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SiC catalysts for cracking of toluene under microwave irradiation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation Yuchuan Zhang, Zhanlong Song*, Yecheng Yan, Xiqiang Zhao, Jing Sun, Yanpeng Mao, Wenlong Wang National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan, 250061, China
article info
abstract
Article history:
In this work, toluene as a model compound of biomass tar and inexpensive Fe supported
Received 1 November 2017
on SiC powder as catalysts were used to investigate the influence of specific microwave
Received in revised form
power levels, Fe content, and space velocity on the microwave cracking of toluene. The
31 January 2018
results revealed that the microwave-aided cracking of toluene was efficient and that the
Accepted 23 February 2018
composition of cracking gas was mainly H2. During the reaction, the instantaneous
Available online xxx
cracking efficiency and the instantaneous hydrogen yield increased initially and then decreased, with the highest values being recorded as 94.4% and 89.8% at 10e12 min of the
Keywords:
reaction. For the whole reaction, the overall cracking efficiency and the overall hydrogen
Toluene
yield reached maximum (86.3% and 84.5%, respectively) when the tests were conducted
Microwave
under the optimum conditions. The catalysts before and after reaction were analyzed by
Catalytic cracking
scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), thermogravim-
Carbon deposition
etry (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). The results confirmed that the deposited carbon included spherical and filamentous carbon and carbon deposition yield could reach 2.3%, which had a negative effect on the cracking of toluene. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent times, the industrial demand for energy has increased substantially but the supply falls far short of the demand. Therefore, finding an abundant renewable energy source to meet this demand has become very important worldwide [1,2]. Biomass has attracted much attention from researchers because it is available from a wide range of sources and is renewable and environmentally friendly [3]. The problem of energy shortage will be alleviated greatly if the biomass energy can be utilized efficiently [4]. Among the biomass utilization methods, biomass gasification technology is preferred as it can be used on a small scale and has a high
energy utilization rate. Moreover, decomposition gases produced by biomass gasification, such as H2 and CO [5], are easy to transport and control [6,7]. Along with the production of decomposition gases and solid carbon, biomass tar is also generated during biomass gasification [8,9]. Biomass tar (hereinafter referred to as tar) is a dark brown mixture with a high aromatic content, mainly toluene, naphthalene, phenol, benzene, and other organic substances [10]. Tar has many disadvantages as it blocks and corrodes the equipment pipeline. Furthermore, the abundant energy in tar cannot be used effectively if the tar is removed completely [11]. Therefore, remove and reuse of tar have become hot topics of research across the globe.
* Corresponding author. E-mail address: [email protected] (Z. Song). https://doi.org/10.1016/j.ijhydene.2018.02.158 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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The methods for the disposal of tar mainly include wet purification, dry purification, thermal cracking, and catalytic cracking [12,13]. Among these methods, catalytic cracking is the most advanced because of low temperature requirements and high efficiency [14]. Catalysts for cracking of tar include natural ore, alkali metal, and nickel-based catalysts [15,16], but their shortcomings are deactivation under some conditions and high cost for purchase and operation [17]. Fe-based catalyst is an attractive metal catalyst which is economical and environmentally friendly [18e20]. Azhar et al. [21] used several kinds of iron oxide catalysts produced under different atmospheres for the cracking of tar and found that the composition of the produced gas differed in each case and this difference had the notable influence on the yield of H2. Nordgreen et al. [22] used metallic Fe and iron oxide as catalysts to crack tar and found that metallic Fe was more effective. Zou et al. [23]cracked toluene (model compounds of tar) with different types of 3Fe8Ni/PG catalysts obtained by changing the calcination temperature and calcination time during the preparation. It was found that the 3Fe8Ni/PG catalysts calcined at 700 C for 2 h had the highest activity. However, in an earlier study, catalytic cracking of tar in the presence of Fe-based compounds under conventional heating was not efficient. This could be due to the following two reasons. First, compared with the expensive metal catalysts, Febased catalysts had inherently lower activities [24]. Second, uneven heating by conventional methods hindered heat transfer between the tar and the catalyst. In comparison with the conventional heating method, microwave heating technology plays an increasingly important role in energy utilization because of its fast and uniform heating characteristics [25e27]. Moreover, when a metal with shape irregularity or sharp edges is subjected to microwave irradiation, discharge phenomenon may take place; the effects of hot spots and plasma accompanied by the metal discharge can accelerate the rate of toluene cracking and compensate for the low activity by inexpensive metal catalysts and make up for the low activity inexpensive metal catalysts [28]. Nevertheless, there are only a few studies on the catalytic cracking of tar with microwave assistance, and most of the research focus has been on the cracking rate of toluene and gas production rate under different working conditions [29]. However, as the evolution behavior of the toluene cracking rate during the entire reaction is not completely clear, it is crucial to understand the cracking mechanism of toluene and design an optimum reactor for cracking toluene. In this article, we investigated the effects of microwave power, Fe content, and space velocity on the toluene cracking rate (X), and the hydrogen production rate (4) on a laboratoryscale fixed bed reactor using Fe-based catalysts. SiC powder was used as carriers in view of its splendid microwave absorption properties. Considering the absolute microwave parameter only applicable to the facility employed in the specific research work, instead, we use the power per 1 g sample as a new criterion, i.e., the specific microwave power (SMP) [30]. Moreover, the performance of cracking of toluene during the reaction was investigated experimentally and the key factors, i.e., temperature variation of the samples at different reaction stages and carbon deposition rate (Y) of the reaction, were analyzed to elucidate their effects on the
catalyst activity. Subsequently, the catalysts were characterized by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), thermogravimetric (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). This is expected to confirm the evolution of carbon deposition during the reaction and help in understanding the cracking mechanism of toluene under microwave irradiation using inexpensive Fe-based catalysts.
Experimental Catalyst and preparation method Fe/SiC catalysts were prepared by an incipient impregnation method [31], in which SiC was used as the support and varying Fe contents (2%, 4%, 6%, 8%, 10%, and 12%) were loaded on SiC as the active ingredient. First, Fe(NO)3$9H2O was chosen as the source of Fe, and SiC powder with a mesh size of 80 was dissolved in an aqueous solution of Fe(NO)3$9H2O.The mixed solution was impregnated overnight with continuous stirring, followed by drying at 160 C for 2 h. Then, the catalysts were calcined at 500 C in an air atmosphere for 3 h to obtain Fe2O3/ SiC catalysts. Finally, the calcined catalysts were reduced insitu for 3 h under H2 (50 ml/min) at 800 C to yield Fe/SiC catalysts. In order to ensure that the calculated Fe content of the catalysts was in accordance with the measured value, six kinds of catalysts with different Fe contents were tested by inductively coupled plasma (ICP) spectroscopy (Optima7000DV). Ferric nitrate solutions, obtained by mixing the catalysts with 3% dilute nitric acid solution, were compared with the standard solution to obtain the Fe content in the sample. The results are shown in Table 1.
