Development of O2 and NO Co-Doped Porous Carbon as a High-Capacity Mercury Sorbent

Insights into the enhanced mechanism of selenium-doped iron nitride carbon catalysts for elemental mercury removal in flue gas

This study developed a novel selenium-doped metal nitride carbon, Fe-NC-Se, via pyrolysis and impregnated hydrothermal methods for elemental mercury removal from coal-fired flue gas. The Fe-NC-Se demonstrated a remarkable mercury removal performance, achieving an average efficiency of 96.98% within 60 min at an optimal Se/Fe ratio of 2:1 and temperature of 110 °C, which was 2.5 times higher than that of the pristine Fe-NC (iron nitride carbon). Notably, Fe-NC-Se maintained an 84% efficiency in a high SO2 environment (1600 ppm), indicating strong resistance to SO2 poisoning. Long-term testing over 24 h showed a consistent removal efficiency of 84.75%, suggesting potential for recyclability. Advanced characterization techniques, including TEM (transmission electron microscopy) and XPS (X-ray photoelectron spectrometer), along with Density Functional Theory calculations, were employed to explore the removal mechanism. Results indicated that selenium doping enhanced surface charge transfer and the reactivity of surface atoms, facilitating mercury oxidation and sequestration. The oxidized Hg2+ was anchored by Se and partially stabilized by C, N, and Fe atoms, enhancing the catalyst's effectiveness. This work not only advances the design of mercury abatement catalysts but also supports the industrial applicability of Fe-NC-Se in flue gas treatment.

Effect of sucrose-based carbon foams as negative electrode additive on the performance of lead-acid batteries under high-rate partial-state-of-charge condition

Lead-acid batteries are noted for simple maintenance, long lifespan, stable quality, and high reliability, widely used in the field of energy storage. However, during the use of lead-acid batteries, the negative electrode is prone to irreversible sulfation, failing to meet the requirements of new applications such as maintenance-free hybrid vehicles and solar energy storage. In this study, in order to overcome the sulfation problem and improve the cycle life of lead-acid batteries, active carbon (AC) was selected as a foaming agent and foam fixing agent, and carbon foams (CF) with layered porous structure was prepared by mixing with molten sucrose. Sucrose as raw material is green and cheap, and the material preparation process is simple. The prepared CF material was then added as an additive to the negative electrode plate, and the electrochemical performance of the electrode plate and the battery was studied. The results proved that the addition of CF could effectively inhibit the sulfate formation of the negative electrode plate, with the 1.0 % CF negative electrode plate showing the best electrochemical performance. Specifically, according to the result of battery cycle testing, the simulated battery with CF had a cycle life of 3642 times, which was 2.87 times that of the blank group and 2.39 times of the AC group. Meanwhile, rate testing showed that the simulated battery with CF could maintain a high capacity even under high-rate discharge conditions.

Multimedia Mercury Recovery from Coal-Fired Power Plants Utilizing N-Containing Conjugated Polymer Functionalized Fly Ash

To recover multimedia mercury from coal-fired power plants, a novel N-containing conjugated polymer (polyaniline and polypyrrole) functionalized fly ash was prepared, which could continuously adsorb 99.2% of gaseous Hg0 at a high space velocity of 368,500 h-1 and nearly 100% of aqueous Hg2+ in the solution pH range of 2-12. The adsorption capacities of Hg0 and Hg2+ reach 1.62 and 101.36 mg/g, respectively. Such a kind of adsorbent has good environmental applicability, i.e. good resistance to coexisting O2/NO/SO2 and coexisting Na+/K+/Ca2+/Mg2+/SO42-. This adsorbent has very low specific resistances (6 × 106-5 × 109 Ω·cm) and thus can be easily collected by an electrostatic precipitator under low-voltage (0.1-0.8 kV). The Hg-saturated adsorbent can desorb almost 100% Hg under relatively low temperature (<250 °C). Characterization and theoretical calculations reveal that conjugated-N is the critical site for adsorbing both Hg0 and Hg2+ as well as activating chlorine. Gaseous Hg0 is oxidized and adsorbed in the form of HgXClX(ad), while aqueous Hg2+ is adsorbed to form a complex with conjugated-N, and parts of Hg2+ are reduced to Hg+ by conjugated-N. This adsorbent can be easily large-scale manufactured; thus, this novel solid waste functionalization method is promising to be applied in coal-fired power plants and other Hg-involving industrial scenes.

