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MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification
Applied Catalysis A: General 278 (2005) 207–212 www.elsevier.com/locate/apcata
Comparison of Co/MgO and Ni/MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification Takeshi Furusawaa,b,1, Atsushi Tsutsumib,* a
CREST, JST (Japan Science and Technology Agency), Honcho 4-1-8, Kawaguchi 332-0012, Japan b Department of Chemical System Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Received 1 June 2004; received in revised form 14 September 2004; accepted 22 September 2004 Available online 13 November 2004
Abstract The catalytic performances of 12 wt.% Co/MgO catalyst pre-calcined at 873 K and of Ni catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, and CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K with all Ni loading values. The catalytic performance data showed that Co/MgO catalyst had higher activity (conv., 23%, 3 h) than any kinds of Ni/MgO catalysts tested in this study, under lower steam/carbon mole ratio (0.6) and higher concentration of fed naphthalene (3.5 mol%) than those used in the other works. The steam reforming of naphthalene proceeded when there was a stoichiometric ratio between the carbon atoms of naphthalene and H2O over Co catalyst; however, the activation of excess H2O happened over the Ni catalyst and this phenomenon can lead to having lower activity than Co catalyst. We concluded that these observations should be attributed to different catalytic performances between Co/MgO and Ni/MgO catalysts. # 2004 Elsevier B.V. All rights reserved. Keywords: Cobalt catalyst; Nickel catalyst; Steam reforming; Naphthalene; Biomass; Tar
1. Introduction Biomass gasification offers the potential for producing a fuel gas that can be used for power generation systems or synthesis gas applications. However, there still have drawbacks to commercialize this process. Despite extensive research efforts, tar formation is still a major problem in biomass gasification systems [1–6]. The condensable compounds present in tar may cause problems in down-
* Corresponding author. Tel.: +81 3 5841 7336; fax: +81 3 5841 7270. E-mail addresses: [email protected] (T. Furusawa), [email protected] (A. Tsutsumi). 1 Present address: Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan. Tel.: +81 28 689 6163; fax: +81 28 689 6163. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.09.035
stream equipment, making catalytic hot gas cleaning a necessary step in most gasification applications [5,7–17]. Most of the researches have focused on the dolomite (CaMg(CO3)2) [8,9] and Ni catalysts [7,10–17] as an effective catalyst for catalytic hot gas cleaning. So far, use of a guard bed with a calcined dolomite to decrease the tar content at the inlet of the Ni catalyst bed to a level below 1– 2 g m3; the tar content at the outlet of Ni catalyst reached 2–10 mg m3 and no deactivation was observed with time on-stream of up to 65 h [15]. However, the material was still not enough to be developed commercially [15,16]. Cobalt catalysts have been reported as effective catalyst systems for the steam gasification of wood [18] and biomass [19], partial oxidation of methane [20], CO2 reforming of methane [21,22], and steam reforming of ethanol [23,24]. If one considers the test reaction to find out new catalyst for tar
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removal, the steam reforming of naphthalene is a most suitable test reaction because naphthalene is one of the most stable tar compounds and it is difficult to decompose. In this point of view, the steam reforming of naphthalene have been studied over several kinds of Ni catalysts [25–27] to develop new catalyst for tar removal. To our knowledge, however, cobalt catalysts have never been studied with regard to the catalytic performance for the steam reforming of naphthalene. In previous research [28], we prepared several kinds of Co/MgO catalysts and we investigated the catalytic performances for the steam reforming of naphthalene. The results showed that 12 wt.% Co/MgO catalyst pre-calcined at 873 K exhibited the best performance (conv. 23%, 3 h) and we concluded that this Co catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed. However, we should be confirmed either this Co catalyst is more excellent system than Ni catalyst or not. Thus, the purposes of this study are: (1) to prepare the Ni/MgO catalysts, which are known to be efficient system for the reforming reaction, and (2) to compare the catalytic performances for the same reaction between Co/MgO catalyst and Ni/MgO catalyst in detail.
allowed to pass through the catalyst bed at a rate of 100 mL min1. Nitrogen was used to carry the naphthalene vapors into the reactor. The flow rate of nitrogen was 20 mL min1 carrying 1.0 g h1 of naphthalene corresponding to 100 times as high as the actual concentration contained in real biomass gasification tar. The water was pumped at a flow rate of 0.9 g h1 in order to achieve a GHSVof 3000 h1, based on the apparent bulk density of the catalyst bed (0.5 g cm3), for 1 g of catalyst. Unreacted naphthalene and H2O were recovered in the ice-cold trap, and the amount of unreacted naphthalene was calculated by subtracting the weight of cotton filter before reaction from that after reaction. The non-condensible gases were analyzed by gas chromatography (HP6890 equipped with a TCD detector and a molecular sieve 5A column for separation of H2, N2, CH4, CO and porapak Q column for separation of CO2 C2–C3 analysis). All the catalysts were tested under the same experimental conditions: GHSV = 3000 h1; residence time 2 s; water to carbon of naphthalene molar ratio 0.6; and atmospheric pressure. After the reaction, the reactants were switched with Ar at the reaction temperature; subsequently the catalysts were cooled down to room temperature and used for further characterization.
2. Experimental
2.3. Characterizations
2.1. Preparation of the catalysts
H2-Temperature Programmed Reduction measurements were performed on calcined samples under flowing 10% H2 diluted with Ar using an AMI-200 (ALTAMIRA) apparatus to investigate the phases after calcination. XRD measurements were performed on a Rigaku powder diffractrometer with Cu Ka radiation. The XRD diffraction patterns were taken in the 2u range of 10–808 at a scan speed of 28 min1. The exposed metal surface area of the reduced catalyst was determined by CO pulse adsorption at room temperature using AMI-200, assuming a 1/1 stoichiometry. After reduction of calcined catalyst (0.2 g), the reduced catalyst was cooled to room temperature with flowing Ar (30 mL min1) and CO was pulsed (58 mL per pulse) over the catalyst until no further adsorption of CO was observed. The CO left after CO adsorption was determined quantitatively with a TCD. The amount of coke on the catalysts was quantitatively determined by Temperature Programmed Oxidation with 5% O2/He. The catalyst after catalytic test was transferred to a U-shaped reactor (i.d. 4 mm) and was supported by means of two plugs of quartz wool. The deposited coke was oxidized to CO or CO2; the consumption of O2 was detected by TCD. Under the same conditions, TPO was also performed on the freshly reduced catalyst without exposing the sample to the atmosphere. The net amount of deposited coke was calculated by subtracting the amount of O2 consumption over the freshly reduced catalyst from that over the tested catalyst.
