Hideo Sobukawa

This review summarizes our achievements involved in developing oxygen storage materials in our laboratory, especially, our contributions to the enhancement of the oxygen storage capacity and the thermal stability improvement. In the 1980s, the first generation ceria-zirconia solid solution (CZ) was invented as a remarkable oxygen storage material for automotive catalysts. This material mainly consisted of about 20　mol% zirconia as the doping compound to ceria. To facilitate the valence change process from Ce⁴⁺ to Ce³⁺ causes to the enhancement of the oxygen storage capacity (OSC). In the 1990s, based on the benefit from the new technology which could dissolve more than 20 mol% zirconia in ceria, the second generation CZ were developed. Subsequently, the third generation CZ (known as ACZ) was developed that further improved the thermal stability. Based on the concept of "Diffusion Barrier", ACZ was synthesized by adding alumina to CZ, in which the alumina acting as a diffusion barrier layer inhibited the coagulation of CZ and A at high temperature. By using oxygen storage materials, the automotive catalysts could efficiently reduce NOx emission from automobiles. As a promoter of automotive catalysts, our progress with oxygen storage materials has made a great contribution to automotive catalyst development. Finally, a brief comment on the future trends related to oxygen storage materials is presented.

Ceria-zirconia solid solution is essential as a catalytic promoter in three-way catalysts used to clean automobile exhaust gas in order to meet recent severe emission controls. There are many synthesis methods reported in recent years for the synthesis of ceria-zirconia solid solution. However, few reports exist concerning synthesis methods that use mechanical energy of hard milling. The only previous report is by the present authors' and concerns a wet milling process for synthesizing solid solution that is expected to provide a material with high specific surface area. The formation mechanism must be elucidated, because few previous reports exist concerning the phenomena of forming solid solution between whole oxides at around room temperature by the wet milling process. It is necessary to clarify the formation mechanism in order to apply this process to make solid solutions between oxides other than the ceria-zirconia system. The formation mechanism of ceria-zirconia solid solution was examined through the analysis of the phenomena of the formation of the solid solutions by the wet milling process. The mechanism was so-called "mechanical alloying", a phenomenon in which materials are mixed at the atomic level by mechanical energy. The crystallite size of the ceria-zirconia solid solution by the wet milling process finally reached 20 nm, which is thought to be the "critical particle size" at which the fracture mode changes from brittle to ductile.

Yoshiyuki Sakamoto, Yoshimi Kizaki,
Tomoyoshi Motohiro

The amount of oxygen storage and release from catalysts on a millisecond scale (MS-OSC) is believed to play an essential role in automotive three-way catalysts. We have developed a new method to evaluate the MS-OSC based on the rapid detection of reaction products on catalyst surfaces immediately after CO and O₂ injections using pulsed valves by time-of-flight mass spectrometry.

CO and O₂ were passed over the surface of a slurry-coated catalyst on a planar cordierite substrate initially under high vacuum (10^-7Pa). This allowed the MS-OSC to be directly evaluated as the amount of CO₂ without gas diffusion effects. We applied this MS-OSC measurement technique to the following three catalysts: Pt/CeO₂-ZrO₂ (CeO₂:ZrO₂ = 1:1); Pt/CeO₂-ZrO₂ (CeO₂:ZrO₂ = 5:1); and Pt/CeO₂-ZrO₂-Y₂O₃ (CeO₂:ZrO₂:Y₂O₃ = 46.5:46.5:7.0). Pt/CeO₂-ZrO₂-Y₂O₃ was reported to improve the transient catalytic activity on a millisecond scale, which could not be explained by conventional thermo-gravimetric OSC measurements on the scale of seconds. Among these three catalysts, Pt/CeO₂-ZrO₂-Y₂O₃ was found to have the highest MS-OSC. This is supported by a previously reported set of engine test results.1)

　

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