Borohydride hydrolysis reaction

Liu BH, Li ZP (2009) A review hydrogen generation from borohydride hydrolysis reaction. J Power Sources 187 527-534... 

Hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. The reaction of sodium borohydride has been the most studied to date ... 

The fact that a plot of H2 volumes initially generated vs. time gave a straight line is indicative of pseudo zero order kinetics. For borohydride hydrolysis, Kaufman and Sen3 and Holbrook and Twist4 also found zero order kinetics. Zero order kinetics for Reaction [1] imply that hydrolysis is independent of the concentrations of the reacting chemical species. This can be explained by assuming that the initial reaction step probably involves a surface catalyzed reaction, most likely BH" adsorption on the catalyst. Since the number... 

The kinetics of the hydrolysis reactions (43) and (44) was studied in detail by means of polarography [158], and the respective pseudo-first-order rate constants and k2 were determined. In 0.2M KOH at 75°C, the rate constants were k = 3.2 x 10 s and At2 = 1.43 X 10 s , indicating that the second step of hydrolysis (44) is nearly 500 times faster than the first step (43). For this reason, the efficiency of borohydride in enhancing the plating reaction is very low, typically on the order of a few %, and most borohydride is lost by hydrolysis. (The actual efficiency depends on the ratio of substrate area to bath volume, i.e., the loading factor.)... 

Since the hydrolysis reactions proceed more rapidly at a lower pH, the plating rate is faster at lower KOH concentrations. To avoid spontaneous decomposition of the bath, however, alkalinity of at least 0.1 M must be used. The effect of the KOH concentration on the deposition rate of a borohydride bath is illustrated in Fig. 30. 

However, beeause this ehemieal is synthesized from the (eostly) sodium borohydride, and beeause its own hydrolysis reaction rate is too high, sulfurated borohydride is not a suitable alternative to hydrosulfite. But interestingly, this study suggests the need for milder and more selective reductive chemicals chemicals able to selectively and rapidly react with carbonyl groups, and not with water to any extent. 

If the above hydrolysis reaction occurs without the presence of catalysts, the hydrogen generation rate is quite slow (-0.6 mL min when 1.2 mmol sodium borohydride is dissolved in 10 ml distilled water [5]). To speed up the hydrogen generation reactions various catalysts can be used such as noble metals (Ru, Pt, Rh, Pd, Pt-Ru, Pt-Pd alloys) and non noble transition metals or related compounds (Cu, Co, Ni, Co-B, Co-P-B, Co-W-B, Ni-SiO, Fe-Ni alloys) [3]. 

Hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. The reaction of sodium borohydride has been the most studied to date. In the first embodiment, a slurry of an inert stabilizing liquid protects the hydride from contact with moisture and makes the hydride pumpable. At the point of use, the slurry is... 

This voltage is noticeably higher than that obtained from hydrogen, and at eight electrons per molecule it indicates a fuel of remarkable potency. Unfortunately, the voltages actually obtained are not so different from a hydrogen fuel cell, because the catalysts that promote the direct borohydride oxidation of equation 5.4 also promote the hydrolysis reaction ... 

Sodium borohydride hydrolysis in aqueous solution can be represented in terms of the overall stoichiometric equation (Eq. 11.1) where NaBFLj reacts with 4 molecules of water to produce 4 molecules of H2 [33]. Although the reaction of NaBH4 hydrolysis has been studied since the discovery of sodium borohydride by Stock in 1933 [34], the theoretical, calculated energy of the reaction is often cited in an incompatible manner to the application, and real experimental data are very scarce [35-38]. The thermodynamic features of the catalyzed hydrolysis in fact are not yet well understood, as the evolved energy depends on the physical state and the hydration degrees of borohydride and metaborate and on-side reactions. 

Chemical Hydrides Chemical hydrides are manufactured hydrogen-containing materials that are chemically reacted with water, releasing heat and hydrogen gas. The hydrogen release process occurs upon a hydrolysis reaction with water. For example, the sodium borohydride reaction is [9]... 

Because of hydrolysis reaction, 7 electrons are utilized per molecule of borohydride electro-oxidation instead of theoretically 8 electrons (Amendola et al. 1999). 

Figures 18 to 20 show that the equilibrium cell voltage increases with the increase in fuel concentration. Although the cell performance increases initially but it does not increase proportionally with further increase in fuel concentration. This is because the increase in fuel concentration leads to the decrease in hydroxyl ion mobility. The hydrolysis reaction dominates with the increase in sodium borohydride concentration and thus the performance increases rather slowly. Further at higher concentration of NaBH4, viscosity of the fuel-electrolyte mixture increases leading to the rapid increase in concentration polarization at higher current densities and the performance decreases (Fig. 20). The maximum power density of 16.2 and 13.8 mW cm" were obtained for 3 M methanol and ethanol concentrations while 22.5 mW cm" for 2 M sodium borohydride. The fuel cell was operated at 25°C, 3 M KOH concentration and with 1 mg cm " of anode catalyst (Pt-black) loading catalyst and 3 mg cm" of cathode (Mn02) loading, respectively.

In this preparation, phenyi-2-nitropropene is reduced to phenyl-2-nitropropane with sodium borohydride in methanol, followed by hydrolysis of the nitro group with hydrogen peroxide and potassium carbonate, a variety of the Nef reaction. The preparation is a one-pot synthesis, without isolation of the intermediate. 

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