Cobalt-supported-on-MgO catalyst was prepared by impregnating MgO (JRC-MGO-41000A, 14–16 m2/g) with aqueous solution of Co(NO3)26H2O (Kanto Co.), followed by drying at 383 K overnight. The material was then calcined in air for 8 h at 873 K. The amount of Co loading on the catalyst was controlled to be 12 wt.%. Nickel-supportedon-MgO catalysts were prepared by impregnating MgO with aqueous solutions of Ni(NO3)26H2O (Kanto Co.), followed by drying at 383 K overnight and calcined in air at 873 or 1173 K. The amount of Ni loading on the catalysts was controlled within 4–36 wt.% by changing the concentration of aqueous solutions of Ni(NO3)26H2O. The calcined catalysts are denoted as XX wt.% Ni(O)/MgO (873 or 1173 K), and the reduced catalysts are denoted as Ni/MgO (873 or 1173 K). The temperature inside the parentheses indicates the calcination temperature. 2.2. Catalytic tests Catalytic tests were conducted by using an atmospheric flow experimental system. The setup and the experimental method were described previously [28] in detail. Here, they will be shown briefly. For the catalytic tests, all the catalysts were reduced in H2/Ar (50 vol%/50 vol%) mixture. They were heated at 10 K min1 rate to 1173 K and were held at 1173 K for 30 min. After reduction, the feed gases (C10H8/ H2O/N2/Ar = 3.5 mol%/21 mol%/20.0 mol%/balance) were
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3. Results 3.1. Characterization of prepared catalysts
Table 1 Exposed Ni metal surface area of the reduced Ni/MgO catalysts precalcined at various temperatures ((m2/g-catalyst) 100) Ni loading (wt.%)
Fig. 1 shows the profiles of H2-TPreduction of various Ni loading amounts (4–36 wt.%) of Ni(O)/MgO (873 K) calcined catalysts and of 12 wt.% Co(O)/MgO (873 K) calcined catalyst. In the previous research [28], the peaks at 615 and 780 K were assigned to reduction of large crystalline Co3O4 to CoO and to reduction from CoO to Co metal, respectively, and one shoulder peak at 960 K was attributed to MgCo2O4 (Fig. 1d). We also have reported that these phases seem to be reduced to Co metal completely after reduction of catalyst at 1173 K. In the case of Ni catalysts (Fig. 1a–c), two reduction peaks at around 700 and 1150 K were observed over all Ni loading amounts of catalysts. According to Freni et al. [24], the peak at around 700 K should be assigned to the reduction of ‘‘unreacted’’ NiO located on the MgO surface or to the reduction of some form of Ni2+ ions having square-pyramidal coordination in the outermost layer of the MgO structure. And the peak at 1150 K can be assigned to the Ni2+ ions located deep in the MgO lattice. It was observed that the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice increased with metal loading, indicating that the higher the Ni loading, the higher the amount of NiO–MgO solid solution formed during the calcination. In the case of Ni(O)/MgO (1173 K) catalysts (not shown here), only a reduction peak at 1150 K was observed over all Ni loaded catalysts. The fraction of reducible NiO over Ni(O)/MgO catalysts calcined at 873 K was higher than that over catalysts calcined at 1173 K, indicating that the NiO–MgO solid solution formed over the latter catalysts existed more stably. Furthermore, it was also found from the results of XRD measurements for Ni(O)/MgO (1173 K) catalysts (not shown here) that the positions of bands were shifted to higher angles with the higher metal loading, indicating that
Fig. 1. The profiles of H2-TP reduction of various Ni loading amounts of Ni(O)/MgO catalysts calcined at 873 K under 10% H2 + Ar at 30 mL min1, ramp rate 10 K min1: (a) 4 wt.%, (b) 12 wt.%, (c) 36 wt.% Ni(O)/MgO, and (d) 12 wt.% Co/MgO.
209
Calcination temperature (K) 873
4 12 12a 36
– 81.9 50.8 461
a
12 wt.% Co/MgO catalysts.
973
1073
1173
17.8
5.7 3.5
28.9 25.0 – 41.8
the higher percentage of NiO–MgO solid solutions formed during calcination. Table 1 summarizes the results of the exposed metal surface areas determined by CO chemisorption for reduced catalysts. For the Ni/MgO (873 K) catalysts, the Ni surface area increased with an increase in Ni loading. This can be explained by the results of H2-TPR profiles for these catalysts (Fig. 1), which showed that the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice increased with Ni loading. On the other hand, for the Ni/MgO (1173 K) catalysts, the Ni surface areas were almost the same over all Ni loaded catalysts. In the case of Ni/MgO (1173 K), the increase in the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice with Ni loading was also observed. However, the Ni surface areas were almost the same, indicating that the sintering of Ni metal during reduction step can occur over the more highly Ni loaded catalyst. It was concluded from these observations that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases, and that a few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K for all Ni loading. 3.2. Catalytic performances of Co and Ni catalysts for the steam reforming of naphthalene Fig. 2 shows the catalytic performances of 12 wt.% Co/ MgO (873 K) and several kinds of Ni/MgO catalysts reduced at 1173 K for the steam reforming of naphthalene as a function of time on-stream. The decrease of the activity for a very short time on-stream was observed over 4 wt.% Ni/ MgO catalysts due to the lack of Ni metal (Table 1). On the other hand, it can be seen from the catalytic performances of 12 wt.% Co/MgO catalyst and 12–36 wt.% Ni/MgO catalysts that the carbon conversions to gas remained for 3 h under the reaction condition tested. It was found that 12 wt.% Co/MgO catalyst showed a higher carbon conversion to gas than all Ni catalysts tested in this study. The results of TPO (not shown here) showed that the amounts of coke over all Ni catalysts reached less than 0.08 wt.% g1 h1. It is well known that the activity of Ni catalyst decreased due to deposition of coke [29–31]; however, the amounts of coke deposited over all Ni catalysts were quite low in this study. Thus, it can be supposed from
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Fig. 2. Catalytic performances of various Ni loading amounts of Ni/MgO catalysts pre-calcined at 873, and 1173 K followed by reduction at 1173 K as a function of time on-stream under the flow of C10H8 (3.5 mol%), H2O (21 mol%), N2 (20 mol%) and Ar (balance) with SV = 3000 h1 at T = 1173 K: (*) 12 wt.% Co/MgO (873 K), ( ) 12 wt.% Ni/MgO (873 K), (~) 12 wt.% Ni/MgO (1073 K), (*) 12 wt.% Ni/MgO (1173 K), (&) 36 wt.% Ni/MgO (873 K), (^) 36 wt.% Ni/MgO (1173 K), (4) 4 wt.% Ni/MgO (873 K), (^) 4 wt.% Ni/MgO (1173 K).
