Turning fulvic acid into silver loaded carbon nanosheet as a regenerable sorbent for complete Hg0 removal in H2S containing natural gas

Chemical Engineering Journal 379 (2020) 122265

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Turning fulvic acid into silver loaded carbon nanosheet as a regenerable sorbent for complete Hg0 removal in H2S containing natural gas

T

Dingyuan Zhanga, Huawei Liua, Juan Wanga, Mingzhu Zhanga, Wenrui Zhanga, Shaojie Chenb, ⁎ Peng Lianga, Huawei Zhanga, a

College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Qingdao 266590, PR China

b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

regenerable and H S resistance Hg • Aremoval sorbent was synthesized 2

0

using fulvic acid.

Ag nanoparticles were confined • The and embedded in the carbon nanosheets.

special core-shell structure of Ag/ • The Fvs is beneficial for the tolerance to H2S.

nanoconfinement of carbon layer • The is conducive to regeneration performance.

A R T I C LE I N FO

A B S T R A C T

Keywords: Natural gas Ag nanoparticles Hg0 removal H2S resistance Regeneration

The migration and agglomeration of Ag nanoparticles during high temperature regeneration processes were generally considered the key contributor for the decreasing of Hg0 removal performance over Ag-based sorbents. In this work, a complexation pathway was developed for controllable synthesis of Ag loaded carbon nanosheet (Ag/Fvs) using fulvic acid as template and carbon source. The elemental Ag nanoparticles were highly dispersed and embedded in the carbon nanosheet with an average diameter of 7.12 nm and formed a special core-shell structure. The synthesized sorbent achieved a complete Hg0 removal in H2S containing natural gas at ambient temperature. At 1% breakthrough, the Hg0 capture capacity of Ag/Fvs was as high as 1.36 mg·g−1, which is much higher than the sample prepared by traditional impregnation method (I-Ag/Fvs, 0.98 mg·g−1) and other existing commercial sorbents. More importantly, the spent Ag/Fvs could be easily regenerated by thermal treating process, its Hg0 capture capacity only slightly decreased by 5.8% after 4 cycles. We considered the carbon layer prevented Ag nanoparticles from poisoning by H2S, and more active sulfur sites for Hg0 capture could be formed on porous carbon nanosheets by chemical adsorption of H2S, which is beneficial for the strong tolerance to H2S. The excellent regeneration performance of Ag/Fvs mainly attributed to the nanoconfinement of carbon layer, which is conducive to prevent the agglomeration of Ag nanoparticles during high temperature regeneration processes. This work represented a practical and efficient pathway to utilize the cheap fulvic acid for applications of Hg0 removal in H2S containing natural gas.

⁎

Corresponding author. Tel: + 86 13806399945. E-mail address: [email protected] (H. Zhang).

https://doi.org/10.1016/j.cej.2019.122265 Received 4 June 2019; Received in revised form 12 July 2019; Accepted 15 July 2019 Available online 18 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 379 (2020) 122265

D. Zhang, et al.

previous work, an effective and regenerable Hg0 removal sorbent was successively synthesized by the nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecular sieve, the prepared sorbent achieved a complete removal of Hg0 in ambient temperature and its capture capacity was as high as 6.64 mg·g−1 [25]. However, the introducing of H2S has an obviously adverse influence on Hg0 capture of Ag/MCM-41 sorbent. Hence, developing novel sorbent with excellent regenerable performance for Hg0 removal in H2S containing natural gas is highly desirable but remains challenging. The fulvic acids (FAs) are originated from the degradation and transformation of plant residues which can be depicted as a series of phenol rings connected by alkyl chains with abundant of carboxyl, hydroxyl, carbonyl and methoxy groups. It can take cation exchange and complexation reaction with Ag+ and allow the uniform distribution of Ag+ in the organic substance [26–28]. By using FAs as a source of carbon, we expect to obtain a composite material with relatively regular distribution of Ag nanoparticles that were embedded in the carbon supporter to form a core-shell structure. The nanoconfinement of carbon nanoshells can effectively prevent the migration and agglomeration of Ag nanoparticles during thermal regeneration processes and the poison of Ag nanoparticles by H2S, which is beneficial for the excellent regeneration and H2S resistant performance. Therefore, a novel Ag loaded carbon nanosheet (Ag/Fvs) was controllable synthesized using FAs as template. For comparison, a conventional impregnation method was also employed to prepare I-Ag/Fvs sample. Their potential for Hg0 removal in H2S containing natural gas and regeneration performance were investigated in detail. Furthermore, the characterizations of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) were used to reveal the physical and chemical properties of sorbents and the Hg0 removal mechanism.

