Preparation of Fe-doped Carbon Catalyst for Methane Decomposition

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Preparation of Fe-doped Carbon Catalyst for Methane Decomposition to Hydrogen Jiaofei Wang, Lijun Jin, Yang Li, and Haoquan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02394 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Industrial & Engineering Chemistry Research

Preparation of Fe-doped Carbon Catalyst for Methane Decomposition to Hydrogen Jiaofei Wang, Lijun Jin, Yang Li, Haoquan Hu* State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China * Corresponding author: Tel.: +86-411-84986157; E-mail: [email protected].

ABSTRACT Fe-doped carbon catalysts were prepared from Shenmu subbituminous coal with addition of Fe(NO3)3 by KOH activation, and used for catalytic methane decomposition. The effects of Fe amount and carbonization/activation temperature on the structure and catalytic performance of resultant catalysts were investigated. The results showed that ferric nitrate mixed with coal could be directly reduced to Fe metal during the carbonization/activation process without hydrogen reduction process. Methane conversion over Fe-doped carbon catalysts significantly increases as the increase of Fe addition amount. When Fe addition amount is 30 wt.%, the resultant Fe-doped carbon has the highest catalytic activity and methane conversion increases from initial 20% to 58% at the reaction time of 9 h. Low carbonization temperature leads to high initial conversion. The active sites of the Fe-doped carbon catalysts prepared at higher temperature mainly come from metal Fe particles, thus leading to lower initial catalytic activity but better stability. KEYWORDS: Fe; Mesoporous carbon; Methane decomposition; Hydrogen

1. INTRODUCTION Recently, catalytic methane decomposition (CMD), as a simple and promising process for hydrogen production, received considerable attention because no by-products CO and CO2 are produced, thus extra

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water-gas shift reaction and gas separation required in traditional hydrogen production processes can be avoided. Various metal catalysts, such as Fe1-3, Ni4-9 and Co10-12, and carbon catalysts13-18 (including activated carbon (AC), carbon black, carbon fibers, etc.) were used for CMD. Metal catalyst generally exhibits high catalytic activity and requires a low reaction temperature. However, its easy deactivation owing to the carbon deposition on the metal restricts its application. Carbon materials, owing to their high surface area and pore volume, can accommodate more carbon deposit during the reaction and exhibit better tolerance to sulfur and other poisonous impurities in the feedstock and resistance to higher temperature in CMD, but have low activity for CMD compared to metal catalysts. Therefore, metal catalysts supported on carbon materials are expected to improve the catalytic activity and stability by utilizing the advantages of both metal and carbon catalysts19-23. In our previous studies, a novel method to in-situ prepare Ni-doped carbon by adding Ni(NO3)2 to the coal liquefaction residue was developed24. The results showed that active Ni could be directly obtained via carbon reducibility in the carbonization process, thus simplifying the preparation process. Compared with Ni-based catalyst, Fe catalyst has relatively lower catalytic activity in methane decomposition, but is of resistance to much higher temperature25,26. Because methane decomposition is an endothermic reaction, high reaction temperature promotes the reaction. Therefore, it is possible to show higher catalytic activity for Fe-based catalysts at high reaction temperature. Jin et al.27 studied the AC supported Fe-Al2O3 catalysts in methane decomposition, and found that the loading amount and Fe/Al2O3 weight ratio affect the textural properties and catalytic activities of catalysts. Ferric nitrate on AC can be in-situ reduced to Fe metal by the reducibility of carbon at 870 oC. Coal is usually used as the carbon precursor of AC owing to high carbon content and wide resource. Considering high temperature for the AC preparation and the reduction of ferric nitrate on AC, it is possible to get a Fe-doped carbon by one-


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step through simply mixing Fe(NO3)3 with coal and activation agent in preparation of AC. However, few works have been involved about this simple method to prepare Fe-doped carbon from coal and Fe precursor directly without hydrogen reduction process for methane decomposition. In this work, we aimed to prepare Fe-doped carbon by directly adding Fe(NO3)3 to coal via KOH activation. The effects of Fe addition amount and carbonization/activation temperature on the morphology and surface texture of resultant catalysts were examined, and the catalytic performance of resultant catalysts in CMD was also investigated.

