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The Morphologically Controlled Synthesis and Application of Mesoporous Alumina Spheres

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05 July 2023

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07 July 2023

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Abstract
The control of alumina morphology is crucial yet challenging for its various applications. Un-fortunately, traditional methods for preparing alumina particles suffer from several limitations such as irregular morphology, poor dispersibility, and restricted application areas. In this study, we develop a novel method for preparing spherical mesoporous alumina using chitin and Plu-ronic P123 as mixed templates. The effects of reaction temperature, time, and the addition of mixed templates on the phase structure, micromorphology, and optical absorption properties of the samples were investigated. The experimental results indicate that lower temperature and shorter reaction time facilitated the formation of spherical mesoporous alumina with excellent CO2 adsorption capacity. The periodic density functional theory (DFT) calculations demonstrate that both (110) and (100) surfaces of γ-Al2O3 can strongly adsorb CO2. The difference in the amount of CO2 adsorbed by Al2O3 is mainly due to the different surface areas, which give dif-ferent number of exposed active sites. This approach introduces a novel strategy for utilizing bi-ological compounds to synthesize spherical alumina and greatly enhances mesoporous alu-mina's application efficiency in adsorption fields. Moreover, the study explored the electro-chemical performance of the synthesized product using cyclic voltammetry and discovered im-proved loading of electrocatalysts and enhanced electrocatalytic activity.
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Subject: Chemistry and Materials Science  -   Materials Science and Technology

1. Introduction

In recent years, alumina has gained significant attention in high-value applications such as adsorbent [1,2,3], ceramic [4,5], catalyst and catalyst carrier [6,7,8,9], etc. The application of alumina is not only dependent on particle size but also on particle shape. Different shapes of alumina, such as rod-like [10], fibrous [11], plate-like [12], and spherical [13], have been wildly used. Seyed [14] got massive γ-aluminum with particle sizes between 0.5 μm and 0.9 μm, but the morphology was irregular. Dabbagh, et al. and Feng, et al [15,16] prepared rod-like, fibrous, and spherical alumina, but the process was complex and hard to control, resulting in poor dispersibility. Lv et al. [17] synthesized spherical alumina with a highly spherical shape and uniform particle size by using the oil-ammonia drop method, but the particle size was large, which limited its application area. Using aluminum isopropoxide as precursor and Pluronic P123 (P123) as template, Wu et al. [18] used aluminum isopropoxide as a precursor and P123 as a template to synthesize organized mesoporous alumina with a hierarchical structure. However, these methods suffer from limitations such as irregular morphology, poor dispersibility, and limited application areas.
Mesoporous materials are highly valued due to their higher specific surface area, organized pore structure, narrow pore size distribution, and continuous pore size, which make them important for adsorption and separation, as well as catalytic reactions [19] Among various shapes of alumina, spherical alumina has high fluidity, making it less prone to gathering and producing channeling during catalytic processes, which significantly enhances catalyst activity. Therefore, the preparation of mesoporous alumina can greatly improve the application efficiency of mesoporous alumina spheres in adsorption, separation, and catalysis [20].
Chitin ((C8H13O5N)n) is a naturally occurring biopolymer that is highly organized and abundant in the exoskeletons of crustaceans and insects. The primary chitin fibrils' structural arrangement varies among arthropod species, with some having helical structures called the Bouligand structure, which enhances photonic and mechanical properties. This structure resembles cholesteric lyotropic liquid crystals. The hierarchical structure of chitin fibrils makes them an excellent natural template for developing new materials. The use of biomass as a porous material can be biodegradable, achieve biocompatibility, and achieve green and sustainable development [21,22,23]. Pluronic-P123 (P123) is a soft template with a symmetric triblock copolymer comprising poly (ethylene oxide) (PEO) alternating with poly (propylene oxide) (PPO), PEO-PPO-PEO. Its phases vary depending on the concentration and combination of solvents [24,25,26,27] and it is often used as a crystal structure modifier in the preparation of mesoporous materials [28].
In this study, mesoporous alumina spheres were successfully prepared using alumina hydrate (AlOOH) as a precursor, (NH2)2CO as a precipitant, as well as Chitin powder and P123. The preparation process, which involved evaporation induced self-assembly (EISA), was green, low cost, and pollution-free. This method significantly improved the catalyst loading firmness and service life of mesoporous alumina spheres. The effects of synthesis temperature, time, and the addition of mixed templates on the structure and morphology of the products were investigated. The CO2 adsorption performance of spherical mesoporous alumina and the electrochemical performance of supported SnO2 were also evaluated.