Experimental system and process The experimental system consists of a quartz reactor, gas supply system, homeothermic water bath, microwave oven, condenser, etc., as shown in Fig. 1. The microwave oven (Media X3-233A) can continuously work under the power settings of 900 W, 700 W, 500 W, and 300 W without the usual intermittent heating. The quartz reactor has an inner diameter of 20 mm and a height of 110 mm. For each trial, about 10 g of samples were evenly placed on a multi orifice plate at a distance of 2 cm above the bottom of the quartz reactor, and the reaction time was 15 min. Nitrogen gas (N2) was used as the carrier gas to transport the toluene vapor into the microwave oven for the catalytic cracking reaction. Before the experiment, the whole reaction system was purged with N2 at a flow rate of 200 ml/min to ensure that the pipeline was filled with inert gas, and the toluene liquid was
Table 1 e Calculated and measured values of varying Fe contents. Item Calculated values Measured values
Fe content (%) 2 1.89
4 4.20
6 6.13
8 8.21
10 10.05
12 12.11
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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Fig. 1 e Schematic diagram of the experimental system.1:Nitrogen cylinder; 2:mass flow controller; 3:homeothermic water bath; 4:evaporation flask; 5:heating tape; 6:temperature controller; 7:quartz reactor; 8:microwave oven; 9:toluene condenser; 10:gas chromatograph analyzer.
preheated using a water bath at constant temperature to facilitate its volatilization. During the reaction, the entire pipeline was covered with heating tape at 160 C to avoid the condensation of toluene vapor. After the reaction, the unreacted toluene was collected in the toluene condenser, and cracking gas was collected in the gas bag. A gas chromatography (GC) system was employed to analyze the components of the cracking gas. In order to ensure that the toluene vapor concentration in the reaction at different space velocities was kept constant, several sets of pre-experiments were carried out before the formal experiment. The mass of escaping toluene at different space velocities was adjusted by changing the temperature of the water bath, and the space velocity was proportional to the mass of toluene vapor. The results showed that the concentration of toluene vapor was 0.08 g/min at space velocities of 637 h1, 956 h1, and 1274 h1 with water bath temperatures of 88 C, 80 C, and 75 C, respectively. In order to minimize errors, each test was repeated at least three times and the average results were reported. The bed temperature of the catalysts was precisely measured by a K-type thermocouple with 1 mm diameter. In order to avoid the impact of microwave radiation on the measurements of the thermocouple, the microwave oven was turned off when the temperature was measured, and then the thermocouple was quickly inserted into the center of the catalysts, and the temperature was recorded after the display was relatively stable (within 6 s for all the measurements). To make sure that thermocouple measurement could represent the actual temperature of the catalysts, the quartz reactor was wrapped closely by silica wool to prevent the temperature from reducing.
Evaluation indicators In this article, the toluene cracking efficiency (X), hydrogen yield (4), and carbon deposition yield (Y) were used to evaluate the cracking reaction performance of toluene by the following equations. X ¼ ðmin mout Þ=min 100%
(1)
where min represents the mass of toluene into the reactor and mout represents the mass of unreacted toluene.
4 ¼ fH2
100 fN2 100%
(2)
where fH2 and fN2 represent the volume percentage ratios of H2 and N2 in the total gas, respectively. H2 and CH4 are the ideal combustible gases produced in the reaction, and CH4 produced in the test is very limited (<1%). Therefore, it is reliable to use 4 to characterize the performance of toluene cracking. Y ¼ ðm1 m2 Þ=m1 100%
(3)
where m1 and m2 represent the masses of the catalysts before and after the reaction, respectively.
Measurement methods The cracking gas was analyzed by GC (PE Clarus 500) with three detectors (2 TCD: 200 C, FID: 250 C). The H2, CH4, C2H2, C2H4, C2H6, CO, and CO2 gases were measured by GC to accurately quantify the components. In order to calculate Y, the weight of the catalysts before and after the reaction was measured with a balance to 0.001 g accuracy first and then Eq. (1) was verified using a thermogravimetric (TG) analyzer (TGA/DSC1/1600HT). The TG test was carried out by heating the temperature to 900 C at the rate of 30 C/min and insulated for 30 min in an air atmosphere to analyze the weight loss of the catalysts [32]. In order to characterize the micrographic changes of the catalysts before and after the reaction, they were subjected to magnification (300e20000 times) analysis using SEM (ZEISS SUPRA™55). The elemental composition of the catalyst surface was analyzed by EDS (OXFORD INCAx-act). To analyze the change of composition between fresh and spent catalysts, XRD and TPO were carried out. XRD were obtained using a Rigaku D/max 2500 PC, and the sample was scanned from 5 to 85 with the speed of 0.4 min1. TPO (JEM2100F) was conducted with the heating rate of 30 C/min and the flow of 5.02% O2 in H
Results and discussion Many chemical reactions may occur during the reaction of toluene cracking. On the whole, toluene was cracked into
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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macromolecular hydrocarbon and H2 first (Eq. (4)), and some of the macromolecular hydrocarbon continued cracking into carbon and H2 (Eq. (5)) [15,23], as shown below: nC7H8 / mCxHy þ pH2
(4)
CxHy / xC þ y/2H2
(5)
Influence of different factors on catalytic cracking of toluene Three factors (power level, Fe content and space velocity) were analyzed to observe their influences on cracking of toluene. In this Section, X means the overall cracking efficiency during the whole reaction time, and 4 means the overall hydrogen yield during the whole reaction time.
Power level Four power levels (90 W/g, 70 W/g, 50 W/g, and 30 W/g) were selected to observe the influence of SMP on the catalytic cracking of toluene, and the results are illustrated in Fig. 2. As observed from Fig. 2, X and 4 increased first and then decreased with an increase in SMP. When SMP increased from 30 W/g to 70 W/g, X changed from 65.4% to 86.3% and 4 increased from 67.3% to 84.5%. The reason maybe that the catalytic cracking reaction is endothermic, and a high SMP value means a high temperature that is beneficial to the decomposition of toluene [33]. After the reaction, the catalysts became powdery and a few catalyst particles adhered together. However, when SMP increased from 70 W/g to 90 W/g, X decreased from 86.3% to 74.1%, with the 4 values also decreasing from 84.5% to 77.3%. Fig. 3 shows the morphology of the catalysts after independent reactions at SMP values of 70 W/g and 90 W/g. From the picture, the shape of catalysts after reaction at 70 W/g was powdery and it was the same as the shape before reaction. When the SMP value increased to 90 W/g, the catalysts absorbed more power and this rapidly increased the temperature to the apparent sintering point. The concept “sintering temperature” is usually not a fixed temperature value for one material, and it depends on many factors such as roasting condition and pressure. In this
Fig. 2 e Effects of SMP on X and 4.
