NH3-SCR Research Articles

Promoting effect of Si on MnOx catalysts for low-temperature NH3-SCR of NO: Enhanced N2 selectivity and SO2 resistance

Manganese-based catalysts have excellent catalytic performance for the effective removal of NOx in low-temperature flue gas through selective catalytic reduction with NH3 (NH3-SCR). However, the catalysts suffer from poor SO2 resistance and N2 selectivity in the actual process, which still need to be solved. In this study, MnOx-SiO2 mixed oxide catalysts were designed and prepared by coprecipitation method to improve the SO2 resistance and N2 selectivity in NH3-SCR process. In the presence of 100 ppm SO2, MnOx-SiO2(0.72) catalyst showed excellent sulfur resistance with the conversion of NOx reached 95 % even after introducing SO2 for 8 h. The doping of SiO2 greatly improved the specific surface area and surface acidity of the catalyst, which was conducive to the adsorption of NH3. Meanwhile, the redox performance of the catalyst was decreased, improving the N2 selectivity. The study on the sulfur resistance mechanism revealed that the doping of SiO2 reduced the adsorption of SO2 on the surface of catalyst, protecting the active components and avoiding the accumulation of sulfates, thus achieving good sulfur resistance performance. This work provided an environmentally friendly NH3-SCR catalyst by inhibiting SO2 adsorption to have good catalytic performance and SO2 resistance.

Promoting effect of N2 calcination on FeCeOx for selective catalytic reduction of NOx with NH3

AbstractBACKGROUNDSelective catalytic reduction of NOx with NH3 (NH3‐SCR) technology is effective for NOx removal. Acidity and redox ability are two significant characteristics for NH3‐SCR catalysts. This study investigated the effects of N2 calcination on the NH3‐SCR performance of FeCeOx, inspired by the promotion of active oxygen species with inert atmosphere calcination. The structure‐activity relationship and reaction mechanism of the two catalysts (Fe9CeOx‐N2 and Fe9CeOx‐air) were concluded based on the results of characterization.RESULTSFe9CeOx‐N2 showed better NOx conversion and SO2 resistance than Fe9CeOx‐air, especially in low and medium temperature range. For fresh catalysts, a higher amount of Oα, Fe3+ and Ce3+ promoted the redox ability of Fe9CeOx‐N2, and further improved the NOx adsorption and activation. Additionally, the greater number of acid sites of Fe9CeOx‐N2 was another reason for its competitive performance. For SO2‐poisoned catalysts, the primary reaction pathway was the reaction between pre‐adsorbed NH3 species and NOx. The higher amount of acidity from metal sulfate contributed to its better SO2 tolerance of Fe9CeOx‐N2.CONCLUSIONHigher redox ability and acidity led to better NH3‐SCR activity and SO2 tolerance of Fe9CeOx‐N2. This study provides a feasible preparation method for enhancing the NOx conversion and SO2 tolerance of NH3‐SCR catalysts. © 2023 Society of Chemical Industry.

V2O5 supported on NbTiOx: A novel NH3-SCR catalyst with high performance induced by V-Nb-Ti interaction

The traditional vanadia-based catalysts frequently employed for selective catalytic reduction of NOx with NH3 (NH3-SCR) have been criticized for their restricted operating temperature window and subpar low-temperature activity. In the present work, a series of NbTiOx solid solutions were used as supports to prepare V/NbxTi1-x (1.0 wt% V2O5) catalysts. The novel 1V/Nb0.15Ti0.85 catalyst presented superior catalytic performance and excellent resistance to SO2 in the SCR reaction, reaching above 95% NOx conversion in a wide temperature range from 270 to 485 °C. In comparison to pure TiO2, Nb-Ti-O solid solution support exhibited different pore structures and pore sizes, contributing to much larger specific surface area. Meanwhile, the surface Nb-OH and Nb = O bonds could serve as new acid sites to facilitate more NH3 adsorption. More importantly, there was a strong redox-acid interaction (V-Nb-Ti) for 1V/Nb0.15Ti0.85 catalyst, which induced the formation of highly redox-active V sites and promoted the activation of adsorbed ammonia, facilitating the NH3-SCR reaction. Unlike 1V/Ti, NOx species adsorbed on 1V/Nb0.15Ti0.85 participated in NH3-SCR reaction below 250 °C, further promoting the low-temperature activity. This study provides a novel vanadium-based catalyst with a low vanadium loading and high SCR activity for practical NOx abatement.

