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A technique for creating a new molecule that
structurally and chemically replicates the active part of the widely used
industrial catalyst molybdenite has been developed by researchers with the U.S.
Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
This technique holds promise for the creation of catalytic materials that can
serve as effective low-cost alternatives to platinum for generating hydrogen
gas from water that is acidic.
Christopher Chang and Jeffrey Long, chemists who hold
joint appointments with Berkeley Lab and the University of California (UC)
Berkeley, led a research team that synthesized a molecule to mimic the
triangle-shaped molybdenum disulfide units along the edges of molybdenite
crystals, which is where almost all of the catalytic activity takes place.
Since the bulk of molybdenite crystalline material is relatively inert from a
catalytic standpoint, molecular analogs of the catalytically active edge sites
could be used to make new materials that are much more efficient and
cost-effective catalysts.
“Using molecular chemistry, we’ve been able to
capture the functional essence of molybdenite and synthesize the smallest
possible unit of its proposed catalytic active site,” says Chang, who is also
an investigator with the Howard Hughes Medical Institute (HHMI). “It should now
be possible to design new catalysts that have a high density of active sites so
we get the same catalytic activity with much less material.”
Says Long, “Inorganic solids, such as molybdenite,
are an important class of catalysts that often derive their activity from
sparse active edge sites, which are structurally distinct from the inactive
bulk of the molecular solid. We’ve demonstrated that it is possible to create
catalytically active molecular analogs of these sites that are tailored for a
specific purpose. This represents a conceptual path forward to improving future
catalytic materials.”
Chang and Long are the corresponding authors of a
paper in the journal Science describing this research titled “A
Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation.”
Other authors are Hemamala Karunadasa, Elizabeth Montalvo, Yujie Sun, and Marcin
Majda.
Molybdenite is the crystalline sulfide of
molybdenum and the principal mineral from which molybdenum metal is extracted.
Although commonly thought of as a lubricant, molybdenite is the standard
catalyst used to remove sulfur from petroleum and natural gas for the reduction
of sulfur dioxide emissions when those fuels are burned. Recent studies have
shown that in its nanoparticle form, molybdenite also holds promise for
catalyzing the electrochemical and photochemical generation of hydrogen from water.
Hydrogen could play a key role in future renewable energy technologies if a
relatively cheap, efficient and carbon-neutral means of producing it can be
developed.
Currently, the best available technique for
producing hydrogen is to split water molecules into molecules of hydrogen and
oxygen using platinum as the catalyst. However, with platinum going for more
than $2,000 an ounce, the market is wide open for a low cost alternative
catalyst. Molybdenite is far more plentiful and about 1/70th the cost of
platinum, but poses other problems.
“Molybdenite has a layered structure with multiple
microdomains, most of which are chemically inert,” Chang says. “High-resolution
scanning tunneling microscopy studies and theoretical calculations have
identified the triangular molybdenum disulfide edges as the active sites for
catalysis; however, preparing molybdenite with a high density of functional
edge sites in a predictable manner is extremely challenging.”
Chang, Long and their research team met this
challenge using a pentapyridyl ligand known as PY5Me2 to create a
molybdenum disulfide molecule that, while not found in nature, is stable and
structurally identical to the proposed triangular edge sites of molybdenite. It
was shown that these synthesized molecules can form a layer of material that is
analogous to constructing a sulfide edge of molybdenite.
“The electronic structure of our molecular analog
can be adjusted through ligand modifications,” Long says. “This suggests we
should be able to tailor the material’s activity, stability, and required
over-potential for proton reduction to improve its performance.”
In 2010, Chang and Long and Hemamala Karunadasa,
who is the lead author on this new Science paper, used the PY5Me2
ligand to create a molybdenum-oxo complex that can effectively and efficiently
catalyze the generation of hydrogen from neutral buffered water or even sea
water. Molybdenite complexes synthesized from this new molecular analog can
just as effectively and efficiently catalyze hydrogen gas from acidic water.
“We’re now looking to develop molecular analogs of
active sites in other catalytic materials that will work over a range of pH
conditions, as well as extend this work to photocatalytic systems” Chang says.
Adds Long, “Our molecular analog for the
molybdenite active site might not be a replacement for any existing catalytic
materials but it does provide a way to increase the density of active sites in
inorganic solid catalytic materials and thereby allow us to do more with less.”
