Coal combustion is one of the main sources of anthropogenic mercury emission around the world. Mercury and its compounds, especially methyl mercury species, are of high toxicity and bioaccumulation . Three forms of mercury exist in coal combustion flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+) and particle-bound mercury (HgP). Hg2+ can be easily removed by the wet flue gas desulfurization (WFGD) due to its water solubility. HgP can be captured along with the fly ash by the electrostatic precipitators (ESP). However, Hg0 is difficult to control because it is extremely volatile and water-insoluble . Therefore, Hg0 is the dominant form of mercury that released to the atmosphere. One of the feasible ways to remove Hg0 is to use catalytic oxidation method to convert Hg0 to Hg2+.
The SCR catalyst was recognized as a promising catalyst for Hg0 oxidation, as it can be used for NO reduction simultaneously. In particular, the V2O5-based commercial SCR catalysts for NO removal have been reported as Hg0 oxidation catalysts , . Since the activities of V2O5-based commercial SCR catalysts are significantly affected by flue gas temperature, the catalytic activities are unstable due to the load variation of the boiler. Meanwhile, as the catalysts are placed before the ESP, they are deactivated easily due to the high concentration of dust in the flue gas , . It has been found that, relative to its effects on the efficiency of NO reduction, the catalyst deactivation could more significantly reduce Hg0 oxidation efficiency . In this case, there is a need for a novel catalyst that could perform well for Hg0 oxidation at lower temperature, which can be located after the cold side ESP. Meanwhile, Huaneng group, a huge power generation company in China, has issued a strict NOx emission standard (less than 50μg/m3) for the coal-fired power plants. The existing commercial SCR catalysts cannot meet the emission standards. Therefore, to save the cost, the catalyst used for Hg0 oxidation at lower temperature was required to remove the NO escaped from upstream simultaneously.
Fortunately, the low toxic MnOx catalysts have been proved to be the co-benefit catalysts for NO and Hg0 removal at lower temperature, and they exhibit excellent performance under certain conditions , , . However, the Hg0 oxidation efficiencies of these catalysts strongly depended on HCl concentration in the flue gas, a factor that cannot be controlled in the power plants. The catalysts exhibited excellent mercury oxidation activities at ∼250°C, which were not the typical flue gas temperature after the cold-side ESP. Considering the above limitations, Mn-based oxides could be modified to be used for Hg oxidation at lower temperature subject to less influence of HCl concentration.
The Mn-based perovskite oxides seem an attractive candidate for Hg0 oxidation due to its excellent thermal stability, superior redox properties and great versatility , . The general formula of perovskite oxides is ABO3. The A-site ions fitted into the dodecahedral interstices are always large rare-earth or alkaline-earth metal ions. The B-site ions are always the transition metal ions which occupy oxygen octahedrons and play a dominant role in the catalytic process. It is generally believed that the properties of the B-site ions, the concentration of oxygen species, and the existence of lattice defects are responsible for the good catalytic activities of the perovskite-type oxides . A-site cations in the perovskite oxides play a role of structure-stabilizing and partial substitution of A-site cations can modify the physicochemical properties of perovskite oxides. The incorporation of a low valence cation in the A-site decreases the valence that is compensated by the creation of oxygen vacancies and generation of more higher valence B-site cations. The most common Mn-based perovskite catalyst is LaMnO3, which has been investigated as a low temperature SCR catalyst recently .
Few researches investigated catalytic effects of LaMnO3 for Hg0 oxidation. Hence, some tests need to be carried out to investigate whether LaMnO3 can be served as an efficient catalyst for Hg0 oxidation. The abundant active oxygen species on the catalyst surface may be conducive for the formation of oxy-chloride or active chlorine species which are active for Hg0 oxidation. In addition, it has been reported that the perovskites exhibits better catalytic performance when the A-site ions La3+ are partially substituted by Sr2+ . This is because partial substitution of La3+ with Sr2+ can generate mixed-valence manganese (Mn3+ and Mn4+) and more active oxygen species. The shift between Mn3+ and Mn4+ is beneficial for the generation of labile oxygen vacancies and bulk oxygen species . The redox couple may also benefit the Hg0 oxidation due to the Mars-Maessen mechanism in the absence of HCl. It seems likely that the LaMnO3 modified by partial substitution of La3+ by Sr2+ could oxidize Hg0 with low concentration of O2 and HCl in the flue gas when low rank coals are burned.
