Nickel–platinum nanoparticles immobilized on graphitic carbon nitride as highly efficient catalyst for hydrogen release from hydrous hydrazine

Abstract

Here we demonstrate that the combination of NiPt alloy nanoparticles with a graphitic carbon nitride (g-C₃N₄) support facilitates H₂ production from hydrous hydrazine in an alkaline solution under moderate conditions. Of all the heterogeneous catalysts tested, Ni₃₇Pt₆₃/g-C₃N₄ shows superior catalytic performance with a maximum initial turnover frequency (TOF) of 570 h⁻¹ at 323 K.

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In light of the upcoming energy crisis and the increasingly severity of environmental pollution it is desirable to use sustainable, renewable and clean energies.¹ Hydrogen, whose only by-product is water, is an environmentally attractive and ideal energy carrier, especially for electricity generation in low-temperature fuel cells.^2–5 However, despite the numerous studies conducted over several decades, the hydrogen economy has not been fulfilled due to the challenges in safely and efficiently storing hydrogen.^6–9 Chemical hydrogen storage is a promising solution because it can be conveniently transported and has a considerable hydrogen content.^10–15 Hydrous hydrazine, such as hydrazine monohydrate (H₂NNH₂·H₂O), is considered a promising candidate for storing hydrogen. It is advantageous due to its high hydrogen storage capacity (8.0 wt%), easy recharging as a liquid (the current infrastructure for liquid fuel cells can be utilized), and its environment-friendliness, i.e. the only byproduct is nitrogen via a complete decomposition reaction: H₂NNH₂ → N₂(g) + 2H₂(g).^16–21 Nevertheless, to efficiently liberate hydrogen, the undesired reaction 3H₂NNH₂ → 4NH₃(g) + N₂(g) must be avoided.^22–27 To solve this problem, one must develop highly efficient, selective, and economic catalysts for producing hydrogen from hydrous hydrazine in practical applications.

For implementing this target, much attention has been devoted to synthesize metal catalysts for improving the hydrogen release from H₂NNH₂·H₂O, among which noble metals, e.g., Pt, Pd, Rh, or their bimetallic alloy catalysts^28–34 have proved to be efficient. In light of this, many studies have been devoted to developing monometallic and polymetallic nanoparticles (NPs), where materials such as Al₂O₃, CeO₂, graphene oxide, carbon black, etc. act as the support^35–44 and metals such as Pt,^45–50 Pd,^33,51 and other elements^{19–21,52,53} are the active ingredients. In particular, binary NiPt alloy NPs are efficient catalysts for hydrous hydrazine decomposition in alkaline solutions. The dispersion of the metal NPs throughout the solution significantly influences the performance of the catalysts;^45–49 therefore, to improve their activity one must reduce the clustering of NPs via the use of proper supports.

Recently, graphitic carbon nitride (g-C₃N₄) has received a great deal of attention as a metal-free candidate to supplement traditional carbon materials due to its many appealing properties: low cost, semiconductivity, high chemical and physical stability, basicity, energy-storage capacity, and nontoxicity.^54,55 g-C₃N₄ has been comprehensively investigated for use in gas storage, fuel cells, photocatalysis, and heterogeneous catalysis.^54–57 It has emerged as one of the more promising supports for metal NPs because the nitrogen-rich structure of its amine-bridged heptazine units provides many adsorption sites for grafting metal ions on its surface.⁵⁸ Furthermore, to the best of our knowledge, no NPs have yet been immobilized on g-C₃N₄ supports for the catalyzing H₂NNH₂·H₂O dehydrogenation. Here we present the first instance of NiPt alloy nanoparticles (NPs) supported on g-C₃N₄ and show their excellent catalytic performance in the dehydrogenation of H₂NNH₂·H₂O under moderate conditions.

g-C₃N₄ was prepared via thermal condensation of melamine powder.⁵⁴ Melamine powder (5 g) was put in a covered alumina crucible, heated to 823 K at a rate of 5 K min⁻¹ and maintained at 823 K for 4 h in a nitrogen flow (30 mL min⁻¹). The final yellow product was collected and ground into powder with an agate mortar.

For the preparation of NiPt/g-C₃N₄ catalysts with variable Ni/Pt ratios (1 : 0, 1 : 4, 2 : 3, 3 : 2, 4 : 1, 0 : 1), 0.2 g of g-C₃N₄ was impregnated by a mixed solution containing 0.4 mmol of metal ions with variable molar ratios of Ni²⁺ and Pt⁴⁺, and the solution was diluted with distilled water to 20 mL. After stirring for 24 h at 298 K, a solution containing sodium borohydride (NaBH₄) and sodium hydroxide (NaOH) was added to the above-mentioned mixture while stirring vigorously at 273 K. The catalysts were extracted by centrifugation, washed with ethanol, and dried in vacuum at 373 K for 12 h.

