The co-effect of Sb and Nb on the SCR performance of the V2O5/TiO2 catalyst

Interface sites on vanadia-based catalysts are highly active for NOx removal under realistic conditions

TiO2-supported V2O5 catalysts are commonly used in NOx reduction with ammonia due to their robust catalytic performance. Over these catalysts, it is generally considered that the active species are mainly derived from the vanadia species rather than the intrinsic structure of V-O-Ti entities, namely the interface sites. To reveal the role of V-O-Ti entities in NH3-SCR, herein, we prepared TiO2/V2O5 catalysts and demonstrated that V-O-Ti entities were more active for NOx reduction under wet conditions than the V sites (V=O) working alone. On the V-O-Ti entities, kinetic measurements and first principles calculations revealed that NH3 activation exhibited a much lower energy barrier than that on V=O sites. Under wet conditions, the V-O-Ti interface significantly inhibited the transformation of V=O to V-OH sites thus benefiting NH3 activation. Under wet conditions, meanwhile, the migration of NH4+ from Ti site neighboring the V-O-Ti interface to Ti site of the V-O-Ti interface was exothermic; thus, V-O-Ti entities together with neighboring Ti sites could serve as channels linking NH3 pool and active centers for activation of NH4+. This finding reveals that the V-O-Ti interface sites on V-based catalysts play a crucial role in NOx removal under realistic conditions, providing a new perspective on NH3-SCR mechanism.

Characteristics of deactivation and thermal regeneration of Nb-doped V2O5-WO3/TiO2 catalyst for NH3-SCR reaction

This study investigated the effect of Nb doping into V₂O₅-WO₃/TiO₂ (VWT) catalyst for removing NO_xvia the SCR (selective catalytic reduction) by NH₃. The experimental results exhibited that Nb can improve the reactivity of the VWT catalyst at low temperatures. The addition of Nb also enhanced the tolerance to SO₂ and H₂O. The de-NO_x efficiency of the V₂O₅-WO₃-Nb₂O₅/TiO₂ (VWNbT) catalyst was increased up to 12% over that of the VWT catalyst at 240 °C when the catalyst was poisoned for 24 h. The prepared catalysts were characterized by FT-IR, XRD, XPS, and N₂ physisorption, elemental analysis. The results showed that the ammonium bisulfate (ABS) was less formed in the VWNbT than in the VWT. Moreover, evolved gas analysis was performed to examine the thermal decomposition behavior of the poisoned catalyst, and confirmed that the ABS deposited on the catalyst was sufficiently decomposed between about 300 and 400 °C. In particular, to most effectively recover the characteristics and activity of the catalysts, thermal treatment at a temperature of 400 °C is suitable.

Simultaneous removal of NO and dichloromethane (CH2Cl2) over Nb-loaded cerium nanotubes catalyst

Herein, a series of niobium oxide supported cerium nanotubes (CeNTs) catalysts with different loading amount of Nb₂O₅ (0-10 wt.%) were prepared and used for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) in the presence of CH₂Cl₂. Commercial V₂O₅-WO₃-TiO₂ catalyst was also prepared for comparison. The physcial properties and chemical properties of the Nb₂O₅ loaded cerium nanotubes catalysts were investigated by X-ray diffractometer, Transmission electron microscope, Brunauer-Emmett-Teller specific surface area, H₂-temperature programmed reduction, NH₃-temperature programmed desorption and X-ray photoelectron spectroscopy. The experiment results showed that the loading amount of Nb₂O₅ had a significant effect on the catalytic performance of the catalysts. 10 wt.% Nb-CeNTs catalyst presented the best NH₃-SCR performance and degradation efficiency of CH₂Cl₂ among the prepared catalysts, due to its superior redox capability, abundant surface oxygen species and acid sites, the interaction between Nb and Ce, higher ratio of Nb⁴⁺/(Nb⁵⁺+ Nb⁴⁺) and Ce³⁺/(Ce³⁺ + Ce⁴⁺), as well as the special tubular structure of cerium nanotube. This study may provide a practical approach for the design and synthesis of SCR catalysts for the simultaneously removal NO_x and chlorinated volatile organic compounds (CVOCs) emitted from the stationary industrial sources.

