Effects of Microwave Irradiation on the Structure of Zinc Oxide

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Effects of Microwave Irradiation on the Structure of Zinc Oxide Sorbents for High Temperature Coal Gas Desulfurization Yang Li, Yu Feng, Qiucheng Zhou, Mengmeng Wu, and Jie Mi* Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Shanxi, People’s Republic of China ABSTRACT: To investigate the effects of microwave irradiation on the structure properties of ZnO sorbents, fractal dimensions and desulfurization kinetics of sorbents prepared by microwave and conventional heating respectively have been studied in the present paper. The performances for desulfurization were tested in a fixed-bed reactor, and the sorbents were characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Fractal dimensions were calculated based on the Frenkel-Halsey-Hill theory, and desulfurization kinetics of sorbents was acquired by the equivalent grain model. The analysis date revealed that the fractal dimensions and sulfur capacity of sorbents prepared by microwave heating are 2.744 and 10.77%, while those of the sorbents prepared by conventional heating are 2.719 and 8.633%. Desulfurization kinetics studies presented that the apparent chemical reaction activation energy and the diffusion activation energy of sorbents heated by the microwave method are 6.756 kJ·mol−1 and 17.57 kJ·mol−1, and those of sorbents heated by the conventional method are 7.980 kJ·mol−1 and 23.09 kJ·mol−1. The consequences interpret that microwave irradiation leads ZnO sorbents to form an abundant three-dimensional pore structure and an irregular surface which facilitate gas transfer in the desulfurization process. Hence, the reaction between H2S gas and ZnO more readily proceeds for sorbents heated by microwave. extremely irregular surface, respectively.21,22 Fractal dimensions usually are obtained from small-angle X-ray scattering (SAXS),23 micrographs,24 small angle neutron scattering (SANS),25 and gas adsorption.26 In which, gas adsorption is widely used to analyze the fractal features of surface. The Frenkel-Halsey-Hill (FHH) model which depicts the multilayer adsorption coverage is an important approach of adsorption methods and is frequently used to measure the fractal dimension derived from its convenience and applicability.27 The FHH model which only requires one adsorption isotherm to calculate the fractal dimensions can be conducted easily.28 For the desulfurization process, which is a typical noncatalytic gas−solid reaction, the unreacted shrinking core model, the grain model, and the equivalent grain model have been engaged to discuss the apparent reaction kinetics.29−31 Different from the two former models, the effects of solid grain diffusion are emphasized in the equivalent grain model which is appropriate for porous material.31,32 In this work, the equivalent grain model was chosen to describe desulfurization kinetics of sorbents. In this paper, for the purpose of investigating the effects of a microwave on the structure properties of ZnO sorbents for H2S removal, the ZnO sorbents were prepared in conventional and microwave environments, respectively. The performances of sorbents for high temperature coal gas desulfurization were studied in a fixed bed, and the structure properties were also characterized. The fractal dimensions of sorbents were calculated by the FHH method, and apparent kinetics was researched according to the equivalent grain model.

1. INTRODUCTION Coal clean utilization plays a necessary and important role in the coal chemical industry for the situation that coal occupies a pivotal position in the energy field.1,2 Gasification is considered as an efficient and advanced technique; meanwhile it is an inevitable trend in coal utilization.3−5 However, the H2S consisting of the coal gas must be removed due to its negativity to the environment and corrosiveness to the equipment.6 Therefore, a desulfurization process is imperative in the utilization of coal.7,8 Coal gas desulfurization based on the gas−solid reaction between H2S and metal oxide is a procedure of mass and heat transfer.9,10 The efficiency of sorbents used for desulfurization largely depends on the pore structure and morphology which have significant influence on the distribution of active sites and gas diffusion.11,12 Traditionally, the internal pore is assumed to be symmetrical and smooth, and the effects of surface area and pore properties on the performance of sorbents are also usually investigated relatively separately which are contrary to the fact that the textures of sorbents are irregular and interrelated. Hence, it starves for a method to describe the heterogeneity of sorbents structure. The fractal theory was introduced by Mandelbrot13 and first applied to describe the complexity of porous media by Avnir and Pfeifer.14,15 With the development of research,16−18 the fractal theory is found to be a useful tool for characterizing the structure and morphology properties. Fractal dimension is an intrinsic characteristic of the surface, and it reflects the roughness of structure. For a fractal material, the factors such as surface area and pore size distribution are contributed to the fractal dimension simultaneously.19 Thus, the fractal dimension is a comprehensive modulus of the surface roughness and irregularity.20 The values of fractal dimension lie between 2 and 3, and two boundary values mean perfectly smooth surface and © XXXX American Chemical Society

