Heterogeneous Mercury Oxidation by HCl over CeO2 Catalyst

Article pubs.acs.org/JPCC

Heterogeneous Mercury Oxidation by HCl over CeO2 Catalyst: Density Functional Theory Study Bingkai Zhang,†,‡ Jing Liu,*,† and Fenghua Shen† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China

‡

S Supporting Information *

ABSTRACT: CeO2-based catalysts have been regarded as potential materials for Hg removal due to high catalytic performance, nontoxicity, and low cost. Density functional theory calculations were performed to investigate the mercury oxidation mechanism by HCl over a CeO2 catalyst. The thermodynamic stability analysis suggests that the stoichiometric CeO2(111) is the most stable surface. The protonated CeO2 surfaces takes place at low oxygen partial pressures, and the chlorinated CeO2 surfaces can stably exist under low HCl concentrations. The adsorption energies and geometries show that Hg0 is physically adsorbed on oxygen sites of the CeO2(111) surface and HCl is chemically adsorbed on the CeO2(111) surface. HCl can dissociate on the CeO2(111) surface with a low barrier. The Hg oxidation is most likely to proceed with the Eley−Rideal mechanism at the first step (Hg → HgCl), followed by the Langmuir− Hinshelwood mechanism at the second step (HgCl → HgCl2). In the whole Hg oxidation reaction, the formation of HgCl2 is the rate-determining step. The low energy barriers for the oxidation reaction of Hg on CeO2 make it an attractive alternative catalyst for Hg oxidation. HCl.16−18 The Hg oxidation across the CeO2/TiO2 catalyst in the simulated flue gas was reported to be higher than 90% when the temperature is from 200 to 250 °C.16 The overall reaction of Hg0 catalytic oxidation is generally reported as the following equation:16

1. INTRODUCTION Coal combustion are the primary source of anthropogenic mercury (Hg) emissions around the world.1,2 In the U.S., the Mercury and Air Toxics Standards (MATS) require coal-fired power plants to control 91% of the Hg emission to the air.3 In addition to mercury, nitrogen oxide (NOx) emission from coal combustion is another environmental concern. Since 2005, the selective catalytic reduction (SCR) of NOx by ammonia has been widely applied in U.S. utilities.4 In China, more and more power plants have installed or are installing SCR and wet flue gas desulfurization (WFGD) systems to control the emission of NOx and sulfur oxide (SOx). A cobenefit of the SCR system is that the SCR catalyst can oxidize the vapor phase mercury from the elemental form (Hg0) to the oxidized form (Hg2+).5−7 The Hg2+ is soluble, which can be easily captured in a downstream WFGD system.8 Accordingly, a catalytic oxidation of Hg0 into Hg2+, followed by the WFGD system, is thought to be the most convenient and economical option to reduce Hg emissions and is also recommended by the U.S. Environment Protection Agency (EPA).3 Therefore, several metal oxide catalysts used for SCR of NOx have been reported for their oxidation of Hg0, such as V2O5,9,10 MnO2,11 and CeO2.12,13 Among these metal oxides, CeO2-based catalysts have received a lot of attention because of their catalytic oxidation activity of Hg0,12−14 nontoxicity, and low-cost.15 Recent experimental studies indicated that CeO2-based catalysts show an excellent low temperature Hg oxidation activity,12−14 and its efficiency depends on the concentration of © 2015 American Chemical Society

Hg 0 + 2HCl + 1/2O2 → HgCl2 + H 2O

(1)

However, the detailed mechanisms for Hg oxidation over a CeO2-based catalyst, which is essential for developing the model of the reaction and the kinetic parameters, are still not clear. Three possible mechanisms have been proposed for Hg catalytic oxidation, and they are the Eley−Rideal (E-R) mechanism, Langmuir−Hinshelwood (L-H) mechanism, and Mars−Maessen mechanism (M-M).19 In the E-R mechanism, one of the reactants (Hg0 or HCl) is adsorbed on the catalyst, and then the adsorbed reactant reacts with the other gas-phase reactant to form HgCl2. In the L-H mechanism, two reactants (Hg0 and HCl) are adsorbed on the catalyst’s surface and react with each other, forming HgCl2. In the M-M mechanism, Hg would react with a lattice oxidant of the catalyst that is replenished from the gas phase (either O2 or Cl2) to form HgO or HgCl2 directly. However, the reaction mechanisms of Hg adsorption and oxidation on the CeO2 surface is still not clear. There is a lack of theoretical studies for Hg and CeO2 Received: January 21, 2015 Revised: June 7, 2015 Published: June 10, 2015 15047

DOI: 10.1021/acs.jpcc.5b00645 J. Phys. Chem. C 2015, 119, 15047−15055

Article

The Journal of Physical Chemistry C

Figure 1. Slab models of CeO2(111) surface: (a) CeO2 unit cell; (b) top view of CeO2(111) with four different adsorption sites; (c) side view of CeO2(111) surface; (d) two different CeO2(111) surface configurations. The red and yellow spheres represent the O and Ce atoms, respectively.

