A Low Temperature Fabrication and Photoactivity of Al2TiO5 in Cinnamic Acid Degradation
2019 Volume 60 Issue 9 Pages 2022-2027
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2019 Volume 60 Issue 9 Pages 2022-2027
Aluminum titanate was fabricated by sol-gel method using aluminium nitrate, titanium isopropoxide as precursors and citric acid as a complexing reagent. Effects of calcination temperature, citric acid content and calcination duration on the formation of Al2TiO5 (ATO) phase were investigated by X-ray diffraction. The results showed that the ATO-rich mixture with traces of anatase TiO2 was obtained at low calcination temperature (700°C). The properties of obtained ATO were characterized by various methods such as FTIR, N2 isotherm adsorption, DRS, SEM, TEM, and TGA. The point of zero charge of sample was also determined by salt addition method. Owning band gap energy of 3.42 eV the nanostructured ATO could be activated by UV light to become photocatalyst. Indeed, at favorable reaction conditions, detected from the experiment, the obtained ATO gave the photodegradation efficiency of cinnamic acid (CA) approximately 90% after 6 h. ATO also showed high stability and easily separated from solution, consequently after 9 times of reusing the CA degradation extent lightly decreased about 20%.
Fig. 11 Recycling of ATO-700-3 catalyst in CA photodegradation.
Semiconductor photocatalysis was used to be an ideal green technology for the remediation of environmental pollution due to its outstanding performance to degrade a wide range of toxic compounds without the requirement of chemicals and high temperature.1)
Microporous structured ATO was applied to purify drinking water, remove natural turbidity and pathogenic micro-organisms.2) ATO has the wide-band gap, vigorous transfer of electrons and efficient separation of photogenerated electrons with holes.3) Thus, ATO could be considered a novel candidate for photocatalytic reaction.
Most recent studies hardly reported synthesis of ATO and unconnected with photoactivity of pseudobrookite-type ATO. Zheng et al.4) prepared ATO ceramics via high-temperature sintering of α-Al2O3 and rutile TiO2 from 1450 to 1600°C. ATO with average particle size of 100 nm was formed at 1320°C from boehmite and titanium hydroxide through sol-gel technique.5) By sol-gel method, Sobhani et al.6) also successfully synthesized ATO nanoparticles with the size of 70 nm over 900°C using aluminum chloride and titanium tetrabutoxide. Another work,7) nanostructured ATO-rich powder was prepared by the sol-gel method at 900°C from titanium butoxide and aluminum chloride owning the band gap energy of 2.89 eV, the specific surface area of 21.1 m2g−1 and methylene blue degradation of 56.3% after 2 h. In general, ATO was virtually synthesized by sol-gel method at high temperature (≥900°C). Thus, a technique for the formation of ATO at low temperature has been interested.
In this work, the fabrication of nanostructured ATO at lower temperature was investigated by the sol-gel technique and its photocatalytic activity and stability were explored in more detail for the disintegration of cinnamic acid (CA) solution. Finally, ATO will be introduced as an inexpensive and easily separated photocatalyst for water purification.
The materials for ATO fabrication were aluminum nitrate nonahydrate (Al(NO3)3·9H2O, Xilong, 99.7 mass%), titanium isopropoxide (Ti(OC3H7)4, Merk, 97.0 mass%), monohydrate citric acid (C6H8O7·H2O, Merk, 99.5 mass%) and ethanol absolute (Xilong, 95.0 mass%).
ATO catalyst was obtained by sol-gel method as follows. Firstly, 3.75 grams Al(NO3)3·9H2O and m grams C6H8O7·H2O (m = 0.00, 1.05, 2.10, 3.15 and 4.20 grams) was dissolved with 5 mL ethanol and stirred for 1 h to form a homogenous solution. Nextly, 3 mL of Ti(OC3H7)4 was added dropwise and stirred for 1 h to form a colorless transparent gel. The synthesized gel was dried at 60°C for 24 h. Finally, the dried gel was calcined at different temperatures (600, 650, 700, 750 and 800°C) at a heating rate of 10°Cmin−1 for various durations (1, 2, 3, 4 and 5 h) to reach ATO sample.