Fig. 3 e Morphology of the catalysts after the reaction: (a) 70 W/g; (b) 90 W/g.
experiment, the sintering temperature of the catalysts under the SMP of 90 W/g was 1280 C. Furthermore, since microwave radiation can simultaneously penetrate the catalysts and be absorbed by them, the temperature gradient of the catalysts within the microwave field was extremely low. Under these circumstances, the catalysts were condensed into a uniform and dense [34] block structure (Fig. 3b). As the micropore structure of the catalysts was blocked, the surface area in contact with toluene was reduced, as a result of which X and 4 values also decreased. Moreover, when 4 reached the maximum value (84.5%), the yield of other gaseous components, such as CH4, C2H2, and C2H4 was <2%, and the yield of CH4 was ~0, which was inconsistent with the results from earlier research findings. For example, Mani et al. [33] cracked toluene with the pine bark biochar catalysts and the CH4 yield reached a maximum value of 10%; Zou et al. [35] used hematite derived from natural limonite as catalysts to crack toluene, and CH4 could be distinctly detected from cracking gas. It can be explained that the Fe/SiC catalysts increase temperatures up to >1000 C, which is sufficient for micromolecular hydrocarbons (such as CH4) to decompose into C and H2 and increase the degree of cracking of toluene [36].
Fe content Six different Fe contents of catalysts (0%, 2%, 4%, 6%, 8%, 10%, and 12%) were studied under the following experimental conditions: SMP ¼ 70 W/g, and space velocity ¼ 637 h1. The results are shown in Fig. 4. From the data in Fig. 4, X and 4 followed an increasing trend with respect to the increase in Fe content before 10%, and then kept steady. While the Fe content increased from 0% to 10%, X increased from 41.2% to 86.3% and 4 increased from 36.6% to 84.5%. These observations are attributed to three main reasons. First, for the catalyst, a higher Fe content implies a greater number of active sites, which lead to a greater probability of more cracking for toluene. Second, Fe is an excellent substance for absorbing microwave radiation, higher Fe contents can enhance the microwave power the catalysts absorb, and in turn, increases the temperature. Third, the interaction between Fe and the microwave influences the cracking reaction to a certain extent. The microwave radiation assists in the activation of a large number of free electrons from the iron atoms, which settle on the
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Fig. 4 e Effects of Fe content on X and 4.
Fig. 6 e X and 4 at every stage of the reaction.
micro-probe of the catalyst particles. The micro-discharge phenomenon is initiated when the free electrons reach a certain number, and this phenomenon induces the hot spot effect, which enhances the degree of catalytic cracking of toluene [37]. Furthermore, the micro-discharge phenomenon can trigger the plasma effect, and the highly active free radicals produced by the plasma promote the cracking reaction [28,38]. Therefore, increasing the Fe content can enhance the catalytic cracking of toluene effectively. When Fe contents increased from 10% to 12%, X and 4 stopped rising and kept steady (X from 86.3% to 86.2%, 4 from 84.5% to 84.7%). It could be explained that the degree of the toluene cracking was quite high when Fe contents reached 10% and it might not have a significant effect when Fe contents continued rising from 10%. So 10% was considered as the optimal Fe content for this test.
space velocity is equal to reducing the contact time of toluene and catalysts, and the unreacted toluene was carried out of the reactor without enough residence time for the decomposition reaction. However, compared with X, the range of decrease in 4 was small. It is speculated that the CeH bond is more unstable than the CH3eC6H5 bond and CeC bond of the benzene ring, and can be broken easily [39]. Hence, hydrocarbons such as CH4 and C2H4 can be decomposed to C and H2 preferentially than cleavage of the benzene ring, and consequently, the degree of toluene cracking is higher. From the aforementioned analysis, it can be speculated that X and 4 should increase sequentially if the space velocity continues to decrease. However, the negative effect is that the quantity of toluene vapor carried out by N2 is lower at lower space velocity and it increases the difficulty of the test. Therefore, 637 h1 is chosen as the optimal space velocity in this experiment.
Space velocity In this work, experiments were conducted with space velocity values of 637 h1, 956 h1, and 1274 h1 and the results are shown in Fig. 5. From the plot, increasing space velocity was detrimental to X and 4 on the whole. When increasing space velocity from 637 h1 to 1274 h1, X decreased from 86.3% to 66.7% and 4 decreased from 84.5% to 76.6%. This is because increasing
Fig. 5 e Effects of space velocity on X and 4.
Study on the process characteristics of catalytic cracking of toluene In order to investigate the characteristics of X, 4, Y, and temperature variation during every stage of the reaction, Fe/SiC with 10% Fe content was selected as the catalyst. SMP (70 W/g) and space velocity (637 h1) parameters were fixed to study the changes of different factors during the reaction. In this Section, X, 4, and Y were all instantaneous values during the reaction period. The time from 0 to 6 min refers to the preheating stage where the temperature of the catalysts relatively low (550 C) (Fig. 7), and X and 4 are both ~0. The toluene vapor was injected into the reactor at 6 min, and the whole experiment lasted for 20 min. The color of catalysts changed from partially dark red to completely bright red during 6e12 min, followed by a gradual change to completely black during 12e20 min. In comparison with the temperature changes observed in Fig. 7, the color of the catalysts was closely related with temperature. The color became brighter when the temperature increased and the color turned dark when there was a drop in temperature. Fig. 6 shows the changes in X and 4 at every stage of the reaction. The X and 4 parameters increased first and then
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Fig. 7 e Temperature and Y at every stage of the reaction. decreased correspondingly. From 6 to 12 min, X and 4 continued to increase and reached the highest level of 94.4% and 89.8% during 10e12 min. Then, both X and 4 decreased and during the last stage of the reaction, dropped to 68.7% and 71.9%, respectively. As the catalytic cracking of toluene is an endothermic reaction, temperature plays a pivotal role in the reaction. From Fig. 7, it is observed that the temperature increased from 580 C to 1131 C during 6e12 min, and toluene could absorb enough power to undergo decomposition. Then, the temperature decreased from 1131 C to 865 C during 12e20 min, and the X and 4 values also decreased. Carbon deposition is one of the most important factors that reduce the lifetime of catalysts. The carbon deposition may
block the active sites of the catalysts, and then the active component (Fe) may lose the effect of catalysis [40]. For the catalytic cracking of toluene, plenty of carbon produced during the reaction was attached to the surface of the catalysts. Fig. 7 showed the changes of Y during the reaction, and other two black lines in Fig. 7 was the temperature of the reaction with and without injecting toluene. When there was no toluene in the reactor, the temperature rose at first and was steady later. This was because in the absence of carbon decomposition by toluene, the catalysts could convert the microwave energy into thermal energy and then maintain the balance between the microwave and thermal energy. When toluene was decomposed by the catalysts, Y increased continuously and the temperature reduced when Y reached 1.8%. It can be speculated that carbon deposition is detrimental to the microwave absorbed by the catalysts when it is added up to a certain point.