H, Cl, and O atoms. A double numerical basis set with polarization functions and a real space cutoff of 4.5 Å were used. Spin-polarized geometry optimization was also carried out in all calculations. The k points of a (4 × 4 × 4) Monkhorst− Pack grid were used for bulk lattice optimizations. The crystal structure of CeO2 is of a cubic fluorite structure with a space group Fm3̅mm, as shown in Figure 1a. The optimized unit cell parameters (a = b = c = 5.476 Å) were within +1.2% error of the experimental determined lattice constants,30 which suggests that the calculations are reliable. The CeO2(111) surface is the most thermodynamically stable bulk termination and catalytically active surface.31−33 There are two kinds of oxygen atoms: the surface oxygen atom Os and the subsurface oxygen Osub, as shown in Figure 1b. The surface was modeled by nine-layer slabs, and the bottom three layers of atoms were fixed in the bulk positions, while the top layers of atoms were allowed to relax, as shown in Figure 1c. The surface slab was constructed by stacking three units of oxygen-cerium-oxygen layers for a total of nine atomic layers. Thus, the slab is symmetric in the z direction, which resulted in a zero dipole moment in the z direction of the surface. Two surface cells of CeO2(111) were modeled: a p(2 × 2) surface cell for a coverage of 1/4 ML and a p(3 × 3) surface cell for a coverage of 1/9 ML, as shown in Figure 1d. The vacuum region between slabs is 12 Å in order to avoid interactions between slabs. Converged (3 × 3 × 1) Monkhorst−Pack grids were employed for two coverage surfaces. The equilibrium geometries of HgCl and HgCl2 molecules were all examined in a large cell of 10 × 10 × 10 Å. The calculated bond distances of HgCl and HgCl2 are 2.527 and 2.312 Å, respectively, and a Cl− Hg−Cl bond angle of 180°. These values agree reasonably well with the experimental data.34 For the adsorption of HgCl,

interaction. The adsorbed state for Hg or HCl over a CeO2based catalyst is uncertain. Furthermore, the reaction pathway and the activated energy are still unclear. Quantum chemical calculations, especially the density functional theory (DFT), have been regarded as an important tool to understand catalysis reaction. DFT studies have been widely performed to elucidate the adsorption and oxidation reaction of Hg to the surface of a catalyst, such as carbonaceous surface,20 metal surface,21,22 and metal oxide surface.23−26 However, to the best of the authors’ knowledge, no theoretical investigations have been performed on the reaction mechanism of Hg with HCl on CeO2 catalyst. Therefore, a detailed investigation of Hg oxidation on the CeO2 surface is necessary. The objective of the work is to study the mechanism of Hg oxidation with HCl over the CeO2 surface using the DFT method. The structure and stability of CeO2 surfaces were investigated by ab initio atomistic thermodynamics. The interactions of Hg species and HCl with CeO2 were studied by the analyses of adsorption energies, adsorption geometries, and electronic structures. The energy profiles of the different oxidation pathways are given. Finally, the oxidation mechanism of Hg over the CeO2 catalyst is presented.

2. COMPUTATIONAL METHODS 2.1. Density Functional Theory Calculations. The density functional theory calculations were carried out using DMol3 code.27 The GGA (generalized gradient approximation) scheme28 and PBE (Perdew, Burke, and Ernzerhof) exchange correlation functional29 were used to describe the exchangecorrelation effects. A density functional semicore pseudopotential method was used for core electrons of Ce and Hg, and an all-electron method was used for core electrons of the 15048


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Figure 2. Surface free energies of eight CeO2 surfaces, as determined by ab initio thermodynamics under a wide range of partial pressures of O2 at 500 and 1000 K.

thus can be calculated for any one phase due to chemical equilibrium. Thus, the relationship between μCe and μO is established by bulk oxide per formula unit, Gbulk CeO2

HgCl2, and HCl molecules, both perpendicular and parallel configurations were considered. The adsorption energy (Eads) was calculated according to the equation Eads = E(adsorbate − substrate) − (Eadsorbate + Esubstrate)

bulk μCe + 2μO = GCeO 2

(2)

For gas-phase O2, H2, H2O, and HCl, the species chemical potential can be written as

where E(adsorbate−substrate), Eadsorbate, and Esubstrate are the total energies of the adsorbate/substrate system, the isolated adsorbate, and the substrate, respectively. All transition states (TSs) along the reaction pathway were searched using the linear synchronous transit/quadratic synchronous transit (LST/QST) combined with the conjugate gradient (CG) refinements method.35,36 Transition states were identified by the number of imaginary frequency with one negative frequency mode. 2.2. Ab Initio Thermodynamics Calculations. In the Hg oxidation experiments, the surface structure and reactivity of the CeO2 surface may be influenced by flue gas composition and temperature. The surface chemistry can be examined using the ab initio thermodynamic method by predicting the relative thermodynamic stability as a function of temperature (T) and pressure (p).37 Thus, in this work, the ab initio thermodynamic method was applied to study the thermodynamic stability of different CeO2 surfaces, including oxygen-vacancy, protonation, hydroxylation, and chlorination. The detailed description of the ab initio thermodynamic method was presented by previous studies;37,38 thus, only the relevant explanations are given. The stability of different CeO2 surfaces in the flue gas environment at a given temperature and pressure is evaluated based on the surface free energy γ(T, p). The surface free energy is defined as γ({pi }, T ) =

1 [G − 2A

∑ Niμi (pi , T )] i

(4)

μO =

1 gas GO 2 2

(5)

μH =

1 gas GH 2 2

(6)

μO + 2μH = G Hgas2O

(7)

gas μCl + μH = G HCl

(8)

For the CeO2 system, eq 3 can be rewritten as γ (T , p) =

1 bulk Eslab − NCeGCeO (T , p) 2 2

{

+ (2NCe − NO)μO(T , p) − μH O(T , p) 2

NH 2

} (9)

The relationship between the chemical potential of gas species and pressure follows the equation μi (T , p) = μi (T , p0 ) +

