SiO2

Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

Environmental and Carbon Dioxide Issues

Activated carbon-assisted fabrication of cost-efficient ZnO/SiO2 desulfurizer with characteristic of high loadings and high dispersion Chao Yang, Jian Wang, Hui-Ling Fan, Yongfeng Hu, Jiasheng Shen, Ju Shangguan, and Baojun Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00532 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Activated carbon-assisted fabrication of cost-efficient ZnO/SiO2 desulfurizer with characteristic of high loadings and high dispersion Chao Yanga, Jian Wanga, Huiling Fan*a, Yongfeng Hub, Jiasheng Shenc, Ju Shangguana Baojun Wanga

Corresponding author e-mail: [email protected] a State Key Laboratory of Coal Science and Technology, Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, West Yingze Street Number 79, Taiyuan 030024, People’s Republic of China b Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan S7N 2V3, Canada c School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, People's Republic of China

ABSTRACT: With the aid of activated carbon, a series of mesoporous ZnO/SiO2 sorbents are fabricated by sol-gel method and characterized by XRD, FTIR, N2 adsorption and desorption, SEM-EDX, TEM and PL spectra. The dynamic tests for H2S capture show that the prepared materials with the aid of AC exhibit excellent performance for H2S capture. The highest sulfur capacity of ZSC-X reaches to 160.95mgS/g sorbent and the corresponding ZnO utilization is up to 69.02%. The enhanced performance is attributed to AC, which has double roles in this study: anchor and confine ZnO to get highly dispersed small nanoparticles; produce more oxygen vacancies in ZnO to facilitate lattice diffusion. The formation mechanism of well dispersed ZnO nanoparticles is also elucidated. The simply fabricating method of sorbents with high ZnO loading and high utilization is of significance for industrial application of ZnO in H2S removal at room temperature.

KEYWORDS: high ZnO loadings, confined formation, high utilization, ZnO nanoparticles

1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs), as a green power source, offer a great potential for high efficiency applications and realization of zero emissions.1, 2 As known, the most important component of PEMFCs is the membrane electrode assemblies (MEAs), mainly composed of the anode and cathode catalyst layers (CLs), gas diffusion layers (GDLs), and proton exchange membrane (PEM). Foreign contaminants (CO, CO2, H2S) in the hydrogen fuels can severely poison the anode and cathode 1

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalysts, or the membrane, which will dramatically degrade the performance of the MEA.3-5 Extensive studies have shown that even a trace amount of H2S (ca. 0.1 ppmv) can significantly degrade the cell performance due to catalyst poisoning.4, 6, 7 In addition, the H2S can corrode the pipeline and damage the environment during actual operation.8-10 Accordingly, it is essential to limit the H2S impurities below 0.1ppmv in the hydrogenous fuels (mainly including: natural gas, syngas, reformate gas) at low temperature to improve the PEMFCs durability and reliability. Due to the strong chemical affinity between H2S and metallic cations, one of the best options is to use metal oxide based sorbents for H2S removal to meet the needs of PEMFCs.11, 12 ZnO-based adsorbents have attracted much attention in recent years due to its great potential in reducing H2S concentrations below 100 ppb at room temperature.13-16 Unfortunately, at room temperature, ZnO suffers from low reactivity at low sulfur concentrations and low ZnO utilization.17 Enhancing the reactivity of ZnO to H2S at room temperature remains as a major hindrance for commercializing ZnO. Currently, a generally accepted method is to mix ZnO with inert substances, such as SiO2,18-21 to realize the requirement of high surface area, high dispersion, small crystal grain size and developed porosity, which can partially compensate the slower sulfidation kinetics present at low temperatures.22 For instance, Liu et al fabricated highly dispersed ZnO/SiO2 gel composites with developed porosity by sol–gel method for H2S removal at room temperature.21 The highest H2S adsorption capacity of this material reached up to 90.73 mgS/g sorbent at a 30wt% ZnO loading, and a corresponding utilization of 76.9%. Although the sulfur capacity and utilization was enhanced significantly, the sulfur capacity was still comparatively lower because of low ZnO loadings. Moreover, it also caused excessive EtOH waste because the obtained alcogel require EtOH immersion during the preparation. Moreover, high ZnO loading leads to the decrease of the adsorption capacity with respect to ZnO agglomeration and blockage of pore channels. In addition, other mesoporous SiO2 materials (such as ordered mesoporous sieves) supported with ZnO also have additional disadvantages, such as high cost. Therefore, a facile ZnO/SiO2 fabrication method with high ZnO loading, high ZnO dispersion, and high H2S removal efficiency but economically efficient is urgently needed. Activated carbon (AC), known for its highly porous and large surface area characteristics,23, 24 is an ideal exotemplate for the creation of high surface area materials.25 Single metal oxides (such as Co3O4, ZnO et al) with high surface areas can be easily obtained by impregnating AC.26-28 In addition, inspired by the work that confined formation of ultrathin ZnO nanorods in reduced graphene oxide,29 2


Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

we designed a new preparation method for ZnO/SiO2 composites with the aid of AC. The results show that high-quality, mesoporous ZnO/SiO2 nanocomposites could be fabricated, possessing high loadings, high dispersion and yet, a small ZnO grain size. The evaluations reveal that the composites in fixed bed possesses an H2S trapping capacity of up to 160.95 mgS/g sorbent and a ZnO utilization of 69.02%, affirming the combined advantages of high ZnO loadings, high H2S capture ability and high ZnO utilization. This preparation method will provide new insight for the facile fabrication of high efficiency adsorbents for H2S removal at room temperature operation.

2. EXPRIMENTAL SECTION 2.1. Materials Methanol (AR, 99.5%), and ethylene glycol (AR, 98%) were acquired from Tianjin Guangfu Fine Chemical Industry Research Institute. Zinc nitrate hexahydrate (AR, 99.0%), tetraethyl orthosilicate (reagent grade, 98%), ethanol absolute (AR, 99.7%) and concentrated nitric acid (AR, 65%) were obtained from Tianjin Kermel Chemical Reagent Co. Ltd. Activated carbon was purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. All the chemicals were used without further purification. . 2.2 Sorbent Preparation. A series of ZnO/SiO2 sorbents were synthesized by AC-assisted sol-gel method. The AC were ground into granular particles with size between 0.106mm-0.125mm. Typically, zinc nitrate hexahydrate (0.04mol) was dissolved in a mixture of ethylene glycol (EG) and methanol (40 vol %) to obtain a zinc nitrate solution (2 M). Tetraethyl orthosilicate (TEOS) and AC were added into a mixed solution of ethanol, H2O and HNO3 at a molar ratio of n(TEOS+AC):n(EtOH):n(H2O):n(HNO3), 1:3:1:0.03, the molar number of AC and TEOS was fixed at 0.04 mol. Stirred for 1h to obtain a mixed SiO2 and AC sol. Then, the as-prepared zinc precursor solution was added into the mixed sol, and further stirred for 1h. Subsequently, the sol was held in a closed chamber at 30oC for 8h and dried at 120oC overnight to acquire a gel. Lastly, the gel was calcinated at 300oC for 2h and then elevated at a rate of 1oC/min until 500oC. A detailed preparation process is shown in Scheme 1. The formulated sorbents are indexed as ZSC-X, where ZSC are the abbreviation for ZnO, SiO2 and AC, respectively. X is the used to specify the amount of AC added, where X=

×100%. The samples after

desulfurization will be abbreviated as ZSC-XE. In addition, pure SiO2 sample was also synthesized by 3


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mentioned procedure, but without AC and zinc precursor addition.

