Synergistic Photocatalytic Degradation of Phenol Using Precious Metal Supported Titanium Dioxide with Hydrogen Peroxide

Abstract

A series of precious metal catalysts (M/titanium dioxide [TiO₂], M = Ru, Rh, Pd, Ag, Ir, Pt, or Au) were prepared by a light deposition method, and for the first time, synergistic effects of photocatalytic degradation of phenol (20 mg/L) under UV irradiation (365 nm) by M/TiO₂ with electron capture agent hydrogen peroxide (H₂O₂) have been investigated. Results showed that H₂O₂ plays a great synergistic role on M/TiO₂, and the photocatalytic activity of M/TiO₂ is closely related to its work function. Ag loading greatly enhanced activity of TiO₂ due to the normal reduction mechanism and the “activated” mechanism between H₂O₂ and Ag nanoparticles. Under the optimum conditions of Ag loading at 0.5 wt.%, Ag/TiO₂ concentration at 0.1 g/L, H₂O₂ concentration at 50 mmol/L, and pH value of the reaction solution at 5, the phenol can be degraded by 65% within 3 h, which is 45% more than that in the TiO₂ photocatalytic system. Kinetics of the degradation of phenol can be characterized as pseudo-first order. Such synergistic photocatalytic degradation by precious metal supported TiO₂ with H₂O₂ provides a new approach for the degradation of refractory pollutants in waste water.

Introduction

Phenol is commonly used to synthesize a variety of drugs and chemical products in the pharmaceutical, petrochemical, coal, paper, pulp, and chemical refinement industries. It has a tremendous impact on water sources and the aquatic ecosphere when it is directly discharged into bodies of water (Pradhan et al., 2012; Sun et al., 2014). It has been recognized as one of the most harmful pollutants known to man when it exists in natural environments. Therefore, the degradation of phenol has received great attention. Many advanced oxidation processes (AOPs) have been developed to remove phenol such as Fenton (Djeffal et al., 2014), Fenton oxidation (Minella et al., 2014), and Photo-Fenton process (Bauer et al., 1999). However, these AOPs have major limitations such as: (i) the need for acidification to avoid iron precipitation (optimum pH for the Photo-Fenton process is near 2.8) and final neutralization; (ii) excess iron must be removed to comply with discharge limits (Brazil: 15 mg/L; Colombia: 10 mg/L; Paraguay: 5 mg/L; and Portugal: 2 mg/L (into water bodies) or 5 mg/L (for irrigation)); (iii) the formation of strong and stable complexes between ferric ions and pollutants, limiting the reduction of Fe³⁺, decreasing the decomposition of hydrogen peroxide (H₂O₂) in the Fenton's reaction, and the overall efficiency of the process; (iv) the formation of ferric complexes with chloride and sulfates, such as FeCl⁺, FeCl²⁺ and FeCl₂⁺, and FeSO₄⁺ and Fe(SO₄)₂⁻, which are much less photoactive than FeOH²⁺, and also the formation of less oxidizing species Cl•, Cl₂•⁻, and SO₄•⁻ compared with HO• radicals (Dias et al., 2014).

In recent years, heterogeneous photocatalysis has generated attention for the treatment of phenol in wastewater (Fabbri et al., 2006; Zhang et al., 2009). Photocatalytic oxidation is one of the most attractive technologies, because it can mineralize the organic pollutants to form H₂O and CO₂. In addition, the photocatalytic technology produces fewer toxic intermediates. Among the various photocatalysts, titanium dioxide (TiO₂) is generally used due to its high photocatalytic activity and stability (Khraisheh et al., 2013). Nevertheless, the main problem of TiO₂ photocatalysis lies in the separation of photogenerated electron-hole pairs. To overcome this problem, different routes have been proposed such as: (1) combining the photocatalyst with another semiconductor (Sivakumar et al., 2013) or conductive substrate such as graphene (Wang et al., 2011) or carbon nanotube (Chai et al., 2012); (2) doping transition metal ions (Rathinavelu et al., 2013; Pang et al., 2014) or rare earth metallic elements (Tian et al., 2014, 2015) to the catalyst and depositing nanometal particles on the surface of the catalyst (Zhang et al., 2013); and (3) adding some sacrificial agent in the reaction system (Harir et al., 2008; Shi et al., 2011; Sivakumar et al., 2013). Among the sacrificial agents, H₂O₂ has been utilized widely (Harir et al., 2008; Sivakumar et al., 2013). However, to the best of our knowledge, the studies about the synergistic photocatalytic degradation activity by precious metal supported TiO₂ with H₂O₂ have rarely been reported.

