Synthesis of Porous Cobalt Oxide and Its Performance for H2S

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Synthesis of Porous Cobalt Oxide and Its Performance for H2S Removal at Room Temperature Jian Wang, Chao Yang, Ying-Rui Zhao, Hui-Ling Fan, Zhong-De Wang, Ju Shangguan, and Jie Mi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02934 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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Industrial & Engineering Chemistry Research

Synthesis of Porous Cobalt Oxide and Its Performance for H2S Removal at Room Temperature Jian Wang, † Chao Yang, † Ying-Rui Zhao, † Hui-Ling Fan,*, † Zhong-De Wang, † Ju Shangguan † and Jie Mi† †

State Key Laboratory of Coal Science and Technology, Co-founded by Shanxi Province and

the Ministry of Science and Technology, Institute for Chemical Engineering of Coal, Taiyuan University of Technology, West Yingze Street Number 79, Taiyuan 030024, People’s Republic of China

*Corresponding Author. Tel.: 0086-351-6010530; fax: 0086-351-6010530; E-mail: [email protected]

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ABSTRACT

Two types of cobalt oxide-silica composites were prepared by sol–gel method using different alcohol (n-butyl alcohol and ethylene glycol) in sol precursors. The corresponding adsorbents with 3DOM structure were also fabricated via colloidal crystal template method. Physicochemical properties of the materials were characterized by means of numerous techniques, and the performance for hydrogen sulfide (H2S) removal were evaluated at room temperature. It was found that using n-butyl alcohol in sol precursors could achieve a material (SCN57-500) with plentiful ordered mesoporous in grains and thus had very large surface area (314.5m2/g). Whereas the sample obtained from ethylene glycol (SCE57-500) only possessed smaller surface area. After introducing 3DOM structure into the bulk counterpart, the surface area improved remarkably as in the case of 3D-SCE57-500, which is in consistent with our previous studies. However, 3D-SCN57-500 showed a decreased surface area compared with SCN57-500. The loss of surface area was due to the sharply decreased pores in grains, which is originated from sintering when the hard template was burning out during preparing process. It can be deduced that the increased surface area after introducing 3DOM structure was ascribed to the excellent dispersion, i.e. the new formed very small nanoscale grains. The results of adsorption experiments show that although SCN57-500 owned highest surface area among these four adsorbents, however it exhibited a poorest performance for H2S capture. Both 3DOM samples possessed much favorable breakthrough H2S capacity compared to the counterparts without 3DOM structure. The 2


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results indicated that size of crystalline or dispersion and not the surface area is what contributes to the reactivity of the adsorbent. The well-ordered and interconnected macropores play important role as well. The breakthrough sulfur capacity of 3D-SCE57-500 could reach as high as 189mg/g with Co3O4 utilization of 63%. The involved reactions for H2S removal by Co3O4 in experimental conditions were suggested.

KEYWORDS

cobalt oxide–silica adsorbent, 3DOM structure, desulfurization, room temperature， n-butyl alcohol, ethylene glycol

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1. Introduction Sulfur removal from industry gas streams has been an important issue since sulfur contaminants, especially hydrogen sulfide, contribute mainly to the formation of acid rain,1–4 and poison the catalysts used in industry. Several treatment techniques are used for the removal of H2S; adsorption, oxidation reaction, and catalytic combustion. Among these techniques, adsorption has been widely used due to its high efficiency, reliability, and wide range of working temperatures.5

Materials such as Fe-, Zn-, Mo-, Mn-, V-, Ca-, Sr-, Ba-, Co-, Cu-, and W- based oxides can thermodynamically remove H2S.6 Thus far, among these oxides, ZnO-, CuO-, Fe2O3-, MnO2-, CeO2-, and cobalt oxide-based adsorbents has attracted great attention for the removal of H2S, and most studies focus on high working temperature.7–9 Compared to high temperature desulfurization, low temperature desulfurization is favorable because of the lower cost of materials and operation. However, at low operation temperature, sulfidation kinetics is greatly decreased, and the adsorption reaction of metal oxide with H2S is less effective, producing a higher diffusion resistance.9,10 Cobalt oxide and cobalt oxide-based adsorbents have shown have good performance for H2S removal at low temperature.11 Baird et al. studied various cobalt-containing metal oxides as adsorbents for H2S removal at low temperatures. Materials investigated in their research include cobalt–zinc oxides,12 cobalt– iron oxides,13 and Co–Zn–Al oxides;14 and they reported that the performance of adsorbents 4


