Dynamics of the Interaction Between Ceria and Platinum During Redox Processes

Abstract

The work is focused on understanding the dynamics of the processes which occur at the interface between ceria and platinum during redox processes, by investigating an inverse catalytic model system made of ceria epitaxial islands and ultrathin films supported on Pt(111). The evolution of the morphology, structure and electronic properties is analyzed in real-time during reduction and oxidation, using low-energy electron microscopy and spatially resolved low-energy electron diffraction. The reduction is induced using different methods, namely thermal treatments in ultra-high vacuum and in H2 as well as deposition of Ce on the oxide surface, while re-oxidation is obtained by exposure to oxygen at elevated temperature. The use of two different epitaxial systems, continuous films and nanostructures, allows determining the influence of platinum proximity on the stabilization of the specific phases observed. The factors that limit the reversibility of the observed modifications with the different oxidation treatments are also discussed. The obtained results highlight important aspects of the cerium oxide/Pt interaction that are relevant for a complete understanding of the behavior of Pt/CeO2 catalysts.

Structure of Reduced Cerium Oxide Ultrathin Films on Pt(111): Local Atomic Environment and Long‐Range Order

“…The formation of the alloy at the step edge of the islands reduces their lateral size. This behavior has been already observed in previous works by LEEM [ 31 ] and AFM. [ 37 ] The decrease of the lateral size of cerium oxide islands is also reflected in the reduction of the first shell intensity in the Fourier transform (Figure 2a) after the reducing treatment.…”

“…[ 19 ] It must be noted that the formation of oxygen vacancies may be favored at the edge sites of the cerium oxide islands, present in the annealed film, because of their reduced coordination and of the proximity of Pt atoms, which possibly induce a decrease of the formation energy of oxygen vacancies by charge transfer [ 29 ] or by interatomic exchange processes. [ 31 ] The 2 ML as‐grown sample has a much lower density of Pt‐ceria edge sites compared to the 2 ML epitaxial sample. The morphology and the interaction with the substrate are likely to play a more important role than the surface defectivity, effectively contributing to the observed higher reducibility of the 2 ML epitaxial film compared to the 2 ML as‐grown one.…”

“…To explain the apparent inconsistency, we hypothesize the formation of a CePt alloy with the reducing treatment at 1023 K. In the literature the formation of metallic CePt alloys has already been observed by LEED, LEEM, and STM/AFM after a significant film reduction. [ 31,35–37 ] In order to explore this possibility, the EXAFS data of the 2 ML samples after reduction were fitted using a Ce–Pt scattering path with a weight A, in addition to the Ce–O one with a weight 1‐A, with A used as a free fitting parameter. We chose three possible candidates as the most probable alloys formed by the high temperature treatment: CePt, [ 38 ] CePt 3 , [ 39 ] and CePt 5 .…”

“…[ 6,13,14,17,19 ] In some cases, the reconstruction was observed only on ultrathin films and ascribed to a lower oxygen vacancy formation energy in different sites of the film‐substrate coincidence cell. [ 13,16,18 ] This is the case of the (5 × 5) superstructure, observed for ultrathin cerium oxide films on Rh(111) [ 11 ] and Pt(111) [ 20 ] and possibly also of the (3 × 3) [ 18 ] and 9/4(√3 × √3), [ 13 ] observed on Pt(111). In other cases, well defined phases, metastable in the bulk, were stabilized on different substrates and at different thickness.…”

“…[ 9 ] Other structures with surface periodicity ascribed to “quasistable” bulk phases like the (3 × 3) one [ 22 ] were obtained on different substrates like Cu(111), [ 17 ] Ru(0001), [ 15 ] and Pt(111). [ 13,20 ] At high Ce 3+ concentrations in some cases the cubic C‐type phase, identified by a (4 × 4) surface periodicity, was observed. [ 14,15,17,23 ] The structural modifications were often shown to be reversible by treatments in oxidizing conditions, which transform the films back to the fluorite CeO 2 phase.…”