Effect of the Mechanism of H2S on Elemental Mercury Removal Using

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Effect Mechanism of H2S on Elemental Mercury Removal Using MnO2 Sorbent during Coal Gasification Zhen Wang, Jing Liu, Yingju Yang, Sen Miao, and Fenghua Shen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03092 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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Effect Mechanism of H2S on Elemental Mercury Removal Using MnO2 Sorbent during Coal Gasification Zhen Wang, Jing Liu*, Yingju Yang, Sen Miao, Fenghua Shen

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

ABSTRACT:

A combination of experiments and density functional theory (DFT) calculations

was employed to investigate the detailed reaction mechanism of elemental mercury (Hg0) with H2S over MnO2 surface. MnO2 sorbent was prepared by a low-temperature sol-gel auto-combustion method and used to capture Hg0 from simulated syngas. The experiment results show that MnO2 possess superior Hg0 removal capacity, over 85% Hg0 removal efficiency is achieved in the temperature range of 80-200 °C. Appropriate concentration of H2S promotes Hg0 removal by forming active sulfur species on MnO2 surface. The computational results indicate that Hg0 and HgS are chemically adsorbed on MnO2(110) surface with the adsorption energies of −69.50 and −286.33 kJ/mol, respectively. H2S undergoes dissociative chemisorption on MnO2(110) surface and forms active sulfur species for Hg0 transformation. Both Eley–Rideal (E–R) and Langmuir– Hinshlwood (L–H) mechanisms are responsible for heterogeneous Hg0 reaction with H2S over MnO2. The E–R mechanism (7.16 kJ/mol) that gaseous Hg0 reacts with active surface sulfur species is kinetically more favorable than L–H mechanism (42.00 kJ/mol) due to its much lower energy barrier. After Hg0 heterogeneous reaction, the most stable mercury compound of HgS formed on MnO2 surface, which is verified by the temperature-programmed desorption and X-ray photoelectron spectroscopy experimental results. 1

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KEYWORDS: Mercury removal; H2S; MnO2; Reaction mechanism; Density functional theory

1. INTRODUCTION Mercury pollution has drawn considerable public concern because of its bioaccumulation in the ecosystems and food chain, toxicity on human health, and durability in the atmosphere.1 Coal processing and utilization industry are the primary sources of anthropogenic mercury emissions to the atmosphere.2 Fortunately, the first international legally binding treaty named Minamata Convention on Mercury was adopted by more than 140 governments in October 2013. This means that the removal of mercury from coal-utilizing facilities will become increasingly urgent. Coal gasification as a source of electric power generation has attracted increasing interest due to its high efficiency and alleviated effects on the environment relative to the direct coal combustion. However, syngas generated from coal gasification includes Hg0, which is very difficult to remove due to its water insolubility and chemical inertness. Worse still, the proportion of Hg0 in syngas (approximately 80 µg/m3)3 is much higher than that in coal-fired flue gas because the reducing environment is unfavorable for Hg0 oxidation4. Therefore, the capture of Hg0 during coal gasification has faced more challenges. Unfortunately, compared with the wide researches on the removal of Hg0 in the coal-fired flue gas, studies on the capture of Hg0 from syngas are still very limited. The capture of Hg0 by sorbent is one of the most potential control methods for reducing Hg0 emissions from coal derived flue gas. To date, mercury sorbents are mainly divided into four kinds: activated carbons,5-8 noble metals,9-12 metal oxides,13-16 and metal sulfides17-19. Activated carbons, especially carbons modified by sulfur and halogens, are found to be effective for the capture of Hg0.20-22 However, the wide application of activated carbons is limited by the high operation costs 2


