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carbon catalyst for methane reforming with carbon dioxide to syngas
Journal of Industrial and Engineering Chemistry 20 (2014) 1677–1683
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of preparation methods on the properties of cobalt/carbon catalyst for methane reforming with carbon dioxide to syngas Guojie Zhang *, Yannian Du, Ying Xu, Yongfa Zhang * Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, PR China
A R T I C L E I N F O
Article history: Received 25 April 2013 Accepted 17 August 2013 Available online 24 August 2013 Keywords: CH4–CO2 reforming Syngas Cobalt Carbon catalyst
A B S T R A C T
The effect of different preparation methods on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. The catalysts were prepared by the impregnation and characterized by the XRD and scanning electron microscopy (SEM). The result indicated that the active components of cobalt/ carbon catalyst prepared by using ultrasonic wave distributed evenly, activity was high and the loading time was short. The Co/Carbon catalyst prepared by incipient-wetness impregnation, 10 wt% loading and 300 8C calcination, achieved the best activity. Furthermore, the effect of reaction temperature, air speed and CH4/CO2 ratio on the catalyst activity and CO/H2 ratio in products was investigated. It was found that the conversion of CO2 and CH4 increased with the increasing of reaction temperature. However, the conversion of CO2 and CH4 increased first and then decreased with the increasing of air speed. With the increasing of CH4/CO2 in feed gas, both the catalyst activity and the CO/H2 ratio in products decreased. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction CH4–CO2 reforming can effectively convert CO2 and CH4 (the main greenhouse gas) into synthesis gas. This not only provides a feasible approach for resource utilization and environmental management, but also its synthesis gas is also chemical raw materials needed for many manufacturing processes [1]. Thus CH4–CO2 reforming has a wide application prospect. Developing a practical catalyst with high activity and good stability, it is not only the research hotspot of catalyst field, but also the key of achieving CH4–CO2 to synthesis gas reforming industrialized [2–5]. Current research results indicated that the noble metal catalyst has good activity and carbon deposition resistance to the reforming reaction [6,7]. However, researches have been devoted to use non-noble metals, such as Ni and Co, instead of noble metals due to the high cost of noble metal catalyst [3,8–16]. For the current research situation, Ni/Al2O3 was the most studied metal active component and carrier [17–20]. However, this catalyst is easy to lose activity because of the serious carbon deposition. Besides, metal Ni and Al2O3 produced NiAl2O4 compound during their reaction, which was not only disadvantageous to the
* Corresponding authors. Tel.: +86 351 601 8676; fax: +86 351 601 8676. E-mail addresses: [email protected], [email protected] (G. Zhang), [email protected] (Y. Zhang).
synthesis generation and difficult for restoration and regeneration, thus influencing its industrialization [17,18,20]. To solve this problem, many domestic and foreign scholars had carried out a lot researches [3–20]. Researches on the development of new catalyst and the activation mechanism of CH4 and CO2 had achieved important progress [21–23]. Our research group also had thrown great endeavor on these aspects [7,24–28]. However, deep researches on how to prevent catalyst deactivation by carbon deposition and improve the stability of catalyst are still required. Cobalt oxide was an alkaline oxide rich in oxygen. Metal catalyst taking cobalt oxide as an assistant or active component showed a good catalytic activity in automobile tail gas clean-up, water-gas shift reaction, CO oxidation, CH4 partial oxidation, synthesis gas production through combustion, and other reactions [29–32]. Compared to the Ni-based catalyst, there’s less research on Co-based catalyst. Applied carriers generally were Al2O3, SiO2, molecular sieve, etc., few reports on the Co/Carbon catalyst [4,6,10,21–23,33]. In this paper, the effect of different preparations on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. These catalysts were prepared by the impregnation method and characterized by the XRD and scanning electron microscopy (SEM). Furthermore, the effect of reaction temperature, air speed and CH4/CO2 ratio on the catalyst activity and CO/H2 ratio in products was investigated.
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.08.016
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2. Experimental 2.1. Catalyst preparation The catalysts were prepared by successive aqueous incipient wetness impregnations. Carbon used in the experiment was prepared by pyrolysis of coal at 1150 8C for 1.5 h and crushing the catalyst mass to 30–60 mesh-size particles [7]. The proximate analysis and ultimate analysis of carbon catalysts were shown in Table 1. The carbon sample was impregnated with a cobalt salt (Co(NO3)26H2O) for 12 h under atmospheric pressure. For ultrasonic incipient-wetness impregnation, the carbon sample was impregnated with a cobalt salt (Co(NO3)26H2O) or 4 h under the use of ultrasonic wave, then impregnated with a cobalt salt (Co(NO3)26H2O) for 2 h under atmospheric pressure. The catalyst was then dried at 110 8C for 4 h and calcined at different temperature for 4 h. 2.2. Catalytic activity measurements The catalytic CH4–CO2 reforming experiments were carried out at a normal pressure in a fixed-bed reactor. High-purity gases were used throughout all experiments. The volumes of CH4, CO2 and N2 were exactly controlled into the reactor by mass flow controllers. About 2 g of the catalyst was used in reaction runs. The apparatus consisted of a fixed-bed quartz reactor (2.0 cm inner diameter) inside an electric furnace. The temperatures at the top and the bottom of the catalyst bed were monitored with a platinumrhodium thermocouple, and the difference between the two temperatures was less than 2 8C for catalyst beds. Before the activity tests, the catalysts were reduced with the addition of 10% H2/N2 flow (50 cm3 min 1) at a constant heating rate of 5 8C min 1 from room temperature to 950 8C, this temperature was kept for 60 min then cooled to the reaction temperature, the reaction run was started by introducing CH4/CO2 mixture into the reactor. After reaching steady-state conditions, the mixture of reactant gases and products was periodically analyzed by a gas chromatography. Prior to analysis, the effluent was passed through a water-trap at 0 8C to remove the reaction water. The error of each data point was less than 3%, and all experiments with large errors were rejected. The steady-state activity was calculated from the constant conversion value at each reaction temperature and each steam/gas ratio. The content of gaseous products in the reactor effluent was analyzed with a gas Chromatography -960 equipped with a TCD detector. Methane and CO2 intake can be measured using a mass flow meter. And the output of product gas flow can be measured using soap liquid meter. No catalyst deactivation was observed during reactions. After the catalyst had served the reaction for a specified period of time, the reaction feed was switched to inert nitrogen, followed by cooling in nitrogen flow of the reactor to room temperature at which the used catalyst was unloaded for various characterizations. 2.3. Catalyst characterization The crystalline structures of the catalysts were determined by X-ray powder diffraction (XRD) with a computer-controlled
