Magnesium and Doped Magnesium Nanostructured Materials for Hydrogen Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 307

absolute mechanism of catalysis is elusive; however, they propose the surface
reaction is a first order process and that must take into account certain surface
conditions prior to modeling [ 40 , 66 ]. In their paper, Kalisvaart et al. describe a
neutron reflectometry experiment that studies the effects of vanadium and chro-
mium doping on Mg hydrogenation kinetics. It was found that with pure thin film
Mg, the deuterium forms an MgD 2 blocking layer on the surface of the film, which
slows the kinetics of hydrogenation. With the addition of catalysts, no such layer
formed, and the deuterium was able to diffuse into the film without inhibition.
Although this study was performed on thin films in the micrometer range, it further
confirms the surface role of different catalysts is highly important [ 54 ]. The ability
for the catalyst to remain on the surface may also come into play. Although
discussion about the surface condition does not take place, Lillo-Rodenas et al.
found that carbon-supported nickel had superior kinetics to pure nickel. The role of
the carbon is unclear, but based on their DSC measurements they surmise that the
carbon support may stabilize the catalytic role of nickel over many cycles (i.e., no
degradation in catalytic activity) [ 61 ]. More recently, Callini et al. performed an
in-depth investigation on magnesium nanoparticles decorated with nickel by
thermal evaporation. The nanoparticles were exposed to oxygen in order to form a
protective MgO layer, which was suggested not to hinder diffusion of H 2 . Using
in situ powder XRD during different simultaneous annealing and hydrogen
exposure experiments, they were able to confirm the presence of the nickel in the
form of Mg 2 Ni prior to hydrogenation and Mg 2 NiH 4 after hydrogenation. The
increased rate of the hydrogen sorption kinetics was clearly attributed to the for-
mation of Mg 2 Ni from Mg 2 NiH 4 . Nonetheless, a lowered hydrogen capacity of
4.4 % was a definite downside of this doped material, attributed to the formation of
the Mg 2 NiH 4 (3.6 wt.% hydrogen) and MgO (0 wt.% hydrogen) [ 96 ].
With regards to MgO formation, Ares-Fernandez et al. investigated the possible
catalytic effect of MgO addition prior to milling. They found that the MgO
addition resulted in smaller Mg particle formation, referring to MgO as a lubricant
for the sliding tribological interfaces. They rule out the possibility that MgO, as
have others, [ 97 ] is acting as a catalyst in the sorption processes, confirming it
actually decreases sorption kinetics [ 98 ]. With this confirmation, they suggest that
the main reason for increased sorption rates for the particles is a result of particle
size; contrarily, a recent theoretical paper by Shevlin et al. has proposed that
nanostructuring does not decrease dehydrogenation enthalpies unless the nano-
cluster contains less than three magnesium atoms, much smaller than those
reported by Ares-Fernandez et al. This particular paper will be discussed more in
the next section [ 99 ]. Another 2013 paper written by Ma et al. formulates a
niobium gateway model to explain the superior catalytic ability of Nb 2 O 5 in MgH 2
dehydrogenation. The gateway model begins by NbH 2 (confirmed by XRD of a 50
wt.% XRD) decomposing rapidly because of its thermodynamic instability. This is
followed by formation of Nb, which then instigates diffusion of hydrogen to form
an NbH x solid solution. This solid solution allows for the flow of hydrogen from
MgH 2 to the outside of the particle until MgH 2 is exhausted. They rule out the
possibility

of NbO acting as a catalyst by investigating Nb 2 O 5 , Nb, and