1. Introduction The mercury is an extremely hazardous pollutant which can cause worldwide environmental contamination due to its biogeochemical cycling and then threatens human health directly [1–7]. The mercury concentrations in natural gas obviously vary with the producing area, typically range from 0.1 to 5000 mg·m−3 and exist predominantly as Hg0 form [8]. Due to the high annual output of natural gas, the Hg0 in natural gas should be removed before combustion to reduce emissions. Furthermore, during low temperature treatment process of natural gas, the Hg0 can cause corrosion and crack of the Al heat exchangers through three mechanisms of amalgamation, amalgam corrosion and liquid metal embrittlement [9,10], which has led to numerous explosion accidents in natural plants such as Skikda gas plant of Algeria and Groningen gas field of Netherlands [11]. Therefore, various countries formulate corresponding policies to limit Hg content of natural gas such as Netherlands (28 μg·m−3) and Germany (30 μg·m−3). Thus, it is necessary to remove Hg0 in natural gas to avoid mercury-caused disaster events [12,13]. Recently, a growing number of researches were concentrated on Hg0 removal in natural gas. Several effective sorbents such as Halide- and sulfur-impregnated activated carbon, nano-sized sulfide metals and immobilized ionic liquids have been extensively identified [14–17]. The noble metals including silver, gold, palladium and platinum loaded materials have been demonstrated as potential Hg0 removal sorbents in natural gas due to the highly dispersed noble metal nanoparticles can easily amalgamate with Hg0 at room temperature, the formed amalgam alloys have low solubility and can be decomposed at higher temperatures for regeneration. It is generally considered that the Hg0 capture performance of noble metals loaded sorbents was significantly associated with the valence state, particle size and dispersity of noble metal nanoparticles. Many efforts have been carried out for the controllable synthesis of noble metal based nanocomposites as regenerable and high capacity Hg0 removal sorbents. Xu et al. [18] reported silver nanoparticles depositing within the channels of SBA-15 matrix as a novel mercury removal sorbent, the silver ions were reduced by thermal reduction in inert atmosphere, a Hg0 capture capacity as high as 13.2 mg·g−1 was achieved by the sorbent at 1% breakthrough. Yan et al. [19] prepared Ag supported UiO-66 sorbent for mercury removal from flue gas, the Ag particles were well dispersed on the surface of the UiO-66 with average size of 20 nm. Ballestero et al. [20] revealed a new method for gold nanoparticles deposition onto a honeycomb structured carbon monolith to obtain Au loaded sorbent, the reduction potential of the carbon material was used to reduce Au3+ to Au0 and then received the average particle size of 23 nm, The synthesized sorbent exhibits a high Hg removal efficiency in the temperature range of 50 °C–150 °C. However, previous studies often suffered from poor regeneration performance and inefficient in the presence of H2S. For instance, the Ag/4A zeolite nanocomposite was reported in our recent work [21], its Hg0 adsorption efficiency was higher than 96% after 200 min adsorption at 30 °C, while the Hg0 capture capacity declined 27% after five regeneration cycles. Han et al. [22] reported that the Pd supported activated carbon sorbents (Pd/AC) showed almost complete Hg0 removal in H2 atmosphere, and the Hg0 removal performance decreased about 30% after the first regeneration cycle. We consider that the decreasing of Hg0 removal performance after regeneration was mainly due to the migration and agglomeration of noble metal nanoparticles during thermal treatment processes. Therefore, it is significantly important to improve the stability of noble metal nanoparticles at high temperatures to ensure the excellent regeneration performance of sorbents. On the other hand, H2S is an inevitable component of natural gas and typically range from 4 to 1000 ppm, which is generally considered has an inhibitory influence for Hg0 capture over noble metal based sorbents [23]. Takahashi et al. [24] found that the Ag+ in Ag-Y zeolite migrated out of the cation sites to become Ag2S after H2S exposure, indicating the deactivation effect of H2S on Ag based sorbents. In our

