Fast SCR Reaction Research Articles

Cu-OFF/ERI Zeolite: Intergrowth Structure Synergistically Boosting Selective Catalytic Reduction of NOx with NH3.

Cu-SSZ-13 has been commercialized for selective catalytic reduction with ammonia (NH3-SCR) to remove NOx from diesel exhaust. As its synthesis usually requires toxic and costly organic templates, the discovery of alternative Cu-based zeolite catalysts with organotemplate-free synthesis and comparable or even superior NH3-SCR activity to that of Cu-SSZ-13 is of great academic and industrial significance. Herein, we demonstrated that Cu-T with an intergrowth structure of offretite (OFF) and erionite (ERI) synthesized by an organotemplate-free method showed better catalytic performance than Cu-ERI and Cu-OFF as well as Cu-SSZ-13. Structure characterizations and density functional theory calculations indicated that the intergrowth structure promoted more isolated Cu2+ located at the 6MR of the intergrowth interface, resulting in a better hydrothermal stability of Cu-T than Cu-ERI and Cu-OFF. Strikingly, the low-temperature activity of Cu-T significantly increased after hydrothermal aging, while that of Cu-ERI and Cu-OFF substantially decreased. Based on in situ diffuse reflectance infrared Fourier transform spectra analysis and density functional theory calculations, the reason can be attributed to the fact that NH4NO3 formed on the CuxOy species within ERI polymorph of Cu-T underwent a fast SCR reaction pathway with the assistance of Brønsted acid sites at the intergrowth interfaces under standard SCR reaction conditions. Significantly, Cu-T exhibited a wider temperature window at a catalytic activity of over 90% than Cu-SSZ-13 (175-550 vs 175-500 °C for fresh and 225-500 vs 250-400 °C for hydrothermal treatment). This work provides a new direction for the design of high-performance NH3-SCR catalysts in terms of the interplay of the intergrowth structure of zeolites.

An in-depth revelation of a novel facilitation mechanism of H2O on the NH3-SCR reaction of Mn-Ce/TiO2 catalyst with exposed anatase {001} facets

H2O typically has a detrimental impact on the NH3-SCR reaction, exacerbating the sulfur poisoning of the catalysts. This study thoroughly explores the unusual facilitating mechanism of H2O (gas) (g) at low concentrations (volume fraction less than 3.5 vol%) on the NH3-SCR reaction of a Mn-Ce/TiO2 (M−C/Ti) catalyst with preferential exposure of the anatase {001} facets (M−C/Ti-001) through comprehensive experimental characterization and DFT (Density functional theory) calculations. The optimum water vapor promotion concentration of 2.5 vol% was experimentally determined. It generated new Brønsted acid sites after H2O adsorption on the anatase {001} facets, increasing the activity of the Lewis acid sites and the NH3 adsorption capacity on Brønsted acid sites (B-NH4+). B-NH4+ species can react with NO2 in a fast SCR reaction, significantly enhancing the overall reaction rate. The low content of H2O (g) contributes to the inert linear nitrite involved in the NH3-SCR reaction, which is favourable for the NH3-SCR reaction. In addition, the Brønsted acid sites effectively inhibited the adsorption of SO2 onto the surface of the M−C/Ti-001 catalyst, enhancing its sulfur resistance. This study provides new insights into the promotion mechanism of M−C/Ti-001 catalysts by low concentration of water vapor.

Enhanced Low-Temperature Activity and Hydrothermal Stability of Ce-Mn Oxide-Modified Cu-SSZ-39 Catalysts for NH3-SCR of NOx

