6. Photocatalytic degradation under visible light irradiation of oxalic acid by TiO 2
6.2.1 Experimental procedure for oxalic acid
Concentration of oxalic acid is determined by titration method with potassium permanganate (Figure30). The permanganate solution was prepared as 0.02 equivalent per liter of KMnO4. 150 mg of TiO2 sample was weighed and mixed with 200 ml of oxalic acid solution (0.0005 mol/L) in a beaker whose wall was covered by aluminum foil to avoid the influence of surrounding room light on photocatalysis experiments.
The initial concentration of oxalic acid used in the present study was 45.015 ppm
Figure 30. Scheme of the experimental setup for titration.
The solution with TiO2 was then stirred for 1 hour at stirring rate of 500 rpm. After mixing, first sample was taken by a syringe and filtered out TiO2 particles through a 0.45 µm syringe filter (Acrodisc® PSF Syringe Filters). Then the TiO2 solution was kept continuously under the visible light irradiation (AULTT® CEL-HXF300), as shown in Figure 31. Three samples were taken at 1, 2, and 4 hours during the photocatalysis.
Figure 31. Scheme of the experimental setup of the visible light irradiation device AULTT® CEL-HXF300.
After filtration, 25 ml clear solution was obtained and mixed with 1 ml sulfuric acid and 50 ml water. The mixed solution was heated to 70-80 °C since the reaction is slow below 60 °C [75]. Then the titration was carried out with the KMnO4 solution.
The reaction is autocatalytic: the first drop of KMnO4 to the titrated sample reacts very slowly, this step has to be handled carefully. Before adding a new permanganate solution dose as a droplet, the colour of the solution should totally disappear. When the colour of the solution remained pink longer time than 30 seconds, the titration was finished and the volume of consumed KMnO4 solution was determined.
6.2.2 Experimental procedure for oxalic acid
TiO2 samples of 150 mg were weighted in 200 ml beaker which were wrapped in aluminium foil to isolate the beaker better for the visible light measurements with the source device. Then 200 ml of formic acid solution (0.0217 mmol/L) was added to the beaker. The initial concentration of oxalic acid used in the present study is 10 ppm.
The solution with TiO2 was then stirred for 1 hour under stirring rate of 500 rpm.
After mixing, first sample was taken by a syringe and TiO2 particles was filtered out
solution was kept continuously irradiating under the visible light. Four samples were taken every 30 minutes under the visible light irradiation. Each irradiation experiment lasted totally 2 hours. After filtering, the concentration of formic acid was analyzed by Ion Chromatography (Dionex ICS-1100).
6.3 Results and discussion
6.3.1 Results and discussion for oxalic acid
The oxalic acid degradation rates with different TiO2 samples obtained by sol-gel method are shown in Figure 32.
Figure 32. Degradation rates of oxalic acid for the TiO2 sample made in the present study.
Degradation of oxalic acid with the commercial TiO2 were carried out twice. Based on the results shown in Figure 32, there was no exact trend for degradation of oxalic acid. The fastest degradation rate was performed with the commercial TiO2. After 4 hours degradation under the visible light the concentration of oxalic acid was 2.88 ppm. For produced TiO2, the fastest degradation rate of oxalic acid was obtained with the sample produced without PEF. It seems that the pulsed electric field does not have strong effect on oxalic acid degradation for the TiO2 samples precipitated in the
minutes. After 4 hours irradiating under visible light the concentration of oxalic acid with TiO2 sample calcined at 400 °C and 500 °C were 42.49 ppm and 43.21 ppm, respectively. Therefore, the degradation rate of oxalic acid with the sample calcined at 400 °C was faster than the sample calcined at 500 °C. The specific surface area for the sample calcined at 400 °C and 500 °C were 41.53 m2/g and 25.10 m2/g, 963 Hz 12 min 500 °C (43.21 ppm). As shown in Figure 32, the effects of calcination temperature on the degradation of oxalic acid were not significant.
