Formation of Reversible Adducts by Adsorption of Oxygen on Ce-ZrO2: An Unusual η2 Ionic Superoxide

The interaction of oxygen with Ce-ZrO2 (Ce ions diluted in the zirconia matrix) is a key aspect for many applications of this material. In this work, for the first time, an unusual reversible interaction of the O-2 molecule with the surface of this solid is reported. The joint use of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations has allowed a full description of the reversible O-2 adsorption phenomenon that involves a full electron transfer from Ce3+ centers to the adsorbed O-2 forming the superoxide anion (O-2(center dot-)). The process can be completely reversed by removing the O-2 gas from the atmosphere as confirmed by the calculated values of both the adsorption energy (0.45 eV) and the activation energy (negligible) of the process. This behavior is typical of a solid oxygen carrier. However, at variance of the reported cases of molecular and solid oxygen carriers that usually form eta(1) end-on adducts, in the present case a side-on, eta(2) Ce-O-2 surface complex is observed. Comparison of structural data and DFT results also indicates that Ce doping occurs in the lattice of tetragonal zirconia while this is not observed in the monoclinic phase.