Micelle-Assisted Synthesis of Al2O3 ·CaO Nanocatalyst: Optical Properties and Their Applications in Photodegradation of 2,4,6-Trinitrophenol

Calcium oxide (CaO) nanoparticles are known to exhibit unique property due to their high adsorption capacity and good catalytic activity. In this work the CaO nanocatalysts were prepared by hydrothermal method using anionic surfactant, sodium dodecyl sulphate (SDS), as a templating agent. The as-synthesized nanocatalysts were further used as substrate for the synthesis of alumina doped calcium oxide (Al2O3 ·CaO) nanocatalysts via deposition-precipitation method at the isoelectric point of CaO. The Al2O3 ·CaO nanocatalysts were characterized by FTIR, XRD, TGA, TEM, and FESEM techniques. The catalytic efficiencies of these nanocatalysts were studied for the photodegradation of 2,4,6-trinitrophenol (2,4,6-TNP), which is an industrial pollutant, spectrophotometrically. The effect of surfactant and temperature on size of nanocatalysts was also studied. The smallest particle size and highest percentage of degradation were observed at critical micelle concentration of the surfactant. The direct optical band gap of the Al2O3 ·CaO nanocatalyst was found as 3.3 eV.


Introduction
Metal oxide nanoparticles play effective role in degradation of hazardous chemicals. Their highly intrinsic surface area and catalytic properties have made them destructive adsorbents. These metal oxides not only adsorb hazardous chemicals on their surface but also destroy them into smaller and less harmful by-products [1,2]. They can destroy a wide range of such chemicals, for example, chlorobenzenes, organosulfurs, organophosphates, and nitroaromatics. These chemicals are present or used in synthesis of explosive materials, insecticides, pollutants, and chemical warfare agents [3][4][5][6][7][8][9][10][11][12][13][14][15]. Their increased consumption and improper disposal have become serious environmental risk. Some of them are penetrating into the ground water through soil, causing serious environmental and health problems [16].
In the current study, CaO and Al 2 O 3 ⋅CaO nanocatalysts were synthesized using hydrothermal method by varying the concentration of sodium dodecyl sulphate (SDS), an anionic surfactant. The objective of this study is to assess the photodegradation of selected ordnance compound by CaO and Al 2 O 3 ⋅CaO nanocatalysts. The ordnance compound of concern, 2,4,6-TNP (2,4,6-trinitrophenol or picric acid), was considered of priority for this study. The effects of temperature and surfactant were studied for the synthesis of nanocatalysts, and catalytic properties of nanocatalysts were assessed for degradation of 2,4,6-TNP.

Characterization.
The CaO nanocatalysts obtained were subjected to thermogravimetric analysis (TGA) by using SDT Q600 TGA. Structural analysis of CaO and Al 2 O 3 ⋅CaO nanocatalysts was done using Fourier Transform Infrared (FTIR)-MIDAC 2000 with KBr powder and powder X-ray diffractometer (XRD) using PANalytical MPD X'PERT PRO. The diffraction patterns were compared using the standard database from International Centre for Diffraction Data (ICDD). The morphology and particle size of nanocatalysts was determined by FEI quanta 200 F Field Emission Scanning Electron Microscope (FESEM) and Philips CM12, 80 kV, Transmission Electron Microscope (TEM). HPLC analysis was performed on Shimadzu model LC 20-AT instrument equipped with diode array detector (SPD-M20A, Shimadzu).

Synthesis of CaO Nanocatalysts by Hydrothermal Method.
CaO nanocatalysts were synthesized by changing the experimental parameters, that is, temperature and concentration of surfactant, to study their effect on particle size and catalytic activity. Synthesis method of CaO is depicted in Figure 1(a).
The mixture containing 0.15 M of CaCl 2 and 0.008 M sodium dodecyl sulfate (SDS) was magnetically stirred at ambient temperature. The precursor to surfactant molar ratio was taken as 1 M : 0.05 M. Sodium hydroxide (0.30 M) solution was added dropwise and the reaction solution was stirred for 30 minutes. After stirring, the reaction suspension was placed in a Teflon line autoclave (hydrothermal bomb) and kept in an oven for 4 h at the desired temperatures (250, 180, 160, and 140 ∘ C).
After 4 h, the autoclave was removed from the oven and allowed to cool for 2 h at ambient temperature. The precipitates of Ca(OH) 2 were separated and washed 3 times with methanol and 2 times with deionized water to remove any reactant, ions, or surfactant and neutralize their pH by using centrifugation machine at the speed of 13000 rpm. The precipitates were dried and calcined at 600 ∘ C in a furnace with air flow for 3 h [40].
Similar process (Figure 1(a)) was adopted to study the effect of surfactant on CaO nanocatalysts. Only the molar concentration of SDS (0.004, 0.006, 0.008, 0.01, and 0.012 M) was varied to be close-and far from its critical micelle concentration (CMC) value which is 8.1 mM [41].

