Ruthenium immobilized on Al₂O₃ pellets as a catalyst for hydrogen generation from hydrolysis and methanolysis of sodium borohydride

Abstract

Ru immobilized Al₂O₃ pellets were synthesized as a catalyst for hydrogen generation in NaBH₄ solutions. Under the Ru/Al₂O₃ catalysis, the hydrogen generation rate from the hydrolysis and methanolysis of NaBH₄ in alkaline solution was explored as a function of NaBH₄ concentration and temperature. Meanwhile, the borate byproducts derived from the NaBH₄ hydrolysis were characterized by XRD and NMR. The results showed that the hydrogen generation rate depended on NaBH₄ concentration for the hydrolysis, but did not for the methanolysis. 10 wt% NaBH₄ hydrolysis containing 1 wt% NaOH catalyzed with 0.5 g Ru/Al₂O₃ could generate hydrogen with a rate of about 65.5 ml min⁻¹ g⁻¹. The productivity and activation energy of hydrogen generation from the NaBH₄ hydrolysis were 93% and 41.8 kJ mol⁻¹, and from the NaBH₄ methanolysis were 97% and 51.0 kJ mol⁻¹, respectively. The XRD and NMR analyses showed that the species from the hydrolysis byproducts varied with the NaBH₄ concentration could be the borax (Na₂B₄O₇·H₂O) and sodium borate hydrate (NaBO₂·H₂O).

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1. Introduction

Air pollution from burning substances containing hydrocarbons and fossil-fuels has deteriorated sharply in decades. Novel clean and renewable energy sources are being developed for solving this problem. It is well known that the hydrogen is the preferred fuel source for providing clean energy through proton exchange membrane (PEM) fuel cells. Hydrogen gas is an anodic fuel for PEM fuel cells in which the only byproduct is water. The controllable generation and purity of hydrogen has made the hydrolysis of chemical hydrides widely studied.¹ Among the chemical hydrides, NaBH₄ contains a theoretical hydrogen content of 10.8 wt% and has many merits including stability, nonflammable in alkaline solution and recycled byproducts.^2–4 The self-hydrolysis of NaBH₄ could release hydrogen spontaneously in water:^5–8

NaBH₄ + 2H₂O → 4H₂ + NaBO₂ + Heat (−300 kJ mol⁻¹) (1)

However, NaBH₄ often needs a suitable catalyst to encourage the hydrogen generation in alkaline solution. Numerous catalysts, such as noble metal Ru,^9–11Rh,^12,13Pt,^14,15PtRu,¹⁶ and transition metal Co,^17,18Ni^19,20 and Fe²¹ have been investigated. Among metal elements, ruthenium (Ru) is the most efficient catalyst to liberate the hydrogen form NaBH₄ solutions; Brown and Brown have observed that those metallic elements exerted a catalytic effect on NaBH₄ hydrolysis with an order: Ru, Rh > Pt > Co > Ni > Os > Ir > Fe ≫ Pd.⁷

On the other hand, the hydrolysis of borohydride is not efficient at low temperature which may be inconvenient for automotive and portable applications in high latitude regions. Lo et al.²² reported that NaBH₄ could generate hydrogen in methanol at low temperatures (253–323 K). NaBH₄ is not only soluble in water (0.55 g g⁻¹ of H₂O at 298 K), but also soluble in low molecular weight alcohols (0.164 g g⁻¹ of CH₃OH at 293 K);^23–25 4 moles hydrogen were generated from 1 mole NaBH₄ methanolysis:

NaBH₄ + 4CH₃OH → 4H₂ + NaB(OCH₃)₄ (2)

Since the composite concept of catalysts is appropriate for easy solid–liquid separation, the controllable initiation or halt of hydrogen generation is therefore highly appreciated in practical applications. Based on the high efficiency of using Ru to catalytically liberate the hydrogen from alkaline NaBH₄, this study used an Al₂O₃ pellet to immobilize the metallic Ru. Meanwhile, through the Ru/Al₂O₃ catalysis, the properties of NaBH₄ hydrolysis and methanolysis were compared, including the effects of NaBH₄ concentrations, temperatures (273–303 K), and solvents (water and methanol). In addition, the characterization of borate byproduct collected after NaBH₄ hydrolysis was provided by using XRD and NMR analyses.