2. Experimental section 2.1. Sorbent preparation The synthesis strategy of Ag/Fvs is illustrated in Fig. 1. Firstly, the raw fulvic acids was purified using alkali-solution method to remove excessive inorganic salts. Secondly, the oxygen containing functional groups such as –OH and –COOH of FAs chelate the Ag+ cations in the AgNO3 solution, allowing the high dispersion and uniform distribution of Ag+ cations in the macromolecular structure of FAs. The third step is the conversion of the Ag-FAs solution to Ag-FAs aerogel via the freezedrying process. The freeze-drying process is tended to form a threedimensional structure, which is beneficial for the mass transfer in the mercury removal process according to our previous research [29]. Finally, the Ag-FAs aerogel underwent a carbonization process at 400 °C in N2 atmosphere, where the Ag+ cations were reduced to elemental Ag nanoparticles and formed Ag/Fvs carbon nanosheets with a typical core-shell structure. For the preparation of I-Ag/Fvs sample, the purified FAs was firstly freeze-dried to form aerogel solid, then carbonized at 400 °C in N2 atmosphere to obtain Fvs carbon nanosheets. The Ag+ cations were loaded on the surface of Fvs carbon nanosheets by co-impregnation method and then reduced by NaBH4 aqueous solution in room temperature. All the used materials and the detailed preparation procedures were shown in SI. 2.2. Characterization of the sorbents To better understand the physical and chemical properties of sorbents, we used a variety of analytical methods to characterize the samples. X-ray diffraction (XRD)pattern was recorded by a Rigaku D/ 2

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Fig. 1. Schematic illustration for the preparation of Ag/Fvs carbon nanosheets.

MAX-2500/PC X-ray diffractometer employing Cu-Ka (λ = 0.5406 nm, 50KV, 300 mA) radiation in the range of 10°–80° with a step size of 5°·min−1. X-ray photo electron spectroscopy (XPS) analysis were carried out on a Perkin-Elmer PHI 5000C ESCA spectrometer equipped with MgKα (1253.60 eV) radiation. The microstructures of the materials were characterized by field emission scanning electron microscopy (SEM, JEOL, JSM-6700F, Japan) and transmission electron microscopy (HRTEM, JEOL, JEM-2100F, Japan).

flow rate, and m is the weight of the sorbent. The temperature programmed desorption of Hg0 (Hg-TPD) was carried out as follows: 200 mg of spent sorbent was placed in a fixedbed reactor and heated from ambient temperature to 500 °C in N2 flow with the heating rate of 10 °C·min−1. The Hg0 releasing curves were obtained by online measurement of RA-915 mercury analyzer.

2.3. Experimental setup and mercury removal activity tests

3.1. Characterization of samples

The Hg0 removal activity tests of sorbents were performed in a fixed-bed quartz reactor (i.d. = 10 mm) under atmospheric pressure, as shown in Fig. S1. In a typical Hg0 adsorption test, 200 mg of sorbent was placed in the reactor, which was located inside a temperature controlled tubular furnace. The CH4 gas stream containing 100–300 ppm of H2S and 70 ± 1 μg·m−3 of Hg0 generated by a Hg0 permeation device (VICI Metronics) was introduced to the reactor with a total flow rate of 1 L·min−1. A mercury analyzer (RA-915 Lumex Inc. Russia. Limit of detection: 2 ng·m−3) was used to detect the inlet and outlet concentrations of Hg0 online. The vent gas was treated with activated carbon before discharge into the atmosphere to avoid air pollution. All experiments were repeated three times and standard deviation was within 1%, indicating the reliable of the experimental data. At the beginning of experiments, the system firstly established stable and basic Hg0 mass balance. The long-term evaluation of Hg0 capture at 30 °C and the space velocity of 50,000 h−1 was also conducted to measure the Hg0 adsorption capacity of sorbents. In this study, the capacity represents the mass of Hg0 absorbed by unit mass of sorbents once Hg0 removal efficiency reaches to 99%. According to following Eqs. (1) and (2), the Hg0 removal efficiency and adsorption capacity were calculated respectively:

