Mars-Maessen Mechanism Research Articles

Structure-activity relationships, product species distribution and the mechanism of effect of multi-component flue gas on Hg0 adsorption and oxidation over CuO/ACs.

A series of Cu-doped activated cokes (CuO/ACs) were synthesized via an impregnation method and applied for the removal of elemental mercury (Hg0). Structure-activity relationships between Hg0 removal and CuO/AC surface characteristics were identified. Hg0 removal over CuO/AC occurs through a combination of physisorption and chemisorption and is mainly dominated by chemisorption. It was found that 1 nm micropores facilitate Hg0 physisorption. Hg0 could weakly adsorb onto an O-terminated crystal layer, whereas strongly adsorb onto Cu-terminated single highly dispersed, clustered and bulk CuO (110) crystal planes via the Mars-Maessen mechanism. Product distributions and mechanisms of Hg0 adsorption and oxidation over the CuO/AC catalyst under multi-component flue gases are also discussed. O2 enhances both physisorption and chemisorption toward Hg0 by 38%. Inhibition of Hg0 removal by SO2 originates from the competitive adsorption and deactivation of CuO cation vacancies, whereas the impact is weakened by O2 through generating 20% of physically adsorbed mercury product species. NO and O2 promote Hg0 chemisorption efficiency by 93% to form Hg(NO3)2. HOCl and/or Cl2 produced by HCl can oxidize 100% of Hg0 to HgCl2, and the catalytic oxidation efficiency is approximately 29%, but O2 slightly lowers the Hg0 catalytic oxidation efficiency by 8%. The affinity ability between various flue gases and Hg0 follows the order O2 < NO < HCl.

Unrefined low-grade pyrolusite for elemental mercury removal from flue gas: Experimental and DFT insights

Dual characteristics of oxidation capacity and economic advantage endow the unrefined low-grade pyrolusite (LGP) as a potential adsorbent/catalyst. This paper successfully applied it to remove Hg0 from flue gas, by which 98.6% of removal efficiency was obtained without chemical modification or external oxidant. Effects of various flue gas components on the Hg0 removal were investigated, and the synchronous improvement in efficiency and stability by HCl was revealed. Also, its reusability was evaluated and the decrease in removal efficiency after 5th cycles is less than 6%. Product analysis indicated that over 90% of Hg0 was chemisorbed as HgO without HCl, but the presence of HCl reversed the pathway to form HgCl2 that easily escape from the surface, with a proportion of about 95%. By combing experiments, characterizations and theoretical calculations, the mechanism was preliminarily speculated: (i) without HCl, O2 and the main components of LGP (MnOx, Fe2O3, etc.) involved in the catalytic oxidation according to Mars-Maessen mechanism; (ii) HCl tended to dissociate and adsorb on the MnO2 to form the chlorinated surface, and the mercury removal followed both L-H and E-R mechanism, although E-R was dominant; (iii) the formation of HgCl lowered the energy barrier of HgCl2 desorption, thereby expediting the oxidation-removal of Hg0.

Potential of iron-based composites derived from sucrose foam for mercury removal and safe recovery

Although various methods for removal of elemental mercury from gas streams have been proposed, the control of mercury emissions to the environment from anthropogenic sources, mainly coal-fired power plants, remains unresolved in many countries worldwide. Meanwhile, the Minamata Convention and the IED of the European Commission continue to impose restrictions for its control and mitigation. In this work, a new material based on a sucrose foam impregnated with iron oxide nanoparticles has been developed for the adsorption of elemental mercury from flue gases allowing the regeneration of the material and the recovery of elemental mercury. Of the different iron oxides supported on the sucrose foam, αFe2O3 nanoneedles were found to be the most efficient in capturing mercury through a Mars-Maessen mechanism and the forming of HgO. The adsorbents developed proved to be effective in the presence of SO2 and H2O, with the O2 present in the gas playing a key role in the regeneration of the material. From the results found sucrose foam-based αFe2O3 composite could be an attractive alternative to traditional non-regenerable activated carbons, reducing costs and being environmentally sustainable.