Insights into the simultaneously enhanced activity, selectivity, and H2O resistance of cobalt modified MnCeOx/TiO2 catalyst for selective catalytic reduction of NOx with NH3

The promotional effect of cobalt (Co) modification on MnCeOx/TiO2 catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) was investigated extensively in this study. A series of CoMnCeOx/TiO2 catalysts with different Co contents were prepared by the facile impregnation method and characterized by XRD, N2 sorption, XPS, H2-TPR, NH3/NO-TPD, and in situ DRIFT spectroscopy. The results indicate that the modification of MnCeOx/TiO2 by Co could significantly improve its low-temperature activity and N2 selectivity, and broaden operation temperature window (>95% NO conversion at 100–225 °C), and that the optimal mole ratio of Co to Ti in the catalyst is 0.4. Furthermore, the introduction of Co to MnCeOx/TiO2 could obviously enhance H2O resistance. The characterization results reveal that the addition of Co to the MnCeOx/TiO2 catalyst could enhance the dispersion of MnOx and CeOx, and increase the relative ratio of Mn4+ and Ce3+ on the catalyst surface. In addition, the surface area of MnCeOx/TiO2 is enlarged evidently after doping of Co. From the results of H2-TPR, NH3/NO-TPD, and NO oxidation, CoMnCeOx/TiO2 exhibits stronger reducibility, larger adsorption capacity for NH3 and NO, and higher NO to NO2 conversion than MnCeOx/TiO2, thus obtaining the higher SCR activity, N2 selectivity, and H2O tolerance. In situ DRIFT spectroscopy tests indicate that the NH3-SCR reaction on both MnCeOx/TiO2 and CoMnCeOx/TiO2 follow simultaneously the Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Promoting the catalytic activity and SO2 resistance of CeO2 by Ti-doping for low-temperature NH3-SCR: Increasing surface activity and constructing Ce3+ sites

CeO2-based oxides are promising catalysts for low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), but the poor SO2 poisoning resistance hampers its further application. Herein, we developed a Ti-doped CeO2 (Ti-CeO2) catalyst with high specific surface area by calcining TiSO4-doped Ce-MOF precursor for NO removal. On Ti-CeO2, abundant Ti3+ sites are generated by the thermally reducing carbon in the organic ligand during calcination, the electronic interaction between Ti3+ and Ce4+ leads to the generation of many more exposed Ce3+ sites, effectively preventing the sulphuration of surface Ce species to Ce2(SO4)3. Besides, the residual sulfate species provides plentiful Brønsted acid sites for NH3 adsorption. Therefore, both the catalytic performance and SO2 resistance of CeO2 are significantly enhanced after Ti-doping. Remarkably, above 80% NO conversion is obtained between 220 and 450 °C on Ti0.10-CeO2 (the molar ratio of Ti:Ce is 0.10 in the synthesis process), and the NO conversion maintains around 76.0% for 170 h in the presence of 100 ppm SO2 and 5 vol% H2O at 250 °C. The reaction on CeO2 and Ti-CeO2 follow the Langmuir-Hinshelwood (L-H) mechanism, but abundant Ce4+ in CeO2 having high oxidation ability leads to the generation of plentiful stable nitrate or nitrite to occupy the active sites. In contrast, the over oxidation of NO and SO2 adsorption are inhibited because of Ti-doping, resulting in superior catalytic performance in the SCR reaction.

