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Single-phase Ni3Sn alloy alkali-leached for hydrogen production from methanol decomposition
Renewable Energy 78 (2015) 357e363
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Single-phase Ni3Sn alloy alkali-leached for hydrogen production from methanol decomposition Pan Wei, Wei Xia, Jian Zhu Li, Haifei Long, Jindan Chen, Ting Li, Meiqiang Fan* Department of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2014 Accepted 9 January 2015 Available online
Methanol decomposition over alkali-leached Ni3Sn powder at 513e793 K was investigated. Compared with untreated Ni3Sn, alkali-leached Ni3Sn had high catalytic activity and selectivity toward H2 and CO production above 633 K. A maximum H2 production rate of 100 103 mol h1 g-Cat1 and H2 selectivity above 95% were attained over alkali-leached Ni3Sn at 793 K. Alkali-leached Ni3Sn presented good catalytic activity for 45 h of reaction at 713 K, whereas Ni3Sn had none. The activation energy was calculated, and its values rapidly decreased from Ni3Sn to alkali-leached ones. The improvement was attributed to the formation of Ni nanoparticles less than 100 nm in diameter in the alkali-leaching process, which had high activity for methanol decomposition. The improved catalytic activity favored the gradual formation of fine Ni3Sn particle during the reaction, which served as the active sites for methanol decomposition when the catalytic activity decreased because of carbon deposition on the Ni surface. Results demonstrated that alkali-leached Ni3Sn was a promising potential catalyst for hydrogen production from methanol. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Methanol decomposition Intermetallic compounds and alloys Hydrogen production
1. Introduction Hydrogen production from methanol decomposition is attracting considerable attention as methanol has many advantageous properties such as low cost, high H/C molar ratio, and so on [1e3]. A great deal of effort has been given in the development of an efficient low cost catalyst for methanol decomposition. Ni-based catalysts are common and their performance has been significantly improved in the past 30 years [4e6]. However, the formation of methane from the side reaction of hydrogen and carbon monoxide products decreases the efficiency of hydrogen production. Therefore, the design of better Ni-based catalysts that do not cause methanation is necessary. A number of intermetallic compounds have excellent catalytic activity and high selectivity toward desired products. One of the catalysts is Ni3Sn [7], which has good selectivity toward hydrogen production from methanol. In the NieSn system, there are three stable intermetallic compounds, namely, Ni3Sn, Ni3Sn2, and Ni3Sn4. Their catalytic activity decreases as the Sn content of the alloy increases [8]. The activity and selectivity of Ni3Sn-based catalysts
* Corresponding author. E-mail address: [email protected] (M. Fan). http://dx.doi.org/10.1016/j.renene.2015.01.023 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
have been widely studied and used in many applications including aqueous-phase reforming of hydrocarbon. Dumesic [9] reported that addition of Sn to Ni improved its selectivity toward hydrogen production by ethylene glycol reforming, and evidence for the formation of Ni3Sn alloy associated with alumina was obtained by Mossbauer spectroscopy. Saadi [10] observed that the high hydrogen selectivity was due to the lower binding activity of the Ni3Sn surface toward CO or C than H2 under the same conditions. On the basis of X-ray photoelectron spectroscopy measurements of Sn-doped Ni/Al2O3, Padeste [11] suggested that Sn segregated to the surface and acted as an inhibitor of catalytic carbon deposition by reducing carbon solubility in Ni. However, the catalytic activity of the Ni3Sn intermetallic compound has seldom been reported to date, which may be attributed to the decreased activity after addition of Sn [12]. Alkali leaching is effective in improving the catalytic activity of some intermetallic compounds. NiAl3, Ni2Al3, and Ni3Al had high catalytic activity after leaching aluminum in a concentrated NaOH solution. Metallic Sn can be dissolved in an alkaline solution; thus, catalytic activity of alkali-leached Ni3Sn is expected to improve. In the presented study, we examined the catalytic activity of alkali-leached Ni3Sn powder for hydrogen production from methanol, which is important in fuel-cell and small-scale hydrogen plants. In this study, the reported Ni3Sn intermetallic compounds
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Fig. 1. Change in methanol conversion of products for Ni3Sn powder as a function of reaction temperature. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkalileached in 20 wt% NaOH for 24 h at 423 K.
