Effect of V2O5 Additive on the SO2 Resistance of a Fe2O3/AC Catalyst for NH3-SCR of NOx at Low Temperatures

Combined Experimental and Density Functional Theory Study on the Mechanism of the Selective Catalytic Reduction of NO with NH3 over Metal-Free Carbon-Based Catalysts

Metal-free carbon-based catalysts are attracting much attention in the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR). However, the mechanism of the NH3-SCR reaction on carbon-based catalysts is still controversial, which severely limits the development of carbon-based SCR catalysts. Herein, we successfully reconstructed carbon-based catalysts through oxidation treatment with nitric acid, thereby enhancing their low-temperature activity in NH3-SCR. Combining experimental results and density functional theory (DFT) calculations, we proposed a previously unreported NH3-SCR reaction mechanism over carbon-based catalysts. We demonstrated that C-OH and C-O-C groups not only effectively activate NH3 but also remarkedly promote the decomposition of intermediate NH2NO. This study enhances the understanding of the NH3-SCR mechanism on carbon-based catalysts and paves the way to develop low-temperature metal-free SCR catalysts.

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

With the looming goal of carbon neutrality and increasingly stringent environmental protection policies, gas purification in coal-fired power plants is becoming more and more intense. To achieve the NOx emission standard when coal-fired power plants are operating at full load, wide-temperature denitrification catalysts that can operate for a long time in the range of 260–420°C are worthy of study. This review focuses on the research progress and deactivation mechanism of selective catalytic reduction (SCR) denitration catalysts applied to a wide temperature range. With the increasing application of SCR catalysts, it also means that a large amount of spent catalysts is generated every year due to deactivation. Therefore, it is necessary to recycle the wide temperature SCR denitration catalyst. The challenges faced by wide-temperature SCR denitration catalysts are summarized by comparing their regeneration processes. Finally, its future development is prospected.

Highly efficient removal of toluene over Cu-V oxides modified γ-Al2O3 in the presence of SO2

Developing efficient catalysts with good resistance to complex flue gas is essential for VOCs removal in coal-fired flue gas. In this study, by exploring the effect of transition metal oxide additive, metal loading and bimetallic synergy on toluene oxidation in coal-fired flue gas, 10Cu-3V/γ-Al₂O₃ is identified as the optimal catalyst. It achieves 90% of CO₂ generation at 350 ℃, which is decreased by ca. 46 ℃ compared with 13Cu/γ-Al₂O₃. And it also exhibits good resistance to H₂O and good stability. ICP-OES, N₂ adsorption-desorption isotherms, XRD, TEM, XPS, EPR and H₂-TPR analyses were applied to characterize the catalyst composition and physicochemical properties. Doping V into 13Cu/γ-Al₂O₃ not only leads to better dispersity of CuO and homogeneous elements distribution that benefits to produce more active centers, but also constitutes the redox cycle of V⁵⁺ + Cu⁺ ↔ V⁴⁺ + Cu²⁺ which induces more surface chemical oxygen (O_sur). Moreover, since SO₂ is the main inhibiting factor in toluene oxidation, the SO₂ poisoning mechanism was illustrated by XPS, TG and in situ DRIFT analyses in depth.

SO2 Tolerance of Rice Hull Ash Based Fe-Cu Catalysts for Low-Temperature CO-SCR of NO

Rice husk ash (RHA) has potential as a supporter of catalysts. In this research, we studied the activity and SO2 tolerance of RHA-based Fe-Cu oxide in the reduction of NO by CO. Characterization methods were employed to study the properties of the catalysts and their SO2 tolerance. Activity and SO2 resistance were also tested at different temperatures. We recommend two catalysts with SO2 resistance ability: Fe0.67Cu0.33/RHA (the highest catalytic activity) and Fe0.8Cu0.2/RHA. The NO removal rate hardly changed with the addition of SO2 and was kept at about 100%. However, the CO conversion rate decreased with increasing SO2 at the lower reaction temperatures, which may be due to the formation of sulfites. Fortunately, the deactivation was reversible and can be reduced with an increase in the reaction temperature. The results of our research may help promote the application of CO-SCR.

