Catalyst-enhanced hydrothermal generation of highly pure compressed hydrogen gas from iron micro-powders

Abstract

In this communication, the hydrothermal reaction of micro-sized iron powders and water has been demonstrated to be a facile and efficient process for the generation of highly pure compressed hydrogen gas. With the assistance of acid and Pd catalysts, the generation rate and conversion could be significantly enhanced.

---

Hydrogen energy is considered as one of the alternative energies having the most potential because of its environmental friendliness, renewability, and wide sources.^1–4 A lot of methods have been developed for the generation of hydrogen gas, including fossil fuel reforming, hydride decomposition, light/dark fermentation, and water splitting.^3–10 Among them, water splitting has received increasing attention because water can be regenerated and is in abundance on the earth. The typical water splitting techniques for hydrogen gas generation include electrolysis,¹¹ thermal decomposition,¹² thermochemical cycles,^13,14 solar water splitting,^3,15,16 and the oxidation–reduction of metals and water.^17–27 For the oxidation–reduction of metals and water, Al and Fe are usually used.^{17–21,24–27} As compared to other methods, this method is particularly attractive owing to the lower cost, lower energy consumption, better efficiency, and less harmful byproducts.

The steam-iron process has been known as a conventional hydrogen gas generation technique.^24–27 By the reaction of iron powder and water vapor, pure hydrogen gas can be obtained according to the following equation:²⁵

3Fe + 4H₂O → Fe₃O₄ + 4H₂ (1)

However, the use of water vapor as the reactant needs higher energy consumption and leads to the process complexity. Recently, we developed a novel and facile hydrothermal process for the generation of compressed hydrogen gas.²⁸ As compared to the steam-iron process, this process has the advantages of low temperature, simplicity, and high purity. Furthermore, the produced hydrogen gas was in the compressed form. This was favorable for its storage and utilization.

It was mentionable that, in our previous work, the surface oxidation of iron powders might hinder the further reaction of water and inner iron and hence led to the incomplete conversion. So, the nano-sized iron powders exhibited much higher hydrogen gas generation rate and conversion than the micro-sized iron powders. However, the nano-sized iron powders were relatively unstable and their storage and safety might be a serious problem in practical application. Furthermore, their cost is much higher than the micro-sized iron powders. In the view point of practical application, micro-sized iron powders might be the better choice. Thus, it is interesting and necessary to develop an improved hydrothermal generation process of hydrogen gas for micro-sized iron powders with higher generation rate and conversion.

It is well known that the presence of acid can remove the surface oxide of iron powders and accelerate the corrosion rate. So, in this study, we attempted to enhance the hydrothermal generation rate of hydrogen gas by the use of acid catalyst at first. Secondly, it has been reported that Pd catalyst could catalyze the reduction of water with zinc,²² and the bimetallic nanoparticles of zero iron and Pd showed the higher hydrogen gas generation rate than pure zero iron.²⁹ So, the use of Pd catalyst to enhance the hydrothermal generation of hydrogen gas by micro-sized iron powders was also attempted.

The flat iron powders of 45 μm were used owing to their larger specific surface area than spherical powders.²⁸ Pd catalyst was deposited on the surface of iron powders via the galvanic replacement reaction (i.e., Pd has higher reduction potential than Fe) by putting 10 g of iron powders into 250 mL of PdCl₂ solution (0.5, 1.0, 2.0 or 4.0 mM) and then sonicating for 1 min. The hydrothermal generation of hydrogen gas was carried out in a Teflon-lined stainless steel autoclave as described in our previous work.²⁸ Typically, 10 g of iron powders (with or without Pd catalyst deposition) and 40 mL of pure water or HCl solutions (0.00125, 0.0025, 0.005, 0.01, and 0.02 M) were put into the 100 mL of cylinder reactor and then the temperature was raised to 120 °C. The pressure variation during the reaction was monitored by a pressure detection system. After reaction, the reactor was naturally cooled to room temperature and then the highly pure hydrogen gas could be obtained by discharging to the connected collection bottle. The solid product was washed with deionized water several times and then dried in a vacuum oven for the followed characterization.