exhibited increased methanol decomposition and suppressed methanation. 2. Experimental Ni3Sn (Ni-25 M% Sn) ingot was prepared by melting the mixture of stoichiometric amounts of Ni and Sn in a high-frequency vacuum melting furnace. The ingot was cut by a planer, pulverized by a hammer and then passed through a 75 mm sieve. Three grams of powder were stirred into 100 mL 20 wt.% NaOH aqueous solution at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h. After leaching, the
NaOH solution was subjected to inductively coupled plasma (ICP) analysis to collect the amount of leached Sn. The powder was filtered out, rinsed in deionized water several times and then airdried at 323 K for 5 h. Catalytic activity experiments were performed in a conventional fixed-bed flow. The reactor was made of a quartz tube with an 8 mm diameter. The catalyst weight was 0.4 g and methanol flow rate was 100 mL/min combined with 30 mL/min N2. The catalyst was reduced at 773 K for 1 h in a gaseous mixture of 5 mL/min N2 and 30 mL/min H2. The catalytic performances were evaluated in temperatures within 513e793 K by measuring the outlet composition of gaseous products with two gas chromatographs equipped with thermal conductivity detector (GL Science, GC 323). H2O and CH3OH were removed from the effluent gas via a cold trap after the composition and total flow rate of the outlet gases were determined by a soap bubble meter. The crystalline structure of the alkali-leached Ni3Sn before and after the reaction was characterized by X-ray diffraction (XRD) with Rigaku RINT 2500 using a CuKa source. The surface morphology and the composition of the samples were examined using scanning electron microscopy (SEM; JEOL, JSM-7000F) equipped with energy dispersive X-ray spectroscopy (EDS). The BrunauereEmmetteTeller surface area was determined with a surface area analyzer (Micromeritics, ASAP2020) using Kr as adsorbent. 3. Results and discussion 3.1. Catalytic activity Fig. 1 shows methanol conversion over Ni3Sn before and after alkali leaching as a function of the reaction temperature. The
Fig. 2. Change in production rates products for Ni3Sn powder as a function of reaction temperature. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
P. Wei et al. / Renewable Energy 78 (2015) 357e363
methanol conversion by Ni3Sn was low (8.5%), even at 793 K; however, the methanol conversion at 793 K over alkali-leached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h were 35.2%, 50.7%, and 56.8%, respectively. The values were at least four times higher than that of plain Ni3Sn. Moreover, methanol conversion over Ni3Sn increased slowly below 793 K. By contrast, the methanol conversion over alkali-leached Ni3Sn increased rapidly in the temperature range of 593e793 K. Fig. 2 shows the production rates of H2, CO, CH4, CO2, and H2O as a function of the reaction temperature of Ni3Sn before and after alkali leaching. The effect of leaching on the rate of product formation was significant. Ni3Sn had limited catalytic activity without leaching. The rate of H2 and CO production gradually increased with increasing reaction temperature. The maximum H2 production rate of 21 103 mol h1 g-Cat1 was obtained at 793 K. However, the H2 and CO production rates over the alkali-leached Ni3Sn increased rapidly under the same conditions. The H2 production rate at 793 K over alkali-leached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h was 107.5, 186.8, and 215.2 mol h1 g-Cat1, respectively. The values were at least five times higher than that over Ni3Sn alone, which indicates that the catalytic activity of Ni3Sn was markedly enhanced by alkali leaching, and the increase in activity is proportional to the duration of leaching. The production rates of the minor by-products CO2, CH4, and H2O over the alkali-leached Ni3Sn were still low in the whole temperature range, and their values slightly increased with increasing leaching extent compared with those over Ni3Sn. The leaching effect can also be confirmed by the evolution of apparent activation energy, which was calculated using the experimental rate of methanol decomposition at different
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temperatures. The temperature dependence of the rate constant, k, is expressed by the following Arrhenius equation:
k ¼ k0 exp E Rg T
(1)
The rates of methanol decomposition and hydrogen production are considered identical because methanol almost decomposed to H2 and CO at 673e793 K. The relationship between In [H2] and 1000/T is shown in Fig. 3. The slope was Ea/R and the apparent activation energy Ea of methanol decomposition over alkalileached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h was 61.1, 52.8, 51.9, and 50.0 kJ mol1, respectively. The activation energy of Ni3Sn clearly decreased with the increase in the extent of leaching. Thus, the alkali-leached Ni3Sn was more catalytically active than Ni3Sn in methanol decomposition. Fig. 4 shows methanol conversion over Ni3Sn before and after alkali leaching as a function of reaction time at 713 K. Methanol conversion over Ni3Sn was very low within 45 h of reaction at 713 K. The largest value obtained was 2.7% after 45 h of reaction. However, the methanol conversions over alkali-leached Ni3Sn at 423 K for 5 h and 423 K for 24 were 24.3% and 16.1%, respectively, under the same conditions. There was a progression of methanol conversion over alkali-leached Ni3Sn, whereas a slow and continuous increase in methanol conversion was observed over Ni3Sn. Over alkali-leached Ni3Sn at 423 K for 5 h, there was a rapid initial increase in methanol conversion with a maximum value of 29.4% followed by a gradual decrease in conversion, which suggests that activation and deactivation occurred in succession during the reaction. Thus, methanol conversion can be maintained at 24%. The alkali-leached Ni3Sn solution at 423 K for 24 h had the highest
