Excellent low-temperature NH3-SCR NO removal performance and enhanced H2O resistance by Ce addition over the Cu0.02Fe0.2CeyTi1-yOx (y = 0.1, 0.2, 0.3) catalysts

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.

Ce(SO4)2/α-Fe2O3 selective catalytic reduction of NOx with NH3: preparation, characterization, and performance

To achieve a low-cost, high-activity denitrification catalyst with excellent water and sulfur resistance, goethite and Ce(SO₄)₂·4H₂O were used to prepare Ce(SO₄)₂/α-Fe₂O₃ composite catalyst by the impregnation way and investigated the effect of Ce(SO₄)₂ on the properties of goethite. Ce(SO₄)₂/α-Fe₂O₃ with various preparation conditions for denitration was systematically discussed, and its structure and properties were characterized by XRD, BET, TEM, XPS, H₂-TPR, and NH₃-TPD methods. The results showed that Ce(SO₄)₂/α-Fe₂O₃ over the Ce/Fe molar ratio of 0.02 and calcination temperature of 350 ℃ had excellent catalytic activity, resistance to sulfur, and water properties and stability. When NO_x initial concentration was 500 ppm, gas hourly space velocity was 36,000 h^-1 and its reaction temperature was 300 ℃; the NO_x conversion efficiency was maintained at over 95% along with 300 ppm SO₂ and nearly 100% couple with 10% H₂O. Its superior performance was mainly attributed to the enhancement of the surface adsorbed oxygen and acidity of α-Fe₂O₃ by cerium sulfate. The multiple advantages of Ce_0.02/α-Fe₂O₃(350) made it feasible for practical engineering application.

Effects of Cu doping on electrochemical NOx removal by La0.8Sr0.2MnO3 perovskites

Electrochemical removal of nitrogen oxides (NO_x) by perovskite electrodes is a promising method due to its low cost, simple operation and no secondary pollution. In this study, a series of La_0.8Sr_0.2Mn_1-xCu_xO₃ (x = 0, 0.05, 0.1 and 0.15) perovskites are fabricated as the improved electrodes of solid electrolyte cells (SECs) for NO_x removal and the effects of Cu doping are investigated systematacially. Multiple characterization methods are carried out to analyze the physicochemical properties of perovskites firstly. Then the performances of cells based on various perovskites are evaluated by the measurements of electrochemical properties and NO_x conversions. The results show that the Cu-doped electrode has more surface oxygen vacancies and a better redox property, thus having a higher NO_x conversion and smaller polarization resistance. The electrode based on La_0.8Sr_0.2Mn_0.9Cu_0.1O₃ has the maximum 70.8% NO_x conversion and the lowest 36.3 Ω cm² R_p value in the atmosphere of 1000 ppm NO at 700 °C. First-principle calculation reveals that the Cu-doped electrode is easier to form surface oxygen vacancy, while the surface oxygen vacancy plays an important role on electron transfer between electrode and NO_x molecule. This study not only provides a new strategy to enhance the electrode performance for NO_x removal in SECs but reveals the fundamental effect of Cu doping on the properties of La_0.8Sr_0.2MnO₃ perovskites.

Investigation of low-temperature performances of hybrid catalysts with different chain length OHC reductants

In the present work, NO_x conversion efficiency of the hybrid catalysts at low temperatures was investigated. ANP-TVM and ANP-TVC-TVM hybrid catalysts for OHC-SCR performance were prepared by the impregnation method. The properties of catalysts were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) and X-ray diffraction (XRD) analyses. The NO_x conversion ratios with real diesel exhaust gases were performed using oxygenated hydrocarbon reductants such as ethanol, propanol, n-butanol and n-penthanol. Performances of the hybrid catalysts at different engine loads and low temperatures were investigated. It was determined that ANP-TVC-TVM gave better results at all temperatures and loads. In general, the performance of ANP-TVC-TVM hybrid catalyst was superior with ethanol reductant except for at 1 kW engine load. The maximum NO_x conversion ratio was 90.6% on the ANP-TVC-TVM hybrid catalyst with n-butanol at 1 kW engine load and 300°C.

Low-Temperature SCR Catalyst Development and Industrial Applications in China

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH4HSO4 on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH3-SCR, and with the renewed demand for deeper NOx treatments, new technologies with lower energy consumption and more functions need to be developed.

