•Co3O4/CeO2-rod exhibited the high catalytic activity of CH2Br2 oxidation.•CeO2 morphology strongly affected Co3O4-CeO2 interactions and structures of Co3O4/CeO2 catalysts.•Co3+ species, surface-adsorbed oxygen and oxygen vacancies were most abundant on Co3O4/CeO2-rod.•The reaction mechanism for CH2Br2 oxidation over Co3O4/CeO2 catalysts was proposed.Brominated hydrocarbons are a typical pollutant in purified terephthalic acid (PTA) exhaust gas, which is harmful for human health and the environment once released to the atmosphere. In this study, three Co3O4/CeO2 catalysts with different CeO2 morphologies (rod, plate, and cube) were prepared and were used for low temperature dibromomethane (CH2Br2) oxidation, which was used as the model compound for brominated hydrocarbons. The experimental results showed that Co3O4/CeO2-rod achieved significantly higher catalytic activity, with a T-90 of approximately 312 °C for CH2Br2 oxidation and higher selectivity to CO2 than Co3O4/CeO2-plate and Co3O4/CeO2-cube. All of the Co3O4/CeO2 catalysts investigated were stable for at least 30 h at 500 ppm CH2Br2 and 10% O2 at a GHSV of 75,000 mL/(g·h)−1, and the final products were COx, Br2, and HBr, without the formation of other Br-containing organic byproducts. The higher catalytic activity of Co3O4/CeO2-rod was attributed to the high content of Co3+, more surface-adsorbed oxygen, and more oxygen vacancies in their exposed {1 0 0} and {1 1 0} planes. In addition, Co3O4 had a stronger interaction with CeO2-rod, making it superior for CH2Br2 oxidation. Moreover, on the basis of the analysis of products and in situ DRIFTS studies, a credible reaction mechanism for CH2Br2 oxidation over Co3O4/CeO2 catalysts was proposed.Download high-res image (96KB)Download full-size image

Ti-modified Co3O4 catalysts with various Co/Ti ratios were synthesized using the co-precipitation method and were used in catalytic oxidation of dibromomethane (CH2Br2), which was selected as the model molecule for brominated volatile organic compounds (BVOCs). Addition of Ti distorted the crystal structure and led to the formation of a Co-O-Ti solid solution. Co4Ti1 (Co/Ti molar ratio was 4) achieved higher catalytic activity with a T90 (the temperature needed for 90% conversion) of approximately 245 °C for CH2Br2 oxidation and higher selectivity to CO2 at a low temperature than the other investigated catalysts. In addition, Co4Ti1 was stable for at least 30 h at 500 ppm CH2Br2, 0 or 2 vol% H2O, 0 or 500 ppm p-xylene (PX), and 10% O2 at a gas hourly space velocity of 60,000 h−1. The final products were COx, Br2, and HBr, without the formation of other Br-containing organic byproducts. The high catalytic activity was attributed to the high Co3+/Co2+ ratio and high surface acidity. Additionally, the synergistic effect of Co and Ti made it superior for CH2Br2 oxidation. Furthermore, based on the analysis of products and in situ DRIFTs studies, a receivable reaction mechanism for CH2Br2 oxidation over Ti-modified Co3O4 catalysts was proposed.Download high-res image (210KB)Download full-size image

•V-Mo-Ti was more active than V-W-Ti for the catalytic oxidation of elemental mercury.•The dope of Ag can improve markedly the mercury oxidation efficiency of V-Mo-Ti.•The catalytic mechanisms were different at various temperature ranges.V2O5-MoO3-TiO2 (V-Mo-Ti) is often used as a selective catalytic reduction catalyst for NOx from coal-fired flue gas. The performance of a V-Mo-Ti catalyst for the oxidation of elemental mercury (Hg0) was investigated. It was found that Mo was resistant toward sulfur dioxide and can enhance the Hg0 adsorption capacity. Ag was employed to enhance the Hg0 oxidation reaction and can enlarge reaction temperature window. Doping with Ag can significantly enhance the oxidation of Hg0, and adding only 0.5% Ag can keep Hg0 oxidation efficiency to approximately 90% with 5 ppm HCl, with an increase of 20–40% compared to that of V-Mo-Ti catalyst. Besides, the reaction temperature window of catalyst was enlarged from 150 to 400 °C. TEM and XPS characterization data indicated that Ag nanoparticles were loaded on the Mo/V-Ti carrier, maintaining Ag-Mo/V-Ti at a higher oxidation state. Furthermore, the TPR and Deacon reaction tests suggested that the Ag dopant might enhance the redox behavior, which facilitates the Deacon or semi-Deacon reactions for HCl activation. In addition, Hg0 desorption and breakthrough experiments and mercury valence state change experiments were carried out to investigate the Hg0 catalytic oxidation mechanisms at various temperature ranges.Download high-res image (56KB)Download full-size image

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/Ce0.6Zr0.4O2 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/Ce0.6Zr0.4O2 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.

