A Clean Air Act At the Quantum Scale

A clean air act at the quantum scale takes place inside state-of-the-art fuel cells. Environmentally friendly technologies have become a necessity for dealing with increasing levels of air pollution. For example, catalytic converters reduce the amount of toxic species in automobile exhausts.

Meanwhile, researchers are intensively developing potential new sources of low-emission power generation, such as solid-oxide fuel cells, which use a solid material to supply migrating ions for an electricity-producing chemical reaction.

To a large extent, many of these devices exploit an amazing property of solid cerium oxide (CeO₂) to release oxygen under oxygen-poor conditions. To shed oxygen, cerium ions in cerium oxide gain electrons, and a series of "reduced" compounds, with Ce₂O₃ as an end product, is formed.

In turn, the final product Ce₂O₃ easily takes up oxygen under oxygen-rich conditions. However, the fundamental microscopic origin of this phenomenon has not been elucidated--until now.

Researchers from Chalmers and Uppsala Universities in Sweden now offer a detailed quantum-mechanical description of how this reaction occurs.

The pivotal transition from CeO₂ to Ce₂O₃, they show, results from the formation of an oxygen vacancy, in which an oxygen atom leaves a spot it normally occupies on the cerium oxide crystal lattice. In order to vacate the CeO₂ lattice, oxygen has to leave behind two electrons, so that it can transform from an ion with a charge of -2 to a free oxygen atom.

Quantum effects make this process possible. They enable the electrons to "localize" on two nearby cerium ions, which initially have a charge of +4. Gaining these electrons allows the two Ce ions each to acquire a charge of +3 and allow a series of "reduced" compounds and eventually Ce₂O₃ to form.

This makes the oxygen storage-and-release ability of solid cerium oxide a remarkable example of the quantum process of electron localization, directly manifesting itself in a macroscopic property used in many modern environmental friendly applications. (N.V. Skorodumova, S.I. Simak, B.I. Lundqvist, I.A. Abrikosov, B. Johansson, Physical Review Letters, 14 Oct 2002; contact Sergei Simak, Uppsala University, 011-46-18-471-5739, Sergei.Simak@fysik.uu.se).