Hydrogen is an environmentally friendly energy source, with a very high energy density and emitting only water after its oxidation [1-3]. In spite of these prominent advantages, the storage and transportation of H2 is still challenging [2, 4]. Therefore, many hydrides which can store hydrogen readily under mild conditions have been developed [5, 6]. For example, NaBH4 is a typical safe hydride with reliable stability [7]. It possesses impressive hydrogen storage capacity [8], with the hydrolysis of 1 g NaBH4 producing 0.21 g H2:
(1) |
Although the amount of H2 provided by NaBH4 is considerable, it has been well known since its original discovery that the aqueous NaBH4 solution has remarkable stability to hydrolysis [1, 9]. Thus, appropriate catalysts with excellent catalytic activity for promoting NaBH4 hydrolysis are highly required.
Herbert C. Brown and Charles A. Brown investigated the catalytic activity of various metal catalysts on the hydrolysis reaction of NaBH4, and found that Ru released H2 more rapidly than Pt, Pd, Fe, Co, Ni, etc. [10]. In the last years, great progress has been made on developing catalysts for NaBH4 hydrolysis [3, 11-18]. And many catalysts based on non-noble metals [19-32] or non-metal materials [33, 34] with impressive catalytic activity have been reported, while their performance is still below their Ru-based counterparts [35-41]. However, as a noble metal, the scarcity and high cost of Ru are obstacles on the way to industrial applications [3]. It is therefore highly desirable to improve furtherly the catalytic performance of Ru catalysts for NaBH4 hydrolysis. For this reason, Ru catalysts have been widely investigated for decades [3]. Ru nanoparticles, which have high catalytic activity for NaBH4 hydrolysis due to high proportion of surface Ru atoms, have attracted more attention of researchers [42-47]. However, it is well-known that nanoparticles tend to aggregate during catalysis process, resulting in loss of catalytic activity. Therefore, Ru nanoparticles were often deposited on varieties of substrates, such as ZIF-67 nanoparticles [42], graphene [43], Al2O3 pellets [44], nickel foam [45], TiO2 nanoparticles [46] and PEDOT [47], to improve their stability in NaBH4 hydrolysis. These substrates provided promising choices for Ru catalysts, nevertheless, new substrates with special structures, high surface area, and easy fabrication with low cost are still desperately desired.
Chitin is the second most abundant naturally produced biopolymer behind cellulose. It is biodegradable and renewable. Nanofibrous chitin materials could be fabricated through a dissolution-precipitation method with NaOH/urea aqueous solution as solvent [48, 49]. This kind of nanofibrous chitin materials has nanoporous structure and high surface area, providing excellent substrates for metal catalysts. In this work, chitin-supported Ru (Ru/chitin) catalysts composed of ultrasmall Ru nanoparticles supported on chitin nanofibrous substrate (Fig. S1 in Supporting information) were fabricated through a cheap and easy method. The structure of the catalysts and their Ru content were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma optical emission spectrometry (ICP-OES). The effects of Ru content of the catalysts, NaBH4 dosage, and reaction temperature on NaBH4 hydrolysis were discussed by measuring H2 generation rates under different conditions. The experiment results showed that the fabricated catalysts could provide a H2 generation rate as high as 55.29 L min -1
As shown in Fig. S2 (Supporting information), the chitin substrate used in this work had distinct nanoporous structure. The Brunauer-Emmett-Teller (BET) surface measurement showed that it had a surface area as high as 168 m2/g, providing huge space for Ru nanoparticles to anchor.
To fabricate the Ru catalysts, Ru3+ was firstly adsorbed onto the surface of nanoporous chitin substrate, and then was reduced to Ru nanoparticles. By controlling the amount of Ru3+ used during the fabrication process, Ru/chitin catalysts with Ru content of 0.07 wt%, 0.18 wt%, 0.29 wt%, 0.59 wt% and 0.93 wt% were obtained. These catalysts were identified as Ru-0.07, Ru-0.18, Ru-0.29, Ru-0.59, and Ru-0.93 in the following text, respectively.
