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Steam reforming of tar derived from lignin over pompom-like potassium-promoted iron-based catalysts formed on calcined scallop shell
Bioresource Technology 139 (2013) 280–284
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Steam reforming of tar derived from lignin over pompom-like potassium-promoted iron-based catalysts formed on calcined scallop shell Guoqing Guan a,⇑, Malinee Kaewpanha a, Xiaogang Hao b,⇑, Ai-min Zhu c, Yutaka Kasai d, Seiji Kakuta d, Katsuki Kusakabe e, Abuliti Abudula a a
North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China c Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China d Industrial Research Institute, Aomori Prefectural Industrial Technology Research Center, 4-11-6 Second Tonyamachi, Aomori 030-0113, Japan e Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Kumamoto 860-0082, Japan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Pompom-like K-promoted iron-based
catalysts were prepared on calcined shell. A mechanism for the formation of pompom-like microspheres was proposed. An excellent catalytic activity were exhibited for the steam reforming of tar.
a r t i c l e
i n f o
Article history: Received 17 January 2013 Received in revised form 1 April 2013 Accepted 2 April 2013 Available online 23 April 2013 Keywords: Lignin Tar reforming Calcined scallop shell Iron-based catalyst Potassium
a b s t r a c t In order to understand the improvement effect of potassium (K) on the catalytic activity of iron-loaded calcined scallop shell (CS) for the steam reforming tar derived from biomass, various K precursors were applied for the catalyst preparation. It is found that pompom-like iron-based particles with a mesoporous structure were easily formed on the surface of calcined scallop shell (CS) when K2CO3 was used as K precursor while no such kind of microsphere was formed when other kinds of K precursors such as KOH and KNO3 were applied. The optimum K-loading amount for the preparation of this catalyst was investigated. Based on the experimental results obtained, a mechanism for the formation of these microspheres was proposed. This pompom-like potassium-promoted iron-based catalyst showed a better catalytic activity and reusability for the steam reforming of tar derived from lignin. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen production by the gasification of biomass instead of fossil fuel is considered a potentially attractive and alternative ⇑ Corresponding authors. Tel.: +81 17 762 7756; fax: +81 17 735 5411. E-mail addresses: [email protected] (G. Guan), [email protected] (X. Hao). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.007
resource to meet the future demands of a hydrogen economy, mainly as the feedstock for fuel cells (Huber et al., 2006; Piatkowski et al., 2011; Trane et al., 2012; Yin, 2012). However, due to a large amount of tar being generated during the gasification process, and the unreformed tar condensing at low temperatures, plugging and corrosion of pipelines can result. It is expected that the tar produced can be completely captured and simultaneously also converted to useful gases such as H2, CO and CH4 in the
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gasification process. Catalytic steam reforming of tar is one of the effective methods for transformation of tar and has been widely investigated (Acharya et al., 2010; Di Felice et al., 2010; Han and Kim, 2008; Li et al., 2009a,b). Various natural mineral catalysts, such as dolomite and olivine, and manufactured catalysts such as Ni, Co, Fe and some noble metals supported on common materials such as Al2O3, MgO, CaO and SiO2 have been developed for this purpose (Delgado et al., 1997; Di Felice et al., 2011; Li et al., 2009a,b; Sutton et al., 2001; Park et al., 2010; Zhang et al., 2011). In order to promote the conversion efficiency and coking resistance, bimetallic and tri-metallic catalysts were synthesized by addition of other transition metals or rare earth metals such as Pt, Rh, La, or Ce to the above catalysts (Asadullah et al., 2001; Bona et al., 2008; Constantinou and Efstathiou, 2011; Nishikawa et al., 2008; Polychronopoulou and Efstathiou, 2006; Rapagná et al., 2002). In the previous study, Fe-based catalyst was supported on calcined scallop shell (CS) and used for the steam reforming of tar derived from biomass. It was found that the transfer of a small amount of potassium from the biomass to the catalysts promoted the catalytic activity of the original catalyst (Guan et al., 2012). Also, when a small amount of K was doped on CS or Fe-loaded CS using KNO3 as K-precursor for the steam reforming of tar derived from dealkaline lignin, it was found that hydrogen production rate was greatly increased (Guan et al., 2013). Alkali and alkaline earth elements have been identified to have good catalytic effects on the pyrolysis of biomass (Wang et al., 2010). Several studies also indicated that K plays a key role in the formation of active sites for the surface carbon gasification reaction (Devoldere and Froment, 1999; Hirano, 1986; Jiang et al., 2012; Li et al., 2010; Li and Shanks, 2011; Mckee, 1983; Shekhah et al., 2004). In order to investigate in details the effects of K on the catalytic performance for the steam reforming of tar, K-doped Fe-based catalysts were supported on the CS using different K precursors in this study. Interestingly, it was found that pompom-like microspheres with a mesoporous structure were formed on the surface of CS if potassium carbonate (K2CO3) was used as the K-precursor. To the best of our knowledge, this is the first report on the preparation of such mesoporous spheres for Fe-based materials via a facile method without any templates. Furthermore, when it was applied for the steam reforming of tar derived from lignin, excellent catalytic activities were observed for the production of H2. 