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

The catalytic treatment of motor vehicle exhaust gas has been a feature of all passenger cars in the USA since the 1975 model year. It has since been adopted in all the developed countries as the principal means of con-trolling the emissions from vehicles powered by Otto-cycle internal com-bustion engines. These include passenger cars, light trucks, and heavy-duty trucks. Catalysts have been developed for motorcycles and even smaller power plants. The so-called three-way catalyst (TWC) has been in use since the 1979 model year. The name reflects the simultaneous treatment by this catalyst of the two reducing pollutants, carbon monoxide and uncombusted hydrocarbons, and the oxidizing pollutant, oxides of nitrogen.

Why Rhodium in Automotive Three-Way Catalysts?

Ni catalysts supported on gamma-Al2O3, CeO2 and CeO2-A1(2)O(3) systems were tested for catalytic CO2 reforming of methane into synthesis gas. Ni/CeO2-Al2O3 catalysts showed much better catalytic performance than either CeO2- or gamma-Al2O3-supported Ni catalysts. CeO2 as a support for Ni catalysts produced a strong metal-support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/gamma-Al2O3 catalysts for this reaction. A weight loading of 1-5 wt% CeO2 was found to be the optimum. Ni catalysts with CeO2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO2-promoted catalysts are attributed to the oxidative properties of CeO2. (C) 1998 Elsevier Science B.V. All rights reserved.

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Metal nanoparticles hold great promise for heterogeneous catalysis due to their high dispersion, large concentration of highly undercoordinated surface sites, and the presence of quantum confinement effects, which can drastically alter their reactivity. However, the poor thermal stability of nano-sized particles limits their use to low temperature conditions and constitutes one of the key hurdles towards industrial application. The present perspective paper briefly reviews the mechanisms underlying nanoparticle sintering, and then gives an overview of emerging approaches towards stabilizing metal nanoparticles for heterogeneous catalysis. We conclude by highlighting the current needs for further developments in the field.

Stabilizing metal nanoparticles for heterogeneous catalysis

CeO2–Al2O3 mixed oxides with different CeO2 loading (in the range of 0.5–12 wt.%) were prepared by wetness impregnation of alumina with aqueous solution of di-ammonium hexanitrate cerate (NH4)2[Ce(NO3)6]. The samples after calcination at 773 and 1073 K were characterized by different techniques, using X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). It is shown that different types of cerium oxide species are formed on the surface depending on the amount of CeO2 and temperature of calcination of the samples. XRD showed the formation of nanocrystallites of ceria on alumina surface when the amount of CeO2 is higher than 6 wt.%; at lower concentrations ceria was found to be amorphous. The ceria loading lower than 6 wt.% stabilizes the textural properties of alumina. At loading of 1 wt.% of CeO2 XPS spectra reveals the presence of a strong interaction between ceria and alumina leading to a formation of superficial CeAlO3-like phase. TPR results show that well dispersed CeO2 particles present on the surface of alumina form CeAlO3 at temperature of reduction in the range of 873–993 K, while for the reduction of CeO2 crystallites, a higher temperature of reduction of 1190 K is needed.

