Experimental and DFT studies of the role of H2S in Hg0 removal from syngas over CuMn2O4 sorbent

Journal Pre-proofs Experimental and DFT studies of the role of H2S in Hg0 removal from syngas over CuMn2O4 sorbent Zhen Wang, Jing Liu, Yingju Yang, Yingni Yu, Xuchen Yan, Zhen Zhang PII: DOI: Reference:

S1385-8947(19)33031-1 https://doi.org/10.1016/j.cej.2019.123616 CEJ 123616

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Chemical Engineering Journal

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22 October 2019 22 November 2019 25 November 2019

Please cite this article as: Z. Wang, J. Liu, Y. Yang, Y. Yu, X. Yan, Z. Zhang, Experimental and DFT studies of the role of H2S in Hg0 removal from syngas over CuMn2O4 sorbent, Chemical Engineering Journal (2019), doi: https:// doi.org/10.1016/j.cej.2019.123616

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Experimental and DFT studies of the role of H2S in Hg0 removal from syngas over CuMn2O4 sorbent Zhen Wang, Jing Liu*, Yingju Yang, Yingni Yu, Xuchen Yan, Zhen Zhang

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

ABSTRACT: Elemental mercury (Hg0) and hydrogen sulfide (H2S) are two typical toxic pollutants in coal-derived syngas. The specific role of H2S in Hg0 elimination over CuMn2O4 sorbent and the involved reaction mechanism were systematically studied by experimental and theoretical methods. The synthesized CuMn2O4 sorbent was tested for Hg0 removal under simulated syngas and exhibited superior Hg0 capture performance (up to 95.6% at 200 °C). In the absence of H2S, both H2 and CO inhibited Hg0 removal over CuMn2O4, but the Hg0 removal efficiency was greatly improved after the introduction of 400 ppm H2S. H2S played a key role in Hg0 elimination in syngas by generating reactive sulfur species upon CuMn2O4. Density functional theory (DFT) calculations indicated that Hg0 and HgS were strongly chemisorbed upon CuMn2O4 surface with the adsorption energies of −129.84 and −220.21 kJ/mol, respectively. H2S was dissociatively adsorbed on CuMn2O4 and generated active sulfur species. Both H2S-pretreatment experiments and DFT calculations demonstrated that Hg0 reaction with H2S over CuMn2O4 occurred via a Langmuir–Hinshlwood mechanism, where chemisorbed Hg0 reacted with active sulfur species to form surface-bonded HgS. Furthermore, XPS and TPD analyses certified

*

Corresponding author. Tel.: +86 27 87545526; fax: +86 27 87545526. E-mail address: [email protected] (J. Liu).

that the formation of active sulfur species and HgS upon the spent CuMn2O4 sorbents. Keywords: Hg0 capture; Syngas; H2S; CuMn2O4; Effect mechanism; Density functional theory

1. Introduction Mercury pollution can cause pernicious effects on human health and ecosystem owing to its hypertoxicity, high volatility, bioaccumulation and durability [1]. In August 2017, the Minamata Convention on Mercury entered into force, committing its contracting parties to take action to control mercury emission [2]. Mercury emission during coal utilization process (i.e., coal combustion and gasification) is regarded as one of the dominant anthropogenic mercury emission sources [3]. The emitted mercury mainly exists in three forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate mercury (Hgp), among which Hg0 is the most difficult species to be removed due to its high volatility and water insolubility [4, 5]. Recently, coal gasification for advanced power generation has attracted increasing attention owing to its high efficiency, extensive raw materials, abundant by-products, and environmentally friendly relative to traditional combustion technologies [6, 7]. However, Hg0 is difficult to be oxidized under reducing atmosphere and thus the Hg0 concentration in coal-derived syngas is much higher than that in coal-fired flue gas [8]. Furthermore, the higher concentration of Hg0 would expedite the corrosion of downstream aluminum components, which has caused catastrophic industrial accidents in plants [9]. Consequently, there are more challenges for removing Hg0 from syngas than coal-fired flue gas. However, most of the previous reported works have concentrated on Hg0 elimination from coal-fired flue gas. Therefore, the effective reduction of Hg0 emission from syngas has become an urgent issue.

