Environmental Engineering Reference
In-Depth Information
Table 3
Different pressure dependence functions, F(P), used by investigators
F(P)
F(P)
h
i n
j
j
P eq P
P eq P
P 0
; P 0 ¼ 1 atm
; P 0 ¼ 1 atm
P 0
P eq
P
P
P eq
Ln
Ln
p
P
P
ð
Þ
ð
Þ
P eq P
P eq P b
P eq
P eq
P 1 = 2 P 1 = 2
P 1 = 2
0
P 1 = 2
eq for bulk
eq ; P 0 ¼ initial pressure
P eq P
P P eq
n
; F
n
P eq P for surface control
P eq
P
F
P
P eq
Reproduced with permission from Ron 1999
P system pressure; P b reaction bed pressure; P eq desorption/absorption equilibrium pressure; Not
all forms of F(P) that have been suggested are presented here
metals and metal oxides as catalysts to improve kinetic performance. Under-
standing the physical nature of the dopants in the magnesium requires an under-
standing of the structure, size, and shape of the nanoparticles. Using techniques
such as x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scan-
ning electron microscopy (SEM), transmission electron microscopy (TEM) among
others, researchers have attempted to understand the physical nature of these do-
pants. Having said that, the following sections will mainly concern themselves
with understanding the kinetic and mechanistic abilities of the different dopants.
Although direct comparison of the results is inherently difficult because of dif-
ferences in the apparatuses, methods and calculations, and kinetic modeling (or
lack thereof), the effects of additional chemical species on sorption kinetics are
undoubtedly apparent. In light of the inability to directly compare the results of
different doping papers with respect to each other, this section will attempt to
highlight the findings in this area.
4.1 Dopant Effects on Hydrogen Sorption Kinetics
One of the earliest reports on the effects of alloying (*15-30 % Cu) on the
hydrogen storage properties of sub-bulk, micron-sized, magnesium was in 1979
when Karty et al. studied the hydrogen sorption kinetics of a Mg/Mg 2 Cu micro-
structured eutectic alloy via a pressure sweep method. Using the Johnson-Mehl-
Avrami-Kolmogorov equation, an adapted version of the JMA equation discussed
earlier, they were able to obtain suggestive evidence that the alloying of Mg with
Cu reduced both the hydrogen desorption and adsorption activation energies with
respect to measured activation for the pure Mg system. They propose that the
Mg 2 Cu's role in decreasing these activation energies is through offering an
alternative pathway for diffusion of hydrogen into and out of the magnesium [ 91 ].
Search WWH ::




Custom Search