Adsorption and oxidation of elemental mercury over Ce-MnOx/Ti-PILCs

Remove elemental mercury from simulated flue gas by CeO2-modified MnOx/HZSM-5 adsorbent

In this study, we synthesized a Ce-modified Mn/HZSM-5 adsorbent via the ultrasound-assisted impregnation for Hg0 capture. Given the addition of 15% CeO2, ~ 100% Hg0 efficiency was reached at 200 °C, suggesting its promotional effect on Hg0 removal. The doped Ce introduced additional chemisorbed oxygen species onto the adsorbent surfaces, which facilitated the oxidation of Hg0 to HgO. Even though adding CeO2 led to a weakened adsorbent acidity, yet it appeared that this negative affect could be completely overcome by the enhanced oxidative ability, which finally endowed Ce-modified Mn/HZSM-5 with a satisfactory Hg0 removal performance within the whole investigated temperature range. During the Hg0 capture process, chemisorption was predominant with Mn4+operating as the main active site for oxidizing Hg0 to Hg2+. Finally, the 15Ce-Mn/HZSM-5 adsorbent exhibited good recyclability and stability. However, its tolerance to H2O and SO2 appeared relatively weak, suggesting that some modification should be conducted to improve its practicality.

Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti₃C₂T_x carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti₃C₂T_x by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti₃C₂T_x stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti₃C₂T_x and COOH-Ti₃C₂T_x. Compared to Ti₃C₂T_x, COOH-Ti₃C₂T_x has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti₃C₂T_x is lower. For both Ti₃C₂T_x and COOH-Ti₃C₂T_x, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg²⁺ concentration of 50 ppm and pH of 6, COOH-Ti₃C₂T_x has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg²⁺ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti₃C₂T_x (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.

Insights into the Adsorption, Alloy Formation, and Poisoning Effects of Hg on Monometallic and Bimetallic Adsorbents

The removal of elemental mercury (Hg⁰) from coal-derived syngas at high temperatures is desired to improve the thermal efficiency of the coal-to-chemical processes. First-principles density functional theory (DFT) calculations for Hg⁰ adsorption are performed using different exchange correlation functionals (PBE, optPBE-vdW, and optB88-vdW). Gibbs free energy (ΔG) calculations are further performed to evaluate the feasibility of Hg⁰ adsorption on various exposed planes of metal nanoparticles and to obtain bimetallic compositions for Hg⁰ removal at various temperatures. Pd and Pt are shown to be suitable for Hg⁰ adsorption at high temperatures (473 K), whereas Rh and Ru are effective only until 373 K. The bimetallic adsorbents comprising Ag or Au along with Rh, Ru, Pd, or Pt are identified for Hg⁰ removal at high temperatures (473 K). The increase in Hg⁰ adsorption strength on various bimetallic surfaces is correlated to the upward shift in the d-band center. Further, calculations predict the tendency of Hg to segregate toward the surface of amalgams and disturb the perfect planar geometry of the Pd, Pt, Rh, Ru, Ir, Cu, Ag, and Au surfaces to form a noncrystalline Hg-rich amalgam surface. An analysis of the binding of various adsorbates (H, O, N, and S) shows that the adsorption becomes significantly weaker on various sites in close proximity to pre-adsorbed Hg. Moreover, for specific combinations of the adsorbate, surface composition, and the site location, the adsorption does not take place on the proximal sites. These results are complemented by the partial density of states calculations, which show changes in the electronic properties of the amalgam surface, thus explaining the poisoning effect of Hg on metallic catalysts.

Development of LnMnO3+σ perovskite on low temperature Hg0 removal

LnMnO_3+σ (Ln = La, Pr, Nd, Sm, Eu, Gd or Dy) perovskites synthesized by sol-gel method were employed for gaseous elemental mercury (Hg⁰) removal from coal-fired flue gas. Characterization results revealed the structure of the perovskites presented a phase transition process from rhombohedral system to O- and O'-orthorhombic structure with the change of A-site rare earth elements. The perovskites showed satisfactory Hg⁰ removal capacity in a narrow temperature range of 100-150°C. NdMnO_3+σ with an O-O' orthorhombic structure presented the best Hg⁰ removal performance, which markedly depends on four factors: crystal structure, oxygen vacancy density, Mn⁴⁺/Mn³⁺ ratio and surface element segregation. The Hg⁰ removal mechanism was illustrated based on the mercury temperature programmed desorption experiment and X-ray photoelectron spectroscopy characterization. Both chemisorption and catalytic oxidation played a role in the Hg⁰ removal process. Chemisorption dominated the Hg⁰ removal, due to the slow catalytic oxidation rate at low temperature. This work preliminarily established the relation between the structure of rare earth manganese perovskite and Hg⁰ removal performance.