⎛ p⎞ 1 kBT ln⎜ 0 ⎟ 2 ⎝p ⎠

(10)

where μi(T,p0) was obtained from NIST-JANAF tables, and kB is the Boltzmann constant. The independent variables in eq 9 are μO and μH2O. At the oxygen-rich limit, gas-phase O2 begins to condense on the surface; at the oxygen-poor limit, O2 begins to decompose from the bulk CeO2. We defined the relative chemical potential of oxygen to be ΔμO = μO − (1/2)Ggas O2 . Thus, the range of ΔμO is defined by the following relationship 39

(3)

Here, G is the Gibbs free energy of the surface neglecting atomic vibrations. The Gibbs free energy of the surface is obtained from the DFT total energy of the surface at 0 K, Eslab. A is the surface area. μi(pi,T) is the chemical potential, i, in this work, i = Ce, O, H, and Cl. Ni is the total number of atoms in component i of the system. μi is the same in each phase and 15049


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Figure 3. Adsorption configurations of Hg0 on the CeO2(111) surface. 1A, Os-top site; 1B, Ce-top site; and 1C, Os-bridge. The red, yellow, and blue spheres represent the O, Ce, and Hg atoms, respectively.

1 bulk bulk (GCeO2 − GCe − GOgas2 ) < ΔμO < 0 2

surfaces of CeO2 was studied, as shown in Figure S2 (Supporting Information), and the corresponding surface structures are shown in Figure S3 (Supporting Information). As shown in Figure S2, in the oxygen-poor region (OPR), the protonated surfaces have lower surface energies, suggesting that protonated surfaces are more stable under reduction conditions. In the oxygen-rich region (ORR), the stability of protonated surfaces decreases with oxygen partial pressures, suggesting that pronation surfaces of CeO2 take place at low oxygen partial pressures. For the molecular adsorption of H2O on the CeO 2 surface (“H 2 O CeO 2 (111)” and “H 2 O CeO2(110)” shown in Figure S2), the two structures have higher surface free energies and are not stable under SCR flue gas conditions, which agrees well with the experimental result that CeO2-based catalysts have resistance to water vapor.41 However, under SCR flue gas conditions (400 K < T < 600 K, pO2 ≈ 0.05 atm, pH2O ≈ 0.09 atm), the order of stability for CeO2(111) and (110) surfaces is protonated surfaces > stoichiometric surfaces > hydroxylated surfaces. The protonated surfaces that involved H atoms interacting with Os atoms are the most stable. Therefore, the presence of H2O in the flue gas may affect the CeO2 surface by protonation, which will influence the catalytic reactivity of CeO2. In addition, the thermodynamic stability studies of oxygen-vacant, protonated, and hydroxylated surfaces indicate that the (111) surface of CeO2 is more stable than the (110) surface. Thus, in the following studies, the calculations of chlorinated surfaces focus on the CeO2(111) surface. The role of HCl in the oxidation of Hg may depend on the nature of the catalyst’s surface. The thermodynamic stability of two chlorinated surfaces is shown in Figure S4 (Supporting Information), and the corresponding surface structures are shown in Figure S5 (Supporting Information). The region on the left of the vertical green dotted line in Figure S4 shows that the surface free energies of chlorinated surfaces are lower than those of the stoichiometric surface under flue gas conditions (T > 300 K, 20 ppm of HCl), suggesting that the chlorine species can stably exist over the CeO2 catalyst. Therefore, the stability is in the order of chlorinated > stoichiometric > oxygen-vacancy surface. Considering the low HCl concentration in flue gas, the chlorinated and stoichiometric surfaces can coexist under flue gas conditions. It is important to note that the stability of reduced, protonated, hydroxylated, and chlorinated surfaces is

(11)

bulk gas where Gbulk CeO2 − GCe − GO2 is the Gibbs free energy of the CeO2 2 formation of oxide, ΔGf . The calculated value for ΔGCeO is f −10.10 eV. Thus, eq 11 is defined as −5.05 < ΔμO < 0 eV. On the other hand, the presence of HCl may lead to surface chlorination under flue gas conditions. Equation 9 can be rewritten as

γ (T , p) =

1 bulk {Eslab − NCeGCeO ( T , p) 2 2 + (2NCe − NO)μO(T , p) − NHClμHCl − NHμH }

(12)

Here, the independent variable is μHCl, and μO is defined by eq 7.

3. RESULTS AND DISCUSSION 3.1. Structure and Stability of CeO2 Surfaces. During oxidation reactions on the CeO2 catalyst, the labile oxygen vacancies and bulk oxygen species can be easily formed. It is necessary to investigate the stability of oxygen-vacancy CeO2 surfaces as a function of temperature and oxygen partial pressure. Figure 2 shows the thermodynamic stabilities of stoichiometric and oxygen-vacancy surfaces of CeO2(111) and (110), and Figure S1 (Supporting Information) shows the corresponding surface structures. Figure 2 shows that that CeO2(111) surface (44.16 meV/Å2) is more stable than the (110) surface (61.06 meV/Å2), which is consistent with the previous study.40 Under the SCR flue gas conditions (400 K < T < 600 K, pO2 ≈ 0.05 atm), the stoichiometric CeO2(111) surface is more stable than oxygen-vacancy structures. For the CeO2(111) surface, a decrease in μO (i.e., a decrease in the partial pressure of O2 or an increase in temperature) would favor the oxygen vacancy formation. The CeO2(111) surface with 1/4 ML oxygen-vacancy forms more easily than that of 1/ 2 ML oxygen-vacancy, but the difference is not obvious. A similar phenomenon occurs on the CeO2(110) surface. H2O in the flue gas may alter the structure of the CeO2 surface by hydroxyl and hydrogen groups since the mass fraction of H2O is about 9 wt % in flue gas. Thus, the thermodynamic stability of 10 protonated and hydroxylated 15050