Representative SEM micrographs of Ag/Fvs and I-Ag/Fvs samples are presented in Fig. 2a–d. It is clearly that both samples displayed a typical structure of 3D nanosheets, some interconnected macropores could be observed, which is beneficial for the mass transfer and the adsorption reaction at a high space velocity. Fig. 2d showed that considerable Ag particles are uniformly present on the surface of I-Ag/Fvs sample, indicating the successful loading of Ag nanoparticles by coimpregnation method. However, the SEM image of Ag/Fvs sample in Fig. 2c represented a relatively smooth surface, and few distinguishable crystalline Ag particles could be observed. We consider that the majority of Ag nanoparticles were encapsulated in the carbon nanosheets. As expected, it can be seen from the HRTEM image in Fig. 2e that considerable spherical Ag nanoparticles were evenly dispersed into the carbon nanosheet with a typical core-shell structure, which is formed by the chelation the Ag+ cations into the macromolecular structure of Fvs. The Ag nanoparticles were conjoined by carbon with no obvious aggregation and the particle size were ranged from 5 to 20 nm with an average size of 7.12 nm. As shown in Fig. 2e, the measured interplanar spacing for the lattice fringes of 0.24 nm and 0.19 nm matched the (1 1 1), and (2 0 0) lattice planes of elemental Ag nanoparticles [30,31]. The EDS mapping in Fig. 2g–j further confirmed the presence of C, Ag and O atoms and exhibited the uniform loading of Ag nanoparticles in carbon nanosheets, the calculated Ag content was about 2.12 wt%. The XRD patterns of Ag/Fvs, I-Ag/Fvs and blank Fvs carbon nanosheets are shown in Fig. 3. Compared to the blank Fvs sample, four characteristic diffraction peaks located at 38.12°, 44.27°, 64.42° and 77.47° appeared in patterns of Ag/Fvs and I-Ag/Fvs samples, which corresponded to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of facecentered cubic crystalline structure of Ag [19,32]. The results further confirmed the existence of Ag nanoparticles in the prepared Ag/Fvs and

η=

Cin - Cout × 100% Cin

3. Results and discussion

(1)

t

q=

Cin Lt − ∫0 Cout Ldt m

(2)

where η is the Hg0 removal efficiency, Cin and Cout represent the Hg0 at the inlet and the outlet of the reactor, respectively. q represents the Hg0 adsorption capacity of the sorbents, t is the adsorption time, L is the gas 3

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Fig. 2. (a and c) SEM images of Ag/Fvs; (b and d) SEM images of I-Ag/Fvs; (e–f) TEM images of Ag/Fvs; (g–j) SEM images and EDS elemental mappings of Ag/Fvs.

