Environmental Engineering Reference
In-Depth Information
Twenty years later, the next notable study, from Liang et al., compared the
sorption kinetics of MgH
2
that was ball milled with different transition metals
(MgH
2
5 % at. TM where TM = Ti, V, Mn, Fe, and Ni) [
5
]. Using the CV model
equation (Table
2
), they were able to plot the desorption kinetics curves at dif-
ferent temperatures and obtain activation energies for the desorption (Fig.
12
)[
5
].
They were, 62.3, 67.6, 71.1, 88.1, and 104.6 kJ mol
-1
for the MgH
2
—vanadium,
iron, titanium, nickel, and manganese nanocomposites, respectively. These values
are much lower than the calculated pure balled milled MgH
2
desorption activation
energy of 120 kJ mol
-1
. They did not calculate the adsorption activation energies;
however, they report increased rate of absorption compared to pure ball-milled
MgH
2
, for all nanocomposites at 10 MPa pressure and temperatures of 473, 373,
and 303 K. A mechanism of catalysis was not specified, but they ruled out changes
in the Mg-H bond thermodynamics by measuring PCIs for all systems [
5
,
68
].
Oelerich and coworkers investigated a series of MgH
2
/Me
x
O
y
nanocomposites
that were synthesized through high energy ball milling (Me
x
O
y
= Sc
2
O
3
, TiO
2
,
V
2
O
5
,Cr
2
O
3
,Mn
2
O
3
,Fe
3
O
4
, CuO, Al
2
O
3
, and SiO
2
). Only the transition metal
oxides acted as catalysts in the hydrogen adsorption/desorption processes
at *573 K. As a result, the authors suggest that the ability of a metal to take on
different oxidation states may play a part in the role of the catalyst. The best
performing catalysts were Fe
3
O
4
and Cr
2
O
3
which allowed for rapid adsorption at
room temperature and pressure of 8.4 bar, and desorption at *473 K and pressure
of 0 bar [
45
,
46
,
53
]. The team also compared the catalytic activity of V, V
2
O
5
, VC,
and VN. They found that the vanadium already bonded with oxygen, nitrogen, or
carbon resulted in the best performance, again supporting the theory that the oxi-
dation state plays an important role. Additionally, they found that exposure of the
MgH
2
-V to small amounts of oxygen increased the catalytic activity of the vana-
dium. This further evidences that oxygen, or more specifically the oxidation state of
vanadium is also playing a role in the catalytic mechanism [
53
]. Not long after,
Barkhodarian et al. compared alloying NbO, NbO
2
, and Nb
2
O
5
with those resear-
ched by Oelerich et al. (Fig.
13
)[
80
]. They found that 0.5 mole% Nb
2
O
5
was a
superior catalyst in both adsorption and desorption of hydrogen, with a tentative
(based on JMA) lowered desorption activation energy of 62 kJ mol
-1
. In their
numerous papers on the subject, Barkhordarian et al. present thorough discussion on
the differences in the kinetic models that can be used. While using the different
approaches discussed in
Sect. 3
to analyze their kinetics, they ultimately conclude
that the mechanism of the catalytic process is unclear, but they agree with Oelerich
and coworkers [
45
,
46
,
53
] that the ability of the transition metals to take on different
electronic states must play a major role. It was proposed that the transition metal
goes through electronic exchange reactions with the hydrogen molecule, which
could accelerate the overall reaction. They claim there are five distinct steps in the
reaction of metals with hydrogen: physisorption, chemisorption, surface penetra-
tion, diffusion, and hydride formation. The slowest of these steps will be the one that
causes slow kinetics; they found that the rate limiting step changes with varying
amounts of their catalyst, Nb
2
O
5
[
80
]. Tentatively, the different transition metal
catalysts were concluded to influence both the chemisorption and diffusion steps.
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