The materials used in the experiment were as follows: aluminum nitrate (AR), ethanol (AR), propylene oxide (AR), ammonia (concentration of 15%–28%), lanthanum nitrate (AR), cerium nitrate (AR) and barium hydroxide (AR). All chemicals were purchased from the Chengdu KeLong Chemical Reagent Plant. The deionized water used in this study was produced in the laboratory. The formula for synthesizing activated alumina is shown in Table 1. The dosages of lanthanum nitrate, cerium nitrate, and barium hydroxide used in the experiment are presented in Tables 2–Tables 4, respectively.

Aluminum nitrate, lanthanum nitrate, cerium nitrate, and barium hydroxide were added to the mixed solution of ethanol and water. For the mixed doping sample, the two-phase mixture was prepared according to the two-phase ratio of 1:1 and the total addition was 1%, and the three-phase mixture sample was doped according to the three-phase ratio of 1:1:1 and the total addition was 1%. The mixed solution was continuously stirred until all particles were completely dissolved in the solution. Propylene oxide was slowly added to the beaker containing the mixed solution. Then, the mixed solution was stirred at high speed for 5–6 min to ensure propylene oxide was evenly distributed in the solution. Afterward, ammonia water was slowly dropped into the beaker and stirred for 10–11 min until the solution became colorless and transparent. Then the mixed solution in the beaker was allowed to stand at room temperature, aging for 24 h, to form a wet gel. The obtained wet gel was placed into an oven and dried at 60°C for 24 h. Afterward, the dried gel was transferred into a resistance furnace and calcined at 550°C for 4 h at a heating rate of 5°C/min. Then, the calcined product was cooled to obtain modified Al₂O₃. Then, the heat treatment aging test was performed at 1,100°C for 4 h, and the changes in specific surface area and phase were investigated.

The X-ray diffractometer (XRD, DX-2700B) was used to test the diffraction pattern of the synthesized activated alumina. The scanning step was 0.04°, and the scanning angle 2θ ranged from 5° to 80°.

The specific surface area of the synthesized activated alumina was measured by a fully automatic physical adsorption instrument (BET, Micromeritics ASAP2460). Before the measurement, the sample was dried at 300°C for 4 h to remove the water and air in the sample, and then the nitrogen adsorption experiment was performed at 77 K. BET was used to calculate the specific surface area.

The molecular structure and chemical bonds of the synthesized activated alumina were analyzed using a Fourier transform infrared spectrometer (FT-IR, NICOLET 6700). The sample was mixed with potassium bromide at a mass ratio of 1:100 and ground evenly in an agate mortar. The samples obtained after mixing were pressed into thin sheets and tested in the range of 4,000–400 cm⁻¹.

The microstructure and pore distribution of the synthesized alumina were examined using a scanning electron microscope (SEM, ZEISS EV0 MA15) and transmission electron microscope (TEM, ZEISS Libra 200 FE). A small amount of ground sample was added to anhydrous ethanol and ultrasonically dispersed for 30 min. Then one drop of ethanol mixed solution was gently dropped onto a clean glass sheet using a dropper. After the ethanol was volatilized, the sample on the glass plate was collected with conductive adhesive and sprayed with gold. Afterward, the sprayed sample was examined using SEM. In addition, one drop of ethanol was added to the mixture onto the copper gate, and the sample was examined using TEM after ethanol volatilization.

