SiC catalyst

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In this work, the properties of the CH4-CO2 reforming reaction over the Fe/SiC catalyst during the whole process were studied under microwave irradiation and ...

Science of the Total Environment 639 (2018) 1148–1155

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Process of CH4-CO2 reforming over Fe/SiC catalyst under microwave irradiation Fusen Zhang a,b, Zhanlong Song a,b,⁎, Junzhi Zhu a,b, Li Liu c, Jing Sun a,b, Xiqiang Zhao a,b, Yanpeng Mao a,b, Wenlong Wang a,b a b c

Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan 250061, China National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan 250061, China School of Information Science and Engineering, Shandong Normal University, Jinan 250014, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Microwave dry reforming process was divided into three reaction stages. • Conversion and selectivity in the slow reaction stage were both N95%. • 50 h test showed that the catalyst activity did not reduce significantly. • Fe-based catalyst has good catalytic activity under microwave irradiation.

a r t i c l e

i n f o

Article history: Received 12 January 2018 Received in revised form 20 April 2018 Accepted 26 April 2018 Available online xxxx Keywords: Microwave Fe-based catalyst SiC foamed ceramics CH4-CO2 reforming

a b s t r a c t In this work, the properties of the CH4-CO2 reforming reaction over the Fe/SiC catalyst during the whole process were studied under microwave irradiation and the reaction process was analyzed by mass spectrometry and Fourier transfer infrared spectrometry in real time. The effects of microwave power on the gas composition, conversion of reactants, and selectivity of products in the reaction were investigated. It was found that the microwave dry reforming reaction can be divided into a rapid reaction stage, slow reaction stage, and reaction equilibrium stage. The conversion of reactants and selectivity of products in the slow reaction stage were both higher than 95% under 90 W/g. In the long-term (~50 h) stability test, a combination of SEM, XRD, BET, and TG analyses found that the catalyst activity did not reduce significantly and the amount of carbon deposits (which was mainly Cγ) was negligible (~0.78 wt%). The results indicate that the cheap Fe-based catalyst has good catalytic activity and stability under microwave irradiation and hence has a promising application. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Syngas has been widely used as an important chemical raw material. CO2 dry reforming of CH4 is considered one of the most effective ways to produce syngas. Because syngas a produced with an adequate H2/CO ratio for the production of liquid hydrocarbons. Besides, the process is ⁎ Corresponding author at: Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan 250061, China. E-mail address: [email protected] (Z. Song).

https://doi.org/10.1016/j.scitotenv.2018.04.364 0048-9697/© 2018 Elsevier B.V. All rights reserved.

recycling of the two important greenhouse gases, which is important for reducing environmental pollution and carbon emissions. Noble metal catalysts are mostly used for reforming reactions (Mei et al., 2014). Noble metal catalysts have higher catalytic activity and good stability, but the scarcity and high cost exclude them in industrial scale use. Non-noble metals (Ni, Co, Fe, and so on) have become the focus of research because of their low cost and easy availability (Fouskas et al., 2014; Selvarajah et al., 2016; Bradford and Vannice, 1999; Tokunaga and Ogasawara, 1989; Zahra et al., 2014). Bradford and Vannice (1999) found that the catalytic activities of noble metals

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carbon deposition occurred during the microwave reforming reaction. The conversion of CH4 and CO2 reached 85% in the microwave field, while the reactants conversion was only approximately 20% during conventional heating.

(Rh, Ru, etc.) are the best; however, Ni, Co, and Fe also have catalytic activities. The order of catalytic activity is Ni N Co ≫ Cu ≫ Fe. Tokunaga and Ogasawara (1989) also found that the Fe-based catalyst was the worst and the reactant conversions were not N20% under the same reaction conditions. Fe-based catalysts have lower catalytic activity and are easy to deactivate. It is generally not recommended as the active component of CH4-CO2 reforming catalysts. Therefore, there are few studies on Fe-based catalysts. It is important for the reforming reaction to be able to improve the activity and stability of Fe-based catalysts by a method that can maximize the advantages (cheap, easy to obtain, and non-toxic) of Fe-based catalysts. Normally, the dry reforming reaction takes place at high temperatures. The high temperature process requires a longer start-up time (to heat the catalyst bed) and efficient heat exchangers (to achieve higher energy efficiency). Therefore, a reaction process with a start-up speed is required. Microwave heating (Jones et al., 2002) is a unique heating method that has the advantages of selective heating, volumetric heating, and fast heating, et al.