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linked strongly to unique morphological features. Factors such as method used to prepare the precursors result in different surface areas and characteristics of the oxides and therefore lead to an improved H2S uptake at ambient temperature. Another recent study indicated that mesoporous cobalt oxide (Co3O4) materials could reach 100% theoretical sulfur capacity at 150°C, and the breakthrough sulfur capacities at 25°C was shown to be as high as 13.4 g S/100g adsorbent with a utilization of 25.5%.15 Besides Co3O4, other forms of cobalt containing adsorbent such as CoOOH and Co(OH)2 were also used for desulfurization at room temperature,16,17 and the adsorption of H2S on these materials were found to be enhanced by the presence of moisture. The presence of water can accelerate H2S dissociation into HS- and S2- and result in a higher H2S adsorption capacity at ambient conditions.18 Three-dimensionally ordered macroporous (3DOM) materials with plenty of uniform large pore size and well defined periodic structure has attracted much attention,19 which also exhibit excellent catalytic performance.20 Previously, our group investigated the fabrication of 3DOM adsorbents including Fe3O4 (surface area=34.26 m2/g),21 ZnO–SiO2 (surface area=357 m2/g),22 γ-Fe2O3–SiO2 (surface area=125 m2/g).23 These 3DOM adsorbents possessed much favorable breakthrough H2S capacity compared to the corresponding bulk counterparts.21–23 The presence of 3DOM structure can provide good mass transfer to the reactant molecules and easy accessibility to the active sites. Moreover, it can also improve the dispersion of metal oxide that leads to the formation of very small grains of active component and therefore improves the adsorption–reaction kinetics of metal oxide with H2S.22 In this paper, we introduce 3DOM structure to cobalt oxide for H2S removal. Until now, the most popular synthetic method of 3DOM metal oxides materials was from alkoxide–based precursors that 5



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usually use ethylene glycol–methanol as the solution.24,25 Considering that the resulting morphology might be different because of the route used to prepare the precursors, we also used n-butyl alcohol–methanol as a solution to prepare 3DOM cobalt oxide–silica composites. The 3DOM samples produced were compared in terms of physical and chemical characters, as well as the desulfurization performance, and very surprising and interesting results were found. Besides, this work investigates the mechanism of Co3O4 reactive adsorption H2S in wet conditions at room temperature, which has never been mentioned in the previous studies.

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2. Experimental Section 2.1.Chemicals.

Polyvinylpyrrolidone (AR, K13–18), potassium persulfate (AR, 99.5%), tetraethyl orthosilicate (reagent grade 98%), ethylene glycol (AR, 98%), methanol (AR, 99.5%) and ethanol absolute (GR, 99.8%) were obtained from Aladdin Chemistry Co. Ltd. Cobalt nitrate hexahydrate (AR, 99%). n-butyl alcohol and styrene (AR, 98%) were purchased from Tianjin Damao Chemical reagent Factory. Hydrochloric acid (AR, 99%) was purchased from Guangzhou Chemical reagent Factory. All the chemicals were used without further purification.

2.2.Synthesis of Adsorbents 3DOM materials was synthesized by colloidal crystal template method.21-23 The well-arrayed monodispersed polystyrene (PS) microspheres and the assembly template are described elsewhere in detail.21 In a typical synthesis of 3DOM cobalt oxide–silicon composites, the procedures mentioned as below. First, silicon sol was prepared by mixing tetraethyl orthosilicate (TEOS), hydrochloric acid, distilled water and anhydrous ethanol (EtOH) at a molar ratio of 1:0.3:1.8:3.9. Then cobalt nitrate hexahydrate was dissolved in a mixture of methanol (volumetric ratio 40%) and ethylene glycol (or n-butyl alcohol) to 7