Hg0 Removal by V2O5 Modified Palygorskite in Simulated Flue Gas at Low Temperature

The V2O5-modified palygorskite (V2O5/PG catalysts) were prepared and used for Hg0 removal in simulated flue gas at low temperature. It was found that the V2O5/PG catalyst had excellent performance for Hg0 removal at 150 °C. O2 exhibited a positive effect on Hg0 removal over V2O5/PG, while SO2 and H2O showed an inhibiting effect. However, Hg0 removal efficiency showed a promotion trend in the presence of H2O, SO2, and O2. The Brunauer–Emmett–Teller (BET) method, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were applied to characterize the physicochemical properties of the V2O5/PG catalyst. Mercury temperature-programmed desorption (Hg-TPD) experiments were also conducted to identify the mercury species adsorbed on the V2O5/PG catalyst, and the pathway of Hg0 removal over V2O5/PG was also discussed. The used V2O5/PG catalyst after Hg0 removal was regenerated, and its capability for Hg0 removal can be completely recovered. The V2O5/PG-Re-300 °C catalyst showed excellent performance and good stability for Hg0 removal after regeneration.

Solvent-free liquid-phase selective catalytic oxidation of toluene to benzyl alcohol and benzaldehyde over CeO2–MnOx composite oxides

A novel CeO2–MnOx composite oxide was prepared by the improved sol–gel method. The synergistic catalysis of Mn3+/Mn2+ and Ce4+/Ce3+ was responsible for the good catalytic performance in the liquid phase solvent-free selective oxidation of toluene.

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.

Different Design Strategies for Metal Sulfide Sorbents to Capture Low Concentrations of Gaseous Hg0 in Coal-Fired Flue Gas and High Concentrations of Gaseous Hg0 in Smelting Flue Gas

Capturing gaseous Hg⁰ using regenerable metal sulfides is a promising technology to recover gaseous Hg⁰ from both coal-fired flue gas (CFG) and smelting flue gas (SFG) for the centralized control. Gaseous Hg⁰ concentration in SFG is 2-3 orders of magnitude higher than that in CFG; therefore, the design strategy of metal sulfides for capturing gaseous Hg⁰ from CFG is quite different from that from SGF. In this work, the structure-activity relationship of metal sulfides to capture Hg⁰ was investigated according to the remarkable difference in MoO₃ loading on sulfureted FeTiO_x to capture low/high concentrations of gaseous Hg⁰. The rate of Hg⁰ adsorption onto metal sulfides was mainly related to the amounts of adsorption sites and S₂^2- on the surface, the affinity of adsorption sites to gaseous Hg⁰, and the gaseous Hg⁰ concentration. Meanwhile, the capacity for Hg⁰ adsorption was approximately equal to the less of the amount of adsorption sites and S₂^2- on the surface. Furthermore, capturing low concentrations of gaseous Hg⁰ from CFG required the metal sulfide sorbents having more adsorption sites with strong affinity to gaseous Hg⁰, while capturing high concentrations of gaseous Hg⁰ from SFG required the sorbents with enough adsorption sites.

Role of impurity components and pollutant removal processes in catalytic oxidation of o-xylene from simulated coal-fired flue gas

Volatile organic compounds (VOCs) emitted from coal-fired flue gas of thermal power plants have reached unprecedented levels due to lack of understanding of reaction mechanisms under industrial settings. Herein, inhibition mechanisms for catalytic oxidation of o-xylene in simulated coal-fired flue gas are elucidated with in-situ and ex-situ spectroscopic techniques considering the presence of impurity components (NO, NH₃, SO₂, H₂O). MnCe oxide catalysts prepared at Mn: Ce mass ratios of 6:4 are demonstrated to promote 87% o-xylene oxidation at 250 °C under gas hourly space velocities of 60,000 h^-1. Reaction intermediates on the catalyst surface are revealed to be o-benzoquinones, benzoates, and formate and they were stably formed under O₂/N₂ atmospheres. When either NO or NH₃ was introduced into the simulated flue gas, the formed species shifted toward formate in minutes, which indicated that changes in catalyst surface chemistry are directly related to impurity components. Presence of NH₃ in the simulated flue gas inhibited o-xylene oxidation by reducing Mn and lowering Brønsted acidity of the catalyst. Impurity components associated with pollutant removal processes (Hg⁰ oxidation and selective catalytic reduction of NO) lowered o-xylene removal efficiency. Presence of o-xylene in the flue gas had little effect on the efficiency of pollutant removal processes. Layered catalytic beds located downstream from Hg⁰/NO pollutant removal processes are proposed to lower VOC emissions from coal-fired flue gases of thermal power plants in industrial settings.

Removal of elemental mercury by photocatalytic oxidation over La2O3/Bi2O3 composite

La₂O₃/Bi₂O₃ photocatalysts were prepared by impregnation of Bi₂O₃ with an aqueous solution of lanthanum precursor followed by calcination at different temperatures. The composite materials were used for the first time for the photocatalytic removal of Hg⁰ from a simulated flue gas under UV light irradiation. The results showed that the sample containing 6 wt.% La₂O₃ and calcined at 500°C has the highest dispersion of the active sites, which was promoted by the strong interaction with the support (i.e., the formation of Bi-O-La species). Since they are fully accessible on the surface, the material also exhibits excellent optical properties while the heterojunction formed in La₂O₃/Bi₂O₃ promotes the separation and migration of photoelectron-hole pairs and thus Hg⁰ oxidation efficiency is enhanced. The effects of the various factors (e.g., the reaction temperature and composition of the simulated flue gas (i.e., O₂, NO, H₂O, and SO₂)) on the efficiency of the Hg⁰ photocatalytic oxidation were investigated. The results demonstrated that O₂ and SO₂ enhanced the efficiency of the reaction while the reaction temperature, NO, and H₂O had an inhibitory effect.