The mass spectrometer can detect the change in gas composition in real time and the response is fast and accuracy. After the calibration, the components can be quantitatively analyzed so that the concentration change of each component can be measured more accurately, providing a quick, convenient, and accurate method for subsequent analysis and calculation. Fourier transform infrared (FTIR) spectroscopy is used to determine the molecular composition and changes in the reaction in real time and helps analyze reaction process. Mass spectrometry and Fourier transform infrared spectrometry can be used for real-time monitoring and comparative analysis of the CH4-CO2 reforming reaction and to provide technical support for optimizing the process. Based on the above analysis, the Fe catalysts supported on SiC foam ceramics were used to the CH4-CO2 reforming reaction under microwave irradiation. The coupling effect between the microwaves and catalyst was used to promote the reforming reaction. The reaction process was analyzed by an on-line mass spectrometer and Fourier transform infrared spectrometer to discuss the possible reactions at different stages and to develop improvement measures for the reaction. Meanwhile, the catalysts were characterized by scanning electron microscope (SEM) coupled with energy dispersive spectrometer (EDS), BrunauerEmmett-Teller (BET), X-Ray Diffraction (XRD), and Thermogravimetric Analysis (TGA) to determine their properties and the feasibility of using cheaper Fe-based catalysts for the reforming reaction.

1) As an excellent microwave-absorbing material with high heat conductivity (Xu et al., 2000; Zhang et al., 2013), SiC foamed ceramic can achieve a volumetric heating of the catalyst and realize the even temperature distribution, which is conducive to avoiding sintering and improving reaction performance. 2) At the same time, the induced electrical charge caused by microwave remains at the surface of the materials (such as SiC, Fe). The kinetic energy of some electrons may increase enabling them to jump out of the materials, which is perceived as sparks or electric arcs formation. But, at a microscopic level, these hot spots are actually plasmas with free radicals, which can enhance the reforming reaction (Fidalgo et al., 2008; Durka et al., 2011; Wang et al., 2016; Chen et al., 2012; Wang et al., 2017; Menéndez et al., 2010). 3) The carbon deposited during the reaction is a good microwave absorbing material which can form hot spots (Fidalgo et al., 2008; Durka et al., 2011; Menéndez et al., 2010; Li, 2012) in the microwave field and causing them to participate in the reaction process and realize rapid directional elimination of the carbon deposits. 4) The unique effect of microwave (non-thermal effects) is considered to reduce the reaction temperature and the activation energy, which has been proven experimentally (Li, 2012; Sun et al., 2012). Xu et al. (2000) studied the effects of conventional heating and microwave heating (600 W) on the reforming reaction over an 8 wt% NiOfoam catalyst. It was found that the discharge and hot-spot of the ceramic foam enhanced the reforming reaction and no significant

2. Material and methods 2.1. Catalyst preparation The catalyst used in the experiments was Fe/SiC. It was prepared as follows: 78 g SiC powder, 20 g Al2O3 powder, and 2 g MgO powder were mixed together. Then, 5 wt% polyacrylamide (dispersing agent) and 1 wt% polyvinyl alcohol (low temperature binder) were uniformly added in the mixture. Next, 1 wt% carboxymethyl cellulose (a stabilizer) and 5 wt% silica sol (high temperature adhesive) were added to a certain amount of deionized water to make a solution (Zhang et al., 2015). The mixture powder was mixed with the solution and stirred until the ceramic slurry had good rheological properties. The polyurethane organic foam was immersed in the slurry to adsorb the slurry and to ensure that the ceramic slurry could be adsorbed evenly on

6

11 T

I-13

FTIR

P-44

5

`

1

4

9

P-47

LC-D

8 1

2

3

10

MS

7 Fig. 1. Experimental system: (1) CH4; (2) CO2; (3) Ar; (4) mass flow meter; (5) gas mixing device; (6) thermocouple; (7) microwave oven; (8) quartz reactor; (9) catalyst bed; (10) MS; (11) FTIR.

