Elemental Mercury Oxidation over Fe-Ti-Mn Spinel: Performance, Mechanism, and Reaction Kinetics

Different roles of FeS and FeS2 on magnetic FeSx for the selective adsorption of Hg2+ from waste acids in smelters: Reaction mechanism, kinetics, and structure-activity relationship

Magnetic FeSx was developed as a high-performance sorbent for selectively adsorbing Hg2+ from waste acids in smelters. However, further improvement of its ability for Hg2+ adsorption was extremely restricted due to the lack of reaction mechanisms and structure-activity relationships. In this study, the roles of FeS and FeS2 on magnetic FeSx for Hg2+ adsorption were investigated with alternate adsorption of Hg2+ without/with Cl-. The structure-activity relationship of magnetic FeSx for Hg2+ adsorption and the negative effect of acid erosion were elucidated using kinetic analysis. FeS can react with Hg2+ with 1:1 stoichiometric ratio to form HgS, while FeS2 can react with Hg2+ in the presence of Cl- with novel 1:3 stoichiometric ratio to form Hg3S2Cl2. The rate of magnetic FeSx for Hg2+ adsorption was related to the instantaneous amounts of FeS and threefold FeS2 on magnetic FeSx and the amount of Hg2+ adsorbed. Meanwhile, its capacity for Hg2+ adsorption was related to the initial sum of FeS amount and threefold FeS2 amount on the surface and their ratios by acid erosion. Then, magnetic FeSx-400 was devised with adsorption rate of 2.12 mg g-1 min-1 and capacity of 1092 mg g-1 to recover Hg2+ from waste acids for centralized control.

Hg0 Conversion over Sulfureted HPMo/γ-Fe2O3 with HCl at Low Temperatures: Mechanism, Kinetics, and Application in Hg0 Removal from Coal-Fired Flue Gas

Recently, sulfureted metal oxides have been developed for the catalytic oxidation of Hg⁰ to HgCl₂ using HCl as an oxidant at low temperatures, and they exhibit excellent Hg⁰ removal performance. Owing to the lack of reaction mechanisms and kinetics, further improvement in their performance for Hg⁰ conversion is extremely restricted. In this study, the reaction mechanism of Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ with HCl at low temperatures was investigated using Hg balance analysis and transient reaction. The chemical adsorption of Hg⁰ as HgS and the catalytic oxidation of Hg⁰ to HgCl₂ both contributed to Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃. Meanwhile, the formed HgCl₂ can adsorb onto sulfureted HPMo/γ-Fe₂O₃. Then, the kinetics of Hg⁰ conversion, Hg^t adsorption, and HgCl₂ desorption were developed, and the kinetic parameters were gained by fitting the Hg balance curves. Subsequently, the inhibition mechanism of H₂O and SO₂ on Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ was determined by comparing the kinetic parameters. The kinetic model suggested that both HgCl₂ resulting from Hg⁰ oxidation and unoxidized Hg⁰ can be completely adsorbed on sulfureted HPMo/γ-Fe₂O₃ with a moderate mass hourly space velocity. Therefore, sulfureted HPMo/γ-Fe₂O₃ can be developed as a reproducible sorbent for recovering Hg⁰ emitted from coal-fired power plants.

Mercury removal using various modified V/Ti-based SCR catalysts: A review

Growing levels of mercury pollution has made countries urgently need a suitable mercury treatment technology. Among various technologies, heterogeneous oxidative mercury removal via different modified V/Ti-based SCR catalysts is considered as a promising approach due to excellent economic value and removal efficiency. Although various related modification experiments have been worked in recent years, the research on the performance, including activity and resistance, and mechanism of catalysts still needs to be improved, so it is necessary to summarize these experiments to guide further work. This article will review many modifications start from the V/Ti catalyst. Not only the performance of these catalysts, but also a lot of speculation about the mercury removal mechanism are include in our research. In addition, the characteristics of some modified catalysts have been linked with their oxidation mechanism and structural changes by comparing many studies, and finally attributed to some special properties of the corresponding modifiers. We expect this study will clarify the research progress of modified V/Ti-based SCR catalysts in mercury removal, and guide future modification so that some properties of the catalyst can be improved in a targeted manner.

Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.

Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature

Manganese alumina composited oxides (MnAlOx) catalysts with ordered mesoporous structure prepared by evaporation-induced self-assembly (EISA) method was designed for the selective catalytic reduction (SCR) of NOx with NH3 at low temperature. The effect of calcination temperature of MnAlOx catalysts was investigated systematically, and it was correlated with SCR activity. Results showed that with an increase in calcination temperature, the SCR activity of MnAlOx catalysts increased. When the calcination temperature was raised up to 800 °C, the NOx conversion was more than 90% in the operation temperature range of 150~240 °C. Through various characterization analysis, it was found that MnAlOx-800 °C catalysts possessed enhanced redox capacities as the higher content of Mn4+/(Mn3+ + Mn4+). Moreover, the improved redox properties could contribute to a higher NOx adsorption and activation ability, which lead to higher SCR performance of MnAlOx-800 °C catalysts. In situ DRIFTs revealed that the adsorbed NO2 and bidentate nitrate are the reactive intermediate species, and NH3 species bonded to Lewis acid sites taken part in SCR progress. The SCR progress predominantly followed E–R mechanism, while L–H mechanism also takes effect to a certain degree.

Novel Counteraction Effect of H2O and SO2 toward HCl on the Chemical Adsorption of Gaseous Hg0 onto Sulfureted HPW/γ-Fe2O3 at Low Temperatures: Mechanism and Its Application in Hg0 Recovery from Coal-Fired Flue Gas

In this work, sulfureted phosphotungstic acid-grafted γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was investigated as a regenerable monolithic sorbent to recover gaseous Hg⁰ upstream of wet flue gas desulfurizations (FGDs), and the effects of HCl, SO₂, and H₂O on the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ were investigated with Hg balance analysis and kinetic analysis. Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was remarkably promoted in the presence of HCl, and most Hg⁰ was catalytically oxidized to HgCl₂. Moreover, the chemical adsorption of Hg⁰ was notably restrained as the key species for Hg⁰ transformation to HgS (i.e., S₂^2-) was rapidly oxidized by Cl*. However, the effect of HCl on Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was almost counteracted by H₂O and SO₂ as they competed with physically adsorbed Hg⁰ and S₂^2- for the consumption of Cl*. Therefore, the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ in the presence of SO₂ and H₂O was slightly inhibited by HCl, and only a small amount of HgCl₂ was formed. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited a moderate ability for gaseous HgCl₂ adsorption. As a result, sulfureted HPW/γ-Fe₂O₃ showed excellent performance in recovering Hg⁰ from the flue gas upstream of the FGDs for the centralized control of Hg⁰ emitted from coal-fired plants.

Study on the Preparation of Magnetic Mn–Co–Fe Spinel and Its Mercury Removal Performance

In this study, the manganese-doped manganese–cobalt–iron spinel was prepared by the sol–gel self-combustion method, and its physical and chemical properties were analyzed by XRD (X-ray diffraction analysis), SEM (scanning electron microscope), and VSM (vibrating sample magnetometer). The mercury removal performance of simulated flue gas was tested on a fixed bed experimental device, and the effects of Mn doping amount, fuel addition amount, reaction temperature, and flue gas composition on its mercury removal capacity were studied. The results showed that the best synthesized product was when the doping amount of Mn was the molar ratio of 0.5, and the average mercury removal efficiency was 87.5% within 120 min. Among the fuel rich, stoichiometric ratio, and fuel lean systems, the stoichiometric ratio system is most conductive to product synthesis, and the mercury removal performance of the obtained product was the best. Moreover, the removal ability of Hg0 was enhanced with the increase in temperature in the test temperature range, and both physical and chemical adsorption play key roles in the spinel adsorption of Hg0 in the medium temperature range. The addition of O2 can promote the removal of Hg0 by adsorbent, but the continuous increase after the volume fraction reached 10% had little effect on the removal efficiency of Hg0. While SO2 inhibited the removal of mercury by adsorbent, the higher the volume fraction, the more obvious the inhibition. In addition, in an oxygen-free environment, the addition of a small amount of HCl can promote the removal of mercury by adsorbent, but the addition of more HCl does not have a better promotion effect. Compared with other reported adsorbents, the adsorbent has better mercury removal performance and magnetic properties, and has a strong recycling performance. The removal efficiency of mercury can always be maintained above 85% in five cycles.

Nanosized ZnIn2S4 supported on facet-engineered CeO2 nanorods for efficient gaseous elemental mercury immobilization

Nanosized ZnIn₂S₄ supported on facet-engineered CeO₂ nanorods were prepared by solvothermal method to effectively capture gaseous elemental mercury from flue gas. The CeO₂/ZnIn₂S₄ sorbent exhibited excellent mercury removal performance (>90%) in a wide temperature range from 60 to 240 ℃ and showed much higher mercury adsorption capacity than pure CeO₂ due to the enlarged specific surface area and abundant active oxygen and sulfur sites on the surface. It was found that CeO₂/ZnIn₂S₄ has good resistance to SO₂, NO and H₂O. At the optimal 120 ℃, the equilibrium Hg⁰ adsorption capacity of CeO₂/ZnIn₂S₄ can reach 19.172 mg/g, which is superior to the reported series of benchmark materials. X-ray photoelectron spectroscopy and temperature programmed desorption of mercury confirmed that the adsorbed mercury existed on the surface as HgO and HgS, indicating that catalytic oxidation and chemisorption occurred on the surface of the adsorbent. The adsorption energy of Hg⁰ on the CeO₂ (110) and ZnIn₂S₄ (110) surfaces calculated with density functional theory (DFT), further confirms that the surface activated oxygen and sulfur sites are the most stable adsorption sites. Furthermore, the good regeneration capability of CeO₂/ZnIn₂S₄ makes it more promising for Hg⁰ capture in practical applications.

