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.

Catalytic Properties of Ceria and CeO2-Containing Materials

A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Computer modeling of surfaces and defects on cerium dioxide

Pd/CeO2–TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents

New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO2 systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L3 edges), and infrared spectroscopy (DRIFTS). Cu-CeO2 nanoparticles prepared by a novel reversed microemulsion method (doped Ce1-xCuxO2 sample) and an impregnation method (impregnated CuOx/CeO2 sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-Ovacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the Ovacancy sites or the Cu-Ovacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO2 catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.

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In Situ Studies of the Active Sites for the Water Gas Shift Reaction over Cu−CeO2 Catalysts: Complex Interaction between Metallic Copper and Oxygen Vacancies of Ceria

The diffuse reflectance spectra (UV–VIS) of bulk and silica-supported CeO2(⩽1.10% Ce) are presented. Very small supported cerium oxide particles (10–15 A) are obtained, which exhibit distinct spectral features assigned to surface cerium oxygen species.

Faraday communications. From bulk CeO2 to supported cerium–oxygen clusters: a diffuse reflectance approach

CO adsorption on palladium–ceria catalysts prepared from Pd acetylacetonate has been investigated by IR and UV–VIS spectroscopies and by volumetric measurements. After calcination and evacuation at 673 K, the samples contain small PdO particles which are immediately reduced upon adsorption of CO at room temperature. For samples prereduced by H2, electron-donor centres (Ce3+ ions, oxygen vacancies) are created during the pretreatment. These centres are responsible for increased CO adsorption by ceria in the presence of palladium, which leads to apparent CO: Pd values greater than unity and their interaction with palladium induces a carrier–to–metal electron transfer. Plasmon absorption in reducible carriers hinders the detection of species adsorbed on the supported metal.

Spectroscopic study of CO adsorption on palladium–ceria catalysts

The interactions of hydrogen with palladium–ceria catalysts have been studied by volumetric and magnetic susceptibility measurements The ratio Hirr : Pdtotal, where Hirr stands for irreversibly adsorbed hydrogen, lies in the range 25–35; this result is ascribed to hydrogen spillover from palladium to ceria The uptake of hydrogen by ceria leads to its complete surface reduction at room temperature, whereas bulk reduction begins at 473 K Magnetic measurements show that: (i) a large amount of hydrogen is reversibly adsorbed on ceria, (ii) evacuation at 373 K induces the reverse migration of hydrogen (back spillover), which leads to the reoxidation of Ce3+ ions The reoxidation ratio depends on the outgassing temperature wheras the amount of irreversibly formed Ce3+ increases with the reduction and the outgassing temperatures However, the estimation of the amount of hydrogen retained by ceria from the quantity of Ce3+ formed does not allow determination of the metal fraction exposed (MFE) by hydrogen adsorption at 373 K

Palladium–ceria catalysts: reversibility of hydrogen chemisorption and redox phenomena

The dehydrogenation of n-butane over supported chromium oxide catalysts has been investigated at 500 °C by insitu UV–VIS diffuse reflectance spectroscopy–on-line activity measurements as a function of the reaction time (0–60 min), the support composition (SiO2:Al2O3 ratio) and the Cr loading (0–8 wt.%). The catalytic reaction is characterized by an initial induction period, during which the supported chromium oxides are reduced to Cr3+/2+, and CO/CO2 and cracking products (C1, C2 and C3) are formed. After this initial induction period, but-1-ene and but-2-ene are produced with a selectivity up to 90%. The catalytic activity increases with increasing Cr content and Al2O3 content of the support, and is maximum after 5–15 min on stream. A relation between the dehydrogenation activity and the amount of Cr3+ formed upon the reduction of Cr6+ has been established.

In situ UV–VIS diffuse reflectance spectroscopy–on-line activity measurements

In situ FTIR spectra of adsorbed CO on CrO3/SiO2 catalysts have been investigated at elevated temperatures to elucidate the nature of the species formed during the reduction process with CO. The results show the formation of three species: bicarbonates, monodentate carbonate and inorganic carboxylate. The reduction of CrO3/SiO2, which occurs through the formation of surface bicarbonate and carboxylate compounds, starts near 100°C.

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Abdelhamid Bensalem

Papers

Faraday communications. From bulk CeO2 to supported cerium–oxygen clusters: a diffuse reflectance approach

Spectroscopic study of CO adsorption on palladium–ceria catalysts

Palladium–ceria catalysts: reversibility of hydrogen chemisorption and redox phenomena

In situ UV–VIS diffuse reflectance spectroscopy–on-line activity measurements

Nature of adsorbed species during the reduction of CrO3/SiO2with CO InsituFTIR spectroscopic study