Enhanced low-temperature NH3-SCR performance by g-C3N4 modified Ce-OMS-2 catalyst

The xCN/Ce-OMS-2 (x is the nominal weight percentages of g-C3N4, x = 5.0, 10.0, and 15.0 wt%) catalysts were prepared via an in-situ synthesis method. Physicochemical properties of the as-prepared materials were measured by means of the XRD, TEM, SEM, BET, XPS, H2-TPR, NH3-TPD, and in-situ DRIFTS techniques, and their catalytic activities for the selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperatures were evaluated. Among all the catalysts, the 10CN/Ce-OMS-2 catalyst showed the widest temperature span for NH3-SCR reaction and the best resistance to SO2 poisoning. This may result from that the modification of g-C3N4 promoted the electron transfer in catalyst, not only increased the concentration of active Mn4+ species and chemisorbed oxygen species, but also improved the surface acidity of the catalyst. In-situ DRIFTS results further showed that the modification of g-C3N4 increased the amount of acid sites and enhanced the formation of NH3 or NO intermediates (i.e., monodentate and bidentate nitrate), which enhanced the reaction activity of the catalyst.

Unraveling the synergy between MnOx and CeO2 in MnOx-CeO2 SCR catalysts based on experimental and DFT studies

MnOx-CeO2 catalysts have shown great potential as selective catalytic reduction of NOx by NH3 (NH3-SCR) at temperature below 300 °C. While the improved catalytic activity due to the synergy between MnOx and CeO2 in MnOx-CeO2 catalysts is widely acknowledged, the synergistic mechanism in NH3-SCR reaction remains elusive due to the lack of direct evidences. In this study, we report a comprehensive study of how MnOx and CeO2 interacted during NH3-SCR in two designed MnOx-CeO2 catalysts with different Ce4+ concentration through both experimental and theoretical approaches. Our results reveal that electron transfer from Mn2+ to Ce4+ induced the formation of surface oxygen vacancy on the CeO2, leading to the synergistic promotion of low-temperature catalytic performance by establishing Mn-redox and Ce-redox cycles to activate NH3 and O2, respectively. This work provides a distinct mechanism diagram of the synergy between MnOx and CeO2, which sheds light on the rational design of low-temperature NH3-SCR catalysts.

Excellent NH3-SCR activity and significant sulfur resistance over novel core–shell Cu-SSZ-13@meso-CeO2 catalyst

A core–shell Cu-SSZ-13@meso-CeO2 catalyst with both microporous and mesoporous pores was successfully synthesized using a self-assembly technique in this research. The catalyst was tested via NH3–SCR of NOx removal from diesel exhausts. The Cu-SSZ-13@meso-CeO2 catalyst was found to have significantly improved activity at high temperatures, wider temperature range, improved hydrothermal stability, and anti-sulfur performance. After the introduction of meso-CeO2, a significant synergistic effect between the Cu2+/Ce4+ ions on the core–shell layer was observed, and the amounts of Cu2+–2Z species and acidic sites increased, which may be conducive to the superior catalytic performance of Cu-SSZ-13@meso-CeO2 contrasted to Cu-SSZ-13. Subsequent to hydrothermal treatment, Cu-SSZ-13@meso-CeO2 maintains a superior CHA-type structure, additional L-acidic sites, and isolated Cu2+ ions while being protected by a meso-CeO2 shell. In addition, in the presence of SO2, the meso-CeO2 shell in Cu-SSZ-13@meso-CeO2 reacts with SO2 to produce cerium sulfate, which serves as a protective component for the copper species on Cu-SSZ-13. Moreover, the reaction mechanism studies have indicated that NO reduction primarily adheres to the E–R mechanism on Cu-SSZ-13, whereas both L–H and E–R mechanisms contribute to Cu-SSZ-13@meso-CeO2. SO2 has a less significant inhibitive effect on the adsorption of NH3 over Cu-SSZ-13@meso-CeO2 compared with that over Cu-SSZ-13. The inhibitive influence of SO2 on Cu-SSZ-13@meso-CeO2 is primarily owing to the inhibitory adsorption of NO and the reduction of the L–H reaction path, with little effect on the E–R reaction mechanism, which is significantly affected in Cu-SSZ-13. The Cu-SSZ-13@meso-CeO2 catalyst demonstrated exceptional NH3–SCR performance, a broad temperature range, and super hydrothermal stability as well as anti-sulfur performance, indicating great application prospects for NOx removal from diesel vehicle exhausts.