The crystalline structures of iron powders before and after reaction were characterized by X-ray diffraction (XRD, Shimadzu model RX-III) using CuKα radiation (λ = 0.1542 nm) at an acceleration voltage of 40 kV and a current of 40 mA. The changes in the morphology and size were observed by a high resolution field emission scanning electron microscope (HR-FESEM, JSM-6700F) operating at 10.0 kV. The Pd and Cl contents in the powders were analyzed by the energy dispersive X-ray spectroscope (EDX, Oxford INCA 400). The iron content in the solid product was determined by dissolving 0.1 g of solid product in 20 mL HCl solution and then analyzing the iron content using inductively coupled plasma-optical emission spectrometer (Horiba Jobin Yvon ULTIMA 200-2 ICP-OES). The gas collected in the collection bottle was characterized by gas chromatography (GC, Shimadzu GC-2014).

As shown in Fig. 1, at 120 °C and 20 wt% of iron powders, it was found that the generation rate of hydrogen gas and conversion could be significantly enhanced by the addition of acid and the increase of acid concentration. When acid concentration was above 0.01 M, the enhancement effect reached the maximum. After 20 h, the pressure could be raised up to 75 bar. Because the amount of HCl added was much lower than that of water, the increased pressure was not due to the decomposition of HCl. According to the eqn (1), 10 g of iron powders could produce 0.239 mol of hydrogen gas after complete reaction with 40 mL of water. However, even in the case of 0.02 M HCl, the maximum hydrogen gas generation via the complete decomposition of HCl from 40 mL of aqueous solution was only 0.0004 moles. So, the strategy to enhance the hydrogen generation rate and conversion of micro-sized iron powders by the addition of acid is indeed effective.

Fig. 1 Variation of pressure with time during the hydrothermal generation of hydrogen gas at 120 °C in water and HCl solutions with different concentrations. HCl concentration: 0 (a), 0.00125 (b), 0.0025 (c), 0.005 (d), 0.01 (e), and 0.02 M (f).

Fig. 2 shows the corresponding XRD patterns of iron powders after reaction at 120 °C and different HCl concentrations for 20 h. It was found that, with the increase of HCl concentration, the characteristic peaks for Fe(110) and Fe(200) at 2θ = 44.80 and 65.15° gradually disappeared. In the meanwhile, the characteristic peaks of Fe₃O₄ at 2θ = 30.22, 35.54, 37.12, 43.22, 53.48, 57.08, 62.60 and 74.6° which related to the (220), (311), (222), (400), (421), (511), (440) and (533) planes were enhanced gradually. This confirmed that the conversion of Fe to Fe₃O₄ increased with the increase of HCl concentration. When HCl concentration increased to 0.01 M or above, the oxidation of Fe was almost complete.

Fig. 2 XRD patterns of iron powders after reaction at 120 °C in HCl solutions with different concentrations for 20 h. HCl concentration: 0.00125 (a), 0.0025 (b), 0.005 (c), 0.01 (d), and 0.02 M (e).

Fig. 3 indicates the corresponding SEM images of iron powders after reaction at 120 °C and different HCl concentrations for 20 h. It was found that, as compared to the iron powders before reaction (Fig. S1†), a lot of smaller particles were formed on the surface of flat fragments in the absence of HCl owing to the oxidation of iron powders. With the increase of HCl concentration, larger particles were formed. When HCl concentration increased from 0.05 to 0.2 M, the size increase of larger particles became less remarkable but less small particles were present. This could be attributed to the increase of conversion and suggested that the resulting small iron oxide particles might aggregate to larger particles under the hydrothermal condition with high pressure hydrogen gas.

Fig. 3 SEM images of iron powders after reaction at 120 °C in water and HCl solutions with different concentrations for 20 h. HCl concentration: 0 (a), 0.00125 (b), 0.0025 (c), 0.005 (d), 0.01 (e), and 0.02 M (f).