2. Materials and methods

2.1 Materials

Sodium borohydride (NaBH₄, 95%, Riedel-de Haën), sodium hydroxide (NaOH, 99%, Riedel-de Haën), sodium metaborate tetrahydrate (NaBO₂·4H₂O, 98%, Alfa Aesar), ruthenium(III) chloride hydrate (RuCl₃·nH₂O, Aldrich) and methanol (CH₃OH, ECHO) were all analytical grade and were used without further purification. Al₂O₃ pellets (Alfa Aesar) pre-calcined at 873 K for 6 h were used as the support for immobilizing Ru; the calcined Al₂O₃ was mixed with an appropriate quantity of RuCl₃·nH₂O dissolved in distilled water, and then soaked at room temperature for 24 h. Afterwards, the Ru/Al₂O₃ precursor was filtered and then loaded in oven at 393 K with N₂ atmosphere to remove water. Finally, the dried precursor was calcined at 823 K for 3 h and reduced in hydrogen (40% H₂/Ar) at 973 K for 6 h to obtain the Al₂O₃ supported zerovalent Ru⁽⁰⁾ catalysts.

2.2 Hydrogen generation

The hydrogen generation experiment was performed in a three necked flask in which the dissolved NaBH₄ and NaOH were mixed with the catalyst in 15 ml water. The hydrogen generation rate and activity energy using the Ru/Al₂O₃ catalysts were determined by recording the volume of hydrogen released periodically. The temperature of the water bath was maintained with a stability of ± 0.1 °C using a thermostatic circulator.

2.3 Characterization of the Ru/Al₂O₃ catalysts and spent NaBH₄

The surface morphology of the Ru/Al₂O₃ catalysts was examined using a scanning electron microscope (SEM, JOEL JXA-840, HITACHI S4100), whereas the elemental compositions on the catalyst surface were analyzed by the attached energy dispersive spectrometer (EDS, LINKS AN10000/85S). The crystallization phase of byproducts from spent NaBH₄ after hydrogen generation was probed by X-ray powder diffraction obtained at room temperature using a Rigaku RX III powder diffractometer with a Cu Kα radiation. The scanning speed is 0.08 degree per 1 s counting time, while the generator tension was 40 kV/30 mA. Moreover, the chemistry of spent NaBH₄ was measured with a nuclear magnetic resonance (NMR) which was recorded in D₂O at 500 MHz on a Bruker AV-500 NMR spectrometer.

3. Results and discussion

Fig. 1 indicates that the size of the Ru/Al₂O₃ catalyst is about a millimetre in thickness and the surface morphology under higher magnification shows the coverage with Ru flakes. The EDS analysis of the catalysts listed in Table 1 suggests that about 5 wt% of the Ru was coated on the surface of the Al₂O₃. The hydrogen generation from NaBH₄ hydrolysis and methanolysis using Ru/Al₂O₃ catalysts without alkaline were compared in Fig. 2. According to eqn (1) and (2), 1 mole NaBH₄ hydrolysis/methanolysis would generate 4 moles hydrogen gas. The volumes of hydrogen generated with different solvents in 1.0 wt% NaBH₄ without NaOH at 298 K are almost linearly proportional to the reaction time which indicated their stable catalytic activity. Before the reaction finished, the rate of hydrogen generation decreased due to the low concentration of NaBH₄. The final volumes of hydrogen generated by NaBH₄ hydrolysis and methanolysis were similar. The hydrogen productivity was calculated according to the following equation, where Vr is the real volume of hydrogen, Vt is the theoretical volume of hydrogen, and W_NaBH4 and M_NaBH4 are the weight and molecular weight of NaBH₄, respectively:

Productivity (%) = (Vr/Vt) × 100 (3)

Vt = (W_NaBH4/M_NaBH4) × (4/1) × 24.5 (L mol⁻¹ at 298 K) (4)

Fig. 1 Scanning electron micrographs of Ru/Al₂O₃ catalysts (a) 25×, (b)50 000×.

