Low-temperature NH3-SCR Activity Research Articles

Recent Progress on Low-Temperature Selective Catalytic Reduction of NOx with Ammonia.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Synergistic effect of Cerium and Niobium enhances the low-temperature NH3-SCR activity of Fe-containing solid waste

To develop environment-friendly low temperature NH3-SCR catalyst and realize the resource utilization of industrial solid waste, this research prepared a series NH3-SCR catalysts by using Fe-containing solid waste loading CeO2 and Nb2O5. It was found that the Ce2Nb1/Fe-Si catalyst exhibited the highest NH3-SCR activity, reaching about 93 % NOx conversion and 99 % N2 selectivity at 300 °C due to the synergistic effect between Ce and Nb species. The characterization results showed that the interaction between Ce and Nb was stronger than that between Nb and Fe species, promoting the NH3 adsorption and redox cycle of Ce4+ + Nb4+ ↔ Ce3+ + Nb5+, Ce3+ + Fe3+ ↔ Fe2+ + Ce4+, which increased the contents of Fe2+ and surface adsorbed oxygen. Moreover, the co-existence of Ce and Nb species would increase the NOx adsorption ability and promote the oxidation of NO to NO2, which was beneficial for “Fast SCR” reaction. Finally, the introduction of CeO2 and Nb2O5 did not changed the reaction mechanism, followed by a Langmuir-Hinshelwood mechanism.

Interface Induced by Hydrothermal Aging Boosts the Low-Temperature Activity of Cu-SSZ-13 for Selective Catalytic Reduction of NOx.

Hitherto, sulfur poisoning and hydrothermal aging have still been the challenges faced in practical applications of the Cu-SSZ-13 catalyst for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust. Here, we elaborately design and conduct an in-depth investigation of the synthetic effects of hydrothermal aging and SO2 poisoning on pristine Cu-SSZ-13 and Cu-SSZ-13@Ce0.75Zr0.25O2 core@shell structure catalysts (Cu@CZ). It has been discovered that Cu@CZ susceptible to 750 °C with 5 vol % H2O followed by 200 ppm SO2 with 5 vol % H2O (Cu@CZ-A-S) could still maintain nearly 100% NOx conversion across the significantly wider temperature region of 200-425 °C, which is remarkably broader than that of the Cu-SSZ-13-A-S (300-400 °C) counterpart. The experimental results show that the hydrothermal aging process results in the migration of highly active Cu species within the cage of Cu-SSZ-13 to the CZ surface, forming CuO/CZ with abundant interfaces, which significantly enhances the adsorption and subsequent activation of NO, leading to the generation of reactive N2O3 and HONO intermediates. Moreover, density functional theory (DFT) calculations reveal that the H of the HONO* species can function as Brønsted acid sites, effectively adsorbing NH3 to generate the active NH4NO2* intermediate, which readily decomposes into N2 and H2O. Furthermore, this pathway is the rate-determining step with an energy barrier of 0.93 eV, notably lower than that of the "standard SCR" pathway (1.42 eV). Therefore, the formation of the new CuO/CZ interface profoundly boosts the low-temperature NH3-SCR activity and improves the coresistance of the Cu@CZ catalyst to sulfur poisoning and hydrothermal aging.

Structural properties and low-temperature NH3-SCR activity of CeO2-MnOx mixed oxides catalyst in the microwave field

The NH3-SCR technology, based on CeO2-MnOx catalysts, is of great significance for NOx removal at low temperatures. Microwave irradiation, as an effective heating approach, can enhance the low-temperature NH3-SCR activity of catalysts. Herein, the structure-activity relationship of CeO2-MnOx oxides catalysts was explored under microwave irradiation. The MnOx catalyst and Ce-Mn(1:1) mixed oxide catalyst exhibited good low-temperature de-NOx performance in the microwave field. With the increase of microwave irradiation intensity and time, the structure and phase composition of MnOx material underwent significant changes. CeO2 played a key role in maintaining the structural stability of the Ce-Mn(1:1) catalyst. In addition, microwave irradiation affected the redox properties and acidity of the catalyst, regulating the catalytic performance. Suitable microwave operation conditions were conducive to increasing the number of acidic sites and enhancing the redox properties, which improved the low-temperature catalytic activity of the catalyst.