2. Methods 2.1. Materials and catalyst preparation In this study, the catalyst support, i.e., CS, was prepared by calcinations of dried scallop shell chips (Aomori, Japan) with an average size of approximately 4 4 2 mm3 in air at 800 °C for 2 h. 0.2– 5 wt% K-doped 2 wt% Fe-based pompom-like catalyst particles were supported on CS by the incipient wetness impregnation method using a mixed aqueous solution of Fe(NO3)39H2O (WAKO, Japan) and K2CO3 (Wako, Japan). After impregnation and aging for approximately 2 h, the slurry was dried overnight at 80 °C. Finally, the supported catalysts were calcined in air at 650 °C for 3 h before storage and further use. As a reference, KNO3 (Wako, Japan) and KOH (Wako, Japan) were also used as K precursors. Powders of dealkaline lignin (Tokyo Chemistry Industry Co., Ltd., Japan, pH 4.12) with minor alkaline elements, and alkaline lignin (Kanto Chemical, Japan, pH 9.3) with approximately 0.53 wt% of Na and 0.015 wt% of K were pressed, crushed and sieved to 0.5–1 mm particle size before used. 2.2. Characterization of catalyst The morphology and elemental mapping of as-prepared catalyst were characterized with a scanning electron microscope (SEM,
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SU6600, Hitachi) equipped with energy dispersive spectrometer (EDS). The crystal structure of as-prepared catalyst was determined by X-ray diffraction (XRD 610, Shimadzu, Japan). The radiation used was CuKa and the operating potential was 30 kV, current 30 mA, and the scanning rate 4°/min. 2.3. Catalytic steam reforming experiment Catalytic steam reforming of tar derived from lignin was performed in a fixed bed reactor with an inner diameter of 19 mm, which was precisely described in our previous study (Guan et al., 2012). 0.3 g of lignin and 4.5 g of catalyst were located separately into the reactor with quartz wool, and the reaction was performed at a stable temperature with a heating rate of 25 °C/min. Steam was generated by bubbling of 90 °C water (pH2O = 70 kPa) with 50 cm3/min of Ar gas flow. The tar, derived from the steam pyrolysis of lignin placed on the catalyst, was carried by the Ar gas into the catalyst layer, and reformed with steam over the catalyst. The gases produced were passed through two cold traps and a filter, and collected in a gas bag. The reaction time for all experiments was fixed at 2 h. The gases produced were analyzed using a gas chromatograph (Agilent 7890A GC system). 3. Results and discussion 3.1. Catalyst characterization During the experiments, it was found that pompom-like particle (Fig. S-1(b) and (c)) was easy to be formed on the surface of 2 wt%Fe-based CS when the loading amount of K was between 0.2 and 2.1 wt% in the case of K2CO3 as the precursor. If the loading amount was too high or too low, no pompom-like particle was obtained (Fig. S-1(a) and (d)). As shown in Fig. S-1, one can see that uniform particles with a size of approximately 10 lm and a microporous structure were formed on the surface of CS in the case of the loading amount of 0.5 and 1.5 wt%K. For this kind of particle, if the surface morphology was enlarged, it is obvious that the sphere was mainly composed of nanorods as well as nanosheets (Fig. S-2(a)-iv and (b)-iv). With the increase in the concentration of K, the sphere size remained almost unchanged but the porous structure changed to some extent. However, when the K concentration increased higher, pompom-like particle disappeared (Fig. S-1(d)). It is possible that the vigorous hydrolysis of K2CO3 in the solution hindered the formation of the sphere particles. Such porous pompom-like morphology is expected to have an advantage if utilized for the adsorption and decomposition of biomass tar. EDS analysis results indicated that the main composition of the pompom-like sphere was Fe element but K element spread over almost the entire CS support (Fig. S-3 in electronic annex of this paper). In the XRD patterns (Fig. S-4 in electronic annex of this paper), similar to iron supported CS, the crystallographic phases of iron oxide and Ca2Fe2O5 in K-doped Fe-based catalyst were observed. Ca2Fe2O5 may be formed at the interfaces of the pompom-like spheres and CS due to the libration of Ca2+ ions into the alkaline solution, which might play a beneficial role towards providing adhesion and support of the spheres on the surface of CS. However, it is found that the peak strength of Ca2Fe2O5 in the product decreased (Fig. S-5 in electronic annex of this paper) with the increase in K concentration as K2CO3 used as precursor, and compared with other K precursors, the peak strength of Ca2Fe2O5 in the case of K2CO3 showed the highest (Fig. S-6 in electronic annex of this paper). On the other hand, as shown in Fig. S-7 (in electronic annex of this paper), when only K or Fe was supported on CS or KNO3 or KOH was used to replace K2CO3 as the K-precursor, no pompom-like sphere was found to be formed on CS. Similarly, if
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Scheme 1. Proposed formation process of pompom-like K-promoted Fe-based catalyst on CS. (a) Nuclei formation; (b) crystal growth; (c) spherical shape formation; (d) pompom-like catalyst formation on CS by calcination.
