Density Functional Theory Study of Elemental Mercury Adsorption on

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Density Functional Theory Study of Elemental Mercury Adsorption on Fe2O3[104] and Its Effect on Carbon Deposit during Chemical Looping Combustion Junjiao Zhang, Wu Qin,* Changqing Dong,* and Yongping Yang National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Energy Engineering, North China Electric Power University, Beijing 102206, People’s Republic of China ABSTRACT: This study investigated the adsorption of elemental mercury (Hg0) on perfect and reduced Fe2O3[104] surfaces as well as analyzed the synergistic effect of Hg0 on the catalytic decomposition of CO on the reduced surfaces using density functional theory calculations. This study aimed to clarify the correlations among Hg0 adsorption, CO decomposition, and Fe2O3 reduction degree. Theoretical results indicated that Hg0 underwent Fe top site adsorption to Fe bridge site adsorption as Fe2O3 was reduced into iron. This phenomenon increased Eads with a local extreme of −2.063 eV for Hg0−Fe2O1.364. The adsorption of Hg0 decreased Eads for CO adsorption on the reduced Fe2O3 surfaces but promoted charge transfer from the surface to the adsorbed CO molecule. This event activated the C−O bond, favoring its decomposition. A kinetic model for catalytic CO decompositions with and without Hg0 further revealed the synergistic effect of Hg0 on carbon deposition and the relationship between the reaction rate and degree of chemical looping combustion.

1. INTRODUCTION Chemical looping combustion (CLC) is a new combustion and carbon capture technology that has received great attention because of its efficient use of energy and inherent separation of CO2.1−3 CLC produces heat and energy using oxygen carriers to supply oxygen instead of air for the combustion of fuel, reduction of NOx emissions, and generation of CO2 and H2O not diluted with N2.4−10 These properties render CLC highly desirable for low-cost coal combustion and carbon capture.11−13 The selection of an adequate oxygen carrier is crucial in CLC. Among the widely used metal oxides of Fe, Ni, Co, Cu, Mn, and Cd, iron-based oxygen carriers are the most attractive option because of their low cost and environmental compatibility. Using these advantages of iron-based oxygen carriers, researchers prepared loaded and multicomponent ironbased oxygen carriers to enhance oxygen transfer capacity and reactivity, avoid carbon deposition, and realize good repeatability.14−23 Considering the synthesis of metal oxides containing a large percentage of exposed facets with high catalytic reactivities, 24−26 we have recently prepared Fe 2 O 3 [104] and demonstrated its high reactivity and regeneration ability for the CLC of CO27,28 and lignite.29 Despite the interest in coal CLC using Fe2O3[104] as an efficient oxygen carrier, mercury emission from combustion sources poses a major concern. Mercury exists in coal at the range of 0.02−1.0 mg/kg30 and assumes three forms in flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp). Mercury is released into flue gas as Hg0 during coal combustion,31 emitted into the atmosphere, and then bioaccumulated through the food chain, which affects human health and generates long-lasting effects.32 Mendiara et al.33 first studied the release of mercury from coal CLC and then quantitatively tested the emissions from the fuel and air reactors of the CLC system. However, the interaction between © XXXX American Chemical Society

mercury and the gradually reduced oxygen carrier in CLC promotes the evolution of mercury. The interaction also changes the behavior of oxygen carriers in oxidizing the fuel molecule and generating carbon deposits. However, the correlations among the mercury−oxygen carrier interaction, CLC−fuel redox reaction, and reduction degree in CLC remain unknown. Considering our previous studies, the present work elucidated the detailed adsorption mechanism of Hg0 on perfect and reduced Fe2O3[104] surfaces as well as revealed the synergistic effect of Hg0 on carbon deposition during CLC using density functional theory (DFT) calculations. A pure Fe2O3[104] surface and a series of reduced Fe2O3[104] surfaces of different oxidation states were modeled to detect Hg0 adsorption. Then, catalytic CO decomposition reactions with and without Hg0 on the reduced Fe2O3[104] surfaces were discussed to understand the synergy of Hg0. Results provided insights into the process-tuning effect of Hg0 on carbon deposition during CLC. This study may serve as a reference for CLC optimization.

