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Hg0 Capture over CoMoS/γ-Al2O3 with MoS2 Nanosheets at Low Temperatures Haitao Zhao,† Gang Yang,† Xiang Gao,‡ Cheng Heng Pang,† Samuel W. Kingman,§ and Tao Wu*,† †

Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo 315100, P. R. China ‡ College of Energy Engineering, Zhejiang University, Hangzhou 310027, P. R. China § Department of Chemical and Environmental Engineering, The University of Nottingham, Nottingham NG7 2RD, The U.K. S Supporting Information *

ABSTRACT: CoMoS/γ-Al2O3 sorbent was prepared via incipient wetness impregnation (IWI) and sulfur-chemical vapor reaction (S-CVR) methods and tested in terms of its potential for Hg0 capture. It was observed that the CoMoO/γAl2O3 showed a Hg0 capture efficiency around 75% at a temperature between 175 and 325 °C while CoMoS/γ-Al2O3 achieved almost 100% Hg0 removal efficiency at 50 °C. The high removal efficiency for CoMoS/γ-Al2O3 remained unchanged for 2000 min in the test. Its theoretical capacity for Hg0 capture was found to be 18.95 mg/g based on the Elovich model. The ability of this material for Hg0 capture is atributed to the MoS2 nanosheets coated on surface of the maro- and meso-pores of γ-Al2O3. These MoS2 are two-dimensional transition-metal dichalcogenide (2D TMDC) assembled with unsulfided cobalt atoms at the edges. It is believed that these MoS2 nanosheets provided dense active sites for Hg0 capture. The removal of Hg0 at low temperatures was achieved via the combination of Hg0 with the chalcogen (S) atoms on the entire basal plane of the MoS2 nanosheets with coordinative unsaturated sites (CUS) to form a stable compound, HgS.

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INTRODUCTION There have been increasing concerns on the emission of mercury to the atmosphere because of its potent detrimental impacts on biological systems, especially its potential lethal impacts on human health.1 Microbial methylation of mercury bioaccumulated at the higher ends of the aquatic (both fresh and marine) food chain can result in severe neurotoxicity and present a severe threat to the welfare of human beings.2,3 The emission of elemental mercury has therefore attracted worldwide attention, which leads to more stringent legislations on its emission control.4,5 The largest anthropogenic source of mercury emission is associated with coal burning, which accounts for approximately 475 tonnes of mercury emission to the atmosphere per annum.6 In general, mercury is emitted to the environment in three forms: elemental (Hg0), oxidized (Hg2+), and particlebound (Hg(p)). Most of the oxidized and particle-bound mercury can be captured by using existing conventional air pollution control devices (APCDs).7 However, elemental mercury, which accounts for approximately 80% of total mercury emission,1 is extremely difficult to be removed since it is highly volatile in air and insoluble in water.1,8 Research on the control of elemental mercury emission at coal-fired power plants has bloomed since 2005.1 Technologies being explored so far can be classified into two main categories, that is, sorbent injection and catalytic Hg0 oxidation. © XXXX American Chemical Society

The injection of powdered activated carbon (PAC) is one of the sorbent injection technologies that have been commercially deployed by some coal-fired power plants in the US.1,8 The capacity of PAC in mercury removal could be enhanced if the activated carbon is impregnated with sulfur9,10 due to the high affinity of sulfur to mercury. However, the presence of mercuryladen carbon in fly ash normally alters its properties and compromises the suitability of fly ash as additives in cement manufacturing.11 In addition, the mercury-laden carbon also showed negative impacts on the usability of gypsum generated at downstream in the wet flue gas desulfurization (FGD) process and affected the operations of air pollution control devices.12 Therefore, it is necessary to develop noncarbon sorbents for mercury emission control at coal-fired power plants. So far, a number of sorbents, such as Au, PdAu, PdAg, PdCu, have been studied as potential cement-friendly sorbents.11,13,14 It is also found that catalysts for selective reduction of NOx (SCR-NOx) could promote the oxidation of Hg0 into Hg2+ provided that there was enough HCl present in flue gas.15,16 However, because of the partial adsorption of HCl on the Received: September 7, 2015 Revised: November 3, 2015 Accepted: December 21, 2015

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DOI: 10.1021/acs.est.5b04278 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Table 1. Textual Properties of γ-Al2O3, CoMoO/γ-Al2O3, and CoMoS/γ-Al2O3 sample

surface area (m2/g)

total pore volume (cm3/g)

average pore width (nm)

macropores (%)

mesopores (%)

micropores (%)

γ-Al2O3 CoMoO/γ-Al2O3 CoMoS/γ-Al2O3

188 181 177

0.49 0.39 0.37

10.3 8.2 8.2

6 6 6

88 83 83

6 11 11

surface of ash particles in flue gas, the concentration of HCl was normally lower than the desired level.17 It is therefore expected that the performance of mercury oxidation of the SCR catalyst is highly dependent on the types of coal utilized at coal-fired power plants.1 In recent years, much research has been carried out to develop cost-effective technologies for the removal of Hg0 from coal-fired flue gas.9−11,13 However, there are still needs to develop new materials for Hg0 capture that are robust and effective in the removal of elemental mercury from different gases under various operating conditions, and to understand the reaction pathways for Hg0 removal. In this study, a novel cement-friendly noncarbon sorbent, CoMoS/γ-Al2O3, was prepared and evaluated in terms of its potential for Hg0 capture. In addition, physical and chemical properties of fresh and spent CoMoS/γ-Al2O3 materials were characterized in detail to study the reaction mechanism for Hg0 removal.

