Molecular-Level Insights into Effect Mechanism of H2S on Mercury

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Materials and Interfaces

Molecular-level insights into effect mechanism of H2S on mercury removal by activated carbon Fenghua Shen, Jing Liu, Dawei Wu, Chenkai Gu, and Yuchen Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01182 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Industrial & Engineering Chemistry Research

Molecular-level insights into effect mechanism of H2S on mercury removal by activated carbon Fenghua Shen, Jing Liu*, Dawei Wu, Chenkai Gu, Yuchen Dong

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China Abstract:

A systematic theoretical study on the effect of H2S on mercury (Hg) adsorption on

activated carbon was conducted by density functional calculation. The form and amount of sulfur species formed by H2S adsorption dominate the pattern of Hg adsorption on activated carbon surface. C-S species is more active to improve Hg adsorption than C-S-C species. C-S species can improve the chemisorption of Hg by enhancing the activity of neighboring C sites. Increasing the amount of C-S species can further enhance the interaction of Hg with C sites, and leads to the formation of HgS. The adsorption of HgS on activated carbon surface is highly thermally favorable, and belongs to chemisorption. C-S species is able to facilitate the adsorption of HgS, and makes HgS adsorbing more likely in molecular adsorption pattern. HgS complex is the most stable form of Hg adsorption on activated carbon surface with H2S. Keywords: Mercury removal; Hydrogen sulfide; Effect mechanism; Activated Carbon; Coal gasification

1. INTRODUCTION Coal gasification is regarded as a promising coal utilization technology to provide energy in a more efficient and eco-friendly way compared with traditional combustion technologies. However, mercury (Hg) will be released into the coal derived fuel gas during the process of gasification, and exists mainly in elemental form.1-3 The elemental mercury is toxic to human and is difficult to be 1

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removed because of its high volatility and insolubility in water.4-6 Moreover, the long distance transportation of elemental mercury in the atmosphere makes its toxic having global-scale impacts.7, 8

The removal of mercury from coal gasification has currently become a pressing problem crying

out for a solution. The using of activated carbon as sorbent is a practical method for removing Hg from coal derived flue gas.9, 10 Various agents, such as elemental sulfur and halogens, were often used to impregnate the activated carbon, in order to improve their Hg removal ability.11-13 However, such impregnated activated carbons are more expensive compared with the raw activated carbons.14 Hydrogen sulfide (H2S) is an acidic pollutant gas, which needs to be removed from the coal derived flue gas.15,

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It has been found experimentally that activated carbon is able to remove H2S

efficiently.17, 18 H2S can be catalytically oxidized on activated carbon surface with the formation of various sulfur species.19, 20 These sulfur species are beneficial for the adsorption of Hg on activated carbon surface because of the generation of new active sites.21 It is thus reasonable to assume that H2S can be used as agent to treat raw activated carbons, in order to enhance their Hg removal ability, thereby reducing the cost of sorbents production. There have been numerous experimental efforts to investigate the adsorption of Hg on activated carbon with H2S. Feng et al.21 studied the correlation of Hg uptake ability and sulfur content of activated carbon by using H2S as sulfurizing agent. They found that higher sulfur content did not necessarily result in higher Hg removal, and suggested that most of the sulfur species did not react with Hg directly. Contrary to these conclusions, Uddin et al.22, 23 investigated the adsorption of Hg on activated carbon in the presence of H2S, and found that H2S could improve Hg removal ability of raw activated carbon significantly. They explained that Hg could react with sulfur species on carbon surface directly, which formed by H2S adsorption, with the forming of HgS. 2


