The rapid depletion of traditional energy sources (crude oil, natural gas, coal, etc.) forces scientists to work on problems of development of alternative energy sources. An unlimited amount of hydrogen is a very attractive candidate as an ecologically friendly source of energy. Hydrogen, used in fuel cells as fuel and obtained by catalytic reforming of natural gas or crude oil, contains carbon monoxide. Even a small amount of CO poisons electrodes containing Pt and Pd and negatively effects performance of fuel cell. Storage and transportation of hydrogen are also an issue. Storage of hydrogen under high pressure, in a liquid form or cryoadsorbed form cannot satisfy demands of safety. The most promising way to store hydrogen is to keep it bound in hydrides of different metals [1] - [9] .

In this work we present the possibility of utilization of sodium hypophosphate (NaH₂PO₂) as a source of hydrogen for fuel cells. In a process of chemical nickel-plating at temperatures of 80˚C - 95˚C, hydrogen is generated due to interaction of NaH₂PO₂ with water. This process takes place with high rate only on metals which exhibit catalytic activity at a high temperature. For this purpose, the surface is usually treated with solution of SnCl₂ and activated with compounds of Pd²⁺. Sn²⁺ ions on the surface reduce Pd²⁺ ions to metal Pd, thin catalytic layer of which uniformly covers the treated surface. For example, total reaction of reduction of Ni²⁺ ions reduction on surfaces activated by metal palladium can be presented as [10] - [16] :

Therefore selection of commercially competitive and efficient catalysts, which will be active at low temperature for hydrogen generation from sodium hypo- phosphite, is problem.

The goal of the present work is synthesis of highly active catalyst CoO∙B₂O₃ for generation of hydrogen from solution of NaH₂PO₂.

Hydrolysis of NaH₂PO₂ is performed in solution of 2 M NaH₂PO₂ + 2 M NaOH in presence of 0.1 g of CoO∙B₂O₃ catalyst (dried at temperature 105˚C) until the full evolution of hydrogen.. With increase of temperature of solution, rate of hydrogen evolution is significantly increased: in 60 min at 30˚C, 620 ml of hydrogen was evolved, while at 80˚C the same volume of hydrogen was evolved in 4 min. The rate of generation of hydrogen increased almost 20 times (Figure 4).

Catalytic activity of CoO∙B₂O₃ was determined in a process of hydrolysis of NaH₂PO₂ (solution temperature 30˚C) in presence of catalysts annealed at 105˚C, 280˚C, 350˚C and 480˚C. Efficiency of hydrogen evolution reached 95% on a catalyst annealed at 280˚C. Generation of hydrogen on a catalyst annealed at 350˚C was finished in 25 min and the efficiency was 78%. The same time (25

min) was necessary to generate hydrogen with 74% at a catalyst annealed at 280˚C (Figure 5).

Figure 6 and Figure 7 show results of experiments conducted with catalysts annealed at 280˚C and 350˚C and with rest of conditions as above. The change of temperature of solution from 30˚C to 80˚C accelerated generation of hydrogen about 22 times (from 14 mL/min to 315 mL/min) in the first case (catalyst annealed at 280˚C) and 16 times (from 24 mL/min to 390 mL/min) in the second case (catalyst annealed at 280˚C).

Reactions rate constants, energy of activation and temperature coefficients of reforming rates of sodium hypophosphate reforming rates on CoO・B₂O₃, catalyst

annealed at 350˚C were determined. Using data of five parallel experiments we built the plot showing dependence of natural log of difference of hydrogen volumes on time, where―hydrogen volume at a normal temperature and pressure (NTP) (ml), allocated until the full decomposition of NaH₂PO₂,―hydrogen volume (ml) (NTP), generated in time point τ (Figure 8). Straight lines not crossing the origin of coordinate axes were obtained. Tangent of slop angle of the straight line to an axis of time is equal to a reaction rate constant:

At each temperature, k^30˚ = 8.53 × 10⁻⁴ s⁻¹; k^40˚ = 1.62 × 10⁻⁴ s⁻¹; k^50˚ = 3.06 × 10⁻³ s⁻¹; k^60˚ = 5.06 × 10⁻³ s⁻¹; k^80˚ = 1.39 × 10⁻² s⁻¹ were calculated.

Using Equations (6) and (7) and rate constants calculated at two temperatures, temperature coefficient and activation energy of chemical reaction were calculated as γ = 0.917; E_A = 49.59 kJ∙mol⁻¹.

γ―temperature coefficient of rate of chemical reaction; k₁ and k₂ rate constants at T₁ and T₂, respectively; R―universal gas constant; E_A energy of activation.

Catalyst CoO∙B₂O₃ is synthesized and identified by chemical and X-ray analysis. Thermal stability of the sample is estimated and temperature of conversion is determined.

Synthesized compound CoO∙B₂O₃ was used as a catalyst during reforming of sodium hypophosphate. Samples annealed at 280˚C - 350˚C demonstrated best catalytic activity. Reforming of NaH₂PO₂ was conducted in a solution of 2 M (NaH₂PO₂ + NaOH) at range of temperatures 30˚C - 80˚C; rate constants k^30˚ = 8.53 × 10⁻⁴ s⁻¹; k^40˚ = 1.62 × 10⁻⁴ s⁻¹; k^50˚ = 3.06 × 10⁻³ s⁻¹; k^60˚ = 5.06 × 10⁻³ s⁻¹; k^80˚ = 1.39 × 10⁻² s⁻¹, temperature coefficient γ = 0.917 and activation energy E_A = 49.59 kJ∙mol⁻¹ are calculated.

The presented project has been fulfilled with the financial support from “Shota Rustaveli National Science Foundation” (Grants № AR/10/3-171/14). Any idea in this publication is possessed by the authors and may not represent the opinion of “Shota Rustaveli National Science Foundation”.

Nikoleishvili, P., Gorelishvili, G., Kveselava, V., Tsurtsumia, G., Nioradze, N., Kurtanidze, R. and Dzanashvili, D. (2017) Hydrogen Generation by Reforming of Sodium Hypophosphite on Cobalt-Boron Oxides Containing Catalyst. Green and Sustainable Chemistry, 7, 85-93. https://doi.org/10.4236/gsc.2017.71007