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Applied Catalysis A: General 310 (2006) 138–144 www.elsevier.com/locate/apcata
Hydrogen production from methanol using a SiC fiber-containing paper composite impregnated with Cu/ZnO catalyst Shuji Fukahori a, Hirotaka Koga a, Takuya Kitaoka a,*, Akihiko Tomoda b, Ryo Suzuki b, Hiroyuki Wariishi a a
Department of Forest and Forest Products Sciences, Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, Fukuoka 812-8581, Japan b R&D Division, F.C.C. Co. Ltd., Shizuoka 431-1304, Japan Received 7 March 2006; received in revised form 19 May 2006; accepted 23 May 2006 Available online 7 July 2006
Abstract Copper-zinc oxide catalyst powders were successfully impregnated into paper-based composites (catalyst paper) of ceramic and silicon carbide (SiC) fibers, prepared using an established wet papermaking process. The catalyst powders were homogeneously scattered over the fiber-mix networks tailored within the catalyst paper. Samples of catalyst paper were subjected to the methanol steam reforming (MSR) process below 300 8C to produce hydrogen gas for fuel cell applications. During this process, the catalyst paper samples exhibited a higher methanol conversion efficiency and lower carbon monoxide concentration than those produced either by a commercial pellet-type Cu/ZnO catalyst or the original powdered Cu/ZnO catalyst. The high heat conductivity of the SiC fibers enhanced the catalytic performance, especially contributing to the suppression of the reverse water gas shift reaction. The heat transfer and the heat distribution inside the catalyst paper were improved by the SiC fibers. The MSR efficiency per catalyst weight was greatly influenced by the addition of pulp fibers, which made the catalyst paper porous. Results indicated that these in-paper void structures are suitable for the MSR reaction. Flexible catalyst paper is a promising, easy-to-handle material for practical MSR applications, due to the controllable heat and pore characteristics involved in the MSR performance. # 2006 Elsevier B.V. All rights reserved. Keywords: Methanol steam reforming; Catalyst paper; Porous structure; Silicon carbide; Fiber; Heat transfer
1. Introduction The growing interest in fuel cell systems as clean generators has received much attention, as they are extremely effective and environmentally friendly when compared to conventional fossil-fuel based thermal power generation systems, which are exhaustible and frequently produce atmospheric pollutants. In particular, polymer electrolyte fuel cell (PEFC) systems using hydrogen as an energy source have attracted much attention due to their low operating temperature and high power density [1,2]. However, hydrogen is not a natural resource per se, and thus its efficient production from hydrogen-containing matter is a key factor for realizing practical fuel cell applications [3–10]. Of the various gas reforming methods, methanol steam reforming (MSR), which involves the
* Corresponding author. Tel.: +81 92 642 2993; fax: +81 92 642 2993. E-mail address: [email protected] (T. Kitaoka). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.05.032
following chemical reaction: CH3OH + H2O ! 3H2 + CO2, is promising because unlike other reforming processes, the MSR reaction progresses at a relatively low temperature [3,5,7]. Moreover, methanol can be thermochemically and biotechnologically produced from biomass and related waste [11], which gives hydrogen PEFC systems the major advantage of being carbon-neutral. Catalytic reforming of solid catalysts is one of the mainstream methods for producing hydrogen. In this case, although powdered catalysts are highly effective, they are inconvenient to handle and frequently cause high pressure drops in the reaction flow system. Thus, molded solid catalysts such as spheres, pellets and rods, with a typical size of 5– 10 mm, are practical alternatives; however, all of these examples show a serious decrease in the catalytic performance as compared with those of the original powdered catalysts, due to intra-particle diffusion limitations [7]. Therefore, effective fabrication methods have been actively investigated in order to develop novel catalyst materials for practical applications.
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In our previous study, we successfully prepared paper-like composites (‘‘catalyst paper’’) from ceramic fibers, and impregnated them with the Cu/ZnO catalyst [12]. It was demonstrated that catalyst paper was effective in converting methanol to hydrogen and carbon dioxide in the MSR process. Furthermore, only a small amount of carbon monoxide (CO), a known catalytic poison of fuel electrode catalysts, was generated by the MSR treatment with catalyst paper, as compared to the amount produced using the original powdered catalyst. Such results suggested that the porous structure of the catalyst paper impacted greatly on the improved methanol conversion and the reduced CO formation, probably due to the rapid transportation of reactants and intermediate products inside the catalyst paper [12]. In this work, a novel catalyst paper composite was prepared from silicon carbide (SiC) fibers having high heat conductivity; SiC fibers are expected to contribute to the effective heat transfer inside the catalyst paper. Samples of the SiC-fibercontaining catalyst paper were subjected to the MSR process, and their catalytic performance was compared with that exhibited by the related catalyst powders and pellets. In addition, the observed CO formation behavior through the reverse water gas shift (WGS) reaction, and the effect of paper void structures on the MSR performance, are also discussed. 2. Materials and methods 2.1. Materials Commercial Cu/ZnO catalyst pellets (MDC-3; cylindrical ¨ D-CHEMIE, Ltd.) shape; 3 mm diameter and 3 mm height; SU were used; they were pulverized in part into 100-mesh pass powders using a bowl mill. Pulp fibers, used as a tentative supporting matrix in the wet papermaking process, were obtained by beating commercial bleached hardwood kraft pulp to 300 ml of Canadian Standard Freeness [13] with a Technical Association of the Pulp and Paper Industry (TAPPI) standard beater [14]. Ceramic fibers (CMLF208; ca. 0.8 mm diameter; heat conductivity; ca. 1.0 W m1 K1; Nippon Sheet Glass, Ltd.) were cut into ca. 0.5 mm lengths using a four-flute end mill. Fibrous silicon carbide whiskers (size; ca. 0.5 mm diameter and 30 mm length; heat conductivity; ca. 25.5 W m1 K1) were purchased from Tateho Chemical Industry, Ltd. Two types of flocculant: polydiallyldimethylammonium chloride (PDADMAC, molecular weight (Mw): ca. 3 105; charge density (CD): 5.5 meq/g; Aldrich, Ltd.) and anionic polyacrylamide (A-PAM, HH-351; Mw: ca. 4 106; CD: 0.64 meq/g; Kurita, Ltd.) were used as retention aids. An alumina sol (Snowtex 520, Nissan Chemicals, Ltd.) was used as a binder for enhancing the physical strength of the catalyst paper.