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and narrow temperature windows. Noble metal sorbents exhibit excellent Hg0 removal capacity in broad temperature windows, but they are very expensive for large-scale applications.23 Some metal oxides, such as Fe2O3-, CeO2-, and ZnO-based sorbents have been demonstrated good performance on the removal of Hg0 from syngas under low temperature (< 150 °C).24-26 Nevertheless, the Hg0 removal efficiency will be significantly decreased as the reaction temperatures increased to 200 °C. However, Hg0 capture in syngas is expected at elevated temperature (approximately 200 °C) to improve the overall thermodynamic efficiency for power generation gasification systems.6,10 Therefore, it is significant to develop an effective adsorbent that is beneficial for Hg0 capture at the temperatures required for syngas purification. In recent years, MnO2-based sorbents have been identified as the promising Hg sorbents due to their excellent capture capability and long persistent activity on the removal of Hg0 from coal-fired flue gas.27-30 However, it is unclear whether MnO2 can effectively remove Hg0 from syngas since the reducing environment is not favorable for Hg0 oxidation. In addition, during coal gasification process, another toxic byproduct H2S is released along with mercury, and it has drawn much concern due to its higher content (0.2-1%)3 and harmful effects on equipment as well as the environment and human health26. MnO2-based sorbents have also been found to be desulfurizers for the removal of H2S from syngas.31-33 However, the adsorption mechanism of H2S on MnO2 surface is unknown. Moreover, the effect mechanism of H2S on Hg0 adsorption and transformation on the MnO2 surface during coal gasification are still indistinct. These drive us to test the performance of MnO2 on Hg0 capture under syngas and to reveal the effect mechanism of H2S on Hg0 removal by MnO2.

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In the current study, the influences of the reaction temperature and H2S on Hg0 removal by MnO2 sorbent were systematically investigated. The temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were used to determine the species adsorbed on MnO2 surface. Furthermore, density functional theory (DFT) calculations were conducted to investigate the effect mechanism of H2S on Hg0 adsorption and transformation over MnO2(110) surface. To the best of the authors’ knowledge, this is the first study involving the effect mechanism of H2S on Hg0 removal by MnO2 during coal gasification.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Sorbent Preparation. The MnO2 sorbent was synthesized employing the low-temperature sol-gel auto-combustion technique. The suitable amount of citric acid was dissolved in 200 mL deionized water under magnetic stirring, followed by the addition of 5 mL alcohol. The calculated amount of 50 wt % Mn(NO3)2 solution was dropwise added to the above citric acid solution. The resulting homogeneous solution was sustained at 60 °C for the complexation reaction. After reaction for 1 h, the mixed solution was continuously stirred and evaporated at 90 °C until the viscous sol–gel was formed. The obtained wet sol–gel was dried at 100 °C for 12 h and then calcined in a muffle furnace at 400 °C for 4 h. Finally, the collected product was grounded and filtered to particle size of 200 mesh. 2.2. Sorbent Characterization. X-ray diffraction (XRD) pattern was recorded by a Shimadzu XRD-7000 diffractometer between 20° and 70° with a step of 5° min–1 operating at 40 kV and 30 mA using Cu Kα radiation (λ = 0.15418 nm). The XRD spectra of synthesized MnO2 sample is shown in Figure 1. There are 4


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three intense characteristic peaks corresponds to MnO2, indicating that the prepared sample mainly exists in the form of MnO2 crystalline phase. BET surface area and pore structure were measured by a nitrogen adsorption apparatus (Micromeritics ASAP 2020) at liquid-nitrogen temperature (77 K). The synthesized sorbent was degassed for 2 h at 180 °C prior to measurement. The BET surface area and pore volume of the sample were measured as 12.38 m2/g and 0.06 cm3/g, respectively. X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) with Al Kα radiation (1486.6 eV, 300 W) as the excitation source was employed to determine the binding energies of Mn 2p, O 1s, Hg 4f, and S 2p. The C 1s peak at 284.6 eV was used to calibrate the charge effect. 2.3. Experimental Apparatus and Procedures. All of the Hg0 removal experiments were performed using a bench-scale fixed-bed reactor, as shown in Figure 2. A Hg0 permeation device was used to provide a constant concentration of Hg0 vapor (about 70 µg/m3), which was introduced into the mixture syngas using N2 flow. Other syngas compositions including CO, H2, H2S, HCl, and the balance N2 were fed into the reactor. The concentration of gas-phase Hg0 was measured by a portable online mercury analyzer (Lumex RA-915M Zeeman). A total gas flow rate of 1 L/min was maintained during the experiments, which corresponded to a gas volume hourly space velocity (GHSV) of approximately 5 ×104 h−1. 0.05 g sorbent sample and 1.95 g quartz sand were used in each experiment. Quartz sand, an inert material, was applied as diluent to mix with MnO2 sample. Quartz sand enhanced the height of heterogeneous reaction region, thereby increasing the residence time of syngas. The accumulation Hg0 removal efficiency ( ηa ) over the MnO2 sorbent is defined as the following equation:

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ηa

∫ =

t

0

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t

Hg in0 dt − ∫ Hg 0out dt 0

∫

t

0

× 100%

(1)