Effect of K-Modified Blue Coke-Based Activated Carbon on Low Temperature Catalytic Performance of Supported Mn-Ce/Activated Carbon

To clarify the K modified effects over activated carbon (AC) supported Mn-Ce oxide catalysts, several Mn-Ce/AC and xK-Mn-Ce/AC mixed oxide catalysts prepared via an impregnation method supported on AC were investigated for low-temperature selective catalytic reduction (SCR) of NO with NH₃ in the simulated sintering flue gas. The Mn-Ce/AC catalyst with a K loading of 8% showed the highest catalytic activity, corresponding to 92.1% NO conversion and 92.5% N₂ selectivity at 225 °C with a space velocity of 12,000 h^-1. Furthermore, the 0.08K-Mn-Ce/AC catalyst exhibited better resistance to SO₂ and H₂O than Mn-Ce/AC, which could convert 72.3% and 74.1% of NO at the presence of 5% SO₂ and H₂O, respectively. After K modification, the relative ratios of Mn⁴⁺/Mn ⁿ⁺ as well as Ce³⁺/Ce ⁿ⁺ and surface adsorbed oxygen increased. Additionally, the reduction performance of the catalyst was improved obviously, and both acid strength and quantity of acid sites increased significantly after the K species were introduced in Mn-Ce/AC. Especially, the NO adsorption capacity of the catalyst was enhanced, which remarkably promoted the denitration efficiency and SO₂ resistance. The SCR of NO with NH₃ on K-Mn-Ce/AC catalysts followed the L-H mechanism.

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 review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance

The development of high-efficiency catalysts is the key to the low-temperature NH₃-SCR technology. The introduction of SO₂ and H₂O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH₃-SCR technology. This review introduces the necessity of NO_x removal, explains the mechanisms of H₂O and SO₂ poisoning on NH₃-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H₂O and SO₂, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO₂/H₂O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

Recent Progress on Improving Low-Temperature Activity of Vanadia-Based Catalysts for the Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been successfully applied to abate NOx from diesel engines and coal-fired industries on a large scale. Although V2O5-WO3(MoO3)/TiO2 catalysts have been utilized in commercial applications, novel vanadia-based catalysts have been recently developed to meet the increasing requirements for low-temperature catalytic activity. In this article, recent progress on the improvement of the low-temperature activity of vanadia-based catalysts is reviewed, including modification with metal oxides and nonmetal elements and the use of novel supports, different synthesis methods, metal vanadates and specific structures. Investigation of the NH3-SCR reaction mechanism, especially at low temperatures, is also emphasized. Finally, for low-temperature NH3-SCR, some suggestions are given regarding the opportunities and challenges of vanadia-based catalysts in future research.

One-pot synthesis of layered mesoporous ZSM-5 plus Cu ion-exchange: Enhanced NH3-SCR performance on Cu-ZSM-5 with hierarchical pore structures

Hierarchical ZSM-5 zeolite with meso- and micro-pore structures was successfully prepared through a facile one-pot hydrothermal synthesis method using bifunctional template. After copper ion-exchange, it was applied for the selective catalytic reduction of NO with NH₃ (NH₃-SCR). Compared with conventional Cu-ZSM-5 catalyst containing only micropores, the hierarchical catalyst with ca. 2 wt.% Cu loading displayed significantly improved catalytic performance. Particularly, the hierarchical zeolite catalyst also displayed excellent hydrothermal stability and sulfur resistance that exhibited great potential in practical application. Characterization techniques such as XRD, N₂ physisorption, temperature programmed desorption/reduction (TPD/TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were comprehensively used to reveal the relationship between zeolite structure and catalytic properties. It was concluded that the hierarchically porous structure could not only improve the mass transfer of reactant/product but also provide larger specific surface area, higher surface acidity, larger NO adsorption capacity. And we found that bidentate nitrate species was more active in Cu-ZSM-5-meso than Cu-ZSM-5-C, which were all beneficial to the NH₃-SCR reaction. This work can provide a guideline to design other high performance hierarchical zeolites with different crystalline structures (such as CHA, LTA) for efficient catalytic NOx removal processes.

Bipyridinium-Based Ionic Covalent Triazine Frameworks for CO2, SO2, and NO Capture

The exploitation of novel porous materials for capturing/adsorption of harmful gases is considered a very promising approach to deal with air pollution. Herein, bipyridinium-based ionic covalent triazine frameworks (ICTFs) were synthesized via ZnCl₂-catalyzed ionothermal polymerization. The as-prepared ICTFs had a satisfactory total pore volume and specific surface of approximately 0.4582 cm³ g^-1 and 1000 m² g^-1, respectively. Moreover, the specific surface area, pore size and distribution, and total pore volumes of ICTFs could be adjusted via ion-exchange of the anion. The obtained ICTFs were explored as the adsorbent for the separation/adsorption of the mixed gases (SO₂, CO₂, NO, and N₂), and they showed the strong adsorption ability for CO₂ (2.75 mmol g^-1), SO₂ (9.22 mmol g^-1), and NO (4.05 mmol g^-1) at 1 bar and 298 K. This unique design provides a new insight to prepare high-efficiency porous materials for CO₂, SO₂, and NO capture.

