SCR” reaction at low temperatures and inhibiting the unselective oxidation of NH 3 at high temperatures.

Effect of synthesis methods on Fe–Ce/Ti catalysts for selective catalytic reduction: the physicochemical properties and catalytic activity

The Fe–Ce/Ti catalysts with different molar ratios of Fe and Ce were synthesized by the precipitation method and impregnation method and used for the SCR reaction. BET, XRD, NH3-TPD, XPS, H2-TPR, UV–Vis, TEM, SEM–EDS mapping techniques were performed to explore the physicochemical properties of catalysts synthesized by different methods. As a result, the catalysts synthesized by precipitation method yields higher catalytic activity than that of the catalysts prepared by the impregnation method. The precipitated catalysts possess larger specific surface areas, higher ratios of Ce3+/Ce4+ and Fe3+/Fe2+ and more surface chemisorbed oxygen, as well as the enhanced surface acid sites and redox ability. Besides these, it holds a high dispersion of active species and more uniform smaller nanoparticles. All mentioned above will result in a higher catalytic performance of catalysts.

Simultaneous removal of NOx and chlorobenzene on V2O5/TiO2 granular catalyst: Kinetic study and performance prediction

The synergetic abatement of multi-pollutants is one of the development trends of flue gas pollution control technology, which is still in the initial stage and facing many challenges. We developed a V2O5/TiO2 granular catalyst and established the kinetic model for the simultaneous removal of NO and chlorobenzene (i.e., an important precursor of dioxins). The granular catalyst synthesized using vanadyl acetylacetonate precursor showed good synergistic catalytic performance and stability. Although the SCR reaction of NO and the oxidation reaction of chlorobenzene mutually inhibited, the reaction order of each reaction was not considerably affected, and the pseudo-first-order reaction kinetics was still followed. The performance prediction of this work is of much value to the understanding and reasonable design of a catalytic system for multi-pollutants (i.e., NO and dioxins) emission control.

Evaluation of Co/SSZ-13 Zeolite Catalysts Prepared by Solid-Phase Reaction for NO-SCR by Methane.

Co/SSZ‐13 zeolites were prepared by heating the finely dispersed mixture of NH4‐SSZ‐13 and different cobalt salts up to 550 °C. Investigations by thermogravimetry – differential scanning calorimetry – mass spectrometry provided new insight into details of the solid‐state reaction. Formation of Co carrying hydrate melt or volatile species was shown to proceed from chloride, nitrate, or acetylacetonate Co precursor salts upon thermal treatment. This phase change allows the transport of the Co species into the zeolite pores. The reaction of the NH4 + or H+ zeolite cations and the mobile Co precursors generates vapor or gas products, readily leaving the zeolite pores, and cobalt ions in lattice positions suggesting that solid‐state ion‐exchange is the prevailing process. The obtained catalysts are of good activity and N2 selectivity in the CH4/NO‐SCR reaction. The thermal treatment of acetate or formate salts give solid intermediates that are unable to get in contact and react with the cations in the zeolite micropores. These catalysts contain mainly Co‐oxide clusters located on the outer surface of the zeolite crystallites and have poor catalytic performance.

Solvent-free synthesis of aluminosilicate SSZ-39 zeolite

Aluminosilicate SSZ-39 zeolite shows the excellent catalytic performance in selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) reaction, but their synthesis is still not efficient due to the use of a large amount of water as a solvent under hydrothermal condition. Herein, for the first time, we report a solvent-free synthesis of aluminosilicate SSZ-39 zeolite without addition of water in the presence of N,N-dimethyl-3,5-dimethylpiperidine (DMDMP) cation as an organic structure-directing agent. Various characterizations, including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry-differential thermal analysis (TG-DTA), N 2 sorption, inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, solid nuclear magnetic resonance (NMR) and NH 3 -temperature-programmed desorption (NH 3 -TPD), show that the aluminosilicate SSZ-39 zeolite displays high crystallinity, uniform sheet-like morphology, large surface area, four-coordinated aluminum species, strong acidic sites and excellent hydrothermal stability. Compared with the conventional synthesis for aluminosilicate SSZ-39 zeolite, the solvent-free synthesis of aluminosilicate SSZ-39 zeolite has the obvious advantages such as high product yield, simple synthesis procedures, and the reduction of pollutants. More importantly, the copper-exchanged aluminosilicate SSZ-39 zeolite shows the good catalytic performance in NH 3 -SCR reaction, which is fully comparable with that of the Cu-SSZ-39 zeolite synthesized under hydrothermal condition from the reported literature. • Aluminosilicate SSZ-39 zeolite is synthesized under solvent-free condition. • This route enhances the utilization efficiency of both the raw materials and space. • The Cu-exchanged SSZ-39 zeolite shows good catalytic performance in NH 3 -SCR reaction. • This route is sustainable.

Structural characteristics of V-based catalyst with Sb on selective catalytic NOx reduction with NH3

In order to remove nitrogen oxides generated in various applications, Sb-added V based catalysts with good performance at low temperatures were used. In order to identify the effects of Sb, NH3-TPD, DRIFTs, TPR and TPRO, XRD, Raman/pL spectroscopy, and XPS were investigated. When V is loaded onto the synthesized Sb/TiO2, it shows very good performance at low temperatures. When Sb and TiO2 were synthesized to begin with, the electron mobility and oxygen storage capacity of the catalyst were improved. When vanadium was supported thereafter, it was confirmed that VSbO4 species, which is a bond structure of V and Sb, was generated. VSbO4 is composed of non-stoichiometric species of V, which can facilitate the transfer of electrons in the SCR reaction. As a result, the oxidation state of unstable vanadium makes it easier to use the gaseous oxygen in the SCR reaction, leading to a faster re-oxidation reaction. This reaction is an important factor showing excellent performance at low temperature.

Unusual positive effect of SO2 on Mn-Ce mixed-oxide catalyst for the SCR reaction of NOx with NH3

A high-performance denitration catalyst based on Mn-Ce mixed-oxide derived from a specific Mn/Ce precursorwas fabricated by solvothermal method. It exhibited outstanding low-temperature activity, which achieved above 90% NOx conversion in the temperature of 150–310 °C. Remarkably, Mn-Ce mixed-oxide demonstrated a much better SO2 tolerance, in which the NOx conversion increased from 92.6% to 97.8% at 150 °C after introducing 200 ppm SO2. The synthesized Mn-Ce mixed-oxide has a porous structure with abundant chemisorbed oxygen spices and exposed active sites. In addition, the increased acidity and redox capacity enhanced the catalytic activity after introducing SO2. Meanwhile, the injection of SO2 separates NH3 adsorption sites from oxidation agents, inhibiting the unwanted oxidization of ammonia to nitrogen oxides. Furthermore, the reaction pathway on Mn-Ce mixed-oxide proved to be the L-H mechanism based on in situ DRIFTS. Thus, the synthesized Mn-Ce mixed oxide demonstrated an excellent denitration performance and the especially high SO2 tolerance, showing great potential for low-temperature NH3-SCR.

Controlling Catalytic Selectivity Mediated by Stabilization of Reactive Intermediates in Small-Pore Environments: A Study of Mn/TiO2 in the NH3-SCR Reaction

Controlling selectivity in heterogeneous catalysis is essential for designing processes that minimize the production of undesired byproducts. For example, TiO2-supported manganese oxide, a promisin...