To confirm the role of HCl as an acid catalyst, the pH values of solutions before and after reaction were measured. In the case of 0.02 M HCl solution, the solution pH increased from 1.82 to 5.54 after reaction for 20 h. The increase of pH revealed the decomposition of HCl occurred during the reaction. Thus, as HCl decomposed gradually, the generation rate of hydrogen gas became slow owing to the presence of less acid catalyst. This accounted for the generation curve of hydrogen gas with time and was consistent with the effect of HCl concentration.

When 0.02 M HCl was replaced by 0.02 M NaCl, it was found that the hydrogen gas generation rate and conversion was almost the same as that in the absence of HCl (Fig. S2†). This revealed that the catalytic effect of HCl was due to the hydrogen ions rather than chloride ions. When 0.02 M HCl was replaced by 0.02 M formic acid or acetic acid, it was found that the hydrogen gas generation rate and conversion were similar to that in the presence of 0.02 M HCl as shown in Fig. 4. The corresponding XRD patterns (Fig. 5) also indicated that iron powders have been converted into iron oxide. These results confirmed the role of HCl as an acid catalyst and revealed that the use of acid catalysts could effectively enhance the generation of hydrogen gas.

Fig. 4 Variation of pressure with time during the hydrothermal generation of hydrogen gas at 120 °C in 0.02 M HCl (a), acetic acid (b), or formic acid (c) solutions.

Fig. 5 XRD patterns of iron powders after reaction at 120 °C in 0.02 M HCl (a), acetic acid (b), or formic acid (c) solutions.

Although the use of acid catalysts could enhance the generation of hydrogen gas, the time required for the reaction at 120 °C was above 20 h even HCl concentration was 0.01 or 0.02 M. This was still longer than that for the iron powders of 100 nm (about 5 h) in the absence of acid catalysts as observed in our previous work.²⁸ So, further improvement was expected. Because Pd has been reported to be an efficient catalyst for the hydrogen gas generation,^22,29 the use of Pd catalyst to further enhance the hydrothermal generation of hydrogen gas by micro-sized iron powders was attempted.

When the PdCl₂ concentrations for the galvanic replacement reaction were 0.5, 1.0, 2.0 and 4.0 mM, the EDX analysis revealed that the amounts of Pd catalyst deposited were 0.17, 1.00, 1.99 and 3.92 wt%, respectively. From the investigation on the generation of hydrogen gas by the 1.0 mM PdCl₂-treated iron powders in the pure water and the HCl solutions with different concentrations (i.e., 0.00125, 0.0025, 0.005, 0.01, and 0.02 M), it was found that, in the absence of acid catalyst, the deposition of Pd catalyst not only did not enhance but even slightly lower the generation rate of hydrogen gas (Fig. S3†). However, as shown in Fig. 6, the generation rate and conversion of hydrogen gas were enhanced with increasing the HCl concentration. As compared to the result in Fig. 1, it was obvious that the deposition of Pd on the surface of iron powders led to the significant increase of hydrogen gas generation rate. The time required for the reaction at 120 °C and 0.01 or 0.02 M HCl could be reduced from above 20 h to about 6 h, close to that for the iron powders of 100 nm. This revealed that the combination of Pd and acid catalysts made the utilization of micro-sized iron powders in the hydrothermal generation of hydrogen gas become more efficient and practicable. As for the catalytic mechanism by Pd catalyst, it was suggested that the adsorption and activation of water molecules on Pd surface might be helpful for the more efficient decomposition of H₂O by Fe to form H₂ and Fe₃O₄ in the presence of acid.²²

Fig. 6 Variation of pressure with time during the hydrothermal generation of hydrogen gas by the 1.0 mM PdCl₂-treated iron powders at 120 °C in HCl solutions with different concentrations. HCl concentration: 0.00125 (a), 0.0025 (b), 0.005 (c), 0.01 (d), and 0.02 M (e).