an aqueous solution of Fe(NO3)3 was mixed with K2CO3 solution, and then dried and calcinated at the same conditions, no pompom-like sphere was observed (Fig. S-8 in electronic annex of this paper). Based on the above results, a process for the formation of the mesostructured pompom-like K-promoted Fe-based catalyst on CS is proposed in Scheme 1. Firstly, in the aqueous solution, OH ion will be formed on the surface of CS, and react with Fe3+ ion to form due to Fe(OH)3 precipitates, which will serve as nuclei for the aggregation of Fe(OH)3 precipitate particles generated in the liquid phase due to the hydrolysis of K2CO3. On the other hand, Ca2+ ion could be liberated into the alkaline impreganation solution during the addition of the Fe and K containing mixture; the Ca and Fe species then precipitate to form a double-layer hydroxide (e.g., CaxFey(OH)z(CO2)deH2O) (Wu et al., 2012), which could be the reason why the mixed oxides such as CaFe2O4 was formed as indicated in Figs. S-4–7. Secondly, the heat from the reaction of CaO with H2O may accelerate the decomposition of carbonate salts and lead to the formation of CO2 microbubbles on the surface of CS. These bubbles should escape from the surface with the aid of heat and drive the precipitate particles to assemble around the bubble and form the initial spheres. With the reaction proceeding further, more CO2 escapes from the solution, and more OH ions are pro-
Fig. 1. Comparison of gas and char yields over different catalysts in 2 h at 610 °C for the steam reforming of dealkaline lignin (A: no catalyst; B: CS; C: 2.0 wt%Fe/CS; D: 0.5 wt%K(K2CO3)/CS; E: 0.5 wt%K(KNO3)–2.0 wt%Fe/CS; F: 0.5 wt%K(KOH)– 2.0wt%Fe/CS); G: 0.5wt%K(K2CO3)–2.0wt%Fe/CS).
duced in the solution, resulting in more Fe(OH)3 precipitates and double-layer hydroxides being generated, and the spheres that have been formed growing layer by layer through the Ostwald ripening mechanism. During the drying process, the formed spheres could self-assembly along the surface of CS driven by the evaporation of water. It is found that the spheres, which only composed of bar-like particles, have been formed on the surface of CS after dried at 80 °C (Fig. S-9), but the images are different with the spheres after calcined (Fig. S-2). During calcination process, the remaining K2CO3 and nitride salts in the spheres arranged on the surface of CS is decomposed gradually so that porous structures are formed in the spheres, and finally the well-defined pompom-like sphere layer is obtained on CS. 3.2. Catalytic activities of pompom-like K–Fe/CS catalyst Fig. 1 compares gas and char yields when different catalysts were used for the steam reforming of dealkaline lignin. In general, lignin thermally decomposes over a broad temperature range (200–500 °C), and the tar obtained is composed mainly of aromatic hydrocarbons, phenolics, hydroxyl-phenolics and guaiacyl/syringyl-type coumpounds (Brebu and Vasile, 2010). As shown in Fig. 1, one can see that the addition of K in the Fe-based CS catalyst
Fig. 2. Effects of K loading amount (on 2 wt%Fe/CS) on the yields of gases for the steam reforming of dealkaline as well as alkaline lignin (D: dealkaline lignin; A: alkaline lignin; reaction temperature: 610 °C; reaction time: 2 h).