2. Experimental Section

2.1. Synthesis of Mesoporous Alumina Spheres

The typical synthesis process involves the following steps: Firstly, 6.6 g of hydrated alumina is added to ultrapure water with a resistivity of 18 MΩ·cm, and dissolved in a volumetric flask to prepare a 0.6 mol/L hydrated alumina solution. Subsequently, 3 g of Chitin and 1g of P123 are dissolved in 40 mL of isopropanol, followed by the addition of 2.4 g of urea, and stirring until fully dissolved. Next, 30 mL of the prepared hydrated alumina solution is added and vigorously stirred at room temperature. After 30 minutes, the solution is transferred into an autoclave with a PTFE liner, and reacted at 140 °C for 3 hours. Following this, the product is rapidly cooled to room temperature using a water bath. The synthesized samples were purified by repeated redispersion in ultrapure water followed by filtration to remove any unreacted reagents. The samples were washed with 500 mL of ultrapure water and 200 mL of absolute ethanol respectively for 3-5 times. The obtained material is then dried under vacuum at 80 °C, and finally calcined at 700 °C for 2 hours. The resulting product is a white spherical mesoporous alumina powder.
The hydrothermal reaction temperature is 120 ℃, 140 ℃, 160 ℃, and 180 ℃. The hydrothermal reaction time is 3 h, 9 h, and 15 h. The Chitin: P123 weight ratio is maintained in the range of 0-4. Tables 1 is the process parameters for preparing alumina and spherical mesoporous alumina, respectively. The resultant samples were denoted as RmTnHs (m= Chitin:P123 weight ratio, n (reaction temperature)= 120, 140, 160 or 180 °C, respectively; s(reaction time)= 3, 9 or 15 h, respectively).

2.2. Physical characterization

The physical properties of the synthesized spherical mesoporous alumina materials were determined using various techniques. The X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert PRO diffractometer with Cu Kα radiation (λ= 1.54056 Å) over the 2θ range of 10-80°. The N2 adsorption-desorption isotherms were measured at -196 °C using a Micromeritics Tristars 3000 analyzer analyzer. The CO2 adsorption isotherms of the synthesized spherical mesoporous alumina materials were measured using a Micromeritics Tristars 3000 analyzer instrument at 25 °C. Prior to the measurements, the samples were degassed at 180 °C under vacuum for 6 hours to remove any moisture and adsorbed gases. The CO2-adsorption isotherms were obtained at various pressures ranging from 0 to 1 bar. The CO2 uptake capacity of the materials was calculated from the amount of gas adsorbed at equilibrium and the mass of the sample. The specific surface area (SSA) and pore volume were calculated using the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) analysis, respectively. The scanning electron microscopy (SEM) images were obtained using a KYKY-2800B microscope. Transmission electron microscopy (TEM) measurements were carried out on Tecnai G2 F20 operated at 200 kV.

2.3. Electrochemistry characterization

The electrochemical performance of the synthesized spherical mesoporous alumina materials was evaluated using a three-electrode system with a platinum wire as the counter electrode, a mercury/mercury oxide electrode as the reference electrode, and the synthesized material as the working electrode. The electrochemical tests were carried out using a CHI660D electrochemical workstation in a 5 mol/L KOH electrolyte solution. Cyclic voltammetry (CV) was carried out in the potential range of 0 to 0.7 V vs. Hg/HgO at a scan rate of 0.025 V/s-0.1 V/s to investigate the electrochemical stability of the synthesized materials.