Figure 33 shows the degradation rate of oxalic acid of TiO2 particles which made in the previous study carried out by Mr. Mäkinen. It can be seen that the initial oxalic acid degradation rates were generally higher for samples that were produced under the influence of pulsed electric field.
In figure 33 it presented that the sample with PEF 50 Hz 24 minutes, the result of oxalic acid degradation rate by photolysis is lower than the result of oxalic acid degradation rate with sample PEF 949 Hz 12 min. The disappearance of oxalic acid that was observed under visible light conditions the TiO2 was more active due to photolysis and photocatalytic process.
Except of commercial TiO2 samples, the fastest degradation rate for oxalic acid is the sample 50 Hz 24 minutes. The degradation rate with sample of 949 Hz and 12 minutes was also obviously fast. The photocatalytic activities of TiO2 samples vary in the following order: Commercial powder (2.88 ppm) > PEF 50 Hz 24min (12.60
ppm) > PEF 949 Hz 12 min (25.92 ppm) > PEF 294 Hz 24 min (30.61 ppm) > PEF 949 Hz 24 min (37.81 ppm) >No PEF (39.61 ppm) > PEF 294 Hz 12 min (40.33 ppm) > PEF 50 Hz 12 min (40.69 ppm).
Figure 33. Degradation rates of oxalic acid for the TiO2 sample made in the previous study.
To compare the TiO2 particles of previous study (Mäkinen) and current study, Figure 34 shows the results of the combination of Figure 32 and 33.
In Figure 34, TiO2 particles made in the current study and influenced by pulsed electric field via sol-gel method are named “New TiO2”. By comparing TiO2 samples produced in the current study to the previous study of Mäkinen, the fastest degradation was obtained with the sample made without pulsed electric field in the current study made. On the other hand, the concentration of oxalic acid was still higher than the third fast in previous study of Mäkinen. It seems that TiO2 produced in current study the photocatalysis was not that active than in previous study made.
The photocatalytic activities of TiO2 samples varied in the following order:
Commercial powder (2.88 ppm) > PEF 50 Hz 24 min(12.60 ppm) > PEF 949 Hz 12 min (25.92 ppm) >New No PEF (36.01 ppm) > PEF 949 Hz 24 min (37.81
ppm) >New PEF 294 Hz 24 min (38.89 ppm) > New PEF 50 Hz 24 min (39.61 ppm) >
Figure 34. Degradation rate of oxalic acid for the TiO2 samples made in the previous study by Mäkinen and the present work.
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6.3.2 Result and discussion for formic acid
Degradation of formic acid by TiO2
The degradation rates of formic acid with different TiO2 samples which made by current study using pulsed electric field via sol-gel method are shown in Figure 35. In order to compare the photocatalytic activity of prepared materials with standard, commercial TiO2 powder Aeroxide® (Evonic Degussa GmbH) was used. Also considering the effect of calcination temperature, two temperatures (400 °C and 500 °C) were tested for sample 963 Hz 12 min. Calculation of initial formic acid degradation rates is shown in Appendix II.
Figure 35. Formic acid degradation rates for TiO2 particles current study made.
By using sample with PEF 963 Hz 12 minutes, TiO2 was more active under the visible light. This means that the initial concentration of formic acid was higher than after visible light treatment with sample by using PEF 963 Hz 12 minutes. With commercial TiO2 powder, the concentration of formic acid was almost zero after 2 hours irradiating of visible light. With low frequency long pulsed electric field treatment time such as sample 50 Hz 24 minutes, the degradation rate is the secondly highest. If two samples which have different calcination temperature (400 °C and
formic acid concetration/ppm
Time/h
500 °C) are only compared, the degradation rate for sample at 500˚C was higher than that at 400°C.
According to Figure 35, all of the TiO2 samples produced with pulsed electric field via sol-gel method have significantly lower initial formic acid decomposition rates than the commercial TiO2 powder. The photocatalytic activities of TiO2 samples vary in the following order: Commercial powder (0 ppm) > PEF 963 Hz 12 min (1.14 results obtained by current made TiO2 particles. Either high frequency short treatment time (949 Hz 12minutes) or low frequency long time (50 Hz 24min) can increase the degradation rate of formic acid.
