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Photoproduction of iodine with nanoparticulate semiconductors and insulators.

Karunakaran C, Anilkumar P, Gomathisankar P - Chem Cent J (2011)

Bottom Line: Their optical edges have been obtained by UV-visible diffuse reflectance spectra.The photocatalytic activities of these oxides and also those of SiO2 and SiO2 porous to oxidize iodide ion have been determined and compared.Use of acetonitrile as medium favors the photogeneration of iodine.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India. karunakaranc@rediffmail.com.

ABSTRACT
The crystal structures of different forms of TiO2 and those of BaTiO3, ZnO, SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2 and Al2O3 nanoparticles have been deduced by powder X-ray diffraction. Their optical edges have been obtained by UV-visible diffuse reflectance spectra. The photocatalytic activities of these oxides and also those of SiO2 and SiO2 porous to oxidize iodide ion have been determined and compared. The relationships between the photocatalytic activities of the studied oxides and the illumination time, wavelength of illumination, concentration of iodide ion, airflow rate, photon flux, pH, etc., have been obtained. Use of acetonitrile as medium favors the photogeneration of iodine.

No MeSH data available.


Formation of iodine with illumination timea and variation of iodine-formation rate with iodide ion-concentrationb. a0.020 g catalyst loading, 0.050 M KI solution (25 mL), 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1; b0.020 g catalyst loading, 25 mL M KI solution, 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1.
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Figure 4: Formation of iodine with illumination timea and variation of iodine-formation rate with iodide ion-concentrationb. a0.020 g catalyst loading, 0.050 M KI solution (25 mL), 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1; b0.020 g catalyst loading, 25 mL M KI solution, 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1.

Mentions: Figure 4 is the time profile of photoformation of iodine. It shows that iodine-generation on TiO2 anatase, TiO2 Hombikat and TiO2 P25 slackens in 15 min whereas that on ZnO and WO3 does so at 30 and 60 min, respectively. The other oxides exhibit sustainable photocatalysis at least up to 2 h of illumination. The slackening of iodine-formation with illumination time is not unknown. Photoformation of iodine on Ag-TiO2 [7], Pt-TiO2 [10], phthalocyanine sensitized TiO2 [11] and immobilized TiO2 [5] or ZnO [5,6] show such behavior. Since the iodine generation on TiO2 anatase, TiO2 Hombikat and TiO2 P25 are not slackened at least up to 15 min and on the other oxides at least up to 30 min, the reaction rates have been obtained by measuring the iodine formed in 15 and 30 min on anatase, Hombikat and P25 TiO2 and the rest of the oxides, respectively. All the nanooxides show sustainable photocatalysis. The recycled oxides without any pre-treatment provide identical results (results not listed). Figure 4 also displays the iodine-formation rate at different concentrations of iodide ion. SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2 and SiO2 show linear increase of reaction rate with [I-] indicating first-order kinetics. The other oxides exhibit saturation kinetics revealing Langmuir-Hinshelwood kinetic model [4,7]. The generation of iodine at different airflow rates is displayed in Figure 5. Iodine-formation is enhanced with increased airflow and the variation conforms to Langmuir-Hinshelwood kinetics. Moreover, oxygen is essential for the photoformation of iodine. Iodine is not formed in nitrogen-purged iodide ion solution illuminated with any of the studied oxide (data not listed). The dependence of generation of iodine on the light intensity is also displayed in Figure 5. The photocatalysis lacks linear dependence on photon flux. Less than first power dependence of rates of surface-photocatalyzed reactions on light intensity at high photon flux is known [4,7]. The dependence of photocatalytic iodine generation on the pH of the medium is shown in Figure 6. The pH of the slurry was adjusted by the addition of small volume of NaOH or HCl solution. Except TiO2 rutile and BaTiO3 all other oxides slow down the iodine generation with increase of pH. Rutile TiO2 and BaTiO3 are less sensitive to pH variation. The adsorption of ionic species on the semiconductor depends also on the surface excess charge on the semiconductor crystals. At pH higher than the point of zero charge (PZC), the semiconductor surface is negatively charged resulting in electrostatic repulsion between iodide ion and the semiconductor crystal. Hence, the concentration of iodide ion at the surface and in the double layer is likely to be lesser than that in the bulk of the solution. The adsorption isotherm turns linear leading to a first order kinetics of photocatalysis. The PZC for TiO2, BaTiO3, SnO2, ZnO, WO3, CuO, Fe2O3, Fe3O4, and ZrO2 are 5.8, 9.0, 4.3, 8.8, 0.4, 9.5, 8.6, 6.5 and 6.7, respectively [9]. Examination of Figure 6 reveals, for some oxides at least (TiO2, SnO2, Fe2O3, Fe3O4 and ZrO2), uniform trend in the photocatalysis at pH higher as well as lower than the PZC. A possible explanation is the modification of the PZC values by the ions present in the solution [12,13]. For example, the PZC of TiO2 is reported to change from 6.4 to 4.5 [12]. Hence it is possible that the PZC values of TiO2, SnO2, Fe2O3, Fe3O4 and ZrO2 in the slurry fall outside the range of measured pH. The catalyzed oxidation of iodide ion carried out separately with light of wavelength 254 and 365 nm reveals that UVC light is more effective than UVA light to generate iodine (Table 2). A possible reason for the larger formation of iodine under UVC radiation than UVA radiation is that the generated iodine also absorbs at 365 nm. That is, the liberated iodine may act as an inner filter by absorbing part of the UVA illumination thereby decreasing the intensity of impinging radiation on the nanoparticles. In the case of ZrO2, 254 nm-illumination will bring in band gap excitation. This may lead to the larger iodine-formation. Table 2 also shows that with majority of oxides studied the photogeneration of iodine is more in the immersion reactor than in the tubular reactor. BaTiO3, CuO, Fe3O4, ZrO2 and SiO2 porous are the exceptions. These oxides fail to disperse uniformly throughout the volume of the KI solution (250 mL) in the immersion reactor. It is evident from Table 2 that baring the said five oxides the process is not limited to micro-level.


