For the sintering flue gas in steel industry, selective catalytic reduction (SCR) of NOx using NH3 at low temperature in the presence of heavy metals is still a challenge. In this study, the toxicity effects of lead salt species (PbCl2, Pb(NO3)2, PbSO4, PbCO3) on Cu-exchanged zeolite X doped with Mn-Ce catalysts (MCCX) were investigated. All catalysts exhibited poor low-temperature SCR performance within 75-225 degrees C after Pb poisoning, and the poisoning effects of PbCl2, Pb(NO3)2, PbSO4 and PbCO3 on MCCX catalyst varied. The PbSO4 had the most serious poisoning on MCCX catalyst, while PbCO3 had poisoning effect on catalysts even at 300 degrees C. Lead species poisoning had bad impact on the structure of zeolite X on MCCX catalyst, and caused some agglutination of Cu species. Likewise, the Pb species might primarily occupy the active site of Mn and Cu species and influence the redox property of catalysts, thereby obstructing the redox and oxidation cycles on catalysts surface. In addition, the PbCl2 and PbNO3 species predominantly affected the Bronsted acid sites and occupied the Cu or Mn active sites, inhabiting the absorption and conversion of nitrites and nitrates on the catalyst surface. Moreover, PbSO4 and PbCO3 species mainly impacted the adsorption of NO + O2 and the conversion of nitrites and nitrates on the catalysts.

The purification of gaseous pollutants, especially for nitrogen oxides, from the combustion of fossil fuels has become a concern matter at present. Selective catalytic reduction (SCR) technology is proved an effective method to remove nitrogen oxides. The performance of catalysts, such as the improvement of anti-poisoning ability and low-temperature catalytic efficiency, become the main direction of current extensive research. Carbonaceous materials were widely applied in low-temperature catalysts for removal of NOx due to their various structures and rich surface functional groups, and had potential to expand the function to remove multipollutant. Herein, different carbonaceous materials, activated methods and carbon-based NH3-SCR catalysts were concluded, and the corresponding reaction mechanisms were discussed. In addition, the removal of double or three contamination components with carbon-based catalysts were also summarized and discussed. Finally, further development directions for carbonaceous material in pollutants control were proposed.

In this study, a series of zeolite-X-supported different crystal phases of MnO2 (α-MnO2, β-MnO2, γ-MnO2, and σ-MnO2) catalysts were prepared via a solid-state diffusion method and high-heat treatment method to explore their low-temperature NH3-SCR performance. All of the catalysts featured typical octahedral zeolite X structures and manganese dioxides species of various crystal types dispersed across the support surface. Throughout the entire temperature range of the reaction, γ-MnO2/X catalyst had the highest NO conversion. Additionally, β-MnO2/X, γ-MnO2/X, and σ-MnO2/X catalysts had nearly 100% of N2 selectivity, whereas the α-MnO2/X catalyst had the lowest N2 selectivity (about 90%) below 125 °C. Moreover, the γ-MnO2/X catalyst demonstrated superior acidity capacity and reduction ability compared with the other three catalysts. All the catalysts contained the essential intermediates NH2NO and NH4NO3 species, which are essential to the SCR reaction. More acid sites and nitrate species existed on the γ-MnO2/X catalyst than on the other catalysts, thereby boosting the SCR reaction.

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

It is indeed a challenging problem to simultaneously remove NO and CO from the steel sintering flue gas, in which a bifunctional catalyst has proven to be an efficient solution for removing both pollutants at low temperature. In this study, four different crystalline phases of MnO2 (& alpha;-, & beta;-, & gamma;-, and & delta;-) catalysts were synthesized via a facile hydrothermal method, and the effects of their crystal structure, morphology, and physicochemical properties on the catalytic performance for NO reduction and CO oxidation were elucidated. The results indicated that & gamma;-MnO2 catalyst exhibited the best catalytic activity, achieving 90% NO removal efficiency and 82% CO conversion rate at 175 degrees C. Reaction kinetics confirmed that & gamma;-MnO2 catalyst exhibited a lower Ea for both NO reduction and CO oxidation compared to & alpha;-MnO2, & beta;-MnO2 and & delta;-MnO2 catalysts. Meanwhile, the interaction of between NH3-SCR and CO catalytic oxidation reactions over the catalysts was also studied. Intriguingly, it was found that the presence of CO enhanced the catalytic activity of & gamma;-MnO2 catalyst in the NH3-SCR reaction. The results of NO + O2-TPD and in situ DRIFTS experiments revealed that CO contributed to the adsorption and oxidation of NO, thus promoting the L-H pathway over & gamma;-MnO2 catalyst. Finally, a possible mechanism model for simultaneous removal of NO and CO over & gamma;-MnO2 catalyst was proposed.