Mechanistic investigation of elemental mercury adsorption over silver-modified vanadium silicate: A DFT study

Ag-modified vanadium silicate (EVS-Ag) has been regarded as a superior sorbent for elemental mercury (Hg⁰) capture from coal-fired flue gas. However, the atomic-level reaction mechanism which determines Hg⁰ adsorption capacity of EVS-Ag sorbent remains elusive. Reaction mechanism and active sites of Hg⁰ adsorption over EVS-Ag sorbent were studied using density functional theory (DFT) calculations systematically. DFT calculation results indicate that silver exchange shows little effects on the geometric structure of EVS-10 sorbent. Hg⁰ adsorption on EVS-10 and EVS-Ag surfaces is controlled by the physisorption and chemisorption mechanisms, respectively. Ag₂ cluster is determined to be the most active site of Hg⁰ adsorption over Ag-modified EVS sorbent. The adsorption energy of Hg⁰ on Ag₂ cluster is -51.93 kJ/mol. The orbital hybridization and electron sharing between Ag and Hg atoms are responsible for the strong interaction between EVS-Ag surface and Hg⁰. HgO prefers to adsorb on Ag₂ cluster of EVS-Ag sorbent, and yields an energy release of 306.21 kJ/mol. HgO desorption from EVS-Ag sorbent surface needs a higher external energy, and occurs at the relatively higher temperatures. O₂ molecule promotes Hg⁰ adsorption over EVS-Ag sorbent. HgO species can be easily formed during Hg⁰ adsorption over EVS-Ag sorbent in the presence of O₂.

Removal of Mercury from Coal-Fired Flue Gas and Its Sulfur Tolerance Characteristics by Mn, Ce Modified γ-Al2O3 Catalyst

Mercury pollution in the atmospheric environment is a matter of international concern. Mercury in coal-fired flue gas is the first human mercury emission source and has become the focus of national mercury pollution control. The catalytic performance of zerovalent mercury (Hg0) in coal-fired flue gas was studied by using manganese-cerium-aluminum oxide as catalyst. The effects of metal loading ratio, reaction temperature, calcination temperature, and O2 and SO2 concentration on the efficiency of Hg0 catalytic removal were investigated, and the Mn-Ce/γ-Al2O3 catalysts before and after the reaction were characterized by BET, SEM, XRD, and XPS to analyze the physicochemical properties of the samples. The results show that the mercury removal efficiency of the composite catalyst with Mn, Ce, and Al as the active component is higher than that of the single metal catalyst. The catalytic activity of Mn0.1Ce0.02Al is the best, the optimum reaction temperature is 150°C, the optimum calcination temperature is 400°C, and the O2 concentration in the conventional flue gas condition satisfies the effective oxidation of Hg0; SO2 in the flue gas can seriously inhibit the oxidation of Hg0.

Photocatalytic removal of elemental mercury via Ce-doped TiO2 catalyst coupling with a novel optical fiber monolith reactor

Reduction of mercury emission from coal combustion is a serious task for public health and environmental societies. Herein, Ce-doped TiO₂ (Ce/TiO₂) catalyst coupling with a novel optical fiber monolith reactor was applied to efficiently remove elemental mercury (Hg⁰) from coal-fired flue gas. Under the optimal operation condition (i.e., 1.5 mW/cm² UV light, 90 °C), above 95% of Hg⁰ removal efficiency was attained over the optical fiber monolith reactor coating with 3.40 g/m² Ce/TiO₂ catalyst. The effects of flue gas compositions on Hg⁰ removal performance were clarified systematically. Gaseous O₂ replenished the surface oxygen, hence maintaining the production of free radicals and promoting the removal of Hg⁰. SO₂, HCl, and NO inhibited Hg⁰ removal in the absence of O₂ due to the competitive adsorption and consumption of free radicals. However, SO₂ and HCl significantly enhanced Hg⁰ removal with the participation of O₂, while NO exhibited obviously inhibitory effect even with the assistance of O₂. H₂O also decreased the Hg⁰ oxidation capacity owing to the competitive adsorption and reduction of HgO. The optical fiber monolith reactor exhibited much superior Hg⁰ removal capacity than the powder reactor. Utilization of Ce/TiO₂ catalyst coupling with an optical fiber monolith reactor provides a cost-effective method for removing Hg⁰ from coal-fired flue gas.