In the present study, the synergistic photocatalytic degradation of phenol has been investigated in aqueous conditions by precious metal supported TiO₂ with addition of H₂O₂ under UV irradiation, and the effects of a series of experimental parameters on the degradation efficiency have also been discussed.

Experimental

Preparation of photocatalyst

All chemicals were analytical grade and were used without further purification. Anatase TiO₂ (Hehai Nanometer Science and Technology Co. Ltd.; Surface area ≥250 m²/g, 98.5% anatase, grain size ≤30 nm) was used in all experiments. Metal was deposited on the TiO₂ surface (M/TiO₂, M = Pt, Pd, Rh, Au, Ir, Ag, and Ru) by a photodeposition method (Lee et al., 2009). RuCl₃ · 3H₂O (Kunming Boren Precious Metals Co. Ltd.; 99.9%), RhCl₃ · nH₂O, HAuCl₄ · 4H₂O, PdCl₂, H₂IrCl₆ · 6H₂O, H₂PtCl₆ · 6H₂O, and AgNO₃ (Shanghai July Chemical Co. Ltd.; 99.9%) were used as the metal precursors. In a typical procedure, 0.2 g TiO₂ and 500 mL deionized water (Millipore Milli-Q) were added into 1-L Pyrex glass beaker and the slurry was dispersed ultrasonically for 15 min. Then, it was placed under the 125 W low pressure mercury lamp (Shenzhen Stone-lighting Opto Device Co. Ltd.; λ_max = 365 nm, 64.9 μW/cm²) and was irradiated for 30 min to remove adsorbed organic impurities from the particle surface. The system was then purged with nitrogen gas at a flow rate of 60 mL/min for 20 min. Thereafter, 20 mL methanol was added to the system, acting as a hole scavenger. The metal precursor was added to the required loading (0.05, 0.1, 0.5, 1.0, 3.0, and 5.0 wt.%) while the pH value was adjusted to 3 using diluted perchloric acid. In the following procedure, the slurry was illuminated for 2 h with the M/TiO₂ particles recovered and washed with deionized water thrice. The washed particles were dried in an oven at 80°C for 10 h and then stored in a desiccator.

Photocatalytic reaction

The photocatalytic reaction system used in this study is shown in Fig. 1. The temperature for all the photocatalytic reactions was kept at 25°C by the circular water outside the Pyrex cell. Typically, the TiO₂ particles (20 mg) were suspended in a 20 mg/L phenol (Sinopharm Chemical Reagent Co. Ltd.; 99.9%) aqueous solution (200 mL) in a 500-mL cylindrical Pyrex vessel, followed by the addition of 2 mmol H₂O₂. The pH of solutions was measured using a digital pH meter (8682, AZ Instrument Corp.,) and adjusted by HCl (0.1 mol/L) or NaOH (0.1 mol/L). The system was then purged with nitrogen gas at a flow rate of 60 mL/min for 20 min. Solution was then stirred in dark for 90 min to reach the adsorption equilibrium. The experiments lasted for 60 min, and the lamp was the same as that used in the photocatalyst preparation. Two milliliter solution was drawn to measure the concentration of phenol at 15 min interval of irradiation. The liquid sample was centrifuged at 3,000 rpm for 10 min and, subsequently, filtered to separate TiO₂ particles. The analyses of phenol were performed in a high-performance liquid chromatograph (LC2000; Shanghai Techcomp Instrument Ltd.) using a Symmetry C18 column and a mobile phase consisting of 70% methanol (chromatographic pure) and 30% water. The LC conditions are as follows: flow rate of mobile phase at 1.0 mL/min, oven temperature at 20°C, and the UV detection wavelength at 277 nm.

FIG. 1.

Photocatalytic reaction system.

Characterization

Brunauer-Emmett-Teller (BET) surface area was evaluated by N₂ adsorption in a constant volume adsorption apparatus (Bel sorp II; Bayer Japan Co. Ltd.). UV-vis diffuse reflectance spectroscopy (DRS) was recorded using a spectrophotometer (UV-2501PC; Shimadzu) in the spectral range of 200–800 nm. The surface morphology of the M/TiO₂ particles was obtained using a scanning electron microscope (SEM; S-4800; Hitachi). The chemical compositions of the sample were tested by an energy dispersive X-ray detector (EDX; EMax-250; Horiba).