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Table 1 Possible reactions in the CO2-CH4 reforming reaction system. Number

Reaction

ΔH298(kJ/mol)

1 2 3 4 5

CH4 + CO2 = 2H2 + 2CO CO2 + H2 = CO + H2O CH4 = C + 2H2 2CO = C + CO2 C + CO2 = 2CO

247.1 41.2 74.6 −172.5 172.5

hole walls and the foam voids were clearly visible. The formed bisque was dried at room temperature (25 °C) for 24 h and dried at 120 °C for 4 h, which made the water slowly seeped out. Then it was moved into a calciner and the temperature was elevated from room temperature to 500 °C in 5 h, then to 1200 °C in 3 h, and maintained for a period before removing the catalyst carriers after cooling (Zhao et al., 2014; Shang et al., 2016). To impregnate metal onto the carrier (Schwarz et al., 1995), an aqueous solution of the metal precursor (Fe(NO3)2·9H2O) was used to support Fe on the carriers. The carriers were soaked in the solution for 12 h at room temperature and dried in an oven at 120 °C for 12 h in air. Then, they were calcined in a muffle furnace at 500 °C for 5 h. Before the reforming reaction, the catalysts were reduced at 550 °C for 1 h in 10% H2 + 90% N2. Finally, Fe/SiC catalysts were prepared. 2.2. Experimental system and process The experimental system consisted of a quartz reactor, gas supply system, microwave oven, and measurement units (including Fourier transform infrared (FTIR) spectroscopy analyzer (Nicolet, 20SX) and mass spectrometer (MS) (AMETEK Dycor LC-D200M)), as shown in Fig. 1.

Before the reaction, a 10 g of the catalyst was placed on a multiorifice plate, which was 4 cm above the bottom of the quartz reactor with a diameter of 5.2 cm diameter. The air in the reactor was evacuated using argon for 20 min. The reaction gas (CH4:CO2:Ar = 1:1:2, space velocity 200 h−1) was controlled using the mass flow meter and the gas was thoroughly mixed and then pumped into the quartz reactor. To quantify the results, we defined the specific microwave power (SMP) as the ratio of the microwave power to the quality of the catalyst. After the reaction, the gas entered the MS, which was calibrated based on the change in the concentration of each component; the R2 of fitting curve was not b0.9935 (Zhu, 2017) and FTIR was used for analysis to study the characteristics of the reaction process. The conversion of CH4 and CO2 (XCO2 and XCH4, %) and selectivity of H2 and CO (RH2 and RCO, %) were calculated using the following equations (Li, 2012): n    o  100% X CO2 ¼ 1− φCO2;out  φAr;in = φAr;out  φCO2;in

ð1Þ

n    o  100% X CH4 ¼ 1− φCH4;out  φAr;in = φAr;out  φCH4;in

ð2Þ

n  o  100% RH2 ¼ φH2 = 2 φCH4;in  φAr;out =φAr;in −φCH4;out

ð3Þ

  RCO ¼ φCO =f φCH4;in  φAr;out =φAr;in −φCH4;out   þ φCO2;in  φAr;out =φAr;in −φCO2;out g  100%

Here, φi, in and φi, out respectively represent the volume fractions of the inlet and outlet gas components.

40

40 φ/%

60

φ/%

60

CO2 CH4

20

1

2

CO2

20

Ar

3

CH4 Ar H2

3

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1

H2 CO

CO

0

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400 600 Time/s

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(a)

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60 CH4 Ar CO2

CH4

Ar

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φ/%

φ/%

CO2 H2

H2 CO

CO

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0

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(b)

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400

600 Time/s

(c)

800

1000

1200

ð4Þ

0

200

400

600 Time/s

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1200

(d)

Fig. 2. Effect of power on CH4-CO2 reforming reaction process. ((a) SMP = 90 W/g; (b) SMP = 72 W/g; (c) SMP = 45 W/g; (d) SMP = 27 W/g).

F. Zhang et al. / Science of the Total Environment 639 (2018) 1148–1155

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100

1

0

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600 800 Time/s

75 X/%

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RCO RH2

RH2 and RCO/%

X/%

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XCH4 XCO2

XCH4 XCO2

75

RH2 RCO

50

50 2

1

25

25

0 1000 1200

0

3

RH2 and RCO/%

100

100 75

25

0

200

400

(a)

600 800 Time/s

0 1000 1200

(b)

Fig. 3. Effect of microwave power on conversion and selectivity of reforming reaction over Fe/SiC catalyst. ((a) SMP = 90 W/g; (b) SMP = 72 W/g).