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Table 1. Adsorption Energies (kJ/mol), Geometric Parameters (Å), and Mulliken Charge (e) for Hg and HCl Adsorption on the CeO2(111) Surface (3 × 3) surface cell

(2 × 2) surface cell

Eads

RHg−X

QHg

Eads

RHg−X

QHg

3.190 3.793 3.080/3.160 RH−O/RCl−O

0.03 0.01 0.03 RH−Cl

−6.89 −3.04 −8.37 Eads

3.483 3.711 3.084/3.175 RH−O/RCl−O

0.02 0.01 0.03 RH−Cl

0.991 1.010

2.122 1.978

−175.06 −169.46

1.002 1.015

2.105 1.977

Hg

1A 1B 1C

−8.32 −3.15 −9.01 Eads

HCl

3A 3B

−181.50 −173.12

S6) with an adsorption energy of −21.73 kJ/mol. Therefore, when Cl is preadsorbed on the surface, the Hg adsorption energies are in general greater than that on the clean surface and Hg prefers to bind to surface Cl sites, suggesting an effective attraction between the Hg and chorine. 3.3. HCl Adsorption and Dissociation on CeO2(111) Surface. HCl is the most important reactant for Hg0 oxidation reaction due to its predominant concentration compared to Cl2 (∼1% of HCl concentration) in flue gas.42 From the results of thermodynamic stability studies, chlorinated surfaces can stably exist under the flue gas conditions. The studies of adsorption and dissociation of HCl over a CeO2 surface will help one to understand the reactivity of the surface and the whole Hg oxidation reaction. All possible adsorption sites and adsorption orientations (including parallel and perpendicular) were considered. The stable adsorption configurations of HCl on the CeO2(111) surface are shown in Figure 4, and the

studied under an individual flue gas and compared with the results of stoichiometric surfaces. Further calculations for H2O and HCl adsorption on the CeO2(111) surface suggest that the adsorption energy of H2O (−51.07 to −61.06 kJ/mol) is significantly less than that of HCl (−173.12 to −181.50 kJ/ mol). On the other hand, the partial pressure of H2O is much larger than that of HCl in the flue gas, which may promote the formation of protonated and hydroxylated surfaces. On the basis of the above analyses, it is suggested that there is a competition between different surfaces. 3.2. Hg0 Adsorption on CeO2(111) Surface. Elucidating gas−surface interactions involved in the Hg oxidation reaction is the first step toward Hg oxidation mechanism. The adsorption of Hg0 on the stoichiometric CeO2(111) surface was studied because the ab initio thermodynamic studies suggest that the stoichiometric CeO2(111) surface can stably exist under flue gas conditions. Figure 3 shows Hg adsorption configurations on the CeO2(111) surface. The calculated adsorption energies and the corresponding geometric parameters for Hg0 adsorbed on the adsorption sites are listed in Table 1. The stability of Hg adsorption on the CeO2(111) surface is in the trend of 1B < 1A < 1C from the adsorption energies results. The most stable adsorption structure for Hg is 1C at the Os bridge site with an adsorption energy of −9.01 kJ/ mol, suggesting a weak interaction. The surface coverage has little effect on Hg adsorption energies. The Mulliken charge of Hg is ranging from 0.01 to 0.03 e, suggesting that few electrons are transferred from Hg to the substrate, which is consistent with the result of adsorption energies. Therefore, the adsorption mechanism of Hg0 on CeO2 is physisorption. The X-ray photoelectron spectroscopy (XPS)18 results of CeO2based catalysts indicated that Hg0 is mainly physically adsorbed on CeO2-based catalysts. The calculated results of Hg adsorption on the CeO2 surface agree well with the experimental results. In addition, the Cl adsorption over the CeO2(111) surface is shown in 2A of Figure S6 (Supporting Information) since the chlorinated surface can exist under SCR flue gas conditions. It was found that Cl is adsorbed on the Os top site and the adsorption energy is −105.61 kJ/mol, suggesting a strong interaction. Three different configurations of Hg adsorbed on oxygen or chlorine sites were optimized, as shown in Figure S6. The adsorption of Hg is defined as Eads = E(surface-Cl-Hg) − E(surface-Cl) − E(Hg). In particular, the configuration with Hg adsorbed on the next-nearest oxygen site (2B in Figure S6) has an adsorption energy of −9.62 kJ/mol, while the configuration with Hg adsorbed on the adjacent oxygen (2C in Figure S6) has an adsorption energy of −76.62 kJ/mol. In this case, the Cl−Os bond is broken with a distance of 3.168 Å and the Cl− Hg bond is formed with a distance of 2.557 Å. The third configuration is Hg adsorbed on the Cl-top site (2D in Figure

Figure 4. Adsorption configurations of HCl on the CeO2(111) surface. The red, yellow, white, and green spheres represent the O, Ce, H, and Cl atoms, respectively.

adsorption energies and geometric parameters are given in Table 1. The most stable structure for HCl adsorption on the CeO2(111) surface is 3A with an adsorption energy of −181.50 kJ/mol, showing a chemical adsorption. In 3A, the H−Cl bond is elongated from 1.284 Å (free HCl) to 2.122 Å, and the bond length of H−O is 0.991 Å, which suggests that the H−Cl bond is broken after HCl adsorption on the surface. The stable configuration 3B is similar in geometric structure to 3A. In addition, the adsorption energies of HCl on the CeO2(111) surface is slightly higher at low coverage, indicating a decreasing interaction between HCl molecules. Heterogeneous catalytic reaction involves adsorption of reactant, activation, and reaction of adsorbed species. Thus, 15051