•Fe2O3 modified SCR catalyst can improve Hg0 oxidation efficiency markedly.•Fe2O3/SCR (1%Fe, wt.) was found to be an optimal content for the oxidation of Hg0.•SO2 had a slight inhibition effect on Hg0 oxidation.To improve the ability of commercial selective catalytic reduction (SCR) catalyst to catalyze the oxidation of gaseous elemental mercury, Fe2O3 was introduced. Modifying with Fe2O3 can significantly enhance the elemental mercury oxidation ability of SCR catalyst. Fe2O3/SCR prepared by an impregnation method was employed as mercury oxidation catalysts in the simulated flue gas, and the role of Fe2O3 was investigated. The temperature window was from 150 to 450 °C. In this study, Fe2O3/SCR (1% Fe, wt.) was found to be an optimal catalyst with a mercury oxidation efficiency of higher than 90% at 350 °C using a simulated flue gas. The catalysts were characterized by X-ray diffraction (XRD), Brunauer Emmet Teller (BET) measurements, and X-ray photoelectron spectroscopy (XPS). The results indicated that the Fe2O3 was well-dispersed on the surface of SCR. The surface areas and catalytic oxidation activity were not consistent patterns, and the diameter of the mercury atom was much smaller than the pore diameter of the Fe2O3/SCR catalysts. Loading content of Fe2O3 was a very important factor for the removal of mercury. HCl was the most effective flue gas component responsible for the Hg0 oxidation. However, SO2 had a slight inhibition effect on Hg0 oxidation. Furthermore, change experiment of a mercury valence state was performed. And the mechanism of Hg0 oxidation was also discussed.

Mn-based perovskite oxide was used as the active site for elemental mercury (Hg0) removal from coal-fired flue gas. Ce1−xSnxO2 binary oxides were selected as the catalyst supports for LaMnO3 to enhance the catalytic oxidation and adsorption performance. Ce0.7Sn0.3O2 had the best Hg0 removal performance among the as-prepared Ce1−xSnxO2 binary oxides; the Hg0 removal efficiency was 95.2% at 350 °C. LaMnO3 had better performance at low temperatures (<200 °C). LaMnO3/Ce0.7Sn0.3O2 enlarged the reaction temperature window and enhanced the Hg0 removal efficiencies. The correlation between the physicochemical properties and the catalytic removal performance was investigated by XRD, BET surface area measurements, Raman spectroscopy, H2-TPR and XPS analysis. With the addition of Ce–Sn binary oxides as catalyst support, the surface areas of LaMnO3 was enlarged, the reducibility was enhanced and the oxygen mobility was improved. In addition, the Hg0 removal mechanism was illustrated on the basis of the experimental results. The roles of Ce, Sn and LaMnO3 were also discussed in this study.

Brominated hydrocarbons are a typical pollutant in exhaust gas from the synthesis process of Purified Terephthalic Acid (PTA), and may cause various environmental problems once emitted into the atmosphere. Dibromomethane (DBM) was employed as the model compound in this study, and a series of Co3O4/TiO2 catalysts with various Co contents were prepared for the catalytic oxidation of DBM. The prepared catalysts were characterized by XRD, BET, SEM, TEM, XPS, H2-TPR and NH3-TPD. Among the prepared catalysts, CoTi-5 (5 wt% Co/TiO2) showed the highest catalytic activity, with T90 at about 346 °C, which was mainly attributed to the enrichment of well-dispersed Co3O4 and the high surface Co3+/Co2+ ratio, as it could provide more surface active sites and active oxygen species. The kinetic study showed that the reaction order of DBM was pseudo first-order and the reaction order of oxygen was approximately zero-order. A plausible DBM reaction mechanism over Co3O4/TiO2 catalysts was also proposed based on the results of in situ FTIR and the analysis of gas products by GC-MS. The reaction process started with the adsorption on surface oxygen vacancies, breakage of C–Br bonds and partial dissociation of C–H bonds with the formation of intermediate species, and then the intermediate species were further oxidized to form CO and CO2.