Results and Discussion

Characterization of catalysts

BET surface areas of M/TiO₂ are listed in Table 1. With the introduction of precious metals on the surface of TiO₂, there is no apparent change in BET surface areas among M/TiO₂ and TiO₂. This result shows that the pore microstructure properties of these samples are similar, indicating that the change of specific surface area is not a main factor to affect the photoactivity.

Table 1.

Specific Surface Area and Loading of Metal of M/Titanium Dioxide

Photocatalyst	A (m²/g)	Loading amount of metal (EDX)/wt.%
TiO₂	253.72	0
0.5 wt.% Ru/TiO₂	251.41	0.51
0.5 wt.% Rh/TiO₂	256.17	0.48
0.5 wt.% Pd/TiO₂	255.94	0.49
0.5 wt.% Ag/TiO₂	247.98	0.47
0.5 wt.% Ir/TiO₂	250.25	0.48
0.5 wt.% Pt/TiO₂	257.42	0.47
0.5 wt.% Au/TiO₂	257.73	0.51

EDX, energy dispersive X-ray detector; TiO₂, titanium dioxide.

Optical properties of M/TiO₂ are shown in Fig. 2. The absorption properties of M/TiO₂ are similar to those of pure TiO₂ in the UV region. Only when the incident light wavelength is above 375 nm, the absorption performance of Au/TiO₂ is stronger than others, which is due to the photon absorption corresponding to Au particles (Chiarello et al., 2010). DRS results show that these precious metals are deposited on TiO₂ surface. The main wavelength of the light source used in our experiments is about 365 nm, and thus, there are no significant differences in the absorption properties of M/TiO₂ in this region.

FIG. 2.

UV-Vis diffuse reflectance spectra of different catalyst samples.

SEM images of TiO₂ samples are shown in Fig. 3. From Fig. 3a, we can see that the TiO₂ particles exhibit flat sheet morphology with a wide distribution of diameters ranging from 50 to 100 nm. The surface morphologies of 0.5 wt.% M/TiO₂ (in Fig. 3b–f) and TiO₂ (Fig. 3a) are similar, and no metal particles are observed on 0.5 wt.% M/TiO₂.

FIG. 3.

(a) TiO₂. (b) 0.5 wt.%Ru/TiO₂. (c) 0.5 wt.%Rh/TiO₂. (d) 0.5 wt.%Pd/TiO₂. (e) 0.5 wt.%Ag/TiO₂. (f) 0.5 wt.%Ir/TiO₂. (g) 0.5 wt.%Pt/TiO₂. (h) 0.5 wt.%Au/TiO₂. TiO₂, titanium dioxide.

The loading amounts of metals characterized by EDX are listed in Table 1. As shown in Table 1, the content of the metal deduced from the EDX spectrum is close to the theoretical value 0.5 wt.%, which provides direct proof that the metal is deposited on nano-TiO₂ surface. As there is no observation of metal particles in SEM image, the light deposition method can effectively disperse precious metals on the TiO₂ surface and inhibit their aggregation. For the C elements, it may be due to the methanol residue used in the preparation process.

Overall, specific surface area, absorption property, and surface morphology are not main factors to affect the photoactivity. That is to say, it is the nature of the precious metal itself that affects the activity of the catalyst.

Synergistic activity of M/TiO₂ with H₂O₂

Degradation curves of phenol under different conditions are shown in Fig. 4. The efficiency of direct photolysis of phenol under UV light is low. In addition, phenol can hardly be degraded in the presence of H₂O₂ alone. Furthermore, the degradation rate of phenol is limited when adding either UV or TiO₂ and H₂O₂. Thus, it is difficult to degrade phenol under all the experimental conditions mentioned above.

FIG. 4.

Degradation of phenol under different conditions.