2

Hg0 removal by using sorbents is a feasible technology for reducing Hg0 emission from syngas. So far, a large number of sorbents, such as activated carbons [10-15], metal oxides [16-21], and noble metals [22, 23] have been exploited to remove Hg0. Nevertheless, the lower Hg0 adsorption capacity and poor regenerability of activated carbons resulted in high operation cost. It has been found that some metal oxides based sorbents can effectively capture Hg0 from syngas at low temperature (< 160 °C) [16, 17, 24]. However, in order to improve the heat efficiency of the integrated gasification combined cycle, Hg0 elimination from syngas is desirable at higher temperature (≥ 200 °C) [12, 22]. Noble metals, particularly palladium based sorbents have been developed for Hg0 removal from syngas, and exhibited good performance at the temperature above 200 °C [22, 23], but their industrial application is limited by the high price. Thus, it is significant to develop novel sorbent with low cost but excellent high-temperature activity for Hg0 removal from syngas. In recent years, manganese-based oxides with spinel AB2O4 structure have received more and more interest in the field of gaseous pollutants removal because of their unique structural and physicochemical characteristics [25-28]. Benefiting from the remarkable tunability of A and B cations, spinels could be rationally regulated and equipped with targeted physicochemical properties (for example, morphologies, defects, electron mobility, and redox behavior) for enhanced performance [29]. In addition, the cations at A and B sites can mutually shift, leading to surface-abundant oxygen defects and mobile-electron environment [30-32]. These characteristics are beneficial for Hg0 adsorption and oxidation. In our previous work [33], a CuMn2O4 spinel sorbent was developed for the capture of mercury from coal-fired flue gas and exhibited good performance at a wide temperature range of 50–350 ℃. However, it is unknown whether CuMn2O4 sorbent can efficiently capture Hg0 in syngas

3

since the reducing atmosphere is adverse to oxidize Hg0. Except for Hg0, hydrogen sulfide (H2S), another notorious toxic pollutant, also exists in coal-derived syngas and requires to be captured. On the other hand, H2S has been employed as reagent to modify raw materials with the aiming of improve their Hg0 capture performance [34-37]. This provides a potential for removing Hg0 and H2S in syngas simultaneously. The fundamental comprehending of the effect mechanism of H2S on Hg0 capture from syngas is important for developing sorbents with superior Hg0 removal performance. However, the specific role of H2S on Hg0 elimination over CuMn2O4 sorbent and the involved reaction mechanism have not been studied. In the present work, a spinel-type CuMn2O4 sorbent was synthesized via low-temperature sol-gel auto-combustion approach and tested for Hg0 removal under simulated syngas. The effects of reaction temperature and H2S on Hg0 elimination over CuMn2O4 sorbent were examined. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) were applied to characterize the adsorbed mercury compound on used CuMn2O4 sorbent. Furthermore, the reaction mechanism between Hg0 and H2S upon CuMn2O4 was studied for the first time by experiments and DFT calculations.

2. Experimental and calculation section 2.1. Sorbent preparation and characterization CuMn2O4 sorbent was synthesized by a low-temperature sol-gel auto-combustion method. Typically, 0.06 mol citric acid monohydrate and 5 ml alcohol were added in 200 ml distilled water with continuous stirring. Then 0.02 mol Cu(NO3)2·3H2O and 0.04 mol Mn(NO3)2·4H2O were dissolved into the solution. The obtained solution was stirred under 60 °C for 1 h and then evaporating under 90 °C to obtain sticky

4

sol-gel. Subsequently, the wet gel was dried at 100 °C for 12 h and calcined at 400 °C for 4 h. Finally, the residual solid product was ground and sized to 200 mesh for use. The surface area of the as-synthesized sorbent was measured using a nitrogen adsorption apparatus (ASAP-2020, Micromeritics) based on Brunauer-Emmett-Teller (BET) adsorption method. The phase composition of the sample was analyzed by X-ray diffraction (XRD-7000, Shimadzu, Japan) using Cu Kα (λ = 1.5406 nm) radiation. The diffraction pattern was recorded from 10° to 80° (2θ) with a scanning rate of 5 °/min. The morphology of the sample was examined using scanning electron microscopy (SEM, JOEL7100F). The surface chemical properties of the sample were analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000), which was equipped with Al Kα (hv = 1486.6 eV) excitation source. The binding energy was corrected using C1s peak at 284.6 eV.