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monitored during the TPDD and TPSR experiments using an online mercury analysis system (Tekran 3300RS, USA). In-situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was used to study characteristics of surface acid sites on the samples by using NH3 as the base probe molecule. The IR spectra were recorded on a Fourier transform infrared spectrometer (Nicolet, USA) equipped with a Praying Mantis reaction chamber (Harrick, USA). The sample was initially heated to 200 °C in N2 to remove impurities adsorbed. The background spectrum was collected in N2 atmosphere (and subtracted from the sample spectra). The amount of adsorbed NH3 was characterized after the adsorption process reached a steady state and then with N2 purging until another steady state was reached. Similar experimental procedures are described elsewhere.18 Apparatus and Procedure. To assess Hg0 capture performance of the samples prepared in this study, tests were conducted in an experimental system specially designed for this purpose. The sample was loaded into a dual fixed-bed reactors system. One of the reactors was used as a reference. Teflon pipes and quartz reactors were chosen to avoid adsorption of mercury onto piping system, gas sampling line, and equipment. N2 gas with a flow rate of 1500 mL/min, which contained around 30 μg/m3 of gas phase Hg0 generated by a mercury generator (Tekran 2537, USA), was introduced into the reactor. An individual sample (about 2 g) was tested in a temperature range of 25−450 °C under atmospheric pressure. The concentration of Hg0 and Hg2+ at the outlet ([Hg0]out and [Hg2+]out) was continueously monitored by using the mercury analysis system (Tekran 3300RS, USA). The instrument was calibrated using the mercury generator (Tekran 2537, USA), and Hg 0 concentration at the inlet ([Hg0]in) was checked before each experiment. Mercury removal efficiency was therefore determined by using the equation shown below:

EXPERIMENTAL SECTION

Preparation of Samples. In this study, a commercial γAl2O3 (V-SK Co., Ltd., size range: 1.18 mm ≤ x ≤ 1.70 mm, surface area: 188 m2/g) was selected as the support. Metal precursors, such as Co(NO3)2·6H2O and (NH4)6Mo7O24· 4H2O (analytical grade, Sinopharm Chemical Reagent Co, Ltd.), were dissolved in deionized water. Oxidized state material (CoMoO/γ-Al2O3) was prepared using incipient wetness impregnation (IWI), followed by calcination, while the sulfided state material (CoMoS/γ-Al2O3) was prepared by the sulfurization of CoMoO/γ-Al2O3 via sulfur-chemical vapor reaction (S-CVR) method.18 The preparation procedure is summarized in Supporting Information (SI) Figure 1S. For comparison purposes, a similar IWI method was adopted to load sulfur only onto the support to prepare S/γ-Al2O3 material by using dichloromethane (CH2Cl2) as the solvent to dissolve sulfur instead of using deionized water. Characterization of Samples. The textural properties of samples prepared in this study were characterized by using a Micromeritics ASAP 2020. Specific surface area of the samples was determined by using the N2 BET method while pore volumes and pore widths were measured following BJH procedures.18 Samples were also analyzed using an AXIS ULTRADLD Multifunctional X-ray Photoelectron Spectroscope with an Al Kα radiation source at room temperature and under a vacuum pressure of 10−7 Pa.19 Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images were captured using a JEM 2100 microscope operated at 200 kV to characterize the samples.19 Temperature-programmed surface reaction (TPSR) (bare surface method20) and temperature-programmed decomposition desorption (TPDD) techniques were adopted to study the characteristics of Hg0 capture and its desorption. The material was heated from 25 to 500 °C at a heating rate of 1 °C/min. Concentration of both Hg0 and Hg2+ was continuously

Hg 0 removal efficiency (%) =

[Hg 0]in − [Hg 0]out [Hg 0]in

× 100 (1)

The long-term evaluation of Hg0 capture was conducted by weighing a mass increment of the sample in a specially designed mercury penetration tube. The temperature was maintained at 50 °C in an oil bath. The schematic procedure is shown in Supporting Information Figure 2S.

