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• O 3 accelerates the electron transfer between Mn4+/Mn3+ in the Mn-TiO 2 catalyst. • O 3 synergizes with the Mn-TiO 2 catalyst to increase the mercury removal efficiency to 98%. • O 3 enables the oxidation adsorption of Hg0 in ESP temperature window (120–150 °C). • After the addition of O 3 , O latt becomes the primary active oxygen species. Ozone, known for its strong oxidizing properties, is commonly utilized in water pollution control. However, the catalytic ozonation of gaseous pollutants also shows potential as an emission reduction technology. The challenge lies in the high concentrations of ozone required in the catalytic process, which increases costs and complexity, hindering widespread adoption. Industrial electrostatic precipitators (ESPs) powered by high-voltage sources can produce ozone during operation, potentially causing environmental pollution if not managed. This study suggests utilizing a catalyst in the low-concentration ozone environment generated by an ESP for the catalytic oxidation of Hg0 in flue gas. The removal efficiency of Hg0 by either the catalyst alone or ozone alone (O 3 : 10 ppm) is modest at 43 % and 8 % respectively. However, when combined, the catalyst and ozone demonstrate a remarkable removal efficiency of over 98 %, significantly outperforming their individual capabilities. Furthermore, this process exhibits consistent high efficiency in a wide temperature range of 50–150 ℃. The study reveals that in the absence of O 3 , adsorbed oxygen acts as the primary oxidant, limiting the oxidation of Hg0. The introduction of O 3 enhances electron transfer within the Mn-Ti catalyst, optimizing the electronic structure of oxygen and enhancing the oxidizing capacity of lattice oxygen, resulting in superior Hg0 removal efficiency. These findings offer valuable insights for the catalytic ozonation of Hg0. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn modified Ce-Sn catalyst was studied for simultaneous Hg0 oxidation and NO reduction. • CeO 2 contributed to NH 3 -SCR activity and SnO 2 was more sensitive to mercury adsorption and oxidation. • Electronic interactions between Mn-Ce-Sn resulted in the desired low-temperature performance. The synergistic catalytic removal of NO and Hg0 under low temperatures is still a great challenge. Here, we report a Mn-modified Ce-Sn catalyst for simultaneous removal of NO and Hg0 at low temperature. The optimum Mn 0.2 /Ce 0.33 Sn 0.67 exhibited more than 95 % NO conversion and N 2 selectivity above 86 % at 150–250 ℃ as well as Hg0 removal of nearly 100 % within 150–300 ℃. SnO 2 -promoted Ce-Sn solid solution enhanced the surface proportion of Ce3+, the number of surface chemisorbed oxygen and the electronic interaction within Ce-Sn linkage. Mn modification further reasonably promoted redox cycling at low temperatures and improved acidic distribution of catalyst. Under the cooperation of Ce-Sn and Mn sites, the presence of Hg0 could substantially facilitate the reduction of NO at 250–300 ℃ and the SCR atmosphere had a weak inhibitory effect on Hg0 removal. During the SCR process, the NH 4 + and NH 2 species that directly reacted with gaseous NO following the Eley − Rideal mechanism and the improved reactivity of NH 2 with nitrates following the Langmuir − Hinshelwood mechanism enhanced the NO conversion efficiency at low temperature. Meanwhile, Mn modification could alleviate competitive adsorption of NH 3 on Hg0 and promote the formation of more chemically adsorbed mercury species. This work will provide guidance for the design of low-temperature catalysts for the simultaneous removal of NO and Hg0 in flue gas. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Mn/Cu co-doping mesoporous carbon was synthesized via template method for mercury removal. • The MnCu 0.1 -C sorbent displayed high mercury removal performance with main product of HgO. • Mn doping on the mesoporous carbon greatly improved the mercury removal ability of sorbent. • Cu doping improved the SO 2 tolerance of sorbent due to a larger affinity of Cu to sulfur. • DFT analysis displayed that Hg0 was mainly chemisorbed on the surface of MnCu 0.1 -C sorbent. Mn/Cu co-doping mesoporous carbon sorbent (MnCu x -C) was synthesized via using hard template method. Thymol blue (C 27 H 30 O 5 S) was used as carbon source and MCM-41 was applied as template agent. Copper nitrate trihydrate (Cu(NO 3) 2 ·3H 2 O) and manganese acetate tetrahydrate (MnC 4 H 6 O 4 ·4H 2 O) was in-situ co-doped with carbon source as Cu/Mn metals precursor. Mercury removal on the MnCu x -C sorbent was investigated on a fixed bed experimental apparatus. The physical and chemical characteristics of the MnCu x -C sorbent were investigated by using BET, SEM, FTIR, XRD and XPS, respectively. Combing with mercury programmed temperature desorption (Hg-TPD) and density functional theory (DFT), the mechanism of mercury adsorption on the MnCu x -C sorbent was revealed. The results showed that the synthesized MnCu 0.1 -C sorbent displayed high mercury removal performance at 150℃, which was considered to be a potential sorbent for mercury removal from industrial flue gas. Mn doping on the mesoporous carbon greatly improved the mercury removal ability of sorbent with mercury capture rate increasing from 6 % to 89 %. Doping of Cu can effectively improve the SO 2 tolerance of the MnCu 0.1 -C sorbent for mercury removal. With different concentration of SO 2 existing in flue gas, the MnCu 0.1 -C sorbent still kept high mercury removal ability. O 2 in flue gas promoted mercury removal on the MnCu 0.1 -C sorbent with main product of HgO. In the presence of SO 2 , a small amount of HgSO 4 formed on the MnCu 0.1 -C sorbent. The DFT analysis further displayed that Hg0 was mainly chemisorbed on the surface of MnCu 0.1 -C sorbent with formation of HgO. [ABSTRACT FROM AUTHOR]

• Cigarette butts combustion is a proper sulfation method for MnCe-based catalyst. • After sulfation, Hg removal efficiency increased from 66.46 % to 88.76 % at 150 °C. • After sulfation, the O ad concentration on the catalyst surface almost doubled. MnO x -CeO 2 is a promising catalyst for Hg0 removal from coal flue gas at ∼150 °C but exhibits insufficient catalytic activity. Even though sulfation enhances the catalytic activity of MnO x -CeO 2 , gas-phase sulfation leads to SO 2 poisoning and catalyst deactivation. In this study, we developed a liquid-phase sulfation strategy involving the direct combustion of cigarette butts soaked in a solution of sulfate and metal precursors to directly synthesize sulfated MnCe catalyst (SMC). SMC was characterized via N 2 adsorption–desorption isotherms, SEM-EDS, XRD, XPS, and Hg-TPD. Both the MnCe catalyst and SMC exhibited good Hg0 oxidation efficiencies (66.46 % and 88.76 %, respectively) at 150 °C. The enhanced performance of SMC was attributed to the generation of abundant chemically adsorbed oxygen species on the SMC surface during sulfation. Moreover, the synergistic effect between the Ce4+ and Mn4+ species contributed to the improved catalytic performance of SMC. This strategy not only provides a one-step synthesis of sulfated catalysts but also has potential applications in Hg0 removal from coal flue gas in coal-fired power plants. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co 3 O 4 nanosheets with different amount of oxygen vacancies were synthesized. • Higher reduction level can over reduce Co 3 O 4 leading to lower oxygen vacancies. • The over reduced Co 3 O 4 will adsorb O 2 forming lattice oxygen not chemisorbed one. • Suitable reduction condition give more oxygen vacancies and improve Hg0 removal. Oxygen vacancy is very important for the Hg0 oxidation process due to it can capture and activate gaseous oxygen forming surface chemisorbed oxygen to oxidize Hg0 to HgO. Precise control of oxygen vacancies amount is a big challenge in this field. For Co 3 O 4 , oxygen vacancy can be generated by Co3+ reduction to Co2+ and Co3+ mainly exist in (2 2 0) facet. Thus, in the present work, Co 3 O 4 nanosheets preferentially growing (2 2 0) facet with more Co3+ were synthesized and CO was used as reduction agent to reduce Co 3 O 4 at different reduction condition. After that, Co 3 O 4 nanosheets with different amount of oxygen vacancies were obtained. XPS, Raman