2.2. Hg0 removal performance tests The Hg0 removal performance of the prepared sorbent was tested in a bench-scale fixed-bed reactor, as presented in Fig. 1. 50 mg sorbent mixed with 1950 mg quartz sand was used in all experiments. As an inert substance, quartz sand was employed as diluter to mix with CuMn2O4 sorbent. Quartz sand could increase the height of reaction region, thus increasing the residence time of syngas. The syngas compositions containing N2, H2, CO, H2S, and HCl were fed by cylinder gases and were controlled by mass flow controllers. The total flow rate was maintained at 1 L/min, and the corresponding gas hourly space velocity (GHSV) was about 5×104 h−1. A stable feed of gaseous Hg0 (about 50 μg/m3) was provided by a mercury permeation tube. An online mercury analyzer (Lumex RA-915M) was used to

5

measure the Hg0 concentration. The instantaneous Hg0 removal efficiency (i ) over CuMn2O4 sorbent was calculated using the following equation:

i 

Hg in0  Hg 0out 100% Hg in0

(1)

The accumulation Hg0 removal efficiency (a ) in 60 min was calculated using the following equation:

a

 

t

0

t

Hg in0 dt   Hg 0out dt 0



t

0

0 in

Hg dt

 100%

(2)

where t is the adsorption time, Hg in0 and Hg 0out are the instantaneous Hg0 concentration at inlet and outlet of the reactor, respectively.

2.3. Computational methods The DFT calculations were conducted using the CASTEP program with the Perdew-Wang 1991 (PW91) exchange−correlation function [38]. The relativistic effects on valence electrons of Cu, Mn and Hg atoms were treated using Vanderbilt Ultrasoft pseudopotentials [39]. The one-electron valence states were expanded in a basis of plane waves with a kinetic energy cutoff of 340 eV. The energy cutoff of calculation system was tested before its utilization. To balance computational accuracy and time, the energy cutoff was set to 340 eV for all calculations. The convergence criterion based on the force (0.05 eV/Å), energy (2.0 × 10−5 eV/atom), self-consistent field (2.0 × 10−6 eV/atom), and displacement (0.002 Å) was applied for calculations. Brillouin zones were sampled with 4 × 4 × 4 and 3 × 2 × 1 Monkhorst-Pack [40] k-point meshes for the bulk and surface of CuMn2O4, respectively. CuMn2O4 with space group Fd3m is a normal spinel structure with Cu2+ cations in tetrahedral sites and Cu3+ cations in

6

octahedral sites [26]. CuMn2O4(100), a typical low-index surface, was used to investigate mercury adsorption and transformation over CuMn2O4 sorbent. Since Hg0 in syngas is dilute, the interaction between neighboring Hg atoms is negligible and thus a big surface model is needful. Consequently, CuMn2O4(100) surface was built by periodically repeated p(2 × 1) surface supercell with eight atomic layers, and the bottom four layers were fixed. The thickness of vacuum space was set to 16 Å, which has been demonstrated to be large enough to prevent the interactions between two periodic slabs [4, 28, 31]. CuMn2O4(100) surface model and the possible active sites were shown in Fig. 2. The adsorption energy (Eads) was calculated by the following equation:

Eads  E(CuMn 2O4 -adsorbate)  ( ECuMn 2O4  Eadsorbate )