these results that the lack of Ni metal surface and the deposition of coke were not the major reasons for having lower activity for the steam reforming of naphthalene over 12–36 wt.% Ni catalysts than over Co catalyst. The further discussion will be shown later. Table 2 summarizes the results of product composition over 4–36 wt.% Ni/MgO (873 and 1173 K), and 12 wt.% Co/MgO (873 K) catalysts reduced at 1173 K for the steam reforming of naphthalene under certain reaction conditions. The percentage of reacted H2O (%) was calculated as follows: the percentage of reacted H2 O ð%Þ ¼
reacted H2 O ðmol min1 Þ 100 ð%Þ; introduced H2 O ðmol min1 Þ reacted H2 O ðmol min1 Þ
¼ H2 derived from reacted H2 O ¼ A B where A is the total product H2, which was determined by GC (mol min1), B is the H2 derived from reacted C10H8,
which was calculated from the sum of product CO, CO2, and CH4 (mol min1). In the case of Ni catalysts, it was found that CH4 and C2H4 were produced over 4 wt.% Ni/MgO catalysts. However, none was obtained over higher Ni loaded catalysts, indicating that the reforming reactions of CH4 and C2 products with H2O or CO2 also occurred over the latter catalysts. For the more highly Ni loaded catalysts (12– 36 wt.%) and for 12 wt.% Co/MgO catalyst, these catalysts showed high carbon conversions to gas and high percentages of reacted H2O. Additionally, it also found that a high percentage of reacted H2O was obtained over the catalysts (12–36 wt.%), of which the amount of coke deposited was low (<0.08 wt.% g1 h1). Furthermore, the main products were H2 and CO2, with less CO over all the catalysts, indicating that the product gas can be used for the fuel cell system, where cannot be operated in the presence of excess CO [32].
3.3. Characterization of tested catalysts by XRD measurement Fig. 3 shows the XRD patterns of 12 wt.% Co/MgO (873 K) catalyst after calcination (Fig. 3a), after reduction (Fig. 3b), and after catalytic test (Fig. 3c) using Cu Ka as the X-ray source. Five major peaks at 37, 43, 62.3, 74.7, and 78.68 for 2u are identified as MgO and it was found that the intensities of these peaks were increased after reduction (Fig. 3b) and catalytic test (Fig. 3c). It was also found that the peaks at 31, 44.5, 59, and 658 assigned to Co3O4 and MgCo2O4 disappeared after reduction of catalyst. Instead of these peaks, the peaks at 44.8 and 75.98 were observed over the catalysts reduced at 1173 K and after catalytic test; these were identified as Co metal. The intensities of these peaks assigned to Co metal remained even after catalytic test, indicating that Co metal phase can exist under reaction conditions tested in this study. These phenomena should be attributed to high and stable activity for the steam reforming of naphthalene over this catalyst. Fig. 4 shows the XRD patterns of 12 wt.% Ni/MgO (873 K) catalyst after calcination (Fig. 4a), after reduction
Table 2 Conversion, product composition, and the percentage of reacted H2Oa over all Ni loaded catalysts (873 and 1173 K), and 12 wt.% Co/MgO (873 K) catalyst Ni loading (wt.%)
Carbon conversion to gas (%)
H2 (mol%)
CO2 (mol%)
CO (mol%)
CH4 (mol%)
C2H4 (mol%)
Reacted H2O (%)
0 4 4b 12 12b 12c 36 36b
3.7 4.3 3.0 7.9 9.4 23.3 5.7 4.9
62.5 64.8 66.7 77.2 76.7 70.0 75.9 74.8
26.7 26.2 23.8 19.9 19.4 27.5 21.6 22.7
7.0 3.8 4.9 2.9 3.9 2.4 2.5 2.5
3.0 4.5 3.9 0 0 0.1 0 0
0.8 0.7 0.7 0 0 0 0 0
9.3 11.4 10.0 39.3 41.5 57.6 26.1 20.3
a b c
Reaction conditions: steam/carbon = 0.6, T = 1173 K, SV = 3000 h1, t = 2 s, reaction time 2 h. Catalysts pre-calcined at 1173 K. 12 wt.% Co/MgO (873 K) catalyst.
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4. Discussion
Fig. 4. XRD patterns of 12 wt.% Ni/MgO catalysts (a) after calcination at 873 K, (b) after reduction at 1173 K, and (c) after catalytic test.
