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Novel Water-Gas-Shift Reaction Catalyst from Iron-Loaded Victorian Brown Coal† Jianglong Yu,‡,§,⊥ Fu-Jun Tian,‡ and Chun-Zhu Li*,‡,⊥ Department of Chemical Engineering, Monash UniVersity, PO Box 36, VIC 3800, Australia, Shenyang Institute of Aeronautical Engineering, 52 Huanghe Bei AVe, Shenyang (110034), China, and CRC for Clean Power from Lignite, Australia ReceiVed August 20, 2006. ReVised Manuscript ReceiVed NoVember 19, 2006
This paper reports on the catalytic characteristics of a novel char-supported iron catalyst for the watergas-shift reaction. The catalyst was prepared from gasification of iron-loaded Victorian brown coal. The catalyst samples were analyzed using a variety of techniques such as X-ray diffraction (XRD) and transmission electron microscopy (TEM). Water-gas-shift reaction experiments were carried out on a fixed-bed quartz reactor in the temperature range of 300-450 °C. The results indicated that iron highly dispersed in the char matrix with a particle size smaller than 50 nm demonstrated a high catalytic activity. Carbon material in the char was able to effectively prevent the agglomeration of nano-iron particles and maintained a high catalytic activity of the catalyst during water-gas-shift reaction. The effects of gasification conditions on the catalytic activity of the char catalyst were also investigated.
Introduction Global environmental problems such as green-house gas emission and air pollutants (NOx, SOx, and fine particulates) have been caused by direct combustion of fossil fuels such as coal and oil. Maintaining a high standard of living for human beings in the future requires the development of sustainable technology for energy production. Among the few options such as using renewable energy, hydrogen energy and improving the efficiency of current technologies of coal utilization, etc., hydrogen energy is expected to play an important role in the future energy supply system. To the use of hydrogen as an energy carrier in the future, the large scale production of hydrogen is essential. The water-gas-shift reaction, i.e., CO + H2O ) CO2 + H2, is probably the most important reaction not only during the production of hydrogen by gasification of carbonaceous materials such as coal and biomass but also during the reforming of hydrocarbons such as methane. The use of catalysts is then always required at sufficiently low temperatures to achieve reasonable reaction rates for the water-gas-shift reaction.1-8 Among the catalysts being developed, the supported† Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * Corresponding author. Fax: +61 3 9905 5686. E-mail:
[email protected]. ‡ Monash University. § Shenyang Institute of Aeronautical Engineering. ⊥ CRC for Clean Power from Lignite. (1) Rhodes, C.; Hutchings, G. J.; Ward, A. M. Catal. Today 1995, 23, 43-58. (2) Idakiev, V.; Tabakova, T.; Naydenov, A.; Yuan, Z.-Y.; Su, B.-L. Appl. Catal. B: EnViron. 2005, 63, 178-186. (3) Ou, X.-J.; Cheng, J.-Y.; Wang, H.-M.; Xiao, Y. J. Nat. Gas Chem. 1999, 8, 231-237. (4) Li, Y.; Li, X.; Chang, L.; Wu, D.; Fang, Z.; Shi, Y. Catal. Today 1999, 51, 73-84. (5) Li, Y.; Chang, L. Ind. Eng. Chem. Res. 1996, 35, 4050-7. (6) Luo, M.; Devilliers, D.; O’Brien, R. J.; Bao, S.; Davis, B. H. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 2004, 49, 172-174. (7) Song, H.-Y.; Yang, P.; Hua, N.-P.; Du, Y.-K. Gongye Cuihua 2002, 10, 10-14.