Promotional removal of gas-phase Hg0 over activated coke modified by CuCl2

Impregnating CuCl₂ on AC (activated coke) support to synthesize xCuCl₂/AC showed superior activity with higher 90% Hg⁰ removal efficiency at 80-140 °C, as well as a lower oxygen demand of 2% O₂ for Hg⁰ removal. The acceleration on Hg⁰ removal was observed for NO and SO₂. The BET, SEM, XRD, XPS, TPD, and FT-IR characterizations revealed that the larger surface area, sufficient active oxygen species and co-existence of Cu⁺ and Cu²⁺ may account for the efficient Hg⁰ removal. In addition, the low demand of gaseous O₂ was contributed to higher content of active oxygen and formed active Cl. After adsorbing on Cu sites, Cl sites, and surface functional groups, the Hg⁰_(ads) removal on xCuCl₂/AC was proceeded through two ways. Part of Hg⁰_(ads) was oxidized by active O and formed Hg⁰, and the other part of Hg⁰ combined with the active Cl, which was formed by the activation of lattice Cl with the aid of active O, and formed HgCl₂. Besides, the Hg²⁺ detected in outlet gas through mercury speciation conversion and desorption peak of HgCl₂ and Hg⁰ further proved it. As displayed in stability test and simulated industrial application test, CuCl₂/AC has a promising industrial application prospect.

Atomically Dispersed Manganese on a Carbon-Based Material for the Capture of Gaseous Mercury: Mechanisms and Environmental Applications

A novel, atomically dispersed carbon-based sorbent was synthesized by anchoring manganese atoms with N atoms for the capture of gaseous elemental Hg (Hg⁰). Oxygen atoms were also introduced into the synthesis process to adjust the oxidizing ability of the Mn atoms. High-valence Mn (Mn⁴⁺) anchored by the O and N atoms (Mn-O/N-C) in the carbon-based materials provided more exposed active sites. The mercury removal efficiency of the composite exceeded 99%. The composite with a Mn loading of 0.9 wt % exhibited high affinity for Hg⁰, and the capacity for Hg⁰ adsorption within 275 min at room temperature reached 16.95 mg·g^-1. The Mn utilization was ∼56.61%, which is much larger than that of reported Mn-based oxide sorbents. The atomic-level distribution of Mn was well evidenced by aberration-corrected high-angle annular darkfield scanning transmission electron microscopy. Density functional theory calculations were conducted to evaluate the energy for adsorption of Hg⁰ on Mn-O/N-C. The results indicated that the amount of N and O atoms in the Mn coordination environment determined the Hg⁰ adsorption energy, and the presence of five optimized Mn adsorption structures in Mn-O/N-C was confirmed by Hg temperature-programmed desorption analysis. These materials may be utilized for mercury removal from disposal sites with high concentrations of mercury, broken mercury-containing lamps, or mercurial thermometers. The strategy of atomic dispersion during synthesis of the materials and adjusting the oxidizing ability in the single-atom strategy may be helpful for the development of environmentally benign functional materials.

Catalytic oxidation of Hg0 with O2 induced by synergistic coupling of CeO2 and MoO3

Based on Volcano plotting, the controlled combination of weak and strong bond strengths of a bimetallic catalyst has the potential to maximize the catalytic effect via the formation of intermediate bond strength between the reactant and the two different types of active sites. In this study, a rational design approach was adopted to couple MoO₃ and CeO₂ to maximize the catalytic oxidation of Hg⁰ using oxygen as the oxidizing agent. It is found that CeO₂ displayed a relatively strong bond strength with Hg⁰ while MoO₃ has relatively weak bond strength with Hg⁰; the pre-doping of MoO₃ results in the transformation of CeO₂ from clusters to the form with additional exposed CeO₂ (111) surface; the CeO₂ and MoO₃ show synergistic effect on the formation of Brønsted acid sites. Moreover, the results show that there is an overlap between the Hg⁰ desorption region of MoO₃ and the Hg⁰ adsorption region of CeO₂ (with adjusted optimum bond strength with Hg⁰), which contributes to the catalytic reaction of Hg⁰ by O₂. Therefore, this study reveals that the synergistic effects of the coupling of CeO₂ and MoO₃ induced the reaction between Hg⁰ and O₂, which is otherwise difficult.

Preparation of microwave-activated magnetic bio-char adsorbent and study on removal of elemental mercury from flue gas

In this study, novel activated magnetic bio-char adsorbents were proposed to remove the element mercury (Hg⁰) from flue gas. Microwave activation and Mn-Fe mixed oxides impregnation assisted by ultrasound treatment were applied on the modification of renewable cotton straw chars. The influence of different preparation methods, loading value of Mn-Fe, molar ratio of Mn/Fe, calcining temperature, reaction temperature and individual flue gas ingredients (O₂, NO, SO₂ and H₂O) on removal of Hg⁰ was investigated in a fixed bed system. The characterization results reveal that microwave activation is advantageous for the development of the pore structure, and ultrasound treatment can optimize the dispersion of Mn and Fe active ingredients. MnFe4%(3/10)/CSWU700 adsorbent exhibits the optimal Hg⁰ removing performance. O₂, NO, low concentration of SO₂ (<600 ppm) and low concentration of H₂O (<2%) are found to be favourable for the capture of Hg⁰, while high concentrations of SO₂ and H₂O inhibit the removal of Hg⁰. Chemical adsorption acts a pivotal part in the process of Hg⁰ removal. Mn and Fe active ingredients are consumed in large quantities during the Hg⁰ capture. In addition, chemisorbed oxygen (O_β) also plays an indispensable in the oxidation process of Hg⁰. Furthermore, the magnetic adsorbent MnFe4%(3/10)/CSWU700 presents a good regeneration performance and adsorption capacity.