Hg in0 dt

where Hgin0 and Hg 0out represent the Hg0 concentration in the inlet and outlet of the reactor, respectively. t represents the accumulated time of each set of experiments, and t is 60 min in this work. 2.4. Computational Methods. All calculations were carried out using spin-polarized DFT method in CASTEP program code.34 The exchange correlation potential was treated within the generalized gradient approximation of Perdew-Burke-Ernzerhof functional (GGA-PBE).35 The electronic wave functions were expanded in a plane wave basis set with a cutoff energy of 340 eV. The tolerance criteria for the force, energy, self-consistent field (SCF), and displacement convergence of optimal configuration were 0.05 eV/Å, 2.0 × 10−5 eV/atom, 2.0 × 10−6 eV/atom, and 0.002 Å, respectively. The Brillouin zone of the MnO2 bulk lattice and the MnO2 surface were sampled with 5 × 5 × 8 and 4 × 3 × 1 Monkhorst-Pack36 k-point meshes, respectively. MnO2(110) surface is the most thermodynamically stable bulk termination and catalytically active surface.37 The p(3×2) surface supercell can lead to a minimal Hg-Hg interaction. Moreover, the coverage effects on adsorption has been examined in our previous studies.27 The results indicate that p(3×2) surface supercell can be used to simulate the MnO2 surface for heterogeneous reaction. Therefore, the MnO2(110)-p(3 × 2) surface supercell was used in this study to investigate the Hg0 transformation process, as shown in Figure 3. There are six different adsorption sites (Os top, Obr top, Obr bridge, Mn5 top, Mn5 bridge, and hollow) on the MnO2(110) surface. The adsorption energy ( Eads ) of adsorbate on MnO2 surface can be calculated as the follows:

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Eads =E(MnO2 −adsorbate) – ( EMnO2 + Eadsorbate )

(2)

where E(MnO2 −adsorbate) , EMnO2 , and Eadsorbate represent the total energies of the adsorption system, the optimized MnO2 slab model, and the isolated adsorbate molecules, respectively. A higher negative

Eads value corresponds to a stronger interaction. Moreover, complete linear synchronous transit/quadratic synchronous transit (LST/QST) was performed to search all transition states and determine the activation energy barrier of the reactions.38 All of the searched transition state (TS) structures were verified by vibrational frequency calculation and the true TS should correspond to an imaginary frequency with a normal mode. The activation energy barrier (Ebarrier) is defined as:

Ebarrier = E(transition state) – E(intermediate)

(3)

where E(transition state) and E(intermediate) represent the total energies of transition state and intermediate (IM), respectively.

3. RESULTS AND DISCUSSION 3.1. Experimental Results 3.1.1. Hg0 Removal by MnO2 Sorbent. The performance of the prepared MnO2 for Hg0 removal was examined from 80 to 200 °C under different atmospheres: pure N2, N2 plus 400 ppm H2S, and simulated syngas (40% CO, 30% H2, 400 ppm H2S, 10 ppm HCl and balanced with N2). Hg0 removal efficiency over MnO2 sorbent is shown in Figure 4. In the case of pure N2 atmosphere, the synthesized MnO2 sorbent exhibited medium Hg0 removal efficiency. It is clearly that ηa increased with increasing reaction temperature from 80 to 160 °C, suggesting a chemisorption mechanism. However, further elevating the reaction temperature to 200 °C resulted in a decrease of ηa because the higher temperature restrained Hg0 7


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adsorption. At 160 °C, the MnO2 sorbent exhibited the best Hg0 removal performance with a medium ηa value of 71.5%. H2S is another toxic byproduct during coal gasification. It is therefore to investigate the effect of H2S on Hg0 capture. The red line of Figure 4 presents the Hg0 removal efficiency as a function of temperature for MnO2 under N2 plus 400 ppm H2S atmosphere. The value of ηa increased from 82.3% to 90.5% as the reaction temperature increased from 80 to 160 °C and then decreased from 90.5% to 71.2% when the temperature further elevated to 200 °C. Compared with Hg0 capture under N2 atmosphere, the Hg0 removal efficiencies were greatly enhanced at the temperature range of 80 to 200 °C when 400 ppm H2S was added to the N2 flow, indicating that H2S can facilitate Hg0 removal over MnO2 sorbent. Hg0 removal experiments were further conducted under simulated syngas to examine the performance of the synthesized MnO2 sorbent in real syngas. As shown in the blue line of Figure 4, MnO2 exhibited high capacity for the removal of Hg0 from simulated syngas, over 90% ηa was obtained at the temperature range of 80-160 °C. Even at evaluated temperature (200 °C), the Hg0 removal efficiency still maintains at a high value of 87.5%. In addition, the GHSV of 5 ×104 h−1 used in this study is higher than that of the real coal-derived syngas feed.39 The higher GHSV results in a shorter residence time for Hg0 adsorption over MnO2 sorbent and thus restrained Hg0 capture. Therefore, the prepared MnO2 sorbent exhibits superior Hg0 removal capacity in the temperature range of 80-200 °C from coal-derived syngas feed due to the longer residence time.

