Manganese oxide nanorod catalysts for low-temperature selective catalytic reduction of NO with NH3

MnO_x nanorod catalysts were successfully synthesized by two different preparation methods using porous SiO₂ nanorods as the template and investigated for the low-temperature selective catalytic reduction (SCR) of NO with NH₃. The catalysts were characterized by scanning electron microscopy, transmission electron microscopy, nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, and NH₃ temperature-programmed desorption. The results show that the obtained MnO_x-P nanorod catalyst prepared by redox precipitation method exhibits higher NO removal activity than that prepared by the solvent evaporation method in the low temperature range of 100–180 °C, where about 98% NO conversion is achieved over MnO_x(0.36)-P nanorods. The reason is mainly attributed to MnO_x(0.36)-P nanorods possessing unique flower-like morphology and mesoporous structures with high pore volume, which facilitates the exposure of more active sites of MnO_x and the adsorption of reactant gas molecules. Furthermore, there is a lower crystallinity of MnO_x, higher percentage of Mn⁴⁺ species and a large amount of strong acid sites on the surface. These factors contribute to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.

Compared with MnO_x(0.36)-E nanorods, MnO_x(0.36)-P nanorods possess unique flower-like morphology and mesoporous structures with high pore volume, contributing to the excellent low-temperature SCR activity of MnO_x(0.36)-P nanorods.     

Synthesis of W-modified CeO2/ZrO2 catalysts for selective catalytic reduction of NO with NH3

In this paper, a series of tungsten–zirconium mixed binary oxides (denoted as W_mZrO_x) were synthesized via co-precipitation as supports to prepare Ce_0.4/W_mZrO_x catalysts through an impregnation method. The promoting effect of W doping in ZrO₂ on selective catalytic reduction (SCR) performance of Ce_0.4/ZrO₂ catalysts was investigated. The results demonstrated that addition of W in ZrO₂ could remarkably enhance the catalytic performance of Ce_0.4/ZrO₂ catalysts in a broad temperature range. Especially when the W/Zr molar ratio was 0.1, the Ce_0.4/W_0.1ZrO_x catalyst exhibited the widest active temperature window of 226–446 °C (NO_x conversion rate > 80%) and its N₂ selectivity was almost 100% in the temperature of 150–450 °C. Moreover, the Ce_0.4/W_0.1ZrO_x catalyst also exhibited good SO₂ tolerance, which could maintain more than 94% of NO_x conversion efficiency after being exposed to a 100 ppm SO₂ atmosphere for 18 h. Various characterization results manifested that a proper amount of W doping in ZrO₂ was not only beneficial to enlarge the specific surface area of the catalyst, but also inhibited the growth of fluorite structure CeO₂, which were in favor of CeO₂ dispersion on the support. The presence of W was conducive to the growth of a stable tetragonal phase crystal of ZrO₂ support, and a part of W and Zr combined to form W–Zr–O_x solid super acid. Both of them resulted in abundant Lewis acid sites and Brønsted acid sites, enhancing the total surface acidity, thus significantly improving NH₃ species adsorption on the surface of the Ce_0.4/W_0.1ZrO_x catalyst. Furthermore, the promoting effect of adding W on SCR performance was also related to the improved redox capability, higher Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio and abundant surface chemisorbed oxygen species. The in situ DRIFTS results indicated that nitrate species adsorbed on the surface of the Ce_0.4/W_0.1ZrO_x catalyst could react with NH₃ due to the activation of W. Therefore, the reaction pathway over the Ce_0.4/W_0.1ZrO_x catalyst followed both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanisms at 250 °C.

Interaction of W with Zr improved NH₃-SCR performance via enhancing redox and surface acidity.     