Shimadzu XRD-6000 apparatus equipped with a monochromator for the Cu Ka radiation, operating at 40 kV and 30 mA. The patterns were recorded in steps of 0.018 with the scanning rate at 88/min from 10 to 808 under atmospheric pressure. The catalysts powders were characterized by thermogravimetric analysis by using Netzsch STA409C instrument, which measures mass changes when temperature increases. SEM images of the catalyst were performed with a scanning electron microscope JSM-4800 (Japan, JEOL Ltd.). Gold was sputtered onto the catalysts to ensure sufficient conductivity. 3. Results and discussion 3.1. Effects of the impregnation method on the catalyst activity The catalyst prepared through excessive impregnation method was recorded as 1#, while the catalyst prepared through incipientwetness impregnation method was recorded as 2#. The effect of different impregnation methods on the catalyst activity was shown in Fig. 1. It revealed that the activity of catalyst prepared through incipient-wetness impregnation method was higher than that of catalyst prepared through excessive impregnation method, viewed from the conversion of reaction feed gas and selectivity of synthesis gas. This may be because that the catalyst prepared by using the excessive impregnation method left quite part of active substances in the impregnation residue. In other words, the active component of the catalyst prepared through excessive impregnation is lower than the metal components carried by the catalyst prepared through incipient-wetness impregnation. Therefore, catalyst made by excessive impregnation achieved lower activity and selectivity than that made by incipient-wetness impregnation. Incipient-wetness impregnation was generally used to make catalyst without strict requirement on particle size since it could conveniently control the load capacity of active component and the load capacity was easy to be calculated. However, it has various shortcomings, such as poor dispersity of active component, uneven distribution of particle size, etc. To overcome these shortcomings, researchers had made a lot of improvements on it, among which the most common one is to increase the disperse uniformity by using ultrasonic wave. 3.2. Effect of ultrasound on the catalyst activity 3.2.1. Effect of ultrasound Currently, it is a very hot research topic in both domestic and foreign countries by making catalyst through ultrasound wave whose main motive power comes from the ‘‘cavitation’’ of ultrasound wave [34]. The breakdown of cavitation bubble nuclear brings a local high temperature, high pressure and strong impact wave as well as micro jet, which provides a new unique physical and chemical environment for the chemical reaction that is difficult to be achieved or impossible to be achieved under general conditions. To explore the effect of ultrasound treatment on the activity of Co/Carbon catalyst, the activity of metal/Carbon catalyst treated by incipient-wetness impregnation (2#) and ultrasound incipient-wetness impregnation (3#, vibration frequency: 40 kHZ) was investigated, as shown in Fig. 2.
Table 1 Proximate and ultimate analysis. Sample
DT coal Carbonaceous DT: Datong.
Ultimate analysis (%, ad)
Proximate analysis (%, daf)
M
A
V
C
H
N
S
O (diff)
3.10 1.20
12.20 13.30
29.00 4.50
87.70 94.60
4.96 1.47
1.27 0.99
0.42 0.17
5.36 2.36
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Fig. 1. The influence of impregnation method on the catalytic activity.
According to Fig. 2, the activity and selectivity of metal/carbon catalyst treated by ultrasonic incipient-wetness impregnation wave were increased. This was mainly because that ‘‘cavitation’’ effect was produced during the ultrasonic incipient-wetness impregnation wave, which destroyed the soft agglomeration of active substances, and enabled the active substances to distribute more evenly on the surface of the carrier. Thus, the specific surface area and active sites of the active substances increased, the activity of the catalyst increased. The SEM of catalysts made through incipient-wetness impregnation and ultrasonic incipient-wetness impregnation was shown in Figs. 3 and 4. It was clearly demonstrated that the active substances carried on the surface of catalyst made through incipient-wetness impregnation method were in a mess and had poor dispersity with some active substances having small particles and some active substance particles integrated into large particles, while the active precursors of catalyst made through ultrasonic incipient-wetness impregnation formed rice-shaped small particles which were distributed evenly on the surface of carbon material. Bianchi [35] also discovered that the active component made under ultrasonic radiation achieved more even distribution on the carrier and the catalyst made on this basis was witnessed with higher activity and better stability. 3.2.2. Effect of ultrasonic vibration time and frequency When the catalyst prepared by ultrasound, different vibration time and frequency produced different ‘‘cavitation’’ effects, which produced different influencing the active substance distribution of
Fig. 2. The influence of ultrasonic wave on the catalytic activity.
Fig. 3. The SEM of sample 6-2# (2000).
the catalyst on the carrier surface. The effects of different vibration time and frequency on the catalyst activity were shown in Figs. 5 and 6. According to Fig. 5, the ultrasonic vibration time affected the selectivity of the synthesis gas slightly, but affected the conversion of feed gas significantly. The catalyst achieved the best activity when the ultrasonic vibration time was 10 min long. This was mainly because that short ultrasonic vibration time can only produce insufficient ‘‘cavitation’’ effect to destroy the soft agglomeration of the catalyst precursors, thus failed to disperse the active components completely. However, over longer ultrasonic vibration time will produce ‘‘cavitation’’ which will destroy the pore structure of carbon material, thus decreasing the specific surface area of the carrier as well as the pore volume [34,35]. It can be seen from Fig. 6 that the ultrasonic strength affected the catalyst activity and selectivity. In the beginning, the conversion of feed gas and the selectivity of synthesis gas increased continuously with the increasing of ultrasonic strength, and the catalyst achieved the best activity at the vibration frequency of 40 kHZ. However, after the vibration frequency exceeded 40 kHZ, the conversion of CH4 and CO2 and the selectivity of synthesis gas decreased with the increasing of vibration
Fig. 4. The SEM of sample 6-3# (2000).
