Hg0 Oxidation Research Articles

Synergistic effect of copper and vanadium species on Hg0 removal of Cu-SCR catalyst: A DFT and experimental study

The synergistic effect of Cu and V species on the improved Hg0 removal performance of Cu-SCR catalyst was analyzed in depth by experiments and theoretical calculations based on density functional theory (DFT). Possible CuVOx/TiO2(001) surface models were constructed for Hg adsorption and oxidation calculations in the presence of O2. According to the results of partial density of states (PDOS) and charge populations, Cu transfers electrons to V through bridge O, and the strong hybridization of Cu 3d and O 2p orbitals facilitate the generation of O−. Although copper doping slightly promotes the Hg adsorption and oxidation ability of VOx species, the O− bonded with Cu provides the optimal adsorption sites and active centers. Strongly hybridized with CuO, Hg easily transfers charge to O− and generate HgO to desorb from the surface, and the oxygen vacancy thus formed can be supplemented by gaseous O2. Characterizations including Raman, fourier transform infrared spectroscopy (FTIR), H2 temperature program reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) verifies the formation of Cu-O-V structure and the higher involvement of CuOx in Hg0 oxidation, and Hg0 temperature program desorption (Hg0-TPD) were conducted to verify the reaction mechanism. This study reveals the good potential of copper species in Hg0 removal, and provides a reference for designing new materials for mercury control.

Together, Not Separately, OH and O3 Oxidize Hg(0) to Hg(II) in the Atmosphere

Mercury, a highly toxic metal, is emitted to the atmosphere mostly as gaseous Hg(0). Atmospheric Hg(0) enters ecosystems largely through uptake by vegetation, while Hg(II) largely enters ecosystems in oceans and in rainfall. Consequently, the redox chemistry of atmospheric mercury strongly influences its fate and its global biogeochemical cycling. Here we report on the oxidation and reduction of Hg(I) (BrHg and HOHg radicals) in reactions with ozone and how the electronic structure of these Hg(I) species affects the kinetics of these reactions. The oxidation reactions lead to YHgO· + O2 (Y = Br and OH), while the reduction reactions produce Hg(0), OY, and O2. According to our calculations with CCSD(T), NEVPT2, and CAM-B3LYP-D3BJ, the kinetics of both oxidation reactions are very similar and much faster than their reduction counterparts. Past modeling of field data has supported the idea that OH and/or O3 (rather than Br) dominates Hg(II) production in the continental boundary layer. Almost all models invoking OH- and ozone-initiated oxidation of Hg(0) assume that these reactions produce Hg(II) in one step, despite the lack of plausible mechanisms. The two-step mechanism of formation of HOHg followed by its reaction with ozone helps reconcile modeling results with mechanistic insights.

New frontiers in heterogeneous Hg0 oxidation by O2 over catalytic single-Fe site on boron-vacancy of h-BN: A density functional theory study

The single-atom catalysts (SACs) have shown tremendous potential in heterogeneous catalytic process, because of their high atomic utilization efficiency and controllable characteristic of various active sites. The accurate structure design at atomic scale of SACs brings new opportunities and challenges for the cost-effective Hg0 oxidation and the study in molecularly catalytic mechanism by the oxidant of O2. Herein, a series of single transition metal atom doped (Mn, Fe, Co, Ni, Cu and Zn) on h-BN vacancy (M/V-BN) were well designed and the interaction behaviours were further investigated via Density Functional Theory (DFT) method. The mercury oxidation profiles and mechanism in the existence of O2 had been clarified on the optimized single atom catalytic site. The results show that the six kinds of TM atoms can exist stably on the surface of boron-vacancy (VB-BN) monolayer and have the potential to activate the new two-dimensional material (h-BN) with high surface reactivity. Among them, the most stable configuration Fe/VB-BN exhibited the best affinity to Hg0 with an adsorption energy of -1.697 eV, and the Fe-Hg bond was the shortest of 2.747 Å. The covalent interaction of Fe-Hg bond is demonstrated by the overlaps between Fe d-orbitals and Hg p-orbital. The chemisorption mechanism dominates the adsorption processes of different mercury species (Hg0, HgO, HgCl, HgBr and HgS), with the adsorption energy ranging from -1.2 to -4.2 eV. The Hg0 oxidation over Fe/VB-BN by O2 mainly follows the Eley-Rideal mechanism, and undergoes three key steps, i.e., O2 molecule dissociation, formation of O-Hg-O structure and desorption of oxidized Hg products. O2 was firstly adsorbed on the unsaturated Fe site and strongly activated by the metal center, which contributes to the high Hg0 catalyst oxidation activity. The generation of O-Hg-O structure is the public stage of mercury products desorption. The rate-determining step of Hg0 catalytic oxidation was dependent on Hg2O2 cluster desorption with an energy of 2.91 eV, which was much smaller than the desorption barrier of traditional catalysts. As a whole, the outcomes demonstrate that Fe/VB-BN catalyst is one promising candidate for catalytic oxidation of Hg0 by O2, which enables cleaner coalfired power plant operations.

