**4. Dye-sensitized photocatalytic degradation of phenol and phenolic compounds**

### **4.1. Theory of dye sensitization**

The process of expanding the spectral sensitivity of semiconductor materials with a dye to the visible spectra is known as dye sensitization. Dye is typically adsorbed onto the semiconductor surface by chemical adsorption process. Chemisorbed dye molecules act as spectral sensitizer that upon excitation with visible light inject an electron into the conduction band of the semiconductor [27]. To undergo successful electron injection, the dye molecule should include few basic properties regarding energy levels, ground-state redox potential, and surface anchoring group. Carboxylic and phosphoric acid groups form strong covalent bonds with semiconductor and provide fast electron transfer rate [58]. Recent studies mention the use of a group of sensitizers such as poly(aniline), poly(thiophene), porphyrins, coumarin, phthalocyanines, eosin Y, alizarin red S, and carboxylate derivatives of anthracene [11, 26, 27]. Among the photosensitizers, transition metal-based sensitizers have shown best results in dyesensitization process. Transition metals such as Fe (II), Ru (II), and Os (II) form d6 complex and undergo intense charge transfer absorption across the entire visible range [59]. However, metal-based sensitizers are not environment friendly, and thus researchers are now focusing on the use of natural dyes as an alternative for dye-sensitization process [59–62]. Several semiconductor photocatalysts have been studied for dye sensitization including TiO2, SrTiO3, ZnO, SnO2, and Cu2O [63, 64]. Among them, TiO2 is the best photocatalyst in terms of (i) cost, (ii) availability, (iii) toxicity, (iv) stability against photocorrosion, and (v) electronic energy band structure [65, 66].

### **4.2. Dye-sensitized photocatalytic phenol degradation mechanism**

Dye-sensitized photodegradation of phenol under visible light began through excitation of the dye molecule from its ground state to the excited state, which then assists the electron transfer to the conduction band of the semiconductor. The oxidized dye molecule (dye+ ) can interact with either phenol or an electron donor to return back to its ground state [7]. Chowdhury et al. [67] used eosin Y (EY) as the sensitizer which provided TiO2/Pt a significant visible-light activity via dye sensitization. Eosin Y contains both hydroxyl and carboxyl end groups, which actually assists the dissociative surface adsorption of eosin Y onto the surface hydroxyl (Ti– OH2 + ) sites of EY-sensitized TiO2/Pt [11]. Triethanolamine (TEOA) was used as an electron donor, which was consumed through an irreversible oxidation by extending the lifetime of eosin Y during phenol degradation (Eqs. (11) and (12)) [7].

$$\text{(CH}\_2-\text{CH}\_2-\text{OH)}\_3\text{N}: + \text{EY}^+ = \text{(CH}\_2-\text{CH}\_2-\text{OH)}\_3\text{N}^+ + \text{EY} \tag{11}$$

$$\left(\text{CH}\_2-\text{CH}\_2-\text{OH}\right)\_3\text{N}^+=\left(\text{CH}\_2-\text{CH}\_2-\text{OH}\right)\_2-\text{N}-\text{CH}\_2-\text{CH}\_2-\text{OH}+\text{H}^+\tag{12}$$

(acid-base equilibrium of TEOA)

(8)

(9)

where *k*<sup>1</sup> is a proportionality constant, *A* is the illuminated area of the photoreactor window, *C*photocat is the photocatalyst concentration, *ε* is the light absorption coefficient of the photocatalyst, *I*<sup>0</sup> is the incident light intensity, and *β* is a constant. They reported *β* values of 0.84, 0.72,

Photocatalytic degradation reaction requires the use of electron acceptor to reduce the charge carrier recombination. Oxygen is the most common electron acceptor because of its availability, higher solubility, and nontoxic nature. The partial pressure of oxygen is adjusted by mixing the oxygen stream with nitrogen stream by maintaining the total flow rate of gas at a constant value. The photocatalytic reaction of phenols will terminate if sufficient oxygen is not available in the solution [15]. Chen and Ray [14] showed the improvement of 4-NP photodegradation rate with increasing oxygen partial pressure. The photodegradation rate constant reached approximately 70% of its maximum value at oxygen partial pressure of 0.2 atm. The effect of oxygen partial pressure on the photodegradation of 4-NP is described by a noncompetitive

> 2 2 2 2 <sup>1</sup><sup>+</sup> ¥ *O O*

*O O*

where *kp* is the kinetic constant for 4-NP degradation, *KO*2 is the adsorption constant of dissolved

The process of expanding the spectral sensitivity of semiconductor materials with a dye to the visible spectra is known as dye sensitization. Dye is typically adsorbed onto the semiconductor surface by chemical adsorption process. Chemisorbed dye molecules act as spectral sensitizer that upon excitation with visible light inject an electron into the conduction band of the semiconductor [27]. To undergo successful electron injection, the dye molecule should include few basic properties regarding energy levels, ground-state redox potential, and surface anchoring group. Carboxylic and phosphoric acid groups form strong covalent bonds with semiconductor and provide fast electron transfer rate [58]. Recent studies mention the use of a group of sensitizers such as poly(aniline), poly(thiophene), porphyrins, coumarin, phthalo-

**4. Dye-sensitized photocatalytic degradation of phenol and phenolic**

*K P* (10)

*p*

oxygen on photocatalyst, and *pO*2 is the partial pressure of dissolved oxygen.

