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J. Phys. Chem. C 2008, 112, 15886–15892
New Insights into Catalytic Hydrolysis Kinetics of Sodium Borohydride from Michaelis-Menten Model Hong-Bin Dai, Yan Liang, Lai-Peng Ma, and Ping Wang* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China ReceiVed: December 18, 2007; ReVised Manuscript ReceiVed: July 9, 2008
Catalytic hydrolysis kinetics is a subject of great interest in the study of sodium borohydride (NaBH4)-based hydrogen generation system. In the present paper, we report our coupled experimental/model analysis studies on the catalytic hydrolysis kinetics of NaBH4 with the presence of Co-B catalyst. The effects of NaBH4 concentration and Co-B catalyst amount on the hydrolysis kinetics of NaBH4 were experimentally examined and analyzed in terms of the Michaelis-Menten model. It was found that the catalytic hydrolysis reaction of NaBH4 follows first-order kinetics at low NaBH4 concentration and zero-order kinetics at high NaBH4 concentration. The hydrolysis kinetics is first-order with respect to catalyst amount. These findings provide valuable insights into the catalytic hydrolysis kinetics of NaBH4. catalyst
1. Introduction
NaBH4 + 4H2O 98 NaB(OH)4(NaBO2 • 2H2O) +
On-board hydrogen storage remains a grand scientific and technical challenge in commercialization of the hydrogenpowered vehicles.1 Compared to pressurized tanks and cryogenic liquid hydrogen, reversible hydrogen storage materials hold greater promise to provide viable on-board hydrogen sources, owing to their high volumetric hydrogen capacity, favorable energy efficiency, and safety advantage. However, decades of extensive efforts on metal/alloy hydrides, nanostructured carbon, and complex hydrides have led to no viable system that can reversibly store >6 wt % hydrogen at temperatures that are relevant to the practical operation of polymer electrolyte membrane fuel cell.1-3 Recently, on-demand hydrogen generation from chemical hydrides, in conjunction with off-board spent fuel regeneration, attracts considerable interest as an important alternative for on-board hydrogen storage. In contrast to the reversible hydrogen storage means, which requires simultaneous satisfaction of both dehydrogenation and rehydrogenation constraints, irreversible chemical hydrogen storage allows us to deal with hydrogen discharging and recharging processes separately. This may enhance the possibility to achieve high-performance hydrogen release and efficient hydride regeneration. Among the chemical hydrides of interest, sodium borohydride (NaBH4) receives the most extensive attention owing to its combined advantages of high hydrogen capacity (with a theoretical value of 10.8 wt %), good storability and reaction controllability, low reaction-initiating temperature, and the environmentally benign hydrolysis product (borax, NaBO2).4,5 In an early example, Millenium Cell Inc. successfully demonstrated a NaBH4-based Hydrogen on Demand system in the Daimler Chrysler’s Natrium fuel cell minivan.6 * Author to whom correspondence should be addressed. E-mail:
[email protected]; Fax: +86 24 2389 1320.
4H2 v + ∼ 210 kJ (1) The NaBH4 aqueous solution can be safely stored by adding a few weight percent of NaOH stabilizer. As hydrogen is required, the hydrolysis reaction following eq 1 can be greatly accelerated using catalysts at ambient temperature.7 A number of noble or non-noble transition metals/alloys have been identified to be catalytically active toward the hydrolysis reaction of NaBH4, including Ru,5 Pt,8,9 Pt-Ru,9,10 Pt-Pd,11 Raney Ni and Co,12 Co and Ni borides,13-20 fluorinated Mg2Ni alloy,21 and so forth. Using these heterogeneous metal catalysts allows us to readily control the hydrogen generation (HG) process of the stabilized NaBH4 fuel solution. Catalytic hydrolysis reaction of NaBH4 is a complicated process, which involves solid-phase dissolution, liquid-phase transfer of the reactant and byproduct, and the reaction occurring at the catalyst surface, particularly in a solution environment with varied temperatures. Better mechanistic understanding of the hydrolysis kinetics of NaBH4 with the presence of catalysts is clearly of significance for hydrogen generator design and the selection of process conditions to pursue optimal system performance. Nevertheless, despite extensive efforts, the study of catalytic hydrolysis kinetics lacks progress. Even the hydrolysis reaction order with respect to NaBH4 concentration is still a subject of great controversy. Many literature reports claimed that the hydrolysis reaction follows zero-order kinetics with respect to NaBH4 concentration, on the basis of the observed linear increase of HG volume with increasing reaction time at fixed NaBH4 concentrations.5,8,9,12-14,16,22 Alternatively, determination of the HG rate at varied initial NaBH4 concentra¨ zkar et al.23 and Guella et al.24 tions gave multifarious results. O suggested zero-order kinetics for NaBH4 in their studies using Ru nanoclusters and Pt/C catalysts, respectively. On the basis of the combined experimental study and model analysis of the systems with the presence of Pt-Pd/CNT paper or Ru/C catalysts, Pen`oa-Alonso et al.11 and Shang et al.,25 independently, reported a predominant first-order kinetics. Additionally, several
10.1021/jp805115m CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