Fig. 3. Arrhenius plot for methanol decomposition catalyzed by alkali-leached Ni3Sn. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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products CH4, CO2, and H2O had low production rates. At the beginning of the reaction, the maximum value obtained was below 5 103 mol h1 g-cat1 followed by a rapid decrease in rate. Selectivity of H2 and the other carbon-containing reaction products CO, CH4, CO2 were calculated according to the methods in the paper [13]. The results are shown in Fig. 6. The selectivity of main products H2 and CO over the alkali-leached Ni3Sn had an obvious increase at the initial period, compared to those of Ni3Sn. Their values were up to approximately 95% and kept at the value in the whole reaction. On the contrary, the selectivity of minor products CH4, CO2 and H2O over the alkali-leached Ni3Sn had a sharp decrease in the beginning and kept at low values in the while reaction. Fig. 4. Change in methanol conversion for the alkali-leached Ni3Sn powders as a function of reaction time at 713 K a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt % NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
methanol conversion at 43.4% in the beginning, and the methanol conversion decreased rapidly within 2 h of reaction followed by a gradual decrease. The trend in the production rates of H2 and CO in Fig. 5 was similar to that of methanol conversion. The production rates of H2 and CO over alkali-leached Ni3Sn at 423 K for 5 h and 423 K for 24 h presented high values above 40 103 and 20 103 mol h1 gcat1, respectively, in the whole reaction region. The values were at least six times higher than those over Ni3Sn. The calculated H2/CO molar ratio over alkali-leached Ni3Sn was close to 2; thus, the minor
3.2. Surface characterization The ICP results of the NaOH solution after leaching showed that Sn metal was selectively leached from Ni3Sn powder, but the leached amount of Sn was very minimal (Table 1). The largest amount of leached Sn was estimated at 0.64 wt% of Sn concentration in the precursor Ni3Sn (Ni-25 wt% Sn) after 24 h of leaching at 423 K. Nickel was not collected from the solution. The specific surface area had a slight increase after alkali leaching (Table 1) and the value increased less than twice by leaching. However, there was no increase in the specific surface area with increased leaching. Fig. 7 shows XRD results of Ni3Sn before and after leaching. All the diffraction peaks were identified as those of the Ni3Sn (Hexagonal, P63/mmc) single-phase powder preparation. No
Fig. 5. Production rates of gaseous products in methanol decomposition at 713 K for the alkali-leached Ni3Sn powder as a function of reaction time. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
P. Wei et al. / Renewable Energy 78 (2015) 357e363
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Fig. 6. Selectivity of H2, CO, CH4, H2O and CO2 during methanol decomposition for the alkali-leached Ni3Sn powder as a function of reaction time. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
significant differences were observed in the XRD patterns of Ni3Sn before and after alkali leaching. The slight changes cannot be detected by XRD, but these were excellently shown in SEM images of the Ni3Sn surface (Fig. 8). Before alkali leaching, Ni3Sn had different particle sizes and numerous small grains were attached on the surface of large particles. After alkali leaching, a considerable number of nanoparticles below 100 nm in diameter were observed. Tables 2 and 3 show EDS results of the Ni3Sn surface before and after alkali leaching. The Ni/Sn molar ratio of spots on the Ni3Sn surface was limited to approximately 3. However, the Ni/Sn molar ratio of spots on alkali-leached Ni3Sn surface deviated from 3 and some values were even higher than 4. The results show that leaching of Sn was only limited to the very thin surface layer of the
Ni3Sn powder, leaving the bulk as it was. In the case of the nickel catalyst f.c.c., Ni phase was generated on the surface as a result of alkali leaching. Combined with the results in Fig. 1, the improvement in catalytic activity of alkali-leached Ni3Sn probably resulted from Ni nanoparticles generated on the Ni3Sn surface. Higher leaching extent resulted in more Ni nanoparticles and, thereby, higher catalytic activity.
Table 1 Results of ICP and BET surface area of Ni3Sn before and after 45 h of reaction at 713 K. Pretreatment process
Ni3Sn as-prepared Ni3Sn in 100 ml 20 wt% NaOH for 5 h at 363 K Ni3Sn in 100 ml 20 wt% NaOH for 5 h at 423 K Ni3Sn in 100 ml 20 wt% NaOH for 24 h at 423 K
ICP results-amount of leached elements (wt%)
Specific surface area (m2/g)
Ni
Sn
Before reaction
After reaction
0 0
0 0.36
0.07 0.13
0.09
0
0.48
0.13
4.1
0
0.64
0.13
4.7
Fig. 7. XRD patterns of the Ni3Sn powder before and after alkali leaching. a, Ni3Sn asprepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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Fig. 8. Surface SEM images and EDS of Ni3Sn powder before and after alkali leaching. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
The C and O contents over alkali-leached Ni3Sn were significantly higher than those over Ni3Sn surface after 45 h of reaction (Fig. 9, Table 4, and Table 5). The surface of alkali-leached Ni3Sn apparently has active sites of Ni nanoparticles and Ni3Sn. Ni nanoparticles had high catalytic activity in methanol decomposition, and it had a significantly higher ability than Ni3Sn to stimulate the formation of C and NiO in methanol decomposition [9]. The specific surface area of alkali-leached Ni3Sn after 45 h of reaction was 30-fold of Ni3Sn under the same conditions (Table 1), which reflected that C and NiO were mainly produced on the Ni surface. The coke formation of C and NiO can physically cover the Ni surface and lead to a decrease in Ni activity [9,14,15]. However, high production rates stimulated by Ni nanoparticles resulted in the formation of more fine Ni3Sn particles during the reaction, which might improve the catalytic activity. Therefore, the decreased activity of Ni and the increased activity of Ni3Sn occurred simultaneously during the reaction. The former was determined over alkali-leached Ni3Sn at the beginning of the reaction, and the latter increased gradually during the reaction (Figs. 4e6). The activity changes depended on the molar ratio of Ni and Ni3Sn on the surface of the alkali-leached Ni3Sn. Large Ni/Ni3Sn molar ratios result in a high initial activity
Table 2 EDS results of Ni3Sn before alkali leaching.