2. EXPERIMENTAL 2.1. Materials Shenmu coal (SM), a bituminous coal from Shaanxi (China), as the carbon precursor, was crushed and sieved to a size below 150 μm before use. Fe(NO3)3∙9H2O and KOH (Shantou Xilong Chemical Technology Co., China) were used as the Fe source and the activating agent, respectively. 2.2. Catalyst preparation About 10 g coal was first mixed with certain amount of Fe(NO3)3∙9H2O in a mixture solvent containing 100 ml deionized water and 20 ml ethanol, and stirred in a sealed beaker for 6 h at 60 oC. The mixture was then evaporated and dried at 100 oC to remove the solvent, and the resultant mixture containing coal and Fe(NO3)3 was named as xFe-SM, here x is the amount of Fe added in the coal before carbonization (x= mass ratio of Fe/coal×100%), varying from 5 to 30. Thereafter, 5 g xFe-SM was physically mixed with the desired amount of KOH so as to keep the mass ratio of KOH/coal being 2/1, and carbonized/activated in a horizontal furnace with a N2 flow rate of 110 ml/min according to the temperature program shown in Figure S1. After cooling down, the samples were washed with deionized water only to avoid the loss of Fe. The resultant catalyst was expressed as xFe-AC.



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2.3. Characterization To determine the content of Fe metal in the catalyst, the Fe-doped carbon catalysts were analyzed by inductively coupled plasma (ICP). X-ray diffraction (XRD) patterns of the samples were obtained by a D/MAX-2400 with a Cu Kα radiation at 30 kV and 30 mA. Temperature-programmed reduction (H2-TPR) was performed in a conventional apparatus equipped with a thermal conductivity detector. About 0.05 g sample was preheated at 400 oC for 30 min before cooling to 120 oC under Ar, then heated to 950 oC at a heating rate of 5 oC/min under pure H2 atmosphere. XPS spectra was performed on an ESCALAB 250Xi spectrometer (Thermo-Fisher, England) using Al Kα X-rays source (hν=1486.6 eV), and operated at a power of 150 W with energy step size of 0.05 eV and a pass energy of 20 eV. The textural properties of the samples were measured by N2 adsorption at 77 K with a physical adsorption apparatus (ASAP 2420). The surface area and pore information were obtained by Brunauer-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods, respectively. The micropore volume (Vmic) and microporosity were calculated by using t-plot method and the ratio of Vmic to total pore volume (Vt). Thermogravimetric (TG) analysis was used to investigate the pyrolysis characteristics of coal and the mixture xFe-SM under a N2 flow of 60 mL/min in a TG analyzer (Mettler Toledo TGA/SDTA851e). Scanning electron microscopy (SEM, QUANTA 450) was applied to record the morphology of samples before and after CMD reaction. A TOF-MS, whose detail information was described elsewhere28,29, was used to simulate H2 reduction process of the catalyst, and analyze the gases generated from the heating process with a heating rate of 5 o

C /min from room temperature to 850 oC in H2 atmosphere with little amount of He.

2.4. Methane decomposition reaction CMD was conducted in a vertical fixed-bed reactor at 850 oC and atmospheric pressure. The reactor charged with 0.2 g catalyst was first heated to 850 oC under N2 (99.999 vol.%) with a flow rate of 40


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mL/min, and then the mixture gas of 10 mL/min methane and 40 mL/min nitrogen was introduced instead of pure N2. The total volumetric hourly space velocity was set at 15,000 mL/(h·gcat). The gas products were analyzed by an online gas chromatograph (Techcomp, GC7890II) equipped with a thermal conductivity detector (packed with 5A molecular sieve) and a flame ionization detector (GDX502 packed column). Methane conversion and hydrogen output rate were calculated by the following formulas, respectively:

XCH4  (FCH4 ,in -FCH4 ,out ) / FCH4 ,in 100%

(1)

Y H 2 , out  2( FC H 4 , in  FCH 4 , out ) / m

(2)

where X, Y, F, m represents the methane conversion, hydrogen output rate, gas flowrate and catalyst mass, respectively.

3. RESULTS AND DISCUSSION 3.1. Effect of Fe on properties of resultant Fe doped carbon Figure 1 shows the TG curves of raw coal and the mixture of coal with different amount of Fe addition. Obviously, coal with addition of Fe exhibits quite different pyrolysis characteristics from raw coal. There may be three processes during the pyrolysis of the mixture, including decomposition of Fe(NO3)3, coal pyrolysis and the reaction between iron oxides from Fe(NO3)3 decomposition and char from coal pyrolysis. It is observed that the weight loss of the mixture increases with the addition amount of Fe, indicating that the weight loss caused by decomposition of Fe(NO3)3 and the reaction between iron oxides from Fe(NO3)3 and chars from coal pyrolysis play an more and more important role than that caused by coal pyrolysis. Besides the peak at about 100 oC ascribed to the water removal, there exist other three peaks in the TG curves of xFe-SM. The peak at around 200 oC is attributed to the decomposition of Fe(NO3)3 into Fe2O3, which becomes larger with the increasing amount of Fe added to the coal. Compared with raw coal, xFe-



