The regulation mechanism of transition metal doping on Hg0 adsorption and oxidation on Ce/TiO2(001) surface: A DFT study.

The mercury oxidation performance of Ce/TiO2 catalyst can be further enhanced by transition metal modifications. This study employed density functional theory (DFT) calculations to investigate the adsorption and oxidation mechanisms of Hg0 on Ce/TiO2(001) and its transition metal modified surfaces. According to the calculation results, Ru-, Mo-, Nb-, and Mn-doping increased the affinity of the Ce/TiO2(001) surface towards Hg0 and HCl, thereby facilitating the efficient capture and oxidation of Hg0. The increased adsorption energy (Eads) of the intermediate HgCl on the modified surfaces could promote its conversion to the final product HgCl2. The modification of transition metals impeded the desorption of the final products HgCl2 and HgO, but it did not serve as the rate-determining step. The oxidation of Hg0 by lattice oxygen and HCl followed the Mars-Maessen and Langmuir-Hinshelwood mechanisms, respectively. HCl exhibited higher mercury oxidation ability than lattice oxygen. The reactivity of lattice oxygen could be further improved by doping transition metals, their promotion order was Ru > Nb > Mo > Mn. In a HCl atmosphere, Mn modification could significantly reduce the energy barrier for HCl activation and HgCl2 formation, providing the optimal enhancement for the mercury oxidation ability of Ce/TiO2 catalyst. The screening method of transition metal modified components based on surface adsorption reaction and oxidation energy barrier was proposed in this study, which provided theoretical guidance for the development of CeTi based catalysts with high mercury oxidation activity.

Anthropogenic short-lived halogens increase human exposure to mercury contamination due to enhanced mercury oxidation over continents.

Mercury (Hg) is a contaminant of global concern, and an accurate understanding of its atmospheric fate is needed to assess its risks to humans and ecosystem health. Atmospheric oxidation of Hg is key to the deposition of this toxic metal to the Earth's surface. Short-lived halogens (SLHs) can provide halogen radicals to directly oxidize Hg and perturb the budget of other Hg oxidants (e.g., OH and O3). In addition to known ocean emissions of halogens, recent observational evidence has revealed abundant anthropogenic emissions of SLHs over continental areas. However, the impacts of anthropogenic SLHs emissions on the atmospheric fate of Hg and human exposure to Hg contamination remain unknown. Here, we show that the inclusion of anthropogenic SLHs substantially increased local Hg oxidation and, consequently, deposition in/near Hg continental source regions by up to 20%, thereby decreasing Hg export from source regions to clean environments. Our modeling results indicated that the inclusion of anthropogenic SLHs can lead to higher Hg exposure in/near Hg source regions than estimated in previous assessments, e.g., with increases of 8.7% and 7.5% in China and India, respectively, consequently leading to higher Hg-related human health risks. These results highlight the urgent need for policymakers to reduce local Hg and SLHs emissions. We conclude that the substantial impacts of anthropogenic SLHs emissions should be included in model assessments of the Hg budget and associated health risks at local and global scales.

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.

Mercury/oxygen reaction mechanism over CuFe2O4 catalyst.

CuFe2O4 is regarded as a promising candidate of catalyst for Hg0 oxidation in industrial flue gas. However, the microcosmic reaction mechanism governing mercury oxidation on CuFe2O4 remains elusive. Herein, experiments and quantum chemistry calculations were conducted for understanding the chemical reaction mechanism of oxygen-assisted mercury oxidation on CuFe2O4. CuFe2O4 shows the optimal catalytic activity towards mercury oxidation at 150 ºC. The reactivity difference of different lattice oxygen species is associated with its atomic coordination environment. The lattice oxygen coordinating with two octahedral Cu atoms and a tetrahedral Fe atom shows higher catalytic activity towards mercury oxidation than other lattice oxygen atoms. The inverse spinel structure of CuFe2O4 is favorable for O2 activation due to the Jahn-Teller effect, thereby promoting mercury oxidation. O2 molecule preferably adsorbs on iron active site and dissociates into active oxygen species. Hg0 oxidation is a three-step reaction process: Hg0 adsorption, Hg(ads) → HgO(ads), and HgO desorption. The energy barrier of mercury oxidation by chemisorbed oxygen is lower than that of mercury oxidation by lattice oxygen. The chemisorbed oxygen preserves higher reactivity towards mercury oxidation than lattice oxygen. Hg(ads) → HgO(ads) is the rate-determining step of mercury oxidation by chemisorbed oxygen because of the higher energy barrier of 116.94 kJ/mol. This work could provide the theoretical guidance for the diversified structure design of highly-efficient catalysts used for elemental mercury oxidation.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury.