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Understanding the effect mechanisms of H2S on Hg adsorption on activated carbon is essential for the design of more effective sorbents for the simultaneous removal of Hg and H2S from coal derived gas. Such interactions can be investigated by applying quantum chemical methods. Padak et al.24, 25 illuminated the adsorption of Hg-Cl species on carbon surface by applying density functional theory (DFT). In our previous study,26 the effect mechanism of SO2 on Hg adsorption on carbon surface has been clarified. The available theoretical studies prove that DFT is a useful tool for clarifying the effects of flue gas on Hg adsorption on carbon surface at molecular level, and solving problems beyond the limitation of experiment. However, no theoretical study involving the effect of H2S on Hg adsorption on activated carbon surface has been carried out. The role of H2S in Hg adsorption on activated carbon surface is still unclear. In the present study, a theoretical study of the effect mechanism of H2S on Hg adsorption on activated carbon surface was performed. The adsorption of Hg and HgS on activated carbon surface was examined. The reaction mechanism of Hg with H2S for the forming of HgS was investigated. This is, to the best of the authors' knowledge, the first theoretical study on the effect mechanism of H2S on Hg adsorption on activated carbon surface. This work helps to clarify the ability of activated carbon in simultaneously removing Hg and H2S from the syngas of coal gasification. 2. COMPUTATIONAL METHODS All of the calculations were performed by applying Gaussian 03 software package.27 Density functional theory (DFT) has been employed, and full geometric optimizations and energy calculations were performed at B3PW91 level of theory. Effective core potential (ECP) was used to replace inner electrons of Hg, as a result of the large amount of electrons in heavy elements. It has been validated that the B3PW91/RCEP60VDZ combination can provide reasonably accurate results

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comparing to experimental data.28, 29 Therefore, B3PW91/RCEP60VDZ was employed for the Hg atom in this study. The 6-31G(d) basis set was used for non-metal elements (C, S and H). The total adsorption energy can be calculated as the following equation: Eads = E(AB) – (E(A)+E(B)) where E(A) is the energy of the adsorbate, E(B) is the energy of the activated carbon surface, and E(AB) is the total energy of the whole adsorption system. Adsorption of the adsorbate is exothermic if Eads is negative. Normally, if the adsorption energy is less than -30 kJ/mol, the interaction belongs to physisorption. If the adsorption energy is higher than -50 kJ/mol, the interaction belongs to chemisorption.30, 31

3. RESULTS AND DISCUSSION 3.1. Modeling the activated carbon surface Solid-state

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C NMR characterization experimental data showed that carbon had chemical

structures consisting of 3-7 benzene rings.32 It was found that the reactivity of the active sites strongly depended on its local shape rather than on the size of the cluster models.33, 34 Therefore, a single graphite layer (seven-ring zigzag: C25H9) cluster model was used to simulate the activated carbon (AC) surface. The upper side carbon atoms in this model were unsaturated to simulate the active sites and the carbon atoms on the other sides were terminated with hydrogen atoms. The optimized bond lengths (average C-C: 1.41 Å, C-H: 1.09 Å) and bond angles (average ∠C-C-C: 121°, ∠C-C-H: 120°) were obtained for this cluster model, which were in good agreement with the experimental data (C-C: 1.42 Å, C-H: 1.07 Å, ∠C-C-C: 120°, ∠C-C-H: 120°)33. Experimental results showed that the dissociative adsorption of H2S leads to the forming of various sulfur species on activated carbon surface.35 Such H2S derived sulfur species will act as active sites for Hg adsorption.36 In our previous theoretical study,37 the adsorption and evolution 4


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processes of H2S on activated carbon surface have been clarified. C-S and C-S-C are the most stable sulfur species formed by the dissociative adsorption of H2S on activated carbon. Shi et al.17 analyzed the adsorption of H2S on activated carbon by using XPS, and they found that C-S groups can be formed by H2S adsorption. Therefore, different models of C-S and C-S-C are used to simulate the active sulfur sites on activated carbon surface, as shown in Fig.1. The optimization of single S atom at different sites on the activated carbon surface yields the structures AC-S1 and AC-S2. The C-S bond length in AC-S1 is 1.662 Å. Two C-S bonds are formed in AC-S2 with bond lengths of 1.845 and 1.833 Å, respectively. Other structures consisting of two S atoms on the surface, including AC-SS1, AC-SS2 and AC-SS3, are also considered. In order to evaluate the strength of the interaction between atoms, Mulliken population analyses were performed. A larger positive value represents a stronger covalent bond. A bond population close to zero means no interaction occurs between the two atoms.25 The change in the bond population of a bond means bonding electrons have been transferred and the bond has been weakened or strengthened.25, 38 The bond populations of S atom adsorption on AC are listed in Table 1. The values of the C-S bond populations in these structures are highly positive, indicating that S atoms in these structures are strongly bonded with surface C atoms. In addition, the adsorption of Hg on AC yields the structure AC-Hg, as shown in Fig 1. The adsorption energy of Hg on AC is -17.8 kJ/mol, which belongs to physisorption. 3.2. Hg adsorption on activated carbon-S The adsorption of Hg on activated carbon surface with sulfur was investigated, as shown in Fig. 2. The adsorption of Hg on different sites of AC-S1 is considered, and two stable structures are obtained, as shown in Fig. 2(a). The most stable structure of Hg on AC-S1 is B with adsorption energy of -56.9 kJ/mol, which is in the range of chemisorption. The adsorption energy of Hg in B is 5



