Hydrogen Generation from the Hydrolysis of Sodium Hypophosphite Using CoB2O4 Catalyst
Authors: Nikoleishvili Paata , Gorelishvili Giorgi , Kveselava Valentina , Kurtanidze Rusudan , Gogoli Davit , Sharabidze Davit 
Published: February 2020

The goal of the work involves a radically new approach – establishment of possibility of the use of sodium hypophosphite (NaH2PO2) as hydrogen source and study of the process of its reforming; synthesis of corresponding catalysts and their testing for generation of hydrogen. It is well known that in the process of chemical nickel plating at 80-95 0C, in the course of an interaction between sodium hypophosphite and water the formed atomic hydrogen reduces nickel on the surface of the plated metal. Nickel plating with the participation of sodium hypophosphite is self-catalytic [1]. Overall catalytic reaction of the interaction between hypophosphite and water may be presented as follows

H2PO 2‾ + H2O → H2PO 3‾ + H2                                                                                             (1)

It is well known that in the absence of nickel-ions in the hypophosphite solution, and with the use of so-called Raney nickel catalyst, an intensive separation of hydrogen is possible at high temperatures (80-95 0C), but at room temperature the process practically stops. It wassynthesisCoB2O4 catalyst, which can produces hydrogen from hypophosphite solution at room temperature and its use in fuel cell are possible.

The hydrogen separation at catalytic interaction between water and hypophosphite proceeds by electrochemical mechanism (the process occurs by electron transfer), according to which the hypophosphite-ion presents the distorted tetrahedron in the center of which a phosphorus atom is located and at the ends of tetrahedron two hydrogen and two oxygen atoms are placed. Hypophosphite-ion is adsorbed at metal (catalyst) surface, hydrogen atom of which is connected with catalyst surface by covalent bond, attenuates P-H bond and finally cleaves it; hydrogen is replaced by hydroxide-ion from water (2, 3) which causes hypophosphite transformation in phosphite. Electron, atomic hydrogen and proton, formed by the reaction (4), produce molecular hydrogen (reaction 6):

H2O = H+ + OH-                                                                                                                       (2)

H2PO2
-ads + OH-ads = H2PO3
-ads+ Hads + ē                                                                                     (3)

Hads + H+ads + e- = H2                                                                                                                (4)

To synthesized a boron containing cobalt catalyst, [2,3], in an ultrasonic setup where a chemical beaker with 1 M CoSO4 solution was placed. 20 mL of 0.6 M NaBH4 solution were added drop wise into the beaker at the room temperature under the conditions of intensive mixing. The formed pulp was washed by distilled water. After drying, the deposit was annealed at the temperature of 450°C and finely dispersed black powder was obtained. In X-ray phase analysis of the sample, small peaks were observed in the X-ray diffraction pattern. They corresponded, according to the ASTM standard, to СoB2O4 (2.88,2.44, 2.03, 1.86 Å). In the work the hydrogen evolution from aqueous solution of sodium hypophosphite was studied at 10-30 0C by CoB2O4 catalyst In case of 2M NaH2PO2 solution hydrogen evolution rate was 3.4-3.6 ml sec-1 g-1, which was enough for 0.6 W H2–O2 fuel cell operation.

References

[1] Electroless plating: fundamentals and applications edited by G. O. Mallory, J. B. Hajdu

[2] Wu, C., Wu, F., Bai, Y., Yi, B., and Zhang, H., Mater.Lett., 2005, vol. 59, p. 1748.

[3] P.O. Nikoleishvili, at all, Russian Journal of Electrochemistry, 2015, Vol. 51, No. 7, pp. 665–671.