Noble-metal-free NiFeMo nanocatalyst for hydrogen generation from the decomposition of hydrous hydrazine

Abstract

Noble-metal-free NiFeMo nanoparticles without any surfactant or support have been facilely synthesized and successfully applied as a highly efficient catalyst for the rapid and complete decomposition of hydrous hydrazine (a promising hydrogen storage/generation material) for hydrogen generation at a mild temperature. The surfactant/support-free nanoparticles possess good dispersion and small particle size. Moreover, upon the incorporation of Mo and Fe, the catalytic activity and hydrogen selectivity of the present trimetallic catalyst are remarkably improved compared with its mono-/bi-metallic counterparts.

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Storing and generating hydrogen (H₂) safely and efficiently remain as great challenges toward the fuel cell based H₂ economy.^1–7 Recently, hydrous hydrazine (N₂H₄·H₂O) has been considered a promising candidate as a H₂ storage/generation material due to its high H₂ content (8.0 wt%), relatively low cost and easy recharging as a liquid (the existing liquid-based fuel distribution infrastructure can be used).^8,9 Importantly, the decomposition of N₂H₄·H₂O does not generate any solid by-products; the only by-product of complete decomposition of N₂H₄·H₂O is nitrogen (N₂): H₂NNH₂ → N₂(g) + 2H₂(g),^10–34 which is environmentally friendly. For H₂ generation, the incomplete and undesirable side reaction (3H₂NNH₂ → 4NH₃(g) + N₂(g)) must be avoided.^35–40 Therefore, it is important to develop effective catalysts for the selective decomposition of N₂H₄·H₂O to H₂.

Up to now, many efforts have been made on the synthesis and application of noble-metal containing nanocatalysts for H₂ generation from N₂H₄·H₂O under mild conditions (298–323 K).^10–25 However, most of these catalysts show moderate activities.^10–19 Additionally, the high costs and limited resources of noble metals restrict their large-scale applications.^27,38 To solve this problem, non-noble-metal nanomaterials have gained more and more research interest for the selective decomposition of N₂H₄·H₂O.^26–34 Generally, minimization and high dispersion of the nanoparticles are the key factors to obtain more surface reactive sites to elevate the activities of the non-noble-metal nanomaterials. Thus, various surfactants or particle supports have been applied during the syntheses of these nanocatalysts.^26–33 For example, it is found that, using hexadecyltrimethylammonium bromide (CTAB) as a surfactant, NiFe alloy nanoparticles could catalyze the complete dehydrogenation of N₂H₄·H₂O at 343 K.²⁶ Meanwhile, supported non-noble-metal nanocatalysts such as Ni–Al₂O₃–HT,²⁷ NiFe-alloy/MgO,²⁹ Ni₃Fe/C³² and FeB/MWCNTS³³ are active for the same reaction at room temperature. However, the usage of surfactants sometimes decreases the catalytic performance due to more or less occupation of the active sites on the surface of the nanocatalysts by surfactants.²⁶ For supported catalysts, the extra addition of supports brings complicated preparation processes, such as multi-step synthesis, long reaction time and strict reaction conditions (high temperature, inert gas protection and post processing), leading to great difficulties in both equipment and operation requirements.^27,29,32,33 More importantly, most of the catalytic activities of the reported non-noble-metal nanomaterials for the dehydrogenation of N₂H₄·H₂O are still very low.^26–31 Based on the above reasons, exploring a facile strategy to obtain the well dispersed non-noble-metal nanocatalyst with excellent catalytic activity and 100% H₂ selectivity for H₂ generation from N₂H₄·H₂O at mild temperature is of good interest but is still a big challenge.

Herein, the noble-metal-free Ni_0.6Fe_0.4Mo nanoparticles (NPs) have been prepared through a facile one-step synthetic route at room temperature under an ambient atmosphere within 10 minutes. The as-prepared NPs have a small particle size and high dispersion without the assistance of a surfactant/support. As expected, the Ni_0.6Fe_0.4Mo nanocatalyst exhibits 100% H₂ selectivity and superior catalytic activity for the decomposition of N₂H₄·H₂O at 323 K.