significantly higher than that in AC-Hg, indicating that C-S species is able to facilitate the chemisorption of Hg on activated carbon surface. C-Hg bond is formed by Hg adsorption in B with a bond length of 2.266 Å. The adsorption energy of Hg in A is -33.6 kJ/mol, which is lower than that in B. Feng et al.36 studied the Hg removal ability of activated carbon in the presence of H2S, and they suggested that some sulfur species formed by H2S adsorption were able to enhance Hg adsorption on activated carbon. The calculation result is consistent with the experimental data. Fig. 2(b) illustrates the stable structures formed by Hg adsorption on different sites of AC-S2, including C and D. The adsorption energies of Hg in C and D are -30.4 and -46.9 kJ/mol, respectively. This means that C-S-C species can also enhance the adsorption of Hg on activated carbon surface. However, the adsorption of Hg on AC-S2 is less exothermic than that on AC-S1. Therefore, C-S species is more active to improve Hg adsorption than C-S-C species. Moreover, Hg tends to adsorb on surface C sites rather than react with S atom in AC-S1 and AC-S2. Feng et al.21 suggested that not all of the sulfur species on activated carbon surface could react with Hg directly. The calculation result agrees with the experimental data. Fig. 2(c) displays the stable structures of Hg on surface with two C-S. With the increasing of the amount of C-S species on carbon surface, S-Hg bond can be formed by the reaction of Hg with S atom. Structures E and F are formed by Hg adsorbing on AC-SS1. The adsorption energy of Hg is -158.5 kJ/mol in E, and -57.7 kJ/mol in F, both belong to chemisorption. The bond lengths of S-Hg in E and F are 2.561 and 2.481 Å, respectively. The adsorption of Hg on AC-SS1 is more exothermic than that on AC-S1. This means that more non-adjacent C-S species can further facilitate the chemisorption of Hg, and lead to the formation of HgS complex on activated carbon surface. Uddin et al.22, 23 suggested experimentally that HgS would be formed by the adsorptions of Hg and H2S on activated carbon. The calculation result is consistent with the experimental data. The 6


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adsorption energy of Hg is lower than that of H2S on carbon surface (-664.9 kJ/mol). H2S is likely to preferentially adsorb on surface by forming sulfur sites. The main route for Hg adsorption on carbon surface with H2S is that H2S adsorbs on surface to form sulfur species first, and then Hg bonds with surface S and C atom. The Eley-Rideal mechanism is the most possible reaction process for the formation of HgS by the reaction of Hg with two C-S. The adsorption of Hg on AC-SS2 and AC-SS3 yield the same configuration G, with adsorption energy of -20.2 kJ/mol. The adsorption energy of Hg in G is close to that in AC-Hg, which may owe to the formation of S-S bond. Moreover, a high concentration of H2S in flue gas may inhibit Hg adsorption since H2S will compete with Hg for the limited active sites on carbon surface. The bond populations of Hg adsorption on AC-S1, AC-S2, AC-SS1, AC-SS2 and AC-SS3 are listed in Table 2. The highly positive values of the S-Hg bond populations confirm the formation of HgS complex in E and F. The bond populations of C-S and C-Hg in F are 0.320 and 0.313, respectively. This indicates that HgS is strongly bonded with surface C sites, namely that HgS on carbon surface is stable. The bond populations of Hg with S and S’ in E are 0.151 and 0.180, respectively, indicating that Hg is interacting with the two sulfur atoms. This accounts for the higher Hg adsorption energy in E than that in F. However, Hg does not interact with S atom in the other structures since their S-Hg bond populations are close to zero. Schematic energy profiles are examined to elucidate the adsorption and evolution processes of Hg on activated carbon surface. Fig. 3 shows the schematic energy profiles of different pathways of Hg adsorption on AC-S surface including reactants, intermediates and products. The energies of the optimized structures are relative to the reactants. The formation of intermediate E is more exothermic than other structures. This means that E is the most stable complex. The desorption pathway of Hg from intermediate E is highly endothermic, and thus not likely to occur. However, 7