(Fig. 4b), and after catalytic test (Fig. 4c) using Cu Ka as the X-ray source. Five major peaks at 37, 43, 62.3, 74.7, and 78.68 for 2u are identified as MgO and/or MgNiO2; the intensities of these peaks remained even after reduction and catalytic test. However, the positions of these peaks were slightly shifted to higher angles after the catalytic test (Fig. 4c), indicating that the catalyst was slightly oxidized by H2O during the reaction. The peaks at 44.6 and 75.98 were observed over the catalysts reduced at 1173 K and after catalytic test; these were identified as Ni metal. The intensities of these peaks assigned to Ni metal were increased after catalytic test, indicating that some Ni metal phase formed by reactant gas under reaction conditions tested in this study. It was supposed from these observations that there is a balance between the formation of Ni metal by reactant gas (C10H8) over the catalyst and the mild oxidation of catalyst by H2O. When the former factor was strongly affected by the catalyst, a gradual increase of the activity of Ni/MgO catalyst (Fig. 2) with time on-stream was observed.
Fig. 3. XRD patterns of 12 wt.% Co/MgO catalysts (a) after calcination at 873 K, (b) after reduction at 1173 K, and (c) after catalytic test.
If one assumes that the steam reforming of C10H8 occurred in terms of the carbon atoms of C10H8:reacted H2O = 1:1.87 (C + 1.87H2O = 0.13CO + 0.87CO2 + 1.87H2) in the case of 12 wt.% Ni/MgO (873 K) catalyst (Table 2), the percentage of reacted H2O can be estimated to be 24.6% by calculating 7.9 (average carbon conversion to gas (%)) 1.87/0.6 (introduced steam/carbon mole ratio). It was found that the percentage of reacted H2O reached 39.3% during the reaction (2 h) shown in Table 2, indicating that the formation of excess O atoms during the reaction led to the oxidation of catalyst by H2O, as was confirmed by XRD measurement (Fig. 4). For the 12 wt.% Co/MgO (873 K) catalyst, if one assumes that the steam reforming of C10H8 occurred in terms of the carbon atoms of C10H8:reacted H2O = 1:1.92 (C + 1.92H2O = 0.08CO + 0.92CO2 + 1.92H2) in the case of 12 wt.% Co/ MgO (873 K) catalyst (Table 2), the percentage of reacted H2O can be estimated to be 74.5% by the method mentioned above. It was found that the percentage of reacted H2O reached 57.6% during the reaction (2 h) shown in Table 2, indicating that the steam reforming of C10H8 seems to be sufficient over 12 wt.% Co/MgO (873 K) catalyst. Furthermore, it was also found that the Co metal phase existed during the reaction (Fig. 3c). Thus, these results showed that 12 wt.% Co/MgO (873 K) catalyst exhibited high and stable activity for the steam reforming of naphthalene due to the existence of Co metal and low amount of coke deposition. So far, it was found that the 12 wt.% Co/MgO catalyst showed higher carbon conversion to gas than Ni catalysts under reaction conditions (steam/carbon mole ratio; 0.6, T; 1173 K, t; 2 s, SV; 3000 h1) in this study. In most cases, the steam reforming of hydrocarbons using Ni catalysts [7,10– 17] has been carried out at steam/carbon mole ratios from 2 to 6 to avoid the coke formation over the catalysts. Additionally, the steam reforming of naphthalene had been studied on Ni–Cr/Al2O3MgOLa2O3 catalyst with 1.6 as the steam/carbon mole ratio [25], on nickel-activated candle filter with about 10 as steam/carbon ratio [26], and on commercial nickel-based catalysts with 4.2 as steam/carbon ratio [27]. These works [25–27] have showed that high naphthalene conversions (90–100%) were obtained over these catalysts using shorter contact time (t; 0.55–1.0 s) and lower reaction temperature (T; 1023–1163 K) than this study. However, it was found in this study that low naphthalene conversion (Table 2) was obtained over 12 wt.% Ni/MgO (873 K) catalyst under reaction conditions tested and it can be supposed that this difference were due to using lower steam/carbon mole ratio and higher concentration of fed naphthalene than the other works. In terms of these suggestions, it should be noted that 12 wt.% Co/MgO (873 K) catalyst showed high and stable activity (conv. 23%, 3 h) with low steam/carbon mole ratio. This result pointed out an excellent advantage of the Co catalyst, because the low steam/carbon ratio required less energy for pumping, heating and recycling excess water, thus increasing overall
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biomass gasification system energy efficiency [33]. It was concluded that this Co catalyst is promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed. However, the activity is still not enough to be developed. Naphthalene conversion might be improved if a lower concentration of naphthalene were fed into the reactor where the catalyst was loaded. In the future, experiments have to be conducted under same reaction conditions as were used in other works (for example, 2–6 for steam/carbon mole ratio and 1000 ppm V for the concentration of fed naphthalene).
5. Conclusions The results of characterization (TPR, XRD, CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K for all Ni loading values. As for the catalytic performance, it was found that the 12 wt.% Co/MgO (873 K) catalyst showed a higher catalytic performance (conv. 23%, 3 h) for the steam reforming of naphthalene than several kinds of Ni catalysts tested in this study. In the case of Ni catalysts, it was found that the catalyst pre-calcined at 873 K showed higher activity than that pre-calcined at 1173 K, due to Ni metal surface area. When one compares Co catalyst and Ni catalyst, one finds that the steam reforming of naphthalene proceeded in stoichiometric ratio between the carbon atoms of naphthalene and H2O over Co catalyst; however, the activation of excess H2O happened over Ni catalyst and this phenomenon can lead to its having lower activity than Co catalyst. Both catalysts showed lower activity in this study than in the other works due to low steam/carbon ratio (0.6) and high concentration of fed naphthalene, corresponding to 100 times as much as that in real biomass gasification tar. From these results, it was concluded that 12 wt.% Co/MgO (873 K) catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed.
Acknowledgement This study was financially supported by a ‘‘Core Research for Evolutional Science and Technology’’ grant from the Japan Science and Technology Agency (JST). The authors appreciate this financial support.