The chemical characteristics of fine particulate matter (PM2.5) emitted from commercial cooking were explored in this study. Three typical commercial restaurants in Shanghai, i.e., a Shanghai-style one (SHS), a Sichuan-style one (SCS) and an Italian-style one (ITS), were selected to conduct PM2.5 sampling. Particulate organic matter (POM) was found to be the predominant contributor to cooking-related PM2.5 mass in all the tested restaurants, with a proportion of 69.1% to 77.1%. Specifically, 80 trace organic compounds were identified and quantified by gas chromatography/mass spectrometry (GC/MS), which accounted for 3.8%–6.5% of the total PM2.5 mass. Among the quantified organic compounds, unsaturated fatty acids had the highest concentration, followed by saturated fatty acids. Comparatively, the impacts of other kinds of organic compounds were much smaller. Oleic acid was the most abundant single species in both SCS and ITS. However, in the case of SHS, linoleic acid was the richest one. ITS produced a much larger mass fraction of most organic species in POM than the two Chinese cooking styles except for monosaccharide anhydrides and sterols. The results of this study could be utilized to explore the contribution of cooking emissions to PM2.5 pollution and to develop the emission inventory of PM2.5 from cooking, which could then help the policymakers design efficient treatment measures and control strategies on cooking emissions in the future.

•AMnO3 (A = Sr2+, La3+ and Ce4+) perovskite oxides were synthesized for Hg0 removal.•The Hg0 capacities increased in the order of SrMnO3 < CeMnO3 < LaMnO3.•O2 enhanced the Hg0 capacities over AMnO3 perovskite oxides.•Temperature swing adsorption (TSA) process was used for Hg0 adsorption and regeneration.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 6.22 mg/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.

•The IrO2/Ce0.6Zr0.4O2 (PVP) catalyst displayed excellent performance for NH3 oxidation.•SO2 could inhibit the NH3 removal, but also improved the N2 selectivity to 100%.•The mechanism of NH3 oxidation follows two pathways at different temperature region.•The internal SCR mechanism was the dominant reaction pathway over IrO2 modified catalyst.The slip ammonia from selective catalytic reduction (SCR) of NOx in coal-fired flue gas can cause degeneration of the utilities and environmental issues like aerosol. To achieve selective catalytic oxidation (SCO) of slip ammonia to N2, novel IrO2 modified Ce–Zr solid solution catalysts were synthesized and tested under various conditions. It was found that IrO2/Ce0.6Zr0.4O2 (PVP) catalyst displayed outstanding catalytic activity for slip ammonia and the removal efficiency was higher than 98%. Interestingly, the effect of SO2 on NH3 oxidation was bifacial, which the presence of SO2 could result in slight deactivation of catalyst but also improve the N2 selectivity of oxidized products to as high as 100% with coexistence of SO2 and NH3. The mechanism of NH3-SCO process over IrO2/Ce0.6Zr0.4O2 (PVP) was evaluated through various techniques, and the results demonstrated that NH3 oxidation could follow both NH mechanism and internal SCR (iSCR) mechanism at different temperature regions. And the dominant pathway is the iSCR mechanism, in which adsorbed ammonia is firstly activated and reacts with oxygen atoms to form the HNO intermediate. Then, the HNO could be oxidized with atomic oxygen from O2 to form NO species. Meanwhile, the formed/adsorbed NO could interact with NH2 to N2 with N2O as by-product, and the presence of SO2 can effectively inhibit the production of N2O and NO. Also, the mechanism of SO2 effects was also evaluated and determined reasonably.The mechanism of NH3-SCO over IrO2/Ce0.6Zr0.4O2 (PVP) catalyst.

•HCl showed a positive effect on the oxidation of Hg0 in ESP electric field.•The reaction mechanism between HCl and Hg0 in ESP electric field was investigated.•A novel discharge activation reactor was employed to improve Hg0 removal.Gas-phase oxidation of elemental mercury (Hg0) from flue gas by simulated electrostatic precipitators (ESP) electric field was explored in this paper. In order to enhance the removal efficiency of Hg0 by ESP electric filed, a novel discharge activation reactor was designed and employed. The influence of HCl concentration, temperature, fly ash, and flue gas components on the Hg0 removal were also considered, respectively. The Hg0 removal efficiency increased with the increase of HCl concentration, temperature and discharge voltage. It has also been found that O2, H2O and fly ash showed a light promotion on the removal of Hg0 while NO and SO2 had a slight inhibition effect on Hg0 oxidation. Furthermore, the novel discharge activation reactor could improve the generation of reactive chemical species, such as Cl· or Cl2, which facilitated the mercury removal. At the reaction temperature of 413 K, about 60% Hg0 could be removed from simulated flue gas under 25.0 kV in presence of 10.0 ppmv HCl. When the novel discharge activation reactor was used, the Hg0 removal efficiency increased to about 80% at the same experimental conditions. It appeared to be a promising technique to enhance the removal of Hg0 by ESP.