followed by a drastic decline. The Ni particles were generated from alkali leaching. Thus, the Ni/Ni3Sn molar ratio depended on leaching extent, which was evidenced with the variations in activity of the different alkali-leached Ni3Sn. Therefore, a suitable leaching extent was determined to obtain optimal alkali-leached Ni3Sn activity. 4. Conclusion The catalytic activity of Ni3Sn before and after alkali leaching was studied for hydrogen production from methanol in the temperature range of 513e793 K. The results showed that Ni3Sn before alkali leaching had a limited catalytic activity in methanol decomposition, but its activity can be significantly improved by alkali leaching. The alkali-leached Ni3Sn samples presented both higher catalytic activity and H2 selectivity in methanol decomposition than those of Ni3Sn below 793 K. The catalytic activity over alkali-leached Ni3Sn increased rapidly with increased leaching extent. The improvement was mainly attributed to the Ni nanoparticles produced on the Ni3Sn surface in the alkali-leaching process and the fine Ni3Sn particles generated during the reaction. The results exhibited that the alkali-leached Ni3Sn is a promising catalyst for hydrogen production.
Table 3 EDS results of Ni3Sn alkali-leached at 423 K for 24 h.
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
56.6 57.8 59.6 57.7 52.8 57.1 54.9 52.9 59.9
19.4 18.6 19.7 19.8 17.9 20.0 17.7 18.0 20.2
14.0 14.0 8.9 11.9 14.6 12.3 15.8 14.8 7.7
10.0 9.6 11.6 10.6 14.7 10.6 11.6 14.3 12.2
2.9 3.1 3.0 2.9 2.9 2.9 3.1 2.9 3.0
1 2 3 4 5 6 7 8 9
48.7 47.6 52.6 54.2 53.7 48.1 46.3 44.8 48.8
12.5 11.8 13.9 12.5 12.0 12.7 11.9 11.7 12.9
19.0 21.5 21.2 19.6 18.2 26.3 26.1 27.9 22.5
19.8 19.1 12.3 13.7 16.1 12.9 15.7 15.6 15.7
3.9 4.0 3.8 4.3 4.5 3.8 3.9 3.8 3.8
P. Wei et al. / Renewable Energy 78 (2015) 357e363
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Fig. 9. SEM images and EDS of alkali-leached Ni3Sn after 45 h of reaction at 713 K a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 24 h at 423 K.
Table 4 EDS results of Ni3Sn after 45 h of reaction at 713 K.
References
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
37.7 40.9 41.7 40.0 46.5 39.3 39.5 40.7 35.1
12.1 12.7 13.7 13.1 15.0 12.5 13.0 13.0 12.0
42.0 42.3 41.9 42.8 34.7 45.6 44.6 43.7 46.4
8.2 4.1 2.7 4.1 3.9 2.6 2.9 2.6 6.1
3.1 3.2 3.0 3.0 3.1 3.1 3.0 3.1 2.9
Table 5 EDS results of alkali-leached Ni3Sn after 45 h of reaction at 713 K. The sample was alkali-leached at 423 K for 24 h. Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
23.1 22.6 23.9 22.2 30.9 28.7 30.2 30.3 29.9
5.6 6.0 6.0 6.1 8.8 7.6 8.4 8.6 8.0
56.1 56.1 59.2 59.6 51.6 53.8 48.3 49.2 49.4
15.2 15.3 10.9 12.1 8.7 9.9 13.1 11.9 12.7
4.1 3.8 4.0 3.7 3.5 3.7 3.6 3.5 3.7
Acknowledgments This work was financially supported by research fund of key laboratory for advanced technology in environmental projection of Jiangsu province and Guangxi Key Laboratory of Information Materials (Guilin University of Electronic Technology), China (Project No. 1210908-02-K).
[1] Brown JC, Gulari E. Hydrogen production from methanol decomposition over Pt/Al2O3 and ceria promoted Pt/Al2O3 catalysts. Catal Commun 2004;5:431e6. [2] Alenzi N, Liao WS, Cremer PS, Torres VS, Wood TK, Cheng ZD. Photoelectrochemical hydrogen production from water/methanol decomposition using Ag/TiO2 nanocomposite thin films. Int J Hydrogen Energy 2010;35: 11768e75. [3] Nakagawa K, Hashida T, Kajita C, Ikenaga N, Kobayashi T, Gamo MN, et al. Catal Lett 2002;80:161e4. [4] Xu Y, Kameoka S, Kishida K, Demura M, Tsai AP, Hirano T. Catalytic properties of Ni3Al intermetallic for methanol decomposition. Mater Trans 2004;45:3177e9. [5] Ohashi T, Wang Y, Shimada S. Preparation and high catalytic performance of hollow BN spheres-supported Ni for hydrogen production from methanol. J Mater Chem 2010;20:5129e35. [6] Tsoncheva T, Mavrodinova V, Ivanova L, Dimitrov M, Stavrev S, Minchev C. Nickel modified ultrananosized diamonds and their application as catalysts in methanol decomposition. J Mol Catal A: Chem 2006;259:223e30. [7] Penkova A, Bobadilla L, Ivanova S, Dominguez MI, Romero-Sarria F, Roger AC, et al. Hydrogen production by methanol steam reforming on NiSn/MgO-Al2O3 catalyst: the role of MgO addition. Appl Catal A: General 2011;392:184e91. [8] Onda A, Komatsu T, Yashima T. Characterization and catalytic properties of NiSn intermetallic compounds in acetylene hydrogenation. Phys Chem Chem Phys 2000;2:2999e3005. [9] Shabaker JW, Huber GW, Dumesic JA. Aqueous-phase reforming of oxygenated hydrocarbons over Sn-modified Ni catalysts. J Catal 2004;222:180e91. [10] Saadi S, Hinnemann B, Helveg S, Appel CC, Abild-Pedersen F, Norskov JK. Firstprinciples investigations of the Ni3Sn alloy at steam reforming conditions. Surf Sci 2009;603:762e70. [11] Padeste C, Trimm DL, Lamb RN. Characterization of Sn doped Ni/Al2O3 steam reforming catalysts by XPS. Catal Lett 1993;17:333e9. [12] Omata K, Yamada M. Prediction of effective additives to a Ni/active carbon catalyst for vapour-phase carbonylation of methanol by an artificial neural net work. Ind Eng Chem Res 2004;43:6622e5. [13] Bokx PKD, Balkenende AR, Geus JW. The mechanism and kinetics of methane formation by decomposition of methanol on a NiSnO2 catalyst. J Catal 1989;117:467e84. [14] Istadi I, Anggoro D, Amin NAS, Ling DHW. Catalyst deactivation simulation through catbon deposition in carbon dioxide reforming over Ni/Cao-Al2O3 catalyst. Bull Chem React Eng Catal 2011;6(2):129e36. [15] Yao SL, Yang CH, Tan YS, Han YZ. Deactivation and regeneration of an activated carbon-supported nickel catalyst for methanol carbonylation in the vapour phase. Catal Commun 2008;9:2107e11.