Fig. 2 Effect of (a) hydrolysis and (b) methanolysis on hydrogen generation in the presence of 1.0 wt% NaBH₄ at 298 K using 0.2 g Ru/Al₂O₃ catalysts.

Table 1 Surface compositions present in the Ru/Al₂O₃ catalysts by EDS

Element	Weight (%)	Atomic (%)
a Pt is sputtered on sample prior to SEM/EDS analysis to increase the electric conductivity of the sample.
O	44.71	62.84
Al	42.05	35.05
Ru	5.43	1.21
Pt ^a	7.81	0.90

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The hydrogen productivities of the hydrolysis and methanolysis of NaBH₄ are 93% and 97%, respectively. Furthermore, the initial hydrogen generation rates (ml min⁻¹ g⁻¹Ru/Al₂O₃ cat.) of the NaBH₄ hydrolysis and methanolysis independently calculated by the mathematical linear regression are 34.9 and 204.3 ml min⁻¹ g⁻¹Ru/Al₂O₃ cat., respectively. Both the productivity and the hydrogen generation rate of NaBH₄ methanolysis without NaOH addition were better than those for NaBH₄ hydrolysis.

3.1 The effect of NaBH₄ concentration

The effect of NaBH₄ concentration on the hydrogen generation rate at 298 K using 0.5 g Ru/Al₂O₃ catalysts was shown in Fig. 3. As the NaBH₄ concentration increased to 7.5 wt%, the generation rate could be increased to 64.5 ml min⁻¹ g⁻¹ (Fig. 3 (a)); however, the generation rate decreased from 65.5 to 31 ml min⁻¹ g⁻¹ (Fig. 3 (b)) with increasing the NaBH₄ from 10 to 30 wt%. Fig. 4 shows viscosities and temperatures as a function of NaBH₄ concentration. The excessive NaBH₄ (more than 10 wt%) would increase the solution viscosity, but did not significantly change the solution temperature. Solution viscosity affecting the hydrogen generation from NaBH₄ hydrolysis has been previously reported in the literature.^17,25,26 A higher viscosity solution, due to the excessive NaBH₄, was shown to inevitably retard the aqueous mass transfer and reduce the hydrolysis efficiency. Fig. 5 shows the hydrogen generation rates of methanolysis which almost maintained at around 6 ml min⁻¹ g⁻¹ as the NaBH₄ concentration ranged from 1 to 10 wt%. Accordingly, Fig. 6 compares the hydrogen generation rate of NaBH₄ hydrolysis with that of methanolysis as a function of NaBH₄ concentration in NaOH solution. The hydrogen generation obviously depended on the NaBH₄ concentration for the hydrolysis, but did not for the methanolysis. Meanwhile, the NaOH played the role of stabilizer for NaBH₄ hydrolysis which greatly reduced the hydrogen generation rate of NaBH₄ methanolysis from 204.3 to 6 ml min⁻¹ g⁻¹. NaBH₄ methanolysis has been known to be a good first-order reaction, of which the rate constant would be remarkably reduced by the trace amount of alkaline.²⁴

Fig. 3 The effect of (a) low and (b) high concentrations of NaBH₄ on hydrogen generation in the hydrolysis of NaBH₄ containing 1 wt% NaOH at 298 K using 0.5 g Ru/Al₂O₃ catalysts.

Fig. 4 The solution viscosity and temperature as a function of NaBH₄ concentration in water.

Fig. 5 The effect of various concentrations of NaBH₄ on hydrogen generation from the methanolysis of NaBH₄ containing 1 wt% NaOH at 298 K using 0.5 g Ru/Al₂O₃ catalysts.