and PL results show that, the amount of oxygen vacancies increase firstly and then decrease with the increase of reduction temperature and time. CO can reduce Co3+ to Co2+ forming oxygen vacancies. However, when the reduction temperature and time are at a high level, Co 3 O 4 will be over reduced to CoO leading to a decrease of oxygen vacancies. DFT confirm that, when Co 3 O 4 is over reduced, O 2 will hard to adsorb on Co 3 O 4 surface and lattice oxygen will be formed even after O 2 adsorption. Co 3 O 4 -Te 300 Ti 2 has an optimal reduction degree leading to higher oxygen vacancy concentration and better O 2 adsorption and activation ability. Therefore, Co 3 O 4 -Te 300 Ti 2 has a high Hg0 oxidation efficiency which is over 96 % in a wide temperature range of 150–300 °C at a high GHSV of 180,000 h−1. [ABSTRACT FROM AUTHOR]

• The removal of Hg0 by HEMS was systematically investigated for the first time. • The HEMS display obviously better Hg0 capture ability than the quaternary sulfide counterpart. • Ce-HEMS has excellent mercury removal performance at the elevated temperature. • Ce increases the HEMS of unsaturated sulfur species and high valence state metal cations. Regulating the surface coordination environment and improving the formation of active species represent the key factor in designing high-performance metal sulfide-based sorbents for gaseous Hg0 capture. This paper reported the Hg0 adsorption by high entropy metal sulfides for the first time, and clarified the mechanisms involved in the evolutions of active species as well as the Hg0 adsorption. The multiple metal components of the high entropy metal sulfides have great significance in improving the surface chemistry. The addition of Ce helps not only the formation of chemically homogeneous structure, but also the generation of high valence state metal cations and unsaturated sulfur species, thereby improving obviously the Hg0 removal ability. The synthesized CeNiCoMnCuS x has a high Hg0 adsorption capacity of 10.52 mg·g−1 at 80 %-breakthrough, and a Hg0 adsorption rate of 4.31 μg·g−1·min−1. The high valence state metal cations (Ni3+, Co3+, Mn4+, Cu2+ and Ce4+) and unsaturated S x 2− and S 2 2− species can directly oxidize Hg0 and convert it into HgS. In comparison, the quaternary sulfide counterpart (NiCoMnCuS x) cannot form a main phase structure, and has obviously lower percentages of high valence state metal cations and unsaturated sulfur species. This work provides a new space for developing metal sulfides for Hg0 adsorption. [ABSTRACT FROM AUTHOR]

[Display omitted] • The re-oxidation of Hg0 by Cl, HCl, ClO, Cl 2 hinders the HTR reaction. • A novel alkaline dechlorination agent is developed for adsorbing Cl-related species. • Two-stage filling dechlorination agent can meet the requirements of Hg-CEMS. • DFT indicates dechlorination agent eliminate Cl, HCl, ClO at elevated temperatures but not Cl 2. The high temperature reduction (HTR) of flue gas divalent mercury to elemental mercury is the core technology for the mercury continuous emission monitoring system (Hg-CEMS). In this work, the high temperature reduction of HgCl 2 to Hg0 was systematically investigated. The results show that HgCl 2 can entirely decompose to Hg0 at 800 °C, but Cl, ClO, Cl 2 , and HCl can easily oxidize Hg0 at low temperature. A new efficient alkaline dechlorination agent for eliminating these Cl species was proposed firstly. The dechlorination agent can efficiently capture the HCl, Cl, and ClO but fail to completely adsorb Cl 2. Consequently, a scheme was created that a pair of dechlorination agents were filled in inlet and outlet of the HTR furnace respectively. It features that the upper layer removes HCl and prevents the generation of Cl 2 because HCl is the precursor of Cl 2 , and the lower layer adsorbs the Cl and ClO generated by the HTR process. The feasibility of the method was successfully verified in both a fixed-bed reactor and a Hg-CEMS pretreatment device. DFT calculation indicates that the adsorption of Cl, ClO, and HCl on CaO falls into the category of strong chemisorption while Cl 2 is weakly chemisorbed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The