Figure 36. Formic acid degradation rates for TiO2 particles made in previous study.
0
formic acid concetration/ppm
Time/h
The photocatalysis rates with TiO2 samples produced in previous study by pulsed electric field assisted sol-gel method and without PEF impact vary in the following order: Commercial powder (0ppm) > PEF 949 Hz 12 min (0.40 ppm) > PEF 50 Hz 24 min (0.35 ppm) > PEF 294 Hz 12 min (3.61 ppm) > PEF 50 Hz 12 min (4.12 ppm) >
PEF 949 Hz 24 min (4.89 ppm) > PEF 294 Hz 12 min (5.28 ppm) > No PEF (5.95 ppm).
Comparison of degradation results of formic acid for previous study made TiO2
particles and current study made TiO2 particles is shown in Figure 37.
Figure 37. Comparison of formic acid degradation rates for TiO2 particles by current study made and previous study made.
0
formic acid concetration/ppm
Time/h
In Figure 37, TiO2 particles made by current study using pulsed electric field via sol-gel method are named “New TiO2”. One sample (50 Hz 24minutes) was made without adjusting pH. It means that only distilled water (18 MΩ cm-1) was added during the TiO2 synthesis process. The results show that the degradation rate of formic acid with the sample without adjusting pH was lower than the sample when pH was adjusted. It can be seen from Figure 37 that the degradation rates of formic acid with sample 949 Hz 12 minutes and sample 50 Hz 24 minutes were the highest in previous study. Moreover, formic acid degraded by the samples 963 Hz 12 minutes and 50 Hz 24 minutes made faster than the samples made with other conditions in current study.
It can be concluded that higher degradation rate of formic acid under the visible light was obtained either by high frequency short treatment time or low frequency long
Figure 38. Comparison of degradation rate of formic acid for the TiO2 samples made in the previous study by Mäkinen under the UV-light and the present work under the visible light.
Figure 38 shows the comparison of degradation of formic acid with the samples made in the previous study by Mäkinen under the UV-light and current study under the visible light. According to the comparison shown in Figure 38, it can be clearly seen that the degradation rates of formic acid were much faster with the UV light than visible light. Under the influence of pulsed electric field, formic acid was degraded fastest with the sample 949 Hz 24 minutes under the UV-light. The degradation rate was 2.25 ppm/min. However, the fastest degradation rate of formic acid was only 0.37 ppm/min with the sample 49.9 Hz 24 minutes under the visible light.
Degradation of formic acid by copper doped TiO2
Some Cu doped TiO2 nanoparticles were also tested with pollutant formic acid. The Cu doped TiO2 nanoparticles were synthesized with different concentration of Cu (5%, 9% and 13%) by sol-gel method. Firstly 5 ml titanium isopropoxide was taken and mixed with 25 ml 2-proponal. Copper chloride (CuCl2) and distilled water were used to prepare copper chloride solutions. These solutions were mixed under constant stirring rate. Ammonium hydroxide was used to make the pH of solution as 7. Then
0
degradation of formic acid,ppm/min
UV-‐light visible light
the mixed solution was stirred using a magnetic stirrer at ambient temperature for 6 hours. After 6 hours, the white and gray gels were obtained. Then the ageing process started. The gels were centrifuged and washed several times with deionized water and ethanol to remove nitrate impurities. The precipitates were dried at 100 ºC for 8 hours followed by calcination at 450 ºC for 3 hours.
Since there is only a small amount of Cu doped TiO2 nanoparticles, 100 mg samples were weighted and put into 200 ml of formic acid solution (0.0217 mmol/l). The degradation rate of formic acid as a function of time is shown in Figure 39.
