Fast and Slow Reduction Behaviors of CeO₂ Nanoparticles by H₂-TPR Kinetics and Electronic Current Increase

抄録

We studied the influence of the surface and crystal lattice about both H₂-oxidation reaction and electronic properties of CeO₂ nanoparticles (NPs). The oxygen evolution phenomena in CeO₂ NPs were observed in reducing (hydrogen or vacuum) atmosphere. In the catalytic hydrogen temperature-programmed reduction experiments (H₂-TPR) of CeO₂, nonstoichiometric CeO₂ produces active oxygen species which react with H₂ to form H₂O. The CeO₂ NPs showed the main TPR temperature of the lattice oxygen at around 700 °C (slow reaction). A low temperature peak appeared at around 500 °C, which is originated from the surface of CeO₂ (fast reaction). The active oxygen formation was characterized by different activation energies which were newly determined as 77±18 kJmol^-1 (on surface) and 131±16 kJmol^-1 (in lattice) for NPs, respectively. Regarding with the electric current response of CeO₂ NP, we found the relatively fast peak-out peak and then the gradual increase at 300 °C under the low oxygen pressure of 5×10^-3 Pa (vacuum). These were both related to the fast and slow release of oxygen on the surface and then in the lattice of CeO₂ NPs. The comparative TPR and electric current observations revealed the common reduction behavior on surface and in bulk region of CeO₂ NPs.