2.3. Characterization of catalyst

3. Results and discussion

X-ray diffractometer D8 Advance of Bruker AXS were used for the phase analysis of the catalysts. A Cu target Kα (λ = 0.15418 nm) was the emission source, the tube voltage and tube current were 40 kV and 40 mA, respectively, and data were collected at 20 °C–90 °C at an angular scanning rate of 0.02°/step. An Autosorb-iQ automated surface area and pore size analyzer (Quantachrome Instruments) was used to carry out N 2 adsorption-desorption experiments at −196 °C to measure the specific surface area of the catalyst and pore volume before and after reaction. Before the nitrogen absorption-desorption test, the calcined catalyst was pretreated in vacuum for 12 h at 120 °C. Finally, the specific surface area of the catalyst was calculated according to the BET model. The catalysts were characterized using SEM (Zeiss, suprass) and EDS (Oxford, INCAx-act) before and after the reaction. Changes in components of the catalysts were analyzed before and after reaction. TGA (METTLER, TGA/DSC1/1600HT) was carried out up to 800 °C at a heating rate of 30 °C/min in air. The type of carbon deposition was identified according to the oxidation temperatures of different types of carbon deposition. Quantitative analysis of coke deposition was also conducted.

The CH4-CO2 reforming process is a very complex reaction. The possible reactions (Fan et al., 2009; Meric et al., 2017; Fidalgo and Menéndez, 2013) are given in Table 1. Reaction 1 is the main reaction and is favored by high temperatures and low pressures. The process selectivity is affected by the reverse water gas shift reaction (RWGS, reaction 2) (Gadalla and Bower, 1988), which makes the selectivity of CO is higher than H2 and increases with the increase of the reaction temperature (Bradford and Vannice, 1999). At temperatures lower than 973 K, carbon deposits may be formed from the Boudouard reaction (reaction 4) and it will be inhibited at high temperatures. Therefore, decomposition of CH4 (reaction 3) is the main reaction for the formation of carbon deposits at high temperatures. Carbon deposition can be eliminated by the CO2 gasification of carbon reaction (reactions 5) at high temperatures. Therefore, the most common reactions at high temperatures and atmospheric pressure are the CH4-CO2 reforming reaction, CH4 pyrolysis reaction, and the CO2 gasification of carbon reaction. 3.1. Effect of microwave power on reforming reaction CO2 dry reforming of CH4 to syngas is an endothermic process hence temperature is an important factor influencing the reforming effect.

a

h

b

c

i

f g

de 1 min

5 min

4200

(A)

3600

3000 2400 1800 Wavenumber/(cm-1)

1200

600

(B)

Fig. 4. FTIR diagram of the renormalization reaction. (SMP = 90 W/g; a: C\ \H stretching vibration peak of alkane; b: C_O vibration peak of CO2; c: absorption peak of CO; d: C_C stretching vibration peak; e: C\ \H flexural vibration peak of alkane; f: C\ \O stretching vibration peak of alcohol; g: O\ \H outside bending vibration peaks; h: O\ \H stretching vibration peak; i: C\ \H stretching vibration peak of olefin).

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Table 2 Composition of different points of fresh catalyst in Fig. 6.

1.2

Item

C

O

Mg

Al

Si

Ca

Fe

Total

1

1 2

0.46 22.16

1.85 35.29

0 2.88

3.36 2.93

10.84 33.87

0.73 0

82.77 0.22

100 100

0.8

60 CH4 CO2

40

20

RH2/RCO 0

20 40 Time on stream(h)

RH2/RCO

X/%

80

0.4

0.0

Fig. 5. Long-term performance of Fe/SiC.