Hg atom may desorb from the other intermediates, since the desorption pathways are less endothermic. Moreover, the desorption pathway of HgS from intermediate E is highly endothermic, revealing that HgS complex formed on activated carbon surface is very stable. The adsorption of Hg depends on the species and amount of sulfur on surface. In general, H2S is able to facilitate the chemisorption of Hg on activated carbon surface, which is in agreement with the previous experimental results.21-23 On one hand, H2S can enhance the activity of surface C sites to facilitate the chemisorption of Hg. On the other hand, HgS can be formed by the reaction of Hg with H2S on activated carbon surface. Zhang et al.39 investigated the role of H2S in Hg adsorption on activated carbon, and they found that HgS would be formed by the interaction of Hg with H2S on activated carbon surface. The calculation result agrees with the experimental data. 3.3. HgS adsorption on activated carbon The Gibbs energy of HgS formation though the reaction of Hg with H2S at room temperature is -56.3 kJ/mol.40 This indicates that the reaction of Hg with H2S is feasible in thermodynamics. In addition, HgS is a final complex formed by the reaction of Hg with sulfur on activated carbon. Therefore, it is necessary to investigate the adsorption of HgS on activated carbon surface, which is important for further clarify the role of H2S in Hg adsorption. The adsorption of HgS molecule on AC was examined by allowing HgS to approach different sites with bond axis parallel (side-on mode) or vertical (S-down mode and Hg-down mode) to the surface. Five possible surface intermediates, including 1A, 1B, 1C, 1D and 1E are obtained, as presented in Fig. 4. The corresponding bond populations are listed in Table 3. It is clear that dissociative adsorption is favored when HgS approaches the surface in side-on and S-down mode. In 1A, C-Hg and C-S bonds are formed. The bond length of C-Hg is 2.271 Å, and the bond length of C-S is 1.679 Å. The adsorption energy of HgS is -403.4 kJ/mol, which belongs to chemisorption. 8


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In 1B, S-Hg bond of HgS molecule is broken, and then Hg and S are adsorbed separately on surface C sites. In 1C (S-down mode), after the dissociation of S-Hg bond, S is bonded with surface C site, whereas Hg leaves from the surface. Molecular adsorption is favored when HgS approaches the surface in Hg-down mode. In 1D and 1E, HgS is adsorbed on surface with the formation of C-Hg bond. The adsorption energy of HgS is -218.9 kJ/mol in 1D, and -221.8 kJ/mol in 1E. In 1D, the bond length of C-Hg is 2.099 Å. In 1E, Hg is interacting with two C atoms, with bond lengths of 2.328 and 2.325 Å, respectively. Moreover, the bond populations of S-Hg in 1D and 1E are 0.289 and 0.304, respectively. This suggests that S-Hg bond doesn’t dissociate after the adsorption of HgS. Fig. 5 illustrates the schematic energy profiles of different pathways of HgS adsorption on AC. It appears that the stability of these structures follows the order of 1B > 1A > 1C > 1E > 1D. This indicates that the dissociative adsorption pattern is more thermodynamically favorable for HgS adsorption on AC. Despite the formation of complexes 1D and 1E is not as exothermic as 1B, there is possibility for 1D and 1E formation as well. It seems from Fig. 5 that once HgS adsorbing in dissociative adsorption manner, Hg may desorb from the surface. However, there is still a quite possibility that HgS remains as stabilized complex on AC when HgS adsorbing in molecular adsorption manner. Li et al.41 reported experimentally that Hg could react with S by forming stable HgS on carbon surface. The calculation result is consistent with the experimental result. 3.4. Co-adsorption of HgS on activated carbon The co-adsorption of HgS on carbon surface was further studied, and the co-adsorption structures are shown in Fig. 6. 2A represents the surface intermediate formed by HgS co-adsorption in Hg-down mode. The co-adsorption energy of HgS in 2A is -690.1 kJ/mol. 2B illustrates the surface intermediate formed by HgS co-adsorption in parallel mode. The co-adsorption energy of

