- Email: [email protected]
Catalytic characteristics of carbon black for decomposition of ethane
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Catalytic characteristics of carbon black for decomposition of ethane Sang Yup Lee a, Mi So Kim a, Jung Hun Kwak a, Gui Young Han a, Jong Hyeok Park a, Tae Jin Lee b, Ki June Yoon a,* a b
School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea School of Chemical Engineering and Chemical Technology, Yeungnam University, Gyeongsan 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
The catalytic activities of rubber, color and conductive carbon black catalysts for decompo-
Received 17 September 2009
sition of ethane were investigated in the temperature range from 973 to 1173 K. Signifi-
Accepted 10 February 2010
cantly higher ethane conversion and lower ethylene selectivity were obtained in the
Available online 13 February 2010
presence of carbon black catalysts compared with non-catalytic decomposition, resulting in much higher hydrogen yields. This indicates that carbon black catalysts are effective catalysts for dehydrogenation of ethane to hydrogen and ethylene, as well as for the subsequent decomposition of ethylene to hydrogen and solid carbon. However, more methane was produced in the presence of carbon black catalysts than in non-catalytic decomposition. A reaction mechanism was proposed for the catalytic decomposition of ethane. The hydrogen yield increased with an increase in the specific surface area of the nonporous rubber and color carbon black catalysts with a surface area of up to approximately 100 m2/g. However, the hydrogen yield over the carbon black catalysts with higher surface areas, including the conductive carbon black catalysts with very high surface areas, did not increase significantly. The carbon black catalysts exhibited stable activity for ethane decomposition and hydrogen production for 36 h despite carbon deposition. Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
The ability of various carbon black (CB) catalysts to decompose ethane was investigated in this study, because various studies have proposed that CB is a stable catalyst for methane decomposition [1–7]. Ethane by itself is seldom produced commercially, but ethane is the second most abundant component of natural gas. For example, one natural gas product from Abu Dhabi contains as much as 16% ethane. Since the infrastructure of natural gas is well-developed, ethane decomposition facilitated by CB catalysts can potentially be used for hydrogen production together with methane decomposition.
Methane decomposition using CB as a catalyst has many advantages compared with the conventional steam reforming. The produced clean carbon can be commercialized, and additional treatments such as the water gas shift reaction, COx removal, and catalyst regeneration are not necessary, which simplifies the process significantly [7–12]. In addition, the heat required for methane decomposition (DH0 = 75.6 kJ/ mol) can be supplied by burning 10–15% of the produced hydrogen; no CO2 is therefore emitted [2]. In previous studies from our laboratory [1–3], the methane decomposition activities of various commercial CB products, such as rubber, color, and conductive CB products, have been investigated. The catalytic characteristics of the CB products,
* Corresponding author: Fax: +82 31 290 7272. E-mail address: [email protected] (K.J. Yoon). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.02.011
CARBON
including their stability, activation energies and reaction orders, were investigated in detail. Despite carbon deposition, all the CB catalysts showed stable or quasi-stable catalytic activity, which suggests that the number of active sites remains almost constant during methane decomposition. Subsequently, acetylene chemisorption was studied to determine the nature of the CB active sites and to understand the basis for the stable activity of CB catalysts. A good correlation was observed between the methane decomposition rate and the acetylene chemisorption or consumption, and reasonable models for the chemisorption processes and maintenance of the number of active sites were proposed [4]. Few studies, however, have investigated ethane decomposition for hydrogen production using non-carbon catalysts. Solomosi et al. [13] performed ethane decomposition using Rh/ZSM-5 as a catalyst at relatively low temperatures (523– 773 K) and reported that the zeolite decreased the reaction temperature and suppressed coke formation. The maximum conversion of ethane was 23% and the main products were hydrogen, methane and ethylene. Considerable amounts of propane and BTX (benzene, toluene and xylenes) were also detected at the initial stage. However, BTX was not detected at any temperature after 20 min and propane was also not detected above 623 K. These results indicate that BTX and propane can exist as intermediates during the initial period of reaction but decompose easily into methane and ethylene. Wang et al. [14] used a Re/HZSM-5 catalyst in a Pd-based membrane reactor at mild temperatures from 773 to 858 K. At 793 K, ethane conversion was the highest (36%) and the selectivities of C3 and BTX were 9% and 55%, respectively. A Ni/SiO2 catalyst was also studied for ethane decomposition at 723–923 K [15]. This catalyst showed high initial activity but deactivated rapidly. The authors of this study suggested that the optimal reaction temperature was around 773 K because the catalyst deactivated more rapidly at higher temperatures. However, to the best of our knowledge, no studies have investigated the use of CB catalysts for ethane decomposition. In this study, as part of our research focus on the decomposition of natural gas, various commercial CB samples, such as rubber, color, and conductive CB products, were employed as the catalysts for ethane decomposition, and their activity, selectivity and stability were characterized. These CB samples have different specific surface areas and morphology. Longterm tests for two representative CB catalysts were also performed. On the basis of the product distributions, a reaction scheme for ethane decomposition was proposed.
All the CB catalysts were dried at 373 K for 24 h in air before the reaction test and characterization. Ethane decomposition was carried out in a vertical, fixed-bed, 8 mm I.D. quartz-tube flow reactor heated by an electric tube furnace (Lindberg Blue M, US). The tube was narrowed somewhat in the middle and rock wool was placed there to support the CB particles. Ethane (99.95%, Duckyang Energen, Korea) and Ar (MS Dongmin Specialty Gas, Korea) were used without further purification. The standard reaction conditions were a catalyst charge of 0.1 g, an ethane flow rate of 25 cm3 (STP)/ min and a volumetric hourly space velocity (VHSV) of 15,000 cm3/h gCB (gCB denotes the mass in grams of beforereaction dried CB). The reaction temperature ranged from 973 to 1173 K in 50 K intervals. Because the stainless steel sheath of the thermocouple is a good catalyst for hydrocarbon decomposition [1], the thermocouple was removed before the reaction experiments after the desired temperature was reached under Ar flow. The first sampling and analysis was usually done 5 min after the ethane flowed, because it took time to flush the Ar gas initially present and for the reaction system to reach steady state. Afterwards, the sampling and analysis was done every 15 min for 2 h. The product gas was analyzed by gas chromatography (Younglin M600D, Korea), using a CarboxenTM 1006 PLOT column (Supelco, US) with Ar carrier and a thermal conductivity detector. The amounts of hydrogen, methane, ethylene, and ethane in the product gas were determined using calibrated data. The long-term test was carried out at 1073 K for 36 h.