To improve the ability of CeO2/TiO2 catalysts to catalyze the oxidation of gaseous elemental mercury, silver was introduced. Doping with Ag can significantly enhance the Hg0 oxidation ability of CeO2/TiO2. In addition, the temperature window was widened (from 150 to 450 °C). The catalysts were characterized by TEM, XRD, XPS and H2-TPR. The results indicated that silver nanoparticles can be loaded on the TiO2 support. The catalysts had better crystallization and higher redox ability after addition of silver. Silver existed mostly in its metallic state, which can keep Ce in a higher Ce(IV) state. HCl was oxidized into active Cl by CeO2 and then was adsorbed on the silver nanoparticles. In addition to the HCl and Hg0 breakthrough experiments, a Hg0 desorption experiment and a Cl2 yield experiment were conducted to study the catalytic mechanisms of elemental mercury oxidation over various temperature ranges; these experiments indicated that the reaction followed the Langmuir–Hinshelwood mechanism at low temperature, and the Eley–Rideal mechanism and homogeneous gas-phase reaction at high temperature. Furthermore, a mercury valence state change experiment was performed, which indicated that HCl was the major catalytic oxidization component.

MnOx/graphene composites were prepared and employed to enhance the performance of manganese oxide (MnOx) 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 MnOx particles could be readily deposited on graphene nanosheets via hydrothermal process described here. Graphene appeared to be an ideal support for MnOx particles and electron transfer channels in the catalytic oxidation of Hg0 at a high efficiency. Thus, MnOx/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 MnOx. The mechanism of Hg0 capture is discussed, and the main Hg0 capture mechanisms of MnOx/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 MnOx, which is beneficial for catalytic oxidation. Furthermore, MnOx/graphene exhibited an excellent regenerative ability, and is a promising sorbent for capturing Hg0.

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. Ce0.5Mn0.5Oy 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 MnOy. 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.

AgI–TiO2 was employed for the removal of elemental mercury (Hg0) from flue gas, and extra elemental silver (Ag) was introduced to enhance the catalytic activity and stability. AgI–TiO2 displayed an excellent effect on Hg0 catalytic oxidation, and the Hg0 oxidation efficiency was almost 100% with only 5 ppm HCl at 350 °C, which was better than that of KI–Ti. Adding Ag to AgI–TiO2 can notably prolong the period of high efficiency, and the Hg0 oxidation efficiency was still above 90% after 10 h with only 2% Ag added. Doping with silver could suppress the decomposition of AgI and the loss of iodine, which maintains the stability of the catalyst performance. In addition, HCl was readily adsorbed and activated by the silver. The iodine in Ag(2%)–AgI–Ti mainly acted as an accelerant for Hg0 oxidation by facilitating formation of the intermediate Hg–I*; then, chlorine can further convert the intermediate to HgCl2 as the final product. In addition, the thermogravimetric (TG) analysis proved that Ag(2%)–AgI–Ti showed a good stability at high temperature. Furthermore, the ion chromatogram tests also showed the chemical stability of AgI–Ti in the presence of Ag.

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.Keywords: cation exchange; mercury removal; nanocrystals; size-dependent; ZnS sorbents

The slip ammonia from selective catalytic reduction (SCR) of NOx 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/Ce0.6Zr0.4O2(polyvinylpyrrolidone (PVP)) catalyst displayed significant catalytic activity and the slip ammonia was almost completely removed with the coexistence of NOx 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/Ce0.6Zr0.4O2(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.

To improve the catalytic oxidation ability for gaseous elemental mercury (Hg0), silver was introduced to V2O5–TiO2 catalysts. The catalysts were prepared by an impregnation method with various additives to obtain well distributed silver nanoparticles on the carrier. It was found that doping silver onto V2O5–TiO2 can significantly improve the catalytic oxidation efficiency of Hg0, and the redox temperature range for Hg0 oxidation was enlarged markedly (150–450 °C). The addition of polyvinylpyrrolidone (PVP) during the preparation of the catalysts can improve the dispersion of silver nanoparticles more effectively, which resulted in a higher Hg0 oxidation efficiency up to 90%. However, the oxidation of Hg0 on the catalyst was slightly inhibited due to the larger silver nanoparticles when the ionic liquid (IL) [bmim][BF4] was used as the additive. The characterization results indicated that V can be induced to a higher oxidation state in the presence of silver nanoparticles, and the transformation trend of TiO2 from the anatase to rutile phase caused by Ag can be minimized in the presence of PVP or ILs. Meanwhile, the mechanisms of the elemental mercury oxidation at various temperature ranges were discussed.