In-situ growth of Co/Zn bimetallic MOF on GO surface to prepare GO supporting Co@C single-atom catalyst for Hg0 oxidation

Oxidation of Hg0 to Hg2+ is an efficient way to remove Hg0 from flue gas. Chemisorbed oxygen as the main active site plays an important role in Hg0 oxidation process. Single-atom catalyst has an excellent ability of transforming gaseous oxygen into chemisorbed oxygen. In the present work, graphene oxide (GO) was used as support to in-situ grow Co/Zn bimetallic MOF on GO surface. After pyrolysis, a GO supported Co@C single-atom catalyst was obtained. SEM and TEM results show that GO supporting gives a high dispersion for calcined MOF particles. HAADF-STEM results prove that most of the Co atoms in calcined MOF particles stay as single-atom status. EXAFS indicated that the Co atoms in the catalyst was coordinated with N, and the coordination was dominated by CoN2. CoN2 can capture and activate gaseous oxygen forming more chemisorbed oxygen. Thanks to the high dispersion of calcined MOF and higher chemisorbed oxygen concentration, the Hg0 oxidation efficiency of the obtained catalyst (Co2.5-SAs@NC/GO-1) can reach above 90 % in the temperature range of 120–180 °C with 0.075 g loading amount at the GHSV of 180, 000 h−1. The probable reaction pathway was analyzed by an adsorption–desorption experiment and XPS over the used and regenerated Co2.5-SAs@NC/GO-1.

Simultaneous catalytic oxidation mechanism of NO and Hg0 over single-atom iron catalyst

Among the many pollutants emitted from coal-fired power plants, elemental mercury (Hg0) and nitrogen oxides (NOx) are widely concerned and regarded as a global environmental problem. Catalytic oxidation is an effective method to remove the above power plant pollutants. Although the recent pioneering experiment found that single-atom catalysts (SACs) exhibit excellent performance for the oxidation of NO and Hg0, the reaction mechanism for the simultaneous removal of NO and Hg0 has not been thoroughly revealed. In this work, a single-atom iron catalyst supported by a double vacancy doped graphene substrate with four N atoms (Fe1N4) was used to explore the reaction mechanism of O2 simultaneous catalytic of NO and Hg0. By using DFT method and energy span model, all possible paths of simultaneous catalytic oxidation of NO and Hg0 are enumerated. Among the ten paths we found, Termolecular Eley-Rideal (TER) mechanism has the lowest energy barrier. The rate determining reaction barrier is 1.59 eV. Disregarding the contribution of molecular collisions, TER path is more kinetically favorable for the integrated removal of NO and Hg0. Comprehensive consideration of intermolecular collisions, the catalytic oxidation reaction follows the E-R mechanism. The study deepened the understanding of the reaction mechanism of simultaneous removal of NO and Hg0.