As shown in Figs. 1a and b, the fabricated Ru/chitin catalysts had distinct nanoporous structure. However, the morphology of the fabricated catalysts was a little different from the raw substrate material (Fig. S2 in Supporting information). It was probably caused by slight aggregation during catalyst fabrication. The backbones of the catalysts had a crosslinked fiber-like structure. The fiber diameter was from tens to hundreds of nanometers, and the size of the nanopores among the fibers was from tens to hundreds of nanometers as well. It was consistent with the adsorption average pore diameter of 66 nm obtained from BET test. This kind of nanoporous fibrous structure had advantages in high surface area and in high porosity. High surface area insured enough space for Ru nanoparticles to anchor, and high porosity guaranteed fast substance exchange between inside and outside of the catalysts. Both the two factors worked together, providing the catalysts with better performance in catalyzing NaBH4 hydrolysis. To confirm the successful anchoring of Ru nanoparticles on the substrate, HAADF-STEM and TEM investigation were carried out and the results were shown in Figs. 1c and d, from which distinct ultrasmall Ru nanoparticles with average diameter as small as 2.4 nm can be clearly observed. The lattice fringes in the inset of Fig. 1d indicated the planes with interplanar spacing of 2.3 Å, corresponding to (100) planes of Ru metal nanoparticles. Furthermore, in XRD pattern of the catalysts (Fig. 2a), the only peaks from 5° to 30° belonged to the chitin substrate [50], while there were no significant reflections associated with metallic Ru species. This could further confirm the small size of Ru nanoparticles. Small nanoparticles could provide more surface atoms, making their catalytic activities improved.
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Fig. 1. (a, b) SEM images of Ru-0.07 catalyst. The crosslinked fiber-like backbones of the catalyst provide it with nanoporous structure. (c) HAADF-STEM image and (d) high resolution TEM images of Ru-0.07 catalyst. Well-distributed ultrasmall Ru nanoparticles can be clearly observed. Several nanoparticles are circled for conspicuousness in (d). The lattice fringes in the inset of (d) show interplanar spacing of 2.3 Å, corresponding to (100) planes of Ru metal nanoparticles. |
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Fig. 2. (a) XRD pattern of Ru/chitin catalysts (top) and standard XRD pattern of metallic Ru (bottom). The chitin peaks from 5° to 30° are distinct, while no reflections associated with metallic Ru species can be observed. (b, c) XPS analysis of Ru/chitin catalysts: (b) Full survey XPS spectrum shows very small peaks of Ru and distinct peaks of C, N and O; (c) Partial scan shows specific peaks of Ru 3p. |
Figs. 2b and c have shown the XPS spectra of the Ru/chitin catalysts. Fig. 2b shows the full survey XPS spectrum of Ru/chitin catalysts with peaks of C, N, O and Ru. The peaks of C, N and O were from nanoporous chitin substrate, whereas the very small peaks of Ru were from Ru nanoparticles. Fig. 2c demonstrates specific 3p peaks of Ru species in the catalysts [51]. Although the intensity was very low, it indeed verified the existence of Ru species in the catalysts.
The rate of H2 generation from NaBH4 hydrolysis was used to evaluate the catalytic activities of the obtained Ru/chitin catalysts. The experiments were carried out in a home-built set-up (Fig. S3 in Supporting information). The catalysts and NaBH4 solution were mixed in a flask with a final reaction mixture weight of 10 g, and the generated H2 was collected and measured with a glass burette.
