[0076] [Example 1]
[0077] (1) Production of lanthanum-containing cerium oxide with reversible oxygen deficiency
[0078] (A) Synthesis of cerium oxide containing lanthanum
[0079] Powder of composite oxide of cerium oxide-lanthanum oxide is used. This composite oxide is an oxide prepared so that the ratio of the number of moles of lanthanum to the total amount of the number of moles of cerium and lanthanum becomes 0.2. 50 g of the composite oxide was allowed to stand still in a heating furnace, and heated while circulating air for firing. The heating was started from room temperature and heated at a temperature increase rate of 5°C/min. After reaching 1000°C, the temperature was maintained for 2 hours. The air flow rate is set to 0.5L/min. Then, it was allowed to cool naturally to obtain lanthanum-containing ceria without reversible oxygen defects. In the measurement using XRD, no La 2 O 3 Diffraction peaks, only observed from CeO 2 Diffraction peaks. From this result, it was confirmed that lanthanum was dissolved in cerium oxide.
[0080] (B) Synthesis of lanthanum-containing cerium oxide with reversible oxygen deficiency
[0081] The rare earth element-containing cerium oxide (50 g) obtained in the preceding item (a) was allowed to stand still in an atmosphere-controlled heating furnace, and heated and reduced while circulating 100% by volume of hydrogen. The heating was started from room temperature and heated at a temperature increase rate of 5°C/min. After reaching 600°C, the temperature was maintained for 3 hours. Then let it cool naturally. The flow rate of hydrogen is set to 1.5L/min. In this way, lanthanum-containing cerium oxide with reversible oxygen defects is obtained.
[0082] (2) Evaluation of conversion from carbon dioxide gas to carbon monoxide gas
[0083] Will use Figure 4 The tube furnace of the device shown is set in a glove box under a nitrogen atmosphere. In the tubular furnace, 8.5 g of lanthanum-containing cerium oxide powder having reversible oxygen defects obtained in the preceding paragraph (1) was allowed to stand still. First, close the valve V5, and all other valves are opened to suck the vacuum in the tubular furnace. In this state, the valve V1 was closed to heat the tubular furnace to 600°C. Then close the valves V2 and V3 and stop the suction in the tubular furnace. The valve V4 was closed to supply carbon dioxide gas (100% by volume) into the tubular furnace. The supply volume is set to 280mL (0°C, 1atm converted value). Then the valve V1 was closed and left for 1 hour. Then the valve V2 was opened, and the nitrogen gas was further supplied into the tubular furnace until the gas recovery bag bulged slightly. Next, the valve V2 is closed, and the gas recovery bag is heat-sealed and separated from the tube at the same time. In this state, the temperature of the tubular furnace is lowered and cooled to room temperature. After the cooling is completed, the valve V1 is opened to supply nitrogen gas into the tubular furnace. The supply is carried out until the pressure in the tubular furnace reaches atmospheric pressure. Finally, open the valves V3 and V5, and use nitrogen to squeeze out the carbon monoxide in the tubular furnace. The recovered reacted gas was qualitatively and quantitatively performed with a gas chromatograph, and the conversion from carbon dioxide to carbon monoxide at 600°C was evaluated according to the following criteria. Unlike this evaluation, except that the heating temperature of the tubular furnace was lowered to 400°C, the conversion from carbon dioxide to carbon monoxide at 400°C was evaluated in the same manner as the above-mentioned method. These results are shown in Table 1 below.
[0084] ○: More than 0.5% of carbon dioxide is converted to carbon monoxide.
[0085] ×: Less than 0.5% of carbon dioxide is converted to carbon monoxide.
[0086] (3) Reaction temperature with carbon dioxide
[0087] The temperature at which the lanthanum-containing cerium oxide used in Example 1 reacts with carbon dioxide (hereinafter referred to as "T CO2 ".). The results are shown in Table 1 below.
[0088] Using a differential thermogravimetric simultaneous measuring device (TG/DTA) (EXSTAR6000 manufactured by SII), 30 mg of lanthanum-containing cerium oxide without reversible oxygen defects was heated to 700°C in a reducing gas atmosphere, and then the state at 700°C Keep for 30 minutes to restore. The circulation rate of the reducing gas was set to 300 mL/min, and the heating rate was set to 20°C/min. As the reducing gas, a mixed gas of hydrogen and nitrogen (4% by volume of hydrogen and 96% by volume of nitrogen) was used. In this way, lanthanum-containing cerium oxide with reversible oxygen defects is obtained. Then, after cooling down to room temperature at a cooling rate of 40°C/min, carbon dioxide gas (100% by volume) was flowed, and the temperature was raised again at the above-mentioned heating rate to react lanthanum-containing cerium oxide with reversible oxygen deficiency and carbon dioxide gas. The flow rate of carbon dioxide gas was set to 300 mL/min. The mass change of lanthanum-containing cerium oxide with reversible oxygen defects was measured, and the temperature at which the mass increase due to the bonding of oxygen was observed was set as T CO2. T CO2 Such as Figure 5 As shown, it is set as the temperature at the intersection of the tangent line L1 before the mass increase starts and the tangent line L2 after the mass increase in the TG curve.
[0089] (4) The generation temperature of oxygen defects
[0090] The temperature at which the lanthanum-containing cerium oxide without reversible oxygen defects used in Example 1 generates reversible oxygen defects (hereinafter referred to as "T red "). The results are shown in Table 1 below.