SO2- and H2O-tolerant catalytic reduction of NOx at low temperatures through shielding ceria-based catalyst from poisoning: An efficient protection mechanism

Improving SO2/H2O resistance at low temperatures is a key challenge for industrial applications of NH3-SCR catalysts. Herein, a novel catalyst with remarkable low-temperature NH3-SCR activity and strong SO2/H2O resistance was designed by loading MoOx protective layer over porous kit-CeO2. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with basic characterization techniques were utilized to further elucidate the underlying mechanisms, and a dynamic balance between the deposition and decomposition of ammonia bisulfate was captured over porous kit-CeO2. Highly dispersed MoOx species pre-adsorbed with NH3 significantly curtailed the adsorption of NOx molecules, and the catalytic reaction over MoOx/kit-CeO2 primarily followed by the Eley-Rideal mechanism. At 250 °C, in the presence of SO2 and H2O, the Ce-O-Mo pairing site was almost unaffected and the super SO2- and H2O-tolerant property originated from the facilitated decomposition of ammonium bisulfate (ABS) and the shielding effect of Mo protective layer on SO2/H2O. This study provided an important guidance to the application of ceria-based catalysts for environmental control within low-temperature operations in non-power industries such as glass, steel, cement manufacturing, etc.

Regulating the distribution of iron active sites on γ-Fe2O3via Mn-modified α-Fe2O3 for NH3-SCR

Developing below 150 °C highly activity and broad reaction window catalysts has been challenging by using Fe-based catalysts for NH3-SCR. The octahedral Fe3+ (Fe3+Oh) sites exist in hematite (α-Fe2O3) and maghemite (γ-Fe2O3) are inactive for NH3-SCR due to the redox circle of Fe2+→Fe3+ reliance on the distribution of iron sites in crystal structure. Here we modified the α-Fe2O3 crystal structure by substituting part inactive Fe3+Oh sites with catalytically active Mn3+Oh sites, which promoted the formation of γ-Fe2O3 to generate the active tetrahedral Fe3+ (Fe3+Td) sites and enhanced the magnetism of the Fe1−yMnyOx. The strong Fe-O-Mn interaction established by crystal coordinative configurations not only boosted the formation of NO2 but also facilitated the Brønsted acid circle. Surprisingly, the Fe0.85Mn0.15Ox exhibited the superior low temperature NH3-SCR activity, with NOx conversion above 100% at 100–275 °C under a GHSV of 60000 h−1.

Unveiling the remarkable deNOx performance of MnMoVOx catalysts via dual regulation of the redox and acid sites

Developing NH3-SCR catalysts possessing excellent NO conversion, N2 selectivity, and alkali-tolerance at low-temperatures remains a great challenge. MnOx-based catalysts have attracted attention due to their exceptional low-temperature NH3-SCR activity. However, their strong oxidative capabilities often lead to excessive NH3 oxidation, causing a narrow operating temperature window and low N2 selectivity. In this study, we employed Mo and V to simultaneously fine-tune the acid and redox sites of MnOx, effectively suppressing the excessive NH3 oxidation at medium-high temperatures. The adjusted Mn0.90Mo0.09V0.01Ox catalyst demonstrated excellent low-temperature activity, a significantly broader active temperature window, and robust resistance to alkali metal poisoning. Multiple characterization results indicate that this dual acid-redox sites regulation strategy appropriately weakens the oxidative capacity while notably enhances surface acidity of the catalyst. Furthermore, the combination of in situ DRIFTS and DFT calculations reveal that following the adjustments of Mo and V on MnOx, new Brønsted acid sites are generated. Besides, the regulated catalyst evidently inhibits NO adsorption and nitrate species formation, thus promoting the reaction exclusively through the E-R mechanism, resulting in boosted N2 selectivity. This study also demonstrates that the optimum NH3-SCR performance of catalyst is achieved when oxidation ability and acid sites are harmoniously balanced, rather than following a "stronger is better" trend. In addition, through effective dual-active site regulation, the K-poisoning catalyst retains satisfactory catalytic activity. Thereby, the proposed dual-active sites regulation strategy in this study offers beneficial insights for the development of high-performance denitration catalysts.