Scheme 1. The preparation process of samples with or without adding AC

2.3 Material Characterizations The crystalline phases of the fabricated samples were determined on a Rigaku D/max-2500 diffractometer using Cu Kα radiation. The spectrum between 5 and 80° was measured for wide angle XRD studies (scan rate = 8°/min). N2 adsorption-desorption isotherms were collected using a Nova 2000e instrument. The BET surface areas were calculated by the standard Brunauer-Emmett-Teller method. The Barrett-Joyner-Halenda (BJH) model was used to calculate pore size distributions and total pore volumes. Fourier transform infrared (FTIR) spectra were studied on a 670 FT-IR spectrophotometer (Thermo Nicolet, USA) (600-4000 cm-1). The morphology of sorbents was observed by a Nanosem430 scanning electron microscopy (SEM). Energy dispersive X-ray (EDX) analysis was conducted using an Oxford X-max80 EDX analyzer operating at an electron accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images were collected on Tecnai G2F20 electron microscopy. Photoluminescence (PL) measurements was performed at room temperature using a He– Cd laser with a wavelength of 320 nm as the excitation source. 2.4. Performance Tests. The H2S capture ability of formulated ZCS-X samples was studied using a fixed bed by removing H2S from a gas stream in moist N2. The typical processes are listed as follows: Firstly, adsorbents were ground into 40−60 mesh particle size and loaded into a U-tube reactor (inner diameter 6 mm) with bed height of 2 cm. Subsequently, the loaded sorbents were pre-humidified with moist N2 (ca. 3% moisture) by bubbling N2 in water at 30°C for 90 min. Lastly, a consistent stream of 850 mg/m3 H2S in moist N2 was passed through the U-tube reactor, maintaining a total flow rate of 100 ml/min. The inlet and outlet 4


Page 4 of 19

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

H2S concentrations were determined by gas chromatography equipped with a flame photometric detector (FPD). The adsorptive performance of sorbents was evaluated based on the breakthrough capacity (mg S/g sorbent), which can be calculated by the equation: *

/

∗ "# $, %& ' %()* + 10/ ∗ ∗ "01#

Where N is the total flow rate (100 ml/min), Cin and Cout are the H2S concentration in the inlet and outlet respectively (mg/m3), MS and MH2S are equal to 32g/mol and 34g/mol, respectively, t is the breakthrough time (min), m is the mass of the loaded adsorbents (g). The breakthrough point was defined as the concentrations of H2S at the outlet, greater than 0.15 mg/m3. The utilization efficiency of ZnO was calculated by the followed equation: Utilization of ZnO

=>?@&A=*BC D)CE)@ FB?BF&*G AH#/HD(@I=* *J=(@=*&FBC D)CE)@ FB?BF&*G AH#/HD(@I=*

.

3. RESULTS AND DISCUSSION 3.1 XRD and FT-IR Analysis

Fig.1. XRD pattern of samples before (a) and after (b) desulfurization.

Phase composition of the synthesized sorbents can be determined by XRD, as contrasted in Fig.1a. It can be seen that the apparent diffraction peaks are consistent with the wurtzite ZnO structures,30 indicating the zinc precursor was decomposed and converted to wurtzite ZnO after calcination. No other peaks corresponding to SiO2 or ZnSiO3 were found, which suggests that the Si species have also dispersed into its amorphous form in the composites. The broadened yet weakened characteristic peaks imply that the ZnO particles can disperse well into the silicon network. Similarly, the dispersion role of silicon network was also observed in the work done by Hossein Barani.31 Additionally, no apparent difference can be observed with the introduction of AC. 5


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Phase transformation occurred when sorbents were used for adsorbing hydrogen sulfide. As shown in Fig.1b, the wurtzite ZnO was converted to the cubic sphalerite ZnS, demonstrating that the reaction ZnO(s) +H2S(g)→ZnS(s) +H2O(g) occurred during the adsorption. An interesting phenomenon is that the peak intensity of ZnS first increases with the addition of AC and then decreases when 10% AC was incorporated into the precursors, suggesting that H2S capture ability of ZnO is enhanced with small amount of AC introduction

. Figure. 2. FT-IR spectra of SiO2 and the selected samples before and after desulfurization.