Catalyst characterization The reacted catalysts were characterized by SEM-EDS, XRD, TG and TPO in order to analyze the behavior of carbon deposition.
SEM-EDS analysis Fig. 8aed showed the photographs of catalysts before and after the reaction. From the pictures, the catalyst surface before the reaction was covered by a spherical layer whose diameter was ~0.2 mm. Subsequently, this substance was characterized by EDS and the results are shown in Fig. 9 and
Fig. 8 e SEM photographs of catalysts (a) before reaction, Mag ¼ 300X; (b) before reaction, Mag ¼ 10000X; (c) after reaction, Mag ¼ 300X; (d) after reaction, Mag ¼ 10000X. Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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generate carbon nanotubes under a microwave field. Most importantly, the conversion of carbon nanotubes had a significant increase with the increase in microwave power. Zeng et al. [43] conducted the rapid pyrolysis of methane with the assistance of microwave radiation. There were numerous carbon nanotubes used in the production and the rapid heating of microwave was favorable for the transformation from carbon black to carbon nanostructures [44].
XRD analysis The XRD analyses of catalysts before and after reaction were conducted and the results are shown in Fig. 10. From the picture, the principal components of the fresh catalysts were SiC (2q ¼ 34.5 , 35.3 , 37.8 , 59.7 , and 72.2 ) and Fe (2q ¼ 44.3 and 45.2 ). Compared with the fresh catalysts, two new diffraction peaks at 41.1 and 42.7 appeared and that was confirmed to be C for the spent samples, and it could be considered as the carbon deposition of the catalysts. Beyond that, for the spent catalysts, the peak intensity of SiC at 59.7 and 72.2 was weaker than those in fresh catalysts, and a new component FeSi was detected at the peak of 51.2 . It could be explained that a small amount of Fe was reacted with SiC
Fig. 9 e EDS spectra at points 1 and 2 in Fig. 8.
Table 2. The Fe content was 94.29% in this spherical portion. As a result, this substance was probably expected to be the simple substance Fe. The surface of the catalysts after the reaction became random and the sphere was surrounded by clumps and filaments. Point 2 was characterized by EDS and the results are shown in Fig. 9b, the carbon content was 88.2% and the Fe content was only 7.12%. Comparison with Fig. 9a showed that the metallic Fe, which is the active substance, was almost completely surrounded by carbon deposited from the reaction, thus lowering the activity of the catalysts. In Fig. 8d, there appeared two different types of carbon on the surface of the catalysts: spherical carbon ((1) in Fig. 8d) and filamentous carbon ((2) in Fig. 8d). Spherical carbon was irregular with a diameter of approximately 0.2 mm; the relatively small particles could aggregate together into bigger particles of ~1 mm in size. The filamentous carbon traversed the surface with different lengths and 50 nm diameter. They are intertwined with each other and attached to the surface of the catalysts. Observing filamentous charcoal under the SEM, the shape and diameter were similar to carbon nanotubes [41]. Bajpai et al. [42] processed the graphite and carbon fiber with microwave and found that these substances were more likely to
Fig. 10 e XRD patterns of fresh and spent catalysts.
Table 2 e The composition of catalysts at points 1 and 2 in Fig. 8. Element Percentage of atom at point 1 (%) Percentage of atom at point 2 (%)
C
Si
Fe
O
Others
Total
1.42
2.68
94.29
0.29
1.32
100
88.20
2.63
7.12
0.82
1.23
100
Fig. 11 e TG curve of carbon generated.
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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Fig. 12 e O2-TPO profiles of fresh and spent catalysts.
when toluene was cracked (Fe þ SiC/FeSi þ C). But the peak intensity of FeSi was very weak and it could be negligible for the Fe reduction.
TG analysis In order to verify the quantity of carbon deposition, the catalysts after the reaction were analyzed by TG. The weight loss of carbon was recorded and the results are shown in Fig. 11. From this illustration, the weight loss curve of carbon in catalysts was mainly divided into two stages. When the temperature was less than 450 C, there was a slight weight loss of the catalysts (0.7%). The main reason is that the catalytic carbon is likely to absorb water and this stage is the weight loss of water in catalysts. In order to verify this speculation, the catalysts were studied by the dehydration treatment. The moisture content of the samples was 0.65%, as confirmed by the weight loss in the first stage. When the temperature was >450 C, there was a significant increase in the rate of weight loss of carbon. The weight loss rate was 2.25% at 1200 s, which may be attributed to the weight loss of carbon by combustion. The carbon content of the samples obtained by TG was consistent with the results (2.3%) obtained by calculating the sample mass before and after reaction by using high precision electronic balance.
TPO analysis To analyze the species of carbon generated in the cracking reaction, TPO measurement was undertaken for both fresh and spent catalysts, and the results are shown in Fig. 12. The curve profiles in the picture represented the O2 uptake of catalysts. SiC would not be oxidized in this TPO analysis because the initial oxidizing temperature of SiC is 1500 C [45]. For fresh catalysts, there was only one broad peak in the range
of 600e750 C, with maxima at 668 C, and this peak corresponded to the oxidation of Fe0 (4Fe þ 3O2 / 2Fe2O3) [46]. Compared with the fresh catalysts, two new peaks at 372 C and 506 C were detected in the TPO profiles of spent catalysts. That would be considered as the two kinds of carbon produced by toluene cracking. Amorphous and polymeric carbon in clumps could be oxidized at between 300 and 380 C [47,48], which corresponded with the minor peak at 372 C. By contrast, the predominant peak at 506 C was confirm to filamentous carbon which was oxidized at between 500 and 640 C [49]. These two kinds of carbon deposition could be observed in Fig. 8.
Conclusions (1) Low space velocity and a high Fe content could improve toluene cracking efficiency (X) and carbon deposition yield (4); improving specific microwave power could enable X and 4 to increase and then drop. The highest overall X (86.3%) and 4 (84.5%) were obtained when the SMP was 70 W/g, space velocity was 637 h1, and Fe content was 10%. (2) For the entire reaction period, the instantaneous X and 4 initially exhibited an increase and followed by a decrease trend. The highest values could reach 94.4% and 89.8%, respectively. (3) Decomposition reaction of toluene will generate carbon, which existed as spherical carbon and filamentous carbon. The carbon deposition content of the reacted samples was 2.3%, and carbon deposition was one of the most important factors that reduce the lifetime of catalysts.
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(4) Microwave radiation could improve the catalytic cracking degree of toluene effectively, which provided a new method for effective cracking of tar and its utilization.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 51576118 and 51506116) and Young Scholars Program of Shandong University (Grant No. 2016WLJH37).