Considering the special application performance as catalyst support, the specific surface area of Al₂O₃ prepared under different conditions was tested. Figure 1 shows the specific surface area of pure activated alumina and samples doped with La, Ce, and Ba calcined at 550°C and aged at 1,100°C for 4 h. The specific surface areas of pure synthesized Al₂O₃ obtained after its calcination and high-temperature aging at 550°C and 1,100°C were 298.5764 and 21.3124 m²/g, respectively, indicating that the specific surface area of Al₂O₃ after high-temperature aging was significantly reduced. The specific surface area of all modified samples was smaller than that of pure Al₂O₃ after calcination at 550°C. When the high-temperature stabilizers La, Ce, and Ba were added to the sample, the specific surface area of the sample showed a downward trend with increasing content. After the sample with 1% of La, Ce, and Ba contents was aged at a high temperature of 1,100°C, the specific surface area became the largest. When the contents of La, Ce, and Ba were more than 1%, the specific surface area of the sample decreased in a “cliff-like” manner, and the specific surface area of the sample doped with La significantly decreased. It is worth noting that when the specific surface area of Ce-2 and Ba-8 changes significantly, this is because the addition of Ce-2 and Ba-8 has a significant effect on the size of primary Al₂O₃ crystallization particles, thus affecting the pore structure. Overall, the modification effect of Ba was the best, and the specific surface area of all the samples was higher than that of pure Al₂O₃ after they were aged at a high temperature of 1,100°C.The specific surface area of “La + Ba” mixed sample after high-temperature aging at 1,100°C for 4 h was the largest (119.2512 m²/g). The high-temperature stability of other mixed-doped samples has not been further improved. The specific surface area of the other groups after high-temperature aging at 1,100°C was lower than that of the samples when each component was doped separately, indicating that there was an interaction between the doped elements.

Figure 2 shows the XRD pattern of pure Al₂O₃ calcined at 550°C (Figure 2A) and aged at 1,100°C for 4 h (Figure 2B). After the pure Al₂O₃ was calcined at 550°C, only the diffraction characteristic peak of γ-Al₂O₃ (PDF # 10-0425) was observed in the spectrum. After the sample was aged at a high temperature of 1,100°C, only the diffraction characteristic peak of α-Al₂O₃ (PDF # 82-1467) was observed in the spectrum, indicating that γ-Al₂O₃ transformed into α-Al₂O₃ after high-temperature treatment at 1,100°C, thus decreasing the specific surface area of pure Al₂O₃.

Figure 3 shows the XRD pattern of La-doped samples calcined at 550°C (Figure 3A) and aged at 1,100°C for 4 h (Figure 3B). As shown in Figure 3A, only diffraction characteristic peaks of γ-Al₂O₃ (PDF#10-0425) and La₂O₃ (PDF#74-2430) were observed in the spectrum after the calcination of the samples at 550°C. No significant decrease was observed in the diffraction peaks of La₂O₃ as La content increased. This phenomenon can be attributed to low calcination temperature and crystallization degree of La₂O₃. As shown in Figure 3B, γ-Al₂O₃ (PDF#10-0425) (Chen et al., 2018), La₂O₃ (PDF#74-2430), and LaAlO₃ (PDF#74-2430) were observed in the sample after it was aged at 1,100°C. The diffraction characteristic peak at 1,100°C was sharper, indicating that high temperature could promote the formation and growth of crystals.

When the content of La was less than 4%, the intensity of the characteristic peak of γ-Al₂O₃ gradually weakened, and the diffraction characteristic peak corresponding to the (311) crystal plane shifted to the left because the introduction of La element led to the distortion of the Al₂O₃ lattice (Gong et al., 2001). With the increased La content, La reacted with activated alumina to form LaAlO₃ (Jing et al., 2020); thus, the corresponding characteristic peak intensity gradually increased. When the content of La was more than 4%, the characteristic peak of LaAlO₃ gradually increased, and the BET test results showed that the corresponding specific surface area was less than 2 m²/g, which was due to the thermally stable product of La atoms covering the surface of Al₂O₃ (BETTMAN et al., 1989).

Figure 4 shows the XRD patterns of Ce-doped samples after calcination at 550°C (Figure 4A) and aging at 1,100°C for 4 h (b). After the calcination of Ce-doped samples at 550°C, the characteristic peaks of γ-Al₂O₃ (PDF#10-0425) and CeO₂ (PDF#78-0694) are presented in Figure 4A. The result showed that the diffraction characteristic peak of CeO₂ was significantly enhanced with increasing Ce content. When the Ce content was 16%, a clear diffraction characteristic peak appeared, indicating that the crystallization degree of CeO₂ in alumina increased; Figure 4B shows the characteristic peak of α-Al₂O₃ (PDF82-1467) after aging the samples at 1,100°C.

