Catalysts for Hydrogen Evolution by Hydrotermolysis and Photocatalytic Hydrolysis of Ammonia Borane
Today, hydrogen storage is one of the most important issues that hinder the development of a hydrogen economy. Many studies searching for effective storage materials are in progress but none has reached the technical criteria that could make them viable from the technological and commercial points of view. According to many researchers, hydrogen generators based on catalytic hydrolysis of hydrides are offered as promising compact and portable devices that were employed with proton-exchange fuel cells. Ammonia borane (NH3BH3, AB), a promising stable H2 storage material, is the object of intense studies now. NH3BH3 contains 19.6 wt% H. Today, most of the published papers about NH3BH3 deal with improvement of catalytic materials.
In this study, a catalytic hydrothermolysis of AB was investigated. In this process the highly exothermal hydrolysis of a part of NH3BH3 (1st step) is coupled to its thermolysis (2nd step). This leads to the high efficiency of this process. Note that catalytic route of hydrothermolysis of AB has not been studied in the literature yet.
It was shown that addition of metal chloride solution (M = Co, Ni, Cu, Fe) to a solid-state bed of AB particles leads to formation in situ a catalytically active nanosized phase. Measurements of the temperature of the reaction layer together with the amount of evolved hydrogen and ATR-FTIR study of products have shown that at external heating 85 С there was acceleration of the 1st step, which resulted in a stronger heating of the reaction layer and a start of 2nd step. The H2 generation rate and H2 yield depend on the temperature of external heating, the mole ratios AB/MCl2 and AB/H2O and the nature of M. High values of H2 storage capacity (7.5 wt%) and average H2 generation rate (39 ml∙gcomposition-1min-1) were achieved at 85 С in this study. Note that organization of hydrogen production by supplying a limited quantity of catalyst precursor solution to the NH3BH3 provides for stability of NH3BH3 in the cartridge.
Nowadays, one more process of interest is the photocatalytic hydrolysis of AB under visible and UV light. In this study, TiO2 (anatase), KxH2-xTi6O13 and Ag/TiO2 were used as photocatalysts. The kinetics of hydrogen generation were studied. It was found that anatase and polytitanate nanostructures being in contact provide for higher activity compared with the phases used separately. Ag content in Ag/TiO2 was optimized and the high value of photonic efficiency of the process (50.6 %) was reached. The fact established at the first time that it is possible to control the hydrogen generation from AB by the light irradiation (λ=365, 450 nm) if Ag content in Ag/TiO2 catalyst is less than 0.1 wt.%.
This work was supported by Ministry of Science and Higher Education of the Russian Federation (project АААА-А17-117041710089-7).