Vanadium-density-dependent reactivity for simultaneous removal of NOx and Hg0 over V2O5/TiO2 catalyst

Vanadium-based catalysts have been widely used in the simultaneous removal of NOx and Hg0. However, how vanadium species affect the reactivity of Hg0 oxidation and coverage-dependent behaviors of vanadium species are still unknown. Herein, we synthesized a series of V2O5/TiO2 catalysts with different V loadings and investigated the reaction characteristics of vanadium species (i.e., sub-monolayer, monolayer, and crystalline) for the catalytically simultaneous abatement of NOx and Hg0. At low V loading, the ratio of Oads/Olatt decreased gradually due to the VOx species occupying the defect sites of TiO2. The reaction rate of Hg0 oxidation is proportional to the square of the VOx density up to monolayer coverage. A maximum amount of V5+ is achieved at monolayer coverage and the TOF shows a volcanic trend with the increase of V loading. This study is an in-depth attempt to explore the effects of different vanadium species on the simultaneous removal of NOx and Hg0, and it is expected to provide theoretical guidance for the effective design of state-of-the-art vanadium-based catalysts for multipollutant control.

Co3O4 with ordered pore structure derived from wood vessels for efficient Hg0 oxidation

Catalytic oxidation of Hg 0 to HgO is an efficient way to remove Hg 0 from coal-fired flue gas. The catalyst with ordered pore structure can lower mass transfer resistance resulting in higher Hg 0 oxidation efficiency. Therefore, in the present work, wood vessels were used as sacrificial template to obtain Co 3 O 4 with ordered pore structure. SEM and BET results show that, when the mass concentrations of Co(NO 3 ) 2 ·6H 2 O was 20%, the obtained catalyst (Co 3 O 4 [20%Co(NO 3 ) 2 ]) possesses better pore structure and higher surface area. It will expose more available surface active sites and lower the mass transfer resistance. Furthermore, XPS results prove that Co 3 O 4 [20%Co(NO 3 ) 2 ] has the highest ratio of chemisorbed oxygen which plays an important role in Hg 0 oxidation process. These results lead to a better Hg 0 oxidation efficiency of Co 3 O 4 [20%Co(NO 3 ) 2 ], which is about 90% in the temperature range of 200 to 350 °C. Furthermore, Co 3 O 4 [20%Co(NO 3 ) 2 ] has a stable catalytic activity, and its Hg 0 oxidation efficiency maintains above 90% at 250 °C even after 90 h test. A probable reaction mechanism is deduced by the XPS results of the fresh, used and regenerated catalyst of Co 3 O 4 [20%Co(NO 3 ) 2 ]. Chemisorbed oxygen can react with Hg 0 forming HgO with the reduction of Co 3+ to Co 2+ . And lattice oxygen and gaseous oxygen can supplement the consumption of chemisorbed oxygen to oxidize Co 2+ to Co 3+ .

Cubic SAPO-34 supported MnOx for low-temperature gaseous mercury oxidation and adsorption from coal-fired flue gas

Gaseous elemental mercury (Hg0) from industrial flue gas is difficult to be synergistically removed by existing flue gas purification equipment. In this paper, Mn-based oxides were used for the modification of SAPO-34 molecular sieve to enhance Hg0 oxidation and adsorption performances. The results showed that when 5 % Mn dropped on the surface of SAPO-34, it had the highest Hg0 removal efficiency and exhibited a better Hg0 removal performance at lower temperature (<150 °C). SO2 would compete for active adsorption sites of Hg0, while H2O vapor had little influence on Hg0 removal. The Hg0 removal mechanism was discussed based on experiment and characterization results and density functional theory calculation. The larger surface area and pore volume of Mn/SAPO-34 benefited the physical adsorption process. The surface MnOx particles enhanced the oxidation of Hg0 to Hg2+ and the oxygen site can further bond with Hg2+ to form stable HgO. In addition, the larger desorption peak of HgO (487 °C) and the more negative adsorption energy (-1.21 eV) revealed the dominant role of chemical adsorption of Hg0 over Mn/SAPO-34. Thus, the Mn/SAPO-34 is a potential catalyst for Hg0 removal from industrial flue gas.

Controllable Disordered Copper Sulfide with a Sulfur-Rich Interface for High-Performance Gaseous Elemental Mercury Capture.