Cation migration in siliceous CHA probed with DFT simulation and its catalytic implication

The migration or diffusion of cations in siliceous CHA with an approximate Si/Al ratio of 11 has been investigated to explore the impact of their location on catalytic performance, using DFT simulation. The migration of protons within the CHA cage and the resulting total energy were calculated to determine the activation energy for this migration. As the distance between the Al site and the nearby framework oxygen decreased, the activation energy for site-to-site migration decreased as well. Eventually, the proton was stabilized near the Al site, where the activation energy for reverse migration reached its maximum at 1.60 eV. The migration of Na cations and Cu cations was also examined. The activation energy for Na cations was only 0.49 eV, which can be attributed to multiple coordination and explains the facile migration. On the other hand, Cu+2 migration had an activation energy of 1.56 eV, while Cu+1 migration had an activation energy of 1.06 eV. Consequently, under the given reaction conditions, the facile migration of the catalytic component Cu+1 can be expected. Thus, CHA containing Na cations exhibits improved low-temperature activity in NH3 SCR, despite the potential for thermal degradation at high temperatures.

Promoted NH3-SCR activity and hydrothermal stability of Cu-SSZ-50 catalyst synthesized by one-pot method

Nitrogen oxide (NOx) is one of the most critical contaminants in the air, and the control of NOx emission from diesel vehicles is very important. Cu-based small-pore zeolites have already been applied for NOx abatement on diesel vehicles. Among the small-pore zeolites, Cu-SSZ-50 catalysts with good NH3-SCR catalytic activity were believed to have potential for application. In this study, a one-pot synthesis method for Cu-SSZ-50 catalysts was developed for the first time, using the co-templates of Cu-TEPA and 2,6-dimethyl-N-methylpyridinium hydroxide. In this synthesis method, Cu-SSZ-50 with various Cu contents can be obtained by adjusting the amount of Cu-TEPA without the need for a further after-treatment process. The addition of Cu-TEPA affected the framework atoms and Cu species, and a lower Si/Al ratio and more SCR active Cu species were obtained. The synthesized catalyst with a Cu/Al ratio of 0.40 exhibited over 90% NOx conversion between 200 °C and 450 °C for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Meanwhile, over 80% NOx conversion could be obtained from 250 °C to 450 °C after hydrothermal aging at 750 °C for 16 h. In addition, both L-H and E-R mechanisms were proven to exist for the one-pot-synthesized Cu-SSZ-50 by in situ DRIFTS experiments. The simple synthesis procedure, excellent catalytic activity and hydrothermal stability brighten the prospects for the application of Cu-SSZ-50.

Catalytic performance of Cu/Hβ catalysts for selective catalytic reduction of NO with NH3

A series of Cu(x)/Hβ catalysts were prepared by the impregnation method and the effect on the performance of the catalysts for the selective catalytic reduction of NO with NH3 (NH3-SCR) was investigated. Characterization techniques such as XRD, N2 adsorption-desorption, NH3-TPD, NO-TPD, H2-TPR, EDS and XPS were used to investigate the physical and chemical properties of the catalysts and the reason of the decrease of catalyst activity in the presence of SO2. It was shown that the catalyst Cu(3)/Hβ, the Cu loading was 3% and Cu(2)/Hβ, the Cu loading was 2%, exhibited the better catalytic activity when the initial reaction material contained no SO2 and SO2, and t95 was 169 and 225 °C, respectively. The analysis results of the catalyst before and after the reaction showed that the main reason for the decrease of catalyst activity in the presence of SO2 was that the ammonium sulfur salt which was formed by the reaction of SO2 and NH3 in the low reaction temperature covers the catalyst active center.

Unveiling the Origin of Selectivity in the Selective Catalytic Reduction of NO with NH3 over Oxide Catalysts.