(3)

where E(CuMn 2O4 -adsorbate) , Eadsorbate , and ECuMn 2O4 are the gross energies of the CuMn2O4-adsorbate system, the isolated adsorbates, and the optimized CuMn2O4(100) model, respectively. The larger negative Eads value, the stronger adsorption strength of adsorbates. The physisorption energy is lower than –30 kJ/mol, whereas the chemisorption energy is higher than –50 kJ/mol [41]. Additionally, complete linear synchronous transit/quadratic synchronous transit (LST/QST) was applied to find all transition states. The obtained transition state (TS) configurations were identified when (1) the forces on the atoms vanish and (2) the energy is a maximum along the reaction coordinate but a minimum with respect to all of the other degrees of freedom. The energy barrier (Ebar) was calculated by the following equation: Ebar = ETS – EIM where ETS and EIM are the energy of TS and intermediate (IM), respectively.

7

(4)

3. Results and discussion 3.1. Characterization of sorbent The BET surface area of the as-prepared CuMn2O4 sorbent was 26.35 m2/g and the total pore volume was 0.16 cm3/g. Fig. 3a shows the XRD pattern and SEM image of the sample. The diffraction peaks indicative of Cu-Mn spinel, Mn3O4, and CuO were detected, and the Cu-Mn spinel was the predominant phase. SEM image of CuMn2O4 shows homogeneous granular structure with average particle size of 54 nm. The XPS spectra of Cu 2p, Mn 2p, and O 1s are shown in Fig. 3(b–d). The results suggested that the surface Cu and Mn atoms existed in flexible valence states of Cu+/Cu2+ and Mn2+/Mn3+/Mn4+. The oxygens on CuMn2O4 surface mainly existed in lattice oxygen (Oα) and chemisorbed oxygen (Oβ).

3.2. Performance of CuMn2O4 sorbent for Hg0 removal 3.2.1. Effect of reaction temperature. The effect of reaction temperature on Hg0 removal under simulated syngas (20% H2, 20% CO, 400 ppm H2S, 10 ppm HCl) over the as-prepared CuMn2O4 sorbent was investigated and displayed in Fig. 4a. In the temperature window of 40−200 ℃, reaction temperature exhibited no obvious effect on Hg0 capture, over 95% Hg0 removal efficiency (a ) was obtained. Nevertheless, a of CuMn2O4 sorbent declined from 95.6% to 60.5% with further increasing reaction temperature from 200 to 240 °C. It is worth noting that the GHSV of 5 ×104 h−1 is greater than that of actual syngas feed [42]. The lower GHSV leads to a longer duration time for Hg0 adsorption and thus facilitates the capture of Hg0 from syngas. Additionally, we have compared the high-temperature performance of Hg0 capture from syngas

8

over CuMn2O4 to the previously reported sorbents, and found that the a of CuMn2O4 at 200 °C was much greater than that of CeO2/TiO2 (< 40% at 200 °C) [16], FexCeyOz/semi-coke (< 50% at 200 °C) [17], CexCoyOz/TiO2 (~ 60% at 200 °C) [43], Fe2O3(N)/TiO2 (~ 70% at 150°C) [44], and coal-based AC (< 80% at 160°C) [12]. Consequently, it can be concluded that the as-prepared CuMn2O4 sorbent has superior high-temperature Hg0 capture performance under simulated syngas. 3.2.2. Role of H2S in Hg0 removal Hg0 removal experiments over CuMn2O4 sorbent under different H2S concentrations and atmospheres at 200 °C were performed to elucidate the role of H2S in Hg0 elimination. Fig. 4b shows the influence of H2S concentration on Hg0 removal. Without H2S, about 73.6% a was obtained. Mars-Maessen mechanism [45], where adsorbed Hg0 reacts with Oα and/or Oβ to produce HgO on sorbent surface, could be used to interpret Hg0 capture by CuMn2O4 under pure N2 atmosphere. The value of a increased to 81.8% and 97.4% when 50 and 400 ppm H2S were added into N2 atmosphere, respectively. No obvious change of a was observed after H2S content was further elevated to 800 ppm, suggesting that 400 ppm H2S was sufficient for promoting Hg0 removal over CuMn2O4. It has been reported that surface oxygens on sorbent could promote the transformation of H2S to surface active sulfur species [12, 24], which is more active than surface oxygens for Hg0 adsorption. Moreover, H2 and CO, two well-known reducing gases, are the major components of syngas. Thus, the role of H2S in Hg0 capture under reducing atmosphere was investigated. As shown in Fig. 4c and 4d, both H2 and CO inhibited Hg0 removal over CuMn2O4 in the absence of H2S. The introduction of 20% H2 and 20% CO into pure N2 flow resulted in the decline of a from 73.6% to 44.6% and 17.1%, respectively. As strong reducing agents, H2 and CO can easily react with surface oxygens of CuMn2O4