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RESULTS AND DISCUSSION Characterization of Materials. The textural properties of γ-Al2O3, CoMoO/γ-Al2O3, and CoMoS/γ-Al2O3 are summarized in Table 1. The virgin γ-Al2O3 has the largest surface area (188 m2/g), a total pore volume of 0.49 cm3/g, and an average pore width of 10.3 nm. The BJH pore distribution analysis showed that this support was mainly dominated by mesopores (88%). As for the calcined CoMoO/γ-Al2O3, the total pore volume and the average pore width were both 20.4% lower than those of the virgin γ-Al2O3 due to the IWI processes. The surface area, however, decreased only by 3.7%. It is also clear B


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Environmental Science & Technology

sulfide) during the sulfurization process. Therefore, MoS2 formed on the surface of sulfided CoMoS/γ-Al2O3. Compared with the fresh CoMoS/γ-Al2O3, no deviation in the valence states of Mo was found in the spent samples that had been used for Hg0 capture for 2000 min and 2000 h, respectively, as shown in Figure 3Sa. Regarding the Co 2p XPS spectra, as shown in Figure 3Sb, very similar peak shapes were observed for calcined, sulfided, and spent sulfided CoMoS/γ-Al2O3. It is clear from Table 2 that there is no significant change in the concentration of Co species in the sorbent. Normally, it is difficult to determine the state of Co since Co of different valence states may present simultaneously in the same sample.22 However, the oxidized phase of molybdenum on the surface of γ-Al2O3 normally shows a stabilizing effect on the oxidized phase of cobalt, while the sulfided phase of molybdenum retards the sulfidation of cobalt.23 Hence, there are negligible changes in valence states of CO species in CoMoS/γ-Al2O3 after the sulfidation process. Moreover, although CoMoS/γ-Al2O3 experienced Hg0 capture for up to 2000 h, as shown in Figure 3Sb, no change in valence states of Co was found. It is speculated that the unsulfided Co atoms acted as promoter atoms and self-assembled at the edges of MoS2 nanosheets to build up a well-dispersed Co−Mo−S structure, which is similar to hydrodesulphurization (HDS) catalysts.24 In this study, the morphology of fresh and spent samples was also characterized using a high-resolution transmission electron microscopy (HRTEM). The HRTEM images are shown in Figure 1. The lattice pattern of chemical clusters could be clearly recognized by raising the magnification of the TEM images. As shown in Table 2, there was a significant amount of Mo (9.7 wt %) and Co (1.0 wt %) being loaded onto the γAl2O3, which should normally be detectable under HRTEM. However, no characteristic features for the calcined CoMoO/γ-

from Table 1 that CoMoO/γ-Al2O3 still had a relatively high proportion of mesopores (83%). By contrast, no significant change was found for sulfided CoMoS/γ-Al2O3 after the S-CVR process. There was a merely 2.2% reduction in surface area while pore distribution remained almost the same as calcined CoMoO/γ-Al2O3. On the basis of the textual properties of these materials, it is obvious that active components were well dispersed as thin films on the surface of the pores (mostly macro- and mesopores) in the γ-Al2O3 support. The average thickness was estimated to be 1 nm since the total pore volume and average pore width were reduced simultaneously without any significant change in surface area. The capillary condensation in the mesoporous structure of γ-Al2O3, as indicated by the N2 adsorption−desorption isotherms,18 might have contributed to the uniform loading of metal precursors during the IWI processes. The distribution of chemical species on the surface of the pores was further investigated using XPS analysis. Table 2 lists Table 2. Concentration of Different Elements on the Surface of the Fresh and Spent Sample element concentration wt % sample/product

Al 2p

O 1s

Mo 3d

Co 2p

S 2p

γ-Al2O3 CoMoO/γ-Al2O3 CoMoS/γ-Al2O3 CoMoS#/γ-Al2O3a

48.0 43.1 42.2 43.7

52.0 46.2 42.0 43.0

9.7 9.6 8.6

1.0 1.2 1.2

5.0 3.5

a Notation: (#) The spent sample experienced Hg0 capture at 50 °C (same as what followed).

the concentration of elements that are present on the surface of the pores of fresh and spent materials together with fresh support. It can be seen that for calcined CoMoO/γ-Al2O3 there were around 9.7 wt % of Mo and 1.0 wt % of Co being loaded onto the support while there were around 5.0 wt % of sulfur species being loaded on the sulfided CoMoS/γ-Al2O3. Compared with the calcined CoMoO/γ-Al2O3, CoMoS/γAl2O3 contained a similar amount of Mo and Co and approximately 4.0−5.0 wt % of S species, which replaced O species in the CoMoO/γ-Al2O3. The XPS analysis also showed that for the spent CoMoS#/γAl2O3, the weight percentage of Mo and S species (on the surface) decreased from 9.6 and 5.0 wt % to 8.6 and 3.5 wt %, respectively, while the weight percentage of other species remained unchanged. It is therefore reasonable to conclude that both S and Mo species participated in Hg0 capture and acted as active centers for the removal of Hg0. To further understand the transformation of valence states of Mo and Co atoms in the sorbents, XPS spectra of Mo 3d and Co 2p for CoMoO/γ-Al2O3, CoMoS/γ-Al2O3, and spent CoMoS/γ-Al2O3 were analyzed in Figure 3S (Supporting Information). Figure 3S is the Mo 3d and Co 2p XPS spectra of fresh and spent CoMoS/γ-Al2O3 materials, which show the valence states of Mo and Co species. According to the XPS database (version 4.1) of NIST Chemistry WebBook,21 the peaks at the binding energy (BE) of approximately 235.8 eV, 233.0 eV, 232.8 eV, 228.8 eV, and 227.3 eV are assigned to Mo6+ 3d3/2, Mo6+ 3d5/ 2, Mo4+ 3d3/2, Mo4+ 3d5/2, and S 2s, respectively. The results indicated that Mo species experienced the transformation of the oxidation state from Mo6+ (metal oxide) to Mo4+ (metal