Revealing insights into the correlations between structure and performance of CrOx monometallic oxide for Hg0 capture

This study explored the impacts of the synthesis methods on the Hg0 elimination ability of CrOx adsorbents. In the sample series, the adsorbent prepared using the sol-gel method (labeled as Cr(S)) presented the best Hg0 capture performance; its Hg0 removal efficiency stayed above 88% at 150 and 200 °C and even rose to ∼97% at 250 °C in the entire 5-h Hg0 capture process. The presence of bountiful surface acid sites aided the sufficient adsorption of Hg0 and an enhancement in the oxidizability primarily accelerated the subsequent Hg0 oxidation step, which constituted the central reason for the satisfactory Hg0 capture capability of Cr(S). The Mars-Maessen mechanism assisted in understanding the Hg0 capture process over Cr(S), in which chemisorption was dominant with Cr6+ and Oα serving as the active sites and HgO formed as the final oxidation product.

Revealing Insights into the Mechanism of Hg0 Removal over MnO2 Hollow Sphere Adsorbents: Effect of Calcination Temperatures

In this study, MnO2 hollow spherical adsorbents were synthesized using the template approach, and the impacts of calcination temperatures on their physicochemical parameters and Hg0 removal ability were investigated. The adsorbent calcined at 300 °C (labeled as Mn300) demonstrated the highest performance; at 200 °C, the Hg0 removal efficiency reached over 90%. The presence of surface acid sites promoted the physisorption of Hg0 and compensated for the relatively weak oxidative ability, which made much of the Hg0 susceptible to capture on the adsorbent surface and consequently explained Mn300’s superior Hg0 capture capacity. The Mars–Maessen mechanism helped to understand the Hg0 capture pathways over Mn300, with Mn4+ and chemisorbed oxygen serving as the active species to oxidize Hg0 to HgO. Finally, Mn300 demonstrated reasonable recyclability and a rather weak gas-component adaptability, indicating that additional modification is required to improve the practical use of the adsorbent.

The novel magnetic adsorbent derived from MIL-100 (Fe) loading with bimetallic Cu and Mn oxides for efficient Hg0 removal from flue gas

A novel magnetic adsorbent derived from MIL-100(Fe) loading with bimetallic Cu and Mn oxides, namely CuMnOx/MIL-100(Fe), was easily synthesized and used to remove elemental mercury (Hg0) from flue gas. Different mass loadings of CuMnOx (3%, 6%, and 9%) were impregnated on MIL-100(Fe) and calcinated at 200–500 °C to form CuMnOx/MIL-100(Fe), which was used to inspect Hg0 removal efficiencies at different reaction temperatures (40–200 °C). The Hg0 removal performance was studied through a variety of characterizations, including XRD, BET, SEM-EDS, TGA, XPS, VSM, H2-TPR, and Hg-TPD. 6%CuMnOx/MIL-100(Fe) calcined at 500 °C had a superior Hg0 removal efficiency of 92.4% and capacity of 1.16 mg/g at 120 °C, indicating that high-temperature calcination and bimetal loading were capable of improving the catalytic activity of MIL-100(Fe) and thereby improving the removal efficiency of Hg0 at a relatively low operating temperature of 120 °C. Under different flue conditions, the increase in O2 and NO concentrations can obviously promote the removal efficiency of Hg0, while SO2 and H2O had an opposite trend. The Hg0 removal efficiency remained above 90% after 8 cycles, which exhibits potential for industrial application. Furthermore, the Hg0 removal mechanism was attributed to the reaction with chemisorbed oxygen or lattice oxygen to form HgO on CuMnOx/MIL-100(Fe), which follows the Langmuir-Hinshelwood and Mars-Maessen mechanism.