Applications of agricultural residue biochars to removal of toxic gases emitted from chemical plants: A review

Crop residues are representative agricultural waste materials, massively generated in the world. However, a large fraction of them is currently being wasted, though they have a high potential to be used as a value-added carbon-rich material. Also, the applications of carbon-rich materials from agricultural waste to industries can have economic benefit because waste-derived carbon materials are considered inexpensive waste materials. In this review, valorization methods for crop residues as carbon-rich materials (i.e., biochars) and their applications to industrial toxic gas removals are discussed. Applications of crop residue biochars to toxic gas removal can have significant environmental benefits and economic feasibility. As such, this review discussed the technical advantages of the use of crop residue biochars as adsorbents for hazardous gaseous pollutants and greenhouse gases (GHGs) stemmed from combustion of fossil fuels and the different refinery processes. Also, the practical benefits from the activation methods in line with the biochar properties were comprehensively discussed. The relationships between the physico-chemical properties of biochars and the removal mechanisms of gaseous pollutants (H₂S, SO₂, Hg⁰, and CO₂) on biochars were also highlighted in this review study. Porosity controls using physical and chemical activations along with the addition of specific functional groups and metals on biochars have significantly contributed to the enhancement of flue gas adsorption. The adsorption capacity of biochar for each toxic chemical was in the range of 46-76 mg g^-1 for H₂S, 40-182 mg g^-1 for SO₂, 80-952 μg g^-1 for Hg⁰, and 82-308 mg g^-1 CO₂, respectively. This helps to find suitable activation methods for adsorption of the target pollutants. In the last part, the benefits from the use of biochars and the research directions were prospectively provided to make crop residue biochars more practical materials in adsorption of pollutant gases.

Controllable Disordered Copper Sulfide with a Sulfur-Rich Interface for High-Performance Gaseous Elemental Mercury Capture

Copper sulfide (CuS) has received increasing attention as a promising material in gaseous elemental mercury (Hg⁰) capture, yet how to enhance its activity at elevated temperature remains a great challenge for practical application. Herein, simultaneous improvement in the activity and thermal stability of CuS toward Hg⁰ capture was successfully achieved for the first time by controlling the crystal growth. CuS with a moderate crystallinity degree of 68.8% showed a disordered structure yet high thermal stability up to 180 °C. Such disordered CuS can maintain its Hg⁰ capture activity stable during longtime test at a wide temperature range from 60 to 180 °C and displayed strong resistance to SO₂ (6%) and H₂O (8%). The significant improvement can be attributed to the synergistic effect of a moderately crystalline nature and a unique sulfur-rich interface. Moderate crystallinity guarantees the thermal stability of CuS and the presence of abundant defects, in which copper vacancy enhances significantly the Hg⁰ capture activity. The sulfur-rich interface enables CuS to provide plentiful highly active S_x^2- sites for Hg⁰ adsorption. The interrelation between structure, reactivity, and thermal stability clarified in this work broadens the understanding toward Hg⁰ oxidation and adsorption over CuS and provides new insights into the rational design and engineering of advanced environmental materials.