Cu-SSZ-39 zeolite with an AEI structure exhibits excellent hydrothermal stability and can be a potential alternative to Cu-SSZ-13 zeolite SCR catalysts for NOx removal in diesel vehicles. However, the inferior low-temperature performance of Cu-SSZ-39 leads to substantial NOx emissions during the cold-start period, impeding its practical application. In this study, Ce-Mn oxide-modified Cu-SSZ-39 catalysts (CeMnOx/Cu-SSZ-39) and references (CeO2/Cu-SSZ-39 and MnOx/Cu-SSZ-39) were prepared by the ion-exchange of Cu ions followed by impregnation of the oxide precursors, with the aim of enhancing the NH3-SCR performance at low temperatures. The modified catalysts exhibited improved low-temperature activity and hydrothermal stability compared to the unmodified counterpart. In particular, CeMnOx/Cu-SSZ-39 showed the highest activity among the three catalysts and achieved NOx conversions above 90% within the temperature range of 180 °C to 600 °C, even after undergoing hydrothermal aging at 800 °C. Experimental results indicated that the synergistic effect between Ce and Mn in CeMnOx improves the redox properties and acidity of the catalyst due to the presence of Ce3+, Mn4+, and abundant adsorbed oxygen species, which facilitate low-temperature SCR reactions. Furthermore, the interaction of CeMnOx with Cu-SSZ-39 stabilizes the zeolite framework and hinders the agglomeration of Cu species during the hydrothermal aging process, contributing to its exceptional hydrothermal stability. The kinetics and NO oxidation experiments demonstrated that CeMnOx provides access to fast SCR reaction pathways by oxidizing NO to NO2, resulting in a significant increase in low-temperature activity. This study provides novel guidelines for the design and preparation of Cu-SSZ-39 zeolite with outstanding SCR performance over a wide temperature range.

Co3O4 as an efficient passive NOx adsorber for emission control during cold-start of diesel engines

The Co3O4 nanoparticles, dominated by a catalytically active (110) lattice plane, were synthesized as a low-temperature NOx adsorbent to control the cold start emissions from vehicles. These nanoparticles boast a substantial quantity of active chemisorbed oxygen and lattice oxygen, which exhibited a NOx uptake capacity commensurate with Pd/SSZ-13 at 100 °C. The primary NOx release temperature falls within a temperature range of 200-350 °C, making it perfectly suitable for diesel engines. The characterization results demonstrate that chemisorbed oxygen facilitate nitro/nitrites intermediates formation, contributing to the NOx storage at 100 °C, while the nitrites begin to decompose within the 150-200 ºC range. Fortunately, lattice oxygen likely becomes involved in the activation of nitrites into more stable nitrate within this particular temperature range. The concurrent processes of nitrites decomposition and its conversion to nitrates results in a minimal NOx release between the temperatures of 150–200 °C. The nitrate formed via lattice oxygen mainly induces the NOx to be released as NO2 within a temperature range of 200–350 °C, which is advantageous in enhancing the NOx activity of downstream NH3-SCR catalysts, by boosting the fast SCR reaction pathway. Thanks to its low cost, considerable NOx absorption capacity, and optimal release temperature, Co3O4 demonstrates potential as an effective material for PNA applications.

Engineering bifunctional Pd/CeNbTiOx catalyst for the simultaneous removal of NO and CO under oxygen-rich conditions at low temperature

The simultaneous elimination of CO and NO has practical significance for the control of air pollutants in the flue gas operating temperature range of 240–320 °C. CeNbTiOx catalyst was prepared by evaporation-induced self-assembly method (EISA), and the 0.3 % Pd/CeNbTiOx catalyst was obtained by appropriate Pd anchoring at the defects of CeNbTiOx. CO and NO removal efficiency were above 90 % from 240 to 320 °C, which exhibited satisfactory N2 selectivity. The CO and NO removal efficiency of the 0.3 % Pd/CeNbTiOx catalyst exceeded 92 % during stability and water resistance. Oα and Oβ were involved in the oxidation of NO and CO, respectively, promoting the fast SCR and CO oxidation reaction within the ordered mesoporous of the 0.3 % Pd/CeNbTiOx catalyst. In addition, the efficiency of CO and NO elimination had been further improved due to the enhancement of defects after resistance of H2O. Therefore, this work provided a new and efficient way to control CO and NO by the efficient using of oxygen species.