3.
Results and discussion
Selectivity was defined as follows based on the carbon balance: Selectivity of A ð%Þ ¼
¼
moles of C2 H6 converted to A 100 total moles of C2 H6 converted
ðmoles of A producedÞ ð# of carbon in 1 molecule of AÞ 2ðtotal moles of C2 H6 convertedÞ 100
Therefore, the sum of the product selectivities of the carbon-containing compounds is 100%. The hydrogen yield was calculated by dividing the moles of hydrogen produced by three times the total moles of ethane fed: H2 yieldð%Þ ¼
3.1.
2.
2031
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
moles of H2 produced 100 3ðtotal moles of C2 H6 fedÞ
Non-catalytic thermal decomposition
Experimental
The CB catalysts examined in this study are listed in Table 1. They include rubber, color, and conductive CB products with wide-ranging specific surface areas. All the rubber CB catalysts except one were in pelletized type (P); for N330, a fluffy type (N330(F)) in addition to N330(P) was tested [2]. The N2 Brunauer–Emmet–Teller surface areas of fresh CB catalysts were measured using an ASAP 2020 (Micromeritics, US) volumetric surface area analyzer, and they were in good agreement with those reported by the suppliers [2,3].
To compare catalytic decomposition results, homogeneous thermal (non-catalytic) decomposition experiments were first carried out in the temperature range of 873–1323 K with the rock wool present only in the middle of the quartz-tube reactor. Hydrogen, methane, ethylene, and solid carbon (C(s)) were detected as products. Below 923 K, ethane conversion was negligible. The results in the range from 973 to 1173 K are shown in Figs. 1–4. In this range, carbon powders were collected on the rock wool but were barely seen at the bottom of the reactor tube. Above 1223 K, a considerable quantity of
2032
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
Table 1 – CB catalysts examined. CBa
Use Rubber black
Color black
Conductive black a b c d
DCC N103 (P) DCC N330 (P) DCC N330 (F) DCC N550 (P) DCC N774 (P) HI-900L (F) HI-20L (F) HI-170 (F) EC-600JD (F) BP-2000 (P)
Manufacturer OCId, Korea
Degussa, Germany
Ketjen Black, Japan Cabot, US
Particle diameter 11–19 26–30 26–30 40–48 61–100 15 28 58 15 15
b
(nm)
Surface areac (m2/gCB) 160 70 77 40 29 256 85 23 1369 1475
(F) for fluffy and (P) for pelletized type. Data from the suppliers. Measured in this study. Formerly DC chemical.
tar and carbon film formed on the reactor wall in addition to significant amounts of carbon particles on the rock wool and at the bottom of the reactor tube. Therefore, the optimal temperature range for ethane decomposition was concluded to be between 973 and 1173 K.
The ethane conversions in the presence of CB catalysts are shown in Fig. 1. Compared with non-catalytic decomposition, the ethane conversions in the presence of CB catalysts were all higher across the entire temperature range evaluated. This indicates that CB catalysts are catalytically effective for ethane decomposition. The differences in conversion were pronounced at 1023 and 1073 K, where the non-catalytic ethane conversions were moderately high. As the surface area of the CB catalysts increased, so did the conversion as a whole.
The methane selectivities in the presence of the CB catalysts are presented in Fig. 2. Two different trends were observed when comparing the results for non-catalytic and catalytic decomposition. The first trend was that the methane selectivity of CB catalysts was higher than that obtained by non-catalytic decomposition in most cases. The other was that methane selectivity increased with temperature for non-catalytic decomposition, while it decreased at and above 1123 K for several CB catalysts and became comparable to or smaller than the methane selectivity obtained by non-catalytic decomposition at 1173 K. These results show that CB catalysts facilitate the formation of methane from ethane and at the same time effectively decompose the methane produced, especially at high temperatures. This will be discussed in more detail in Section 3.4. The ethylene selectivities in the presence of CB catalysts were significantly lower than those of non-catalytic decom-
Fig. 1 – Ethane conversion vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
Fig. 2 – Methane selectivity vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
3.2.
Comparison of the activity and selectivity
CARBON
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
Fig. 3 – Ethylene selectivity vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
position (Fig. 3). This clearly shows that CB catalysts can effectively decompose ethylene to C(s) and hydrogen. As the surface area of the CB catalysts increased, the ethylene selectivity became lower. The ethylene selectivity increased with temperature at low temperatures up to 1023 K in most cases. This can be explained as follows. Ethane conversion increases about threefold from 973 to 1023 K, which results in the production of much more ethylene, but a smaller proportion of the produced ethylene decomposes due to the still low temperature. However, above 1023 K, the ethylene selectivity decreases rap-
Fig. 4 – Hydrogen yield vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
2033
idly with temperature, indicating that ethylene decomposes rapidly. It is important to note that the ethylene yield (not shown here for brevity), which is the ethylene selectivity multiplied by the ethane conversion, was much higher at 1023 and 1073 K than at 973 K, and the maximum ethylene yield appeared at around 1073 K for each CB. The hydrogen yields are presented in Fig. 4. The hydrogen yields in the presence of the CB catalysts were much higher than those obtained by non-catalytic decomposition. This is in agreement with the higher ethane conversion and the lower ethylene selectivity in the presence of the CB catalysts, because hydrogen is produced by dehydrogenation of ethane and subsequent decomposition of the produced ethylene. In contrast, the formation of methane requires consumption of the produced hydrogen. However, although the methane selectivity of the CB catalysts was higher, it was so small that its effect on the hydrogen yield was insignificant. The hydrogen yield increased again as the specific surface area of the CB catalysts increased. In order to examine the effect of surface area on the catalytic activity of CB catalysts, the ethane conversion, ethylene selectivity, and hydrogen yield at a fixed temperature were plotted against the specific surface areas of fresh CB catalysts (Fig. 5). The selected temperature was 1023 K, as at this temperature the catalytic effects were the most pronounced compared with non-catalytic decomposition. Data for the surface area of zero correspond to non-catalytic decomposition. As the surface area increased up to ca. 100 m2/gCB, ethane conversion increased and ethylene selectivity decreased, resulting in a marked increase in hydrogen yield. However, for CB catalysts with a surface area higher than this, the increase in hydrogen yield was not significant, even for conductive CB catalysts with a surface area greater than 1300 m2/gCB. This indicates that the number of active sites on CB catalysts
Fig. 5 – Ethane conversion, ethylene selectivity and hydrogen yield at 1023 K according to the specific surface area of fresh CB catalysts (VHSV = 15,000 cm3/h gCB).