Catalytic performance and sulfur resistance of OMS-2 modified by copper for mercury removal at low temperature

Cu-doped manganese oxide molecular sieve catalysts (Cu-OMS-2) are prepared by hydrothermal synthesis method, and their Hg0 removal ability and sulfur resistance are investigated experimentally under simulated flue gas conditions. It indicated that appropriate proportion of Cu doped could increase both the content of Mn4+ active sites on the catalyst surface and the BET specific surface area. The maximum specific surface area of these catalysts is 260.49 m2/g, with the Cu/Mn molar ratio of 0.025. A highest Hg0 removal efficiency of 93.6 % is achieved by the catalyst with Cu/Mn molar ratio of 0.05. SO2 in flue gas significantly inhibits the Hg0 removal ability of OMS-2-H120, but its suppression ability is relatively weak after the addition of Cu to OMS-2-H120. The Hg0 removal efficiency of Cu0.05-OMS-2 can reach 92 % even in the presence of 1000 ppm SO2. The doping of Cu on OMS-2 enhances SO2 adsorbed by CuO, inhibiting the reaction of SO2 with the active center on the catalyst surface. It reduces the generation of sulfate and enhances the sulfur resistance ability of Cu0.05-OMS-2. The process of Hg0 oxidation by O2 over Cu0.05-OMS-2 is found to follow Mars-Maessen mechanism based on the characteristics of fresh and spent catalysts. Elemental mercury is firstly chemisorbed on the Cu0.05-OMS-2 catalyst surface and then oxidized.

Efficient MnFeW/Ti catalyst for simultaneous removal of NO and Hg0 in flue gas at low temperature: Effect of NO on Hg0 oxidation and reaction mechanism

The MnFeW/Ti catalyst was synthesized and used for simultaneously removing NO and Hg0 from flue gas at low temperature and its catalytic performance has been tested. The MnFeW/Ti catalyst showed over 80% NOx conversion and 88% Hg0 removal between 180 and 300 °C at a high GHSV of 100000 h−1. NO obviously enhanced the catalytic capability for Hg0 removal and NH3 had almost no effect on Hg0 removal because NH3 did not affect the adsorption of NO and Hg0. Hg balance results revealed that Hg0 removal over MnFeW/Ti catalyst proceeded via both chemisorption and catalytic oxidation, which is attributed to the different bonding strength between Hg0 and active sites. The Hg0 oxidation process was also investigated through transient reaction experiments along with Hg-TPD and XPS characterization. Hg0 oxidation over MnFeW/Ti catalyst obeys both Mars–Maessen mechanism in which active oxygen reacts with adsorbed Hg0 and Langmuir − Hinshelwood mechanism comprising the reaction of formed NO2 with adsorbed Hg0. HgO, resulted from the reaction between NO2 as an active intermediate and adsorbed Hg0, is liable to desorb from MnFeW/Ti catalyst.

Complete catalytic reaction of mercury oxidation on CeO2/TiO2 (001) surface: A DFT study

CeO2/TiO2 catalyst is a promising material for realizing the integration of denitrification and mercury removal to reduce mercury emissions. Oxidation mechanism of Hg0 on CeO2/TiO2 (001) surface in the presence of HCl and O2 was studied by density functional theory (DFT). The results indicated that Hg0 was physically adsorbed on CeO2/TiO2 (001) surface. As an important intermediate, HgCl was adsorbed on the surface of CeO2/TiO2 (001) utilizing enhanced chemisorption, while the adsorption energy of HgCl2 was only −57.05 kJ/mol. In the absence of HCl, mercury oxidation followed the Mars-Maessen mechanism with a relatively high energy barrier, and the product (HgO) was difficult to desorb, which hindered the reaction process. When HCl existed, reactive chlorine (Cl*) would be produced by the dissociation of HCl, and the mercury oxidation would follow the Langmuir-Hinshelwood mechanism. The co-existence of HCl and O2 had no significant effect on the adsorption of Hg0, but reduced the reaction energy barrier and the final product (HgCl2) was more easily desorbed from the catalyst surface. In addition, two complete cyclic reaction pathways for catalytic oxidation of Hg0 on CeO2/TiO2 (001) surface were constructed to clarify the detailed reaction process.

