Oxygen evolution properties of CeO2-ZrO2 powders as automotive exhaust sub-catalysts and the phase diagrams

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 Arf H-2 + H2O gas mixtures. A closed-system oxygen analyzer developed by one of the authors was used to establish a constant ratio of H-2/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 O-2 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 jo-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 nm, 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 jo-T-t curves (2) and (3) were essentially different from those of the Ist run. This came from the appearance of cubic phi' phase, (Ce2Zr3O10?), which had not been reported previously. (iv) : By analyzing the jo-T-t curves, two kinds of qualitative phase diagrams of the ZrO2-CeO2-CeO1.5, 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 similar to 65 mol% after the appearance of the phi' phase. It was inferred that oxygen vacancies played a part in the growth of phi' 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 phi was obtained. The phi phase turned to phi' phase once deoxidized and successively oxidized. (vi): The structure of phi' 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: phi->tpyrochlore->phi', and the evolution rate of oxygen became fastest, along with a change in the phase diagram. (vii): Through the phase transitions, CeO2-ZrO2 powders with ZrO2, 45 similar to 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.