In order to do comparison, commercial TiO2 powder and one sample produced under the influence of PEF which also has highest degradation rate (949Hz 12minutes) were selected for further studies. The same TiO2 sample amount of 100 mg was used for the photocatalysis experiments. The degradation rate of formic acid decreased with the increase of the content of Cu. The concentration of formic acid was even higher at 3 hours than that at 2 hours. It seems that Cu doped TiO2 does not have significant effect on formic acid degradation. The photocatalytic activities of Cu doped TiO2
nanoparticles vary in the following order: Commercial TiO2 powder (0 ppm) > PEF 949 Hz 12 min (2.66 ppm) > No Cu doped (36.87 ppm) > 5 % Cu doped (9.15 ppm) >
Formic Acid concentration/ppm
Time/h
7. Conclusion
In the present study, titanium dioxide particles were produced using sol-gel method under the influence of pulsed electric field. The aim was to study the visible light irradiation in photocatalysis processes when the sol-gel method of TiO2 nanocrystals were affected by pulsed electric field. It was expected that specific surface area, calcination temperature and pH had influence on the photocatalytic activities of produced crystals.
Different pulse frequencies and treatment times were used to investigate the influence of PEF. The pulsed electric field experiments were carried out with three different between specific surface area and frequency of the pulsed electric field nor between specific surface area and the treatment time of pulsed electric field. With short treatment time and low frequency of pulsed electric field treatment, the specific surface area could be increased from 29 m2/g (PEF 50 Hz 12 minutes) to 34 m2/g (PEF 963 Hz 12 minutes). Comparing the TiO2 particles which were made in previous study by Mäkinen with pulsed electric field via sol-gel method, the specific surface area was also increased. It increased from 21 m2/g (PEF 50 Hz 12 minutes) to 39 m2/g (PEF 949 Hz 12 minutes). When the time of pulsed electric field treatment was longer, the specific surface area did not change significantly. It was from 32.87 m2/g (PEF 50 Hz 24 minutes) to 32.23 m2/g (PEF 963 Hz 24 minutes). Thus for the TiO2 particles made in previous study by Mäkinen it decreased from 46 m2/g (PEF 50 Hz 24 minutes) to 23 m2/g (PEF 949 Hz 24 minutes). This is notable because specific surface area is one of the key factors that determine the photocatalytic performance of a
photocatalyst. Based on the experimental results, it could be seen that degradation rates of the oxalic acid and formic acid were faster with high specific surface area under the visible light irradiation, especially for formic acid.
According to the experimental results, it was also noticed that the calcination temperature in the sol-gel process has a significant effect on photocatalytic performance. From low temperature (400 °C) to high temperature (500 °C), the specific surface area of TiO2 precipitated decreased from 41 m2/g to 25 m2/g. The results for degradation rate of oxalic acid and formic acid are different. After 4 hours irradiating the concentration of oxalic acid with the samples calcined at 400 °C and 500 °C were 42.49 ppm and 43.21 ppm, respectively. So the degradation rate of oxalic acid calcined with lower temperature (400 °C) was faster than the sample obtained at higher calcination temperature (500 °C). But for the concentration of formic acid, with 400˚C calcination was 4.51ppm and 500˚C calcination was 4.18 ppm. So the degradation rate was opposite with oxalic acid. The higher calcination temperature (500 ˚C) was faster than low calcination temperature (400 ˚C).
Produced particles were used as photocatalysts for the degradation of oxalic acid and formic acid under the visible light irradiation. Degradation rate of oxalic acid and formic acid was chosen to be the indicator of photocatalytic performance. The photocatalytic activities of samples produced by the sol-gel method were also compared to photocatalytic activities of the TiO2 particles made in previous study at the same conditions. Moreover, the photocatalytic activity of commercial TiO2 was also carried out to make a reference for comparison.
For oxalic acid, results from the photocatalytic experiments show that pulsed electric field did have an effect on the photocatalytic activity of TiO2 particles. By using pulsed electric field via sol-gel method to made TiO2 particles, under the visible light treatment to degraded oxalic acid. The degradation rate of oxalic acid with the sample
made in current study was not obvious compared with the sample made in previous study. In the current study, the fastest degradation was obtained using 294 Hz 24 minutes (38.89 ppm) treatment, the second fastest was gotten with the sample 50 Hz 24 minutes (39.61 ppm). Moreover the degradation rate for TiO2 particles which made by previous study was much more active. Low frequency long treatment time (PEF 50 Hz 24 minutes) or high frequency short time treatment time (949 Hz 12minutes) was appropriate to produce TiO2 particles which have higher photocatalytic activity under visible light. Specific surface area was higher for samples precipitated with either low frequency long time or high frequency short time.