iron catalyst is economically attractive, has the potential to clean tarry materials,9 and allows simultaneous removal of nitrogen species10-14 and sulfur species. However, extensive study is necessary in order to understand the catalytic characteristics and behavior of this kind of catalyst. This paper reports on the development of a novel char-supported nano-iron catalyst which was prepared from gasification of cheap brown coal loaded with FeCl3. Experimental Section Catalyst Preparation. Loy Yang brown coal from Victoria, Australia, was used to prepare the char-supported catalysts in this study. Its ultimate analyses (on a dry basis (db)) are as follows: C, 69.9%; H, 5.3%; N, 0.61%; S, 0.26%. The coal features very low contents of inorganic species with an ash yield of 1.1 wt %. The main ash-forming species are Na, Mg, and Ca as well as some Fe and Al which exist in raw coal mainly as ion-exchangeable carboxylates or salt (e.g., NaCl).15 The raw coal was crushed to a size range of 106-150 µm and was washed using a 0.2 M H2SO4 aqueous solution to remove the majority of ion-exchangeable metallic species in the coal. Ferric chloride was then used for iron impregnation. The acid-washed coal was mixed with a 0.2 M ferric chloride aqueous solution and stirred in an inert gas atmosphere for 24 h. The different iron-loading levels were achieved by controlling the pH value of the solution by adding a 0.1 N ammonia (8) Jedynak, A.; Sentek, J.; Kowalczyk, Z.; Pielaszek, J.; Stolecki, K. Pol. J. Chem. 2000, 74, 1803-1806. (9) Yu, J.; Tian, F.-J.; McKenzie, L. J.; Li, C.-Z. Process Saf. EnViron. Prot., EFCE (Part B) and IChemE 2006, 84, 125-130. (10) Asami, K.; Mori, H.; Watanabe, T.; Ohtsuka, Y. Sekitan Kagaku Kaigi Happyo Ronbunshu 1992, 29, 166-169. (11) Asami, K.; Mori, H.; Watanabe, T.; Ohtsuka, Y. Sekitan Kagaku Kaigi Happyo Ronbunshu 1993, 30, 145-148. (12) Ohtsuka, Y. Sekiyu Gakkaishi 1998, 41, 182-192. (13) Ohtsuka, Y.; Asami, K. Coal Sci. Technol. 1991, 18, 139-151. (14) Ohtsuka, Y.; Asami, K.; Watanabe, T.; Mori, H. Proceedings of the Annual International Pittsburgh Coal Conference, 1994; pp 224-229. (15) Hayashi, J.-i.; Li, C.-Z. Eds. Structure and Properties of Victorian Brown Coal. In AdVances in the Science of Victorian Brown Coal; Li, C.Z., Ed.; Elsevier: Oxford, 2004; pp 11-84.
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396 Energy & Fuels, Vol. 21, No. 2, 2007 water solution during loading. The slurry was then filtered and washed using deionized water. Samples with two levels of iron loading were prepared. Sample “S1” has an iron content of 1.13% (wt, db), and sample “S2” has an iron loading level of 2.66% (wt, db). The char-supported catalysts were then prepared from the partial gasification of the above iron-loaded coal samples in 15% (v) steam in argon using a fluidized-bed/fixed-bed quartz reactor16 which was heated in an electrical furnace at 1070 K. The coal sample was fed into the reactor through a water-cooled feeding tube. The completion of the coal feeding was taken as the start of the char gasification, and the reactor was lifted out of the furnace and weighed. The weight was assigned as W0. The weight losses of chars after that were compared with W0 on a catalyst free basis were defined as char conversion. After the char was gasified for 0, 6, 15, 30, and 60 min, the reactor was lifted out of the furnace and the gasification was stopped with a char conversion of 0%, 9.8%, 23.5%, 35.1%, and 45.6%, respectively. The char samples were then collected and used as the catalysts for the water-gas-shift reaction. Water-Gas-Shift Reaction Experiments. The catalysts prepared from the above gasification process were transferred to the fixed-bed quartz reactor with a diameter of 30 mm to test their catalytic activity for water-gas-shift reactions. The schematic diagram of the experimental setup is reported elsewhere.9 In each experiment, 0.7-1.0 g of catalyst (accurately weighed) was charged into the reactor. A thermocouple was used to measure the temperature of the catalyst bed. The experiments were carried out over the temperature range of 300-425 °C using a gas mixture containing 2% CO and 10% steam in helium. Steam was generated in situ by feeding water directly into the reactor. The gas stream enters the reactor from the top with a flow rate ranging from 0.95 to 3.55 L‚min-1. As a comparison with the char-supported iron catalyst, a commercial pure magnetite nanopowder (Sigma-Aldrich Pty, Ltd) was used in a number of experimental runs. Analyses of Product Gas and Characterization of Catalyst Samples. Gaseous products from the outlet of the quartz reactor passed through ice-water-cooled bubblers and were collected using 3 L sampling bags. The contents of H2, CO, and CO2 were then determined using a GC equipped with a molecular sieve column, a Porapack N column, and a thermal conductivity detector (TCD). Char samples collected from gasification and after water-gas-shift reactions were subjected to X-ray diffraction (XRD) analysis. Some samples were analyzed using a transmission electron microscopy (TEM) instrument and an energy dispersion spectrometer (EDS). The morphology of the catalyst samples was also observed under a scanning electron microscope (SEM).