An investigation on the promoting effect of Pr modification on SO2 resistance over MnOx catalysts for selective reduction of NO with NH3

Pr-modified MnOx catalyst was synthesized through a facile co-precipitation process, and the results showed that MnPrOx catalyst exhibited much better selective catalytic reduction (SCR) activity and SO2 resistance performance than pristine MnOx catalyst. The addition of Pr in MnOx catalyst led to a complete NO conversion efficiency in 120-220 °C. Moreover, Pr-modified MnOx catalyst exhibited a superior resistance to H2O and SO2 compared with MnOx catalyst. After exposing to SO2 and H2O for 4 h, the NO conversion efficiency of MnPrOx catalyst could remain to 87.6%. The characterization techniques of XRD, BET, hydrogen-temperature programmed reduction (H2-TPR), ammonia-temperature programmed desorption (NH3-TPD), XPS, TG and in situ diffuse reflectance infrared spectroscopy (DRIFTS) were adopted to further explore the promoting effect of Pr doping in MnOx catalyst on SO2 resistance performance. The results showed that MnPrOx catalyst had larger specific surface area, stronger reducibility, and more L acid sites compared with MnOx catalyst. The relative percentage of Mn4+/Mnn+ on the MnPrOx-S catalyst surface was also much higher than those of MnOx catalyst. Importantly, when SO2 exists in feed gas, PrOx species in MnPrOx catalyst would preferentially react with SO2, thus protecting the Mn active sites. In addition, the introduction of Pr might promote the reaction between SO2 and NH3 rather than between SO2 and Mn active sites, which was also conductive to protect the Mn active sites to a great extent. Since the presence of SO2 in feed gas had little effect on NH3 adsorption on the MnPrOx catalyst surface, and the inhibiting effect of SO2 on NO adsorption was alleviated, SCR reactions could still proceed in a near-normal way through the Eley-Rideal (E-R) mechanism on Pr-modified MnOx catalyst, while SCR reactions through the Langmuir-Hinshelwood (L-H) mechanism were suppressed slightly.

Remarkable enhancement in the N2 selectivity of NH3-SCR over the CeNb3Fe0.3/TiO2 catalyst in the presence of chlorobenzene

The simultaneous removal of NO_x and dioxins is the frontier of environmental catalysis, which is still in the initial stage and poses several challenges. In this study, a series of CeNb₃Fe_x/TiO₂ (x = 0, 0.3, 0.6, and 1.0) catalysts were prepared by the sol-gel method and examined for the synergistic removal of NO_x and CB. The CeNb₃Fe_0.3/TiO₂ catalyst exhibits an optimum catalytic performance, with an NO_x conversion greater than 95% at 260-380 °C. It also exhibits an optimal CB oxidation activity, in which CB promoted both the NO_x conversion and N₂ selectivity below 250 °C. Moreover, the more favorable ratios of Ce⁴⁺ to Ce³⁺ and plentiful surface-adsorbed oxygen species are the reasons why CeNb₃Fe_0.3/TiO₂ catalyst has better catalytic activity than other catalysts at the lower temperature. Simultaneously, owing to the modulation of Fe to the redox properties of Ce and Nb, the large number of oxygen vacancies and acid sites was generated, and the CeNb₃Fe_0.3/TiO₂ catalyst is beneficial to NO_x reduction and CB oxidation. Furthermore, the results of in situ DRIFTS study reveal the NH₃-SCR reactions over CeNb₃Fe_0.3/TiO₂ catalysts are mainly conformed to by the L-H mechanism (< 350 °C) and E-R mechanism (> 350 °C), respectively, and the multi-pollutant conversion mechanism in the synergistic reaction was systematically studied.

Study on NH3-SCR of Cerium-based Substances in Rare Earth Concentrates from Bayan Obo

In this paper, CePO4 and CeCO3F were prepared by hydrothermal synthesis based on the ratio of bastnaesite to monazite in the process mineralogy of Baiyun Ebo rare earth concentrate. A comparison of the two treatments, ball milling and ball milling sulphation, revealed that the denitrification efficiency of the catalysts treated with ball milling sulphation increased by 20 percentage points, compared to those treated without sulphation, with denitrification efficiencies of up to 80%. The surface properties, redox properties and catalytic mechanism of the samples before and after the sulphation treatment were analyzed, by using XRD, NH3-TPD, H2-TPR, XPS and in situ IR characterization. The results showed that the CeF3 diffraction peaks in the XRD patterns disappeared in the sulphated samples, NH3-TPD showed that the adsorption capacity of NH3 on the surface of the samples was enhanced, and the introduction of sulphuric acid provided a large number of acidic sites on the catalyst surface, among which the Lewis acidic sites might be more favorable for the promotion of SCR reaction. The acidification of sulphuric acid greatly increases the redox capacity of the catalyst, and the interconversion between Cen+ was enhanced. XPS showed a significant increase in the amount of adsorbed oxygen on the surface of the sample. The presence of -NO2, an important intermediate in the L-H mechanism, was also detected by IR analysis. reactant species during the L-H mechanism reaction were monodentate nitrate, bridged nitrate and NH4 + species produced by NH3 adsorption on the Brønsted acidic site of the catalyst surface.

