PHOTOCATALYTIC ACTIVITY EVALUATION

2.4.1 Photocatalytic Water Oxidation and Reduction

The photocatalytic oxygen evolution reaction from water was carried out in a 100 ml two neck double-walled quartz round-bottomed flask with a water circulation through the outer jacket to maintain the temperature within the reactor. The necks of the reactor were sealed with rubber septum to prevent leakage of the gas produced. The reaction conditions were

Chapter 2 Anindya Sundar Patra maintained at room temperature by continuous water circulation. A 500W tungsten halogen lamp was used as the light source to conduct the photocatalytic reaction. The emission profile of the lamp was confined in between the wavelength ranges 195 – 1100 nm. In a typical photocatalytic experiment, 25 mg of catalyst was dispersed in 50 mL of water (in case of La1−xSrxMnO3 (x = 0.0 – 0.5) photocatalysts, we have used 100 mg of catalyst in 25 mL of water).

A 100 mL two-neck double walled quartz round bottomed flask was used for all the photocatalytic H2 evolution reactions. The reactor was sealed with rubber septum to prevent gas leakage. To conduct the photocatalytic reaction, a 300 W tungsten halogen lamp was used as the light source with emission profile in between the wavelength ranges 195 – 1100 nm.

In every photocatalytic experiment, typically 25 mg of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixture as a sacrificial reagent for hole scavenging was dispersed in 50 mL of water using a magnetic stirrer.

The reactor was purged with nitrogen for 10 minutes at a flow rate of 200 mL per minute monitored by a rotameter and consequently, the system was evacuated by a vacuum pump in order to remove the dissolved oxygen and any other gases from inside the reactor. This process was repeated several times before irradiating the system by a tungsten halogen lamp placed 15 cm away from the reactor. During the irradiation, the water suspension was constantly stirred to confirm uniform exposure of the catalyst to the light source. The gas sample was collected every 15 minutes up to 1 hour by a 1mL gastight syringe and was analyzed by gas chromatography, using a thermal conductivity detector (TCD), Molesieve column with nitrogen as the carrier gas. In case of La1−xSrxMnO3 (x = 0.0 – 0.5) photocatalysts, we have purged N2 gas for 30 minutes and collected the evolved gas by inverted burette method. No considerable amount of gas evolution from the photoreactor was observed in absence of either photocatalyst or light irradiation which confirms the role of the photocatalyst in water oxidation/reduction.

The apparent quantum yield (AQY) of the photocatalysts were measured under the same reaction condition and it was calculated using the following equation: ⁴

AQY = Number of reacted electrons

Number of incident photons × 100%

= Number of moles of O₂/2 × Number of moles of H₂ produced in 1 hour

Number of incident photons in 1 hour × 100%

Anindya Sundar Patra Chapter 2

31 2.4.2 Schematic Diagram of Used Photocatalytic Reactor Setup

2.4.3 Photocatalytic Methyl Orange (MO) Dye Degradation

The dye degradation efficiency of the catalysts was analyzed by the monitoring the absorbance of MO dye solution. The experiments were carried out in the same reactor in which photocatalytic water oxidation was performed. MO solution at an acidic pH of 2.5 was made by adding the calculated amount of concentrated hydrochloric acid to it. A 50 mL of 10^-5 M of prepared MO solution was taken inside the reactor and 50 mg of catalyst was added to it and stirred continuously for 15 minutes in dark to attain the adsorption-desorption equilibrium. Hence, the dye mixture was illuminated by the aforesaid 500W tungsten halogen lamp from a distance of 15 cm. Continuous water circulation through the outer jacket of the reactor was carried out to maintain the reaction temperature at room temperature. After illuminating the reactor, 1 ml of the dye solution was taken out from the reactor in every 5 minutes and centrifuged to settle down all the catalyst particles and the dye solution was further filtered through a 0.45µm syringe filter. The electronic absorption spectra of the supernatant dye solution were recorded in the range of 200 – 800 nm. The photocatalytic degradation efficiency was calculated as follows,

Efficiency(%) = (C₀− C)

C₀ × 100

Where, C0 is the initial MB dye concentration and C is the MO dye concentration in the filtrates at certain time after light irradiation. ⁵

1 10

2 8

5 6

7 9

4 3

1. N₂cylinder 2. Rotameter

3. Double walled reactor 4. Photocatalytic suspension 5. Magnetic stirrer 6. Cooling water inlet 7. Cooling water outlet 8. Light source 9. Temperature sensor 10. Gas chromatograph

Chapter 2 Anindya Sundar Patra 2.4.4 Photocatalytic Methylene Blue (MB) Dye Degradation

Photocatalytic methylene blue (MB) dye degradation experiments were performed in a 100 mL round-bottom quartz flask by illuminating with a 300 W tungsten–halogen lamp kept 15 cm away from the reactor. 50 mg of the photocatalyst was dispersed in 50 mL of 10^-5 M aqueous MB solution for the photocatalytic dye degradation analysis. The pH of the dye solution was adjusted to 13 by adding required amount of aqueous NH3 solution. In order to achieve adsorption-desorption equilibrium among the dye, catalyst particles, dissolved oxygen, and atmospheric oxygen, the mixture was stirred for 30 minutes in the dark. During 1 h light irradiation, 2 mL of solution was collected from the photo-reactor in every 15 minutes. The collected dye solutions were centrifuged for 5 minutes to settle the suspended photocatalysts. The electronic absorption spectra of the supernatant dye solution were recorded in the range of 200 – 800 nm. By monitoring change in absorbance at 664 nm, the degradation of MB dye was determined. The photocatalytic degradation efficiency was calculated as follows,

Efficiency(%) =(C₀− C)

C₀ × 100

Where, C0 is the initial MB dye concentration and C is the MB dye concentration in the filtrates at certain time after light irradiation. ⁵