Accordingly, we hypothesize that the La1−xSrxMnO3 (x=0/0.2/0.4/0.6) perovskite catalysts could exhibit remarkable catalytic performance on Hg0 oxidation with low concentration of O2 and HCl in the flue gas at low temperatures. In the present work, the Mn-based perovskite oxides were synthesized and characterized to investigate the catalytic activity of the oxides on Hg0 oxidation. In order to improve the catalytic activity of the LaMnO3, the La3+ ions were partially substituted by Sr2+ ions to increase the surface defects so as to generate more oxygen vacancies and active oxygen species. The catalytic performance of the oxides under conditions such as low concentration of O2 and HCl at low temperature was also investigated, and the Hg0 oxidation mechanism was identified. The effects of SO2, H2O, NO and NH3 on the Hg0 oxidation efficiency were studied. Hg0 oxidation efficiency under SCR atmosphere and the SCR activity of the selected catalysts (La0.6Sr0.4MnO3) were considered as well. It is demonstrated that the synthesized perovskite oxides may serve as an environmentally friendly catalyst for simultaneous removal of NO and Hg0 from coal combustion flue gas at low temperature.
Preparation of catalysts
The La1−xSrxMnO3 (x=0/0.2/0.4/0.6) catalysts were prepared by complexation method using citric acid as a complexation agent . They are denoted as LSx, where L is the raw LaMnO3, S is the doped Sr, and x represents the mole ratio of Sr/(Sr+La). Stoichiometric amounts of lanthanum nitrate, strontium nitrate and manganese nitrate were dissolved in deionized water. Citric acid was separately dissolved in distilled water and added to the mixed solution. The amount of the citric acid was equal to
Characterization of catalysts
The N2 adsorption–desorption isotherms and pore size distributions of the LSx catalysts were presented in Fig.2(a)and(b), respectively. As seen in Fig.2(a), the catalysts showed N2 adsorption–desorption isotherms of type II with a hysteresis loop of type H3 in the relative pressure range of 0.7–1.0. This type of the isotherm was characteristic of macroporous materials  and this type of hysteresis loop indicated the formation of mesopores. Moreover, a certain amount of N2 were adsorbed
In this study, the catalytic effects of La1−xSrxMnO3 (x=0/0.2/0.4/0.6) on Hg0 oxidation were investigated. It was found that La0.6Sr0.4MnO3 exhibited the strongest catalytic activity for Hg0 oxidation at 100–200°C. HCl had a prominent enhancement effect on Hg0 oxidation in the presence of O2, and 10 ppmv HCl was sufficient for enhancing the oxidation process. The results suggest that the catalyst could be used in power plants burning low-chlorine coals. The catalyst could help oxidize the Hg0
This work is supported by the National Natural Science Foundation of China (51476064, U1261204) and the National Basic Research Program of China (2013CB228501). The support of the Analytical and Testing Center at the Huazhong University of Science and Technology is also appreciated. Mr. Ho Simon Wang has helped improve the linguistic presentation of the manuscript.
Cited by (95)
Perovskite surface regulation to enhance the catalytic performances on SCR and Hg<sup>0</sup> oxidation at low temperature
2023, Chemical Physics Impact(Video) Water Oxidation Catalysis with Atomically Defined Active Sites on Nanostructured (...) | 2022NSSA
LaMnO3 is one of the most common Mn-based perovskites which has been studied as a low-temperature catalyst for NOx reduction. However, the Hg0 oxidation performance of LaMnO3 is not ideal. An acid treatment method was adopted to regulate the LaMnO3 surface to expose more active sites. The obtained MnO2/ LaMnO3 sample showed enhanced SCR and Hg0 oxidation performances. The NO conversion rate and Hg0 removal efficiency of MnO2/ LaMnO3 could be over 85% in the temperature range of 150−250°C. The results suggested that the catalyst might be used to remove NO and Hg0 simultaneously at low temperature in coal-fired flue gas.