The FT-IR spectra of as-prepared and spent sorbents are shown in Fig 2. In terms of pure SiO2, there are three adsorption bands at 795, 1080 and 1210 cm-1, which are corresponding to the characteristic peaks of the ring structure of the SiO4 tetrahedra, Si-O-Si symmetric stretching mode and asymmetric vibrational Si–O- stretching, respectively.21, 32, 33 The band appeared at around 960 cm-1 is attributed to the stretching vibration of Si-OH.34 The additional adsorption bands around at 1633 and 3440 cm-1 are indexed as H2O adsorption peaks. The former is due to the H-OH bending vibration of adsorbed H2O in pores or on the surface while the latter is ascribed to the O-H asymmetrical stretching vibration of structural water. Compared to pure SiO2, the bands at 960 cm-1 is intensified after compositing with ZnO. It is reported that the intensity of the band at 960 cm-1 is also related to the degree of dispersion of the metal oxide within the SiO2 matrix.33, 35, 36 Hence, we deduce that these changes may be caused by a more homogenous dispersion of ZnO in the SiO2 matrix. In addition, no obvious vibrations related to AC were found in the sorbent spectra after AC addition, which confirms the complete combustion of AC during the heating treatment. After desulfurization, the intensity of the band at 960 cm-1 became weaker, the band at 795 cm-1 was strengthened, and no other obvious changes were observed. These results indicate that the features 6


Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

of mesopores formed by ZnO/SiO2 composites were maintained after adsorbing H2S.21 There were no ZnS peaks observed after desulfurization, which may be overlapped by the Si-OH bands at 960 cm-1.21, 32

3.2 Texture Characterizations

Fig.3. N2 adsorption and desorption isotherms (a) and PSDs (b) of samples before and after desulfurization.

Figure 3 shows the N2 adsorption and desorption isotherms of the sorbents and pore size distributions (PSDs) calculated by the BJH method. It is evident that all the samples produce a typical IV isotherm according to the IUPAC classification,37 and a sharp inflection at the relative pressure range of 0.7–0.9. The typical H2 type hysteresis loop suggests the presence of high quality mesopores with an ink-bottle-like pore structure within the prepared materials.36 The corresponding PSDs (shown in Fig 3b) clearly show the fabricated materials possess multimodal PSDs with pore sizes centered on 2-30 nm. Adding AC to precursors influence the quantity and size of the mesopores. A decrease in the pore size is observed when the amount of AC is below 10%. However, further addition of AC does not lead to a further decrease in the pore size, probably because of the collapse of the pores formed by AC combustion as a result of the reduced proportion of SiO2 in composites. The detailed texture information of the composites is listed in Table 1. It is apparent that the surface area and pore volumes initially increased and then decrease with further AC addition. After desulfurization, the H2 type hysteresis loop of the material narrowed, indicating the amount of mesopores have decreased. It is thought that the formed mesopores may be blocked due to the material expansion when new ZnS products, with larger volumes, were formed.21 In comparison with ZSC-7, the pore size distribution of ZSC-7E decreased significantly from 2-30 nm to 2-10 nm, suggesting the reaction might occur in mesopores. This result is consistent with that of the SBA-15 7


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 19

supported ZnO NPs.19 From Table 1, we can clearly see that the surface area and pore volume of ZSC-XE dramatically decreased after exposing ZnO to H2S. The surface area of ZSC-7E is 124.9 m2/g, which is 49.7 m2/g less than ZSC-7, yet two times larger than the reduction of ZSC-0E (21.9 m2/g), indicating that the ZSC-7 adsorbed more H2S molecule than ZSC-0 Table 1. Texture Parameters of the Samples Before and After Desulfurization SBET

Vt

WBJH

3

(cm /g)

(nm)

148.2

0.436

7.105

ZSC-5

164.8

0.5

7.061

ZSC-7

174.6

0.539

7.033

ZSC-10

156.1

0.469

6.476

ZSC-30

129.9

0.426

7.798

ZSC-0E

126.3

0.258

7.208

ZSC-7E

124.9

0.268

7.134

Sample

2

(m /g)

ZSC-0

SBET, BET specific area; Vt, total pore volume; WBJH, pore size determined from BJH desorption data.