Received: January 17, 2017 Revised: May 25, 2017 Published: June 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b00188 Energy Fuels XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION 2.1. Preparation of Sorbents. The ZnO sorbents were prepared by the solid-state method and calcined by microwave or conventional heating. (CH3COO)2Zn·2H2O and H2C2O4·2H2O were mixed sufficiently in a planetary ball mill. After having been dried for 10 h at 80 °C, the mixture was dehydrated at 170 °C for 4 h in a muffle furnace to gain the precursor ZnC2O4. Then, the ZnC2O4 was mixed with modified semicoke, clay binders, and 5 wt % of CeO2 in a planetary ball mill. The amount of ZnC2O4 was measured to produce a definite weight of ZnO which occupied 30 wt % of the sorbents. Afterward, the deionized water was added to the mixture, and the paste obtained was extruded into cylinders (Φ3 × 3 mm) before drying at 80 °C. The dried sorbents were calcined by microwave and conventional heating and designated as MC and CC, respectively. 2.2. Desulfurization Tests. The desulfurization performance of sorbents was tested in a fixed-bed quartz reactor. Simulated coal gas with a composition of H2S (2800 ppm), CO2 (12 vol %), H2 (27 vol %), CO (39 vol %), H2O (10 vol %), and N2 (balance) was used for desulfurization. The amount of sample for each test is 10 g. The desulfurization tests were carried out at 500 °C. The gas flow rate of inlet gas was 333.3 mL·min−1, and it was controlled by mass flowmeters. The concentration of H2S in outlet gas was analyzed by gas chromatography (GC-950, Shanghai Haixin chromatography Instrument) equipped with a flame photometry detector. The test was terminated when the concentration of H2S in outlet gas reaches 500 ppm and the sorbents were considered to be broken through. The sulfur capacity (SC) was calculated by the following equation: ⎛ g‐sulfur ⎞ M SC⎜ ⎟ = WHSV × s × Vg ⎝ 100 g ⎠

∫0

t

Figure 1. Breakthrough curves and sulfur capacities of sorbents calcined by microwave and conventional heating.

(C in − Cout)dt × 10−4 (1)

In eq 1, WHSV (L·h−1·g−1) is the weight hourly space velocity, Ms (32.06 g·mol−1) is the molar weight of S, Vg (24.5 L·mol−1) is the molar volume of H2S at 1 atm and 25 °C, Cin and Cout (ppm) stand for the concentration of H2S in inlet and outlet gases, and t (min) is the breakthrough time. 2.3. Characterization of the Sorbents. The structures of the sorbents before and after sulfurization were characterized by X-ray diffraction (XRD, D/max-2500, Rigaku) with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 10° to 80° at a speed of 8° per minute. Scanning electron microscopy (SEM, MAIA 3 LMH, Tescan) was used to observe the morphology of the samples. The X-ray photoelectron spectroscopy (XPS) spectra of the sorbents were analyzed by a photoelectron spectrometer (ESCALab220i-XL, VG Scientific) with Al Kα source at 300 W. The surface area and pore structure were investigated by nitrogen adsorption (Quadrasorb SI/MP-2, Quantachrome Instruments) at 77 K. All samples were outgassed in vacuum at 200 °C previously. The surface areas of sorbents were calculated by using the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was computed by the Barrett−Joyner−Halenda (BJH) method. 2.4. Calculation Method of the Fractal Dimension. The fractal dimensions (D) of sorbents were determined from analyses of nitrogen gas adsorption isotherms and calculated by the Frenkel-HalseyHill (FHH) method. The FHH equation is as follows33

θ ∼ [ln(P0/P)]−1/ m

Figure 2. XRD patterns of fresh and sulfurized sorbents calcined by microwave and conventional heating. where θ is the adsorption quantity, and P/P0 is the relative pressure; in eq 2 m = s /3 − D