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Figure 1. XRD spectra of coal and char using the S2 sample: (a) char after 0 min of gasification after the coal was fed into the reactor at 1073 K; (b) char after gasification at 1073 K for 15 min; (c) char after gasification at 1073 K for 30 min; (d) char after gasification at 1073 K for 60 min; (e) char catalyst after water-gas-shift reaction. In the spectra, “R” represents R-Fe, “γ” represents γ-Fe, and “M” represents magnetite (Fe3O4).
Figure 2. CO consumption rate during the water-gas-shift reaction catalyzed by the char-supported nano-iron catalysts prepared from the steam gasification of the “S2” sample at 800 °C for different lengths of time: (O) 6; (]) 15; (4) 15 (repeated); (×) 30; (0) 60 min.
Results and Discussion The consumption rates of CO, expressed as moles per gram of Fe per second, during the water-gas-shift reaction using charsupported catalysts were measured over the temperature range of 300-425 °C. The CO consumption rate is calculated by [(C0 - Ct) × 100)/C0∆w]‚1/dt, where Ct is the CO molar concentration at any time t in the gas stream measured by GC, ∆w is the weight of iron in the catalysts loaded into the reactor, and C0 is the initial CO molar concentration in the gas stream. The charsupported iron catalysts have shown excellent catalytic activity for the water-gas-shift reaction. The activation energy calculated from the experimental data is between 85 and 95 kJ‚mol-1 similar to that of the Fe3O4/Cr2O3 catalyst reported previously.1 The effects of the iron-loading level on the CO consumption rate during the water-gas-shift reactions were reported in our previous study.9 The results indicated that the observed CO consumption rate was proportional to the amount of iron in the char and the dispersion of iron particles in the carbon matrix (16) Yu, J.; Chow, M. C.; Tian, F.-J.; McKenzie, L. J.; Li, C.-Z. Fuel 2005, 85, 127-133.
Figure 3. Effects of the char conversion level during steam char gasification at 800 °C on the subsequent activity for the water-gasshift reaction at 400 (0) and 415 °C (4).
was the determining factor affecting the catalytic activity of the char-supported iron catalyst. It is therefore important to prevent the agglomeration of iron particles during the preparation of such char-supported iron catalysts and during water-gasshift reactions. Chemical Form of Iron in the Char-Supported Iron Catalyst. Our experimental results showed that the charsupported iron catalysts in this study have an activation energy similar to that of the pure magnetite nanopowder (which was ∼90 kJ‚mol-1 from the measurement in this study). This suggested that the active catalyst phase in the iron catalysts supported on the char was magnetite. Further investigation was then carried out to examine the forms of iron in the catalysts
Iron-Loaded Victorian Brown Coal Catalyst
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Figure 4. TEM images of chars of different conversion after steam gasification at 800 °C for different lengths of time: (a) 15; (b) 30; (c) 60 min.
