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An oxygen molecule (yellow, top
right) splits when encountering a vacancy on a titanium oxide
surface. One atom fills the vacancy and the other can move a
couple spaces away (bottom right). |
The team was a bit surprised by the unequal
sharing of resources among the oxygen atoms.
"It is unique that one atom stays in place and the other one is mobile
and probably gets most of the energy," says lead scientist Igor
Lyubinetsky, who performed the work at the The Environmental Molecular
Sciences Laboratory, a DOE national scientific user facility located
at PNNL, with funding by DOE's Office of Basic Energy Sciences. Their
work will be published as the cover article in the Journal of Physical
Chemistry C on February 21, 2008, and previously appeared online
January 5, 2008.
Researchers have yet to determine if this short-lived extra mobility
plays a role in chemical reactions, but understanding the basic
chemistry might be important in processes that break down pollutants
or split water to generate hydrogen.
Previous research has revealed much about how oxygen molecules
interact with metals. For example, when molecular oxygen (O2)
hits a platinum surface, the platinum helps split the molecule apart
and each oxygen atom zips over the surface in opposite directions,
eventually sticking to the metal. Chemists call the pumped up atoms
"hot" because the extra energy released by the breaking and reforming
bonds gives the atoms their boost.
Titanium dioxide is not only a popular catalyst, but it also serves as
a great model oxide to study basic chemistry. PNNL scientists, led by
Lyubinetsky, wanted to know if molecular oxygen behaved on titanium
dioxide the way it behaves on metals such as platinum. Oxides have
different properties than metals: Rust, for example, is iron oxide,
which flakes off from iron metal.
To find out, the team started with a slice of titanium oxide crystal,
oriented so that titanium and oxygen atoms line up on the surface in
alternating strips, forming grooves of titanium troughs between oxygen
rows. By heating the sample, the team created imperfections on the
surface, or spots where an oxygen atom vacated its row. Using scanning
tunneling microscopy, the researchers found that molecular oxygen only
broke apart when it encountered a vacancy, indicating that oxygen
molecules bounce along flawless titanium oxide surfaces and don't
react, as expected from previous results.
The team also expected one of the atoms to make the vacancy its home,
and the second to situate itself right next to its former partner.
Instead, the chemists found that the second oxygen behaved like a
"hot" atom and was free to move one or two crystal lattice spaces away.
Out of 110 molecules the team counted, more than three quarters of the
hot atoms hopped one or two spaces away before becoming mired on the
surface.
"This is one of the first time chemists have looked at oxygen on metal
oxides at the atomic level, and this finding was unexpected," says
Lyubinetsky.
But a skittering atom requires some sort of energy to propel it, so
the researchers explored how a splitting oxygen molecule divvied up
its energetic resources. The team found that a free oxygen atom at
room temperature (about 20 C or 68 F) is virtually immobile on a
titanium oxide surface. However, previous calculations have suggested
that the energy is released from the rearrangement of the bonds --
from within the oxygen molecule and between the oxygen atom and
titanium surface - and the team has concluded this might be the source
of the hot atom's burst after its partner anchored itself in the
vacancy: the calculated energy was about two to three times that
required to get an immobilized oxygen unstuck. Lyubinetsky postulates
that the hot oxygen atom uses this energy to move around on the
titanium oxide surface.
The scientists are trying to better understand the mechanism because
it might be significant in basic catalytic chemistry.
"This finding may be important in surface reactivity. We don't know
yet," Lyubinetsky says. The chemical event could, for example, be
affected by the extra energy the oxygen atom possesses. The effect
might also play into whether surface oxygen atoms interfere with the
chemistry between the catalyst and other reagents.
In any event, the result will keep chemists tango-ing with new
questions for a long time.
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