Photocatalytic degradation of phenol over M/TiO₂ are shown in Fig. 5. Compared with the experimental results without adding H₂O₂, the photocatalytic activity has been significantly improved when adding H₂O₂, which is consistent with the results reported by Mahmoodi (2013). The enhancement of degradation by the addition of H₂O₂ is due to the increased production of valuable hydroxyl radicals (Reaction 1) (Sivakumar et al., 2013) and inhibition of the e_CB⁻/h_VB⁺ recombination process. Therefore, H₂O₂ plays a great synergistic role on M/TiO₂ in the photocatalytic degradation of phenol. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm H}_2{ \rm O}_2 + { \rm e}_{ \rm CB}{}^{ -} \rightarrow { \rm HO} \bullet + { \rm OH}^{ -} \tag{1}\end{align*} \end{document}

It is also noted that, when there is no H₂O₂, the sequence of the catalytic activity is Pd/TiO₂ > Pt/TiO₂ > Rh/TiO₂ ≈ TiO₂ > Au/TiO₂ ≈ Ag/TiO₂ > Ru/TiO₂ > Ir/TiO_2, while with the addition of H₂O₂, the sequence is Ag/TiO₂ > Pd/TiO₂ > Pt/TiO₂ > Rh/TiO₂ > TiO₂ > Au/TiO₂ > Ru/TiO₂ > Ir/TiO₂. These two sequences are similar except for Ag/TiO₂. The reasons will be discussed in the following section.

Many researchers have evaluated the impacts of different precious metals on the catalytic activity in terms of molar loading (Michaelson, 1977; Fu et al., 2008). The activities per mole metal of photocatalytic degradation of phenol over 0.5 wt.% M/TiO₂ are shown in Fig. 6. In the absence of H₂O₂, the sequence of the activity per mole metal is Pt/TiO₂ > Au/TiO₂ ≈ Pd/TiO₂ > Rh/TiO₂ > Ag/TiO₂ > Ir/TiO₂ > Ru/TiO₂. This sequence is consistent with that reported by Fu et al. (2008). This is mainly related to the work function of the noble metals (Michaelson, 1977). In the presence of H₂O₂, the sequence of the production per mole metal turns out to be Pt/TiO₂ > Ag/TiO₂ ≈ Au/TiO₂ > Pd/TiO₂ > Rh/TiO₂ ≈ Ir/TiO₂ > Ru/TiO₂. This sequence is similar to that of no H₂O₂ except for Ag/TiO₂. It is worth noting that the catalytic activity of Ag/TiO₂ is promoted greatly and better than Pd/TiO₂, Au/TiO₂, and Rh/TiO₂ when H₂O₂ is added. In addition, the work functions of Pd, Au, and Rh are higher than Ag (Michaelson, 1977). For the high activity of Ag/TiO₂, this may be related to the normal reduction mechanism and the “activated” mechanism between H₂O₂ and Ag nanoparticles (Flaètgen et al., 1999; Campbell et al., 2009). As reported, Ag nanoparticles can accelerate the reduction reaction of H₂O₂, leading to the electrons being quickly transferred to H₂O₂, which will allow it to avoid the recombination of photogenerated electron-hole pairs efficiently. As a result, the catalytic activity of Ag/TiO₂ is high when adding H₂O₂. In addition, in regards to the noble metals that show inhibition effects on degradation compared with TiO₂ as shown in Fig. 5, this may be due to the high metal loading so that these noble metals have become the electron-hole recombination centers (An et al., 2009).

FIG. 5.

Photocatalytic degradation of phenol over M/TiO₂.

FIG. 6.

Activity per mole metal of photocatalytic degradation of phenol over M/TiO₂.

From the above discussion, the factors such as the precious metal loading, the catalyst concentration, and the H₂O₂ concentration have a great influence on the synergistic effect of photocatalytic degradation phenol by M/TiO₂ with H₂O₂.

Effects on activity

Effects of the loading

The effects of Ag loading are shown in Fig. 7. As we can see, the degradation efficiency first increases with the loading of Ag, then decreases sharply. The highest catalytic activity is achieved when ∼0.5 wt.% Ag is loaded. The result shows that the loading of Ag has an obvious promoting effect on the photocatalysis of TiO₂, in accordance with the report by Martins et al. (2009). This is mainly because the active site of the TiO₂ surface is limited when the Ag loading is too small, while excessive Ag nanoclusters cover the surface of the TiO₂ to form a recombination center when the Ag loading is too large.

FIG. 7.

Effect of Ag loading amount on degradation of phenol.

