The paper is a review concerning the properties of CeO₂– and ZrO₂-based mixed oxides; their structural and textural properties are specifically focused. There is quite wide interest concerning the properties of these mixed oxides since they are employed in different areas, ranging from catalytic applications to ceramics, fuel cell technologies, gas sensors, ionic conducting materials, biomaterials, and even sunscreen cosmetics.

“The paper attempts to rationalize some fundamental properties of CeO2-ZrO2 mixed oxides, and, in particular, it addresses some of the open issues concerning their structural properties”

Our laboratory has a well-assessed experience in the investigation of these materials and we have published more than 70 papers on the topic. Despite the huge amount of work that has been published on these materials over the past 10 years or so, there have been a number of open questions. We tried to address some of them in the paper. We believe that the rationale, which is provided in the paper for some of the properties of the CeO₂-ZrO₂ mixed oxides, caught the interest of several researchers.

The paper attempts to rationalize some fundamental properties of CeO₂-ZrO₂ mixed oxides, and, in particular, it addresses some of the open issues concerning their structural properties. It is shown that the "same" materials, i.e., material with the same chemical composition, can have different structural properties according to the particle dimension.

We show that when the powders are prepared in the form of nano-particles with a grain size smaller than ca. 15 nm, the structural properties of the material do change with respect to conventional materials with particles in the micrometer range, as used, for example, in the field of ceramics. We also discuss how the thermal stability can be conferred to these materials by designing their textural properties, such as the way in which nano-particles agglomerate, leading to a specific pore structure.

The crucial point of the paper is to make the reader understand that even for a conceptually extremely simple mixed oxide such as CeO₂-ZrO₂ materials, when the particle dimension decreases from micro- to nano-dimension, the properties of the system can become very complex.

Maximum care must be taken when characterizing and using these materials before a rationale, based on the simple concepts of chemical compositions, can be derived for any of their properties.

Nano-materials and nano-technology are foreseen to strongly impact our future. We discuss in the paper the effects of decreasing the dimensions of the particles in the CeO₂-ZrO₂-based materials from a micro- (1/1,000,000 m) to nano-dimension (1/1,000,000,000 m), showing how some of their properties are affected by this process.

The importance of the findings are related to the fact that these mixed oxides are extensively used as nano-materials in several applications, the most important being the automotive catalytic converters.

Our interest in the development of CeO₂- and ZrO₂-based materials dates back to the beginning of the 1990s when we were interested in the investigation of the so-called three-way catalysts (TWCs), which are used in automotive catalytic converters. By that time, a major issue was that of improving thermal stability of these catalysts and, in particular, that of the CeO₂-based component, which is a critical ingredient of the TWCs (see below).

It was known that CeO₂ typically sintered above 700-800°C leading to loss of its surface area and deactivation of the catalysts. We have addressed this issue in an unconventional way by preparing a series of fully sintered Rh/CeO₂-ZrO₂ mixed oxides by treating them at 1600°C. To our surprise, these samples became redox-active at low temperatures as long as an appropriate amount of ZrO₂ was inserted into the CeO₂ lattice. The possibility of using the oxygen from the bulk of the solid solution at moderate temperatures suggested that intrinsically thermally-stable systems could have been obtained.

CeO₂-ZrO₂-based materials have been critical ingredients of virtually all automotive catalysts since 1994. Their role in the TWCs is related to their redox properties, since their capability to adsorb and release oxygen under fluctuating exhaust conditions allows one to increase the effectiveness of the TWCs by enlarging the operating window of the engine-out air/fuel ratio.

Over the last 10 years, our attempts to develop a rational approach to the understanding of some of the intriguing properties of the CeO₂-ZrO₂ mixed oxides, led, in cooperation with industry, to the development of novel products, leading to more efficient depollution catalysts, particularly in the field of TWCs. The tuning of the redox capability in a desired interval of temperatures and/or the design of the textural properties and hence thermal stability of the catalyst, have become feasible on a ton-scale industrial production.