using XRD, and the results are shown in Figure 1. On the basis of previous Mo¨ssbauer spectroscopic studies,17,18 the ironloading procedure in this study would add iron into the brown coal mainly in the form of highly dispersed FeOOH although some iron would exist in coal as ion-exchangeable cations.15 These highly dispersed forms of iron are not visible under XRD.9,16 After gasification in steam at 800 °C for less than 15 min, the XRD patterns of the char showed the presence of magnetite coexisting with the reduced forms of Fe (i.e., R-Fe and γ-Fe), as shown in Figure 1a and b. During the char gasification, carbon is believed to be a strong reducing agent for iron.19 Especially during the early char gasification stage, the carbon in the char has a very high reactivity.15 Therefore, some iron was maintained in the metallic forms. These reducedforms of iron were transformed into magnetite during further char gasification in steam. After gasification for longer than 15 min at 800 °C, only the magnetite pattern appeared in the XRD spectra of the char-supported catalysts, as is shown in Figure 1c and d. TEM analysis suggests that the magnetite particles were larger during this stage. Our previous TEM study9 also revealed that, after the watergas-shift reaction experiments, the iron catalyst was distributed in the char matrix as nanoparticles with a particle size smaller than 50 nm. No apparent growth in the particle size after the (17) Cook, P. S.; Cashion, J. D. Fuel 1987, 66, 661-8. (18) Asami, K.; Sears, P.; Furimsky, E.; Ohtsuka, Y. Fuel Process. Technol. 1996, 47, 139-151. (19) Furimsky, E.; Sears, P.; Suzuki, T. Energy Fuels 1988, 2, 634639.
water-gas-shift reaction was observed under TEM. However, the transformation of the metallic iron to Fe3O4 and subsequent crystallization at the reaction temperature of the water-gasshift reaction was evident by comparing the XRD patterns in Figure 1e with Figure 1a and b. It is therefore certain that magnetite was the stable bulk phase in the char-supported catalysts during the water-gas-shift reaction. The previous results9 showing that the char catalysts used for 10 to 20 h had a catalytic reactivity rather similar to that of the fresh char catalysts virtually suggest that the high extent of dispersion of the catalyst nanoparticles is the major factor for the high reactivity of the char-supported iron catalyst. Under the present experimental conditions, the carbon is unlikely to be consumed at low temperatures during water-gas-shift reactions. The iron nanoparticles separated by the carbon materials have little chance to be agglomerated. Therefore, the high reactivity of the iron catalyst is well-maintained. The above results agree well with the extensive past studies on the use of iron oxides as the water-gas-reaction catalysts.1,3-8 These studies have revealed that the catalytically active species is usually magnetite (Fe3O4) and the catalyst is classified as the high temperature shift catalyst for operations at temperatures higher than 300 °C. The kinetics of the water-gas-shift reaction catalyzed by iron-containing catalysts has been reported under various conditions.20-22 The mechanism of the catalysis has also (20) van der Laan, G. P.; Beenackers, A. A. C. M. Appl. Catal. A: Gen. 2000, 193, 39-53. (21) Dry, M. E.; Shingles, T.; Boshoff, L. J. J. Catal. 1972, 25, 99104.