Cost-Effective Manganese Ore Sorbent for Elemental Mercury Removal from Flue Gas

Mercury capture from flue gas remains a challenge for environmental protection due to the lack of cost-effective sorbents. Natural manganese ore (NMO) was developed as a cost-effective sorbent for elemental mercury removal from flue gas. NMO sorbent showed excellent Hg⁰ removal efficiency (>90%) in a wide temperature window (100-250 °C) under the conditions of simulated flue gas. O₂, NO, and HCl promoted Hg⁰ removal due to the surface reactions of Hg⁰ with these species. SO₂ and H₂O slightly inhibited Hg⁰ removal under the conditions of simulated flue gas. O₂ addition could also weaken the inhibitory effect of SO₂. NMO sorbent exhibited superior regeneration performance for Hg⁰ removal during ten-cycle experiments. Quantum chemistry calculations were used to identify the active components of NMO sorbent and to understand the atomic-level interaction between Hg⁰ and sorbent surface. Theoretical results indicated that Mn₃O₄ is the most active component of NMO sorbent for Hg⁰ removal. The atomic orbital hybridization and electrons sharing led to the stronger interaction between Hg⁰ and Mn₃O₄ surface. Finally, a chemical looping process based on NMO sorbent was proposed for the green recovery of Hg⁰ from flue gas. The low cost, excellent performance, superior regenerable properties suggest that the natural manganese ore is a promising sorbent for mercury removal from flue gas.

Outstanding Performance of Recyclable Amorphous MoS3 Supported on TiO2 for Capturing High Concentrations of Gaseous Elemental Mercury: Mechanism, Kinetics, and Application

Hg⁰ capture by sorbents was a promising technology to control Hg⁰ emission from coal-fired power plants and smelters. However, the design of a high performance sorbent and the predicting of the extent of Hg⁰ adsorption were both extremely limited due to the lack of adsorption kinetics and structure-activity relationship. In this work, the adsorption kinetics of gaseous Hg⁰ onto MoS₃/TiO₂ was investigated and kinetic parameters were obtained by fitting breakthrough curves. According to the kinetic parameters, the removal efficiency, the adsorption rate and the capacity for Hg⁰ capture were accurately predicted. Meanwhile, the structure-activity relationship of metal sulfides for gaseous Hg⁰ adsorption was built. The chemical adsorption rate of gaseous Hg⁰ was found to mainly depend on the amount of surface adsorption sites available for the physical adsorption of Hg⁰, the amount of surface S₂^2- available for Hg⁰ oxidation and gaseous Hg⁰ concentration. As MoS₃/TiO₂ showed a superior performance for capturing high concentrations of Hg⁰ due to the large number of surface adsorption sites for the physical adsorption of gaseous Hg⁰, it has promising applications in recovering Hg⁰ from smelting flue gas.

Synergetic degradation of VOCs by vacuum ultraviolet photolysis and catalytic ozonation over Mn-xCe/ZSM-5

Volatile organic compounds (VOCs) are one of the most important precursors to form the fine particulate matter and photochemical smog, and should be strictly controlled. Vacuum ultraviolet (VUV) photolysis has provided a facile and an effective way to remove VOCs due to its powerful oxidation capability under mild reaction conditions. However, VUV irradiation would generate ozone which brings about secondary pollution. In this study, ZSM-5 supported Mn-Ce mixed oxides (Mn-xCe/ZSM-5) were fabricated as efficient catalysts for ozone catalytic oxidation (OZCO) process, which were applied in combination with VUV photolysis to remove O₃ byproduct and simultaneously facilitate toluene oxidation. The results indicated that the Mn-3Ce/ZSM-5 catalyst considerably enhanced the catalytic degradation efficiency up to 93% for the gas-phase toluene, one of the hazardous VOCs. Meanwhile, almost all the O₃ by-product could be eliminated in the process. It was found that the strong interaction of the MnOCe bond and the variable chemical valence of Mn and Ce based species in the mixed oxides would tune the redox capacity of Mn-xCe /ZSM-5. An increase in surface Ce³⁺ species and surface density of oxygen vacancies would benefit the adsorption and catalytic transformation of O₃ which eventually form the reactive oxygen species over Mn-xCe/ZSM-5.

Highly Dispersed Mn–Ce Binary Metal Oxides Supported on Carbon Nanofibers for Hg0 Removal from Coal-Fired Flue Gas

Highly dispersed Mn–Ce binary metal oxides supported on carbon nanofibers (MnOx–CeO2/CNFs(OX)) were prepared for Hg0 removal from coal-fired flue gas. The loading value of the well-dispersed MnOx–CeO2 was much higher than those of many other reported supports, indicating that more active sites were loaded on the carbon nanofibers. In the present study, 30 wt % metal oxides (15 wt % MnOx and 15 wt % CeO2) were successfully deposited on the carbon nanofibers, and the sorbent yielded the highest Hg0 removal efficiency (>90%) within 120–220 °C under a N2/O2 atmosphere. An increase in the amount of highly dispersed metal oxides provided abundant active species for efficient Hg0 removal, such as active oxygen species and Mn4+ cations. Meanwhile, the carbon nanofiber framework provides the pathway for charge transfer during the heterogeneous Hg0 capture reaction processes. Under a N2+6%O2 atmosphere, a majority of Hg0 was removed via chemisorption reactions. The effects of flue gas composition were also investigated. O2 replenished the active oxygen species on the surface and thus greatly promoted the Hg0 removal efficiency. SO2 had an inhibitory effect on Hg0 removal, but NO facilitated Hg0 capture performance. Overall, carbon nanofibers seems to be a good candidate for the support and MnOx–CeO2/CNFs(OX) may be promising for Hg0 removal from coal-fired flue gas.