Catalytic Hydrolysis Kinetics of Sodium Borohydride recent studies using noble or non-noble transition metal catalysts claimed or implied fractional or negative reaction orders with respect to NaBH4 concentration.5,14,26-29 In our recent efforts to address these inconsistent and contradictory kinetics study results, we performed combined experiment study and Michaelis-Menten (M-M) model analysis of the hydrolysis kinetics of NaBH4 over a Co-B catalyst. The M-M model was initially proposed in 1913 to account for enzyme kinetics, and was later generally accepted as an important basis for many catalysis models.30 In the present study, we used this model to analyze the hydrolysis kinetics behavior of NaBH4 with the presence of metal catalyst. Our study showed that the hydrolysis kinetics is first-order with respect to catalyst amount, and that the hydrolysis reaction order with respect to NaBH4 depends on the NaBH4 concentration. These findings provide insights into the catalytic hydrolysis kinetics of NaBH4. 2. Experimental Details 2.1. Catalyst Preparation. In the present study, the Co-B catalyst was selected due to its good catalytic activity, low cost, easy preparation, and its ferromagneticity that allows ready magnetic separation of the catalyst powder from the fuel solution. The Co-B catalyst was prepared in air by using a chemical reduction method, which involves the usage of two solutions with the following compositions: solution A contains 50 g L-1 CoCl2 · 6H2O, 80 g L-1 of NH4Cl, 45 mL L-1 of NH3 · H2O (25 wt %); the reducing solution B contains 40 g L-1 of NaBH4. All the chemical reagents are of analytical grade and were used as received. In a typical preparation process, 100 mL of solution A was first placed in a beaker at 25 °C, and then, an equal volume of solution B was dropwise added into solution A. The mixture was kept under magnetic stirring until the bubble generation ceased (which generally took about 30 min). The black reaction product was separated from the solution by vacuum filtration, followed by washing with deionized water thoroughly to remove the Cl-, NH4+, and Na+ ions. The samples were finally dried in vacuum at 30 °C for 48 h, and ready for use. In the hydrolysis kinetics measurements, the Co-B catalyst powder was attracted on the magnetic stirring bar, and then immersed into the fuel solution. 2.2. Catalyst Characterization. The Co-B catalyst was examined by powder X-ray diffraction (PXRD, Rigaku D/MAX2500, Cu KR radiation) and scanning electron microscope (SEM, LEO Supra 35) equipped with an energy dispersive X-ray (EDX) analysis unit (Oxford). The specific surface area of the catalyst samples was measured by N2 adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method (Micromeritics ASAP 2010). To minimize the measurement error, each sample was measured three times, from which an average value was given. The composition of the catalyst was analyzed by using inductively coupled plasma-atomic emission spectrometry (ICPAES, Iris Intrepid). 2.3. Hydrolysis Kinetics Measurement. The hydrolysis kinetics measurements were carried out in air. The flask reactor containing the NaBH4 solution was placed in a thermostat that was equipped with a water circulating system to maintain the reaction temperature, typically within (0.5 °C. In case of the measurements using larger amount of Co-B catalyst (over 5 mg), the temperature of the fuel solution was measured and carefully controlled within (2 °C by adding ice-water blend. The NaBH4 solution was magnetically stirred at 800 rpm to promote an interface-controlled reaction between the solution and the catalyst. In a typical measurement, the NaBH4 solution containing 0.5 M NaOH was preheated and held at a designated
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15887 SCHEME 1: Kinetics Model for the Hydrogen Generation from Catalytic Hydrolysis of NaBH4 in Terms of Michaelis-Menten Model
temperature, and then, the Co-B powder catalyst attracted on a magnetic stirring bar was dropped into the NaBH4 solution to initiate the hydrolysis reaction. The H2 amount was measured by monitoring the water displaced from a graduated burette (with a volume of 5.2 L) as the reaction proceeded. 3. Analysis Method A critical feature of the M-M model is the assumption that the overall reaction is composed of two elementary steps; the reactant first interacts with the catalyst and forms a complex intermediate, which then decomposes to produce reaction products and the “regenerated” catalyst.30-33 Interestingly, a similar hypothesis actually dominates the current mechanistic understanding of the catalytic hydrolysis of NaBH4.9,11,24,34-36 Therefore, the M-M model may find its important application in understanding and predicting the catalytic hydrolysis behavior of NaBH4. It is suggested that the catalytic hydrolysis reaction of NaBH4 proceeds following Scheme 1.11,34,36 The metal catalyst M (Co-B in the present case) reacts with BH4-, with a rate constant k1, and forms an intermediate metal borohydride complex MBH4. MBH4 may then dissociate reversibly to BH4and M with a rate constant k-1, or react with H2O to produce B(OH)4-, H2, and the “regenerated” catalyst M, with a rate constant k2. In the steady state, the concentration of the intermediate complex stays the same even if the concentrations of the reactants and products are changing. This occurs when the rates of formation and breakdown of the MBH4 complex are equal. We then have
d[MBH4] ) k1[BH4 ][M] - k-1[MBH4] - k2[MBH4] ) 0 dt (2) By rearranging eq 2, we obtain
k1[BH[BH4 ][M] 4 ][M] [MBH4] ) ) k-1 + k2 KM
(3)
where KM, called the Michaelis constant, is defined as
KM )
k-1 + k2 k1
(4)
The concentration of unoccupied metal sites [M] is equal to the total metal site concentration [M0] minus the concentration of the metal borohyride complex [MBH4].