Development of a model based strategy for catalyst screening to control NOx and N2O emissions in NH3-SCR process towards Eu7 readiness

Transient kinetic models were used to compare different commercial NH3-SCR catalytic monoliths under World Harmonized Transient Cycle (WHTC) conditions for NOx abatement performance, N2O formation and NH3 slip. Towards this goal, kinetic models for four Cu-zeolite, two Vanadia and one Fe-zeolite formulation were developed using experimental data of NH3 adsorption, desorption, NH3 oxidation, NO oxidation, Standard SCR and Fast SCR reactions. Models were in very good agreement with experimental data. Cu and Fe-zeolite formulations showed high deNOx activity with cycle efficiencies > 90% and > 95% respectively, for cold and warm WHTC runs, whereas Vanadia formulations outshined with negligible N2O formation. Simulations were performed to assess the variation of tailpipe NOx as a function of reactor volume. A plateau in deNOx performance was observed for all formulations as a function of the reactor volume which was around 30 and 50 L for cold and warm WTHC runs, respectively. This was found to be due to the decreasing temperature and fractional coverage of adsorbed NH3 along the axial distance of the reactors. Models were also used to develop hybrid reactor in series designs. Successive SCR reactors consisting of different formulations were simulated. The most successful design in terms of high deNOx performance and low N2O formation was with a reactor using a Vanadia catalyst followed by a reactor with Cu-zeolite catalyst. This resulted in a very similar deNOx performance but with a significantly lower N2O emissions as compared to a reactor with the same total volume consisting of Cu-zeolite.

Optimized local geometry and electronic structure of MoO3/CeO2 catalyst by adding copper cations for boosted nitrogen oxide reduction performance

The construction of highly active centers over the CeO2-based catalyst for the low-temperature NH3 selective catalytic reduction of nitrogen oxides (NH3–SCR) technology in non-power industry is one of the challenges in the application field. Herein, the SCR performance of MoO3/CeO2 catalyst has been remarkably enhanced by introducing Cu to induce the generation of active MoO structure. Advanced spectroscopy characterizations reveal that the adding of Cu into MoO3/CeO2 catalyst could create unsaturated sites on CeO2 for Mo anchor, and the enhanced electrons transfer from Mo to Cu would cause the formation of a new terminal MoO with highly distorted octahedral geometry, which is a new Lewis acid site for coordinated NH3 production. Meanwhile, the added Cu creates the adsorbed site for gaseous NO and the formed Mo-O-Cu pair center facilitates the transformation of ionic NO2– generated from NO adsorption to NO2, which is conducive to the fast SCR reaction process.

Study on the Performance of the Zr-Modified Cu-SSZ-13 Catalyst for Low-Temperature NH3-SCR.

Cu-SSZ-13 and Zr-modified Cu-SSZ-13 catalysts with different Zr/Cu mass ratios were prepared by ion-exchange and impregnation methods, respectively. The NH3-SCR performance tests were performed using the catalyst performance evaluation device to investigate the effects of different Zr/Cu mass ratios on the catalyst ammonia-selective catalytic reduction (NH3-SCR) performance. X-ray diffraction, ICP-OES, BET, NH3 temperature-programed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectrometry, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were used to characterize the catalysts. The results show that the prepared Cu-SSZ-13 catalyst had good catalytic activity. Zr introduction was carried out on this basis. The results showed that proper Zr doping improved the catalytic activity at low temperatures and widened the high-temperature stage, with an optimal activity stage at a Zr/Cu mass ratio of 0.2. The NO x conversion efficiency was close to 100% at 200 °C and over 80% at 450 °C. The active species were well dispersed on the catalyst surface, and the metal modification did not change the crystal structure of the zeolite. The NH3-TPD results showed that the Zr-modified catalyst had more abundant acid sites, and the H2-TPR results indicated that the Cu species on the catalyst had excellent reducibility at low temperatures. The interaction between Cu and Zr could regulate the Cu+ and Cu2+ proportion on the catalyst surface, which facilitated the increase in the Cu+ for fast SCR reaction at low temperatures. With abundant acid sites and both SCR reactions following the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism on the catalyst surface at a low temperature of 150 °C, more abundant acid sites and reaction paths created favorable conditions for NH3-SCR reactions at low temperatures.