Low Temperature deNOx Catalytic Activity with C2H4 as a Reductant Using Mixed Metal Fe-Mn Oxides Supported on Activated Carbon

The selective catalytic reduction of NOx (deNOx) at temperatures less than or at 200 °C was investigated while using C2H4 as the reductant and mixed oxides of Fe and Mn supported on activated carbon; their activity was compared to that of MnOx and FeOx separately supported on activated carbon. The bimetallic oxide compositions maintained high NO conversion of greater than 80–98% for periods that were three times greater than those of the supported monometallic oxides. To examine potential reasons for the significant increases in activity maintenance, and subsequent deactivation, the catalysts were examined by using bulk and surface sensitive analytical techniques before and after catalyst testing. No significant changes in Brunauer-Emmett-Teller (BET) surface areas or porosities were observed between freshly-prepared and tested catalysts whereas segregation of FeOx and MnOx species was readily observed in the mono-oxide catalysts after reaction testing that was not detected in the mixed oxide catalysts. Furthermore, x-ray diffraction and Raman spectroscopy data detected cubic Fe3Mn3O8 in both the freshly-prepared and reaction-tested mixed oxide catalysts that were more crystalline after testing. The presence of this compound, which is known to stabilize multivalent Fe species and to enhance oxygen transfer reactions, may be the reason for the high and relatively stable NO conversion activity, and its increased crystallinity during longer-term testing may also decrease surface availability of the active sites responsible for NO conversion. These results point to a potential of further enhancing catalyst stability and activity for low temperature deNOx that is applicable to advanced SCR processing with lower costs and less deleterious side effects to processing equipment.

Byproduct Analysis of SO2 Poisoning on NH3-SCR over MnFe/TiO2 Catalysts at Medium to Low Temperatures

The byproducts of ammonia-selective catalytic reduction (NH3-SCR) process over MnFe/TiO2 catalysts under the conditions of both with and without SO2 poisoning were analyzed. In addition to the NH3-SCR reaction, the NH3 oxidation and the NO oxidation reactions were also evaluated at temperatures of 100–300 °C to clarify the reactions occurred during the SCR process. The results indicated that major byproducts for the NH3 oxidation and NO oxidation tests were N2O and NO2, respectively, and their concentrations increased as the reaction temperature increased. For the NH3-SCR test without the presence of SO2, it revealed that N2O was majorly from the NH3-SCR reaction instead of from NH3 oxidation reaction. The byproducts of N2O and NO2 for the NH3-SCR reaction also increased after increasing the reaction temperature, which caused the decreasing of N2-selectivity and NO consumption. For the NH3-SCR test with SO2 at 150 °C, there were two decay stages during SO2 poisoning. The first decay was due to a certain amount of NH3 preferably reacted with SO2 instead of with NO or O2. Then the catalysts were accumulated with metal sulfates and ammonium salts, which caused the second decay of NO conversion. The effluent N2O increased as poisoning time increased, which was majorly from oxidation of unreacted NH3. On the other hand, for the NH3-SCR test with SO2 at 300 °C, the NO conversion was not decreased after increasing the poisoning time, but the N2O byproduct concentration was high. However, the SO2 led to the formation of metal sulfates, which might inhibit NO oxidation reactions and cause the concentration of N2O gradually decreased as well as the N2-selectivity increased.

Improved NO x Reduction in the Presence of SO2 by Using Fe2O3-Promoted Halloysite-Supported CeO2-WO3 Catalysts

Currently, selective catalytic reduction (SCR) of NO _x with NH₃ in the presence of SO₂ by using vanadium-free catalysts is still an important issue for the removal of NO _x for stationary sources. Developing high-performance catalysts for NO _x reduction in the presence of SO₂ is a significant challenge. In this work, a series of Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalysts were synthesized by a molten salt treatment followed by the impregnation method and demonstrated improved NO _x reduction in the presence of SO₂. The obtained catalyst exhibits superior catalytic activity, high N₂ selectivity over a wide temperature range from 270 to 420 °C, and excellent sulfur-poisoning resistance. It has been demonstrated that the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst increased the ratio of Ce³⁺ and the amount of surface oxygen vacancies and enhanced the interaction between active components. Moreover, the SCR reaction mechanism of the obtained catalyst was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. It can be inferred that the number of Brønsted acid sites is significantly increased, and more active species could be produced by Fe₂O₃ promotion. Furthermore, in the presence of SO₂, the Fe₂O₃-promoted halloysite-supported CeO₂-WO₃ catalyst can effectively prevent the irreversible bonding of SO₂ with the active components, making the catalyst exhibit desirable sulfur resistance. The work paves the way for the development of high-performance SCR catalysts with improved NO _x reduction in the presence of SO₂.

