Probing Oxygen Vacancies on Ceria

Sherlock Holmes solved some murder mysteries by combing crime scenes looking for anything out of place. Like the fabled detective, surface scientists are solving puzzles in chemistry by scrutinizing crystal surfaces--hunting for atoms that are out of place.

Oxygen vacancies and other crystal imperfections have been fingered as hot spots for reactivity on oxide surfaces. On ceria (cerium oxide, CeO₂), the vacancies are readily formed and filled. That property has made ceria a critical component of automotive catalysts and a key catalyst support for hydrogen production and other types of reactions.

Although oxygen vacancies are recognized as players in surface catalysis, the nature of their role has remained unclear. Now, researchers in Trieste, Italy, have begun unraveling the vacancy mystery by determining the structure of the defects, as well as their concentration, distribution, and mobility on ceria surfaces (Science 2005, 309, 752).

The team includes Friedrich Esch at the National Laboratory for Advanced Technology & Nanoscience of the National Institute for the Physics of Matter (INFM), Stefano Fabris at the Democritos National Simulation Center of INFM, and their coworkers at the University of Trieste and other institutions.

To investigate vacancy-dependent processes, the team subjected ceria samples to high-temperature treatments, which induced controlled formation of oxygen vacancies. They then mapped the crystal surface with atomic-scale detail by using scanning tunneling microscopy. The experimental work was coupled with quantum mechanical calculations to interpret the results and develop mechanistic models.

At room temperature, the vacancies are immobile, the team reports. At higher temperatures, additional oxygen atoms are emitted from the surface, leaving behind vacancies that tend to form straight lines--clusters of vacancies at adjacent lattice positions.

On the basis of simulations that reproduce surface features within a tiny fraction of an angstrom, Esch, Fabris, and coworkers conclude that a common defect that they refer to as a double linear surface vacancy actually consists of two surface vacancies and another vacancy just below the surface. As a result of the defect's crystal structure and the difference in valences between cerium and oxygen, the missing oxygen atoms expose pockets of Ce³⁺ ions surrounded by Ce⁴⁺ ions.

Another finding is that a subsurface vacancy is required to nucleate vacancy clusters. The group also shows that zirconium, which is commonly used to promote ceria-based catalysts, may perform its job by sidestepping the subsurface vacancy requirement.

University of Washington chemistry professor Charles T. Campbell and Charles H. F. Peden, a scientist at Pacific Northwest National Laboratory, remark that the "state-of-the-art" methods employed by the group "provide much-needed atomic-level structural detail." Their comments appear in the same issue of Science.

Campbell and Peden note that the exposed Ce³⁺ ions may serve as potent surface sites for catalysis and photocatalysis. They add that the vacancy clusters may protect ceria-supported metal catalysts from sintering and may serve another function in which they direct metal nanoparticles to form wires and other shapes that promote catalytic reactions.