9

and thus suppressed Hg0 elimination. Nevertheless, H2 and CO exhibited no obvious effects on Hg0 capture in the presence of 400 ppm H2S. Based on the above results, it can be concluded that H2S played a key role in Hg0 elimination from syngas by CuMn2O4 sorbent.

3.3. Mechanism for Hg0 removal over CuMn2O4 in the presence of H2S In order to reveal the Hg0 capture mechanism by CuMn2O4 sorbent in the presence of H2S, the H2S pretreatment experiments were conducted. CuMn2O4 sorbent was firstly pretreated under N2 + 400 ppm H2S atmosphere at 200 ℃ for 30 min and then swept by pure N2 for 30 min. After that, the H2S-pretreated CuMn2O4 sorbent was used for Hg0 removal under N2 atmosphere at different reaction temperature. As presented in Fig. 5, the H2S-pretreated CuMn2O4 sorbent showed much higher Hg0 removal performance than the fresh CuMn2O4 sorbent, which suggested that H2S pretreatment yielded reactive sulfur substance on CuMn2O4 surface and hence promoted Hg0 elimination. Moreover, a of H2S-pretreated CuMn2O4 sorbent increased with decreasing reaction temperature from 200 to 150 ℃. Normally, the lower temperature is favorable for Hg0 adsorption. Since reaction rates generally decrease as reaction temperature decreases, less Hg0 should be captured at lower temperature if gaseous Hg0 can react with active surface sulfur species. Consequently, it can be concluded that Hg0 reaction with H2S on CuMn2O4 follows the Langmuir–Hinshlwood mechanism, in which H2S is adsorbed on sorbent surface and generates active sulfur species to react with adsorbed Hg0. To further understand the Hg0 removal mechanism over CuMn2O4 sorbent, the surface chemistries of H2S-pretreated and Hg0-pretreated (saturated by Hg0 under N2 + 400 ppm H2S atmosphere at 200 ℃ and then swept by pure N2 flow for 60 min to remove the weakly adsorbed species) CuMn2O4 sorbents

10

were analyzed by XPS. The S 2p XPS spectra of H2S-pretreated CuMn2O4 sorbent is displayed in Fig. 6a. The spectra was fitted into three peaks, and the peaks at 162.44 and 163.68 eV were ascribed to Sn2− [46], which is eminent for its very strong affinity with Hg0. This demonstrated that H2S was adsorbed and transformed into active sulfur species on CuMn2O4 surface. In addition, the peak at 168.61 eV was assigned to SO42−, indicating the formation of manganese and/or copper sulfates. Fig. 6b presents the Hg 4f spectra of Hg0-pretreated CuMn2O4 sorbent. The peaks at 100.43 and 105.38 eV were characteristic of Hg 4f7/2 and Hg 4f5/2 spin−orbit doublet of Hg2+, respectively, and the Hg2+ species could be assigned to HgS and HgO since their Hg 4f characteristic peaks are very close [5]. This certified that Hg0 was chemically adsorbed and converted into oxidized mercury species on CuMn2O4 sorbent surface. Furthermore, the mercury species on spent CuMn2O4 sorbent was further confirmed by performing TPD experiments. Fig. 6c shows the Hg-TPD curve of spent CuMn2O4 sorbent (pretreated by Hg0 under N2 + 400 ppm H2S atmosphere at 200 ℃ for 60 min). As shown, no Hg0 was released at the temperature lower than 235 ºC, which suggested that Hg0 was chemisorbed on the spent CuMn2O4 surface. Only one intensively Hg0 desorption peak at 310 ℃ was detected, which was corresponded to the decomposition of cinnabar (red HgS) [47]. Therefore, it can be concluded that Hg0 was chemisorbed and transformed into HgS on CuMn2O4 sorbent surface in the presence of H2S.