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achieve the cobalt nitrate solution (1.5M). After that, mixed cobalt nitrate solution and silica sol to get final precursor solution with different Co content. 5.0g PS hard template was thoroughly soaked in the above precursor solution for 7h and the excessive liquid was removed by Vacuum filtration. After the wet template was dried in air at room temperature overnight, the dried materials was well ground and calcined under flowing air in a muffle furnace at a heating rate of 1oC /min from RT to 300 oC for 1h and then at 500 oC for 4h.. In this work, materials without 3DOM structure, which was named as SCE57-500 and SCN57-500, whereas N indicates n-butyl alcohol instead of E (ethylene glycol) for cobalt nitrate solution preparation. The corresponding 3DOM samples was named as 3D-SCN57-500 and 3D-SCE57-500, respectively. 3DOM materials (ethylene glycol in sol precursors) with the weight ratios of Co3O4 of 40%, 57%, 75% and 100% were obtained, and were labeled as 3D-SCEx-500 where x denotes Co3O4 weight ratio in percent. In addition, the 3DOM materials with 57 wt % Co3O4 calcined at different temperature named as 3D-SCE57-y, where y indicates calcination temperature. The suffix −S is added to the name of the samples spent for H2S removal. 2.3.Performance Tests. Dynamic tests were carried out at room temperature (30 oC) to determine the breakthrough capacity of adsorbents for H2S removal under wet conditions. Adsorbent samples (60-80 mesh) were evaluated in a continuous–flow fixed-bed glass microreactor (i.d= 6mm) and a bed height of 2 cm. Prior to the experiments run in moist conditions, samples were prehumidified with moist N2 (ca. 3% moisture) for 2 h. moist N2 containing 500 mg/m3 H2S was then passed through the column of adsorbent at 100 ml/min. The 8


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breakthrough of H2S was monitored using a gas chromatograph (Haixin, GC-920) equipped with a photometric detector (FPD). The bed was considered as breakthrough when the outlet concentration reached 1mg/m3. The breakthrough sulfur capacity was calculated in the amount of adsorbed sulfur (in milligrams) per g of adsorbents.

3. Results and Discussion 3.1. Characterization of the Synthesized Adsorbents

Figure 1 shows the X-ray diffraction (XRD) measurement of the as-prepared cobalt oxide–silica composites with and without a 3DOM structure. The corresponding sulfide samples were also present. By comparing to standard Co3O4 (JCPDS PDF#42-1467), the XRD patterns of fresh samples clearly reveal that the crystal structure should be indexed to cubic Co3O4 phase, and the diffraction peaks can be well indexed to (111), (220), (311), (222), (400), (422), (511), (400), and (533) crystal faces. Besides, based on Figure 1a and Figure S1, the samples obtained by ethylene glycol show a preferable dispersion than that of n-butyl alcohol samples, and the diffraction peaks of samples without a 3DOM structure are stronger than those of corresponding 3DOM materials. The low angle diffraction lines (Figure S1) were observed only for SCN57-500, SCE57-500, and 3D-SCE57-500, indicating ordered structure among these samples. After the adsorption of H2S, in the used adsorbents the peaks of Co3O4 become weak and with no obvious diffraction peaks of sulfide phase (Figure 1b). Figure S2 shows the XRD patterns of the sample with different ratios of cobalt oxide to silica and shows that 3D-SCE40-500, which has the lowest content of cobalt, has very weak 9



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peaks corresponding to cubic Co3O4 and a broad peak at 2θ of 23.5°, corresponding to amorphous silica.22 With the increased cobalt oxide weight ratio, the feature peaks of Co3O4 appear and their intensities enhance. Obviously, introduction of silica could significance improve the dispersion of cobalt oxide. In addition, Figure S3 shows that the intensities of the peaks gradually increase as the calcination temperature increases, suggesting that the nanoparticles grow gradually.

Fig 1. XRD patterns of fresh and sulfide cobalt oxide–silica composites

Figure 2 shows the scanning electron microscopy (SEM) images of 3D-SCE57-500 and 3D-SCN57-500. Both samples displayed a high-quality 3DOM architecture, with highly periodically arrayed uniform macropores interconnected with small windows with a diameter of ~36 nm. For fresh 3D-SCE57-500 samples, the macropore sizes and the wall thicknesses observed from the SEM images are about 140 nm and 30 nm, respectively. Whereas for 3D-SCN57-500, they are about 170 nm and 20 nm, respectively. Such difference indicates that the precursor of 3D-SCE57-500 could immerse into the template more easily and form tougher 3DOM structures than that of 3D-SCN57-500, demonstrating that ethylene glycol plays a key role in the fabrication of 3DOM materials.24 10