Figure 1. HRTEM morphologies of (a) CoMoO/γ-Al2O3, (b) CoMoS/γ-Al2O3, (c) CoMoS#/γ-Al2O3, and (d) the schematic structure of (b) and (c). C


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Figure 2. TPSR analysis of the (a) CoMoO/γ-Al2O3 sample and (b) CoMoS/γ-Al2O3 sample. (The space velocity was 45 000 h−1 and the inlet concentration of Hg0 was 30 μg/m3. The samples were heated from 50 to 450 °C with a heating rate of 1 °C/min.)

Dynamic Transient Behavior of Hg0 Capture Materials. Our previous research showed that both Mo sulfide and Co sulfide showed better performance in Hg0 capture than their respective oxides (as shown in Figure 4S),29 while Co could also be used as an excellent promoter for Hg0 capture.30 Therefore, in this study, the CoMoS/γ-Al2O3 was selected as a promising candidate for Hg0 capture and was studied systematically in comparison with its oxidized form (CoMoO/γ-Al2O3). To evaluate the performance of the calcined CoMoO/γAl2O3 and sulfided CoMoS/γ-Al2O3 in terms of Hg0 capture, dynamic transient behaviors were further studied as shown in Figure 2. It can be seen that the oxidized state material, CoMoO/γ-Al2O3, showed low effectiveness on Hg0 removal at temperatures below 100 °C. However, about 30 μg/m3 of Hg0 was completely captured by the sulfided material, CoMoS/γAl2O3, at temperatures below 150 °C. The outlet concentration of Hg0 over the CoMoS/γ-Al2O3 suddenly increased when the temperature was further raised to above 150 °C while that over CoMoO/γ-Al2O3 decreased sharply. It is clear that Hg2+ was released over CoMoO/γ-Al2O3 at around 200 °C. Hg0 removal efficiency of CoMoO/γ-Al2O3 was greater than 75% at a temperature between 200 and 300 °C, in which the concentration of Hg2+ reached a peak. This suggests that CoMoO/γ-Al2O3 has a catalytic effect on mercury removal and facilitates the oxidization of elemental mercury to the oxidized state (Hg2+). The oxidization of elemental mercury reached a peak at around 275 °C and became negligible when temperature was above 375 °C. From Figure 2, it is obvious that for CoMoS/γ-Al2O3 the at 200 °C, the amount of Hg0 desorbed reached the first peak. When temperature was continuously increased to high levels, there was a trough for the outlet concentration of Hg0, which was very close to the inlet Hg0 concentration. After that, the adsorbed Hg0 continued to be desorbed from CoMoS/γ-Al2O3 resulting in a second peak. At temperatures above 450 °C, the outlet concentration of Hg0 was the same as the inlet Hg0 concentration, indicating that CoMoS/γ-Al2O3 has no effect on Hg0 capture after 450 °C. It is also evident that the concentration of Hg2+ remains unchanged over a wide temperature range. This indicates that sulfided materials have no catalytic effect on elemental mercury removal and adsorption dominates the removal of elemental mercury using the CoMoS/γ-Al2O3 as the Hg0 capture material.