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HgS in 2B is -895.6 kJ/mol. The co-adsorption of HgS on carbon surface is highly exothermic process, and the structures formed by such co-adsorption are very stable. 3.5. HgS adsorption on activated carbon-S The adsorption of HgS on AC-S were further examined in the similar manner, and five stable surface intermediates 3A, 3B, 3C, 3D and 3E were obtained, as shown in Fig. 7. The bond populations of HgS adsorption on AC-S are given in Table 3. In 3E, HgS is bonded with surface C sites with the formation of C-Hg and C-S bond. The bond lengths of C-Hg and C-S are 2.171 and 1.762 Å, respectively. The adsorption energy of HgS in 3E is -510.6 kJ/mol, which belongs to chemisorption. The bond population of S’-Hg is 0.181 in 3E, indicating a strong interaction of Hg with S’ atom. This implies that HgS is adsorbed molecularly in 3E. In 3A and 3B, HgS adsorbs in molecular adsorption pattern on surface by forming new C-Hg bond. The adsorption energies of HgS in 3A and 3B are -288.7 and -245.7 kJ/mol, respectively. Moreover, dissociative adsorption occurs in 3C and 3D with adsorption energies of -429.8 and -451.6 kJ/mol, respectively. Compared with the adsorption of HgS on AC, the adsorption of HgS on AC-S are more exothermic. This implies a positive effect of C-S species on HgS adsorption. Moreover, HgS is more likely to adsorb molecularly on surface with the presence of C-S species. This means that C-S species makes HgS complex more stable on activated carbon surface. Moreover, the bond population of S-Hg in 3E is highly positive, indicating a strong interaction of Hg atom with S atom. Fig. 8 shows the schematic energy profiles of different pathways of HgS on AC-S. The adsorption energy of HgS in 3E is higher than that of the other intermediates. This indicates that the surface complex 3E is the most stable structure to be formed. The desorption of Hg from 3E is an endothermic process with an endothermicity of 158.5 kJ/mol, indicating that 3E prefers to remain as a stable surface complex. However, pathway for the desorption of Hg from intermediate 3D is less endothermic process with an endothermicity of 20.9 kJ/mol. This implies that Hg may desorb from 10


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3D. Furthermore, the desorption of HgS from the intermediates 3A and 3B is highly endothermic. This shows that once HgS adsorbing molecularly, it will remain as stable complex on activated carbon surface. Based on the above analysis, it is sufficient to point out that HgS is chemically adsorbed on activated carbon surface in molecular or dissociative adsorption pattern. The dissociative adsorption of HgS on surface may lead to the desorption of Hg. The molecularly adsorbed HgS will remain as stable complex on surface. C-S species is able to enhance the stability of HgS complex on surface. Moreover, a suitable amount of H2S adsorption is important for the positive effect of H2S on Hg adsorption and the formation of HgS complex on activated carbon surface. A higher concentration H2S can hinder Hg adsorption since H2S will compete with Hg for the limited active carbon sites on surface. Vitolo et al.42,

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studied experimentally the effects of H2S on Hg removal ability of

activated carbons. They found that sulfur species formed by H2S adsorption made raw activated carbon achieving an Hg removal ability comparable to a sulfurized activated carbon. Meanwhile, H2S adsorption on activated carbon was initially beneficial to Hg adsorption, whereas excessive H2S adsorption inhibited Hg adsorption. 4. CONCLUSION Density functional theory calculations were conducted to provide molecular-level understanding of effect mechanisms of H2S on Hg adsorption on activated carbon surface. The form and amount of sulfur species govern the pathway of Hg adsorption on activated carbon surface. H2S has a positive effect on Hg adsorption on activated carbon. C-S species can enhance the activity of surface C sites, and thus facilitates the chemisorption of Hg on surface. When increasing the amount of C-S species, Hg will react with S atom leading to the forming of HgS. The adsorption of HgS on AC belongs to chemisorption. HgS is likely to adsorb dissociatively, and Hg atom may 11



desorb from the surface. H2S can improve the chemisorption of HgS on activated carbon surface. Molecular adsorption is the most possible pathway for HgS adsorption on AC-S. The chemisorbed HgS is likely to remain as stable complex on AC-S. The most stable form of Hg adsorption on activated carbon with H2S is HgS. However, a higher concentration of H2S will hinder Hg adsorption since H2S will compete with Hg for the limited active sites on surface.