The addition of Ce inhibited the transformation of γ-Al₂O₃ to α-Al₂O₃. The crystallization degree of CeO₂ was related to the content of CeO₂. The crystallization degree of CeO₂ increased when the content of CeO₂ was below 8%. By comparing the samples with Ce content of 1, 2, and 4%, the diffraction characteristic peaks corresponding to (511) of γ-Al₂O₃ containing Ce content of 2% and 4% increased significantly, indicating that Ce element had the effect of inducing Al₂O₃ lattice distortion. Compared with the spectrum at 550°C, the crystallinity of CeO₂ crystal was higher with the increase in temperature under the same dosage condition, indicating that high temperature contributed to the formation of CeO₂. The above results show that Ce atoms mainly exist in the form of CeO₂, and the presence of Al₂O₃ does not interfere with the formation of CeO₂ (Lu et al., 2003; Pang et al., 2020).

Figure 5 shows the XRD pattern of Ba-doped samples aged at 550°C (Figure 5A) and 1,100°C (Figure 5B) for 4 h. After calcination of Ba-doped samples at 550°C, the characteristic peaks of γ-Al₂O₃ (PDF#10-0425), BaO (PDF#74-1228), and BaCO₃ (PDF#71-2394) are shown in Figure 5A. After the aging of Ba-doped samples at 1,100°C, the characteristic peaks of γ-Al₂O₃ (PDF#10-0425), BaO (PDF#74-1228), and BaAl₂O₄ (PDF#72-0387) are shown in Figure 5B. The result showed BaCO₃ decomposed after it was aged at 1,100°C. When the Ba content was less than 4%, the characteristic peak intensity of γ-Al₂O₃ gradually decreased with the increasing Ba content. A weak characteristic peak of BaAl₂O₄ was observed at 4% addition of Ba, which indicated that Ba²⁺ existed in the lattice gap of alumina after the decomposition of BaCO₃ (Chen et al., 2021), resulting in the distortion of alumina lattice and the interaction with alumina to form BaAl₂O₄ (Jing et al., 2021). At the same time, compared with the XRD patterns of samples with 1% and 4% Ba content, the characteristic peaks corresponding to γ-Al₂O₃ (311) and (222) rapidly disappeared. When the Ba content was 4%, the corresponding characteristic peaks disappeared, but (440) and (400) still exhibited sharp diffraction characteristic peaks, which further indicated that the introduction of Ba atoms led to the distortion of alumina crystals. When the Ba content was more than 4%, the characteristic peak of BaAl₂O₄ appeared in the sample, and the characteristic peak of α-Al₂O₃ (PDF#82-1467) was not observed. The BaAl₂O₄ and α-Al₂O₃ were both hexagonal close-packed structures. Only the characteristic peak of BaAl₂O₄ appeared after the sample was calcined at a high temperature, indicating that the crystal structure of the former was more stable. The addition of Ba inhibited the transformation of γ-Al₂O₃ to α-Al₂O₃. The specific surface area of the sample with 16% of Ba content was much higher than that of La and Ce samples with the same content owing to a large specific surface area of BaAl₂O₄ (Yang et al., 2015; Yadav and Sharma, 2019).

Figure 6 shows the XRD pattern of the samples calcined at 550°C (Figure 6A) and aged at 1,100°C for 4 h (Figure 6B), with the sample containing 1% content of La, Ce, and Ba. The characteristic peaks of γ-Al₂O₃ (PDF#10-0425), La₂O₃ (PDF#74-2430), and CeO₂ (PDF#78-0694) after calcination at 550°C are shown in Figure 6A. Most of the aluminas retained their amorphous structures after calcination. After the samples were aged at 1,100°C, the characteristic peaks of α-Al₂O₃ (PDF#82-1467), γ-Al₂O₃ (PDF#10-0425), LaAlO₃ (PDF#85-0848), CeO₂ (PDF#78-0694), and BaAl₂O₄ (PDF#72-0387) are shown in Figure 6B. After La and Ce were co-doped, the characteristic peak of LaAlO₃ containing 1% content of La was observed, and the characteristic peak of CeO₂ was significantly enhanced, indicating the existence of an interaction between La and Ce (Tang et al., 2018), which could promote the formation of LaAlO₃ and CeO₂. After the incorporation of Ba and Ce, α-Al₂O₃ diffraction peaks appeared, indicating that the inhibition of Ba and Ce on the phase transition of Al₂O₃ is interfered with each other, which may be related to the electron arrangement of the them. After La and Ba were doped, the diffraction characteristic peaks of BaAl₂O₄ were observed in the spectrum, and the diffraction characteristic peaks of (311) and (222) corresponding to γ-Al₂O₃ were more visible than those of single doping of Ba. This phenomenon indicated that the introduction of Ba atoms distorted the alumina crystal, and the doped La atoms could induce the distortion and promote the formation of BaAl₂O₄. The XRD patterns of “La + Ce” doped with three elements were similar to that of “La + Ce” doped. However, the diffraction characteristic peak of α-Al₂O₃ was more visible; as a result, the specific surface area of this group was the lowest, indicating mutual interference between the elements when the three elements were doped, which reduced the inhibitory effect of the crystal transformation of alumina.