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a

b

c

d

e

f

Fig 2. SEM images of (a, b, c) 3D-SCE57-500, (d, e, f) 3D-SCN57-500

Figure 3 shows the high-resolution transmission electron microscopy (HRTEM) images as well as selected-area electron diffraction (SAED) patterns of the cobalt oxide–silica composites samples. The figure shows that SCN57-500 displayed an irregular morphological with uneven particle size ranging from 55 to 121 nm (Figure 3a–c). Mesopores with a diameter of 2–5 nm can be found on the surface of large particles. Comparatively, SCE57-500 shows quite uniformed small cubic particles with an average size of 36 nm, in which very few mesopores can be observed (Figure 3f). Obviously, using ethylene glycol can help obtain well distributed particles. However, n-butyl alcohol can help to form a mesoporous material, which may be meaningful in the field where small mesoporous material are needed. A work from

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Steven L. Suib et al. reported that n-butyl alcohol

could serve as an interface modifier for fabrication mesoporous materials.

The images in Figure 3g–l show good-quality 3DOM structures existed in both 3D-SCN57-500 and 3D-SCE57-500 samples, which agrees with SEM results. The nanocrystals are well spread at the framework with the size range of 20–25 nm and 10–15 nm 11



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for 3D-SCN57-500 and 3D-SCE57-500 samples, respectively. Clearly, SEM and TEM results indicate that ethylene glycol and n-butyl alcohol play different roles in the fabrication of 3DOM materials. Davis et al. suggested that the introduced ethylene glycol could react with metal nitrates to form glyoxylate anion,27 which could function as a ligand to coordinate with the metal nitrates and convert to metal oxide without melting.24–26 That is the reason why the samples obtained by ethylene glycol show a preferable dispersion than that of n-butyl alcohol samples.

In addition, Figure 3i and l show that the exposed planes were (111) and (220) (lattice spacing (d value) = 0.24 nm)29, 30 for 3D-SCN57-500 and (111) (lattice spacing (d value) = 0.467nm) and (311) (lattice spacing (d value) = 0.24 nm) planes for 3D-SCE57-500.29 The observation of multiple bright electron diffraction rings in the SAED patterns (insert of Figure 3c, f, i, and l) suggests that the nanocrystals in both samples have Co3O4 polycrystalline structures.

Figure 3m, n, and o shows different magnification HRTEM images of sulfide adsorbent (3D-SCE57-500S). The figures show that after the adsorption of hydrogen sulfide, the ordered 3DOM structure was retained and was still integrated. No lattice fringes could be found in the nanoparticles, and the corresponding SAED pattern (insert of Figure 3l) shows no evidence of crystallinity with only diffuse rings present, indicating the sulfide cobalt are in amorphous form.

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Fig 3. HRTEM images of SCN57-500(a, b, c), SCE57-500 (d, e, f), 3D-SCN57-500 (g, h, i), 3D-SCE57-500(j, k, l) and 3D-SCE57-500S (m, n, o) 13



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Figure 4 shows the nitrogen adsorption–desorption isotherms and pore size distribution of cobalt oxide–silica composites, and the texture parameters are summarized in Table 1. For the SCN57-500 and SCE57-500 samples, a type IV nitrogen adsorption isotherm was observed, indicating the existence of an ordered mesoporous structure,15 SCN57-500 showed the highest surface area (314.5 m2/g), which was much more than the surface area of SCE57-500 (137.2 m2/g)。As revealed in the HRTEM images, the plentiful mesoporous in grains are responsible for this large area.

The corresponding 3DOM samples displayed a type II isotherm with a type H3 hysterics loop in the relative pressure (p/p0) range of 0.8–1.0, indicating the existence of macropores.22, 30

The inflection point of all 3D-SC samples except 3D-SCE100-500 showed a hysteresis

loop in the relative pressure range of 0.2–0.8, which is linkable to capillary condensation occurring in the mesoporous, indicating that the textural mesoporous existed.31,32 Based on the comparative BET result of SCE57-500 and 3D-SCE57-500 shown in Table 1, it can be seen that the 3DOM structure can improve the surface area of material, which is in consistent with the result from our earlier work. 21–23 The increased surface area is mainly due to the reduced grain size in 3DOM materials, as revealed in the TEM and XRD results. However, when BET surface areas of SCN57-500 and 3D-SCN57-500 are compared, a contrary result was found. As shown in