AUTHOR INFORMATION Corresponding Author *Tel: +86 27 87545526; fax: +86 27 87545526; e-mail address: [email protected]. ORCID Jing Liu: 0000-0001-6520-9612 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Key R&D Program of China (2016YFB0600604) and National Natural Science Foundation of China (51606078).

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desorption of mercury species over activated carbon sorbents for mercury removal from coal-derived fuel gas. Energ Fuels 2009, 23, (10), 4710-4716. (23) Morimoto, T.; Wu, S.; Azhar Uddin, M.; Sasaoka, E., Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2S. Fuel 2005, 84, (14), 1968-1974. (24) Padak, B.; Brunetti, M.; Lewis, A.; Wilcox, J., Mercury binding on activated carbon. Environ. Prog. 2006, 25, (4), 319-326. (25) Padak, B.; Wilcox, J., Understanding mercury binding on activated carbon. Carbon 2009, 47, (12), 2855-2864. (26) 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. (27) Frisch, M.; Trucks, G.; Schlegel, H.; Scuseria, G.; Robb, M.; Cheeseman, J.; Montgomery Jr, J.; Vreven, T.; Kudin, K.; Burant, J., Gaussian 03, ReVisions B. 04 and C. 02. Gaussian Inc., Pittsburgh PA 2003. (28) Liu, J.; Qu, W.; Yuan, J.; Wang, S.; Qiu, J.; Zheng, C., Theoretical studies of properties and reactions involving mercury species present in combustion flue gases. Energ Fuels 2010, 24, (1), 117-122. (29) Liu, J.; Cheney, M. A.; Wu, F.; Li, M., Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. J. Hazard. Mater. 2011, 186, (1), 108-113. (30) Qu, W.; Liu, J.; Shen, F.; Wei, P.; Lei, Y., Mechanism of mercury-iodine species binding on carbonaceous surface: Insight from density functional theory study. Chem. Eng. J. 2016, 306, 704-708. (31) Yang, Y.; Liu, J.; Zhang, B.; Liu, F., Mechanistic studies of mercury adsorption and oxidation 15



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Table 1 Mulliken bond populations for activated carbon surfaces with sulfur. Bond

AC-S1

AC-S2

AC-SS1

AC-SS2

AC-SS3

C(12)-C(11)

0.466

0.474

0.463

0.475

0.472

C(11)-C(8)

0.505

0.426

0.506

0.487

0.457

C(8)-C(9)

0.240

0.385

0.225

0.258

0.257

C(9)-C(10)

0.238

0.310

0.237

0.284

0.317

C(10)-C(22)

0.457

0.354

0.427

0.284

0.317

C(22)-C(25)

0.319

0.342

0.405

0.257

0.257

C(25)-C(29)

0.414

0.408

0.326

0.487

0.457

C(29)-C(31)

0.527

0.477

0.401

0.475

0.472

C(9)-S

0.475

0.237

0.487

0.388

0.267

0.388

0.267

C(11)-S

0.234

C(22)-S’ C(29)-S’

0.400

S-S’

0.100

18


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Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Mulliken bond populations for Hg adsorption on AC-S. Bond

A

B

C

D

E

F

G

C(12)-C(11)

0.464

0.468

0.472

0.471

0.478

0.520

0.472

C(11)-C(8)

0.477

0.509

0.427

0.425

0.479

0.440

0.457

C(8)-C(9)

0.236

0.255

0.381

0.379

0.325

0.259

0.253

C(9)-C(10)

0.264

0.234

0.314

0.324

0.269

0.405

0.313

C(10)-C(22) 0.456

0.478

0.334

0.394

0.396

0.353

0.311

C(22)-C(25) 0.315

0.256

0.326

0.310

0.393

0.429

0.271

C(25)-C(29) 0.414

0.422

0.448

0.389

0.357

0.312

0.441

C(29)-C(31) 0.479

0.504

0.475

0.517

0.467

0.379

0.459

0.471

0.227

0.231

0.361

0.320

0.269

0.230

0.234

C(9)-S

0.446

C(11)-S C(22)-S’