Compared with the reactions uncatalyzed or catalyzed by bare nanoporous chitin, NaBH4 hydrolysis catalyzed by Ru/chitin catalysts had far outstripping H2 generation rates (Fig. S4 in Supporting information). For different Ru/chitin catalysts, the Ru content of the catalysts had distinct effect on their catalytic activities. Catalyzed NaBH4 hydrolysis experiments were carried out to disclose the difference among the catalytic activities. For the experiments of Ru-0.07 catalyst, 100 mg catalyst was used for each experiment, while for the other catalysts, the amount of Ru species was controlled the same as in the experiments of Ru-0.07. As shown in Fig. 3, although the amount of Ru was the same for each catalyst, their catalytic activities were distinctly different. The catalytic activity increased dramatically with the decreasing of Ru content in the catalysts. Ru-0.07 catalyst, with Ru content of 0.07 wt%, had the best catalytic activity, giving a H2 generation rate of 55.29 L min -1
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Fig. 3. H2 generation from NaBH4 hydrolysis catalyzed by Ru/chitin catalysts with different Ru content. (a) Plots of H2 volume versus time for reactions catalyzed by different catalysts. (b) H2 generation (HG) rate per gram Ru decreases dramatically with the increase of Ru content in the catalysts. Experiment conditions: 10 g reaction mixture, 50 mg NaBH4, 30 ℃. |
The effect of NaBH4 concentration on the H2 generation rate was shown in Fig. S6 (Supporting information). From the slopes of the lines, it can be clearly seen that the catalytic activity of Ru-0.07 was not decreased when amount of NaBH4 used increased from 25 mg to 500 mg. The H2 generation rate actually increased slightly with the increasing of NaBH4 amount, especially from 125 mg to 500 mg. When NaBH4 amount reached 1 g (10 wt% concentration), the initial H2 generation rate was high too; however, it dropped down in a short time after the reaction started (brown stars in Fig. S6). When the NaBH4 concentration was too high, the solution became too viscous. The byproduct of hydrolysis, NaBO2, could not diffuse quickly, so it deposited on the catalysts, blocking the way for reactants to reach the catalysts [38, 42]. As a result, the H2 generation rate decreased dramatically. Additionally, when the amount of NaBH4 used was from 25 mg to 500 mg, the final volume of H2 generated was proportional to the amount of NaBH4 used. It suggests that the change of NaBH4 amount in this range did not have significant negative effect on the catalytic activity of the catalysts for NaBH4 hydrolysis.
Another important factor which drastically influenced H2 generation rate was temperature. The H2 generation tests catalyzed by Ru-0.07 were carried out at different temperature from 30 ℃ to 70 ℃. It can be clearly observed from Fig. 4a that H2 generation was accelerated sharply with temperature increasing. Compared with the H2 generation rate of 55.29 L min -1
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Fig. 4. (a, b) H2 generation from NaBH4 hydrolysis catalyzed by Ru-0.07 at different reaction temperature: (a) Plots of H2 volume versus time for reactions catalyzed by Ru-0.07 catalyst at different reaction temperature; (b) Arrhenius plot for Ru-0.07 catalyzed NaBH4 hydrolysis. Experiment conditions: 10 g reaction mixture, 50 mg NaBH4, 100 mg Ru-0.07. (c, d) The durability of Ru-0.07 in cycle use: (c) Ru-0.07 catalyst shows very little activity loss after used for 20 cycles; (d) TEM image of Ru-0.07 catalyst after used for 20 cycles. Experiment conditions: 10 g reaction mixture, 50 mg NaBH4, 100 mg Ru-0.07, 30 ℃. |
(2) |
where T is H2 generation reaction temperature in Kelvin (K), k is the corresponding rate constant of reactions, R is the gas constant, and A is pre-exponential factor. Based on the results of H2 generation reactions catalyzed by Ru-0.07 at different temperature, the Ea was calculated at about 39.16 kJ/mol (Fig. 4b), which is comparable with the activation energy of many NaBH4 hydrolysis reactions catalyzed by other Ru catalysts (Table S1) [38-43]. It indicates that Ru-0.07 had remarkable catalytic activity for H2 generation from NaBH4 hydrolysis.
