Effect of Activated Carbon Supports on Removing H2S from Coal

Dec 28, 2014 - Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology,. Taiyuan ...
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Effect of Activated Carbon Supports on Removing H2S from Coal-Based Gases using Mn-Based Sorbents Jiancheng Wang,† Fenglong Ju,† Lina Han,†,‡ Haochen Qin,† Yongfeng Hu,§ Liping Chang,† and Weiren Bao*,† †

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, P. R. China ‡ College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, P. R. China § Canadian Light Source, 101 Perimeter Road, Saskatoon, Saskatchewan, Canada S7N0X4 ABSTRACT: A series of MnOx/AC sorbents were prepared by a sub- or supercritical water impregnation method at different temperatures using three types of active carbons (ACs) as supports. The desulfurization activities of the MnOx/AC sorbents were studied, and the physicochemical properties of fresh and used sorbents were characterized by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray absorption near edge structure. The results indicate that the AC composition and preparing temperature affect the desulfurization activity of prepared sorbent by changing the form and dispersion of the active component. MnOx/AC sorbent prepared at 350 °C and active carbon AC(Z) from walnut shell as support presents the best desulfurization activity, holding above 22 h under the desulfurization level of nearly entire removing H2S in hot coal-based gas with breakthrough sulfur capacity of 7.24 g of S/100 g of sorbent. It is found that the valence of Mn loaded on the different AC supports was various, which related directly to the activity and capacity of the H2S removal from hot coal-based gases. The species, content, and the dispersion of the manganese oxide on the surface of the supports were dominated by AC properties. In addition, Mn3O4 is the main active component for the H2S removal over MnOx/ACs sorbents.

1. INTRODUCTION The development of clean coal technologies is critical to secure the energy supply and reduce the environmental impact of developing countries such as China who heavily relies on coal as the primary energy source. Coal gasification has been under development in China, and it was viewed (or considered) as one of the novel alternative clean coal technologies.1 In the coal gas, sulfur is mainly present as hydrogen sulfide (H2S) and accounts for above 90% of gaseous S-containing compounds. The presence of H2S leads to serious corrosion problems (pipes, compressors, etc.) and catalyst poison in the chemical industry.2,3 To avoid equipment corrosion and infringement of emission legislation, the sulfur compounds in coal gas have to be removed prior to the coal utilization. Hot gas desulfurization (HGD) over metal oxides is a key technology, and it can improve the thermal efficiencies and reduce capital costs required due to the procedure of fuel cooling and heating again.4,5 In recent years, many different materials have been investigated as sorbents for removal of H2S from gases under different temperatures and for different reactors,6,7 and various metal oxides and their mixtures as sorbents have been investigated for hot coal gas desulfurization. For example, oxides of Fe, Zn, Cu, Mn, and Co have been studied as recyclable desulfurizers for the direct removal of H2S.8−12 However, the search for sorbents with high H2S removal efficiency, high sulfur loading capacity, good ability of regeneration, and sufficient strength is still under development.13,14 Key parameters involved in achieving acceptable reactivity in the moderate temperature range include porosity, pore size distribution, and surface area, which provide an indication of the immediate availability of inner sorbent area for reaction with H2S.15 © XXXX American Chemical Society

In the past several years, material synthesis in the media of supercritical water has been the subject of a wide variety of research. This is largely due to its enhanced properties under supercritical conditions (i.e., high density, high diffusivity, low viscosity, etc.) as well as the ability to control its properties by varying the pressure and temperature.16 The high reaction rates and low solubility of metal oxide in supercritical water can lead to high supersaturation and the deposition of fine metal oxide particles from aqueous solutions of metal salts.17 The preparations of composite materials, such as manganese oxide, silver, and lead oxide, loaded on alumina by the supercritical water impregnation method as efficient and feasible examples have been reported.11,16,18 Iron oxide nanoparticles on activated carbon (AC) have successfully been synthesized in supercritical water.17,19 In contrast to the conventional impregnation in the liquid phase, the formation of metal complexes such as MnAl2O4 and Pb2Al2O5 was observed during the impregnation using supercritical water, and the formation of these metal complexes was conclusive evidence that the particles were not simply formed on the support surface, but rather through interactions with the support material itself.16 In addition, sorbent/catalyst could be prepared without the use of toxic or noxious solvents, and the post-treatments such as processes of drying and calcining are not needed.11,16,20 Thus, supercritical water impregnation (SCWI), a solvent-free and environmentally benign procedure, would be expected as a new preparation method for supported sorbent/catalysts. Received: August 11, 2014 Revised: December 23, 2014