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paper. A fiber suspension containing catalyst powders was mixed with PDADMAC (0.5% total solids), alumina sol and APAM (0.5% total solids), in that order. The mixture was poured into a pulp fiber suspension, and then various types of catalyst paper with SiC fibers of 0–60% (w/w) per total fiber content were prepared according to the TAPPI Test Methods [21]. Following pressing at 350 kPa for 5 min, the wet sheets were dried in an oven at 105 8C for 30 min. One paper composite (2 104 mm2) before calcination consisted of catalyst powders (1.5 g), ceramic fibers (2.0–5.0 g), SiC fibers (3.0–0.0 g), alumina sol binder (0.1 g) and pulp (0.10–1.0 g). The paper composites obtained were thermally treated at 350 8C for 24 h to remove the organic components, and then thermo-welded to improve the physical strength. 2.3. MSR process and performance test Ten circular catalyst paper samples, each with an area of 8 102 mm2 and thickness of 1 mm, were stacked on top of each other (8 103 mm3) and placed in a stainless steel thermoregulatory reaction cylinder. Similarly, catalyst powder or pellets with total volumes of 8 103 mm3 were placed inside the reaction cylinder. When either the powder or pellets were used, inert silica–alumina powders with the same particle sizes were mixed in to adjust the occupied volume to 8 103 mm3. A schematic diagram of the gas flow system for the MSR process is given in our previous study [12], but is omitted in this article. The MSR reactants were introduced into the reactor at a 1.5:1.0 molar ratio of water to methanol (steam/ carbon, S/C ratio = 1.5) at a constant flow rate of 2150 h1. The amounts of Cu/ZnO catalyst were adjusted to 0.6 g in each case. Prior to the MSR reaction, the catalyst samples were reduced in situ with hydrogen at 250 8C for 1.5 h, followed by a complete purge of all the flow lines with nitrogen for 30 min. The gas composition during the catalytic reaction was monitored at reaction temperatures ranging from 200 to 300 8C using two online gas chromatographs (GCs) and one offline GC. The exhaust gases generated in the MSR reaction were passed through a cold trap in an ice bath. The unreacted methanol and water residues were separated from the gaseous components and then quantified using an offline GC-FID (GC-17A, Shimadzu, Ltd.) fitted with a Supel-Q Plot column (0.53 mm 30 m, Shimadzu, Ltd.) at a furnace temperature of 60 8C. The major gaseous products of hydrogen and carbon dioxide were determined online using GC-TCD, fitted with a Porapak-Q column (3 mm 2 m, Shinwa Chemical Industries, Ltd.), while CO, which was isolated as a minor by-product, was measured by first converting it to CH4 with an online methanizer (MTN-1, Shimadzu, Ltd.), and then measuring the quantity of CH4 produced using a GC-FID fitted with a Porapak-Q column. All measurements were repeated at least four times and standard deviations were obtained.
2.2. Preparation of catalyst paper 2.4. Reverse WGS reaction test Essential catalyst paper preparation details were obtained from previous related studies [12,15–20]. Briefly, ceramic and SiC fibers were used as fiber matrix components in the catalyst
Catalyst paper or catalyst powders were first set in the reaction cylinder in a similar manner to the MSR reaction, and
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then reduced in situ with hydrogen at 250 8C for 1.5 h. After a complete purge of all the flow lines with nitrogen, an imitative reformed gas consisting of 75% hydrogen and 25% carbon dioxide by volume was introduced into the reactor at reaction temperatures ranging from 200 to 300 8C. The formation of CO by the reverse WGS reaction was monitored by online GC. 2.5. Other analyses The Cu/ZnO content in the catalyst paper was determined by atomic absorption analysis using a Shimadzu AA-6600F apparatus, whereby the concentration of Cu2+ eluted using 35% nitric acid was quantified. Other inorganic material was measured gravimetrically after calcination at 700 8C for 20 min, taking weight loss into consideration. The surface observation of the catalyst paper was performed using scanning electron microscopy (SEM) (JSM-5600 apparatus, JEOL, Ltd.), where the electron accelerating voltage used was set at 10 kV. Mercury intrusion analysis was carried out using a PoreMaster 33P (YUASA IONICS, Ltd.) to evaluate the pore diameter and porosity of the catalyst paper. The pore volume and pore size distribution were obtained from the pressure-cumulative mercury intrusion volume curve normalized by the catalyst weight. 3. Results and discussion 3.1. SiC fiber-containing catalyst paper prepared by a papermaking technique The optical and SEM images of the SiC fiber-containing catalyst paper are shown in Fig. 1. The catalyst paper was successfully prepared by a typical wet papermaking technique using a dual polymer retention system, whose mechanism was reported in our previous studies [12,15–20]. The Cu/ZnO powders were impregnated into a flexible, cardboard-like material (Fig. 1a) in which the catalyst particles were scattered on the fiber-mix matrix of ceramic and SiC fibers, as shown in Fig. 1b. The pore size distribution profiles of the SiC paper composites with different pulp dosages were measured by mercury intrusion analysis and are displayed in Fig. 2. The micrometer-scale pore structures of the SiC catalyst paper varied with the pulp dosage; the average pore size increased, while the pore size distribution broadened with increasing pulp dosage. The total pore volumes also increased similarly, where those of the SiC paper samples with pulp dosages of 0.10, 0.25, 0.50 and 1.0 were 2.08 0.14, 2.10 0.06, 2.12 0.12 and 2.18 0.07 mm3 mg1, respectively. These paper composites were thermally treated to remove the pulp fibers, such that the final compositions were the same in each sample. However, the use of pulp fibers as a tentative matrix in paper forming made a clear contribution to the pore structures of the catalyst paper, where the porosity reached up to ca. 50% (v/v). The MSR performance of these catalyst paper samples is shown in a later section. Interestingly, in the case where the pulp fiber content of catalyst paper amounted to 2.0 g, the total pore volume reached 2.44 0.19 mm3 mg1. These results indicate that the porous
Fig. 1. Optical and SEM images of SiC fiber-containing catalyst paper.