Copper sulfide (CuS) has received increasing attention as a promising material in gaseous elemental mercury (Hg0) capture, yet how to enhance its activity at elevated temperature remains a great challenge for practical application. Herein, simultaneous improvement in the activity and thermal stability of CuS toward Hg0 capture was successfully achieved for the first time by controlling the crystal growth. CuS with a moderate crystallinity degree of 68.8% showed a disordered structure yet high thermal stability up to 180 °C. Such disordered CuS can maintain its Hg0 capture activity stable during longtime test at a wide temperature range from 60 to 180 °C and displayed strong resistance to SO2 (6%) and H2O (8%). The significant improvement can be attributed to the synergistic effect of a moderately crystalline nature and a unique sulfur-rich interface. Moderate crystallinity guarantees the thermal stability of CuS and the presence of abundant defects, in which copper vacancy enhances significantly the Hg0 capture activity. The sulfur-rich interface enables CuS to provide plentiful highly active Sx2- sites for Hg0 adsorption. The interrelation between structure, reactivity, and thermal stability clarified in this work broadens the understanding toward Hg0 oxidation and adsorption over CuS and provides new insights into the rational design and engineering of advanced environmental materials.

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.

Mechanochemical preparation of well-structured copper sulfide for elemental mercury sequestration from coal combustion flue gas

Among various metal sulfides, copper sulfide (CuS) was found to be the most efficient one for the abatement of elemental mercury (Hg0) pollution in coal combustion flue gas because of the enrichment of under-coordinated sulfur along the 〈001〉 direction. Unlike saturated sulfur, the under-coordinated sulfur exhibits a dual role in oxidizing and immobilizing Hg0, but how to effectively synthesize (001) surface exposed CuS through a facile method has long been challenging in the Hg0 controlling community. In this work, a feasible synthesis strategy based on mechanical ball-milling procedure was proposed to synthesize (001) surface exposed CuS under mild conditions. Owing to the controllable nucleus formation and favorable crystalline growth manner under mechanochemical condition, the as-obtained CuS-based sorbent via ball-milling was primarily consisted of well-structured nanosheet containing abundant under-coordinated sulfur ligands, ensuring the adequate exposure of active sites. Consequently, the CuS as obtained exhibited the Hg0 adsorption capacity and uptake rate as high as 86.22 mg g−1 and 112.04 μg g−1 min−1, far surpassing those benchmark metal sulfides as reported in previous studies. Notably, the Hg0 uptake rate of the CuS nanosheet surpasses that of CuS nanoparticle with irregular agglomeration by approximate ten folds. The under-coordinated sulfur, especially polysulfide (Sx2-), was demonstrated to be the primary ligand accounting for the oxidation and sequestration of Hg0 over the surface of CuS nanosheet. This facile synthesis logic with scale-up potential may also be applicable in the preparation of other metal sulfides in addition to CuS, marking an effective attempt stepping towards the industrial application of metal sulfides for Hg0 immobilization from coal combustion flue gas.

Enhancing the oxidation efficiency of elemental mercury in the presence of NO and SO2 by using Cu(II)O doped TiO2 photothermal catalysts: Kinetics and mechanisms

This study aimed to apply sol–gel synthesized Cu(II)O doped TiO2 to enhance the oxidation of gaseous elemental mercury (Hg0) in the flue gases containing NO and SO2 at the temperatures of 120-180℃ under the irradiation of near-ultraviolet (near-UV). The influences of SO2 and NO on the oxidation efficiency of Hg0 were investigated by in-situ diffuse reflection infrared spectroscopy (DRIFT) and density functional theory (DFT). Experimental results showed that an optimal 5 %Cu(II)O/TiO2 had the oxidation efficiencies of Hg0 from 40 to 100 % at 120-180℃. The oxidation efficiencies of Hg0 were 85 and 70 % at 120℃, and 40 and 5 % at 180 °C, in the presence of solely SO2 and NO, respectively. Surface characterization showed that Cu(II)O was evenly dispersed over the surface of TiO2 catalysts. NO could inhibit the oxidation of Hg0 since it consumed the chemisorbed oxygen (Oα) and compete with Hg0 at the surface active sites. In contrast, SO2 could promote the oxidation efficiency of Hg0 due to the formation of HgSO4 on the surface of Cu(II)O/TiO2 catalysts. In the exposure of NO/SO2, NO and SO2 can be adsorbed on the catalyst surface reacting with ̇OH. The photothermal oxidation reactivity of Hg0 was inhibited due to the competitive adsorption between NO/SO2 and Hg0. The Langmuir-Hinshelwood (L-H) kinetic model successfully simulated the oxidation rate of Hg0. Moreover, DFT was further applied to estimate the binding energies of different gas molecules confirming the competence for active sites on the surface of Cu(II)O/TiO2 between NO/SO2 and Hg0.