Fig. 6 The rates of hydrogen generated by the (a) hydrolysis and (b) methanolysis of NaBH₄ containing 1 wt% NaOH at 298 K using 0.5 g Ru/Al₂O₃ catalysts.

3.2 Effect of temperature

Fig. 7 shows effect of temperature in the range 278–308 K on the hydrogen generation rate of NaBH₄ hydrolysis using the Ru/Al₂O₃ catalyst in alkaline solution. The hydrogen generation rate, that increased with increasing temperature, (Fig. 7 (a)) could be regarded as a zero-order reaction and could be written as:where k_NaBH4 is the hydrogen generation rate (mol min⁻¹ g⁻¹) of NaBH₄, k₀ is the pre-exponential parameter (mol min⁻¹ g⁻¹), E_a is the activation energy of the reaction, R is the gas constant and T is reaction temperature in Kelvins.²⁷

Fig. 7 (a) The effect of temperature on the volume of hydrogen generation in NaBH₄ hydrolysis and (b) the Arrhenius plot obtained from different temperatures in the presence of 12.5 wt% NaBH₄ containing 1 wt% NaOH using 0.5 g Ru/Al₂O₃ catalysts.

Fig. 7 (b) shows the Arrhenius plot of NaBH₄ hydrolysis in which the activation energy could be derived from the slope of ln k linearly reciprocal to absolute temperature (1/T). In addition, the hydrogen generation rate of NaBH₄ methanolysis in 273–308 K [in Fig. 8 (a)] also represents a similar trend to that of NaBH₄ hydrolysis. The calculated activation energies of the NaBH₄ hydrolysis and methanolysis reactions were 41.8 and 51.0 kJ mol⁻¹, respectively (Fig. 7 (b) and Fig. 8 (b)). Table 2 summarizes the metallic catalysts used for NaBH₄ hydrolysis in the literature and suggests that the Ru/Al₂O₃ catalyst in this study exhibits lower activation energy and is highly efficient for use in hydrogen generation.

Fig. 8 (a) The effect of temperature on volume of hydrogen generation in NaBH₄ methanolysis and (b) the Arrhenius plot obtained from different temperatures in the presence of 10 wt% NaBH₄ containing 1 wt% NaOH using 0.5 g Ru/Al₂O₃ catalysts.

Table 2 The activation energies of various catalysts

Catalyst	Condition		Activation energy (kJ mol^−1)	Reference
	[NaBH4]	[NaOH]
Co	0.1 mol dm^3	2 mol dm^3	75.0	Kaufman, 1985^28
Ni			71.0
RANEY® Ni			63.0
Ru/LiCoO2	10.0 wt (%)	5.0 wt (%)	68.5	Liu, 2008^29
Ni–Ru nanocomposite	5.0 wt (%)	5.0 wt (%)	52.7	Liu, 2009^5
Ru/IRA-400	20.0 wt (%)	10.0 wt (%)	47.0	Amendola, 2000^9
	7.5 wt (%)	1.0 wt (%)	56.0
Ni powder Co powder	10.0 M	10.0 wt (%)	62.7	Liu, 2006^19
RANEY® Ni	10.0 M	20.0 wt (%)	41.9
RANEY® Co			50.7
RANEY® Ni50Co50			53.7
			52.5
Ru/Al2O3	12.5 wt (%)	10 wt (%)	14.8	This paper

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3.3 Characterization of byproducts after NaBH₄ hydrolysis

It is worthy to provide the definite information about the characterization of byproducts in spent NaBH₄ solution as its recycling and reuse after hydrogen generation is an issue. Fig. 9 shows the XRD analysis of white crystals collected in the solution after the NaBH₄ hydrolysis (1 and 10 wt%) catalyzed with Ru/Al₂O₃. According to eqn (1), sodium metaborate (NaBO₂) is the mainly byproduct after NaBH₄ hydrolysis. As compared with the pure borax, however, the crystallization database fitting indicated that the byproduct probably belonged to the Na₂B₄O₇·10H₂O or Na₂B₄O₅(OH)₄·8H₂O phase. White crystals were also found for the 12.5–30 wt% NaBH₄ hydrolysis; as shown in Fig. 10, the diffraction peaks of the byproduct well fit with the sodium borate hydrate (NaBO₂·2H₂O). This result suggested that the byproduct compound from NaBH₄ hydrolysis would depend on the NaBH₄ concentration.