2. THEORETICAL METHODS Corundum-type α-Fe2O3 (hematite), the most common form of crystalline iron oxide, was modeled. The model contains iron and oxygen atoms arranged in a trigonal−hexagonal scalenohedral structure with space group R3̅c and lattice parameters a = b = 5.0356 Å and c = 13.7489 Å, with six formula units per unit cell.34 A four-layered Fe2O3[104]-p(2 × 1) super surface with 22 Fe atoms and 33 O atoms was modeled by cleaving the Fe2O3 crystalline cell along the [104] axis. A series of reduced Fe2O3[104]-p(2 × 1) surfaces was obtained by gradually removing the O atoms on the outer surface layer. Five cycle annealing calculations were performed under constant Received: August 26, 2015 Revised: March 4, 2016

A

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electronic energy Eel, to yield G ≅ Eel,slab.51 Similarly, the Gibbs free energy for the bulk system is approximated as follows: Eel,bulk ≅ G = NFeμFe + NOμO. To connect the chemical potential of Fe, O, and bulk Fe2O3 and to set up the poor limit of μO, the equilibrium with the bulk is considered as

pressure and temperature (NPT) until equilibrium states were reached, where the initial temperature is 300 K and the mid-cycle temperature is 1073 K (the temperature for real CLC). Further geometric optimization using DFT calculations was then conducted. Surface free energies were calculated to confirm the thermodynamic stability of the surface models, including Fe2O3, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe (Figure 1). The Hg0 adsorption and carbon deposition on these surfaces were investigated.

μFebulkO = 2μFe + 3μO

(2)

2 3

This formula represents the limit as the decomposition of the material when μO is extremely low. The rich limit of μO is considered as a gasphase equilibrium 1 1 EO ≅ μO = μO 2 2 2 2

(3)

where EO2 is the calculated total energy of the O2 molecule. In addition to eq 2 for the lower limit of μO, we also considered the limit in terms of reducing hematite. Moreover, we considered the limit at which α-Fe2O3 reduces to Fe2O2.636, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe. The adsorption energy for the studied systems was calculated using the following equation:

Eads = Ecompound − Eslab − Ex

(4)

where Ecompound, Eslab, and Ex are the total energies for the substrate with adsorbate, the substrate, and the adsorbate, respectively. A more negative adsorption energy represents a stronger interaction.

3. RESULTS AND DISCUSSION 3.1. Stoichiometric Surfaces. We modeled the perfect and reduced surfaces of different oxidation states (Figure 1). Table 1 summarizes their surface energies after energy minimization.

Figure 1. Top and side views of the perfect four-layer Fe2O3[104]-p(2 × 1) surface and the reduced surfaces: Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe (purple, Fe; red, O). In reference to works focusing on accurate calculations of hematitebased systems,35−37 all calculations were performed using a plane-wave pseudo-potential method based on dispersion-corrected density function theory (DFT-D) in Cambridge Sequential Total Energy Package (CASTEP).38 Ultrasoft Vanderbilt pseudo-potentials39 were used to describe core orbitals, and the exchange correlation potential was calculated within the generalized gradient approximation using the Perdew−Burke−Ernzerhof scheme.40,41 To correct the strong electronic correlation of the Fe 3d electrons, the DFT + U method (U = 5.0 eV) was applied in the form proposed by Dudarev et al.35,36 A plane-wave expansion with a cutoff of 300 eV was employed with second-order smearing and 0.1 eV width. The magnetic configuration (+ − − +) was set for Fe atoms in the rhombohedral unit cell of Fe2O3 to render the optimized cell at the lowest total energy.42−45 Up- and down-spin directions with respect to the z axis were designated by + and − , respectively. The Brillouin zone integration of the surfaces was calculated using 4 × 4 × 1 Monkhorst−Pack k-point meshes. The convergence criteria for the structure optimization and energy calculation were set to (a) an energy tolerance of 2.0 × 10−5 eV/ atom, (b) an SCF tolerance of 2.0 × 10−6 eV/atom, (c) a maximum force tolerance of 0.5 eV/nm, and (d) a maximum displacement tolerance of 0.0002 nm. The formulations of the linear and quadratic synchronous transit methods were used to search for the transition states in each reaction.46 The surface free energy γ is calculated in accordance with the firstprinciples thermodynamic framework of Reuter et al.47−49 and in the style detailed by Lo et al.50 for hematite. At P = 0 and T = 0, the surface free energy is defined as 1 1 ⎛ 1 ⎜Eslab − NFeμFe O (Eslab − E bulk ) = 2 3 ⎝ 2A 2A 2 ⎛3 ⎞ ⎞ + ⎜ NFe − NO⎟μO⎟ ⎝2 ⎠ ⎠