Design and construction of an O-Au-O coordination environment in Au single atom-doped Ti4+ defected TiO2 for an enhanced oxidative ability of lattice oxygen for Hg0 oxidation

Transition metal oxide (TMO)-decorated TiO2 catalysts excel at low-temperature Hg0 oxidation. However, TMOs are typically found in large nanoparticles (NPs), resulting in a lower atom utilization efficiency. The current paper presents the design of an Au single atom (SA)-doped Ti4+ defected TiO2 (Au SAC-Ti1-xO2) and investigations concerning the influence of an O-Au-O coordination environment on Hg0 oxidation. Au SAC-Ti1-xO2 was designed using an O-Au-O coordination environment and the O-Au-O orbital interaction was elucidated using density functional theory (DFT). The enhanced oxidative capability of lattice oxygen atoms and the interaction between Hg0 and oxygen atoms were investigated. The O-Au-O coordination environment containing Au SAC Ti1-xO2 was then synthesized and characterized using X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), etc. Finally, Hg0 oxidation catalytic activity was determined. Due to the increasing population of Au-O antibonding orbital interactions in the valence band, the bond order of O-Au-O in Au SAC Ti1-xO2 decreased significantly, thereby enhancing the oxidation ability of its adjacent oxygen atoms. Au SA and ultra-small Au clusters coexisted on a Ti1-xO2 catalyst, and the Hg0-oxidation efficiency at 150–200 °C was 14 times and 1.5 times greater than those of TiO2 and 0.1 % Au/TiO2, respectively. Furthermore, Au SAC Ti1-xO2 was more resistant to the inhibition of SO2 and NO. It provides an efficient method for developing highly active SA catalysts for Hg0 oxidation.

Effects of Coal-Fired Flue Gas Components on Mercury Removal by the Mechanochemical S-Modified Petroleum Coke.

In this work, the effects of coal-fired flue gas components (O2, CO2, SO2, and NO) on the Hg0 removal by the promising mercury removal adsorbent mechanochemical S-modified petroleum coke were characterized and analyzed in terms of the Hg0 removal efficiency, mercury adsorption capacity, and mercury mass balance. The results show that the mechanochemical S-modified petroleum coke with a theoretical sulfur content of 21% (named TSC-21) is the best candidate for mercury removal based on the Hg0 removal efficiency, Hg0 removal capacity, and difference ratio of Hg0 removal capacity (anti-interference ability) in the basic and full-component simulated flue gas atmosphere (N2 + O2 + CO2, N2 + O2 + CO2 + SO2 + NO). The maximum value (MV) and stable value (SV) of the Hg0 removal efficiency of TSC-21 in the basic simulated flue gas atmosphere are 99.25% (MV) and 91.17% (SV), respectively. O2, CO2, and NO all promote the Hg0 removal by the adsorbent, but they benefit the Hg0 oxidation while inhibiting the Hg0 adsorption. The promoting effect of O2 on the Hg0 removal by TSC-21 is affected by the reaction time, which is especially obvious after 1 min. The presence of SO2 inhibits the oxidation and adsorption of Hg0, which in turn reduces the Hg0 removal performance of the adsorbent. The improving effects on the oxidative escape of Hg0 by CO2 is higher than that by NO and O2. TSC-21 acts more as an oxidant than an adsorbent for Hg0 removal.