Microwave power can change the temperature of the bed, strength of the discharge (Chen et al., 2012; Wang et al., 2017), and further influence the synergy between the microwave and the catalyst. Therefore, it is of great significance to study the influence of microwave power on the reforming reaction. 3.1.1. Effect of SMP on the gas components Fig. 2 shows the trend of the main gas concentration in the reaction system with varying microwave power. As can be seen from Fig. 2, under high microwave power, the time to reach the stable conversion was less and extremum of the conversion was higher. When the SMP was 27 W/g, the corresponding curves of CH4 and CO2 were basically unchanged, which indicates that no reforming reaction occurred. It was mainly because the electromagnetic field in the microwave cavity was weak and the catalyst bed temperature was low leading to few activated molecules in the reaction system. When the SMP was 45 W/g, the catalyst bed temperature rose, which led to the reforming reaction; the CO and H2 in the component increased gradually and reached the maximum value (about 20% each) at about 10 min. And the reactants conversions were about 25%. When SMP was 72 W/g or 90 W/g, the reforming process could be divided into a rapid reaction stage, slow reaction stage, and reaction equilibrium stage (Fig. 2 (a, b) and Fig. 4 (A)), but the reaction intensity and reaction times of the different reaction stages were varied. When the SMP was 90 W/g, the reactants conversions reached their maximum at 330 s and the concentration of the reactants decreased to about 0.5%. When the SMP was 72 W/g, the rapid reaction stage was delayed by about 100 s, the concentration of reactants reduced to a minimum at 600 s. This is mainly because the maximum bed temperature and the

Fig. 6. SEM of fresh catalyst.

discharge intensity (Chen et al., 2012; Wang et al., 2017) were decreased with the decrease of the microwave irradiation, and caused the delay of the reforming process and the decrease of the extremum conversion. 3.1.2. Effect of SMP on conversion and selectivity Fig. 3 shows the conversions of CH4 and CO2 and the selectivity curves of H2 and CO when the SMP was 90 W/g and 72 W/g. 1) Rapid reaction stage: SiC foamed ceramic is a strong microwaveabsorbing material. The catalyst bed temperature increased rapidly in the microwave field. Under the condition of discharge and high temperatures, CH4 and CO2 reacted violently in a short time. As seen from Fig. 2 (a), when the SMP was 90 W/g, the reactant concentration decreased rapidly from approximately 25% to 1% in 120 s and the conversions of reactants exceeded 90%. When the SMP was 72 W/g, the reactant concentration dropped to approximately 6%, the conversion was approximately 80%, and the product selectivity exceeded 80% at 200 s. The production of C2 compounds was evident in Fig. 4 (B); the reaction produced small amounts of unsaturated olefins (d, i peaks), saturated alkanes (a, e peaks), and alcohols (h, f peaks) (Pawar et al., 2015). In the CH4-CO2 reforming reaction, CH4 adsorption and dissociation produced large amounts of H+ + and CH+ x and some CHx reacted to form C2 compounds, which promoted CH4 conversion (Erdohelyi et al., 1993; Niu et al., 2016). Therefore, the reaction time of this stage can be reduced by increasing the microwave power and the catalyst bed temperature under allowable conditions. 2) Slow reaction stage: The degree of the reforming reaction continued to increase, but the growth of the conversions slowed down. When the SMP was 90 W/g, the conversions increased to a maximum value of approximately 97% in 330 s. It reached an extreme conversion of approximately 90% at 600 s with an SMP of 72 W/g. The reactants conversions and products selectivity continued to increase, but the growth of the conversions was lower than that in the rapid reaction stage. The main reasons were as follows: First, the temperature of the catalyst bed increased continuously, favoring the reaction. In this stage, the catalyst bed reached a maximum temperature of 810 °C and the high temperature was favorable for the reforming

Fig. 7. SEM image of catalyst after 20 h reaction.