Table 1. Calculated Surface Free Energies for Perfect and Reduced α-Fe2O3[104] Surfaces γ (J m−2)

Fe2O3

Fe2O2

Fe2O1.364

Fe2O0.909

Fe2O0.182

Fe

1.27

0.88

1.09

1.28

1.35

1.54

The surface energies of Fe2O3, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe are 1.27, 0.88, 1.09, 1.28, 1.35, and 1.54 J m−2, respectively. These values are close to those obtained by de Leeuw et al.52 for the highly stable Fe3O4[001] (0.96 J m−2), Fe3O4[011] (1.37 J m−2), and Fe3O4[111] (1.10 J m−2) but are higher than that obtained for Fe3O4[001] (∼0.9 J m−2).53 The Fe3O4[001] and [11̅ 2] surfaces in hematite have similar surface energies; the surface energy of [1̅12] is lower than that of Fe3O4[001] by only ∼0.1 J/m2.53 These results indicate that Fe2O3, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe modeled in the present work are thermodynamically stable. In addition, these surfaces are chemically active, which can be explained by the fact that metal oxides with high index facets possess more highly reactive surfaces than those with low index facets.54,55 3.2. Hg 0 Adsorption on Perfect and Reduced Fe2O3[104] Surfaces. The perfect four-layer Fe2O3[104] slab has a rectangular surface with side lengths of 16.82 and 5.42 Å, which consist of 22 Fe and 33 O atoms (Figure 1). The Fe2O3[104] surface contains a four-folded Fe atom, a fivefolded Fe atom, a six-folded Fe atom, a two-folded O atom, and a three-folded O atom. This result indicates that CLC facilitates the gradual reduction of the perfect Fe2O3[104] surface to a series of defected surfaces and finally to an Fe22 slab. The chemical environments of atoms on the perfect and reduced surfaces differ, and many possible active sites for Hg0 adsorption exist. Four random Hg0 adsorption models on different sites of the perfect and reduced surfaces were

γ=

(1)

where Eslab and Ebulk are the total energies of the slab and bulk systems with the same number of atoms as in the slab, respectively, and A is the surface area of the unit cell. The Gibbs free energy, G = Eel,slab + Evib + Eother,internal + PV − TS, is approximated by the dominant term, i.e., the B

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and silver nanoparticles.57 Thus, Hg0 adsorption would inevitably alter the properties of the reduced Fe2O3[104] surfaces and, hence, affect the adsorption of CO. Figure 4

considered. Figure 2 illustrates the energies for Hg0 adsorption (Eads) on Fe2O3, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe.

Figure 2. Eads for Hg0 adsorption on four sites of the perfect and reduced Fe2O3[104] surfaces (pink line represents the average Eads).

Figure 4. Eads of CO on the reduced Fe2O3 surfaces with and without Hg0.