3.1.2. Effect of H2S Concentration. To further investigate the effect of H2S concentration on the performance of MnO2 sorbent, Hg0 adsorption experiments were conducted under different H2S concentration at 160 °C. Figure 5 8


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shows the impact of the H2S content on the Hg0 removal efficiency. The value of ηa increased from 85.9% to 96.9% as the concentration of H2S increased from 200 to 800 ppm, indicating that the appropriate increase of H2S concentration is beneficial for Hg0 removal over MnO2. However, the further increase of H2S concentration from 800 to 1000 ppm resulted in a decrease of ηa from 96.9% to 88.1%. This may be attributed to the formation of rings or long linear chains of sulfur species, which is not active for Hg0 adsorption,40 on the MnO2 surface under higher H2S concentration level. Moreover, the formed rings or long linear chains of sulfur species tend to block pores and cover surface active regions, which further prevents Hg0 capture. Therefore, H2S possesses both stimulative and inhibitive effects on Hg0 removal by MnO2 sorbent, and the specific effect depends on the H2S content in the syngas.

3.1.3. Analysis of mercury species in the spent samples To identify the Hg0 removal mechanism of MnO2 sorbent in the presence of H2S, the XPS spectra of Hg 4f, S 2p, and Mn 2p for fresh and pretreated (saturated by Hg0 under 400 ppm H2S balanced in N2 and then purged with pure N2 for 1 hours) MnO2 samples were characterized. The XPS spectra of Hg 4f is shown in Figure 6(a). The binding energies of the Hg 4f peaks were located at 100.7 and 104.3 eV, which can be ascribed as HgS36 and HgO15, respectively. In addition, no adsorbed Hg0 was observed on the sample surface, because the peak (approximately 99.9 eV)41 of Hg 4f corresponding to Hg0 was not detected in the XPS profiles. Figure 6(b) shows the XPS spectra of S 2p. Five peaks of S 2p spectra were observed in the pretreated sample. The three peaks at lower binding energies of 160.5, 161.4, and 162.8 eV were characteristic of S2−, which indicates that the existence of HgS. The peak at 164.1 eV was assigned to S8,42 which verified the previous inference that the formation of rings or long linear chains of sulfur species. The peak at higher 9


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binding energy of 168.2 eV was characteristic of S6+, suggesting the formation of sulfate. The sulfate was speculated to exist in the form of manganese sulfate (MnSO4) since the peak (approximately 101.0 eV)43 of S 2p corresponding to mercury sulfate was not observed in the XPS profiles. The XPS spectra of Mn 2p are shown in Figure 6(c) and Figure 6(d). For the fresh MnO2 sorbent, the peaks at 642.7, 641.3, and 640.1 eV were attributed to Mn4+, Mn3+, and Mn2+, respectively. For the pretreated MnO2 sorbent, the peaks of Mn4+, Mn3+, and Mn2+ were observed at 643.0, 641.7, and 640.6 eV, respectively. The ratio of Mn4+ decreased from 36.0% (fresh sorbent) to 33.2% (pretreated sorbent), and the ratio of Mn2+ increased from 27.4% (fresh sorbent) to 30.7% (pretreated sorbent), implying that the reduction of Mn4+ to Mn2+ on the surface of the sorbent contributed to the oxidation of Hg0 and S2−. Based on the above analysis, it can be inferred that the transformation of Hg0 to HgS and HgO on MnO2 surface in the presence of H2S is the predominant way to remove Hg0 from syngas. To further examine the mercury species on the MnO2 sorbent and investigate their desorption characteristics, TPD experiments were implemented in pure N2 condition with a flow rate of 0.5 L/min. The heating rate was set to 10 °C/min. Prior to TPD experiments, two group of fresh MnO2 samples were used for Hg0 adsorption under pure N2 and N2 + 400 ppm H2S, respectively. As shown in Figure 7, MnO2 sorbent after adsorption under pure N2 atmosphere obviously release Hg0 from 190 to 360 °C and peaked at 255 °C. It has been reported that HgO decompose at 200−380 °C and peaked at 260 °C.44 Thus, the peak at 255 °C could be assigned to HgO. In the case of adsorption under N2 + 400 ppm H2S atmosphere, the Hg-TPD curve presented two desorption peaks at approximately 260 and 290 °C. The desorption peak at 260 °C could be attributed to the decomposition of both HgO and metacinnabar (black HgS), since the experimental findings44 10