The trade-off between activity and selectivity is a century-old puzzle in catalysis. In the selective catalytic reduction of NO with NH3 (NH3-SCR), various typical oxide catalysts exhibit distinct characteristics of activity and selectivity: Mn-based catalysts show outstanding low-temperature activity and poor N2 selectivity, mainly caused by N2O formation, while Fe- and V-based catalysts possess inverse characteristics. The underlying mechanism, however, has remained elusive. In this study, by combining experimental measurements and density functional theory calculations, we demonstrate that the distinct difference in the selectivity of oxide catalysts is determined by the gap in the energy barriers between N2 formation and N2O formation from the consumption of the key intermediate NH2NO. The gaps in the energy barriers follow the order of α-MnO2 < α-Fe2O3 < V2O5/TiO2, in correspondence with the order of N2 selectivity of the catalysts. This work discloses the intrinsic link between the target reaction and side reactions in the selective catalytic reduction of NO, providing fundamental insights into the origin of selectivity.

Unexpected promotional effects of HCl over CeO2-based catalysts for NOx reduction against alkali poisoning

In the flue gas of stationary NOx emission source, HCl was deemed to be one of the poisons of NOx reduction catalysts since it can react with ammonia to form NH4Cl and adhere to oxide surface, reducing the redox property and blocking the active sites of catalysts for selective catalytic reduction of NOx with NH3 (NH3-SCR). Herein, unexpected promotional effects of HCl over CeO2-based catalysts for NOx reduction against alkali poisoning have been unraveled. The modification of gaseous HCl on the surface of CeO2 constructed Cl-Ce-O-Ce-OH and Cl-Ce-O-Ce3+ sites, providing additional Brønsted and Lewis acid sites to facilitate ammonia adsorption. It is worth noting that HCl can not only directly neutralize alkali metal to reduce alkalinity, but also increase the acidity to offset the encroachment of acid sites by alkali metal poisoning. The lower NO adsorption energy and weaker oxidation property of Cl-Ce-O-Ce3+ sites also prevented the accumulation of inert nitrate on the surface of alkali metal poisoned catalyst. This work unveils multi-promotion mechanisms for the enhanced catalytic activity and anti-alkali poisoning of gaseous HCl modified cerium-based NOx reduction catalysts, providing a new thinking based on pollutant interaction for further development of highly efficient NOx reduction catalysts in multipollutant flue gas.

Reaction mechanism of selective catalytic reduction of NO with NH3 on Fe3O4 (1 1 1) surface: Experimental and DFT studies

Iron-based catalysts have vast potential for application prospect in selective catalytic reduction of NOx with NH3 (NH3-SCR). These catalysts offer excellent SCR activity at medium and high temperatures, N2 selectivity and environmental performance. Among these catalysts, Fe3O4 catalyst is highly active and significant in the denitration reaction due to the presence of both ferrous and ferric ions. This study utilized density functional theory (DFT) in combination with experimental measurements to investigate the reaction pathway of NH3-SCR on the Fe3O4(111) surface. The research found that the surface was favorable for the adsorption of NO molecules and followed the NO activation mechanism. Upon activation of NO at the Feoct site, it reacted with NH3 to form NH2NO, This compound then underwent decomposition into N2 and H2O. The presence of O2 on the catalyst facilitated the surface dehydrogenation reaction and completed the SCR cycle reaction. Based on the DFT results, the formation of NH2NO intermediate was the rate-determining step of SCR reaction. The Fe3O4(111) surface followed the L-H mechanism, which was in line with the experimental results. This study provided novel insights into the SCR reaction process and can be useful in developing new iron-based SCR catalysts.

