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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Studies of Reaction between Sodium Borohydride and Methanol, Water, and Their Mixtures Chih-ting F. Lo,†,‡ Kunal Karan,*,†,‡ and Boyd R. Davis‡,§
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Department of Chemical Engineering and Department of Mining Engineering, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6, and Queen’s-RMC Fuel Cell Research Centre, Kingston, Ontario, Canada K7L 5L9
This paper reports the kinetics of hydrogen generation from the reaction between sodium borohydride and methanol, water, and their mixtures over a temperature range between -20 and +50 °C. Hydrogen generation was found to obey a first-order rate law with respect to sodium borohydride concentration for each of the four reacting mixtures of methanol, “nearly dry” methanol (2:1 water to sodium borohydride mole ratio), “wet” methanol (10:1 water to sodium borohydride ratio), and water, with activation energies of 53.0 ( 3.4, 52.3 ( 9.5, 36.1 ( 2.8, and 86.6 ( 8.0 kJ/mol, respectively. Methanolysis of sodium borohydride was shown to be a feasible method for low-temperature hydrogen generation. However, this noncatalytic reaction system exhibited large lag time and slow reaction kinetics at low temperatures. Our study indicates that the reaction system based on sodium borohydride and the nearly dry methanol can be a potential high gravimetric density hydrogen storage system. 1. Introduction Sodium borohydride is currently proposed as a suitable hydrogen storage material for fuel cell applications. This is because it has a high hydrogen storage density and it can conveniently release hydrogen by the hydrolysis reaction.1-5 However, the hydrolysis of borohydride is not feasible at temperatures below 0 °C, an important requirement for automotive and portable applications in cold countries such as Canada. Further, in a batch mode of operation, the hydrolysis reaction is self-inhibiting, because the pH increases as the borohydride reacts to generate hydrogen, and the rate is an extremely slow at pH 12 or higher.6 Although high activity catalysts have been identified to boost the reaction kinetics significantly,7-16 their high cost may adversely affect the economics of the system. Nonetheless, there are several advantages of the hydrolysis reaction. The most important one is the ability to utilize water produced in the fuel cell operation instead of carrying it onboard. This is an extremely attractive process in that the hydrogen storage capacity can be as high as 21 wt % based on the amount of hydrogen generated per unit mass of sodium borohydride consumed. For such a high storage capacity, the ideal hydrolysis reaction is described as reaction 1:
NaBH4 + 2H2O f NaBO2 + 4H2
(1)
If the hydrogen storage capacity is calculated with respect to the theoretical reaction byproduct, anhydrous sodium borate, the storage capacity is still as high as 12 wt %. The difference * To whom correspondence should be addressed. Tel.: (613) 5333095. Fax: (613) 533-6637. E-mail:
[email protected]. † Department of Chemical Engineering, Queen’s University. ‡ Queen’s-RMC Fuel Cell Research Centre. § Department of Mining Engineering, Queen’s University.
in the calculation is due to the fact that sodium borate is heavier than sodium borohydride. A majority of proposed systems for hydrogen generation are based on utilization of sodium borohydride stabilized by sodium hydroxide (3-5 wt %) in an aqueous solution at approximately 20 wt %.7,17-21 However, one serious technical problem with the hydrolysis of a base-stabilized aqueous system of sodium borohydride is the formation of hydrated sodium borate as the reaction byproduct according to the reaction
NaBH4 + 6H2O f NaBO2‚4H2O + 4H2
(2)
The formation of the hydrated byproduct severely compromises the overall storage density, which can be calculated to be close to 4.2 wt % by taking the amount of hydrogen produced divided by the total weight of the reactants. Further, an important design criterion for these base-stabilized aqueous systems is to ensure that the reaction byproduct, hydrated sodium borate, is dissolved in water. This is because the precipitated sodium borate can deposit on the equipment, rendering the cleanup process to be cumbersome. This problem, along with the requirement of a tank for the storage of aqueous sodium borate solution resulting from the consumption of sodium borohydride, further reduces the hydrogen storage density to less than 3 wt %. To avoid the formation of hydrated reaction product, an alternative to the use of water as a reactant is the use of a primary alcohol. In fact, sodium borohydride is known to be reactive to low-molecular-weight primary alcohols such as methanol, ethanol, and ethylene glycol, as well as acidic alcohols.1,4,22,23 Among the primary alcohols, methanol has the highest reactivity toward sodium borohydride. Methanol is also the lightest alcohol, which makes it a suitable alternative to water
10.1021/ie0608861 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007
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Figure 1. Schematic diagram of the experimental apparatus used for kinetic studies.