Fig. 2. Pore size distribution of SiC fiber-containing catalyst paper: pulp dosage 0.10 g (blue), 0.25 g (orange), 0.50 g (green) and 1.0 g (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
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structures of the catalyst paper were appropriately controllable. Such micrometer-scale pores derived from the layered fiber network are characteristically found in catalyst paper. Moreover, it has already been reported in our previous study that the porous structure is important for the MSR performance [12]. Recently, the micrometer-scale pores in solid catalyst materials have become a major point of interest in the catalytic efficiency, and the unique pore-dependent effects were investigated with regard to the heat exchanging properties in the catalyst materials, and the mass transfer of reactants onto the active catalyst surfaces [22–24]. Therefore, here we focus on the effects of the catalyst paper structure as a catalytic reaction field. 3.2. MSR performance of SiC catalyst paper Fig. 3 shows the correlation between SiC fiber content and MSR performances at a reaction temperature of 280 8C. The Cu/ ZnO catalyst content was adjusted in each paper sample. In the case of catalyst paper prepared using just ceramic fibers as the fiber component, the methanol conversion efficiency was ca. 75%, while the CO concentration was ca. 4300 ppm. The MSR performances of the catalyst paper on the other hand, were noticeably influenced when SiC fibers were added; the maximum methanol conversion efficiency (>80%) and minimal CO concentration (ca. 3000 ppm) was achieved when the SiC fiber content was 20% and 40%, respectively. Methanol conversion is closely related to hydrogen production, and the 80% conversion observed here corresponds to 12 mmol min1 g-catalyst1 of 75% (v/v) hydrogen [12]. The porosity of the catalyst paper remained almost unchanged (ca. 50%) with respect to the amount of SiC fibers added, and thus such high MSR performances might be attributed to the improved heat transfer resulting from the added SiC fibers, which are known to have a higher heat conductivity than ceramic fibers. For reasons unknown, a greater SiC fiber content in the catalyst paper had a negative impact on the degree of methanol conversion and CO concentration. From a practical viewpoint however, it was reasonably estimated that
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catalyst paper with a SiC fiber content of 20% exhibited good MSR performance. The MSR performances of 20% SiC-containing catalyst paper with a pulp dosage of 0.50 g produced using the wet papermaking process are compared with those obtained using powdered catalyst and catalyst pellets, as shown in Fig. 4. In all cases, the methanol conversion efficiency increased linearly with an increase in the MSR reaction temperature. On the other hand, the CO concentration rose drastically above 250 8C. The SiC paper demonstrated a higher methanol conversion efficiency and lower CO concentration than those produced by either the powdered catalyst or pellets over a wide temperature range (220–300 8C). Fig. 4c displays the tradeoff relationship between methanol conversion efficiency and CO concentration in the MSR process. In general, a higher methanol conversion induces a higher CO concentration. SiCcontaining Cu/ZnO catalyst paper demonstrated an extremely high performance as compared with either the powdered catalyst or the catalyst pellets. Indeed, even when the same quantity of catalyst was used, within the same occupied volume in the reactor, at the same reactant supply rates, it was evident that the SiC paper exhibited the highest MSR performance. In the gas reforming with Cu-based catalysts, a serious deactivation frequently occurs. In this study, both catalyst powders and pellets gradually deteriorated in the MSR process for 3 h; a large oscillation in the gas flow rate was observed with large variations in the internal temperature of the reactor, resulting in unstable hydrogen production and thermally induced deactivation. On the other hand, the SiC catalyst paper demonstrated much more stable MSR behavior, presumably because the reformed gas quickly diffused through the paper void and then the MSR reaction might proceed efficiently, as similarly reported in our previous study on the autothermal reforming process with catalyst paper [25]. As shown in Fig. 3, the MSR performance of the catalyst paper was improved by adding SiC fibers. The MSR process is an endothermic reaction, which proceeds by supplying thermal energy from outside of the reaction cylinder. The SiC fibers in the catalyst paper appeared to conduct heat effectively, such that sufficient heat was provided to the Cu/ZnO particles inside the catalyst paper, affording high methanol conversion efficiency. Furthermore, the CO concentration was found to decrease when the SiC fiber content in the catalyst paper was less than 40% of the total fiber used. Many researchers have reported on the mechanisms involved in the MSR reaction [3,5,7], and it is generally agreed upon that CO is mainly generated from hydrogen and carbon dioxide through the endothermic reverse WGS reaction (H2 + CO2 ! CO + H2O) [7]. Therefore, the lower CO concentration at a high methanol conversion would seem to imply that the suppression of CO formation occurs through the reverse WGS reaction. 3.3. CO formation behavior through reverse WGS reaction
Fig. 3. Relationship between the SiC fiber content of the catalyst paper and MSR performances.
The CO formation behavior through the reverse WGS reaction was investigated in detail by comparing the MSR performances of the powdered catalyst and catalyst paper, both
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Fig. 4. MSR performances: (a) methanol conversion, (b) CO concentration, (c) relationship between methanol conversion and CO concentration; SiC paper (squares), catalyst powder (triangles) and catalyst pellet (crosses).
with and without SiC fibers. The imitative reformed gas, consisting of 75% hydrogen and 25% carbon dioxide by volume, was introduced into the catalyst layer at a flow rate of 200 mL min1, and the concentration of CO produced by the reverse WGS reaction was monitored by GC. This gas flow rate corresponds to the flow in the MSR process used to achieve a methanol conversion of 75% in this study. Fig. 5 shows the CO concentration at different reaction temperatures ranging from
Fig. 5. CO formation behaviors through reverse water gas shift reaction; SiC paper (squares), catalyst paper without SiC fibers (circles) and catalyst powder (triangles).