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Single-phase Ni3Sn alloy alkali-leached for hydrogen production from methanol decomposition Pan Wei, Wei Xia, Jian Zhu Li, Haifei Long, Jindan Chen, Ting Li, Meiqiang Fan* Department of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2014 Accepted 9 January 2015 Available online
Methanol decomposition over alkali-leached Ni3Sn powder at 513e793 K was investigated. Compared with untreated Ni3Sn, alkali-leached Ni3Sn had high catalytic activity and selectivity toward H2 and CO production above 633 K. A maximum H2 production rate of 100 103 mol h1 g-Cat1 and H2 selectivity above 95% were attained over alkali-leached Ni3Sn at 793 K. Alkali-leached Ni3Sn presented good catalytic activity for 45 h of reaction at 713 K, whereas Ni3Sn had none. The activation energy was calculated, and its values rapidly decreased from Ni3Sn to alkali-leached ones. The improvement was attributed to the formation of Ni nanoparticles less than 100 nm in diameter in the alkali-leaching process, which had high activity for methanol decomposition. The improved catalytic activity favored the gradual formation of fine Ni3Sn particle during the reaction, which served as the active sites for methanol decomposition when the catalytic activity decreased because of carbon deposition on the Ni surface. Results demonstrated that alkali-leached Ni3Sn was a promising potential catalyst for hydrogen production from methanol. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Methanol decomposition Intermetallic compounds and alloys Hydrogen production
1. Introduction Hydrogen production from methanol decomposition is attracting considerable attention as methanol has many advantageous properties such as low cost, high H/C molar ratio, and so on [1e3]. A great deal of effort has been given in the development of an efficient low cost catalyst for methanol decomposition. Ni-based catalysts are common and their performance has been significantly improved in the past 30 years [4e6]. However, the formation of methane from the side reaction of hydrogen and carbon monoxide products decreases the efficiency of hydrogen production. Therefore, the design of better Ni-based catalysts that do not cause methanation is necessary. A number of intermetallic compounds have excellent catalytic activity and high selectivity toward desired products. One of the catalysts is Ni3Sn [7], which has good selectivity toward hydrogen production from methanol. In the NieSn system, there are three stable intermetallic compounds, namely, Ni3Sn, Ni3Sn2, and Ni3Sn4. Their catalytic activity decreases as the Sn content of the alloy increases [8]. The activity and selectivity of Ni3Sn-based catalysts
* Corresponding author. E-mail address: [email protected] (M. Fan). http://dx.doi.org/10.1016/j.renene.2015.01.023 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
have been widely studied and used in many applications including aqueous-phase reforming of hydrocarbon. Dumesic [9] reported that addition of Sn to Ni improved its selectivity toward hydrogen production by ethylene glycol reforming, and evidence for the formation of Ni3Sn alloy associated with alumina was obtained by Mossbauer spectroscopy. Saadi [10] observed that the high hydrogen selectivity was due to the lower binding activity of the Ni3Sn surface toward CO or C than H2 under the same conditions. On the basis of X-ray photoelectron spectroscopy measurements of Sn-doped Ni/Al2O3, Padeste [11] suggested that Sn segregated to the surface and acted as an inhibitor of catalytic carbon deposition by reducing carbon solubility in Ni. However, the catalytic activity of the Ni3Sn intermetallic compound has seldom been reported to date, which may be attributed to the decreased activity after addition of Sn [12]. Alkali leaching is effective in improving the catalytic activity of some intermetallic compounds. NiAl3, Ni2Al3, and Ni3Al had high catalytic activity after leaching aluminum in a concentrated NaOH solution. Metallic Sn can be dissolved in an alkaline solution; thus, catalytic activity of alkali-leached Ni3Sn is expected to improve. In the presented study, we examined the catalytic activity of alkali-leached Ni3Sn powder for hydrogen production from methanol, which is important in fuel-cell and small-scale hydrogen plants. In this study, the reported Ni3Sn intermetallic compounds
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P. Wei et al. / Renewable Energy 78 (2015) 357e363
Fig. 1. Change in methanol conversion of products for Ni3Sn powder as a function of reaction temperature. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkalileached in 20 wt% NaOH for 24 h at 423 K.