Table 1, the surface area of 3D-SCN57-500 is only 159.5m2/g,

which is much less than that of SCN57-500. As mentioned that when the 3DOM structure is introduced，the particle size could be reduced greatly (average size from 93 to 23 nm). The strong exothermic effect caused by hard template burning during the preparation sintered the localized grain, and eliminated the mesopores, and therefore decreased the surface area. 14


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Fig 4. Nitrogen adsorption and desorption isotherms of cobalt oxide–silica composites (inset is the corresponding pore size distribution).

Table 1. Structure parameters of cobalt oxide–silica composites

Sample

SBET (m2/g)

SCN57-500 SCE57-500 3D-SCN57-500 3D-SCE57-500 3D-SCE100-500 3D-SCE57-500S 3D-SCN57-500S

314.5 137.2 159.5 275.4 20.1 104.3 55.2

Vt (cm3/g) 0.296 0.168 0.319 0.490 0.145 0.242 0.154

WBJH (nm) 3.506 3.723 1.999 3.523 2.471 3.474 1.935

Figure 5 compares the nitrogen adsorption–desorption curves of 3D-SCE57-500 samples before and after sulfidation, and the sulfide sample produced a similar isotherm to that of the fresh sample but with a remarkable decrease of nitrogen adsorption. BET surface area decreased from 275.4 m2/g to 104.3 m2/g, and numerous pores were blocked, as seen from the pore size distribution (insert of Figure 5). 15



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Fig 5. Nitrogen adsorption and desorption isotherms of 3D-SC57-500 before and after sulfidation at 30 oC

3.2. Desulfurization Performance

3.2.1 H2S Breakthrough Performance of the Adsorbents Prepared by Different Route

Fig 6. H2S breakthrough curves for SCN57-500, SCE57-500, 3D-SCN57-500, 3D-SCE57-500 (a) and corresponding sulfur sorption capacities and Co3O4 utilization levels (b).

The prepared adsorbents were tested at room temperature (30°C) in the presence of about 3% moisture for the H2S removal. Before this test, it has been approved that the contribution of 3DOM-SiO2 to H2S removal could be ignored. The measured H2S 16


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breakthrough curves are shown in Figure 6. The figure shows the quite remarkable difference between adsorbents SCN57-500 and SCE57-500 and their corresponding 3DOM sample in the performance for H2S removal. SCN57-500 exhibited very poor desulfurization performance, as its breakthrough time was only 20 minutes with a very low breakthrough capacity of 12 mg/g. adsorbent SCE57-500 was a little better, but it was still poor in H2S removal with a breakthrough capacity of 28 mg/g and Co3O4 utilization of 9%. However, a much longer breakthrough time could be obtained for both 3DOM samples. For 3D-SCN57-500, the breakthrough capacity and Co3O4 utilization was 75 mg/g and 25%, whereas 3D-SCE57-500 exhibited the highest H2S breakthrough capacity (189 mg/g) and the highest Co3O4 utilization (63%). Obviously, both the 3DOM structure and precursor composites have a dramatic influence on H2S removal performance. As stated above, the reactivity of the metal oxide for H2S capture severely decreases when temperature decreases, especially at room temperature. To enhance the sulfur capture capacity at low temperature, many studies have focused on improving the surface areas of the adsorbent.11,15 In this work, we got a adsorbent SCN57-500 that ranks highest among these four adsorbents and possesses a very large surface area of 314.5 m2/g. However, it exhibits incredibly poor capacity for H2S removal. Nitrogen adsorption/desorption and TEM results show that it is a porous material with plenty mesopores in the particles. Although it has a large surface area, the average grain size (93 nm) is much larger, as revealed by XRD and TEM, which means the dispersion is considerably worse. Comparing to SCN57-500, SCE57-500 has a low surface area but has a smaller average grain size (36 nm) with better dispersion, exhibiting a better desulfurization performance. It indicates that the size of crystalline or dispersion and not the surface area is 17