3. Results and Discussion

Synthesis of Mesoporous Alumina Spheres The synthesis of spherical macroporous alumina materials has been achieved through the hydrothermal method, using hydrated alumina (AlOOH) as the precursor, along with Chitin and P123 as the templates, and urea as a precipitant, while adapting the evaporation induced self-assembly (EISA) method, as depicted in Figure 1. By systematically varying the reaction conditions, the impact of reaction temperature and time, as well as the Chitin:P123 weight ratio, on the morphology and properties of the resulting spherical mesoporous alumina materials was investigated. The Detailed process parameters are shown as fowling table. During the calcining process at 700 °C, the organic components, including Chitin and P123, underwent decomposition and evaporation, creating voids within the alumina matrix. This decomposition and evaporation of organic materials resulted in the formation of well-defined mesopores within the macroporous alumina microspheres.
Table 1. Detailed process parameters for preparing spherical mesoporous alumina.
Table 1. Detailed process parameters for preparing spherical mesoporous alumina.
Sample Weight ratio
(Chitin:P123)		 Reaction temperature (°C) Reaction time (h)
R2:1T120H3 2:1 120 3
R0:1T120H9 0:1 120 9
R4:1T120H15 4:1 120 15
R1:2T140H3 1:2 140 3
R3:1T140H9 3:1 140 9
R1:1T140H15 1:1 140 15
R0:4T160H3 0:4 160 3
R2:3T160H9 2:3 160 9
R3:1T160H15 3:1 160 15
R3:1T180H3 3:1 180 3
R1:2T180H9 1:2 180 9
R0:4T180H15 0:4 180 15
The effect of synthesis temperature and time on the morphology of spherical mesoporous alumina. Figure 2a displays the X-ray diffraction (XRD) map of the precursor synthesized under various temperatures and durations. The precursors of R2:1T120H3 and R0:1T120H9 are primarily amorphous products. However, as the synthesis temperature increases to 160 °C and 180 °C, The precursors of R0:4T160H3 and R3:1T180H3 start to crystallize and become AlOOH (JCPDS 01-072-0359). This finding suggests that the synthesis temperature has a significant impact on the crystallization of the precursor. At lower hydrothermal temperatures, the system energy is insufficient to facilitate the formation and transformation of crystals, leading to the amorphous state of the samples. In contrast, when the hydrothermal temperature increases, individual diffraction peaks emerge, and their intensity and width grow, indicating the increased crystallinity of the particles. The precursors of R3:1T140H9 and R1:1T140H15 are synthesized at 140 °C for 9 h and 15 h, respectively, providing sufficient energy for crystallization. Nevertheless, the diffraction peak intensity and width of precursors R3:1T140H9 and R1:1T140H15 are smaller compared to those of precursors R0:4T160H3 and R3:1T180H3 synthesized at higher temperatures. This difference implies that the crystallinity of particles in precursors of R3:1T140H9 and R1:1T140H15 is weaker than that in precursors R0:4T160H3 and R3:1T180H3. By controlling the growth rate of crystal faces, the crystal orientation can be influenced, enabling control over the morphology and crystal structure of products.
Figure 2b shows the mesoporous alumina XRD image after calcination at 600 °C. The amorphous precursor of R2:1T120H3 and R0:1T120H9 prepared at a lower synthesis temperature transforms into γ-Al2O3 under calcination at 700 °C; sample R0:4T160H3, R3:1T180H3, R3:1T140H9 and R1:1T140H15 are γ-Al2O3 (JCDPS 10-0425). With the increase of hydrothermal temperature, the position of each diffraction peak remains the same. However, the intensity and width of the diffraction peak grow larger, indicating an increase in the crystallinity of particles.
Figure 2c, d illustrates the SEM images of the mesoporous alumina samples prepared under different synthesis conditions. The images reveal that the temperature significantly influences the sample morphology, and 140 °C promotes the formation of spherical particles. When synthesized at 120 °C, the sample particles are massive and spherical (Figure 2c), while at 140 °C, the particles are regularly spherical with a smooth surface and particle sizes ranging between 50 nm and 200 nm (Figure 2d). As the synthesis temperature increases, more particles begin to crystallize and form crystalline solids. While these solids can completely transform into γ-Al2O3 at 700 °C, the amorphous precursor requires more energy to undergo phase transformation at this temperature. Thus, crystallization is relatively slow, which favors the preparation of spherical mesoporous alumina (Figure 2a, b). However, at higher synthesis temperatures, the Chitin powder used as a template tends to carbonize and lose some of its templating function. As a result, the sample transforms into irregular and flocculent particles that agglomerate (Figure S1a-d). Moreover, longer synthesis times are also not conducive to the development of spherical alumina particles (Figure S1c, d). Such conditions promote the development of flocculent, strip, and irregular particles, which hinder the normal growth of spherical particles.