3.4. DFT studies of Hg0 adsorption and transformation on CuMn2O4 3.4.1. Hg0, H2S and HgS binding upon CuMn2O4(100) surface DFT calculations were carried out to elucidate the microcosmic mechanism of Hg0 reaction with H2S over CuMn2O4 sorbent at atomic level. On account of the above experimental results, Hg0 and H2S

11

are the reactants, while HgS is the final product. Thus, the adsorption mechanisms of Hg0, H2S and HgS on CuMn2O4(100) surface were firstly investigated. During the calculations, all potential active adsorption sites and adsorption directions were considered. Fig. 7 shows the adsorption energies and structural parameters of the stable configurations for Hg0, H2S and HgS adsorption. For Hg0 adsorption on CuMn2O4(100) surface, Hg0 is preferably bonded to Mn5f site (shown in Fig. 7a), forming a Mn–Hg bond with a bond length of 2.973 Å. The corresponding adsorption energy is −129.84 kJ/mol, indicating a strong chemisorption. In Fig. 7b, Hg0 is adsorbed on Cu2f site with the formation of a Cu–Hg bond (2.753 Å). This configuration yielded a binding energy of −121.94 kJ/mol, which is close to that of Fig. 7a, suggesting that the surface Cu2f site is also active for Hg0 adsorption. Therefore, it can be concluded that chemisorption mechanism is responsible for Hg0 adsorption on CuMn2O4 surface. This is consistent with the previous XPS and TPD experimental conclusions that Hg0 was chemisorbed on CuMn2O4 sorbent. Fig. 7c shows the most stable structure for H2S adsorption, which yields an adsorption energy of −164.67 kJ/mol, suggesting a strong interaction between H2S and CuMn2O4 sorbent surface. In this configuration, H2S is dissociated on CuMn2O4(100) and separates into S and H atoms. The H atoms strongly bond to two adjacent O3f sites and generate two surface OH groups. Meanwhile, the S atom intensely interacts with surface Cu2f, leading to the formation of a Cu–S bond with a bond length of 2.081 Å. The above XPS analysis indicates that H2S can be adsorbed on CuMn2O4 sorbent to generate surface active sulfur species. Therefore, the DFT calculation agrees well with the experimental result. In the most stable configuration for HgS adsorption (Fig. 7d), HgS molecule adsorbs on CuMn2O4(100) surface in parallel direction. The Hg atom of HgS strongly bonds to surface Cu2f atom,

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while the S atom strongly interacts with surface Mn5f atom. After HgS adsorption, Cu–Hg and Mn–S bonds are formed with the bond lengths of 2.664 Å and 2.179 Å, respectively. The adsorption energy of this configuration is as high as −220.21 kJ/mol, implying that the strong chemisorption mechanism is responsible for HgS adsorption on CuMn2O4 surface. This agrees well with the above XPS and TPD experimental observations that the peaks characteristic of HgS were detected in the spent CuMn2O4 sorbents. 3.4.2. Hg0 reaction with H2S upon CuMn2O4(100) surface On account of the above calculations, both Hg0 and H2S are strongly chemisorbed on CuMn2O4(100) surface. It can be inferred that the heterogeneous Hg0 reaction with H2S over CuMn2O4 surface follows the Langmuir–Hinshlwood mechanism, which is in good agreement with the above H2S pretreatment experimental findings. Moreover, the detailed process of Hg0 reaction with H2S on CuMn2O4(100) was identified by calculating the energy barrier and reaction heat. The energy profile of Hg0 transformation on CuMn2O4(100) and the corresponding optimized configurations of intermediate (IM), transition state (TS), and final state (FS) are presented in Fig. 8. The reaction pathway includes two steps: (1) Hg0 and H2S co-adsorption and (2) Hg(ads) →HgS(ads). For the first step, both gaseous Hg0 and H2S are chemisorbed on CuMn2O4(100) to form IM. This step is spontaneous and exoergic by 265.31 kJ/mol. In IM, Hg0 is adsorbed upon surface Cu2f site with the formation of Cu–Hg bond. Meanwhile, H2S molecule dissociates on the surface to produce an active S atom. The second step is the reaction of chemisorbed Hg with the surface active S atom via IM → TS → FS. This step is endoergic by 7.79 kJ/mol and the energy barrier is 17.38 kJ/mol. As the distance between S and Hg atoms reduces (6.285 Å (IM) → 4.307 Å (TS) → 2.725 Å (FS)), the adsorbed Hg is finally transformed into surface-bonded