200 to 300 8C. Here, the CO concentration decreased in the following order: powdered catalyst, catalyst paper without SiC fibers, and SiC fiber-containing catalyst paper, which is in agreement with the order observed for the MSR reaction, where the hydrogen and carbon dioxide initially generated from methanol and water, reacted to form CO. On the other hand, the imitative reformed gas was directly subjected only to the reverse WGS reaction, such that a significant volume of CO was formed. In our MSR system, it was confirmed that the CO generated through thermal decomposition of methanol, with or without catalysts present, was negligible (data not shown). As such, it is possible that the paper structure induced CO formation. By comparing the effects of SiC fiber content in the catalyst paper, we found that smaller amounts of CO were generated when the catalyst paper contained SiC fibers. Moreover, this further suppression of the reverse WGS reaction by the SiC fiber components with high heat conductivity was confirmed in this model case. Using mercury intrusion analysis, we confirmed that there were practically no differences in porosity and pore size distribution between the catalyst paper samples prepared with and without SiC fibers (data not shown). Thus, it was clarified that the SiC fibers contributed not only to the high methanol conversion, but also to the suppression of the reverse WGS reaction. It is considered that longer hydrogen and carbon dioxide residence times around the catalyst surfaces, and the occurrence of heterogeneous heat distribution, must stimulate
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the reverse WGS reaction [7]. The micrometer-scale pores inside the catalyst paper presumably provide the appropriate flow path for the reaction products, allowing them to pass smoothly through the catalyst layer, resulting in the suppression of CO formation. Moreover, the presence of SiC fibers in the catalyst paper also improved heat distribution, and thus the reverse WGS reaction appeared to be further repressed. 3.4. Effect of paper voids on MSR performance The MSR behavior was investigated with respect to the paper void structures in the SiC fiber-containing catalyst paper. Fig. 6 displays the temperature-dependent correlation between the amounts of pulp fiber added during the wet papermaking process and the MSR behavior. SiC fiber-containing paper samples were calcined before use in order to remove the organic pulp fibers and to improve the paper strength by binder treatment. Therefore, when the pulp dosage was increased, the apparent porosity of the catalyst paper also increased, with a subsequent increase in the number of burned marks due to the pulp fibers; however, the final components were identical in each sample. SiC paper samples with pulp dosages of 0.10, 0.25, 0.50 and 1.0 g per handmade sheet were prepared. Both methanol conversion efficiency and CO concentration were found to decrease with increasing pulp content at the same reaction temperature. This result indicates that an excess of voids in the paper brought about a decrease in the MSR
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efficiency; the final MSR efficiency was equivalent to that obtained when a simple mixture of catalyst powders and fiber components was employed [12]. An excess of large pores might induce the localized gas flow and reduce the opportunities for reactants to contact with the catalyst surfaces. Fig. 6c shows the relationship between methanol conversion and CO concentration when modified by SiC fiber-containing paper samples with various pore sizes. The SiC fiber-containing paper with a pulp dosage of 0.1 g exhibited high methanol conversion, although large amounts of CO formed at high reaction temperatures. On the other hand, when a larger pulp dosage of 1.0 g was employed, the SiC fiber-containing paper generated smaller volumes of CO, although in this case methanol conversion efficiency was quite low. From the results in Fig. 6c, the optimum value was obtained for SiC fiber-containing paper samples with a pulp dosage of 0.25 g, indicating a higher methanol conversion at lower CO concentration. It is understood that the MSR reaction occurs on the surface of the Cu/ ZnO catalyst particles in the paper composites. However, here, both the heat conductivity of the SiC fibers and the porous paper structure significantly affected the MSR performance, even when similar amounts of catalyst were employed. These factors are controllable by varying both the types of inorganic fibers used and the amounts of organic pulp fibers introduced. Thus, the catalyst paper is allowed to provide an appropriate environment for the reforming catalyst. Here, the catalyst paper prepared by a simple papermaking technique is proposed for use as a practical catalyst-based material.
Fig. 6. Correlation between pulp dosage and MSR performances: (a) methanol conversion, (b) CO concentration and (c) relationship between methanol conversion and CO concentration; SiC paper with pulp dosage 0.10 g (diamonds), 0.25 g (squares), 0.50 g (triangles) and 1.0 g (circles).
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4. Conclusion Paper-based Cu/ZnO catalyst composites (catalyst paper) were successfully prepared by a conventional wet papermaking technique using ceramic and SiC fibers. The porous SiC fibercontaining catalyst paper exhibited higher methanol conversion efficiency and lower CO concentration, as compared with catalysts prepared from either powders or pellets. It is likely that the SiC fibers enhanced heat transfer inside the catalyst paper, contributing to the high methanol conversion. It was also clarified that the SiC catalyst paper most likely suppressed the undesirable reverse WGS reaction as a result of the uniform heat distribution inside the catalyst paper. Furthermore, the porous structure of the catalyst paper has a great impact on the MSR efficiency, where an optimum paper void is indicated. Both the heat property and void structure were controllable to some extent by the wet papermaking process. Therefore, catalyst paper with both practical convenience and high MSR performance is a promising candidate for catalyst-based materials. Acknowledgements This research was financially supported by the Industrial Technology Research Grant Program in 2003 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors wish to thank Ms. K. Hayashida for her assistance with the sample preparation and instrumental analyses. References [1] K.A. Starz, E. Auer, Th. Lehmann, R. Zuber, J. Power Sources 84 (1999) 167–172.