Nitrogen oxides (NOx) and carbon monoxide (CO) coexisted in numerous industrial waste gases. And selective catalytic reduction of NOx with ammonia (NH3-SCR) and CO catalytic oxidation were considered as the effective technologies for simultaneous elimination of NOx and CO, respectively. However, an efficient bifunctional catalyst was crucial for achieving simultaneous removal of NOx and CO. Herein, we developed a series of Cumodified CeFe catalysts for simultaneous removal of NOx and CO. The results demonstrated that 5Cu-CeFe catalyst presented the best catalytic performance, achieving approximately 81% NOx removal efficiency and nearly 99% CO conversion rate at 200 degrees C under a gas houely space velocity of 90,000 h-1. The XPS analysis demonstrated that copper oxides played a critical role in promoting the formation of Ce3+ and O beta, thereby facilitating the NH3-SCR catalytic performance and CO catalytic oxidation. Moreover, the copper oxide modification of CeFe catalyst attained a trade-off between the surface acidity and redox ability, resulting in improving adsorption and activition capacity of the reactant gases. In situ DRIFTS experiments revealed that the NH3-SCR process primarily obeyed the Eley-Rideal (E-R) pathway over both CeFe and 5Cu-CeFe catalysts, while LangmuirHinshelwood (L-H) pathway hardly participated in NH3-SCR process. The presence of Cu+-CO species and surface lattice oxygen was crucial for CO oxidation, and both Mars-van Krevelen (M-K) and L-H pathways might be involved in the CO catalytic oxidation process over 5Cu-CeFe catalyst.

The increase of solid wastes exerted great pressure on the environment and even damaged human health. Given this, the resource utilization of solid wastes became an inevitable trend of the industrial development. However, many researchers recognized that solid wastes with the component of Fe2O3, SiO2, Al2O3, TiO2, etc., could be utilized as the low-cost materials for preparing NH3-SCR catalysts, which could solve both the accumulation of solid wastes and the emission of NOx in industrial processes. Herein, some typical solid wastes, such as blast furnace slag (BFS), red mud (RM), coal fly ash (CFA), Fenton sludge (FS), etc., were extensively studied for their potential in NH3-SCR catalysts. Although the presence of some harmful components in solid wastes led to catalysts deactivation, they could still be excellent supports applied on NH3-SCR catalysts after proper pretreatment. In fact, these catalyst supports derived from solid wastes provided larger specific surface areas, thereby enhancing the NH3 adsorption and redox capacity and facilitating the active ingredients loaded uniformly. Meanwhile, some useful impurities in solid wastes might also facilitate the NH3-SCR performance, achieving denitrification efficiency comparable to other catalysts made from costly materials. Moreover, most of these NH3-SCR catalysts performed significant SO2 and/or H2O tolerance via metal oxides-doping, which would expand their potential for practical application in industry. Finally, the prospects for the application of solid wastes in NH3-SCR catalysts were also proposed.

In this study, a composite rapid soil stabilizer (CRSS) was developed based on a calcium sulfoaluminate cement (CSA) to enhance the rapid solidification performance of sludge soil and augment its deficient engineering properties. The appropriate admixture type and dosage were determined through a double mixing test, and the stabilizer formula was optimized via response surface analysis. The solidification products and micromorphology were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the solidification mechanism was discussed. The results showed that the unconfined compressive strength (UCS) of the CRSS-solidified sludge cured for 1 day reached 9 MPa, which was approximately 5 times higher than that of sludge solidified by early strength composite portland cement (PC 42.5R). Gypsum, lithium salt, and ionic soil stabilizer addition significantly affected the strength of the CSA-solidified sludge.

Sludge is a kind of soil with very poor engineering properties and difficult to solidify rapidly. Through unconfined compressive strength test, consolidated drained triaxial test, micro test and mercury intrusion test, the strength development law of sludge solidified by composite rapid soil stabilizer (CRSS) and the influence of the content of soil stabilizer on the strength are studied, and the micro mechanism of strength development is preliminarily discussed. The results show that the strength of CRSS-solidified sludge increases logarithmically with the curing age, the early strength develops rapidly, and the unconfined compressive strength can reach 3 MPa after curing for 1 h; with the increase of stabilizer content, the triaxial shear stress-strain curve of solidified sludge gradually develops from strain hardening type to strain softening type, and the failure deviatoric stress and cohesion increase linearly. The microstructure shows that the hydration products of stabilizer can cement soil particles, fill soil pores and enhance the integrity of soil. The mercury intrusion test results further reveal the mechanism that the cohesion of solidified sludge increases linearly with the increase of the content of stabilizer through the connection of the most probable pore size. CRSS-solidified sludge has significant advantages of fast hardening and high strength, which provides a theoretical basis for the application of rapid solidification of sludge.

