Environmental Engineering Reference
In-Depth Information
Fig. 5 X-ray diffraction
patterns of magnesium
nanoparticles with varying
sizes. Reproduced with
permission from Norberg
2011
MgH
2
-LiAlH
4
, but rather will focus on dopants in small percentages (10 % or
less). Information on composites can be found elsewhere [
35
-
38
]. A variety of
dopants can be incorporated through many of the previously mentioned synthesis
methods. Several papers have been written on a series of transition metals [
5
,
39
-
43
] or transition metal oxides [
44
-
46
] while others have focused on specific
elemental dopants, the most common being Al, [
47
,
48
] Si, [
47
,
49
] Ti, [
50
,
51
]V
and its corresponding oxides, [
52
,
53
] Cr, [
54
] Fe, [
50
,
55
-
57
] Ni, [
17
,
50
,
58
-
63
]
Ge, [
64
] Nb and corresponding oxides [
52
,
54
,
57
,
60
,
65
-
68
], and Pd [
59
]. More
discussion of these dopants and their effects on sorption kinetics will be discussed
in
Sects. 3
and
4
.
3 Evaluating the Kinetics of Hydrogen Sorption
and Desorption
There are several different methods by which the kinetics of hydrogenation and
dehydrogenation can be determined. Non-isothermal methods, also known as
Temperature Programmed Desorption (TPD), involve the use of either Thermo-
gravimetric Analysis (TGA) or Differential Scanning Calorimetry (DSC). Iso-
thermal approaches include the measurements of Gravimetric-Composition-
Isotherms (GCI) and Pressure-Composition-Isotherms (PCI's). GCI measure the
change in mass of a sample under constant temperature and pressure conditions.
Conversely, a Sievert's apparatus is used with dynamic pressure and constant
temperature to measure PCI's [
69
,
70
]. This is a volumetric method which
determines the amount of hydrogen absorbed or desorbed by a sample through the
ideal gas law, PV = nRT, by measuring the change in pressure in a system with
known volume and temperature [
71
].
A downside of TPD methods is that they can only be used to analyze desorption
of hydrogen from a material, not the adsorption. However, since the measurements
are
done
with
a
TGA
or
DSC
instrument,
common
facility
instruments,
Search WWH ::
Custom Search