(2)

where s is an FHH exponent which is considered as a parameter to characterize the shape of the adsorption isotherm in the multilayer region, and generally s is assumed equal to the theoretical value 3 in calculation. The fractal FHH equation is usually used as

Table 1. Conditions of Desulfurization Kinetics Experiment conditions

date

temperature/K sample weight/g gas composition H2S concentration/ppm

573, 673, 773, 873 0.5 H2S and N2 balance 1000

(3)

⎡ ⎛ ⎛ P ⎞⎞⎤ ⎛V ⎞ ln⎜ ⎟ = C + AD⎢ln⎜ln⎜ 0 ⎟⎟⎥ ⎣ ⎝ ⎝ P ⎠⎠⎦ ⎝ Vm ⎠

(4)

where C is a constant, V is the volume of adsorbed gas at equilibrium pressure, Vm is the adsorption volume in a monolayer, AD is an B


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Table 2. Surface Element Content and Energy Positions of O 1s Fitting Peaks binding energy of O 1s, eV

element content, %

sample

adsorbed oxygen

oxygen deficient

lattice oxygen

Zn, atomic %

O, atomic %

MC CC

532.4 532.6

531.0 531.4

529.5 530.3

8.32 3.20

37.41 22.46

Then the passing gas was converted to H2S gas balanced by N2. The detection of H2S concentration in the outlet gas was maintained 60 min. The conditions of the experiment are shown in Table 1. The elimination of external diffusion and internal diffusion has been considered by changing the flow rate varied from 300 mL/min to 600 mL/min and the particle size varied from 0.106 mm to 0.250 mm. For the equivalent grain model, the desulfurization reaction was segmented into two stages: chemical reaction control and intraparticle diffusion control.30−32 In the control region of the chemical reaction, the equivalent grain model was expressed as

Figure 3. Zn 2p XPS spectra of sorbents calcined by microwave and conventional heating.

G(x) = 1 − (1 − x)1/3

(8)

ρ0 R 0

(9)

ksC0

In the control region of intraparticle diffusion t = B1 + B2 F(x)

(5)

(10)

ρ0 D0

B1 =

is applied in the FHH equation and usually used for low pressure. While at high surface coverage, the interface is controlled by surface tension in the context of the capillary condensation regime, and AD is given by

AD = (D − 3)

(7)

A=

exponent which is dependent on fractal dimension D and the mechanism of adsorption, and P0 is the gas saturation pressure. During the beginning of the multilayer buildup, the interface is controlled by van der Waals force, and the equation AD = (D − 3)/3

t = AG(x)

(11)

(1 − ε)C0

B2 =

ρ0 R 02(1 − ε) 6DeC0

(12)

F(x) = 1 − 3(1 − x)2/3 + 2(1 − x)

(6) 21,26,34

(13)

x is the conversion rate of sorbents, and it was computed by the formula as follows:

and has a larger range of application. In this study, eq 6 is selected to calculate the fractal dimension. Therefore, according to the adsorption isotherm obtained from the N2 gas adsorption date, the value of AD can be acquired from the slope of plot ln(V/Vm) vs ln[ln(P0/P)] and further used to calculate the fractal dimension, D. 2.5. Desulfurization Kinetics Tests. The devices and detectors of H2S concentration applied in the kinetics experiment were the same as those used in the desulfurization test. The sample was heated to the target temperature in N2 atmosphere with a heating rate of 10 °C/min.

x=

ω − ω0 ω1 − ω0

(14)

In accordance with the Arrhenius equation ks = ks0 exp−Ea / RT

(15)

De = De0 exp−Ed / RT

(16)

Figure 4. O 1s XPS spectra of sorbents calcined by microwave and conventional heating. C


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Figure 5. SEM images of fresh and sulfurized sorbents calcined by microwave and conventional heating: a, MC; b, sulfurized MC; c, CC; d, sulfurized CC. complete reaction, ω (g) is the weight of sorbents at t (min) time, and G(x) and F(x) are characteristic functions of chemical reaction control and intraparticle diffusion control, respectively.

