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

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 M5,4 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.