Porous ZIF-7 with the sodalite (SOD) cage structure (ZIF, Zeolitic imidazolate framework) were synthesized by the solvothermal method. Synthesized material was characterized by powder X-ray diffraction (PXRD), thermal gravity (TG), scanning electron microscopy (SEM) and N2 adsorption analysis. ZIF-8 with the SOD structure and a little larger pore window was synthesized in a similar way and was characterized for comparisons. Thermal stability and structural stability of ZIF-7 were tested through PXRD analysis, and the capability of the material for CO2 capture from simulated flue gas was investigated through physical adsorption method. The results showed that CO2 adsorption capacity on ZIF-7 was about 48 mL·g−1 while the capacity on ZIF-8 was about 18 mg·g−1 (at 12°C and 0.98 P/P0 relative pressure). Furthermore, the impact of flue gas components on adsorption capacity of ZIF-7 and the selectivity of CO2 against N2 on ZIF-7 was also investigated in this work.

In this work, a catalytic membrane using Mn/Mo/Ru/Al2O3 as the catalyst was employed to remove elemental mercury (Hg0) from flue gas at low temperature. Compared with traditional catalytic oxidation (TCO) mode, Mn/Al2O3 membrane catalytic system had much higher removal efficiency of Hg0. After the incorporation of Mo and Ru, the production of Cl2 from the Deacon reaction and the retainability for oxidants over Mn/Al2O3 membrane were greatly enhanced. As a result, the oxidization of Hg0 over Mn/Al2O3 membrane was obviously promoted due to incorporation of Mo and Ru. In the presence of 8 ppmv HCl, the removal efficiency of Hg0 by Mn/Mo/Ru/Al2O3 membrane reached 95% at 423 K. The influence of NO and SO2 on Hg0 removal were insignificant even if 200 ppmv NO and 1000 ppmv SO2 were used. Moreover, compared with the TCO mode, the Mn/Mo/Ru/Al2O3 membrane catalytic system could remarkably reduce the demanded amount of oxidants for Hg0 removal. Therefore, the Mn/Mo/Ru/Al2O3 membrane catalytic system may be a promising technology for the control of Hg0 emission.

Fe–Ti–V spinel showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 250–400 °C, and an external magnetic field can effectively prevent the emission of a vanadium based catalyst to the environment due to its magnetization.

Nonstoichiometric Fe−Ti spinel (Fe3-xTix)1-δO4 has a large amount of cation vacancies on the surface, which may provide active sites for pollutant adsorption. Meanwhile, its magnetic property makes it separable from the complex multiphase system for recycling, and for safe disposal of the adsorbed toxin. Therefore, (Fe3-xTix)1-δO4 may be a promising sorbent in environmental applications. Herein, (Fe3-xTix)1-δO4 is used as a magnetically separable sorbent for elemental mercury capture from the flue gas of coal-fired power plants. (Fe2Ti)0.8O4 shows a moderate capacity (about 1.0 mg g−1 at 250 °C) for elemental mercury capture in the presence of 1000 ppmv of SO2. Meanwhile, the sorbent can be readily separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed mercury.Keywords (keywords): capture capacity; cation vacancy; elemental mercury; Fe−Ti spinel; magnetic sorbent

A series of nanosized (Fe3-xMnx)1-δO4 (x = 0, 0.2, 0.5, and 0.8) were synthesized for elemental mercury capture from the flue gas. Cation vacancies on (Fe3-xMnx)1-δO4 can provide the active sites for elemental mercury adsorption, and Mn4+ cations on (Fe3-xMnx)1-δO4 may be the oxidizing agents for elemental mercury oxidization. With the increase of Mn content in the spinel structure, the percents of Mn4+ cations and cation vacancies on the surface increased. As a result, elemental mercury capture by (Fe3-xMnx)1-δO4 was obviously promoted with the increase of Mn content. (Fe2.2Mn0.8)1-δO4 showed an excellent capacity for elemental mercury capture (>1.5 mg g−1 at 100−300 °C) in the presence of SO2 and HCl. Furthermore, (Fe2.2Mn0.8)1-δO4 with the saturation magnetization of 45.6 emu g−1 can be separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed Hg. Therefore, nanosized (Fe2.2Mn0.8)1-δO4 may be a promising sorbent for the control of elemental mercury emission.