Al2O3 can be identified in Figure 1a. This could occur only if the metal oxides are well dispersed as thin films on the surface of the pores (otherwise they should be detected under the HRTEM). It is in a good agreement with the characterization of BET surface area and BJH pore distribution, in which impregnation of Mo and Co did not affect surface area significantly and the average pore width. Compared with the micrographs of calcined CoMoO/γ-Al2O3, the regular fringe pattern with a spacing of 0.27 nm was identified from the HRTEM image (Figure 1b) of the sulfided CoMoS/γ-Al2O3. This type of regular fringe pattern is the characteristic pattern of (001) basal plane of (100) MoS2 crystal structure which confirms the existence of MoS2 nanosheets in the CoMoS/γAl2O3 materials. Furthermore, dislocated and disordered atomic arrangement on the basal plane can be observed by careful investagetion of this HRTEM image, which suggests that these MoS2 nanosheets are defect-rich ones.25 It is generally believed that these defects are the coordinative unsaturated sites (CUS) (i.e., sulfur vacancies sites) that play an essential role for catalytic reaction.18 On the basis of the BET, BJH, XPS, and HRTEM results, it can be concluded that MoS2 was dispersed evenly and formed MoS2 nanosheets on the surface of macro- and meso-pores in the γ-Al2O3. Such a MoS2 nanosheet sturcture, a typical example of TMDC, has strong in-plane bonds (predominantly covalent in nature) but weak out-of-plane interactions (caused by van der Waals forces).26,27 This property enables the layers to exfoliate into two-dimensional (2D) single layers, similar as graphene. The schematic structure of CoMoS/γ-Al2O3 is presented in the top left corner of Figure 1b. In addition, comparing the HRTEM micrograph of the spent CoMoS#/γ-Al2O3 (Figure 1c) with that of fresh CoMoS/γAl2O3 (Figure 1b), the fringe pattern identified for the fresh sample is not evident in the spent sample. It is speculated that basal plane of MoS2 played some role in Hg0 capture, because of such, the pattern was destroyed. On the basis of these findings, the stereoscopic schematic structures of the fresh (top) and spent (bottom) samples was proposed and illustrated in Figure 1d. In this study, mercury species was not detected either from the HTREM micrographs of the spent samples or from the TEM-EDS analysis. However, this is normal as the concentration of mercury species captured on the spent samples was too low to be directly detected either by TEM or XPS analyses, the same as what was reported by others.28 D


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Figure 3. TPDD analysis of the (a) CoMoO/γ-Al2O3 sample and (b) CoMoS/γ-Al2O3 sample. The space velocity was about 45 000 h−1 and the inlet was about 250 °C. The samples were exposed in Hg0 with concentration of 30 μg/m3 (at 250 °C for CoMoO/γ-Al2O3 and at 50 °C for others) and then heated from 50 to 450 °C with a heating rate of 1 °C/min in N2.

Figure 4. (a) Time-dependent activities of Hg0 conversion over CoMoS/γ-Al2O3 at 25 °C, 50 °C, 75 °C, 100 and 125 °C, (b) temperature dependent activities of Hg0 conversion over CoMoS/γ-Al2O3, CoMoO/γ-Al2O3, S/γ-Al2O3, and γ-Al2O3. (The space velocity was 45 000 h−1, the inlet concentration of Hg0 was 30 μg/m3 and 180 min tests for steady-state analysis of Hg0 conversion efficiencies by using eq 1).

To further understand the characteristics of Hg0 capture and desorption processes, TPDD analysis was carried out, the results of which are shown in Figure 3. In the TPDD analysis of CoMoO/γ-Al2O3 sample (Figure 3a), the adsorption process was conducted at 250 °C since it was found previously that the removal of Hg0 became significant at temperatures above 200 °C. In the TPDD, it was found that during desorption a similar amount of Hg0 and Hg2+ (around 0.65 μg/m3) was desorbed between 50 and 250 °C. The temperature was above 250 °C, the concentration of Hg2+ increased while the concentration of Hg0 decreased. A peak of HgT (total mercury) appeared at a temperature around 375 °C, which was mainly due to the increase of Hg2+. Figure 3b shows that mercury species captured by CoMoS/γAl2O3 started to desorb slowly at 75 °C and then rapidly at approximately 110 °C. The rate of desorption reached a peak at around 150 °C. Another Hg0 desorption peak was observed at a temperature around 260 °C when temperature was raised to high levels. Meanwhile, in the experiment, it was observed that there was yellow crystal (S) formed at the lower part of the reactor where temperature was much lower than that of the reactor. The result was further confirmed by EDS test (shown in Figure 5S (SI)). It is therefore clear that both Hg0 and S species were desorbed simultaneously; and the two peaks that appear in Figure 3b correspond to the decomposition of β-HgS (the first peak) and α-HgS (the second peak).

Generally, sulfur has a high affinity toward mercury and can react with mercury to form HgS. It is speculated that on the CoMoS/γ-Al2O3 the gas phase Hg0 was immobilized as HgS on the (001) basal plane of (100) MoS2 at a temperature below 180 °C (i.e., in region i in Figure 2b). Since Hg0 was trapped in CoMoS/γ-Al2O3, it was not detected at the outlet of the reactor in the form of Hg2+. However, the HgS was decomposed into Hg0 and desorbed from the CoMoS/γ-Al2O3 when the temperature was raised to higher levels (i.e., in regions ii and iii in Figure 2b). By calculating the absolute area with the baseline of 30 μg/m3 in the gas phase, it was found that the area of region i (adsorption region) was almost the same as the total area of regions ii and iii (desorption regions). The calculated amount of Hg0 captured was 2330 μg/g, which was close to that of desorbed Hg0 (2310 μg/g). This suggests that Hg0 captured below 180 °C was almost completely decomposed in a temperature range between 180 and 450 °C. To summarize, it can be concluded that CoMoO/γ-Al2O3 and CoMoS/γ-Al2O3 had different reaction pathways and temperature windows for the removal of Hg0. More specifically, CoMoS/γ-Al2O3 showed higher efficiency in Hg0 capture at lower temperatures, at which Hg0 capture efficiency for CoMoO/γ-Al2O3 was negligible. However, the catalytic effects of CoMoO/γ-Al2O3 at temperatures above 175 °C were significant. E


