4.2. Photo-oxidation of Pb2+ ion to PbO2 on hematite in acid solution under visible light irradiation

The photo-oxidation treatment of Pb2+ ions in aqueous solution was examined by using hematite for the purpose of elimination of them from the solution [25, 24]. The hematite in this case was prepared from thermal oxidation of iron plate at 600C for 3 h in air. The cell consisting of the hematite working electrode in 0.1 M HNO3–10 μM Pb(NO3)2 and of the graphite counter electrode in 0.1 M H2SO4–10 mM Ce(SO4)2 aqueous solution was used as a photocell performing without applied voltage. The flow of photocurrent was observed by irradiation of visible light to the hematite electrode. Figure 23 shows the dependence of photocurrent on irradiation time in this cell and also in the cell with the graphite electrode solution of 0.1 M H2SO4–10 mM Fe2(SO4)3 aqueous solution. In this case, the hematite and graphite electrodes acted as a photoanode and a cathode, respectively. It is clear that the presence of Ce4+ in the cathode solution was effective for the performance of the cell based on the hematite photoanode. Since the standard equilibrium potential of Ce4+/Ce3+ system of 1.44 V vs. NHE is more positive than that of Fe3+/Fe2+ system of 0.771 V vs. NHE, Ce4+ may act as a stronger electron acceptor. Figure 24 shows the SEM image of the surface of the hematite electrode before and after irradiation for 6 h in 0.1 M HNO3–10 μM Pb(NO3)2 solution. The photodeposition of many particles was observed on the surface of hematite. The XRD peak due to PbO2 was confirmed on the hematite after irradiation. This suggests the occurrence of the following photoelectrochemical reactions in Eqs. (30)–(32). The photo-generated hole (h<sup>+</sup> ) and electron (e) pair was separated to oxidize Pb2+ to PbO2 by hole at the hematite photoanode and reduce Ce4+ to Ce3+ by electron at the graphite cathode according to Eqs. (30) and (31). The total reaction is represented as Eq. (32). With regard to the standard Gibbs free energy change, ΔG<sup>0</sup> , for reaction (32), the positive value 48.2 kJ/mol could be evaluated by using the standard equilibrium potential of Pb4+/Pb2+, 1.69 V and that of Ce4+/Ce3+, 1.44 V vs. NHE. This means that the reaction (32) is not spontaneous reaction and the light energy causes it to proceed. The hematite prepared from thermal oxidation of iron showed the value of Efb of 0.31 V vs. NHE in the 0.1 M aqueous HNO3 solution. Therefore, the position of valence band edge of hematite

Figure 22b shows the relationship between concentration of citric acid and irradiation time as well as between concentration of acetonedicarboxylic acid and irradiation time. The UV light irradiation accelerated the photo-oxidation of citric acid to acetonedicarboxylic acid. Table 2 summarizes the values concerning photocurrent quantum efficiency, quantum efficiency and current efficiency for the photo-oxidation of citric acid to acetonedicarboxylic acid on the

Nh<sup>ν</sup> Qelec (C) Nelec ηelec (%) ΔC (μmol) Nmole ηmole (%) Ieff (%) 2.47 <sup>10</sup><sup>20</sup> 1.91 1.19 <sup>10</sup><sup>19</sup> 4.82 6.14 3.70 <sup>10</sup><sup>18</sup> 1.50 62

Table 2. Values in relation to the photoanodic oxidation on the hematite electrode in 0.1 M aqueous Na2SO4 solution containing initial concentration of 200 μM citric acid under irradiation of UV light (wavelength: 360 nm, intensity:

Figure 22. A chromatograms of the citric acid solution before and after irradiation of UV light (wavelength: 360 nm,

citric acid, concentration of acetonedicarboxylic acid and irradiation time. Hematite was prepared by the same method as

) to the hematite electrode at 1.0 V vs. Ag/AgCl for 9 hours, b: Relationship among concentration of

intensity: 4.2 mW/cm2

168 Iron Ores and Iron Oxide Materials

that in Figure 21.