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Fig. 5. The influence of ultrasonic vibration time on the catalytic activity.
Fig. 7. The influence of metal loading on the CH4 conversion.
strength. The ultrasonic vibration frequency reflects the capacity of ultrasonic energy output. Lower vibration frequency enables ultrasonic vibration to produce lower sonic pressure to spread in liquid, liquid molecules to produce lower vacuum to pull-apart into cavity and cavity crack to produce smaller impact, thus making it impossible to well disperse the active precursors. With the increasing of vibration frequency, the cavity pulled by the liquid molecules gets close to vacuum and millions of small cavitation bubbles break and produce strong impact, enabling the carried metal active substances distributed evenly on the carbon surface or pore canals as small particles. When the vibration frequency exceeds 40 kHZ, cracks produce over large strong impact under the continuous increasing of vibration frequency, which destroys the pore structure of the carbon material, finally decreasing the specific surface area of the catalyst. Therefore, the activity of the catalyst decreases [36].
with the increasing of reforming reaction temperature. The catalyst achieved the best activity at the reforming reaction temperature of 900 8C and load capacity of 10 wt%, the conversion of CH4 and CO2 was 89.8% and 93.8% respectively. When the loading content was 5 wt% and 15 wt%, the conversion of CH4 and CO2 was 60.3% and 80.1% as well as 73.3% and 84.2% respectively. Within the whole investigated temperature range, the metal load activity of methane conversion was ordered as 10 wt% > 15 wt% > 5 wt%. At low cobalt loadings, cobalt metal suspended on carbon integrates into the skeletal structure. And no Co cluster formation. At cobalt loading more than 10 wt%, excessive Co cannot incorporate into the skeletal structure of catalyst, which will form Co agglomerates on the catalyst surface, and blocks the micro-pores of Co/Carbon catalyst.
3.3. Effect of loading on the catalyst activity
To investigate the interaction between carbon material and active metals, it firstly applied thermogravimetric analysis to study the variation trend of cobalt nitrate and Co/Carbon catalyst, as shown in Figs. 9 and 10. It can be seen from the DTA curve of Fig. 9 that there were two main endothermic peaks before 800 8C, locating at 200 8C and 280 8C respectively, which were water (surface water and crystal water) evaporation and decomposition of cobalt nitrate correspondingly. Furthermore, the TG curve also revealed that the weight remained basically unchanged after the
The loading of metal active components is an important influencing factor of catalyst activity. The effects of different metal load capacities on the CH4–CO2 reforming reaction were shown in Figs. 7 and 8. It was found that different metal load capacities affected the reforming conversion differently, but three loading content produced the same trend and the conversion increased
Fig. 6. The influence of ultrasonic vibration frequency on the catalytic activity.
3.4. Effect of calcination temperature on the catalyst activity
Fig. 8. The influence of metal loading on the CO2 conversion.
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Table 2 Effect of calcination temperature on the Co/carbonaceous catalytic activity. Sample
Conversion (%)
Selectivity (%)
CH4
CO2
CO
H2
200 8C 300 8C 600 8C
71.28 89.78 75.24
80.15 93.84 82.39
91.28 96.78 90.88
85.96 93.65 87.35
decomposition of cobalt nitrate. According to the DTA in Fig. 10, there were three endothermic peaks. Compared to the endothermic peaks in Fig. 9, it can be concluded that the endothermic peak before 200 8C was the dehydration peak of both external and internal water of the catalyst, and the endothermic peak before 300 8C was the decomposition peak of cobalt nitrate. Compared to the thermogravimetric curve of cobalt nitrate, the thermogravimetric curve of the Co/Carbon catalyst produced another endothermic peak at 600 8C, which may be because that the substances in the carbon material reacted with cobalt oxide and thus generated new oxidation species. The effect of the different calcination temperatures on the catalyst activity was shown in Table 2. It was found that with increasing calcination temperatures, the conversion of CH4 and CO2 and selectivity of synthesis gas increased first. But further increase in calcinations temperatures to certain temperatures high enough, the conversion of CH4 and CO2 decreased with increasing of calcination temperature. It was mainly because the calcination was an endothermic reaction. The high calcination temperature was beneficial to the progression of decomposition. As a result of thermal decomposition (calcination), volatile compounds were eliminated, pores were formed. Thus, the surface area of a catalyst can be increased. However, it may be caused by the sintering of metal at too high temperature. Furthermore, with the increasing of the calcination temperature, it may develop crystal transformation
during the calcination. It indicates that the calcination temperature has a major influence on the properties of catalyst. Too low or too high a calcination is unfavorable for the sintering process. The XRD spectrogram of catalyst under different calcination temperatures was shown in Fig. 11. It was found that the XRD diffraction peaks of original carbon material mainly were the 002 peak and 100 peak of the carbon material as well as CaO (37.6 and 548) and SiO2 diffraction peaks. The XRD diffraction peaks comparison of Co/Carbon catalyst and original carbon catalyst discovered the strength of 0 0 2 peak, 1 0 0 peak and CaO peak reduced or disappeared, which was mainly because the loaded cobalt metallic was well distributed on the carbon material, thus covering the these substance peaks. After calcination under 200 8C, two new peaks produced at 44 and 588 except for the CaO peak, which were identified as Co3O4 diffraction peak and Co2O3 diffraction peak through comparison. When the calcination temperature was increased to 300 8C, Co2O3 diffraction peak disappeared basically, while the Co3O4 diffraction peak strengthened slightly. Further increasing the calcination temperature to 600 8C, it can be seen from the XRD spectrogram that both Co3O4 diffraction peak and Co2O3 diffraction peak disappeared after the calcination, but a new cobalt diffraction peak was produced (44.51 and 51.838). It can infer by combining with the thermogravimetric curve that with the increasing of calcination temperature, cobalt nitrate decomposed to produce Co3O4 and Co2O3 firstly and then with the further increasing of calcination temperature, Co2O3 was restored to Co3O4 and simple cobalt. The catalyst activity increased with the increasing of temperature. However, when further increasing the calcination temperature, cobalt metal produced sintering and the catalyst activity started to decrease. This was mainly because too high calcination temperature not only may collapse the pore canal of catalyst carrier to certain extent and destroy the skeleton structure to reduce the pore diameter of the catalyst, but also will aggregate the loaded active components to enlarge the crystal grain of the active components.