Dual 2-dimensional CuSe/g-C3N4 nano-heterostructure for boosting immobilization of elemental mercury in flue gas

With the rapid development of Hg0 treatment methods in flue gas, the key challenge at the current stage is the trade-off between adsorbent performance, treatment cost and environmental friendliness. A series of dual 2-dimensional CuSe/g-C3N4 nano-heterostructures (CSCN) were successfully synthesized through an incipient wetness impregnation method followed by in-situ selenization, and further used for gaseous elemental mercury immobilization. The results had shown that the as-synthesized CSCN with CuSe loading ratio of 20% (20CSCN) achieved the maximum mercury removal performance. Up to 95.09% of Hg0 removal efficiency was achieved by 20CSCN under rigorous experimental conditions with the gas hourly space velocity and initial Hg0 concentration of 240000 h−1 and 180 ug/m3, respectively. After a 10 h adsorption test, the equilibrium adsorption capacity of 20CSCN reached 9.113 mg/g. Owing to the generation of dual 2-dimensional heterojunctions, the mercury removal performance of 20CSCN was superior to the contrast 20CSCN(mix) prepared by simply mechanically mixing. Besides, the acidic gases (NO and SO2) in the flue gas will adversely affect the Hg0 capture performance of CSCN, while O2 and CO2 exhibited a neutral effect on the Hg0 capture. Based on the analysis of XPS and TPD results, the chemical reaction rate was the main rate-controlling step and the chemisorption dominated the conversion of Hg0 on heterojunctions, which was in accordance with the Mars-Maessen mechanism. The active Se ligands on the surface of the heterojunction, as the main activity center of chemical reaction, could rapidly capture Hg0 in the flue gas to form stable HgSe. This can effectively avoid the secondary pollution caused by the re-release of Hg0 from Hg-containing waste exposed to the environment, because of its low Hg-leaching characteristic.

Influence of Mo doping on mercury capture and SO2 tolerance of MoxFe6Mn1-xOy magnetic sorbent

Molybdenum (Mo) was doped to Fe-Mn magnetic sorbent (named MoxFe6Mn1-xOy) through co-precipitation method to enhance the SO2 tolerance for mercury capture. The influence of Mo doping on mercury capture and SO2 tolerance of MoxFe6Mn1-xOy sorbent was studied in a fixed-bed experimental device. Temperature programmed mercury desorption (Hg-TPD) test was also conducted to analyze the mercury capture product on the used sorbent. Combing with sorbent characterization, the mechanism of mercury capture and resistance to SO2 poisoning of MoxFe6Mn1-xOy sorbent was explored. The results show that the magnetic Mo0.2Fe6Mn0.8Oy sorbent is an excellent mercury capture sorbent with high SO2 tolerance. O2 co-existence with SO2 weakens the inhibition from SO2 on mercury capture, but H2O intensifies SO2 poisoning. Mo existing in Mo0.2Fe6Mn0.8Oy sorbent is not active sites for mercury capture, but Mo doping modifies the pore channel and uniformly disperses the Mn4+ active sites. Mo doping can also promote the oxidation of SO2 to SO3, which is beneficial to mercury capture. More importantly, after Mo doping, the SO2 tolerance of Mo0.2Fe6Mn0.8Oy sorbent is significantly improved because of a lager affinity of sulfur to Mo. The Hg-TPD test indicates that HgO is the main mercury capture product. Therefore, it can be determined that most of Hg0 is adsorbed on MnO2 in Mo0.2Fe6Mn0.8Oy via Mars-Maessen mechanism. The existence of SO2 promotes part of Hg2SO4 formation and the co-existence of H2O leads to more Hg2SO4 formation.

Nanosized Cu-In spinel-type sulfides as efficient sorbents for elemental mercury removal from flue gas