13

HgS. The overall reaction pathway is exoergic by 257.52 kJ/mol, and the reaction energy barrier is as low as 17.38 kJ/mol. Therefore, the reaction between Hg0 and H2S to form HgS over CuMn2O4 sorbent is thermodynamically and kinetically favorable. The strong chemisorption of Hg0 and H2S as well as the easy reaction between Hg0 and H2S on CuMn2O4(100) can be used to explain why CuMn2O4 is effective for Hg0 removal from syngas.

4. Conclusion A series of experiments and DFT calculations were carried out to study the specific role of H2S in Hg0 capture by CuMn2O4 sorbent and the involved reaction mechanism. The as-prepared CuMn2O4 sorbent showed as high as 95.6% Hg0 removal efficiency at 200 °C under simulated syngas. Under N2 atmosphere, both H2 and CO consumed surface oxygens and thus suppressed Hg0 removal over CuMn2O4. Nevertheless, they showed no obvious influences on Hg0 capture with H2S assistance. H2S greatly promoted Hg0 elimination by producing active sulfur species on CuMn2O4 sorbent. DFT calculations suggested that Hg0 and HgS adsorption upon CuMn2O4 surface were controlled by strong chemisorption mechanism. H2S was dissociatively chemisorbed upon CuMn2O4(100) with an adsorption energy of −164.67 kJ/mol, and produced active surface sulfur species, which was confirmed by

XPS

analysis.

According

to

H2S-pretreatment

experiments

and

DFT

calculations,

Langmuir–Hinshlwood mechanism was proposed to interpret the reaction between Hg0 and H2S upon CuMn2O4. After the reaction, a stable product of HgS was generated and adsorbed on CuMn2O4 surface, which was proved by TPD and XPS experimental results.

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Acknowledgments This work was supported by Fundamental Research Funds for the Central Universities (2019kfyRCPY021), Program for HUST Academic Frontier Youth Team (2018QYTD05) and Postdoctoral Science and Technology Activity Foundation of Hubei Province (G63). The authors also thank the Analytical and Testing Center of Huazhong University of Science and Technology for the measurement.

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List of Figures Captions

Fig. 1. Schematic diagram of the experimental system for Hg0 removal. Fig. 2. Slab models of CuMn2O4(100) surface. Cu2f denotes 2-fold coordinated Cu. Mn5f denotes 5-fold coordinated Mn. O3f and O4f denote 3-fold and 4-fold coordinated O, respectively. Fig. 3. (a) XRD pattern (red line) and SEM image of CuMn2O4 sorbent. (b–d) XPS spectra of Cu 2p, Mn 2p, and O 1s for CuMn2O4. Fig. 4. (a) Effect of reaction temperature on Hg0 removal over CuMn2O4 sorbent under simulated syngas. (b) Effect of H2S concentration on Hg0 removal at 200 ºC. (c, d) Role of H2S in Hg0 removal under reducing atmosphere at 200 ºC. Fig. 5. Hg0 breakthrough curves of H2S-pretreated CuMn2O4 sorbent under N2 atmosphere. Fig. 6. (a) S 2p XPS spectra of H2S-pretreated CuMn2O4 sorbent. (b) Hg 4f XPS spectra of Hg0-pretreated CuMn2O4 sorbent. (c) Hg-TPD curve of CuMn2O4 after Hg0 capture under N2 + 400 ppm H2S atmosphere. Fig. 7. Adsorption energies and structural parameters of the stable adsorption configurations. (a, b) Hg0, (c) H2S, and (d) HgS adsorption on CuMn2O4(100) surface. Fig. 8. The energy and geometrical diagram of the heterogeneous Hg0 reaction with H2S on CuMn2O4(100) surface.