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800

100

C

O

Na

Mg

Al

Si

Ca

Fe

Total

1 2

4.49 21.2

40.18 40.29

1.08 0

6.29 0

14.16 0.82

26.17 37.34

0.81 0

5.84 0.4

100 100

reaction. In Fig. 4 (B) on 5 min line, the peak areas of CH4 and CO2 were significantly smaller than that on 1 min line, indicating that the intensity of reformation was stronger, in agreement with Fig. 3 (a). Second, the side reaction occurred. Water vapor (Fig. 4 (B), g peak) and CO (Fig. 2 (a) and Fig. 3 (a) clearly showed that the yield and selectivity of CO gradually exceeded that of H2 in this stage) were increase and it indicated that the RWGS reaction (Bradford and Vannice, 1999) occurred which would affect the reforming reaction. Third, the amount of C2 compounds reduced. It can be seen from Fig. 4 (B) that the peak areas of unsaturated olefins (d, i peak) and saturated alkanes (a, e peak) at 5 min line were smaller than those at 1 min line. This means that the amount of C2 compounds reduced and the yields of CO and H2 increased further. Therefore, the heat storage capacity of the catalyst should be increased to maintain a higher reaction temperature and extend the duration of this stage, thus maintaining a high conversion and selectivity. 3) Reaction equilibrium stage: Reactant conversion and product selectivity decreased before eventually stabilizing. When the SMP was 90 W/g, the conversions of reactants reduced to approximately 88%, while the final stable conversions were approximately 85% when the SMP was 72 W/g. First, the heat dissipation to the surrounding environment of the reactor caused the catalyst temperature to reduce to 770 °C, thereby reducing the reforming reaction intensity. Second, because the SiC carrier was acidic and it was not favorable for the adsorption of CO2. MgO was added to improve the overall alkalinity of the catalyst to promote the adsorption of CO2 to eliminate carbon deposition (Hu et al., 2011). But CH4 pyrolysis reaction was occurred easily (Niu et al., 2016; Polo-Garzon et al., 2016). With the continuation of the reforming process, carbon deposition increased gradually. The type of carbon accumulated by the original Cα and Cβ converted to inert Cγ, thus reducing the catalyst activity (Pino et al., 2014; Bartholomew, 2001) and causing decreases in reactants conversions and product selectivity. In addition, when the SMP was 90 W/g, the selectivity of CO was higher than that of H2, while the selectivity of CO and H2 was mostly identical under the SMP of 70 W/g. This is mainly because the catalyst bed temperature was higher at a high microwave power and the intense

0.78wt% 600

TG / %

Item

400

99

Tem perature/ o C

Table 3 Compositions of different points of catalyst after 20 h reaction in Fig. 7.

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200

98 0

300

600

900

1200

1500

0 1800

Time/s Fig. 9. TGA curve of catalyst after 50 h reaction.

RWGS reaction (Bradford and Vannice, 1999) was led to a higher selectivity of CO than H2. 3.1.3. Long-term performance test of catalysts Fig. 5 shows the stability of the catalyst in the long-term reaction. The catalyst still had high catalytic activity after 50 h of the reforming reaction and the conversions of reactants remained at 85%. Because silicon carbide is a hydrophobic material, it can remove the water adsorbed on the surface (Aw et al., 2015) during the reforming reaction and inhibit the RWGS reaction. Meanwhile, carbon deposits become hot spots participating in the reforming reaction under microwave irradiation to reduce the deposition of carbon so it can maintain high conversions and product selectivity. SiC has a high thermal conductivity that makes the temperature distribution more uniform and can improve the sintering resistance of the catalyst (Zhang et al., 2013). Therefore, no active component sintering was found in the XRD and SEM diagrams. 3.2. Catalyst characterization 3.2.1. SEM-EDS analysis Fig. 6 shows the SEM image of the fresh catalyst. There were two different substances on the surface of the catalyst and analyzed by EDS. The results are given in Table 2. As shown in Fig. 6 and Table 2, on the surface of the catalyst showed dark gray matter (Point 2, the main components were Si, C, and O, which were the components of catalyst carrier) was the catalyst carrier

Fig. 8. SEM image of catalyst after 50 h of reaction.

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ƽ凬

Intensity

Spent Catalyst Fresh Catalyst

Ʒ ƽƾ

SiC SiO2 ƾ: Fe2O3 ƹ: Fe3C ƿ:Fe Ʒ凬

Ʒ

Ʒ ƹ

ƿ ƽ ƹ

Ʒ

ƾ

Ʒ ƽ

Ʒ

Ʒ

Ʒ

ƹ ƿƷ

ƿ

ƾƿ ƽ

Ʒ

1

SiC

100

2

%

SiC

50 0 20

40

60

80

2theta Fig. 10. XRD spectra of fresh catalyst and the spent catalyst after 50 h reaction.