sulfur species in co-pyrolysis chars were regulated through SO 2 activation. • The BET surface area and pore volume were greatly increased after SO 2 activation. • More sulfur- and oxygen-containing groups were generated after SO 2 activation. • The sulfurized co-pyrolysis chars exhibited excellent resistance to H 2 O and SO 2. In this study, novel S-doped sorbents synthesized via one-step co-pyrolysis of biomass and scrap tires under SO 2 atmosphere were developed for elemental mercury (Hg0) removal from coal-fired flue gas. The presence of SO 2 could regulate the sulfur species in S-doped sorbents and improve their Hg0 removal performance. Sample characterization exhibited that SO 2 activation increased the sulfur content and decreased the carbon content in sorbents due to the occurrence of carbothermal reduction reaction. The specific surface area, pore volume, and micropore content in S-doped sorbents were obviously improved compared with raw sorbents. After SO 2 activation, partial inorganic sulfide was converted into sulfur-containing functional groups and more oxygen-containing functional groups were generated on the sorbents. The S-doped sorbents presented more excellent performance for Hg0 removal compared with raw sorbents. The optimal pyrolysis temperature and SO 2 concentration in activation atmosphere were 700 °C and 10%, respectively. It was surprising that SO 2 facilitated Hg0 removal and the S-doped sorbents showed good water-resistance due to the presence of ZnS. O 2 and NO had positive effects on Hg0 removal. A close fitting to the pseudo-second-order model was obtained for the behavior of Hg0 removal over S-doped sorbents. Aliphatic sulfur, sulfoxide, carbonyl and carboxyl/ester groups functioned as active components for Hg0 removal. [ABSTRACT FROM AUTHOR]

• Two-dimensional layered Bi 2 WO 6 nanosheets were prepared using a hydrothermal method. • C 2 /2D-BWO exhibits a photocatalytic mercury removal efficiency of up to 86.3 %. • The effects of SO 2 and NO in flue gas on Hg oxidation were also studied. • The experiments and DFT calculations showed that CQDs improved the photoactivity of 2D-BWO. Photocatalytic technology has the advantages of environmental friendliness, low energy consumption, and fast reaction speed, which can be applied in the field of visible light mercury removal. High stability and narrow bandgap are fundamental factors for ideal semiconductor photocatalysts. A novel method of hydrothermal synthesis of CQDs/2D-BWO composite catalyst to improve the oxidation efficiency of elemental mercury was proposed. The morphology, physicochemical properties, and optical performance of the materials were fully characterized. The experiment of Hg oxidation efficiency under visible light showed that C 2 /2D-BWO exhibited a Hg removal efficiency of was as high as 86.3 % under visible light and adequate cyclic stability. The effects of SO 2 and NO on Hg oxidation were also studied, and the reasons for the improved of photocatalytic activity of CQDs on 2D-BWO were analyzed based on DFT calculations. The reaction mechanism of the catalytic oxidation of Hg by C 2 /2D-BWO was proposed, and C 2 /2D-BWO was found to be an efficient and stable visible light catalyst. [ABSTRACT FROM AUTHOR]

• Various synthetic methods for metal sulfides are introduced and compared. • Performances, merits, and flaws of metal sulfides are summarized and commented. • Layered metal sulfides, MoS 2 , WS 2 and SnS 2 , are the most promising mercury sorbents. • Effects of gas ingredients on the performances of metal sulfides are summed up. • Mercury capture mechanisms for metal sulfides are outlined and discussed. Mercury is largely emitted into the atmosphere during fossil fuel combustion or gasification, which causes a long-term contamination problem and poses great threats to human health. Currently, novel metal sulfides have been extensively investigated in capturing gaseous mercury because of their intense Hg0 affinities, large mercury uptakes, and distinguished tolerance to sulfur dioxide. This article overviews the recent progress on elemental mercury capture by diverse metal