The synthesis of the NiPt/g-C₃N₄ is displayed in Scheme 1. The NiPt/g-C₃N₄ catalysts were reproducibly prepared by this simple impregnation method, followed by reduction by the NaBH₄ and NaOH solution at 273 K.⁴⁹ After centrifugation and washing with copious amounts of water, NiPt/g-C₃N₄ was obtained, characterized using various techniques, and was used for the dehydrogenation of hydrous hydrazine. The molar ratio of the NiPt NPs in the catalysts was modified by tuning the initial composition of NiCl₂ and H₂PtCl₆, and determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). As shown in Table S1 (ESI†), the Ni/g-C₃N₄, Ni₂₄Pt₇₆/g-C₃N₄, Ni₃₇Pt₆₃/g-C₃N₄, Ni₅₈Pt₄₂/g-C₃N₄, Ni₈₂Pt₁₈/g-C₃N₄, and Pt/g-C₃N₄ catalysts were synthesized using metal precursors at Ni/Pt molar ratios of 1 : 0, 1 : 4, 2 : 3, 3 : 2, 4 : 1, and 0 : 1, respectively. The catalytic reactions were performed in alkaline solutions of hydrazine at 323 K, as shown in Fig. 1 and Table S2 (ESI†). The performances of the catalysts were distinctly improved when Pt was alloyed to Ni. Ni₃₇Pt₆₃/g-C₃N₄ showed the best catalytic performance and a maximum turnover frequency (TOF) of 570 h⁻¹ at 323 K, which is much higher than most of literature values for NiPt catalysts (Table 1).^45–49 The 100% hydrogen selectivity of the decomposition reaction of hydrous hydrazine over Ni₃₇Pt₆₃/g-C₃N₄ was verified using mass spectrometry (Fig. S1, ESI†). Pt/g-C₃N₄ was nearly catalytically inactive, but catalytic activity increased by alloying and increasing the composition of Ni, which highlights the existence of synergistic effects on the molecular level from alloying NiPt NPs.

Scheme 1 Schematic illustration of the synthesis of the NiPt/g-C₃N₄ catalysts.

Fig. 1 Time course plots for the decomposition of hydrous hydrazine over NiPt/g-C₃N₄ with NaOH (0.5 M) at 323 K (catalyst = 0.100 g; N₂H₄·H₂O = 0.1 mL).

Table 1 Comparison of the decomposition of hydrous hydrazine over different catalysts in this study and previous reports

Catalyst	T (K)	TOF (h^−1)	E a (kJ mol^−1)	Reference
Ni37Pt63/g-C3N4	323	570	36.6	This work
Ni0.99Pt0.01	323	12	49.95	59
Ni0.9Pt0.1/Ce2O3	298	28.1	42.3	36
Ni84Pt16/graphene	323	415	40	44
Ni88Pt12@MIL-101	323	375.1	51.29	43
Ni3Pt7/graphene	323	416	49.36	45
Ni60Pt40/CeO2	303	293	58.2	39
Ni64.1Mo11.5B24.4–La(OH)3	323	30	55.1	19
Ni–Al2O3-HT	303	3.9	49.3	17
NiIr0.059/Al2O3	303	12.4	38.6	24
NiPt0.057/Al2O3	303	16.5	34	30
Ni30Fe30Pd40 NPs	323	21.5	40.0	33
Ni85Ir15@MIL-101	323	464	66.9	53
Rh58Ni42@MIL-101	323	344	33	44
Ni3Rh7/NPC-900	323	156	—	40
Pt0.6Ni0.4/PDA-rGO	323	2056	33.39	48

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The dehydrogenation of hydrous hydrazine catalyzed by the Ni₃₇Pt₆₃/g-C₃N₄ was conducted at different temperatures (298–343 K) to acquire the activation energy (E_a) of this reaction. The values of the rate constant (k) were acquired from the slope of the linear part of the graph of dehydrogenation reaction versus time, at different temperatures (T) in Fig. 2(a). The Arrhenius plot (ln k vs. 1/T) for this catalyst is displayed in Fig. 2(b). E_a is determined to be 36.6 kJ mol⁻¹, which is smaller than most of literature value for this reaction over NiPt catalysts (Table 1). Furthermore, the recyclability of the Ni₃₇Pt₆₃/g-C₃N₄ catalyst was investigated during the decomposition of hydrazine hydrate at 323 K (Fig. S2, ESI†). After the four times cycle run, the catalytic activity of catalyst decreased slightly and hydrogen selectivity remained stable. As shown in Fig. S3,† the reason for the decrease in the dehydrogenation performance can be attributed to agglomeration of the particles, which is consistent with the literature findings.^47,49

Fig. 2 (a) Time course plots for hydrogen generation by the decomposition of hydrous hydrazine by Ni₃₇Pt₆₃/g-C₃N₄ at, 298 K, 313 K, 323 K, 333 K and 343 K. (b) Plot of ln k versus 1/T during the hydrous hydrazine decomposition over Ni₃₇Pt₆₃/g-C₃N₄ at different temperatures (catalyst = 0.100 g; N₂H₄·H₂O = 0.1 mL).