Research Status and Prospect on Vanadium-Based Catalysts for NH₃-SCR Denitration

Selective catalytic reduction of NO_x with NH₃ is one of the most widely used technologies in denitration. Vanadium-based catalysts have been extensively studied for the deNO_x process. V₂O₅/WO₃(MoO₃)TiO₂ as a commercial catalyst has excellent catalytic activity in the medium temperature range. However, it has usually faced several problems in practical industrial applications, including narrow windows of operation temperatures, and the deactivation of catalysts. The modification of vanadium-based catalysts will be the focus in future research. In this paper, the chemical composition of vanadium-based catalysts, catalytic mechanism, the broadening of the temperature range, and the improvement of erosion resistance are reviewed. Furthermore, the effects of four major systems of copper, iron, cerium and manganese on the modification of vanadium-based catalysts are introduced and analyzed. It is worth noting that the addition of modified elements as promoters has greatly improved the catalytic performance. They can enhance the surface acidity, which leads to the increasing adsorption capacity of NH₃. Surface defects and oxygen vacancies have also been increased, resulting in more active sites. Finally, the future development of vanadium-based catalysts for denitration is prospected. It is indicated that the main purpose for the research of vanadium-based modification will help to obtain safe, environmentally friendly, efficient, and economical catalysts.

Characteristics of Vanadium-Based Coal Gasification Slag and the NH3-Selective Catalytic Reduction of NO

In order to realize the resource utilization of coal gasification slag (CGS) and to effectively control the emission of nitrogen oxides (NOx) in coke oven gas, the effect of the reaction conditions and vanadium loading over the CGS catalysts was carried out for the selective catalytic reduction (SCR) of NO by NH3. The various vanadium loaded CGS catalysts were prepared using impregnation methods. The addition of 1% vanadium to the CGS catalyst (V1/CGS) significantly enhanced the NO conversion at a wide temperature range of 180–290 °C. The catalysts were characterized by N2 adsorption/desorption, X-ray photoelectron spectroscopy, H2-temperature programmed reduction, NH3-temperature programmed desorption, Inductively coupled plasma optical emission spectrometer (ICP-OES), thermo gravimetric analyses (TGA), Fourier Transform infrared spectroscopy (FTIR), Scanning electron microscope-Energy dispersive spectrometer (SEM-EDS), and X-ray powder diffraction (XRD). The experimental results show the following: That (1) the NO removal efficiency of the sample CGS3 was the best, and it could be up to 100% under the experimental conditions; (2) The NO removal efficiency of the catalysts was higher in the atmosphere with SO2 than that without SO2; (3) The XRD results indicated the active component of vanadium was homogeneously dispersed over CGS and the active component of catalyst was V2O5 according to the XPS results. In particular, the NH3-TPD spectra of the vanadium loaded CGS catalyst showed that vanadium produced more acid sites, and the Lewis acid sites on the vanadium species were the active sites for the catalytic reduction of NO at 240–290 °C.

The selective catalytic reduction of NO over Ce0.3TiOx-supported metal oxide catalysts

A Ce_0.3TiO_x oxide carrier was synthesized via a sol-gel process, and Ce_0.3TiO_x supported metal (M=Cd, Mn, Fe, W, Mo) oxide catalysts were prepared by the method of incipient-wetness impregnation. The catalysts were characterized by means of X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and Temperature-programmed reduction with H₂ (H₂-TPR). The catalytic activities for de-NO_x were evaluated by the NH₃-SCR reaction. Among all the catalysts tested, the 2wt.% Cd/Ce_0.3TiO_x catalyst exhibited the best NH₃-SCR performance, with a wide temperature window of 250-450°C for NO conversion above 90%. Moreover, the catalyst showed N₂ selectivity greater than 99% from 200 to 450°C.

Modification of MnCo2Ox catalysts by NbOx for low temperature selective catalytic reduction of NO with NH3

Among the NbOx modified MnCo₂Ox catalysts by co-precipitation, MnCo₂Nb_0.5Ox presented high activity, N₂ selectivity and H₂O resistance for NH₃-SCR reaction at low temperatures.