4.2 mW/cm2

) for 9 h.

Figure 23. Dependence of photocurrent on irradiation time in the cell consisting of the hematite 0.1 M HNO3–10 μM Pb (NO3)2 and of the graphite cathode in 0.1 M H2SO4–10 mM Ce(SO4)2 solution (a) and also in the cell of the photoanode in the same solution and of the cathode in 0.1 M H2SO4–10 mM Fe2(SO4)3 solution (b). Hematite was prepared from thermal oxidation of iron at 600�C for 3 h in air.

could be regarded approximately as 2.3 V vs. NHE in the solution (pH = 1) by referring to the band gap energy of 2.0 eV. The occurrence of photo-oxidation of Pb2+ to PbO2 deposition on hematite may be supported by this positive potential of valence band edge. For 0.1 M aqueous HNO3 solution containing 10 μM Pb(NO3)2, the removal rate of Pb2+ ions due to visible light irradiation was checked by atomic absorption analysis. In the case of using Ce4+ electron acceptor without applied voltage, 8.9% of the initial concentration of Pb2+ was removed from the solution after 6 h irradiation. By holding the potential of hematite at 1.50 V vs. Ag/AgCl, a marked increase in the removal rate was confirmed. The values of 27.0, 38.7 and 53.2% were obtained after 2, 4 and 6 h irradiation, respectively.

$$\text{Pb}^{2+} + 2\text{H}\_2\text{O} + 2\text{h}^+ \rightarrow \text{PbO}\_2 + 4\text{H}^+ \tag{30}$$

$$\text{2C}\text{e}^{4+} + \text{2e}^{-} \rightarrow \text{2C}\text{e}^{3+} \tag{31}$$

4.3. Photo-polymerization of aniline on hematite and characteristics of polyaniline/hematite

Figure 24. SEM images of the surface of the hematite electrode before and after visible light irradiation for 6 h in 0.1 M HNO3–10 μM Pb(NO3)2 solution. Hematite was prepared from thermal oxidation of iron at 600C for 3 h in air.

Photoelectrochemistry of Hematite

171

http://dx.doi.org/10.5772/intechopen.73228

The photo-polymerization of aniline was carried out by the photoelectrochemical cell consisting of the separated parts of hematite photoanode in 0.1 M HClO4–0.1 M aniline and the graphite cathode in 0.1 M H2SO4–10 mM Ce(SO4)2 aqueous solutions under visible light irradiation [27]. This hematite was prepared from thermal oxidation of iron at 600C for 3 h in air. These electrolytic solutions were connected by a KCl salt bridge. The deposition of many particles was observed on the surface of the hematite electrode after irradiation. The photoanodic polymerization of aniline due to the photo-generated hole could proceed on the hematite electrode without applied voltage by using Ce4+ ions as an electron acceptor. Because the potential for the bottom of conduction band is positive as understood from the positive Efb value of hematite, the presence of a strong oxidizing agent as an electron acceptor may be necessary for occurrence of photoelectrochemical reaction on the hematite without application

The polyaniline/hematite electrode has a unique property. Figure 25 (a–c) shows the Mott-Schottky plots of the hematite electrode, the polyaniline electrode prepared from anodic deposition of polyaniline film on the glassy carbon and the polyaniline/hematite electrode in 0.1 M aqueous HClO4 solution, respectively, at the frequency of 1 kHz. On the hematite

electrode under visible light irradiation

of electric power.

$$\text{Pb}^{2+} + 2\text{H}\_2\text{O} + 2\text{Ce}^{4+} \rightarrow \text{PbO}\_2 + 4\text{H}^+ + 2\text{Ce}^{3+} \tag{32}$$

Figure 24. SEM images of the surface of the hematite electrode before and after visible light irradiation for 6 h in 0.1 M HNO3–10 μM Pb(NO3)2 solution. Hematite was prepared from thermal oxidation of iron at 600C for 3 h in air.