2034
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
is not proportional to the surface area, but tends to increase less and less as the surface area increases [2–4,7].
3.3.
Long-term tests
Long-term tests were performed with N330(F) and BP-2000 as representative CB catalysts, as these two catalysts had higher catalytic activity than the other CB catalysts evaluated. The chosen reaction temperature was 1073 K, as at this temperature ethane conversion is high and the catalytic effects are pronounced. The results are presented in Figs. 6 and 7. For both CB catalysts, ethane conversion was stable for 36 h. The methane selectivity in the presence of BP-2000 was very stable, but that in the presence of N330(F) increased slowly and became considerably higher than that in the presence of BP-2000. In contrast, the ethylene selectivity in the presence of both CB catalysts decreased gradually. Consequently, the hydrogen yield in the presence of BP-2000 increased, but it decreased slowly in the presence of N330(F). Nevertheless, the hydrogen yields for both CB catalysts were fairly stable. This will be discussed in more detail in Section 3.4. As the produced carbon is deposited, the surface area per mass of the original CB plus the deposited carbon decreases, but the mass of whole carbon in the reactor increases with time. As a consequence, the total surface area of carbon in the reactor varies during reaction, as reported in a previous study [1]. Despite this, the nearly constant catalytic activity of whole carbon again suggests that the total number of active sites remains constant for each CB [1,4,6].
3.4.
Proposed reaction mechanism
The ethane decomposition results indicate that both non-catalytic thermal decomposition and catalytic decomposition are significant, suggesting that ethane decomposition pro-
Fig. 7 – Ethylene selectivity and hydrogen yield in long-term tests with N330(F) and BP-2000 at 1073 K (VHSV = 15,000 cm3/h gCB).
ceeds by the two mechanisms operating in parallel. Because the thermal decomposition of ethane proceeds by the wellknown free-radical chain reaction mechanism, its process will be briefly described as follows. In the initiation steps, an ethane molecule decomposes to produce two methyl radicals, and then the methyl radical reacts with an ethane molecule to produce a methane molecule and an ethyl radical. In the propagation steps, the ethyl radical decomposes to an ethylene molecule and a hydrogen radical, and the hydrogen radical reacts with an ethane molecule to produce a hydrogen molecule and an ethyl radical. Finally, ethylene decomposes to C(s) and hydrogen through several steps. The mechanism of catalytic decomposition is proposed as follows, where the active site is represented by *: (i) Ethylene formation: CH3 CH3 þ ! CH3 CH2 þH–
ð1Þ
CH3 CH2 þ ! CH2 @CH2 þ H–
ð2Þ
2H– ! H2 þ 2
ð3Þ
(ii) Decomposition of ethylene: CH2 @CH2 þ 2 ! CH2 @CH– þH–
ð4Þ
CH2 @CH– ! CH@CH– þH–
ð5Þ
CH@CH– þ ! –CH@CH–
ð6Þ
–CH@CH– ! H2 þ –C @C –ð¼ CðsÞÞ
ð7Þ
2H– ! H2 þ 2
ð8Þ
Overall reaction : CH2 @CH2 ¼ 2 ! 2H2 þ –C @C –ð¼ CðsÞÞ Fig. 6 – Ethane conversion and methane selectivity in longterm tests with N330(F) and BP-2000 at 1073 K (VHSV = 15,000 cm3/h gCB).
(iii) Other possible elementary steps: CH3 þH– ! CH4 þ
ð9Þ
CARBON
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
CH3 þ ! CH3 –
ð10Þ
CH3 – þH– $ CH4 þ 2
ð11Þ
CH3 – þ $ CH2 – þ H–
ð12Þ
During catalytic decomposition, an ethyl radical can be formed from ethane and then transformed to ethylene with the aid of a catalyst, as shown in Steps 1 and 2. These steps are in accordance with the higher ethane conversion seen with the catalyst present. The processes for the formation of H2 and C(s) from ethylene are proposed in Steps 4–8. The key point in these steps is that the adsorbed acetylenic species are indispensable intermediates. Although the acetylenic species may be formed through other pathways than Steps 4 and 5 [16], it is simply postulated here that the hydrogen atoms are abstracted sequentially. Steps 6 and 7 are proposed based on a previous study on acetylene chemisorption on CB catalysts [4], in which acetylene adsorbs in a bridged form on the edge of the hexagonal rings and then the hydrogen atoms are rapidly abstracted at a sufficiently high temperature (>873 K), forming new hexagonal rings and leaving behind active sites on the new edge. The species –C*=C*– in Step 7 denotes the newly formed active sites on the new graphene layer edges of the CB catalysts. These steps are similar to those in the hydrogen-abstraction–C2H2-addition mechanism that has been proposed by other previous studies [17,18]. As a result, a repetitive reaction sequence of abstraction of hydrogen atoms followed by addition of C2 species predicts the growth of graphene layers [4,18]. In fact, the growth of graphene layers has been observed by the transmission electron microscopy during decomposition of hydrocarbons over CB catalysts; the growing graphene layers stack in parallel and form conical protrusions [2,3,19]. As mentioned in the Section 1, this proposal of the mechanism can also explain maintenance of the number of active sites despite carbon deposition. Consequently, Steps 4–8 can explain the more rapid decomposition of ethylene and the higher hydrogen yield when CB catalysts are present. Other possible elementary steps include the formation of methane (Steps 9–11) and the decomposition of methane or methyl radicals (Steps 10–12). The methyl radicals formed by CAC bond breaking can react directly with adsorbed hydrogen or they can adsorb and then react with adsorbed hydrogen to produce methane. This can explain the higher methane selectivity of CB catalysts compared with non-catalytic decomposition (Fig. 2). However, at sufficiently high temperatures (>1123 K), methane can decompose rapidly when CB catalysts are present, and hence the methane selectivity becomes lower than that observed for non-catalytic decomposition. The species ÆCH2–* in Step 12 may subsequently undergo many possible elementary steps [16], such as addition of methyl and ethyl radicals or an ethylene molecule; the resulting CnHm–* intermediates are expected to eventually produce new graphene layer edges and H2 molecules after hydrogen abstraction. However, a detailed mechanistic description of these steps is beyond the scope of this study. The difference in the stability of the methane selectivity of N330(F) and BP-2000, shown in Fig. 6, is considered to originate from their original structures and the structures of deposited carbon. Active sites are formed on the deposited carbon [1–4,7], but their characteristics can differ from those of the
2035
original CB catalyst. Scanning electron spectroscopy and transmission electron spectroscopy were used in a previous study [2] to demonstrate that N330(F) is nonporous and that the deposited carbon forms many protrusions on the particles. The results in Figs. 2 and 6 suggest that the active sites on the protrusions produce more methane from ethane or become gradually less active for methane decomposition as the protrusions grow. The reasoning for this is that the methane production by CB requires one single methyl radical or adsorbed methyl group and a very mobile adsorbed hydrogen atom, while methane decomposition requires multiple sites, as shown in Steps 11 and 12. The active sites on the protrusions may be less effective for methane decomposition. On the contrary, BP-2000 is highly porous, and deposited carbon fills in the pores first, thereby making the particles of BP-2000 gradually nonporous, but with no formation of protrusions for a while [3]. Hence, the methane selectivity of BP-2000 remains constant over time. In contrast, the ethylene selectivity of both N330(F) and BP2000 decreased slowly over time, as shown in Fig. 7. This suggests that ethylene decomposition is less sensitive to the structures of the original CB and the deposited carbon than methane decomposition. This is probably because ethylene, which has an unsaturated bond, adsorbs more easily than methane, or that some active sites are effective only for adsorption of ethylene while others can adsorb both ethylene and methane. The slow increase in the ethylene decomposition rate indicates that the number of active sites for ethylene adsorption increases gradually with increasing amounts of deposited carbon.