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Comparison of Co/MgO and Ni/MgO catalysts for the steam reforming of naphthalene as a model compound of tar derived from biomass gasification Takeshi Furusawaa,b,1, Atsushi Tsutsumib,* a
CREST, JST (Japan Science and Technology Agency), Honcho 4-1-8, Kawaguchi 332-0012, Japan b Department of Chemical System Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Received 1 June 2004; received in revised form 14 September 2004; accepted 22 September 2004 Available online 13 November 2004
Abstract The catalytic performances of 12 wt.% Co/MgO catalyst pre-calcined at 873 K and of Ni catalysts for the steam reforming of naphthalene were investigated. The results of characterizations (TPR, XRD, and CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K with all Ni loading values. The catalytic performance data showed that Co/MgO catalyst had higher activity (conv., 23%, 3 h) than any kinds of Ni/MgO catalysts tested in this study, under lower steam/carbon mole ratio (0.6) and higher concentration of fed naphthalene (3.5 mol%) than those used in the other works. The steam reforming of naphthalene proceeded when there was a stoichiometric ratio between the carbon atoms of naphthalene and H2O over Co catalyst; however, the activation of excess H2O happened over the Ni catalyst and this phenomenon can lead to having lower activity than Co catalyst. We concluded that these observations should be attributed to different catalytic performances between Co/MgO and Ni/MgO catalysts. # 2004 Elsevier B.V. All rights reserved. Keywords: Cobalt catalyst; Nickel catalyst; Steam reforming; Naphthalene; Biomass; Tar
1. Introduction Biomass gasification offers the potential for producing a fuel gas that can be used for power generation systems or synthesis gas applications. However, there still have drawbacks to commercialize this process. Despite extensive research efforts, tar formation is still a major problem in biomass gasification systems [1–6]. The condensable compounds present in tar may cause problems in down-
* Corresponding author. Tel.: +81 3 5841 7336; fax: +81 3 5841 7270. E-mail addresses: [email protected] (T. Furusawa), [email protected] (A. Tsutsumi). 1 Present address: Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan. Tel.: +81 28 689 6163; fax: +81 28 689 6163. 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.09.035
stream equipment, making catalytic hot gas cleaning a necessary step in most gasification applications [5,7–17]. Most of the researches have focused on the dolomite (CaMg(CO3)2) [8,9] and Ni catalysts [7,10–17] as an effective catalyst for catalytic hot gas cleaning. So far, use of a guard bed with a calcined dolomite to decrease the tar content at the inlet of the Ni catalyst bed to a level below 1– 2 g m3; the tar content at the outlet of Ni catalyst reached 2–10 mg m3 and no deactivation was observed with time on-stream of up to 65 h [15]. However, the material was still not enough to be developed commercially [15,16]. Cobalt catalysts have been reported as effective catalyst systems for the steam gasification of wood [18] and biomass [19], partial oxidation of methane [20], CO2 reforming of methane [21,22], and steam reforming of ethanol [23,24]. If one considers the test reaction to find out new catalyst for tar
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removal, the steam reforming of naphthalene is a most suitable test reaction because naphthalene is one of the most stable tar compounds and it is difficult to decompose. In this point of view, the steam reforming of naphthalene have been studied over several kinds of Ni catalysts [25–27] to develop new catalyst for tar removal. To our knowledge, however, cobalt catalysts have never been studied with regard to the catalytic performance for the steam reforming of naphthalene. In previous research [28], we prepared several kinds of Co/MgO catalysts and we investigated the catalytic performances for the steam reforming of naphthalene. The results showed that 12 wt.% Co/MgO catalyst pre-calcined at 873 K exhibited the best performance (conv. 23%, 3 h) and we concluded that this Co catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed. However, we should be confirmed either this Co catalyst is more excellent system than Ni catalyst or not. Thus, the purposes of this study are: (1) to prepare the Ni/MgO catalysts, which are known to be efficient system for the reforming reaction, and (2) to compare the catalytic performances for the same reaction between Co/MgO catalyst and Ni/MgO catalyst in detail.
allowed to pass through the catalyst bed at a rate of 100 mL min1. Nitrogen was used to carry the naphthalene vapors into the reactor. The flow rate of nitrogen was 20 mL min1 carrying 1.0 g h1 of naphthalene corresponding to 100 times as high as the actual concentration contained in real biomass gasification tar. The water was pumped at a flow rate of 0.9 g h1 in order to achieve a GHSVof 3000 h1, based on the apparent bulk density of the catalyst bed (0.5 g cm3), for 1 g of catalyst. Unreacted naphthalene and H2O were recovered in the ice-cold trap, and the amount of unreacted naphthalene was calculated by subtracting the weight of cotton filter before reaction from that after reaction. The non-condensible gases were analyzed by gas chromatography (HP6890 equipped with a TCD detector and a molecular sieve 5A column for separation of H2, N2, CH4, CO and porapak Q column for separation of CO2 C2–C3 analysis). All the catalysts were tested under the same experimental conditions: GHSV = 3000 h1; residence time 2 s; water to carbon of naphthalene molar ratio 0.6; and atmospheric pressure. After the reaction, the reactants were switched with Ar at the reaction temperature; subsequently the catalysts were cooled down to room temperature and used for further characterization.
2. Experimental
2.3. Characterizations
2.1. Preparation of the catalysts
H2-Temperature Programmed Reduction measurements were performed on calcined samples under flowing 10% H2 diluted with Ar using an AMI-200 (ALTAMIRA) apparatus to investigate the phases after calcination. XRD measurements were performed on a Rigaku powder diffractrometer with Cu Ka radiation. The XRD diffraction patterns were taken in the 2u range of 10–808 at a scan speed of 28 min1. The exposed metal surface area of the reduced catalyst was determined by CO pulse adsorption at room temperature using AMI-200, assuming a 1/1 stoichiometry. After reduction of calcined catalyst (0.2 g), the reduced catalyst was cooled to room temperature with flowing Ar (30 mL min1) and CO was pulsed (58 mL per pulse) over the catalyst until no further adsorption of CO was observed. The CO left after CO adsorption was determined quantitatively with a TCD. The amount of coke on the catalysts was quantitatively determined by Temperature Programmed Oxidation with 5% O2/He. The catalyst after catalytic test was transferred to a U-shaped reactor (i.d. 4 mm) and was supported by means of two plugs of quartz wool. The deposited coke was oxidized to CO or CO2; the consumption of O2 was detected by TCD. Under the same conditions, TPO was also performed on the freshly reduced catalyst without exposing the sample to the atmosphere. The net amount of deposited coke was calculated by subtracting the amount of O2 consumption over the freshly reduced catalyst from that over the tested catalyst.