adsorption capacities were defined as the mass of Hg0 adsorbed per unit mass of the sorbents when the Hg0 breakthrough reaches 1%. As shown in Fig. 4b, the Hg0 removal efficiency of blank Fvs rapidly decreased to 28.9% after 3 h, confirming its poor Hg0 removal performance. However, the Hg0 removal efficiencies of Ag/Fvs and I-Ag/Fvs samples remained over 99% during the 10 h test. At 1% breakthrough, the calculated Hg0 capture capacities of Ag/Fvs and I-Ag/Fvs are 1.36 mg·g−1 and 0.98 mg·g−1, respectively. It is evident that the Hg0 capture capacity of Ag/Fvs is superior to I-Ag/Fvs and other reported Ag based sorbents such as (Ag/MC) 0.9 mg·g−1 and (Ag nanoparticle-based magnetic HZSM-5) 22.3 μg·g−1 [33,34], which is mainly attributed to the smaller average Ag particle size in Ag/Fvs sample is beneficial for Hg0 removal. The excellent stability and low temperature activity of Ag/Fvs ensured it to be a promising sorbent for Hg0 capture from natural gas at room temperature. 3.2.2. Regeneration performance Based on the Hg-TPD results, the spent samples were regenerated by thermal treatment at 400 °C for 1 h in N2 atmosphere. The Hg0 removal performance of regenerated sample was investigated at 30 °C and space velocity of 50,000 h−1. The Hg0 removal efficiencies and adsorption capacities of regenerated samples were determined after reaction time of 2 h and 70 h, respectively. The Hg0 removal efficiencies and capacities of fresh and regenerated sorbents are shown in Fig. 5. Evidently, the Hg0 removal efficiencies of regenerated Ag/Fvs and I-Ag/Fvs are almost the same as the fresh samples, which indicated that the regenerated sorbents can remain complete removal of Hg0 in 1 h of adsorption. However, it can be seen from the long-term tests that the Hg0 capture capacities of regenerated Ag/Fvs and I-Ag/Fvs are quite distinct. There is only a slight loss in the Hg0 capture capacity of regenerated Ag/Fvs, which decreased negligibly from 1.36 mg·g−1 to 1.28 mg·g−1. While the Hg0 capture capacity of I-Ag/Fvs decreased significantly from 0.98 mg·g−1 to 0.80 mg·g−1 after 4 cycles. Generally, the Ag/Fvs showed a superior regeneration performance than I-Ag/Fvs. Fig. 6 showed the HRTEM images of fresh and spent Ag/Fvs (a) and I-Ag/Fvs (b) samples. It can be seen that the Ag nanoparticles on the surface of I-Ag/Fvs sample agglomerated evidently after 4 regeneration cycles. The average size of Ag nanoparticles increased significantly from 23.6 nm in fresh I-Ag/Fvs to 34.3 nm in spent sample, indicating the instability of Ag nanoparticles under high temperatures of thermal regeneration process, which is the major contributor for the poor regeneration performance of I-Ag/Fvs sample. On the contrary, the agglomeration of Ag nanoparticles in Ag/Fvs sample was not observed, and the average size of Ag nanoparticles remained about 7.63 nm after 4 regeneration cycles, which is mainly due to the nanoconfinement of carbon nanoshells of Ag/Fvs sample effectively prevented the Ag nanoparticles from migration and agglomeration during thermal regeneration process. It has been proved that the Hg0 removal efficiencies and capacities of noble metal based sorbents are closely related to the

Fig. 3. XRD patterns of Ag/Fvs, I-Ag/Fvs and blank Fvs samples.

I-Ag/Fvs sorbent, which are consistent with the characterization of HRTEM. In addition, considering the XRD spectra of Ag/Fvs, it is evidently that the peaks are similar with those of I-Ag/Fvs sample, while the width of Ag characteristic peaks increased significantly, which implied that the average Ag particle size of Ag/Fvs is much smaller than that of in I-Ag/Fvs sample. 3.2. The Hg0 removal performance of samples 3.2.1. The mercury capture efficiency and capacity The Hg0 capture efficiencies of samples at temperatures ranging from 30 to 120 °C and space velocity of 50,000 h−1 are shown in Fig. 4a, all the removal efficiencies were measured after 2 h of reaction. It can be seen that the blank Fvs showed a limited Hg0 removal efficiency, which is only 48.9% at 30 °C, indicating the absence of Hg0 adsorption active sites on the surface of blank Fvs sample. Loading of Ag nanoparticles on carbon nanosheets significantly enhanced Hg0 capture performance. Both of Ag/Fvs and I-Ag/Fvs exhibited almost complete Hg0 removal in the temperature range of 30–60 °C, the dramatic improvement suggested that the highly dispersed Ag nanoparticles play a crucial role in the capture of Hg0 by Ag-Hg amalgamation mechanism. When the adsorption temperature increased from 60 °C to 120 °C, the Hg0 removal efficiencies of Ag/Fvs and I-Ag/Fvs decreased to 81.2% and 78.9%, respectively. The adverse effect of higher temperatures on Hg0 removal performance indicated that the prepared Ag/Fvs and I-Ag/Fvs are not suitable for Hg0 removal in high temperature flue gas and syngas. The samples were further exposed to a continuous Hg0 flow at 30 °C for long-term stability tests. The 4

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Fig. 4. (a) The Hg0 removal efficiencies of samples in the temperature range of 30–120 °C and (b) Long-term stability tests of samples at 30 °C.

particle size and dispersity of noble metal particles [21,25]. Therefore, we can draw the conclusion that the favorable thermal stability and non-agglomeration of Ag nanoparticles under high temperatures are beneficial for the excellent regeneration performance of Ag/Fvs sample.

respectively, indicating the Ag/Fvs was relatively less affected by H2S. Generally, the Ag/Fvs exhibited better resistance to H2S than I-Ag/Fvs, which ensured the sorbent is potential suitable for treatment of H2S containing natural gas.