N-doped graphene/CoFe2O4 catalysts for the selective catalytic reduction of NOx by NH3

In this paper, CoFe₂O₄/graphene catalysts and N-doped graphene/CoFe₂O₄ (CoFe₂O₄/graphene-_N) catalysts were prepared using the hydrothermal crystallization method for the selective catalytic reduction of NO_x by NH₃. The results of the test showed that CoFe₂O₄/graphene catalysts exhibited the best denitrification activity when the loading was at 4% and the conversion rate of NO_x reached 99% at 250–300 °C. CoFe₂O₄/graphene-_N catalysts presented a better denitrification activity at low temperature than CoFe₂O₄/graphene catalysts, and the conversion rate of NO_x reached more than 95% at 200–300 °C. The intrinsic mechanism of CoFe₂O₄/graphene-_N catalysts in promoting SCR activity was preliminarily explored. The physicochemical properties of the samples were characterized using XRD, TEM, N₂ adsorption, XPS, NH₃-TPD, and H₂-TPR. The results indicated that nitrogen doping can improve the dispersion of CoFe₂O₄, and it also increased the acidic sites and the redox performance conducive to improving the denitrification activity of the catalysts. In addition, CoFe₂O₄/graphene-_N catalysts demonstrated a better resistance to water and sulfur than CoFe₂O₄/graphene catalysts.

N-doped graphene/CoFe₂O₄ presented better denitrification activity than CoFe₂O₄/graphene due to the more uniform distribution of CoFe₂O₄ and acidic sites etc.     

Enhanced resistance to calcium poisoning on Zr-modified Cu/ZSM-5 catalysts for the selective catalytic reduction of NO with NH3

Ca/ZrCu/ZSM-5 catalysts containing different Zr contents were prepared by incipient wetness impregnation. The catalysts were tested for the selective catalytic reduction (SCR) of NO_x with ammonia and characterized by N₂-BET, N₂O titration, XRD, NH₃-TPD, H₂-TPR, and XPS techniques. In the temperature range of 100–170 °C, after calcium impregnation, NO_x conversion over the Cu/ZSM-5 catalyst decreased by 11.3–24.3%, while that over Zr_0.10/Cu/ZSM-5 only decreased by 3.8–12.2%. The improvement of the calcium poisoning resistance of the ZrCu/ZSM-5 catalyst is mainly attributed to an increase in the dispersion and the surface concentration of Cu. Moreover, the addition of zirconium promotes the reduction of CuO by decreasing the interaction between CuO and CaO, which also contributes to the improvement of resistance to CaO poisoning. The apparent activation energy and turnover frequency for the SCR reaction over the Ca/Zr_xCu/ZSM-5 catalysts were calculated and discussed.

After calcium impregnation, NO_x conversion over the Cu/Z catalyst decreased by 11.3–24.3%, while that over Zr_0.10/Cu/Z only decreased by 3.8–12.2%.     

Deactivation by HCl of CeO2–MoO3/TiO2 catalyst for selective catalytic reduction of NO with NH3

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃ was investigated with BET, XRD, NH₃-TPD, H₂-TPR, XPS and catalytic activity measurements. The results showed that HCl had an inhibiting effect on the activity of the CeO₂–MoO₃/TiO₂ catalyst. The deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst could be attributed to pore blockage, weakened interaction among ceria, molybdenum and titania, reduction in surface acidity and degradation of redox ability. The Ce³⁺/Ce⁴⁺ redox cycle was damaged because unreactive Ce³⁺ in the form of CeCl₃ lost the ability to be converted to active Ce⁴⁺ in the SCR reaction. In addition, a decrease in the amount of chemisorbed oxygen and the concentrations of surface Ce and Mo was also responsible for the deactivation by HCl of the CeO₂–MoO₃/TiO₂ catalyst.

The effect of HCl on a CeO₂–MoO₃/TiO₂ catalyst for the selective catalytic reduction of NO with NH₃.     

Novel CeMoxOy-clay hybrid catalysts with layered structure for selective catalytic reduction of NOx by NH3

A facile method is to prepare novel CeMo_xO_y-clay hybrid catalysts with layered structures by using organic cation modified clay as support. During the preparation process, cerium cations and molybdate anions are easily adsorbed and impregnated into the interlamellar space of the organoclay, and after calcination they undergo transformation to highly dispersed CeMo_xO_y nanoparticles within the interlamellar space of the clay. As expected, the prepared CeMo_0.15O_x-OC-T catalysts with layered structures had high selective catalytic reduction (SCR) activity such as high NO_x conversion of >90% in the wide temperature range of 220–420 °C. Meanwhile, they also exhibit high stability and tolerance to water vapor (5 vol%) and SO₂ (200 ppm), demonstrating that these novel catalysts could serve as a good alternative for NH₃-SCR in practical application.