Fig. 9 X-Ray diffraction patterns of (a) pure borax and byproducts after the hydrolysis of (b) 1wt% NaBH₄ and (c) 10 wt% NaBH₄. containing 1wt% NaOH using 0.5 g Ru/Al₂O₃ catalysts at 298 K.

Fig. 10 X-Ray diffraction patterns of byproducts after the hydrolysis of (a) 12.5 wt% NaBH₄ (b) 15 wt% NaBH_4, (c) 20 wt% NaBH₄, and (c) 30 wt% NaBH₄ containing 1 wt% NaOH using 0.5 g Ru/Al₂O₃ catalysts at 298 K.

In Fig. 11, the chemistry of the byproducts collected from NaBH₄ hydrolysis with various concentrations is analyzed by ¹¹B NMR spectra. Compared to the boron trifluoride diethyl etherate (BF₃·C₄H₁₀O) the chemical shifts were referenced to the pure Na₂B₄O₇·10H₂O sample at −9.6 ppm and the pure NaBO₂·4H₂O at −15.5 ppm, respectively. A main signal corresponding to the byproduct from 10 wt% NaBH₄ hydrolysis is observed at −8 ppm, which is relatively similar to the pure borax (Na₂B₄O₇·10H₂O). For the byproducts from higher NaBH₄ concentrations (15–30 wt%), the shifts at around −16 ppm could be assigned to the sodium borate hydrate (NaBO₂·4H₂O). The chemistry of the byproducts from NaBH₄ hydrolysis identified by the ¹¹B NMR spectra was in good agreement with that of the XRD analysis.

Fig. 11 ¹¹B NMR spectra of byproducts after the hydrolysis of (a) 10 wt% (b) 15 wt% (c) 20 wt% (d) 30 wt% NaBH₄ containing 1 wt% NaOH and 0.5 g Ru/Al₂O₃ catalysts.

4. Conclusion

This study confirmed that the productivity and hydrogen generation rates of the NaBH₄ methanolysis (204.3 ml min⁻¹g⁻¹) without NaOH are better than those of the NaBH₄ hydrolysis (34.9 ml min⁻¹g⁻¹). The productivities of hydrogen are 93% and 97% for NaBH₄ hydrolysis and methanolysis, respectively. In alkaline solution (1 wt% NaOH), however, the hydrogen generation rate depended on the NaBH₄ concentration for the NaBH₄ hydrolysis, but did not for the NaBH₄ methanolysis. By employing 0.5 g Ru/Al₂O₃ to catalyze the NaBH₄ hydrolysis, the hydrogen generation rate increased from 24.6 to 65.5 ml min⁻¹ g⁻¹ as the NaBH₄ concentration increased from 1 to 10 wt%, and decreased to 31 ml min⁻¹g⁻¹ with further increasing the NaBH₄ to 30 wt%. Moreover, hydrogen generation rate could only be maintained at around 6 ml min⁻¹ g⁻¹ with increasing NaBH₄ from 1 to 10 wt%. On the other hand, the activation energies of the NaBH₄ hydrolysis and methanolysis catalyzed with Ru/Al₂O₃ were 41.8 kJ mol⁻¹ and 51.0 kJ mol⁻¹ at 273–303 K, respectively, which were lower than those reported in the literature. Both the XRD and NMR analyses showed that the byproduct compounds from NaBH₄ hydrolysis depend on the NaBH₄ concentration. The byproduct was borax (Na₂B₄O₇·H₂O) for low NaBH₄ concentrations (10 wt%) and sodium borate hydrate (NaBO₂·H₂O) for high NaBH₄ concentrations (15–30 wt%).

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