and the bright part (Point 1, the main component was Fe) was the active component of the catalyst, which was distributed evenly on the surface of the catalyst carrier. It should also be emphasized that some forms of carbon result in loss of catalytic activity and some do not. In the reaction process, three different forms of carbon deposition (Cα, Cβ and Cγ) will be produced. Cα is an adsorbed atomic carbon contained CHx fragments (York et al., 2007) and can react to form Cβ. Cα and Cβ have a higher reactivity with CO2 and will participate in the reaction. The amorphous forms of carbons formed at low temperatures (e.g. Cα and Cβ) are converted to graphitic forms (Cγ) in the shape of vermicular filaments at high temperatures (Bartholomew, 2001). The researcher (Xu et al., 2011) found that carbon filaments may be multi-wall carbon nanotubes (MWCNTs), which has high graphitization and is not easy to eliminate. So Cγ usually refers to the inert carbon. The inert carbon gradually accumulates and surrounds the active components, which makes the catalyst deactivated. Fig. 7 shows SEM images of the catalyst after 20 h of the reforming reaction. There were some filamentous carbon deposits in surface of the catalyst, identified as Cγ (Bartholomew, 2001). Local amplification of the carbon deposits clearly showed that the filaments were surrounded by fine filamentous carbon deposits which were developing. EDS analysis was carried out on the fully developed carbon deposits (Point 1) and the developing carbon deposits (Point 2); the results were given in Table 3. It can be seen from Fig. 7 and Table 3 that there was a small amount of the active component, Fe, but it was not clearly visible in the SEM

0.0024

Fresh Catalyst Spent Catalyst

0.0012

dv(cm

2

-1 · ·

-1

g nm )

0.0018

0.0006 0.0000 0

7

14 21 Pore width(nm)

28

35

Fig. 11. Change of catalyst pore volume before and after reaction for 50 h.

image. It was presumed that the active component was wrapped by the filamentous carbon or that carbon reacted with the active component to form a compound (Fe3C was found in the XRD analysis). However, the carbon content in the developing carbon deposits was obviously higher than that in the developed carbon deposits and there was no active component. Therefore, it was inferred that the carbon deposit (Pino et al., 2014; Guo et al., 2004) or iron carbide (Riedel et al., 2003) became active site to promote the reaction under microwave irradiation. Fig. 8 is the SEM diagram of the catalyst surface after 50 h of the reforming reaction. Compared to the previous catalysts, more carbon deposits were produced on the catalyst surface. The forms of carbon deposition included amorphous carbon (Cα, Cβ) and filamentous carbon (Cγ) (Bartholomew, 2001). In addition, it was found that some positions of filamentous carbon interacted with amorphous carbon. Local amplification of filamentous carbon (Fig. 8b) shows that its surface was uneven, similar to the accumulation and connection of many globules. It is speculated that the Cα and Cβ were changed to Cγ (Bartholomew, 2001).

3.2.2. TG analysis Fig. 9 shows the TG diagram of the catalyst after 50 h of reaction. It can be seen from the TG diagram that the amount of carbon deposition was approximately 0.78 wt% after 50 h of reaction. The process of weight loss can be divided into three stages. The first stage was from room temperature to 250 °C, where the catalyst undergone no reaction. The second stage was 250 °C to 500 °C, wherein the mass of the catalyst decreases slightly because of the elimination of Cα, Cβ, (Fig. 8, the two forms of carbon deposition were found in the SEM diagram) and catalyst-surface CHx fragments (Selvarajah et al., 2016). The third stage was above 500 °C, most of the carbon deposition (Cγ) was eliminated in this stage and the amount of carbon deposition (Cγ) was approximately 90% of the total carbon deposition. The TGA and SEM analysis results were identical. The form of carbon deposit was mainly Cγ in the CH4-CO2 reforming reaction over Fe-based catalyst. This kind of carbon should be eliminated at higher temperature. The amount of carbon deposition was less than in many other experimental studies. SEM-EDS analysis shows that there was more oxygen in different parts of the catalyst; these oxygen components could oxidize the carbon deposition and improve the anti carbon deposition performance of the catalyst under microwave irradiation. In addition, when the furnace temperature was about 500 °C, the quality of the catalyst increased slightly because the Fe was oxidized during the heating process, resulting in a slight recovery in quality (Pawar et al., 2015).

3.2.3. XRD analysis Fig. 10 shows the XRD diagram of the catalyst before and after the reaction. The catalyst compositions included SiC, SiO2, active components Fe and Fe2O3, etc. Although the main component before and after reaction was SiC, the crystal form of SiC had changed. The main parameters of SiC in the fresh catalyst were a = 3.802, b = 3.082, and c = 15.118, recorded as SiC1. The main parameters of SiC in the spent catalyst were a = 3.079, b = 3.079, and c = 67.990, recorded as SiC2. According to the change in diffraction peaks at some positions (for example, SiC and SiO2 diffraction peaks changed at 2θ values of 44.5° and 75.3°), it was presumed that the catalyst carrier participated in the reaction during the reforming process, leading to the change of crystal structure. It was also found that the Fe and Fe2O3 diffraction peaks also changed after the reaction, mainly due to the oxidation of Fe and the reduction of Fe2O3. In addition, Fe3C was found in XRD analysis at 2θ of 26.6°, 38.1°, and 54.5°. The diffraction peak intensity increased after reaction, which was related to the formation of Fe3C. The results were consistent with that in Fig. 7, which further confirmed the existence of Fe3C.

