Elsevier

Journal of Catalysis

Volume 220, Issue 1, 15 November 2003, Pages 35-43
Journal of Catalysis

Flame-made nanocrystalline ceria/zirconia: structural properties and dynamic oxygen exchange capacity

https://doi.org/10.1016/S0021-9517(03)00235-5Get rights and content

Abstract

Ceria/zirconia nanocrystals were prepared by flame-spray synthesis. Application of carboxylic acid-derived solvents allowed the continuous production of homogeneous mixed oxides with narrow particle-size distribution. The high preparation temperature favored the formation of a highly crystalline solid solution of the constituents. Structural and textural properties of the ceria-zirconia nanoparticles were characterized by high-resolution electron microscopy, nitrogen adsorption, and X-ray diffraction. The redox properties of the nanoparticles were investigated by temperature-programmed reduction and by dynamic oxygen exchange capacity (OEC) measurements using H2, CO, and propene as reductants. For CO and H2 a similar reduction was found, whereas propene afforded a considerably higher degree of reduction. While the flame-made powders had very stable specific surface area (up to 80 m2/g) even after severe calcination (2 h at 900 °C in air) their OEC was lower than in corresponding ceria-zirconia prepared by coprecipitation. This difference is traced to the high crystallinity and low defect concentration of the flame-made material.

Introduction

The current new generation of three-way catalysts (TWC) uses ceria/zirconia as key components for its high dynamic oxygen exchange capacity [1]. In the treatment of noxious gases from car exhaust, the ceria switches between its two major oxidation states Ce(III) and Ce(IV) [2]. The temperature in a typical automotive exhaust rapidly changes and very high temperature causes severe thermal stress. Therefore, stability is a major issue in TWC research as reflected in a wealth of scientific papers. It is well established that formation of ceria-zirconia solid solution greatly enhances reducibility of the ceria [2]. Different production methods, however, lead to different molecular mixing of ceria and zirconia and varying contents of defects and cracks in the material. Maximum stability is found for intensively mixed powders forming a stable solid solution of the constituents.

Coprecipitation of ceria/zirconia can lead to mixed oxide powders with extremely high specific surface area [3], [4]. Unfortunately, the temperature stability of as-prepared oxides is characterized by a severe loss of surface area at elevated temperature. Preparation at high temperature may produce an oxide with increased stability. This has prompted several authors to prepare ceria and ceria containing oxides by flame methods which led to a series of patents [5], [6], [7], [8], [9], [10]. These methods either use expensive precursors, such as alkoxides [8], [9], demanding atomization devices [7], [11] and tend to produce inhomogeneous powders containing large lumps [10]. Aruna et al. investigated the ceria/zirconia synthesis by combusting mixtures of redox compounds (urea) and oxidizing metal precursors (nitrates) [12]. This high temperature preparation yielded a high surface area product with excellent phase mixing. It is a batch process, however, and explosion hazard causes great difficulties. Laine et al. prepared ceria/zirconia by flame spray synthesis but the specific surface area of the product powder stayed low, typically at 10 to 15 m2/g [13], [14]. Sutorik et al. [15] very recently produced ceria/zirconia mixed oxides by flame spray synthesis showing formation of solid solutions, but obtained rather low specific surface area while no tests on the oxygen exchange capacity were undertaken. It was shown recently that complex mixed oxides with high specific surface areas could be prepared at even higher temperatures by using continuous flame and flame-spray pyrolysis [16].

Here, we report the use of continuous flame-spray pyrolysis for production of high specific surface area ceria/zirconia from low-cost precursors [17], [18] and compared them to samples prepared by coprecipitation [19]. Since volatile organic components (VOC) are a major component in automotive exhaust gas, the role of propene is investigated as a reductant in OEC measurements beside CO and hydrogen.

Section snippets

Preparation of mixed oxides

Ceria/zirconia mixed oxide powders were produced by flame-spray pyrolysis in a laboratory scale setup [17]. Cerium (III) acetate hydrate (Aldrich, >99.7%) and zirconium tetraacetylacetonate (Aldrich, 99%) were mixed according to the product composition and dissolved in a lauric/acetic acid mixture (1/1 by weight), heated to full dissolution resulting in a total metal concentration of 0.15 M. In a preliminary study [18], the lauric/acetic acid mixture proofed to be the most suitable carrier

Structural properties

Spraying corresponding mixtures of cerium and zirconium containing precursor solutions into a methane/oxygen flame continuously produced ceria/zirconia nanoparticles. Fig. 1 (left side) depicts transmission electron micrographs of as-prepared ceria/zirconia Ce0.5Zr0.5O2. The product consists of well-crystalline, sharp-edged nanoparticles of 4–10 nm size [18]. Fig. 1 (right side) further shows the same powder after sintering at 900 °C for 2 h in air. Regularly shaped particles of high

Flame-spray synthesis

Fig. 1 shows homogeneous nanostructured ceria/zirconia. No large lumps of ceria could be detected, corroborating good dispersion of the metal precursor and subsequent precipitation in the flame. For pure ceria, high combustion heat of the carrier liquid is crucial for obtaining a homogeneous material [17]. A droplet entering the flame may continuously release precursor solution or an enrichment of the metal may take place if the carrier liquid (fuel) leaves the droplet first. In the latter

Conclusions

The production of ceria/zirconia nanocrystals by flame-spray pyrolysis affords mixed oxides of high specific surface area and improved thermal stability. Control of the major process parameters: combustion energy delivery, metal concentration, and carrier liquid (solvent) composition allow production of nanometer-sized ceria/zirconia crystals over a broad range of conditions. Product homogeneity and phase stability are directly related to a homogeneous delivery of both constituent precursors

Acknowledgements

We thank S. Veith for the pore-size analysis and Dr. F. Krumeich for the high-resolution transmission electron microscopy investigations. Financial support by ETH Gesuch Nr. 19/01-1 and the Swiss Commission for Technology and Innovation, Top Nano 21, Nr. 5978.2 is kindly acknowledged.

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