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indicate that they have the same decomposition peaks. Another desorption peak at around 290 °C was assigned to cinnabar (red HgS).45 Additionally, the Hg-TPD analysis reveals again that mercury sulfate was not formed on the used sorbent because the decomposition peak (approximately 580 °C)45 corresponding to HgSO4 was not observed in the Hg-TPD curve. Furthermore, the desorption peak intensity of the spent sample used for Hg0 adsorption in N2 + 400 ppm H2S was much higher than that in pure N2, indicating that H2S significantly improves the capacity of MnO2 sorbent for Hg0 removal, which agrees well with the previous Hg0 removal experiments.

3.2. Computational Results In order to provide insight into the effect mechanism of H2S on Hg0 removal by MnO2 sorbent during coal gasification, DFT calculations were performed to comprehend the adsorption of Hg0, H2S and HgS. All possible adsorption sites on the MnO2(110) surface were considered. Both parallel and perpendicular adsorption orientations were taken into account for polyatomic molecules (H2S and HgS) adsorption. The influence of H2S on Hg0 removal performance of MnO2 can be studied by comparing the adsorption energies of Hg0 on clean and sulfurized MnO2(110) surfaces. The predominant reaction mechanism of Hg0 with H2S over MnO2 surface was determined on account of the reaction energy barrier.

3.2.1. Hg0 and H2S Adsorption on MnO2(110) Surface. The stability of Hg0 adsorption on MnO2(110) surface is in the trend of Mn5 top < Obr top < Obr bridge, and the adsorption energy of Hg0 ranges from −51.55 to −69.50 kJ/mol. The most active site for Hg0 adsorption is the Obr bridge site with an adsorption energy of −69.50 kJ/mol, and Mulliken charge transfer of 0.31 e, respectively. Thus, it can be concluded that Hg0 is chemically adsorbed on MnO2 and prefers to interact with Obr atoms. The previous Hg0 removal experiments show that ηa 11


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increased with increasing reaction temperature (80−160 °C) and the adsorption behavior is normally described as chemisorption mechanism. In addition, the TPD analysis indicate that HgO is formed on MnO2 surface after Hg0 adsorption under pure N2. Therefore, the theoretical calculations are in good agreement with the experimental results. Previous experimental results indicate that H2S plays an important role in the capture of Hg0 from syngas. H2S may be turned into active sulfur species on MnO2 surface and further facilitate Hg0 removal. Therefore, the adsorption of H2S on MnO2(110) surface was examined to make out the reactivity of sorbent and the Hg0 heterogeneous reaction. Two optimized configurations of H2S on MnO2(110) surface are obtained and presented in Figure 8. The adsorption energies, bond lengths and Mulliken charges are given in Table 1. The most stable configuration for H2S adsorption is 1A with an adsorption energy of −254.12 kJ/mol and Mulliken charge transfer of 1.18

e, suggesting a chemisorption. In 1A, the S atom of H2S strongly interacts with Mn5 as well as Os atom of MnO2 and forms Mn–S and O–S bonds with the bond lengths of 2.348 and 1.714 Å, respectively. In addition, the H atoms of H2S migrate to the two adjacent Obr top sites and form two surface hydroxyls. The two H–S bond lengths are largely extended from 1.355 (free H2S) to 2.148 and 2.827 Å, respectively, implying the dissociation of H2S. Similar to 1A, H2S in configuration 1B is also dissociative adsorption with an adsorption energy of −220.72 kJ/mol and Mulliken charge transfer of 0.80 e, respectively. In 1B, the S atom intensely bonds to the Mn5 as well as Obr atom of MnO2 and forms Mn–S and O–S bonds with the bond lengths of 2.294 and 1.610 Å, respectively. Meanwhile, the H atoms migrate to the two adjacent Os top sites and form two surface hydroxyls.

