## 3 Results and Discussion  
### Effect of Precipitant Type on the Morphology of Alumina Precursors  
The morphology of alumina exhibits an inheritance relationship with its precursor morphology. To investigate the influence of the precipitant type on the morphology of the aluminum oxide, XRD and SEM analyses were conducted on the precursor products prepared with the three different precipitants. Using hexahydrate aluminum chloride asthe aluminum source, the experimental procedures were the same as those outlined in Section 2.2. The test results are presented in Figures 1 and 2, respectively. The XRD spectra results indicate that aluminum oxide precursors prepared with sodium bicarbonate and potassium bicarbonate as precipitants do not exhibit any additional peaks. However, aluminum oxide precursor prepared with ammonium bicarbonate as the precipitant primarily consists of ammonium aluminate carbonate, with additional peaks attributed to \(\gamma\)-AlOOH. The detailed peak positions and crystal planes in the XRD spectrum are shown in Table 1.  
The average crystal size (D) of different precursors of aluminum oxide prepared in different addition sequences was determined using Scherrer's formula, D = k\(\lambda\)/(Bcos\(\theta\)), where k is the constant value of 0.9; \(\lambda\) is the wavelength of X-ray radiation; B and \(\theta\)  
Figure 1: XRD patterns of alumina precursors prepared from different precipitants. (**a**) Sodium bicarbonate; (**b**) ammonium bicarbonate; (**c**) potassium bicarbonate. Figure 2: SEM images of alumina precursors prepared from different precipitants. (**a**) Sodium bicarbonate; (**b**) ammonium bicarbonate; (**c**) potassium bicarbonate. are, respectively, the full width at half maximum (FWHM) and half of the diffraction angle (20) detected on the crystal face in the XRD spectrum. The precursor prepared with sodium bicarbonate is sodium basic aluminate carbonate, with an average crystal grain size of 19.96 nm; the precursor prepared with ammonium bicarbonate is sodium basic aluminum ammonium carbonate, with an average crystal grain size of 20.03 nm; the precursor prepared with potassium bicarbonate is boehmite, with an average crystal grain size of 22.53 nm. The average grain size of alumina precursors prepared with different precipitants is shown in Table 2.  
Figures 1 and 2 show that, when sodium bicarbonate was used as the precipitant, the prepared precursor exhibited a rod-shaped sodium basic aluminum carbonate (NaAl(OH)2 CO3) with an average length of 800 nm. During the reaction process, sodium bicarbonate first reacts with aluminum chloride to form boehmite (\(\gamma\)-AlOOH). Subsequently, sodium bicarbonate further reacts with boehmite, ultimately yielding sodium basic aluminum carbonate, as indicated by the chemical reaction in Equations (1) and (2). \[AlCl_{3}+3 NaHCO_{3}=\gamma\text{-}AlOOH\downarrow+3 NaCl+3 CO_{2}\uparrow+H_{2}O \tag{1}\]  
\[\gamma\text{-}AlOOH+NaHCO_{3}=NaAl(OH)_{2}CO_{3} \tag{2}\]  
When ammonium bicarbonate was used as the precipitant, the prepared precursor appeared as irregular rod-shaped sodium basic aluminum ammonium carbonate (NH4 Al(OH)2 CO3). The crystal length was similar to that of sodium basic aluminum carbonate, but the diameter increased, and the surface exhibited a rough texture. This is because, compared to sodium basic aluminum carbonate, sodium basic aluminum ammonium carbonate showed characteristic peaks of the (111) crystal plane with higher intensity, indicating preferential growth of the (111) crystal plane in the sodium basic aluminum ammonium carbonate crystals. Therefore, the crystals of sodium basic aluminum ammonium carbonate had a larger diameter than did those of sodium basic aluminum carbonate [21]. During the reaction process, ammonium bicarbonate directly reacted with aluminum chloride to produce sodium basic aluminum ammonium carbonate.