4.
Conclusions
As part of our ongoing research into decomposition of natural gas, the catalytic activities of rubber, color and conductive CB catalysts for ethane decomposition were investigated. Significantly higher ethane conversion and lower ethylene selectivity were obtained in the presence of CB compared with noncatalytic thermal decomposition, resulting in much higher hydrogen yields. This indicates that CB is catalytically effective for dehydrogenation of ethane to hydrogen and ethylene as well as subsequent decomposition of ethylene to hydrogen and C(s). However, more methane was produced in the presence of CB catalysts. Based on the product distributions, a reaction mechanism for the CB catalytic decomposition of ethane was proposed. The hydrogen yield increased with an increase in the specific surface area of fresh CB catalysts up to ca. 100 m2/g. However, for CB catalysts with surface areas greater than this, the increase in hydrogen yield was not significant, even for conductive CB catalysts with a surface area greater than 1300 m2/g. This indicates that the number of active sites on CB catalysts is not proportional to the surface area, but increases less and less as the surface area increases. Despite carbon deposition, representative CB catalysts exhibited stable activity for ethane decomposition and hydrogen production for 36 h, which suggests that the number of active sites remained nearly constant for each CB. However, methane selectivity remained constant or slowly increased,
2036
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
while ethylene selectivity decreased very slowly. This was discussed with respect to the different structures of the original CB and the deposited carbon.
Acknowledgements This work was supported by the ‘‘National R&D Organization for Hydrogen and Fuel Cells,’’ component of the New and Renewable Energy R&D Program (2004-N-HY12-P-03-0000) of the Korea Energy Management Corporation under the Ministry of Knowledge Economy of Korea. The authors would also like to thank OCI for supplying carbon black samples.
R E F E R E N C E S
[1] Lee EK, Lee SY, Han GY, Lee TJ, Jun JH, Yoon KJ, et al. Catalytic decomposition of methane over carbon blacks for CO2-free hydrogen production. Carbon 2004;42:2641–8. [2] Ryu BH, Lee SY, Lee DH, Han GY, Lee TJ, Yoon KJ. Catalytic characteristics of various rubber-reinforcing carbon blacks in decomposition of methane for hydrogen production. Catal Today 2007;123:303–9. [3] Lee SY, Ryu BH, Han GY, Lee TJ, Yoon KJ. Catalytic characteristics of specialty carbon blacks in decomposition of methane for hydrogen production. Carbon 2008;46:1978–86. [4] Lee SY, Kwak JH, Han GY, Lee TJ, Yoon KJ. Characterization of active sites for methane decomposition on carbon black through acetylene chemisorption. Carbon 2008;46:342–8. [5] Muradov NZ. How to produce hydrogen from fossil fuels without CO2 emission. Int J Hydrogen Energy 1993;18(3):211–5. [6] Muradov N. Hydrogen via methane decomposition: an application for decarbonization of fossil fuels. Int J Hydrogen Energy 2001;26:1165–75.
[7] Muradov N, Smith F, T-Raissi A. Catalytic activity of carbons for methane decomposition reaction. Catal Today 2005;102:225–33. [8] Fulcheri L, Schwob Y. From methane to hydrogen, carbon black and water. Int J Hydrogen Energy 1995;20(3):197–202. [9] Poirier MG, Sapundzhiev C. Catalytic decomposition of natural gas to hydrogen for fuel cell applications. Int J Hydrogen Energy 1997;22(4):429–33. [10] Gaudernack B, Lynum S. Hydrogen from natural gas without release of CO2 to the atmosphere. Int J Hydrogen Energy 1998;23(12):1087–93. [11] Steinberg M. Fossil fuel decarbonization technology for mitigating global warming. Int J Hydrogen Energy 1999;24:771–7. [12] Moliner R, Suelves I, La´zaro MJ, Moreno O. Thermocatalytic decomposition of methane over activated carbons: influence of textural properties and surface chemistry. Int J Hydrogen Energy 2005;30:293–300. [13] Solomosi F, Szoke A, Ovari L. Decomposition of ethane and its reaction with CO2 over Rh/ZSM-5 catalyst. J Catal 1999;186:269–78. [14] Wang L, Murata K, Inaba M. Production of pure hydrogen and more valuable hydrocarbons from ethane on a novel highly active catalyst system with a Pd-based membrane reactor. Catal Today 2003;82:99–104. [15] Chin SY, Chin Y-H, Amiridis MD. Hydrogen production via catalytic cracking of ethane over Ni/SiO2 catalysts. Appl Catal A 2006;300:8–13. [16] Wauters S, Martin GB. Computer generation of a network of elementary steps for coke formation during the thermal cracking of hydrocarbons. Chem Eng J 2001;82:267–79. [17] Frenkalch M. Reaction mechanism of soot formation in flames. Phys Chem Chem Phys 2002;4:2028–37. [18] Frenklach M, Ping J. On the role of surface migration in the growth and structure of graphene layers. Carbon 2004;42:1209–12. [19] Yoon SH, Park NK, Lee TJ, Yoon KJ, Han GY. Hydrogen production by thermocatalytic decomposition of butane over a carbon black catalyst. Catal Today 2009;146:202–8.