Fig. 10. The TG and DTA of Co/Carbon.
Fig. 11. XRD patterns of catalysts calcined at different temperature.
Fig. 9. The TG and DTA of Co(NO3)26H2O.
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increasing of CH4/CO2 ratio, the CO/H2 ratio in the synthesis gas decreased, which valued 1.4 when CH4/CO2 was 0.5 and decreased to nearly 1 when CH4/CO2 increased to 1. When CH4/CO2 was increased to 2, the CO/H2 ratio in the output synthesis gas was close to 0.4 basically. Based on the above mentioned data, it can deduce that it is possible to adjust the inlet CO2/CH4 ratio in order to obtain synthesis gas with appropriate CO/H2 ratio. Besides, the early study carried out by the research group indicated that carbon deposition generated by the reaction during low temperature achieved high activity and was easy to produce a gasification reaction with CO2. Therefore, to reduce the catalyst inactivation caused by carbon deposition as much as possible, it can increase the CO2 proportion in inlet air appropriately to increase the conversion of methane and prolong the service life of the catalyst.
3.5.2. Effect of CH4/CO2 The relationship between different CH4/CO2 ratios and CO/H2 was shown in Fig. 13. In view of the analysis on the chemical equation of CO2–CH4 reforming reaction (CH4 + CO2 = 2CO + 2H2), the CO/H2 ratio in synthesis gas shall be 1:1. However, it can be discovered from Fig. 13 that CO/H2 changed with the variation of CH4/CO2 ratio. When CH4/CO2 < 1, CO/H2 > 1, which was mainly because that excessive CO2 produced gasification reaction with the easy-gasified substances in the carbon material. When CH4/ CO2 > 1, CO/H2 < 1, which was mainly because that CH4 produced a cracking reaction except for participating in the reforming reaction. Furthermore, Fig. 13 also revealed that with the
3.5.3. Effect of airspeed Airspeed refers to the gas quantity treated by unit volume of catalyst in unit time under given conditions. Quicker allowed the airspeed represents higher catalyst activity and larger processing capacity of the unit. However, for a given unit, the raw material stays short on the catalyst and achieves shallow reaction depth when the airspeed increases. Contrarily, slow airspeed represents a short reaction time. It is beneficial for increasing the conversion of reaction to slow down the airspeed. However, under the same treatment quantity, slower airspeed requires more catalysts and larger reactor volume, which are uneconomic. The variation law of conversion of CH4 and CO2 with the increasing of airspeed from 150 ml g 1 h 1 to 750 ml g 1 h 1 was shown in Fig. 14. It revealed that in the beginning, the conversion of CH4 and CO2 increased with the increasing of airspeed, reaching the maximum at the airspeed of 360 ml g 1 h 1 (90.1 and 92.8%, respectively). Then, the conversion started to decrease with the increasing of airspeed. The airspeed was slow in the beginning and the reforming reaction was a heterogeneous catalytic reaction under the effect of carbon material, which was mainly controlled by diffusion. Therefore, the conversion of reforming reaction increased with the increasing of airspeed. When the airspeed reached a certain value, the diffusion produced decreasing the impact and the reaction was mainly controlled by chemical reaction. At this moment, the time for gas participated in the reaction to contact with catalyst reduced with the increasing of airspeed, thus decreasing the conversion. On the other hand, the reaction gas absorption capacity of carbon catalyst reduced due to the increased airspeed, thus decreasing the reaction probability of gas that participated in the reaction. This was macroscopically manifested by the decreased conversion.
Fig. 13. Influence of CH4/CO2 on the CO/H2.
Fig. 14. Effect of space velocity on the conversion of CH4 and CO2.
Fig. 12. Influence of temperature on the conversion of CO2 and CH4.
3.5. Effect of process conditions on the Co/C catalyst activity 3.5.1. 3.5.1Effect of temperature The effect of reaction temperature on the conversion of CH4 and CO2 over Co/Carbon catalyst was shown in Figs. 7, 8 and 12. It was found that the temperature affected the conversion of CH4 and CO2 basically same. The conversion increased with the increasing of reaction temperature. The conversion of CH4 increased from 8.1% at 600 8C to 97.5% at 950 8C and the conversion of CO2 increased from 10.6% at 600 8C to 96.9% at 950 8C. This was mainly because that the CO2–CH4 reforming reaction is a strong endothermic reaction and the increasing of temperature can contribute to the increasing of reaction conversion.