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obviously promoted the catalytic activity for the steam reforming of these complex compounds. As a result, the H2 production yield increased to a great extent while production yields of CO, CH4, and CO2 also increased in the case of K-doped Fe-based CS catalyst. It should be noted that a higher enhancement effect appeared when K2CO3 was used as the K precursor. This should be attributed to the special pompom-like structure, which may absorb more tar and provide more active sites for the tar reforming. As indicated in Fig. 1, in this case, the total H2 amount increased approximately 59% when compared with that in the case of 2 wt%Fe/CS. Many studies have identified that addition of alkali metal salts or oxides such as K2CO3 or K2O can enhance the steam reforming reactions (Devoldere and Froment, 1999; Di Felice et al., 2011; Guan et al., 2013; Hirano, 1986; Jiang et al., 2012; Li and Shanks, 2011; Mckee, 1983; Shekhah et al., 2004). It is suggested that these alkali elements may alter the ionization potential of the surface carbon atoms and hinder the deposition of carbon on the surface of catalyst (Mckee, 1983). Fig. 2 shows production yields of H2, CO, CH4 and CO2 when different concentrations of K-doped 2 wt%Fe/CS catalysts were used for the reforming of tar derived from dealkaline as well as alkaline lignins for 2 h at 610 °C. With the increase in K loading amount, more gases were produced. It should be noted that hydrogen yield increased approximately 31.7%, 32.4% and 22.2% for 0.5 wt%, 1 wt% and 1.5 wt% K-loaded 2 wt%Fe/CS catalysts, respectively, when alkaline lignin was used. This should be attributed to some alkaline elements such as Na and K, which were contained in alkaline lignin, and these alkaline elements had in-situ catalytic activity for the reforming of tar and char in the lignin. 3.3. Stability and reusability of pompom-like K–Fe/CS catalyst Fig. 3 shows gas and char yields when 1 wt%K–2 wt%Fe/CS was used again with and without regeneration for the steam reforming of tar derived from dealkaline as well as alkaline lignin. It is found that the activity of the catalyst decreased when it was used again in the case of without regeneration for two kinds of lignin. However, the catalytic activity recovered after the used catalyst was calcined in air at 610 °C for 1 h. Furthermore, in the case of reforming tar derived from alkaline lignin, the regenerated catalyst showed a little high catalytic activity. As indicated above, almost no alkaline elements are contained in dealkaline lignin, and therefore, it can be considered that no alkaline will be evaporated and move to the catalyst layer with the tar. However, for alkaline lignin, alkaline elements could move to the surface of the catalyst with the tar (Guan et al., 2012), resulting in the enhancement of catalytic activity for the original catalyst.
Fig. 3. Stability and reusability of as-prepared pompom-like catalyst for the steam reforming of dealkaline as well as alkaline lignin (F: first run; S: second run; R: regenerated catalyst used catalyst: 1 wt%K–2.0 wt%Fe/CS; reaction temperature: 610 °C; reaction time: 2 h).
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The accumulation of carbon deposits on the catalyst during the reaction is considered one of the main reasons for the deactivation of Fe-based catalyst for tar reforming. In the present study, the oxide K in the catalyst could react with H2O to form KOH which reacted further with carbon to form K2CO3, which could decompose, release CO2 and leave behind K-oxides (Hirsch et al., 1982; Mühler et al., 1990). Therefore, the addition of K is also expected to prevent or remove carbon deposits and maintain a high catalytic activity. Considering the composition of the catalyst matrix, formation of KFeO2 and Ca2Fe2O5 species is also a possible route to modify Febased catalysts. It was found that the presence of KFeO2 was necessary for maintaining a high and long-term activity for Fe-based catalyst (Hirsch et al., 1982; Mühler et al., 1990; Shekhah et al., 2004) while the presence of Ca2Fe2O5 could have catalytic reactivity for the decomposition of polyaromatic tar (Huang et al., 2012). Furthermore, for the present catalysts, formation of KFeO2 in the spheres could prevent elemental K from evaporating during the reaction.
4. Conclusions In summary, a facile method has been found to fabricate pompom-like K-promoted Fe-based microspheres with a mesoporous structure on the surface of CS without any templates. This method may also be applied for the preparation of other kinds of microspheres. Based on the experimental results obtained, a mechanism for the formation of these microspheres was proposed and further investigated. The generation of CO2 on the surface of CS seemed to play an important role in the formation of the pompom-like spheres. This kind of catalyst exhibited an excellent catalytic activity and reusability for the steam reforming of tar derived from lignin. Acknowledgements This work is supported by Japan Science and Technology Agency (JST), Japan and Aomori City Government.