Si/Al Ratio Determines the SCR Performance of Cu-SSZ-13 Catalysts in the Presence of NO2.

A comparative study was performed to investigate the NH3-selective catalytic reduction (SCR) reaction activity of Cu-SSZ-13 zeolites having Si/Al ratios (SARs) of 5, 18, and 30. Remarkably, the Cu-SSZ-13 zeolite catalysts exhibited completely opposite behaviors as a function of SAR under standard SCR (SSCR) and fast SCR (FSCR) reaction atmospheres. Under SSCR conditions, the NOx conversion increased as expected with the decreasing SAR. Under FSCR conditions, however, the NOx conversion decreased as the SAR decreased, contrary to expectations. In this study, based on characterization of the catalysts by X-ray diffraction, transmission electron microscopy, electron paramagnetic resonance, H2-temperature-programmed reduction, temperature-programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy, together with theoretical calculations, the authors found that the amount of Brønsted acid sites goes up while the SAR goes down, leading to an increase in the accumulation of NH4NO3 under FSCR reaction conditions. Moreover, the accumulated NH4NO3 is of greater stability for those low SAR Cu-SSZ-13 catalysts. These two reasons cause the FSCR performance of Cu-SSZ-13 to decrease with a decrease in SAR. As a result, the NO2 effect on SCR activity changes from promotion to inhibition as the SAR decreases.

Insight into the promotional mechanism of Cu modification towards wide-temperature NH3-SCR performance of NbCe catalyst

A simple strategy of Cu modification was proposed to broaden the operation temperature window for NbCe catalyst. The best catalyst Cu 0.010 /Nb 1 Ce 3 presented over 90% NO conversion in a wide temperature range of 200−400 o C and exhibited an excellent H 2 O or/and SO 2 resistance at 275 o C. To understand the promotional mechanism of Cu modification, the correlation among the “activity-structure-property” were tried to establish systematically. Cu species highly dispersed on NbCe catalyst to serve as the active component. The strong interaction among Cu, Nb and Ce promoted the emergence of NbO 4 and induced more Brønsted acid sites. And Cu modification obviously enhanced the redox behavior of the NbCe catalyst. Besides, EPR probed the Cu species exited in the form of monomeric and dimeric Cu 2+ , the isolated Cu 2+ acted as catalytic active sites to promote the reaction: Cu 2+ -NO 3 - +NO(g)→Cu 2+ -NO 2 - +NO 2 (g). Then the generated NO 2 would accelerate the fast-SCR reaction process and thus facilitated the low-temperature deNO x efficiency. Moreover, surface nitrates became unstable and easy to decompose after Cu modification, thus providing additional adsorption and activation sites for NH 3 , and ensuring the improvement of catalytic activity at high temperature. Since the NH 3 -SCR reaction followed by E-R reaction pathway efficaciously over Cu 0.010 /Nb 1 Ce 3 catalyst, the excellent H 2 O and SO 2 resistance was as expected.

Simultaneous removal of NO and elemental mercury from coal-fired flue gas using natural ferruginous manganese ore at low temperature

Natural ferruginous manganese ore (FMO) was developed as a cost-effective NH3-SCR catalyst for the simultaneous removal of NO and elemental mercury (Hg0) from coal-fired flue gas. The chemical composition, pore structure, crystal structure and surface chemistry of FMO were characterized by different methods. The results showed that FMO was constructed with sheet-like structures. There were massive slit-shaped pores in the FMO. Manganese and iron oxides existed in an amorphous form. After calcination, the NO removal performance of FMO was obviously decreased due to the decomposition of MnO2. A high NH3/NO ratio was conductive to NO removal but excessive NH3 was unnecessary for NO removal. NO could promote Hg0 removal whereas Hg0 had little effect on NO removal. The effects of different SCR atmosphere (inlet, middle layer, and outlet), reaction temperature (100 °C–400 °C), and flue gas composition (O2 and SO2) on NO/Hg0 removal was also studied. At the optimal temperature (200 °C), the NO removal efficiency was greater than 80% and the Hg0 removal efficiency was close to 100%. To some extent, FMO presented preferable SO2 poisoning resistance probably because of the presence of iron oxide. The mechanism of NO/Hg0 removal over FMO was further described. Both fast SCR and conventional SCR reactions happened over FMO during NO removal. Mn4+, Fe3+, and lattice oxygen were first consumed and then replenished by gaseous O2. FMO served as a catalyst for Hg0 oxidation during a long period of denitration process.