Performance of Mn-Fe-Ce/GO-x for Catalytic Oxidation of Hg0 and Selective Catalytic Reduction of NOx in the Same Temperature Range

A series of composites of Mn-Fe-Ce/GO-x have been synthesized by a hydrothermal method. Their performance in simultaneously performing the catalytic oxidation of Hg0 and the selective catalytic reduction of nitrogen oxides (NOx) in the same temperature range were investigated. In order to investigate the physicochemical properties and surface reaction, basic tests, including Brunauer-Emmett-Teller (BET), XRD, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were selected. The results indicate that the active components deposited on graphene play an important role in the removal of mercury and NOx, with different valences. Especially, the catalyst of Mn-Fe-Ce/GO-20% possesses an excellent efficiency in the temperature range of 170 to 250 °C. Graphene has a huge specific surface area and good mechanical property; thus, the active components of the Mn-Fe-Ce catalyst can be highly dispersed on the surface of graphene oxide. In addition, the effects of O2, H2O, NO and SO2 on the removal efficiency of Hg0 were examined in flue gas. Furthermore, the regeneration experiments conducted by thermal methods proved to be promising methods.

Multiform Sulfur Adsorption Centers and Copper-Terminated Active Sites of Nano-CuS for Efficient Elemental Mercury Capture from Coal Combustion Flue Gas

Nanostructured copper sulfide synthesized with the assistance of surfactant with nanoscale particle size and high Brunauer-Emmett-Teller surface area was for the first time applied for the capture of elemental mercury (Hg⁰) from coal combustion flue gas. The optimal operation temperature of nano-CuS for Hg⁰ adsorption is 75 °C, which indicates that injection of the sorbent between the wet flue gas desulfurization and the wet electrostatic precipitator systems is feasible. This assures that the sorbent is free of the adverse influence of nitrogen oxides. Oxygen (O₂) and sulfur dioxide exerted a slight influence on Hg⁰ adsorption over the nano-CuS. Water vapor was shown to moderately suppress Hg⁰ capture efficiency via competitive adsorption. The simulated adsorption capacities of nano-CuS for Hg⁰ under pure nitrogen (N₂), N₂ + 4% O₂, and simulated flue gas reached 122.40, 112.06, and 89.43 mgHg⁰/g nano-CuS, respectively. Compared to those of traditional commercial activated carbons and metal sulfides, the simulated adsorption capacities of Hg⁰ over the nano-CuS are at least an order of magnitude higher. Moreover, with only 5 mg loaded in a fixed-bed reactor, the Hg⁰ adsorption rate reached 11.93-13.56 μg/g min over nano-CuS. This extremely speedy rate makes nano-CuS promising for a future sorbent injection technique. The anisotropic growth of nano-CuS was confirmed by X-ray diffraction analysis and provided a fundamental aspect for nano-CuS surface reconstruction and polysulfide formation. Further X-ray photoelectron spectroscopy and Hg⁰ temperature-programmed desorption tests showed that the active polysulfide, S-S dimers, and copper-terminated sites contributed primarily to the extremely high Hg⁰ adsorption capacity and rate. With these advantages, nano-CuS appears to be a highly promising alternative to traditional sorbents for Hg⁰ capture from coal combustion flue gas.

Low-temperature co-purification of NOx and Hg0 from simulated flue gas by CexZryMnzO2/r-Al2O3: the performance and its mechanism

In this study, series of Ce_xZr_yMn_zO₂/r-Al₂O₃ catalysts were prepared by impregnation method and explored to co-purification of NO_x and Hg⁰ at low temperature. The physical and chemical properties of the catalysts were investigated by XRD, BET, FTIR, NH₃-TPD, H₂-TPR, and XPS. The experimental results showed that 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ yielded higher conversion on co-purification of NO_x and Hg⁰ than the other prepared catalysts at low temperature, especially at 200-300 °C. 91% and 97% convert rate of NO_x and Hg⁰ were obtained, respectively, when 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ catalyst was used at 250 °C. Moreover, the presence of H₂O slightly decreased the removal of NO_x and Hg⁰ owing to the competitive adsorption of H₂O and Hg⁰. When SO₂ was added, the removal of Hg⁰ first increased slightly and then presented a decrease due to the generation of SO₃ and (NH₄)₂SO₄. The results of NH₃-TPD indicated that the strong acid of 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃ improved its high-temperature activity. XPS and H₂-TPR results showed there were high-valence Mn and Ce species in 10% Ce_0.2Zr_0.3Mn_0.5O₂/r-Al₂O₃, which could effectively promote the removal of NO_x and Hg⁰. Therefore, the mechanisms of Hg⁰ and NO_x removal were proposed as Hg (ad) + [O] → HgO (ad), and 2NH₃/NH₄⁺ (ad) + NO₂ (ad) + NO (g) → 2 N₂ + 3H₂O/2H⁺, respectively. Graphical abstract ᅟ.