Mercury emitted from human activities has received increasing attention because of its extreme toxicity, persistence and bioaccumulation. The development of highly-efficient sorbent with abundant active sites that exhibit high affinity toward Hg0 is the key challenge for elemental mercury capture at low temperature. Herein, Cu-In spinel-type sulfides were synthesized through a hydrothermal synthesis. The Hg0 removal performance of CuxIn2-xS2 sorbents was evaluated in the temperature range of 75 °C to 175 °C. The synthesized CuxIn2-xS2 sorbents showed excellent performance for Hg0 removal at low temperatures, which perfectly matches the optimal temperature of flue gas at the downstream of desulfurization system. Hg0 removal efficiency of CuxIn2-xS2 sorbents significantly improved as the Cu proportion increased. CuInS2 sorbent showed superior mercury removal performance, the mercury removal efficiency reached 99.6% at 125 °C. O2 and NO showed a slight inhibition on Hg0 capture. The coexistence of SO2 and H2O showed no obvious negative effects on Hg0 removal. The CuInS2 sorbent displayed a superior tolerance to SO2 and H2O. TPD and XPS analyses demonstrated that the adsorbed mercury mainly existed in the form of mercuric sulfides (HgS). Hg0 adsorption over CuInS2 sorbent occurred via the Mars-Maessen mechanism. In this mechanism, Hg0 vapor was physically adsorbed on CuInS2 sorbent and then converted to HgS. This study provides future potential for applying CuxIn2-xS2 sorbents to capture gaseous mercury at low temperature.

SO2 Tolerance and Mechanism of Elemental Mercury Removal from Flue Gas by a Magnetic Recyclable Fe6Mn0.8Ce0.2Oy Sorbent

A Ce-doped Fe–Mn magnetic sorbent (Fe₆Mn₀.₈Ce₀.₂Oy) was developed using the coprecipitation method. The effects of O₂, H₂O, flue gas temperature, SO₂ concentration, and Ce doping on the SO₂ tolerance of the Fe₆Mn₀.₈Ce₀.₂Oy sorbent for mercury removal were explored in a fixed-bed system. Combined with mercury temperature-programmed desorption (Hg-TPD) and X-ray photoelectron spectroscopy (XPS) analysis, the mechanism of mercury removal by the Fe₆Mn₀.₈Ce₀.₂Oy sorbent in the presence of SO₂ was analyzed. The results show that the Fe₆Mn₀.₈Ce₀.₂Oy sorbent is a promising sorbent for circulating mercury removal with high SO₂ tolerance. Ce doping significantly enhances the SO₂ tolerance of the Fe₆Mn₀.₈Ce₀.₂Oy sorbent for mercury removal by protecting the Mn⁴⁺ active sites from SO₂ poisoning. This is because Ce has a stronger affinity with SO₂ compared to that of Mn, and most of SO₂ will preferentially react with CeO₂ instead of MnO₂ to form Ce₂(SO₄)₃. O₂ in flue gas improves the SO₂ tolerance of the Fe₆Mn₀.₈Ce₀.₂Oy sorbent by timely supplementing the oxygen vacancies. H₂O coexisting with SO₂ in flue gas further reduces the mercury removal rate because of the competitive adsorption between H₂O and mercury. Mercury is mainly adsorbed by MnO₂ in the Fe₆Mn₀.₈Ce₀.₂Oy sorbent by the Mars–Maessen mechanism, and SO₂ in flue gas causes the generation of a small amount of Hg₂SO₄ and HgSO₄.

Effects of CO and CO2 on the Removal of Elemental Mercury over Carbonaceous Surfaces.

Coal gasification is a popular method for the optimization of coal utilization and the reduction of environmental pollutant emissions. However, the reductive atmosphere of its products is disadvantageous for removing elemental mercury (Hg0). Activated cokes (AC) was employed in this work for mercury capture in a reducing atmosphere. The high-temperature heating decreases the mercury-removal capability of carbon sorbents because the carbonaceous surface is becoming oxygen-depleted and micropore-decreased after the heating treatment. The mechanism of mercury adsorption in pure nitrogen follows the Mars–Maessen mechanism over the carbon sorbents. To identify the effects of carbon monoxide (CO) and carbon dioxide (CO2) on Hg0 removal, the Hg0-adsorption and thermal desorption experiments were carried in a fixed-bed reaction system. CO inhibits both the chemisorption and physisorption of Hg0. CO2 competes for the active sites, lactone groups and hydroxyl groups, and occupies the micropores, which is beneficial to adsorb Hg0 physically. When CO and CO2 coexisted, the removal efficiencies show steadier than those in monocomponent gas (only CO or CO2). CO2 can resist the negative effect of CO on Hg0 removal, to some extent, because CO2 can inhibit the oxidation and disproportionation of CO. This experimental study provides practical guidance for the development of mercury-removal technology with carbon materials in the coal gasification plant.