Formation of sulfur oxide groups by SO2 and their roles in mercury adsorption on carbon-based materials

The presence of SO₂ display significant effect on the mercury (Hg) adsorption ability of carbon-based sorbent. Yet the adsorption and oxidation of SO₂ on carbon with oxygen group, as well as the roles of different sulfur oxide groups in Hg adsorption have heretofore been unclear. The formation of sulfur oxide groups by SO₂ and their effects on Hg adsorption on carbon was detailed examined by the density functional theory. The results show that SO₂ can be oxidized into SO₃ by oxygen group on carbon surface. Both C-SO₂ and C-SO₃ can improve Hg adsorption on carbon site, while the promotive effect of C-SO₂ is stronger than C-SO₃. Electron density difference analyses reveal that sulfur oxide groups enhance the charge transfer ability of surface unsaturated carbon atom, thereby improving Hg adsorption. The experimental results confirm that surface active groups formed by SO₂ adsorption is more active for Hg adsorption than the groups generated by SO₃.

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.

Spherical-shaped CuS modified carbon nitride nanosheet for efficient capture of elemental mercury from flue gas at low temperature

Mercury (Hg⁰) pollution poses a huge threat to human health and the environment due to its high toxicity, long persistence and bioaccumulation in the environment. Most of the traditional Hg⁰ adsorbents have a low reaction rate, high operating cost, especially poor resistance to SO₂, which limited their practical application. In this work, nanosheet g-C₃N₄ was used as the support and modified by CuS to capture flue gas mercury. Take advantage of the large specific surface area of g-C₃N₄ to increase the BET of the composite and decrease the use of CuS. The effects of CuS loading, reaction temperature, and common components in the coal-fired flue gas on the mercury removal performance were studied respectively. The experimental outcomes showed that the 10CuS/g-C₃N₄ (10CuS/CN) reaches as high as almost 100% Hg⁰ removal efficiency under the temperature of 40-120 ℃. Meanwhile the common components like SO₂, NO, HCl and H₂O have no obvious inhibition effects on Hg⁰ removal efficiency of the 10CuS/CN adsorbent. S_x^2- and Cu²⁺ as the primary bonding sites shows a synergy effect on Hg⁰ removal. 10CuS/CN is a promising material for Hg⁰ removal under various flue gas conditions, which is expected to be a substitute for traditional adsorbents.

Co9S8 nanoparticles-embedded porous carbon: A highly efficient sorbent for mercury capture from nonferrous smelting flue gas

In this study, a novel Co₉S₈ nanoparticles-embedded porous carbon (Co₉S₈-PC) was designed as an effective reusable sorbent for Hg⁰ capture from smelting flue gas. Some flue gas components can create more active sites on Co₉S₈-PC for Hg⁰ adsorption, but compete with Hg⁰ for the same sulfur sites over nano Co_1-xS/Co₃S₄ (CoS) and Co_1-xS/Co₃S₄ embedded porous carbon (CoS-PC), which can be ascribed to the difference in crystal structure between Co₉S₈ and Co_1-xS/Co₃S₄. Therefore, Co₉S₈-PC shows much better Hg⁰ capture ability than CoS and CoS-PC under smelting flue gas. O₂, SO₂ and HCl improve Hg⁰ adsorption on Co₉S₈-PC mainly through creating Co³⁺ site, but H₂O has neglectable effect on Hg⁰ capture. Co₉S₈-PC shows a remarkably large Hg⁰ adsorption capacity of 43.18 mg/g, which is greatly higher than the representative metal sulfides for Hg⁰ removal from smelting flue gas. During Hg⁰ adsorption, Co³⁺ is the primary site to directly interact with Hg⁰, and the adsorbed mercury exists as HgS. Co₉S₈-PC exhibits an excellent recyclability for capturing Hg⁰, which is mainly assigned to the replenishment of consumed Co³⁺ site by O₂, SO₂ and HCl. Therefore, Co₉S₈ nanoparticles-embedded porous carbon is an efficient, sustainable and highly recyclable sorbent for Hg⁰ recovery from smelting flue gas.