Comparison of Elemental Mercury Oxidation Across Vanadium and Cerium Based Catalysts in Coal Combustion Flue Gas: Catalytic Performances and Particulate Matter Effects

This paper discussed the field test results of mercury oxidation activities over vanadium and cerium based catalysts in both coal-fired circulating fluidized bed boiler (CFBB) and chain grate boiler (CGB) flue gases. The characterizations of the catalysts and effects of flue gas components, specifically the particulate matter (PM) species, were also discussed. The catalytic performance results indicated that both catalysts exhibited mercury oxidation preference in CGB flue gas rather than in CFBB flue gas. Flue gas component studies before and after dust removal equipment implied that the mercury oxidation was well related to PM, together with gaseous components such as NO, SO₂, and NH₃. Further investigations demonstrated a negative PM concentration-induced effect on the mercury oxidation activity in the flue gases before the dust removal, which was attributed to the surface coverage by the large amount of PM. In addition, the PM concentrations in the flue gases after the dust removal failed in determining the mercury oxidation efficiency, wherein the presence of different chemical species in PM, such as elemental carbon (EC), organic carbon (OC) and alkali (earth) metals (Na, Mg, K, and Ca) in the flue gases dominated the catalytic oxidation of mercury.

The synthetic evaluation of CuO-MnOx-modified pinecone biochar for simultaneous removal formaldehyde and elemental mercury from simulated flue gas

A series of low-cost Cu-Mn-mixed oxides supported on biochar (CuMn/HBC) synthesized by an impregnation method were applied to study the simultaneous removal of formaldehyde (HCHO) and elemental mercury (Hg⁰) at 100-300° C from simulated flue gas. The metal loading value, Cu/Mn molar ratio, flue gas components, reaction mechanism, and interrelationship between HCHO removal and Hg⁰ removal were also investigated. Results suggested that 12%CuMn/HBC showed the highest removal efficiency of HCHO and Hg⁰ at 175° C corresponding to 89%and 83%, respectively. The addition of NO and SO₂ exhibited inhibitive influence on HCHO removal. For the removal of Hg⁰, NO showed slightly positive influence and SO₂ had an inhibitive effect. Meanwhile, O₂ had positive impact on the removal of HCHO and Hg⁰. The samples were characterized by SEM, XRD, BET, XPS, ICP-AES, FTIR, and H₂-TPR. The sample characterization illustrated that CuMn/HBC possessed the high pore volume and specific surface area. The chemisorbed oxygen (O_β) and the lattice oxygen (O_α) which took part in the removal reaction largely existed in CuMn/HBC. What is more, MnO₂ and CuO (or Cu₂O) were highly dispersed on the CuMn/HBC surface. The strong synergistic effect between Cu-Mn mixed oxides was critical to the removal reaction of HCHO and Hg⁰ via the redox equilibrium of Mn⁴⁺ + Cu⁺ ↔ Mn³⁺ + Cu²⁺.

Design of 3D MnO2/Carbon sphere composite for the catalytic oxidation and adsorption of elemental mercury

Three-dimensional (3D) MnO₂/Carbon Sphere (MnO₂/CS) composite was synthesized from zero-dimensional carbon spheres and one-dimensional α-MnO₂ using hydrothermal method. The hierarchical MnO₂/CS composite was applied for the catalytic oxidation and adsorption of elemental mercury (Hg⁰) from coal-fired flue gas. The characterization results indicated that this composite exhibits a 3D urchin morphology. Carbon spheres act as the core and α-MnO₂ nano-rods grew on the surface of carbon spheres. This 3D hierarchical structure benefits the enlargement of surface areas and pore volumes. Hg⁰ removal experimental results indicated that the MnO₂/CS composite has an outstanding Hg⁰ removal performance due to the higher catalytic oxidation and adsorption performance. MnO₂/CS composite had higher than 99% Hg⁰ removal efficiency even after 600min reaction. In addition, the nano-sized MnO₂/CS composite exhibited better SO₂ resistance than pure α-MnO₂. Moreover, the Hg-TPD results indicated that the adsorbed mercury can release from the surface of MnO₂/CS using a thermal decomposition method.

Support effect of Mn-based catalysts for gaseous elemental mercury oxidation and adsorption

Mn-Based catalysts with a Mn loading of 4 wt% were prepared using an impregnation method.