If a catalyst is going to be used in practical application, the reusability must be satisfying. In the present study, the reusability of Ru-0.07 was evaluated by measuring its activity in cycle use. After each NaBH4 hydrolysis reaction, the catalyst was collected by centrifugation, washed thoroughly with water, and dispersed in water again for next use. As shown in Fig. 4c, Ru-0.07 catalyst did not show significant activity loss after 20 cycles of experiments. The calculated H2 generation rate at 20th cycle was 90.8% of that at the 1st cycle. The TEM image (Fig. 4d) has shown that there were still distinct ultrasmall nanoparticles well-distributed on the chitin substrate in the Ru-0.07 catalyst which had been used for 20 times. The good reusability can be partially ascribed to the stability of the catalyst itself, while the thorough wash of the catalyst can be another reason. As mentioned above, the byproduct of NaBH4 hydrolysis, NaBO2, which remains on the surface of catalysts, can block the way for reactants to reach the catalysts, resulting in decreasing of catalytic activity. Indeed, in our experiments, if the catalysts were not washed, the H2 generation rate could decrease distinctly. Fortunately, the chitin substrate had a surface very hydrophilic. During washing with water, mass exchange on the catalyst surface was easy and smooth. Therefore, the catalyst could be washed thoroughly after each use, bringing back the original catalytic activity successfully.
Through a cheap and easy method, chitin-supported Ru catalysts, composed of ultrasmall Ru nanoparticles supported on nanoporous chitin substrate, with Ru content from 0.07 wt% to 0.93 wt% were successfully fabricated. SEM investigation showed that the catalysts had nanoporous structure and crosslinked fiber-like backbones. TEM investigation showed that Ru nanoparticles were ultrasmall and well distributed on chitin substrate. By comparing the catalysts with different Ru content, it was found that the catalytic activity for NaBH4 hydrolysis gradually increased with the decreasing of Ru content. Ru-0.07 catalyst (0.07 wt% Ru content) showed a H2 generation rate of 55.29 L min -1
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
AcknowledgmentThis work was supported by Chengdu University of Technology (No. 10912-2019KYQD07208).
Appendix A. Supplementary dataSupplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.11.042.
[1] |
S.S. Muir, X.D. Yao, Int. J. Hydrogen Energy 36 (2011) 5983-5997. DOI:10.1016/j.ijhydene.2011.02.032 |
[2] |
A.M. Abdalla, S. Hossain, O.B. Nisfindy, et al., Energy Convers. Manage. 165 (2018) 602-627. DOI:10.1016/j.enconman.2018.03.088 |
[3] |
P. Brack, S.E. Dann, K.G.U. Wijayantha, Energy Sci. Eng. 3 (2015) 174-188. DOI:10.1002/ese3.67 |