A

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(Rigaku, Japan), a graphite monochromator and Cu Kα radiation sources (λ = 0.154056 nm), and the tube voltage and current was 40 kV and 100 mA, respectively. The scan rate was 8° /min for 2θ values of 10−80°. The crystalline phases of samples were measured by transmission electron microscopy (TEM) at the room temperature with a JEOL2100f microscope operating at 200 kV (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) was conducted for surface analyses, using an ESCALAB220i-XL spectrometer (VG Scientific Ltd., UK) equipped with a monochromated Al Kα. Energy calibration was performed by using the C 1s peak at 284.6 eV. The pore structure was determined via nitrogen adsorption at 77 K using a ASAP2020 analyzer (Micromeritics Instrument Corporation, America). The surface area was measured by the Brunauer−Emmett− Teller (BET) equation. The metal content in the samples was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Atomcan-16, America). The X-ray absorption near edge structure (XANES) measurement of the Mn species on the sorbents was performed at the Canadian Light Source, using the Soft X-ray Micro characterization Beam line (SXRMB). Si(111) double crystal monochromotor was used, giving an energy resolving power of 10,000.24 The sample was mounted using double-sided, conducting carbon tape, and loaded into a vacuum chamber (base pressure of 1 × 10−8 Torr). Mn K-edge XAS spectra were recorded in the surface sensitive total electron yield (TEY) by monitoring the drain sample current, and the bulk sensitive fluorescence yield (FLY) with a silicon(Li) drifted detector. Representative Mn oxide or sulfate compounds included manganese monoxide (MnO), manganese(II, III) oxide (Mn3O4), manganese sesquioxide (Mn2O3), manganese dioxide (MnO2), manganese(II) sulfide (MnS). All reference compounds were obtained from Aldrich and the purities were over 97%.

In our previous studies, AC supported manganese oxide sorbents have been prepared by the SCWI, and the effect of the operating conditions and manganese precursor on the synthesis of sorbents have been studied.21,22 In this work, three types of ACs made from different materials were chosen as supports, the MnOx/AC sorbents were prepared by SCWI at different temperature (300, 350, and 400 °C) and X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray absorption near edge structure (XANES) were employed to characterize the fresh and used sorbents. The properties of AC supports and their influences on the desulfurization activity over the MnOx/AC sorbents prepared by supercritical water impregnation were investigated.

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. The ACs from three different materials (AC(M) and AC(X) are prepared using coal, and AC(Z) using walnut shell), grinded into the particle sizes of 8−10 mesh (1.70−2.36 mm), were selected as the supports. The sorbents were prepared according to the method reported by our group.21,22 Preparation of the MnOx/ AC sorbent was accomplished in a 300 mL stainless-steel autoclave, with a pressure gauge and a thermocouple. The reactor was heated in an externally heated furnace equipped with a temperature controller. Mn(NO3)2 solutions of 0.46 mol/L were used as the precursor of active component. A total of 100 mL of precursor solution and 20 g of ACs were added to the autoclave. The autoclave was then heated to the predetermined temperature (300, 350, and 400 °C) at a rate of 5 °C/min and was maintained for 30 min, which was defined as the impregnation temperature. The rest processes of sample treatments are the same as previously reported.22 The remaining solid sample in the reactor was the sorbent, which was named as “MnOx/AC (a) Tb” with “a” and “b” standing for the activated carbon type and the impregnation temperature, respectively. 2.2. Desulphurization Tests. The sulfidation tests of the MnOx/ AC sorbents were carried out at atmospheric pressure using a vertical fixed-bed quartz reactor with a perforated sintered plate installed in the middle of the reactor. About 6.5 g of sorbent was used for each experiment. All sulfidation tests were carried out at 350 °C with a space velocity of 2000 h−1. The typical reactant gas mixture consisted of 45 vol % H2, 32 vol % CO, 6 vol % CO2, 5 vol % H2O, 500 ppm of H2S and N2 balance gas. The flow rate of H2S was exactly controlled by a mass flow controller, and the concentrations of H2S were analyzed using the gas chromatographs (GC950, Shanghai Haixin Chromatographic Instrument Co., Ltd.) equipped with a flame photometry detector (FPD, GDX-303). The test was stopped when the desulfurization efficiency was less than 99.8%, i.e., H2S content in the outlet gas over 1 ppm. The computation details for the desulfurization efficiency, the sulfur capacity, and the use rate of the sorbent have been reported in our previous work,23 and the data obtained in this work are calculated similarly. The computational formulas for the efficiency of H2S removal (η), the sulfur capacity (Sc), and the utilization rate of the sorbent used in this study are as follows:

η(%) =

C0 − C × 100 C0

Sc (g of S/100 g of sorbent) =

3. RESULTS AND DISCUSSION 3.1. Desulfurization Activities of the Sorbents. Three types of activated carbons (AC(M), AC(X), and AC(Z)) were used as supports in order to investigate the effect of the properties of AC supports on the desulfurization activity. The MnOx/AC sorbents, using Mn(NO3)2 as precursor, were prepared by SCWI at temperatures of 300, 350, 400 °C respectively, while being impregnated in sub- or supercritical water for 30 min. The sulfidation breakthrough curves of the prepared sorbent are shown in Figure 1. The results show that these sorbents synthesized by the supercritical water impregnation method presented very high precision removing H2S, nearly not detecting H2S in outlet gas before the breakthrough of sorbent bed. The MnOx/AC sorbents have similar change trends of the desulfurization. The breakthrough times of the MnOx/AC(M), MnOx/AC(X), and MnOx/AC(Z) sorbents prepared at 350 °C were longer than that of the sorbents prepared at 300 and 400 °C. It can also be seen that the breakthrough time of the MnOx/AC(Z) sorbent was longer than those of the MnOx/AC(X) and MnOx/AC(M) sorbents prepared at the same temperature. And the desulfurization activities of the MnOx/AC(Z)350 sorbent can be maintained at 100% for above 22 h. The above results indicated that the MnOx/AC sorbents used AC(Z) as support was superior to those used AC(X) and AC(M) as supports. The sulfur capacities of the MnOx/AC sorbents are shown in Figure 2. It can be seen that the sulfur capacities of the MnOx/ AC sorbents were very different, with the MnOx/AC sorbents prepared at 350 °C being better than those prepared at 300 and 400 °C. And the sulfur capacity of the MnOx/AC(Z)350 sorbents can reach 7.24 g of S/100 g, and it was the highest among all sorbents.

(1)

∑ ηit i

32vspVbjC0 22.4G

× 100

(2)

−1

where vsp is the space velocity of gas (h ), Vbj is the volume of sorbent in the reactor (L), ti is the adsorption time in the i sampling time before breakthrough (h), G is the weight of sorbent in the reactor (g), C0 is the inlet H2S concentration (ppmv), and C is the outlet H2S concentration in the i time sampling before breakthrough (ppmv). 2.3. Characterization of the Sorbent. X-ray diffraction (XRD) was employed to investigate the crystal structures of the sorbents. The instrument was equipped with a D/max2500 diffractometer B

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Figure 1. Desulfurization efficiencies of the MnOx/AC sorbents prepared under different conditions (sulfidation conditions: 350 °C, 2000 h−1, 45% H2, 32% CO, 6% CO2, 5% H2O, 500 ppm of H2S, N2 balance gas).

Compared with the desulfurization activity of the MnOx/AC sorbents shown in Figures 1 and 2, it can be correlated that the sorbent with lower SiO2 content in its AC support had higher desulfurization activity, while the desulfurization activity of sorbent was lower when the content of SiO2 in its AC support was higher. The supercritical water presented the properties of high density, high diffusivity, low viscosity, high nucleation etc., such that Si in AC support partly reacted with the Mn precursor to form Mn2SiO4-like species.21 So, it is inferred that the different Si contents may result in different Mn oxide species or dispersion on support and further influence the corresponding desulfurization activities. It is also found from Table 2 that the K2O and Na2O content of AC(Z) was 12.28% and 24.06%, respectively, and those of AC(M) and AC(X) were very low. AC(Z) was selected in order to investigate whether the alkali metal K+ and Na+ ion on the AC supports had a catalytic effect on the desulfurization reaction. A total of 1 g of KNO3 or 1 g of NaNO3 was added in the stainless-steel autoclave to prepare two kinds of sorbents with additional alkali metal K+ or Na+ ion, and then their desulfurization activities were tested. The desulfurization results are shown in Figure 3. The desulfurization efficiency of AC(Z) with Na+ is slightly improved, but the efficiency of AC(Z) with K+ is reduced (only 13.5 h). Therefore, it can be concluded that K+ exhibited an inhibition effect on desulfurization activity to some degree. 3.2. Characterizations of MnOx/AC Sorbents. In order to explore the relationship between the desulfurization activity and the pore structure of sorbents, the BET surface areas of the MnOx/AC sorbents prepared at 350 °C were tested, and the

Figure 2. Sulfur capacities of the MnOx/AC sorbents prepared under different conditions.

