Regenerable Sorbent with a High Capacity for Elemental Mercury Removal and Recycling from the Simulated Flue Gas at a Low Temperature

To remove and recycle elemental mercury from flue gas, a serial of Ce–Mn binary metal oxides was prepared and tested as the regenerable sorbents for mercury capture. Ce_0.5Mn_0.5O_<i>y</i> showed the best performance at 100 °C (about 5.6 mg g^–1 adsorption capacity), and Ce–Mn binary metal oxides could adsorb more elemental mercury than MnO_<i>y</i>. Furthermore, it was found that captured mercury can be released from the sorbent in the form of elemental mercury by heating to 350 °C. Meanwhile, the sorbent can be regenerated and repeatedly used. Powder X-ray diffractometer (PXRD), transmission electron microscopy (TEM), hydrogen temperature-programmed reduction (H₂-TPR), ammonia temperature-programmed desorption (NH₃-TPD), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption methods were employed to characterize the sorbents. A model based on mercury temperature-programmed desorption (Hg-TPD) data was built to calculate mercury desorption activation energy from the sorbent. Additionally, the impacts of the temperature and flue gas components on the adsorption capacity were investigated. NO had negligible impact on mercury adsorption, while the presence of SO₂ slightly inhibited the capability of sorbents for mercury capture. The results indicated that Ce–Mn binary metal oxides are a promising sorbent for the mercury removal and recycling from flue gas

MnO_<i>x</i>/Graphene for the Catalytic Oxidation and Adsorption of Elemental Mercury

MnO_<i>x</i>/graphene composites were prepared and employed to enhance the performance of manganese oxide (MnO_<i>x</i>) for the capture of elemental mercury (Hg⁰) in flue gas. The composites were characterized using FT-IR, XPS, XRD, and TEM, and the results showed that the highly dispersed MnO_<i>x</i> particles could be readily deposited on graphene nanosheets via hydrothermal process described here. Graphene appeared to be an ideal support for MnO_<i>x</i> particles and electron transfer channels in the catalytic oxidation of Hg⁰ at a high efficiency. Thus, MnO_<i>x</i>/graphene-30% sorbents exhibited an Hg⁰ removal efficiency of greater than 90% at 150 °C under 4% O₂, compared with the 50% removal efficiency of pure MnO_<i>x</i>. The mechanism of Hg⁰ capture is discussed, and the main Hg⁰ capture mechanisms of MnO_<i>x</i>/graphene were catalytic oxidation and adsorption. Mn is the main active site for Hg⁰ catalytic oxidation, during which high valence Mn (Mn⁴⁺ or Mn³⁺) is converted to low valence Mn (Mn³⁺ or Mn²⁺). Graphene enhanced the electrical conductivity of MnO_<i>x</i>, which is beneficial for catalytic oxidation. Furthermore, MnO_<i>x</i>/graphene exhibited an excellent regenerative ability, and is a promising sorbent for capturing Hg⁰

Novel Effective Catalyst for Elemental Mercury Removal from Coal-Fired Flue Gas and the Mechanism Investigation

Mercury pollution from coal-fired power plants has drawn attention worldwide. To achieve efficient catalytic oxidation of Hg⁰ at both high and low temperatures, we prepared and tested novel IrO₂ modified Ce–Zr solid solution catalysts under various conditions. It was found that the IrO₂/Ce_0.6Zr_0.4O₂ catalyst, which was prepared using the polyvinylpyrrolidone-assisted sol–gel method, displayed significantly higher catalytic activity for Hg⁰ oxidation. The mechanism of Hg⁰ removal over IrO₂/Ce_0.6Zr_0.4O₂ was studied using various methods, and the Hg⁰ oxidation reaction was found to follow two possible pathways. For the new chemisorption–regeneration mechanism proposed in this study, the adsorbed Hg⁰ was first oxidized with surface chemisorbed oxygen species to form HgO; the HgO could desorb from the surface of catalysts by itself or react with adsorbed HCl to be release in the form of gaseous HgCl₂. O₂ is indispensable for the chemisorption process, and the doping of IrO₂ could facilitate the chemisorption process. In addition, the Deacon reaction mechanism was also feasible for Hg⁰ oxidation: this reaction would involve first oxidizing the adsorbed HCl to active Cl species, after which the Hg⁰ could react with Cl to form HgCl₂. Additionally, doping IrO₂ could significantly improve the Cl yield process. In summary, the novel IrO₂ modified catalyst displayed excellent catalytic activity for elemental mercury oxidation, and the proposed reaction mechanisms were determined reasonably

[MoS₄]^2– Cluster Bridges in Co–Fe Layered Double Hydroxides for Mercury Uptake from S–Hg Mixed Flue Gas