Insight into the low-temperature DeNOx performance of copper-based oxides: Perspective from the valence state distribution

Understanding the effect of CuOx valence state specifically is significant for constructing high-performance Cu based oxide catalyst for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR). In this work, CuOx with different valence state including Cu2O, Cu2O-CuO and CuO particles are synthetized as model catalysts, which present same variation laws in catalytic performance at 180 ℃, 210 ℃ and 240 ℃. NH3-SCR catalytic performance of Cu2O-CuO heterojunction is the best, CuO exhibits good catalytic activity, while Cu2O possesses excellent N2 selectivity. Experimental results combined with density functional theory (DFT) calculations reveal that Cu2+ favor to form monodentate nitrate and possess strong ability of activation and dehydrogenation of NH3. The energy barrier for the form of NH2* is 79.5 kJ/mol, which is much lower than that for Cu+. While Cu+ is beneficial to NH3 and NOx adsorption and the form of bidentate nitrate, which have a positive impact on improving the selectivity of N2. Moreover, the collaboration of Cu2+ and Cu+ facilitates the redox capability and the form of surface adsorbed oxygen species, thus the reaction progress is further promoted. The SCR reaction catalyzed by three samples are dominated mainly by Langmuir-Hinshelwood (L-H) mechanism at 180 ℃. While at 240 ℃, the reaction over Cu2O follows Eley-Rideal (E-R) mechanism, during which the rate-limiting step is the internal hydrogen transfer procedure (NH2NO*→NHNOH*). Over CuO, the reaction follows L-H mechanism, the process of atomic isomerism (NHNOH*+ *→N2*+H2O*) is the rate-determining step. The energy barriers are 136 and 146 kJ/mol, respectively. This work may supply important theoretical support for the preparation of high-performance Cu-based oxide catalysts.

Enhanced NH3-SCR activity at low temperatures over MnOx supported on two-dimensional TiO2 derived from ZIF-8

ZIF-8 was used as a sacrificial template to synthesize porous anatase TiO2 (TiO2(Z)), consisting of agglomerated 2D flake-like particles. TiO2(Z) exhibited a high specific surface area (259 m2 g−1), in which the external surface area was significantly larger than that of the micropores because of its 2D flake-like structure. The catalytic performance of Mn/TiO2(Z) in selective catalytic reduction with NH3 (NH3-SCR) reaction was superior to that of MnOx catalysts supported on commercially available anatase TiO2 (TiO2(G); G5 and TiO2(H); Hombikat 8602). A large portion of MnOx was introduced into the micropores of Mn/TiO2(H) and Mn/TiO2(G), whereas MnOx was mainly located on the external surface of TiO2(Z) in the form of clusters. In particular, the cause of the high catalytic activity of Mn/TiO2(Z) was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy, demonstrating that bridged nitrate species formed on MnOx played an essential role in the low-temperature NH3-SCR reaction. These bridged nitrate species were likely formed over the MnOx clusters rather than the highly dispersed MnOx. Because TiO2(Z) exhibited a high external surface area, MnOx clusters were produced mainly over the external surfaces, resulting in high catalytic activity at low temperatures in the NH3-SCR reaction. In addition, it was demonstrated that high N2 selectivity of Mn/TiO2(Z) resulted from the migration of intermediate ammonium nitrate species, which was stabilized in a micropore structure.

A superior CeO2 @TiO2 catalyst with high dispersion of CeO2 for selective catalytic reduction of NOx with NH3

NOx emissions have raised many environmental problems and posed a serious threat to human health. Development of excellent catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR) is desirable. In this study, by ingeniously encapsulating Ce ions into MIL-125 channels, CeO2 @TiO2 catalysts were prepared by annealing the Ce/MIL-125 precursor. Due to the porous framework structure of the MIL-125, a large amount of Ce ions were adsorbed into the pores, thus achieving high dispersion of CeO2 in the composites. All the CeO2 @TiO2 catalysts exhibit a NO conversion of nearly 100% at 350 ℃ and a wide temperature range larger than 200 ℃. The CeO2 @TiO2 (2/3) catalyst shows high specific surface area and porous structure. Meanwhile, a large amount of Ce3+ sites, enriched surface oxygen species, outstanding adsorption of NOx species and high surface acidity for CeO2 @TiO2 (2/3) catalyst account for the excellent NH3-SCR properties with a wide temperature range (200 ℃ ∼ 450 ℃) and excellent H2O and SO2 resistance.

