Cu²⁺-induced room temperature hydrogen release from ammonia borane

Abstract

Mechanical stirring of ammonia borane with CuCl₂ in the solid state resulted in the release of hydrogen at room temperature through the intermediacy of [NH₄]⁺[BCl₄]⁻.

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Broader context

Hydrogen is a very promising candidate that is tipped to meet future energy demands. Developing methodologies to store and release hydrogen around ambient conditions is one of the major hurdles facing the so-called hydrogen economy. Ammonia borane (H₃N·BH₃) with its striking 19.6 weight% hydrogen is being widely investigated as the material to meet these challenges. The present study reveals a simple but very effective way to release hydrogen from ammonia borane at temperatures as low as room temperature. At a loading of only 5 mol% CuCl₂ ∼9 wt% of hydrogen is released in 6.5 h at ∼80 °C, the temperature at which proton exchange membrane (PEM) fuel cells operate. The reaction for hydrogen generation proceeds through a new intermediate [NH₄]⁺[BCl₄]⁻ which excludes the formation of borazine byproduct. The simplicity and cost effectiveness of this procedure makes this reaction scheme extremely attractive for real time applications.

Ammonia borane, AB a compound with 19.6 wt% of hydrogen content is under intense investigation as a chemical hydrogen storage medium in recent times.¹ Ammonia borane is a stable white crystalline solid under ambient conditions. Hydrogen can be released from ABviathermolysis or by catalytic dehydrogenation in protic solvents. Metal catalyzed AB hydrolysis or methanolysis of AB has been studied using both precious and first row transition metals.²Regeneration of AB from final products possessing B–O fragments, of hydrolysis or methanolysis reactions, is a highly energy intensive process. Metal complexes such as [Ir(H)₂(POCOP)] (POCOP = η³-1,3-(OP-tert-Bu)₂C₆H₃), Ni–NHC (NHC = N-heterocyclic carbene), Cp₂Ti derivatives, [Rh(1,5-COD)Cl]₂, and Re–nitrosysls have been found to be very effective in dehydrogenation of AB in non-aqueous media.³ The drawback in these cases is that the solvent contributes to the weight of the system leading to an overall lower hydrogen wt% of the system. In this regard, hydrogen release from ABvia solid state reactions is more practicable for mobile applications. The decomposition of neat AB is a multi-step process. Even though AB liberates hydrogen below 100 °C, the kinetics are sluggish with longer induction periods. Rapid release of one equivalent of hydrogen occurs upon melting of AB (around 110 °C). The second equivalent of hydrogen can be liberated from AB above 130 °C.⁴ Realization of faster hydrogen release rates of two moles of H₂ (∼14 wt%) in a proton-exchange membrane fuel cell operating at a temperature below 80 °C is crucial for real time on-board applications.^1aIn-situ¹¹B NMR spectral studies on neat AB at 88 °C show that the formation of diammoniate of diborane (DADB) is the critical step in the decomposition of AB.⁵ Corroborating this, recently it has been shown that addition of DADB to AB decreases the induction time significantly.⁶ Tom Autery and co-workers used mesoporous silica (SBA-15) for enhanced hydrogen release rates.⁷ Ionic liquids which can stabilize the ionic DADB have also been found to be effective in dehydrogenation of AB.⁸LiH and NaH are found to destabilize AB by producing amido boranes.⁹ But very recently, Fijałkowski and Grochala have shown that thermolysis of sodium amidoborane liberates substantial amount of ammoniavia the [NaNH₃]⁺[BH₃(NHNa)BH₃]⁻ intermediate.¹⁰ In this context, chemical modification of AB for low temperature hydrogen release with high purity is highly desirable.

