Marta Boaro

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.

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.

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.

Study on Redox, Structural and Electrical Properties of Ce x Zr1 − x O2 for Applications in SOFC Anodes

The effect of redox cycles on the reducibility, structural, and electrical properties of dense pellets of ceria-zirconia solid solutions has been investigated by temperature programmed reduction studies, X-ray diffraction technique, Raman spectroscopy, and electrochemical impedance spectroscopy. Similarities and differences among the compositions with high (Ce 0.80 Zr 0.20 O 2 ), intermediate (Ce 0.50 Zr 0.50 O 2 ), and low (Ce 0.20 Zr 0.80 O 2 ) amounts of ceria were evaluated in view of their application as active components for solid oxide fuel cell (SOFC) anodes. Cycles of reduction and reoxidation, respectively, at 1273 and 873 K, promote the reduction of all the compositions investigated, shifting it at a lower temperature. Moreover, redox treatments cause a significant change in the crystal structure of Ce 0.50 Zr 0.50 O 2 , ultimately leading to the formation of cubic pyrochlore phases as the reduction temperature increases. The correlations among structural changes, redox, and electrical properties of these materials are discussed.

Redox behavior of gold supported on ceria and ceria-zirconia based catalysts

Abstract A series of gold-based catalysts were prepared by deposition precipitation or incipient wetness impregnation on Ce x Z 1– x O 2 solid solutions (0.28≤ x ≤1.00). The morphological and structural characterization of these catalysts were carried out with X-ray diffraction, transmission electron microscopy (TEM) analysis and physical adsorption technique, and their redox properties were studied by temperature programmed reduction using both H 2 and CO as probe molecules. Two cycles of oxidation/reduction were carried out in order to evaluate the effects of redox aging and gold sintering on the oxygen exchange capability. As observed with other noble metals, gold enhanced and promoted the ceria reduction at lower temperatures. Reduction by CO was shown to be dependent on the fine dispersion of gold and to be negatively affected by the ageing process more than reduction with hydrogen. This might have implications in reactions like water gas shift and CO-PROX which involve CO as a main reactant.

Water splitting reaction on Ce0.15Zr0.85O2 driven by surface heterogeneity

Ce0.15Zr0.85O2 was investigated in the two-step water splitting reaction. High-temperature treatment in N2 induced compositional and structural heterogeneities which contributed to a six-fold increase of H2 yield after the first cycle. Ceria surface enrichment and the formation of a ceria–zirconia oxynitride phase positively affected the reduction and oxidation steps.

Influence of Different Palladium Precursors on the Properties of Solution-Combustion-Synthesized Palladium/Ceria Catalysts for Methane Combustion

A series of Pd/CeO2 catalysts was prepared by solution combustion synthesis (SCS) using different Pd precursors. The powders were characterized by complementary techniques such as BET surface area measurements, X‐Ray diffraction analysis, X‐Ray photoelectron spectroscopy, temperature‐programmed reduction, temperature‐programmed oxidation, and high‐resolution TEM. The results obtained evidenced the formation of a Pd‐Ce solid solution on all SCS samples. This solid solution is in the form of an ordered supercell structure only in the SCS catalyst prepared from palladium nitrate. This was correlated to the heat of reaction between Pd(NO3)2 and the fuel. The samples were tested for methane catalytic combustion, and the reaction rates on all SCS samples were approximately twice that of the impregnated counterpart, irrespective of the precursor. This was attributed to the presence of the Pd‐O‐Ce solid solution, which gave rise to strong Pd–ceria interactions.