Table 1 shows the proximate and ultimate analyses of the three types of ACs. The ash content of AC(M) was the highest and that of AC(Z) was the lowest, as 9.44% and 2.66%, respectively. The ash composition of the three types of ACs is given in Table 2. It is found that the SiO2 content of the ash of AC(M) was the highest (62.91%), and that of AC(Z) was the lowest (29.72%). The absolute content of SiO2 on the AC supports can be calculated by the content of ash and the content of SiO2 from the proximate analysis (Tables 1 and 2). The absolute contents of SiO2 on AC(M), AC(X), and AC(Z) supports were 5.94%, 3.85%, and 0.79%, respectively.

Table 1. Proximate and Ultimate Analyses of Three Kinds of ACs proximate analysis (wt %)

a

ultimate analysis (wt %) (ad)

sample

Mad

Aad

Vdaf

C

H

N

S

Oa

AC(M) AC(X) AC(Z)

2.01 5.59 3.63

9.44 6.81 2.66

3.65 3.64 3.41

83.70 82.27 85.56

1.09 1.04 1.07

0.74 0.76 0.80

0.60 0.53 0.29

2.42 3.00 2.58

By difference.

Table 2. Ash Composition Analyses of Three Kinds of ACs ash composition (wt %) sample

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

K2O

Na2O

P2O5

AC(M) AC(X) AC(Z)

62.91 56.47 29.72

15.82 17.02 3.96

6.83 8.52 5.78

4.23 6.19 8.15

2.88 3.15 5.70

1.89 2.42 2.19

1.35 1.65 5.15

0.24 0.69 12.28

2.36 2.57 24.06

1.01 0.73 2.49

C

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results are shown in Figure 4 and Table 3. It can be seen from Figure 4 that the pore volumes of the MnOx/AC(Z) sorbent were slight larger than those of the MnOx/AC(M) and MnOx/AC(X) sorbents. The BET surface areas of the MnOx/ AC sorbents prepared at 350 °C are shown in Table 3. It is found that the BET surface area of the MnOx/AC(Z)350 sorbent is the largest. From our previous study,22 it is found that the AC support with the higher specific surface area has better desulfurization activity. Compared with the desulfurization results, the desulfurization activity of AC(Z)T350 is the best with the maximum surface area (921.7 m2/g), and the desulfurization activity of AC(M)T350 and AC(X)T350 are lower with a lower surface area (829.3 m2/g and 823.6 m2/g, respectively). This may be one of the reasons that the MnOx/ AC(Z)350 sorbent had the high desulfurization efficiency and sulfur capacity. The XRD patterns of the fresh and used sorbents prepared by different ACs at different temperatures are displayed in Figure 5. It can be found from Figure 5a that all the diffraction peaks of the fresh MnOx/AC sorbents used different ACs as support could be attributed to the structure of Mn3O4 [ICDD PDF# 80-0382]. It can be concluded that the manganese nitrate precursor was converted to the crystalline Mn3O4 in supercritical water. From Figure 5b it can be seen that the used sorbents had obvious diffraction peaks of MnS [ICDD PDF# 88-2223], and the diffraction peaks assigned to Mn3O4 disappeared.25 Together with the MnS peaks, it should be noted that the diffraction peaks assignable to crystalline MnO [ICDD PDF #78-0424] were also observed in Figure 5b. This suggests that Mn3O4 on the MnOx/AC sorbents is the active component, and it was transformed in the presence of H2S to MnS and MnO in reducing atmosphere, in agreement with the literature.25 In order to further investigate the dispersion of the Mn species on the MnOx/AC sorbents, the TEM images of the MnOx/AC sorbents prepared at 350 °C were recorded and analyzed. As shown in Figure 6, the black dots in the images were the Mn species. It can be clearly seen that the dispersion of the Mn species on the MnOx/AC sorbents was significantly different. The sequence of the dispersion of the Mn species is AC(Z) > AC(X) > AC(M). The diameter of some particles in AC(M) was obviously greater than 150 nm, and some aggregation of active component can be found. The dispersion of the Mn species on AC(Z) was relative even, and the particle