In order to overcome the shortcomings of the traditional catalytic oxidation (TCO) mode for the conversion of the trace level of elemental mercury (Hg0) in flue gas, we put forward a novel and unique assembly that integrated membrane delivery with catalytic oxidation systems (MDCOs), which combined the controlled delivery of oxidants with the catalytic oxidation of Hg0. The results show that the demanded HCl for Hg0 conversion in the MDCOs was less than 5% of that in the TCO mode, and over 90% of Hg0 removal efficiency can be obtained in the MDCOs with less than 0.5 mg m−3 of HCl escaped. Meanwhile, the inhibition of SO2 to Hg0 catalytic conversion in the MDCOs was also less significant than in the TCO. The MDCOs have high retainability for HCl, which is quite favorable to Hg0 conversion and HCl utilization. The reaction mechanism on mercury conversion in the MDCOs is discussed. The MDCOs appear to be a promising method for emission control of elemental mercury.

Catalytic conversion of elemental mercury (Hg0) to its oxidized form has been considered as an effective way to enhance mercury removal from coal-fired power plants. In order to make good use of the existing selective catalytic reduction of NOx (SCR) catalysts as a cobenefit of Hg0 conversion at lower level HCl in flue gas, various catalysts supported on titanium dioxide (TiO2) and commercial SCR catalysts were investigated at various cases. Among the tested catalysts, ruthenium oxides (RuO2) not only showed rather high catalytic activity on Hg0 oxidation by itself, but also appeared to be well cooperative with the commercial SCR catalyst for Hg0 conversion. In addition, the modified SCR catalyst with RuO2 displayed an excellent tolerance to SO2 and ammonia without any distinct negative effects on NOx reduction and SO2 conversion. The demanded HCl concentration for Hg0 oxidation can be reduced dramatically, and Hg0 oxidation efficiency over RuO2 doped SCR catalyst was over 90% even at about 5 ppm HCl in the simulated gases. Ru modified SCR catalyst shows a promising prospect for the cobenefit of mercury emission control.

A stoichiometric nanosized Mn–Fe spinel (Fe2.2Mn0.8O4) was synthesized using a coprecipitation method. After the thermal treatment at 400 °C under air, chemical heterogeneity deriving from the oxidation kinetic difference between Fe2+ and Mn2+/Mn3+ was observed in (Fe2.2Mn0.8)1-δO4. XPS and TEM analyses both pointed a Mn enrichment (especially Mn4+ cation) on the particle’s surface. Furthermore, the percent of cation vacancy on the surface increased obviously due to the enrichment of Mn4+ cation on the surface. As a result, the capacity of (Fe2.2Mn0.8)1-δO4-400 for elemental mercury capture was generally much better than those of MnOx/γ-Fe2O3, (Fe2.2Mn0.8)1-δO4-200 and Fe2.2Mn0.8O4. Furthermore, the saturation magnetization of (Fe2.2Mn0.8)1-δO4 obviously increased after the thermal treatment under air at 400 °C, which made it easier to separate the sorbent and adsorbed mercury from the fly ash for recycling, regeneration, and safe disposal of the adsorbed mercury. Therefore, (Fe2.2Mn0.8)1-δO4-400 may be a promising sorbent for elemental mercury capture.

A novel magnetic Fe–Ti–V spinel catalyst showed an excellent performance for elemental mercury capture at 100 °C, and the formed HgO can be catalytically decomposed by the catalyst at 300 °C to reclaim elemental mercury and regenerate the catalyst.

In order to facilitate the removal of elemental mercury (Hg0) from coal-fired flue gas, catalytic oxidation of Hg0 with manganese oxides supported on inert alumina (α-Al2O3) was investigated at lower temperatures (373−473 K). To improve the catalytic activity and the sulfur-tolerance of the catalysts at lower temperatures, several metal elements were employed as dopants to modify the catalyst of Mn/α-Al2O3. The best performance among the tested elements was achieved with molybdenum (Mo) as the dopant in the catalysts. It can work even better than the noble metal catalyst Pd/α-Al2O3. Additionally, the Mo doped catalyst displayed excellent sulfur-tolerance performance at lower temperatures, and the catalytic oxidation efficiency for Mo(0.03)−Mn/α-Al2O3 was over 95% in the presence of 500 ppm SO2 versus only about 48% for the unmodified catalyst. The apparent catalytic reaction rate constant increased by approximately 5.5 times at 423 K. In addition, the possible mechanisms involved in Hg0 oxidation and the reaction with the Mo modified catalyst have been discussed.