Preparation of Alumina Supported CaO Nanocatalysts by Deposition Precipitation
Method. Synthesis of alumina doped CaO nanocatalysts was carried out by deposition precipitation method [32]. AlCl 3 ⋅6H 2 O and CaO nanocatalysts were used as precursors. 15 mM solution of AlCl 3 ⋅6H 2 O was prepared in 9 mL deionised water and 50 mg of CaO nanocatalysts was added at different time intervals to attain pH 12.3 (isoelectric point) [42]. Meanwhile, the reaction was constantly stirred on magnetic stirring plate. Teflon line autoclave was filled with reaction solution and kept in the oven for 4 h at the same temperature as that of CaO nanocatalysts precursor. Similarly, the precipitates were washed and calcined. The experimental setup is displayed in Figure 1(b).

Results and Discussion
3.1. Thermogravimetric Analyses. Figure 2 shows TGA/DSC profile of the Ca(OH) 2 synthesized with SDS at 180 ∘ C for 4 h via hydrothermal treatment. A significant weight loss (16.25%) is observed in the temperature range 375-450 ∘ C, which can be attributed to the thermal decomposition of Ca(OH) 2 . The observed weight loss in the range is smaller than the theoretical value (24.3%) calculated on the assumption of total dehydration of Ca(OH) 2 to CaO [40].
The results indicate that a nearly complete conversion of Ca(OH) 2 to CaO took place below 600 ∘ C. This implies that the surfactant used in the fabrication of CaO nanocatalysts had been almost removed at around 450 ∘ C. (Figure 3) at 3427 cm −1 and 3453 cm −1 can be attributed to the stretching and bending vibrations of hydrogen-bonded surface OH groups (physisorbed water). It reveals that only a slight amount of water molecules is retained in the fabricated CaO and Al 2 O 3 ⋅CaO samples. The appearance of strong IR absorption band at 424 cm −1 may be attributed to the lattice vibrations of CaO [43]. The IR absorption bands at 1415 cm −1 and 1439 cm −1 are due to the symmetric stretching vibration of unidentate carbonate. Weak absorption band at 875 cm −1 further demonstrates the presence of carbonate species. This is due to exposure of highly reactive surface area of CaO to air during calcination which resulted in the formation of considerable amount of CO 2 and H 2 O, which are adsorbed on the surface of CaO in the form of free -OH and carbonate species. This indicates that surface -OH and lattice oxygen of CaO do provide oxygen which is more assessable on high surface area samples (Figure 3(a)) [44,45]. In Figure 3    where is the mean crystallite size, is the grain shape dependent constant 0.89, is the wavelength of the incident beam in nm, is the Bragg reflection angle, and is the line broadening at half the maximum intensity in radians.

Fourier Transform Infrared Analyses. FTIR peaks
The 2 values of Al 2 O 3 ⋅CaO nanocatalysts ( Figure 5) were compared with the ICDD database to identify the phase purity and composition formed. Aluminium forms Ca 3 Al 2 O 6 phase (PDF# 00-006-0495,) with the calcium oxide nanoparticles at 600 ∘ C due to strong Al-O interaction [47,48]. The essential peaks at 2 = 20.9 ∘ , 21.

Optical Properties of Al
Optical properties of Al 2 O 3 ⋅CaO nanocatalysts were analyzed by UV-Vis absorption measurement at room temperature and using deionised water as blank. The sample was prepared by dispersing 3.6 mg of Al 2 O 3 ⋅CaO nanocatalysts in 10 mL of deionised water and stirring by magnetic stirrer for 15 min. A homogeneous suspension solution was prepared and subjected for assessment for optical properties. Figure 6 describes typical absorption spectra of Al 2 O 3 ⋅CaO nanocatalysts, which shows the shifting of absorption edges to the shorter wavelength (blue shift).
Equation (2) was used to calculate optical absorption coefficient from absorption data: where is the theoretical density of Al concentration of suspension solution, and is the molecular weight of Al 2 O 3 ⋅CaO nanocatalysts. Using Urbach's equation (3), the density of the localized tail state ( = 1.87 eV) in the forbidden energy gap was determined by plotting ln versus ℎ] as shown in Figure 7: Here, 0 a is constant and ℎ] is the energy of photons. The optical band gap for direct transition was determined by plotting ( ℎ]) 2 versus ℎ] using where is constant and nature of transition has been assumed to have values 1/2, 2, 3/2, and 3 for direct, indirect, forbidden direct, and forbidden indirect transitions, respectively [50,51]. The direct optical band gap energy is determined by extrapolating the linear portion of the curve in Figure 8; the intersection of the extrapolation gives the value of 3.3 eV, which is much less than band gap energy of Al 2 O 3 (7.2 eV) [52].
Proposed behavior of Al 2 O 3 ⋅CaO nanocatalyst towards organic pollutant due to band gap energy is illustrated in Figure 9.