The Effect of Chitin:P123 Weight Ratio on Spherical Morphology. During the synthesis process, the amount of additive used has a significant impact on the morphology and structure of the final product.[29,30,31] Table 1 displays the test results of spherical mesoporous alumina prepared using different Chitin:P123 weight ratios (Chitin quality:P123 quality).
When Chitin/P123 weight ratio is 0 (R= 0:4), the predominant types of pores observed in the samples are ink-bottle type pores with a large opening and small diameter, as well as uneven crack-like pores, as shown in Figure 3a. The nitrogen adsorption isotherm (see Figure 3f) exhibits a type IV curve with a hysteresis loop between H2 and H3 in the mid-dle to high voltage range of 0.5 p/p0 to 0.9 p/p0. The adsorption amount in this range is limited to only 0.4 to 1 mmol/g. This is primarily due to the irregular stacking of the strip-shaped samples, which results in a low number of mesoporous structures being dis-tributed. As shown in Figure 3g, the pore sizes are mainly distributed in the range of approximately 7 nm.
As the weight ratio increases to 3, 4 (R= 3:1, 4:1), the particle size of the sample becomes larger and the spherical shape becomes more regular (see Figure 3d,e). The nitrogen adsorption isotherms show that in the middle-high pressure area, nitrogen adsorption capacity has further increased to about 7-8 mmol/g, as shown in Figure 3f,g. The pore size has further increased to about 9.4 nm and the pore volume has increased to 0.7 cm3/g. The type IV curve of nitrogen adsorption isotherms shows a hysteresis loop between type H1 and H2. The formation of inkbottle pores becomes more prominent, and the mesoporous structure becomes more developed. However, with the further increase of weight ratio, the nitrogen adsorption capacity gradually decreases, indicating that excessive chitin may hinder the formation of the mesoporous structure. Therefore, the optimal weight ratio of Chitin/P123 is 3, 4 (R= 3:1, 4:1).
As shown in Figure 3d, at a weight ratio of 3, the sample formed regular, uniform, and monodisperse microspheres and the predominant pore structure is a regular cylindrical shape with uniform size and shape. The nitrogen adsorption isotherms for this sample show a steep type IV curve and an H1 hysteresis loop (see Figure 3f), indicating that the sample has a uniform meso-porous structure. and pore sizes are mainly distributed between 7 nm and 9 nm. Notably, the pore size has increased by 8.6 nm compared to the previous weight ratio, and the pore volume has also increased by 0.3 cm3/g.
In summary, the Chitin/P123 weight ratio has a significant effect on the morphology and structure of the mesoporous alumina sample. When the weight ratio is 0 (R= 0:4), the sample has irregular strips with few mesoporous structures. As the weight ratio increases to 3(R= 3:1), the sample forms regular and uniform microspheres with a uniform mesoporous structure, which leads to an increase in pore size, pore volume, and BET surface area. However, when the weight ratio is further increased to 4 (R= 4:1), the sample exhibits adhesion and aggregation, and the pore size, pore volume, and BET surface area decrease. This is due to the increase in viscosity of the solution, which affects the template space steric effect and weakens homogeneous nucleation.
Using the method, the experiment successfully prepared spherical mesoporous alumina with uniform pore size, good dispersion, and identical morphology, with a Chitin/P123 weight ratio of 3 (R= 3:1). The amorphous precursor was prepared at a lower temperature of 140 °C and for a shorter time of 3 hours. Upon calcination at 600 °C, inorganic Al3+ interacted slowly with and connected to the organic micelle interface via electrostatic interaction. This process allowed the aluminum ions to cover the entire particle surface and achieve consistent growth rates for all surfaces, ultimately resulting in the formation of spherical particles via homogeneous nucleation. The particle surface also formed a uniform mesoporous layer, resulting in the complete transformation of the precursor into spherical mesoporous Al2O3, as shown in Figure S2.
Figure S2 displays TEM images of spherical mesoporous alumina materials. The images clearly show that the alumina has a uniform and regular spherical form, as well as good dispersion. A 10 nm mesoporous layer uniformly covers the surface of the alumina particles, and the mesopore size is consistent throughout. The nitrogen sorption isotherms and corresponding pore size distributions of the spherical mesoporous alumina materials are also presented in Figure S2.
CO2 adsorption on spherical mesoporous alumina. Figure 4a displays the CO2 gas adsorption isotherm curves of spherical mesoporous alumina, which was synthesized with varying mass ratios of Chitin to P123 at a temperature of 273 K. With its large specific surface area and pore volume, spherical mesoporous alumina exhibits a high potential for gas adsorption. In this study, we investigated the adsorption capacity of spherical mesoporous alumina, synthesized with different ratios of template agents (Chitin:P123), for carbon dioxide at 273 K.