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SM samples have less weight loss rate with the increasing amount of Fe addition in the range of 400 to 500 oC, the main temperature range of coal pyrolysis, because of the decreasing amount of coal in the mixture xFe-SM. Moreover, when the Fe addition amount in coal is up to 15 wt.%, the temperature corresponding to the maximum weight loss rate in the range of 400 to 500 oC decreases, implying that the Fe addition may have a little catalytic effect on coal pyrolysis. The weight loss above 800 oC is mainly caused by the reaction between Fe2O3 and char produced from coal pyrolysis. The difference in pyrolysis characteristics between raw coal and the coal with Fe addition may affect the carbonization/activation process of coal significantly, and further the texture properties and catalytic performance of the resultant Fe-doped carbons. The addition amount of Fe in coal and the corresponding Fe content determined by ICP in the resultant catalysts are listed in Table 1. During the carbonization/activation process, there may present several reactions, including coal pyrolysis, the reaction between coal and KOH, decomposition of Fe(NO3)3 to iron oxides, and the reaction between iron oxides and coal or carbon. In the preparation of Fe-doped carbon, the amount of Fe is almost no change, but the coal was seriously consumed through pyrolysis, reaction with KOH and iron oxides. Therefore, the content of Fe in the resultant Fe-doped carbon catalyst is higher than the content in the original coal/ Fe(NO3)3 mixture. The difference increases with the increasing addition amount of Fe in coal. Figure 2 shows the XRD patterns of Fe-doped carbons with different Fe content. Clearly, for all the Fedoped carbons, only peaks at 44.7o and 65.0o ascribed to Fe metal are found besides a small peak of carbon. No peaks of iron oxides were detected, indicating that almost all the iron oxides were reduced during the carbonization. The peak of carbon disappears when the amount of Fe addition is up to 30 wt.%. Generally, carbon materials show the peaks at about 26o and 43o in XRD patterns, corresponding to C(002) and C(101)


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lattice planes, respectively. The intensity ratio of the two peaks, C(101)/C(002), can provide the information about the order of graphitic layers in carbonaceous materials30. Lower ratio means higher order of the structure. As listed in Table S1, the prepared samples become more disordered with the Fe addition amount, which is attributed to the increasing extent of reaction between carbon and iron oxides during carbonization/activation process. Figure 3a illustrates H2-TPR curves of the AC and Fe-doped carbon samples. It seems that no remarkable differences present among the Fe-doped carbons, and two main peaks appear for all the samples at similar temperature. Compared with AC, the Fe-doped carbons present the curves with a small peak around 570 oC, which may be attributed to the reduction of a little amount of unreduced Fe3O4 or FeO or the reaction of H2 with oxygen-containing groups generated during the activation process despite no peaks appear in the XRD patterns. XPS patterns of Fe2p regions for different Fe-doped carbons are shown in Figure S2. Two main peaks, assigned to Fe2p3/2 (712.5 eV) and Fe2p1/2 (725.9 eV), were seen for three Fe-doped carbons. As for Fe2p3/2, the peaks at 710.7 eV and 712.7 eV represent the existence of iron oxides such as FeO and Fe3O431,32. The existence of iron oxides can explain the hydrogen consumption around 570 oC during H2-TPR experiment. It can be seen that both the Fe-doped samples and the AC sample without Fe doping have a sharply consumption of H2 above 800 oC, indicating that the peak above 800 oC was most likely attributed to the H2 consumption of carbon rather than that of Fe species such as iron oxides. To clarify it further, the evolution gases were checked by TOF-MS analysis. Before testing, 30Fe-AC was firstly heated to 850 oC in vacuum to remove the small molecules and volatiles in carbons, and then cooled to 350 oC. After that the sample was gradually heated to 850 oC at a heating rate of 5 o

C/min in H2 atmosphere with little amount of He as a reference. From Figure 3b, the ion content of H2O

decreased and CO increased at above 650 °C, but no CH4 or other gases were detected. It is considered



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that the hydrogenation of oxygen-containing surface groups in carbon happens at above 650 °C 33, which may release CO and H2O. The hydrogenation of unsaturated hydrocarbons also happens at above 800 oC, which may lead to the consumption of H2. Table 2 shows the textural properties of Fe-doped carbon and AC produced from raw coal. Compared with AC, the total and microporous surface areas of Fe-doped carbons gradually decrease with the increase of Fe addition amount. However, the increase of external surface area and mesoporosity are mainly ascribed to the reaction between iron oxides and carbon. Especially, 10Fe-AC sample has more than 1100 m2/g of external surface area and mesoporosity of 95%. However, too much addition of Fe results in the decrease of specific surface area of the resultant catalysts because more carbon is consumed by the iron oxides from Fe(NO3)3 decomposition during carbonization, leading to the enlarging or collapsing of some pores, which is also confirmed by the change of total pore volume. For example, the pore volume of 30FeAC is 0.29 cm3/g, about one third of that of 5Fe-AC. The doping of Fe leads to the increase of average pore size of carbon catalysts.