Sulfur monobromide (S2Br2) was employed as a task-specific oxidant to capture and stabilize elemental mercury from coal-fired flue gas. Its performances on the removal of Hg0 were investigated with respect to the gas-phase reaction and particle-involved reactions. It was found that the gas-phase reaction between Hg0 and S2Br2 was rapid, and the determined second-rate constant was about 1.2(±0.2) × 10−17cm3 molecules−1 s−1 at 373 K, which was about 30 times higher than that with sulfur monochloride. The pilot tests showed that the presence of fly ash in flue gas can accelerate the removal of Hg0 significantly. It was predicted that about 90% of Hg0 removal efficiency can be obtained with 0.6 ppmv S2Br2 and 30 g/m3 fly ash in flue gas, and the unburned carbon in fly ash played an important role for Hg0 removal. The fates of S2Br2 and mercury in the process were evaluated, and the product analysis and leaching tests indicated that mercuric sulfide was the main product of the converted Hg0 by the direct reaction and consequent series reactions, which is more stable and less toxic than other mercury species. Also, the surplus S2Br2 in flue gas could be captured and neutralized effectively by the alkali components in fly ash or FGD liquor, and its hydrolysis products (elemental sulfur and sulfide) were also helpful to the stabilization of mercury. The result indicated that S2Br2 is a promising oxidant for elemental mercury (Hg0) oxidation and stabilization for mercury emission control.

The equilibria and kinetics of the reaction between bromine (Br2) and chlorine (Cl2) to form bromine chloride (BrCl) at various temperatures were determined. BrCl was employed to oxidize elemental mercury (Hg0) under simulated flue gas conditions. The removal of Hg0 from the gas phase by a homogeneous gas-phase oxidation reaction and the heterogeneous reactions involving flyash were investigated. The second-order gas phase rate constant was determined to be 2.3(±0.2) × 10−17 cm3·molecules−1·s−1 at 373K. The reaction of Hg0/BrCl was significantly accelerated in the presence of flyash, and the estimated Hg0 removal efficiency in the presence of 0.6 ppmv BrCl and 20 g/m3 flyash was up to 90%. Unexpectedly, the major product was found to be HgCl2, rather than HgBr2, indicating that bromine in part acted as the accelerant in Hg0 oxidation in BrCl/Br2/Cl2 system by facilitating the formation of intermediates. As a result, bromine consumption is much less than if only bromine gas is utilized alone. These results were helpful not only for understanding the mechanism of Hg0 removal in coal-fired flue gas but also in any atmosphere in which bromine and chlorine species coexist.

MnOx/Al2O3 catalysts (i.e., impregnating manganese oxide on alumina) were employed to remove elemental mercury (Hg0) from flue gas. MnOx/Al2O3 was found to have significant adsorption performance on capturing Hg0 in the absence of hydrogen chloride (HCl), and its favorable adsorption temperature was about 600 K. However, the catalytic oxidation of Hg0 became dominant when HCl or chlorine (Cl2) was present in flue gas, and the removal efficiency of Hg0 was up to 90% with 20 ppm of HCl or 2 ppm of Cl2. In addition, the catalysts with adsorbed mercury could be chemically regenerated by rinsing with HCl gas to strip off the adsorbed mercury in the form of HgCl2. Sulfur dioxide displayed inhibition to the adsorption of Hg0on the catalysts, but the inhibition was less to the catalytic oxidation of Hg0, especially in the presence of Cl2. The analysis results of XPS and pyrolysis−AAS indicated that the adsorbed mercury was mainly in the forms of mercuric oxide (HgO) and the weakly bonded speciation, and the ratio of them varied with the adsorption amount and manganese content on catalysts. The multifunctional performances of MnOx/Al2O3 on the removal of Hg0 appeared to be promising in the industrial applications.

Sulfur-containing organic compounds, such as mercaptans, are widely distributed in petroleum. A novel refining method for the degradation of mercaptans in petroleum was put forward and investigated by means of γ-irradiation in this paper. Dodecanethiol (DCT) dissolved in dodecane was employed as the simulation of petroleum. The results showed that the chemical rate constants for the degradation reaction could be described with the first-order rate model. Irradiation with low-dose rate gave rise to a high removal efficiency and a high energy efficiency when the radiation dose was the same. The degradation process was improved greatly by oxygen, and the demanded irradiation dose was reduced markedly. Some chemicals were selected as radio-sensitizers to improve the degradation of dodecanethiol. The degradation processes were enhanced by acetone and carbon tetrachloride, and the degradation efficiencies increased by over 25%, respectively. The main degradation product of dodecanethiol was didodecyl disulfide. Meanwhile, the amount of degradation for dodecane was slight. It would be a potential alternative to degradation of mercaptan compounds in petroleum by γ-irradiation.