A facile method is to prepare novel CeMo_xO_y-clay hybrid catalysts with layered structures by using organic cation modified clay as support.     

High N2 selectivity in selective catalytic reduction of NO with NH3 over Mn/Ti–Zr catalysts

A series of Mn-based catalysts were prepared by a wet impregnation method for the selective catalytic reduction (SCR) of NO with NH₃. The Mn/Ti–Zr catalyst had more surface area, Lewis acid sites, and Mn⁴⁺ on its surface, and showed excellent activity and high N₂ selectivity in a wide temperature range. NH₃ and NO oxidation was investigated to gain insight into NO reduction and N₂O formation. The formation of N₂O was primarily dominated by the reaction of NO with NH₃ in the presence of O₂via the Eley–Rideal mechanism. An intimate synergistic effect existed between the Mn-based and the Ti–Zr support. It was demonstrated that the Ti–Zr support greatly promoted the catalytic performance of Mn-based catalysts.

The Ti–Zr support greatly promotes the catalytic performance of Mn-based catalysts.     

Promotion effect of urchin-like MnOx@PrOx hollow core–shell structure catalysts for the low-temperature selective catalytic reduction of NO with NH3

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃. The structural properties of the catalyst were characterized by FE-SEM, TEM, XRD, BET, XPS, H₂-TPR and NH₃-TPD analyses, and the performance of the low-temperature NH₃-SCR was also tested. The results show that the catalyst with a molar ratio of Pr/Mn = 0.3 exhibited the highest NO conversion at nearly 99% at 120 °C and NO conversion greater than 90% over the temperature range of 100–240 °C. Also, the MnO_x@PrO_x catalyst presented desirable SO₂ and H₂O resistance in 100 ppm SO₂ and 10 vol% H₂O at the space velocity of 40 000 h⁻¹ and a testing time of 3 h test at 160 °C. The excellent low-temperature catalytic activity of the catalyst could ultimately be attributed to high concentrations of Mn⁴⁺ and adsorbed oxygen species on the catalyst surface, suitable Lewis acidic surface properties, and good reducing ability. Additionally, the enhanced SO₂ and H₂O resistance of the catalyst was primarily ascribed to its unique core–shell structure which prevented the MnO_x core from being sulfated.

A MnO_x@PrO_x catalyst with a hollow urchin-like core–shell structure was prepared using a sacrificial templating method and was used for the low-temperature selective catalytic reduction of NO with NH₃.     

Low-temperature selective catalytic reduction of NO with NH3 over an FeOx/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.

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.     

Effect of initial support particle size of MnOx/TiO2 catalysts in the selective catalytic reduction of NO with NH3

A series of manganese-based catalysts supported by 5–10 nm, 10–25 nm, 40 nm and 60 nm anatase TiO₂ particles was synthesized via an impregnation method to investigate the effect of the initial support particle size on the selective catalytic reduction (SCR) of NO with NH₃. All catalysts were characterized by transmission electron microscopy (TEM), N₂ physisorption/desorption, X-ray diffraction (XRD), temperature programmed techniques, X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared transform spectroscopy (DRIFTS). TEM results indicated that the particle sizes of the MnO_x/TiO₂ catalysts were similar after the calcination process, although the initial TiO₂ support particle sizes were different. However, the initial TiO₂ support particle sizes were found to have a significant influence on the SCR catalytic performance. XPS and NH₃-TPD results of the MnO_x/TiO₂ catalysts illustrated that the surface Mn⁴⁺/Mn molar ratio and acid amount could be influenced by the initial TiO₂ support particle sizes. The order of surface Mn⁴⁺/Mn molar ratio and acid amount over the MnO_x/TiO₂ catalysts was as follows: MnO_x/TiO₂(10–25) > MnO_x/TiO₂(40) > MnO_x/TiO₂(60) > MnO_x/TiO₂(5–10), which agreed well with the order of SCR performance. In situ DRIFTS results revealed that the NH₃-SCR reactions over MnO_x/TiO₂ at low temperature occurred via a Langmuir–Hinshelwood mechanism. More importantly, it was found that the bridge and bidentate nitrates were the main active substances for the low-temperature SCR reaction, and bridge nitrate adsorbed on Mn⁴⁺ showed superior SCR activity among all the adsorbed NO_x species. The variation of the initial TiO₂ support particle size over MnO_x/TiO₂ could change the surface Mn⁴⁺/Mn molar ratio, which could influence the adsorption of NO_x species, thus bringing about the diversity of the SCR catalytic performance.