We recently reported Co²⁺-, Ni²⁺- and Cu²⁺- assisted AB hydrolysis for hydrogen generation.^2e It was demonstrated that metal atoms/clusters formed in-situ during the hydrolysis were the real catalysts. Herein, we report our studies on the Co²⁺-, Ni²⁺- and Cu²⁺-induced H₂ release from AB in the solid state.¹¹

Mechanical stirring of CoCl₂, NiCl₂ and CuCl₂ with AB in the solid state resulted in significant hydrogen evolution at 60 °C. Fig. 1a shows a comparison of Co²⁺-, Ni²⁺-and Cu²⁺- (M²⁺/AB = 0.15) induced H₂ evolution from AB at 60 °C. As evident from the figure, Cu²⁺-induced H₂ evolution was found to be more efficient than that of Co²⁺ and Ni²⁺. Moreover, in the case of Cu²⁺, liberation of H₂ started almost instantaneously, whereas in the case of Co²⁺ and Ni²⁺, H₂ release was slower during the initial periods of thermolysis. For a 4 h reaction time, the Co²⁺-, Ni²⁺- and Cu²⁺-induced reaction generated ∼1.1, 0.5, and 2.0 equivalents of hydrogen respectively. Gislon and co-workers reported ball milling of AB with 1 wt% of H₂PtCl₆ that resulted in the liberation of about 1 mole of hydrogen at 138 °C in 1.25 h.¹² Ammonia borane loaded with 5 wt% Li/CMK-3 framework released ∼7 wt% (excluding Li/CMK) of hydrogen around 60 °C.¹³ Even though lithium and sodium amido boranes were shown to liberate two moles of hydrogen with good kinetics, recent studies have shown that these materials release a substantial amount of ammonia along with H₂. Hydrogen released from AB induced by Cu²⁺ (Cu²⁺/AB = 0.15) was found to be more than that of Co²⁺ and Ni²⁺; 2 moles of H₂ were liberated in 4 h at 60 °C, which is well below the fuel cell operating temperature. Considerable amounts of H₂ release was noted even at room temperature. The CuCl₂–AB mixture was stirred at room temperature for 24 h. The room temperature hydrogen release for various Cu²⁺ concentrations is shown in Fig. 1b. To the best of our knowledge, this is the best system realized to date for the release of hydrogen from AB in the solid state. In this context the results obtained here are significant; around 8 wt% of hydrogen was obtained in the Cu²⁺-induced reaction at 60 °C, in addition to our findings that the H₂ release can take place at room temperature.‡

Fig. 1 (a) Hydrogen evolution from M²⁺–AB mixtures at 60 °C (M²⁺ = Co²⁺, Ni²⁺, and Cu²⁺; M²⁺/AB = 0.15); (b) Cu²⁺-induced hydrogen evolution from AB at room temperature for various Cu²⁺ concentrations.

Volumetric H₂ measurements for H₂ release at 60 °C were carried out by varying Cu²⁺ concentration (Fig. 2a) and also by varying the temperature (Fig. 2b) for a fixed Cu²⁺ concentration (Cu²⁺/AB = 0.05). Increase in Cu²⁺ concentration or temperature resulted in faster H₂ release from AB.

Fig. 2 (a) Hydrogen generation from Cu²⁺–AB composite at 60 °C for various Cu²⁺ concentrations; (b) Hydrogen generation from Cu²⁺–AB composite (Cu²⁺/AB = 0.05) at different temperatures.

Fig. 3a shows a comparison between the thermal behavior of neat AB and the Cu²⁺/AB combination. Neat ammonia borane showed two weight losses as expected around 100 °C and 150 °C.⁴ The Cu²⁺/AB system showed an initial weight loss of around 10 wt% below 60 °C, which was assigned to the loss of water from CuCl₂ and also H₂ release from AB. Evidence for this assignment was obtained from mass profiles of m/z = 2 (Fig. 3b) and m/z = 17,18 (see ESI†). Exposure of anhydrous CuCl₂ to the atmosphere although brief, (1–2 min) during sample preparation and loading, in the TGA-MS instrument could not be avoided thereby enabling the sample to pick up some water from the atmosphere. The 10 wt% loss suggests the presence of approximately one water molecule per CuCl₂. For Cu²⁺/AB = 0.05, a maximum of 0.016 equivalent of AB can undergo hydrolysis reaction, which is almost negligible. No detectable borate species were observed by IR and NMR spectroscopies, suggesting that the H₂ evolution was solely due to thermolysis of AB and ruled out the hydrolysis pathway. Attempts to detect ammonia which has a m/z value very close to that of water were unsuccessful. No change in the pH of water in a bubbler through which the evolved gas was continuously purged, was noted. Borazine which is a major impurity in the decomposition of neat AB was not observed in the mass profile of m/z = 80. These observations rule out NH₃ and borazine evolution and show that the H₂ evolved is highly pure.