Fig. 7 shows the generation curve of hydrogen gas at 120 °C and 0.02 M HCl by the iron powders treated with different concentrations of PdCl₂. Obviously, the treatment of iron oxide with 1.0 mM PdCl₂ led to a larger initial generation rate than that with 0.5 mM PdCl₂. However, the initial generation rate was not further enhanced when PdCl₂ concentration was raised to 2.0 and 4.0 mM. This revealed that the deposition of Pd catalyst on the surface of iron powders via the treatment with 1.0 mM PdCl₂ was enough for the enhancement of hydrogen gas generation rate. Furthermore, it was noted that the maximum hydrogen gas pressure obtained decreased slightly with increasing the PdCl₂ concentration. This might be due to the absorption of hydrogen gas by the deposited Pd catalyst.³⁰ Thus, the optimal PdCl₂ concentration for the deposition of Pd catalyst on the surface of iron powders was 1.0 mM in this work.

Fig. 7 Variation of pressure with time during the hydrothermal generation of hydrogen gas by the iron powders treated with different concentrations of PdCl₂ at 120 °C in 0.02 M HCl solution.

Fig. 8 shows the typical XRD patterns of 1.0 mM PdCl₂-treated iron powders after reaction for 20 h at 120 °C in the pure water and 0.02 M HCl solution. It was obvious that only a small part of iron was converted to iron oxide in the absence of acid catalyst. However, in 0.02 M HCl solution, almost all iron could be converted to iron oxide. This was consistent with the corresponding generation curves of hydrogen gas as indicated in Fig. 6 and S3.† Because of low content, no characteristic peaks of Pd catalyst were observed. By dissolving the solid product in HCl solution and analyzing the iron content using ICP-OES, the conversion of iron powders after reaction for 20 h at 120 °C in 0.02 M HCl solution could be estimated to be about 100%. This was in agreement with the above XRD analysis and demonstrated that the combination of Pd and acid catalyst indeed could enhance effectively the hydrogen gas generation rate and lead to the almost complete conversion.

Fig. 8 XRD patterns of 1.0 mM PdCl₂-treated iron powders after reaction at 120 °C in water (a) and 0.02 M HCl solution (b).

Fig. 9 indicates the SEM images of 1.0 mM PdCl₂-treated iron powders after reaction for 20 h at 120 °C in the pure water and HCl solutions with different concentrations. In general, the phenomenon was similar to that without the deposition of Pd catalyst as shown in Fig. 3. However, the aggregated particles with the deposition of Pd catalyst were smaller.

Fig. 9 SEM images of 1.0 mM PdCl₂-treated iron powders after reaction at 120 °C in water and HCl solutions with different concentrations for 20 h. HCl concentration: 0 (a), 0.00125 (b), 0.0025 (c), 0.005 (d), 0.01 (e), and 0.02 M (f).

To further confirm the purity of hydrogen gas and examine the form of chloride ions after HCl decomposition, the resulting hydrogen gas and iron oxide powders after reaction for 20 h at 120 °C in 0.02 M HCl solution were characterized by GC and EDX, respectively. The GC analysis revealed that the water content was only about 0.247%. This demonstrated that the obtained hydrogen gas indeed had a high purity (i.e., 99.75%). Furthermore, from the EDX analysis, the weight ratio of Cl/Fe was determined to be 0.00288. This was quite close to the theoretical value (i.e., 0.00284) in the original reaction mixture (i.e., 10 g Fe and 40 mL of 0.02 M HCl solution). This implied that chloride ions were retained in the solid powders after reaction, probably in the form of ferrous chloride or ferric chlorite. Owing to the very low content, their characteristic peaks in the XRD pattern were not observed.

Conclusions

In conclusion, the highly pure compressed hydrogen gas has been successfully generated via the hydrothermal reaction of iron micro-powders and water. By the assistance of acid and Pd catalyst, the generation rate could be significantly enhanced to a level of nano-sized iron powders. Also, the conversion could be raised to about 100%. This made the hydrothermal generation of hydrogen gas using micro-sized iron powders become more efficient and practicable. Such a facile process is expected to be useful in the developments of hydrogen generation techniques, hydrogen energy-related devices, and hydrogen gas-related chemical processes owing to its advantages of low temperature, simplicity, high efficiency, high purity, and high pressure.

Acknowledgements

This work was supported by the National Science Council of the Republic of China (grant number NSC 103-ET-E-006-004-ET), to which the authors wish to express their thanks.