We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII-organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.

Flame synthesized nanoscale catalyst (CuCeWTi) with excellent Hg0 oxidation activity and hydrothermal resistance.

In view of poor hydrothermal resistance of impregnation prepared catalysts (Cu5Ce5W9Ti-I), this paper aims to enhance thermal and hydrothermal resistance of Cu/Ce based catalysts for Hg0 oxidation via flame synthesis technology. The result found that the flame synthesis method could form nanoscale Cu10Ce10W9Ti-F particles with smaller lattice size (8-25 nm), more stable carrier structure and more oxygen vacancies. The inter-doping and inter-substitution of Ce, Cu and Ti oxides created more oxygen vacancies (Ce3+) and L-sites (O2-). Furthermore, the carrier TiO2 of Cu10Ce10W9Ti-F existed in form of highly thermostable rutile rather than anatase. High Hg0 oxidation efficiency (MOE) of 83.9-99.7% at 100-450 °C proved excellent oxidation activity of Cu10Ce10W9Ti-F catalyst. Moreover, the thermal and hydrothermal treatment (700 °C) only decreased MOE by less than 5% since L-sites kept fine thermostability of Cu10Ce10W9Ti-F. The flame synthesis was proven to be a promising catalyst preparation method to enhance thermal and hydrothermal resistance.

Insight into the effect of facet-dependent surface and oxygen vacancies of CeO2 for Hg removal: From theoretical and experimental studies.

The reaction mechanisms of Hg oxidation on CeO2(111) and (110) surface are clarified by a group of designed experiments and density functional theory (DFT) calculations. CeO2 nanorods and nanoparticles with exposure (110) and (111) faces were prepared by hydrothermal methods, and their morphological properties were investigated using XRD, XPS and HRTEM. Combined experimental and DFT results, the nanorods show better activity than nanoparticles. The total oxidation of Hg can be partially prohibited by the high barriers for the incorporated chlorine activation at reduced surfaces, due to the strong electronic repulsion of heavily accumulated charges. The energy barrier profiles suggest Hg oxidation is much more favorable on CeO2(110) surface than that on CeO2(111) surface. In the Hg oxidation via HCl and O2, the role of O2 is not only replenishment of lattice oxygen, but also could generate surface oxygen as active center for HCl active. The complete catalytic cycle can be identified as four parts: (i) HCl activated by lattice oxygen, (ii) Hg oxidation on defect surface, (iii) HCl activated by adsorbed oxygen and (iv) Hg oxidation on stoichiometric surface. The results of this study provide deep insights into the effects of CeO2 nanocatalyst morphology on the Hg oxidation.

A complete catalytic reaction scheme for Hg0 oxidation by HCl over RuO2/TiO2 catalyst.

RuO2-based catalysts have attracted great attention in mercury emission control region due to their outstanding catalytic activity and long-term stability. Quantum chemistry calculation was performed to uncover the atomic-scale reaction mechanism of Hg0 oxidation by HCl over RuO2/TiO2 catalyst. The results indicate that Hg0 adsorption on RuO2/TiO2(110) surface is controlled by a weak chemisorption mechanism. The 5-fold coordinated surface Ru atom is identified as the active center for Hg0 adsorption. HgCl molecule serves as an intermediate connecting reactant state to product state. The weak interaction between HgCl2 and catalyst surface is favorable for product desorption. HCl activation is an O-assisted surface reaction process in which HCl is oxidized into active Cl atom for Hg0 oxidation. The heterolytic cleavage of HCl molecule occurs without noticeable activation energy barrier. Hg0 oxidation by HCl over RuO2/TiO2 catalyst proceeds through two independent reaction channels. The dominant reaction channel of Hg0 oxidation is identified as a four-step process. Finally, a complete catalytic cycle that can produce the correct stoichiometry was proposed to understand the heterogeneous reaction mechanism of Hg0 oxidation over RuO2/TiO2 catalyst. The catalytic cycle consists of HCl activation, mercury oxidation and surface reoxidation. Mercury oxidation is the rate-determining step of the catalytic cycle.

Bio-oxidation of elemental mercury during growth of mercury resistant yeasts in simulated hydrosphere.

