Mareo Kimura

The application of CeZr oxide solid solution to oxygen storage promoters in automotive catalysts

The complex oxides in the CeO2ZrO2 system were examined for the improvement of oxygen storage capacity in automotive catalysts. The formation of CeZr oxide solid solution improved the thermal stability and activity of CeO2. The CeZr addition enhanced the removal activity for CO, NOx and hydrocarbons under dynamic air-fuel ratio condition. The automotive catalyst was designed and developed through research on the oxides in the CeO2ZrO2 system.

Research on new DeNOx catalysts for automotive engines

Abstract The performance and durability of Cu-ZSM-5 catalysts for selective reduction of NOx in an oxidizing atmosphere were studied. These catalysts exhibited substantial NOx conversion performance in a leanburn engine exhaust and the simulated gas. However, the temperature dependence, SV performance, and thermal stability must still be improved.

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

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. Kissingers 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. Kissingers 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

Thermal stability and characterization of γ-Al2O3 modified with rare earths

Rare earth modification was effective for improving the thermal stability of γ-Al2O3. It inhibited the grain growth of transition Al2O3 and the formation of α-Al2O3. The surface area was 12 m2 g−1 for pure Al2O3, 40 m2 g−1 for ceriummodified Al2O3 and 51–59 m2 g−1 for Al2O3 modified with lanthanum and others, heated at 1200°C for 5 h. The activation energy of α transformation, evaluated by differential thermal analysis, was 581–583 kJ mol−1 for pure and cerium-modified Al2O3 and 635–655 kJ mol−1 for Al2O3 modified with lanthanum, samarium and ytterbium. Lanthanum inserted into the crystal lattice of Al2O3 with spinel structure, whereas cerium existed on the Al2O3 surface as a form of CeO2. The difference in states between La3+ and Ce4+ can explain that the modification of lanthanum is more effective than that of cerium for improving the thermal resistance of γ-Al2O3.

Design of the intelligent catalyst for Japan ULEV standard

We have reported the innovation of “An intelligent catalyst” which has the function for self-regeneration of Pd realized through the solid solution and segregation of Pd in a perovskite crystal. In this paper, the issues and the solutions for a practical perovskite catalyst for the Japan ULEV standards are discussed.

Regeneration of Palladium Subsequent to Solid Solution and Segregation in a Perovskite Catalyst: An Intelligent Catalyst

The object of this study was to provide a function for self-regeneration of precious metals in a usage ambience without auxiliary treatment. The strategy was to control the catalytic active site of those crystalline ceramics known as perovskite-type oxides at the atomic level in order to create the new, needed function. Three series of Pd-containing perovskite catalyst systems were prepared by coprecipitation of Pd with La, Fe, and Co using the alkoxide method. It was confirmed that Pd formed a solid solution of the perovskite-type oxide. And Pd in the perovskite crystal structure exhibited an abnormal oxidation number or higher binding energy than the normal bivalence, and it was presumed to be the reason for increasing the catalytic activity. The results of dissolution analysis for the aged Pd-perovskite catalyst suggested that Pd was not only dispersed on the surface of the perovskite crystal, but was present also in the solid solution of the perovskite crystal. The formation of a solid solution in this Pd-perovskite crystal was affected by the B site elements. And Pd in LaFe0.54Co0.36Pd0.10O3 system was more durable than in LaCo0.90Pd0.10O3 or in LaFe0.90Pd0.10O3. Furthermore, the formation of Pd solid solution into these perovskite crystals also depended on atmospheres and temperatures. It appeared that a high state of dispersion was maintained as Pd repeatedly forms solid solutions in the perovskite crystal or segregates out from the crystal depending on the fluctuation of redox conditions and temperatures in automotive catalyst ambience. We named such a catalyst, wherein a precious metal regenerates itself while in operation and remains highly active without requiring any auxiliary treatment, “an intelligent catalyst”.

Thermal stability and characterization of γ-Al2O3 modified with lanthanum or cerium

Etude par resonance paramagnetique electronique dans des echantillons contenant 0,5 a 10% mol. de La ou Ce. La stabilite thermique est meilleure par addition de La que par addition de Ce. Les ions de La avec leur large rayon ionique inhibe la diffusion des ions et empeche la transformation de la phase γ a la phase α

Preparation and characterization of zirconium dioxide catalyst supports modified with rare earth elements

Abstract ZrO2 catalyst supports modified with rare earth elements were prepared by coprecipitation from an aqueous solution of zirconium oxychloride and rare earth chlorides. The crystallization of amorphous hydrous ZrO2 was inhibited by doping with rare earths; the crystallization temperature was elevated as the amount and ionic radius of the rare earth modifiers was increased. Only modification using cerium had no effect on the crystallization process. The behavior of cerium was different from that of other rare earth elements with valency + 3. A metastable cubic phase was formed for ZrO2 modified with 10 mol.% lanthanum, neodymium and samarium by heating at 600°C. X-ray diffraction and Raman data indicated that the metastable phase had large microstrain and short-range ordering similar to tetragonal symmetry. Rare earth modified ZrO2 showed a large surface area and good thermal stability as a catalyst support. The carbon monoxide oxidation activity of iron was enhanced by modification with neodymium of ZrO2 supports. The results suggest the effectiveness of rare earth modified ZrO2 as catalyst supports.

Reduction reaction of lanthanum-added cerium dioxide with carbon monoxide

The reaction of lanthanum-added cerium dioxide with carbon monoxide was examined by using a CO pulse reaction method and high-temperature X-ray diffraction (XRD). The lanthanum addition enhanced the activity of cerium dioxide for oxidizing carbon monoxide under moderately reducing conditions. Isothermal XRD observation at 500 and 700 °C indicated that the reduction reaction of CeO2 and La-added CeO2 with CO progressed in CO-N2 flowing gas. The kinetics of the reaction CeO2 + x/2CO → Cex1−x/4+Cex3+O2−x/2 + x/2CO2 + x/2V0 was analysed by Janders model: [1−(1−x)1/3]2=kt.

Development of Thermal Resistant Three-Way Catalysts

Automotive catalysts with a good thermal durability have been developed by modifying the composition of additives. On the basis of the experimental results, the authors have designed optimal Pt/Rh/Ce three-way catalysts, which have showed significantly improved thermal durability and performance