Cobalt-supported-on-MgO catalyst was prepared by impregnating MgO (JRC-MGO-41000A, 14–16 m2/g) with aqueous solution of Co(NO3)26H2O (Kanto Co.), followed by drying at 383 K overnight. The material was then calcined in air for 8 h at 873 K. The amount of Co loading on the catalyst was controlled to be 12 wt.%. Nickel-supportedon-MgO catalysts were prepared by impregnating MgO with aqueous solutions of Ni(NO3)26H2O (Kanto Co.), followed by drying at 383 K overnight and calcined in air at 873 or 1173 K. The amount of Ni loading on the catalysts was controlled within 4–36 wt.% by changing the concentration of aqueous solutions of Ni(NO3)26H2O. The calcined catalysts are denoted as XX wt.% Ni(O)/MgO (873 or 1173 K), and the reduced catalysts are denoted as Ni/MgO (873 or 1173 K). The temperature inside the parentheses indicates the calcination temperature. 2.2. Catalytic tests Catalytic tests were conducted by using an atmospheric flow experimental system. The setup and the experimental method were described previously [28] in detail. Here, they will be shown briefly. For the catalytic tests, all the catalysts were reduced in H2/Ar (50 vol%/50 vol%) mixture. They were heated at 10 K min1 rate to 1173 K and were held at 1173 K for 30 min. After reduction, the feed gases (C10H8/ H2O/N2/Ar = 3.5 mol%/21 mol%/20.0 mol%/balance) were
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3. Results 3.1. Characterization of prepared catalysts
Table 1 Exposed Ni metal surface area of the reduced Ni/MgO catalysts precalcined at various temperatures ((m2/g-catalyst) 100) Ni loading (wt.%)
Fig. 1 shows the profiles of H2-TPreduction of various Ni loading amounts (4–36 wt.%) of Ni(O)/MgO (873 K) calcined catalysts and of 12 wt.% Co(O)/MgO (873 K) calcined catalyst. In the previous research [28], the peaks at 615 and 780 K were assigned to reduction of large crystalline Co3O4 to CoO and to reduction from CoO to Co metal, respectively, and one shoulder peak at 960 K was attributed to MgCo2O4 (Fig. 1d). We also have reported that these phases seem to be reduced to Co metal completely after reduction of catalyst at 1173 K. In the case of Ni catalysts (Fig. 1a–c), two reduction peaks at around 700 and 1150 K were observed over all Ni loading amounts of catalysts. According to Freni et al. [24], the peak at around 700 K should be assigned to the reduction of ‘‘unreacted’’ NiO located on the MgO surface or to the reduction of some form of Ni2+ ions having square-pyramidal coordination in the outermost layer of the MgO structure. And the peak at 1150 K can be assigned to the Ni2+ ions located deep in the MgO lattice. It was observed that the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice increased with metal loading, indicating that the higher the Ni loading, the higher the amount of NiO–MgO solid solution formed during the calcination. In the case of Ni(O)/MgO (1173 K) catalysts (not shown here), only a reduction peak at 1150 K was observed over all Ni loaded catalysts. The fraction of reducible NiO over Ni(O)/MgO catalysts calcined at 873 K was higher than that over catalysts calcined at 1173 K, indicating that the NiO–MgO solid solution formed over the latter catalysts existed more stably. Furthermore, it was also found from the results of XRD measurements for Ni(O)/MgO (1173 K) catalysts (not shown here) that the positions of bands were shifted to higher angles with the higher metal loading, indicating that
Fig. 1. The profiles of H2-TP reduction of various Ni loading amounts of Ni(O)/MgO catalysts calcined at 873 K under 10% H2 + Ar at 30 mL min1, ramp rate 10 K min1: (a) 4 wt.%, (b) 12 wt.%, (c) 36 wt.% Ni(O)/MgO, and (d) 12 wt.% Co/MgO.
209
Calcination temperature (K) 873
4 12 12a 36
– 81.9 50.8 461
a
12 wt.% Co/MgO catalysts.
973
1073
1173
17.8
5.7 3.5
28.9 25.0 – 41.8
the higher percentage of NiO–MgO solid solutions formed during calcination. Table 1 summarizes the results of the exposed metal surface areas determined by CO chemisorption for reduced catalysts. For the Ni/MgO (873 K) catalysts, the Ni surface area increased with an increase in Ni loading. This can be explained by the results of H2-TPR profiles for these catalysts (Fig. 1), which showed that the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice increased with Ni loading. On the other hand, for the Ni/MgO (1173 K) catalysts, the Ni surface areas were almost the same over all Ni loaded catalysts. In the case of Ni/MgO (1173 K), the increase in the peak intensity (1150 K) assigned to the Ni2+ ions located deep in the MgO lattice with Ni loading was also observed. However, the Ni surface areas were almost the same, indicating that the sintering of Ni metal during reduction step can occur over the more highly Ni loaded catalyst. It was concluded from these observations that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases, and that a few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K for all Ni loading. 3.2. Catalytic performances of Co and Ni catalysts for the steam reforming of naphthalene Fig. 2 shows the catalytic performances of 12 wt.% Co/ MgO (873 K) and several kinds of Ni/MgO catalysts reduced at 1173 K for the steam reforming of naphthalene as a function of time on-stream. The decrease of the activity for a very short time on-stream was observed over 4 wt.% Ni/ MgO catalysts due to the lack of Ni metal (Table 1). On the other hand, it can be seen from the catalytic performances of 12 wt.% Co/MgO catalyst and 12–36 wt.% Ni/MgO catalysts that the carbon conversions to gas remained for 3 h under the reaction condition tested. It was found that 12 wt.% Co/MgO catalyst showed a higher carbon conversion to gas than all Ni catalysts tested in this study. The results of TPO (not shown here) showed that the amounts of coke over all Ni catalysts reached less than 0.08 wt.% g1 h1. It is well known that the activity of Ni catalyst decreased due to deposition of coke [29–31]; however, the amounts of coke deposited over all Ni catalysts were quite low in this study. Thus, it can be supposed from
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Fig. 2. Catalytic performances of various Ni loading amounts of Ni/MgO catalysts pre-calcined at 873, and 1173 K followed by reduction at 1173 K as a function of time on-stream under the flow of C10H8 (3.5 mol%), H2O (21 mol%), N2 (20 mol%) and Ar (balance) with SV = 3000 h1 at T = 1173 K: (*) 12 wt.% Co/MgO (873 K), ( ) 12 wt.% Ni/MgO (873 K), (~) 12 wt.% Ni/MgO (1073 K), (*) 12 wt.% Ni/MgO (1173 K), (&) 36 wt.% Ni/MgO (873 K), (^) 36 wt.% Ni/MgO (1173 K), (4) 4 wt.% Ni/MgO (873 K), (^) 4 wt.% Ni/MgO (1173 K).