Silica-Silver Nanocomposites as Regenerable Sorbents for Hg0 Removal from Flue Gases

Silica-silver nanocomposites (Ag-SBA-15) are a novel class of multifunctional materials with potential applications as sorbents, catalysts, sensors, and disinfectants. In this work, an innovative yet simple and robust method of depositing silver nanoparticles on a mesoporous silica (SBA-15) was developed. The synthesized Ag-SBA-15 was found to achieve a complete capture of Hg⁰ at temperatures up to 200 °C. Silver nanoparticles on the SBA-15 were shown to be the critical active sites for the capture of Hg⁰ by the Ag-Hg⁰ amalgamation mechanism. An Hg⁰ capture capacity as high as 13.2 mg·g^-1 was achieved by Ag(10)-SBA-15, which is much higher than that achievable by existing Ag-based sorbents and comparable with that achieved by commercial activated carbon. Even after exposure to more complex simulated flue gas flow for 1 h, the Ag(10)-SBA-15 could still achieve an Hg⁰ removal efficiency as high as 91.6% with a Hg⁰ capture capacity of 457.3 μg·g^-1. More importantly, the spent sorbent could be effectively regenerated and reused without noticeable performance degradation over five cycles. The excellent Hg⁰ removal efficiency combined with a simple synthesis procedure, strong tolerance to complex flue gas environment, great thermal stability, and outstanding regeneration capability make the Ag-SBA-15 a promising sorbent for practical applications to Hg⁰ capture from coal-fired flue gases.

Design of MnO2/CeO2-MnO2 hierarchical binary oxides for elemental mercury removal from coal-fired flue gas

MnO₂/CeO₂-MnO₂ hierarchical binary oxide was synthesized for elemental mercury (Hg⁰) removal from coal-fired flue gas. CeO₂ in-situ grow on the surface of carbon spheres, and that CeO₂@CSs acted as precursor for porous MnO₂/CeO₂-MnO₂. XRD, Raman, XPS, FT-IR, and H₂-TPR were selected for the physical structural and chemical surface analysis. The results indicated that the composite has sufficient surface oxygen and hierarchical porous structure. The Hg⁰ removal experiments results indicated that MnO₂/CeO₂-MnO₂ exhibited excellent Hg⁰ removal performance, with an 89% removal efficiency of total 300min at 150°C under 4% O₂. MnO₂ was the primary active site for Hg⁰ catalytic oxidation. The porous structure was beneficial for gaseous mercury physically adsorption. In addition, CeO₂ enhanced the oxygen capture performance of the composite and the oxidation performance for MnO₂. Moreover, the effects of O₂, SO₂ and H₂O were also tested in this study. O₂ promoted the Hg⁰ removal reaction. While SO₂ and H₂O can poison the MnO₂ active site, resulted in a low Hg⁰ removal efficiency.

Transformation of para arsanilic acid by manganese oxide: Adsorption, oxidation, and influencing factors

Aromatic organoarsenic compounds tend to transform into more mobile toxic inorganic arsenic via several processes, and can inadvertently spread toxic inorganic arsenic through the environment to water sources. To gain insight into the transformation mechanisms, we herein investigated how the process of para arsanilic acid (p-ASA) transformation works in detail on the surface of adsorbents by comparing it with phenylarsonic acid (PA) and aniline, which have similar chemical structures. In contrast to the values of 0.23 mmol g^-1 and 0.68 mmol g^-1 for PA and aniline, the maximum adsorption capacity was determined to be 0.40 mmol g^-1 for p-ASA at pH 4.0. The results of FTIR and XPS spectra supported the presence of a protonated amine, resulting in a suitable condition for the oxidation of p-ASA. Based on the combined results of UV-spectra and UPLC-Q-TOF-MS, we confirmed that the adsorbed p-ASA was first oxidized through the transfer of one electron from p-ASA on MnO₂ surface to form a radical intermediate, which through further hydrolysis and coupling led to formation of benzoquinone and azophenylarsonic acid, which was identified as a major intermediate. After that, p-ASA radical intermediate was cleaved to form arsenite (III), and then further oxidized into arsenate (V) with the release of manganese (Mn) into solution, indicating a heterogeneous oxidation process.

Simultaneous removal of NO and Hg0 over Ce-Cu modified V2O5/TiO2 based commercial SCR catalysts

A series of novel Ce-Cu modified V₂O₅/TiO₂ based commercial SCR catalysts were prepared via ultrasonic-assisted impregnation method for simultaneous removal of NO and elemental mercury (Hg⁰). Nitrogen adsorption, X-ray diffraction (XRD), temperature programmed reduction of H₂ (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalysts. 7% Ce-1% Cu/SCR catalyst exhibited the highest NO conversion efficiency (>97%) at 200-400°C, as well as the best Hg⁰ oxidation activity (>75%) at 150-350°C among all the catalysts. The XPS and H₂-TPR results indicated that 7% Ce-1% Cu/SCR possess abundant chemisorbed oxygen and good redox ability, which was due to the strong synergy between Ce and Cu in the catalyst. The existence of the redox cycle of Ce⁴⁺+Cu¹⁺↔Ce³⁺+Cu²⁺ could greatly improve the catalytic activity. 7% Ce-1% Cu/SCR showed higher resistance to SO₂ and H₂O than other catalysts. NO has a promoting effect on Hg⁰ oxidation. The Hg⁰ oxidation activity was inhibited by the injection of NH₃, which was due to the competitive adsorption and oxidized mercury could be reduced by ammonia at temperatures greater than 325°C. Therefore, Hg⁰ oxidation could easily occurred at the outlet of SCR catalyst layer due to the consumption of NH₃.