as a reactant for hydrogen release. The overall reaction between methanol and sodium borohydride can be described as follows:
NaBH4 + 4CH3OH f NaB(OCH3)4 + 4H2
(3)
The methanolysis reaction is spontaneous and has been found in previous studies to exhibit rapid reaction kinetics at 60, 25, and 0 °C.1,22,24 The rate constant for the noncatalytic methanolysis reaction at 0 °C has been estimated to be greater than that of the hydrolysis reaction.1 Although the methanolysis of sodium borohydride as a hydrogen generation process appears attractive, this system has a 4.9 wt % gravimetric storage capacity because methanol is heavier than water. Thus, neither base-stabilized hydrolysis nor methanolysis of sodium borohydride will likely meet the U.S. Department of Energy (DOE) criterion of 9.0 wt % hydrogen storage system density for automotive fuel cells by 2015. For the ease of process operation in both automotive and portable applications, it would be advantageous to develop an alternative reaction scheme where the formation of hydrated borate product could be avoided. Such a scheme is possible if reactants other than pure water are employed. This study proposes that by exploiting the advantages of each of the two processes, a reaction between sodium borohydride and a methanol-water mixture can result in increased overall hydrogen storage density. It is hypothesized that by using a mixture of water and methanol, the hydrolysis of sodium borohydride stated in eq 1 may be achieved when water acts as a limiting reactant, provided methanol acts as an inert solvent in the process. This is possible provided that methanol has a low reactivity with sodium borohydride in the presence of water. As an added benefit, the presence of methanol lowers the freezing temperature of the solvent mixture to enable cold startup. Furthermore, if the hydrolysis reaction proceeds to completion as stated in eq 1 and does not produce hydrated sodium borate, the gravimetric storage capacity can be as high as 21 wt %, less the mass of the methanol when water is fed from the fuel cell. Previous literature on the reaction kinetics of sodium borohydride in the water-methanol mixture is limited. In fact, the only kinetic data available are reported by Davis and Gottbrath,22 who suggested that the addition of water increases the rate of
methanolysis reaction at both 0 and 25 °C. However, this work did not confirm whether the reaction was mainly methanolysis or hydrolysis. It only concludes that the rate of disappearance of borohydride increased in the water-methanol mixture. The other report of a methanol-water system for sodium borohydride was described in the U.S. Patent application by Zaluski et al.,25 who indicated that the use of a water-methanol mixture lowers the operating temperature of the borohydride system. In this paper we report the results of a kinetic study conducted over the expected operating temperature range of -20 to +50 °C for reactions between sodium borohydride and each of the following four solvents: anhydrous methanol, nearly dry methanol (2:1 H2O:NaBH4 molar ratio), wet methanol (10:1 H2O:NaBH4 molar ratio), and water. 2. Experimental Methods 2.1. Materials. NaBH4 (99.8%+ purity VenPure AF granules, Rohm and Haas) and anhydrous methanol (ultradry 99.995%+, Fisher Scientific) were used as reactants. These materials were stored in a glovebox to prevent hydration. For kinetic experiments, sodium borohydride was used in a powder form. 2.2. Apparatus and Procedure. The experimental setup is shown in Figure 1. The reactor consisted of a modified 100 mL three-neck round-bottom glass flask. The middle neck was modified to allow dispensing of sodium borohydride powder into the reactor without exposing the reactants to air. A second neck was used for temperature monitoring, and the third neck was used to purge the system with argon gas and to allow the gas produced during the reaction to leave the reactor and be collected in the inverted graduate cylinder. The reaction temperature was controlled by an external heater/cooler. Briefly, the experimental procedure involved filling the reactor with 18 mL of solvent, loading 0.04 g of sodium borohydride in the form of powder or pellet, as appropriate, in the modified reactor, and sealing the reactor. The amount of solvent was in large excess compared to the amount of sodium borohydride: approximately 90 times higher than the theoretical molar ratio. Subsequent flushing of the reactor with argon for at least 20 min helped minimize the moisture in the vessel. After thermal equilibration was achieved, sodium borohydride was
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Figure 2. Typical kinetic data showing fractional conversion of NaBH4 as a function of time. (Data for methanolysis of NaBH4 at 20 °C.) (a) Lag phase; (b) exponential phase; (c) final phase.