Simultaneous removal of NO and Hg0 from flue gas over MnSmCo/Ti catalyst at low temperature

The MnSmCo/Ti catalyst was designed for the simultaneous removal of NO and Hg0 at low temperature. The MnSmCo/Ti catalyst exhibited 80% NO conversion, almost 100% N2 selectivity and 100% Hg0 removal in the absence of HCl within the temperature range of 150–250 °C with a gas hourly space velocity of 100,000 h−1. The influence of flue gas components on Hg0 oxidation was investigated and the NO and O2 are mainly responsible for Hg0 oxidation. Hg balance analysis revealed that the Hg0 removal was achieved through chemisorption and catalytic oxidation. Furthermore, the Hg0 oxidation mechanism was explored using transient reaction along with temperature-programmed desorption of mercury and X-ray photoelectron spectroscopy. Hg0 oxidation proceeds through Mars–Maessen mechanism in which adsorbed Hg0 is bound to MnOx to form weakly bonded HgMnOx+1 species and follows Langmuir−Hinshelwood mechanism, where adsorbed Hg0 reacts with active NO2 to generate HgO.

Removal of Elemental Mercury from Flue Gas Using Cobalt-Containing Biomaterial Carbon Prepared from Contaminated Iris sibirica Biomass.

Iris sibirica biomass (ISBM) used for cobalt (Co) pollution remediation was prepared by one-step pyrolysis and employed to remove elemental mercury (Hg0) from flue gas. Results showed that the ISBM pyrolyzed at 700 °C (ISBM700) exhibited good Hg0 removal performance (about 86%) at 150 °C. The existence of NO and O2 facilitated the removal of Hg0, while SO2 and water vapor inhibited it. Characterization analysis (including N2 adsorption–desorption, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry) showed that ISBM700 has a relatively higher specific surface area, a quantity of lattice oxygen derived from well-dispersed amorphous-phase CoOx, and abundant oxygen functional groups. A Mars–Maessen mechanism is thought to be involved in the Hg0 removal process. The adsorbed Hg0 could be oxidized to HgO by the surface oxygen species derived from CoOx, and then, the consumed surface oxygen species can be replenished by O2. Therefore, the Co-contaminated I. sibirica biomass (CCIB) from phytoremediation could be utilized for Hg0 removal after being pyrolyzed instead of any chemical modification.

Effect of Molybdenum on the Activity Temperature Enlarging of Mn-Based Catalyst for Mercury Oxidation

The MnO2/TiO2 (TM5) catalyst modified by molybdenum was used for mercury oxidation at different temperatures in a fixed-bed reactor. The addition of molybdenum into TM5 was identified as significantly enlarging the optimal temperature range for mercury oxidation. The optimal mercury oxidation temperature of TM5 was only 200 °C, with an oxidation efficiency of 95%. However, the mercury oxidation efficiency of TM5 was lower than 60% at other temperatures. As for MnO2–MoO3/TiO2 (TM5Mo5), the mercury oxidation efficiency was above 80% at 200–350 °C. In particular at 250 °C, the mercury oxidation efficiency of TM5Mo5 was over 93%. Otherwise, the gaseous O2, which could supplement the lattice oxygen in the catalytic reaction, played an important role in the process of mercury oxidation over TM5Mo5. The results of X-ray photoelectron spectroscopy (XPS) suggested that mercury oxidized by O2 over TM5Mo5 followed the Mars–Maessen mechanism.

Elemental mercury oxidation over manganese oxide octahedral molecular sieve catalyst at low flue gas temperature