SEM images of the Fe-doped carbons are presented in Figure 4. Compared with the AC, some particles could be observed on the surface of Fe-ACs, and increase with amount of Fe addition, suggesting that the particles observed are Fe particles, which can be confirmed by the EDS results in Figure S3. In addition, some Fe particles gradually aggregate to form larger ones, which can also be observed in Figure S3. For 30Fe-AC, the aggregated metals block some pores in carbon, leading to the sharp decline of surface area and pore volume, which is accordant with the N2 adsorption results in Table 2. Interestingly, a special and regular Fe metal morphology is found on both 15Fe-AC and 30Fe-AC, which didn’t appear for other Fe catalysts in literatures as we know2, 24-27.


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3.2. Effect of Fe addition amount on catalytic performance of resultant Fe-doped carbon Figure 5 shows the methane conversion and hydrogen output rate over carbon catalysts with different amount of Fe addition. The methane was mainly decomposed to hydrogen, and no C2H4 or other hydrocarbons was detected. The similar trend of methane conversion and hydrogen output rate with time was obtained. Clearly, the activity of Fe-doped carbon increases with Fe addition amount. When the Fe addition amount is 5 wt.%, the methane conversion and hydrogen output rate decrease with reaction time, which is similar to AC catalyst. But further increasing the Fe addition amount up to 10 wt.%, the resultant catalyst exhibits quite different behaviors, i.e., methane conversion and hydrogen output rate increase at first 30 min (as shown in Figure S4), which may be attributed to the transitional period required before Fe particles being developed to their full capacities24,27. Then the conversion and hydrogen output rate decrease in the sequent 2 h, which is related to the pore blocking by deposited carbon from CH4 decomposition. After 2 h of reaction, the activities of Fe-doped carbons with different Fe addition amount show different change with time. When the Fe addition amount is less than 15 wt.%, the activities of Fedoped carbons decrease with time after 2 h; however, little change of the activity happens for the catalyst with Fe addition amount of 20 wt.%. Methane conversion over 30Fe-AC increases with time, and reaches 58% at 550 min of reaction time. This increase was also found in Ni-doped carbon24, which was attributed to the higher dispersity and smaller size of Ni particles formed with time on stream. To understand the increase of methane conversion and hydrogen output rate with time over 30Fe-AC, methane decomposition reaction was carried out over 30Fe-AC for different time, and the spent catalysts were analyzed. As seen from Table S2, the surface area and pore volume of the catalyst almost disappear after 180 min reaction, saying that the carbon in catalyst is soon deactivated because of pore blocking by the carbon deposition in the initial reaction period. But the little change in pore volume and surface area



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of the catalyst from 180 min to 550 min indicates that the carbon in catalyst mainly serves as support for active Fe in this period. Figure 6 gives the XRD patterns of spent 30Fe-AC after different reaction time. Obviously, the peak intensity at about 26o ascribed to carbon gradually increases with the reaction time. In addition, new diffraction peaks assigned to Fe species (Fe3C) appear on the patterns of spent 30Fe-AC catalyst at reaction time of 550 min, but no peaks of Fe3C appear at reaction time of 180 min. This suggests that some Fe particles gradually form Fe3C via the reaction of Fe with carbon during the methane decomposition. Figure 7 illustrates the SEM images of 30Fe-AC reacted after different reaction time. Many particles present in carbon surface, which were identified as Fe metal particles by the EDS images in Figure S5. Different from the fresh 30Fe-AC, Fe particles on the spent 30Fe-AC become smaller. And the aggregated Fe particles seem crushed and better dispersion of Fe metal is obtained, which can also be seen from the SEM and EDS images in Figure S3 and S5. It is analyzed that the increasing carbon deposition maybe gradually crush Fe particles to form smaller and more irregular morphology. Smaller particles and better dispersion of Fe metals led to higher catalytic reactivity and increasing methane conversion with time for 30Fe-AC. Interestingly, more carbon fibers were also observed on the surface of 30Fe-AC after reaction of 550 min compared with that after reaction of 180 min. The methane decomposition may include several following steps34: The first is that the methane decomposes on the surface of iron catalyst to form surface carbon and atomic hydrogen and the latter desorbing subsequently as H2. The second is that the surface carbon diffuses into the catalyst particles and forms Fe3C that is a metastable state at high temperature, and will crack to form graphitized carbon and αFe, the latter being active to methane decomposition35-37, finally the Fe metal is gradually covered by the carbon. In our work, the cracking of Fe3C leads to the formation of smaller Fe particles, which can be