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3.4. HgCl and HgCl2 Adsorption on CeO2(111) Surface. Recent studies have suggested that HgCl is likely an intermediate link in the Hg oxidation reaction.21,38 Thus, it is necessary to calculate the adsorption of HgCl on the CeO2 surface to understand the role of HgCl during Hg oxidation. The stable configurations of HgCl on the CeO2(111) surface are shown in Figure S7 (Supporting Information), and the adsorption energies and geometries are shown in Table 2. The stability of the surface complex is in the order of 4A > 4B > 4C on the CeO2(111) surface. The most stable configuration for HgCl adsorption is 4A with Hg-down orientation on the Os-top site, indicating that HgCl prefers binding to oxygen sites of the CeO2(111) surface. In 4A, the adsorption energy of HgCl is −146.57 kJ/mol, suggesting a strong chemisorption behavior. The bond lengths of Hg−O and Hg−Cl are 2.021 and 2.308 Å, respectively. In addition, the adsorption energies increase at lower coverage for all configurations due to the increased repulsive interaction between HgCl molecules. The adsorbed HgCl will change the electronic structural character of surface atoms, making them less reactive for another HgCl. The strong adsorption of HgCl suggests that this intermediate species may exist as a surface-bound intermediate on the surface and react with the surface Cl species to form HgCl2. HgCl2 is the product in the Hg oxidation reaction.20 A strong or weak interaction between HgCl2 and surface may have a great influence on the catalytic activity of CeO2. The stable configurations for HgCl2 adsorption on the surface are shown in Figure S8 (Supporting Information), and the adsorption energies and geometries are listed in Table 2. It is found that the most stable configuration for HgCl2 on the CeO2(111) surface is 5B, which is on the Os-top site with the side-on mode, as shown in Figure S8. In 5B, the adsorption energy is −46.55 kJ/mol, suggesting that adsorption of HgCl2 is between chemical and physical adsorption. The distance between Hg and the surface O atom is 2.509 Å. The angle between Hg and Cl decreases from 180.0° (gaseous HgCl2) to 170.1°, and the bond lengths between Hg and Cl increase from 2.313 Å (gaseous HgCl2) to 2.431 and 2.432 Å. Again, adsorption energies are a little higher at low surface coverage due to a decreased repulsive interaction between HgCl2 molecules on the CeO2(111) surface. 3.5. Reaction Mechanism of Hg Oxidation by HCl on CeO2 Surface. The Hg oxidation pathway on the CeO2 surface starts from gaseous Hg0 and the HCl predissociated surface to form IM2 and IM2′, and this step is barrierless. Figure 6 shows the reaction pathways and energy profiles of Hg oxidation on the CeO2(111) surface via (a) IM2 and (b) IM2′, respectively. Two pathways for Hg oxidation on the CeO2 surface via IM2 and IM2′ were investigated and are shown in Figure 6. The pathway 1 goes through two steps: (1) Hg → HgCl and (2)

after studying the adsorption of HCl, the dissociation of HCl (i.e., activation of HCl) on the CeO2(111) surface was investigated. The possible dissociation of a HCl molecule on the CeO2(111) surface was investigated in the following manners: (1) Os−H and Os−Cl (where H and Cl are both bonded to the Os sites), (2) Os−H and Ce−Cl (where H and Cl are bonded to the Os and Ce sites, respectively); and (3) Os−H and Osub−Cl (where H and Cl are bonded to the Os and Osub sites, respectively). After geometry optimization, the three manners are all energetically favored, resulting in negative adsorption energies. The first manner (H and Cl are both bonded to the Os sites) is the most stable with the lowest energy. The energy profile including intermediate, transition state, and final state for HCl dissociation is shown in Figure 5.

Figure 5. Energy profile of HCl dissociation on the CeO2(111) surface.

The energy barrier is 19.57 kJ/mol, and the dissociation reaction is exothermic by 40.66 kJ/mol. The results are in agreement with the in situ Fourier transform infrared spectroscopy (FTIR) experiment, which suggested that surface chlorination occurs via dissociative HCl adsorption and formation of OH groups.43 The relatively low energy barrier (19.57 kJ/mol) suggests that HCl dissociation on the CeO2(111) surface is likely to occur at low temperature, which is consistent with the experimental findings that CeO2 catalysts maintain high activity below 300 °C.16 On the basis of the above analyses, it can be concluded that HCl easily dissociates on the CeO2(111) surface and transforms into surface hydroxyl and surface chlorine species, which verifies the previous experimental predictions.43

Table 2. Adsorption Energies (kJ/mol) and Optimized Parameters (Å) for HgCl and HgCl2 Adsorption on the CeO2(111) Surface p(3 × 3) surface cell HgCl

HgCl2

4A 4B 4C 5A 5B

Eads

RHg−O

−146.57 −131.82 −42.41

2.021 2.357

p(2 × 2) surface cell

RCl−O

RHg−Cl

Eads

RHg−O

−139.48 −127.61 −35.81

2.057 2.374

2.421

2.308 2.544 2.640

RCl−O

RHg−Cl

2.447

2.312 2.517 2.666

Eads

RHg−O

RHg−Cl

θHgCl2

Eads

RHg−O

RHg−Cl

θHgCl2

−36.48 −46.55

2.522 2.509

2.341/2.343 2.431/2.432

172.4 170.1

−32.37 −43.13

2.531 2.529

2.371/2.372 2.387/2.384

173.0 172.8

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Figure 6. Reaction pathways and energy profiles of Hg oxidation on the CeO2(111) surface via (a) IM 2 and (b) IM 2′, respectively.

