Stability of Ultrathin Ceria Films on Pt(111) Exposed to Air and Treated in Redox Cycles

Abstract

The stability of cerium oxide (ceria) is a major topic in the field of heterogeneous catalysis. When exposed to a reactive environment or treated in a redox step, ceria is prone to changes of its surface morphology, atomic structure and composition, with a strong impact onto its catalytic properties. Here, we investigate the stability of Pt(111)-supported ultra-thin ceria films upon air exposure and during redox cycles under ultra-high vacuum (UHV) conditions. Scanning probe microscopy, X-ray photoemission spectroscopy and low energy electron diffraction show that upon air-exposure a clean ceria film surface gets contaminated by hydroxyls and carbon-containing species whereas a following annealing in molecular oxygen at around 650°C removes such contaminants and allows to restore the original surface morphology and structure. When clean films are oxidized in a background oxygen pressure (reduced in UHV) at ∼ 650°C , the film coverage increases (decreases). The decrease of the film coverage upon reduction is probably due to a release of cerium atoms, which form an alloy with the platinum substrate that acts as a reservoir for cerium atoms. Due to the alloying, the surface work function (WF) of Pt(111) decreases by φ CePt 5 /Pt(111)-Pt ≈ −0.20 ± 0.05 eV, as observed by Kelvin probe force microscopy. Upon oxidation, the released cerium is used to form new ceria. With respect to WF changes of the Pt(111) surface by the ceria film, a decrease is found for the oxidized film (φ CeO2/Pt-Pt ≈ −0.55 ± 0.05 eV) as well as for the reduced film (φ rCeria/Pt-Pt ≈ −0.60 ± 0.05 eV).

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

“…The formation of the same alloy was observed for metallic Ce layers deposited on the Pt(111) surface and annealed in vacuum at intermediate temperatures in previous papers. [ 35,36,43 ] Evidences of mass exchange processes between cerium oxide films and the Pt substrate under reducing conditions have also been observed by scanning microscopies [ 37 ] and low energy electron microscopy [ 20 ] and they are fully consistent with the results of the EXAFS analysis illustrated in Section 2.1.…”

“…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. This drop in intensity is quantified by the increase of the apparent DW factor (as reported in Table 2).…”

“…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 .…”

Dynamics of the Interaction Between Ceria and Platinum During Redox Processes

“…A phase with the same surface periodicity (and LEED dynamic) is observed also on a 1.5 MLE cerium oxide films fully covering the Pt surface on which the reduction is induced by cerium deposition at 680 K (Figure 3F), and also after reduction of a 1.8 MLE film (grown at 1,040 K) induced by H 2 reduction of a 1.8 MLE film (grown at 1,040 K) induced by H 2 exposure of at 680 K (Figure 6). The formation of a surface alloy on thermally reduced ceria films on Pt(111) was also hypothesized based on atomic and Kelvin probe force microscopy experiments (Gasperi et al, 2018). These evidences indicate that surface alloy formation is a process which also occurs at intermediate temperatures in reducing conditions.…”

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

“…A very important difference with respect to the ceria/Pt(111) case is that the CPD contrast of the support is always homogeneous and constant, in particular after a complete H 2 reduction. Therefore, we can assume that there is no local alloying effect between Cu and Ce such that destinctive regions of pure Cu(111) and regions of the alloy are created as in the case of ceria/Pt(111) [23]. A strong support comes from the morphology of the ceria NPs, where neither the coverage nor the height of the NPs noticeable changes upon a redox step unlike on ceria/Pt(111) [23].…”