Characterization of ceria-coated alumina carrier

Transition alumina, such as 7-A1203, 0-A1203 and 6-A1203, has been widely used as a catalyst support. The surface area of the support significantly decreases at high temperature operations, for example in purifying automotive exhaust gas [1]. In this case, it is important to maintain the support with high surface area free of sintering or phase transition, otherwise precious metals (platinum, rhodium etc.) supported on alumina will consequently sinter. Cerium and other rare earth elements have been used as a promoter to improve the catalytic activities for purifying carbon monoxide, nitrogen oxides and hydrocarbons emitted from automotive engines [2-4]. It has been suggested that the addition of cerium plays serveral roles, including the catalysis of the water-gas shift reaction, the oxygen storage in the lattice for oxidation catalysis under rich air-fuel conditions, and the inhibition of the growth of noble metal particles. The present letter describes the influence of the addition of cerium to 7-A1203 catalyst supports on the phase transition of 7-A1203 to ~-A1203 and the sintering of transition alumina below the transition temperature. A series of cerium-added alumina was prepared by the impregnation technique. Powdered 7-A1203 with surface area 170 m 2 g ~ was used as a support, and an aqueous cerium nitrate was used as an impregnation solution. The samples were then dried at 110 ° C for 8 h and calcined at 600°C for 3 h. The starting alumina contained 0.05 wt % impurities, mainly iron and trace silicon, calcium and sodium. The cerium contents of the samples were 0.5, 1, 2, 3, 5 and 10mol% by the CeO2 to the total moles of oxides (CeO2 + A1203). The samples were further calcined in air at a temperature between 1 000 ° C and 1 200 ° C for 5 h in order to examine the thermal stability of the alimina support. The surface area of the samples was derived by application of the BET equation, from nitrogen adsorption data obtained at 77K by a standard volumetric procedure. The phase transition temperature was measured with a DTA apparatus at heating rates of 2.5, 5, 10 and 20Kmin ~ using 0¢-A1203 powder as a reference material. Kissinger's plots [5] were applied to the evaluation of the activation energy of the phase transition from active alumina to c¢-A12 03. Phase analysis was done by the powder X-ray diffraction (XRD) technique. The amount of ~-A1203 in the samples was determined by the XRD intensity of the (1 00) diffraction line of 0~-Al203. The standard data were obtained from the XRD data of the mixtures of u-A1203 powder and 0-A1203 powder which were formed by calcining 7-A1203 at 1 300°C and l 000°C for 10 h. The samples containing 0.01 mol % gadolinium were prepared by the same procedure as above, but using an aqueous solution of gadolinium nitrate. Fig. 1 shows the Surface area of alumina supports containing cerium with different molar concentrations, calcined in air at 1 000 ° C, l 100°C and 1 200°C for 5 h. The thermal stability of alumina was improved by the addition of only 0.5 to 2 mol % cerium, and a gradual decline in surface area was found with increasing cerium content. XRD analysis of the samples calcined in air at 1 200°C indicated that 0.5 mol % Ce-A1203 consisted of 0-, ~and ~-A1203, and that pure A1203 completely transformed to ~-A1203 (Fig. 2). CeO2 phase was found in the samples with more than 2 mol % cerium. These results clearly show that the addition of cerium improves the thermal stability of alumina at 1 200°C by preventing the transformation to ~-A1203. However, the overloading of cerium decreases the surface area of the alumina because of the growth of CeO2 particles. The XRD data of the samples calcined at 1 100°C indicated the formation of 0and ~-A12 03. The decrease in surface area was inhibited by the cerium loading at 1 100 ° C. Kissinger's plots of the data obtained by DTA for both pure AI203 and 0.5 tool % Ce-AI203 and given in Fig. 3 as the relation of ln[(dT/dt)Tn72] with l/Tm. (dT/dt) is the heating rate, and Tm represents the maximum rate temperature of the phase transition. The activation energy of the phase transition was calculated at 581kJmol ~ for pure AI203 and 582 kJ tool ~ for 0.5 mol % Ce-A1203. Although these values are rather high compared with those in the literature [6, 7], it can be found that the alumina in this study has the constant activation energy of the phase transition regardless of the cerium addition. Scanning electron microscopy (SEM) observation of the samples calcined at ! 200 ° C revealed the formation of ~-A1203 particles, 0.1 to 0.3 #m in diameter. The particle size was independent of the content of cerium in the samples. These results can be explained by assuming that the cerium addition decreased the ~-A1203 transformation by influencing the nucleation process of ~-A1203 but not its growth process. Fig. 4 compares the 1 250 ° C isothermal transformation data for pure A1203 with those for 0.5mol % Ce-A1203. The data were analysed using the following empirical kinetic equation

Effect of cerium addition on the thermal stability of gamma alumina support

Etude de la preparation des poudres d'oxyde de zirconium par impregnation du charbon actif par du zirconium puis par pyrolyse a 550°C dans l'air

M. Kimura

Papers

Effect of cerium addition on the thermal stability of gamma alumina support

Preparation of zirconia powder by the pyrolysis of active carbon