between them, and pathway 1 (Hg → HgCl → HgCl2) is dominant because of its lower energy barriers. Overall, the results of Hg oxidation pathways on the CeO2 surface show that the Hg oxidation reaction on the CeO2 surface prefers the pathway: Hg → HgCl → HgCl2, rather than a pathway directly oxidizing Hg to HgCl2. The rate-limiting step is the second step HgCl → HgCl2, i.e., the formation of HgCl2. On the basis of the above analysis, gaseous Hg reacts with a surface Cl generated through a surface motivated HCl dissociation to form HgCl; it seems that Hg oxidation occurs via the E-R mechanism. However, in the following step (HgCl → HgCl2), HgCl and Cl are coadsorbed on the CeO2(111) surface, as shown in IM3 and IM3′ of Figure S9 (Supporting Information), indicating that the step is via the L-H mechanism. Therefore, the Hg oxidation is most likely to proceed with the E-R mechanism at the first step (Hg → HgCl), followed by the L-H mechanism at the step of HgCl → HgCl2. Moreover, by comparing the energy barriers of HgCl2 formation on CeO2(111) to that on Au(111),21 Pd(100),36 and V2O5/TiO2(001)44 surfaces, it is found that the energy barriers of HgCl2 formation on CeO2(111) (59.39 and 50.11 kJ/mol in the case via IM2 and IM2′, respectively) are slightly higher than that on the Au(111) surface (from 33 to 55 kJ/ mol), but lower than that on the Pd(100) surface (67.53 kJ/ mol), and also lower than that on the V2O5/TiO2(001) surface (91.53 kJ/mol). Au and Pd noble metals have been proven to be very active to facilitate the oxidation of Hg0 to Hg2+ in various experimental studies.45,46 The V2O5/TiO2 has been extensively used as SCR catalyst and has also been proven to oxidize Hg0 to Hg2+. Therefore, low energy barriers for the Hg oxidation reaction over the CeO2 surface will make it an attractive alternative for Hg oxidation in current SCR catalysts.

HgCl → HgCl2. The pathway 2 goes through one step: Hg → HgCl2. The energies of the optimized intermediate (IM), transition state (TS), and final state (FS) are relative to the initial adsorption configuration. The related structures of intermediates, transition states, and final states are shown in Figure S9 (Supporting Information). In the case of the Hg oxidation reaction via IM2, the pathway 1 for Hg oxidation is via IM2 → TS2 → IM3 → TS3 → FS2, including two steps: (1) Hg → HgCl and (2) HgCl → HgCl2. The first step is exothermic by 47.79 kJ/mol with an energy barrier of 21.77 kJ/mol. During this step, the distance between Hg and Cl decreases gradually: 3.920/3.895 Å in IM2, 2.876/ 3.796 Å in TS2, and 2.480/3.796 Å in IM3, indicating the formation of HgCl. The second step (HgCl → HgCl2) is via the transition state TS3 and is endothermic by 12.70 kJ/mol. The energy barrier for this step is 59.39 kJ/mol higher than that in the first step, suggesting the rate-limiting step for the Hg oxidation reaction. In this step, a HgCl2 molecule is formed with Hg bound to the Os-top site and two Cl atoms turned upward. The whole reaction is exothermic by 35.09 kJ/mol. Pathway 2 (Hg → HgCl2) is via IM2 → TS4 → FS2. The energy barrier for pathway 2 is 78.05 kJ/mol higher than those of pathway 1, suggesting an unfavorable oxidation pathway. The result from the case of Hg oxidation via IM2 suggests that the Hg oxidation reaction tends to occur through the pathway 1 due to its lower energy barriers. In the case of the Hg oxidation reaction via IM2′, the two pathways for Hg oxidation are similar to those in the case via IM2. For the pathway 1, the first step (Hg → HgCl) is exothermic by 39.73 kJ/mol with an energy barrier of 28.45 kJ/ mol. The second step (HgCl → HgCl2) is endothermic by 9.46 kJ/mol with an energy barrier of 50.11 kJ/mol, suggesting the rate-limiting step. For the pathway 2, directly oxidizing Hg to HgCl2, the energy barrier is 56.92 kJ/mol, which is slightly higher than that of pathway 1. The result from the case of Hg oxidation via IM2′ suggests that the reaction may happen through the two pathways due to a small barrier difference

4. CONCLUSIONS The Hg oxidation mechanism by HCl over a CeO2 catalyst was studied using the density functional theory method. The stoichiometric CeO2(111) is the most stable surface. Hg0 is physically adsorbed on oxygen sites of the CeO2(111) surface. 15053