exhibited increased methanol decomposition and suppressed methanation. 2. Experimental Ni3Sn (Ni-25 M% Sn) ingot was prepared by melting the mixture of stoichiometric amounts of Ni and Sn in a high-frequency vacuum melting furnace. The ingot was cut by a planer, pulverized by a hammer and then passed through a 75 mm sieve. Three grams of powder were stirred into 100 mL 20 wt.% NaOH aqueous solution at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h. After leaching, the
NaOH solution was subjected to inductively coupled plasma (ICP) analysis to collect the amount of leached Sn. The powder was filtered out, rinsed in deionized water several times and then airdried at 323 K for 5 h. Catalytic activity experiments were performed in a conventional fixed-bed flow. The reactor was made of a quartz tube with an 8 mm diameter. The catalyst weight was 0.4 g and methanol flow rate was 100 mL/min combined with 30 mL/min N2. The catalyst was reduced at 773 K for 1 h in a gaseous mixture of 5 mL/min N2 and 30 mL/min H2. The catalytic performances were evaluated in temperatures within 513e793 K by measuring the outlet composition of gaseous products with two gas chromatographs equipped with thermal conductivity detector (GL Science, GC 323). H2O and CH3OH were removed from the effluent gas via a cold trap after the composition and total flow rate of the outlet gases were determined by a soap bubble meter. The crystalline structure of the alkali-leached Ni3Sn before and after the reaction was characterized by X-ray diffraction (XRD) with Rigaku RINT 2500 using a CuKa source. The surface morphology and the composition of the samples were examined using scanning electron microscopy (SEM; JEOL, JSM-7000F) equipped with energy dispersive X-ray spectroscopy (EDS). The BrunauereEmmetteTeller surface area was determined with a surface area analyzer (Micromeritics, ASAP2020) using Kr as adsorbent. 3. Results and discussion 3.1. Catalytic activity Fig. 1 shows methanol conversion over Ni3Sn before and after alkali leaching as a function of the reaction temperature. The
Fig. 2. Change in production rates products for Ni3Sn powder as a function of reaction temperature. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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methanol conversion by Ni3Sn was low (8.5%), even at 793 K; however, the methanol conversion at 793 K over alkali-leached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h were 35.2%, 50.7%, and 56.8%, respectively. The values were at least four times higher than that of plain Ni3Sn. Moreover, methanol conversion over Ni3Sn increased slowly below 793 K. By contrast, the methanol conversion over alkali-leached Ni3Sn increased rapidly in the temperature range of 593e793 K. Fig. 2 shows the production rates of H2, CO, CH4, CO2, and H2O as a function of the reaction temperature of Ni3Sn before and after alkali leaching. The effect of leaching on the rate of product formation was significant. Ni3Sn had limited catalytic activity without leaching. The rate of H2 and CO production gradually increased with increasing reaction temperature. The maximum H2 production rate of 21 103 mol h1 g-Cat1 was obtained at 793 K. However, the H2 and CO production rates over the alkali-leached Ni3Sn increased rapidly under the same conditions. The H2 production rate at 793 K over alkali-leached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h was 107.5, 186.8, and 215.2 mol h1 g-Cat1, respectively. The values were at least five times higher than that over Ni3Sn alone, which indicates that the catalytic activity of Ni3Sn was markedly enhanced by alkali leaching, and the increase in activity is proportional to the duration of leaching. The production rates of the minor by-products CO2, CH4, and H2O over the alkali-leached Ni3Sn were still low in the whole temperature range, and their values slightly increased with increasing leaching extent compared with those over Ni3Sn. The leaching effect can also be confirmed by the evolution of apparent activation energy, which was calculated using the experimental rate of methanol decomposition at different
359
temperatures. The temperature dependence of the rate constant, k, is expressed by the following Arrhenius equation:
k ¼ k0 exp E Rg T
(1)
The rates of methanol decomposition and hydrogen production are considered identical because methanol almost decomposed to H2 and CO at 673e793 K. The relationship between In [H2] and 1000/T is shown in Fig. 3. The slope was Ea/R and the apparent activation energy Ea of methanol decomposition over alkalileached Ni3Sn at 363 K for 5 h, 423 K for 5 h, or 423 K for 24 h was 61.1, 52.8, 51.9, and 50.0 kJ mol1, respectively. The activation energy of Ni3Sn clearly decreased with the increase in the extent of leaching. Thus, the alkali-leached Ni3Sn was more catalytically active than Ni3Sn in methanol decomposition. Fig. 4 shows methanol conversion over Ni3Sn before and after alkali leaching as a function of reaction time at 713 K. Methanol conversion over Ni3Sn was very low within 45 h of reaction at 713 K. The largest value obtained was 2.7% after 45 h of reaction. However, the methanol conversions over alkali-leached Ni3Sn at 423 K for 5 h and 423 K for 24 were 24.3% and 16.1%, respectively, under the same conditions. There was a progression of methanol conversion over alkali-leached Ni3Sn, whereas a slow and continuous increase in methanol conversion was observed over Ni3Sn. Over alkali-leached Ni3Sn at 423 K for 5 h, there was a rapid initial increase in methanol conversion with a maximum value of 29.4% followed by a gradual decrease in conversion, which suggests that activation and deactivation occurred in succession during the reaction. Thus, methanol conversion can be maintained at 24%. The alkali-leached Ni3Sn solution at 423 K for 24 h had the highest