They can be converted to the form as follows:

E ln ks = ln ks0 − a RT ln De = ln De0 −

Ed RT

(17)

3. DISCUSSION 3.1. Desulfurization Tests. Figure 1 shows the breakthrough curves and sulfur capacities of all sorbents. As we can see, sample MC has a longer breakthrough time of 15.0 h and a larger sulfur capacity of 10.77%, while the performance of sample CC for H2S removal is 12.0 h breakthrough time and 8.633% sulfur capacity. The H2S concentration in the outlet gas of the CC sample elevated more quickly than the MC sample; this circumstance may arise from the fact that the active component loaded on the surface of the CC sample has been consumed and the sulfurization reaction made the pore structure be undermined and blocked. H2S gas was hard to contact with the ZnO inside the sorbent. 3.2. XRD. The XRD patterns of all sorbents before and after sulfurization are illustrated in Figure 2. In all XRD patterns of fresh sorbents, the characteristic peaks of ZnO were observed which proved that the active component ZnO was prepared by the solid-state method mentioned in this paper. The positions of ZnO characteristic peaks did not shift which means that the crystal structure has not transformed under the condition of microwave irradiation. In comparison with sample CC, sample MC occupies stronger ZnO peaks which indicate that the ZnO in sample MC has a higher crystallinity.35 Crystallite sizes of ZnO were calculated based on

(18)

Synthesizing eq 7 to 18, two new formulas

ln

ln

k C E 1 = ln s0 0 − a A ρ0 R 0 RT

(19)

E 6De0C0 1 = ln − d B2 RT ρ0 R 02(1 − ε)

(20)

were obtained. The slopes and intercepts of fitted curves ln(1/A) vs 1/T and ln(1/B2) vs 1/T can be utilized to acquire the Ea, Ed, ks0, and De0. In these equations, the reaction order for desulfurization is 1,32 ρ0 is the density of metal oxide in the sorbents (2.083 × 105 g·m−3), R0 is the particle radius (7.5 × 10−5 m), C0 is the mass concentration of H2S in the inlet gas (1.518 g·m−3), ε is the porosity according N2 adsorption (the MC sample is 0.0917, the CC sample is 0.0694), R is the gas constant (8.314 J·mol−1·K−1), T is the reaction temperature (K), Ea is the apparent chemical reaction activation energy (kJ·mol−1), Ed is the intraparticle diffusion activation energy (kJ·mol−1), ks0 is the chemical reaction rate constant (m·s−1), ks is the frequency factor of the chemical reaction rate constant (m·s−1), D0 is the correlation coefficient with B1, De is the diffusion coefficient (m2·s−1), De0 is the frequency factor of the diffusion coefficient (m2·s−1), ω0 (g) is the initial weight and ω1 (g) is the weight after the D


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Figure 6. FHH plots of fresh and sulfurized sorbents calcined by microwave and conventional heating.

Table 3. Fractal Dimensions and Structure Properties of Sorbents before and after Desulfurization sample

surface area, m2·g−1

pore volume, cm3·g−1

average pore diameter, nm

fractal dimension D

R2

MC sulfurized MC CC sulfurized CC

81.40 55.35 71.30 42.64

0.132 0.093 0.100 0.097

8.888 7.180 7.497 7.447

2.744 2.672 2.719 2.446

0.9944 0.9933 0.9933 0.9901

the Scherrer formula.36 The results show that the MC sample has lower crystallite size (14.34 nm) than the CC sample (18.91 nm). ZnO crystals of sample MC grew under an even heating environment which was provided by microwave irradiation. The homogeneous growth of crystalline grain restrained the aggregation, and consequently the active components have a more favorable dispersion on the support surface.37 The smaller size of the ZnO particle is beneficial for desulfurization on account of it bringing more occasions of contact between H2S and active components. After sulfurization, the XRD pattern of sample CC shows that ZnO still appearing in the sorbent means that the desulfurization was incomplete. It may be due to the fact that the generation of ZnS leads to the plugging of pores and collapse of the structure which develops more inaccessible pore space,38 so it is difficult for the H2S gas to enter the interior of the sorbent and react with internal ZnO. 3.3. XPS. The Zn 2p spectra of MC and CC sorbents were presented in Figure 3. The binding energy of Zn 2p3/2 for the MC sorbent is 1021.4 eV, lower than that of the CC sorbent whose binding energy is 1022.1 eV, which is assigned to Zn2+.39