Fig. 3 (a) TG profiles of neat AB (solid line) and Cu²⁺/AB composite (dashed line); (b) Synchronous MS profiles of m/z = 2 (H₂), m/z = 80 (borazine). The ramp rate is 2 °C min⁻¹.

In order to probe the reaction further, we recorded ¹¹B MAS-NMR spectrum of Cu²⁺/AB combination after thermolysis at 60 °C for 1.5 h. The NMR spectrum (Fig. 4) showed four broad peaks centered around δ −28, 3, 12, and 23.

Fig. 4 ¹¹B MAS NMR spectrum of Cu²⁺/AB composite after 1.5 h of thermolysis at 60 °C.

The latter two peaks can be assigned to B(H)N₂ and BN(H)₂ of polymeric amino borane (PAB).¹⁴ We tentatively assign the peak centered around δ 3 to [NH₄]⁺[BCl₄]⁻ based on the following: (a) the ¹¹B NMR resonance for H₃N·BCl₃ was observed at δ 4.1 by Hu and Geanangel¹⁵ and (b) Landesman and Williams found that the chemical shifts of BCl₃.NR₃ and BCl₄⁻ to be very similar.¹⁶

The broad peak around δ −28 can be assigned to µ-aminodiborane [B₂H₅(µ-NH₂)].^15,17 Also a shoulder was observed around δ −2 which can be assigned to the Cl₂BHNH₃ species.¹⁵ The powder XRD pattern (see ESI†) of the final product showed reflections corresponding to Cu(0) and very weak reflections assignable to NH₄Cl. All of these observations are consistent with eqn (1).

H₃N·BH₃ + 2CuCl₂ → [NH₄]⁺[[BCl₄]⁻ + H₂ + 2Cu(0) (1)

We believe that the formation of [NH₄]⁺[BCl₄]⁻ is the nucleation step in the Cu²⁺-induced decomposition of AB. [NH₄]⁺[BCl₄]⁻ can propagate the reaction by reacting further with AB and can form intermediates of the type ClBH₂NH₃ and Cl₂BHNH₃. The latter species was observed in the ¹¹B MAS-NMR spectrum. Also, the observation of µ-aminodiborane corroborates the formation of ClBH₂NH₃ as an intermediate. Similar type of intermediates were observed when BCl₃ was treated with AB in ether solution.¹⁵ To test the catalytic activity of in-situ generated Cu(0) nanoparticles, Cu nanoparticles (∼20 nm) were added to neat AB and thermolysis was carried out at 60 °C. Only 0.47 moles of H₂ were liberated in 17 h from this reaction. Solid state decomposition of neat AB involves formation of the diammoniate of diborane (DADB) in the nucleation step. Addition of CuCl₂ to AB provides an alternative path for the decomposition of ABvia the intermediate [NH₄]⁺[BCl₄]⁻. This eliminates the route to borazine formation and changes the overall feature of hydrogen release.

In conclusion, addition of CoCl₂, NiCl₂ or CuCl₂ salts to neat AB resulted in significant destabilization of AB. Cu²⁺-induced dehydrogenation of AB released a considerable amount of hydrogen even at room temperature. For Cu²⁺/AB = 0.05, 2 moles of H₂ were liberated in 6.5 h, which is equal to 9 wt% of the system. Intermediacy of [NH₄]⁺[BCl₄]⁻ eliminates the formation of borazine impurity thereby affording pure H₂. These features and the additional cost effectiveness of the first row transition metal salts used for AB dehydrogenation make this reaction scheme extremely attractive.

Acknowledgements

We are grateful to the Board of Research in Nuclear Sciences, Department of Atomic Energy, India for the financial support. SBK thanks the Council of Scientific & Industrial Research, India for a fellowship.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, powder XRD. See DOI: 10.1039/b909448m

‡ Caution:Milling of anhydrous CuCl₂ and ammonia borane can result in eruption of fire.

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