Insights into the co-doping effect of Fe3+ and Zr4+ on the anti-K performance of CeTiOx catalyst for NH3-SCR reaction

A novel K-resistant Fe³⁺ and Zr⁴⁺ co-doped CeTiO_x catalyst was first prepared by co-precipitation method for the ammonia-selective catalytic reduction (NH₃-SCR) of NO_x. On the premise of retaining the outstanding catalytic activity of CeTiO_x catalyst, Fe³⁺ and Zr⁴⁺ co-doping efficiently improves its K-resistance with superior NO_x conversion up to 84% after K-poisoning. Specially, the grain growth during the second calcination after K poisoning is successfully inhibited by Fe³⁺ and Zr⁴⁺ co-doping. Consequently, the large specific surface area with increased acid sites and efficiently retained reducibility over K-poisoned FeZrCeTiO_x catalyst are realized, which prompt NH₃ activation and NO oxidation, further benefit NH₃-SCR. Besides, NH₃-SCR reaction over CeTiO_x and FeZrCeTiO_x catalysts follows a possible L-H mechanism, and K-poisoning makes no change to it. Finally, a reasonable anti-K poisoning mechanism of FeZrCeTiO_x catalyst is proposed: the excellent K-resistance is attributed to part of Fe and Zr are sacrificed to form Fe-O-K and Zr-O-K species protecting the active site Ce-O-Ti from K-poisoning, as well as the additional reducibility and surface acidity brought from Fe-O species with Zr prompting its uniform distribution.

Improvement of the activity and SO2 tolerance of Sb-modified Mn/PG catalysts for NH3-SCR at a low temperature

A series of MnM/palygorskite (PG) (M = La, W, Mo, Sb, Mg) catalysts was prepared by the wetness co-impregnation method for low-temperature selective catalytic reduction (SCR) of NO with NH₃. Conversion efficiency followed the order Sb > Mo > La > W > Mg. A combination of various physico-chemical techniques was used to investigate the influence of Sb-modified Mn/PG catalysts. MnSb_0.156/PG catalyst showed highest NO conversion at low temperatures in the presence of SO₂ which reveals that addition of Sb oxides effectively enhances the SCR activity of catalysts. A SO₂ step-wise study showed that MnSb_0.156/PG catalyst displays higher durable resistance to SO₂ than Mn/PG catalyst, where the sulfating of active phase is greatly inhibited after Sb doping. Scanning electron microscopy and X-ray diffraction results showed that Sb loading enhances the dispersion of Mn oxides on the carrier surface. According to the results of characterization analyses, it is suggested that the main reason for the deactivation of Mn/PG is the formation of manganese sulfates which cause the permanent deactivation of Mn-based catalysts. For Sb-doped Mn/PG catalyst, SO_x ad-species formed were mainly combined with SbO_x rather than MnO_x. This preferential interaction between SbO_x and SO₂ effectively shields the MnO_x as active species from being sulfated by SO₂ resulting in the improvement of SO₂ tolerance on Sb-added catalyst. Multiple information support that, owing to the addition of Sb, original formed MnO_x crystallite has been completely transformed into highly dispersed amorphous phase accounting for higher SCR activity.

Enhanced activity and alkali metal resistance in vanadium SCR catalyst via co-modification with Mo and Sb

Enhanced surface acidity contributed significantly to the catalytic activity and alkali metal resistance of the optimum catalysts.

Recent advances in layered double hydroxides (LDHs) derived catalysts for selective catalytic reduction of NOx with NH3

In recent years, layered double hydroxides (LDHs) derived metal oxides as highly efficient catalysts for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) have attracted great attention. The high dispersibility and interchangeability of cations within the brucite-like layers make LDHs an indispensable branch of catalytic materials. With the increasingly stringent and ultra-low emission regulations, there is an urgent need for highly efficient and stable low-medium temperature denitration catalysts in markets. In this contribution, we have critically summarized the recent research progress in the LDHs derived NH₃-SCR catalysts, including their ability for NO_x removal, N₂ selectivity, active temperature window, stability and resistance to poisoning. The advantages and defects of various types of LDHs-derived catalysts are comparatively summarized, and the corresponding modification strategies are discussed. In addition, considering the importance of the catalyst's resistance to poisoning in practical applications, we discuss the poisoning mechanism of each component in flue gases, and provide the corresponding strategies to improve the poisoning resistance of catalysts. Finally, from the perspective of practical applications and operation cost, the regeneration measures of catalysts after poisoning is also discussed. We hope that this work can give timely technical guidance and valuable insights for the applications of LDHs materials in the field of NO_x control.