[2] V. Mehta, J.S. Cooper, J. Power Sources 114 (2003) 32–53. [3] S. Nagano, H. Miyagawa, O. Azegami, K. Ohsawa, Energy Conversion Manage. 42 (2001) 1817–1829. [4] S. Dunn, Int. J. Hydrogen Energy 27 (2002) 235–264. [5] J. Agrell, H. Birgersson, M. Boutonnet, J. Power Sources 106 (2002) 249– 257. [6] E. Newson, T.B. Truong, Int. J. Hydrogen Energy 28 (2003) 1379– 1386. [7] H. Purnama, T. Ressler, R.E. Jentoft, H. Soerijanto, R. Schlo¨gl, R. Schoma¨cker, Appl. Catal. A: Gen. 259 (2004) 83–94. [8] A. Karim, J. Bravo, D. Gorm, T. Conant, A. Datye, Catal. Today 110 (2005) 86–91. [9] C.Z. Yao, L.C. Wang, Y.M. Liu, G.S. Wu, Y. Cao, W.L. Dai, H.Y. He, K.N. Fan, Appl. Catal. A: Gen. 297 (2006) 151–158. [10] L. Pan, S. Wang, Int. J. Hydrogen Energy 30 (2005) 973–979. [11] C.N. Hamelinck, A.P.C. Faaij, J. Power Sources 111 (2002) 1–22. [12] S. Fukahori, T. Kitaoka, H. Tanaka, A. Tomoda, R. Suzuki, H. Wariishi, Appl. Catal. A: Gen. 300 (2006) 155–161. [13] Tech. Assoc. Pulp Paper Ind. Test Methods, 1997, T227. [14] Tech. Assoc. Pulp Paper Ind. Test Methods, 1997, T200. [15] H. Ichiura, T. Kitaoka, H. Tanaka, J. Mater. Sci. 37 (2002) 2937– 2941. [16] H. Ichiura, T. Kitaoka, H. Tanaka, Chemosphere 50 (2003) 79–83. [17] H. Ichiura, T. Kitaoka, H. Tanaka, Chemosphere 51 (2003) 855–860. [18] H. Ichiura, T. Kitaoka, H. Tanaka, J. Mater. Sci. 38 (2003) 1611– 1615. [19] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Environ. Sci. Technol. 37 (2003) 1048–1051. [20] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Appl. Catal. B: Environ. 46 (2003) 453–462. [21] Tech. Assoc. Pulp Paper Ind. Test Methods, 1997. T205. [22] S.R. Mukai, H. Nishihara, H. Tamon, Micropor. Mesopor. Mater. 63 (2003) 43–51. [23] R. Takahashi, S. Sato, T. Sodesawa, K. Arai, M. Yabuki, J. Catal. 229 (2005) 24–29. [24] H. Nishihara, S.R. Mukai, D. Yamashita, H. Tamon, Chem. Mater. 17 (2005) 683–689. [25] H. Koga, S. Fukahori, T. Kitaoka, A. Tomoda, R. Suzuki, H. Wariishi, Appl. Catal. A: Gen. 309 (2006) 263–269.
Hydrogen production from methanol using a SiC fiber-containing paper composite impregnated with Cu/ZnO catalyst Shuji Fukahori a, Hirotaka Koga a, Takuya Kitaoka a,*, Akihiko Tomoda b, Ryo Suzuki b, Hiroyuki Wariishi a a
Department of Forest and Forest Products Sciences, Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, Fukuoka 812-8581, Japan b R&D Division, F.C.C. Co. Ltd., Shizuoka 431-1304, Japan Received 7 March 2006; received in revised form 19 May 2006; accepted 23 May 2006 Available online 7 July 2006
Abstract Copper-zinc oxide catalyst powders were successfully impregnated into paper-based composites (catalyst paper) of ceramic and silicon carbide (SiC) fibers, prepared using an established wet papermaking process. The catalyst powders were homogeneously scattered over the fiber-mix networks tailored within the catalyst paper. Samples of catalyst paper were subjected to the methanol steam reforming (MSR) process below 300 8C to produce hydrogen gas for fuel cell applications. During this process, the catalyst paper samples exhibited a higher methanol conversion efficiency and lower carbon monoxide concentration than those produced either by a commercial pellet-type Cu/ZnO catalyst or the original powdered Cu/ZnO catalyst. The high heat conductivity of the SiC fibers enhanced the catalytic performance, especially contributing to the suppression of the reverse water gas shift reaction. The heat transfer and the heat distribution inside the catalyst paper were improved by the SiC fibers. The MSR efficiency per catalyst weight was greatly influenced by the addition of pulp fibers, which made the catalyst paper porous. Results indicated that these in-paper void structures are suitable for the MSR reaction. Flexible catalyst paper is a promising, easy-to-handle material for practical MSR applications, due to the controllable heat and pore characteristics involved in the MSR performance. # 2006 Elsevier B.V. All rights reserved. Keywords: Methanol steam reforming; Catalyst paper; Porous structure; Silicon carbide; Fiber; Heat transfer
1. Introduction The growing interest in fuel cell systems as clean generators has received much attention, as they are extremely effective and environmentally friendly when compared to conventional fossil-fuel based thermal power generation systems, which are exhaustible and frequently produce atmospheric pollutants. In particular, polymer electrolyte fuel cell (PEFC) systems using hydrogen as an energy source have attracted much attention due to their low operating temperature and high power density [1,2]. However, hydrogen is not a natural resource per se, and thus its efficient production from hydrogen-containing matter is a key factor for realizing practical fuel cell applications [3–10]. Of the various gas reforming methods, methanol steam reforming (MSR), which involves the
* Corresponding author. Tel.: +81 92 642 2993; fax: +81 92 642 2993. E-mail address: [email protected] (T. Kitaoka). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.05.032
following chemical reaction: CH3OH + H2O ! 3H2 + CO2, is promising because unlike other reforming processes, the MSR reaction progresses at a relatively low temperature [3,5,7]. Moreover, methanol can be thermochemically and biotechnologically produced from biomass and related waste [11], which gives hydrogen PEFC systems the major advantage of being carbon-neutral. Catalytic reforming of solid catalysts is one of the mainstream methods for producing hydrogen. In this case, although powdered catalysts are highly effective, they are inconvenient to handle and frequently cause high pressure drops in the reaction flow system. Thus, molded solid catalysts such as spheres, pellets and rods, with a typical size of 5– 10 mm, are practical alternatives; however, all of these examples show a serious decrease in the catalytic performance as compared with those of the original powdered catalysts, due to intra-particle diffusion limitations [7]. Therefore, effective fabrication methods have been actively investigated in order to develop novel catalyst materials for practical applications.