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From the above analysis, it can be concluded that H2S undergoes dissociative adsorption on MnO2(110) surface and turns into surface hydroxyl and manganese−sulfur−oxygen complexes. These conclusions are in consistent with the previous experimental findings46.

3.2.2. Hg0 Adsorption on Sulfurized MnO2(110) Surface. The influence of H2S on Hg0 removal by MnO2 was further investigated by comparing the Hg0 adsorption on clean and sulfurized MnO2(110) surface. Figure 9 shows the optimized configurations of Hg0 adsorption on sulfurized MnO2(110) surface, and table 2 summarizes the corresponding adsorption energies, bond lengths and Mulliken charges. The most stable structure for Hg0 adsorption is 2A with an adsorption energy of −103.40 kJ/mol and Mulliken charge transfer of 0.63

e, respectively. In 2A, Hg0 strongly interacts with the previously adsorbed S and forms Hg–S bond with the bond length of 2.315 Å, suggesting the formation of HgS. This is in consistent with the TPD and XPS experimental results that HgS is formed in the presence of H2S. In 2B, Hg0 also bonds to the adsorbed S and forms Hg-S bond with an adsorption energy of −81.59 kJ/mol and Mulliken charge transfer of 0.62 e, respectively. The adsorption energy and Mulliken charge transfer of Hg0 on sulfurized MnO2(110) surface are higher than those on clean MnO2(110) surface, indicating that H2S facilitates the chemisorption of Hg0. This is in line with the previous experimental results that ηa increased greatly with the addition of 400 ppm H2S in pure N2 flow. In addition, the bond length of Hg–Obr for Hg0 adsorption on sulfurized MnO2(110) surface (2.114 Å) is shorter than that of Hg0 adsorption on MnO2(110) surface (2.540 Å), implying that the adsorbed H2S enhances the activity of its neighbor atoms for Hg0 adsorption. Consequently, the above analysis verifies the previous experimental observations that gaseous H2S can be dissociatively adsorbed on MnO2 surface to form active sulfur species for Hg0 transformation. 13


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3.2.3. HgS Adsorption on MnO2(110) Surface. HgS can be formed and adsorbed on sorbent surface in the presence of H2S from the above experiments. Therefore, the adsorption mechanism of HgS on MnO2(110) surface was investigated to better understand Hg0 transformation process. The optimized configurations of HgS adsorption are shown in Figure 10, and the adsorption energies, bond lengths and Mulliken charges are listed in Table 3. The stability of the surface complex is in the order of 3D > 3C > 3A > 3B. The most stable structure for HgS adsorption is 3D with an adsorption energy of −286.33 kJ/mol and Mulliken charge transfer of 0.80 e, implying a strong chemisorption. In 3D, HgS is molecularly adsorbed on MnO2(110) surface in parallel orientation, resulting in the formation of Mn–Hg and S–O bonds with the bond lengths of 2.858 and 1.565 Å, respectively. Similar to 3D, HgS in 3C is also molecular adsorption in parallel orientation with Hg bonding to Obr atom and S bonding to Mn5 atom. The adsorption energy and Mulliken charge transfer of HgS are −259.72 kJ/mol and 0.61 e, respectively, which are slightly lower than those of HgS in 3D. In the case of Hg-down orientation on Obr top site (3A) and Obr bridge site (3B), the adsorption energies are −243.56 kJ/mol and −185.59 kJ/mol, suggesting that perpendicular adsorption is also favorable for HgS adsorption. Based on the above analyses, we can conclude that HgS molecule can stably exist on MnO2 surface. This agrees well with the previous XPS and TPD experimental results in which the peak corresponding to HgS was detected on spent MnO2 sorbent.