Effects of CO2 on the low-temperature NH3-SCR performance of CeOx-biochar catalyst

Low-temperature NH3-SCR of NOx technology was recognized as a promising way to control NOx emissions from industrial kilns at low temperatures. Except for SO2 and H2O, lots of CO2 exist in the flue gas of industrial kilns. However, there are few researches on the effects of CO2 on the low-temperature NH3-SCR activity. To investigate the effects of CO2 on the low-temperature NH3-SCR performance and mechanism of CeOx-biochar catalyst, a series of experiments employed. The results showed that the presence of CO2 had an obvious inhibitory effect on the NH3-SCR performance of CeOx-biochar0.6 at 150 °C. There was competitive adsorption between CO2 and NO on the surface of CeOx-biochar0.6. CO2 was mainly adsorbed on CeO2, which hindered the oxidation of NO and the adsorption of NOx on the catalyst surface. Meanwhile, although the presence of CO2 could improve NH3 adsorption capacity of CeOx-biochar0.6 due to the formation of NH4HCO3 and (NH4)2CO3, the existence of carbonates and bicarbonates blocked the formation of active intermediates NH4NO2 and NH2NO for NH3-SCR reaction. As a result, the NOx conversion of CeOx-biochar0.6 was dropped from 96.9% to 40.8% at 200 °C. In view of the bicarbonate and carbonate species was unstable at high temperatures, the inhibitory effect of CO2 on NH3-SCR reaction was gradually weakened with the increase of reaction temperature. The NH3-SCR activity of CeOx-biochar0.6 was seldom affected by CO2 at 300 °C. Finally, a CO2 poisoning mechanism for NOx removal by NH3-SCR at low temperature was proposed.

Enhancement low-temperature NH3-SCR activity of the Fe-Mn-Mo/TiO2 catalyst and its DFT calculations and kinetics

The 5 %Fe-3 %Mn-3 %Mo/TiO2 catalyst was prepared by impregnation method, and the effects of Mn, Mo doping on the structure and NH3-SCR performance of Fe-based catalysts were investigated. The 5 %Fe-3 %Mn-3 %Mo/TiO2 catalyst showed highest NH3-SCR activity (over 80 % NO conversion at 200–400 °C) and best long-time stability. Especially, the conversion rate of NO was 97 % at 250–300 °C. The introduction of Mn, Mo increased the surface acidity and redox ability of the 5 %Fe/TiO2 catalyst and improved the dispersion of active components (Fe). DFT calculations showed that the Mo-MnFe2O4 (311) surface over 5 %Fe-3 %Mn-3 %Mo/TiO2 catalyst had the strongest adsorption capacity for NO and NH3, which was more favorable for the SCR process, and this result was consistent with the NH3-SCR activity test. Kinetics results indicated the apparent activation energy of the 5 %Fe-3 %Mn-3 %Mo/TiO2 catalyst decreased by 39 % when compared to 5 %Fe/TiO2. It was evident that the addition of Mn and Mo reduced the apparent activation energy of the catalyst.

Engineering CeZrOx-Cu/SSZ-13 coupled catalysts to synergistically enhance the low-temperature NH3-SCR activity

Improving the low-temperature (<200 °C) NOx conversion of Cu/SSZ-13 NH3-SCR catalysts is crucial for achieving zero NOx emissions from diesel engines. This is primarily due to the limited oxidation capacity of active Cu species limits NOx conversion at low temperatures, particularly following the hydrothermal treatment at high temperatures (>650 °C) and high water vapor (10 vol.%). In this study, CeZrOx-Cu/SSZ-13 (CZ/CSZ) coupled catalysts were synthesized to construct additional active sites for NO oxidation on the CSZ surface, thus improving the low-temperature NOx conversion through synergistic interaction. The results reveal that when the CSZ and CZ mass ratio is 1:3, the NOx conversion at 150 °C is improved by 11% after hydrothermal treatment at 800 °C for 16 h. The coupled CZ oxidizes NO at low temperatures and generates abundant HONO species on CZ/3CSZ. These HONO species formed on CZ transfer to CSZ and rapidly react with BAS-NH3 to produce easily decomposable NH4NO2. This efficient synergistic mechanism circumvented the rate-limitation caused by Cu species, thus enhancing the low-temperature activity. Additionally, the coupled CZ obviously reduces the amount of H2O adsorption on CZ/3CSZ and effectively mitigates the damage of Si-OH-Al sites by H2O molecules during the hydrothermal treatment. The improved structural stability enables the maintenance of isolated Cu2+ and stable acid sites, ensuring the rapid response of synergistic reactions, ultimately resulting in a significantly enhanced in the low-temperature SCR activity even after the hydrothermal treatment. This work would provide a novel strategy for designing the next generation of commercial NH3-SCR catalysts.