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confirmed by the SEM and EDS images as shown in Figure S5, so we consider that the Fe particles firstly cracking to smaller ones before the Fe metal is covered by the carbon deposits. The lower Fe doping, the lower active sites present in the Fe-doped carbon. When the Fe doping is low, the Fe particles is soon crushed to smaller ones and covered by the carbon deposits because of the low amount of active Fe. While the Fe amount increases to high level, more carbon is needed to form Fe3C, and the aggregated Fe particles will be smaller through the carbide cycle35, thus the higher activity with time on stream during the reaction of 550 min24. The 30Fe-AC may also deactivate after a certain period as that happens for low Fe doping catalysts, however, under the reaction condition in our fixed bed reactor, the amount of carbon deposit product from methane decomposition is so large that blocks the reactor, thus the reaction has to stop although the activity of the catalysts is still high. 3.3. Effect of carbonization/activation temperature of Fe-doped catalyst on the catalytic performances Besides the Fe addition amount, the carbonization/activation temperature is another factor to affect the properties and catalytic characteristic of resultant Fe-doped carbon. High temperature can accelerate the reaction of iron oxides with the carbon, but also cause the metal sintering. In order to examine the effect of carbonization/activation temperature, different temperature y being 750 to 900 oC is chosen at the Fe addition amount of 30 wt.%, and the other preparation conditions are the same as mentioned at section 2.2. The resultant Fe-doped carbons are expressed as 30Fe-AC-y. Figure S6 illustrated the XRD patterns of Fe-doped carbon samples prepared at different carbonization/activation temperatures. When the carbonization/activation temperature is 750 oC, the peaks attributed to Fe metal appear and no peaks of iron oxides present in XRD patterns, suggesting that iron oxides from decomposition of ferric nitrate can be in-situ reduced by the carbon at above 750 oC. No peaks belonging to carbon are detected, meaning that few amount or highly disordered carbon was present in the



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resultant Fe-doped carbons after carbonization/activation and reduction of Fe oxides at above 750 oC. The texture properties of Fe-doped catalysts prepared at different carbonization/activation temperature are listed in Table 3. Obviously, the increase of the carbonization/activation temperature leads to the decrease of specific surface area and pore volume, but the increase of mesoporosity and pore size. When the carbonization/activation temperature increases from 750 oC to 900 oC, the mesoporosity of resultant Fe-doped carbon changes from 51% to 96%, but the total pore volume decreases from 0.52 to 0.03 cm3/g. Therefore, too high carbonization/activation temperature (i.e. 900 oC) is unfavorable to the carbon structure and probably causes the collapse and/or damage of some pores. Figure 8 illustrates the effect of carbonization/activation temperature on methane conversion and hydrogen output rate over Fe-doped carbons. When the carbonization/activation temperature is 800 oC, the obtained catalyst shows the best activity in 9 h; while the catalyst prepared at 900 oC presents extreme low methane conversion and hydrogen output rate, which is related with its low specific surface area, volume and/or its metal agglomeration. Additionally, the performance of methane conversion with time on stream is different among the Fe-doped carbon catalysts prepared by different carbonization/activation temperature. Except for 30Fe-AC-900, a transitional period in first 1 h also appears for the other three samples. Different from the increasing activity on 30Fe-AC-800 and 30Fe-AC-850, the activity of 30FeAC-750 declines with time after 2 h. Although 30Fe-AC-750 has the highest specific surface area and pore volume, its catalytic activity and stability are lower than those of 30Fe-AC-800 and 30Fe-AC-850. Therefore, it seems that low carbonization temperature leads to high initial conversion from carbon, and the active sites of the catalysts prepared at higher temperature mainly come from the metal Fe particles, thus leading to lower initial catalytic activity but better stability.