Support particle size could influence the surface Mn⁴⁺/Mn ratio of catalysts, promoting the reactivity of bridge nitrate, therefore enhancing SCR performance.     

Insights into the promotion role of phosphorus doping on carbon as a metal-free catalyst for low-temperature selective catalytic reduction of NO with NH3

The catalytic reduction of NO with NH₃ (NH₃-SCR) on phosphorus-doped carbon aerogels (P-CAs) was studied in the temperature range of 100–200 °C. The P-CAs were prepared by a one-pot sol–gel method by using phosphoric acid as a phosphorus source followed by carbonization at 600–900 °C. A correlation between catalytic activity and surface P content is observed. The P-CA-800_vac sample obtained via carbonization at 800 °C and vacuum treatment at 380 °C shows the highest NO conversion of 45.6–76.8% at 100–200 °C under a gas hourly space velocity of 500 h⁻¹ for the inlet gas mixture of 500 ppm NO, 500 ppm NH₃ and 5.0 vol% O₂. The coexistence of NH₃ and O₂ is essential for the high conversion of NO on the P-CA carbon catalysts, which can decrease the spillover of NO₂ and N₂O. The main Brønsted acid sites derived from P-doping and contributed by the C–OH group at edges of carbon sheets are beneficial for NH₃ adsorption. In addition, the C₃–P O configuration seems to have the most active sites for favorable adsorption and dissociation of O₂ and facilitates the formation of NO₂. Therefore, the simultaneous presence of acidic groups for NH₃ adsorption and the C₃–P O active sites for NO₂ generation due to the activation of O₂ molecules is likely responsible for the significant increase in the NH₃-SCR activity over the P–CAs. The transformation of C₃–P O to C–O–P functional groups after the reaction is found, which could be assigned to the oxidation of C₃–P O by the dissociated O*, resulting in an apparent decrease of catalytic activity for P-CAs. The C–O–P based functional groups are also active in the NH₃-SCR reaction.

P species can effectively enhance the catalytic activity of carbon aerogels for NO reduction at low temperature.     

Synthesis of oxygen functionalized carbon nanotubes and their application for selective catalytic reduction of NOx with NH3

Oxygen functionalized carbon nanotubes synthesized by surface acid treatment were used to improve the dispersion properties of active materials for catalysis. Carbon nanotubes have gained attention as a support for active materials due to their high specific surface areas (400–700 m² g⁻¹) and chemical stability. However, the lack of surface functionality causes poor dispersion of active materials on carbon nanotube supports. In this study, oxygen functional groups were prepared on the surface of carbon nanotubes as anchoring sites for decoration with catalytic nanoparticles. The oxygen functional groups were prepared through a chemical acid treatment using sulfuric acid and nitric acid, and the amount of functional groups was controlled by the reaction time. Vanadium, tungsten, and titanium oxides as catalytic materials were dispersed using an impregnation method on the synthesized carbon nanotube surfaces. Due to the high density of oxygen functional groups, the catalytic nanoparticles were well dispersed and reduced in size on the surface of the carbon nanotube supports. The selective catalytic reduction catalyst with the oxygen functionalized carbon nanotube support exhibited enhanced NO_x removal efficiency of over 90% at 350–380 °C which is the general operating temperature range of catalysis in power plants.

Oxygen functionalized carbon nanotubes synthesized by surface acid treatment were used to improve the dispersion properties of active materials for catalysis.     

