自動車排ガス用助触媒CeO₂-ZrO₂系粉末の酸素放出特性と状態図

抄録

CeO₂-ZrO₂ powders with various ZrO₂ compositions, which were prepared by co-precipitation, were subjected to an evolved-oxygen gas-analysis. The samples were heated from 373 to 1323 K in Ar+H₂+H₂O gas mixtures. A closed-system oxygen analyzer developed by one of the authors was used to establish a constant ratio of H₂/H₂O and also detect the oxygen evolved from the samples. After the amount of evolved oxygen was recorded as a function of temperature, the samples were oxidized at 873 K in O₂ gas, then the 2nd deoxidation run continued by heating again from 373 K. The heating-deoxidation run was repeated three times, and the following conclusions were derived. (i): For CeO₂ powders, a peak on the J_O-T-t curve (1) was observed around 1073 K, and did not shift even on the 2nd and 3rd heating-deoxidation runs. (ii): On the 1st heating-deoxidation run, as the content of ZrO₂ was increased, a peak appeared around 873 K and became higher; the sub-catalyst at low temperatures was improved monotonously with the addition of ZrO₂. (iii): After the deoxidation and successive oxidation, the shapes of J_O-T-t curves (2) and (3) were essentially different from those of the 1st run. This came from the appearance of cubic φ′ phase, (Ce₂Zr₃O₁₀?), which had not been reported previously. (iv): By analyzing the J_O-T-t curves, two kinds of qualitative phase diagrams of the ZrO₂-CeO₂-CeO_1.5 ternary system were established. The evolution behavior of oxygen from ZrO₂-CeO₂ powders could be explained on the basis of the phase diagrams; it was proved that the oxygen evolution behavior of ZrO₂-CeO₂ powders at low temperatures as sub-catalyst is the best in the composition range of ZrO₂: 45∼65 mol% after the appearance of the φ′ phase.
It was inferred that oxygen vacancies played a part in the growth of φ′ phase, and therefore similar experiments were undertaken with the powders of X_Ce⁄X_Zr=2⁄3 doped with CaO. (v): When it was annealed at 1773 K for 50 h, almost a single phase of tetragonal φ was obtained. The φ phase turned to φ′ phase once deoxidized and successively oxidized. (vi): The structure of φ′ was approximated by a pyrochlore, though there was no oxygen vacancies; Zr and Ce atoms may take regular arrays. (vii): The grain of the powders became fine through the phase transitions: φ→pyrochlore→φ′, and the evolution rate of oxygen became fastest, along with a change in the phase diagram. (viii): Through the phase transitions, CeO₂-ZrO₂ powders with ZrO₂, 45∼65 mol% possess a regeneration property as automotive exhaust sub-catalysts, and may be durable at higher temperatures. (ix): Through the present investigation, it was demonstrated that the evolved-oxygen gas-analysis utilizing the closed-system oxygen analyzer with high sensitivity is highly effective for the evaluation of ternary phase diagram related to oxides and also the detection of plausible compounds.