Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

Abstract

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce x Zr1−x O2/Al2O3, Ce x Zr1−x O2/SiO2 and Ce x Zr1−x O2/TiO2 composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce0.75Zr0.25O2, Ce0.6Zr0.4O2, Ce0.16Zr0.84O2 and Ce0.5Zr0.5O2 phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce x Zr1−x O2 solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce x Zr1−x O2 phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce4+ and Ce3+ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO2 and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce x Zr1−x O2 oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.