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what contributes to the reactivity of the sorbent. Small crystalline size provides more active sites, because more defects are created.33 Furthermore, small particles could have much shorter diffusion paths for sulfur ion immigration to deep layers inside the bulk structure during the desulfurization process.11 In addition, the pore diameter distribution (Figure 4) shows that SCN57-500 has more pores with diameters less than 3–4 nm, which were easily blocked during the prehumidified process, losing its ability to capture H2S. As for sorbents with a 3DOM structure, they displayed a much better dispersion than their corresponding bulk counterpart with nanocrystalline size ranging from 10–15 and 20–25 nm, which are very beneficial for increasing the reactivity for H2S adsorption at low temperature. Additionally, the excellent performance of 3DOM samples was due to the high porosity and well-ordered and connected macropores.34 In this study, because 3D-SCE57-500 has much smaller particle and larger surface area than 3D-SCN57-500, it exhibits better desulfurization performance than the latter.

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3.2.2 Effect of Calcination Temperature and Co/Si Ratio of 3DOM Composites on the Desulfurization Performanc

Fig 7. (a) H2S Breakthrough curves for 3D-SCE57-y treated at different temperature with 57% Co3O4 content and (b) H2S Breakthrough curves for 3D-SCEx-500 treated at 500°C with different Co3O4 content.

Figure 7 shows the breakthrough curves of the samples with different calcination temperature and various Co/Si ratios. Figure 7a shows that the breakthrough time reaches a maximum at a calcination temperature of 500°C, and further increases of temperature results in a decline breakthrough time. Interestingly, 3D-SCE57-400 showed a significant breakthrough time close to that of 3D-SCE57-500. It was confirmed that calcination at low temperature leads to insufficient burning out of PS template (as shown in Figure S4). The retained residual carbon impurities might cover the surface-active sites and partially block the pore channels,22 which results in a worse performance. However, it was reported that these carbon impurities might form C-O-Co bonds, which could cause surface hydroxylation and therefore complement the negative effect of the incomplete calcination.17,

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When the

calcination temperature is over 500°C, Co3O4 aggregation occurred as revealed by XRD results (Figure S3). Therefore 3D-SCE57-700 show a decreased sulfur capacity. 19



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Figure 7b shows the breakthrough curves of 3DOM adsorbents with various Co3O4 weight ratios. Increasing the Co3O4 content from 40% to 100% increased the breakthrough time first and then it decreased; the breakthrough capacity increased from 116 to 189 mg/g then decrease to 12mg/g (Figure S5). Moreover, the Co3O4 utilization reached a maximum at 63% (Co3O4 content 57%) and subsequently decreased. Because 3D-SCEx-500 are composites of silica and cobalt oxide, the presence of silica in the sorbents improves the dispersion of active sites and improves the desulfurization performance. However, higher silica content could cover the active sites, leading to reduced contact of cobalt oxide and H2S.36 When the Co3O4 content increases excessively, silica cannot reduce active site aggregation, and thus results in a poor desulfurization performance, as occurred for 3D-SCE100-500.

3.3 Involved Reactions for H2S Removal by Co3O4 in Moisture Conditions

Fig 8. XPS spectra of Co 2p (a) and S 2p (b) for 3D-SCE57-500S

Although several studies have reported on cobalt oxide-based sorbents for H2S removal, the reactions involved were not discussed, probably because of the complicated changes

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regarding the chemical state of cobalt. To investigate the possible reactions of the prepared cobalt containing sorbent during desulfurization, X-ray photoelectron spectroscopy (XPS) and Thermogravimetric-mass spectroscopy (TG-MS) analysis were performed. XPS were used to reveal the state of surface elements of 3D-SCE57-500 after desulfurization, As shown in Figure 8a, multiple peaks can be observed in the Co 2P spectrum, and the two peaks around 782 eV and 798.2 eV are ascribed to Co 2p1/2 and Co 2p3/2, respectively. 37 The energy separation between the Co 2p1/2 and Co 2p3/2 peaks is 16 eV, with concurs with the literature on cobalt sulfide.38 After fitting, the components located at 778.8 eV and 794.8 eV, and satellite peaks at 786.8 eV and 803.3 eV, are characteristic of CoO, and the presence of CoO can be explained by surface absorbed hydroxide species as the cobalt ion has a very strong affinity to oxygen in air.37,39 In addition, the peaks at 780.2 eV and 796.2 eV are attributed to CoSOH;40 which is oxidized from CoS in the presence of hydroxyl.41 Another two components located at 782.2 eV and 798.2 eV are thought to be CoS.37 Figure 8b shows the S 2p peak of 3D-SCE57-500S and that the peaks at 161.9 eV are attributed to CoS.37 In addition, the peaks at 162.5 eV are assigned to S2−, with higher binding energy assigned to CoSOH;42 and the peaks at 163.5 are attributed to element sulfur.15 The last high energy doublet at 168.9 eV can be shifted to sulfate which was formed by oxidation of element sulfur when exposure to air.15,37