CuOx and WOx modified CeO2-TiO2 with various morphologies for the selective catalytic reduction of NOx with NH3

CuOx and WOx modified CeO2-TiO2 (CeTi) with three various morphologies were fabricated and applied for selective catalytic reduction of NOx by NH3 (NH3-SCR). Catalytic performance showed that the loading of CuOx and WOx can enhance the low-temperature and high-temperature NH3-SCR activity of CuOx-WOx/CeO2-TiO2 (CuW/CT), respectively. CuOx-WOx/CeO2-TiO2 nanopolyhedrons (CuW/CT-NP), CuOx-WOx/CeO2-TiO2 nanorods (CuW/CT-NR) and CuOx-WOx/CeO2-TiO2 nanocubes (CuW/CT-NC) predominately exposed {111}, {110} and {100} crystal planes, respectively. The NH3-SCR performance indicated that CuW/CT-NP achieved the best NH3-SCR activity, excellent resistance to H2O&SO2 and the widest reaction temperature window with a full conversion temperature (T90) of 178 °C, much lower than that of CuW/CT-NR (210 °C) and CuW/CT-NC (273 °C). XPS, Raman, H2-TPR and NH3-TPD results confirmed that the redox and acidic properties of CuW/CT catalysts were closely related to the exposed facets of CeTi supports. The in situ DRIFTS results indicated that both NH3 and NOx adsorbed on the catalysts surface were active species. CuW/CT-NP exhibited stronger adsorption capacity for NOx and NH3 compared with CuW/CT-NR and CuW/CT-NC. A large number of active NH3 and nitrate species were formed on the surface of CuW/CT-NP, and the Langmuir Hinshelwood (L-H) pathway accelerated the NH3-SCR reaction, thereby enhancing the NH3-SCR performance.

Comparison of precursors for the synthesis of Cu-SSZ-39 zeolite catalysts for NH3-SCR reaction

Cu-SSZ-39 zeolites are noted for excellent NH3-SCR activity and hydrothermal stability for applications on diesel vehicles. In this study, Cu-SSZ-39 zeolites were prepared using four precursors. The results showed that the Cu-SSZ-39 prepared using ZSM-5 as precursor (Cu2.5-SSZ-39-Z) has high atomic utilization and the largest particle size, comparing with those using Y, beta and colloidal silica. The Si/Al ratio of Cu2.5-SSZ-39-Z is 9.0, and over 32% of Cu ions are Cu(OH)+ species. Cu2.5-SSZ-39-Z exhibited the best low-temperature NH3-SCR activity and hydrothermal stability. Over 90% of NOx conversion was achieved in the temperature range of 200–550 oC; meanwhile, over 90% NOx conversion could be obtained between 225 oC and 450 oC after hydrothermal aging at 850 oC for 16 h or 900 oC for 5 h. The excellent activity and hydrothermal stability make ZSM-5 the preferred precursor for industrial preparation of Cu-SSZ-39 zeolite.

Facile one-pot synthesis of Fe-UZM-35 catalysts for ammonia selective catalytic reduction

Owing to the narrow crystallization field, the synthesis of the large-pore zeolite UZM-35 with MSE topology starting from amorphous reagents in the presence of dimethyldipropylammonium ions as an organic structure-directing agent is often non-reproducible. Here we report the synthesis of UZM-35 via an interzeolite conversion route using USY zeolites, as the sole source of Al and Si, and a small amount of previously synthesized UZM-35 as seed crystals. We also present the one-pot synthesis of Fe-UZM-35 with the best low-temperature NH3-SCR activity among the iron-exchanged zeolites reported so far. These synthesis routes showed high reproducibility, together with considerably faster crystallization kinetics (≤1.5 vs 7 days) compared to conventional UZM-35 synthesis. When the Fe loading level was properly adjusted, the NH3-SCR activity of one-pot-synthesized Fe-UZM-35 was found to be essentially the same as that of post-synthetically exchanged Fe-UZM-35 due to the similarity in the nature of their iron active sites.

Mechanistic Insight into the Promotion of the Low-Temperature NH3-Selective Catalytic Reduction Activity over MnxCe1-xOy Catalysts: A Combined Experimental and Density Functional Theory Study.

CeO2 has attracted much attention in the field of selective catalytic reduction of NO with NH3 (NH3-SCR). However, poor low-temperature activity and a narrow operation window restrict the industrial application of Ce-based oxide catalysts. Herein, the low-temperature NH3-SCR activity of Ce-based oxide catalysts was dramatically improved by Mn doping, and the mechanism was elucidated at the atomic level by experimental measurements and density functional theory calculations. We found that the addition of Mn significantly promoted the formation of surface oxygen vacancies. The oxygen vacancies easily captured O2 in air and formed active oxygen species (superoxide and peroxide) on the surface. The surface active oxygen species efficiently oxidized NO into NO2 and then facilitated the "fast SCR" reaction. This study provides atomic-level insights into the promotion of the NH3-SCR activity over Mn-Ce composite oxides and is beneficial for the development of low-temperature Ce-based catalysts.

