Hydrogen would command a key role in future renewable energy technologies,
experts agree, if a relatively cheap, efficient and carbon-neutral means of
producing it can be developed. An important step towards this elusive goal has
been taken by a team of researchers with the U.S.
Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley
Lab) and the University of California, Berkeley. The team has discovered
an inexpensive metal catalyst that can effectively generate hydrogen gas from
water.
"Our new proton reduction catalyst is based on a molybdenum-oxo metal complex
that is about 70 times cheaper than platinum, today's most widely used metal
catalyst for splitting the water molecule," said Hemamala Karunadasa, one of
the co-discoverers of this complex. "In addition, our catalyst does not require
organic additives, and can operate in neutral water, even if it is dirty, and
can operate in sea water, the most abundant source of hydrogen on earth and
a natural electrolyte. These qualities make our catalyst ideal for renewable
energy and sustainable chemistry."
From left, Jeffrey Long, Christopher Chang and Hemamala Karunadasa have discovered an inexpensive metal that can generate hydrogen from neutral water, even if it is dirty, and can operate in sea water. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Karunadasa holds joint appointments with Berkeley Lab's Chemical Sciences
Division and UC Berkeley's Chemistry Department. She is the lead author
of a paper describing this work that appears in the April 29, 2010 issue of
the journal Nature, titled "A molecular molybdenum-oxo catalyst for generating
hydrogen from water." Co-authors of this paper were Christopher Chang
and Jeffrey Long, who also hold joint appointments with Berkeley Lab and UC
Berkeley. Chang, in addition, is also an investigator with the Howard Hughes
Medical Institute (HHMI).
Hydrogen gas, whether combusted or used in fuel cells to generate electricity,
emits only water vapor as an exhaust product, which is why this nation would
already be rolling towards a hydrogen economy if only there were hydrogen wells
to tap. However, hydrogen gas does not occur naturally and has to be produced.
Most of the hydrogen gas in the United States today comes from natural gas,
a fossil fuel. While inexpensive, this technique adds huge volumes of carbon
emissions to the atmosphere. Hydrogen can also be produced through the electrolysis
of water – using electricity to split molecules of water into molecules
of hydrogen and oxygen. This is an environmentally clean and sustainable method
of production – especially if the electricity is generated via a renewable
technology such as solar or wind – but requires a water-splitting catalyst.
Nature has developed extremely efficient water-splitting enzymes – called
hydrogenases – for use by plants during photosynthesis, however, these
enzymes are highly unstable and easily deactivated when removed from their native
environment. Human activities demand a stable metal catalyst that can operate
under non-biological settings.
Metal catalysts are commercially available, but they are low valence precious
metals whose high costs make their widespread use prohibitive. For example,
platinum, the best of them, costs some $2,000 an ounce.
"The basic scientific challenge has been to create earth-abundant molecular
systems that produce hydrogen from water with high catalytic activity and stability,"
Chang says. "We believe our discovery of a molecular molybdenum-oxo catalyst
for generating hydrogen from water without the use of additional acids or organic
co-solvents establishes a new chemical paradigm for creating reduction catalysts
that are highly active and robust in aqueous media."
The molybdenum-oxo complex that Karunadasa, Chang and Long discovered is a
high valence metal with the chemical name of (PY5Me2)Mo-oxo. In their studies,
the research team found that this complex catalyzes the generation of hydrogen
from neutral buffered water or even sea water with a turnover frequency of 2.4
moles of hydrogen per mole of catalyst per second.
Long says, "This metal-oxo complex represents a distinct molecular motif
for reduction catalysis that has high activity and stability in water. We are
now focused on modifying the PY5Me ligand portion of the complex and investigating
other metal complexes based on similar ligand platforms to further facilitate
electrical charge-driven as well as light-driven catalytic processes. Our particular
emphasis is on chemistry relevant to sustainable energy cycles."
This research was supported in part by the DOE Office of Science through Berkeley
Lab's Helios Solar Energy Research Center, and in part by a grant from
the National science Foundation.