This is an
interesting advance in membrane technology and at least allows hydrogen and
helium separation convincingly. Of course
just about everything else will not pass easily. There are plenty of chemical processes that
can be enhanced with something like this.
An experiment
I would like to try is to use it to reduce the free hydrogen content of
distilled water while exploring methods to reduce the energetics of the water. In such a situation H3O3 may be a decay
product throwing off a free hydrogen and increasing the stable H3O3 fraction to
a very high level. Near freezing
temperatures could work very well. In
fact slow crystallization would increase the internal free hydrogen to
accelerate crystal growth as the hydrogen escapes.
Very
useful work in terms of hydrogen chemistry.
Laying Down a Discerning Membrane
Oct. 4, 2013 — One of the
thinnest membranes ever made is also highly discriminating when it comes to the
molecules going through it. Engineers at the University of South Carolina have
constructed a graphene oxide membrane less than 2 nanometers thick with high
permeation selectivity between hydrogen and carbon dioxide gas molecules.
The selectivity is based
on molecular size, the team reported in the journalScience. Hydrogen and helium pass relatively easily through the
membrane, but carbon dioxide, oxygen, nitrogen, carbon monoxide and methane
permeate much more slowly.
"The hydrogen
kinetic diameter is 0.289 nm, and carbon dioxide is 0.33 nm. The difference in
size is very small, only 0.04 nm, but the difference in permeation is quite
large" said Miao Yu, a chemical engineer in USC's College of Engineering
and Computing who led the research team. "The membrane behaves like a
sieve. Bigger molecules cannot go through, but smaller molecules can."
In addition to
selectivity, what's remarkable about the USC team's result is the quality of
the membrane they were able to craft on such a small scale. The membrane is
constructed on the surface of a porous aluminum oxide support. Flakes of
graphene oxide, with widths on the order of 500 nm but just one carbon atom
thick, were deposited on the support to create a circular membrane about 2
square centimeters in area.
The membrane is
something of an overlapping mosaic of graphene oxide flakes. It's like covering
the surface of a table with playing cards. And doing that on a molecular scale
is very hard if you want uniform coverage and no places where you might get
"leaks." Gas molecules are looking for holes anywhere they can be
found, and in a membrane made up of graphene oxide flakes, there would be two
likely places: holes within the flakes, or holes between the flakes.
It's the spaces between
flakes that have been a real obstacle to progress in light gas separations.
That's why microporous membranes designed to distinguish in this molecular
range have typically been very thick. "At least 20 nm, and usually thicker,"
said Miao. Anything thinner and the gas molecules could readily find their way
between non-uniform spaces between flakes.
Miao's team devised a
method of preparing a membrane without those "inter-flake" leaks.
They dispersed graphene oxide flakes, which are highly heterogeneous mixtures
when prepared with current methods, in water and used sonication and
centrifugation techniques to prepare a dilute, homogeneous slurry. These flakes
were then laid down on the support by simple filtration.
Their thinnest result
was a 1.8-nm-thick membrane that only allowed gas molecules to pass through
holes in the graphene oxide flakes themselves, the team reported. They found by
atomic force microscopy that a single graphene oxide flake had a thickness of
approximately 0.7 nm. Thus, the 1.8-nm-thick membrane on aluminum oxide is only
a few molecular layers thick, with molecular defects within the graphene oxide
that are essentially uniform and just a little too small to let carbon dioxide
through easily.
The advance has a range
of potential applications. With widespread concerns about carbon dioxide as a
greenhouse gas, the efficient separation of carbon dioxide from other gases is
a high research priority. Moreover, hydrogen represents an integral commodity
in energy systems involving, for example, fuel cells, so purifying it from gas
mixtures is also an active area of interest.
Yu also notes that the
dimensions of the molecular sieve are on the order of the size of water, so,
for example, purifying the copious amounts of tainted water produced by
hydraulic fracturing (fracking) is another possibility.
Being able to reduce
membrane thickness -- and by an order of magnitude -- is a big step forward, Yu
said. "Having membranes so thin is a big advantage in separation
technology," he said. "It represents a completely new type of
membrane in the separation sciences."
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