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Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation Yuchuan Zhang, Zhanlong Song*, Yecheng Yan, Xiqiang Zhao, Jing Sun, Yanpeng Mao, Wenlong Wang National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan, 250061, China
article info
abstract
Article history:
In this work, toluene as a model compound of biomass tar and inexpensive Fe supported
Received 1 November 2017
on SiC powder as catalysts were used to investigate the influence of specific microwave
Received in revised form
power levels, Fe content, and space velocity on the microwave cracking of toluene. The
31 January 2018
results revealed that the microwave-aided cracking of toluene was efficient and that the
Accepted 23 February 2018
composition of cracking gas was mainly H2. During the reaction, the instantaneous
Available online xxx
cracking efficiency and the instantaneous hydrogen yield increased initially and then decreased, with the highest values being recorded as 94.4% and 89.8% at 10e12 min of the
Keywords:
reaction. For the whole reaction, the overall cracking efficiency and the overall hydrogen
Toluene
yield reached maximum (86.3% and 84.5%, respectively) when the tests were conducted
Microwave
under the optimum conditions. The catalysts before and after reaction were analyzed by
Catalytic cracking
scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), thermogravim-
Carbon deposition
etry (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). The results confirmed that the deposited carbon included spherical and filamentous carbon and carbon deposition yield could reach 2.3%, which had a negative effect on the cracking of toluene. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent times, the industrial demand for energy has increased substantially but the supply falls far short of the demand. Therefore, finding an abundant renewable energy source to meet this demand has become very important worldwide [1,2]. Biomass has attracted much attention from researchers because it is available from a wide range of sources and is renewable and environmentally friendly [3]. The problem of energy shortage will be alleviated greatly if the biomass energy can be utilized efficiently [4]. Among the biomass utilization methods, biomass gasification technology is preferred as it can be used on a small scale and has a high
energy utilization rate. Moreover, decomposition gases produced by biomass gasification, such as H2 and CO [5], are easy to transport and control [6,7]. Along with the production of decomposition gases and solid carbon, biomass tar is also generated during biomass gasification [8,9]. Biomass tar (hereinafter referred to as tar) is a dark brown mixture with a high aromatic content, mainly toluene, naphthalene, phenol, benzene, and other organic substances [10]. Tar has many disadvantages as it blocks and corrodes the equipment pipeline. Furthermore, the abundant energy in tar cannot be used effectively if the tar is removed completely [11]. Therefore, remove and reuse of tar have become hot topics of research across the globe.
* Corresponding author. E-mail address: [email protected] (Z. Song). https://doi.org/10.1016/j.ijhydene.2018.02.158 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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The methods for the disposal of tar mainly include wet purification, dry purification, thermal cracking, and catalytic cracking [12,13]. Among these methods, catalytic cracking is the most advanced because of low temperature requirements and high efficiency [14]. Catalysts for cracking of tar include natural ore, alkali metal, and nickel-based catalysts [15,16], but their shortcomings are deactivation under some conditions and high cost for purchase and operation [17]. Fe-based catalyst is an attractive metal catalyst which is economical and environmentally friendly [18e20]. Azhar et al. [21] used several kinds of iron oxide catalysts produced under different atmospheres for the cracking of tar and found that the composition of the produced gas differed in each case and this difference had the notable influence on the yield of H2. Nordgreen et al. [22] used metallic Fe and iron oxide as catalysts to crack tar and found that metallic Fe was more effective. Zou et al. [23]cracked toluene (model compounds of tar) with different types of 3Fe8Ni/PG catalysts obtained by changing the calcination temperature and calcination time during the preparation. It was found that the 3Fe8Ni/PG catalysts calcined at 700 C for 2 h had the highest activity. However, in an earlier study, catalytic cracking of tar in the presence of Fe-based compounds under conventional heating was not efficient. This could be due to the following two reasons. First, compared with the expensive metal catalysts, Febased catalysts had inherently lower activities [24]. Second, uneven heating by conventional methods hindered heat transfer between the tar and the catalyst. In comparison with the conventional heating method, microwave heating technology plays an increasingly important role in energy utilization because of its fast and uniform heating characteristics [25e27]. Moreover, when a metal with shape irregularity or sharp edges is subjected to microwave irradiation, discharge phenomenon may take place; the effects of hot spots and plasma accompanied by the metal discharge can accelerate the rate of toluene cracking and compensate for the low activity by inexpensive metal catalysts and make up for the low activity inexpensive metal catalysts [28]. Nevertheless, there are only a few studies on the catalytic cracking of tar with microwave assistance, and most of the research focus has been on the cracking rate of toluene and gas production rate under different working conditions [29]. However, as the evolution behavior of the toluene cracking rate during the entire reaction is not completely clear, it is crucial to understand the cracking mechanism of toluene and design an optimum reactor for cracking toluene. In this article, we investigated the effects of microwave power, Fe content, and space velocity on the toluene cracking rate (X), and the hydrogen production rate (4) on a laboratoryscale fixed bed reactor using Fe-based catalysts. SiC powder was used as carriers in view of its splendid microwave absorption properties. Considering the absolute microwave parameter only applicable to the facility employed in the specific research work, instead, we use the power per 1 g sample as a new criterion, i.e., the specific microwave power (SMP) [30]. Moreover, the performance of cracking of toluene during the reaction was investigated experimentally and the key factors, i.e., temperature variation of the samples at different reaction stages and carbon deposition rate (Y) of the reaction, were analyzed to elucidate their effects on the
catalyst activity. Subsequently, the catalysts were characterized by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), thermogravimetric (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). This is expected to confirm the evolution of carbon deposition during the reaction and help in understanding the cracking mechanism of toluene under microwave irradiation using inexpensive Fe-based catalysts.
Experimental Catalyst and preparation method Fe/SiC catalysts were prepared by an incipient impregnation method [31], in which SiC was used as the support and varying Fe contents (2%, 4%, 6%, 8%, 10%, and 12%) were loaded on SiC as the active ingredient. First, Fe(NO)3$9H2O was chosen as the source of Fe, and SiC powder with a mesh size of 80 was dissolved in an aqueous solution of Fe(NO)3$9H2O.The mixed solution was impregnated overnight with continuous stirring, followed by drying at 160 C for 2 h. Then, the catalysts were calcined at 500 C in an air atmosphere for 3 h to obtain Fe2O3/ SiC catalysts. Finally, the calcined catalysts were reduced insitu for 3 h under H2 (50 ml/min) at 800 C to yield Fe/SiC catalysts. In order to ensure that the calculated Fe content of the catalysts was in accordance with the measured value, six kinds of catalysts with different Fe contents were tested by inductively coupled plasma (ICP) spectroscopy (Optima7000DV). Ferric nitrate solutions, obtained by mixing the catalysts with 3% dilute nitric acid solution, were compared with the standard solution to obtain the Fe content in the sample. The results are shown in Table 1.
