The two atoms of an oxygen molecule severed by a metal catalyst usually behave identically, but new research reveals that on a particular catalyst, one oxygen atom plants itself while the other moves away, probably with energy partially stolen from the stationary one.
Scientists from the Pacific Northwest National Laboratory found this unanticipated behavior while studying how oxygen interacts with reduced titanium oxide surfaces. The chemists are trying to understand how molecular oxygen -- the stuff we breathe -- interacts with metals and metal oxides, which are used as catalysts in a variety of environmental and energy applications.
Researchers worldwide are exploring the use of titanium dioxide, especially in hydrogen production for solar fuel cells.
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 DOE's Environmental Molecular Sciences Laboratory.
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.
Reference: Y. Du, Z. Dohnalek, and I. Lyubinetsky, Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2(110), J. Phys. Chem. C, 2008, 10.1021/jp077677u.
- PHYSICAL SCIENCES
- EARTH SCIENCES
- LIFE SCIENCES
- SOCIAL SCIENCES
Subscribe to the newsletter
Stay in touch with the scientific world!
Know Science And Want To Write?
- Hugh Hefner's Wife Was Not Poisoned By Breast Implants
- Why An Extra Planet Can't Be Hidden Behind The Sun Or Above The South Pole
- SYRINA: A Trojan Horse For Endocrine Disrupting Chemicals?
- The Daily Physics Problem - 5
- Why Has Organic Farming Flatlined?
- The Daily Physics Problem - 2
- The Daily Physics Problem - 4
- "Is nibru coming in July???..."
- "Charles, yes they have found several dwarf planets beyond Neptune. And dozens of smaller objects..."
- "I read the first paragraph and that is pretty objective. Obviously true as well. Critical thinking..."
- "Some folks, say vegans or vegetarians, may experience continual dietary exposure to isoflavones..."
- "Isn't something posted here on a site called science20.com (or is this an oxymoron?) supposed to..."
- Opercular index score: A novel approach for determining clinical outcomes in stroke
- Cataclysm at Meteor Crater: Crystal sheds light on Earth, moon, Mars
- Workforce processes prior to mechanical thrombectomy vary widely, new study finds
- Male frogs have sex on land to keep competitors away
- Cord blood outperforms matched, unrelated donor in bone marrow transplant