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4. CONCLUSIONS

Fe-doped carbon catalysts were in-situ prepared from raw coal with addition of Fe(NO3)3 by KOH activation. The active Fe species could be directly obtained during carbonization and activation process. Fe addition amount and carbonization temperature obviously influence the textural properties of the resultant carbon materials. With increasing amount of Fe addition, the specific surface area of the resultant catalysts decreases, but the hydrogen output rate and methane conversion over Fe-doped carbon catalysts increase significantly. When the Fe addition amount is 30 wt.%, the catalyst shows the highest catalytic activity, and the methane conversion increases with time from initial 20% to 58% at reaction time of 9 h. The catalytic activity of carbon in Fe-doped carbon catalyst is soon deactivated because of pore blocking by the carbon deposition in the initial reaction period, and served as the Fe support in subsequent time. The deposited carbon from methane decomposition promotes the Fe particle size smaller, resulting in the enhancement of dispersion and increasing activity with time. The total specific surface area and pore volume of the catalysts decrease with increasing carbonization temperature. At the carbonization temperature of 800 oC and 850 oC, the obtained Fe-doped carbons exhibit relatively high activity and stability in methane decomposition.

ASSOCIATED CONTENT Supporting Information Intensity ratio of C(101) to C(002) in XRD of catalysts with different Fe contents (Table S1); textural properties of 30Fe-AC after different reaction time (Table S2); carbonization/activation temperature program in preparation of Fe-doped carbons (Figure S1); XPS patterns of the Fe2p regions for different Fe-doped carbons (Figure S2); SEM and EDS images of different Fe-doped carbons (Figure S3), and different spent Fe-doped carbons (Figure S5); CH4 conversion and H2 output rate over Fe-doped carbons



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with different Fe addition amount during the reaction (Figure S4); XRD patterns of Fe-doped carbons from different carbonization temperature (Figure S6) and the pore size distribution of Fe-doped carbons with different amount of Fe and from different carbonization temperature (Figure S7).

ACKNOWLEDGEMENTS This work was financially supported by Joint Fund of NSFC and Xinjiang Provincial Government (No. U1503194), the Joint Fund of Coal-based Low Hydrocarbons by NSFC and Shanxi Provincial Government (No. U1510101), the NSFC (No. 21576046), and Natural Science Fund of Liaoning Province (No. 20170540180).


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(9) Zhou, L.; Guo, Y.; Hideo, K. Unsupported nickel catalysts for methane catalytic decomposition into pure hydrogen. AIChE J 2014, 60, 2907. (10) Italiano, G.; Delia, A.; Espro, C.; Bonura, G.; Frusteri, F. Methane decomposition over Co thin layer supported catalysts to produce hydrogen for fuel cell. Int J Hydrogen Energy 2010, 35, 11568. (11) Jana, P.; de la Peña O’Shea, V. A.; Coronado, J. M.; Serrano, D. P. Cobalt based catalysts prepared by Pechini method for CO2-free hydrogen production by methane decomposition. Int J Hydrogen Energy 2010, 35, 10285. (12) Nuernberg, G. B.; Fajardo, H. V.; Mezalira, D. Z.; Casarin, T. J.; Probst, L. F. D.; Carreño, N. L. V. Preparation and evaluation of Co/Al2O3 catalysts in the production of hydrogen from thermo-catalytic decomposition of methane: Influence of operating conditions on catalyst performance. Fuel 2008, 87, 1698. (13) Shilapuram, V.; Ozalp, N.; Oschatz, M.; Borchardt, L.; Kaskel, S.; Lachance, R. Thermogravimetric analysis of activated carbons, ordered mesoporous carbide-derived carbons, and their deactivation kinetics of catalytic methane decomposition. Ind Eng Chem Res 2014, 53, 1741. (14) Bai, Z.; Chen, H.; Li, B.; Li, W. Catalytic decomposition of methane over activated carbon. J Anal Appl Pyrol 2005, 73, 335. (15) Kameya, Y.; Hanamura, K. Carbon black texture evolution during catalytic methane decomposition. Carbon 2012, 50, 3503. (16) Kim, M. Hydrogen production by catalytic decomposition of methane over activated carbons: kinetic study. Int J Hydrogen Energy 2004, 29, 187. (17) Muradov, N.; Smith, F.; T-Raissi, A. Catalytic activity of carbons for methane decomposition reaction. Catal Today 2005, 102-103, 225. (18) Suelves, I.; Lázaro, M. J.; Moliner, R.; Pinilla, J. L.; Cubero, H. Hydrogen production by methane