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AC trap

3-way valve

H2S CO

Sorbent Computer

H2 HCl Furnace

N2

Online Hg analyzer Hg0 generator

MFC Water bath

NaOH Drying agent Temperature controller

Fig. 1. Schematic diagram of the experimental system for Hg0 removal.

22

Top view

O4f

hollow

O3f

Cu2f

Mn5f

Side view

Cu Mn O CuMn2O4(100) surface

Fig. 2. Slab models of CuMn2O4(100) surface. Cu2f denotes 2-fold coordinated Cu. Mn5f denotes 5-fold coordinated Mn. O3f and O4f denote 3-fold and 4-fold coordinated O, respectively.

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(a)

100 nm

(b) Cu 2p

Cu2+

Cu+ Cu-Mn spinel Mn3O4 CuO (c) Mn 2p

(d) O 1s Mn

Mn2+

Oα

3+

Mn4+ Oβ Satellite

Fig. 3. (a) XRD pattern (red line) and SEM image of CuMn2O4 sorbent. (b–d) XPS spectra of Cu 2p, Mn 2p, and O 1s for CuMn2O4.

24

(a)

(b)

(c)

(d)

Fig. 4. (a) Effect of reaction temperature on Hg0 removal over CuMn2O4 sorbent under simulated syngas. (b) Effect of H2S concentration on Hg0 removal at 200 ºC. (c, d) Role of H2S in Hg0 removal under reducing atmosphere at 200 ºC.

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Fig. 5. Hg0 breakthrough curves of H2S-pretreated CuMn2O4 sorbent under N2 atmosphere.

26

(a) S 2p

(b) Hg 4f

168.61

(c) Hg-TPD

HgS

105.38 163.68 162.44

100.43

Fig. 6. (a) S 2p XPS spectra of H2S-pretreated CuMn2O4 sorbent. (b) Hg 4f XPS spectra of Hg0-pretreated CuMn2O4 sorbent. (c) Hg-TPD curve of CuMn2O4 after Hg0 capture under N2 + 400 ppm H2S atmosphere.

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(b)

(a) Hg

2.753

2.973

Cu

Mn Eads = ̶ 121.94 kJ/mol

Eads = ̶ 129.84 kJ/mol (c)

(d)

34

9 2.

H

S

2.827

Hg

2.650

2.081 2.664

O

S

2.179

H Eads = ̶ 164.67 kJ/mol

Eads = ̶ 220.21 kJ/mol

Fig. 7. Adsorption energies and structural parameters of the stable adsorption configurations. (a, b) Hg0, (c) H2S, and (d) HgS adsorption on CuMn2O4(100) surface.

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0

Hg0 + H2S +CuMn2O4

Cu

Mn

O

Hg

S

H

0.00

Relative Energy (kJ/mol)

-50 2.093

-100

4.307 3.349

2.725 2.128

-150 -200

2.081 S

6.285

2.890

Hg 2.758 TS

-250

IM

FS

−247.93

−257.52

−265.31 -300 Adsorption

Reaction Reaction pathway

Fig. 8. The energy and geometrical diagram of the heterogeneous Hg0 reaction with H2S on CuMn2O4(100) surface.

Highlights  CuMn2O4 exhibited superior performance for Hg0 removal from syngas at 200 ℃.  H2S played a key role in Hg0 capture by forming active sulfur species on CuMn2O4.  Hg0 reaction with H2S over CuMn2O4 occurred via Langmuir–Hinshlwood mechanism.  HgS was demonstrated as the captured mercury species on spent CuMn2O4 sorbent.

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