3.2.4. Reaction Mechanism of Hg0 and H2S over MnO2 Surface. Langmuir-Hinshelwood (L-H) mechanism47,48 and Eley-Rideal (E-R) mechanism49,50 have been proposed for heterogeneous Hg0 reaction. For L-H mechanism, two reactants (Hg0 and H2S) are 14


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adsorbed on the surface of sorbent and react to form HgS. For E-R mechanism, one of the reactants (Hg0 or H2S) is first adsorbed on the sorbent, and then reacts with the other gaseous (or weakly adsorbed) reactant to form HgS. On account of the existence of active sulfur species on MnO2 surface, L–H and/or E–R mechanisms were proposed to explain the Hg0 reaction since mercury (gaseous Hg0 and/or adsorbed mercury) can react with active sulfur species to form HgS. The energy profile of Hg0 reaction to HgS via L–H and E–R mechanisms on MnO2(110) surface and the related optimized structures of intermediate and transition state are shown in Figure 11. The energies of the optimized structures are relative to the intermediates. In the case of L–H mechanism, the heterogeneous Hg0 transformation process is via reactants → IM1 → TS1 → FS. The first step is the co-adsorption of Hg0 and H2S on MnO2(110) to form IM1. This step is barrierless and exothermic by 292.38 kJ/mol. In IM1, Hg0 bonds to the surface Obr atoms and forms Hg–O bonds, indicating the formation of HgO. This is in good agreement with the XPS and TPD analysis results that HgO is formed on the surface of MnO2. The second step is surface bound HgS formation via the IM1 → TS1 → FS. This step is exothermic by 65.14 kJ/mol with an energy barrier of 42.00 kJ/mol. In TS1, the Hg–O bond is formed with a Hg–O bond length of 2.357 Å. The distance between Hg and S atoms decreases gradually: 3.925 Å in IM1, 3.230 Å in TS1, and 2.315 Å in FS. In the case of E–R mechanism, the first step is the adsorption of H2S on MnO2(110) to form H2S pre-dissociated surface IM2, and then Hg0 is adsorbed on IM2 to form IM3. The adsorption process (reactants → IM2 → IM3) is exothermic by 298.89 kJ/mol. In the reaction process for HgS formation (IM3 → TS2 → FS), the adsorbed Hg0 approaches to the surface S atom, and overcomes a minor energy barrier of 7.16 kJ/mol to form the surface bound HgS. In TS2, the Hg–S and Hg–O 15


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bonds are formed. The distance between Hg and S atoms gradually decreases when Hg0 is completely transformed into HgS: 2.700 Å in IM3, 2.546 Å in TS2, and 2.315 Å in FS. The reaction between weakly adsorbed Hg0 and surface S is an exothermic process with a reaction heat of 58.63 kJ/mol. Overall, the lower energy barriers of Hg0 reaction via the two mechanisms indicate that the formation of HgS is easy and both E–R and L–H mechanisms are favorable for Hg0 oxidation. Furthermore, the E–R mechanism is the dominant Hg0 transformation mechanism because its energy barrier (7.16 kJ/mol) is much lower than that of L–H mechanism (42.00 kJ/mol).

4. CONCLUSIONS The heterogeneous mechanism of Hg0 reaction with H2S over MnO2 surface was firstly investigated by experiments and DFT calculations. The prepared MnO2 sorbent exhibited excellent Hg0 removal capacity under simulated syngas. H2S promoted Hg0 capture by forming active surface sulfur species. The XPS and TPD experimental results indicate that the captured mercury species on the spent MnO2 sorbent mainly exist in the form of HgS and HgO. The chemisorption mechanism is responsible for the adsorption of Hg0 and HgS on MnO2(110) surface. H2S is dissociatively adsorbed on the MnO2(110) surface with an adsorption energy of −254.12 kJ/mol and converts into active sulfur species for Hg0 reaction. The heterogeneous Hg0 reaction with H2S over MnO2 can occur through both E–R and L–H mechanisms since their lower reaction energy barriers. The E–R mechanism, in which H2S is previously dissociated on the surface and generates active sulfur species to react with gaseous Hg0, is kinetically more favorable than L–H mechanism due to its much lower energy barrier. The computational results agree well with the experimental data and provide a good insight into Hg0 removal mechanism over MnO2 sorbent in the presence of H2S. In 16


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the future study, the effects of CO, H2 and HCl on Hg0 removal over MnO2 surface will be independently examined.

■ AUTHOR INFORMATION Corresponding Author *Tel: +86 27 87545526; Fax: +86 27 87545526; E-mail: [email protected].

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (51376072, 51661145010), and Natural Science Foundation of Hubei Province (2015CFA046).