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decarbonization: Carbonaceous catalysts. Int J Hydrogen Energy 2007, 32, 3320. (19) Bai, Z.; Chen, H.; Li, B.; Li, W. Methane decomposition over Ni loaded activated carbon for hydrogen production and the formation of filamentous carbon. Int J Hydrogen Energy 2007, 32, 32. (20) Fidalgo, B.; Arenillas, A.; Menéndez, J. Á. Synergetic effect of a mixture of activated carbon+Ni/Al2O3 used as catalysts for the CO2 reforming of CH4. Appl Catal A: Gen 2010, 390, 78. (21) Takehira, K.; Ohi, T.; Shishido, T.; Kawabata, T.; Takaki, K. Catalytic growth of carbon fibers from methane and ethylene on carbon-supported Ni catalysts. Appl Catal A: Gen 2005, 283, 137. (22) Takenaka, S. Formation of filamentous carbons over supported Fe catalysts through methane decomposition. J Catal 2004, 222, 520. (23) Pudukudy, M.; Yaakob, Z.; Akmal, Z. S. Direct decomposition of methane over SBA-15 supported Ni, Co and Fe based bimetallic catalysts. Appl Surf Sci 2015, 330, 418. (24) Zhang, J.; Jin, L.; Li, Y.; Hu, H. Ni doped carbons for hydrogen production by catalytic methane decomposition. Int J Hydrogen Energy 2013, 38, 3937. (25) Ibrahim, A. A.; Al-Fatesh, A. S.; Khan, W. U.; Soliman, M. A.; Al Otaibi, R. L.; Fakeeha, A. H. Influence of support type and metal loading in methane decomposition over iron catalyst for hydrogen production. J Chin Chem Soc-Taip 2015, 62, 592. (26) Pinilla, J. L.; Utrilla, R.; Lázaro, M. J.; Moliner, R.; Suelves, I.; García, A. B. Ni- and Fe-based catalysts for hydrogen and carbon nanofilament production by catalytic decomposition of methane in a rotary bed reactor. Fuel Process Technol 2011, 92, 1480. (27) Jin, L.; Si, H.; Zhang, J.; Lin, P.; Hu, Z.; Qiu, B.; Hu, H. Preparation of activated carbon supported Fe-Al2O3 catalyst and its application for hydrogen production by catalytic methane decomposition. Int J Hydrogen Energy 2013, 38, 10373.



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Table 1 – Fe loading amount of the resultant catalyst samples Sample 5Fe-AC 10Fe-AC 15Fe-AC 20Fe-AC 30Fe-AC a mass

Fe addition amount a (wt.%)

Fe content in the catalyst b (wt.%)

5 10 15 20 30

7 16 23 33 45

ratio of Fe/coal ×100%. b Determined by ICP analysis.


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Table 2 – Textural properties of AC and Fe-doped carbon catalysts Sample

SBET (m2/g)

Smic (m2/g)

Sext (m2/g)

Mesoporositya (%)

Vt (cm3/g)

Vmic (cm3/g)

Dave (nm)

AC 5Fe-AC 10Fe-AC 15Fe-AC 20Fe-AC 30Fe-AC

1535 1501 1189 762 758 425

1055 202 106 67 97 80

480 1299 1125 695 661 345

31 87 95 91 87 81

0.76 0.88 0.81 0.59 0.53 0.29

0.49 0.08 0.04 0.02 0.04 0.03

1.97 2.30 2.63 3.10 2.80 2.73

a

Mesoporisity: Sext/SBET ×100%



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Table 3 –Textural properties of Fe-doped carbons from different carbonization temperature Smic Sext Mesoporositya Vmic Vt SBET Sample 2 2 2 (m /g) (m /g) (m /g) (%) (cm3/g) (cm3/g) 51 30Fe-AC-750 839 415 424 0.52 0.19 30Fe-AC-800 436 119 317 73 0.28 0.05 30Fe-AC-850 425 80 345 81 0.29 0.03 30Fe-AC-900 27 1.5 26 96 0.03 0.00 a Mesoporisity: Sext/SBET ×100%


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Dave (nm) 2.48 2.54 2.73 5.23

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Figure captions Figure 1 – TG (a) and DTG (b) curves of raw coal and the mixture of coal with different amount of Fe addition. (N2 flowrate, 60 mL/min; heating rate, 10 oC /min) Figure 2 – XRD patterns of Fe-doped carbon prepared with different Fe addition amount. Figure 3 – H2-TPR curves of Fe-doped carbon prepared with different Fe addition amount (a) and TOFMS curves of 30Fe-AC in H2 with little amount of He (b). Figure 4 – SEM images of the Fe-doped carbons: (a) AC, (b) 5Fe-AC, (c) 15Fe-AC, (d) 30Fe-AC. Figure 5 –CH4 conversion (a) and H2 output rate (b) over Fe-doped carbons with different Fe addition amount. (Reaction conditions: catalyst, 0.2 g; total flowrate, 50 mL/min; CH4:N2, 1:4 (v/v); space velocity, 15,000 mL/(h∙gcat); reaction temperature, 850 oC) Figure 6 – XRD patterns of fresh and spent 30Fe-AC after different reaction time. Figure 7 – SEM images of 30Fe-AC reacted after different reaction time: (a, d) Fresh 30Fe-AC, (b, e) After 180 min, (c, f) After 550 min. Figure 8 – CH4 conversion (a) and H2 output rate (b) over Fe-doped carbon prepared from different carbonization temperature (Reaction conditions: catalyst, 0.2 g; total flowrate, 50 mL/min; CH4:N2, 1:4(v/v); space velocity, 15,000 mL/(h∙gcat); reaction temperature, 850 oC)