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In our previous study, we successfully prepared paper-like composites (‘‘catalyst paper’’) from ceramic fibers, and impregnated them with the Cu/ZnO catalyst [12]. It was demonstrated that catalyst paper was effective in converting methanol to hydrogen and carbon dioxide in the MSR process. Furthermore, only a small amount of carbon monoxide (CO), a known catalytic poison of fuel electrode catalysts, was generated by the MSR treatment with catalyst paper, as compared to the amount produced using the original powdered catalyst. Such results suggested that the porous structure of the catalyst paper impacted greatly on the improved methanol conversion and the reduced CO formation, probably due to the rapid transportation of reactants and intermediate products inside the catalyst paper [12]. In this work, a novel catalyst paper composite was prepared from silicon carbide (SiC) fibers having high heat conductivity; SiC fibers are expected to contribute to the effective heat transfer inside the catalyst paper. Samples of the SiC-fibercontaining catalyst paper were subjected to the MSR process, and their catalytic performance was compared with that exhibited by the related catalyst powders and pellets. In addition, the observed CO formation behavior through the reverse water gas shift (WGS) reaction, and the effect of paper void structures on the MSR performance, are also discussed. 2. Materials and methods 2.1. Materials Commercial Cu/ZnO catalyst pellets (MDC-3; cylindrical ¨ D-CHEMIE, Ltd.) shape; 3 mm diameter and 3 mm height; SU were used; they were pulverized in part into 100-mesh pass powders using a bowl mill. Pulp fibers, used as a tentative supporting matrix in the wet papermaking process, were obtained by beating commercial bleached hardwood kraft pulp to 300 ml of Canadian Standard Freeness [13] with a Technical Association of the Pulp and Paper Industry (TAPPI) standard beater [14]. Ceramic fibers (CMLF208; ca. 0.8 mm diameter; heat conductivity; ca. 1.0 W m1 K1; Nippon Sheet Glass, Ltd.) were cut into ca. 0.5 mm lengths using a four-flute end mill. Fibrous silicon carbide whiskers (size; ca. 0.5 mm diameter and 30 mm length; heat conductivity; ca. 25.5 W m1 K1) were purchased from Tateho Chemical Industry, Ltd. Two types of flocculant: polydiallyldimethylammonium chloride (PDADMAC, molecular weight (Mw): ca. 3 105; charge density (CD): 5.5 meq/g; Aldrich, Ltd.) and anionic polyacrylamide (A-PAM, HH-351; Mw: ca. 4 106; CD: 0.64 meq/g; Kurita, Ltd.) were used as retention aids. An alumina sol (Snowtex 520, Nissan Chemicals, Ltd.) was used as a binder for enhancing the physical strength of the catalyst paper.
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paper. A fiber suspension containing catalyst powders was mixed with PDADMAC (0.5% total solids), alumina sol and APAM (0.5% total solids), in that order. The mixture was poured into a pulp fiber suspension, and then various types of catalyst paper with SiC fibers of 0–60% (w/w) per total fiber content were prepared according to the TAPPI Test Methods [21]. Following pressing at 350 kPa for 5 min, the wet sheets were dried in an oven at 105 8C for 30 min. One paper composite (2 104 mm2) before calcination consisted of catalyst powders (1.5 g), ceramic fibers (2.0–5.0 g), SiC fibers (3.0–0.0 g), alumina sol binder (0.1 g) and pulp (0.10–1.0 g). The paper composites obtained were thermally treated at 350 8C for 24 h to remove the organic components, and then thermo-welded to improve the physical strength. 2.3. MSR process and performance test Ten circular catalyst paper samples, each with an area of 8 102 mm2 and thickness of 1 mm, were stacked on top of each other (8 103 mm3) and placed in a stainless steel thermoregulatory reaction cylinder. Similarly, catalyst powder or pellets with total volumes of 8 103 mm3 were placed inside the reaction cylinder. When either the powder or pellets were used, inert silica–alumina powders with the same particle sizes were mixed in to adjust the occupied volume to 8 103 mm3. A schematic diagram of the gas flow system for the MSR process is given in our previous study [12], but is omitted in this article. The MSR reactants were introduced into the reactor at a 1.5:1.0 molar ratio of water to methanol (steam/ carbon, S/C ratio = 1.5) at a constant flow rate of 2150 h1. The amounts of Cu/ZnO catalyst were adjusted to 0.6 g in each case. Prior to the MSR reaction, the catalyst samples were reduced in situ with hydrogen at 250 8C for 1.5 h, followed by a complete purge of all the flow lines with nitrogen for 30 min. The gas composition during the catalytic reaction was monitored at reaction temperatures ranging from 200 to 300 8C using two online gas chromatographs (GCs) and one offline GC. The exhaust gases generated in the MSR reaction were passed through a cold trap in an ice bath. The unreacted methanol and water residues were separated from the gaseous components and then quantified using an offline GC-FID (GC-17A, Shimadzu, Ltd.) fitted with a Supel-Q Plot column (0.53 mm 30 m, Shimadzu, Ltd.) at a furnace temperature of 60 8C. The major gaseous products of hydrogen and carbon dioxide were determined online using GC-TCD, fitted with a Porapak-Q column (3 mm 2 m, Shinwa Chemical Industries, Ltd.), while CO, which was isolated as a minor by-product, was measured by first converting it to CH4 with an online methanizer (MTN-1, Shimadzu, Ltd.), and then measuring the quantity of CH4 produced using a GC-FID fitted with a Porapak-Q column. All measurements were repeated at least four times and standard deviations were obtained.
2.2. Preparation of catalyst paper 2.4. Reverse WGS reaction test Essential catalyst paper preparation details were obtained from previous related studies [12,15–20]. Briefly, ceramic and SiC fibers were used as fiber matrix components in the catalyst
Catalyst paper or catalyst powders were first set in the reaction cylinder in a similar manner to the MSR reaction, and
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then reduced in situ with hydrogen at 250 8C for 1.5 h. After a complete purge of all the flow lines with nitrogen, an imitative reformed gas consisting of 75% hydrogen and 25% carbon dioxide by volume was introduced into the reactor at reaction temperatures ranging from 200 to 300 8C. The formation of CO by the reverse WGS reaction was monitored by online GC. 2.5. Other analyses The Cu/ZnO content in the catalyst paper was determined by atomic absorption analysis using a Shimadzu AA-6600F apparatus, whereby the concentration of Cu2+ eluted using 35% nitric acid was quantified. Other inorganic material was measured gravimetrically after calcination at 700 8C for 20 min, taking weight loss into consideration. The surface observation of the catalyst paper was performed using scanning electron microscopy (SEM) (JSM-5600 apparatus, JEOL, Ltd.), where the electron accelerating voltage used was set at 10 kV. Mercury intrusion analysis was carried out using a PoreMaster 33P (YUASA IONICS, Ltd.) to evaluate the pore diameter and porosity of the catalyst paper. The pore volume and pore size distribution were obtained from the pressure-cumulative mercury intrusion volume curve normalized by the catalyst weight. 3. Results and discussion 3.1. SiC fiber-containing catalyst paper prepared by a papermaking technique The optical and SEM images of the SiC fiber-containing catalyst paper are shown in Fig. 1. The catalyst paper was successfully prepared by a typical wet papermaking technique using a dual polymer retention system, whose mechanism was reported in our previous studies [12,15–20]. The Cu/ZnO powders were impregnated into a flexible, cardboard-like material (Fig. 1a) in which the catalyst particles were scattered on the fiber-mix matrix of ceramic and SiC fibers, as shown in Fig. 1b. The pore size distribution profiles of the SiC paper composites with different pulp dosages were measured by mercury intrusion analysis and are displayed in Fig. 2. The micrometer-scale pore structures of the SiC catalyst paper varied with the pulp dosage; the average pore size increased, while the pore size distribution broadened with increasing pulp dosage. The total pore volumes also increased similarly, where those of the SiC paper samples with pulp dosages of 0.10, 0.25, 0.50 and 1.0 were 2.08 0.14, 2.10 0.06, 2.12 0.12 and 2.18 0.07 mm3 mg1, respectively. These paper composites were thermally treated to remove the pulp fibers, such that the final compositions were the same in each sample. However, the use of pulp fibers as a tentative matrix in paper forming made a clear contribution to the pore structures of the catalyst paper, where the porosity reached up to ca. 50% (v/v). The MSR performance of these catalyst paper samples is shown in a later section. Interestingly, in the case where the pulp fiber content of catalyst paper amounted to 2.0 g, the total pore volume reached 2.44 0.19 mm3 mg1. These results indicate that the porous
Fig. 1. Optical and SEM images of SiC fiber-containing catalyst paper.