DFT and Experimental Studies on the Mechanism of Mercury Adsorption on O2-/NO-Codoped Porous Carbon

The utilization of O₂ and NO in flue gas to activate the raw porous carbon with auxiliary plasma contributes to an effective mercury (Hg)-removal strategy. The lack of in-depth knowledge on the Hg adsorption mechanism over the O₂-/NO-codoped porous carbon severely limits the development of a more effective Hg removal method and the potential application. Therefore, the generation processes of functional groups on the surface during plasma treatment were investigated and the detailed roles of different groups in Hg adsorption were clarified. The theoretical results suggest that the formation of functional groups is highly exothermic and they preferentially form on a carbon surface, and then affect Hg adsorption. The active groups affect Hg adsorption in a different manner, which depends on their nature. All of these active groups can improve Hg adsorption by enhancing the interaction of Hg with a surface carbon atom. Particularly, the preadsorbed NO₂ and O₃ groups can react directly with Hg by forming HgO. The experimental results confirm that the active groups cocontribute to the high Hg removal efficiency of O₂-/NO-codoped porous carbon. In addition, the mercury temperature-programmed desorption results suggest that there are two forms of mercury present on O₂-/NO-codoped porous carbon, including a carbon-bonded Hg atom and HgO.

Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl

LaMnO₃-based catalysts with perovskite structure have gained increasing interest for Hg⁰ oxidation owing to their excellent catalytic activity, high thermal stability and unique redox behavior. Understanding the Hg⁰ oxidation behavior on LaMnO₃ will broaden the application of LaMnO₃-based perovskites in Hg⁰ removal field. Density functional theory (DFT) calculations were conducted to examine the catalytic mechanism of Hg⁰ oxidation by HCl on LaMnO₃ surface. The results indicate that Mn-terminated LaMnO₃(010) surface is more active and stable than La-terminated surface. Hg⁰ and HgCl₂ are chemisorbed on LaMnO₃(010) surface. HgCl can be molecularly chemisorbed on LaMnO₃(010) and serve as an intermediate in Hg⁰ oxidation reaction. HCl dissociatively adsorbs on LaMnO₃(010) and generates surface active chlorine complexes. Langmuir-Hinshelwood mechanism, where the chemisorbed Hg⁰ reacts with the dissociatively adsorbed HCl, is responsible for Hg⁰ oxidation by HCl on LaMnO₃(010). Catalytic Hg⁰ oxidation over the surface contains four-steps: Hg⁰ → Hg(ads) → HgCl(ads) → HgCl₂(ads) → HgCl₂, and the second step (Hg(ads) → HgCl(ads)) is the rate-determining step because of its relatively larger energy barrier (0.74 eV).

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.

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.

A complete catalytic reaction scheme for Hg0 oxidation by HCl over RuO2/TiO2 catalyst

RuO₂-based catalysts have attracted great attention in mercury emission control region due to their outstanding catalytic activity and long-term stability. Quantum chemistry calculation was performed to uncover the atomic-scale reaction mechanism of Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst. The results indicate that Hg⁰ adsorption on RuO₂/TiO₂(110) surface is controlled by a weak chemisorption mechanism. The 5-fold coordinated surface Ru atom is identified as the active center for Hg⁰ adsorption. HgCl molecule serves as an intermediate connecting reactant state to product state. The weak interaction between HgCl₂ and catalyst surface is favorable for product desorption. HCl activation is an O-assisted surface reaction process in which HCl is oxidized into active Cl atom for Hg⁰ oxidation. The heterolytic cleavage of HCl molecule occurs without noticeable activation energy barrier. Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst proceeds through two independent reaction channels. The dominant reaction channel of Hg⁰ oxidation is identified as a four-step process. Finally, a complete catalytic cycle that can produce the correct stoichiometry was proposed to understand the heterogeneous reaction mechanism of Hg⁰ oxidation over RuO₂/TiO₂ catalyst. The catalytic cycle consists of HCl activation, mercury oxidation and surface reoxidation. Mercury oxidation is the rate-determining step of the catalytic cycle.