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Catalytic characteristics of carbon black for decomposition of ethane Sang Yup Lee a, Mi So Kim a, Jung Hun Kwak a, Gui Young Han a, Jong Hyeok Park a, Tae Jin Lee b, Ki June Yoon a,* a b
School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea School of Chemical Engineering and Chemical Technology, Yeungnam University, Gyeongsan 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
The catalytic activities of rubber, color and conductive carbon black catalysts for decompo-
Received 17 September 2009
sition of ethane were investigated in the temperature range from 973 to 1173 K. Signifi-
Accepted 10 February 2010
cantly higher ethane conversion and lower ethylene selectivity were obtained in the
Available online 13 February 2010
presence of carbon black catalysts compared with non-catalytic decomposition, resulting in much higher hydrogen yields. This indicates that carbon black catalysts are effective catalysts for dehydrogenation of ethane to hydrogen and ethylene, as well as for the subsequent decomposition of ethylene to hydrogen and solid carbon. However, more methane was produced in the presence of carbon black catalysts than in non-catalytic decomposition. A reaction mechanism was proposed for the catalytic decomposition of ethane. The hydrogen yield increased with an increase in the specific surface area of the nonporous rubber and color carbon black catalysts with a surface area of up to approximately 100 m2/g. However, the hydrogen yield over the carbon black catalysts with higher surface areas, including the conductive carbon black catalysts with very high surface areas, did not increase significantly. The carbon black catalysts exhibited stable activity for ethane decomposition and hydrogen production for 36 h despite carbon deposition. Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
The ability of various carbon black (CB) catalysts to decompose ethane was investigated in this study, because various studies have proposed that CB is a stable catalyst for methane decomposition [1–7]. Ethane by itself is seldom produced commercially, but ethane is the second most abundant component of natural gas. For example, one natural gas product from Abu Dhabi contains as much as 16% ethane. Since the infrastructure of natural gas is well-developed, ethane decomposition facilitated by CB catalysts can potentially be used for hydrogen production together with methane decomposition.
Methane decomposition using CB as a catalyst has many advantages compared with the conventional steam reforming. The produced clean carbon can be commercialized, and additional treatments such as the water gas shift reaction, COx removal, and catalyst regeneration are not necessary, which simplifies the process significantly [7–12]. In addition, the heat required for methane decomposition (DH0 = 75.6 kJ/ mol) can be supplied by burning 10–15% of the produced hydrogen; no CO2 is therefore emitted [2]. In previous studies from our laboratory [1–3], the methane decomposition activities of various commercial CB products, such as rubber, color, and conductive CB products, have been investigated. The catalytic characteristics of the CB products,
* Corresponding author: Fax: +82 31 290 7272. E-mail address: [email protected] (K.J. Yoon). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.02.011
CARBON
including their stability, activation energies and reaction orders, were investigated in detail. Despite carbon deposition, all the CB catalysts showed stable or quasi-stable catalytic activity, which suggests that the number of active sites remains almost constant during methane decomposition. Subsequently, acetylene chemisorption was studied to determine the nature of the CB active sites and to understand the basis for the stable activity of CB catalysts. A good correlation was observed between the methane decomposition rate and the acetylene chemisorption or consumption, and reasonable models for the chemisorption processes and maintenance of the number of active sites were proposed [4]. Few studies, however, have investigated ethane decomposition for hydrogen production using non-carbon catalysts. Solomosi et al. [13] performed ethane decomposition using Rh/ZSM-5 as a catalyst at relatively low temperatures (523– 773 K) and reported that the zeolite decreased the reaction temperature and suppressed coke formation. The maximum conversion of ethane was 23% and the main products were hydrogen, methane and ethylene. Considerable amounts of propane and BTX (benzene, toluene and xylenes) were also detected at the initial stage. However, BTX was not detected at any temperature after 20 min and propane was also not detected above 623 K. These results indicate that BTX and propane can exist as intermediates during the initial period of reaction but decompose easily into methane and ethylene. Wang et al. [14] used a Re/HZSM-5 catalyst in a Pd-based membrane reactor at mild temperatures from 773 to 858 K. At 793 K, ethane conversion was the highest (36%) and the selectivities of C3 and BTX were 9% and 55%, respectively. A Ni/SiO2 catalyst was also studied for ethane decomposition at 723–923 K [15]. This catalyst showed high initial activity but deactivated rapidly. The authors of this study suggested that the optimal reaction temperature was around 773 K because the catalyst deactivated more rapidly at higher temperatures. However, to the best of our knowledge, no studies have investigated the use of CB catalysts for ethane decomposition. In this study, as part of our research focus on the decomposition of natural gas, various commercial CB samples, such as rubber, color, and conductive CB products, were employed as the catalysts for ethane decomposition, and their activity, selectivity and stability were characterized. These CB samples have different specific surface areas and morphology. Longterm tests for two representative CB catalysts were also performed. On the basis of the product distributions, a reaction scheme for ethane decomposition was proposed.
All the CB catalysts were dried at 373 K for 24 h in air before the reaction test and characterization. Ethane decomposition was carried out in a vertical, fixed-bed, 8 mm I.D. quartz-tube flow reactor heated by an electric tube furnace (Lindberg Blue M, US). The tube was narrowed somewhat in the middle and rock wool was placed there to support the CB particles. Ethane (99.95%, Duckyang Energen, Korea) and Ar (MS Dongmin Specialty Gas, Korea) were used without further purification. The standard reaction conditions were a catalyst charge of 0.1 g, an ethane flow rate of 25 cm3 (STP)/ min and a volumetric hourly space velocity (VHSV) of 15,000 cm3/h gCB (gCB denotes the mass in grams of beforereaction dried CB). The reaction temperature ranged from 973 to 1173 K in 50 K intervals. Because the stainless steel sheath of the thermocouple is a good catalyst for hydrocarbon decomposition [1], the thermocouple was removed before the reaction experiments after the desired temperature was reached under Ar flow. The first sampling and analysis was usually done 5 min after the ethane flowed, because it took time to flush the Ar gas initially present and for the reaction system to reach steady state. Afterwards, the sampling and analysis was done every 15 min for 2 h. The product gas was analyzed by gas chromatography (Younglin M600D, Korea), using a CarboxenTM 1006 PLOT column (Supelco, US) with Ar carrier and a thermal conductivity detector. The amounts of hydrogen, methane, ethylene, and ethane in the product gas were determined using calibrated data. The long-term test was carried out at 1073 K for 36 h.
3.