The adsorption capacity of CO2 is highest (1.21 mmol/g) when the ratio of template agents is 3, resulting in the preparation of spherical mesoporous alumina with excellent dispersion and uniform pore size. Notably, the different mass ratios of template agents used in the synthesis produced alumina with distinct shapes, which significantly impacted their adsorption capacities.
Electrochemical performance of porous alumina supported SnO2. The electrochemical performance of porous alumina supported SnO2 was studied using spherical mesoporous alumina-supported SnO2 particle electrodes. The electrodes were prepared by a dip-calcination method, with a loading amount of 4%. Cyclic voltammetry tests were carried out using a three-electrode system with 5 mol/L KOH solution as the electrolyte, a mercury/mercury oxide electrode as the reference electrode, and a platinum electrode as the auxiliary electrode. The prepared electrode sheet was used as the working electrode.
The CV curves of spherical mesoporous alumina-supported SnO2 prepared with different ratios of template agents (Chitin:P123) at a scan rate of 0.075 V/s were shown in Figure 4b. The redox peak in the Figure 6a indicates the pseudocapacitive properties of the material. It was observed that when the ratio of template agent increases, the area of the cyclic voltammetry curve increases, and the specific capacitance increases significantly, indicating that the spherical mesoporous alumina has a better energy storage performance. However, when the ratio of template agent is too large (R= 4), the viscosity is viscosity of the solution increases significantly. This increase in viscosity can affect the steric effect of the template space and weaken the homogeneous nucleation, which can lead to poor dispersion of particles. Poor particle dispersion can result in a poor carrier effect, which can negatively impact the electrochemical performance of the material.
The CV curves at different scan rates when the ratio of template agent (Chitin:P123) is 3 and the loading of SnO2 is 4% were shown in Figure 4c. The CV curves were observed to be different with different scan rates, and with the increase of the test scan rate, the area of the closed graph also increased. This can be attributed to the high surface area and porous structure of the alumina support, which provides a large number of active sites for SnO2 deposition and enhances the catalytic activity of SnO2. Mesoporous alumina sphere can be an excellent electrocatalyst support due to its special mesoporous structure.
Periodic density functional theory (DFT). To gain deep insight on CO2 adsorption, we performed periodic density functional theory (DFT) calculations (see Supplemental Information for computational details). The (110) and (100) surfaces of γ-Al2O3 were selected, as these two surfaces are predominant in γ-Al2O3, proved by XRD in Figure 2b.[1] The optmized structures of Al2O3(110) and Al2O3(100) surfaces are shown in Figure S3. The bare Al2O3(110) surface is terminated with two-coordinated O(II), three-coordinated O(III) and four-coordinated Al(IV), while Al2O3(100) surface is exposed with three-coordinated O(III) and five-coordinated Al(V). Our results indicate that the surface energy of Al2O3(110) is lower than that of Al2O3(100) (1.41 J·m-2 vs 2.45 J·m-2), indicating that the Al2O3(110) surface is more stable.
The adsorption energies and optimized structures of CO2 on Al2O3(110) and Al2O3(100) surfaces are shown in Figure 5 and Figure S4. DFT calculated results show that the formation of carbonate species is the most stable adsorption model, where CO2 binds in abidentate configuration across the O-Al bridge sites with an adsorption energy of -0.93 eV and -0.75 eV on Al2O3(110) and Al2O3(100), respevtively. To obtain the insight into the nature of molecular adsorption behavior, the differences of charge density (Δρ) of the most stable adsorption CO2 on Al2O3 surfaces are calculated. The results are clearly plotted in Figure 5. It is evident that charges accumulate and deplet around the O and C of CO2 on both surfaces, respectively. The net result is the transfer of electrons from Al2O3 surfaces to the adsorbed CO2. The Bader charge analyses proved that the electronic charges transferred from the Al2O3(110) and Al2O3(100) surfaces to CO2 are 0.28 a.u. and 0.29 a.u., respectively. These two surfaces achieve the purpose of CO2 activation by transferring their electrons to the antibonding molecular orbital of CO2. The former is more stable, mainly because there are more electrons near the Fermi level on the atoms of surface active site Al(IV)-O(II) on Al2O3(110) than that of Al(V)-O(III) on Al2O3(100) (see projected density of states in Figure S5). The above results demonstrate that both surfaces can strongly adsorb CO2, which explains the experimental results. The difference in the amount of CO2 adsorbed by Al2O3 synthesized under different conditions is mainly due to the different surface areas, which give different number of exposed active sites. The higher the crystallinity, the higher the surface content of Al2O3(110) [32] .