2.2 Characteristics of ATOThe characteristics of obtained ATO were investigated by powder X-Ray Diffraction (XRD, Bruker D2 Pharser), BET nitrogen adsorption isotherms (Nova 2200e instrument), field emission scanning electron microscopy (SEM, Hitachi S4800), transmission electron microscopy (TEM, Jeol Jem 1400), Fourier transform infrared spectroscopy (FTIR, Bruker Tensor 27) and thermogravimetric analysis (TGA, Setaram LABSYS Evo TG-DSC 1600C). The point of zero charge (PZC) was determined by salt addition method.8) Diffuse reflectance spectroscopy (DRS, Varian Cary 5000 UV-Vis-NIR) was used to determine the diffuse reflectance and optical band gap of the ATO catalyst. The band gap energy (Eg) of synthesized ATO was calculated by Tauc plot.9)
2.3 Photocatalytic activity of ATOThe photocatalytic activity of samples was studied by the batch method as described in Ref. 10). The reaction mixture was stirred in the dark for 40 mins to establish the adsorption/desorption equilibrium before exposing to the UV light irradiation of 36 UVA Engin LZ1-00U600 lamps (λ ≈ 350–400 nm, with the maximum peak at 365 nm). The entire process of decomposition lasted 6 h. Influences of catalyst concentration, initial solution pH and air flow on the photoactivity of ATO were investigated. The reaction solution was separated by filtration and analyzed by UV-visible spectrophotometer on UV–1800 (Shimadzu) at 272 nm.
The recycling test was an appropriate approach to evaluate the stability of the best catalyst. CA solution was removed from the reaction solution after the end of each batch and replaced by fresh one to carry out the next photocatalytic reaction.
Figure 1 illustrates the TG-TGA curve of the synthesized gel dried at 60°C for 24 h. The peak at 186.5°C accompanied with 30.3% weight loss was related to dehydration of the gel. The wide exothermic peak in range 300–600°C accompanied with the weight loss of 36.3% was attributed to the dehydroxylation of Ti(OH)4–AlOOH inorganic moieties and decomposition of nitrates5) to form chemical bonds in the gel.6,11) From 690 to 760°C, there are two sharp peaks at 700.9 and 755.2°C in the TGA curve accompanied with 1.8% weight loss was observed. The similar TGA peak at about 724°C was reported in Ref. 12), which is said to correspond to the crystallization of ATO.
TG-TGA curve of the synthesized gel dried at 60°C for 24 h.
The XRD patterns in Fig. 2 show that the first peak is responsible for the crystallization of ATO amorphous phases and the second one is owing to the decomposition of ATO into TiO2. There was no weight loss at temperature higher than 760°C was observed, indicating most residues degraded at temperature lower than 760°C. XRD patterns of ATO powders calcined in the temperature range of 600–800°C for 3 h in air indicated that no crystalline phases were formed in samples processed at 600 and 650°C. The increase of calcination temperature to 700°C obtained roughly pure ATO accompanied with negligible amount of anatase TiO2. A further increasing to 750°C preferential formation of anatase TiO2 due to ATO decomposition was observed.11) Furthermore, there was formation of rutile TiO2 at 800°C. This is different from the result in previous reports,6,11) ATO sample synthesized from aluminum chloride and titanium tetrabutoxide was still amorphous after calcined at 700°C and the ATO-rich nanocomposite was formed at higher calcination temperature (900–950°C).
XRD patterns of catalysts prepared with R = 0.50 and varying in calcined temperatures: 600°C (a), 650°C (b), 700°C (c), 750°C (d), and 800°C (e).
XRD patterns of ATO samples prepared with different molar ratios of citric acid to metal cations: Al3+ and Ti4+ (R) are illustrated in Fig. 3. The sample hardly contained anatase TiO2 without adding citric acid. At R = 0.25, it is obviously found that most crystal phases were anatase TiO2 and ATO was formed with weak intensity. ATO-rich phases were obtained in sample with R = 0.50. In contrast, with higher molar ratio of citric acid to metal ions at R = 0.75 and 1.00, ATO was disappeared and simultaneously anatase TiO2 was formed. Addition of excess citric acid was unfavorable for the formation of pervoskite13) and suitable for the constituent phases of TiO2. It can be explained that citric acid as a chelating agent facilitates Ti4+ and Al3+ ions in solution to aggregate together. During the citrate sol-gel synthesis, hydroxyl and carboxyl groups of citric acid were easy to form a strong hydrogen bond with Al3+ ions.14)
XRD patterns of catalysts prepared from various molar ratios of citric acid to metal ions (R): R = 0.00 (a), R = 0.25 (b), R = 0.50 (c), R = 0.75 (d), and R = 1.00 (e) and calcined at 700°C for 3 h.