[4] |
A. Schneemann, J.L. White, S. Kang, et al., Chem. Rev. 118 (2018) 10775-10839. DOI:10.1021/acs.chemrev.8b00313 |
[5] |
T.P. Huang, J.X. Zou, N. Zhao, X.Q. Zeng, W.J. Ding, J. Alloys Compd. 791 (2019) 1270-1276. DOI:10.1016/j.jallcom.2019.02.306 |
[6] |
Y.N. Men, J. Su, X.L. Wang, et al., Chin. Chem. Lett. 30 (2019) 634-637. DOI:10.1016/j.cclet.2018.11.010 |
[7] |
L.Z. Ouyang, W. Chen, J.W. Liu, et al., Adv. Energy Mater. 7 (2017) 1700299. DOI:10.1002/aenm.201700299 |
[8] |
D.M.F. Santos, C.A.C. Sequeira, Renew. Sust. Energy Rev. 15 (2011) 3980-4001. DOI:10.1016/j.rser.2011.07.018 |
[9] |
H.I. Schlesinger, H.C. Brown, A.E. Finholt, et al., J. Am. Chem. Soc. 75 (1953) 215-219. DOI:10.1021/ja01097a057 |
[10] |
H.C. Brown, C.A. Brown, J. Am. Chem. Soc. 84 (1962) 1493-1494. DOI:10.1021/ja00867a036 |
[11] |
S.H. Wang, Y.A. Fan, M.Q. Chen, et al., J. Mater. Chem. A 3 (2015) 8250-8255. DOI:10.1039/C5TA00904A |
[12] |
X.C. Shen, Q. Wang, Q.Q. Wu, et al., Energy 90 (2015) 464-474. DOI:10.1016/j.energy.2015.07.075 |
[13] |
A. Zabielaite, A. Balciunaite, I. Stalnioniene, et al., Int. J. Hydrogen Energy 43 (2018) 23310-23318. DOI:10.1016/j.ijhydene.2018.10.179 |
[14] |
J. Guo, Y.J. Hou, B. Li, Y.L. Liu, Int. J. Hydrogen Energy 43 (2018) 15245-15254. DOI:10.1016/j.ijhydene.2018.06.117 |
[15] |
H.K. Cai, L.P. Liu, Q. Chen, P. Lu, J. Dong, Energy 99 (2016) 129-135. DOI:10.1016/j.energy.2016.01.046 |
[16] |
H. Jia, X.J. Liu, X. Chen, et al., Int. J. Hydrogen Energy 42 (2017) 28425-28433. DOI:10.1016/j.ijhydene.2017.09.172 |
[17] |
H.M. Sun, J. Meng, L.F. Jiao, F.Y. Cheng, J. Chen, Inorg. Chem. Front. 5 (2018) 760-772. |
[18] |
N. Patel, A. Miotello, Int. J. Hydrogen Energy 40 (2015) 1429-1464. DOI:10.1016/j.ijhydene.2014.11.052 |
[19] |
X.Y. Zhang, X.W. Sun, D.Y. Xu, et al., Appl. Surf. Sci. 469 (2019) 764-769. DOI:10.1016/j.apsusc.2018.11.094 |
[20] |
M. Bekirogullari, M. Kaya, C. Saka, Int. J. Hydrogen Energy 44 (2019) 7262-7275. DOI:10.1016/j.ijhydene.2019.01.246 |
[21] |
J.Z. Ding, Q. Li, Y. Su, et al., Int. J. Hydrogen Energy 43 (2018) 9978-9987. DOI:10.1016/j.ijhydene.2018.04.077 |
[22] |
Z.T. Gao, C.M. Ding, J.W. Wang, et al., Int. J. Hydrogen Energy 44 (2019) 8365-8375. DOI:10.1016/j.ijhydene.2019.02.008 |
[23] |
L. Hostert, E.G.C. Neiva, A.J.G. Zarbin, E.S. Orth, J. Mater. Chem. A 6 (2018) 22226-22233. DOI:10.1039/C8TA05834B |
[24] |
T.T. Liu, K.Y. Wang, G. Du, A.M. Asiri, X.P. Sun, J. Mater. Chem. A 4 (2016) 13053-13057. DOI:10.1039/C6TA02997C |
[25] |
C. Luo, F.Y. Fu, X.J. Yang, et al., ChemCatChem 11 (2019) 1643-1649. DOI:10.1002/cctc.201900051 |
[26] |
M.X. Ma, L. Wei, F. Jin, Funct. Mater. Lett. 12 (2019) 1850109. DOI:10.1142/S1793604718501096 |
[27] |
A.M. Pornea, M.W. Abebe, H. Kim, Chem. Phys. 516 (2019) 152-159. DOI:10.1016/j.chemphys.2018.08.044 |