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been described in the literature23 with the aid of many analytical techniques (e.g., IR spectroscopy). The mechanism generally involves some iron oxide surface complex intermediates interacting with H2O and CO at high temperatures. The catalysis cycle is realized by the transformation between Fe(II) and Fe(III). The transformation between these two oxidation states of iron during heating has also been reported previously,24 suggesting that the catalytic activity of iron oxide for water-gasshift reactions does not necessarily involve the metallic iron. The absence of metallic iron in the char-supported iron catalysts (Figures. 1c-e) as well as in the magnetite nanopowder does not necessarily mean that the catalyst should have low activity for the water-gas-shift reaction with magnetite as the active catalytic phase in both. This is supported by the abovementioned similarity in the activation energy between the char-supported nano-iron catalysts and the magnetite nanopower. Effects of Char Conversion during Gasification on the Catalytic Activity. The influence of char conversion on the catalytic activity of iron for the water-gas-shift reaction is shown in Figure 2. After char gasification for 6, 15, 30, and 60 min, the char has reached a conversion of 9.8%, 23.5%, 35.1%, and 45.6%, respectively. It is obvious that the CO consumption rate in the catalyst decreased when the char conversion increased. The decreases in the catalytic activity were rather small when the conversion was lower than 35%. When the char conversion was higher than 40%, the catalytic activity dropped significantly, as shown in Figure 3. However, the data in Figure 2 appear to indicate that the activation energy (slopes of the plots in Figure 2) did not change significantly when char conversion was changed. It is therefore believed that the nature of the catalytic sites remained unchanged while increasing the char conversion when the catalysts were prepared. Three aspects may be considered here. First, the reactivity of char decreases with increasing the conversion during gasification. The extent of interaction of carbon and iron would therefore become smaller at a higher conversion level. Second, as carbon in the char is consumed, iron has a high chance to be agglomerated to form larger particles, hence decreasing the dispersion of iron. The decreases in the char-iron interaction with increasing char conversion would further enhance the agglomeration/sintering of Fe3O4 particles. From the TEM images shown in Figure 4, the growth of the nano-iron particles in the char is apparent at (22) Oki, S.; Mezaki, R. J. Phys. Chem. 1973, 77, 447-452. (23) Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces, 1st ed.; John Wiley Sons Ltd.: Chichester, 2003; p 690. (24) Cornell, R. M.; Schwertmann, U. The iron oxides : structure, properties, reactions, occurrence, and uses; VCH.: Weinheim and New York, 1996; p 573.
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the later gasification stage. As the particles grow larger, the reactivity and mobility of iron particles also decreases, leading to a decrease in the catalytic activity. Third, due to carbon deposits such as soot formation at the surfaces of the iron particles during the devolatilisation stage (i.e., the initial thermal decomposition of coal), the access of the reactant gases to the catalyst surface would be rather limited at a low char conversion. The carbon deposits at the surface of the particles will be gasified by steam during gasification. The pore structure, in particular the mesopores and micropores, develops further as steam gasification proceeds. The accessibility would therefore become better as the gasification proceeds. In order to prepare a char-supported iron catalyst with high catalytic activity, it is therefore important to choose an appropriate gasification time to achieve a moderate char conversion without excessive agglomeration/sintering of catalyst particles. However, it is also important to achieve a char conversion level high enough to have more iron catalyst in the catalyst system exposed and not covered by carbon deposits formed during the devolatilisation stage. In this study, 10-15 min at 800 °C is therefore recommended as the optimum gasification time with a char conversion of ∼35%. During our experiments, we have also found that low temperature oxidation of char in oxygen at 400 °C had effects similar to that of char gasification conversion on the reactivity of the char catalyst. Conclusions A novel char-supported iron catalyst prepared from the gasification of iron-loaded Victorian brown coal is highly active for water-gas-shift reactions. The active phase for the catalytic effect is magnetite under the present experimental conditions. The dispersion of the iron particles in the char support is important for its catalytic activity while the carbon material in the char plays an important role in preventing the agglomeration of iron nanoparticles. A char conversion of ∼35% for catalyst preparation during gasification at 800 °C is recommended to achieve a high catalytic activity during the water-gas-shift reaction. Acknowledgment. The authors gratefully acknowledge the financial support from the Cooperative Research Centre (CRC) for Clean Power from Lignite, which is established and supported under the Australian Government’s Cooperative Research Centres program. The authors also acknowledge the partial support of this work by the New Energy and Industrial Technology Development Organisation (NEDO) in Japan. EF060399Y