Effects of the catalyst concentration

The effects of the concentration of Ag/TiO₂ on the degradation of phenol are shown in Fig. 8. When the concentration is 0.10 g/L, the photodegradation efficiency of phenol increases sharply, and decreases significantly when the catalyst concentration continues to increase. This result is consistent with many reports (Harir et al., 2008; Yang et al., 2008; Kashif and Ouyang, 2009). The increase of the number of catalyst particles will facilitate the absorption of photons and adsorption of phenol molecules. A further increase of the catalyst concentration beyond 0.10 g/L may cause a light barrier, light scattering, or screening effects. The excessive opacity of the suspension prevents UV light from illuminating the catalyst. The scattering and screening effects reduce the specific activity of the catalyst. In addition, at high concentrations of the catalyst, particle aggregation may also reduce the catalytic activity. Therefore, the photodegradation efficiency of phenol decreases gradually. In this study, the optimum amount of catalyst is found to be 0.10 g/L for the degradation of phenol.

FIG. 8.

Effect of concentration of Ag/TiO₂ on degradation of phenol.

Effects of H₂O₂ concentration

The effects of the H₂O₂ concentration on the degradation of phenol are shown in Fig. 9. The degradation efficiency of phenol increases rapidly from 7% to 37% when a small amount of H₂O₂ is added (up to 50 mmol/L). If the H₂O₂ concentration is larger than 50 mmol/L, the degradation efficiency decreases gradually. This is because there cannot be sufficient H₂O₂ to capture photogenerated electrons when the concentration of H₂O₂ is low; but at the high concentration of H₂O₂, H₂O₂ reacts with HO• to produce HO₂• with weak oxidizing ability. HO₂• then reacts with HO• to produce H₂O and O₂. The overall result is that the excess H₂O₂ consumes HO•, as in the following two reactions (2 and 3) below (Sivakumar et al., 2013): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm HO} \bullet + { \rm H}_2{ \rm O}_2 \rightarrow { \rm H}_2{ \rm O} + { \rm HO}_2 \bullet \tag{2}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm HO} \bullet + { \rm HO}_2 \bullet \rightarrow { \rm H}_2{ \rm O} + { \rm O}_2 \tag{3}\end{align*} \end{document}

FIG. 9.

Effect of concentration of H₂O₂ on degradation of phenol. H₂O₂, hydrogen peroxide.

Effects of pH

Effects of the pH on the degradation of phenol are shown in Fig. 10. As shown in Fig. 10, the phenol photocatalytic degradations in acid and neutral environments are better than in alkaline environments, which are primarily related to the nature of TiO₂ in solutions of different pH values (Zhao et al., 2004). The pH_zpc (the pH at zero point of charge) of TiO₂ is about 6.8 (Alaton et al., 2002). This implies that when pH value is higher than 6.8, the surface of TiO₂ is covered with negative charges, while positive charges are rich when pH is lower than 6.8. In an alkaline environment, the reaction is slower because of the decrease in adsorption. The semiconductor will repel the negatively charged phenolate, which becomes the dominant organic form in the solution (the pKa of phenol is 9.9) (Akbal and Onar, 2003). Furthermore, in an alkaline environment, it favors the formation of bicarbonate ion and carbonate ions (the pKa₁ and pKa₂ of H₂CO₃ were 6.38 and 10.33), which are effective scavengers of OH⁻, and will reduce the efficiency of degradation processes (Reactions 5 and 6) (Wu et al., 2002). At pH 7, the reaction solution is neutral. As the Reaction (1) occurs, it will bring some amount of OH⁻. It will benefit the production of HO• as the following Reaction (4) (Yang et al., 2008).

FIG. 10.

Effect of pH on degradation of phenol.

Photocatalytic degradation kinetics

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm OH}^{ -} + { \rm h}_{ \rm VB}{}^{ +} \rightarrow { \rm HO} \bullet \tag{4}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm HO} \bullet + { \rm HCO}_3{}^{ -} \rightarrow { \rm H}_2 { \rm O} + { \rm CO}_3 \bullet \bullet^{ -} \tag{5}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm HO} \bullet + { \rm CO}_3{}^{2 -} \rightarrow { \rm OH}^{ -} + { \rm CO}_3 \bullet \bullet^{ -} \tag{6}\end{align*} \end{document}

Photocatalytic degradation kinetics

Kinetic curves of three photocatalytic systems (TiO₂ + UV, H₂O₂ + TiO₂ + UV, and H₂O₂ + Ag/TiO₂ + UV) under the optimal conditions are shown in Fig. 11. The photocatalytic degradation of pollutants on TiO₂ in liquid can be characterized as the first-order kinetics, which is most widely used to describe heterogeneous photocatalytic reactions (Muruganandham et al., 2006; Muruganandham and Swaminathan, 2006). The kinetic curves of three catalytic systems are fitted with the first-order kinetics as listed in Table 2. The three catalytic systems are all consistent with the first-order kinetics (R² in the ranges of 0.9626–0.9953). Moreover, the reaction rate constant k of the system (H₂O₂ + Ag/TiO₂ + UV) is 4.2 times and 1.5 times higher than for the systems (TiO₂ + UV) and (H₂O₂ + TiO₂ + UV), respectively, indicating that adding the electron capture agent and loading the precious metals can effectively improve the photocatalytic reaction.