The degradation rate of formic acid was generally higher for samples that were produced under the influence of pulsed electric field. TiO2 particles were more active for formic acid than oxalic acid under the visible light. According to the results of TiO2 particles made using pulsed electric field via sol-gel method both in current and previous studies, the best condition was obtained by high frequency short time (PEF 963 Hz, 12 min), and the second one with the lowest frequency and longer time (PEF 50 Hz, 24 min). The higher photocatalytic activities of samples that were produced under the influence of pulsed electric field are attributed to larger specific surface areas, especially for formic acid degradation. Moreover, there was one more sample (50 Hz 24 minutes) which was prepared without adjusting pH. The result of degradation was not satisfactory compared to the sample when pH was adjusted.
Four different TiO2 particles with concentration of Cu (0 %, 5 %, 9 % and 13 %) were tested under the visible light irradiation to investigate the photocatalytic activities of formic acid. Results show that without Cu doped TiO2 particles had the highest degradation rate of formic acid compared with copper doped TiO2 samples. Moreover, the degradation rate decreased with the increase of the content of copper in TiO2
samples. Thus copper doped TiO2 particles are not suitable for the degradation of formic acid.
For both oxalic acid and formic acid the commercial TiO2 powder had higher photocatalytic activities compared to the samples that were produced in the current work. It can be explained that the commercial TiO2 has the highest specific surface area which is 45 m2/g. The specific surface areas of samples produced with sol-gel method were in the range of 16 – 40 m2/g. Based on the literature, the commercial TiO2 is a mixture of rutile and anatase crystallites. Crystallite mixtures can have an effect and thus increase photocatalytic activity. Therefore, the commercial TiO2
particles are more active since it has higher specific surface area [75].
Further studies may focus on investigations on the effect of pulsed electric field on TiO2 crystal structures via sol-gel method since different polymorphs have different effects on photocatalytic activities of pollutants.
8. References:
[1] Pelizzetti, E., Serpone, N. 1986. Homogeneous and Heterogeneous Photocatalysis, D. Reidel Publishing Company, Dordrecht,
[2] Serpone, N., Pelizzetti, E. 1989. Photocatalysis—Fundamentals and Applications, John Wiley & Sons, New York.
[3] Kamat, P.V. 1993. Photochemistry of nonreactive and reactive (semiconductor) surfaces. Chemical. Review, vol. 93, pp. 267.
[4] Elvers, B., Hawkings, S. 1996, Ullmann's Encyclopedia of Industrial Chemistry.
5th ed., vol. 27, Weinheim: Wiley-VCH.
[5]Mikhaylin, S., Nikonenko, V., Pourcelly, G. & Bazinet, L. 2014. Intensification of demineralization process and decrease in scaling by application of pulsed electric field with short pulse/pause conditions. Journal of Membrane Science，vol. 468, pp.
[6] Mäkinen, S. 2014. Pulsed electric field assisted sol-gel preparation of TiO2
particles and their photocatalytic properties, Master’s thesis, LUT.
[7] Hashimoto, K., Irie, H. 2005. TiO2 photocatalysis: a historical overview and future prospects, Japanese Journal of Applied Physics. pp. 8269-8285
[8] Yin, W., Chen, S., Gong, X., Yan. 2010. Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction. Applied Physics Letters, vol. 96, pp.
221-223
[9] Fujishima, A, Honda, K. 1972. “Electrochemical photolysis of water at a semiconductor electrode, International Journal of Photoenergy, vol. 238 pp. 37-38.
[10] Zhang, H., Banfield, J.F. 2000. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: Insights from TiO2.
[10] Zhang, H., Banfield, J.F. 2000. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: Insights from TiO2.