4. Conclusions

In summary, using P123 as soft templates, mesoporous alumina sphere with high specific surface area, larger pore size, and pore volume was synthesized. The synthesis conditions, such as temperature, time, and weight ratio of additives, have a significant impact on the morphology and properties of the material. By optimizing these conditions, monodisperse spherical shape and uniform mesoporous structure can be elicited, leading to better CO2 adsorption capacity and improved electrocatalytic effect. This method provides a new method to control pore size and structure, and the material has potential applications in various fields. The periodic density functional theory (DFT) calculations results demonstrate that both (110) and (100) surfaces of γ-Al2O3 can strongly adsorb CO2, which explains the experimental results. The difference in the amount of CO2 adsorbed by Al2O3 synthesized under different conditions is mainly due to the different surface areas, which give different number of exposed active sites. The higher the crystallinity, the higher the surface content of Al2O3(110). The research contributes to the development of mesoporous materials and enriches the diversity in solution phase synthetic chemistry.

Author Contributions

Y.X. and X.T. conceived the project. Y.X., B.Y., Y.H., and X.T. supervised the project. Y.X., L.G., and M.X. carried out the material growth experiment and performed the Electrochemical and CO2 adsorption measurements. Y.X., L.G., M.X., and B.Y. conducted SEM, TEM, Nitrogen sorption and XRD characterizations. All authors contributed to the scientific discussion and writing of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 52062005 and No.52074141). Key Science and Technology Support Project of Guizhou Province(No.[2021]326), Guizhou Provincial Key Laboratory in Higher Education Institutions of Low-Dimensional Materials and Environmental and Ecological Governance(No.[2022]046). The Department of Education of Guizhou Province Natural Science Projects (No. 2022[161]).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the preparation of spherical mesoporous alumina materials.
Figure 1. Schematic diagram of the preparation of spherical mesoporous alumina materials.
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Figure 2. The X-ray diffraction (XRD) patterns of (a) the precursor and (b) mesoporous alumina XRD image under different synthesis conditions. The SEM images of mesoporous alumina under different synthesis conditions. (c) 120 °C, (d) 140 °C.
Figure 2. The X-ray diffraction (XRD) patterns of (a) the precursor and (b) mesoporous alumina XRD image under different synthesis conditions. The SEM images of mesoporous alumina under different synthesis conditions. (c) 120 °C, (d) 140 °C.
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Figure 3. Nitrogen sorption isotherms (a-e: sample number R0:4T140H15, R1:1T140H15, R2:1T140H15, R3:1T140H15, R4:1T140H15, respectively.), inset are corresponding pore size distributions (above), and corresponding SEM images (below) of the spherical.
Figure 3. Nitrogen sorption isotherms (a-e: sample number R0:4T140H15, R1:1T140H15, R2:1T140H15, R3:1T140H15, R4:1T140H15, respectively.), inset are corresponding pore size distributions (above), and corresponding SEM images (below) of the spherical.
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Figure 4. (a) CO2-adsorption isotherms of mesoporous alumina sphere synthesized with different templating agent ratios. Cyclic voltammograms of (b)different templating agent ratios and (c)different scan rates.
Figure 4. (a) CO2-adsorption isotherms of mesoporous alumina sphere synthesized with different templating agent ratios. Cyclic voltammograms of (b)different templating agent ratios and (c)different scan rates.
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Figure 5. The adsorption energies and optimized structures of CO2 on (a) Al2O3(110) and (c) Al2O3(100) surfaces. (c) and (d) are the charge difference of CO2 adsorption on Al2O3(110) and Al2O3(100) surfaces, respectively. The values of the isosurfaces are ±0.003 e/Å3. Yellow and light cyan isosurfaces indicate the accumulation and depletion of charge density.
Figure 5. The adsorption energies and optimized structures of CO2 on (a) Al2O3(110) and (c) Al2O3(100) surfaces. (c) and (d) are the charge difference of CO2 adsorption on Al2O3(110) and Al2O3(100) surfaces, respectively. The values of the isosurfaces are ±0.003 e/Å3. Yellow and light cyan isosurfaces indicate the accumulation and depletion of charge density.
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