Fig. 2. Pore size distribution of SiC fiber-containing catalyst paper: pulp dosage 0.10 g (blue), 0.25 g (orange), 0.50 g (green) and 1.0 g (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
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structures of the catalyst paper were appropriately controllable. Such micrometer-scale pores derived from the layered fiber network are characteristically found in catalyst paper. Moreover, it has already been reported in our previous study that the porous structure is important for the MSR performance [12]. Recently, the micrometer-scale pores in solid catalyst materials have become a major point of interest in the catalytic efficiency, and the unique pore-dependent effects were investigated with regard to the heat exchanging properties in the catalyst materials, and the mass transfer of reactants onto the active catalyst surfaces [22–24]. Therefore, here we focus on the effects of the catalyst paper structure as a catalytic reaction field. 3.2. MSR performance of SiC catalyst paper Fig. 3 shows the correlation between SiC fiber content and MSR performances at a reaction temperature of 280 8C. The Cu/ ZnO catalyst content was adjusted in each paper sample. In the case of catalyst paper prepared using just ceramic fibers as the fiber component, the methanol conversion efficiency was ca. 75%, while the CO concentration was ca. 4300 ppm. The MSR performances of the catalyst paper on the other hand, were noticeably influenced when SiC fibers were added; the maximum methanol conversion efficiency (>80%) and minimal CO concentration (ca. 3000 ppm) was achieved when the SiC fiber content was 20% and 40%, respectively. Methanol conversion is closely related to hydrogen production, and the 80% conversion observed here corresponds to 12 mmol min1 g-catalyst1 of 75% (v/v) hydrogen [12]. The porosity of the catalyst paper remained almost unchanged (ca. 50%) with respect to the amount of SiC fibers added, and thus such high MSR performances might be attributed to the improved heat transfer resulting from the added SiC fibers, which are known to have a higher heat conductivity than ceramic fibers. For reasons unknown, a greater SiC fiber content in the catalyst paper had a negative impact on the degree of methanol conversion and CO concentration. From a practical viewpoint however, it was reasonably estimated that
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catalyst paper with a SiC fiber content of 20% exhibited good MSR performance. The MSR performances of 20% SiC-containing catalyst paper with a pulp dosage of 0.50 g produced using the wet papermaking process are compared with those obtained using powdered catalyst and catalyst pellets, as shown in Fig. 4. In all cases, the methanol conversion efficiency increased linearly with an increase in the MSR reaction temperature. On the other hand, the CO concentration rose drastically above 250 8C. The SiC paper demonstrated a higher methanol conversion efficiency and lower CO concentration than those produced by either the powdered catalyst or pellets over a wide temperature range (220–300 8C). Fig. 4c displays the tradeoff relationship between methanol conversion efficiency and CO concentration in the MSR process. In general, a higher methanol conversion induces a higher CO concentration. SiCcontaining Cu/ZnO catalyst paper demonstrated an extremely high performance as compared with either the powdered catalyst or the catalyst pellets. Indeed, even when the same quantity of catalyst was used, within the same occupied volume in the reactor, at the same reactant supply rates, it was evident that the SiC paper exhibited the highest MSR performance. In the gas reforming with Cu-based catalysts, a serious deactivation frequently occurs. In this study, both catalyst powders and pellets gradually deteriorated in the MSR process for 3 h; a large oscillation in the gas flow rate was observed with large variations in the internal temperature of the reactor, resulting in unstable hydrogen production and thermally induced deactivation. On the other hand, the SiC catalyst paper demonstrated much more stable MSR behavior, presumably because the reformed gas quickly diffused through the paper void and then the MSR reaction might proceed efficiently, as similarly reported in our previous study on the autothermal reforming process with catalyst paper [25]. As shown in Fig. 3, the MSR performance of the catalyst paper was improved by adding SiC fibers. The MSR process is an endothermic reaction, which proceeds by supplying thermal energy from outside of the reaction cylinder. The SiC fibers in the catalyst paper appeared to conduct heat effectively, such that sufficient heat was provided to the Cu/ZnO particles inside the catalyst paper, affording high methanol conversion efficiency. Furthermore, the CO concentration was found to decrease when the SiC fiber content in the catalyst paper was less than 40% of the total fiber used. Many researchers have reported on the mechanisms involved in the MSR reaction [3,5,7], and it is generally agreed upon that CO is mainly generated from hydrogen and carbon dioxide through the endothermic reverse WGS reaction (H2 + CO2 ! CO + H2O) [7]. Therefore, the lower CO concentration at a high methanol conversion would seem to imply that the suppression of CO formation occurs through the reverse WGS reaction. 3.3. CO formation behavior through reverse WGS reaction
Fig. 3. Relationship between the SiC fiber content of the catalyst paper and MSR performances.