Effecting pattern study of SO<inf>2</inf> on Hg<sup>0</sup> removal over α-MnO<inf>2</inf> in-situ supported magnetic composite
2023, Journal of Hazardous Materials
α-MnO2 was in-situ supported onto silica coated magnetite nanoparticles (MagS-Mn) to study the adsorption and oxidation of Hg0 as well as the effecting patterns of SO2 and O2 on Hg0 removal. MagS-Mn showed Hg0 removal capacity of 1122.6μg/g at 150°C with the presence of SO2. Hg0 adsorption and oxidation efficiencies were 2.4% and 90.6%, respectively. Hg0 removal capability deteriorated at elevated temperatures. Surface oxygen and manganese chemistry analysis indicated that SO2 inhibited the Hg0 removal through consumption of adsorbed oxygen and reduction of high valence manganese. This inhibiting effect was observed to be counteracted by O2 at lower temperatures. O2 tended to compete with SO2 for active sites and further create additional adsorbed oxygen sites for Hg0 surface reaction via surface dissociative adsorption rather than replenish the active sites consumed by SO2. The high valence manganese was also preserved by O2 which was essential to Hg0 oxidation. The intervention of O2 in the inhibition of SO2 on Hg0 removal was weakened at temperatures higher than 250°C. Aa a result, Hg0 tends to be catalytic oxidized in the condition of low reaction temperatures and with the presence of O2 over α-MnO2 oriented composites.
Self-template synthesis of CuCo<inf>2</inf>O<inf>4</inf> nanosheet-based nanotube sorbent for efficient Hg<sup>0</sup> removal
2023, Separation and Purification Technology
Adsorption method is an effective way to remove Hg0 from flue gas, and the morphology and active site of sorbents seriously affect the Hg0 removal efficiency. The hierarchical porous nanosheet-based nanotube structure can effectively reduce the mass transfer resistance and provide more active sites. Inspired by this, the organic-inorganic hybrid nanowires of Co-aspartic acid was used as the template, which can hydrolyze in water-alcohol mixed solution and coordinate with Cu2+ based on Kirkendall effect to prepare nanosheet-based nanotube of CuCo2O4 sorbents. By changing the ratio of water to alcohol, the hydrolysis rate of Co-aspartic acid nanowires will be changed resulting in different physicochemical properties for CuCo2O4 sorbents. Characterization results show that when the ratio is 1:2, the sorbent (CuCo2O4-1-2) has the best hierarchical porous nanosheet-based nanotube structure and the best redox properties. CuCo2O4-1-2, which has the widest reaction temperature window, has a high Hg0 removal efficiency of 89% under a high GHSV of 180 000h−1 at 250°C and a good poisoning resistance and stability. The straight nanotube can effectively reduce the gas transfer mass resistance, and the mesoporous nanosheets structure on its surface provide more active sites for the reaction, which improves the ability of CuCo2O4-1-2 sorbent to capture and activate O2. Meanwhile, DFT calculation shows Cu-Co on the surface of CuCo2O4 is the main active site because the bond length of OO can be effectively lengthened when O2 is adsorbed on the Cu-Co site.