3.2.3. Effect of H2S on Hg0 removal performance As an important component of natural gas, H2S was generally considered has inhibitory influence for Hg0 removal over Ag based sorbents. In this study, the influences of H2S on Hg0 removal performance over blank Fvs, Ag/Fvs and I-Ag/Fvs samples were investigated by introducing different concentrations of H2S into the gas stream. As shown in Fig. 7a, the Hg0 removal efficiency of blank Fvs increased dramatically from 63.8% to 81.6% when 100 ppm of H2S at 20 min was introduced, the great enhancement is attributed to the formation of C-S species on the surface of FAs by H2S adsorption, which can react directly with Hg0 by forming HgS via the Eley-Rideal mechanism [35]. The presence of H2S had a negligible effect on Hg0 removal over I-Ag/ Fvs and Ag/Fvs samples in the beginning of 80 min, both reached nearly 100% of Hg0 removal efficiencies. However, when the adsorption time continuously increased, the 1% Hg0 breakthrough of I-Ag/Fvs sample was reached in 38.2 h, while the Ag/Fvs remained a complete removal of Hg0 for 75.2 h and the amount of captured Hg0 was as high as 1.32 mg·g−1 in 1% Hg0 breakthrough. Fig. 7b showed when the H2S concentrations increased from 0 ppm to 300 ppm, the Hg0 capture capacities of Ag/Fvs and I-Ag/Fvs decreased by 4.1% and 66.3%,

3.3. The Hg0 adsorption mechanism The XPS survey spectra of Ag/Fvs and I-Ag/Fvs samples were shown in Fig. S2, the three sharp peaks located at 281.7 eV, 486.30 eV and 533.6 eV corresponded to C1s, Ag3d and O1s, respectively, which is consistent with the XRD and elemental mapping characterization results, indicating the successful loading of Ag nanoparticles on carbon nanosheets [36–38]. Fig. 8a demonstrated the narrow Ag3d XPS spectra of the fresh and spent Ag/Fvs and I-Ag/Fvs samples without the presence of H2S. The two sharp peaks located at 374.4 eV and 368.8 eV could be ascribed as Ag3d5/2 and Ag3d3/2 photoelectron spectra of elemental Ag, respectively [38,39]. After Hg0 adsorption, it is clearly that a slight decreasing of peak intensity and a shift of 0.3 eV-0.4 eV to lower binding energy were observed for the spent samples, suggesting the formation of Ag-Hg amalgam [25,40,41]. The narrow Hg4f spectrum of spent Ag/Fvs sample was displayed in Fig. 8b, the unique peak centered at 101.8° could be assigned to Ag-Hg amalgam. No element mercury peak centered at 99.0 eV was detected in the Hg4f spectrum, which implied that Ag-Hg amalgam is the only absorbed form of spent Ag/Fvs sorbent.

Fig. 5. Regeneration test results for the Ag/Fvs (a) and I-Ag/Fvs (b) after 80 h of adsorption. 5

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Fig. 6. HRTEM images of fresh and regenerated I-Ag/Fvs and Ag/Fvs samples.

confirmed the co-existence of three forms of absorbed sulfur species of sulfide, elemental S and sulfate in the spent samples. The contents of different sulfur species were listed in Table S1 by calculating the peak area. Compared to Ag/Fvs, the content of S2− in I-Ag/Fvs is higher by 11.97%, and the content of elemental S is lower by 8.58%. This observation implied the more content of sulfide in spent I-Ag/Fvs than that of in spent Ag/Fvs sample. The Hg-TPD experiments of spent Ag/Fvs and I-Ag/Fvs samples at 30 °C for 15 min of Hg0 capture in H2S containing flow were carried out in the temperature range of 30–500 °C at a heating rate of 10 °C·min−1 in a N2 atmosphere, the Hg0 capture amount on I-Ag/Fvs and Ag/Fvs samples are 0.98 µg and 0.99 µg, respectively. As shown in Fig. 9c, there is only a peak centered at 218 °C emerging on the Hg-TPD curve of spent I-Ag/Fvs sample, which corresponded to the decomposition of AgHg amalgam [19]. The released Hg0 amount is 0.99 µg via calculation, implying the complete releasing of absorbed mercury. The Hg-TPD result showed that the Ag-Hg amalgam is the unique absorbed mercury on spent I-Ag/Fvs sample. It can be seen from the Hg-TPD curve of spent Ag/Fvs that there is an overlapped peak and the released amount