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Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Steam reforming of tar derived from lignin over pompom-like potassium-promoted iron-based catalysts formed on calcined scallop shell Guoqing Guan a,⇑, Malinee Kaewpanha a, Xiaogang Hao b,⇑, Ai-min Zhu c, Yutaka Kasai d, Seiji Kakuta d, Katsuki Kusakabe e, Abuliti Abudula a a
North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China c Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China d Industrial Research Institute, Aomori Prefectural Industrial Technology Research Center, 4-11-6 Second Tonyamachi, Aomori 030-0113, Japan e Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Kumamoto 860-0082, Japan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Pompom-like K-promoted iron-based
catalysts were prepared on calcined shell. A mechanism for the formation of pompom-like microspheres was proposed. An excellent catalytic activity were exhibited for the steam reforming of tar.
a r t i c l e
i n f o
Article history: Received 17 January 2013 Received in revised form 1 April 2013 Accepted 2 April 2013 Available online 23 April 2013 Keywords: Lignin Tar reforming Calcined scallop shell Iron-based catalyst Potassium
a b s t r a c t In order to understand the improvement effect of potassium (K) on the catalytic activity of iron-loaded calcined scallop shell (CS) for the steam reforming tar derived from biomass, various K precursors were applied for the catalyst preparation. It is found that pompom-like iron-based particles with a mesoporous structure were easily formed on the surface of calcined scallop shell (CS) when K2CO3 was used as K precursor while no such kind of microsphere was formed when other kinds of K precursors such as KOH and KNO3 were applied. The optimum K-loading amount for the preparation of this catalyst was investigated. Based on the experimental results obtained, a mechanism for the formation of these microspheres was proposed. This pompom-like potassium-promoted iron-based catalyst showed a better catalytic activity and reusability for the steam reforming of tar derived from lignin. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen production by the gasification of biomass instead of fossil fuel is considered a potentially attractive and alternative ⇑ Corresponding authors. Tel.: +81 17 762 7756; fax: +81 17 735 5411. E-mail addresses: [email protected] (G. Guan), [email protected] (X. Hao). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.007
resource to meet the future demands of a hydrogen economy, mainly as the feedstock for fuel cells (Huber et al., 2006; Piatkowski et al., 2011; Trane et al., 2012; Yin, 2012). However, due to a large amount of tar being generated during the gasification process, and the unreformed tar condensing at low temperatures, plugging and corrosion of pipelines can result. It is expected that the tar produced can be completely captured and simultaneously also converted to useful gases such as H2, CO and CH4 in the
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gasification process. Catalytic steam reforming of tar is one of the effective methods for transformation of tar and has been widely investigated (Acharya et al., 2010; Di Felice et al., 2010; Han and Kim, 2008; Li et al., 2009a,b). Various natural mineral catalysts, such as dolomite and olivine, and manufactured catalysts such as Ni, Co, Fe and some noble metals supported on common materials such as Al2O3, MgO, CaO and SiO2 have been developed for this purpose (Delgado et al., 1997; Di Felice et al., 2011; Li et al., 2009a,b; Sutton et al., 2001; Park et al., 2010; Zhang et al., 2011). In order to promote the conversion efficiency and coking resistance, bimetallic and tri-metallic catalysts were synthesized by addition of other transition metals or rare earth metals such as Pt, Rh, La, or Ce to the above catalysts (Asadullah et al., 2001; Bona et al., 2008; Constantinou and Efstathiou, 2011; Nishikawa et al., 2008; Polychronopoulou and Efstathiou, 2006; Rapagná et al., 2002). In the previous study, Fe-based catalyst was supported on calcined scallop shell (CS) and used for the steam reforming of tar derived from biomass. It was found that the transfer of a small amount of potassium from the biomass to the catalysts promoted the catalytic activity of the original catalyst (Guan et al., 2012). Also, when a small amount of K was doped on CS or Fe-loaded CS using KNO3 as K-precursor for the steam reforming of tar derived from dealkaline lignin, it was found that hydrogen production rate was greatly increased (Guan et al., 2013). Alkali and alkaline earth elements have been identified to have good catalytic effects on the pyrolysis of biomass (Wang et al., 2010). Several studies also indicated that K plays a key role in the formation of active sites for the surface carbon gasification reaction (Devoldere and Froment, 1999; Hirano, 1986; Jiang et al., 2012; Li et al., 2010; Li and Shanks, 2011; Mckee, 1983; Shekhah et al., 2004). In order to investigate in details the effects of K on the catalytic performance for the steam reforming of tar, K-doped Fe-based catalysts were supported on the CS using different K precursors in this study. Interestingly, it was found that pompom-like microspheres with a mesoporous structure were formed on the surface of CS if potassium carbonate (K2CO3) was used as the K-precursor. To the best of our knowledge, this is the first report on the preparation of such mesoporous spheres for Fe-based materials via a facile method without any templates. Furthermore, when it was applied for the steam reforming of tar derived from lignin, excellent catalytic activities were observed for the production of H2. 