G. Zhang et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 1677–1683
4. Conclusion CO2 reforming of CH4 over different Co/Carbon catalysts is studied at an atmospheric pressure. Different preparation methods of Co/Carbon catalysts play a major role regarding its structural properties and the reduction behavior, and hence the catalytic performance. The variables investigated in this paper are the calcination temperature used during the preparation of the different impregnated catalyst and the metal loading. The result demonstrates that the active components of metal/Carbon catalyst prepared by using ultrasound wave distributed evenly, activity was high and loading time was short. The Co/Carbon catalyst with 10 wt% loadings, precalcined at 300 8C, provide high and stable activities for the CO2 reforming of CH4. However, a too high calcination temperature (e.g. 600 8C) may cause a low activity, and a too high Co loading (15 wt%) may result in an unstable activity. The process conditions show that found that the conversion of CO2 and CH4 increased with the increasing of reaction temperature. However, the conversion of CO2 and CH4 increased first and then decreased with the increasing of air speed. With the increasing of CH4/CO2 in feed gas, both the catalyst activity and the CO/H2 ratio in products decreased. Acknowledgements This work was supported by the National Basic Research Program of China (Grant No. 2005CB221202), the Natural Science Foundation of China (Grant No. 21006066) and Shanxi Provincial Natural Science Foundation (Grant No. 2011021009-2). References [1] A.M. Gadalla, B. Bower, Chem. Eng. Sci. 143 (1988) 3049–3062. [2] M.K. Nikoo, N.A.S. Amin, Fuel Process. Technol. 92 (2011) 678–691. [3] M. Fan, A.Z. Abdullah, S. Bhatia, Int. J. Hydrogen Energy 36 (2011) 4875–4886.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of preparation methods on the properties of cobalt/carbon catalyst for methane reforming with carbon dioxide to syngas Guojie Zhang *, Yannian Du, Ying Xu, Yongfa Zhang * Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, PR China
A R T I C L E I N F O
Article history: Received 25 April 2013 Accepted 17 August 2013 Available online 24 August 2013 Keywords: CH4–CO2 reforming Syngas Cobalt Carbon catalyst
A B S T R A C T
The effect of different preparation methods on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. The catalysts were prepared by the impregnation and characterized by the XRD and scanning electron microscopy (SEM). The result indicated that the active components of cobalt/ carbon catalyst prepared by using ultrasonic wave distributed evenly, activity was high and the loading time was short. The Co/Carbon catalyst prepared by incipient-wetness impregnation, 10 wt% loading and 300 8C calcination, achieved the best activity. Furthermore, the effect of reaction temperature, air speed and CH4/CO2 ratio on the catalyst activity and CO/H2 ratio in products was investigated. It was found that the conversion of CO2 and CH4 increased with the increasing of reaction temperature. However, the conversion of CO2 and CH4 increased first and then decreased with the increasing of air speed. With the increasing of CH4/CO2 in feed gas, both the catalyst activity and the CO/H2 ratio in products decreased. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction CH4–CO2 reforming can effectively convert CO2 and CH4 (the main greenhouse gas) into synthesis gas. This not only provides a feasible approach for resource utilization and environmental management, but also its synthesis gas is also chemical raw materials needed for many manufacturing processes [1]. Thus CH4–CO2 reforming has a wide application prospect. Developing a practical catalyst with high activity and good stability, it is not only the research hotspot of catalyst field, but also the key of achieving CH4–CO2 to synthesis gas reforming industrialized [2–5]. Current research results indicated that the noble metal catalyst has good activity and carbon deposition resistance to the reforming reaction [6,7]. However, researches have been devoted to use non-noble metals, such as Ni and Co, instead of noble metals due to the high cost of noble metal catalyst [3,8–16]. For the current research situation, Ni/Al2O3 was the most studied metal active component and carrier [17–20]. However, this catalyst is easy to lose activity because of the serious carbon deposition. Besides, metal Ni and Al2O3 produced NiAl2O4 compound during their reaction, which was not only disadvantageous to the
* Corresponding authors. Tel.: +86 351 601 8676; fax: +86 351 601 8676. E-mail addresses: [email protected], [email protected] (G. Zhang), [email protected] (Y. Zhang).
synthesis generation and difficult for restoration and regeneration, thus influencing its industrialization [17,18,20]. To solve this problem, many domestic and foreign scholars had carried out a lot researches [3–20]. Researches on the development of new catalyst and the activation mechanism of CH4 and CO2 had achieved important progress [21–23]. Our research group also had thrown great endeavor on these aspects [7,24–28]. However, deep researches on how to prevent catalyst deactivation by carbon deposition and improve the stability of catalyst are still required. Cobalt oxide was an alkaline oxide rich in oxygen. Metal catalyst taking cobalt oxide as an assistant or active component showed a good catalytic activity in automobile tail gas clean-up, water-gas shift reaction, CO oxidation, CH4 partial oxidation, synthesis gas production through combustion, and other reactions [29–32]. Compared to the Ni-based catalyst, there’s less research on Co-based catalyst. Applied carriers generally were Al2O3, SiO2, molecular sieve, etc., few reports on the Co/Carbon catalyst [4,6,10,21–23,33]. In this paper, the effect of different preparations on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. These catalysts were prepared by the impregnation method and characterized by the XRD and scanning electron microscopy (SEM). Furthermore, the effect of reaction temperature, air speed and CH4/CO2 ratio on the catalyst activity and CO/H2 ratio in products was investigated.
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.08.016
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2. Experimental 2.1. Catalyst preparation The catalysts were prepared by successive aqueous incipient wetness impregnations. Carbon used in the experiment was prepared by pyrolysis of coal at 1150 8C for 1.5 h and crushing the catalyst mass to 30–60 mesh-size particles [7]. The proximate analysis and ultimate analysis of carbon catalysts were shown in Table 1. The carbon sample was impregnated with a cobalt salt (Co(NO3)26H2O) for 12 h under atmospheric pressure. For ultrasonic incipient-wetness impregnation, the carbon sample was impregnated with a cobalt salt (Co(NO3)26H2O) or 4 h under the use of ultrasonic wave, then impregnated with a cobalt salt (Co(NO3)26H2O) for 2 h under atmospheric pressure. The catalyst was then dried at 110 8C for 4 h and calcined at different temperature for 4 h. 2.2. Catalytic activity measurements The catalytic CH4–CO2 reforming experiments were carried out at a normal pressure in a fixed-bed reactor. High-purity gases were used throughout all experiments. The volumes of CH4, CO2 and N2 were exactly controlled into the reactor by mass flow controllers. About 2 g of the catalyst was used in reaction runs. The apparatus consisted of a fixed-bed quartz reactor (2.0 cm inner diameter) inside an electric furnace. The temperatures at the top and the bottom of the catalyst bed were monitored with a platinumrhodium thermocouple, and the difference between the two temperatures was less than 2 8C for catalyst beds. Before the activity tests, the catalysts were reduced with the addition of 10% H2/N2 flow (50 cm3 min 1) at a constant heating rate of 5 8C min 1 from room temperature to 950 8C, this temperature was kept for 60 min then cooled to the reaction temperature, the reaction run was started by introducing CH4/CO2 mixture into the reactor. After reaching steady-state conditions, the mixture of reactant gases and products was periodically analyzed by a gas chromatography. Prior to analysis, the effluent was passed through a water-trap at 0 8C to remove the reaction water. The error of each data point was less than 3%, and all experiments with large errors were rejected. The steady-state activity was calculated from the constant conversion value at each reaction temperature and each steam/gas ratio. The content of gaseous products in the reactor effluent was analyzed with a gas Chromatography -960 equipped with a TCD detector. Methane and CO2 intake can be measured using a mass flow meter. And the output of product gas flow can be measured using soap liquid meter. No catalyst deactivation was observed during reactions. After the catalyst had served the reaction for a specified period of time, the reaction feed was switched to inert nitrogen, followed by cooling in nitrogen flow of the reactor to room temperature at which the used catalyst was unloaded for various characterizations. 