[M] ) [M0] - [MBH4]
(5)
Substituting this expression for [M] in eq 3 gives
[MBH4] )
[M0][BH4] KM + [BH4]
(6)
According to Scheme 1, the rate of catalytic hydrolysis of NaBH4 is given by
15888 J. Phys. Chem. C, Vol. 112, No. 40, 2008
r)
-d[NaBH4] d[H2] ) ) k2[MBH4] dt 4dt
Dai et al.
(7)
By substituting the expression for [MBH4] eq 6 into eq 7 and assuming that sodium borohydride is fully ionized in the aqueous solution, i.e., [BH4-] ) [NaBH4], we obtain
r)
k2[M0][NaBH4] KM + [NaBH4]
(8)
4. Results and Discussion First, the as-prepared Co-B catalyst was subjected to a series of structural analyses to characterize its microstructural features. As seen in Figure 1, the XRD pattern of the catalyst sample displays one broad diffraction peak centered around 2θ ) 45°, indicative of its amorphous structure. Figure 2 presents the SEM images of the catalyst sample at low and high magnifications, respectively. The round catalyst particles, with an average size of approximately 300 nm, exhibit considerable aggregation. This is consistent with the relatively small specific surface area (15 m2 g-1) of the catalyst sample that was measured by using the BET method. Interestingly, as seen clearly in Figure 2b, the catalyst particles are entangled by the irregular nanosheets. A similar morphological feature was also observed in the mesoporous Co-B catalyst prepared by using the chemical reduction method.37 Currently, the formation mechanism of the nanosheets, as well as its influence on the hydrolysis kinetics of NaBH4, is still unclear. According to the ICP-AES analysis result, the asprepared catalyst possesses a Co/B atomic ratio of around 2:1. 4.1. Experimental Studies of Catalytic Hydrolysis Kinetics. The present study aims to elucidate the effects of NaBH4 concentration and Co-B catalyst amount on the hydrolysis reaction kinetics of NaBH4. For this purpose, two sets of experiments were designed and conducted. All experiments were carried out using a fixed amount of NaBH4 (0.01 mol) that would theoretically yield 994 mL of hydrogen at 30 °C and 1
Figure 1. Powder XRD pattern of the as-prepared Co-B catalyst.
Figure 2. SEM images of the as-prepared Co-B catalyst: (a) low magnification; (b) high magnification.
Figure 3. HG kinetics curves of the NaBH4 solutions with varied NaBH4 concentration and fixed NaOH concentration (0.5 M) and Co-B catalyst amount (5 mg) at 30 °C. The inset shows the HG kinetics curves at the initial stage.
atm. Here, to minimize the measurement error that may arise from the highly exothermic hydrolysis reaction (around 210 kJ mol-1 NaBH4)7 and, from the effect of NaBO2 byproduct, the maximum NaBH4 concentration was selected to be 1.0 M. The first set of experiments were performed using solutions with fixed NaOH concentration (0.5 M) and Co-B catalyst amount (5 mg), but with varied NaBH4 concentrations. Figure 3 gives the HG profiles of the fuel solutions with NaBH4 concentrations ranging from 0.08 to 1.0 M. It was observed that, even at the minimum NaBH4 concentration of 0.08 M, rapid HG can be initiated immediately upon immersing the catalyst into fuel solution. For the solutions containing higher concentration of NaBH4, the HG volume increased linearly with increasing the reaction time until reaching 100% conversion of NaBH4. In contrast, the HG profiles of the dilute NaBH4 solutions (