[MoS₄]^2– clusters were bridged between CoFe layered double hydroxide (LDH) layers using the ion-exchange method. [MoS₄]^2–/CoFe-LDH showed excellent Hg⁰ removal performance under low and high concentrations of SO₂, highlighting the potential for such material in S–Hg mixed flue gas purification. The maximum mercury capacity was as high as 16.39 mg/g. The structure and physical-chemical properties of [MoS₄]^2–/CoFe-LDH composites were characterized with FT-IR, XRD, TEM&SEM, XPS, and H₂-TPR. [MoS₄]^2– clusters intercalated into the CoFe-LDH layered sheets; then, we enlarged the layer-to-layer spacing (from 0.622 to 0.880 nm) and enlarged the surface area (from 41.4 m²/g to 112.1 m²/g) of the composite. During the adsorption process, the interlayer [MoS₄]^2– cluster was the primary active site for mercury uptake. The adsorbed mercury existed as HgS on the material surface. The absence of active oxygen results in a composite with high sulfur resistance. Due to its high efficiency and SO₂ resistance, [MoS₄]^2–/CoFe-LDH is a promising adsorbent for mercury uptake from S–Hg mixed flue gas

Ultraeffective ZnS Nanocrystals Sorbent for Mercury(II) Removal Based on Size-Dependent Cation Exchange

We report a novel nanocrystals (NCs) sorbent, which shows an extraordinary adsorption capacity to aqueous Hg²⁺ based on cation exchange and allows for the utmost removal of mercury from water. The NCs sorbent was synthesized by direct coating ZnS NCs on the surface of the α-Al₂O₃ nanoparticles. The as-prepared ZnS NCs sorbent can efficiently remove over 99.9% Hg²⁺ in 1 min, and lower the Hg²⁺ concentration from 297.5 mg/L (ppm) to below 1.0 μg/L (ppb) within 5 min. The saturated adsorption capacity of ZnS NCs for Hg²⁺ is about 2000 mg/g, which is close to the theoretic saturated adsorption capacity. The mechanism of Hg²⁺ removal by ZnS NCs sorbent, the influences of pH value and other cations on Hg²⁺ removal were investigated, respectively. Meanwhile, it is found the size-dependent cation exchange plays a critical role in the removal of Hg²⁺ by ZnS NCs. Small size ZnS NCs shows better performance than the big size ZnS NCs in the adsorption capacity and adsorption rate for Hg²⁺. Furthermore, the mercury adsorbed by the ZnS NCs sorbent is readily recycled by extraction with aqueous sodium sulfide

Mechanism of the Selective Catalytic Oxidation of Slip Ammonia over Ru-Modified Ce–Zr Complexes Determined by in Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy

The slip ammonia from selective catalytic reduction (SCR) of NO_<i>x</i> in coal-fired flue gas can result in deterioration of the utilities or even the environmental issues. To achieve selective catalytic oxidation (SCO) of slip ammonia, Ru-modified Ce–Zr solid solution catalysts were prepared and evaluated under various conditions. It was found that the Ru/Ce_0.6Zr_0.4O₂(polyvinylpyrrolidone (PVP)) catalyst displayed significant catalytic activity and the slip ammonia was almost completely removed with the coexistence of NO_<i>x</i> and SO₂. Interestingly, the effect of SO₂ on NH₃ oxidation was bifacial, and the N₂ selectivity of the resulting products was as high as 100% in the presence of SO₂ and NH₃. The mechanism of the SCO of NH₃ over Ru/Ce_0.6Zr_0.4O₂(PVP) was studied using various techniques, and the results showed that NH₃ oxidation follows an internal SCR (iSCR) mechanism. The adsorbed ammonia was first activated and reacted with lattice oxygen atoms to form an −HNO intermediate. Then, the −HNO mainly reacted with atomic oxygen from O₂ to form NO. Meanwhile, the formed NO interacted with −NH₂ to N₂ with N₂O as the byproduct, but the presence of SO₂ can effectively inhibit the production of N₂O

SO₂‑Driven In Situ Formation of Superstable Hg₃Se₂Cl₂ for Effective Flue Gas Mercury Removal

Flue gas mercury removal is mandatory for decreasing global mercury background concentration and ecosystem protection, but it severely suffers from the instability of traditional demercury products (e.g., HgCl2, HgO, HgS, and HgSe). Herein, we demonstrate a superstable Hg3Se2Cl2 compound, which offers a promising next-generation flue gas mercury removal strategy. Theoretical calculations revealed a superstable Hg bonding structure in Hg3Se2Cl2, with the highest mercury dissociation energy (4.71 eV) among all known mercury compounds. Experiments demonstrate its unprecedentedly high thermal stability (>400 °C) and strong acid resistance (5% H2SO4). The Hg3Se2Cl2 compound could be produced via the reduction of SeO32– to nascent active Se0 by the flue gas component SO2 and the subsequent combination of Se0 with Hg0 and Cl– ions or HgCl2. During a laboratory-simulated experiment, this Hg3Se2Cl2-based strategy achieves >96% removal efficiencies of both Hg0 and HgCl2 enabling nearly zero Hg0 re-emission. As expected, real mercury removal efficiency under Se-rich industrial flue gas conditions is much more efficient than Se-poor counterparts, confirming the feasibility of this Hg3Se2Cl2-based strategy for practical applications. This study sheds light on the importance of stable demercury products in flue gas mercury treatment and also provides a highly efficient and safe flue gas demercury strategy