2.3. Catalyst characterization The crystalline structures of the catalysts were determined by X-ray powder diffraction (XRD) with a computer-controlled
Shimadzu XRD-6000 apparatus equipped with a monochromator for the Cu Ka radiation, operating at 40 kV and 30 mA. The patterns were recorded in steps of 0.018 with the scanning rate at 88/min from 10 to 808 under atmospheric pressure. The catalysts powders were characterized by thermogravimetric analysis by using Netzsch STA409C instrument, which measures mass changes when temperature increases. SEM images of the catalyst were performed with a scanning electron microscope JSM-4800 (Japan, JEOL Ltd.). Gold was sputtered onto the catalysts to ensure sufficient conductivity. 3. Results and discussion 3.1. Effects of the impregnation method on the catalyst activity The catalyst prepared through excessive impregnation method was recorded as 1#, while the catalyst prepared through incipientwetness impregnation method was recorded as 2#. The effect of different impregnation methods on the catalyst activity was shown in Fig. 1. It revealed that the activity of catalyst prepared through incipient-wetness impregnation method was higher than that of catalyst prepared through excessive impregnation method, viewed from the conversion of reaction feed gas and selectivity of synthesis gas. This may be because that the catalyst prepared by using the excessive impregnation method left quite part of active substances in the impregnation residue. In other words, the active component of the catalyst prepared through excessive impregnation is lower than the metal components carried by the catalyst prepared through incipient-wetness impregnation. Therefore, catalyst made by excessive impregnation achieved lower activity and selectivity than that made by incipient-wetness impregnation. Incipient-wetness impregnation was generally used to make catalyst without strict requirement on particle size since it could conveniently control the load capacity of active component and the load capacity was easy to be calculated. However, it has various shortcomings, such as poor dispersity of active component, uneven distribution of particle size, etc. To overcome these shortcomings, researchers had made a lot of improvements on it, among which the most common one is to increase the disperse uniformity by using ultrasonic wave. 3.2. Effect of ultrasound on the catalyst activity 3.2.1. Effect of ultrasound Currently, it is a very hot research topic in both domestic and foreign countries by making catalyst through ultrasound wave whose main motive power comes from the ‘‘cavitation’’ of ultrasound wave [34]. The breakdown of cavitation bubble nuclear brings a local high temperature, high pressure and strong impact wave as well as micro jet, which provides a new unique physical and chemical environment for the chemical reaction that is difficult to be achieved or impossible to be achieved under general conditions. To explore the effect of ultrasound treatment on the activity of Co/Carbon catalyst, the activity of metal/Carbon catalyst treated by incipient-wetness impregnation (2#) and ultrasound incipient-wetness impregnation (3#, vibration frequency: 40 kHZ) was investigated, as shown in Fig. 2.
Table 1 Proximate and ultimate analysis. Sample
DT coal Carbonaceous DT: Datong.
Ultimate analysis (%, ad)
Proximate analysis (%, daf)
M
A
V
C
H
N
S
O (diff)
3.10 1.20
12.20 13.30
29.00 4.50
87.70 94.60
4.96 1.47
1.27 0.99
0.42 0.17
5.36 2.36
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Fig. 1. The influence of impregnation method on the catalytic activity.
According to Fig. 2, the activity and selectivity of metal/carbon catalyst treated by ultrasonic incipient-wetness impregnation wave were increased. This was mainly because that ‘‘cavitation’’ effect was produced during the ultrasonic incipient-wetness impregnation wave, which destroyed the soft agglomeration of active substances, and enabled the active substances to distribute more evenly on the surface of the carrier. Thus, the specific surface area and active sites of the active substances increased, the activity of the catalyst increased. The SEM of catalysts made through incipient-wetness impregnation and ultrasonic incipient-wetness impregnation was shown in Figs. 3 and 4. It was clearly demonstrated that the active substances carried on the surface of catalyst made through incipient-wetness impregnation method were in a mess and had poor dispersity with some active substances having small particles and some active substance particles integrated into large particles, while the active precursors of catalyst made through ultrasonic incipient-wetness impregnation formed rice-shaped small particles which were distributed evenly on the surface of carbon material. Bianchi [35] also discovered that the active component made under ultrasonic radiation achieved more even distribution on the carrier and the catalyst made on this basis was witnessed with higher activity and better stability. 3.2.2. Effect of ultrasonic vibration time and frequency When the catalyst prepared by ultrasound, different vibration time and frequency produced different ‘‘cavitation’’ effects, which produced different influencing the active substance distribution of
Fig. 2. The influence of ultrasonic wave on the catalytic activity.
Fig. 3. The SEM of sample 6-2# (2000).
the catalyst on the carrier surface. The effects of different vibration time and frequency on the catalyst activity were shown in Figs. 5 and 6. According to Fig. 5, the ultrasonic vibration time affected the selectivity of the synthesis gas slightly, but affected the conversion of feed gas significantly. The catalyst achieved the best activity when the ultrasonic vibration time was 10 min long. This was mainly because that short ultrasonic vibration time can only produce insufficient ‘‘cavitation’’ effect to destroy the soft agglomeration of the catalyst precursors, thus failed to disperse the active components completely. However, over longer ultrasonic vibration time will produce ‘‘cavitation’’ which will destroy the pore structure of carbon material, thus decreasing the specific surface area of the carrier as well as the pore volume [34,35]. It can be seen from Fig. 6 that the ultrasonic strength affected the catalyst activity and selectivity. In the beginning, the conversion of feed gas and the selectivity of synthesis gas increased continuously with the increasing of ultrasonic strength, and the catalyst achieved the best activity at the vibration frequency of 40 kHZ. However, after the vibration frequency exceeded 40 kHZ, the conversion of CH4 and CO2 and the selectivity of synthesis gas decreased with the increasing of vibration
Fig. 4. The SEM of sample 6-3# (2000).
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Fig. 5. The influence of ultrasonic vibration time on the catalytic activity.