Figure 3. Evaluation curves of the desulfurization activity of AC(Z) before and after added KNO3 and NaNO3 (SCW preparation temperature: 400 °C; sulfidation conditions: 350 °C, 2000 h−1, 45% H2, 32% CO, 6% CO2, 5% H2O, 500 ppm of H2S, N2 balance gas).

Figure 4. Pore-size distribution of the MnOx/AC sorbents prepared at 350 °C.

Table 3. Surface Areas of MnOx/AC Sorbents Prepared at 350 °C and Their Mn 2p and Si 2p Contents sorbent

MnOx/ AC(M)T350

MnOx/ AC(X)T350

MnOx/ AC(Z)T350

surface area (m2/g) Mn 2p (At %) Si 2p (At %)

829.3 0.95 1.10

823.6 1.08 1.07

921.7 1.49

Figure 5. XRD patterns of the MnOx/AC sorbents before (a) and after (b) sulfidation AC(M)T350, AC(X)T350, AC(Z)T350. D

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Figure 6. TEM images of the MnOx/AC sorbents prepared at 350 °C.

at 350 °C. As shown in Figure 7a, the Mn 2p3/2 peaks on the surface of the MnOx/AC sorbents at 642.1, 641.9, and 641.6 eV were assigned to hausmannite,17,27,28 and this is consistent with the results of XRD. Hausmannite contains both manganous and manganic ions, and it belongs to class II of mixed valence compounds, in which there is minimal interaction between different ion sites.17,29,30 On the other side, the peak area in the XPS spectrum can reflect the relative concentration of surface Mn species. It can be seen from Table 3 that the surface Mn content of the MnOx/AC sorbents was quite different when different ACs were used as support. The surface Mn species content of MnOx/AC(Z)T350 was the highest, and its mass percentage is 1.49%. Those of MnOx/AC(M) and MnOx/AC(X) were lower, at 0.95% and 1.08%, respectively. Combined with the evaluation results of the desulfurization experiments, it can be suggested that the MnOx/AC sorbents with the higher content of the surface Mn species has better desulfurization activity. The same trend was found for sorbents prepared at 400 °C. The surface Mn species content of MnOx/AC(Z)T400 was also the highest (0.85%), and that of MnOx/AC(M)400 and MnOx/AC(X)400 was 0.67% and 0.68%, respectively. The desulfurization activity of the MnOx/AC sorbents with the different supports was in good correlation with their surface Mn species content. It has been deduced that the loading content of the Mn species on the different sorbents has little effect on the activity of desulfurization. However, the content and the dispersion of the active Mn species on the surface of the supports could greatly affect the desulfurization activity. The Si species on the surface of the MnOx/AC sorbents prepared at 350 °C were also analyzed by XPS, and the results are presented in Figure 7b and Table 3. The content of Si species on the MnOx/AC(Z)350 sorbents was very low, with almost no detectable Si 2p peak. The Si species on the surface

size of the Mn species was small and under 60 nm. From the above results it can be concluded that the dispersion of the Mn species on the surface of the ACs supports is very different. The smaller crystal size of the active component could result in the more active sites which are favorable for the sulfidation reaction.26 When the dispersion of active component is better and the particle size of active component is smaller, the desulfurization activity of sorbents is higher. Table 4 shows the Mn contents of the MnOx/AC sorbents obtained by ICP. It can be seen that the Mn contents of all the Table 4. MnOx/AC Sorbents Prepared under Different Conditions and Their Mn Contents by ICP sorbent

support

impregnation temperature (°C)

Mn content (%)

MnOx/AC(M)T300 MnOx/AC(M)T350 MnOx/AC(M)T400 MnOx/AC(X)T300 MnOx/AC(X)T350 MnOx/AC(X)T400 MnOx/AC(Z)T300 MnOx/AC(Z)T350 MnOx/AC(Z)T400

AC(M) AC(M) AC(M) AC(X) AC(X) AC(X) AC(Z) AC(Z) AC(Z)