Results and discussion
Selectivity was defined as follows based on the carbon balance: Selectivity of A ð%Þ ¼
¼
moles of C2 H6 converted to A 100 total moles of C2 H6 converted
ðmoles of A producedÞ ð# of carbon in 1 molecule of AÞ 2ðtotal moles of C2 H6 convertedÞ 100
Therefore, the sum of the product selectivities of the carbon-containing compounds is 100%. The hydrogen yield was calculated by dividing the moles of hydrogen produced by three times the total moles of ethane fed: H2 yieldð%Þ ¼
3.1.
2.
2031
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
moles of H2 produced 100 3ðtotal moles of C2 H6 fedÞ
Non-catalytic thermal decomposition
Experimental
The CB catalysts examined in this study are listed in Table 1. They include rubber, color, and conductive CB products with wide-ranging specific surface areas. All the rubber CB catalysts except one were in pelletized type (P); for N330, a fluffy type (N330(F)) in addition to N330(P) was tested [2]. The N2 Brunauer–Emmet–Teller surface areas of fresh CB catalysts were measured using an ASAP 2020 (Micromeritics, US) volumetric surface area analyzer, and they were in good agreement with those reported by the suppliers [2,3].
To compare catalytic decomposition results, homogeneous thermal (non-catalytic) decomposition experiments were first carried out in the temperature range of 873–1323 K with the rock wool present only in the middle of the quartz-tube reactor. Hydrogen, methane, ethylene, and solid carbon (C(s)) were detected as products. Below 923 K, ethane conversion was negligible. The results in the range from 973 to 1173 K are shown in Figs. 1–4. In this range, carbon powders were collected on the rock wool but were barely seen at the bottom of the reactor tube. Above 1223 K, a considerable quantity of
2032
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
Table 1 – CB catalysts examined. CBa
Use Rubber black
Color black
Conductive black a b c d
DCC N103 (P) DCC N330 (P) DCC N330 (F) DCC N550 (P) DCC N774 (P) HI-900L (F) HI-20L (F) HI-170 (F) EC-600JD (F) BP-2000 (P)
Manufacturer OCId, Korea
Degussa, Germany
Ketjen Black, Japan Cabot, US
Particle diameter 11–19 26–30 26–30 40–48 61–100 15 28 58 15 15
b
(nm)
Surface areac (m2/gCB) 160 70 77 40 29 256 85 23 1369 1475
(F) for fluffy and (P) for pelletized type. Data from the suppliers. Measured in this study. Formerly DC chemical.
tar and carbon film formed on the reactor wall in addition to significant amounts of carbon particles on the rock wool and at the bottom of the reactor tube. Therefore, the optimal temperature range for ethane decomposition was concluded to be between 973 and 1173 K.
The ethane conversions in the presence of CB catalysts are shown in Fig. 1. Compared with non-catalytic decomposition, the ethane conversions in the presence of CB catalysts were all higher across the entire temperature range evaluated. This indicates that CB catalysts are catalytically effective for ethane decomposition. The differences in conversion were pronounced at 1023 and 1073 K, where the non-catalytic ethane conversions were moderately high. As the surface area of the CB catalysts increased, so did the conversion as a whole.
The methane selectivities in the presence of the CB catalysts are presented in Fig. 2. Two different trends were observed when comparing the results for non-catalytic and catalytic decomposition. The first trend was that the methane selectivity of CB catalysts was higher than that obtained by non-catalytic decomposition in most cases. The other was that methane selectivity increased with temperature for non-catalytic decomposition, while it decreased at and above 1123 K for several CB catalysts and became comparable to or smaller than the methane selectivity obtained by non-catalytic decomposition at 1173 K. These results show that CB catalysts facilitate the formation of methane from ethane and at the same time effectively decompose the methane produced, especially at high temperatures. This will be discussed in more detail in Section 3.4. The ethylene selectivities in the presence of CB catalysts were significantly lower than those of non-catalytic decom-
Fig. 1 – Ethane conversion vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
Fig. 2 – Methane selectivity vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
3.2.
Comparison of the activity and selectivity
CARBON
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
Fig. 3 – Ethylene selectivity vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
position (Fig. 3). This clearly shows that CB catalysts can effectively decompose ethylene to C(s) and hydrogen. As the surface area of the CB catalysts increased, the ethylene selectivity became lower. The ethylene selectivity increased with temperature at low temperatures up to 1023 K in most cases. This can be explained as follows. Ethane conversion increases about threefold from 973 to 1023 K, which results in the production of much more ethylene, but a smaller proportion of the produced ethylene decomposes due to the still low temperature. However, above 1023 K, the ethylene selectivity decreases rap-
Fig. 4 – Hydrogen yield vs. temperature in the presence of CB catalysts (VHSV = 15,000 cm3/h gCB).
2033
idly with temperature, indicating that ethylene decomposes rapidly. It is important to note that the ethylene yield (not shown here for brevity), which is the ethylene selectivity multiplied by the ethane conversion, was much higher at 1023 and 1073 K than at 973 K, and the maximum ethylene yield appeared at around 1073 K for each CB. The hydrogen yields are presented in Fig. 4. The hydrogen yields in the presence of the CB catalysts were much higher than those obtained by non-catalytic decomposition. This is in agreement with the higher ethane conversion and the lower ethylene selectivity in the presence of the CB catalysts, because hydrogen is produced by dehydrogenation of ethane and subsequent decomposition of the produced ethylene. In contrast, the formation of methane requires consumption of the produced hydrogen. However, although the methane selectivity of the CB catalysts was higher, it was so small that its effect on the hydrogen yield was insignificant. The hydrogen yield increased again as the specific surface area of the CB catalysts increased. In order to examine the effect of surface area on the catalytic activity of CB catalysts, the ethane conversion, ethylene selectivity, and hydrogen yield at a fixed temperature were plotted against the specific surface areas of fresh CB catalysts (Fig. 5). The selected temperature was 1023 K, as at this temperature the catalytic effects were the most pronounced compared with non-catalytic decomposition. Data for the surface area of zero correspond to non-catalytic decomposition. As the surface area increased up to ca. 100 m2/gCB, ethane conversion increased and ethylene selectivity decreased, resulting in a marked increase in hydrogen yield. However, for CB catalysts with a surface area higher than this, the increase in hydrogen yield was not significant, even for conductive CB catalysts with a surface area greater than 1300 m2/gCB. This indicates that the number of active sites on CB catalysts
Fig. 5 – Ethane conversion, ethylene selectivity and hydrogen yield at 1023 K according to the specific surface area of fresh CB catalysts (VHSV = 15,000 cm3/h gCB).
2034
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
is not proportional to the surface area, but tends to increase less and less as the surface area increases [2–4,7].