21



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Fig. 9. TG-MS curves of 3D-SCE57-500S

Figure 9 is the TG-MS curves for the exhausted 3D-SCE57-500 sample, the sample was heated at a heating rate of 10°C/min in 100ml/min of air atmosphere. As the temperature raised from room temperature to 810°C. The DTG curve shows that there are four remarkable weight losses accompanied by gas release during the heating process. The first weigh loss centered at 150°C is the liberation of chemical bounded water molecules, and the gradual slight weight loss below 100°C is ascribed to the loss of water physically absorbed on sorbents.15 The second weight loss was very sharp, occurred at around 150–210°C, and was accompanied with the elution of H2O (170°C), which is quickly followed by a very strong release of SO2 (185°C). The evolution of H2O at 170 can be associated with the decomposition of CoSOH.43

4CoSOH → 4CoS + 4H2O+O2

(1)

CoSOH was formed mainly by CoS oxidation with O2 and H2O. It might also come from

22


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the direction interaction of CoOOH with H2S during sulfidation.16, 43,44 4CoS+O2+2H2O → 4CoSOH

(2)

8CoOOH+H2S+2H2O → 6CoSOH+CoSO4

(3)

In addition, the weight loss at this stage may include the contribution from the release of element sulfur.45

The release of SO2 at 185°C is caused by the oxidation of CoS. However, normally the oxidation of CoS would lead to the formation of CoSO4 and cause a weight gain. 43,46 In our experiment, TG curve show only weigh loss during the whole process. Considering the new formed water molecules from the release of CoSOH, the involved oxidation reactions was thought be be as follows:47–49

2CoS + 2O2 → 2CoO +SO2

(4)

The third noteworthy weight loss at 310°C with a MS-H2O peak is due to the decomposition of CoOOH.50

12CoOOH → 4Co3O4+ O2 + 6H2O

(5)

CoOOH is considered to originate from unstable CoS, which may produce CoOx/Co(OH)2 and then CoOOH when CoS is exposed to a temperature-increased warm air during the TG test.51

2CoO+H2O → 2Co(OH)2

(6)

4Co(OH)2+O2 → 4CoOOH+2H2O

(7) 23



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The large weight loss ranged from 660 to 800°C accompanied with a small MS-SO2 peak is explained by the thermal decomposition of CoSO4.52 It could be formed when element sulfur in sulfided sample touched with air during characterization or storage. And it might also come from the oxidation of CoS during TG test:12,43 S + 3/2O2 + H2O → 2H+ + SO42CoO+O2+SO2 → CoSO4

(8)

(9)

Based on the results from XRD, XPS, and TG-MS, as well as the phenomena observed during the experiment, CoS, CoSO4, CoSOH, CoOOH, and elemental sulfur were produced during the sulfidation at the experimental condition. Therefore, the involved reactions were supposed to be as follows:

Before the tests, the Co3O4 adsorbents in bed were prehumidified with moist N2 (3% moisture), and hydroxylation occurred on the surface:53–55 Co3O4 + 4H2O → 3Co(OH)2 + 2OH-

(10)

Co(OH)2 + OH → CoOOH + H2O + e-

(11)

Then H2S can be absorbed by: 12, 16 Co(OH)2 + H2S → CoS (black color) + 2H2O 8CoOOH + H2S + 2H2O → 6CoSOH + CoSO4 Co3O4 + H2S → 3CoO + S + H2O

(12)

(13)

(14)

