58 The MCPA calibration curve was obtained from running known concentrations of the MCPA standard. An MCPA stock solution was prepared by dissolving 10.00 mg MCPA into 100 mL Millipore water, and then employed to prepare 0.5 – 10 mg L-1.
Equation 3.4 was used to calculate the volume of MCPA solution required to prepare concentrations of 0.5, 2, 4, 6, 8 and 10 mg L-1 respectively.
C1V1 = C2V2 (3.4)
Standard working solutions of 2, 4, 6, 8 and 10 mg L-1 were also prepared from following similar calculations. During analysis, 20 µL of the analyte was injected at 1 mL min-1 flow rate. Analysis was done at the absorption wavelength of MCPA at 290 nm. The concentration of MCPA was quantified from the constructed calibration curve of area versus concentration with a regression co-efficient of 0.9999. For each analysis, the area of the peak (y) appearing at retention time of ≈ 3.85 minutes was converted to concentration through solving for x in equation 3.5.
y = 50077x - 129.16 (3.5)
59 3.3.2 Synthesis of SBA-15
The following method was adapted from Meynen and Vansant [4]. During the preparation of SBA-15, a solution of H2O : HCl (130.18 g : 20.26 g) was prepared in a 400 mL beaker.
To this solution 4.23 g of surfactant P123 was added and the mixture was stirred for 30 minutes at room temperature to ensure that surfactant (P123) was completely dissolved. The mixture was then transferred into a 500 mL three-necked round bottom flask (fixed with a thermometer, and a condenser opened to air) and was refluxed at 50 ˚C for 1 hour. 9.13 g of tetraethyl orthosilicate (TEOS) was added dropwise to the mixture and stirring was continued for a further 6 hours 30 minutes. The magnetic stirring was then stopped and the temperature was increased to 80 ˚C.
The solution was cooled to room temperature after heating at 80 ˚C for 15 hours 30 minutes without stirring.
The product was recovered through filtration under vacuum and washed with 3 portions of 25.00 g de-ionized water. The pure white product was collected and dried in an oven set at 100 ˚C for 3 hours. After drying, the solid product was calcined in a muffle furnace system at 550 ˚C for 6 hours, with heating rate 1 ˚C per minute (˚C min-1). In a typical experiment, 5.35 g was calcined and the recovered mass was 3.12 g. The synthesis of SBA-15 was done in duplicate.
3.3.3 Synthesis of SBA-15 Coated MWCNTs (SBA-CNT Composites)
This section describes the coating of the functionalized CNTs (aCNTs) with SBA-15. Similar method to SBA-15 synthesis was used with the exception of introducing CNTs at the initial stage of the synthesis method.
4.00 g of P123 and 0.50 g aCNTs were weighed and placed in a 400 mL beaker. 130.00 g de- ionized H2O and 20.00 g HCl were then added to the beaker, the mixture was treated with an ultrasonic probe for 20 minutes. The mixture was then transferred into a 500 mL two-necked round bottom flask with a thermometer, and a condenser (opened to air) attached. The mixture was heated to 45 ˚C and 0.19 g TEOS (for an estimated 10 wt. % SBA-15 coating on the CNTs) was added dropwise.
60 Stirring was stopped and the temperature was increased to 80 ˚C and the mixture was left standing at 80 ˚C for 15.5 hours. The solution was allowed to cool to room temperature and the product was recovered through filtration under vacuum and washing with 3 portions of 25.00 g de-ionized water. The black product was dried in an oven at 100 ˚C for 18 hours. After drying, the solid product was calcined in a muffle furnace system at 400 ˚C for 6 hours, with a heating rate of 1 ˚C min-1. The typical yield was 0.38 g.
The same procedure was followed to coat 20 and 30 wt. % SBA-15 on CNTs. For the 20 wt., approximately 0.44 g TEOS was used and for 30 wt. %, 0.79 g TEOS was used. The yields were 0.49 g and 0.57 g for 20 and 30 wt. % SBA-15 respectively. The synthesis of these materials was done in triplicate. The amounts of TEOS used during the respective experiments were calculated according to equations in appendix A 1.2.
3.3.4 Synthesis of Mesoporous Titanium Dioxide
Prior to synthesizing the catalyst, various optimization experiments were carried out using different weight ratios of the surfactant F127. The optimization was carried out in order to evaluate the critical micelle concentrations (CMC) for the surfactant F127 in the solvent system of choice and the effect of the number of moles of the titania precursor (TIP). The CMC experiments were modified from methods reported by Fuguet et al. [5]. In the conductivity experiments, 2.50 g – 25.00 g F127 was dissolved in varying solvent systems of EtOH:H2O at room temperature. Different mass of F127 were completely dissolved in fixed volumes of the solvent systems according to Table 3.2. The conductivity of the solutions was measured and a plot of conductivity versus F127 concentration indicated the CMC of the surfactant ideal for the solvent system Appendix A1.
Table 3.2 Critical micelle concentration experimental compositions.
F127 (g) 2.50 5.00 7.50 10.00 12.50 15.00 17.50 20.00 22.50 25.00 Mass of solvent (g) 97.50 95.00 92.50 90.00 87.50 85.00 82.50 80.00 77.50 75.00
Solvent ratio (g)* 50:50 60:40 60:40 80:20 80:20
*Solvent system was made of H2O:EtOH.