Sb-Containing Metal Oxide Catalysts for the Selective Catalytic Reduction of NOx with NH3

Sb-containing catalysts (SbZrOx (SbZr), SbCeOx (SbCe), SbCeZrOx (SbCeZr)) were prepared by citric acid method and investigated for the selective catalytic reduction (SCR) of NOx with NH3 (NH3-SCR). SbCeZr outperformed SbZr and SbCe and exhibited the highest activity with 80% NO conversion in the temperature window of 202–422 °C. Meanwhile, it also had good thermal stability and resistance against H2O and SO2. Various characterization methods, such as XRD, XPS, H2-TPR, NH3-TPD, and in situ diffuse reflectance infrared Fourier transform (DRIFT), were applied to understand their different behavior in NOx removal. The presence of Sb in the metal oxides led to the difference in acid distribution and redox property, which closely related with the NH3 adsorption and NO oxidation. Brønsted acid and Lewis acid were evenly distributed on SbCe, while Brønsted acid dominated on SbCeZr. Compared with Brønsted acid, Lewis acid was slightly active in NH3-SCR. The competition between NH3 adsorption and NO oxidation was dependent on SbOx and metal oxides, which were found on SbCe while not on SbCeZr.

Effects of MoO3 and CeO2 doping on the decomposition and reactivity of NH4HSO4 on V2O5/TiO2 catalysts

The deposition of NH₄HSO₄ on catalysts is one of the key issues for selective catalytic reduction of NO_x. In this study, NH₄HSO₄ was preloaded on catalysts, and the effects of MoO₃ and CeO₂ doping on the decomposition and reactivity of NH₄HSO₄ on V₂O₅/TiO₂ catalysts are studied. The results show that the introduction of MoO₃ and CeO₂ significantly promoted NO_x conversion on the V₂O₅/TiO₂ catalysts. Doping with MoO₃ could effectively enhance the S and H₂O resistance of the catalysts. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis indicate that it is the strong chemical interactions between NH₄HSO₄ and the catalysts that are adverse to the decomposition of NH₄HSO₄. However, doping with MoO₃ apparently inhibits these interactions, which significantly decrease the decomposition temperature of NH₄HSO₄. In situ FTIR experiments show that the NH₄⁺ in preloaded NH₄HSO₄ could react with gaseous NO on catalysts, and doping with MoO₃ could facilitate the reaction rate.

Selective catalytic reduction of NOx by NH3 over CeVO4-CeO2 nanocomposite

In this study, it was found that the CeVO₄-CeO₂ nanocomposite possessed remarkably selective catalytic reduction (SCR) performance and wider active temperature scope. And, the promotion principle was explored based on BET, XRD, XPS, H₂-temperature-programmed reduction, NH₃-temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform (DRIFT) techniques. The characterization outcomes manifested that the CeVO₄-CeO₂ nanocomposite could inhibit its crystallinity and enhance the concentrations of chemisorbed oxygen species and Ce³⁺, which was advantageous to the SCR process. Moreover, the in situ DRIFT technique manifested that the NH₃-SCR reaction over Ce_0.75V_0.25O_y was enhanced effectively through the mechanism of L-H.

Promotional effects of modified TiO2- and carbon-supported V2O5- and MnOx-based catalysts for the selective catalytic reduction of NOx: a review

This review presents the promotional effects of transition metal modification over TiO₂- and carbon-supported V₂O₅- and MnO_x-based SCR catalysts.