300 350 400 300 350 400 300 350 400

9.53 10.54 8.95 9.09 10.30 11.43 10.91 11.11 10.01

sorbents were in the range of 8.95−11.43%, and there was no significant difference among these sorbents. This suggests that the Mn content was not the major factor influencing the desulfurization activity of the MnOx/AC sorbents. In order to obtain the relevant information on the active components on the surface of sorbents, the XPS technique was employed to characterize the MnOx/AC sorbents prepared

Figure 7. XPS spectra of Mn 2p (a) and Si 2p (b) on the surface of the MnOx/AC sorbents. E

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Energy & Fuels of the MnOx/AC(M)350 and MnOx/AC(X)350 sorbents were similar, and the contents of Si species on these two sorbents were nearly identical, about 1% (Table 3). This is consistent with the content of Si species resulting from the ash composition analysis. Combined with the results of desulfurization, it is found that MnOx/AC(Z) sorbents with the low content of the Si species have good desulfurization activity, and the MnOx/AC(M) sorbents with the high content of the Si species show poor desulfurization activity. It could be deduced that reactions exist between the Mn precursor and the Si species in the support, as it has been reported recently that Li2MnSiO4 and MnSiO4 can be synthesized in the supercritical fluid media.31,32 The formation of these Mn/Si compounds could impact the desulfurization activity of the sorbents in this work. Figure 8 shows the normalized Mn K-edge XANES spectra of reference compounds (MnS, MnO, M2O3, and Mn3O4), the

Figure 9. Mn K-edge XANES spectra of reference and the fresh MnOx/AC sorbents prepared at 350 °C.

the spectra of AC(M) and AC(X) were very similar to the above analyses, and the absorption peak of AC(Z) is higher (peak c). These results indicate that the Mn species on the sorbent prepared by AC(Z) exists mostly as Mn3O4 with a little of MnO and Mn2O3. In order to verify the desulfurization performance of different manganese oxide species, the desulfurization experiments of three types of bulk manganese oxides (MnO, Mn2O3, and Mn3O4) were carried out. For each experiment, about 0.4 g of MnOx and 2 g of quartz sand were loaded into the reactor. Those tests were carried out at 350 °C with a space velocity of 20 000 h−1. The desulfurization performance of pure manganese oxides are shown in Figure 10. From the results it is suggested

Figure 8. Mn K-edge XANES spectra of reference, fresh, and used MnOx/AC(M) sorbents.

fresh and used MnOx/AC (M) sorbents prepared at 300, 350, and 400 °C. Mn K-edge spectrum was dominated by the transition of Mn 1s electron to the Mn 4p unoccupied orbitals, with a weak pre-edge peak (peak a) corresponding to the transition of Mn 1s electron to the Mn 3d and ligand up mixed orbitals.33 It can be seen from the spectra of the reference compounds, the chemical shift of edge jump to the higher energy (arrow d) was clearly observed as Mn oxidation state increases. Comparing the spectra of the MnOx/AC(M) sorbents prepared by AC(M) at different temperatures with the reference compounds, it can be deduced that the Mn species mostly exists in the form of MnO and Mn3O4, with a little of Mn2O3, and the proportion of different Mn oxidation states on the surface of sorbents was strongly influenced by the preparation temperature. These results did not seem to fully agree with those of XRD, the possible reason may be that the XRD is not able to detect the amorphous component of the sample. For the amorphous or crystalline material which is below its limited detection, XRD cannot reveal all the information on the surface and bulk phase of materials. However, XAS can generate corresponding absorption for the low and trace content of materials, so the information about atomic structure obtained by XAS is more comprehensive and accurate. The spectra of sorbents prepared at different temperatures after the sulfidation treatment showed that the edge jump shifted to lower energy, thus implying that the reduction of Mn. It can be concluded that the Mn species on the MnOx/AC(M) sorbents after desulfurization exists as the mixture of MnO and MnS, in agreement with the XRD findings. Figure 9 shows the XANES spectra of the MnOx/AC sorbents prepared by three types of ACs supports at 350 °C;

Figure 10. Evaluation curves of the desulfurization activity of pure manganese oxide references.