dispensed into the solvent. The reaction time was calculated starting from the time sodium borohydride was released into the solvent. The progress of the reaction was monitored by following the amount of hydrogen generated using a volumetric technique, which allowed tracking of the reaction in real time. As shown in Figure 1, an inverted graduated cylinder in water was used for this technique because it allowed stripping of any methanol vapor present in the product stream. The presence of methanol vapor in the product stream was anticipated because of its significant vapor pressure at experimental conditions. In the inverted graduated cylinder setup, water absorbed all methanol vapor that entered the graduated cylinder and ensured that the volume change in the inverted graduated cylinder was due only to hydrogen. A 250 mL graduated cylinder filled with distilled water was used to trap the gas. Each reaction was repeated at least two times to confirm the reproducibility. The pH of the reacting mixture was not maintained at a constant level during the experiments, but was recorded at the end of each run. 2.3. Byproduct Analysis. The byproduct of the hydrogen generation reactions was analyzed with X-ray diffractometry (XRD) for chemical species identification. The main species of interest included sodium borate, hydrated sodium borate, and sodium tetramethoxyborate. For XRD analysis of the byproduct, the mixture remaining in the reactor was dried under argon atmosphere for approximately 24 h to yield solid material. This analysis was performed in air. 3. Results and Discussion 3.1. General Discussion. Figure 2 shows a typical course of a hydrogen generation reaction presented in terms of the fractional conversion of sodium borohydride as a function of time. The fractional conversion of sodium borohydride was calculated by measuring the measured amount of hydrogen generated and then dividing by the total theoretical amount of hydrogen that would be produced upon complete conversion of sodium borohydride. The time evolution of the hydrogen generation process was classified into three phases: a lag phase, an exponential phase, and a final phase. The lag phase was defined as the length of time before the first bubble breaking the surface of water in the inverted graduated cylinder was observed. The exponential phase of the reaction was defined as the time period when a rapid exponentially increasing hydrogen generation rate was observed. During the exponential phase, bubbling was observed to be the most vigorous. The final
Figure 3. Natural logarithm of fraction of unreacted NaBH4 as a function of time. (Data for methanolysis of NaBH4 at 20 °C.)
Figure 4. Arrhenius plot for the rate constants of reaction of NaBH4 with methanol, water, and their mixtures.
stage was defined as the time period when the rate of reaction slowed down significantly. In this study, we arbitrarily defined the final stage as the time period over which the fractional conversion of sodium borohydride reached 0.95. It is useful to mention that all experiments conducted in this work reached completion. All reactions were carried out under isothermal conditions. For experiments conducted with sodium borohydride powder, the powder dissolved instantaneously when introduced into the solution. In case of tests with sodium borohydride pellets, the pellet dissolution was rapid but not instantaneous. 3.2. Kinetics of Hydrogen Generation. For all systems tested, the kinetic data were found to obey a first-order rate law with respect to the sodium borohydride concentration. The rate law was confirmed by the linearity of the data on a plot of natural logarithm of the fraction of unreacted sodium borohydride as a function of time. An example is depicted in Figure 3. In each of the four solvent/reactant systems the rate constant was determined for each temperature from plots similar to Figure 3. The Arrhenius plot generated for each system is shown in Figure 4. It can be clearly seen that the rate constants for each system follow Arrhenius behavior. The activation energy was found to behave in the following order: wet methanol < anhydrous and nearly dry methanol < water. It is interesting to note that the activation energy for reaction of sodium borohydride with nearly dry methanol is close to that for the reaction with anhydrous methanol. This may indicate that the presence of water had little influence on the reaction kinetics.
Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007 5481 Table 1. Summary of the Kinetic Parameters and Byproduct Generated from the Four Solvents and NaBH4 solvent
H2O:NaBH4 (mole ratio)
Arrhenius equation
R2
activation energy (kJ/mol)
reaction byproduct NaB(OCH3)4 NaBO2‚2H2O T > 0 °C; NaB(OCH3)4 T < 0 °C; both products T ) 0 °C NaBO2‚4H2O NaBO2‚4H2O
methanol nearly dry methanol
0 2
ln k ) -6369.5/T + 14.6 ln k ) -6289.2/T + 14.3
0.973 0.988
52.96 ( 3.38 52.29 ( 9.48
wet methanol water
10 945
ln k ) -4345.1/T + 9.2 ln k ) -10415.4/T + 26.1
0.991 0.999
36.13 ( 2.79 86.59 ( 7.97
Table 2. Comparison of Rate Constant and Experimental Conditions for NaBH4 Methanolysis Reaction reference
temp (°C)
calcd rate const (1/s)
analytical method
NaBH4 purity
this work Davis and Gibbrath22 Brown et al.24 this work Davis and Gibbrath4 this work Brown et al.1
0 0 0 25 25 50 60
0.000 128 0.000 434 0.000 537 0.001 36 0.001 31 0.004 48 0.002 85
volumetric iodate iodate volumetric iodate volumetric iodate
99.99%, silicon free 98.20% 99.90% 99.99%, silicon free 98.20% 99.99%, silicon free about 90%
The reaction byproducts were analyzed by XRD to gain further insight into the reaction mechanisms. Depending upon the solvent composition and the reaction temperature, the reaction byproduct was found to be either sodium tetramethoxyborate or hydrated sodium borate. In one case at 0 °C, a mixture of the two products was found. A summary of the kinetic parameters and byproducts is provided in Table 1. The XRD patterns are shown in Figure 5. The XRD analyses of the byproduct(s) for reactions in methanol, wet methanol, and water systems yielded as-expected results, but the results for the nearly dry methanol system were initially unexpected. In experiments with nearly dry methanol at temperatures below 0 °C, sodium tetramethoxyborate was the only species found, which indicated the occurrence of a predominantly methanolysis reaction. At 0 °C, a mixture of sodium borate and sodium tetramethoxyborate suggested that both hydrolysis and methanolysis occur. In experiments above 0 °C, hydrated sodium borate was the only byproduct found, which points to the occurrence of the hydrolysis reaction. However, if different reactions occurred over different temperature ranges, one would expect a change in the slope in the Arrhenius plot. However, the Arrhenius plot for the reaction between nearly dry methanol and sodium borohydride is linear and almost identical to the plot generated for the pure methanolysis system. This is further discussed in section 3.4. 3.3. Kinetics of Methanolysis of NaBH4. The intrinsic kinetics of the methanolysis of sodium borohydride was investigated by reacting powdered sodium borohydride and anhydrous methanol at temperatures ranging from -20 to +50 °C. This is the first time that a systematic study over such a wide range of temperatures using solid sodium borohydride has been presented. The Arrhenius plot for the first-order reactions is presented in Figure 4. Previous studies have reported results for the methanolysis of sodium borohydride at 0, 25, and 60 °C.1,22,24 The rate constants extracted from literature data are presented together with those from our study in Table 2. As in previous studies,1,22,24 our results indicate that the rate of reaction is firstorder with respect to the concentration of sodium borohydride. A complete set of kinetic parameters was not available in the literature, and only conversion data or rate constants are reported here for specific temperatures. The rate constant at 25 °C reported by Davis and Gobbrath22 compares favorably to our data. However, our results differed from those of Brown et al.,1 who reported a rate constant at 60 °C that was 35% lower than our data at 50 °C. This discrepancy is attributed to the difference in sodium borohydride purity since