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Table 1. . The Adsorption Energies (Eads, kJ/mol), Bond Lengths (R, Å) and Mulliken Charges (Q, e) for H2S Adsorption on MnO2(110) Surface Eads

RH-S

RS-X

RH-O

Q

1A

−254.12

2.148/2.827

1.714/2.348

0.996/0.979

1.18

1B

−220.72

3.661/3.731

1.610/2.294

1.000/1.000

0.80

Subscript “X” denotes surface Mn or O atom

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Table 2. The Adsorption Energies (Eads, kJ/mol), Bond Lengths (R, Å) and Mulliken Charges (Q, e) for Hg0 Adsorption on Sulfurized MnO2(110) Surface Eads

RHg-O

RHg-S

QHg

2A

−103.40

2.114

2.315

0.63

2B

−81.59

2.135

2.429

0.62

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Table 3. The Adsorption Energies (Eads, kJ/mol), Bond Lengths (R, Å) and Mulliken Charges (Q, e) for HgS Adsorption on MnO2(110) Surface Eads

RHg-X

RS-X

RHg-S

Q

3A

−243.56

2.110

−

2.305

0.46

3B

−185.59

2.304/2.275

−

2.306

0.44

3C

−259.72

2.156

2.316

2.350

0.61

3D

−286.33

2.858

1.565

2.554

0.80

Subscript “X” denotes surface Mn or O atom

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List of Figures Captions

Figure 1. XRD pattern of the prepared MnO2 sample. Figure 2. Schematic of the fixed-bed Hg0 removal system. Figure 3. Structure of MnO2(110) surface and the active sites. (a) side view of MnO2(110) surface； (b) top view of MnO2(110) surface.

Figure 4. Hg0 removal efficiency over MnO2 sorbent in different atmospheres. Figure 5. Effect of H2S concentration on Hg0 removal over MnO2 sorbent. Figure 6. XPS spectra of Hg 4f, S 2p, and Mn 2p for MnO2 before and after Hg0 adsorption. Figure 7. Hg-TPD profiles after Hg0 capture in N2 and N2 + 400 ppm H2S atomospheres. Figure 8. Adsorption structures of H2S on MnO2(110) surface. Figure 9. Adsorption structures of Hg0 on sulfurized MnO2(110) surface. Figure 10. Adsorption structures of HgS on MnO2(110) surface. Figure 11. The energy and geometrical diagram of the Hg0 reaction with H2S on MnO2(110) surface via L–H mechanism and E–R mechanism.

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Figure 1. XRD pattern of the prepared MnO2 sample.

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condenser

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

3-way valve

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Figure 2. Schematic of the fixed-bed Hg0 removal system.

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Figure 3. Structure of MnO2(110) surface and the active sites. (a) side view of MnO2(110) surface； (b) top view of MnO2(110) surface.

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Figure 4. Hg0 removal efficiency over MnO2 sorbent in different atmospheres.

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Figure 5. Effect of H2S concentration on Hg0 removal over MnO2 sorbent.

29


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104.3

161.4 162.8

100.7

168.2

160.5 164.1

641.3

641.7

642.7

640.6

640.1

643.0

Figure 6. XPS spectra of Hg 4f, S 2p, and Mn 2p for MnO2 before and after Hg0 adsorption.

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Figure 7. Hg-TPD profiles after Hg0 capture in N2 and N2 + 400 ppm H2S atomospheres.

31


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48 2 .1 31 3 .7

7 82 2.

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

3.6 6

1 .6

1

10

Figure 8. Adsorption structures of H2S on MnO2(110) surface.

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2.315

Hg

S 35 2.1

2. 11 4

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

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2.429

1.726

Mn O Hg S H

2A

2B

Figure 9. Adsorption structures of Hg0 on sulfurized MnO2(110) surface.

33


2. 3 11 0

2.1

2.

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 34 of 35

05

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Figure 10. Adsorption structures of HgS on MnO2(110) surface.

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56

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Mn

Hg0 + H2S +MnO2

Hg

O

S

L–H mechanism E–R mechanism

H

1.716 S 3.925 H 2.350

3.230

Hg

2.374

2.599

2.357

2.636

2.315

IM2

2.348

IM1 292.38

254.12 Hg

46 2.5

00

1.827 S

2.438

TS2 291.73

IM3 298.89

2. 7

H

TS1 250.38

4

H

1.714 S

11

Relative Energy (kJ/mol)

0.00

2.

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

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28 2.7

FS 357.52

2.369

2.359

Reaction

Adsorption Reaction pathway

Figure 11. The energy and geometrical diagram of the Hg0 reaction with H2S on MnO2(110) surface via L–H mechanism and E–R mechanism.

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Effect of the Mechanism of H2S on Elemental Mercury Removal Using

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