sulfides, i.e., CuS, ZnS, pyrite-phase metal sulfides, layered metal sulfides, Bi 2 S 3 and In 2 S 3. CuS and ZnS show good performances in a narrow temperature range due to the poor thermal stability and the compact crystal structure, respectively. Bi 2 S 3 and In 2 S 3 are also active in Hg0 adsorption, however, their performances are still unsatisfactory. Most pyrite-phase metal sulfides show moderate Hg0 capture abilities likely due to their compact crystal structures. Loading pyrites onto carriers or creating S vacancies in pyrite lattices seem to be feasible pathways to improve their performances. MoS 2 and WS 2 attained via molten salt method significantly outperform the ones obtained via hydrothermal method at elevated temperatures due to the production of ample sulfur variances and chemisorbed oxygen. Incorporating heteroatoms and expanding the interlayer distance by ions intercalation are effective routes to further strengthen their performances probably due to promoted electron transfer and increased accessibility of active sites, respectively. [ABSTRACT FROM AUTHOR]

• Ti 3 C 2 MXene/Cu 2 Se sorbent was prepared via a Lewis acid etching route. • The Hg0 removal ability of Ti 3 C 2 MXene/Cu 2 Se excelled other metal selenides. • Ti 3 C 2 MXene/Cu 2 Se showed favorable Hg0 removal performance over 100 °C. • Industrial flue gas exerted no effects on Hg0 capture of Ti 3 C 2 MXene/Cu 2 Se. A fundamental obstacle that restricts the use of metal selenides for gaseous elemental mercury (Hg0) immobilization is the failure to take both the optimization of intrinsic structure favorable for Hg0 diffusion and adequate exposure of metal selenide towards Hg0 into consideration. In this work, a Ti 3 C 2 MXene/Cu 2 Se sorbent for Hg0 sequestration was synthesized via a Lewis acid etching route combined with room-temperature selenization pathway. During this strategy, copper chloride plays a dual role, i.e., an etching agent for the Ti 3 C 2 MXene synthesis and a precursor for Cu0 decoration. Cu 2 Se precursors (Cu0) were dispersedly and firmly pre-anchored on the substrate, effectively avoiding the agglomeration and warranting excellent dispersion of Cu 2 Se. Compared with pristine Cu 2 Se, Ti 3 C 2 MXene/Cu 2 Se exhibits advantageous structural characteristics fully boosting Hg0 diffusion kinetics. Meanwhile, the enhanced dispersion of Cu 2 Se from the prefixation of Cu0 largely enhances the exposure and utilization of selenide. Consequently, Ti 3 C 2 MXene/Cu 2 Se shows a Hg0 adsorption capacity of 147.64 mg g−1, which is much higher than those of Cu 2 Se and most other metal selenides. Impressively, its uptake rate of 688.99 μg g−1 min−1 is the highest record rate in metal selenides. Moreover, Ti 3 C 2 MXene/Cu 2 Se still maintained outstanding Hg0 adsorption performance under relatively high reaction temperature with the shelter of Ti 3 C 2 MXene. Typical fuel gas situations including coal combustion flue gas, smelting flue gas, natural gas have negligible influences on its Hg0 adsorption performance, indicating its extensive applicability and flexibility for actual industrial situations. This work also provides valuable hints for the trade-off of structural properties and ligands exposure to design Hg0 sorbents with satisfactory Hg0 sequestration performances. [ABSTRACT FROM AUTHOR]

• Review the pretreatment of natural gas and condensates before Hg detection. • The detection techniques of total mercury and speciated mercury were summarized. • Focus on the adsorbent's acid gas resistance and mercury adsorption selectivity. • Appropriate sampling process and regenerable adsorbents were put forward. The requirement to characterize and remove mercury from natural gas and its condensate span a wide range of subject fields, from environmental science and analytical chemistry to material design and chemical engineering. The first part of this review discusses the mercury species and exposure limits in natural gas and gas condensate. In the second and third parts, the analytical techniques and pretreatment steps for analyzing mercury in natural gas and gas condensate are described separately. In the fourth