Experimental system and process The experimental system consists of a quartz reactor, gas supply system, homeothermic water bath, microwave oven, condenser, etc., as shown in Fig. 1. The microwave oven (Media X3-233A) can continuously work under the power settings of 900 W, 700 W, 500 W, and 300 W without the usual intermittent heating. The quartz reactor has an inner diameter of 20 mm and a height of 110 mm. For each trial, about 10 g of samples were evenly placed on a multi orifice plate at a distance of 2 cm above the bottom of the quartz reactor, and the reaction time was 15 min. Nitrogen gas (N2) was used as the carrier gas to transport the toluene vapor into the microwave oven for the catalytic cracking reaction. Before the experiment, the whole reaction system was purged with N2 at a flow rate of 200 ml/min to ensure that the pipeline was filled with inert gas, and the toluene liquid was
Table 1 e Calculated and measured values of varying Fe contents. Item Calculated values Measured values
Fe content (%) 2 1.89
4 4.20
6 6.13
8 8.21
10 10.05
12 12.11
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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Fig. 1 e Schematic diagram of the experimental system.1:Nitrogen cylinder; 2:mass flow controller; 3:homeothermic water bath; 4:evaporation flask; 5:heating tape; 6:temperature controller; 7:quartz reactor; 8:microwave oven; 9:toluene condenser; 10:gas chromatograph analyzer.
preheated using a water bath at constant temperature to facilitate its volatilization. During the reaction, the entire pipeline was covered with heating tape at 160 C to avoid the condensation of toluene vapor. After the reaction, the unreacted toluene was collected in the toluene condenser, and cracking gas was collected in the gas bag. A gas chromatography (GC) system was employed to analyze the components of the cracking gas. In order to ensure that the toluene vapor concentration in the reaction at different space velocities was kept constant, several sets of pre-experiments were carried out before the formal experiment. The mass of escaping toluene at different space velocities was adjusted by changing the temperature of the water bath, and the space velocity was proportional to the mass of toluene vapor. The results showed that the concentration of toluene vapor was 0.08 g/min at space velocities of 637 h1, 956 h1, and 1274 h1 with water bath temperatures of 88 C, 80 C, and 75 C, respectively. In order to minimize errors, each test was repeated at least three times and the average results were reported. The bed temperature of the catalysts was precisely measured by a K-type thermocouple with 1 mm diameter. In order to avoid the impact of microwave radiation on the measurements of the thermocouple, the microwave oven was turned off when the temperature was measured, and then the thermocouple was quickly inserted into the center of the catalysts, and the temperature was recorded after the display was relatively stable (within 6 s for all the measurements). To make sure that thermocouple measurement could represent the actual temperature of the catalysts, the quartz reactor was wrapped closely by silica wool to prevent the temperature from reducing.
Evaluation indicators In this article, the toluene cracking efficiency (X), hydrogen yield (4), and carbon deposition yield (Y) were used to evaluate the cracking reaction performance of toluene by the following equations. X ¼ ðmin mout Þ=min 100%
(1)
where min represents the mass of toluene into the reactor and mout represents the mass of unreacted toluene.
4 ¼ fH2
100 fN2 100%
(2)
where fH2 and fN2 represent the volume percentage ratios of H2 and N2 in the total gas, respectively. H2 and CH4 are the ideal combustible gases produced in the reaction, and CH4 produced in the test is very limited (<1%). Therefore, it is reliable to use 4 to characterize the performance of toluene cracking. Y ¼ ðm1 m2 Þ=m1 100%
(3)
where m1 and m2 represent the masses of the catalysts before and after the reaction, respectively.
Measurement methods The cracking gas was analyzed by GC (PE Clarus 500) with three detectors (2 TCD: 200 C, FID: 250 C). The H2, CH4, C2H2, C2H4, C2H6, CO, and CO2 gases were measured by GC to accurately quantify the components. In order to calculate Y, the weight of the catalysts before and after the reaction was measured with a balance to 0.001 g accuracy first and then Eq. (1) was verified using a thermogravimetric (TG) analyzer (TGA/DSC1/1600HT). The TG test was carried out by heating the temperature to 900 C at the rate of 30 C/min and insulated for 30 min in an air atmosphere to analyze the weight loss of the catalysts [32]. In order to characterize the micrographic changes of the catalysts before and after the reaction, they were subjected to magnification (300e20000 times) analysis using SEM (ZEISS SUPRA™55). The elemental composition of the catalyst surface was analyzed by EDS (OXFORD INCAx-act). To analyze the change of composition between fresh and spent catalysts, XRD and TPO were carried out. XRD were obtained using a Rigaku D/max 2500 PC, and the sample was scanned from 5 to 85 with the speed of 0.4 min1. TPO (JEM2100F) was conducted with the heating rate of 30 C/min and the flow of 5.02% O2 in H
Results and discussion Many chemical reactions may occur during the reaction of toluene cracking. On the whole, toluene was cracked into
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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macromolecular hydrocarbon and H2 first (Eq. (4)), and some of the macromolecular hydrocarbon continued cracking into carbon and H2 (Eq. (5)) [15,23], as shown below: nC7H8 / mCxHy þ pH2
(4)
CxHy / xC þ y/2H2
(5)
Influence of different factors on catalytic cracking of toluene Three factors (power level, Fe content and space velocity) were analyzed to observe their influences on cracking of toluene. In this Section, X means the overall cracking efficiency during the whole reaction time, and 4 means the overall hydrogen yield during the whole reaction time.
Power level Four power levels (90 W/g, 70 W/g, 50 W/g, and 30 W/g) were selected to observe the influence of SMP on the catalytic cracking of toluene, and the results are illustrated in Fig. 2. As observed from Fig. 2, X and 4 increased first and then decreased with an increase in SMP. When SMP increased from 30 W/g to 70 W/g, X changed from 65.4% to 86.3% and 4 increased from 67.3% to 84.5%. The reason maybe that the catalytic cracking reaction is endothermic, and a high SMP value means a high temperature that is beneficial to the decomposition of toluene [33]. After the reaction, the catalysts became powdery and a few catalyst particles adhered together. However, when SMP increased from 70 W/g to 90 W/g, X decreased from 86.3% to 74.1%, with the 4 values also decreasing from 84.5% to 77.3%. Fig. 3 shows the morphology of the catalysts after independent reactions at SMP values of 70 W/g and 90 W/g. From the picture, the shape of catalysts after reaction at 70 W/g was powdery and it was the same as the shape before reaction. When the SMP value increased to 90 W/g, the catalysts absorbed more power and this rapidly increased the temperature to the apparent sintering point. The concept “sintering temperature” is usually not a fixed temperature value for one material, and it depends on many factors such as roasting condition and pressure. In this
Fig. 2 e Effects of SMP on X and 4.
