Carla de Leitenburg

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.

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.

A novel and simple route to catalysts with a high oxygen storage capacity: the direct room-temperature synthesis of CeO2–ZrO2 solid solutions

High-energy mechanical alloying of pure CeO2 and ZrO2 at room temperature results in the formation of a single phase Ce1–xZrxO2 fluorite structured solid solution in all the examined composition ranges; the compounds are characterized by a high oxygen storage capacity and an excellent reducibility.

Wet oxidation of acetic acid catalyzed by doped ceria

Abstract A series of ceria-containing mixed oxides has been prepared and utilized as catalysts in the oxidation of acetic acid in water. The incorporation of even relatively small amounts of ZrO2 and MnOx or CuO into the fluorite lattice of CeO2 strongly enhances the redox properties of the material with a consequent promotion of the oxidation activity. The best results are obtained with the ternary mixed-oxides CeO2-ZrO2-CuO and CeO2-ZrO2-MnOx which benefit from a synergetic interaction between CeO2 (whose properties are already modified by the presence of Zr) and CuO or MnOx. Analysis of metal ion concentration in the effluents after reaction indicates also a high stability of the mixed-oxide catalysts under the conditions employed.

The direct room-temperature synthesis of CeO2-based solid solutions: a novel route to catalysts with a high oxygen storage/transport capacity

An alternative route for the preparation of CeO2-based solid solution is presented. It is shown that high-energy mechanical alloying is an effective method for the preparation of nanostructured, single phase, binary solid solutions of the type Ce1-xMxO2, with M = Zr, Hf or Tb. Large amounts of oxygen, exceeding those in ceria, are accessible in the solid solutions at low temperatures, while the rapid and complete reoxidation of reduced samples indicates a fast oxygen diffusion and an excellent oxygen storage capacity.

CeO2-based solid solutions with the fluorite structure as novel and effective catalysts for methane combustion

The partial substitution of Ce in CeO2 with isovalent elements like Hf and Zr greatly increases the overall activity of methane combustion measured as light-off and ignition temperature; the presence of a defective fluorite-structured oxide is recognized as a key factor in the activity enhancement.

Oxygen Storage Behavior of Ceria–Zirconia-Based Catalysts in the Presence of SO2

This paper investigates the effect of the presence of SO2 in the dynamics of oxygen storage on ceria and ceria–zirconia. The introduction of SO2 under reaction conditions at T<873 K negatively affects CO conversion under oscillating conditions on all the supports studied, owing to the formation of sulfate species. Deactivation is observed on all supports and activity is recovered only after desorption of SO2, which occurs at 950<T<1000 K, depending on catalyst composition (Ce/Zr ratio) and treatment atmosphere. The amount of sulfur adsorbed is higher over solid solutions, reaching a maximum with CexZr1−xO2 (0.5<x<0.68). However this does not adversely affect activity compared to ceria. In the presence of Rh and Pd, reactivation is favored under reaction conditions. More generally, it appears that the removal of sulfates is facilitated in reductive atmospheres (both hydrogen and CO) over mixed oxides. No differences are observed following regeneration under oxidizing conditions.