The effects of four distinct zinc species (ZnCl2, Zn(NO3)2, ZnSO4, and ZnCO3) on a Mn-Ce co-doped CuX (MCCX)catalyst were investigated and contrasted in the low-temperature NH3-SCR process. Aqueous solutions of ZnCl2, Zn(NO3)2, ZnSO4, and ZnCO3 were used to poison the catalysts. The catalytic activity of all catalysts was assessed, and their physicochemical properties were studied. There was a notable drop trend in catalytic activity in the low temperature range (200 °C) after zinc species poisoning on MCCX catalyst. Interestingly, ZnSO4 and ZnCO3 on MCCX catalyst had more serious effect on catalytic activity than Zn(NO3)2 and ZnCl2 from 150 °C to 225 °C, in which NO conversion of the MCCX-Zn-S and MCCX-Zn-C catalysts dropped about 20–30% below 200 °C compared with the fresh MCCX catalyst. The zeolite X structure was impacted by Zn species doping on the MCCX catalyst, and the Zn-poisoned catalysts had less acidic and lower redox ability than fresh Mn-Ce/CuX catalysts. Through the results of in situ DRIFTS spectroscopy experiments, all catalysts were governed by both Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms, and the possible mechanism for poisoning the Mn-Ce/CuX catalyst using various zinc species was revealed.

In this work, ammonium carbonate, sodium hydroxide, and ammonium hydroxide were selected as the precipitants to prepare bifunctional MnOx catalysts via the PEG-assisted co-precipitation method. SEM analysis revealed that MnOx catalysts prepared with ammonium carbonate (AC) displayed a spherical morphology with relatively large size, whereas the catalysts synthesized with sodium hydroxide (SH) and ammonium hydroxide (AH) exhibited polyhedron shapes with smaller size. The results demonstrated that Mn-AC catalyst presented superior catalytic activity for both NO reduction and CO oxidation, achieving 89% NO conversion at 100 oC and over 80% CO conversion at 200 oC, outperforming Mn-SH and Mn-AH catalysts. Mn-AC catalyst possessed more abundant Mn4+ species and surface chemisorbed oxygen than Mn-AH and Mn-SH catalysts, which served as a significant motivation for the difference in catalytic activity. Additionally, Mn-AC catalyst exhibited stronger surface acidity, which facilitated the adsorption and activation of ammonia species in the NH3-SCR reaction. Furthermore, in situ DRIFTS experiment suggested that the pre-desorption of CO promoted the adsorption and activation of NO. On this basis, the possible mechanism model of three precipitants on bifunctional MnOx catalysts was proposed.

The study of SCR catalysts for nitrogen oxides reduction with NH3 had recently attracted much attention. As a typical heavy metal element, Pb was prone to seriously damage the denitrification performance of SCR catalysts in flue-gas environment. This work systematically reviewed the mechanisms of Pb poisoning and anti-deactivation measures reported these years for SCR catalysts. The mechanisms of Pb poisoning were summarized from both physical and chemical aspects, including the specific surface area, redox ability and surface acidity of the catalysts, and the corresponding reaction paths of the catalysts were also considered during the SCR process. In the meantime, this paper reviewed the anti-deactivation mechanisms and measures currently used to solve the problem of lead poisoning for SCR catalysts. The main methods included adding active substances to improve the redox ability and surface acidity of the catalysts, using ions or anti-poisoning sites to preferentially bind to Pb species, or using the structural characteristics of the carriers to prevent Pb from contacting with active substances. The concrete measures included the modification of catalysts with metal oxides and solid super acids, the change of carriers’ structure, and the dual effect of carriers and active materials. Finally, a prospect of lead poisoning for further research on SCR catalysts was proposed.