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Figure 5. (a) In situ DRIFTS study of surface acid sites (by using NH3 as base probe molecule) on the samples: (i) CoMoS/γ-Al2O3, (ii) S/γ-Al2O3, (iii) CoMoO/γ-Al2O3, and (iv) γ-Al2O3 and (b) relative intensities of the Brønsted and Lewis acid sites.

started to lose its capacity in immobilizing Hg0 as HgS (as shown in Figure 2b) at temperatures above 100 °C, which was mainly due to the accelerated decomposition of HgS (as shown in Figure 3b). The results of steady-state analysis, as shown in Figure 4, confirmed that the CoMoS/γ-Al2O3 sorbent was highly efficient in the removal of Hg0 at low temperatures. To understand the influence of individual components in the CoMoS/γ-Al2O3 on Hg0 removal, several samples were prepared and subsequently studied under various temperatures as shown in Figure 4b. It was found that when the γ-Al2O3 alone was used, negligible Hg0 removal efficiency was recorded between 25 and 450 °C. However, when sulfur was loaded onto γ-Al2O3, the efficiencies increased from 14% to 83% when temperature was raised from 25 and 100 °C, and then decreased rapidly from 72% to 3% when the temperature was further raised from 125 to 150 °C. It is believed that the increased mercury removal efficiency was due to the high surface area provide by γ-Al2O3 to support the loaded sulfur for Hg0 capture. Compared with that for CoMoS/γ-Al2O3, the peak efficiency was 17% lower while the peak temperature was 50 °C higher. By contrast, the calcined CoMoO/γ-Al2O3 started to show mercury capture capability at 125 °C, peaked at a temperature between 175 and 325 °C with a removal efficiency greater than 75%, followed by a rapid deactivation from 350 °C. It is worth mentioning that there was a considerable amount of oxidized mercury (Hg2+) detected continuously from 200 to 325 °C, which was in accordance with the TPSR analysis. It is believed that Hg0 was oxidized to Hg2+ by oxygen from CoMoO/γAl2O3. Both S/γ-Al2O3 and CoMoO/γ-Al2O3 showed capability for Hg0 removal, but peaked at different temperatures with relatively lower removal efficiencies when compared with CoMoS/γ-Al2O3. Therefore, all components, that is, S, O, Co, and Mo, contributed to Hg0 removal in an effect that is synergistic rather than additive. Furthermore, active centers were also investigated by in situ DRIFTS, the results of which are presented in Figure 5. It was reported that Hg0 as a base tends to combine with surface acid sites on the sorbents.34 This is the reason why the NH3 was selected as the probe molecule to characterize the acidicty of active centers by DRIFTS spectra (as shown in Figure 5a). Several peaks in the ranges of 1150−1750 and 2750−3300 cm−1 remained after NH3 adsorption and N2 purging. The peaks around 1230 and 1630 cm−1 were attributed to the

By contrast, TPDD spectra of S/γ-Al2O3 sample (Figure 6S (SI)) showed the first peak at around 225 °C (with only half the amount of Hg0 concentration of CoMoS/γ-Al2O3 being released) and the second distinct peak at around 310 °C. These results indicate that the desorption on CoMoS/γ-Al2O3 was dominated by the decomposition of β-HgS while that of the S/ γ-Al2O3 sample was mainly a result of the decomposition of both β-HgS and α-HgS. It is confirmed that the two peaks shown in Figure 3b correspond to the decomposition of β-HgS and α-HgS, respectively, which is in agreement with the TPDD results of cinnabar and meta-cinnabar reagents.28 On the basis of the above disscussions, it can be concluded that the captured Hg0 bonded on the MoS2 (basal plane) of CoMoS/γ-Al2O3 was mainly in the form of β-HgS at 50 °C. The defects that exist in MoS2 a nanosheet structure with CUS sites significantly lowered the temperatures for Hg0 capture and resulted in the formation of HgS, whereas for CoMoO/γ-Al2O3, it has catalytic effects at higher temperatures, and most of the Hg0 molecules were oxidized to Hg2+ by the oxygen from the metal oxides on CoMoO/γ-Al2O3 when the temperature was raised to be above 250 °C. It is also shown that the immobilized HgS in the CoMoS/γ-Al2O3 could be recovered as elemental Hg0 and S by heating the spent CoMoS/γ-Al2O3 to a temperature higher than 180 °C. However, for CoMoO/γAl2O3, when the immobilized mercury was desorbed, a significant proportion of mercury is released in the form of Hg2+. The Steady-State Analysis of Active Centers. Hg0 captured by the CoMoS/γ-Al2O3 in N2 atmosphere at different temperatures is presented in Figure 4a. The Hg0 removal efficiencies at 25 °C, 50 °C, 75 °C, 100 °C, and 125 °C for 3 h were 90%, 100%, 93%, 81%, and 40%, respectively. It is obvious that the sample showed excellent performance in Hg0 removal at low temperatures such as 25, 50, and 75 °C. With the increase in temperature from 100 to 125 °C, the removal efficiency decreased significantly from 81% to 40%. The effective temperature window for the CoMoS/γ-Al2O3 is similar to those reported for carbon-based sorbents.31,32 For these carbon-based sorbents, mercury removal efficiency was high at low temperature and decreased from approximately 95% to 10% when temperature was raised from around 100 to 175 °C.31,32 According to the dynamic transient behavior analysis carried out in the previous section, the CoMoS/γ-Al2O3 sorbent F