100

a

o

Weight (wt.%)

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

Weight loss rate (wt.%/ C)


90 80

SM 5Fe-SM 10Fe-SM 15Fe-SM 20Fe-SM 30Fe-SM

70 60 50

0

200 400 600 Temperature (oC)

800

Page 24 of 32

0.00

b

-0.05 -0.10

SM 5Fe-SM 10Fe-SM 15Fe-SM 20Fe-SM 30Fe-SM

-0.15 -0.20 -0.25 0

200

400 600 Temperature (oC)

800

Figure 1 –TG (a) and DTG (b) curves of raw coal and the mixture of coal with different amount of Fe addition. (N2 flowrate, 60 mL/min; heating rate, 10 oC /min)


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

 Fe  C 30Fe-AC

Intensity (a.u.)

Page 25 of 32



20Fe-AC 15Fe-AC

10

20



10Fe-AC



5Fe-AC

30

40 50 60 2 (dgree)

70

80

Figure 2 –XRD patterns of Fe-doped carbon prepared with different Fe addition amount



b

a

CH4(16) H2O(18)

30Fe-AC

CO(28)

Intensity (a.u.)

Intensity (a.u.)

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

Page 26 of 32

20Fe-AC 15Fe-AC 10Fe-AC 5Fe-AC AC

100

300

500 700 Temperatrue (oC)

900

400

500

600 700 o Temperature ( C)

800

Figure 3 – H2-TPR curves of Fe-doped carbon prepared with different Fe/coal mass ratio (a) and TOF-MS curves of 30Fe-AC in H2 with little amount of He (b).


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Figure 4 –SEM images of Fe-doped carbons: (a) AC, (b) 5Fe-AC, (c) 15Fe-AC, (d) 30Fe-AC.


AC 5Fe-AC 10Fe-AC

60 Conversion of CH4 (%)

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

50

a

15Fe-AC 20Fe-AC 30Fe-AC

40 30 20 10 0

100

200

300

400

Time (min)

500

600

H2 output rate (mmol/(gcatmin))


3

AC 5Fe-AC 10Fe-AC

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b

15Fe-AC 20Fe-AC 30Fe-AC

2

1

0

100

200

300 400 Time (min)

500

600

Figure 5 –CH4 conversion (a) and H2 output rate (b) over Fe doped carbons with different Fe addition amount. (Reaction conditions: catalyst, 0.2 g; total flowrate, 50 mL/min; CH4:N2, 1:4 (v/v); space velocity, 15,000 mL/(h∙gcat); reaction temperature, 850 oC)


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

 

Intensity (a.u.)

Page 29 of 32

Fe  C Fe3C 

After 550 min  After 180 min

Fresh 30 Fe-AC

10

20

30

40 50 60 2(degree)

70

80

Figure 6 – XRD patterns of 30Fe-AC reacted after different reaction time



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Figure 7 – SEM images of 30Fe-AC reacted after different reaction time: (a,d) Fresh 30Fe-AC, (b,e) After 180 min, (c,f) After 550 min.


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80

Conversion of CH4 (%)

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


a

30Fe-AC-750 30Fe-AC-800 30Fe-AC-850 30Fe-AC-900

60 40 20 0

0

100

200 300 400 Time (min)

500

600

H2 output rate (mmol/(gcatmin))

Page 31 of 32

4

30Fe-AC-750 30Fe-AC-800 30Fe-AC-850 30Fe-AC-900

3

b

2 1

0

100

200 300 400 Time (min)

500

600

Figure 8 – CH4 conversion (a) and H2 output rate (b) over Fe-doped carbon under different carbonization temperature (Reaction conditions: catalyst, 0.2 g; total flowrate, 50 mL/min; CH4:N2, 1:4(v/v); space velocity, 15,000 mL/(h∙gcat); reaction temperature, 850 oC)



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For Table of Contents Only


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Preparation of Fe-doped Carbon Catalyst for Methane Decomposition

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