[0091] Using a differential thermogravimetric simultaneous measuring device (TG/DTA) (EXSTAR6000 manufactured by SII), 30-35 mg of lanthanum-containing cerium oxide that does not have reversible oxygen defects is heated in a reducing gas atmosphere. Set the temperature at which the mass change due to the desorption of oxygen is observed as T red. As the reducing gas, a mixed gas of hydrogen and nitrogen (4% by volume of hydrogen and 96% by volume of nitrogen) was used. The circulation rate of the reducing gas was set to 300 mL/min, and the heating rate was set to 20°C/min. Such as Figure 5 As shown, the temperature at the intersection of the tangent line L3 before the mass reduction starts and the tangent line L4 after mass reduction in the TG curve is set to T red.
[0092] [Examples 2 to 7 and Reference Example 1]
[0093] In place of the composite oxide of cerium oxide-lanthanum oxide, a composite oxide of cerium oxide-praseodymium prepared so that the ratio of the number of moles of praseodymium to the total amount of moles of cerium and praseodymium reached the value shown in Table 1 was used And the firing conditions were set to 1400°C·3 hours instead of 1000°C·2 hours, except that the operation was the same as in Example 1 to obtain the praseodymium-containing cerium oxide of Example 2 without reversible oxygen defects .
[0094] In addition, instead of the cerium oxide-lanthanum oxide composite oxide, a cerium oxide-gadolinium oxide composite prepared so that the ratio of the number of moles of gadolinium to the total number of moles of cerium and gadolinium reached the value shown in Table 1 was used Oxide, and the firing conditions were set to 1400°C·3 hours instead of 1000°C·2 hours, except that the same procedure as in Example 1 was performed to obtain gadolinium-containing gadolinium without reversible oxygen defects of Example 3 Ceria.
[0095] In place of the cerium oxide-lanthanum oxide composite oxide, a cerium oxide-yttrium oxide composite oxide prepared so that the ratio of the number of moles of yttrium to the total number of moles of cerium and yttrium reached the value shown in Table 1 was used , And the firing conditions were set to 1500°C·5 hours instead of 1000°C·2 hours, except that the same operation as in Example 1 was performed to obtain yttrium-containing cerium oxide without reversible oxygen defects of Example 4 .
[0096] Instead of the cerium oxide-lanthanum oxide composite oxide, a cerium oxide-samarium oxide composite oxide prepared so that the ratio of the number of moles of samarium to the total number of moles of cerium and samarium reached the value shown in Table 1 was used In addition, the firing conditions were set to 1500°C·5 hours instead of 1000°C·2 hours, except that the same operation as in Example 1 was carried out to obtain a samarium-containing cerium oxide having no reversible oxygen defects of Example 5 .
[0097] Instead of the cerium oxide-lanthanum oxide composite oxide, a cerium oxide-ytterbium oxide composite oxide prepared so that the ratio of the number of moles of ytterbium to the total number of moles of cerium and ytterbium reached the value shown in Table 1 was used In addition, the firing conditions were set to 1500°C·5 hours instead of 1000°C·2 hours, except that the same operation as in Example 1 was carried out to obtain ytterbium-containing cerium oxide without reversible oxygen defects of Example 6 .
[0098] Instead of the cerium oxide-lanthanum oxide composite oxide, a cerium oxide-scandium oxide composite oxide prepared so that the ratio of the moles of scandium to the total moles of cerium and scandium reached the value shown in Table 1 In addition, the firing conditions were set to 1500°C·5 hours instead of 1000°C·2 hours, except that the same operation as in Example 1 was carried out to obtain scandium-containing cerium oxide with no reversible oxygen defects of Example 7 .
[0099] In the measurement by XRD, the praseodymium-containing cerium oxide of Example 2, the gadolinium-containing cerium oxide of Example 3, the yttrium-containing cerium oxide of Example 4, the samarium-containing cerium oxide of Example 5, and the In the ytterbium-containing cerium oxide of 6 and the scandium-containing cerium oxide of Example 7, no Pr 6 O 11 , Gd 2 O 3 , Y 2 O 3 , Sm 2 O 3 , Yb 2 O 3 And Sc 2 O 3 Diffraction peaks, only observed from CeO 2 Diffraction peaks. From this result, it was confirmed that praseodymium, gadolinium, yttrium, samarium, ytterbium, and scandium were dissolved in cerium oxide in each example.
[0100] Except for the use of the above-mentioned substances, the same operations as in Example 1 were performed to obtain praseodymium-containing cerium oxide, gadolinium-containing cerium oxide, yttrium-containing cerium oxide, samarium-containing cerium oxide, and ytterbium-containing cerium oxide with reversible oxygen defects. And cerium oxide containing scandium. In addition, as Reference Example 1, cerium oxide was used in the same manner as in Example 1 to obtain cerium oxide having reversible oxygen defects.
[0101] For all the praseodymium-containing cerium oxide, gadolinium-containing cerium oxide, yttrium-containing cerium oxide, samarium-containing cerium oxide, ytterbium-containing cerium oxide, and scandium-containing cerium oxide obtained as described above without reversible oxygen defects (Examples 2 to 7) and cerium oxide (Reference Example 1), the same T as in Example 1 was performed CO2 And T red The determination. The results are shown in Table 1 below.
[0102] In addition, all of the praseodymium-containing cerium oxide, gadolinium-containing cerium oxide, yttrium-containing cerium oxide, samarium-containing cerium oxide, ytterbium-containing cerium oxide, and scandium-containing cerium oxide having reversible oxygen defects obtained as described above (Examples 2 to 7) and cerium oxide (Reference Example 1), the same conversion evaluation from carbon dioxide to carbon monoxide as in Example 1 was performed. The results are shown in Table 1 below.