Fig. 3 e Morphology of the catalysts after the reaction: (a) 70 W/g; (b) 90 W/g.
experiment, the sintering temperature of the catalysts under the SMP of 90 W/g was 1280 C. Furthermore, since microwave radiation can simultaneously penetrate the catalysts and be absorbed by them, the temperature gradient of the catalysts within the microwave field was extremely low. Under these circumstances, the catalysts were condensed into a uniform and dense [34] block structure (Fig. 3b). As the micropore structure of the catalysts was blocked, the surface area in contact with toluene was reduced, as a result of which X and 4 values also decreased. Moreover, when 4 reached the maximum value (84.5%), the yield of other gaseous components, such as CH4, C2H2, and C2H4 was <2%, and the yield of CH4 was ~0, which was inconsistent with the results from earlier research findings. For example, Mani et al. [33] cracked toluene with the pine bark biochar catalysts and the CH4 yield reached a maximum value of 10%; Zou et al. [35] used hematite derived from natural limonite as catalysts to crack toluene, and CH4 could be distinctly detected from cracking gas. It can be explained that the Fe/SiC catalysts increase temperatures up to >1000 C, which is sufficient for micromolecular hydrocarbons (such as CH4) to decompose into C and H2 and increase the degree of cracking of toluene [36].
Fe content Six different Fe contents of catalysts (0%, 2%, 4%, 6%, 8%, 10%, and 12%) were studied under the following experimental conditions: SMP ¼ 70 W/g, and space velocity ¼ 637 h1. The results are shown in Fig. 4. From the data in Fig. 4, X and 4 followed an increasing trend with respect to the increase in Fe content before 10%, and then kept steady. While the Fe content increased from 0% to 10%, X increased from 41.2% to 86.3% and 4 increased from 36.6% to 84.5%. These observations are attributed to three main reasons. First, for the catalyst, a higher Fe content implies a greater number of active sites, which lead to a greater probability of more cracking for toluene. Second, Fe is an excellent substance for absorbing microwave radiation, higher Fe contents can enhance the microwave power the catalysts absorb, and in turn, increases the temperature. Third, the interaction between Fe and the microwave influences the cracking reaction to a certain extent. The microwave radiation assists in the activation of a large number of free electrons from the iron atoms, which settle on the
Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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Fig. 4 e Effects of Fe content on X and 4.
Fig. 6 e X and 4 at every stage of the reaction.
micro-probe of the catalyst particles. The micro-discharge phenomenon is initiated when the free electrons reach a certain number, and this phenomenon induces the hot spot effect, which enhances the degree of catalytic cracking of toluene [37]. Furthermore, the micro-discharge phenomenon can trigger the plasma effect, and the highly active free radicals produced by the plasma promote the cracking reaction [28,38]. Therefore, increasing the Fe content can enhance the catalytic cracking of toluene effectively. When Fe contents increased from 10% to 12%, X and 4 stopped rising and kept steady (X from 86.3% to 86.2%, 4 from 84.5% to 84.7%). It could be explained that the degree of the toluene cracking was quite high when Fe contents reached 10% and it might not have a significant effect when Fe contents continued rising from 10%. So 10% was considered as the optimal Fe content for this test.
space velocity is equal to reducing the contact time of toluene and catalysts, and the unreacted toluene was carried out of the reactor without enough residence time for the decomposition reaction. However, compared with X, the range of decrease in 4 was small. It is speculated that the CeH bond is more unstable than the CH3eC6H5 bond and CeC bond of the benzene ring, and can be broken easily [39]. Hence, hydrocarbons such as CH4 and C2H4 can be decomposed to C and H2 preferentially than cleavage of the benzene ring, and consequently, the degree of toluene cracking is higher. From the aforementioned analysis, it can be speculated that X and 4 should increase sequentially if the space velocity continues to decrease. However, the negative effect is that the quantity of toluene vapor carried out by N2 is lower at lower space velocity and it increases the difficulty of the test. Therefore, 637 h1 is chosen as the optimal space velocity in this experiment.
Space velocity In this work, experiments were conducted with space velocity values of 637 h1, 956 h1, and 1274 h1 and the results are shown in Fig. 5. From the plot, increasing space velocity was detrimental to X and 4 on the whole. When increasing space velocity from 637 h1 to 1274 h1, X decreased from 86.3% to 66.7% and 4 decreased from 84.5% to 76.6%. This is because increasing
Fig. 5 e Effects of space velocity on X and 4.
Study on the process characteristics of catalytic cracking of toluene In order to investigate the characteristics of X, 4, Y, and temperature variation during every stage of the reaction, Fe/SiC with 10% Fe content was selected as the catalyst. SMP (70 W/g) and space velocity (637 h1) parameters were fixed to study the changes of different factors during the reaction. In this Section, X, 4, and Y were all instantaneous values during the reaction period. The time from 0 to 6 min refers to the preheating stage where the temperature of the catalysts relatively low (550 C) (Fig. 7), and X and 4 are both ~0. The toluene vapor was injected into the reactor at 6 min, and the whole experiment lasted for 20 min. The color of catalysts changed from partially dark red to completely bright red during 6e12 min, followed by a gradual change to completely black during 12e20 min. In comparison with the temperature changes observed in Fig. 7, the color of the catalysts was closely related with temperature. The color became brighter when the temperature increased and the color turned dark when there was a drop in temperature. Fig. 6 shows the changes in X and 4 at every stage of the reaction. The X and 4 parameters increased first and then
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Fig. 7 e Temperature and Y at every stage of the reaction. decreased correspondingly. From 6 to 12 min, X and 4 continued to increase and reached the highest level of 94.4% and 89.8% during 10e12 min. Then, both X and 4 decreased and during the last stage of the reaction, dropped to 68.7% and 71.9%, respectively. As the catalytic cracking of toluene is an endothermic reaction, temperature plays a pivotal role in the reaction. From Fig. 7, it is observed that the temperature increased from 580 C to 1131 C during 6e12 min, and toluene could absorb enough power to undergo decomposition. Then, the temperature decreased from 1131 C to 865 C during 12e20 min, and the X and 4 values also decreased. Carbon deposition is one of the most important factors that reduce the lifetime of catalysts. The carbon deposition may
block the active sites of the catalysts, and then the active component (Fe) may lose the effect of catalysis [40]. For the catalytic cracking of toluene, plenty of carbon produced during the reaction was attached to the surface of the catalysts. Fig. 7 showed the changes of Y during the reaction, and other two black lines in Fig. 7 was the temperature of the reaction with and without injecting toluene. When there was no toluene in the reactor, the temperature rose at first and was steady later. This was because in the absence of carbon decomposition by toluene, the catalysts could convert the microwave energy into thermal energy and then maintain the balance between the microwave and thermal energy. When toluene was decomposed by the catalysts, Y increased continuously and the temperature reduced when Y reached 1.8%. It can be speculated that carbon deposition is detrimental to the microwave absorbed by the catalysts when it is added up to a certain point.