these results that the lack of Ni metal surface and the deposition of coke were not the major reasons for having lower activity for the steam reforming of naphthalene over 12–36 wt.% Ni catalysts than over Co catalyst. The further discussion will be shown later. Table 2 summarizes the results of product composition over 4–36 wt.% Ni/MgO (873 and 1173 K), and 12 wt.% Co/MgO (873 K) catalysts reduced at 1173 K for the steam reforming of naphthalene under certain reaction conditions. The percentage of reacted H2O (%) was calculated as follows: the percentage of reacted H2 O ð%Þ ¼
reacted H2 O ðmol min1 Þ 100 ð%Þ; introduced H2 O ðmol min1 Þ reacted H2 O ðmol min1 Þ
¼ H2 derived from reacted H2 O ¼ A B where A is the total product H2, which was determined by GC (mol min1), B is the H2 derived from reacted C10H8,
which was calculated from the sum of product CO, CO2, and CH4 (mol min1). In the case of Ni catalysts, it was found that CH4 and C2H4 were produced over 4 wt.% Ni/MgO catalysts. However, none was obtained over higher Ni loaded catalysts, indicating that the reforming reactions of CH4 and C2 products with H2O or CO2 also occurred over the latter catalysts. For the more highly Ni loaded catalysts (12– 36 wt.%) and for 12 wt.% Co/MgO catalyst, these catalysts showed high carbon conversions to gas and high percentages of reacted H2O. Additionally, it also found that a high percentage of reacted H2O was obtained over the catalysts (12–36 wt.%), of which the amount of coke deposited was low (<0.08 wt.% g1 h1). Furthermore, the main products were H2 and CO2, with less CO over all the catalysts, indicating that the product gas can be used for the fuel cell system, where cannot be operated in the presence of excess CO [32].
3.3. Characterization of tested catalysts by XRD measurement Fig. 3 shows the XRD patterns of 12 wt.% Co/MgO (873 K) catalyst after calcination (Fig. 3a), after reduction (Fig. 3b), and after catalytic test (Fig. 3c) using Cu Ka as the X-ray source. Five major peaks at 37, 43, 62.3, 74.7, and 78.68 for 2u are identified as MgO and it was found that the intensities of these peaks were increased after reduction (Fig. 3b) and catalytic test (Fig. 3c). It was also found that the peaks at 31, 44.5, 59, and 658 assigned to Co3O4 and MgCo2O4 disappeared after reduction of catalyst. Instead of these peaks, the peaks at 44.8 and 75.98 were observed over the catalysts reduced at 1173 K and after catalytic test; these were identified as Co metal. The intensities of these peaks assigned to Co metal remained even after catalytic test, indicating that Co metal phase can exist under reaction conditions tested in this study. These phenomena should be attributed to high and stable activity for the steam reforming of naphthalene over this catalyst. Fig. 4 shows the XRD patterns of 12 wt.% Ni/MgO (873 K) catalyst after calcination (Fig. 4a), after reduction
Table 2 Conversion, product composition, and the percentage of reacted H2Oa over all Ni loaded catalysts (873 and 1173 K), and 12 wt.% Co/MgO (873 K) catalyst Ni loading (wt.%)
Carbon conversion to gas (%)
H2 (mol%)
CO2 (mol%)
CO (mol%)
CH4 (mol%)
C2H4 (mol%)
Reacted H2O (%)
0 4 4b 12 12b 12c 36 36b
3.7 4.3 3.0 7.9 9.4 23.3 5.7 4.9
62.5 64.8 66.7 77.2 76.7 70.0 75.9 74.8
26.7 26.2 23.8 19.9 19.4 27.5 21.6 22.7
7.0 3.8 4.9 2.9 3.9 2.4 2.5 2.5
3.0 4.5 3.9 0 0 0.1 0 0
0.8 0.7 0.7 0 0 0 0 0
9.3 11.4 10.0 39.3 41.5 57.6 26.1 20.3
a b c
Reaction conditions: steam/carbon = 0.6, T = 1173 K, SV = 3000 h1, t = 2 s, reaction time 2 h. Catalysts pre-calcined at 1173 K. 12 wt.% Co/MgO (873 K) catalyst.
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4. Discussion
Fig. 4. XRD patterns of 12 wt.% Ni/MgO catalysts (a) after calcination at 873 K, (b) after reduction at 1173 K, and (c) after catalytic test.