Ni_0.6Fe_0.4Mo NPs were synthesized by a surfactant/support-free one-step co-reduction method at 298 K (Scheme 1), in which NiCl₂·6H₂O, Fe₂SO₄·7H₂O and Na₂MoO₄·2H₂O were used as the metal precursors and NaBH₄ was added as the reducing agent. Fig. 1a shows the typical transmission electron microscopy (TEM) image of the as-prepared Ni_0.6Fe_0.4Mo NPs. The NPs are well dispersed with an average particle size of less than 5 nm. The corresponding energy dispersive X-ray (EDX) spectrum displays all the existences of the Ni, Fe and Mo elements (Fig. 1b). The atomic ratio for Ni : Fe : Mo is detected to be 0.298 : 0.199 : 0.503 by EDX, and this is in good agreement with the theoretical value (0.6 : 0.4 : 1). The high-resolution TEM (HRTEM) image reveals the crystalline nature of the Ni_0.6Fe_0.4Mo NPs, and the lattice spacing is measured to be 0.206 nm (Fig. 1c). The X-ray diffraction (XRD) pattern shows that the tri-metallic specimen has a crystalline peak centered at 43.95 (Fig. 1d, black trace), which is between the (111) plane of fcc Ni (JCPDS file: 04-0850), the (111) plane of the fcc Fe structure (JCPDS file: 52-0513) and the (110) plane of the body-centered cubic (bcc) Mo (JCPDS file: 65-7442).^26,41,42 Moreover, after heat treatment at 823 K for 3 h in argon (Ar), this sample is better crystallized into a fcc crystal structure (Fig. 1d, red trace). Compared with the pure fcc Ni, the diffraction peaks are slightly shifted to the lower angles, which may result from the addition of Fe and Mo atoms into the crystal lattice of Ni, and this is consistent with the HRTEM result. Based on the above analyses, the well dispersed Ni_0.6Fe_0.4Mo NPs with an alloy structure have been successfully synthesized through the present facile co-reduction method without support and surfactant at 298 K.

Scheme 1 Schematic illustration for the preparation and application of the NiFeMo nanocatalyst for the decomposition of N₂H₄·H₂O under mild conditions.

Fig. 1 TEM image (a), the corresponding EDX spectrum (b) and HRTEM image (c) of the Ni_0.6Fe_0.4Mo NPs; (d) X-ray diffraction patterns of the Ni_0.6Fe_0.4Mo NPs before (1) and after (2) heat treatment at 823 K for 3 h in an argon atmosphere.

To investigate the effect of Mo on the chemical state of Ni_0.6Fe_0.4Mo, the X-ray photoelectron spectroscopy (XPS) analyses after Ar sputtering have been applied to Ni_0.6Fe_0.4Mo and Ni_0.6Fe_0.4 for comparison. It can be seen from Fig. 2 that the incorporation of Mo into bi-metallic Ni_0.6Fe_0.4 NPs results in a change in the binding energy (BE) of the component elements. After Mo addition, the BE of Ni 2p_3/2 is negatively shifted from 852.6 to 852.1 eV (Fig. 2a). Meanwhile, the BE of Fe 2p_3/2 is also negatively shifted from 708.0 to 707.4 eV (Fig. 2b). Whereas, the BE of Mo 3d_5/2 (228.6 eV, Fig. 2c) is shifted to a higher value relative to that of metallic Mo⁰ (228.0 eV).⁴³ Based on the XPS analyses, it can be seen that, in the alloy of Ni_0.6Fe_0.4Mo, Mo acts as an electron donor for atoms of Ni and Fe. Such electron transfer in Ni_0.6Fe_0.4Mo has the potential to endow itself with high catalytic activity for H₂ generation from N₂H₄·H₂O decomposition.⁴⁴ Moreover, since crystal growth can be limited by co-deposition of multi-elements,²⁸ the addition of Mo results in the smaller particle sizes of Ni_0.6Fe_0.4Mo relative to Ni_0.6Fe_0.4 (Fig. S1†), which may provide more active sites for H₂ generation from the decomposition of N₂H₄·H₂O. In addition, no element of B has been detected in the specimen of Ni_0.6Fe_0.4Mo by XPS (Fig. S2†).

Fig. 2 XPS spectra of (a) Ni 2p and (b) Fe 2p for Ni_0.6Fe_0.4 and the Ni_0.6Fe_0.4Mo nanocatalyst; (c) Mo 3d for the Ni_0.6Fe_0.4Mo nanocatalyst.