0.247

C(29)-S’ S-Hg

0.036

0.026

0.011

0.003

S’-Hg C(11)-Hg

0.320

0.437

0.151

0.201

0.180 0.242

0.025 0.313

C(22)-Hg

0.208

C(29)-Hg

0.209

0.218

0.205

0.298

0.219

0.212

19



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Table 3 Mulliken bond populations for HgS adsorption on AC and AC-S. HgS on AC

HgS on AC-S

Bond 1A

1B

1C

1D

1E

3A

3B

3C

3D

3E

C(12)-C(11)

0.463

0.504

0.465

0.477

0.463

0.467

0.492

0.475

0.472

0.478

C(11)-C(8)

0.478

0.422

0.504

0.444

0.437

0.498

0.443

0.488

0.457

0.479

C(8)-C(9)

0.236

0.257

0.241

0.326

0.370

0.268

0.276

0.258

0.253

0.324

C(9)-C(10)

0.258

0.478

0.239

0.335

0.364

0.245

0.348

0.284

0.313

0.269

C(10)-C(22) 0.459

0.234

0.456

0.376

0.365

0.407

0.401

0.284

0.311

0.396

C(22)-C(25) 0.316

0.255

0.317

0.325

0.369

0.317

0.322

0.258

0.271

0.393

C(25)-C(29) 0.421

0.509

0.414

0.410

0.437

0.451

0.410

0.487

0.441

0.357

C(29)-C(31) 0.486

0.468

0.480

0.476

0.462

0.479

0.473

0.475

0.459

0.467

0.443

0.222

0.385

0.269

0.361

0.388

0.247

C(9)-S

0.446

C(22)-S

0.473 0.471

C(22)-S’ C(29)-S’ S-Hg

0.320 0.040

0.026

0.002

0.289

0.304

S’-Hg C(9)–Hg C(11)-Hg C(22)-Hg

0.208 0.239

0.356

0.069

0.001

0.001

0.315

0.221

0.001

0.150 0.025

0.181

0.211

0.208

0.345 0.211

0.324

C(29)-Hg

0.298 0.212

20


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List of Figures Captions

Fig. 1. Activated carbon (AC) surface with Hg and sulfur. Fig. 2. Adsorption of Hg on AC-S: (a) Hg on AC-S1; (b) Hg on AC-S2; (c) Hg on AC-SS. Fig. 3. Schematic energy profiles of different pathways of Hg adsorption on AC-S. Fig. 4. Adsorption of HgS on AC. Fig. 5. Schematic energy profiles of different pathways of HgS adsorption on AC. Fig. 6. Co-adsorption structures of HgS on activated carbon. Fig. 7. Adsorption of HgS on AC-S. Fig. 8. Schematic energy profiles of different pathways of HgS adsorption on AC-S.

21



Fig. 1. Activated carbon (AC) surface with Hg and sulfur.

22


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Fig. 2. Adsorption of Hg on AC-S: (a) Hg on AC-S1; (b) Hg on AC-S2; (c) Hg on AC-SS.

23



120

Relative Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AC-Hg+S

90 60

AC-S1+HgS

30 0 -30 -60

AC-S+Hg

AC-S+Hg G C D B

A F

-90

-120 E

-150 -180

Reaction Coordinate Fig. 3. Schematic energy profiles of different pathways of Hg adsorption on AC-S.

24


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Fig. 4. Adsorption of HgS on AC.

25



100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

0

AC+HgS

AC+HgS

-100 -200

1D 1E

-300 1C 1A

-400

AC-S1+Hg

1B -500

Reaction Coordinate Fig. 5. Schematic energy profiles of different pathways of HgS adsorption on AC.

26


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. Co-adsorption structures of HgS on activated carbon.

27



Fig. 7. Adsorption of HgS on AC-S.

28


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Page 29 of 30

200 100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AC-S+HgS

AC-S+HgS

0

1A+S 1B+S

-100 -200

3B

-300

3A

AC-SS1+Hg

-400

3C 3D 3E

AC-SS2+Hg

-500

AC-SS3+Hg

-600

Reaction Coordinate Fig. 8. Schematic energy profiles of different pathways of HgS adsorption on AC-S.

29



TOC/Abstract Art

30


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Molecular-Level Insights into Effect Mechanism of H2S on Mercury

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