the sodium borohydride used in Brown’s work was only about 90% pure. As suggested in Davis’s work, the impurities in Brown’s experiments were likely sodium methoxide and sodium carbonate, compounds that can stabilize the methanolysis reaction. This is a reasonable assumption because sodium methoxide and sodium carbonate are byproducts of the sodium borohydride production. The rate constants determined from the work of Davis and Gobbrath22 and Brown et al.24 at 0 °C were similar to each other, yet much higher than those determined in this work. One possible explanation is the different analytical methods used to quantify the amount of sodium borohydride consumed. Both Davis and Gobbrath and Brown et al. determined the sodium borohydride concentration through the iodate method, which assumes that sodium borohydride is instantaneously stabilized as soon as the reacting mixture sample is withdrawn from the reactor. Therefore, one possibility for the discrepancy between our data and that of Brown et al. and Davis and Gobbrath is the lack of an instantaneous quench of the reaction mixture in each of these previous studies. Our experiments, on the other hand, did not require withdrawal of the reacting mixture. The reaction progress was followed by monitoring hydrogen production in real time. 3.4. Kinetics of Reaction between Methanol-Water Mixtures and NaBH4. As stated earlier, two sets of experiments were conducted. In one set, the water content in the methanolwater mixture was exactly stoichiometric for sodium borohy-
Figure 5. X-ray diffraction patterns of products of (a) methanolysis of NaBH4 at 20 °C, (b) nearly dry methanol reaction with NaBH4 at 20 °C, (c) wet methanol reaction with NaBH4 at 20 °C or hydrolysis of NaBH4 at 20 °C, and (d) nearly dry methanol reaction with NaBH4 at 0 °C.
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dride consumption as per reaction 1. This mixture was denoted as “nearly dry methanol” with a water to sodium borohydride molar ratio of 2:1. The second set of experiments were conducted in a methanol-water mixture containing water in excess of that required by the stoichiometry in reaction 2. This mixture was called “wet methanol”. Reactions with both nearly dry and wet methanol were conducted under conditions of excess methanol. The rate of hydrogen generation was observed to follow first-order rate law with respect to sodium borohydride concentration in both cases. The wet methanol system showed the highest rate constants and the lowest activation energy at all temperatures tested. Unlike the nearly dry methanol, there is significantly more water in wet methanol, which promotes the hydrolysis reaction. Hydrolysis of sodium borohydride is a Lewis acid-base reaction. The mixture of water-methanol enhances the reaction because methanol acts as a base, which increases the amount of hydronium ion in the system. Although water in wet methanol enhances the hydrogen production, the additional water also causes the hydration of sodium borate. The reaction byproduct analysis showed that only hydrated sodium borate had formed. Sodium tetramethoxyborate was not detected even at temperatures as low as -15 °C. The reaction of wet methanol with sodium borohydride was conducted to gain an understanding of the reaction when the water content is higher than the amount required stoichiometrically to react all of sodium borohydride but less than the majority of the solvent. Hydration of methanol can occur very easily if methanol is not stored in a water-free environment. The kinetic parameters for the reaction between sodium borohydride and the nearly dry methanol system or the wet methanol system are presented in Figure 4 and Table 2. Interestingly, the experimental reactions between the nearly dry methanol and sodium borohydride yielded rate constants almost identical to that of the methanolysis reaction within a 95% confidence limit. This finding suggests that the hydrogen generation reactions in both systems occur preferentially via the pathway associated with the methanolysis of sodium borohydride, and the small amount of water had no significant effect on the rate of reaction. When the reaction byproducts from the two systems were analyzed, the crystals dried in the nearly dry methanol system were found to comprise material other than the expected byproduct from a methanolysis reaction, sodium tetramethoxyborate. In fact, the byproduct was either sodium borate, sodium tetramethoxyborate, or a mixture of both depending on the temperature. We believe that the formation of sodium borate is due to a subsequent reaction between sodium tetramethoxyborate and water as stated in eq 4.