Recent progress of low-temperature selective catalytic reduction of NOx with NH3 over manganese oxide-based catalysts

Selective catalytic reduction with NH3 (NH3−SCR) was the most efficient approach to mitigate the emission of nitrogen oxides (NOx). Although the conventional manganese oxide-based catalyst had gradually become a kind...

A novel CNTs functionalized CeO2/CNTs-GAC catalyst with high NO conversion and SO2 tolerance for low temperature selective catalytic reduction of NO by NH3

Low-temperature selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) for diminishing SO₂ poisoning remains an issue in flue gas denitrification (DeNO_x). Herein, A novel CNTs functionalized low temperature NH₃-SCR catalyst CeO₂/CNTs-GAC was prepared, which showed high NO conversion activity (100% at 150 °C) and SO₂ resistance. The addition of CNTs restrained SO₂ adsorption but improved the selective adsorption of NO, which restricted the deposition of (NH₄)₂SO₄ and/or Ce₂(SO₄)₃, and resulted in high SO₂ resistance. The addition of CNTs facilitated the diffusion and transportation of NH₃ and NO, and the electron transfer on CeO₂/CNTs-GAC, leading to higher content of Ce³⁺ and adsorbed O species on the CeO₂/CNTs-GAC surface and promoted formation of surface-adsorbed oxygen O_A. Therefore, CeO₂/CNTs-GAC provided abundant NO adsorption and activation sites, facilitating "fast SCR" reaction and enhancing the NH₃-SCR reaction. The proposed CeO₂/CNTs-GAC catalyst exhibited higher NH₃-SCR activity, N₂ selectivity, catalytic durability and SO₂ resistance than CeO₂/GAC.

A Study on Mn-Fe Catalysts Supported on Coal Fly Ash for Low-Temperature Selective Catalytic Reduction of NOX in Flue Gas

A series of Mn0.15Fe0.05/fly-ash catalysts have been synthesized by the co-precipitation method using coal fly ash (FA) as the catalyst carrier. The catalyst showed high catalytic activity for low-temperature selective catalytic reduction (LTSCR) of NO with NH3. The catalytic reaction experiments were carried out using a lab-scale fixed-bed reactor. De-NOx experimental results showed the use of optimum weight ratio of Mn/FA and Fe/FA, resulted in high NH3-SCR (selective catalytic reduction) activity with a broad operating temperature range (130–300 °C) under 50000 h−1. Various characterization methods were used to understand the role of the physicochemical structure of the synthesized catalysts on their De-NOx capability. The scanning electron microscopy, physical adsorption-desorption, and X-ray photoelectron spectroscopy showed the interaction among the MnOx, FeOx, and the substrate increased the surface area, the amount of high valence metal state (Mn4+, Mn3+, and Fe3+), and the surface adsorbed oxygen. Hence, redox cycles (Fe3+ + Mn2+ ↔ Mn3+ + Fe2+; Fe2+ + Mn4+ ↔ Mn3+ + Fe3+) were co-promoted over the catalyst. The balance between the adsorption ability of the reactants and the redox ability can promote the excellent NOx conversion ability of the catalyst at low temperatures. Furthermore, NH3/NO temperature-programmed desorption, NH3/NO- thermo gravimetric-mass spectrometry (NH3/NO-TG-MS), and in-situ DRIFTs (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) results showed the Mn0.15Fe0.05/FA has relatively high adsorption capacity and activation capability of reactants (NO, O2, and NH3) at low temperatures. These results also showed that the Langmuir–Hinshelwood (L–H) reaction mechanism is the main reaction mechanism through which NH3-SCR reactions took place. This work is important for synthesizing an efficient and environmentally-friendly catalyst and demonstrates a promising waste-utilization strategy.

Effect of CaCO3 on catalytic activity of Fe–Ce/Ti catalysts for NH3-SCR reaction

In the present work, fresh and Ca poisoned Fe–Ce/Ti catalysts were prepared and used for the NH₃-SCR reaction to investigate the effect of Ca doping on the catalytic activity of catalysts. And these catalysts were characterized by BET, XRD, Raman, UV-vis DRS, XPS, H₂-TPR, and NH₃-TPD techniques. The obtained results demonstrate that Ca doping could lead to an obvious decrease in the catalytic activity of catalysts. The reasons for this may be due to the smaller specific surface area and pore volume, the decreased ratio of Fe³⁺/Fe²⁺ and Ce³⁺/Ce⁴⁺, as well as the reduced redox ability and surface acidity.

In the present work, fresh and Ca poisoned Fe–Ce/Ti catalysts were prepared and used for the NH₃-SCR reaction to investigate the effect of Ca doping on the catalytic activity of catalysts.