Enhanced adsorption of gaseous mercury on activated carbon by a novel clean modification method
2023, Separation and Purification Technology
Activated carbon is the most widely used gaseous mercury adsorbent. However, lack of active sites on its surface leads to its poor mercury adsorption ability. Various modification methods, including acid-alkali, sulfide, halogenide, metallic oxides, etc., are used to modify activated carbon to enhance its adsorption ability, but there are still secondary pollution or/and solid waste problems. This article tried to use a clean ultraviolet (UV)/H2O2 advanced oxidation process (AOP) to modify activated carbon by generating hydroxyl radicals for enhancing adsorption of Hg0 on activated carbon. The activated carbon modification parameters, main influencing factors, mechanism and kinetics of mercury removal were studied. Studies indicate that UV/H2O2 AOP modification could substantively enhance the Hg0 adsorption over activated carbon. The optimum H2O2 modification concentration is 9%, and the optimum reaction temperature for Hg0 adsorption is 120°C. The optimized Hg0 adsorption capacity reaches 3636.43μg/g. Characterization measurement (e.g., BET, SEM, FTIR, XPS, etc.) demonstrates that UV/H2O2 AOP modification has slight destructive effect on pore structure of activated carbon, but significantly raises the contents of oxygen-containing functional groups (e.g., -OH, CO and CO). It is found that Hg0 adsorption is enhanced by O2 and NO in gas stream, while is inhibited by H2O and SO2 in gas stream. The whole process of Hg0 adsorption is mainly dominated via chemical adsorption step, and the oxygen-containing functional groups (e.g., -OH, CO and CO) and the chemisorbed oxygen (O*) are the main active sites to realize the oxidative removal of Hg0 on the modified activated carbon surface.(Video) Hybrid materials by Molecular Layer Deposition
Performance and mechanism of co-doped Ce–Mn perovskite for degradation of tetracycline via heterogeneous photocatalysis coupled PMS oxidation
2023, Materials Science in Semiconductor Processing
In this work, CeMn1-xMxO3 (M=Cu, Fe and Co) perovskites were synthesized and used as efficient heterogeneous catalyst for degrade of tetracycline chloride (TC-HCl) via photocatalysis coupled peroxymonosulfate (PMS) oxidation under visible-light illumination. Metal doping significantly improved the catalytic activity of perovskites. The CeMn0.6Co0.4O3/PMS/Vis system demonstrated excellent performance that 96.4% of 60mg/L of TC-HCl was degraded within 30min with a high reaction rate constant of 0.136 min−1, which is about 4.3 times higher than that of CeMnO3, and almost 4.4 times higher than that of pure PMS. Results confirmed the addition of Co led to mixed valence states of Mn (III)/Mn (Ⅳ) and Co (II)/Co (III) in the perovskite structure, promoting the electron transfer from the CeMn0.6Co0.4O3 to PMS. Moreover, CeMn0.6Co0.4O3 exhibited good stability over a wide pH range (3.0–11.0). The rapid transformation of Co (II)/Co (III) redox pairs is critical for PMS activation as the limiting step. Superoxide radicals (•O2−), electron (e−) and sulfate radical (SO4−•) were the dominant oxidative species responsible for removing TC-HCl. This work proposed some new insights to understand the heterogeneous reaction mechanism during PMS oxidation and photocatalytic process and promote the development of perovskites catalyst for removal of organic pollutants.
A comprehensive review of the heavy metal issues regarding commercial vanadium‑titanium-based SCR catalyst
2023, Science of the Total Environment
Facing the increasing demand of atmosphere pollutant control, selective catalytic reduction (SCR) technology has been widely applied in various industries for NOx abatement. However, in the condition of complicated flue gas components, the heavy metal issue is a great challenge to the catalyst deactivation and atmospheric pollution control. In this review, with the comprehensive consideration of SCR catalysts in heavy metal-rich flue gas scenarios, the distribution character of heavy metals in SCR system is firstly summarized, then the detailed interaction mechanism between heavy metals and the vanadium‑titanium-based catalyst is discussed. Focusing on the mercury oxidation as well as against arsenic/lead poisoning, certain modification strategies are also concluded to develop novel SCR catalysts with multiple functions. Furthermore, the state-of-the-art technologies regarding the regeneration, the valuable metal recovery, and the harmless treatment of the spent SCR catalyst are also reported. This paper provides theoretical guidance for the manufacture of novel SCR catalysts under multiple scenarios, as well as the synergistic control of NOx and heavy metals.