New Insights into the Decomposition Behavior of NH4HSO4 on the SiO2-Decorated SCR Catalyst and Its Enhanced SO2-Resistant Ability

This article illustrates the detailed decomposition behavior of NH₄HSO₄ on the TiO₂ and TiO₂-SiO₂ supports, along with the effect of SiO₂ addition on the sulfur resistance of the corresponding V₂O₅-based catalysts. For TiO₂ support, sulfate species selectively occupied its surface basic hydroxyl groups, while Si-OH groups functioned as the main sites for the accommodation of NH₄HSO₄ over the TiO₂-SiO₂ mixed support, enabling its surface sulfate species with higher thermal stability. Compared with NH₄ ⁺ on the TiO₂ surface, NH₄ ⁺ on the TiO₂-SiO₂ mixed support was much easier to be consumed during the heating process, hence causing some variations in the decomposition behavior of NH₄HSO₄. Finally, adding SiO₂ enhanced the SO₂ tolerance properties of the catalysts to a certain extent. When exposed to the SO₂-containing flue gas, the deposition of NH₄HSO₄ mainly caused serious deactivation of SiO₂-free catalyst, while the as-accumulated SO₄ ^2- also contributed to the declined activity of SiO₂-added catalyst. These results ensured the potential commercialization of TiO₂-SiO₂-based catalysts in the typical low-temperature selective catalytic reduction systems in the short run and pointed out a strategy to design new catalysts with superior activity and enhanced SO₂-tolerant ability.

Enhancing low-temperature SCR de-NOx and alkali metal poisoning resistance of a 3Mn10Fe/Ni catalyst by adding Co

Alkaline K poisoned and Co-modified catalysts were prepared using Fe and Mn as active components, nickel foam as a carrier, and Co as a trace additive.

A Comparative Study of the NH3-SCR Reactions over an Original and Sb-Modified V2O5–WO3/TiO2 Catalyst at Low Temperatures

Considering the practical requirements for continuous operation under part load condition, the commercial honeycomb selective catalytic reduction (SCR) catalyst was modified with Sb addition. Experiments were performed to investigate the effect of modification on long-time SCR performance under part load condition. Characterizations for the original and modified catalysts were also conducted to analyze the changes of the catalysts. The results indicated that the activity of modified catalyst was obviously enhanced in the temperature range of 275–325 °C and it achieved about 64.5% removal efficiency during the 30 h stability test at 275 °C. The characterization results indicated that the ammonium sulfate was chemically adsorbed on the catalyst surface at low temperatures, which led to the decrease of the specific surface area, pore volume, and V4+/V5+ ratio of the catalysts. These are the reasons for the decrease of the catalyst activity at low temperatures, while the deposition amount of ammonium sulfate was relatively small over the modified catalyst. In addition, the decomposition temperature of the ammonium sulfate was reduced in the modified catalyst compared with the original one. NH4+ ions decomposed at 275 °C by reacting with the NOx in the flue gas, and the dynamic equilibrium of this reaction was achieved on the modified catalyst after a short period of time. Therefore the modified catalyst can be continuously and stably operated at this temperature, and the part load operation of the SCR system in the coal-fired power plant can be realized.

Strategies of Coping with Deactivation of NH3-SCR Catalysts Due to Biomass Firing

Firing of biomass can lead to rapid deactivation of the vanadia-based NH3-SCR catalyst, which reduces NOx to harmless N2. The deactivation is mostly due to the high potassium content in biomasses, which results in submicron aerosols containing mostly KCl and K2SO4. The main mode of deactivation is neutralization of the catalyst’s acid sites. Four ways of dealing with high potassium contents were identified: (1) potassium removal by adsorption, (2) tail-end placement of the SCR unit, (3) coating SCR monoliths with a protective layer, and (4) intrinsically potassium tolerant catalysts. Addition of alumino silicates, often in the form of coal fly ash, is an industrially proven method of removing K aerosols from flue gases. Tail-end placement of the SCR unit was also reported to result in acceptable catalyst stability; however, flue-gas reheating after the flue gas desulfurization is, at present, unavoidable due to the lack of sulfur and water tolerant low temperature catalysts. Coating the shaped catalysts with thin layers of, e.g., MgO or sepiolite reduces the K uptake by hindering the diffusion of K+ into the catalyst pore system. Intrinsically potassium tolerant catalysts typically contain a high number of acid sites. This can be achieved by, e.g., using zeolites as support, replacing WO3 with heteropoly acids, and by preparing highly loaded, high surface area, very active V2O5/TiO2 catalyst using a special sol-gel method.