[28] |
G.R.M. Tomboc, A.H. Tamboli, H. Kim, Energy 121 (2017) 238-245. DOI:10.1016/j.energy.2017.01.027 |
[29] |
F.H. Wang, Y.J. Zhang, Y.A. Wang, et al., Int. J. Hydrogen Energy 43 (2018) 8805-8814. DOI:10.1016/j.ijhydene.2018.03.140 |
[30] |
Y. Wang, W. Meng, D. Wang, et al., Int. J. Hydrogen Energy 42 (2017) 30718-30726. DOI:10.1016/j.ijhydene.2017.10.131 |
[31] |
H.M. Zhang, X.L. Feng, L.N. Cheng, et al., Colloid Surf. A 563 (2019) 112-119. DOI:10.1016/j.colsurfa.2018.12.002 |
[32] |
Y.N. Men, J. Su, C.Z. Huang, et al., Chin. Chem. Lett. 29 (2018) 1671-1674. DOI:10.1016/j.cclet.2018.04.009 |
[33] |
L.L. Lu, H.J. Zhang, S.W. Zhang, F.L. Li, Angew. Chem. Int. Ed. 54 (2015) 9328-9332. DOI:10.1002/anie.201500942 |
[34] |
N. Sahiner, Int. J. Hydrogen Energy 43 (2018) 9687-9695. DOI:10.1016/j.ijhydene.2018.04.050 |
[35] |
N. Cao, W. Luo, G.Z. Cheng, Int. J. Hydrogen Energy 38 (2013) 11964-11972. DOI:10.1016/j.ijhydene.2013.06.125 |
[36] |
J.Y. Guo, C.B. Wu, J.F. Zhang, et al., J. Mater. Chem. A 7 (2019) 8865-8872. DOI:10.1039/C8TA10695A |
[37] |
L. Wen, Z. Zheng, W. Luo, P. Cai, G.Z. Cheng, Chin. Chem. Lett. 26 (2015) 1345-1350. DOI:10.1016/j.cclet.2015.06.019 |
[38] |
Y.S. Wei, Y. Wang, L. Wei, et al., Int. J. Hydrogen Energy 43 (2018) 592-600. DOI:10.1016/j.ijhydene.2017.11.010 |
[39] |
M. Wen, Y.Z. Sun, X.M. Li, et al., J. Power Sources 243 (2013) 299-305. DOI:10.1016/j.jpowsour.2013.05.190 |
[40] |
G. Bozkurt, A. Ozer, A.B. Yurtcan, Energy 180 (2019) 702-713. DOI:10.1016/j.energy.2019.04.196 |
[41] |
Y. Liang, H.B. Dai, L.P. Ma, P. Wang, H.M. Cheng, Int. J. Hydrogen Energy 35 (2010) 3023-3028. DOI:10.1016/j.ijhydene.2009.07.008 |
[42] |
D.D. Tuan, K.Y.A. Lin, Chem. Eng. J. 351 (2018) 48-55. DOI:10.1016/j.cej.2018.06.082 |
[43] |
M.H. Tan, Y. Wang, A. Taguchi, et al., Int. J. Hydrogen Energy 44 (2019) 7320-7325. DOI:10.1016/j.ijhydene.2019.01.245 |
[44] |
Y.H. Huang, C.C. Su, S.L. Wang, M.C. Lu, Energy 46 (2012) 242-247. DOI:10.1016/j.energy.2012.08.027 |
[45] |
Z.M. Huang, A. Su, Y.C. Liu, Energy 51 (2013) 230-236. DOI:10.1016/j.energy.2013.01.020 |
[46] |
K. Mori, K. Miyawaki, H. Yamashita, ACS Catal. 6 (2016) 3128-3135. DOI:10.1021/acscatal.6b00715 |
[47] |
A.D. Chowdhury, N. Agnihotri, A. De, Chem. Eng. J. 264 (2015) 531-537. DOI:10.1016/j.cej.2014.11.108 |
[48] |
B. Duan, X. Zheng, Z.X. Xia, et al., Angew. Chem. Int. Ed. 54 (2015) 5152-5156. DOI:10.1002/anie.201412129 |
[49] |
B. Duan, Y. Huang, A. Lu, L.N. Zhang, Prog. Polym. Sci. 82 (2018) 1-33. DOI:10.1016/j.progpolymsci.2018.04.001 |
[50] |
G. Cardenas, G. Cabrera, E. Taboada, S.P. Miranda, J. Appl. Polym. Sci. 93 (2004) 1876-1885. DOI:10.1002/app.20647 |
[51] |
D.J. Morgan, Surf. Interface Anal. 47 (2015) 1072-1079. DOI:10.1002/sia.5852 |