Transformation of metallic mercury (Hg°) to mercuric ion (Hg2+) in hydrosphere is the entrance of mercury cycle in water environments and leads to toxicological impact of serious global concern. Two yeast strains of Yarrowia (Idd1 and Idd2) isolated from Hg-contaminated sediments were studied for their mediating role in Hg° dissolution and oxidation. Growth of the Yarrowia cells in Hg-free liquid medium, incubated for 5 d in closed air-tight systems containing Hg°, produced extracellular polymeric substances (EPS). Approximately 230 (±5.7) ng and 120 (±6.8) ng of the dissolved Hg° were oxidized to Hg2+ by the cultures of Idd1 and Idd2, respectively, 5 day post-inoculation. Transmission electron microscopy (TEM) and X-ray energy dispersive spectrophotometry (XEDS) analysis of the EPS and cell mass revealed the presence of extracellular Hg nanoparticles, presumably HgS, as an indication of EPS-Hg complexation that is useful for Hg° dissolution and its eventual oxidation to Hg2+ by the cells. Fourier transmission infra-red (FTIR) analyses of the EPS and cell-mass during Hg-oxidation revealed that amine and carbonyl groups were used by EPS for Hg complexation. Our findings provided information about mediatory role played by Yarrowia (Idd1 and Idd2) in hydrosphere in biogeochemical cycling of Hg.

Copper slag as a catalyst for mercury oxidation in coal combustion flue gas.

Copper slag is a byproduct of the pyrometallurgical smelting of copper concentrate. It was used in this study to catalyze elemental mercury (Hg0) oxidation in simulated coal combustion flue gas. The copper slag exhibited excellent catalytic performance in Hg0 oxidation at temperatures between 200 °C and 300 °C. At the most optimal temperature of 250 °C, a Hg0 oxidation efficiency of 93.8% was achieved under simulated coal combustion flue gas with both a high Hg0 concentration and a high gas hourly space velocity of 128,000 h-1. Hydrogen chloride (HCl) was the flue gas component responsible for Hg0 oxidation over the copper slag. The transition metal oxides, including iron oxides and copper oxide in the copper slag, exhibited significant catalytic activities in the surface-mediated oxidation of Hg0 in the presence of HCl. It is proposed that the Hg0 oxidation over the copper slag followed the Langmuir-Hinshelwood mechanism whereby reactive chlorine species that originated from HCl reacted with the physically adsorbed Hg0 to form oxidized mercury. This study demonstrated the possibility of reusing copper slag as a catalyst for Hg0 oxidation and revealed the mechanisms involved in the process and the key factors in the performance. This knowledge has fundamental importance in simultaneously reducing industrial waste and controlling mercury emissions from coal-fired power plants.

Performance evaluation of non-thermal plasma injection for elemental mercury oxidation in a simulated flue gas.

The use of non-thermal plasma (NTP) injection approach to oxidize elemental mercury (Hg(0)) in simulated flue gas at 110°C was studied, where a surface discharge plasma reactor (SDPR) inserted in the simulated flue duct was used to generate and inject active species into the flue gas. Approximately 81% of the Hg(0) was oxidized and 20.5µgkJ(-1) of energy yield was obtained at a rate of 3.9JL(-1). A maximal Hg(0) oxidation efficiency was found with a change in the NTP injection air flow rate. A high Hg(0) oxidation efficiency was observed in the mixed flue gas that included O2, H2O, SO2, NO and HCl. Chemical and physical processes (e.g., ozone, N2 metastable states and UV-light) were found to contribute to Hg(0) oxidation, with ozone playing a dominant role. The deposited mercury species on the internal surface of the flue duct was analyzed using X-ray photoelectron spectroscopy (XPS) and electronic probe microanalysis (EPMA), and the deposit was identified as HgO. The mercury species is thought to primarily exist in the form of HgO(s) by adhering to the suspended aerosols in the gas-phase.

Determination of thiomersal by flow injection coupled with microwave-assisted photochemical online oxidative decomposition of organic mercury and cold vapor atomic fluorescence spectroscopy.

We developed a flow injection (FI) method for the determination of thiomersal (sodium ethylmercurithiosalicylate, C9H9HgNaO2S) based on the UV/microwave (MW) photochemical, online oxidation of organic mercury, followed by cold vapor generation atomic fluorescence spectrometry (CVG-AFS) detection. Thiomersal was quantitatively converted in the MW/UV process to Hg(II), with a yield of 97±3%. This reaction was followed by the reduction of Hg(II) to Hg(0) performed in a knotted reaction coil with NaBH4 solution, and AFS detection in an Ar/H2 miniaturized flame. The method was linear in the 0.01-2 µg mL(-1) range, with a LOD of 0.003 µg mL(-1). This method has been applied to the determination of thiomersal in ophthalmic solutions, with recoveries ranging between 97% and 101%. We found a mercury concentration in commercial ophthalmic solutions ranging between 7.5 and 59.0 µg mL(-1).