Fig. 3. Arrhenius plot for methanol decomposition catalyzed by alkali-leached Ni3Sn. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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products CH4, CO2, and H2O had low production rates. At the beginning of the reaction, the maximum value obtained was below 5 103 mol h1 g-cat1 followed by a rapid decrease in rate. Selectivity of H2 and the other carbon-containing reaction products CO, CH4, CO2 were calculated according to the methods in the paper [13]. The results are shown in Fig. 6. The selectivity of main products H2 and CO over the alkali-leached Ni3Sn had an obvious increase at the initial period, compared to those of Ni3Sn. Their values were up to approximately 95% and kept at the value in the whole reaction. On the contrary, the selectivity of minor products CH4, CO2 and H2O over the alkali-leached Ni3Sn had a sharp decrease in the beginning and kept at low values in the while reaction. Fig. 4. Change in methanol conversion for the alkali-leached Ni3Sn powders as a function of reaction time at 713 K a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt % NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
methanol conversion at 43.4% in the beginning, and the methanol conversion decreased rapidly within 2 h of reaction followed by a gradual decrease. The trend in the production rates of H2 and CO in Fig. 5 was similar to that of methanol conversion. The production rates of H2 and CO over alkali-leached Ni3Sn at 423 K for 5 h and 423 K for 24 h presented high values above 40 103 and 20 103 mol h1 gcat1, respectively, in the whole reaction region. The values were at least six times higher than those over Ni3Sn. The calculated H2/CO molar ratio over alkali-leached Ni3Sn was close to 2; thus, the minor
3.2. Surface characterization The ICP results of the NaOH solution after leaching showed that Sn metal was selectively leached from Ni3Sn powder, but the leached amount of Sn was very minimal (Table 1). The largest amount of leached Sn was estimated at 0.64 wt% of Sn concentration in the precursor Ni3Sn (Ni-25 wt% Sn) after 24 h of leaching at 423 K. Nickel was not collected from the solution. The specific surface area had a slight increase after alkali leaching (Table 1) and the value increased less than twice by leaching. However, there was no increase in the specific surface area with increased leaching. Fig. 7 shows XRD results of Ni3Sn before and after leaching. All the diffraction peaks were identified as those of the Ni3Sn (Hexagonal, P63/mmc) single-phase powder preparation. No
Fig. 5. Production rates of gaseous products in methanol decomposition at 713 K for the alkali-leached Ni3Sn powder as a function of reaction time. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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Fig. 6. Selectivity of H2, CO, CH4, H2O and CO2 during methanol decomposition for the alkali-leached Ni3Sn powder as a function of reaction time. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
significant differences were observed in the XRD patterns of Ni3Sn before and after alkali leaching. The slight changes cannot be detected by XRD, but these were excellently shown in SEM images of the Ni3Sn surface (Fig. 8). Before alkali leaching, Ni3Sn had different particle sizes and numerous small grains were attached on the surface of large particles. After alkali leaching, a considerable number of nanoparticles below 100 nm in diameter were observed. Tables 2 and 3 show EDS results of the Ni3Sn surface before and after alkali leaching. The Ni/Sn molar ratio of spots on the Ni3Sn surface was limited to approximately 3. However, the Ni/Sn molar ratio of spots on alkali-leached Ni3Sn surface deviated from 3 and some values were even higher than 4. The results show that leaching of Sn was only limited to the very thin surface layer of the
Ni3Sn powder, leaving the bulk as it was. In the case of the nickel catalyst f.c.c., Ni phase was generated on the surface as a result of alkali leaching. Combined with the results in Fig. 1, the improvement in catalytic activity of alkali-leached Ni3Sn probably resulted from Ni nanoparticles generated on the Ni3Sn surface. Higher leaching extent resulted in more Ni nanoparticles and, thereby, higher catalytic activity.
Table 1 Results of ICP and BET surface area of Ni3Sn before and after 45 h of reaction at 713 K. Pretreatment process
Ni3Sn as-prepared Ni3Sn in 100 ml 20 wt% NaOH for 5 h at 363 K Ni3Sn in 100 ml 20 wt% NaOH for 5 h at 423 K Ni3Sn in 100 ml 20 wt% NaOH for 24 h at 423 K
ICP results-amount of leached elements (wt%)
Specific surface area (m2/g)
Ni
Sn
Before reaction
After reaction
0 0
0 0.36
0.07 0.13
0.09
0
0.48
0.13
4.1
0
0.64
0.13
4.7
Fig. 7. XRD patterns of the Ni3Sn powder before and after alkali leaching. a, Ni3Sn asprepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
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Fig. 8. Surface SEM images and EDS of Ni3Sn powder before and after alkali leaching. a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 363 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 5 h at 423 K; d, Ni3Sn alkali-leached in 20 wt% NaOH for 24 h at 423 K.