Steady-state kinetic modeling of NH3-SCR by monolithic Cu-CHA catalysts

The kinetic modeling of NH3-SCR with a Cu-ion-exchanged chabazite (Cu-CHA) catalyst in steady-state reactions was investigated by a combination of flow reactor experiments and kinetic modeling with an in-house computational code. Here H-CHA and Cu-CHA monolith cylindrical catalyst specimens were used to identify the reactions at both Brønsted (H+) acid sites and Cu sites. Based on the proposals in the literature for NH3-SCR mechanism, a scheme for SCR and side reactions incorporating 17 reactions is proposed, and some of the reactions were verified by the experimental results here for the adsorption/desorption of NH3, unsteady-state reactions of the adsorbed NH3 with NO2 over the Cu-CHA and H-CHA, and NH3 oxidation over Cu-CHA. The Arrhenius parameters for Standard, Fast, and Slow SCR reactions, NH3 oxidation, NO oxidation to NO2, NO2 decomposition to NO, and NH3 desorption were obtained by kinetic experiments. This report develops a quantitative simulation model for SCR and side reactions, and the model was validated by experimental data for adsorption/desorption of NH3 and NO2, steady-state SCR reactions (Standard, Fast, and Slow SCR), and side reactions (NH3 and NO oxidation). The simulation model developed in this study can predict the steady-state SCR data and adsorption/desorption of NH3 and NO2 under unsteady-state.

Insight into the influence of K on the adsorption performance and reaction pathways of CeO2/TiO2 catalyst

K species can cause severe deactivation of CeO2/TiO2 NH3-SCR catalyst. In this work, the effects of K on the adsorption and activation of reactive species, the evolution of intermediates, and the surface reactions on the CT catalyst were systematically studied using DFT calculations. The adsorption of K caused the recombination of surface electrons, resulting in the active sites Ce and O getting electrons from K and becoming less reactive, which inhibited the adsorption and activation of NH3. The presence of K could promote the adsorption of NO and O2, and the decomposition of O2. But most of the adsorbed NO and O2 would transform into inactive NOx species, and then cover the active sites on the surface. The continuous dehydrogenation reaction of NH3 was suppressed, thereby hindering the formation of NH2. The formation and decomposition of the intermediate NH2NO were both inhibited, which seriously affected the reduction of NO. In addition, the formation of NO2 involved in the fast-SCR reaction was inhibited, which is the main reason for the deactivation of CT catalysts at low temperatures.

N2O Formation during NH3-SCR over Different Zeolite Frameworks: Effect of Framework Structure, Copper Species, and Water

The formation characteristics of N2O were investigated with respect to copper-functionalized zeolites, i.e., Cu/SSZ-13 (CHA), Cu/ZSM-5 (MFI), and Cu/BEA (BEA) and compared with the corresponding zeolites in the H form as references to elucidate the effect of the framework structure, copper addition, and water. Temperature-programmed reduction with hydrogen showed that the CHA framework has a higher concentration of Cu2+ (Z2Cu) compared to MFI and BEA. The characterizations and catalyst activity results highlight that CHA has a framework structure that favors high formation of ammonium nitrate (AN) in comparison with MFI and BEA. Moreover, AN formation and decomposition were found to be promoted in the presence of Cu species. On the contrary, lower N2O formation was observed from Cu/CHA during standard and fast SCR reactions, which is proposed to be due to highly stabilized AN inside the zeolite cages. On the other hand, significant amounts of N2O were released during heating due to decomposition of AN, implying pros and cons of AN stability for Cu/CHA with possible uncontrolled N2O formation during transient conditions. Additionally, important effects of water were found, where water hinders AN formation and increases the selectivity for decomposition to NO2 instead of N2O. Thus, less available AN forming N2O was observed in the presence of water. This was also observed in fast SCR conditions where all Cu/zeolites exhibited lower continuous N2O formation in the presence of water.