CoO + H2S → CoS (black color) + H2O

(15) 24


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4. Conclusions In the present study, we characterized two types of cobalt oxide-silica composites with and without 3DOM structure prepared using different alcohol (n-butyl alcohol and ethylene glycol) in sol precursors. It was found that the composition of precursors had a significant influence on the structure and H2S adsorption capacities of cobalt oxide–silica composites. As for adsorbents without 3DOM structure. SCN57-500 achieved plentiful ordered mesoporous with a diameter of 2–5 nm in grains and a considerable large surface area (314.5m2/g). However, it exhibits incredibly poor H2S capture capacity of 12mg/g. With respect to SCN57-500, SCE57-500 possessed smaller surface area (137.2 m2/g) but a well distributed particles thus obtained a better H2S breakthrough capacity of 28mg/g. It was found that using ethylene glycol can benefit the formation of 3DOM material. The surface area of 3D-SCE57-500 (275.4 m2/g) improved remarkably. However, 3D-SCN57-500 (159.5 m2/g) showed a decreased surface area compared with SCN57-500. The loss of surface area was due to the sharply decreased pores in grains, which is originated from strong exothermic effect caused by hard template burning. It can be concluded that the increased surface area after introducing 3DOM structure was ascribed to the excellent dispersion. The Co3O4 content in 3DOM composites and calcination temperature of 3DOM composites also had great influence on the structure of 3DOM composites and desulfurization performance. 3DOM samples possessed much favorable breakthrough H2S capacity compared to the corresponding bulk counterparts. The results indicated that size of crystalline or dispersion contributes to the reactivity of the adsorbent. The well-ordered and interconnected macropores play important role as well. Even though sulfidation at low temperature is 25



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kinetically slow, the breakthrough sulfur capacity of 3D-SCE57-500 could reach as high as 189mg/g with Co3O4 utilization of 63% at room temperature. 3D-SCE57-500 exhibits better performance compared with reference mesoporous cobalt oxide. Through a combination of XRD and SAED analysis, we found that sulfide cobalt are exist in amorphous form, and regarded as CoS by XPS and TG-MS analysis. In addition, XPS and TG-MS analysis were performed for 3D-SCE57-500S, and the involved reactions for H2S removal by Co3O4 in experimental conditions was suggested. The results demonstrate that at least five distinct sulfidation product (CoS. CoSO4, CoSOH, CoOOH, and elemental sulfur) were produced during sulfidation and storage process.

AUTHOR INFORMATION Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Nature Science Fundamental (21576180).

ASSOCIATED CONTENT

Supporting information

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Low angle PXRD patterns of SCN57-500, SCE57-500, 3D-SCN57-500, 3D-SCE57-500, XRD patterns for 3D-SCEx-500 samples with different Co3O4 content, XRD patterns for 3D-SCE57-y with different calcination temperature, EDX analysis of 3D-SCE57-400, Breakthrough sulfur capacities and Co3O4 utilization levels of 3D-SCEx-500 samples . This material is available free of charge via the Internet at http://pubs.acs.org/.

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Figures: Fig 1. XRD patterns of fresh and sulfide cobalt oxide–silica composites Fig 2. SEM images of (a, b, c) 3D-SCE57-500, (d, e, f) 3D-SCN57-500 Fig 3. HRTEM images of SCN57-500(a, b, c), SCE57-500 (d, e, f),3D-SCN57-500 (g, h, i), 3D-SCE57-500(j, k, l) and 3D-SCE57-500S (m, n, o)

Fig 4. Nitrogen adsorption and desorption isotherms of cobalt oxide–silica composites (inset is the corresponding pore size distribution).

34


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Fig 5. Nitrogen adsorption and desorption isotherms of 3D-SC57-500 before and after sulfidation at 30 oC Fig 6. H2S breakthrough curves for SCN57-500, SCE57-500, 3D-SCN57-500, 3D-SCE57-500 (a) and corresponding sulfur sorption capacities and Co3O4 utilization levels (b).

Fig 7. (a) H2S Breakthrough curves for 3D-SCE57-y treated at different temperature with 57% Co3O4 content and (b) H2S Breakthrough curves for 3D-SCEx-500 treated at 500°C with different Co3O4 content.

Fig 8. XPS spectra of Co 2p (a) and S 2p (b) for 3D-SCE57-500S Fig. 9. TG-MS curves of 3D-SCE57-500S (air atmosphere, 100ml/min; heating rate:10°C/min)

ABSTRACT GRAPHIC

35


Synthesis of Porous Cobalt Oxide and Its Performance for H2S

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