Photoproduction of iodine with nanoparticulate semiconductors and insulators.

Karunakaran C, Anilkumar P, Gomathisankar P - Chem Cent J (2011)

Formation of iodine with illumination timea and variation of iodine-formation rate with iodide ion-concentrationb. a0.020 g catalyst loading, 0.050 M KI solution (25 mL), 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1; b0.020 g catalyst loading, 25 mL M KI solution, 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1.
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Figure 4: Formation of iodine with illumination timea and variation of iodine-formation rate with iodide ion-concentrationb. a0.020 g catalyst loading, 0.050 M KI solution (25 mL), 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1; b0.020 g catalyst loading, 25 mL M KI solution, 7.8 mL s-1 airflow, 22.4 mg L-1 dissolved O2, 365 nm, 25.2 μEinstein L-1 s-1.
Mentions: Figure 4 is the time profile of photoformation of iodine. It shows that iodine-generation on TiO2 anatase, TiO2 Hombikat and TiO2 P25 slackens in 15 min whereas that on ZnO and WO3 does so at 30 and 60 min, respectively. The other oxides exhibit sustainable photocatalysis at least up to 2 h of illumination. The slackening of iodine-formation with illumination time is not unknown. Photoformation of iodine on Ag-TiO2 [7], Pt-TiO2 [10], phthalocyanine sensitized TiO2 [11] and immobilized TiO2 [5] or ZnO [5,6] show such behavior. Since the iodine generation on TiO2 anatase, TiO2 Hombikat and TiO2 P25 are not slackened at least up to 15 min and on the other oxides at least up to 30 min, the reaction rates have been obtained by measuring the iodine formed in 15 and 30 min on anatase, Hombikat and P25 TiO2 and the rest of the oxides, respectively. All the nanooxides show sustainable photocatalysis. The recycled oxides without any pre-treatment provide identical results (results not listed). Figure 4 also displays the iodine-formation rate at different concentrations of iodide ion. SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2 and SiO2 show linear increase of reaction rate with [I-] indicating first-order kinetics. The other oxides exhibit saturation kinetics revealing Langmuir-Hinshelwood kinetic model [4,7]. The generation of iodine at different airflow rates is displayed in Figure 5. Iodine-formation is enhanced with increased airflow and the variation conforms to Langmuir-Hinshelwood kinetics. Moreover, oxygen is essential for the photoformation of iodine. Iodine is not formed in nitrogen-purged iodide ion solution illuminated with any of the studied oxide (data not listed). The dependence of generation of iodine on the light intensity is also displayed in Figure 5. The photocatalysis lacks linear dependence on photon flux. Less than first power dependence of rates of surface-photocatalyzed reactions on light intensity at high photon flux is known [4,7]. The dependence of photocatalytic iodine generation on the pH of the medium is shown in Figure 6. The pH of the slurry was adjusted by the addition of small volume of NaOH or HCl solution. Except TiO2 rutile and BaTiO3 all other oxides slow down the iodine generation with increase of pH. Rutile TiO2 and BaTiO3 are less sensitive to pH variation. The adsorption of ionic species on the semiconductor