Synthesis and Characterization of Cerium Oxide Powder

Example 1

A ceric nitrate solution not less than 90 mol % cerium ions of which were tetravalent was taken so that 20 g of cerium in terms of cerium oxide was contained, and the total volume was adjusted to 1 liter with pure water. Here, the concentration in terms of cerium oxide was 20 g/L. The solution was placed in an autoclave reactor, heated to 100° C., held at this temperature for 24 hours, and allowed to cool in an atmosphere to room temperature.

Then an aqueous ammonia solution was added to neutralize to pH 8 to obtain cerium oxide hydrate in the form of a slurry. The slurry was then subjected to solid-liquid separation with a Nutsche filter, followed by separation of the mother liquor, to obtain a filter cake. The filter cake was calcined at 300° C. for 10 hours in a box-type electric furnace under air atmosphere to obtain cleric oxide, which was then ground in a mortar into ceric oxide powder (referred to as powder (A) hereinbelow). The specific surface area of powder A) was measured by the BET method. Further, the specific surface areas of powder (A) after calcination at 800° C. for 2 hours, at 900° C. for 5 hours, and at 1000° C. for 5 hours, respectively, were measured by the BET method. The tap density and total pore volume of powder (A) were also measured. Further, powder (A) was calcined at 900° C. for 5 hours, and then the OSC of the resulting ceric oxide powder was measured at 400° C. The results of these measurements are shown in Table 1.

Powder (A) was calcined at 1000° C. for 5 hours, and then the TPR measurement was made. The results are shown in FIG. 1. Further, from the TPR curve taken after calcination at 1003° C. for 5 hours, the ratio of the area (S1) defined by the baseline and the TPR curve in the temperature range of 200 to 600° C. to the area (S2) defined by the baseline and the TPR curve in the temperature range of 600 to 1000° C., i.e., the S1/S2 ratio, was determined. The results are shown in Table 1.

Following experiment was made in accordance with the teaching of Example 9 of Jp-7-61863-B.

922 ml of a solution of cerous nitrate containing 150 g/L of CeO₂ and 38 ml of a solution of hydrogen peroxide diluted to 200 ml were placed at room temperature in an autoclave reactor having a useful volume of 2 liters 150 ml of an aqueous 3N ammonia solution were added, while maintaining the temperature at 80° C., until a pH equal to 9.5 was obtained. The reaction medium was maintained at 8° C. for 1 hour to obtain a precipitate. The resulting precipitate was separated with a Nutsche filter, and washed with water.

The entire mass of the thus obtained precipitate was suspended in 150 ml of an aqueous 1N ammonia solution, placed in an autoclave, and treated therein at 160° C. for 4 hours. At the end of this heat treatment, the precipitate was recovered with a Nutsche filter. The obtained ceric oxide powder was subjected to the measurements as in Example 1. The results are shown in Table 2. Further, similarly to Example 9, the ceric oxide powder was further calcined at 500° C. for 5 hours, or at 700° C. for 5 hours, and then the tap density and total pore volume were measured, respectively. The results of these are shown in Table 2. Still further, as in Example 1, the ceric oxide powder obtained by calcining at 300° C. for 10 hours, followed by pulverization in a mortar, was further calcined at 1000° C. for 5 hours, and then the TPR measurement was made. The results are shown in FIG. 1.

Example 2

Ceric oxide powder was prepared in the same way as in Example 1, except that the temperature and duration for holding the prepared ceric nitrate solution under heating were changed as shown in Table 1.

A ceric nitrate solution not less than 90 mol % cerium ions of which were tetravalent was taken so that 20 g of cerium in terms of cerium oxide was contained, and the total volume was adjusted to 1 liter with pure water. Here, the concentration in terms of cerium oxide was 20 g/L. The resulting solution was immediately neutralized with an aqueous ammonia solution to pH 8 without the heat treatment in an autoclave reactor, to thereby obtain cerium oxide hydrate in the form of a slurry. The slurry was then subjected to solid-liquid separation with a Nutsche filter, followed by separation of the mother liquor, to obtain a filter cake. The filter cake was calcined at 300° C. for 10 hours in a box-type electric furnace under air atmosphere, and ground in a mortar into ceric oxide powder. The obtained powder was subjected to the same measurements as in Example 1. The results are shown in Table 2. Further, as in Example 1, the ceric oxide powder obtained by calcining at 300° C. for 10 hours, followed by pulverization in a mortar, was further calcined at 1000° C. for 5 hours, and then the TDR measurement was made. The results are shown in FIG. 1.

Example 3

A filter cake was obtained in the same way as in Example 1. The filter cake obtained was treated in an autoclave reactor, dispersed in water to reslurry, heated to 100° C., held at this temperature for 1 hour, and cooled to room temperature. The slurry was then subjected to solid-liquid separation with a Nutsche filter, to obtain a filter cake. The filter cake was calcined at 300° C. for 10 hours in a box-type electric furnace under air atmosphere, and ground in a mortar, to thereby obtain ceric oxide powder. The resulting powder was subjected to the same measurements as in Example 1. The results are shown in Table 1.

A filter cake was obtained in the same way as in Comparative Example 2. The obtained filter cake was subjected to the heat treatment and calcination in the same way as in Example 3, to obtain ceric oxide powder. The powder was subjected to the same measurements as in Example 1. The results are shown in Table 2.

In Tables 1 and 2, REO concentration is the concentration of cerium in the ceric nitrate solution in terms of cerium oxide. BET(1) is the specific surface area of the ceric oxide powder obtained by calcining at 300° C. for 10 hours, followed by pulverization in a mortar; BET(2) is the specific surface area of the powder in BET(1) further calcined at 800° C. for 2 hours; BET(3) is the specific surface area of the powder in BET(1) further calcined at 900° C. for 5 hours; and BET(4) is the specific surface area of the powder in BET(1) further calcined at 100° C. for 5 hours, all measured by the BET method. The specific surface area is shown in m²/g. Tap density (1) is the tap density of the ceric oxide powder obtained by calcining at 300° C. for 10 hours, followed by pulverization in a mortar; Tap density (2) is the tap density of the powder in Tap density (1) further calcined at 500° C. for 5 hours; and Tap density (3) is the tap density of the powder in Tap density (1) further calcined at 700° C. for 5 hours. The tap density is shown in g/ml. Total pore volume (1) is the total pore volume of the ceric oxide powder obtained by calcining at 300° C. for 10 hours, followed by pulverization in a mortar; Total pore volume (2) is the total pore volume of the powder in Total pore volume (1) further calcined at 500° C. for 5 hours; and Total pore volume (3) is the total pore volume of the powder in Total pore volume (1) further calcined at 700° C. for 5 hours. The total pore volume is shown in ml/g.