Vanadium catalyst based on a tungsten trioxide structure modified with antimony in NH3-selective catalytic reduction for improved low-temperature activity

For NH3-selective catalytic reduction (SCR) of NO, a catalyst that exhibits effective DeNOx performance and SO2 resistance at low temperatures is necessary. In V2O5-WO3/TiO2 systems, a WO3 structure enhanced by Sb addition is effective in improving the SCR performance and SO2 resistance. In this study, the addition of Sb5+ to the WO3 structure increased NH3 adsorption due to Lewis acid formation. Furthermore, the increased electron density on the catalyst surface resulted in the formation of a higher number of WO, which may serve as active sites for SCR, and improved oxygen mobility at low temperatures. This property subsequently caused an increase in the amount of oxygen used in SCR and improved the re-oxidation capacity in the rate-determining step to enhance the DeNOx performance. Furthermore, the NO2– produced during SCR induced the Fast-SCR reaction in V/WSb2Ti. Therefore, V/WSb2Ti produced more N2 than that produced by V/WTi during SCR over a short time, as established using Fourier transform infrared spectroscopy and quadrupole mass spectrometer.

Effect of oxygen vacancies on improving NO oxidation over CeO2 {111} and {100} facets for fast SCR reaction

Different exposed crystal surface CeO2 catalysts (nanopolyhedrons and nanocubes) are synthesized and treated by NH3 to investigate the effect of oxygen vacancies on improving NO oxidation for fast SCR reaction. The experiment results show that N-CeO2-NPs catalyst (treated by NH3) has the most oxygen vacancies of the four catalysts, thus leading to the best oxidation activity of NO, which further promotes the fast SCR reaction. From the XPS and Raman spectra results, it indicates that the more oxygen vacancies are due to the higher concentration of Ce3+ and surface adsorbed oxygen resulting from the NH3 treatment at high temperature. Meanwhile, the test of NO conversion to NO2 shows that the rich oxygen vacancies promote the conversion of NO to NO2, in which N-CeO2-NPs catalyst yields more remarkable promotion of NO oxidation to NO2, respectively. Also, the magnitude of the variety is in accordance with that of the oxygen vacancy content and concentration of Ce3+ ions. Thus, CeO2 with NH3 treatment is found to significantly enhance its NH3-SCR performance which leads to higher NOx removal efficiency of N-CeO2-NPs catalyst with {111} exposed crystal surface.

Enhancement strategies for SCR activity, H2O & SO2 resistances and N2 selectivity on upgraded HMoP/Co/MnCeOx/NF catalysts