The average Eads values for Hg0 adsorption on Fe2O3, Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe are −0.204, − 0.593, − 2.063, − 0.480, − 0.583, and −2.104 eV, respectively. The average Eads changes gradually with the reduction of Fe2O3 from a high to a low oxidation state. The peak value of the average Eads is −2.063 eV, which can be found in the Hg−Fe2O1.364 interaction system. However, the strongest interaction case (with the highest Eads) among the four random adsorption cases on each surface also shows a similar phenomenon to the average Eads that gradually changes with the reduction of Fe2O3. The difference in Eads for each adsorption case on the same surface is more pronounced on the reduced surfaces than on the perfect surface. This result implies that the electronic properties of the adsorption sites on the perfect surface are similar, whereas those on the reduced surfaces considerably differ. The relatively most stable Hg0 adsorption models are shown in Figure 3. In the Hg0−Fe2O3, Hg0−Fe2O2.64, Hg0−Fe2O2, and Hg0−Fe2O0.91 systems, the Hg0 bonds to the top site of the Fe atoms possess distances of 2.990, 2.767, 2.758, and 2.620 Å, respectively. The Fe top site adsorption and Eads in the Hg0− Fe2O3 system are similar to those in the Hg0−Fe2O3[0001] system.56 In the Hg0−Fe2O1.36, Hg0−Fe2O0.18, and Hg0−Fe systems, the Hg0 bonds to the bridge site of the Fe atoms. The stronger interaction in these systems results in higher Eads values than the single-bonded adsorption cases (i.e., Hg0− Fe2O3, Hg0−Fe2O2.64, Hg0−Fe2O2, and Hg0−Fe2O0.91 systems). 3.3. Synergistic Effect of Hg0 Adsorption on CO Adsorption. Mercury adsorption alters the properties of gold

compares the Eads for CO adsorption on the reduced Fe2O3[104] surfaces with and without Hg0. As shown in Figure 4, Hg0 decreases the interaction between CO and the surfaces, thereby reducing the Eads for CO adsorption on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe. To elucidate in detail the effect of Hg0 on CO adsorption, Figure 5 describes the relationship between the reduction

Figure 5. Charge population (Q) for the C atom, O atom, and CO molecule and the length of C−Fe and C−O bonds (a) over the reduced Fe2O3 surfaces and (b) over the reduced Hg0−Fe2O3 surfaces.

Figure 3. Most stable adsorption configurations for the Hg0−Fe2O3, Hg0−Fe2O2.636, Hg0−Fe2O2, Hg0−Fe2O1.364, Hg0−Fe2O0.909, Hg0−Fe2O0.182, and Hg0−Fe systems. C

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Energy & Fuels degree and charge population (Q) on the C atom (QC) and the O atom (QO) as well as the relationship between the reduction degree and bond length (L) for the adsorbed CO molecule (LC−O) and the newly formed Fe−C bond (LC−Fe). The total charge population on the adsorbed CO molecule can be calculated as QCO = QC + QO. In Figure 5a, the QC and QO for the adsorbed CO molecule on Hg0−Fe2O2, Hg0−Fe2O1.364, Hg0−Fe2O0.909, Hg0−Fe2O0.182, and Hg0−Fe are −0.31 and 0.20, −0.34 and 0.13, −0.37 and 0.10, −0.37 and 0.11, and −0.36 and 0.00, respectively, whereas those on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe are −0.29 and 0.21, −0.32 and 0.13, −0.33 and 0.11, −0.36 and 0.11, and −0.36 and 0.06, respectively. The atomic charge population shows that 0.11, 0.21, 0.27, 0.26, and 0.36 e transport from Hg0− Fe2O2, Hg0−Fe2O1.364, Hg0−Fe2O0.909, Hg0−Fe2O0.182, and Hg0−Fe to the adsorbed CO molecule, respectively, whereas 0.08, 0.19, 0.25, 0.25, and 0.30 e transport from Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe to the adsorbed CO molecule, respectively. Although the charge transfer is limited, the presence of Hg0 promotes charge transfer from the surface slabs to the adsorbed CO molecule. In comparison to the LC−O (1.129 Å) calculated for the pure CO molecule, the LC−O values of the adsorbed CO molecule on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe are lengthened to 1.164, 1.174, 1.181, 1.182, and 1.184 Å, respectively. The presence of Hg0 further lengthens LC−O. In Figures 4 and 5, although Hg0 decreases Eads for CO adsorption on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe, Hg0 promotes electron transfer from the reduced surface to the adsorbed CO molecule, which further activates the adsorbed CO molecule. The following section focuses on the synergistic effect of Hg0 adsorption on CO decomposition on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe. 3.4. Synergistic Effect of Hg0 Adsorption on CO Decomposition. CO decomposition on the reduced oxygen carrier contributes to carbon deposition during CLC.28,58 Therefore, we considered the following reaction mechanism on the reduced Fe2O3[104] surfaces:

