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NbO-doped samples. They found that both Nb
2
O
5
and Nb formed NbH
2
, which
allows for the Nb gateway to take place; however, the NbO-doped samples did not
yield the same kinetic rates or NbH
2
species. This study is one of the more recent
studies that straightforwardly attempts to characterize the mechanism of catalysis
for doped Magnesium. However, as mentioned earlier, most of the niobium phases
confirmed to be present in this study were confirmed by XRD of a 50 wt.% dopant
sample, much more than that used in most kinetic studies. Nonetheless, this paper
gives insight into possible mechanisms for catalysis; further proposing the
hypothesis that destabilization of the hydride phase occurs through bond formation
between the catalyst and hydrogen itself [
96
]. Many other groups have worked
with doping magnesium with nickel and Nb
2
O
5
as well as their effects on the
hydrogen sorption kinetics. For example, many of them have tried investigating
the physical nature with high resolution micrographs (Fig.
14
), [
60
,
67
] and the
others can be sought out for more information than already mentioned [
13
,
61
,
62
,
65
,
67
]. As with nickel and Nb
2
O
5
, more studies have since been conducted in
order to further investigate the effects of adding various metals and metal com-
posites to Mg, e.g., Ti, Fe, Ge, Si, Al, and Fe, however, very little new insight has
been given into the nature and mechanism of these catalysts besides reconfirmation
of their respective abilities to improve sorption kinetics [
17
,
48
,
50
,
51
,
55
,
57
,
64
,
100
-
104
].
4.2 Theoretical Modeling Studies on Nature of Dopants
In attempt to investigate and characterize the kinetics and mechanism of the
hydrogen sorption reactions, many groups have tried using different theoretical
approaches to elucidate the nature of the reactions, both catalyzed and non-cata-
lyzed. Song et al. carried out theoretical calculations to explore the effects of
alloying Mg with Cu, Ni, Al, Nb, Fe, and Ti. Using the full-potential linearized
augmented plane-wave method [based on density functional theory (DFT)], they
found that alloying the magnesium should destabilize the MgH
2
by weakening the
Mg-H bond, with increasing effect from Ti, Fe, Nb, Al, Ni to Cu [
105
]. This is
contrary to what Liang et al. had previously reported [
5
], however, the calculations
were not necessarily taking into account particle size [
105
]. Similarly, Cleri and
coworkers did extensive electronic structure and DFT calculations on the effects of
doping with small amounts of Cu, Ni, Fe, Ti, Zr, Pd, Co, Al, Cr, and Nb. Com-
paring their results with experiment, they were able to establish a trend, with a few
exceptions [
39
]. Additionally, Moser et al. studied the stability of Mg-transition
metal hydrides, where TM = Ti, Zr, Hf, V, Nb, and Ta. Their DFT calculations
suggest the H is more strongly bonded to the transition metal, causing a stepwise
dehydrogenation process [
41
]. Kelkar et al. also carried out a computational
study on the effects of pure and aluminum and silicon-doped alpha-, gamma-, and
beta-MgH
2
hydrogen sorption kinetics. They did this using ab initio plane
wave pseudopotential method based on DFT, similar to Song et al. [
47
,
105
].
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