Generation of multi-valence CuxO by reduction with activated semi-coke and their collaboration in the selective reduction of NO with NH3

Multi-valence Cu_xO has been demonstrated to have high activity in the low-temperature selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Here, Cu_xO was loaded onto activated semi-coke (ASC) for SCR, which has shown satisfactory low-temperature SCR activity. By virtue of the reduction property of carbon, the valence of Cu was regulated by simply adjusting the calcination temperature. The high concentration of Cu⁺ generated from the reduction of CuO by ASC during calcination can collaborate to form Cu²⁺/Cu⁺ circulation. After systematic characterization by XPS, H₂-TPD, and NH₃-TPR, it is revealed that abundant acidic sites and surface reactive oxygen species are formed on the surface of the catalysts. Further investigation with in situ DRIFTS confirms that the NH₃-SCR over the as-prepared CuO|Cu₂O-ASC catalysts simultaneously follows the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) pathways, attributed to the synergistic effects of Cu²⁺ and Cu⁺.

In this work, we systematically analyzed the structure–activity relationship and synergistic mechanism of activated semi-coke (ASC)-loaded multi-valence Cu_xO for the low-temperature NO-SCR process by NH₃.     

The promotion effect of manganese on Cu/SAPO for selective catalytic reduction of NOx with NH3

The activity and hydrothermal stability of Cu/SAPO and xMn–2Cu/SAPO for low-temperature selective catalytic reduction of NO_x with ammonia were investigated. An ion-exchanged method was employed to synthesize xMn–2Cu/SAPO, which was characterized by N₂ adsorption, ICP-AES, X-ray diffraction (XRD), NH₃-temperature programmed desorption (NH₃-TPD), NO oxidation, X-ray photoelectron spectrum (XPS), UV-vis, H₂-temperature programmed reduction (H₂-TPR) and diffuse reflectance infrared Fourier transform spectra (DRIFTS). 2Mn–2Cu/SAPO and 4Mn–2Cu/SAPO showed the best SCR activity, in that at 150 °C NO conversion reached 76% and N₂ selectivity was above 95% for the samples. NO oxidation results showed that the 2Mn–2Cu/SAPO had the best NO oxidation activity and the BET surface area decreased as manganese loading increased. XRD results showed that the metal species was well dispersed. NH₃-TPD showed that the acid sites have no significant influence on the SCR activity of xMn–2Cu/SAPO. H₂-TPR patterns showed good redox capacity for xMn–2Cu/SAPO. UV-vis and H₂-TPR showed that the ratio of Mn⁴⁺ to Mn³⁺ increased as manganese loading increased. XPS spectra showed a significant amount of Mn³⁺ and Mn⁴⁺ species on the surface and addition of manganese increased the ratio of Cu²⁺. The promotion effect of manganese to 2Cu/SAPO comes from the generation of Mn³⁺ and Mn⁴⁺ species. Deduced from the DRIFTS spectra, the Elay–Rideal mechanism was effective on 4Mn–2Cu/SAPO.

The activity and N₂ selectivity of Cu/SAPO and xMn–2Cu/SAPO for low-temperature selective catalytic reduction of NO_x with NH₃ were investigated.     

Insight into the synergism between MnO2 and acid sites over Mn–SiO2@TiO2 nano-cups for low-temperature selective catalytic reduction of NO with NH3

The rational synthesis of low-temperature catalysts with high catalytic activity and stability is highly desirable for the selective catalytic reduction of NO with NH₃. Here we synthesized a Mn–SiO₂/TiO₂ nano-cup catalyst via the coating of the mesoporous TiO₂ layers on SiO₂ spheres and subsequent inlay of MnO₂ nanoparticles in the narrow annulus. This catalyst exhibited superior catalytic SCR activities and stability for low-temperature selective catalytic reduction of NO with NH₃, with NO conversion of ∼100%, N₂ selectivity above 90% at a temperature ∼140 °C. The characterization results, such as BET, XRD, H₂-TPR, O₂/NH₃-TPD and XPS, indicated that this nano-cup structure catalyst possesses high concentration and dispersion of Mn⁴⁺ active species, strong chemisorbed O⁻ or O₂²⁻ species and highly stable MnO_X active components over the annular structures of the TiO₂ shell and SiO₂ sphere, and thus enhanced the low-temperature SCR performance.