FIG. 11.

Kinetic curves of photocatalytic degradation of phenol. (Reaction conditions: phenol concentration 20 mg/L, 0.5 wt.% Ag loading, catalyst concentration 0.1 g/L, H₂O₂ concentration 50 mmol/L, and pH = 5).

Table 2.

Dynamic Analysis of Photocatalytic Degradation of Phenol

Catalytic systems	C = Aexp(−kt)	R²	k (min⁻¹)
TiO₂ + UV	C = 21.06exp(−42.4 × 10⁻³t)	0.9862	42.4 × 10⁻³
H₂O₂ + TiO₂ + UV	C = 24.40exp(−118.9 × 10⁻³t)	0.9953	118.9 × 10⁻³
H₂O₂ + Ag/TiO₂ + UV	C = 26.07exp(−176.6 × 10⁻³t)	0.9626	176.6 × 10⁻³

H₂O₂, hydrogen peroxide.

Conclusions

H₂O₂ plays a great synergistic role on the photocatalytic degradation of phenol with M/TiO₂.The photocatalytic activities of M/TiO₂ are mainly related to the work function of these noble metals. Ag loading could greatly enhance the activity of TiO₂ due to the normal reduction mechanism and the “activated” mechanism between H₂O₂ and Ag nanoparticles. Experimental conditions have a significant impact on the catalytic activity. The phenol can be degraded by 65% within 3 h over Ag/TiO₂ under the conditions of 0.5 wt.% Ag loading, 0.5 wt.% Ag/TiO₂ concentration of 0.1 g/L, H₂O₂ concentration of 50 mmol/L, and pH value of the reaction liquid at 5, which are 45% more than that in the TiO₂ photocatalytic system. Furthermore, the degradation kinetics of phenol follows the first-order kinetics, and the determined reaction rate constant for phenol degradation is 176.6 × 10⁻³/min.

Footnotes

Acknowledgments

All the authors gratefully acknowledge support from the Fund for the Research and Development of Science and Technology in Shenzhen (No. JCYJ20150731104949789, JCYJ20150403161923536, CXZZ20130516145955144), the Special fund for the Development of Strategic and New Industry in Shenzhen (No. JCYJ20120613114951217), and the National High Technology Research and Development Program of China (2012ZX07206-002).

Author Disclosure Statement

The authors declare that all statements are correct. No competing financial interests exist.

References

Akbal

, and Onar

A.N.

(2003). Photocatalytic degradation of phenol. Environ. Monit. Assess., 83, 295.

Alaton

I.A.

, Balcioglu

I.A.

, and Bahnemann

D.W.

(2002). Advanced oxidation of a reactive dyebath effluent: Comparison of O₃, H₂O₂/UV-C and TiO₂/UV-A processes. Water Res. 36, 1143.

H.Q.

, Zhou

, Li

J.X.

, Zhu

B.L.

, Wang

S.R.

, Zhang

S.M.

, Wu

S.H.

, and Huang

W.P.

(2009). Deposition of Pt on the stable nanotubular TiO₂ and its photocatalytic performance. Catal. Commun., 11, 175.

Bauer

, Waldner

, Fallmann

, Hager

, Klare

, Krutzler

, Malato

, and Maletzky

(1999). The photo-fenton reaction and the TiO₂/UV process for waste water treatment-novel developments. Catal. Today, 53, 131.

Campbell

F.W.

, Belding

S.R.

, Baron

, Xiao

, and Compton

R.G.

(2009). Hydrogen peroxide electroreduction at a silver-nanoparticle array: Investigating nanoparticle size and coverage effects. J. Phys. Chem. C, 113, 9053.

Chai

, Peng

T.Y.

, Zeng

, and Zhang

X.H.