3.3.
Long-term tests
Long-term tests were performed with N330(F) and BP-2000 as representative CB catalysts, as these two catalysts had higher catalytic activity than the other CB catalysts evaluated. The chosen reaction temperature was 1073 K, as at this temperature ethane conversion is high and the catalytic effects are pronounced. The results are presented in Figs. 6 and 7. For both CB catalysts, ethane conversion was stable for 36 h. The methane selectivity in the presence of BP-2000 was very stable, but that in the presence of N330(F) increased slowly and became considerably higher than that in the presence of BP-2000. In contrast, the ethylene selectivity in the presence of both CB catalysts decreased gradually. Consequently, the hydrogen yield in the presence of BP-2000 increased, but it decreased slowly in the presence of N330(F). Nevertheless, the hydrogen yields for both CB catalysts were fairly stable. This will be discussed in more detail in Section 3.4. As the produced carbon is deposited, the surface area per mass of the original CB plus the deposited carbon decreases, but the mass of whole carbon in the reactor increases with time. As a consequence, the total surface area of carbon in the reactor varies during reaction, as reported in a previous study [1]. Despite this, the nearly constant catalytic activity of whole carbon again suggests that the total number of active sites remains constant for each CB [1,4,6].
3.4.
Proposed reaction mechanism
The ethane decomposition results indicate that both non-catalytic thermal decomposition and catalytic decomposition are significant, suggesting that ethane decomposition pro-
Fig. 7 – Ethylene selectivity and hydrogen yield in long-term tests with N330(F) and BP-2000 at 1073 K (VHSV = 15,000 cm3/h gCB).
ceeds by the two mechanisms operating in parallel. Because the thermal decomposition of ethane proceeds by the wellknown free-radical chain reaction mechanism, its process will be briefly described as follows. In the initiation steps, an ethane molecule decomposes to produce two methyl radicals, and then the methyl radical reacts with an ethane molecule to produce a methane molecule and an ethyl radical. In the propagation steps, the ethyl radical decomposes to an ethylene molecule and a hydrogen radical, and the hydrogen radical reacts with an ethane molecule to produce a hydrogen molecule and an ethyl radical. Finally, ethylene decomposes to C(s) and hydrogen through several steps. The mechanism of catalytic decomposition is proposed as follows, where the active site is represented by *: (i) Ethylene formation: CH3 CH3 þ ! CH3 CH2 þH–
ð1Þ
CH3 CH2 þ ! CH2 @CH2 þ H–
ð2Þ
2H– ! H2 þ 2
ð3Þ
(ii) Decomposition of ethylene: CH2 @CH2 þ 2 ! CH2 @CH– þH–
ð4Þ
CH2 @CH– ! CH@CH– þH–
ð5Þ
CH@CH– þ ! –CH@CH–
ð6Þ
–CH@CH– ! H2 þ –C @C –ð¼ CðsÞÞ
ð7Þ
2H– ! H2 þ 2
ð8Þ
Overall reaction : CH2 @CH2 ¼ 2 ! 2H2 þ –C @C –ð¼ CðsÞÞ Fig. 6 – Ethane conversion and methane selectivity in longterm tests with N330(F) and BP-2000 at 1073 K (VHSV = 15,000 cm3/h gCB).
(iii) Other possible elementary steps: CH3 þH– ! CH4 þ
ð9Þ
CARBON
4 8 ( 20 1 0 ) 2 0 3 0–20 3 6
CH3 þ ! CH3 –
ð10Þ
CH3 – þH– $ CH4 þ 2
ð11Þ
CH3 – þ $ CH2 – þ H–
ð12Þ
During catalytic decomposition, an ethyl radical can be formed from ethane and then transformed to ethylene with the aid of a catalyst, as shown in Steps 1 and 2. These steps are in accordance with the higher ethane conversion seen with the catalyst present. The processes for the formation of H2 and C(s) from ethylene are proposed in Steps 4–8. The key point in these steps is that the adsorbed acetylenic species are indispensable intermediates. Although the acetylenic species may be formed through other pathways than Steps 4 and 5 [16], it is simply postulated here that the hydrogen atoms are abstracted sequentially. Steps 6 and 7 are proposed based on a previous study on acetylene chemisorption on CB catalysts [4], in which acetylene adsorbs in a bridged form on the edge of the hexagonal rings and then the hydrogen atoms are rapidly abstracted at a sufficiently high temperature (>873 K), forming new hexagonal rings and leaving behind active sites on the new edge. The species –C*=C*– in Step 7 denotes the newly formed active sites on the new graphene layer edges of the CB catalysts. These steps are similar to those in the hydrogen-abstraction–C2H2-addition mechanism that has been proposed by other previous studies [17,18]. As a result, a repetitive reaction sequence of abstraction of hydrogen atoms followed by addition of C2 species predicts the growth of graphene layers [4,18]. In fact, the growth of graphene layers has been observed by the transmission electron microscopy during decomposition of hydrocarbons over CB catalysts; the growing graphene layers stack in parallel and form conical protrusions [2,3,19]. As mentioned in the Section 1, this proposal of the mechanism can also explain maintenance of the number of active sites despite carbon deposition. Consequently, Steps 4–8 can explain the more rapid decomposition of ethylene and the higher hydrogen yield when CB catalysts are present. Other possible elementary steps include the formation of methane (Steps 9–11) and the decomposition of methane or methyl radicals (Steps 10–12). The methyl radicals formed by CAC bond breaking can react directly with adsorbed hydrogen or they can adsorb and then react with adsorbed hydrogen to produce methane. This can explain the higher methane selectivity of CB catalysts compared with non-catalytic decomposition (Fig. 2). However, at sufficiently high temperatures (>1123 K), methane can decompose rapidly when CB catalysts are present, and hence the methane selectivity becomes lower than that observed for non-catalytic decomposition. The species ÆCH2–* in Step 12 may subsequently undergo many possible elementary steps [16], such as addition of methyl and ethyl radicals or an ethylene molecule; the resulting CnHm–* intermediates are expected to eventually produce new graphene layer edges and H2 molecules after hydrogen abstraction. However, a detailed mechanistic description of these steps is beyond the scope of this study. The difference in the stability of the methane selectivity of N330(F) and BP-2000, shown in Fig. 6, is considered to originate from their original structures and the structures of deposited carbon. Active sites are formed on the deposited carbon [1–4,7], but their characteristics can differ from those of the
2035
original CB catalyst. Scanning electron spectroscopy and transmission electron spectroscopy were used in a previous study [2] to demonstrate that N330(F) is nonporous and that the deposited carbon forms many protrusions on the particles. The results in Figs. 2 and 6 suggest that the active sites on the protrusions produce more methane from ethane or become gradually less active for methane decomposition as the protrusions grow. The reasoning for this is that the methane production by CB requires one single methyl radical or adsorbed methyl group and a very mobile adsorbed hydrogen atom, while methane decomposition requires multiple sites, as shown in Steps 11 and 12. The active sites on the protrusions may be less effective for methane decomposition. On the contrary, BP-2000 is highly porous, and deposited carbon fills in the pores first, thereby making the particles of BP-2000 gradually nonporous, but with no formation of protrusions for a while [3]. Hence, the methane selectivity of BP-2000 remains constant over time. In contrast, the ethylene selectivity of both N330(F) and BP2000 decreased slowly over time, as shown in Fig. 7. This suggests that ethylene decomposition is less sensitive to the structures of the original CB and the deposited carbon than methane decomposition. This is probably because ethylene, which has an unsaturated bond, adsorbs more easily than methane, or that some active sites are effective only for adsorption of ethylene while others can adsorb both ethylene and methane. The slow increase in the ethylene decomposition rate indicates that the number of active sites for ethylene adsorption increases gradually with increasing amounts of deposited carbon.