61 3.3.4.1 Determination of CMC from Titanium Isopropoxide (0.005 moles) with
[15%] and [25%] F127
After considering the CMC concentration of pluronic F127, the effect of TIP concentration was monitored. In the experiments, 15.00 g and 25.00g of F127 were weighed into respective 400 mL beakers. Ethanol (50.00 g) was added to each beaker and both beakers where treated in an ultrasound probe for 20 minutes and stirred for 10 minutes until the surfactant was fully dissolved. About 1.26 g of TIP was slowly added to the respective solutions and stirring was continued for 10 minutes. De-ionized water (50.00 g) of pH 1.09 was added drop-wise to the respective solutions and stirring was continued for another 1 hour before the respective mixtures were left to stand and age in air without stirring for 24 hours. The respective mixtures were transferred to an oven set at 100 ˚C for 6 days. The samples were then heat treated under Argon flow at 400 ˚C for 6 hours with a heating rate of 1 ˚C min-1. They were further calcined in air at 400 ˚C for a period of 8 hours with a heating rate of 1 ˚C min-1.
The mole ratio of the sol-gel solution of [15%] w/w surfactant was 0.0012:0.005:1.085:2.778 and of the mole ratio of the [25%] w/w surfactant was 0.0020:0.005:1.085:2.778 (F127:TIP:EtOH:H2O).
3.3.4.2 Determination of Titanium Isopropoxide (0.015 moles) with [15%] and [25%] F127
Similar methods as in above were followed to prepare TiO2 using higher concentrations of TIP.
The differences in the experiments were the mole compositions of the solutions. The mole ratio of the sol-gel solution for the [15%] w/w surfactant was 0.0012:0.015:1.085:2.778 and that of the [25%] w/w surfactant was 0.0020:0.015:1.085:2.778 (F127:TIP:EtOH:H2O). After stirring the respective beakers of [15%] and [25%] of F127 solution mixtures for 10 minutes, a slow addition of 3.78 g TIP was made to each solution. The solutions were stirred for a further 10 minutes before an addition of the de-ionized water (pH 1.06) was made. After stirring for 1 hour, the solutions were left to age in air. Similar aging and thermal treated as those in above (section 3.3.4.1) were considered for these samples.
62 The materials prepared from using the [15%] and [25%] of F127 with 0.005 and 0.015 moles of TIP were characterized and the method yielding desirable properties of mesoporous TiO2 identified and these method was employed for the synthesis of the actual TiO2 catalysts.
3.3.5 Synthesis of Mesoporous TiO2 Nanoparticles Catalyst
In the actual catalyst synthesis, 15.01 g of F127 was weighed into a 400 mL beaker. To the beaker 50.00 g of ethanol was added and the mixture was treated with an ultrasonic probe for 20 minutes and stirred for 10 minutes. 1.26 g TIP was slowly added into the solution and stirring was continued for a further 30 minutes. De-ionized water solution (50.00 g) with pH 1.09 was added drop-wise to the stirring solution. Magnetic stirring was continued for a further 1 hour before the mixture was left to stand and age in air without stirring. The mole ratio of the sol-gel solution was 0.0012: 0.005: 1.085: 2.778 (F127: TIP: EtOH: H2O). The solution was left to age at room temperature for 24 hours, and then transferred to age in an oven set at 100 ˚C for 6 days.
The samples were transferred into a crucible quartz boat and then fixed in a glass quartz reactor tube for calcination experiments. The samples were then heat treated under Argon flow at temperatures of 400 ˚C for 6 hours with a heating rate 1 ˚C min-1, then further calcined in air at 400 ˚C for 8 hours at 1 ˚C min-1 heating rate.
3.3.6 Synthesis of TiO2 Nanoparticles Supported on Different Materials
The sol gel method was employed for the synthesis of all titania supported composites. In a typical synthesis, 15.00 g of F127 surfactant was mixed with 0.80 g of the supporting material (i.e CNTs, SBA-15 or 30 wt. % SBA-CNTs) and 50.00 g of ethanol in a 400 mL beaker. The mixture was treated with an ultrasonic probe for 20 minutes before they were stirred for 10 minutes. Thereafter, 0.44 g of TIP (to yield 10 wt. % TiO2) was added in a drop-wise manner.
The amount of TIP to be added was determined from a series of calculations (appendix A 1.2).
50.00g de-ionized water of pH ±1.05 was added drop-wise into the solution mixture after 30 minutes. Stirring was continued for a further 1 hour and the solution was left to age at room temperature for a period of 24 hours, then transferred to age in an oven set at 100 ˚C for 6 days.
63 The same methodology was employed under the same conditions for the synthesis of the 5 and 20 wt. % TiO2/SBA-CNT composites. The variation in the synthesis method was in the weight of the TIP solution that was added into the mixture. Respectively 0.26 g and 0.82 g of TIP were added drop-wise for the 5 and 20 wt. % TiO2 composites respectively.
It is important to note that the mass of TIP calculated was added with 0.110 g of TIP during synthesis because TIP sticks to the walls of apparatus used for measurements and transferring.
This addition was thus done to account for any TIP precursor which might be lost to the walls of the 50 mL beaker used to weigh it in and the pasteur pipette used to add it into the respective solutions.
After aging the respective samples, they were heat treated under Argon flow at 400 ˚C for 6 hours with a heating rate of 1 ˚C min-1, then further calcined in air at 400 ˚C for 8 hours at a heating rate of 1 ˚C min-1. Figure 3.7 illustrates the muffle furnace set up employed during heat treatment of materials in an argon environment. The materials were synthesized in triplicate.
Figure 3. 7 labelled photograph of the experimental setup used for the heat treatment of materials in an inert environment using a tube furnace (LABOFURN).
Gas inlet
Muffle furnace
Temperature Programmer
Gas outlet
64