part, the conventional gas processing operations are introduced together with the limit values of mercury distribution in natural gas processing operations. By analyzing the distribution and transformation of mercury in natural gas processing, the location of the mercury removal unit (MRU) for natural gas plants is optimized. In the fifth part, the different types of adsorbents for mercury removal from natural gas and liquid media are enumerated based on different mechanisms. The wet and acidic resistance of the different adsorbents in removing Hg0 from natural gas and flue gas is introduced. In the sixth part, as the species of mercury in a condensate are more complex than those in natural gas, the adsorption selectivity of each adsorbent for mercury removal from natural gas condensate is listed for comparison. Finally, the remaining challenges for mercury species determination and removal together with the expected future developments for mercury adsorbents are concluded. [ABSTRACT FROM AUTHOR]

• 21 %-ES-SC is the best mercury removal adsorbent with removal efficiency of 97.1 %. • Thiophene/elemental sulfur are main sulfur species for promoting mercury removal. • SO 2 and NO inhibit the mercury removal, while HCl has a promoting effect. • Mercury forms of the used ES- SC are mainly oxidized and residual forms. This work uses widely sourced rice straw as raw material, sulfur containing substances as modifiers, and mechanochemistry as modification methods to prepare the mechanochemical sulfur modified rice straw char mercury removal adsorbent (S-SC). The influence of modifiers, flue gas components, and the environmental stability of mercury removal products were systematically studied. The mechanochemical sulfur modification (elemental sulfur (ES), Na 2 S, NaHS) can improve the mercury removal performance of raw rice straw char (R-SC). It enhances mercury adsorption and capture. The ES is the best sulfur specie whose mercury removal rate firstly increases and then decreases as the amount of sulfur added increases. Sulfur modification promotes mercury forms convert to HgS (red) or even HgSO 4 with higher thermal stability. The thiophene/elemental sulfur are the main sulfur species for promoting mercury removal. SO 2 and NO inhibit mercury removal by ES-SC, while HCl has a promoting effect. As their concentration in the flue gas increases, the proportion of mercury removed by adsorption increases, and the adsorbed mercury transforms into a more thermally stable mercury form. The mercury forms of the spent ES-SC are mainly oxidized and residual form with TCLP leaching rate of zero, which has good environmental leaching stability. [ABSTRACT FROM AUTHOR]

• The synthesis and characterization methods of oxygen vacancy are described systematically. • The mechanism of photocatalyst oxidation of Hg0 with oxygen vacancy was summarized in detail from the electronic level. • The comprehensive prospect provides scientific guidance for the development of photocatalysts containing oxygen vacancy. As a common crystal defect, oxygen vacancy has a significant effect on the optical properties of semiconductor photocatalysts. Improving the photocatalytic activity of photocatalysts by introducing and regulating oxygen vacancy is one of the current research focuses. Meanwhile, oxygen vacancies can capture and activate molecular oxygen, which plays an important role in the photocatalytic oxidation of elemental mercury. In this review, the preparation and characterization methods of oxygen vacancy are reviewed systematically and the mechanism of photocatalytic oxidation of elemental mercury is revealed in detail. In addition, the mechanism of the effect of oxygen vacancy in optimizing the energy band structure, promoting carrier separation, surface reactions and changing the electron transfer path is discussed in depth. Finally, the future development prospects of oxygen vacancies in the field of photocatalytic oxidation of elemental mercury are prospected. This work can provide valuable theoretical reference and scientific basis for the design and research of photocatalysts containing oxygen vacancies. [ABSTRACT FROM AUTHOR]