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Title: The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study

Abstract

Experimental and field measurements have shown that, in the presence of both iron-containing aerosols and sunlight, oxidized mercury species such as HgCl 2 and HgBr 2 undergo reduction to elemental mercury (Hg°), which remains in the atmosphere longer than oxidized mercury species due to its higher volatility. We performed density functional theory (DFT, PW91+U) calculations to elucidate the reduction mechanism for atmospheric HgCl 2 and HgBr 2 to Hg° on several iron-oxide aerosol surfaces relevant in the troposphere. On the OH-Fe-R-terminated α-Fe 2O 3(0001) surface, predicted to be most prevalent under ambient conditions, we show that: (1) the first Hg-X bond is broken via either thermal or photolytic activation depending on the ambient temperature; (2) photons with an energy of 2.69 eV (461 nm) are required to break the second Hg-X bond; and (3) a photo-induced surface-to-adsorbate charge-transfer process can promote Hg° desorption with an excitation energy of 2.59 eV (479 nm). All the calculated excitation energies are below the threshold value of 3.9 eV (320 nm) for photons in the troposphere, suggesting that sunlight can facilitate mercury reduction on iron-oxide aerosol surfaces. In contrast, the gas-phase reduction of HgCl 2 (HgBr 2) involves photoexcitation requiring an energy of 4.98more » (4.45) eV (249 (279) nm); therefore, the energy range of sunlight is not suitable for gas-phase reduction. Our computational results provide the first evidence on the detailed mechanism for the combined role of aerosols and photons in the reduction of HgCl 2 and HgBr 2. In conclusion, our methodology can be adapted to study other photochemical heterogeneous processes in the atmosphere.« less

Authors:
 [1];  [1];  [1];  [2];  [1]
  1. Univ. of Wisconsin, Madison, WI (United States). Dept. of Chemical and Biological Engineering
  2. Univ. of Wisconsin, Madison, WI (United States). Dept. of Chemical and Biological Engineering; Univ. of Wisconsin, Madison, WI (United States). Dept. of Civil and Environmental Engineering
Publication Date:
Research Org.:
Univ. of Wisconsin, Madison, WI (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Chemical Sciences, Geosciences & Biosciences Division
Contributing Org.:
The National Energy Research Scientific Computing Center (NERSC); the Center for Nanoscale Materials (CNM) at Argonne National Laboratory (ANL); and the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user resource located at Pacific Northwest National Laboratory (PNNL).
OSTI Identifier:
1440987
Grant/Contract Number:  
FG02-05ER15731; AC02-05CH11231; AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Applied Catalysis. B, Environmental
Additional Journal Information:
Journal Volume: 234; Journal Issue: C; Journal ID: ISSN 0926-3373
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Density functional theory; Iron oxide; Mercury reduction; Photons; Aerosols

Citation Formats

Tacey, Sean A., Szilvasi, Tibor, Xu, Lang, Schauer, James J., and Mavrikakis, Manos. The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study. United States: N. p., 2018. Web. doi:10.1016/j.apcatb.2018.04.049.
Tacey, Sean A., Szilvasi, Tibor, Xu, Lang, Schauer, James J., & Mavrikakis, Manos. The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study. United States. doi:10.1016/j.apcatb.2018.04.049.
Tacey, Sean A., Szilvasi, Tibor, Xu, Lang, Schauer, James J., and Mavrikakis, Manos. Sun . "The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study". United States. doi:10.1016/j.apcatb.2018.04.049.
@article{osti_1440987,
title = {The role of iron-oxide aerosols and sunlight in the atmospheric reduction of Hg(II) species: A DFT+ U study},
author = {Tacey, Sean A. and Szilvasi, Tibor and Xu, Lang and Schauer, James J. and Mavrikakis, Manos},
abstractNote = {Experimental and field measurements have shown that, in the presence of both iron-containing aerosols and sunlight, oxidized mercury species such as HgCl2 and HgBr2 undergo reduction to elemental mercury (Hg°), which remains in the atmosphere longer than oxidized mercury species due to its higher volatility. We performed density functional theory (DFT, PW91+U) calculations to elucidate the reduction mechanism for atmospheric HgCl2 and HgBr2 to Hg° on several iron-oxide aerosol surfaces relevant in the troposphere. On the OH-Fe-R-terminated α-Fe2O3(0001) surface, predicted to be most prevalent under ambient conditions, we show that: (1) the first Hg-X bond is broken via either thermal or photolytic activation depending on the ambient temperature; (2) photons with an energy of 2.69 eV (461 nm) are required to break the second Hg-X bond; and (3) a photo-induced surface-to-adsorbate charge-transfer process can promote Hg° desorption with an excitation energy of 2.59 eV (479 nm). All the calculated excitation energies are below the threshold value of 3.9 eV (320 nm) for photons in the troposphere, suggesting that sunlight can facilitate mercury reduction on iron-oxide aerosol surfaces. In contrast, the gas-phase reduction of HgCl2 (HgBr2) involves photoexcitation requiring an energy of 4.98 (4.45) eV (249 (279) nm); therefore, the energy range of sunlight is not suitable for gas-phase reduction. Our computational results provide the first evidence on the detailed mechanism for the combined role of aerosols and photons in the reduction of HgCl2 and HgBr2. In conclusion, our methodology can be adapted to study other photochemical heterogeneous processes in the atmosphere.},
doi = {10.1016/j.apcatb.2018.04.049},
journal = {Applied Catalysis. B, Environmental},
number = C,
volume = 234,
place = {United States},
year = {Sun Apr 22 00:00:00 EDT 2018},
month = {Sun Apr 22 00:00:00 EDT 2018}
}

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