2. Methods 2.1. Materials and catalyst preparation In this study, the catalyst support, i.e., CS, was prepared by calcinations of dried scallop shell chips (Aomori, Japan) with an average size of approximately 4 4 2 mm3 in air at 800 °C for 2 h. 0.2– 5 wt% K-doped 2 wt% Fe-based pompom-like catalyst particles were supported on CS by the incipient wetness impregnation method using a mixed aqueous solution of Fe(NO3)39H2O (WAKO, Japan) and K2CO3 (Wako, Japan). After impregnation and aging for approximately 2 h, the slurry was dried overnight at 80 °C. Finally, the supported catalysts were calcined in air at 650 °C for 3 h before storage and further use. As a reference, KNO3 (Wako, Japan) and KOH (Wako, Japan) were also used as K precursors. Powders of dealkaline lignin (Tokyo Chemistry Industry Co., Ltd., Japan, pH 4.12) with minor alkaline elements, and alkaline lignin (Kanto Chemical, Japan, pH 9.3) with approximately 0.53 wt% of Na and 0.015 wt% of K were pressed, crushed and sieved to 0.5–1 mm particle size before used. 2.2. Characterization of catalyst The morphology and elemental mapping of as-prepared catalyst were characterized with a scanning electron microscope (SEM,
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SU6600, Hitachi) equipped with energy dispersive spectrometer (EDS). The crystal structure of as-prepared catalyst was determined by X-ray diffraction (XRD 610, Shimadzu, Japan). The radiation used was CuKa and the operating potential was 30 kV, current 30 mA, and the scanning rate 4°/min. 2.3. Catalytic steam reforming experiment Catalytic steam reforming of tar derived from lignin was performed in a fixed bed reactor with an inner diameter of 19 mm, which was precisely described in our previous study (Guan et al., 2012). 0.3 g of lignin and 4.5 g of catalyst were located separately into the reactor with quartz wool, and the reaction was performed at a stable temperature with a heating rate of 25 °C/min. Steam was generated by bubbling of 90 °C water (pH2O = 70 kPa) with 50 cm3/min of Ar gas flow. The tar, derived from the steam pyrolysis of lignin placed on the catalyst, was carried by the Ar gas into the catalyst layer, and reformed with steam over the catalyst. The gases produced were passed through two cold traps and a filter, and collected in a gas bag. The reaction time for all experiments was fixed at 2 h. The gases produced were analyzed using a gas chromatograph (Agilent 7890A GC system). 3. Results and discussion 3.1. Catalyst characterization During the experiments, it was found that pompom-like particle (Fig. S-1(b) and (c)) was easy to be formed on the surface of 2 wt%Fe-based CS when the loading amount of K was between 0.2 and 2.1 wt% in the case of K2CO3 as the precursor. If the loading amount was too high or too low, no pompom-like particle was obtained (Fig. S-1(a) and (d)). As shown in Fig. S-1, one can see that uniform particles with a size of approximately 10 lm and a microporous structure were formed on the surface of CS in the case of the loading amount of 0.5 and 1.5 wt%K. For this kind of particle, if the surface morphology was enlarged, it is obvious that the sphere was mainly composed of nanorods as well as nanosheets (Fig. S-2(a)-iv and (b)-iv). With the increase in the concentration of K, the sphere size remained almost unchanged but the porous structure changed to some extent. However, when the K concentration increased higher, pompom-like particle disappeared (Fig. S-1(d)). It is possible that the vigorous hydrolysis of K2CO3 in the solution hindered the formation of the sphere particles. Such porous pompom-like morphology is expected to have an advantage if utilized for the adsorption and decomposition of biomass tar. EDS analysis results indicated that the main composition of the pompom-like sphere was Fe element but K element spread over almost the entire CS support (Fig. S-3 in electronic annex of this paper). In the XRD patterns (Fig. S-4 in electronic annex of this paper), similar to iron supported CS, the crystallographic phases of iron oxide and Ca2Fe2O5 in K-doped Fe-based catalyst were observed. Ca2Fe2O5 may be formed at the interfaces of the pompom-like spheres and CS due to the libration of Ca2+ ions into the alkaline solution, which might play a beneficial role towards providing adhesion and support of the spheres on the surface of CS. However, it is found that the peak strength of Ca2Fe2O5 in the product decreased (Fig. S-5 in electronic annex of this paper) with the increase in K concentration as K2CO3 used as precursor, and compared with other K precursors, the peak strength of Ca2Fe2O5 in the case of K2CO3 showed the highest (Fig. S-6 in electronic annex of this paper). On the other hand, as shown in Fig. S-7 (in electronic annex of this paper), when only K or Fe was supported on CS or KNO3 or KOH was used to replace K2CO3 as the K-precursor, no pompom-like sphere was found to be formed on CS. Similarly, if
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Scheme 1. Proposed formation process of pompom-like K-promoted Fe-based catalyst on CS. (a) Nuclei formation; (b) crystal growth; (c) spherical shape formation; (d) pompom-like catalyst formation on CS by calcination.