The CO formation behavior through the reverse WGS reaction was investigated in detail by comparing the MSR performances of the powdered catalyst and catalyst paper, both
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Fig. 4. MSR performances: (a) methanol conversion, (b) CO concentration, (c) relationship between methanol conversion and CO concentration; SiC paper (squares), catalyst powder (triangles) and catalyst pellet (crosses).
with and without SiC fibers. The imitative reformed gas, consisting of 75% hydrogen and 25% carbon dioxide by volume, was introduced into the catalyst layer at a flow rate of 200 mL min1, and the concentration of CO produced by the reverse WGS reaction was monitored by GC. This gas flow rate corresponds to the flow in the MSR process used to achieve a methanol conversion of 75% in this study. Fig. 5 shows the CO concentration at different reaction temperatures ranging from
Fig. 5. CO formation behaviors through reverse water gas shift reaction; SiC paper (squares), catalyst paper without SiC fibers (circles) and catalyst powder (triangles).
200 to 300 8C. Here, the CO concentration decreased in the following order: powdered catalyst, catalyst paper without SiC fibers, and SiC fiber-containing catalyst paper, which is in agreement with the order observed for the MSR reaction, where the hydrogen and carbon dioxide initially generated from methanol and water, reacted to form CO. On the other hand, the imitative reformed gas was directly subjected only to the reverse WGS reaction, such that a significant volume of CO was formed. In our MSR system, it was confirmed that the CO generated through thermal decomposition of methanol, with or without catalysts present, was negligible (data not shown). As such, it is possible that the paper structure induced CO formation. By comparing the effects of SiC fiber content in the catalyst paper, we found that smaller amounts of CO were generated when the catalyst paper contained SiC fibers. Moreover, this further suppression of the reverse WGS reaction by the SiC fiber components with high heat conductivity was confirmed in this model case. Using mercury intrusion analysis, we confirmed that there were practically no differences in porosity and pore size distribution between the catalyst paper samples prepared with and without SiC fibers (data not shown). Thus, it was clarified that the SiC fibers contributed not only to the high methanol conversion, but also to the suppression of the reverse WGS reaction. It is considered that longer hydrogen and carbon dioxide residence times around the catalyst surfaces, and the occurrence of heterogeneous heat distribution, must stimulate
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the reverse WGS reaction [7]. The micrometer-scale pores inside the catalyst paper presumably provide the appropriate flow path for the reaction products, allowing them to pass smoothly through the catalyst layer, resulting in the suppression of CO formation. Moreover, the presence of SiC fibers in the catalyst paper also improved heat distribution, and thus the reverse WGS reaction appeared to be further repressed. 3.4. Effect of paper voids on MSR performance The MSR behavior was investigated with respect to the paper void structures in the SiC fiber-containing catalyst paper. Fig. 6 displays the temperature-dependent correlation between the amounts of pulp fiber added during the wet papermaking process and the MSR behavior. SiC fiber-containing paper samples were calcined before use in order to remove the organic pulp fibers and to improve the paper strength by binder treatment. Therefore, when the pulp dosage was increased, the apparent porosity of the catalyst paper also increased, with a subsequent increase in the number of burned marks due to the pulp fibers; however, the final components were identical in each sample. SiC paper samples with pulp dosages of 0.10, 0.25, 0.50 and 1.0 g per handmade sheet were prepared. Both methanol conversion efficiency and CO concentration were found to decrease with increasing pulp content at the same reaction temperature. This result indicates that an excess of voids in the paper brought about a decrease in the MSR
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efficiency; the final MSR efficiency was equivalent to that obtained when a simple mixture of catalyst powders and fiber components was employed [12]. An excess of large pores might induce the localized gas flow and reduce the opportunities for reactants to contact with the catalyst surfaces. Fig. 6c shows the relationship between methanol conversion and CO concentration when modified by SiC fiber-containing paper samples with various pore sizes. The SiC fiber-containing paper with a pulp dosage of 0.1 g exhibited high methanol conversion, although large amounts of CO formed at high reaction temperatures. On the other hand, when a larger pulp dosage of 1.0 g was employed, the SiC fiber-containing paper generated smaller volumes of CO, although in this case methanol conversion efficiency was quite low. From the results in Fig. 6c, the optimum value was obtained for SiC fiber-containing paper samples with a pulp dosage of 0.25 g, indicating a higher methanol conversion at lower CO concentration. It is understood that the MSR reaction occurs on the surface of the Cu/ ZnO catalyst particles in the paper composites. However, here, both the heat conductivity of the SiC fibers and the porous paper structure significantly affected the MSR performance, even when similar amounts of catalyst were employed. These factors are controllable by varying both the types of inorganic fibers used and the amounts of organic pulp fibers introduced. Thus, the catalyst paper is allowed to provide an appropriate environment for the reforming catalyst. Here, the catalyst paper prepared by a simple papermaking technique is proposed for use as a practical catalyst-based material.
Fig. 6. Correlation between pulp dosage and MSR performances: (a) methanol conversion, (b) CO concentration and (c) relationship between methanol conversion and CO concentration; SiC paper with pulp dosage 0.10 g (diamonds), 0.25 g (squares), 0.50 g (triangles) and 1.0 g (circles).
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4. Conclusion Paper-based Cu/ZnO catalyst composites (catalyst paper) were successfully prepared by a conventional wet papermaking technique using ceramic and SiC fibers. The porous SiC fibercontaining catalyst paper exhibited higher methanol conversion efficiency and lower CO concentration, as compared with catalysts prepared from either powders or pellets. It is likely that the SiC fibers enhanced heat transfer inside the catalyst paper, contributing to the high methanol conversion. It was also clarified that the SiC catalyst paper most likely suppressed the undesirable reverse WGS reaction as a result of the uniform heat distribution inside the catalyst paper. Furthermore, the porous structure of the catalyst paper has a great impact on the MSR efficiency, where an optimum paper void is indicated. Both the heat property and void structure were controllable to some extent by the wet papermaking process. Therefore, catalyst paper with both practical convenience and high MSR performance is a promising candidate for catalyst-based materials. Acknowledgements This research was financially supported by the Industrial Technology Research Grant Program in 2003 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors wish to thank Ms. K. Hayashida for her assistance with the sample preparation and instrumental analyses. References [1] K.A. Starz, E. Auer, Th. Lehmann, R. Zuber, J. Power Sources 84 (1999) 167–172.
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