Herein, the inductive strategy of three precipitants on dual transition metal MnFeOx catalysts for low -temperature NH3-SCR reaction was investigated in detail. The results showed that the anions of precipitants played the leading role in the formation of catalytic phase, in which carbonate induced the transfer of Mn precursor from MnCO3 to the ultimate Mn2O3 phase while that of hydroxyl from Mn3O4 to Mn5O8 phase. Iron coupling would alter the original transition behavior of manganese due to an internal synergistic effect. Furthermore, the cations decided the stability of hydroxyl groups on ammonium and sodium hydroxide pre-cipitants, leading to the difference of morphology in catalysts. The activity results suggested that MnFe-ammonium carbonate (denoted as AC) catalyst possessed the higher catalytic efficiency than other catalysts, with over 90% NO conversion achieved from 75 to 225 degrees C. The abundant Mn4+ and Fe3+ and strong surface acidity of MnFe-AC catalyst were in favor of the redox circle and the adsorption of NH3. Both activation energy and TOF, relied on the amount of acid, confirmed that the precipitants had an effect on the intrinsic catalytic activity of the catalysts such as energy barrier and reaction rate. Transient reaction further revealed that both E-R and L-H mechanisms co-existed on all catalysts, while E-R mechanism contributed more during the SCR process. On this basis, the possible inductive mechanism model of three precipitants on MnFeOx catalyst was proposed.

Abstract The reactive MgO (r-MgO)-microbial curing technology can not only effectively improve the strength of electrolytic manganese residue (EMR) but also repair heavy metal ions. By conducting unconfined compressive strength (UCS) test, heavy metal leaching test, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP), the effects of the type and concentration of sulfate on the strength and repair of heavy metals of the cured EMR under drying–wetting (DW) cycles were studied, and the strength and the evolution mechanism of heavy metal remediation is elucidated. The results indicate the following: With the increase in the number of DW cycles, the dry density of the samples in high concentration of MgSO4 first increased slightly and then decreased gradually and became stable. The dry density of the samples in Na2SO4, low concentration of MgSO4, and water generally showed a downward trend. The pH of the soaking solution slightly decreased, and the leaching concentration of heavy metal Mn2+ ions always remained at a low level. The UCS of the samples in MgSO4 first increased slightly and then decreased gradually, and then became stable. The amount of cured products first increased and then decreased, and the size of internal pores first decreased and then increased. The UCS of the samples in Na2SO4 and water gradually decreased and became stable. The amount of cured products gradually decreased, and the size of internal pores gradually increased. The research results provide a theoretical basis for the evaluation of resistance against the sulfate corrosion of cured EMR under DW cycles.

Composite rapid soil stabilizer (CRSS) comprising calcium sulfoaluminate cement, gypsum, lithium carbonate, and ionic soil stabilizer is characterized by rapid hardening and high strength. In this study, it was used to solidify sludge rapidly, and how the number of wetting–drying (WD) cycles and the CRSS dosage influenced the durability of the solidified sludge was studied. The results show that for a given CRSS dosage but an increasing number of WD cycles, the dry mass and density of the solidified sludge decreased, the water content and pH of the soaking solution increased, the unconfined compressive strength increased slightly, then decreased slowly, finally stabilized. The ettringite in the solidified sludge was transformed from columnar to needle-like. the diffraction peak intensity decreased, and the microscopic pores increased in both size and quantity. With increasing CRSS dosage, the apparent structure was more intact, the strength attenuation was smaller. After 12 WD cycles, with a 20% content of solidified sludge, the dry mass loss was only 2%, the strength loss was only 12%, and the cumulative pore volume did not increase obviously, which was maintained at about 0.20 ml/g, showing the high resistance to WD erosion. Based on the chemical reaction and combined with the macroscopic and microscopic test results, the microscopic evolution mechanism of the durability of solidified sludge under WD cycles—especially the strength evolution mechanism—is revealed clearly. The solidified sludge with CRSS had good durability against WD cycles, thereby laying a theoretical foundation for applications of CRSS.

Poor N2 selectivity and low SO2-tolerant are dominated weaknesses for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR) catalysts, which must be overcome for practical applications. Herein, a Mn-Fe mixed catalyst supported on active carbon (Mn-Fe/AC) for low-temperature NH3-SCR of NO was prepared, and the influence of SO2 on this process was also studied. Both iron and sulfur dioxide exhibited a dual-acting over Mn-Fe/AC catalyst for NH3-SCR of NO. Sulfur dioxide had two effects when SO2 was introduced in the feed gas under SCR conditions, including inhibiting the low temperature activity of the catalysts and improving N2 selectivity which exceeded 90% during the whole testing temperature. The introduction of SO2 could promote the formation of NH2NO which was the main intermediate and then decomposed into N2. Fe species in Mn-Fe/AC catalyst could improve N2 selectivity. In the presence of SO2, it showed considerable SO2 resistance compared to Mn/AC catalyst. On the one hand, iron species improved the selectivity through forming new active sites promoting the formation of NH2NO and suppressing the free NO3- generation. On the other hand, iron existing in Mn-Fe/AC catalyst could decrease the amount of sulfur species adsorbent and SO2 oxidation, thereby exhibiting better SO2 resistance.

