In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition

Chrysanthi Andriopoulou,Deb Harris,Soghomon Boghosian,Hazel Stephenson,Angelos M Efstathiou

doi:10.3390/catal10040462

Abstract

In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped CexZr1−xO2−δ oxygen storage capacity (OSC) mixed metal oxide materials of identical at % composition, which were prepared by the same chemical synthesis route, in which one synthesis parameter of the aqueous chemistry was slightly varied. The Raman fingerprint of the anionic sublattice is very sensitive to O atom relocations within the bulk of the material matrix and to the pertinent defect topology in each case. A protocol of sequential Raman measurements and analysis was proposed to discern subtle differences between the oxygen vacancy and defect topologies of the examined materials. It can be concluded that for two materials under comparison for their structures, identical Raman spectra are obtained only if the procedures followed for their preparation are identical; a slight variation of one single parameter (e.g., in the aqueous chemistry stage) results in discernible differences in the Raman spectra. The proposed procedure can serve as a tool for proving or disproving infringement of IPR (Intellectual Property Rights) protected preparation methods of ceria-based mixed metal oxide materials.

Highlights

Ceria–zirconia mixed oxides (Ce1−x Zrx O2−δ ) constitute materials of great and topical technological interest due to their formidable oxygen storage capacity (OSC) properties as well as to numerous applications in industrial catalysis [1,2,3,4]
For the first time, we used in situ Raman spectroscopy to capture pertinent temperature-dependent snapshots of Raman spectra reflecting the corresponding structural conformation at each temperature for Cex Zr1−x O2−δ mixed oxides made by the citrate sol-gel and urea-coprecipitation methods [10]
The evolution of the sequential in situ Raman spectra obtained under oxidizing conditions at 50, 200, 450, and 50 ◦ C were interpreted in terms of O atom relocations within the lattice [10]

Summary

Introduction

Ceria–zirconia mixed oxides (Ce1−x Zrx O2−δ ) constitute materials of great and topical technological interest due to their formidable oxygen storage capacity (OSC) properties as well as to numerous applications in industrial catalysis [1,2,3,4]. Pertinent applications include (but are not restricted to) promotion of the water gas shift (WGS) and steam reforming reactions, favoring of activity at the interface of the metal/support, promotion of CO oxidation by utilization of lattice oxygen, and most importantly, three-way-catalysis (TWC) where ceria–zirconia based oxides are used to eliminate toxic exhaust gases in vehicles [5,6]. The exceptional redox properties of ceria ensure a great functionality of such materials in grounds of of oxygen storage and delivery under working dynamic conditions [2] by allowing interactions. The more refractory nature of ZrO2 -containing mixed oxides, combined with the spectacular downshift of the reduction temperature (some 200–300 ◦ C below the respective reduction temperature of pure ceria) resulting by the ZrO2 insertion in the ceria matrix, results in materials that are resistant to sintering that may be caused as a consequence of prolonged thermal aging [6,7]

Methods

Results

Conclusion