Metal
borides have attracted the attention of researchers due to
their useful physical properties and unique ability to form high hydrogen-capacity
metal borohydrides. We demonstrate improved hydrogen storage properties
of a nanoscale Mg–B material made by surfactant ball milling
MgB2 in a mixture of heptane, oleic acid, and oleylamine.
Transmission electron microscopy data show that Mg–B nanoplatelets
are produced with sizes ranging from 5 to 50 nm, which agglomerate
upon ethanol washing to produce an agglomerated nanoscale Mg–B
material of micron-sized particles with some surfactant still remaining.
X-ray diffraction measurements reveal a two-component material where
32% of the solid is a strained crystalline solid maintaining the hexagonal
structure with the remainder being amorphous. Fourier transform infrared
shows that the oleate binds in a “bridge-bonding” fashion
preferentially to magnesium rather than boron, which is confirmed
by density functional theory calculations. The Mg–B nanoscale
material is deficient in boron relative to bulk MgB2 with
a Mg–B ratio of ∼1:0.75. The nanoscale MgB0.75 material has a disrupted B–B ring network as indicated by
X-ray absorption measurements. Hydrogenation experiments at 700 bar
and 280 °C show that it partially hydrogenates at temperatures
100 °C below the threshold for bulk MgB2 hydrogenation.
In addition, upon heating to 200 °C, the H–H bond-breaking
ability increases ∼10-fold according to hydrogen–deuterium
exchange experiments due to desorption of oleate at the surface. This
behavior would make the nanoscale Mg–B material useful as an
additive where rapid H–H bond breaking is needed.
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