The Oxygen Squeeze Play
Scientists first to discover tetraoxygen on the surface of a common catalyst
Oxygen molecules fill reactive pockets on the surface of rutile titanium dioxide. When heated, they split apart, with one oxygen staying put and the other filling a nearby vacancy (Top). When oxygen is added at low temperatures, two oxygen molecules fit into the vacancy. When heated, these molecules form a new species, tetraoxygen. Enlarged View
Results: When it gets cold on the surface of a popular catalyst, four oxygen atoms squeeze into a spot designed for just one, according to scientists at Pacific Northwest National Laboratory. Initially, the atoms are paired up as two oxygen molecules or
2 O2. But, when the temperature rises, the two molecules react to form a new species, tetraoxygen or O4.
Why it matters: If researchers want to design a catalyst from scratch or improve an existing one, they need to predict how oxygen will react with the surface, said PNNL's Greg Kimmel, the principal investigator on the project. This very fundamental study revealed new information that will help predict oxygen interactions.
Methods: The researchers exposed a sample of rutile titanium dioxide or TiO2(110) to different amounts of oxygen at very low temperatures. Then, they heated the sample and studied the behavior of the oxygen using electron-stimulated desorption and mass spectrometry techniques.
The researchers began with a catalyst whose surface contained scattered reactive pockets, or vacancies, where oxygen atoms had left the surface. Next, they added (or adsorbed) molecular oxygen to see how it would interact with the reactive pockets. The oxygen was adsorbed at about 25 K or -414 degrees Fahrenheit, which required cooling the sample with liquid helium. In one experiment, they added one oxygen molecule per oxygen vacancy. In a second experiment, they added two oxygen molecules per vacancy.
For both experiments, no oxygen was released from the surface as it was heated. Instead the molecules reacted on the surface, but the type of reactions depended on whether the vacancies had one or two oxygen molecules.
For the sample with one molecule per vacancy, the oxygen molecule began to split apart at 150 K: one oxygen atom stayed put filling the vacancy, and the other adsorbed at nearby site on the surface. By 280 K, all of the oxygen had filled in surface vacancies.
However, when the sample that initially had two O2 per vacancy was heated above 200 K, these molecules transformed to tetraoxygen - a 4-atom arch poking up from the reactive pockets. By 500 K, the tetraoxygen decomposed, filling in nearby reactive pockets. But, to drive off all the oxygen and restore the vacancies, the scientists had to heat the sample to 700 K, about the temperature used to bake bread.
What's next? This fundamental research leads to more questions about the behavior of the common oxygen molecule on the catalyst's surface. The researchers plan to continue their research.
Acknowledgments: This research was done by Greg Kimmel and Nikolay Petrik at PNNL. The DOE Office of Basic Energy Sciences, Chemical Sciences Division funded the research.
Experiments were performed in the electron and photon stimulated desorption laboratory located in DOE's EMSL, a national scientific user facility at PNNL.
The work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.
Reference: Kimmel GA and NG Petrik. 2008. "Tetraoxygen on Reduced TiO2(110): Oxygen Adsorption and Reactions with Bridging Oxygen Vacancies." Physical Review Letters 100, 196102. DOI: 10.1103/PhysRevLett.100.196102.