Recommended articles (6)
Fe–Mn–Ce/ceramic powder composite catalyst for highly volatile elemental mercury removal in simulated coal-fired flue gas
Journal of Industrial and Engineering Chemistry, Volume 25, 2015, pp. 352-358
The Fe–Mn–Ce/CP catalyst prepared by the incipient-wetness impregnation method was investigated for elemental mercury (Hg0) removal in simulated coal-fired flue gas. The incorporation of Fe could remarkably enhance the SO2-resistance of Fe–Mn–Ce/CP catalyst at low temperature, and the existence of water vapor had negative impact on Hg0 removal efficiency. More than 95% of Hg0 could be removed at 100°C under the conditions of 60ppm HCl, 3% O2 (v/v), 5% H2O (v/v) and 400ppm SO2. Hg-TPD results indicated that Hg0 adsorption amount decreased after the addition of H2O or SO2. The XPS results revealed that the incorporation of Fe could enhance the contents of high valence Mnx+ (i.e., Mn3+/Mn4+) and Ce3+ species in the composite catalysts, which are favorable for the oxidation process of elemental mercury. The surface characteristics were not the primary factor determining the catalytic activity. Overall, the catalytic performance of the Fe–Mn–Ce/CP catalyst was closely related to the Fe3+ state, high ratios of (Mn4++Mn3+)/Mn2+ and high content of not fully coordinated cerium species.(Video) 12 MDL - Yang Shao-Horn: The Future of Electrochemistry
Removal of elemental mercury from flue gas using CuOx and CeO2 modified rice straw chars enhanced by ultrasound
Fuel Processing Technology, Volume 170, 2018, pp. 21-31
Rice straw char (RS), a common biomass pyrolysis waste, was modified by Cu-Ce mixed oxides with ultrasound-assisted impregnation to develop a cost-effective sorbent for Hg0 removal from flue gas. A variety of techniques including thermogravimetric analysis, nitrogen adsorption-desorption, scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy were used to characterize the physical and chemical properties of the sorbents. The effects of ultrasound-assisted impregnation, Cu/Ce molar ratio, loading value, calcination temperature, reaction temperature and concentrations of the simulated flue gas components on Hg0 removal were investigated in a fixed-bed reactor. The results indicated that ultrasound-assisted impregnation promoted Hg0 removal. The optimal Cu/Ce molar ratio, loading value, calcination temperature and reaction temperature were 1/5, 0.18mol/L, 260°C and 150°C, respectively. The presence of O2 and NO promoted Hg0 removal, but H2O and SO2 inhibited Hg0 removal. The highest Hg0 removal efficiency obtained was 95.26%; and chemical adsorption was found to play a key role in the removal process. The results of stability and regeneration tests conducted suggest the sorbents could sustain longtime use for Hg0 removal.
Mn-based perovskite oxides for Hg0 adsorption and regeneration via a temperature swing adsorption (TSA) process
Fuel, Volume 182, 2016, pp. 428-436
Mn-Based perovskite oxides combined with a temperature swing adsorption (TSA) process were employed for Hg0 adsorption and regeneration from coal-fired flue gas, in which oxygen directly acted as oxidant to enhance the capture of Hg0. Three kinds of Mn-based perovskie oxides, SrMnO3, LaMnO3 and CeMnO3 were evaluated. The results indicated that the performances for Hg0 removal decreased in the order of LaMnO3>CeMnO3>SrMnO3. LaMnO3 had a Hg0 capacity of 6.22mg/g with 4% O2 at 150°C. O2 significantly enhanced the reaction activity, the Hg0 capacity was increased by approximately 6.5% in the presence of 8% O2. The characterization results indicated that perovskite crystal structure was beneficial for Hg0 capture. The Hg0 removal mechanism was primary ascribed to catalytic oxidation and chemical-adsorption. The abundant of adsorbed oxygen, high ratio of Mn4+/Mn3+ in LaMnO3 lead to the high activity. Moreover, LaMnO3 can be regenerated without the loss of capacity. The released Hg0 could be gathered which prevent from mercury secondary pollution in the environment.