CO + * ↔ CO* CO* + * ↔ C* + O*

Figure 6. Barrier energy diagrams for CO decomposition into C and O on the reduced Fe2O3[104] surfaces with and without the adsorption of Hg0 at the reversible potential.

deposition may occur after Fe2O0.909, corresponding to the weight increase of the thermogravimetric curve of CO− Fe2O3.28 Specifically, Fe-catalyzed CO decomposition crosses a rather low barrier energy, and the amount of carbon deposited is directly proportional to the number of iron sites present on the surface.59 However, the adsorption of Hg0 obviously decreases the reaction barrier for the catalytic decomposition of CO* on each surface at the same oxidation state. The decomposition of CO* occurs around and after Fe2O1.364 in the presence of Hg0. The atomic charge distribution of the system at the transition state of the reaction shows that Hg0 promotes electron transfer from the surface slabs to adsorbed CO through the reaction, corresponding to the atomic charge analysis for the initial state. The effect of Hg0 on the promotion of charge transfer to activate the C−O bond of the adsorbed CO molecule contributes to the acceleration of carbon deposition during CLC. To address the trends in CO* decomposition accurately, a kinetic model was established for treating the different time scales associated with the reduction of Fe2O3 in CLC. We applied a simple phenomenological model to reveal the trends. The net rates for reactions I and II can be expressed as

(I) (II)

r1 = k1θ pCO − k −1θCO *

where ∗ denotes a free step site. The catalytic decomposition of CO may result in a wide range of products. We have neglected the further interaction among C*, O*, and CO that forms CO2 or carbonate species on the reduced surfaces. Considering that CO decomposition can hardly occur on the surface of iron oxide at a high oxidation state,28 we focused on the decomposition of CO on Fe2O2, Fe2O1.364, Fe2O0.909, Fe2O0.182, and Fe with and without the adsorption of Hg0 to reveal the synergistic effect of Hg0 adsorption on CO decomposition. The reaction initiates from the most stable CO adsorption configuration and arrives at the decomposition of CO* into C* and O* as the final state. Transition state search calculations were performed to detect the energy profiles of catalytic CO* decomposition reactions on these reduced surfaces with and without Hg0 adsorption. The results of these calculations are listed in Figure 6. Fe2O2-catalyzed, Fe2O1.364catalyzed, and Fe2O0.909-catalyzed CO* decomposition reactions are energetically close because of the relatively high reaction barrier energies, whereas Fe2O0.182-catalyzed and Fecatalyzed CO* decomposition reactions are highly accessible because of the relatively low reaction barrier energies. Carbon

r2 = k 2θCO − k −2θCθO where {ki} are the forward rate constants for steps I and II and {k−i} are the rate constants for the backward reactions. p and θ denote the pressure and surface coverage, respectively. The rate constants may be expressed as ki =

⎛ −E ⎞ kBT f ≠ exp⎜ i ⎟ ⎝ RT ⎠ h ∏j f j

where Ei is the activation free energy of step i. The degree of freedom for the reaction component j in each reaction step is given by f j = ft fr fv

where f t, f r, and f v are the translational, rotational, and vibrational degrees of freedom, respectively. In the above approximations, step II is the rate-determining step for CO evolution. Hence, the kinetic activity is determined by the barrier energy for the decomposition of CO (ΔECO⧧). Figure 7 illustrates the relationship between r2 and ΔECO⧧ for D

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These phenomena activate the C−O bond of the adsorbed CO molecule and, hence, decrease the barrier energy for its decomposition.