TEM of CaO and Al 2 O 3 ⋅CaO Nanocatalysts.
Representative TEM image of the CaO and Al 2 O 3 ⋅CaO nanocatalysts obtained after hydrothermal treatment is shown in Figure 11. The nanocatalysts exist in coagulated form with the particle size of 16 nm (Figure 11(a)). The particles size is decreased to 3.6 nm after the formation of Al 2 O 3 ⋅CaO nanocatalysts (Figure 11(b)). On the basis of Beer-Lambert law, calibration was done for 2,4,6-TNP at a wavelength of maximum absorptivity, max , 356 nm [53]. The catalytic activity was determined using UV Spectrophotometer (UV-1700 Shimadzu) by measuring the change in absorbance at 356 nm every 60-second interval. Same procedure was adopted to determine catalytic activity of Al 2 O 3 ⋅CaO nanocatalysts against 2,4,6-TNP [54]. The analysis of samples showed a continuous decrease in absorption at max = 356 nm, which was used to track the degradation of 2,4,6-TNP [55]. It is evident that reaction kinetics of both CaO (Figure 12(a)) and Al 2 O 3 ⋅CaO (Figure 12(b)) nanocatalysts with 2,4,6-TNP follows first order. The first order rate constant values, , were determined from the slope of the graphs as shown in Figures 12(a) and 12(b).

Effect of Variation of Temperature on Catalytic Activity of CaO Nanocatalysts.
Catalytic activity of CaO nanocatalysts synthesized by varying hydrothermal treatment temperature (140, 160, 180, and 250 ∘ C) was studied while keeping other experimental parameters constant. It was observed that an increase in temperature (from 140 ∘ C to 180 ∘ C) resulted in increases in rate constant, value (0.0732, 0.0791, and 0.1283 min −1 ), but the catalytic activity decreases to 0.1124 min −1 at 250 ∘ C. This change in the catalytic activity trend suggests that high hydrothermal temperature favors fast reaction which increases the particle size and decreases the surface area and contributes to destructive adsorbent ability.

Effect of Variation of Surfactant Concentration on Catalytic Activity of CaO and Al 2 O 3 ⋅CaO
Nanocatalysts. The synthesis of CaO nanocatalysts under basic conditions is believed to follow the X − I + S − module, where S − is the anionic surfactant, I + is the inorganic precursor, and X − is the counter ion [56]. A generalized mechanism of electrostatic interaction between inorganic precursor, surfactant and counter ions was proposed in Figure 13. When sodium hydroxide is added to the system, Na + and OH − ions are supposed to surround Ca 2+ -DS − . The electrostatic attraction between Ca 2+ and DS − is stronger than that between Na and SD − ions; this behavior enhances the particle formation [57]. Na + joins with Cl − to make NaCl in the mixture system due to the electrostatic repulsion of Cl − and DS − . The OH − ions self-assembled around the micelle, so Ca 2+ ions were attracted towards OH − to form Ca(OH) 2 in the presence of surfactant (templating agent). In the final step of the process, the template was removed by calcination at 600 ∘ C for 3 h to generate pores.
The catalytic activity of CaO and Al 2 O 3 ⋅CaO nanocatalysts (synthesized via hydrothermal treatment at 180 ∘ C and 4 h using different surfactant (SDS)) concentration is shown in Tables Figure 13: Mechanism of micelle assisted formation of OH − Ca 2+ DS − system.
It was observed that the highest values for both CaO and Al 2 O 3 ⋅CaO were found at CMC of SDS in accordance to the small particle size of these nanocatalysts at this concentration. The value increases (particle size decreases) when the nanocatalysts were prepared by using surfactant from 0.004 to 0.008 M as the precursors are well dispersed in the surfactant template. However, further increase in surfactant concentration from 0.008 to 0.012 M decreases the values and increases the particle size due to formation of micelle which coagulates the particles. A parabola is formed showing the relationship between values and surfactant concentration as shown in Figure 14. The sample solutions were filtered and then degassed by sonication before use.
Mobile phase was prepared for HPLC analyses by mixing 70% methanol with 0.1 M acetic acid buffer in the ratio of 97 : 3, v/v. The mobile phase was filtered and then degassed by sonication before use [58]. The data was analyzed by obtaining area under sample peaks at 355 nm. The observed retention time for standard 2,4,6-TNP solution was found 7.425 min as shown in Figure 15.
Each sample solution was injected separately in the HPLC and none of them showed any peak at the wavelength of   Figure 16). GC-MS technique was used to determine the intermediates generated during catalytic degradation of 2,4,6-TNP. Sample was prepared by suspending 5 mg  15 minutes at ambient temperature. The sample solution was filtered before use.
A schematic diagram is proposed as given in Scheme 1 on the basis of GC-MS chromatogram ( Figure 17).

Conclusion
CaO and Al 2 O 3 ⋅CaO nanocatalysts were prepared by varying the temperature and surfactant (SDS) concentration above and below CMC value using hydrothermal and using deposition precipitation method. Catalytic activity of these nanocatalysts was measured against the degradation of 2,4,6-TNP, which proved that the nanocatalysts are effective catalysts. The highest rate constant value, , was observed in those samples which were prepared at CMC value of the   The Scientific World Journal