•Hg-TPD method was used for speciation of mercury species.•Different elements modified MnOx have different mercury binding state.•Understanding mercury existed state was beneficial for designing novel materials.Mercury temperature-programmed desorption (Hg-TPD) method was employed to clarify mercury species over Mn-based oxides. The elemental mercury (Hg0) removal mechanism over MnOx was ascribed to chemical-adsorption. HgO was the primary mercury chemical compound adsorbed on the surface of MnOx. Rare earth element (Ce), main group element (Sn) and transition metal elements (Zr and Fe) were chosen for the modification of MnOx. Hg-TPD results indicated that the binding strength of mercury on these binary oxides followed the order of Sn-MnOx < Ce-MnOx ∼ MnOx < Fe-MnOx < Zr-MnOx. The activation energies for desorption were calculated and they were 64.34, 101.85, 46.32, 117.14, and 106.92 eV corresponding to MnOx, Ce-MnOx, Sn-MnOx, Zr-MnOx and Fe-MnOx, respectively. Sn-MnOx had a weak bond of mercury (Hg-O), while Zr-MnOx had a strong bond (HgO). Ce-MnOx and Fe-MnOx had similar bonds compared with pure MnOx. Moreover, the effects of SO2 and NO were investigated based on Hg-TPD analysis. SO2 had a poison effect on Hg0 removal, and the weak bond of mercury can be easily destroyed by SO2. NO was favorable for Hg0 removal, and the bond strength of mercury was enhanced.Download high-res image (115KB)Download full-size image

To improve the ability of CeO2/TiO2 catalysts to catalyze the oxidation of gaseous elemental mercury, silver was introduced. Doping with Ag can significantly enhance the Hg0 oxidation ability of CeO2/TiO2. In addition, the temperature window was widened (from 150 to 450 °C). The catalysts were characterized by TEM, XRD, XPS and H2-TPR. The results indicated that silver nanoparticles can be loaded on the TiO2 support. The catalysts had better crystallization and higher redox ability after addition of silver. Silver existed mostly in its metallic state, which can keep Ce in a higher Ce(IV) state. HCl was oxidized into active Cl by CeO2 and then was adsorbed on the silver nanoparticles. In addition to the HCl and Hg0 breakthrough experiments, a Hg0 desorption experiment and a Cl2 yield experiment were conducted to study the catalytic mechanisms of elemental mercury oxidation over various temperature ranges; these experiments indicated that the reaction followed the Langmuir–Hinshelwood mechanism at low temperature, and the Eley–Rideal mechanism and homogeneous gas-phase reaction at high temperature. Furthermore, a mercury valence state change experiment was performed, which indicated that HCl was the major catalytic oxidization component.

A novel magnetic Fe–Ti–V spinel catalyst showed an excellent performance for elemental mercury capture at 100 °C, and the formed HgO can be catalytically decomposed by the catalyst at 300 °C to reclaim elemental mercury and regenerate the catalyst.

Fe–Ti–V spinel showed excellent SCR activity, N2 selectivity and H2O/SO2 durability at 250–400 °C, and an external magnetic field can effectively prevent the emission of a vanadium based catalyst to the environment due to its magnetization.

To improve the catalytic oxidation ability for gaseous elemental mercury (Hg0), silver was introduced to V2O5–TiO2 catalysts. The catalysts were prepared by an impregnation method with various additives to obtain well distributed silver nanoparticles on the carrier. It was found that doping silver onto V2O5–TiO2 can significantly improve the catalytic oxidation efficiency of Hg0, and the redox temperature range for Hg0 oxidation was enlarged markedly (150–450 °C). The addition of polyvinylpyrrolidone (PVP) during the preparation of the catalysts can improve the dispersion of silver nanoparticles more effectively, which resulted in a higher Hg0 oxidation efficiency up to 90%. However, the oxidation of Hg0 on the catalyst was slightly inhibited due to the larger silver nanoparticles when the ionic liquid (IL) [bmim][BF4] was used as the additive. The characterization results indicated that V can be induced to a higher oxidation state in the presence of silver nanoparticles, and the transformation trend of TiO2 from the anatase to rutile phase caused by Ag can be minimized in the presence of PVP or ILs. Meanwhile, the mechanisms of the elemental mercury oxidation at various temperature ranges were discussed.