In this study, a series of K species or/and SO2 poisoned Mn-Ce doped CuX (MCCX) catalysts were prepared from waste blast furnace slag (BFS), and the influence mechanism of K species or/and SO2 poisoning on the catalysts in low-temperature NH3-SCR process was investigated. The results demonstrated that MCCX-KCl catalyst had poorer NO conversion than MCCX-K2O below 200 °C, and the co-existence of K species and SO2 considerably inhibited NO conversion. Aggregation of the Cu active component might also be facilitated by co-poisoning with K species and SO2. Furthermore, K species and SO2 co-poisoned catalysts showed fewer isolated Cu2+ species and Mn4+ species than that of K-poisoned catalysts, indicating a decrease in active sites after the addition of SO2. The co-effect of K2O/KCl and SO2 species impaired the redox capacity and decreased the surface acidity than K-poisoned catalysts to a greater extent. Nevertheless, K-poisoning had little effect on the surface intermediates during SCR reaction process, and K-poisoned catalysts followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. However, MCCX-KCl-SO2 and MCCX-K2O-SO2 catalysts contained fewer nitrate species, indicating that the co-effect of K species and SO2 impeded the reaction between NO + O2 species and pre-NH3 species, thereby resulting in poor low-temperature NH3-SCR activity.

Background: Ceria (CeO2) was considered as one of the most promising catalysts due to its oxygen storage ca-pacity and redox property. However, the low activity of CeO2 catalyst in SCR reaction limited its further application on industrial processes.Methods: Transition metal (Mn, Cu and Fe) oxides were impregnated on nanopolyhedron CeO2 (CeO2-NP) cat-alysts to investigate the SCR performance.Significant findings: The Mn-CeO2-NP catalyst displayed the highest NOx conversion (ca. 70% conversion at 300 degrees C), followed by the Cu-CeO2-NP catalyst (ca. 65% conversion at 300 degrees C) and then the Fe-CeO2-NP catalyst (ca. 45% conversion at 300 degrees C). The results of H2-TPR showed that Mn-CeO2-NP and Cu-CeO2-NP catalysts were more efficient in reducing hydrogen at lower temperature than that of Fe-CeO2-NP catalysts. Based on XPS measurements, it was determined that the O alpha/(O alpha+O beta) ratio was Mn-CeO2-NP (ca. 65%)> Fe-CeO2-NP (ca. 59%)> Cu-CeO2-NP (ca. 55%) catalyst. It was also shown by Raman spectroscopy that the Mn-CeO2-NP catalyst had a higher concentration of oxygen vacancies than that of the Cu-CeO2-NP and Fe-CeO2-NP catalysts. Mean-while, more Bronsted acid sites, Lewis acid sites, nitrate and nitrites species were found in the Mn-CeO2-NP catalyst as seen by in situ DRIFT spectra.

Ce–Ti catalysts were considered as a promising replacement for V–Ti based catalysts for selective catalytic reduction (SCR) of nitrogen oxides (NO and NO2) with NH3.

Lead salts poisoning was a great challenge for the catalysts application in selective catalytic reduction (SCR) reaction of NOx with NH3 in stationary sources. Herein, several typical transition metal (Cu, Co and Zr) oxides modified Mn-Ce/AC catalysts were prepared by impregnation method, and the effects of Cu, Co and Zr doping on resistance PbCl2 poisoning for the catalyst were investigated. The addition of three transition metal oxides improved the PbCl2 resistance of Mn-Ce/AC catalyst and maintained excellent low-temperature denitrification activity of the catalyst. The lead resistance performance of the three metal oxides followed: Cu > Co > Zr oxides. The NO conversion of Cu-doping catalyst after PbCl2 poisoning was only ca. 10% lower than that of fresh Mn-Ce/ AC catalyst at 225 C. Cu, Co and Zr doping could decrease the loss of specific surface area caused by poisoning, while Cu-doped catalyst showed lowest crystallinity of active components. Meanwhile, the contents of Mn4+ and chemisorbed oxygen in Cu-doped catalyst were higher than those in Co-or Zr-doped catalysts. Cu, Co and Zr doping improved the surface acidity and redox performance of the poisoned catalysts, with Cu-doped poisoned catalyst exhibiting nearly the same surface acidity and redox performance as the fresh Mn-Ce/AC catalyst. Furthermore, Cu doping could also improve NO adsorption and was in favor to the SCR process. Besides, in situ DRIFTS results showed that the catalytic reaction pathways of all the poisoned or modified catalysts were in accordance with both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Finally, a possible antiPbCl(2) poisoning mechanistic model of (Cu, Co and Zr) oxides modified on Mn-Ce/AC catalyst was proposed.