(Fig. 4b), and after catalytic test (Fig. 4c) using Cu Ka as the X-ray source. Five major peaks at 37, 43, 62.3, 74.7, and 78.68 for 2u are identified as MgO and/or MgNiO2; the intensities of these peaks remained even after reduction and catalytic test. However, the positions of these peaks were slightly shifted to higher angles after the catalytic test (Fig. 4c), indicating that the catalyst was slightly oxidized by H2O during the reaction. The peaks at 44.6 and 75.98 were observed over the catalysts reduced at 1173 K and after catalytic test; these were identified as Ni metal. The intensities of these peaks assigned to Ni metal were increased after catalytic test, indicating that some Ni metal phase formed by reactant gas under reaction conditions tested in this study. It was supposed from these observations that there is a balance between the formation of Ni metal by reactant gas (C10H8) over the catalyst and the mild oxidation of catalyst by H2O. When the former factor was strongly affected by the catalyst, a gradual increase of the activity of Ni/MgO catalyst (Fig. 2) with time on-stream was observed.
Fig. 3. XRD patterns of 12 wt.% Co/MgO catalysts (a) after calcination at 873 K, (b) after reduction at 1173 K, and (c) after catalytic test.
If one assumes that the steam reforming of C10H8 occurred in terms of the carbon atoms of C10H8:reacted H2O = 1:1.87 (C + 1.87H2O = 0.13CO + 0.87CO2 + 1.87H2) in the case of 12 wt.% Ni/MgO (873 K) catalyst (Table 2), the percentage of reacted H2O can be estimated to be 24.6% by calculating 7.9 (average carbon conversion to gas (%)) 1.87/0.6 (introduced steam/carbon mole ratio). It was found that the percentage of reacted H2O reached 39.3% during the reaction (2 h) shown in Table 2, indicating that the formation of excess O atoms during the reaction led to the oxidation of catalyst by H2O, as was confirmed by XRD measurement (Fig. 4). For the 12 wt.% Co/MgO (873 K) catalyst, if one assumes that the steam reforming of C10H8 occurred in terms of the carbon atoms of C10H8:reacted H2O = 1:1.92 (C + 1.92H2O = 0.08CO + 0.92CO2 + 1.92H2) in the case of 12 wt.% Co/ MgO (873 K) catalyst (Table 2), the percentage of reacted H2O can be estimated to be 74.5% by the method mentioned above. It was found that the percentage of reacted H2O reached 57.6% during the reaction (2 h) shown in Table 2, indicating that the steam reforming of C10H8 seems to be sufficient over 12 wt.% Co/MgO (873 K) catalyst. Furthermore, it was also found that the Co metal phase existed during the reaction (Fig. 3c). Thus, these results showed that 12 wt.% Co/MgO (873 K) catalyst exhibited high and stable activity for the steam reforming of naphthalene due to the existence of Co metal and low amount of coke deposition. So far, it was found that the 12 wt.% Co/MgO catalyst showed higher carbon conversion to gas than Ni catalysts under reaction conditions (steam/carbon mole ratio; 0.6, T; 1173 K, t; 2 s, SV; 3000 h1) in this study. In most cases, the steam reforming of hydrocarbons using Ni catalysts [7,10– 17] has been carried out at steam/carbon mole ratios from 2 to 6 to avoid the coke formation over the catalysts. Additionally, the steam reforming of naphthalene had been studied on Ni–Cr/Al2O3MgOLa2O3 catalyst with 1.6 as the steam/carbon mole ratio [25], on nickel-activated candle filter with about 10 as steam/carbon ratio [26], and on commercial nickel-based catalysts with 4.2 as steam/carbon ratio [27]. These works [25–27] have showed that high naphthalene conversions (90–100%) were obtained over these catalysts using shorter contact time (t; 0.55–1.0 s) and lower reaction temperature (T; 1023–1163 K) than this study. However, it was found in this study that low naphthalene conversion (Table 2) was obtained over 12 wt.% Ni/MgO (873 K) catalyst under reaction conditions tested and it can be supposed that this difference were due to using lower steam/carbon mole ratio and higher concentration of fed naphthalene than the other works. In terms of these suggestions, it should be noted that 12 wt.% Co/MgO (873 K) catalyst showed high and stable activity (conv. 23%, 3 h) with low steam/carbon mole ratio. This result pointed out an excellent advantage of the Co catalyst, because the low steam/carbon ratio required less energy for pumping, heating and recycling excess water, thus increasing overall
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biomass gasification system energy efficiency [33]. It was concluded that this Co catalyst is promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed. However, the activity is still not enough to be developed. Naphthalene conversion might be improved if a lower concentration of naphthalene were fed into the reactor where the catalyst was loaded. In the future, experiments have to be conducted under same reaction conditions as were used in other works (for example, 2–6 for steam/carbon mole ratio and 1000 ppm V for the concentration of fed naphthalene).
5. Conclusions The results of characterization (TPR, XRD, CO adsorption) for Ni catalysts showed that Ni metal particles were formed over the catalysts pre-calcined at 873 K with high Ni loading via reduction of NiO–MgO phases. A few Ni metal particles were obtained over the catalysts pre-calcined at 1173 K for all Ni loading values. As for the catalytic performance, it was found that the 12 wt.% Co/MgO (873 K) catalyst showed a higher catalytic performance (conv. 23%, 3 h) for the steam reforming of naphthalene than several kinds of Ni catalysts tested in this study. In the case of Ni catalysts, it was found that the catalyst pre-calcined at 873 K showed higher activity than that pre-calcined at 1173 K, due to Ni metal surface area. When one compares Co catalyst and Ni catalyst, one finds that the steam reforming of naphthalene proceeded in stoichiometric ratio between the carbon atoms of naphthalene and H2O over Co catalyst; however, the activation of excess H2O happened over Ni catalyst and this phenomenon can lead to its having lower activity than Co catalyst. Both catalysts showed lower activity in this study than in the other works due to low steam/carbon ratio (0.6) and high concentration of fed naphthalene, corresponding to 100 times as much as that in real biomass gasification tar. From these results, it was concluded that 12 wt.% Co/MgO (873 K) catalyst is a promising system for the steam reforming of naphthalene derived from biomass gasification as a second fixed catalytic bed.
Acknowledgement This study was financially supported by a ‘‘Core Research for Evolutional Science and Technology’’ grant from the Japan Science and Technology Agency (JST). The authors appreciate this financial support.
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