Novel insight into elemental mercury removal by cobalt sulfide anchored porous carbon: Phase-dependent interfacial activity and mechanisms

Construction high-activity phase of metal sulfides on low-cost carbon represents a judicious way to develop advanced environmental materials, and yet the interrelation between crystal phase of metal sulfides on carbon matrix and interfacial reactivity of the composites towards gaseous elemental mercury (Hg0) has not been explored. Herein, we clarified the mechanisms of crystal phase in determining the Hg0 oxidation and adsorption activity of cobalt sulfide anchored carbon for the first time. Both CoS2 and Co9S8 phases were employed as model and synthesized on the same porous carbon matrix. The sulfur-rich phase of CoS2 anchored carbon has obvious advantages in providing Co3+ and S22- species comparison to Co9S8 anchored carbon, which act as reaction centers for Hg0. The CoS2 anchored carbon shows a high Hg0 adsorption capacity of 16.2 mg·g−1, which is superior to various activated carbons and metal sulfides. Surprisingly, the composite is able to utilize O2 and SO2 in flue gas to regenerate the consumed Co3+ sites, which is ascribed to the use of carbon as matrix. Both experimental and theoretical results confirm that Co sites represent the most active center for Hg0 adsorption. This work provides a new paradigm in design and engineering of high-performance materials in environmental remediation.

Dissociation of Mercuric Oxides Drives Anomalous Isotope Fractionation during Net Photo-oxidation of Mercury Vapor in Air.

The atmosphere is the primary medium for long-distance transport and transformation of elemental mercury (Hg), a potent neurotoxin. The recent discovery of mass-independent fractionation (MIF) of even-mass Hg isotopes (even-MIF, measured as Δ200Hg and Δ204Hg) in the atmosphere is surprising and can potentially serve as a powerful tracer in understanding Hg biogeochemistry. Far-ultraviolet (UVC) light-induced gas-phase reactions have been suspected as a likely cause for even-MIF, yet the mechanism remains unknown. Here, we present the first experimental evidence of large-scale even-MIF caused by UVC-induced (wavelength: 254 nm) Hg oxidation in synthetic air at the pressure (46-88 kPa) and temperature (233-298 K) resembling those of the lower atmosphere. We observe negatively correlated Δ200Hg and Δ204Hg signatures with values as low as -50‰ and as high as 550‰, respectively, in the remaining atomic Hg pool. The magnitude of even-MIF signatures decreases with decreasing pressure with the Δ200Hg/Δ204Hg ratio being similar to that observed in global precipitation. This even-MIF can be explained by photodissociation of mercuric oxides that are photochemically formed in the UVC-irradiated Hg-O2 system. We propose that similar processes occurring in the atmosphere, where mercuric oxide species serve as intermediates, are responsible for the observed even-MIF in the environment.

A kinetic study on mercury oxidation by HCl over typical Mn-based SCR catalysts

A kinetic approach was adopted to analyze the reaction mechanism of elemental mercury (Hg0) oxidation by HCl over three Mn-based low-temperature SCR catalysts, MnOx/TiO2, Fe-MnOx/TiO2 and CeMnO3. Experimental results validated that Hg0 oxidation efficiencies of the catalysts were promoted by HCl at different temperatures. The kinetic models for the heterogeneous Hg0 oxidation were established and verified based on the experimental data. The verification results demonstrated that Hg0 oxidation over the Mn-based catalysts follows Langmuir-Hinshelwood mechanism. The kinetic parameters of reaction rate constant and HCl adsorptionconstant were calculated from the model fitting equations. The reaction rate constant was raised with the increase of temperature, while the HCl adsorptionconstant presented the opposite trend. The Mn-based catalysts with the reaction rate constant of 84.17–335.77 s−1 showed the advantage over some other catalysts such as commercial V2O5-(WO3)/TiO2 and CeO2/TiO2. The kinetic parameters were further improved in the presence of O2. The apparent activation energy for Hg0 oxidation over the catalysts that was derived from the kinetic parameters was 4.13–12.53 kJ/mol, which was much lower than that of the other approaches. The advantageous kinetic parameters and apparent activation energy were favorable for the Mn-based catalysts to act as low-temperature SCR catalyst for synergistic Hg0 removal. The kinetic study results were of fundamental significance for designing the reactor according to the known flue gas conditions, used catalyst and required Hg0 oxidation efficiency.