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(7) Dranga, B. A.; Lazar, L.; Koeser, H. Oxidation catalysts for elemental mercury in flue gases-A Review. Catalysts 2012, 2 (1), 139− 170. (8) Liu, J.; Qu, W.; Joo, S. W.; Zheng, C. Effect of SO2 on mercury binding on carbonaceous surfaces. Chem. Eng. J. 2012, 184, 163−167. (9) Kamata, H.; Ueno, S.-I.; Naito, T.; Yukimura, A. Mercury oxidation over the V2O5(WO3)/TiO2 commercial SCR catalyst. Ind. Eng. Chem. Res. 2008, 47 (21), 8136−8141. (10) Kamata, H.; Ueno, S.-I.; Naito, T.; Yamaguchi, A.; Ito, S. Mercury oxidation by hydrochloric acid over a VOx/TiO2 catalyst. Catal. Commun. 2008, 9 (14), 2441−2444. (11) Qiao, S.; Chen, J.; Li, J.; Qu, Z.; Liu, P.; Yan, N.; Jia, J. Adsorption and catalytic oxidation of gaseous elemental mercury in flue gas over MnOx/alumina. Ind. Eng. Chem. Res. 2009, 48 (7), 3317− 3322. (12) Wen, X.; Li, C.; Fan, X.; Gao, H.; Zhang, W.; Chen, L.; Zeng, G.; Zhao, Y. Experimental study of gaseous elemental mercury removal with CeO2/γ-Al2O3. Energy Fuels 2011, 25 (7), 2939−2944. (13) Tian, L.; Li, C.; Li, Q.; Zeng, G.; Gao, Z.; Li, S.; Fan, X. Removal of elemental mercury by activated carbon impregnated with CeO2. Fuel 2009, 88 (9), 1687−1691. (14) Fan, X.; Li, C.; Zeng, G.; Gao, Z.; Chen, L.; Zhang, W.; Gao, H. Removal of gas-phase element mercury by activated carbon fiber impregnated with CeO2. Energy Fuels 2010, 24 (8), 4250−4254. (15) Nair, J. P.; Wachtel, E.; Lubomirsky, I.; Fleig, J.; Maier, J. Anomalous expansion of CeO2 nanocrystalline membranes. Adv. Mater. 2003, 15 (24), 2077−2081. (16) Li, H.; Wu, C.-Y.; Li, Y.; Zhang, J. CeO2−TiO2 catalysts for catalytic oxidation of elemental mercury in low-rank coal combustion flue gas. Environ. Sci. Technol. 2011, 45 (17), 7394−7400. (17) Wan, Q.; Duan, L.; He, K.; Li, J. Removal of gaseous elemental mercury over a CeO2-WO3/TiO2 nanocomposite in simulated coalfired flue gas. Chem. Eng. J. 2011, 170 (2), 512−517. (18) Hua, X. Y.; Zhou, J. S.; Li, Q.; Luo, Z. Y.; Cen, K. F. Gas-phase elemental mercury removal by CeO2 impregnated activated coke. Energy Fuels 2010, 24 (10), 5426−5431. (19) Presto, A. A.; Granite, E. J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol. 2006, 40 (18), 5601−5609. (20) Liu, J.; Qu, W.; Zheng, C. Theoretical studies of mercurybromine species adsorption mechanism on carbonaceous surface. Proc. Combust. Inst. 2013, 34, 2811−2819. (21) Lim, D. H.; Wilcox, J. Heterogeneous mercury oxidation on Au(111) from First Principles. Environ. Sci. Technol. 2013, 47 (15), 8515−8522. (22) Aboud, S.; Sasmaz, E.; Wilcox, J. Mercury adsorption on PdAu, PdAg and PdCu alloys. Main Group Chem. 2008, 7 (3), 205−215. (23) Suarez Negreira, A.; Wilcox, J. Role of WO3 in the Hg oxidation across the V2O5-WO3-TiO2 SCR catalyst: A DFT study. J. Phys. Chem. C 2013, 117 (46), 24397−24406. (24) Sasmaz, E.; Wilcox, J. Mercury species and SO2 adsorption on CaO(100). J. Phys. Chem. C 2008, 112 (42), 16484−16490. (25) Zhang, B.; Liu, J.; Zheng, C.; Chang, M. Theoretical study of mercury species adsorption mechanism on MnO2(110) surface. Chem. Eng. J. 2014, 256, 93−100. (26) Xiang, W.; Liu, J.; Chang, M.; Zheng, C. The adsorption mechanism of elemental mercury on CuO(110) surface. Chem. Eng. J. 2012, 200, 91−96. (27) Delley, B. From molecules to solids with the DMol approach. J. Chem. Phys. 2000, 113, 7756. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77 (18), 3865. (29) Perdew, J. P.; Burke, K.; Wang, Y. Generalized gradient approximation for the exchange-correlation hole of a many-electron system. Phys. Rev. B 1996, 54 (23), 16533. (30) Gerward, L.; Olsen, J. S.; Vaitheeswaran, G.; Kanchana, V.; Svane, A.; Gerward, L.; Olsen, J. S.; Vaitheeswaran, G.; Kanchana, V.; Svane, A. Bulk modulus of CeO2 and PrO2An experimental and theoretical study. J. Alloys Compd. 2005, 400, 56.

HCl is chemically adsorbed on the CeO2(111) surface, and then dissociates with a low energy barrier. Energy pathway analyses indicate that Hg oxidation prefers the pathway of Hg → HgCl → HgCl2. The first step (Hg → HgCl) proceeds with the Eley−Rideal mechanism. The second step (HgCl → HgCl2) is via the Langmuir−Hinshelwood mechanism. In the whole Hg oxidation reaction, the formation of HgCl2 is the rate-determining step. The results suggest that the CeO2-based catalyst is an attractive alternative catalyst for Hg oxidation. It should be noted that, except HCl, the lattice oxygen may also play a role in mercury oxidation by CeO2. Further studies will be carried out to examine it.

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ASSOCIATED CONTENT

S Supporting Information *

The stoichiometric, oxygen-vacant, protonated, hydroxylated, and chlorinated surfaces of CeO2 studied in the thermodynamic stability work; the adsorption of Cl on clean the CeO2(111) surface and adsorption configuration of Hg on the chlorinated CeO2(111) surface; the adsorption configurations of HgCl and HgCl2 on the CeO2(111) surface; and the structures of intermediates, transition states, and final states in Hg oxidation pathways on CeO2(111) surface are provided in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b00645.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 27 87545526. Fax: +86 27 87545526. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51376072), the National Basic Research Program of China (2014CB238904), and the Natural Science Foundation of Hubei Province (2015CFA046).