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3.2.4. Pore structure and specific surface area analysis Fig. 11 shows the change in pore volume of the catalyst before and after reaction. The fresh catalyst was mainly mesoporous with pore size of 5–8 nm and a small amount of 0–2 nm micropores. After 50 h of reforming reaction, the micropores of the catalyst disappeared and the pore size of the mesopores became smaller; the specific surface area of the catalyst changed from 32.118 m2/g to 27.443 m2/g after the reforming reaction. According to the SEM and TGA results, the main reason for this was that the carbon deposition accumulated on the catalyst surface blocked the pores, which led to the disappearance of micropores and the decrease in the mesopore size. This change was detrimental to the reforming reaction. 4. Conclusion 1) The microwave dry reforming reaction could be divided into a rapid reaction stage, slow reaction stage, and reaction equilibrium stage. In the slow reaction stage, the conversion of reactants and selectivity of products exceeded 95%. The conversion of reactants at the equilibrium stage was maintained at 85% and increased with the increase of the SMP. The optimum conditions were as follows: 12 wt% Fe/SiC, the SMP was 90 W/g, and the space velocity was 200 h−1 in terms of the conversion of reactants and selectivity of products. 2) The catalyst retained high activity after 50 h of reaction: the conversion of reactants was approximately 85%, the H2/CO ratio was close to 1, and the carbon deposition was approximately 0.78 wt%. 3) The results of this study show that Fe-based catalysts supported on SiC foam ceramics had positive effects on the CH4-CO2 reforming reaction under microwaves irradiation. The use of cheap and environmentally friendly Fe catalysts enables coupling between microwaves and the strong absorbing medium (SiC), which is of considerable significance for the dry reforming reaction. Acknowledgments This work was sponsored by the National Natural Science Foundation of China (Grant No. 51576118), the Fundamental Research Funds of Shandong University (Grant No. 2016JC004), China Postdoctoral Science Foundation (Grant No. 2016M602139), and Young Scholars Program of Shandong University (Grant No. 2016WLJH37). References Aw, M.S., Zorko, M., Djinović, P., Pintar, A., 2015. Insights into durable NiCo catalysts on βSiC/CeZrO2 and γ-Al2O3/CeZrO2 advanced supports prepared from facile methods for CH4–CO2 dry reforming. Appl. Catal. B Environ. 164, 100–112. Bartholomew, C.H., 2001. ChemInform abstract: mechanisms of catalyst deactivation. ChemInform 32, 17–60. Bradford, M.C.J., Vannice, M.A., 1999. The role of metal-support interactions in CO2 reforming of CH4. Catal. Today 50, 87–96. Chen, W., Gutmann, B., Kappe, C.O., 2012. Characterization of microwave-induced electric discharge phenomena in metal-solvent mixtures. Chemistryopen 1, 39–48. Durka, T., Stefanidis, G.D., Van Gerven, T., Stankiewicz, A.I., 2011. Microwave-activated methanol steam reforming for hydrogen production. Int. J. Hydrog. Energy 36, 12843–12852. Erdohelyi, A., Cserenyi, J., Solymosi, F., 1993. Activation of CH4 and its reaction with CO2 over supported Rh catalysts. J. Catal. 141, 287–299. Fan, M.S., Abdullah, A.Z., Bhatia, S., 2009. ChemInform abstract: catalytic technology for carbon dioxide reforming of methane to synthesis gas. ChemCatChem 1, 192–208. Fidalgo, B., Domínguez, A., Pis, J.J., Menéndez, J.A., 2008. Microwave-assisted dry reforming of methane. Int. J. Hydrog. Energy 33, 4337–4344. Fidalgo, B., Menéndez, J.A., 2013. Syngas production by CO2 reforming of CH4 under microwave heating. Challenges and oportunities. Syngas: Production, Applications and Environmental Impact, pp. 121–149.

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