Alessandro Trovarelli

Catalytic Properties of Ceria and CeO2-Containing Materials

Abstract Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

The utilization of ceria in industrial catalysis

The main applications of cerium dioxide in industrial catalysis are reviewed, with particular attention to the role played by ceria. The main uses of CeO2 are connected to depollution of noxious compounds from gaseous streams originating from industrial productions and from automobiles (de-SOx in FCC processes; treatment of emissions from spark-ignited and diesel engines), although ceria is also a key component of catalyst formulation for the dehydrogenation of ethylbenzene to styrene. Recent advances in the application of ceria for the removal of organic compound from wastewater through oxidation (catalytic wet oxidation CWO) are also reviewed. # 1999 Elsevier Science B.V. All rights reserved.

The use of temperature-programmed and dynamic/transient methods in catalysis: characterization of ceria-based, model three-way catalysts

Abstract Temperature-programmed (TP) methods have been increasingly used in recent years for the characterization of catalytic materials under conditions similar to those encountered in commercial applications. A large variety of complementary TP techniques can be used with minimum variation of experimental conditions, thus allowing great characterization potential in a single apparatus. Modern analytical and numerical tools allow accurate analysis and modeling of TP profiles, to obtain kinetic and other reaction parameters. Here, we will briefly review the experimental TP methods used for the characterization of the reduction features and the dynamic behavior of oxygen-storage/redox components of auto exhaust catalysts, based mainly on CeO2 and ceria-zirconia.

Some recent developments in the characterization of ceria-based catalysts

Abstract In the present paper some recent developments in the use of ceria and related materials in catalysis are considered. The relevant features of ceria–zirconia and ceria–silica as oxygen storage components are briefly reviewed with a special focus on the characterization and relationship between oxygen uptake/release and structural properties.

Nanofaceted PdO Sites in PdCe Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane†

An open superstructure: A Pd/CeO2 catalyst prepared by solution combustion synthesis is three to five times more active for CH4 combustion than the best conventional palladium-based systems. The catalyst contains an ordered, stable Pd-O-Ce surface superstructure (see picture; cyan arrow is a square-planar Pd site, red arrow is an undercoordinated O atom) and is an example of ultra-highly dispersed, stable PdO within an oxide carrier.

The effect of doping CeO2 with zirconium in the oxidation of isobutane

The preparation of a CeO2ZrO2 mixed oxide of composition Ce0.8Zr0.2O2 with its characterization and its use as a catalyst in the oxidation of isobutane is reported, and compared with the reactivity of pure CeO2. The formation of an homogeneous, fluorite-type solid solution is observed; the material is characterized by a higher reducibility and a higher capacity of oxygen uptake compared to pure CeO2. The catalytic activity in the oxidation of isobutane is not greatly affected by the introduction of ZrO2, but the selectivity to isobutene is significantly enhanced. This enhancement has been attributed to an increased oxygen mobility and to an increased activity for the Ce4+/Ce3+ redox couple, occurring as a consequence of the creation of surface and bulk defects in the solid solution, induced by the introduction of the smaller Zr4+ cation in the fluorite lattice.

Rh–CeO2 interaction induced by high-temperature reduction. Characterization and catalytic behaviour in transient and continuous conditions

The effects of the high-temperature reduction of Rh/CeO2 catalyst on the hydrogenation of CO, CO2, acetone and ethene, and on the hydrogenolysis of ethane, in transient and continuous conditions, have been investigated. The high-temperature reduction (HTR) at 773 K induced a transient Rh–CeO2 interaction in the catalyst which enhances the rate of CO, CO2 and acetone hydrogenation. Temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) show the reduction of Ce4+ to Ce3+ after HTR in the near surface layers. We suggest that the oxygen vacancies on the support (i.e. presence of Ce3+) can interact with the CO moiety promoting its activation.

Structural and Oxygen Storage/Release Properties of CeO2-Based Solid Solutions

Abstract The use of mixed-oxides containing CeO2 as oxygen storage/release components is discussed with special focus on the applications of these materials in auto-exhaust catalytic converters. Ceria easily forms solid solutions with transition-metal/rare-earth oxides over a wide composition range. The incorporation of dopants like Zr4+, Pr3/4+, Tb3/4+ into the cubic fluorite lattice of CeO2 strongly affects the structural and energetic properties of the materials by lowering activation energy for oxygen migration and by increasing reducibility of the cerium cation. This augments both total and kinetic oxygen exchange between the solid and the gas phase under conditions typically encountered in real systems.

The effect of water in the low-temperature catalytic oxidation of hydrogen sulfide to sulfur over activated carbon

Abstract This study investigates the use of low-temperature catalytic oxidation for the removal of H 2 S from tail gases originating from geothermal plants, with special focus on the effect of water on the overall performance of the activated carbon catalyst. It is shown that water strongly influences the reaction rate and the total amount of sulfur that can be adsorbed on the catalyst prior to regeneration. It is suggested that the reaction takes place in a thin water layer, inside the carbon pores, from the reaction of dissolved H 2 S with chemisorbed oxygen.

Electrical and oxygen storage/release properties of nanocrystalline ceria-zirconia solid solutions

The electrical properties of nanocrystalline Ce0.75Zr0.25O2 solid solution in the ceria–zirconia system were investigated using four-point DC conductivity measurements and impedance spectroscopy. Conductivity measurements were carried out as a function of temperature (723–821 K) and oxygen partial pressure (pO2=10−3–1 atm). The results were compared to the properties of bulk oxide of similar composition. Both the nanocrystalline and the coarsened oxide exhibit mixed conduction where the prevailing contribution is electronic. However, the nanocrystalline oxide shows a higher ionic contribution due to the enhancement of anionic vacancy mobility, which is more than one order of magnitude higher. In contrast with pure ceria, the electronic conductivity and activation energy for diffusion is similar in both nanocrystalline and sintered material, which could explain the similar reduction behavior shown by high and low surface area samples. These results are discussed and interpreted in terms of the role of surface area and the related crystallite size in the catalytic and reduction properties of ceria–zirconia solid solutions.