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REFERENCES

(1) Global Mercury Assessment 2013: Sources, Emissions, Releases and Environmental Transport; United Nations Environment Programme: Geneva, Switzerland, 2013. (2) Liu, J.; Abanades, S.; Gauthier, D.; Flamant, G.; Zheng, C.; Lu, J. Determination of kinetic law for toxic metals release during thermal treatment of model waste in a fluid-bed reactor. Environ. Sci. Technol. 2005, 39 (23), 9331−9336. (3) The EPA updated the emission limits for new power plants under the: Mercury and Air Toxics Standards (MATS); EPA: Washington, DC, April 24, 2013. (4) Laudal, D. L. Power Plant Evaluation of the Effect of Selective Catalytic Reduction on Mercury; Electric Power Research Institute (EPRI) Report No. 005400; Electric Power Research Institute: Palo Alto, CA, 2002. (5) Wilcox, J.; Rupp, E.; Ying, S. C.; Lim, D. H.; Negreira, A. S.; Kirchofer, A.; Feng, F.; Lee, K. Mercury adsorption and oxidation in coal combustion and gasification processes. Int. J. Coal Geol. 2012, 90, 4−20. (6) Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal., B 2014, 144, 486−497. 15054


Article

The Journal of Physical Chemistry C (31) Lyons, D. M. Preparation of ordered mesoporous ceria with enhanced thermal stability. J. Mater. Chem. 2002, 12 (4), 1207−1212. (32) Lyons, D. M.; McGrath, J. P.; Morris, M. A. Surface studies of Ceria and mesoporous Ceria powders by solid-state 1H MAS NMR. J. Phys. Chem. B 2003, 107 (19), 4607−4617. (33) Yang, Z.; Woo, T. K.; Baudin, M.; Hermansson, K. Atomic and electronic structure of unreduced and reduced CeO2 surfaces: A firstprinciples study. J. Chem. Phys. 2004, 120 (16), 7741−7749. (34) Kaupp, M.; von Schnering, H. G. Origin of the unique stability of condensed-phase Hg22+. An ab initio investigation of MI and MII species (M = Zn, Cd, Hg). Inorg. Chem. 1994, 33 (18), 4179−4185. (35) Dai, G. L.; Li, Z. H.; Lu, J.; Wang, W. N.; Fan, K. N. Deep oxidations in the oxidative dehydrogenation reaction of propane over V2O5(001): Periodic Density functional theory study. J. Phys. Chem. C 2011, 116 (1), 807−817. (36) Zhang, B.; Liu, J.; Zhang, J.; Zheng, C.; Chang, M. Mercury oxidation mechanism on Pd(100) surface from first-principles calculations. Chem. Eng. J. 2014, 237 (0), 344−351. (37) Negreira, A. S.; Aboud, S.; Wilcox, J. Surface reactivity of V2O5(001): Effects of vacancies, protonation, hydroxylation, and chlorination. Phys. Rev. B 2011, 83 (4), 045423. (38) Suarez Negreira, A.; Wilcox, J. DFT Study of Hg oxidation across vanadia-titania SCR catalyst under flue gas conditions. J. Phys. Chem. C 2013, 117 (4), 1761−1772. (39) Chase, M. W. NIST-JANAF Themochemical Tables, Fourth Edition. J. Phys. Chem. Ref. Data, Monogr. 1998, 1−1951 (Monograph 9). (40) Branda, M. M.; Ferullo, R. M.; Causa, M.; Illas, F. Relative stabilities of low index and stepped CeO2 surfaces from hybrid and GGA+U implementations of density functional theory. J. Phys. Chem. C 2011, 115 (9), 3716−3721. (41) Xu, W.; Chen, Q.; Zhang, T.; Cai, Z.; Jia, X.; Xie, Q.; Ren, Y. Development and application of ultra performance liquid chromatography-electrospray ionization tandem triple quadrupole mass spectrometry for determination of seven microcystins in water samples. Anal. Chim. Acta 2008, 626 (1), 28−36. (42) Galbreath, K. C.; Zygarlicke, C. J. Mercury speciation in coal combustion and gasification flue gases. Environ. Sci. Technol. 1996, 30 (8), 2421−2426. (43) Amrute, A. P.; Mondelli, C.; Moser, M.; Novell-Leruth, G.; López, N.; Rosenthal, D.; Farra, R.; Schuster, M. E.; Teschner, D.; Schmidt, T. Performance, structure, and mechanism of CeO2 in HCl oxidation to Cl2. J. Catal. 2012, 286, 287−297. (44) Zhang, B.; Liu, J.; Dai, G.; Chang, M.; Zheng, C. Insights into the mechanism of heterogeneous mercury oxidation by HCl over V2O5/TiO2 catalyst: Periodic density functional theory study. Proc. Combust. Inst. 2015, 35, 2855−2865. (45) Hrdlicka, J. A.; Seames, W. S.; Mann, M. D.; Muggli, D. S.; Horabik, C. A. Mercury oxidation in flue gas using gold and palladium catalysts on fabric filters. Environ. Sci. Technol. 2008, 42 (17), 6677− 6682. (46) Blythe, G. D. K.; Machalek, T.; Richardson, C.; Richardson, M. Pilot Testing of Mercury Oxidation: Catalysts for Upstream of Wet FGD Systems, Final Report; Cooperative Agreement DE-FC26-01NT41185; EPRI: Palo Alto, CA, U.S. Department of Energy, National Energy Technology Laboratory: Pittsburgh, PA, October 2006.

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Heterogeneous Mercury Oxidation by HCl over CeO2 Catalyst

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