Fig. 7. The influence of metal loading on the CH4 conversion.
strength. The ultrasonic vibration frequency reflects the capacity of ultrasonic energy output. Lower vibration frequency enables ultrasonic vibration to produce lower sonic pressure to spread in liquid, liquid molecules to produce lower vacuum to pull-apart into cavity and cavity crack to produce smaller impact, thus making it impossible to well disperse the active precursors. With the increasing of vibration frequency, the cavity pulled by the liquid molecules gets close to vacuum and millions of small cavitation bubbles break and produce strong impact, enabling the carried metal active substances distributed evenly on the carbon surface or pore canals as small particles. When the vibration frequency exceeds 40 kHZ, cracks produce over large strong impact under the continuous increasing of vibration frequency, which destroys the pore structure of the carbon material, finally decreasing the specific surface area of the catalyst. Therefore, the activity of the catalyst decreases [36].
with the increasing of reforming reaction temperature. The catalyst achieved the best activity at the reforming reaction temperature of 900 8C and load capacity of 10 wt%, the conversion of CH4 and CO2 was 89.8% and 93.8% respectively. When the loading content was 5 wt% and 15 wt%, the conversion of CH4 and CO2 was 60.3% and 80.1% as well as 73.3% and 84.2% respectively. Within the whole investigated temperature range, the metal load activity of methane conversion was ordered as 10 wt% > 15 wt% > 5 wt%. At low cobalt loadings, cobalt metal suspended on carbon integrates into the skeletal structure. And no Co cluster formation. At cobalt loading more than 10 wt%, excessive Co cannot incorporate into the skeletal structure of catalyst, which will form Co agglomerates on the catalyst surface, and blocks the micro-pores of Co/Carbon catalyst.
3.3. Effect of loading on the catalyst activity
To investigate the interaction between carbon material and active metals, it firstly applied thermogravimetric analysis to study the variation trend of cobalt nitrate and Co/Carbon catalyst, as shown in Figs. 9 and 10. It can be seen from the DTA curve of Fig. 9 that there were two main endothermic peaks before 800 8C, locating at 200 8C and 280 8C respectively, which were water (surface water and crystal water) evaporation and decomposition of cobalt nitrate correspondingly. Furthermore, the TG curve also revealed that the weight remained basically unchanged after the
The loading of metal active components is an important influencing factor of catalyst activity. The effects of different metal load capacities on the CH4–CO2 reforming reaction were shown in Figs. 7 and 8. It was found that different metal load capacities affected the reforming conversion differently, but three loading content produced the same trend and the conversion increased
Fig. 6. The influence of ultrasonic vibration frequency on the catalytic activity.
3.4. Effect of calcination temperature on the catalyst activity
Fig. 8. The influence of metal loading on the CO2 conversion.
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Table 2 Effect of calcination temperature on the Co/carbonaceous catalytic activity. Sample
Conversion (%)
Selectivity (%)
CH4
CO2
CO
H2
200 8C 300 8C 600 8C
71.28 89.78 75.24
80.15 93.84 82.39
91.28 96.78 90.88
85.96 93.65 87.35
decomposition of cobalt nitrate. According to the DTA in Fig. 10, there were three endothermic peaks. Compared to the endothermic peaks in Fig. 9, it can be concluded that the endothermic peak before 200 8C was the dehydration peak of both external and internal water of the catalyst, and the endothermic peak before 300 8C was the decomposition peak of cobalt nitrate. Compared to the thermogravimetric curve of cobalt nitrate, the thermogravimetric curve of the Co/Carbon catalyst produced another endothermic peak at 600 8C, which may be because that the substances in the carbon material reacted with cobalt oxide and thus generated new oxidation species. The effect of the different calcination temperatures on the catalyst activity was shown in Table 2. It was found that with increasing calcination temperatures, the conversion of CH4 and CO2 and selectivity of synthesis gas increased first. But further increase in calcinations temperatures to certain temperatures high enough, the conversion of CH4 and CO2 decreased with increasing of calcination temperature. It was mainly because the calcination was an endothermic reaction. The high calcination temperature was beneficial to the progression of decomposition. As a result of thermal decomposition (calcination), volatile compounds were eliminated, pores were formed. Thus, the surface area of a catalyst can be increased. However, it may be caused by the sintering of metal at too high temperature. Furthermore, with the increasing of the calcination temperature, it may develop crystal transformation
during the calcination. It indicates that the calcination temperature has a major influence on the properties of catalyst. Too low or too high a calcination is unfavorable for the sintering process. The XRD spectrogram of catalyst under different calcination temperatures was shown in Fig. 11. It was found that the XRD diffraction peaks of original carbon material mainly were the 002 peak and 100 peak of the carbon material as well as CaO (37.6 and 548) and SiO2 diffraction peaks. The XRD diffraction peaks comparison of Co/Carbon catalyst and original carbon catalyst discovered the strength of 0 0 2 peak, 1 0 0 peak and CaO peak reduced or disappeared, which was mainly because the loaded cobalt metallic was well distributed on the carbon material, thus covering the these substance peaks. After calcination under 200 8C, two new peaks produced at 44 and 588 except for the CaO peak, which were identified as Co3O4 diffraction peak and Co2O3 diffraction peak through comparison. When the calcination temperature was increased to 300 8C, Co2O3 diffraction peak disappeared basically, while the Co3O4 diffraction peak strengthened slightly. Further increasing the calcination temperature to 600 8C, it can be seen from the XRD spectrogram that both Co3O4 diffraction peak and Co2O3 diffraction peak disappeared after the calcination, but a new cobalt diffraction peak was produced (44.51 and 51.838). It can infer by combining with the thermogravimetric curve that with the increasing of calcination temperature, cobalt nitrate decomposed to produce Co3O4 and Co2O3 firstly and then with the further increasing of calcination temperature, Co2O3 was restored to Co3O4 and simple cobalt. The catalyst activity increased with the increasing of temperature. However, when further increasing the calcination temperature, cobalt metal produced sintering and the catalyst activity started to decrease. This was mainly because too high calcination temperature not only may collapse the pore canal of catalyst carrier to certain extent and destroy the skeleton structure to reduce the pore diameter of the catalyst, but also will aggregate the loaded active components to enlarge the crystal grain of the active components.