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Figure 6. Hg0 removal over the CoMoS/γ-Al2O3 at 50 °C during (a) 2000 min test, and (b) 2000 h test with with kinetics study.

HZSM-5 at 250 °C40 but with a lower effective temperatures at around 50 °C. The longer time evaluation of Hg0 capture capacity was also conducted by determination of the mass increment of a sample in a specially designed mercury penetration tube. The temperature was maintained at 50 °C in an oil bath. The results are presented in Figure 7S . It is found that for CoMoS/ γ-Al2O3, the amount of Hg0 captured was 15 mg/g during the 2000 h test. To further understand this chemisorption process, the experimental data were analyzed by using the Elovich model, which is used mainly for the interpretation of experimental data of chemisorption.41 If it is assumed that the amount of adsorbed Hg0 only depends on the fraction of unoccupied surface (δ) at the time, the Elovich model could then be developed into

absorbed NH3 on Lewis acid sites, while peaks at 1450 and 1670 cm−1 were assigned to coordinated NH4+ species on Brønsted acid sites.35 The NH4+ bands at 2820, 3010, and 3175 cm−1 could also be found in the stretching region especially for CoMoS/γ-Al2O3 and CoMoO/γ-Al2O3. It is believed that the bands were contributed by the adsorbed NH3 on the different surface acid sites. To further understand the contributions of Brønsted and Lewis acid sites, areas of the peaks around 1450 and 1630 cm−1 were further calculated by the integration method. The results were illustrated in Figure 5b. The CoMoO/γ-Al2O3 sample had similar Lewis acid sites with the support γ-Al2O3. By contrast, the CoMoS/γ-Al2O3 and S/γ-Al2O3 samples had slightly more Lewis acid sites. For Brønsted acid sites in different materials studied, its intensity is on the order of CoMoS/γ-Al2O3 > CoMoO/γ-Al2O3 > S/γ-Al2O3 > γ-Al2O3. It is clear in Figure 4b that the sulfided CoMoS/γ-Al2O3 sample had the strongest Brønsted and Lewis acid sites with higher Hg0 removal efficiency than S/γ-Al2O3, CoMoO/γ-Al2O3, and γ-Al2O3. Good correlations of the amount of acid sites and Hg0 removal efficiencies were found. It can be seen that Brønsted acid sites play a more significant role in Hg0 capture than Lewis acid sites. However, it was reported that the Lewis acide sites could be converted to Brønsted acid sites due to the existence of unsaturated coordination sites35 and the amount of NH3 adsorbed associated with anion vacancies (or coordinative unsaturated sites).22 In our study, the defect-rich MoS2 ultrathin nanosheets that are rich in unsaturated sites could result in the conversion of Lewis acid sites into Brønsted sites and subsequently exhibit excellent catalytic properties.25 On the basis of our characterization results, it is reasonable to conclude that Brønsted acid sites on the MoS2 nanosheet played important roles in Hg0 capture at low temperatures. Capacities and Kinetics of Hg0 Capture. In this study, it has been shown previously that CoMoS/γ-Al2O3 with MoS2 nanosheet has a great potential in Hg0 capture. The CoMoS/γAl2O3 was therefore further investigated for Hg0 capture at 50 °C. As illustrated in Figure 6a, Hg0 (30 μg/m3) was completely captured. The removal efficiency remained at almost 100% for 2000 min with calculated Hg0 capture capacities of 45.31 μg/g. The Hg0 capture performance is comparable to other types of promising sorbents, such as iron sulfide sorbent with an effective temperature range of 60−100 °C,36,37 zeolite sorbent in the temperature range of 130−200 °C,38 Ag-X zeolite sorbent in the temperature range of 110−150 °C,39 and Cu/

C = Cm(1 − e−δt )