4. CONCLUSION We performed a computational study to investigate the adsorption of mercury on perfect and reduced Fe2O3[104] surfaces as well as elucidated the synergistic effect of adsorbed Hg0 on the adsorption and decomposition of CO on the reduced surfaces. Hg0 can bond to the Fe atoms at the top site of the perfect Fe2O3[104] surface and the reduced Fe2O3[104] surfaces with relatively high oxidation states. Meanwhile, Hg0 bonds to the Fe atoms at the bridge site of the reduced Fe2O3[104] surfaces with relatively low oxidation states. The Eads for Hg0 adsorption generally increases with the reduction of Fe2O3[104] into iron, showing a local extreme Eads of −2.063 eV on Fe2O1.364. After Hg0 is strongly attracted to the reduced surfaces, Hg0 decreases Eads for CO adsorption on the surfaces but promotes the charge transfer through the interface to activate the C−O bond of the adsorbed CO molecule. The enhanced charge transfer decreases the barrier energy for CO decomposition, which promotes carbon deposition on the reduced Fe2O3[104] surfaces in CLC. The kinetic model reveals the synergistic effect of Hg0 on carbon deposition and the relationship between the reaction rate and degree of CLC. The results of the current DFT work can be used to design effective oxygen carriers, control the reduction degree of oxygen carriers in the fuel reactor to enhance the CLC reaction rate, and restrain carbon deposition. Future studies may use DFT modeling to investigate the synergistic effects of multiple elements (e.g., K, Na, and Ca) included in coal on mercury adsorption and related reactions in CLC and to provide insights into other transition metal oxides for commercial interest in CLC.

Figure 7. Reaction rate for CO decomposition on the reduced Fe2O3[104] at θCO = 1, θO = 1, and θC = 1 for step II at 1073 K.

CO decomposition on each surface slab and the relations between r2 and the valence state of Fe atom of the reduced Fe2O3[104] surface. Log10 r2 is in a linear relationship to ΔECO⧧ because the catalytic CO decomposition on these surfaces follows a one-step reaction mechanism. The dashed line model shows that the CO decomposition reaction rate increases with the reduction of oxygen carriers. This phenomenon causes obvious carbon deposition on the oxygen carrier surface, which corresponds to the weight increase in the thermogravimetric curve of the CO−Fe2O3[104]/Al2O3 reaction.28 However, the solid line curve shows that Hg0 adsorption favors the catalytic decomposition of CO. Carbon deposition occurs on the surfaces with relatively higher oxidation states than the cases with Hg0 adsorption. Our dynamics model reveals the promotion of Hg0 for the catalytic decomposition of CO and presents the relationship between the carbon deposition rate and the reduction degree of oxygen carriers in CLC. Figure 8 schematically summarizes the carbon deposits on the reduced Fe2O3[104] surface with and without Hg0 along the reduction reaction coordinate. Less carbon deposits accumulate on the highly reduced Fe2O3[104] surface without Hg0 adsorption, whereas Hg0 promotes carbon deposition on the Hg0-decorative Fe2O3[104] surface. More carbon deposits can be observed on the reduced Hg0-decorative Fe2O3[104] surfaces, and carbon deposition may occur on the reduced surfaces at relatively higher oxidation states, as compared to the reduced Fe2O3[104] surfaces in the absence of Hg0. This result can be attributed to the synergistic effect of Hg0 on the interface electronic interaction between CO and the reduced surfaces and the promotion of charge transfer from the reduced surfaces to the adsorbed CO molecule at the initial and transition states of the catalytic CO decomposition reactions.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86-10-61772031. E-mail: [email protected]. *Fax: +86-10-61772031. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (51206044), the 111 Project (B12034), the Fundamental Research Funds for the Central Universities (2014MS36 and 2014ZD14), and the Beijing Natural Science Foundation (3132017).

Figure 8. Schematic for Hg0 adsorption and carbon deposition on an iron-based oxygen carrier with or without Hg0. E

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DOI: 10.1021/acs.energyfuels.5b01937 Energy Fuels XXXX, XXX, XXX−XXX