*K p <sup>k</sup>*

and 0.82 for the degradation of 4-NP, 4-CP, and phenol, respectively.

408 Phenolic Compounds - Natural Sources, Importance and Applications

*3.4.5. Effect of electron acceptor*

Langmuir kinetic equation as follows:

**compounds**

**4.1. Theory of dye sensitization**

Eosin Y-sensitized phenol degradation mechanism under visible light is described below [7, 68]:

$$\text{TiO}\_2-\text{EY} \stackrel{\text{hv} \in \text{NNO}}{\rightarrow} \text{TiO}\_2-\text{EY}^\* \tag{13}$$

$$\text{TiO}\_2 - \text{EY}^\* \rightarrow \text{TiO}\_2 - \left(\text{EY}^{\oplus} + e\_{\text{CB}}^{-}\right) \tag{14}$$

$$\text{TiO}\_2 - \left(EY^{\ominus} + e\_{\text{CB}}^{-}\right) + \text{PhOH} \rightarrow \text{TiO}\_2 - \left(EY + e\_{\text{CB}}^{-}\right) + \text{PhOH}^{\ominus} \tag{15}$$

$$\text{TiO}\_2 - \left(EY^{\text{\oplus}} + e\_{\text{CB}}^{-}\right) + \text{TEOA} \rightarrow \text{TiO}\_2 - \left(EY + e\_{\text{CB}}^{-}\right) + \text{TEOA}^{\oplus} \tag{16}$$

$$\text{TiO}\_2 - \left(EY + e\_{\text{CB}}^{-}\right) + \text{O}\_2 \rightarrow \text{TiO}\_2 - \mathbb{E}\text{Y} + \text{O}\_2^{\bullet -} \tag{17}$$

$$\text{CO}\_2^{\bullet-} + \text{H}^+ \rightarrow \text{HCO}^\bullet \rightarrow \rightarrow \text{HO}^\bullet \tag{18}$$

$$\text{HCO}^{\bullet} + \text{C}\_{6}\text{H}\_{5}\text{OH} \rightarrow \text{intermediate} + \text{CO}\_{2} \tag{19}$$

$$\text{HCO}^{\bullet} + \text{intermediate} \rightarrow \text{H}\_2\text{O} + \text{CO}\_2\tag{20}$$

### **4.3. Dye-sensitized photocatalytic phenol degradation kinetics**

In dye-sensitized photodegradation under the visible light, the dye molecule is first activated by visible light (λ > 420 nm) and then injects electrons into the conduction band of the semiconductor. Chowdhury et al. [7] described the kinetics of phenol degradation using eosin Y-sensitized TiO2/Pt with a modified Langmuir-Hinshelwood equation as follows:

$$-\left(\frac{d\mathbb{C}\_{Ph}}{dt}\right) = \frac{W\left(k\_r K\_A \mathbb{C}\_{ph}\right)}{V\left(1 + K\_A \mathbb{C}\_{Ph0}\right)} \times I^{\beta} \tag{21}$$

where *W* is the mass of photocatalyst, *CPh* is the phenol concentration at time *t*, *CPh*<sup>0</sup> is the initial phenol concentration, *V* is the volume of the reaction mixture, *KA* is the adsorption equilibrium constant, *kr* is the kinetic rate constant, *I* is the light intensity, and *β* is a constant. The apparent kinetic constant as is defined as follows:

$$K\_{app} = \frac{W}{V} \left(k\_r K\_A\right) \tag{22}$$

Combining Eqs. (21) and (22), they obtained Eq. (23) as follows:

$$-\left(\frac{d\mathbf{C}\_{Ph}}{dt}\right) = \frac{K\_{app}\mathbf{C}\_{Ph}}{1 + K\_A\mathbf{C}\_{Ph0}} \times I^{\beta} \tag{23}$$

Eq. (23) was used to predict the kinetic parameters of phenol photodegradation at different irradiation intensities (range, 25–100 mW cm−2). Based on a parameter estimation using the experimental data, the values of *Kapp*, *KA*, and *β* were obtained for the degradation of phenol. The values of *Kapp* was 8.02 × 10−6 min−1, *KA* was 0.13 L mg−1, and *β* was 2.15 [7].