depends also on the surface excess charge on the semiconductor crystals. At pH higher than the point of zero charge (PZC), the semiconductor surface is negatively charged resulting in electrostatic repulsion between iodide ion and the semiconductor crystal. Hence, the concentration of iodide ion at the surface and in the double layer is likely to be lesser than that in the bulk of the solution. The adsorption isotherm turns linear leading to a first order kinetics of photocatalysis. The PZC for TiO2, BaTiO3, SnO2, ZnO, WO3, CuO, Fe2O3, Fe3O4, and ZrO2 are 5.8, 9.0, 4.3, 8.8, 0.4, 9.5, 8.6, 6.5 and 6.7, respectively [9]. Examination of Figure 6 reveals, for some oxides at least (TiO2, SnO2, Fe2O3, Fe3O4 and ZrO2), uniform trend in the photocatalysis at pH higher as well as lower than the PZC. A possible explanation is the modification of the PZC values by the ions present in the solution [12,13]. For example, the PZC of TiO2 is reported to change from 6.4 to 4.5 [12]. Hence it is possible that the PZC values of TiO2, SnO2, Fe2O3, Fe3O4 and ZrO2 in the slurry fall outside the range of measured pH. The catalyzed oxidation of iodide ion carried out separately with light of wavelength 254 and 365 nm reveals that UVC light is more effective than UVA light to generate iodine (Table 2). A possible reason for the larger formation of iodine under UVC radiation than UVA radiation is that the generated iodine also absorbs at 365 nm. That is, the liberated iodine may act as an inner filter by absorbing part of the UVA illumination thereby decreasing the intensity of impinging radiation on the nanoparticles. In the case of ZrO2, 254 nm-illumination will bring in band gap excitation. This may lead to the larger iodine-formation. Table 2 also shows that with majority of oxides studied the photogeneration of iodine is more in the immersion reactor than in the tubular reactor. BaTiO3, CuO, Fe3O4, ZrO2 and SiO2 porous are the exceptions. These oxides fail to disperse uniformly throughout the volume of the KI solution (250 mL) in the immersion reactor. It is evident from Table 2 that baring the said five oxides the process is not limited to micro-level.

Bottom Line: Their optical edges have been obtained by UV-visible diffuse reflectance spectra.The photocatalytic activities of these oxides and also those of SiO2 and SiO2 porous to oxidize iodide ion have been determined and compared.Use of acetonitrile as medium favors the photogeneration of iodine.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamilnadu, India. karunakaranc@rediffmail.com.

ABSTRACT
The crystal structures of different forms of TiO2 and those of BaTiO3, ZnO, SnO2, WO3, CuO, Fe2O3, Fe3O4, ZrO2 and Al2O3 nanoparticles have been deduced by powder X-ray diffraction. Their optical edges have been obtained by UV-visible diffuse reflectance spectra. The photocatalytic activities of these oxides and also those of SiO2 and SiO2 porous to oxidize iodide ion have been determined and compared. The relationships between the photocatalytic activities of the studied oxides and the illumination time, wavelength of illumination, concentration of iodide ion, airflow rate, photon flux, pH, etc., have been obtained. Use of acetonitrile as medium favors the photogeneration of iodine.

No MeSH data available.