The Ag/Fvs and I-Ag/Fvs samples were continuously exposed to the gas flow containing 100 ppm of H2S for 20 h, and the spent sorbents were characterized using XPS and Hg-TPD to identify the Hg0 removal mechanism in the presence of H2S. As shown in Fig. 9a, two peaks with high intensity were observed in the Ag3d narrow spectra of spent samples, which could be fitted into four peaks at 374.7 eV, 374.0 eV, 368.6 eV and 368.1 eV. The two fitting peaks centered at 374.7 eV and 368.6 eV were assigned to elemental Ag0, and the two fitting peaks centered at 374.0 eV and 368.1 eV were attributed to oxidized Ag+, respectively [42,43]. It is clearly that the calculated contents of Ag0 and Ag+ in spent Ag/Fvs sample are 89.7% and 11.3%, while those in spent I-Ag/Fvs sample are 29.8% and 71.2%, respectively. The content of oxidized Ag+ in spent I-Ag/Fvs is 57.8% higher than that of in spent Ag/Fvs, indicating the Ag0 nanoparticles in I-Ag/Fvs are more easily react with H2S to form Ag+ species. The S2p XPS spectra of spent samples (Fig. 9b) suggested that three species of adsorbed sulfur existed on the surface of the sorbents. The fitting peaks located at 162.8 eV, 163.9 eV and 169.0 eV could be interpreted as S2−, elemental S and S6+, respectively [29,44], which

Fig. 7. Comparison of Hg0 removal performance of the Fvs, I-Ag/Fvs and Ag/Fvs in the presence of H2S. (a) Hg0 removal efficiency of samples at 30 °C, (b) Effect of H2S concentrations on Hg0 capture capacities of samples. 6

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Fig. 8. XPS spectra of the samples: (a) Ag3d, (b) Hg4f.

Fig. 9. XPS spectra of the samples: (a) Ag3d, (b) S2p. (c) The Hg-TPD curves of spent samples.

of Hg0 is 1.00 µg, which is basically equal to the Hg0 capture amount on Ag/Fvs. The desorption peak can be fitted by two peaks centered at 218 °C and 287 °C, which attributed to the decomposition and the release of Ag-Hg amalgam and red HgS, respectively [45]. Thus, we can draw the following Hg0 adsorption mechanism on Ag/Fvs in the presence of H2S. On one hand, the Ag nanoparticles in Ag/Fvs reacted directly with Hg0 to form Ag-Hg amalgam. On the other hand, the H2S in gas flow chemisorbed on the surface of carbon nanosheet and generated C-S species, then the Hg0 was further captured by these active sulfur sites and became stable HgS, which was also demonstrated by Liu et al. [34]. However, due to the less amount of adsorbed HgS and the amorphous form of Ag-amalgam, Fig. S3 showed that Ag2S, Agamalgam and HgS characteristic peaks were not detected by XRD in the spent sample, the same result was also reported by others [46–48]. Based on the results of HRTEM, XPS and Hg-TPD, the Hg0 adsorption mechanisms over I-Ag/Fvs and Ag/Fvs samples in H2S containing natural gas and the regeneration performance of spent samples can be described as Fig. 10. For the I-Ag/Fvs sample, the Ag nanoparticles were the active sites for Hg0 capture and the Ag-Hg amalgam was the unique absorbed mercury form. The H2S in natural gas firstly chemisorbed on the surface of I-Ag/Fvs to form elemental S and S6+ species, parts of absorbed elemental S then prior reacted with Ag nanoparticles and generated Ag2S, no HgS was formed during the adsorption process. The formation of Ag2S led to the reduction of mercury capture active sites, thus had a negative effect on Hg0 removal capacity of I-Ag/Fvs, which decreased from 0.98 mg·g−1 in pure natural gas to 0.86 mg·g−1 in 100 ppm of H2S containing natural gas. After 4 cycles of regeneration, the Hg0 removal capacity of I-Ag/Fvs decreased dramatically by 24.5%, which mainly caused by the migration and agglomeration of Ag