MnCeOx/NF, Co/MnCeOx/NF and HMoP/Co/MnCeOx/NF catalysts were optimized step by step and used for the selective catalytic reduction (SCR) of NOx with NH3. The catalytic performance, water/sulfur resistances and N2 selectivity of the overall catalyst were significantly improved and maintained during above continuous optimization processes. Firstly, MnCeOx loaded 3D monolithic Ni-foam (MnCeOx/NF) catalysts were investigated using response surface methodology (RSM) method with central composite design (CCD). Regression equations and 3D response surface graphs showed that the model-predicted value was highly in line with experiment-actual result for the catalytic performances synchronously corresponding to load and calcination temperature, calcination and reaction temperature, which obtained 95.7% NOx conversion with 76.7% N2-selectivity at 179.5 °C over the optimal 16.5%MnCeOx/NF catalyst calcinated at 432 °C. Calcination temperature has a great influence on SCR activity that a suitable one increased the surface Mn4+, Ce3+ and chemical adsorption oxygen, while a high one visibly decreased NOx conversion due to the rapid weakened oxygen absorption and lattice oxygen content. The coexistence of typical Eley–Rideal and Fast-SCR reaction mechanisms were found over MnCeOx/NF, while the reaction rate of intermediates changed significantly but also reduced N2-selectivity due to the increased rate of side reactions as reaction temperature increasing. Secondly, among Co-, Ni- or Fe-modified catalysts, the optimized Co/MnCeOx/NF obtain an acceptable H2O and SO2 resistance with stable NOx conversion above 72% within 10 h at 175 °C. Furthermore, phosphomolybdic acid modified Co/MnCeOx/NF catalyst maintained more than 80% N2-selectivity at 275 °C along with almost 100% NOx conversion within 175 °C–275 °C. These advantages synergistically accelerate the overall SCR performances of activity, selectivity and resistance of the optimized HMoP/Co/MnCeOx/NF catalyst.

Insight into the Promoting Role of Er Modification on SO2 Resistance for NH3-SCR at Low Temperature over FeMn/TiO2 Catalysts

Er-modified FeMn/TiO2 catalysts were prepared through the wet impregnation method, and their NH3-SCR activities were tested. The results showed that Er modification could obviously promote SO2 resistance of FeMn/TiO2 catalysts at a low temperature. The promoting effect and mechanism were explored in detail using various techniques, such as BET, XRD, H2-TPR, XPS, TG, and in-situ DRIFTS. The characterization results indicated that Er modification on FeMn/TiO2 catalysts could increase the Mn4+ concentration and surface chemisorbed labile oxygen ratio, which was favorable for NO oxidation to NO2, further accelerating low-temperature SCR activity through the “fast SCR” reaction. As fast SCR reaction could accelerate the consumption of adsorbed NH3 species, it would benefit to restrain the competitive adsorption of SO2 and limit the reaction between adsorbed SO2 and NH3 species. XPS results indicated that ammonium sulfates and Mn sulfates formed were found on Er-modified FeMn/TiO2 catalyst surface seemed much less than those on FeMn/TiO2 catalyst surface, suggested that Er modification was helpful for reducing the generation or deposition of sulfate salts on the catalyst surface. According to in-situ DRIFTS the results of, the presence of SO2 in feeding gas imposed a stronger impact on the NO adsorption than NH3 adsorption on Lewis acid sites of Er-modified FeMn/TiO2 catalysts, gradually making NH3-SCR reaction to proceed in E–R mechanism rather than L–H mechanism.

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.

Novel Mn–Ce bi-oxides loaded on 3D monolithic nickel foam for low-temperature NH3-SCR de-NOx: Preparation optimization and reaction mechanism

This study explored the superior citrate method (CM) to synthesize Mn–Ce bi-oxides on 3D monolithic Ni-foam (NF) catalysts for the selective catalytic reduction of NO by NH3 (NH3-SCR). The 17 wt% Mn(7)Ce(3)Ox/NF (CM-17) catalyst shows the NOx conversion of 98.7% at 175 °C and 90% in the presence of 10 vol% H2O. It is revealed that the combination of surface-active oxygen (formed by high-level oxygen vacancies) and strongly oxidized Mn4+ species promots the Fast-SCR reactions, in which Mn4+ species play a leading role in NH3-SCR reaction, and the unsaturated Ni atoms and also Ce3+ species promote electron exchange and thus improve the redox performance. The coexistence mechanisms of Fast-SCR reactions and E-R pathways are observed over Mn-CeOx/NF catalyst, which may be promoted by the Brønsted sites at low temperature. In addition, the heat resistance, stability, 3D monolithic porous structure and excellent physical properties of foam nickel provide a unique growth substrates for catalysts preparation and reaction sites for NOx purification. Therefore, industrial application of Mn–Ce bi-oxides loaded on 3D monolithic is proposed to be achieved through reasonable preparation methods.