NaB(OCH3)4 + 2H2O f NaBO2 + 4CH3OH
(4)
It is possible that reaction 4 occurred either concurrently with reaction 3 or over the 24 h period afterward as the dissolved byproduct in methanol-water mixture was dried under low flow rate argon overnight. This 24 h period could be long enough for reaction 4 to proceed with the moisture in the argon stream. Unfortunately, the experiment data here do not delineate between the two possible pathways. In addition, less or no sodium borate was found at lower temperatures. As the reaction temperature decreases, the kinetics of reaction 4 may also decrease. The XRD analysis indicated that reaction 4 reached completion at temperatures above 0 °C, since no sodium tetramethox-
Figure 6. Proposed reaction scheme for NaBH4 reaction with water, methanol, or water-methanol mixtures.
yborate was found. If all of the sodium tetramethoxyborate was consumed via reaction 4, the only plausible product would be anhydrous sodium borate, since the borohydride:water feed molar ratio was 1:2. In the alternative, if reaction 4 was not completed, the XRD analysis should have contained both hydrated borate and sodium tetramethoxyborate. However, the XRD analysis did not record the presence of sodium tetramethoxyborate. Thus, we believe that, for the conditions of a 2:1 water to sodium borohydride mole ratio, only anhydrous borate should be produced. The hydration of sodium borate could result during either the drying step or the XRD analysis. Anhydrous sodium borate synthesized in-house was added to anhydrous methanol and subjected to overnight drying under low argon flow rate in order to investigate whether hydrated sodium borate would form during the drying process. Dried powder analyzed with XRD technique showed strong peaks of hydrated sodium borate (NaBO2‚2H2O). Based on the hydrogen generation kinetics and byproduct analyses, we propose the reaction scheme between sodium borohydride and the four solvent systems as shown in Figure 6. Hydrolysis is the dominant reaction for both water and wet methanol solvent systems, and the overall reaction proceeds via route a. Methanolysis of sodium borohydride proceeds via route b. Route c was not detected for pure methanolysis reactions. This is likely because tetramethoxyborate reacts slowly with water vapor during the drying process, and any amount of sodium borate produced was not detectable by XRD. Route c represents the in situ reactions 3 and 4, while route b and route d show the two-step-reaction proposal between nearly dry methanol and sodium borohydride. Route e suggests a mechanism for borate hydration from the nearly dry methanol system. We found that the formation of borate or tetramethoxyborate as reaction byproducts did not hinder the rate of reaction. This is advantageous because the reaction byproducts dissolve in methanol. If these products had instead slowed the rate of reaction, problems of separating the products in situ would pose challenges in an actual process system implementation. The solubility of sodium borate and sodium tetramethoxyborate in industrial processes is a subject for further study. 3.5. Lag Time Observation. We made the significant finding that all solvent systems except wet methanol exhibit lag time. Lag times ranged from 5 s to 80 min for almost all reactions studied. The wet methanol system showed the shortest lag time in this experimental work. The maximum lag time recorded was almost 80 min for the powdered sodium borohydride system at -20 °C. The lag time in the nearly dry methanol system was shorter than that in the methanolysis system, but still significantly larger than that in the wet methanol system.
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We also found that reaction lag time decreased exponentially with increasing temperature. The lag times recorded for methanolysis of sodium borohydride powder were similar at temperatures above 10 °C. The associated kinetic data indicate that the rate of reaction was not significantly affected by the lag time and the rate constants were very similar at all temperatures tested for both forms of sodium borohydride. 3.6. Implications for the Use of Methanol-WaterBorohydride Reacting Mixture. Although the reaction in the nearly dry methanol was found to be methanolysis from the kinetic study results, the subsequent byproduct reaction with water, as shown in reaction 4, was able to produce anhydrous sodium borate and methanol. Therefore, methanol was not consumed in the overall reaction. The ability to produce anhydrous sodium borate is the key to increasing the overall hydrogen storage density of systems based on sodium borohydride as the storage media. The reaction in wet methanol would be preferred if the hydration of sodium borate can be prevented. The high methanol to water ratio also enables low-temperature operations. The nearly dry methanol and wet methanol solvents tested in this work both have freezing temperatures below typical winter temperatures. Although the solvent to sodium borohydride ratio of this experimental work is high, we believe that to scale the solvent to sodium borohydride ratio down in order to increase the overall system hydrogen storage density will not be a major problem. A report of the solubility of sodium borate in methanol is the subject of future study. The miscibility between methanol and water in the methanolwater mixture provides additional benefits. For example, this allows the methanol vapor that escapes the hydrogen generation reactor to be readily absorbed back into a water stream during the hydrogen humidification step. As a result, the need for a condenser or membrane to remove and recycle methanol vapor from the product stream prior to entering the fuel cell is reduced or eliminated. Furthermore, it is possible to allow the mix of hydrogen and methanol vapor as the anodic feed for the fuel cell, as methanol is not poisonous to the fuel cell. The production of carbon monoxide, a poison to the polymer electrolyte membrane (PEM) fuel cell, is also unlikely to occur since the activation overpotential for methanol is much higher than that for hydrogen. Finally, robust catalysts have been prepared for the hydrolysis of sodium borohydride that can increase the hydrogen production rate and to release hydrogen on demand. It is likely that these catalysts will also work well in wet methanol and the nearly dry methanol for the hydrogen generating reactions.