Sodium ion intercalation in vanadium oxide promotes low-temperature NH3-SCR activity: Sodium vanadium bronzes (Na0.33V2O5) for NOx removal

NH3-SCR (selective catalytic reduction) is a key technique for converting harmful NOx to harmless N2 and H2O in exhaust gases. Herein, V2O5, Na0.33V2O5 and Na1.2V3O8 are synthesized by a simple chemical process (the oxalate method) as examples of Na-intercalated vanadium oxide (sodium vanadium oxide bronze) for low-temperature NH3-SCR less than 150 °C. Among them, Na0.33V2O5 shows 92% (dry) and 86% (in the presence of 10 vol% water) NO conversions at 150 °C. The reaction rate of Na0.33V2O5 is 6.1-times higher than that of V2O5, indicating that Na intercalation accelerates NH3-SCR cycles. Mechanistic analysis suggests that the reaction mechanism of Na0.33V2O5 is different from those of other V-based catalysts. Although it has been believed that alkali ions inhibit catalytic cycles of NH3-SCR, the results of this study suggest that alkali ions affect the structural, redox and adsorption properties of bulk V2O5, leading to an increase in NH3-SCR activity.

Dual improvement in acid and redox properties of the FeOx/OAC catalyst via APS oxygen-functionalization: High low-temperature NH3-SCR activity, SO2 and H2O tolerance

Carbon-based transition metal catalysts (CTCs) have shown tremendous potential within NOx abatement, while the improvement of low-temperature NH3-SCR activity and SO2 and H2O tolerance remains challenging. Herein, we report an easily-achievable strategy to regulate the active phases of the FeOx/AC catalyst via APS oxygen-functionalization for improving in acid and redox properties and facilitating NH3-SCR activity. For redox sites, APS oxygen-functionalization boosted the metal-support interaction, immobilized γ-Fe2O3 nanoparticles highly dispersed on FeOx/OAC1.5-60-3 surface. Meanwhile, Fe valence and oxygen vacancy of FeOx were tailored by APS oxygen-functionalization, resulting in the formation of active oxygen vacancy-coupled Fe2+. For acid sites, in situ DRIFTs further confirmed that APS oxygen-functionalization enhanced the adsorption and activation properties of NH3 over the oxygen-containing functional groups (OCFGs) on FeOx/OAC1.5-60-3. Due to the improvement of acid and redox properties, FeOx/OAC1.5-60-3 delivered superior NO conversion (>90 %) at 120–250 °C and SO2/H2O resistance in comparison with FeOx/AC. Therefore, this work sheds light on a robust and pragmatic strategy for the rational design of CTCs and provides a new insight in improving catalytic performance of CTCs.

Low-temperature selective catalytic reduction of NO with NH3 over an FeO x /β-MnO2 composite.

A series of Fe-modified β-MnO2 (FeO x /β-MnO2) composite catalysts were prepared by an impregnation method with β-MnO2 and ferro nitrate as raw materials. The structures and properties of the composites were systematically characterized and analyzed by X-ray diffraction, N2 adsorption-desorption, high-resolution electron microscopy, temperature-programmed reduction of H2, temperature-programmed desorption of NH3, and FTIR infrared spectroscopy. The deNO x activity, water resistance, and sulfur resistance of the composite catalysts were evaluated in a thermally fixed catalytic reaction system. The results indicated that the FeO x /β-MnO2 composite (Fe/Mn molar ratio of 0.3 and calcination temperature of 450 °C) had higher catalytic activity and a wider reaction temperature window compared with β-MnO2. The water resistance and sulfur resistance of the catalyst were enhanced. It reached 100% NO conversion efficiency with an initial NO concentration of 500 ppm, a gas hourly space velocity of 45 000 h-1, and a reaction temperature of 175-325 °C. The appropriate Fe/Mn molar ratio sample had a synergistic effect, affecting the morphology, redox properties, and acidic sites, and helped to improve the low-temperature NH3-SCR activity of the composite catalyst.