(2012). Preparation of a MWCNTs/ZnIn₂S₄ composite and its enhanced photocatalytic hydrogen production under visible-light irradiation. Dalton Trans. 41, 1179.

Chiarello

G.L.

, Aguirre

M.H.

, and Selli

(2010). Hydrogen production by photocatalytic steam reforming of methanol on noble metal-modified TiO₂. J. Catal., 273, 182.

Dias

I.N.

, Souza

B.S.

, Pereira

J.H.O.S.

, Moreira

F.C.

, Dezotti

, Boaventura

R.A.R.

, and Vilar

V.J.P.

(2014). Enhancement of the photo-Fenton reaction at near neutral pH through the use of ferrioxalate complexes: A case study on trimethoprim and sulfamethoxazole antibiotics removal from aqueous solutions. Chem. Eng. J., 247, 302.

Djeffal

, Abderrahmane

, Benzina

, Fourmentin

, Siffert

, and Fourmentin

(2014). Efficient degradation of phenol using natural clay as heterogeneous Fenton-like catalyst. Environ. Sci. Pollut. Res., 21, 3331.

10.

Fabbri

, Prevot

A.B.

, and Pramauro

(2006). Effect of surfactant microstructures on photocatalytic degradation of phenol and chlorophenols. Appl. Catal. B Environ., 62, 21.

11.

Flaètgen

, Wasle

, Luèbke

, Eickes

, Radhakrishnan

, Doblhofer

, and Ertl

(1999). Autocatalytic mechanism of H₂O₂ reduction on Ag electrodes in acidic electrolyte: Experiments and simulations. Electrochim. Acta, 44, 4499.

12.

X.L.

, Long

J.L.

, Wang

X.X.

, Leung

D.Y.C.

, and Ding

Z.X.

(2008). Photocatalytic reforming of biomass: A systematic study of hydrogen evolution from glucose solution. Int. J. Hydrogen Energy, 33, 6484.

13.

Harir

, Gaspar

, Kanawati

, Fekete

, Frommberger

, Martens

, Kettrup

, El Azzouzi

, and Schmitt-Kopplin

(2008). Photocatalytic reactions of imazamox at TiO₂, H₂O₂ and TiO₂/H₂O₂ in water interfaces: Kinetic and photoproducts study. Appl. Catal. B Environ., 84, 524.

14.

Kashif

, and Ouyang

(2009). Parameters effect on heterogeneous photocatalysed degradation of phenol in aqueous dispersion of TiO₂. J. Environ. Sci. China, 21, 527.

15.

Khraisheh

, Kim

, Campos

, Al-Muhtaseb

A.H.

, Walker

G.M.

, and AlGhouti

(2013). Removal of carbamazepine from water by a novel TiO₂-coconut shell powder/UV process: Composite preparation and photocatalytic activity. Environ. Eng. Sci., 30, 515.

16.

Lee

S.L.

, Scott

, Chiang

, and Amal

(2009). Nanosized metal deposits on titanium dioxide for augmenting gas-phase toluene photooxidation. J. Nanopart. Res., 11, 209.

17.

Mahmoodi

N.M.

(2013). Photocatalytic degradation of dyes using carbon nanotube and titania nanoparticle. Water Air Soil Pollut. 224, 1612.

18.

Martins

A.F.

, Mayer

, Confortin

E.C.

, Frank

C.D.

(2009). A study of photocatalytic processes involving the degradation of the organic load and amoxicillin in hospital wastewater. J. Clean., 37, 365.

19.

Michaelson

H.B.

(1977). The work function of the element and its periodicity. J. Appl. Phys., 48, 4729.

20.

Minella

, Marchetti

, Laurentiis

E.D.

, Malandrino

, Maurino

, Minero

, Vione

, and Hanna

(2014). Photo-Fenton oxidation of phenol with magnetite as iron source. Appl. Catal. B Environ. 154–155, 102.

21.

Muruganandham

, Shobana

, and Swaminathan

(2006). Optimization of solar photocatalytic degradation conditions of reactive yellow 14 azo dye in aqueous TiO₂. J. Mol. Catal. A Chem., 246, 154.

22.

Muruganandham

, and Swaminathan

(2006). TiO₂-UV photocatalytic oxidation of reactive yellow 14: Effect of operational parameters. J. Hazard. Mater., 135, 78.

23.

Pang

D.D.

, Dong

M.H.

, Ma

X.D.