4.
Conclusions
As part of our ongoing research into decomposition of natural gas, the catalytic activities of rubber, color and conductive CB catalysts for ethane decomposition were investigated. Significantly higher ethane conversion and lower ethylene selectivity were obtained in the presence of CB compared with noncatalytic thermal decomposition, resulting in much higher hydrogen yields. This indicates that CB is catalytically effective for dehydrogenation of ethane to hydrogen and ethylene as well as subsequent decomposition of ethylene to hydrogen and C(s). However, more methane was produced in the presence of CB catalysts. Based on the product distributions, a reaction mechanism for the CB catalytic decomposition of ethane was proposed. The hydrogen yield increased with an increase in the specific surface area of fresh CB catalysts up to ca. 100 m2/g. However, for CB catalysts with surface areas greater than this, the increase in hydrogen yield was not significant, even for conductive CB catalysts with a surface area greater than 1300 m2/g. This indicates that the number of active sites on CB catalysts is not proportional to the surface area, but increases less and less as the surface area increases. Despite carbon deposition, representative CB catalysts exhibited stable activity for ethane decomposition and hydrogen production for 36 h, which suggests that the number of active sites remained nearly constant for each CB. However, methane selectivity remained constant or slowly increased,
2036
CARBON
4 8 ( 2 0 1 0 ) 2 0 3 0 –2 0 3 6
while ethylene selectivity decreased very slowly. This was discussed with respect to the different structures of the original CB and the deposited carbon.
Acknowledgements This work was supported by the ‘‘National R&D Organization for Hydrogen and Fuel Cells,’’ component of the New and Renewable Energy R&D Program (2004-N-HY12-P-03-0000) of the Korea Energy Management Corporation under the Ministry of Knowledge Economy of Korea. The authors would also like to thank OCI for supplying carbon black samples.
R E F E R E N C E S
[1] Lee EK, Lee SY, Han GY, Lee TJ, Jun JH, Yoon KJ, et al. Catalytic decomposition of methane over carbon blacks for CO2-free hydrogen production. Carbon 2004;42:2641–8. [2] Ryu BH, Lee SY, Lee DH, Han GY, Lee TJ, Yoon KJ. Catalytic characteristics of various rubber-reinforcing carbon blacks in decomposition of methane for hydrogen production. Catal Today 2007;123:303–9. [3] Lee SY, Ryu BH, Han GY, Lee TJ, Yoon KJ. Catalytic characteristics of specialty carbon blacks in decomposition of methane for hydrogen production. Carbon 2008;46:1978–86. [4] Lee SY, Kwak JH, Han GY, Lee TJ, Yoon KJ. Characterization of active sites for methane decomposition on carbon black through acetylene chemisorption. Carbon 2008;46:342–8. [5] Muradov NZ. How to produce hydrogen from fossil fuels without CO2 emission. Int J Hydrogen Energy 1993;18(3):211–5. [6] Muradov N. Hydrogen via methane decomposition: an application for decarbonization of fossil fuels. Int J Hydrogen Energy 2001;26:1165–75.
[7] Muradov N, Smith F, T-Raissi A. Catalytic activity of carbons for methane decomposition reaction. Catal Today 2005;102:225–33. [8] Fulcheri L, Schwob Y. From methane to hydrogen, carbon black and water. Int J Hydrogen Energy 1995;20(3):197–202. [9] Poirier MG, Sapundzhiev C. Catalytic decomposition of natural gas to hydrogen for fuel cell applications. Int J Hydrogen Energy 1997;22(4):429–33. [10] Gaudernack B, Lynum S. Hydrogen from natural gas without release of CO2 to the atmosphere. Int J Hydrogen Energy 1998;23(12):1087–93. [11] Steinberg M. Fossil fuel decarbonization technology for mitigating global warming. Int J Hydrogen Energy 1999;24:771–7. [12] Moliner R, Suelves I, La´zaro MJ, Moreno O. Thermocatalytic decomposition of methane over activated carbons: influence of textural properties and surface chemistry. Int J Hydrogen Energy 2005;30:293–300. [13] Solomosi F, Szoke A, Ovari L. Decomposition of ethane and its reaction with CO2 over Rh/ZSM-5 catalyst. J Catal 1999;186:269–78. [14] Wang L, Murata K, Inaba M. Production of pure hydrogen and more valuable hydrocarbons from ethane on a novel highly active catalyst system with a Pd-based membrane reactor. Catal Today 2003;82:99–104. [15] Chin SY, Chin Y-H, Amiridis MD. Hydrogen production via catalytic cracking of ethane over Ni/SiO2 catalysts. Appl Catal A 2006;300:8–13. [16] Wauters S, Martin GB. Computer generation of a network of elementary steps for coke formation during the thermal cracking of hydrocarbons. Chem Eng J 2001;82:267–79. [17] Frenkalch M. Reaction mechanism of soot formation in flames. Phys Chem Chem Phys 2002;4:2028–37. [18] Frenklach M, Ping J. On the role of surface migration in the growth and structure of graphene layers. Carbon 2004;42:1209–12. [19] Yoon SH, Park NK, Lee TJ, Yoon KJ, Han GY. Hydrogen production by thermocatalytic decomposition of butane over a carbon black catalyst. Catal Today 2009;146:202–8.