A novel Mn–SiO₂@TiO₂ nano-cup catalyst with synergy of MnO₂ and acid sites for efficient low-temperature SCR reaction.     

Tuning the catalytic properties of La–Mn perovskite catalyst via variation of A- and B-sites: effect of Ce and Cu substitution on selective catalytic reduction of NO with NH3

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution. In order to investigate the effect of Ce and Cu substitution on the physicochemical properties of perovskite in NH₃-SCR system, a series of La_1−xCe_xMn_1−yCu_yO₃ (x = 0, 0.1, y = 0, 0.05, 0.1, 0.2, 0.4) catalysts were prepared by citrate sol-gel method and employed for NO removal in the simulated flue gas, and the physical and chemical properties of the catalysts were studied using XRD, SEM, BET, XPS, DRIFT characterizations. The Ce substitution on A-site cation of LaMnO₃ can improve the denitration activity of the perovskite catalyst, and La_0.9Ce_0.1MnO₃ displays NO conversion of 86.7% at 350 °C. The characterization results indicate that the high denitration activity of La_0.9Ce_0.1MnO₃ is mainly attributed to the larger surface area, which contributes to the adsorption of NH₃ and NO. Besides, the appropriate Cu substitution on B-site cation of La_0.9Ce_0.1MnO₃ can further improve the denitration activity of perovskite catalyst, and La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ displays the NO conversion of 91.8% at 350 °C. Although the specific surface area of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ is lower than La_0.9Ce_0.1MnO₃, the Cu active sites and the Ce³⁺ contents are more developed, making many reaction units formed on the catalyst surface and redox properties of catalyst improved. In addition, strong metal interaction (Ce⁴⁺ + Mn²⁺ + Cu²⁺ ↔ Ce³⁺ + Mn³⁺/Mn⁴⁺ + Cu⁺) and high concentrations of chemical adsorbed oxygen and lattice oxygen both strengthen the redox reaction on catalyst surface, thus contributing to the better denitration activity of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃. Therefore, appropriate cerium and copper substitution will markedly improve the denitration activity of La–Mn perovskite catalyst. We also reasonably conclude a multiple reaction mechanism during NH₃-SCR denitration process basing on DRIFT results, which includes the Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism.

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution.     

Sulfation effect of Ce/TiO2 catalyst for the selective catalytic reduction of NOx with NH3: mechanism and kinetic studies

Ceria-based catalysts are competitive substitutes for the commercial SCR catalysts due to their high SCR activity and excellent redox performance. For a better understanding of the SO₂ poisoning mechanism over ceria-based catalysts, the sulfation effect of the Ce/TiO₂ catalyst on the SCR activity over a wide reaction temperature range was systematically studied via comprehensive characterizations, in situ DRIFT studies and kinetic studies. The results demonstrated that the NO conversion at 150 °C is significantly inhibited by the formation of cerium sulfites/sulfates due to the inhibited redox properties and excessive adsorption of NH₃, which restrict the dissociation of NH₃ to NH₂, resulting in a much lower reaction rate of E–R reaction over the sulfated Ce/TiO₂ catalyst. With the increase in the reaction temperature, the reaction rate of the E–R reaction significantly increased due to the improved redox properties and weakened adsorption of NH₃. Moreover, the rate of the C–O reaction over the sulfated Ce/TiO₂ catalysts is obviously lower than that of the fresh Ce/TiO₂ catalyst. The promotion of NO conversion over the sulfated catalyst at 330 °C is attributed to both the increase in the reaction rate of E–R reaction and the inhibition of the C–O reaction.

After sulfation treatment, low-temperature SCR activity is severely inhibited due to the much lower reaction rate of E–R reaction.     