Research article(Video) JEE Qualitative Analysis, Metallurgy & d block elements | Solved Questions | JEE Chemistry
Effects of existing energy saving and air pollution control devices on mercury removal in coal-fired power plants
Fuel Processing Technology, Volume 131, 2015, pp. 99-108
The impacts of a low temperature economizer (LTE) on mercury removal across an electrostatic precipitator and influence of load variation on mercury conversion over selective catalytic reduction (SCR) catalysts were determined at two coal-fired boilers. When the LTE was on, the total and elemental mercury removal efficiency increased by 42.87% and 18.85%, respectively, due to the improvement of adsorption and oxidation capacity of the fly ash at lower temperature. Mercury speciation at the inlet and outlet of the SCR system were analyzed, and the impacts of load variation and catalyst aging on Hg0 conversion were discussed. The variable loads resulted in simultaneous changes of the gas hourly space velocity, the ambient temperature, and the oxygen content. The results showed the load ratio was significant for Hg0 conversion by the SCR catalysts and load reduction benefitted Hg0 conversion. When the load ratios were 100%, 75% and 60%, the Hg0 conversion were 61.78%, 65.71% and 72.12%, respectively. Moreover, Hg0 conversion was more significantly affected by the catalyst aging than NOx reduction. Among the three factors, the most important one is the flue gas temperature based on the grey relational analysis.
Study on removal of elemental mercury over MoO3-CeO2/cylindrical activated coke in the presence of SO2 by Hg-temperature-programmed desorption
Chemical Engineering Journal, Volume 371, 2019, pp. 666-678
MoO3-CeO2/cylindrical activated coke samples (MoCeY/AC) synthesized by an impregnation method were employed to investigate elemental mercury (Hg0) removal at 60–210 °C from simulated flue gas without HCl. MoCe0.5/AC with an optimal Mo/Ce molar ratio of 0.5 exhibited an excellent Hg0 removal efficiency (94.74%) at 120 °C, as well as good stability and prominent resistance to SO2 and H2O. The physicochemical property of the samples and the Hg0 removal mechanism were discussed by ICP-AES, SEM, EDX, BET, XRD, H2-TPR, XPS and Hg-TPD. The results of characterizations showed that MoCe0.5/AC possessed the special petal-like outer microstructure, large BET surface area, well-dispersed metal oxides and high reducibility, which was conducive for Hg0 removal. Furthermore, the synergistic effect between Mo6+ and Ce3+ was favorable to the high Hg0 removal performance by providing high valence Ce. According to the Hg-TPD tests, the chemisorption of Hg0 was a major approach for Hg0 removal, while physisorption and catalytic oxidation were just accounted for a tiny fraction. Moreover, the chemisorbed mercury could be validly distinguished into weakly-HgO, strongly-HgO, Oα-HgO and HgSO4 (when SO2 was added). Compared with raw AC, MoCe0.5/AC could enhance the Hg0 oxidation performance and produce Oα-HgO during the Hg0 removal process. In addition, the possible reason for the high SO2 tolerance of MoCe0.5/AC was examined: (i) the preferential combination between sulfate and MoO3 could protect CeO2 for Hg0 removal; (ii) SO2 could contribute to the formations of weakly-HgO and HgSO4. Finally, the regenerability of MoCe0.5/AC was also discussed.
Removal of elemental mercury from flue gas using wheat straw chars modified by Mn-Ce mixed oxides with ultrasonic-assisted impregnation
Chemical Engineering Journal, Volume 326, 2017, pp. 169-181
In this article, Mn-Ce mixed oxides modified wheat straw chars were prepared by an ultrasonic-assisted impregnation method, and were employed to remove elemental mercury (Hg0) from flue gas. Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) were employed to characterize the physicochemical properties of the catalysts. The effects of ultrasonic-assisted impregnation, Mn/Ce molar ratios, calcination temperatures, Mn-Ce loading values, reaction temperatures and main flue gas components such as SO2, O2, NO and H2O on mercury removal using these catalysts were studied in a fixed bed reactor. The results showed that the catalyst with a Mn/Ce molar ratio of 2/1 exhibited high mercury removal activity at 150°C. The optimal Mn-Ce loading value and calcination temperature were 0.12mol/L and 250°C, respectively. The presence of O2 and NO obviously promoted Hg0 removal. Low concentrations of water vapor and SO2 strengthened Hg0 removal, but high concentrations of water vapor and SO2 inhibited Hg0 removal. Finally, the mercury removal mechanism was also discussed based on experimental results and characterization analysis.(Video) Catalysts, 3D-Printing and Supports for Electrochemical Water Splitting in Alkaline Media
Copyright © 2015 Elsevier B.V. All rights reserved.