Catalyst characterization The reacted catalysts were characterized by SEM-EDS, XRD, TG and TPO in order to analyze the behavior of carbon deposition.
SEM-EDS analysis Fig. 8aed showed the photographs of catalysts before and after the reaction. From the pictures, the catalyst surface before the reaction was covered by a spherical layer whose diameter was ~0.2 mm. Subsequently, this substance was characterized by EDS and the results are shown in Fig. 9 and
Fig. 8 e SEM photographs of catalysts (a) before reaction, Mag ¼ 300X; (b) before reaction, Mag ¼ 10000X; (c) after reaction, Mag ¼ 300X; (d) after reaction, Mag ¼ 10000X. Please cite this article in press as: Zhang Y, et al., Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.158
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generate carbon nanotubes under a microwave field. Most importantly, the conversion of carbon nanotubes had a significant increase with the increase in microwave power. Zeng et al. [43] conducted the rapid pyrolysis of methane with the assistance of microwave radiation. There were numerous carbon nanotubes used in the production and the rapid heating of microwave was favorable for the transformation from carbon black to carbon nanostructures [44].
XRD analysis The XRD analyses of catalysts before and after reaction were conducted and the results are shown in Fig. 10. From the picture, the principal components of the fresh catalysts were SiC (2q ¼ 34.5 , 35.3 , 37.8 , 59.7 , and 72.2 ) and Fe (2q ¼ 44.3 and 45.2 ). Compared with the fresh catalysts, two new diffraction peaks at 41.1 and 42.7 appeared and that was confirmed to be C for the spent samples, and it could be considered as the carbon deposition of the catalysts. Beyond that, for the spent catalysts, the peak intensity of SiC at 59.7 and 72.2 was weaker than those in fresh catalysts, and a new component FeSi was detected at the peak of 51.2 . It could be explained that a small amount of Fe was reacted with SiC
Fig. 9 e EDS spectra at points 1 and 2 in Fig. 8.
Table 2. The Fe content was 94.29% in this spherical portion. As a result, this substance was probably expected to be the simple substance Fe. The surface of the catalysts after the reaction became random and the sphere was surrounded by clumps and filaments. Point 2 was characterized by EDS and the results are shown in Fig. 9b, the carbon content was 88.2% and the Fe content was only 7.12%. Comparison with Fig. 9a showed that the metallic Fe, which is the active substance, was almost completely surrounded by carbon deposited from the reaction, thus lowering the activity of the catalysts. In Fig. 8d, there appeared two different types of carbon on the surface of the catalysts: spherical carbon ((1) in Fig. 8d) and filamentous carbon ((2) in Fig. 8d). Spherical carbon was irregular with a diameter of approximately 0.2 mm; the relatively small particles could aggregate together into bigger particles of ~1 mm in size. The filamentous carbon traversed the surface with different lengths and 50 nm diameter. They are intertwined with each other and attached to the surface of the catalysts. Observing filamentous charcoal under the SEM, the shape and diameter were similar to carbon nanotubes [41]. Bajpai et al. [42] processed the graphite and carbon fiber with microwave and found that these substances were more likely to
Fig. 10 e XRD patterns of fresh and spent catalysts.
Table 2 e The composition of catalysts at points 1 and 2 in Fig. 8. Element Percentage of atom at point 1 (%) Percentage of atom at point 2 (%)
C
Si
Fe
O
Others
Total
1.42
2.68
94.29
0.29
1.32
100
88.20
2.63
7.12
0.82
1.23
100
Fig. 11 e TG curve of carbon generated.
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Fig. 12 e O2-TPO profiles of fresh and spent catalysts.
when toluene was cracked (Fe þ SiC/FeSi þ C). But the peak intensity of FeSi was very weak and it could be negligible for the Fe reduction.
TG analysis In order to verify the quantity of carbon deposition, the catalysts after the reaction were analyzed by TG. The weight loss of carbon was recorded and the results are shown in Fig. 11. From this illustration, the weight loss curve of carbon in catalysts was mainly divided into two stages. When the temperature was less than 450 C, there was a slight weight loss of the catalysts (0.7%). The main reason is that the catalytic carbon is likely to absorb water and this stage is the weight loss of water in catalysts. In order to verify this speculation, the catalysts were studied by the dehydration treatment. The moisture content of the samples was 0.65%, as confirmed by the weight loss in the first stage. When the temperature was >450 C, there was a significant increase in the rate of weight loss of carbon. The weight loss rate was 2.25% at 1200 s, which may be attributed to the weight loss of carbon by combustion. The carbon content of the samples obtained by TG was consistent with the results (2.3%) obtained by calculating the sample mass before and after reaction by using high precision electronic balance.
TPO analysis To analyze the species of carbon generated in the cracking reaction, TPO measurement was undertaken for both fresh and spent catalysts, and the results are shown in Fig. 12. The curve profiles in the picture represented the O2 uptake of catalysts. SiC would not be oxidized in this TPO analysis because the initial oxidizing temperature of SiC is 1500 C [45]. For fresh catalysts, there was only one broad peak in the range
of 600e750 C, with maxima at 668 C, and this peak corresponded to the oxidation of Fe0 (4Fe þ 3O2 / 2Fe2O3) [46]. Compared with the fresh catalysts, two new peaks at 372 C and 506 C were detected in the TPO profiles of spent catalysts. That would be considered as the two kinds of carbon produced by toluene cracking. Amorphous and polymeric carbon in clumps could be oxidized at between 300 and 380 C [47,48], which corresponded with the minor peak at 372 C. By contrast, the predominant peak at 506 C was confirm to filamentous carbon which was oxidized at between 500 and 640 C [49]. These two kinds of carbon deposition could be observed in Fig. 8.
Conclusions (1) Low space velocity and a high Fe content could improve toluene cracking efficiency (X) and carbon deposition yield (4); improving specific microwave power could enable X and 4 to increase and then drop. The highest overall X (86.3%) and 4 (84.5%) were obtained when the SMP was 70 W/g, space velocity was 637 h1, and Fe content was 10%. (2) For the entire reaction period, the instantaneous X and 4 initially exhibited an increase and followed by a decrease trend. The highest values could reach 94.4% and 89.8%, respectively. (3) Decomposition reaction of toluene will generate carbon, which existed as spherical carbon and filamentous carbon. The carbon deposition content of the reacted samples was 2.3%, and carbon deposition was one of the most important factors that reduce the lifetime of catalysts.
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(4) Microwave radiation could improve the catalytic cracking degree of toluene effectively, which provided a new method for effective cracking of tar and its utilization.
Acknowledgements This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 51576118 and 51506116) and Young Scholars Program of Shandong University (Grant No. 2016WLJH37).
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