The photocatalytic reaction was further improved by using the diluted CO2 (20 %) in feed to increase the concentration of H2O vapors in the reaction. The optimization step improved the fuel products; however, optimizing the factors, which affect the photocatalytic activity, further increased the photocatalytic efficiency. These factors including UV light sources and their intensities, H2O/CO2 ratios, various catalyst shapes used (powder, pellets, film) along with water vapor play a crucial role for CO2 reduction therefore the water vapor effect as well as the catalyst activation and regeneration is taken into observation. These factors are briefly explained below.
7.1.7.1 Different UV light sources (200W, 300W)
The main factor that directly affects the initiation of a photocatalytic reaction is light, either it is UV or solar light. In photocatalytic reactions, UV lights are of common sources of illumination and usually provide 100-400 nm range of wavelength. Moreover, usage of light with
Chapter 7
125 a shorter wavelength for irradiation, is definitely more effective but not economical. Therefore, the selection of the optimum light for a specific semiconducting material in a photocatalytic reaction is of vital interest. Two different types of UV lamps with similar UVA and UVB ranges but different in intensities and temperatures and with powers of 200 and 300 W have been exploited in the current work to explore and optimize the effect of light on the catalytic activity.
The difference in the production rates and a comparison of the average production rates of the products obtained from optimized Ti-KIT-6-calcined (Si/Ti=100) photocatalyst is shown in Fig.
7.10. The results indicate that the 300 W lamp is more useful in this reaction than the 200 W lamp. The 300 W lamp has more photon power, more UVB intensity and produces a higher temperature, which boosts the CO2 reduction path. Therefore, more fuel products are produced when the 300 W lamp is used than that of 200W lamp.
Figure: 7.10 The UV light source effect on the activity of the optimized
Ti-KIT-6-calcined (Si/Ti=100) photocatalyst: (a, c) 200 W UV lamp, (b, d) 300 W UV lamp,
at standard operating conditions with 20 % CO
2, 0.2 g of photocatalyst, 50 mL/min
flow rate, H
2O/CO
2=0.1.
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126
7.1.7.2 Various UV light intensities effect
Another factor that affects the reaction is the light intensity. As the light intensity increases, the number of generated photons increases, and this leads to more electron-hole pairs being excited with high energy [258] The effect of light intensity on the production rate and average production rate of the products via optimized Ti-KIT-6-calcined (Si/Ti=100) photocatalyst is shown in Fig. 7.11. Three different distances (70, 35 and 10 cm) have been investigated between the lamp and the catalyst in the reactor to acquire different light intensities.
As shown in Fig. 7.11a and d, products formation is lower at a distance of 70 cm. Moreover, the reaction kinetics is slower, due to the low light intensity. However, as can be observed in Fig.
7.11b and e, when the distance is reduced to 35 cm, the light intensity increases and the production rate increases remarkably with faster reaction kinetics and long-term product formation stability.
Figure: 7.11 Effect of different UV light intensities on the activity of the optimized
Ti-KIT-6-calcined (Si/Ti=100) photocatalyst: (a, d) 70 cm, (b, e) 35 cm, (c, f) 10
cm, at standard operating conditions with 20 % CO
2, 0.2 g of photocatalyst, 50
mL/min flow rate, H
2O/CO
2=0.1, 300 W UV lamp.
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127 Fig. 7.11c and f shows that, as the distance is further decreased to 10 cm, the product formation increases and very fast reaction kinetics occurs at the beginning of the reaction process, which is due to the increased light intensity. However, in this case, after 3 h of illumination, the reaction shows a deactivation trend, which could be caused by the rapid saturation of the active sites, due to the fast reaction kinetics and increased temperature from the light.
7.1.7.3 Different H
2O/CO
2ratios
H2O is very crucial component in the CO2 reduction process but an appropriate amount of H2O with CO2 could enhance the photoactivity to obtain the photocatalytic products. However, recent research indicates that H2O vapors have shown a green chemistry approach towards CO2
reduction to fuels; however, reaction is mainly dependent on the H2O/CO2 molar ratio [259].