Figure 4 shows that the O 1s peaks at 530.8 and 531.3 eV are corresponding to MC and CC sorbents, respectively. They were fitted to three distinguishing components, and the energy positions of each fitting peaks were listed in Table 2. The component with the highest binding energy is generally attributed to the species which have relatively loosely bound oxygen, and it adsorbed on the surface weakly, such as adsorbed H2O and O2.40 The secondary component is considered to be the O2− ions in oxygen deficient ZnO.40,41 The component with the lowest binding energy belongs to the O2− lattice oxygen in the ZnO crystal.42,43 The decrease of binding energy for sorbents heated under microwave irradiation indicates that sample MC has a higher electron density of Zn in the periphery of sorbents. Meanwhile, the lower binding energy has an advantage in removal of acid gas H2S in accordance with Lewis acid−base electron theory.44 Table 2 also exhibited the element concentrations on the surface of sorbents, as seen that sample MC has a conspicuously higher content of elements Zn and O than sample CC. Microwave irradiation offers a homogeneous heating field,45 and the crystallite can grow uniformly in consequence. E


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Energy & Fuels The microwave may also have an effect on the chemical bounds of ZnO and lead a migration of elements Zn and O simultaneously.43,46 These variations give rise to better dispersion of active components and higher electron density on the surface of sorbents which both conduce to the reaction between H2S and active components ZnO. 3.4. SEM. The SEM images of fresh and sulfurized sorbents are shown in Figure 5. As shown in Figure 5, compared with sample CC, sample MC exhibited a relatively looser structure and more structural channels which are beneficial to the mass transfer and the dispersion of the active components on the sorbent surface. After the sulfurization process, the agglomerate phenomena of reaction products were observed. The ZnS heaped up on the surface of support and led to the plugging of the pore structure especially for the CC sample, and the surface area decreased correspondingly. These results coincided with the structure analysis. 3.5. Fractal Dimensions. To investigate the effects of microwave irradiation on the structure of MC and CC sorbents, N2 adsorption was employed to characterize the structural properties of sorbents, and the corresponding fractal dimensions were calculated by means of FHH equations. The fitted curves of ln(V/Vm) vs ln[ln(P0/P)] deduced from N2 gas adsorption isotherms were illustrated in Figure 6, and the computed results were tabulated in Table 3. It was found that sample MC sorbents occupied a higher fractal dimension than sample CC (2.744 vs 2.719). This implies that the surface of sorbents heated by microwave consists of a more irregular structure. Microwave heating guided the decomposition of precursor and the formation of structure equably. As a consequence, the pore structure spread evenly in the body of sorbents. The three-dimensional structure formed in the calcining process encourages the diffusion and adsorption of H2S,6,47 and some research demonstrated that the activation energy may rest with the surface fractal dimensions to some extent.48 The superiorities of sorbents prepared in microwave condition arouse a more active reactivity between ZnO active components and H2S gas and then further develop a higher sulfur capacity of sorbents for desulfurization. The fractal dimensions of sample CC declined remarkably after the sulfurization process contrasted with that of sample MC. The violent reduction of the fractal dimension clarified that the surface roughness was reduced because the framework of the structure collapsed seriously, and the pore canals were blocked. The recession of irregularity means that the pore structure of CC sorbents was feeble; this may be in charge of the incompletion of the sulfurization reaction. 3.6. Desulfurization Kinetics. To eliminate the influence of external diffusion and internal diffusion, the ZnO conversion rates at various gas flow rates and particle sizes were tested. The results are shown in Figures 7 and 8. The conversion rate of ZnO increased with the rising of the gas flow rate and diminution of particle size, and it improved rarely when the gas flow rate and particle size attained to 500 mL·min−1 and 0.125−0.150 mm, respectively. The external diffusion and internal diffusion were considered to be eliminated, and the following tests were carried out at these conditions. The conversion rates of sorbents at different reaction temperature were detected, and the results of MC and CC samples were described in Figures 9 and 10. The conversion rates were promoted with the rise of reaction temperature. An inflection was observed at about 30 min.

Figure 7. Conversion rates of ZnO at conditions of various gas flow rates.

Figure 8. Conversion rates of ZnO at conditions of various particle sizes.

Figure 9. Conversion rates of the MC sample at different reaction temperatures.