Sm-doped manganese-based Zr–Fe polymeric pillared interlayered montmorillonite for low temperature selective catalytic reduction of NOx by NH3 in metallurgical sintering flue gas

In this work, Sm-doped manganese supported Zr–Fe polymeric pillared interlayered montmorillonites (Mn/ZrFe-PILMs) were prepared for the low-temperature selective catalytic reduction (SCR) of NO_x with NH₃ in metallurgical sintering flue gas. These pillared interlayered montmorillonite catalysts were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy, nitrogen adsorption–desorption isotherm, ammonia temperature-programmed desorption, and hydrogen temperature-programmed reduction to study the influence of Sm doping on the SCR performance. The ZrFe-PILMs with a Mn/Sm molar ratio of 18 : 2 showed the excellent SCR activity among these catalysts, where a 95.5% NO_x conversion ratio at 200 °C at a space velocity of 20 000 h⁻¹ was obtained. Samarium oxide and manganese oxides were highly dispersed on the ZrFe-PILMs with different Mn/Sm molar ratios by the XRD results and SEM-EDS results. Meanwhile, the Mn–Sm/ZrFe-PILM (18 : 2) had the lowest temperature hydrogen reduction peak by H₂-TPR results, which indicated that it had the lowest active bond energy on its surface. And the NH₃-TPD results expressed that the Mn–Sm/ZrFe-PILM (18 : 2) had the most acidic sites, especially the weakly acidic sites. Therefore, it was found that the introduction of a small amount of Sm (Mn : Sm = 18 : 2) to Mn/ZrFe-PILM can significantly improve catalytic activity by the increased active oxygen component and the surface acidity.

Trace amount of Sm-doped Mn-based Zr–Fe polymeric pillared interlayered montmorillonite promotes low temperature catalytic activity in excess oxygen.     

New insights into the deactivation mechanism of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO with NH3: synergies between arsenic and potassium species

Synergies between arsenic (As) and potassium (K) species in the deactivation of V₂O₅-WO₃/TiO₂ catalyst were investigated. Both arsenic oxide and potassium species presented a serious poisoning impact on catalyst activities, and the extent of poisoning of (As + K) was much stronger than their single superposition. The intrinsic reasons were explored and analyzed by N₂ physisorption, XPS, H₂-TPR, NH₃-TPD, NH₃-DRIFTS and in situ FTIR. Results indicated that BET surface area decreased due to the formation of a dense arsenic coating on the catalyst surface. V–OH active sites were destroyed by arsenic and As–OH acid sites were newly generated. After potassium species were added to arsenic-poisoned catalyst, K⁺ further neutralized the As–OH acid sites, and the amount and stability of both Lewis and BrØnsted acid sites decreased more greatly. Potassium also reacted with intermediate NH₂⁻ when the temperature was elevated to higher than 250 °C, which resulted in more NH₃ consumption and NH₃-SCR reaction inhibition. The extent of deactivation was related to the potassium species when both poisons reacted on the catalyst, and the influence sequence followed AsKS < AsKN < AsKC. As₂O₃ + K₂SO₄ presented the weakest impact among these three poisoned catalysts due to the resistance of SO₄²⁻ to arsenic.

Synergies between arsenic (As) and potassium (K) species in the deactivation of V₂O₅-WO₃/TiO₂ catalyst were investigated.     

Low-temperature selective catalytic reduction of NOx with NH3 over an activated carbon-carbon nanotube composite material prepared by in situ method

Ce-supported activated carbon–carbon nanotube composite (Ce/AC-CNTs) catalyst was prepared by in situ formation of CNTs on AC and then modified by Ce. This Ce/AC-CNTs catalyst was subsequently used for low-temperature selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The NO conversion of Ce/AC-CNTs was 1.41 times higher than that of Ce/AC at 150 °C with good SO₂ tolerance. The catalysts were analyzed by N₂ physisorption, SEM, XRD, NH₃-TPD, XPS, and Raman technologies. The results showed that the introduction of CNTs could form new mesopores and increase the amount of surface chemisorbed oxygen and acid sites, which all contribute to the high NH₃-SCR activity.

A Ce-supported activated carbon-carbon nanotube composite (Ce/AC-CNTs) catalyst was prepared by in situ formation of CNTs on AC and then modified by Ce, and was subsequently used for low-temperature selective catalytic reduction of NO_x with NH₃.