Absence of Ce3+ sites in chemically active colloidal ceria nanoparticles.

The catalytic performance of ceria nanoparticles is generally attributed to active sites on the particle surface. The creation of oxygen vacancies and thus nonstoichiometric CeO(2-δ) has been proposed to result in Ce(3+) sites with unpaired f electrons which can be oxidized to spinless Ce(4+) ions during catalytic reactions. We monitored the Ce electronic structure during the synthesis and catalase mimetic reaction of colloidal ceria nanoparticles under in situ conditions. By means of high-energy resolution hard X-ray spectroscopy, we directly probed the Ce 4f and 5d orbitals. We observe pronounced changes of the Ce 5d bands upon reduction of the particle size and during the catalytic reaction. The Ce 4f orbitals, however, remain unchanged, and we do not observe any significant number of spin-unpaired Ce(3+) sites even for catalytically active small (3 nm) particles with large surface to bulk ratio. This confirms strong orbital mixing between Ce and O, and the Ce spin state is conserved during the reaction. The particles show an increase of the interatomic distances between Ce and O during the catalytic decomposition of hydrogen peroxide. The redox partner is therefore not a local Ce(3+) site, but the electron density that is received and released during the catalytic reaction is delocalized over the atoms of the nanoparticle. This invokes the picture of an electron sponge.