XRD patterns (Fig. 4) of samples calcined at 700°C for various durations show that the richest ATO phase had been appeared in the sample treated for 3 h and a higher peak of anatase TiO2 was formed on the samples with annealing time as a variation of 1, 2, 4, and 5 h.
XRD patterns of catalysts calcined at 700°C varying in durations: 1 h (a), 2 h (b), 3 h (c), 4 h (d), and 5 h (e).
To sum up, the ATO-rich composite could be obtained by the sol-gel method with R = 0.50 and the calcination at 700°C for 3 h and designated as ATO-700-3.
3.2 Characteristics of ATO-700-3XRD pattern of ATO-700-3 (Fig. 5) contains the peaks at 2θ = 18.9, 26.6, 28.1, 33.7, 38.0, 42.0, 42.6, 47.7, 50.7, 54.1, 57.0, 58.3, 62.4, 67.9, and 72.1° assigned to ATO phase.15) Moreover, a minor peak at 2θ = 25.2° was identified to anatase TiO2.16) The crystalline size of ATO calculated by Scherrer equation17) at 2θ = 26.6° was approximately 33.6 nm.
XRD pattern of ATO-700-3 catalyst.
On the FTIR spectra of samples (Fig. 6) a band appeared at 3100–3700 cm−1 connected to stretching vibrations of OH groups and a peak at 1630 cm−1 ascribed to the bending vibration of OH groups in free hydroxyls or adsorbed water were observed.18) They can enhanced the photoactivity of catalyst based on the formation of hydroxyl radicals and the prevention of electron-hole recombination.19) The peaks at 400–800 cm−1 may be attributed to Ti–O and Al–O vibrations.20,21) There are no vibration bands of C=O and C–O at 1700 and 1200 cm−1, related to citric acid bonds,8) indicating that there is no residual citric acid in ATO-700-3.
FTIR spectra of ATO-700-3 catalyst.
On the DRS spectra of ATO-700-3 (Fig. 7), the absorption wavelength was found at 363 nm and the band gap energy was calculated by Tauc plot, reaching 3.42 eV, which is much lower than that was reported in Ref. 22) (Eg = 4.5 eV) but higher than that found by Bakhshandeh.7) According to Bakhshandeh et al., the Eg value of ATO-rich powder, preparated from tetrabutexide and aluminum chloride, dependent on calcination temperature (850–1000°C) is in the range 2.89–3.75 eV. The reduction in the band gap energy of ATO materials was explained by the lack of oxygen creating the trapping levels.11) Thus, in this study, by sol-gel method at relatively low temperature ATO-rich mixture had been synthesized as a semiconductor, which can be activated by UV light to become photocatalyst.
UV-Vis diffuse reflectance spectra (a) and Tauc plot (b) of ATO-700-3 catalyst.
From the SEM image (Fig. 8(a)), it can be noted that catalyst was blocks in the size range of 100–900 nm. Indeed, TEM image (Fig. 8(b)) shows that ATO exists in corn shape, built from particles of size about 20 nm connected together. It can be assumed that in the medium of citric acid nano-sized grains ATO are formed, which are consequently aggregated together during the calcination process at high temperatures. Synthesized ATO-700-3 catalyst had the specific surface area of 18.1 m2g−1 and the average pore diameter about 2.8 nm. The pore diameter of material is much larger than the molecular size of cinnamic acid (0.5 nm × 1.0 nm), so CA can approach all catalytic centers inner the surface of pores.23)
SEM (a) and TEM (b) images of ATO-700-3 catalyst.
The results of adsorption in dark and photolysis of CA showed that CA adsorbed on catalyst and decomposed by UV light (without catalyst) is negligible.