In the initial desulfurization phase, the structure of the sorbent was loose, and the H2S gas was able to thread the layers of the sorbent easily and to make contact with the metal oxide; F


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The linear relationships of G(x) and F(x) with t for MC and CC samples and the fitted curves of ln(1/A) and ln(1/B2) with 1/T were exhibited in Figure 11. The fitted curves exhibited a good linear relationship with the date which demonstrates that the equivalent grain model is appropriate for the kinetics of desulfurization. The chemical reaction activation energy and the intraparticle diffusion activation energy of MC and CC samples were calculated based on the slopes, and ks0 and De0 were acquired from the intercepts of fitting curves. The results were summarized in Table 4. The results show that the apparent chemical reaction activation energy of the MC sample is slightly below that of the CC sample. Particularly, compared with the intraparticle diffusion activation energy of the CC sample, that of the MC sample has a remarkable reduction. The results indicate that H2S gas can thread through the interval of layers and make contact with the active component easier for sorbents heated by microwave. At the same time, the results imply that the effects of microwave irradiation on preparation of ZnO sorbents mainly focus on modification of the pore structure.

Figure 10. Conversion rates of the CC sample at different reaction temperatures.

the rate of desulfurization was mainly controlled by the chemical reaction. As the reaction progresses, gas channels were jammed, and the product layers became thick; the resistance from intraparticle diffusion grew as well. During this period, the rate of desulfurization was affected by gas diffusion.32

4. CONCLUSIONS The ZnO sorbents were prepared through microwave and conventional methods, and their performance for high temperature coal gas desulfurization was tested. The sulfur capacities of sorbents heated by microwave and conventional methods are

Figure 11. Relationships of ln(1/A) and ln(1/B2) with 1/T for MC and CC samples. G


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Table 4. Results of the Chemical Reaction Activation Energy and the Intraparticle Diffusion Activation Energy of Sorbents sample MC

CC

ks, m s−1

T, K 573 673 773 873 573 673 773 873

2.470 3.089 3.602 4.014 2.264 2.882 3.499 4.014

× × × × × × × ×

ks0, m s−1 −2

10 10−2 10−2 10−2 10−2 10−2 10−2 10−1

Ea, kJ mol−1

0.1025

6.756

0.1206

7.980

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mengmeng Wu: 0000-0001-7805-9385 Jie Mi: 0000-0002-9374-2307 Notes

The authors declare no competing financial interest.

■ ■

8.179 1.285 1.986 2.921 4.789 9.577 1.676 2.514

× × × × × × × ×

−8

10 10−7 10−7 10−7 10−8 10−8 10−7 10−7

De0, m2 s−1 7.865 × 10

−6

6.035 × 10−6

Ed, kJ mol−1 17.57

23.09

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10.77% and 8.633%, respectively. The XRD date shows that the microwave results in a smaller size of crystallite. The XPS analysis indicates that the microwave irradiation reduces the binding energy of Zn 2p3/2 from 1022.1 to 1021.4 eV and that of O 1s from 531.3 to 530.8 eV. Simultaneously, the elements concentrations of Zn and O on the surface increase nearly twice the amount after microwave heating. These factors are in favor of the adsorption of the H2S. The fractal dimensions obtained via FHH theory expressed that the sorbents heated by microwave possess higher fractal dimension, which means that the structure of microwave heating sorbents is rough and irregular compared with the sorbents calcined by the conventional method. The rapid decrease of fractal dimensions after desulfurization for sorbents prepared by conventional heating explains that the sorbents prepared by the conventional approach have a fragile structure which was destroyed in the process of desulfurization. Comparatively, the texture of sorbents heated under the microwave condition is relatively stable. The microwave irradiation provides a homogeneous heating field which improves the formation environment of the pore structure. The excellent properties of the surface structure facilitate the mass and heat transfer process in desulfurization. The consequences of desulfurization kinetics show that the apparent chemical reaction activation energy of MC and CC samples is 6.756 kJ·mol−1 and 7.980 kJ·mol−1, and the intraparticle diffusion activation energy is 17.57 kJ·mol−1 and 23.09 kJ·mol−1, respectively. The results suggest that less energy is needed for H2S gas to diffuse in the layers of sorbents, and the reaction between H2S and ZnO is easier to proceed for sorbents prepared by microwave. These characteristics make the sorbents heated by microwave have an outstanding ability of desulfurization.

■

De, m2 s−1

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51272170 and 21506143). REFERENCES

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