an aqueous solution of Fe(NO3)3 was mixed with K2CO3 solution, and then dried and calcinated at the same conditions, no pompom-like sphere was observed (Fig. S-8 in electronic annex of this paper). Based on the above results, a process for the formation of the mesostructured pompom-like K-promoted Fe-based catalyst on CS is proposed in Scheme 1. Firstly, in the aqueous solution, OH ion will be formed on the surface of CS, and react with Fe3+ ion to form due to Fe(OH)3 precipitates, which will serve as nuclei for the aggregation of Fe(OH)3 precipitate particles generated in the liquid phase due to the hydrolysis of K2CO3. On the other hand, Ca2+ ion could be liberated into the alkaline impreganation solution during the addition of the Fe and K containing mixture; the Ca and Fe species then precipitate to form a double-layer hydroxide (e.g., CaxFey(OH)z(CO2)deH2O) (Wu et al., 2012), which could be the reason why the mixed oxides such as CaFe2O4 was formed as indicated in Figs. S-4–7. Secondly, the heat from the reaction of CaO with H2O may accelerate the decomposition of carbonate salts and lead to the formation of CO2 microbubbles on the surface of CS. These bubbles should escape from the surface with the aid of heat and drive the precipitate particles to assemble around the bubble and form the initial spheres. With the reaction proceeding further, more CO2 escapes from the solution, and more OH ions are pro-
Fig. 1. Comparison of gas and char yields over different catalysts in 2 h at 610 °C for the steam reforming of dealkaline lignin (A: no catalyst; B: CS; C: 2.0 wt%Fe/CS; D: 0.5 wt%K(K2CO3)/CS; E: 0.5 wt%K(KNO3)–2.0 wt%Fe/CS; F: 0.5 wt%K(KOH)– 2.0wt%Fe/CS); G: 0.5wt%K(K2CO3)–2.0wt%Fe/CS).
duced in the solution, resulting in more Fe(OH)3 precipitates and double-layer hydroxides being generated, and the spheres that have been formed growing layer by layer through the Ostwald ripening mechanism. During the drying process, the formed spheres could self-assembly along the surface of CS driven by the evaporation of water. It is found that the spheres, which only composed of bar-like particles, have been formed on the surface of CS after dried at 80 °C (Fig. S-9), but the images are different with the spheres after calcined (Fig. S-2). During calcination process, the remaining K2CO3 and nitride salts in the spheres arranged on the surface of CS is decomposed gradually so that porous structures are formed in the spheres, and finally the well-defined pompom-like sphere layer is obtained on CS. 3.2. Catalytic activities of pompom-like K–Fe/CS catalyst Fig. 1 compares gas and char yields when different catalysts were used for the steam reforming of dealkaline lignin. In general, lignin thermally decomposes over a broad temperature range (200–500 °C), and the tar obtained is composed mainly of aromatic hydrocarbons, phenolics, hydroxyl-phenolics and guaiacyl/syringyl-type coumpounds (Brebu and Vasile, 2010). As shown in Fig. 1, one can see that the addition of K in the Fe-based CS catalyst
Fig. 2. Effects of K loading amount (on 2 wt%Fe/CS) on the yields of gases for the steam reforming of dealkaline as well as alkaline lignin (D: dealkaline lignin; A: alkaline lignin; reaction temperature: 610 °C; reaction time: 2 h).