that Mn3O4 has the best desulfurization performance, and Mn2O3 and MnO have poor capacity of the H2S removal. The above results indicated that the Mn oxidation state on the supports, which is related to the properties of AC support, influence greatly the activity and capacity of the H2S removal, and Mn3O4 is the dominant active component. Qualitative analysis of the composition of Mn on the different sorbents was carried out by linear combination fitting of spectra using Athena.34 An example of the linear combination fitting result of MnOx/AC(X)T300 is shown in Figure 11, and the linear combination fitting results of all samples are given in Table 5. As it can be seen from Table 5, the active component on the MnOx/AC sorbents mostly exists as Mn3O4. Though the content of the component active Mn3O4 on the MnOx/AC sorbents prepared at various temperatures was different, there F

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4. CONCLUSION A series of the Mn-based sorbents for the removing H2S from coal-derived gas were prepared by sub- or supercritical water impregnation method at different temperatures (300, 350, and 400 °C) using three types of ACs as supports. The desulfurization activities of the MnOx/AC sorbents were investigated using a fixed-reactor. The XRD, XPS, SEM, and XANES techniques were employed to characterize the fresh and used sorbents. It is found that the MnOx/AC sorbents prepared at 350 °C had the best desulfurization activities, especially, the sorbents with AC(Z) as support. The content and the dispersion of the active Mn species on the surface of the MnOx/AC sorbents could greatly affect the desulfurization activity. The alkali metal K+ ion of the ACs supports had an inhibition effect on the desulfurization activity of the MnOx/ AC sorbents. The active components of the MnOx/AC sorbents were mainly composed of Mn3O4. Different AC supports could affect the type content and distribution of manganese oxide that then influence the activity and capacity of the H2S removal for different sorbents.

Figure 11. Mn XANES fitting curves of reference and MnOx/ AC(X)T300 sorbent.

Table 5. Distributions of the Major Mn Species in Different Sorbents Measured by XANES sorbent

Mn2O3 (%)

Mn3O4 (%)

MnO (%)

MnOx/AC(M)T300 MnOx/AC(M)T350 MnOx/AC(M)T400 MnOx/AC(X)T300 MnOx/AC(X)T350 MnOx/AC(X)T400 MnOx/AC(Z)T300 MnOx/AC(Z)T350 MnOx/AC(Z)T400

6.9 3.8 0.0 8.6 0.9 0.0 18.9 12.5 0.0

80.6 81.2 71.1 76.4 79.6 61.5 72.9 85.2 82.6

12.5 15.0 28.9 15.0 19.5 38.5 8.2 2.3 17.4

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2012CB723105), the National Natural Science Foundation of China (21006067), and the Shanxi Province Natural Science Foundation (2011021008-4). We also thank the CLS staff for the technical support. The CLS is financially supported by NSERC Canada, CIHR, NRC, and the University of Saskatchewan.

was a clear correlation between the intensity of Mn3O4 and the H2S removal performance: the content of Mn3O4 on the MnOx/AC sorbents prepared at 350 °C were the highest, and the desulfurization activity of the MnOx/AC sorbents prepared at 350 °C was the highest. In addition, the desulfurization activity of the MnOx/AC(Z)T350 sorbent was the best; at the same time the Mn3O4 content of the MnOx/AC(Z)T350 was the highest, reaching as high as 85.2%. It can be inferred that the proportion of Mn3O4 in the active components had a significant effect on the desulfurization activity of the MnOx/ AC sorbents; the higher the content of Mn3O4 is, the better the desulfurization performance of its corresponding sorbent is. Obviously, the composition of the AC, such as the mineral forms and the Si content, could affect the formation of Mn products and thus their desulfurization performance. Recent literature described that Mn/Si complexes were formed during sub- or supercritical water impregnation in the presence of AC with high Si contents.21 Lee M et al.35 recently reported that almost pure Mn3O4/AC was prepared from commercially available AC (Norit GAC 1240) by the supercritical technique. Therefore, it is sure that different Mn3O4% in the sorbents could be prepared from different ACs even at the same calcination temperature. It also can be found that the Mn2O3 content of the MnOx/ AC sorbents decreases as the preparation temperature gradually increases. When the temperature reached 400 °C, the Mn2O3 content of the three types of the MnOx/AC sorbents was zero. At the same time, the contents of MnO contents were the highest, and the desulfurization performance of MnO was the worst. Therefore, the desulfurization activity of the MnOx/AC sorbents prepared at 400 °C was much lower than that of the MnOx/AC sorbents prepared at 350 °C. Combining Figure 10 and Table 5, the sample with the higher ratio of Mn3O4 and lower ratio MnO will have the better desulfurization performance.

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