Modified cold-rolled sludge as catalyst for selective catalytic reduction of NOx with NH3 at low temperatures

Iron-based mixed oxides are considered commercially promising environmentally friendly catalysts for selective catalytic reduction with NH3 (NH3-SCR) for removing nitrogen oxides (NOx) because of their high catalytic activity. In this work, cold-rolled sludge with high content of iron species was used as the iron precursor for preparing catalysts by supported MnOx and/or sulfuric acid modifications. The prepared catalyst showed 100 % of NOx conversion in the temperature range of 60–180 ℃ and exhibited good resistance to SO2 and H2O. This is mainly attributed to the synergy between MnOx and iron sulfate, which improved the denitration performance at super low-temperature by improving the textural properties, redox capacity, surface acidity, and surface chemical states of the catalyst. Furthermore, the presence of Fe3+ and SO42- protected the active ingredient from the poisoning of SO2 and improved its sulfur resistance. The super low-temperature reaction mechanism of the catalyst was unraveled by transient reaction experiments and In situ DRIFTS. The results show that both Brønsted and Lewis acid sites are present in 10 %Mn/CRS-S-400, and that the reaction is dominated by the Langmuir–Hinshelwood (L–H) mechanism. This study provides an innovative and cost-effective strategy for the design of NH3-SCR catalysts by sulfation iron-based mixed oxides.

Effects of impregnation sequence on the NH3-SCR activity and hydrothermal stability of a Ce-Nb/SnO2 catalyst

Hydrothermal stability is crucial for the practical application of deNOx catalyst on diesel vehicles, for the selective catalytic reduction of NOx with NH3 (NH3-SCR). SnO2-based materials possess superior hydrothermal stability, which is attractive for the development of NH3-SCR catalyst. In this work, a series of Ce-Nb/SnO2 catalysts, with Ce and Nb loading on SnO2 support, were prepared by impregnation method. It was found that, the NH3-SCR activities and hydrothermal stabilities of the Ce-Nb/SnO2 catalysts significantly varied with the impregnation sequences, and the Ce-Nb(f)/SnO2 catalyst that firstly impregnated Nb and then impregnated Ce exhibited the best performance. The characterization results revealed that Ce-Nb(f)/SnO2 possessed appropriate acidity and redox capability. Furthermore, the strong synergistic effect between Nb and Sn species stabilized the structure and maintained the dispersion of acid sites. This study may provide a new understanding for the effect of impregnation sequence on activity and hydrothermal stability and a new environmental-friendly NH3-SCR catalyst with potential applications for NOx removal from diesel and hydrogen-fueled engines.

Design of Ca-type todorokite catalysts with highly active for the selective reduction of NOx by NH3 at low temperatures

Ca-type todorokite catalysts were designed and prepared by a simple redox method and applied to the selective reduction of NOx by NH3 (NH3-SCR) for the first time. Compared with the Na-type manjiroite prepared by the same method, the todorokite catalysts with different Mn/Ca ratios showed greatly improved catalytic activity for NOx reduction. Among them, Mn8Ca4 catalyst exhibited the best NH3-SCR performance, achieving 90% NOx conversion within temperature range of 70–275°C and having a high sulphur resistance. Compared to the Na-type manjiroite sample, Ca-type todorokite catalysts possessed an increased size of tunnel, resulting in a larger specific surface area. As increased the amounts of Ca doping, the Na content in Ca-type todorokite catalysts significantly decreased, providing larger amounts of Brønsted acid sites for NH3 adsorption to produce NH4+. The NH4+ species were highly active for reaction with NO + O2, playing a determining role in NH3-SCR process at low temperatures. Meanwhile, larger amounts of surface adsorbed oxygen contained over the Ca-doping samples than that over Na-type manjiroite, promoting the oxidation of NO and fast SCR processes. Over the Ca-type todorokite catalysts, furthermore, nitrates produced during the flow of NO + O2, were more active for reaction with NH3 than that over Na-type manjiroite, benefiting the occurrence of NH3-SCR process. This study provides novel insights into the design of NH3-SCR catalysts with high performance.