, and Ouyang

(2014). Photocatalytic decomposition of gas-phase chlorobenzene with transition metal-doped KLaTi₂O₆ under visible light irradiation. Environ. Eng. Sci., 31, 1.

24.

Pradhan

, Murugavelh

, and Mohanty

(2012). Phenol biodegradation by indigenous mixed microbial consortium: Growth kinetics and inhibition. Environ. Eng. Sci., 29, 86.

25.

Rathinavelu

J.R.

, Jung

M.J.

, Im

J.S.

, Seak-Lee

, and Palanivelu

(2013). Electrospinning of polymer-unaided TiO₂ fibers and iron impregnation for sunlight-induced photo-Fenton's degradation of dyes. Environ. Eng. Sci., 30, 653.

26.

Shi

H.X.

, Zhang

T.Y.

, Wang

H.L.

, Wang

, and He

(2011). Photocatalytic conversion of naphthalene to alpha-naphthol using nanometer-sized TiO₂. Chinese J. Catal., 32, 46.

27.

Sivakumar

, Selvaraj

, Ramasam

A.K.

, and Balasubramanian

(2013). Enhanced photocatalytic degradation of reactive dyes over fetio₃/tio₂ heterojunction in the presence of H₂O₂, Water Air Soil Pollut. 224, 1529.

28.

Sun

, Zhang

, Du

J.S.

, Qiao

J.L.

, and Guan

X.H.

(2014). Reinvestigation of the role of humic acid in the oxidation of phenols by permanganate. Environ. Sci. Technol., 47, 14332.

29.

Tian

, Zhu

R.S.

, He

Y.B.

, and Ouyang

(2014). Improving photocatalytic activity for hydrogen evolution over ZnIn₂S₄ under visible-light: A case study of rare earth modification. Int. J. Hydrogen Energy, 39, 6335.

30.

Tian

, Zhu

R.S.

, Song

K.L.

, Ouyang

, and Cao

(2015). The effects of amount of La on the photocatalytic performance of ZnIn₂S₄ for hydrogen generation under visible light. Int. J. Hydrogen Energy, 40, 2141.

31.

Wang

Y.J.

, Lin

, Zong

R.L.

, He

, and Zhu

Y.F.

(2011). Enhanced photoelectric catalytic degradation of methylene blue via TiO₂ nanotube arrays hybridized with graphite-like carbon. J. Mol. Catal. A Chem., 349, 13.

32.

G.Z.

, Katsumura

, Muroya

, Lin

M.Z.

, and Morioka

(2002). Temperature dependence of carbonate radical in NaHCO₃ and Na₂CO₃ solutions: Is the radical a single anion? J. Phys. Chem. A, 106, 2430.

33.

Yang

L.M.

, Yu

L.E.

, and Ray

M.B.

(2008). Degradation of paracetamol in aqueous solutions by TiO₂ photocatalysis. Water Res. 42, 3480.

34.

Zhang

D.D.

, Qiu

R.L.

, Song

, Eric

, Mo

Y.Q.

, and Huang

X.F.

(2009). Role of oxygen active species in the photocatalytic degradation of phenol using polymer sensitized TiO₂ under visible light irradiation. J. Hazard. Mater., 163, 843.

35.

Zhang

G.S.

, Zhang

, Wang

, Minakata

, Chen

Y.S.

, and Crittenden

(2013). Stability of an H₂-producing photocatalyst (Ru/(CuAg)_0.15In_0.3Zn_1.4S₂) in aqueous solution under visible light irradiation. Int. J.Hydrogen Energy, 38, 1286.

36.

Zhao

, Xu

S.H.

, Zhong

J.B.

, and Bao

X.H.

(2004). Kinetic study on the photo-catalytic degradation of pyridine in TiO₂ suspension systems. Catal. Today. 93–95, 857.

Synergistic Photocatalytic Degradation of Phenol Using Precious Metal Supported Titanium Dioxide with Hydrogen Peroxide

Abstract

Abstract

Introduction

Experimental

Preparation of photocatalyst

Photocatalytic reaction

Characterization

Results and Discussion

Characterization of catalysts

Synergistic activity of M/TiO2 with H2O2

Effects on activity

Effects of the loading

Effects of the catalyst concentration

Effects of H2O2 concentration

Effects of pH

Photocatalytic degradation kinetics

Photocatalytic degradation kinetics

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Synergistic activity of M/TiO₂ with H₂O₂

Effects of H₂O₂ concentration