Manganese oxide octahedral molecular sieve (OMS-2) with cryptomelane structure synthesized by a solvent-free method was employed to oxidize gaseous elemental mercury (Hg0) in coal combustion flue gas for the first time. Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) measurement were employed to characterize the catalyst. The OMS-2 with large surface area and abundant pores presented excellent Hg0 oxidation performance at a wide temperature range of 100–250 °C. At the optimal operating temperature of 150 °C, 93% Hg0 oxidation efficiency was obtained under a gas hourly space velocity as high as 1,350,000 h−1. Hg0 oxidation on OMS-2 catalyst was supposed to follow the Mars-Maessen mechanism, where the gaseous Hg0 was firstly adsorbed on the OMS-2 catalyst surface to form adsorbed Hg0, and then the adsorbed Hg0 was oxidized by the lattice oxygen over catalyst surface to form mercury oxide (HgO). Both HCl and NO in the flue gas promoted Hg0 oxidation mainly due to the formation of active species like Cl∗, NO2, which reacted with Hg0 to form volatile mercury species. SO2 inhibited the Hg0 oxidation by reducing the adsorbed HgO into gaseous Hg0 or generating sulfate on the catalyst surface. Water vapor also played an inhibitive role in Hg0 oxidation. However, the SO2 and H2O resistance of OMS-2 catalyst was much superior compared to other commercial catalysts, which makes it promising for industrial application. This knowledge is beneficial for developing economical and efficient Hg0 oxidation catalysts for coal-fired power plants.

Cobalt doped ceria for abundant storage of surface active oxygen and efficient elemental mercury oxidation in coal combustion flue gas

Cobalt doped CeO2 (Co-CeO2) prepared by a single-step hydrothermal method was used for Hg0 catalytic oxidation. The catalysts were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, H2-temperature programmed reduction test, and thermogravimetric measurement. The results show that the exposed planes for the flake-shaped Co-CeO2 mainly were (110) and (100) with low oxygen vacancy formation energies. Abundant oxygen vacancy defects were identified on the Co-CeO2, which further induced plentiful chemisorbed oxygen on the surface. The oxygen storage capacity for the Co-CeO2 was up to 1.43 μmol O2 m−2 at 200 °C, and a large portion of this capacity was for chemisorbed oxygen. Without the introduction of gas-phase O2, Hg0 oxidation probably occurred through a Mars-Maessen mechanism, where active chemisorbed oxygen originated from oxygen vacancy defects reacts with adsorbed Hg0 to form HgO. Active chlorine species was generated from the interactions of chemisorbed oxygen with trace amount gaseous HCl. The active chlorine compensated the slight inhibitive effects of SO2 and H2O, leading to above 90% Hg0 oxidation under simulated low-rank coal burning flue gas atmosphere at a gas hourly space velocity of 160,000 h−1. The abundant active chemisorbed oxygen played important role in and guaranteed an efficient Hg0 oxidation in extremely adverse application environment. These results reveal the role of doping metals on structure-catalytic property relations for CeO2, which opened a new strategy for controlling mercury emission from coal-fired flue gas using CeO2 based catalysts by tuning the morphology and exposed planes.

Reaction kinetic study of elemental mercury vapor oxidation with CuCl2

In this study, the reaction kinetics for a heterogeneous oxidation reaction of elemental mercury (Hg(0)) vapor with CuCl2 was studied in a fixed-bed reactor using 2%(wt) CuCl2/α-Al2O3 between 100 and 180 °C for Hg(0) oxidation after air preheater at a typical coal-fired power plant. The reaction rate expression was first order with respect to Hg(0). However, between 100 and 180 °C, CuCl2 over α-Al2O3 agglomerates and sinters. This sintering effect added significant mass-transfer resistance to the diffusion of Hg(0) vapor, and thus made the conversion of CuCl2 incomplete. Therefore, a grain model was formulated to determine the rate constant by taking into account the mass-transfer resistance. The model constituted a two parameter estimation problem for the determination of the rate constant and product layer diffusivity. The model predictions with the two optimum parameters were in good agreement with the experimental data. The activation energy value determined from the rate constant values was significantly lower than those for other Hg(0) oxidation catalysts under HCl and O2 gases reported in the literature. This result corroborates that CuCl2 can enhance Hg(0) oxidation by lowering the activation energy barrier with the reduction of Cu(2+) to Cu(1+) and supplying thermally stable surface Cl sites following a Mars-Maessen mechanism. CuCl2-based catalyst has potential to be applied after air preheater for Hg(0) oxidation followed by the separation of Hg(2+) in wet flue gas desulfurization (FGD) system or by activated carbon (AC) injection.