The C and O contents over alkali-leached Ni3Sn were significantly higher than those over Ni3Sn surface after 45 h of reaction (Fig. 9, Table 4, and Table 5). The surface of alkali-leached Ni3Sn apparently has active sites of Ni nanoparticles and Ni3Sn. Ni nanoparticles had high catalytic activity in methanol decomposition, and it had a significantly higher ability than Ni3Sn to stimulate the formation of C and NiO in methanol decomposition [9]. The specific surface area of alkali-leached Ni3Sn after 45 h of reaction was 30-fold of Ni3Sn under the same conditions (Table 1), which reflected that C and NiO were mainly produced on the Ni surface. The coke formation of C and NiO can physically cover the Ni surface and lead to a decrease in Ni activity [9,14,15]. However, high production rates stimulated by Ni nanoparticles resulted in the formation of more fine Ni3Sn particles during the reaction, which might improve the catalytic activity. Therefore, the decreased activity of Ni and the increased activity of Ni3Sn occurred simultaneously during the reaction. The former was determined over alkali-leached Ni3Sn at the beginning of the reaction, and the latter increased gradually during the reaction (Figs. 4e6). The activity changes depended on the molar ratio of Ni and Ni3Sn on the surface of the alkali-leached Ni3Sn. Large Ni/Ni3Sn molar ratios result in a high initial activity
Table 2 EDS results of Ni3Sn before alkali leaching.
followed by a drastic decline. The Ni particles were generated from alkali leaching. Thus, the Ni/Ni3Sn molar ratio depended on leaching extent, which was evidenced with the variations in activity of the different alkali-leached Ni3Sn. Therefore, a suitable leaching extent was determined to obtain optimal alkali-leached Ni3Sn activity. 4. Conclusion The catalytic activity of Ni3Sn before and after alkali leaching was studied for hydrogen production from methanol in the temperature range of 513e793 K. The results showed that Ni3Sn before alkali leaching had a limited catalytic activity in methanol decomposition, but its activity can be significantly improved by alkali leaching. The alkali-leached Ni3Sn samples presented both higher catalytic activity and H2 selectivity in methanol decomposition than those of Ni3Sn below 793 K. The catalytic activity over alkali-leached Ni3Sn increased rapidly with increased leaching extent. The improvement was mainly attributed to the Ni nanoparticles produced on the Ni3Sn surface in the alkali-leaching process and the fine Ni3Sn particles generated during the reaction. The results exhibited that the alkali-leached Ni3Sn is a promising catalyst for hydrogen production.
Table 3 EDS results of Ni3Sn alkali-leached at 423 K for 24 h.
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
56.6 57.8 59.6 57.7 52.8 57.1 54.9 52.9 59.9
19.4 18.6 19.7 19.8 17.9 20.0 17.7 18.0 20.2
14.0 14.0 8.9 11.9 14.6 12.3 15.8 14.8 7.7
10.0 9.6 11.6 10.6 14.7 10.6 11.6 14.3 12.2
2.9 3.1 3.0 2.9 2.9 2.9 3.1 2.9 3.0
1 2 3 4 5 6 7 8 9
48.7 47.6 52.6 54.2 53.7 48.1 46.3 44.8 48.8
12.5 11.8 13.9 12.5 12.0 12.7 11.9 11.7 12.9
19.0 21.5 21.2 19.6 18.2 26.3 26.1 27.9 22.5
19.8 19.1 12.3 13.7 16.1 12.9 15.7 15.6 15.7
3.9 4.0 3.8 4.3 4.5 3.8 3.9 3.8 3.8
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Fig. 9. SEM images and EDS of alkali-leached Ni3Sn after 45 h of reaction at 713 K a, Ni3Sn as-prepared; b, Ni3Sn alkali-leached in 20 wt% NaOH for 5 h at 423 K; c, Ni3Sn alkalileached in 20 wt% NaOH for 24 h at 423 K.
Table 4 EDS results of Ni3Sn after 45 h of reaction at 713 K.
References
Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
37.7 40.9 41.7 40.0 46.5 39.3 39.5 40.7 35.1
12.1 12.7 13.7 13.1 15.0 12.5 13.0 13.0 12.0
42.0 42.3 41.9 42.8 34.7 45.6 44.6 43.7 46.4
8.2 4.1 2.7 4.1 3.9 2.6 2.9 2.6 6.1
3.1 3.2 3.0 3.0 3.1 3.1 3.0 3.1 2.9
Table 5 EDS results of alkali-leached Ni3Sn after 45 h of reaction at 713 K. The sample was alkali-leached at 423 K for 24 h. Spot no
Ni (at%)
Sn (at%)
C (at%)
O (at%)
Ni/Sn
1 2 3 4 5 6 7 8 9
23.1 22.6 23.9 22.2 30.9 28.7 30.2 30.3 29.9
5.6 6.0 6.0 6.1 8.8 7.6 8.4 8.6 8.0
56.1 56.1 59.2 59.6 51.6 53.8 48.3 49.2 49.4
15.2 15.3 10.9 12.1 8.7 9.9 13.1 11.9 12.7
4.1 3.8 4.0 3.7 3.5 3.7 3.6 3.5 3.7
Acknowledgments This work was financially supported by research fund of key laboratory for advanced technology in environmental projection of Jiangsu province and Guangxi Key Laboratory of Information Materials (Guilin University of Electronic Technology), China (Project No. 1210908-02-K).
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