Based on the above discussion results, this section discusses the functional group changes on the surface of Al₂O₃ before and after doping and modification with La + Ba As shown in Figure 7, the corresponding infrared spectra of each sample are mainly the vibration peaks of -OH and Al-O (Xu et al., 2022). Among them, the stretching vibration at 3,450 and 1,644 cm⁻¹ corresponds to that of -OH, and the vibration at 1,030 cm⁻¹ corresponds to that of Al-O. Studies have shown that the essence of the sintering phase transition process of Al₂O₃ is its dehydroxylation process (Wu et al., 2021). For the sample treated at 550°C, the peak area of La + Ba doped modified Al₂O₃ sample is larger at 3,450 cm⁻¹, and the strength value is higher. This means that the modified sample has a higher content of γ-Al₂O₃. In the sample treated at 1,100°C, the diffraction peak area at 3,450 cm⁻¹ is smaller, which indicates that the transition from γ-Al₂O₃ to α-Al₂O₃ is irreversible.

Compared with the spectra of modified samples containing La + Ba after aging at 1,100°C, the peak areas of undoped samples at 3,450 and 1,644 cm⁻¹ were smaller, indicating that the modified samples retained more -OH, that is, more γ-Al₂O₃. The results of the infrared analysis showed that the incorporation of La + Ba could hinder the crystal transformation of alumina, which was consistent with the XRD test results. Numerous Al-O vibration peaks at 1,101, 826, 665, 558, and 462 cm⁻¹ were observed in the fingerprint region (Hiremath et al., 2020), indicating the formation of the Al-O-Al bond and the corresponding oxygen ion vacancies in the structure (Feng et al., 2011; Belekar and Dhoble, 2018; Li et al., 2020; Yang et al., 2020). La and Ba atoms affected the diffusion and rearrangement of O²⁻and Al³⁺ by occupying the corresponding lattice vacancies (Arai and Machida, 1996; Ozawa and Nishio, 2016).

Figure 8 shows the SEM image of the sample after aging at 1,100°C for 4 h. The samples before and after aging were tested using TEM to better compare the microstructure difference before and after aging. Figure 9 shows the TEM image of the sample before and after aging. Under the condition of 500 times magnification, the pure Al₂O₃ synthesized using the sol-gel method had an irregular polyhedron shape, and the surface was mainly flat and smooth, with only a few depression marks, and the diameter was about 3–8 μm (Figure 8). The Al₂O₃ modified by doping had a rough surface, with clear gaps and folds, and more holes. The size of the holes was larger than that of the pure Al₂O₃. As shown in Figure 8, the TEM image of pure Al₂O₃ before aging was fog-like. The interior of the pure Al₂O₃ mainly contained fine particles, and each particle was evenly distributed, with abundant pores between the particles. TEM images revealed cracks in the samples after aging, and there were agglomerations between the particles, indicating that the pores of Al₂O₃ were reduced during the transformation from γ phase to α phase after high-temperature aging, thus significantly decreasing specific surface area. The observed cracks were attributed to different orientations between grains. The La and Ba-modified samples displayed thick dark lines in the TEM images before aging, and other samples were similar to the TEM images of pure Al₂O₃ before aging. By comparing the TEM images after aging, the pores in the samples doped with La and Ba were reduced. However, the agglomerated products of the modified samples were significantly larger than those of pure Al₂O₃ samples after calcination, indicating that the transition from γ phase to α phase was insignificant.