The X-ray diffraction (XRD) patterns of the g-C₃N₄, Ni/g-C₃N₄, Pt/g-C₃N₄, Ni₂₄Pt₇₆/g-C₃N₄, Ni₃₇Pt₆₃/g-C₃N₄, Ni₅₈Pt₄₂/g-C₃N₄ and Ni₈₂Pt₁₈/g-C₃N₄ are shown in Fig. S4 (ESI†). A distinct peak centered at 2θ = 27.5° is attributed to the XRD pattern of the g-C₃N₄.⁵⁷ The XRD patterns of the Ni/g-C₃N₄ and Pt/g-C₃N₄ catalysts (Fig. S4(b) and (c), ESI†) are consistent with the literature values (JCPDS, PDF 65-0380 for Ni and PDF 65-2868 for Pt), and confirm the presence of pure Ni and Pt NPs.³⁶ No peaks characteristic of pure Ni and Pt were observed in the XRD patterns of NiPt/g-C₃N₄ with different Ni/Pt molar ratios (Fig. S4(d–g), ESI†); this reveals the existence of a bimetallic phase rather than a mixture of monometallic Ni and Pt NPs.^{34–36,43,45–49} As seen in the literature,^{34–36,45–49} the diffraction peaks of all the NiPt nanocatalysts are partially shifted compared to those of pure Ni and Pt NPs; this highlights that an alloy has formed by expanding the Ni crystal lattice through the substitution of larger Pt atoms for the smaller Ni atoms. Additionally, the morphologies of the as-synthesized Ni₃₇Pt₆₃/g-C₃N₄ catalysts were observed using transmission electron microscopy (TEM). The high-resolution TEM (HRTEM) image in Fig. 3(a–d) demonstrates the crystalline nature of the Ni₃₇Pt₆₃ NPs. The lattice spacing is calculated to be 0.210 nm, which lies between the (111) plane of face-centered cubic (fcc) Ni (0.204 nm) and fcc Pt (0.227 nm), further demonstrating the formation of an alloy structure of NiPt NPs.^{34–36,43,45–49} Fig. S5 (ESI†) shows that the NiPt NPs have a narrow size distribution with a mean particle size of 3.2 ± 0.2 nm.

Fig. 3 (a–d) TEM images of Ni₃₇Pt₆₃/g-C₃N₄ with different magnification.

The elemental chemical states of the Ni₃₇Pt₆₃/g-C₃N₄ were elucidated using X-ray photoelectron spectroscopy (XPS). The high-resolution XPS spectra for C 1s, N 1s, Ni 2p, and Pt 4f in Ni₃₇Pt₆₃/g-C₃N₄ are shown in Fig. S6 (ESI†). The XPS spectra of C 1s are centered at 287.8 and 284.6 eV. The first peak at 287.8 eV is attributed to the sp²-hybridized carbon atoms in the aromatic rings of g-C₃N₄, while the other peak at 284.6 eV is assigned to fortuitous carbon impurities. The N 1s spectrum of Ni₃₇Pt₆₃/g-C₃N₄ shows two distinct peaks at binding energies of 399.8 and 398.1 eV that are attributed to amino groups (C–N–H) and the sp²-hybridized nitrogen atoms of triazine (C=N–C) units, respectively. These assignments are in good agreement with the reported values for g-C₃N₄.⁵⁷ Furthermore, the XPS results (Fig. S6, ESI†) of Ni₃₇Pt₆₃/g-C₃N₄ demonstrate the presence of metallic Ni⁰ with Ni 2p_3/2 at 855.2 eV and Ni 2p_1/2 at 872.7 eV, as well as metallic Pt⁰ with Pt 4f_7/2 71.5 eV and Pt 4f_5/2 74.7 eV.^{34–36,43,45–49} The binding energies for Ni 2p and Pt 4f in Ni₃₇Pt₆₃ NPs were different from those of pure Ni and Pt NPs; thus, the XPS results confirm the alloy structure of Ni₃₇Pt₆₃ NPs in Ni₃₇Pt₆₃/g-C₃N₄. The formation of oxidized Ni centered at 861.1 and 879.3 eV is a result of the reported sample preparation process for XPS measurements.^43,46,49 The results obtained from XRD, HRTEM, and XPS all demonstrate that NiPt NPs form an alloy when deposited on the surface of g-C₃N₄.

In conclusion, well-dispersed NiPt NPs have been successfully immobilized on g-C₃N₄ using a simple co-reduction method, which catalyze the dehydrogenation of hydrous hydrazine in an alkaline solution. The dehydrogenation performance of the catalysts depends on the composition of NiPt in the as-synthesized catalysts, indicating a synergistic molecular-scale alloy effect in bimetallic NiPt NPs. The Ni₃₇Pt₆₃/g-C₃N₄ catalysts demonstrate 100% hydrogen selectivity and remarkable catalytic performance with a maximum TOF value of 570 h⁻¹ at 323 K, which is much higher than most of literature values for NiPt catalysts (Table 1). Designing and developing these high-performance catalysts using g-C₃N₄ as a support may accelerate the use of hydrous hydrazine as a hydrogen storage material.

Acknowledgements

Financial supports from National Natural Science Foundation of China (21376005, 21476001) and the Program for Zhejiang Leading Team of S&T Innovation (2013TD07) are gratefully appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01335j

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