Fig. 10. The TG and DTA of Co/Carbon.
Fig. 11. XRD patterns of catalysts calcined at different temperature.
Fig. 9. The TG and DTA of Co(NO3)26H2O.
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increasing of CH4/CO2 ratio, the CO/H2 ratio in the synthesis gas decreased, which valued 1.4 when CH4/CO2 was 0.5 and decreased to nearly 1 when CH4/CO2 increased to 1. When CH4/CO2 was increased to 2, the CO/H2 ratio in the output synthesis gas was close to 0.4 basically. Based on the above mentioned data, it can deduce that it is possible to adjust the inlet CO2/CH4 ratio in order to obtain synthesis gas with appropriate CO/H2 ratio. Besides, the early study carried out by the research group indicated that carbon deposition generated by the reaction during low temperature achieved high activity and was easy to produce a gasification reaction with CO2. Therefore, to reduce the catalyst inactivation caused by carbon deposition as much as possible, it can increase the CO2 proportion in inlet air appropriately to increase the conversion of methane and prolong the service life of the catalyst.
3.5.2. Effect of CH4/CO2 The relationship between different CH4/CO2 ratios and CO/H2 was shown in Fig. 13. In view of the analysis on the chemical equation of CO2–CH4 reforming reaction (CH4 + CO2 = 2CO + 2H2), the CO/H2 ratio in synthesis gas shall be 1:1. However, it can be discovered from Fig. 13 that CO/H2 changed with the variation of CH4/CO2 ratio. When CH4/CO2 < 1, CO/H2 > 1, which was mainly because that excessive CO2 produced gasification reaction with the easy-gasified substances in the carbon material. When CH4/ CO2 > 1, CO/H2 < 1, which was mainly because that CH4 produced a cracking reaction except for participating in the reforming reaction. Furthermore, Fig. 13 also revealed that with the
3.5.3. Effect of airspeed Airspeed refers to the gas quantity treated by unit volume of catalyst in unit time under given conditions. Quicker allowed the airspeed represents higher catalyst activity and larger processing capacity of the unit. However, for a given unit, the raw material stays short on the catalyst and achieves shallow reaction depth when the airspeed increases. Contrarily, slow airspeed represents a short reaction time. It is beneficial for increasing the conversion of reaction to slow down the airspeed. However, under the same treatment quantity, slower airspeed requires more catalysts and larger reactor volume, which are uneconomic. The variation law of conversion of CH4 and CO2 with the increasing of airspeed from 150 ml g 1 h 1 to 750 ml g 1 h 1 was shown in Fig. 14. It revealed that in the beginning, the conversion of CH4 and CO2 increased with the increasing of airspeed, reaching the maximum at the airspeed of 360 ml g 1 h 1 (90.1 and 92.8%, respectively). Then, the conversion started to decrease with the increasing of airspeed. The airspeed was slow in the beginning and the reforming reaction was a heterogeneous catalytic reaction under the effect of carbon material, which was mainly controlled by diffusion. Therefore, the conversion of reforming reaction increased with the increasing of airspeed. When the airspeed reached a certain value, the diffusion produced decreasing the impact and the reaction was mainly controlled by chemical reaction. At this moment, the time for gas participated in the reaction to contact with catalyst reduced with the increasing of airspeed, thus decreasing the conversion. On the other hand, the reaction gas absorption capacity of carbon catalyst reduced due to the increased airspeed, thus decreasing the reaction probability of gas that participated in the reaction. This was macroscopically manifested by the decreased conversion.
Fig. 13. Influence of CH4/CO2 on the CO/H2.
Fig. 14. Effect of space velocity on the conversion of CH4 and CO2.
Fig. 12. Influence of temperature on the conversion of CO2 and CH4.
3.5. Effect of process conditions on the Co/C catalyst activity 3.5.1. 3.5.1Effect of temperature The effect of reaction temperature on the conversion of CH4 and CO2 over Co/Carbon catalyst was shown in Figs. 7, 8 and 12. It was found that the temperature affected the conversion of CH4 and CO2 basically same. The conversion increased with the increasing of reaction temperature. The conversion of CH4 increased from 8.1% at 600 8C to 97.5% at 950 8C and the conversion of CO2 increased from 10.6% at 600 8C to 96.9% at 950 8C. This was mainly because that the CO2–CH4 reforming reaction is a strong endothermic reaction and the increasing of temperature can contribute to the increasing of reaction conversion.
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4. Conclusion CO2 reforming of CH4 over different Co/Carbon catalysts is studied at an atmospheric pressure. Different preparation methods of Co/Carbon catalysts play a major role regarding its structural properties and the reduction behavior, and hence the catalytic performance. The variables investigated in this paper are the calcination temperature used during the preparation of the different impregnated catalyst and the metal loading. The result demonstrates that the active components of metal/Carbon catalyst prepared by using ultrasound wave distributed evenly, activity was high and loading time was short. The Co/Carbon catalyst with 10 wt% loadings, precalcined at 300 8C, provide high and stable activities for the CO2 reforming of CH4. However, a too high calcination temperature (e.g. 600 8C) may cause a low activity, and a too high Co loading (15 wt%) may result in an unstable activity. The process conditions show that found that the conversion of CO2 and CH4 increased with the increasing of reaction temperature. However, the conversion of CO2 and CH4 increased first and then decreased with the increasing of air speed. With the increasing of CH4/CO2 in feed gas, both the catalyst activity and the CO/H2 ratio in products decreased. Acknowledgements This work was supported by the National Basic Research Program of China (Grant No. 2005CB221202), the Natural Science Foundation of China (Grant No. 21006066) and Shanxi Provincial Natural Science Foundation (Grant No. 2011021009-2). References [1] A.M. Gadalla, B. Bower, Chem. Eng. Sci. 143 (1988) 3049–3062. [2] M.K. Nikoo, N.A.S. Amin, Fuel Process. Technol. 92 (2011) 678–691. [3] M. Fan, A.Z. Abdullah, S. Bhatia, Int. J. Hydrogen Energy 36 (2011) 4875–4886.
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