Novel Promotion of Sulfuration for Hg0 Conversion over V2O5-MoO3/TiO2 with HCl at Low Temperatures: Hg0 Adsorption, Hg0 Oxidation, and Hg2+ Adsorption

In this work, the commercial selective catalytic reduction (SCR) catalyst V₂O₅-MoO₃/TiO₂ was sulfureted with H₂S to improve both its capability for elemental mercury (Hg⁰) removal at low temperatures and its resistance to SO₂ and H₂O. Hg⁰ removal over both V₂O₅-MoO₃/TiO₂ and sulfureted V₂O₅-MoO₃/TiO₂ involved the catalytic oxidation of Hg⁰ to HgCl₂ and the chemical adsorption of gaseous Hg⁰; therefore, the effect of sulfuration on Hg⁰ chemical adsorption and the catalytic oxidation of Hg⁰ to HgCl₂ over V₂O₅-MoO₃/TiO₂ and its resistance to SO₂ and H₂O were investigated using Hg balance analysis. Kinetic analysis showed that the rates of the chemical adsorption and oxidation of Hg⁰ were both in direct proportion to the concentration of physically adsorbed Hg⁰. The physical adsorption of gaseous Hg⁰ on V₂O₅-MoO₃/TiO₂ was remarkably promoted after sulfuration, and the physical adsorption of gaseous Hg⁰ over sulfureted V₂O₅-MoO₃/TiO₂ was scarcely inhibited by SO₂ and H₂O. Therefore, the performance of V₂O₅-MoO₃/TiO₂ for Hg⁰ removal and its resistance to SO₂ and H₂O were both improved after sulfuration. Even more remarkably, sulfureted V₂O₅-MoO₃/TiO₂ can adsorb gaseous HgCl₂, which resulted from Hg⁰ oxidation. Therefore, sulfureted V₂O₅-MoO₃/TiO₂ showed an excellent performance to recover Hg⁰ from coal-fired power plants, which can then be converted to liquid Hg.

Outstanding performance of CuO/Fe–Ti spinel for Hg0 oxidation as a co-benefit of NO abatement: significant promotion of Hg0 oxidation by CuO loading

Conversion of gaseous Hg⁰ to soluble Hg²⁺ using selective catalytic reduction (SCR) catalysts with gaseous HCl as an oxidant as a co-benefit of NO abatement is widely used for resolving Hg pollution from coal-burning power plants.

Significant Enhancement of Gaseous Elemental Mercury Recovery from Coal-Fired Flue Gas by Phosphomolybdic Acid Grafting on Sulfurated γ-Fe2O3: Performance and Mechanism

The existing technologies to control Hg emissions from coal-fired power plants can be improved to achieve the centralized control of Hg⁰ emissions, which continue to pose a risk of Hg exposure to human populations. In this work, MoS_x@γ-Fe₂O₃, formed by the sulfuration of phosphomolybdic acid (HPMo)-grafted γ-Fe₂O₃, was developed as a magnetic and regenerable sorbent to recover gaseous Hg⁰ from coal-fired flue gas as a cobenefit to the use of wet electrostatic precipitators. The thermal stability of γ-Fe₂O₃ was notably enhanced by HPMo grafting; thus, the magnetization of MoS_x@γ-Fe₂O₃ hardly decreased during the application. The kinetic analysis indicates that the chemical adsorption of gaseous Hg⁰ was mainly dependent on the amounts of surface S₂^2- and surface adsorption sites. Although the amount of S₂^2- on sulfurated γ-Fe₂O₃ decreased after HPMo grafting, the amount of surface adsorption sites significantly increased due to the formation of a layered MoS_x structure on the surface. Therefore, the ability of sulfurated γ-Fe₂O₃ to capture Hg⁰ was improved considerably after HPMo grafting. Furthermore, low concentrations of gaseous Hg⁰ in coal-fired flue gas can be gradually enriched by at least 1000 times by MoS_x@γ-Fe₂O₃, which facilitates the recovery and centralized control of gaseous Hg⁰ in flue gas.