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obviously promoted the catalytic activity for the steam reforming of these complex compounds. As a result, the H2 production yield increased to a great extent while production yields of CO, CH4, and CO2 also increased in the case of K-doped Fe-based CS catalyst. It should be noted that a higher enhancement effect appeared when K2CO3 was used as the K precursor. This should be attributed to the special pompom-like structure, which may absorb more tar and provide more active sites for the tar reforming. As indicated in Fig. 1, in this case, the total H2 amount increased approximately 59% when compared with that in the case of 2 wt%Fe/CS. Many studies have identified that addition of alkali metal salts or oxides such as K2CO3 or K2O can enhance the steam reforming reactions (Devoldere and Froment, 1999; Di Felice et al., 2011; Guan et al., 2013; Hirano, 1986; Jiang et al., 2012; Li and Shanks, 2011; Mckee, 1983; Shekhah et al., 2004). It is suggested that these alkali elements may alter the ionization potential of the surface carbon atoms and hinder the deposition of carbon on the surface of catalyst (Mckee, 1983). Fig. 2 shows production yields of H2, CO, CH4 and CO2 when different concentrations of K-doped 2 wt%Fe/CS catalysts were used for the reforming of tar derived from dealkaline as well as alkaline lignins for 2 h at 610 °C. With the increase in K loading amount, more gases were produced. It should be noted that hydrogen yield increased approximately 31.7%, 32.4% and 22.2% for 0.5 wt%, 1 wt% and 1.5 wt% K-loaded 2 wt%Fe/CS catalysts, respectively, when alkaline lignin was used. This should be attributed to some alkaline elements such as Na and K, which were contained in alkaline lignin, and these alkaline elements had in-situ catalytic activity for the reforming of tar and char in the lignin. 3.3. Stability and reusability of pompom-like K–Fe/CS catalyst Fig. 3 shows gas and char yields when 1 wt%K–2 wt%Fe/CS was used again with and without regeneration for the steam reforming of tar derived from dealkaline as well as alkaline lignin. It is found that the activity of the catalyst decreased when it was used again in the case of without regeneration for two kinds of lignin. However, the catalytic activity recovered after the used catalyst was calcined in air at 610 °C for 1 h. Furthermore, in the case of reforming tar derived from alkaline lignin, the regenerated catalyst showed a little high catalytic activity. As indicated above, almost no alkaline elements are contained in dealkaline lignin, and therefore, it can be considered that no alkaline will be evaporated and move to the catalyst layer with the tar. However, for alkaline lignin, alkaline elements could move to the surface of the catalyst with the tar (Guan et al., 2012), resulting in the enhancement of catalytic activity for the original catalyst.
Fig. 3. Stability and reusability of as-prepared pompom-like catalyst for the steam reforming of dealkaline as well as alkaline lignin (F: first run; S: second run; R: regenerated catalyst used catalyst: 1 wt%K–2.0 wt%Fe/CS; reaction temperature: 610 °C; reaction time: 2 h).
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The accumulation of carbon deposits on the catalyst during the reaction is considered one of the main reasons for the deactivation of Fe-based catalyst for tar reforming. In the present study, the oxide K in the catalyst could react with H2O to form KOH which reacted further with carbon to form K2CO3, which could decompose, release CO2 and leave behind K-oxides (Hirsch et al., 1982; Mühler et al., 1990). Therefore, the addition of K is also expected to prevent or remove carbon deposits and maintain a high catalytic activity. Considering the composition of the catalyst matrix, formation of KFeO2 and Ca2Fe2O5 species is also a possible route to modify Febased catalysts. It was found that the presence of KFeO2 was necessary for maintaining a high and long-term activity for Fe-based catalyst (Hirsch et al., 1982; Mühler et al., 1990; Shekhah et al., 2004) while the presence of Ca2Fe2O5 could have catalytic reactivity for the decomposition of polyaromatic tar (Huang et al., 2012). Furthermore, for the present catalysts, formation of KFeO2 in the spheres could prevent elemental K from evaporating during the reaction.
4. Conclusions In summary, a facile method has been found to fabricate pompom-like K-promoted Fe-based microspheres with a mesoporous structure on the surface of CS without any templates. This method may also be applied for the preparation of other kinds of microspheres. Based on the experimental results obtained, a mechanism for the formation of these microspheres was proposed and further investigated. The generation of CO2 on the surface of CS seemed to play an important role in the formation of the pompom-like spheres. This kind of catalyst exhibited an excellent catalytic activity and reusability for the steam reforming of tar derived from lignin. Acknowledgements This work is supported by Japan Science and Technology Agency (JST), Japan and Aomori City Government.
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