Pt-Mediated Reversible Reduction and Expansion of CeO₂ in Pt Nanoparticle/Mesoporous CeO₂ Catalyst: In Situ X‑ray Spectroscopy and Diffraction Studies under Redox (H₂ and O₂) Atmospheres

Abstract

Here, we report the Pt nanoparticle mediated reduction (oxidation) and lattice expansion (contraction) of mesoporous CeO₂ under H₂ (O₂) atmospheres and in the temperature range of 50–350 °C. We found that CeO₂ in the Pt/CeO₂ catalyst was partially reduced in H₂ (and fully oxidized back in O₂) as demonstrated by several in situ techniques: APXPS spectra (4d core levels) for the topmost surface, NEXAFS total electron yield spectra (at the M_5,4 edges) in the near surface regions, and (N)­EXAFS fluorescence spectra (at the L₃ edge) in the bulk. Moreover, XRD and EXAFS showed the reversible expansion and contraction of the CeO₂ unit cell in H₂ and O₂ environments, respectively. The expansion of the CeO₂ cell was mainly associated with the formation of oxygen vacancies as a result of the Pt-mediated reduction of Ce⁴⁺ to Ce³⁺. We also found that pure mesoporous CeO₂ can not be reduced in H₂ under identical conditions but can be partially reduced at above 450 °C as revealed by APXPS. The role of Pt in H₂ was identified as a catalytic one that reduces the activation barrier for the reduction of CeO₂ via hydrogen spillover