Effect of Molybdenum on the Activity Temperature Enlarging of Mn-Based Catalyst for Mercury Oxidation

The MnO2/TiO2 (TM5) catalyst modified by molybdenum was used for mercury oxidation at different temperatures in a fixed-bed reactor. The addition of molybdenum into TM5 was identified as significantly enlarging the optimal temperature range for mercury oxidation. The optimal mercury oxidation temperature of TM5 was only 200 °C, with an oxidation efficiency of 95%. However, the mercury oxidation efficiency of TM5 was lower than 60% at other temperatures. As for MnO2–MoO3/TiO2 (TM5Mo5), the mercury oxidation efficiency was above 80% at 200–350 °C. In particular at 250 °C, the mercury oxidation efficiency of TM5Mo5 was over 93%. Otherwise, the gaseous O2, which could supplement the lattice oxygen in the catalytic reaction, played an important role in the process of mercury oxidation over TM5Mo5. The results of X-ray photoelectron spectroscopy (XPS) suggested that mercury oxidized by O2 over TM5Mo5 followed the Mars–Maessen mechanism.

MIL-100(Fe) supported Mn-based catalyst and its behavior in Hg0 removal from flue gas

A novel magnetic composite catalyst of MnOx loaded on MIL-100(Fe) was prepared for the removal of Hg⁰ from flue gas, via incipient wetness impregnation followed with calcination at 300 °C. The MIL-100(Fe) supported catalyst showed greater capacity of Hg⁰ adsorption and oxidation than Fe₂O₃ supported catalyst at all test temperatures, showing Hg⁰ removal efficiency of 77.4% at 250 °C with high GHSV of 18,000 h^-1. Besides the merit of high BET surface area and developed porous, the ultra-highly dispersed and homogeneous Fe sites on MIL(Fe) significantly promoted Hg⁰ adsorption and oxidation via the synergy effect with MnOx. Furthermore, the catalyst exhibited magnetic property, which allowed easy separation of the catalyst from fly ash with a recovery of 104%. SO₂, H₂O and NH₃ in flue gas were proved inhibited Hg⁰ removal via different mechanisms. SO₂ and H₂O competed and desorbed Hg²⁺ on the surface of catalyst, while NH₃ was more likely to compete adsorption sites with Hg⁰.

Tourmaline-Modified FeMnTiO x Catalysts for Improved Low-Temperature NH3-SCR Performance

Low temperature NH₃ selective catalytic reduction (NH₃-SCR) technology is an efficient and economical strategy for cutting NO _x emissions from power-generating equipment. In this study, a novel and highly efficient NH₃-SCR catalyst, tourmaline-modified FeMnTiO _x is presented, which was synthesized by a simple one-step sol-gel method. We found that the amount of tourmaline has an important impact on the catalytic performance of the modified FeMnTiO _x-based catalysts, and the NO _x conversion exceeded 80% from 160 to 380 °C with the addition of 5 wt % tourmaline. Compared with the pure FeMnTiO _x, the catalytic efficiency at a temperature below 100 °C was increased by nearly 18.9%, and the operation temperature window was broadened significantly. The enhanced catalytic performance of the FeMnTiO _x catalyst was mainly attributed to the small spherical nanoparticles structure around the tourmaline powders, resulting in the increased content of Mn³⁺, Mn⁴⁺, and chemical oxygen on the catalytic surface. These as-developed tourmaline-modified FeMnTiO _x materials have been demonstrated to be promising as a new type highly efficient low temperature NH₃-SCR catalyst.

Synthesis of CrOx/C catalysts for low temperature NH3-SCR with enhanced regeneration ability in the presence of SO2

Chromium oxide nano-particles with an average diameter of 3 nm covered by amorphous carbon (CrO_x/C) were successfully synthesized. The synthesized CrO_x/C materials were used for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR), which shows superb NH₃-SCR activity and in particular, satisfactory regeneration ability in the presence of SO₂ compared with Mn-based catalysts. The as-prepared catalysts were characterized by XRD, HRTEM, Raman, FTIR, BET, TPD, TPR, XPS and in situ FTIR techniques. The results indicated presence of certain amounts of unstable lattice oxygen exposed on the surface of CrO_x nano-particles with an average size of 3 nm in the CrO_x/C samples, which led to NO being conveniently oxidized to NO₂. The formed NO₂ participated in NH₃-SCR activity, reacting with catalysts via a “fast NH₃-SCR” pathway, which enhanced th NH₃-SCR performance of the CrO_x/C catalysts. Furthermore, the stable lattice of the CrO_x species made the catalyst immune to the sulfation process, which was inferred to be the cause of its superior regeneration ability in the presence of SO₂. This study provides a simple way to synthesize stable CrO_x nano-particles with active oxygen, and sheds light on designing NH₃-SCR catalysts with highly efficient low temperature activity, SO₂ tolerance, and regeneration ability.

Novel CrO_x@C catalyst with both remarkable NH₃-SCR activity and satisfactory regeneration ability in the presence of SO₂.