Enhancement of low temperature NH3-SCR activity and SO2 & H2O tolerance over VPO-Co/TiO2: Determining roles of acidity via Co doping

A series of VPO-Co/TiO2 samples were synthesized for selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperature for the first time. The activity testing results suggested that the catalytic performance of VPO-Co/TiO2 was greatly improved after Co was introduced. Among all the catalysts, VPO-Co(0.07)/TiO2 provided the optimal low-temperature SCR activity, with NO conversion 100% and N2 selectivity above 98% at 180–270 °C. Additionally, VPO-Co(0.07)/TiO2 also presented extraordinary SO2 + H2O durability after introducing 200 ppm SO2 and 6 vol% H2O to the SCR reaction at 180 °C. The characterization results revealed that the oxidation capacity and the acidity were both improved with the Co doping. In this VPO-Co/TiO2 system, the increased acidity via the Co doping played a more critical role in raising the SCR activity. The richest surface acidity could offset the adverse effect of moderate oxidation capacity over VPO-Co(0.07)/TiO2, thus resulting in the best de-NOx performance at low temperature. Furthermore, the in-situ diffuse reflectance Fourier transform infrared spectroscopy (in-situ DRIFTS) studies revealed that the SCR reaction over VPO-Co(0.07)/TiO2 was proceeded via a combination of Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) pathways at 180 °C.

Poisoning mechanism of KCl, K2O and SO2 on Mn-Ce/CuX catalyst for low-temperature SCR of NO with NH3

In this study, a series of K species or/and SO2 poisoned Mn-Ce doped CuX (MCCX) catalysts were prepared from waste blast furnace slag (BFS), and the influence mechanism of K species or/and SO2 poisoning on the catalysts in low-temperature NH3-SCR process was investigated. The results demonstrated that MCCX-KCl catalyst had poorer NO conversion than MCCX-K2O below 200 °C, and the co-existence of K species and SO2 considerably inhibited NO conversion. Aggregation of the Cu active component might also be facilitated by co-poisoning with K species and SO2. Furthermore, K species and SO2 co-poisoned catalysts showed fewer isolated Cu2+ species and Mn4+ species than that of K-poisoned catalysts, indicating a decrease in active sites after the addition of SO2. The co-effect of K2O/KCl and SO2 species impaired the redox capacity and decreased the surface acidity than K-poisoned catalysts to a greater extent. Nevertheless, K-poisoning had little effect on the surface intermediates during SCR reaction process, and K-poisoned catalysts followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. However, MCCX-KCl-SO2 and MCCX-K2O-SO2 catalysts contained fewer nitrate species, indicating that the co-effect of K species and SO2 impeded the reaction between NO + O2 species and pre-NH3 species, thereby resulting in poor low-temperature NH3-SCR activity.

Effects of Nb-modified CeVO4 to form surface Ce-O-Nb bonds on improving low-temperature NH3-SCR deNOx activity and resistance to SO2 & H2O

A series of novel ternary CeVO4 modified with different Nb contents (CeNbVO) catalysts were synthesized via alkali-assisted hydrothermal method, and applied for selective catalytic reduction (SCR) of NOx with NH3. The as-prepared CeNbVO catalysts show enhanced NH3-SCR performance compared with the CeVO catalyst. Especially, the optimal CeNbVO-2 with appropriate Nb amount displays the superior NH3-SCR activity with above 90 % NO conversion in the range of 210–420 °C with 5 %vol·H2O and excellent resistance to H2O/SO2. Effects of Nb-modified CeVO4 to form surface Ce-O-Nb bonds on improving low-temperature NH3-SCR activity and resistance to SO2 & H2O were investigated by using various characterization methods, including XRD, Raman spectroscopy, N2 physical adsorption, TEM, H2-TPR, NH3-TPD, O2-TPD, NO-TPD, XPS, kinetic tests and in-situ DRIFTS. Different phases can be obtained with the different Nb amount doping, leading to different SCR activity. The pure CeVO4 phase with the short-range ordered Nbn+–O2–-Cen+ bond is formed by adding a small amount of Nb in CeNbVO-2. The superior catalytic performance of CeNbVO-2 can be ascribed to its improved acidity and suitable oxidation ability, which may counteract the negative effects of nonselective NH3 oxidation to N2O. It is found that the formation of Nbn+–O2–-Cen+ bonds can significantly increase the surface active oxygen species and oxygen vacancy of the catalyst. Additionally, the NH3-SCR reaction mechanism is revealed using in-situ DRIFTS and kinetic studies.