The catalytic performances of Ni_0.6Fe_0.4Mo together with its bi-/mono-metallic counterparts for H₂ generation from N₂H₄·H₂O in the presence of NaOH at 323 K by magnetic stirring are presented in Fig. 3. It can be seen that Ni is the key active element of all the prepared catalysts. Without Ni addition, monometallic Fe and Mo and bimetallic FeMo NPs almost show no activity (Fig. 3b–d). This is consistent with the previous reports that Ni is the representative non-noble-metal towards the decomposition of N₂H₄·H₂O.^11–32 With Ni addition, the activities of Ni, Ni_0.6Fe_0.4, NiMo and Ni_0.6Fe_0.4Mo have been enhanced obviously (Fig. 3a, e–g), in which only the trimetallic Ni_0.6Fe_0.4Mo leads to the complete decomposition of N₂H₄·H₂O within a short time (15 min) at 323 K [n(N₂ + H₂)/nN₂H₄ = 3.0, Fig. 3a]. Noteworthy, a change in the molar ratio of Ni : Fe : Mo to some other values results in a serious decrease in its activity or selectivity (Fig. S3 and S4†). Namely, the optimum molar ratio of Ni : Fe : Mo is determined to be 0.6 : 0.4 : 1.0. After the catalytic reaction over Ni_0.6Fe_0.4Mo, the ultraviolet visible (UV-vis) spectrum indicates no existence of hydrazine in the solution (Fig. S5†). Moreover, the generated gas is identified by mass spectrometry (MS) to be H₂ and N₂ with a H₂/N₂ ratio of 2.0 (Fig. S6†). Therefore, the UV-vis and MS results reveal 100% conversion and selectivity for highly efficient NH₃-free H₂ generation from N₂H₄·H₂O over the as-prepared Ni_0.6Fe_0.4Mo catalyst. NH₃-free H₂ is crucial for fuel cell applications, since the formation of NH₃ may seriously poison the Nafion membrane and the fuel cell catalysts.⁴⁵ The initial TOF over Ni_0.6Fe_0.4Mo is measured to be 28.8 h⁻¹ [eqn (S1)†] at 323 K. To the best of our knowledge, this initial TOF value is much higher than the most widely reported noble-metal-free heterogeneous catalysts for N₂H₄·H₂O decomposition,^26–31 and is even superior to most of those noble-metal-containing catalysts (Table S1†).^10–19 This superior activity of Ni_0.6Fe_0.4Mo may be attributed to the electrical modification of Ni in the alloy structure of Ni_0.6Fe_0.4Mo, and also the small particle size and good dispersion of Ni_0.6Fe_0.4Mo resulting from the present easy synthetic method. It should be noted that magnetic stirring has no negative effect on the catalytic performance of the magnetic-element-containing Ni_0.6Fe_0.4Mo towards N₂H₄ decomposition (Fig. S7†).

Fig. 3 Time-course plots for the decomposition of the hydrazine aqueous solution (0.5 M, 4 mL) to H₂ catalyzed by (a) Ni_0.6Fe_0.4Mo, (b) Fe, (c) Mo, (d) FeMo, (e) Ni, (f) Ni_0.6Fe_0.4 and (g) NiMo NPs with NaOH (1.8 M, 4 mL) at 323 K.

To determine the effect of NaOH, the catalyst promoter in the dehydrogenation of N₂H₄·H₂O,^13,26 different amounts of NaOH were employed for the same catalytic reaction (Fig. S8†). It was found that the selectivity and activity of Ni_0.6Fe_0.4Mo improved with an increase of NaOH amount from 0 to 7.2 mmol, while further increases to the NaOH amount had no obvious impact on the performance of Ni_0.6Fe_0.4Mo.

In order to obtain the activation energy (E_a) of the N₂H₄·H₂O decomposition catalyzed by the Ni_0.6Fe_0.4Mo NPs, the reactions at different temperature (298–323 K) were carried out and the results are shown in Fig. S9.† As expected, the catalytic system shows the improved decomposition kinetics with increasing temperature, and exhibits nearly 100% H₂ selectivity at the examined temperature range (Fig. S9a†). The Arrhenius plot of ln TOF vs. 1/T for this catalyst is plotted in Fig. S9b,† from which E_a was calculated as 50.7 kJ mol⁻¹ [eqn (S2)†], which is very similar to the previously reported values for the catalytic decomposition of N₂H₄·H₂O.^27,28

The recycling stability of Ni_0.6Fe_0.4Mo was tested at 323 K under ambient atmosphere. It can be clearly seen that, the H₂ selectivity and activity almost has no decline after the 3^rd run (Fig. S10 and Table S2†), indicating the good stability of the present tri-metallic Ni_0.6Fe_0.4Mo NPs for H₂ generation from N₂H₄·H₂O at mild temperature.

In summary, a facile methodology for the synthesis of noble-metal-free Ni_0.6Fe_0.4Mo NPs has been proposed through a co-reduction route at room temperature under ambient atmosphere. The resultant Ni_0.6Fe_0.4Mo catalyst without support and surfactant exhibits excellent catalytic performance for the decomposition of hydrazine aqueous solution at mild temperature. The obtained catalyst which has the favourable properties of high performance and low-cost may further encourage the practical application of hydrazine as a promising H₂ generation/storage material.

Acknowledgements

This work is supported in part by the National Natural Science Foundation of China (51471075, 51401084 and 51101070); National Key Basic Research, Development Program (2010CB631001); and Jilin University Fundamental Research Funds.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, results of TEM, EDS, XRD, XPS, UV-vis and MS, and TOF list of different nanocatalysts tested for hydrogen generation from the decomposition of hydrazine are supplied. See DOI: 10.1039/c4ta05360e

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