nanoparticles on the surface of carbon nanosheets in the high temperature regeneration processes. For the Ag/Fvs sample, the Ag nanoparticles were encapsulated by carbon layer to form a special core-shell structure. During Hg0 removal process in H2S containing gas flow, the barrier effect of carbon layer prevented Ag nanoparticles from reacting with absorbed elemental S. The XPS results showed that there was only little amount of Ag2S formed after Hg0 adsorption, and Hg-TPD experiment proved the existence of HgS in spent Ag/Fvs sample, which suggested the absorbed elemental S on the surface of Ag/Fvs would react directly with Hg0 by forming HgS via the Eley-Rideal mechanism. Considering the carbon layer prevented Ag nanoparticles from poisoning by H2S, and more active sulfur sites for Hg0 capture could be formed on porous carbon nanosheets, the Ag/Fvs was suitable for Hg0 removal in H2S containing natural gas. Its Hg0 capture capacity is as high as 1.32 mg·g−1 in 100 ppm of H2S containing gas flow, which is 34.8% higher than I-Ag/Fvs sample. In addition, the Hg0 capture capacity of Ag/Fvs decreased slightly by 5.8% after 4 cycles of regeneration. We considered the nanoconfinement of carbon nanoshells effectively prevented the migration and agglomeration of Ag nanoparticles during high temperature regeneration processes, which ensured its excellent regeneration performance.

4. Conclusions Two kinds of Ag loaded sorbents for Hg0 removal in natural gas were successfully synthesized using fulvic acid as carbon source. For the Ag/Fvs sample synthesized by complexation reaction method, the Ag nanoparticles were confined by nanocarbon layers with an average diameter of 7.12 nm and formed a special core-shell structure. For the I7

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Fig. 10. The Hg0 adsorption mechanisms over I-Ag/Fvs and Ag/Fvs samples in H2S containing natural gas.

Ag/Fvs sample prepared by traditional impregnation and chemical reduction method, the Ag nanoparticles were supported on the surface of carbon nanosheets with an average diameter of 23.6 nm. The Ag/Fvs showed superior Hg0 capture capacity and regeneration performance than I-Ag/Fvs. At 1% breakthrough, the Hg0 capture capacity of fresh Ag/Fvs was 1.36 mg·g−1 and decreased slightly to 1.28 mg·g−1 after 4 cycles of regeneration, which was 38.7% and 60.0% higher than I-Ag/ Fvs. We considered the highly dispersed Ag nanoparticles with smaller particle size are the key contributor for the excellent Hg0 capture capacity of Ag/Fvs, and the confinement of carbon layer is beneficial for the regeneration performance. The H2S in gas stream significantly decreased the Hg0 capture capacity of I-Ag/Fvs due to the reduction of Hg0 capture active sites caused by the formation of Ag2S. However, the H2S had a negligible influence on Hg0 capture capacity of Ag/Fvs, which decreased slightly from 1.36 mg·g−1 in pure natural gas to 1.32 mg·g−1 in 100 ppm of H2S containing gas stream. The excellent resistance to H2S of Ag/Fvs attributed to its special core-shell structure, the carbon layer prevented Ag nanoparticles from poisoning by H2S, and more active sulfur sites for Hg0 capture could be formed on porous carbon nanosheets by chemical adsorption of H2S. Considering the high Hg0 removal efficiency and capacity, excellent regeneration performance and strong tolerance to H2S, the Ag/Fvs can be used as a promising sorbent for Hg0 removal in H2S containing natural gas.

[3]

[4]

[5] [6]

[7]

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[10]

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Acknowledgements [12]

This work was supported by the National Natural Science Foundation of China (21776164), the Key Research and Development Program of Shandong Province (2017GSF17101), the Natural Science Foundation of Shandong Province (ZR2019MEE023), and the SDUST Research Fund (2019TDJH101).

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[14]

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122265.

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