Quantitative depth profiling of Ce3+ in Pt/CeO2 by in situ high-energy XPS in a hydrogen atmosphere

• Main Authors: Kato, Shunsuke, Ammann, Markus, Huthwelker, Thomas, Paun, Cristina, Lampimäki, Markus, Lee, Ming-Tao, Rothensteiner, Matthäus, van Bokhoven, Jeroen A
• Format: Journal Article
• Language: English
• Published: 01.01.2015
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp05643d

The redox property of ceria is a key factor in the catalytic activity of ceria-based catalysts. The oxidation state of well-defined ceria nanocubes in gas environments was analysed in situ by a novel combination of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) and high-energy XPS at a synchrotron X-ray source. In situ high-energy XPS is a promising new tool to determine the electronic structure of matter under defined conditions. The aim was to quantitatively determine the degree of cerium reduction in a nano-structured ceria-supported platinum catalyst as a function of the gas environment. To obtain a non-destructive depth profile at near-ambient pressure, in situ high-energy XPS analysis was performed by varying the kinetic energy of photoelectrons from 1 to 5 keV, and, thus, the probing depth. In ceria nanocubes doped with platinum, oxygen vacancies formed only in the uppermost layers of ceria in an atmosphere of 1 mbar hydrogen and 403 K. For pristine ceria nanocubes, no change in the cerium oxidation state in various hydrogen or oxygen atmospheres was observed as a function of probing depth. In the absence of platinum, hydrogen does not dissociate and, thus, does not lead to reduction of ceria. Where is the Ce 3+ ? Depth-profiling by in situ high-energy XPS provides an answer.