3.3 Morphology Characterization

Fig.4 SEM images of ZSC-0(a,c) and ZSC-7(b,d) and corresponding EDX analysis.

SEM characterization of the selected samples ZSC-0 and ZSC-7 and their corresponding EDX analysis are shown in Fig 4. It is clear that the synthesized sorbents correspond to the monolithic material with small, spherical particles scattered on the surface. There was no noticeable change in the surface morphology after the addition of AC into precursors. EDX analysis indicates that the prepared 8


Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

samples show only three elements: Zn, O and Si. Based on the atomic ratio of the elements, we deduce that two main phases ZnO and SiO2 are involved in the designed material. The higher atomic % of oxygen compared to that of the theoretical amount based on the stoichiometry of ZnO and SiO2, suggests an oxygen-rich surface. As observed in FT-IR, the oxygen-rich surface may be caused by the adsorbed H2O or OH groups.

Fig.5. TEM images of ZSC-0 (a,d), ZSC-7 (b,e,g,h) and ZSC-10 (c,f)

To directly observe the dispersion of ZnO in the SiO2 matrix, the TEM characterization is conducted and shown in Fig 5. Evidently, ZnO nanoparticles (NPs) are aggregated in ZSC-0 (shown in Fig 5a, d) while evenly dispersed in ZSC-7 with a particle sizes distribution between 2-10nm (shown in Fig 5g). The porosity of the designed material is formed from SiO2 networks and ZnO NPs scattered on the surface.32, 38 Some small mesopores could be found when the size of ZnO NPs decrease because of the closed packing. This well explains the shift in the hysteresis loop to lower relative pressure when AC was used in preparation, as previously shown in Fig 3. In addition, the lattice fringes separated by gaps of about 0.202 nm (shown in Fig 5h), correspond to the (101) plane of ZnO, which is consistent with XRD results shown in Fig 1. As for sample ZSC-10, the ZnO NPs aggregate to some extent, leading to the growth of the ZnO particles. The particle size distributions statistically obtained from TEM results are given in Fig 6. The good dispersion of ZnO with the aid of AC is considered to be ascribed to the anchor and confinement effect of AC which possesses characteristics of developed porosity, especially micro- and meso-pores.29, 39 More clearly, AC provides a habitat that is suitable for 9


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

zinc precursor filling, anchoring nucleation and spatially confined growth of ZnO NPs. During calcination, AC was converted into gaseous CO or CO2, creating enough volume for the formation of highly dispersed ZnO NPs/SiO2 nanocomposites.26-28, 40 Fig 6 is presented to elucidate the mechanism of the formation of highly dispersed ZnO NPs.

Fig.6. Schematic illustration of ZnO NPs growth and size distribution with AC introduction or not

It can be considered that without AC addition, Zn species would more likely agglomerate in the SiO2 matrix and grow into larger ZnO NPs after calcination at 500oC. Adding small amounts of AC in the precursors to improve dispersion of the ZnO NPs in four ways. Firstly, some zinc species may enter into the pores of AC, which could reduce the stacking issue of Zn in the SiO2 matrix. Secondly, the Zn2+ can be anchored onto the surface of AC due to a strong coordination interaction between the enriched functional oxygen group and metal ions, which will lead to a very even dispersion. Thirdly, the limited pore size within AC can efficiently prevent the formed ZnO NPs from agglomeration during the crystal growth. Lastly, with combustion of AC at elevated temperatures, some cavities will be created, and the highly dispersed ZnO NPs are obtained in the created pore walls. However, when further amount of AC was added, the total amount of SiO2 in the system was reduced, the cavities formed by AC annealing will collapse, and the evenly dispersed ZnO NPs will be re-agglomerated, leading to the growth of ZnO NPs. All in all, it is of vital importance to optimally control the amount of AC added to achieve a high dispersion of ZnO NPs in the SiO2 matrix. 3.4 Desulfurization Performance and Discussions The dynamic tests were performed to investigate the performance of sorbents for H2S removal. 10


Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig 7 shows the breakthrough curves, the corresponding breakthrough sulfur capacities (BSCs) and the utilization of ZnO. As shown in Fig 7a, all the samples can effectively remove H2S, resulting in a low H2S concentration of below 0.1 ppmv at the outlet. The breakthrough time (BT), corresponding BSCs and ZnO utilizations of ZSC-0 are about 515 min, 104.2 mgS/g sorbent and 46.2% respectively. As for sorbents prepared with the aid of AC, the H2S removal capacity increases as the content of AC increases, up to 7%, reaching the maximum BSCs of 160.95 mgS/g sorbent and realizing a very high ZnO utilization of 69.02%. Further addition of AC decreases the capture ability of the sorbent. It is clear that adding an optimal amount of AC into the precursor (silicon sol) can significantly enhance the performance.

Fig.7. Breakthrough curves (a) and corresponding BSCs as well as ZnO utilization (b) of the ZSC-X

As it is known, the H2S removal by ZnO is typically via a non-catalytic gas-solid reaction. The gaseous H2S molecule firstly reaches the surface of the sorbent via pore diffusion and reacts with the exposed active ZnO sites. When the first layer of ZnO is exhausted, the sulfur species on the surface will permeate into the sublayers for further reactions, which is the so-called lattice diffusion. Therefore, the sorbent parameters related to surface reaction and lattice diffusion will significantly affect the H2S removal performance. It is well acknowledged that the specific surface area and porosity determine the accessibility of active sites on the surface, and thus it is of vital importance for the surface reaction, especially at room temperature.41 Fig.8 shows the surface area and sulfur capacity as a function of AC content (%). Clearly, the increasing the surface area via higher AC (up to 7%) loadings is accompanied with the increase of the BSCs. With further addition of AC, the surface area and BSCs will both decrease. Apparently, surface area is positively related to BSCs in this study, and is indeed a crucial factor to the performance of sorbent. Based on aforementioned characterization results, we can clearly know that the enlarged surface area originates from combustion of AC and smaller ZnO NPs. One hand, 11


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 19

this created small ZnO NPs through the assistance of AC, can create more active sites for H2S capture. On the other hand, the thickness of the potential sulfide shell on the outer layer of the zinc oxide grains were reduced, while the distance for lattice diffusion is also decreased. Therefore smaller ZnO NPs is favorable to achieve an higher overall rate of reaction, reaction extent and enhanced breakthrough capacity.42,43 Additionally, smaller grain sizes also implies larger surface areas. Another role that can’t be ignored is that small grain sizes, especially at the nanoscale, also produce more surface defects due to the unsaturated coordination of surface atoms, which result in an enhancement of H2S removal reactivity.44,45 Therefore, we suggest that the enriched surface defects (especially oxygen vacancies) in ZnO should also be considered in the contribution to the improved sulfidation behavior.

Fig. 8. Surface area and BSCs as a function of AC content

It is reported that the sulfidation reaction is normally confined to within one monolayer of ZnO at room temperature due to the low reactivity, which is also why low sulfur capacity is seen.17 However, the highest BSCs and ZnO utilization (160.95mg S/g sorbent, 69.02%) achieved in this study are far greater than that mentioned in the reference (21.4mg S/g sorbent, 5.42% 17). We actually believe that the reaction is not confined to the surface monolayer. We use the same method used in literature to calculate the average depth of reaction and the results are shown in Fig 9. It is calculated by assuming a stoichiometry of one molecule of H2S for one Zn2+O2- surface ion pair in the surface of ZnO. It also assumes that the ZnO surface contains 1×1019 Zn2+O2- ion pair m2-. Based on above assumptions, the average reaction depth in terms of surface monolayers can be obtained as follows:17, 46 LM +N O M

molecules of U1 + 1 number of X1 Y1Z 1

As per the calculation, the average depth of reaction for all the samples were greater than one layer. Specifically, the depth of reaction for ZSC-7 was up to 1.73 monolayers. These results strongly 12


Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

suggest that the reaction between ZnO and H2S is not confined to the surface, but bulk interaction, is also involved, which significantly influences the overall rate of reaction between oxides and H2S. Bulk sulfidation also involves surface reaction, anion exchange, and replenishment at the sulfide surface. As our previous study suggests, oxygen vacancies (VOs) in ZnO could efficiently promote the exchange rate of S2- and O2-, and thereby improve BSCs.47

Fig. 9. Average depth of reaction of ZSC-X (X=0, 5, 7, 10, 30)

Fig.10. PL spectra of the selected fresh samples

To validate the above mentioned suggestions, we perform the PL spectra at room temperature (shown in Fig.10) to characterize the variation of VOs in our formulated materials. Evidently, three peaks can be observed at around 446nm, 466nm, 547nm. The peaks at around 446nm, 547nm are ascribed to VOs while the peak at about 466 is related to the Zn vacancies.48-51 The concentrations of VOs, similar to BSCs, initially increased and then decreased with increasing amounts of AC. As mentioned above, small NPs not only contribute to a larger surface area, but also beneficial for producing VOs on the surface. In other words, the concentration of VOs in this study correlates positively to the surface area. However, according to Table 1, sorbent ZSC-10 should have lower VOs concentrations than that of ZSC-5. Surprisingly, based on the results of PL spectra, the VOs concentration of ZSC-10 is significantly larger than ZSC-5. This means grain size is not the sole contributor for VOs formation. It is supposed that AC may contribute to obtain more bulk VOs by producing zinc cations with lower oxidation states during AC combustion.52 In summary, the high concentration of VOs in samples after AC addition originated from smaller ZnO NPs and AC combustion, and VOs produced in ZnO results in significant contributions for the enhancement of BSCs after AC addition. 3.5 Comparison of the capture ability of ZSC-X sorbents with those in the literature In Table 2, we compare BSCs and ZnO utilization for our fabricated materials with other 13


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 19

ZnO-based sorbents that are found in literature. From Table 2, all the sorbents in the literature with low ZnO loadings show higher ZnO utilization, which is due to the good dispersion and developed porosity. Unfortunately, higher ZnO loadings result in low BSCs due to worsened dispersions and blocked or collapsed pores. Hence, it is necessary to increase the height of the bed to realize the high-precision during the industrialization, which significantly increases the industrial cost. Undoubtedly, three-dimensionally-ordered macroporous ZnO/SiO2 (3D-SZ) can meet the need of industry. Especially, 3D-SZ-73 can realize BSCs up to 170 mgS/g sorbent, attributing to the well-ordered and interconnected pore network and high surface areas. However, the 3DOM materials has a complex preparation procedure, which will be a challenge for industrialization. In this work, the fabricated ZSC-X sorbents show much better H2S trapping ability than other ZnO/SiO2 adsorbents. Especially, the BSCs of ZSC-7 (the ZnO loading is 59.3wt%) is up to 160.95mgS/g sorbent, and ZnO utilization is 60.9%. Both high ZnO loading and high utilization of these sorbents could better satisfy the need of commercialization. Table 2. Comparison of BSCs of ZnO-Based Sorbents with That in the Literature

adsorbent

Composition

ZnO/SiO2

100ppm

ZnO/SiO2

H2S in N2

ZnO/SiO2

10000ppm

Fe-Mn-ZnO/SiO2

H2S in H2

10Zn/MSU-1

50000ppm

30Zn/MSU-1

H2S in CH4

Z30/K6

800ppm

Z40/K6

H2S in N2

3D-SZ50-500

500ppmH2S

3D-SZ73-500

3%H2O in N2

ZSC-0

800ppm H2S

ZSC-7

3% H2O in N2

Cout(H2S) (mg/m3) ＜1.5

o

T ( C)

20

SiO2

Recommend Documents