Figure 9 shows the influences of operational parameters on the photoactivity of ATO-700-3 catalyst in CA degradation. As it followed from Fig. 9(a), CA degradation was significantly affected by the catalyst dosages. When the catalyst concentration extended from 0.25 to 0.50 and 0.75 gL−1, CA degradation efficiency after 6 h reached by 46.8, 71.0 and 88.9%, respectively. However, a further increase in the catalyst dosage (1.00 and 1.25 gL−1) higher the optimal value (0.75 gL−1) led to a decrease in the CA degradation due to the turbidity of the suspension and light scattering effects.24)
Effects of operational parameters: photocatalyst dosage (a), initial pH solution (b) and air flow (c) on the photocatalytic degradation of CA on ATO-700-3 catalyst.
As can be seen from Fig. 9(b), the solution pH has a critical impact on CA degradation efficiency. The optimum pH was found to be 3.8 and the maximum conversion of 88.9% was achieved within 6 h. When the initial pH of CA solution was increased, there was a remarkable decrease in CA degradation efficiency. Indeed, it was decreased from 88.9 to 19.6% by increasing the initial pH from 3.8 to 9.0. This can be explained that at pH values greater than PZC of ATO-700-3 determined 6.21, the catalyst surface becomes more negatively charged, thus preventing adsorption of the negatively charged CA on the surface. This would be one of the reasons for the reduced degree of degradation at alkaline medium. Conversely, acidic medium would be a favor condition for CA degradation.25) In addition, electrostatic attraction between the positively charged surface and CA may result in an increased degradation. Further experiment was performed at pH 3.8.
It can be seen from Fig. 9(c) that CA conversion on ATO-700-3 catalyst roughly did not fluctuate when the air flow rate increased from 0.1 to 0.5 Lmin−1. In photocatalytic reaction, the role of oxygen was to enhance the separation of the photogenerated electrons and holes through production of $\text{O}_{2}^{ - \bullet }$ radical. However, at the higher oxygen flow rate may hinder the absorbance of UV light to the photocatalyst.26) The results show that the air flow rate is within the optimal range.
It was found from Fig. 10, ATO content in the samples greatly affect the photocatalytic activity. The higher the ATO content the higher photocatalytic performance was observed. The samples calcined at temperatures lower than 700°C showed very low activity because the ATO crystalline phase has not yet formed. The most effective photocatalyst was roughly pure ATO (ATO-700-3) with 88.9% of CA degradation after 6 h. The increase of calcination temperature from 700°C to 750 and 800°C caused the reduction of ATO content that markedly decreased the photoactivity of catalyst. The activity of ATO phase is comparable to that of pure anatase TiO2, with CA conversion after 90 min of reaction reached 50.0% versus 58.5%.27)
Effect of calcination temperature on photocatalytic activity of ATO catalysts.
The recyclability of ATO-700-3 in the suitable conditions as catalyst concentration of 0.75 gL−1, air flow of 0.3 Lmin−1 and initial pH of 3.8 (Fig. 11) showed that a gradually slow decrease in the photocatalytic degradation was observed in each run. After 9 times of recycling, the removal efficiency of CA decreased about 20%, from 84.3% to 67.8%. The mass loss of the photocatalyst during the accumulation and eliminated CA solution from reaction mixture can be a certain reason for a part of this decline.28) Moreover, with specific weight of 1.42 gmL−1, unlike nano TiO2 (1.05 gmL−1), ATO-700-3 was easily separated from water after used.
Recycling of ATO-700-3 catalyst in CA photodegradation.
Nanostructured ATO-rich catalyst was fabricated by sol-gel method from aluminum nitrate and titanium isopropoxide as precursors and citric acid as a complexing reagent at 700°C. With band gap energy of 3.42 eV the obtained ATO is likely to be used as photocatalyst in the decomposition of persistent pollutants. By activated with UV light this catalyst decomposed ∼90% CA after 6 h and 50% after 90 min. The great advantage of ATO was easily recovered and reused, its activity only decreased ∼20% after 9 cycles. ATO is a promising photocatalyst system besides traditional TiO2.
This research was supported by University of Technology – VNUHCM under the grant No. T-KTHH-2018-103.