4. Conclusions A detailed kinetic study of the methanolysis of sodium borohydride and the reactions between sodium borohydride and water-methanol mixtures over a wide range of temperatures was completed for the first time. In addition to investigating hydrogen generation reactions from methanol-based solutions, the hydrolysis of sodium borohydride was also tested and its kinetic behavior studied. The results for all of these experiments indicated that the hydrogen generation reaction followed firstorder rate kinetics with respect to the sodium borohydride concentration, although each system had a different activation energy. Among all, the system with most favorable and rapid kinetics was the wet methanol system. Our data indicate that, for this system, hydrogen generation occurs primarily via the hydrolysis reaction, producing sodium borate tetrahydrate as the
byproduct. Hydrolysis of sodium borohydride, on the other hand, required the largest activation energy. The reaction kinetics between sodium borohydride and methanol or nearly dry methanol solvents were identical, and the rates of reactions were between those of the wet methanol and water systems. Both methanol and nearly dry methanol solvents promoted the methanolysis reaction with sodium borohydride, but the reaction byproduct underwent another reaction with water to produce anhydrous sodium borate at temperatures above 0 °C in the nearly dry methanol system. Hydrogen generation at temperatures below 0 °C was possible in all systems containing methanol, but the reaction rate was relatively slow. In order to use such a reaction as a lowtemperature start-up mechanism, additives or catalysts that increase the rate of reaction and/or decrease the lag time are subjects for further study. With improved catalyst design and optimization, the nearly dry methanol system is a candidate for automotive or portable fuel cells. By producing anhydrous sodium borate as a reaction byproduct and maintaining the methanol content relatively constant, this hydrogen generating system can also utilize recycle water from the fuel cell to eliminate on-board water storage and to increase overall storage capacity. Acknowledgment Financial support from Natural Sciences and Engineering Research Council of Canada (NSERC) and Auto21 Networks of Centres of Excellence is gratefully acknowledged.
Literature Cited (1) Brown, H. C.; Mead, E. J.; Rao, B. C. S. Study of solvents for sodium borohydride and the effect of solvent and the metal ion on borohydride reductions. J. Am. Chem. Soc. 1955, 77, 6209. (2) Pecsok, R. L. Polarographic studies of the oxidation and hydrolysis of sodium borohydride. J. Am. Chem. Soc. 1953, 75, 2862. (3) Davis, R. E.; Swain, C. G. General acid catalysis of the hydrolysis of sodium borohydride. J. Am. Chem. Soc. 1960, 82, 5949. (4) Davis, R. E.; Bromels, E.; Kibby, C. L. Boron hydrides. iii. hydrolysis of sodium borohydride in aqueous solution. J. Am. Chem. Soc. 1962, 84, 885. (5) Kaufman, C. M.; Sen, B. Hydrogen generation by hydrolysis of sodium tetrahydroborate: effects of acids and transition metals and their salts. J. Chem. Soc., Dalton Trans. 1985, 307. (6) Rohm and Haas. Borohydride digest; Rohm and Haas: 2003. (7) Amendola, S. C.; Sharp-Goldman, S. L.; Janjua, M. S.; Spencer, N. C.; Kelly, M. T.; Petillo, P. J.; Binder, M. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int. J. Hydrogen Energy 2000, 25, 969. (8) Brown, H. C.; Brown, C. A. Catalytic hydrogenation. i. reaction of platinum metal salts with sodium borohydride. new active platinum metal catalysts for hydrogenation. Tetrahedron, Suppl. 1966, 149. (9) Suda, S.; Sun, Y. M.; Liu, B. H.; Zhou, Y.; Morimitsu, S.; Arai, K.; Tsukamoto, N.; Uchida, M.; Candra, Y.; Li, Z. P. Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 209. (10) Kojima, Y.; Suzuki, K. i.; Fukumoto, K.; Sasaki, M.; Yamamoto, T.; Kawai, Y.; Hayashi, H. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int. J. Hydrogen Energy 2002, 27, 1029. (11) Kim, J. H.; Lee, H.; Han, S. C.; Kim, H. S.; Song, M. S.; Lee, J. Y. Production of hydrogen from sodium borohydride in alkaline solution: development of catalyst with high performance. Int. J. Hydrogen Energy 2003, 29, 263. (12) Dong, H.; Yang, H.; Ai, X.; Cha, C. Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. Int. J. Hydrogen Energy 2003, 28, 1095.