(2)

where Cm is the maximum of Hg0 adsorbed (mg) on each gram of sample (g), C is the amount of adsorbed Hg0 (mg). According to this equation, it is clear that the amount of adsorbed Hg0 and time have an exponential relationship. The nonlinear mathematical function of y = y0 + A1e−x / t1 was attempted to fit these experimental data. There were three assumptions made: first, the layered MoS2 nanosheets (with the same structure) had the same activity for the adsorption of gas phase Hg0; second, the adsorption mechanism only involved the combination of gas phase Hg0 with sulfur on the layered MoS2 nanosheets (i.e., [Hg·S]); third, the amount of Hg0 adsorbed was less than that of one complete monolayer of Hg0 (i.e., Cm) which occurs after infinite time. The nonlinear fitting of the experimental data with calculated Hg0 coverage ratios are shown in Figure 6b. It can be seen that the Elovich model fits very well with the experimental data with a R-square around 0.996, which also proves that the assumptions made previously are valid. On the basis of Figure 6b, the amount of Hg0 that can be adsorbed by CoMoS/γ-Al2O3 is 18.95 mg/g, which is the theoretical capacity of CoMoS/γ-Al2O3 for Hg0 adsorption. It is the ideal situation that the layered MoS2 nanosheets have been fully covered by a monolayer of the adsorbed Hg0. Mechanisms of Hg0 Capture Process. On the basis of the XPS analysis (Figure 3S), it was found that the peak intensity of Mo6+ 3d5/2 (BE = 233.0 eV) for Mo in CoMoO/γAl2O3 shifted to Mo4+ 3d5/2 (BE = 228.8 eV) in CoMoS/γG



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Al2O3. This means that the existence of Mo in CoMoS/γ-Al2O3 is in the form of MoS2. MoS2 is a typical graphene-like material from the family of transition-metal dichalcogenides (TMDCs), which enable the layers to exfoliate into two-dimensional (2D) single layers.27 Moreover, the representative HRTEM micrograph (Figure 1b) clearly shows the regular fringe pattern with an interplanar spacing of 0.27 nm, which corresponds to the characteristic of (100) basal plane of hexagonal MoS2. The existence of defect in atomic arrangement on the basal plane with rich CUS sites found in this research is consistent with the recent reported MoS2 ultrathin nanosheets.25 The results of BET surface areas and BJH pore distribution analysis provided additional information to demonstrate that the MoS2 nanosheets were well dispersed as thin films onto the macro- and mesopores of γ-Al2O3. It is reported that the edge plane of MoS2 are highly active for catalytic reactions while the basal plane is generally believed to be nonactive.22,42 The reason is that the basal plane has active sites terminated by a close-packed S layer.42 However, basal plane of MoS2 showed high efficiency for Hg0 capture in this study. The reason is that the surface acid sites on the nanosheet played an important role for Hg0 capture, and the ultra-abundant active S atoms (with rich CUS sites) on the 2D TMDC MoS2 nanosheets make the entire basal plane catalytically active.26 It is believed that CoMoS/γ-Al2O3 has a high density of active sites, where the MoS2 nanosheets are assembled with unsulfided cobalt atoms, which are coated on the large surface of the support. The chalcogen atoms (S) on the entire basal plane of the MoS2 nanosheets with surface acid sites can combine with gas phase Hg0 to form HgS at lower temperatures. More specifically, the gas phase Hg0 atoms are mainly combined with chalcogen atoms (S) that are well-dispersed in the MoS2 nanosheets. The MoS2 nanosheets, which consist of some defects and are activated by the assembled unsulfided cobalt atoms, provide more S to combine with elemental mercury.43 Subsequently, Hg0 is immobilizated as stable β-HgS on the nanosheets. Therefore, the overall mechanism of Hg0 capture over CoMoS/γ-Al2O3 is speculated as follows: Hg 0 + CoMoS → CoMo[S ·Hg]

(3)

CoMo[S ·Hg] → CoMo[] + HgS

(4)

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b04278.

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Preparation of samples, experimental procedures, and material characterization results (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 (0) 574 88180269; fax: +86 (0) 574 8818 0175; e-mail: [email protected]; address: 199 Taikang East Road, Ningbo 315100, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Following funding bodies are acknowledged for partially sponsoring this research: Ningbo Bureau of Science and Technology under its Innovation Team Scheme (2012B82011) and Major Research Scheme (2012B10042), Zhejiang Provincial Innovation Team on SOx and NOx Removal Technologies (2011R50017), and Ministry of Science and Technology under its International Cooperation Programme (2012DFG91920). The University of Nottingham Ningbo China is also acknowledged for providing scholarship to the first author.

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REFERENCES

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Where [S·Hg] represents the chemisorption compound and [] is the coordinative unsaturated site. In this study, it is demonstrated that CoMoS/γ-Al2O3 consists of MoS2 nanosheets, a type of graphite-like twodimensional transition metal dichalcogenide (2D TMDC) material. These MoS 2 nanosheets are assembled with unsulfided cobalt atoms coated on the large surface of the support. The CoMoS/γ-Al2O3 showed excellent Hg0 capture performance at 50 °C with a removal efficiency around 100%. The chalcogen atoms (S) present on the entire basal plane of the MoS2 nanosheets react with gas phase Hg0 and immobilize Hg0 as β-HgS. The nonlinear regression result of the Elovich model also proves that the proposed mechanisms are valid. However, future work is needed to evaluate other TMDCs materials for Hg0 capture and to understand the role of Co and other transition metals in Hg0 capture. H


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