Figure: 7.12 Effect of different H
2O/CO
2ratios on the activity of the optimized
Ti-KIT-6-calcined (Si/Ti=100) photocatalyst: (a) 0.1, (b) 0.2, (c) 0.3 (d) 0.4, at
standard operating conditions with 20 % CO
2, 0.2 g of photocatalyst, 50 mL/min
flow rate, 300 W UV lamp.
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128 The effect of different H2O/CO2 ratios on the activity of the optimized Ti-KIT-6-calcined (Si/Ti=100) photocatalyst is shown in Fig. 7.12. Different ratios were explored to achieve the best ratio to maximize the fuel products. Fig. 7.12a-d indicates that CO2 conversion increases as the H2O/CO2 ratio increases from 0.1 to 0.3, whereas, an excessive amount of H2O (H2O/CO2=0.4) has comparatively suppressed the overall reaction, which is in agreement with the literature [155].
7.1.7.4 Various shapes of photocatalysts (powder, pellets and thin film)
Another key parameter to be optimized is the shape of a catalyst that affects product formation as is the potential use of the catalyst for commercial applications. [260, 90]
Photocatalytic activity of CO2 redution towards fuel formation by powder form catalyst is very lower than that of pellet and thin film. The powder form of the catalyst has been compared with that of pellets (500 micron) and thin film, as shown in Fig. 7.13a-f. The pellets and thin film showed comparatively higher activity than the powder shape of the photocatalyst, which is in accordance with the literature [90, 151]. In pellets and film shapes of the photocatalyst, the better exploitation of the surface that is available for the adsorption of CO2 and H2O, and the better penetration of the UV light, which is necessary to initiate a photocatalytic reaction are the main causes of the superior activity. Moreover, better reaction kinetics and better long-term photocatalytic stability have also been observed for the pellets and thin film than for the powder as well as economical point of view, pellets and thin films are more favorable than powder form.
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129
Figure: 7.13 Effect of different catalyst shapes of the optimized Ti-KIT-6-calcined (Si/Ti=100) photocatalyst on the activity: (a, d) powder, (b, e) pellets, (c, f) film, at standard operating conditions with 20 % CO
2, 0.2 g of photocatalyst, 50 mL/min flow rate, H
2O/CO
2=0.1, 300 W UV lamp.
7.1.7.5 Confirmation of water vapor effect on the activity and catalyst deactivation/regeneration
Water vapors have a significant role in CO2 photocatalytic reaction to fuel products. This essential role of water vapor has been confirmed, which is shown in Fig. 7.14. This indicates that the reaction could not pursue in the absence of water vapors as well without UV light. Moreover, as shown in Fig. 7.14, the color of the photocatalyst changed and a slight photocatalytic deactivation was observed. The deactivation mechanism is not still clear, but could be due to the partial saturation of the active sites for adsorption, on the photocatalyst surface, with intermediate product/by-product/mixture formation. Moreover, after formation, some products as CH3OH and CO were supposed to be not desorbing fully from the catalysts surface and could be re-oxidized back into CO2 through the O2 produced during reaction [100]. The deactivated catalyst was naturally dried for 24 h. After the regeneration, this acquired again more than 90 % of the activity, which indicates that most of the adsorbed species were desorbed away. However,
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130 for a more clear understanding about the deactivation, a separate in-depth study is recommended to explore the exact nature and the causes of these deactivating species.
Figure: 7.14 Confirmatory test of effect of water vapor on the activity of the optimized Ti-KIT-6-calcined (Si/Ti=100) photocatalyst, and catalyst deactivation/
regeneration.
7.1.7.6 Stability test of optimized photocatalyst
The final stability test of the optimized Ti-KIT-6-calcined (Si/Ti = 100) photocatalyst under the optimized reaction conditions such as with 0.2 g pellets, with 20% CO2, 50 mL/min flow rate, H2O/CO2 = 0.3, and 300W UV lamp with 35 cm distance was performed to observed the stability of the catalyst for extended reaction time of 10 h instead of 5h of the normal reaction time. It has shown in Fig. 7.15, that there is a slow deactivation trend than the reaction under the standard operating conditions. This indicates that the deactivation is not only due to a single factor but various another reaction conditions participate to deactivate the catalyst. Therefore, it
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131 is not yet clear that which factors contribute in the deactivation process. However, a depth-study for understanding the phenomena should be under consideration.