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Ind. Eng. Chem. Res., Vol. 46, No. 17, 2007
(13) Salgado, J. R. C.; Antolini, E.; Gonzalez, E. R. Preparation of PtCo/C electrocatalysts by reduction with borohydride in acid and alkaline media: the effect on the performance of the catalyst. J. Power Sources 2004, 138, 56. (14) Krishnan, P.; Yang, T. H.; Lee, W. Y.; Kim, C. S. PtRu-LiCoO2s an efficient catalyst for hydrogen generation from sodium borohydride solutions. J. Power Sources 2005, 143, 17. (15) Oezkar, S.; Zahmakiran, M. Hydrogen generation from hydrolysis of sodium borohydride using Ru(0) nanoclusters as catalyst. J. Alloys Compd. 2005, 404-406, 728. (16) Liu, B. H.; Li, Z. P.; Suda, S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J. Alloys Compd. 2006, 415, 288. (17) Amendola, S. C.; Sharp-Goldman, S. L.; Janjua, M. S.; Kelly, M. T.; Petillo, P. J.; Binder, M. An ultrasafe hydrogen generator: aqueous, alkaline borohydride solutions and Ru catalyst. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 1999, 44, 864. (18) Amendola, S. C.; Onnerud, P.; Kelly, M. T.; Petillo, P. J.; SharpGoldman, S. L.; Binder, M. A novel high power density borohydride-air cell. J. Power Sources 1999, 84, 130. (19) Mohring, R. M.; Wu, Y. Hydrogen generation via sodium borohydride. AIP Conf. Proc. 2003, 671, 90. (20) Kojima, Y.; Kawai, Y.; Kimbara, M.; Nakanishi, H.; Matsumoto, S. Hydrogen generation by hydrolysis reaction of lithium borohydride. Int. J. Hydrogen Energy 2004, 29, 1213.
(21) Kojima, Y.; Suzuki, K. i.; Fukumoto, K.; Kawai, Y.; Kimbara, M.; Nakanishi, H.; Matsumoto, S. Development of 10 kw-scale hydrogen generator using chemical hydride. J. Power Sources 2004, 125, 22. (22) Davis, R. E.; Gottbrath, J. A. Boron hydrides. v. methanolysis of sodium borohydride. J. Am. Chem. Soc. 1962, 84, 895. (23) Kadlec, V.; Hanzlik, J. Behavior of sodium tetrahydridoborate in methanol and tetrahydrofuran. Collect. Czech. Chem. Commun. 1974, 39, 3200. (24) Brown, H. C.; Ichikawa, K. Influence of solvent and metal ion on the rate of reaction of alkali metal borohydrides with acetone. J. Am. Chem. Soc. 1961, 83, 4372. (25) Zaluski, L.; Zaluska, A.; Strom-Olsen, J. O. Method of hydrogen generation for fuel cell applications and a hydrogen-generating system, U.S. Patent Appl. 2003/0157018 A1, 2001.
ReceiVed for reView July 10, 2006 ReVised manuscript receiVed February 27, 2007 Accepted March 12, 2007 IE0608861