Candidate primary reference measurement procedure for the amount-of-substance content of a complexing agent expressed as EDTA based on the coulometric titration with electrogenerated Hg ions

The primary reference measurement procedure is a method to realize the definition of the quantity without relation to a reference standard of the same quantity. This paper presents a candidate primary reference measurement procedure for the measurement of the amount-of-substance of a complexing agent expressed as ethylenediaminetetraacetic acid (EDTA) based on coulometric titration. Herein, mercury (Hg) was used as the anode because it does not undergo spontaneous oxidation. Furthermore, a 1:1 complex is formed between the electrogenerated Hg2+ ions and EDTA (in the form of Y4−). Although the electrochemical oxidation of Hg may generate Hg2 2+ ion, Hg2 2+ undergoes disproportionation in the presence of EDTA, thereby forming the desired Hg2+ species. The equivalence point occurs at the maximum of the derivative of the theoretical titration curve. As such, the measurement results were obtained by assuming that the maximum of the derivative of the experimental titration curve was the end point. The developed procedure was employed for the measurement of the amount-of-substance content of the complexing agent in EDTA reference material certified at the Slovak Institute of Metrology. The experimental measurements agreed well with the certified value for the reference material within the measurement uncertainty. The candidate procedure will be useful in certifying EDTA reference material and establishing international equivalence of amount content of the complexing agent.

Synergistic catalytic elimination of NO, mercury and chlorobenzene over WCeMnOx/TiO2-ZrO2: Performance study of pore structure

The catalytic performance of synergistic catalytic elimination of NO, mercury and chlorobenzene for WCeMnOx/TiO2-ZrO2 was improved by controlling the pore structure. The formation of micron-sized macropores made the catalyst structure looser, and the transparent macropores helped the catalyst expose more active sites. At low temperature, -NH2* reacted with monodentate nitrite and bidentate nitrate, and NH4+ reacted with gas phase NO to generate N2 and H2O. At high temperature, NH4+ reacted rapidly with gas phase NO. The reaction process of catalytic oxidation of Hg0 was that gaseous Hg0 was adsorbed on the catalyst surface and then oxidized to Hg2+. For the catalytic degradation of chlorobenzene, phenols and HCl were generated firstly, and then phenols and hydroxyl groups reacted to form maleate, bidentate carbonate and carbonate species. The maleate and bidentate carbonate degraded to form carbonate, and the carbonate degraded to produce CO2 and H2O.

Mechanistic study on the role of VOx in the acidity, oxidizability, and Hg0 capture capability of FeOx-VOx binary mixed oxides

In this article, we have prepared a series of FeOx-VOx mixed oxides using the sol-gel method and evaluated their Hg0 removal performance. Virgin Fe showed poor activity, as evidenced by its low Hg0 capture capacity of ca. 6.7 μg/g. When VOx was introduced, abundant acid sites came out on the adsorbent surfaces, which facilitated the physisorption of Hg0 and overpowered the negative effects of the weakened oxidizability involved in the oxidation of Hg0. Consequently, improved Hg0 adsorption performance was observed as ca. 41.9 μg/g Hg0 capture capacity could be obtained as the molar ratio of V/Fe reached 1/9. Throughout the whole Hg0 capture process, chemisorption dominated with the initial adsorption stage constituting the rate-controlling step. V5+, together with chemical adsorbed oxygen, was directly involved in the oxidation of Hg0, whereas Fe3+ functioned as a promoter-like site to accelerate the regeneration of V5+. Finally, FeOx-VOx adsorbent exhibited elevated Hg0 capture capacity in NO- or SO2-containing flue gas as well as satisfactory recyclability, demonstrating its potential as an adsorbent for practical applications.