References

1. A. Züttel, A. Remhof, A. Borgschulte and O. Friedrichs, Philos. Trans. R. Soc., A, 2010, 368, 3329–3342.
2. M. Ball and M. Wietschel, Int. J. Hydrogen Energy, 2009, 34, 615–627.
3. R. M. Navarro, M. C. Sánchez-Sánchez, M. C. Alvarez-Galvan, F. del Valle and J. L. G. Fierro, Energy Environ. Sci., 2009, 2, 35–54.
4. K. Christopher and R. Dimitrios, Energy Environ. Sci., 2012, 5, 6640–6651.
5. C. Acar and I. Dincer, Int. J. Hydrogen Energy, 2014, 39, 1–12.
6. J. D. Holladay, J. Hu, D. L. King and Y. Wang, Catal. Today, 2009, 139, 244–260.
7. C. C. Cormos, Int. J. Hydrogen Energy, 2011, 36, 5960–5971.
8. B. Choi, D. Panthi, M. Nakoji, T. Kabutomori, K. Tsutsumi and A. Tsutsumi, Int. J. Hydrogen Energy, 2015, 40, 6197–6206.
9. C. Jiao, Z. Huang, X. Wang, H. Zhang, L. Lu and S. Zhang, RSC Adv., 2015, 5, 34364–34367.
10. P. C. Hallenbeck and J. R. Benemann, Int. J. Hydrogen Energy, 2002, 27, 1185–1193.
11. M. Wang, Z. Wang, X. Gong and Z. Guo, Renewable Sustainable Energy Rev., 2014, 29, 573–588.
12. F. Lapicque, J. Lédé and J. Villermaux, Int. J. Hydrogen Energy, 1983, 8, 675–679.
13. M. A. Rosen, Energy, 2010, 35, 1068–1076.
14. L. Xiao, S. Y. Wu and Y. R. Li, Renewable Energy, 2012, 41, 1–12.
15. P. D. Tran, L. H. Wong, J. Barber and J. S. C. Loo, Energy Environ. Sci., 2012, 5, 5902–5918.
16. S. J. A. Moniz, S. A. Shevlin, D. J. Martin, Z. X. Guo and J. Tang, Energy Environ. Sci., 2015, 8, 731–759.
17. V. Rosenband and A. Gany, Int. J. Hydrogen Energy, 2010, 35, 10898–10904.
18. M. Watanabe, J. Phys. Chem. Solids, 2010, 71, 1251–1258.
19. K. Mahmoodi and B. Alinejad, Int. J. Hydrogen Energy, 2010, 35, 5227–5232.
20. S. Elitzur, V. Rosenband and A. Gany, Int. J. Hydrogen Energy, 2014, 39, 6328–6334.
21. M. Pudukudy, Z. Yaakob, B. Narayanan, R. Ramakrishnan and S. Viswanathan, Int. J. Hydrogen Energy, 2012, 37, 7451–7456.
22. O. V. Kravchenko, L. G. Sevastyanova, S. A. Urvanov and B. M. Bulychev, Int. J. Hydrogen Energy, 2014, 39, 5522–5527.
23. S. Mukhopadhyay, G. Rothenberg, H. Wiene and Y. Sasson, New J. Chem., 2000, 24, 305–308.
24. D. P. Gregory and J. B. Pangborn, Annu. Rev. Energy, 1976, 1, 279–310.
25. V. Hacker, R. Fankhauser, G. Faleschini, H. Fuchs, K. Friedrich, M. Muhr and K. Kordesch, J. Power Sources, 2000, 86, 531–535.
26. K. Otsuka, T. Kaburagi, C. Yamada and S. Takenaka, J. Power Sources, 2003, 122, 111–121.
27. M. Rydén and M. Arjmand, Int. J. Hydrogen Energy, 2012, 37, 4843–4854.
28. Y. C. Tsai, L. H. Liu and D. H. Chen, RSC Adv., 2016, 6, 8930–8934.
29. K. F. Chen, S. Li and W. X. Zhang, Chem. Eng. J., 2011, 170, 562–567.
30. L. L. Jewell and B. H. Davis, Appl. Catal., A, 2006, 310, 1–15.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19947j

---