NH3-SCR performance and the resistance to SO2 for Nb doped vanadium based catalyst at low temperatures

Niobium oxide as the promoter was doped in the V/WTi catalyst for the selective catalytic reduction (SCR) of NO. The results showed that the addition of Nb₂O₅ could improve the SCR activity at low temperatures and the 6wt.% additive was an appropriate dosage. The enhanced reaction activity of adsorbed ammonia species and the improved dispersion of vanadium oxide might be the reasons for the elevation of SCR activity at low temperatures. The resistances to SO₂ of 3V6Nb/WTi catalyst at different temperatures were investigated. FTIR spectrum and TG-FTIR result indicated that the deposition of ammonium sulfate species was the main deactivation reason at low temperatures, which still exhibited the reactivity with NO above 200°C on the catalyst surface. There was a synergistic effect among NH₃, H₂O and SO₂ that NH₃ and H₂O both accelerated the catalyst deactivation in the presence of SO₂ at 175°C. The thermal treatment at 400°C could regenerate the deactivated catalyst and get SCR activity recovered. The particle and monolith catalysts both kept stable NO_x conversion at 225°C with high concentration of H₂O and SO₂ during the long time tests.

Rational selection of Fe2V4O13 over FeVO4 as a preferred active site on Sb-promoted TiO2 for catalytic NOX reduction with NH3

Fe₂V₄O₁₃ outperforms FeVO₄ as an active site for NH₃-SCR and resists SO₂/ABS/Na poisons with the inclusion of an Sb promoter.

Effect of the calcination temperature of cerium–zirconium mixed oxides on the structure and catalytic performance of WO3/CeZrO2 monolithic catalyst for selective catalytic reduction of NOx with NH3

The calcined temperature of the carrier obviously affected SCR activity of catalysts, WO₃/Ce_0.68Zr_0.32O₂-500 showed the best low-temperature NH₃-SCR activity due to its more Lewis acid sites and stronger redox property.

Investigation of the promotion effect of WO3 on the decomposition and reactivity of NH4HSO4 with NO on V2O5–WO3/TiO2 SCR catalysts

Electron deviation from catalyst atoms to SO₄²⁻ is the key factor in the behavior of NH₄HSO₄ decomposition on the catalysts.

A novel Ce–Sb binary oxide catalyst for the selective catalytic reduction of NOx with NH3

The synergetic effect between Sb and Ce not only increases the surface acidity of the catalyst but also enhances the redox property, both of which contribute to improving NH₃-SCR activity.

Exploring the role of V2O5 in the reactivity of NH4HSO4 with NO on V2O5/TiO2 SCR catalysts

NH₄⁺ in NH₄HSO₄ is consumed during the reaction with NO, while S-species are stabilized as tridentate SO₄²⁻ on the catalysts.

Effect of water vapor on NH3-NO/NO2 SCR performance of fresh and aged MnOx-NbOx-CeO2 catalysts

A MnOx-NbOx-CeO2 catalyst for low temperature selective catalytic reduction (SCR) of NOx with NH3 was prepared by a sol-gel method, and characterized by NH3-NO/NO2 SCR catalytic activity, NO/NH3 oxidation activity, NOx/NH3 TPD, XRD, BET, H2-TPR and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that the MnOx-NbOx-CeO2 catalyst shows excellent low temperature NH3-SCR activity in the temperature range of 150-300°C. Water vapor inhibits the low temperature activity of the catalyst in standard SCR due to the inhibition of NOx adsorption. As the NO2 content increases in the feed, water vapor does not affect the activity in NO2 SCR. Meanwhile, water vapor significantly enhances the N2 selectivity of the fresh and the aged catalysts due to its inhibition of the decomposition of NH4NO3 into N2O.