Visible-light photocatalytic mechanism of bisphenol-A on nano-Bi₂O₃: a combined DFT calculation and experimental study

Abstract

The photocatalytic degradation of bisphenol-A (BPA) on Bi₂O₃ nanosheets under visible-light irradiation is uncovered and the degradation mechanism is proposed by exploring a reasonable pathway based on theoretical calculations and experiments. For the first time we hypothesized that the photogenerated hole Bi₂O₃˙⁺ took part in the early stage of the photocatalytic reaction. With DFT calculations both in the gaseous phase and aqueous phase, five intermediate products were determined: they are hydroquinone, 4-isopropenylphenol, 4-hydroxyacetophenone, methyl p-hydroxybenzoate and p-hydroxybenzoic acid, respectively. To test and verify our calculations, three intermediate products (m/z 151, m/z 133, and m/z 137) were detected by HPLC and LC-MS analyses. By exploring the function of the catalyst Bi₂O₃, we find that the Bi₂O₃ catalyst does play an important role in the direct oxidation of holes with bismuth-based semiconductors in the process of visible-light photocatalysis. The catalyst Bi₂O₃ segments the BPA in the early stage of the photocatalytic reaction and initiates the subsequent degradation reactions, which also serves as a donor of the charges and single electron. Based on the above theoretical calculations and experimental results, we propose a reasonable pathway of the photocatalytic degradation of BPA by Bi₂O₃. The photogenerated holes Bi₂O₃˙⁺ take part in the reaction, which can efficiently reduce the recombination of holes and electrons and thus overcome the deadly shortcoming of photocatalysis. This is critically important in a photocatalytic reaction. Our results will be helpful for understanding the process of visible-light photodegradation.

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1. Introduction

Bisphenol-A (BPA), known as an endocrine disrupting chemical (EDC), is widely used as the monomer for the production of polycarbonate plastics (e.g., in baby bottles) and as a major component of the epoxy resin in food can linings and dental sealants,¹ and has been spreading over the environment and has deteriorated the generative function of some species of living things on the Earth.² Various methods have been developed to remove BPA from water, including chemical oxidation,^3,4 electrochemical oxidation,^5,6 biological methods,^7–9 and photocatalytic methods.^10–12 Among these methods, photocatalytic oxidation is one of the most promising techniques because of its high mineralization efficiency, extremely efficient degradation rate, and low toxigenicity, ideally producing carbon dioxide and water as the final products.¹³ The popular photocatalyst TiO₂ has been widely used in the degradation of BPA in aqueous solution.^{10,11,14–17} However, it absorbs only ultraviolet light below 387.5 nm, which accounts for about 4% of sunlight.⁵ Therefore, it is desirable to develop visible-light-driven photocatalysts. To date, many bismuth-containing materials, such as Bi₂O₃,^18,19 BiVO₄,²⁰ Bi₂WO₆,²¹ Bi₂MoO₆,²² Bi₁₂TiO₂₀,²³ Bi₂Fe₄O₉,²⁴ Bi₃NbO₇,²⁵ NaBiO₃,²⁶ and BiOX,²⁷ have been found to be highly active visible-light-driven photocatalysts. Bi₂O₃, because of its simple structure, was selected for our investigation.

It has been reported^28,29 that Bi₂WO₆ and Bi_3.84W_0.16O_6.24 have considerable efficiency in degrading BPA, but the degradation mechanism is still unclear. Degradation of BPA using the Bi₂O₃ photocatalyst under visible-light irradiation also has not been reported. The most important is that previous studies on the degradation mechanism of BPA have only achieved some molecular fragments or a pathway by combining these molecular fragments,^12,30–32 but little has been reported about the reaction mechanism or the effects of the photocatalyst during the reaction, especially in theoretical calculations. It is still unclear whether the catalyst really participates in the photoreaction, and what kind of roles it performs.

To fill this deficiency, the present study investigated the photocatalysis mechanism of BPA by DFT calculations in both the gaseous and aqueous phases. In the calculations, we proposed that the photocatalyst participates in the reaction and attempted to model the photogenerated holes Bi₂O₃˙⁺ to investigate the degradation mechanism of BPA by the Bi₂O₃ catalyst, and identified five reasonable intermediates. To test and verify our calculations, we identified the intermediate products during the photocatalysis process by high performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC-MS). Subsequently, among the five intermediate products predicted theoretically, three were detected by LC-MS. Finally, we presented a pathway for the proposed degradation.

2. Modelling and computational methodology

To model the photogenerated holes Bi₂O₃˙⁺, a series of Bi₂O₃ structural models were constructed and optimized (ESI†). The optimized structure is used in our calculation, which possessed the lowest energy compared with others.

Geometry optimizations of the reactants, products, intermediates, and transition state structures of the reactions between BPA and the Bi₂O₃ catalyst were performed using the DFT method within the Gaussian 09 package.³³ Although the DFT method suffers slightly from spin contamination, it is appropriate for open-shell systems such as the reaction system under present consideration.³⁴ Our DFT calculations were performed using the hybrid B3LYP function, which combines Hartree, Fock and Becke exchange terms with the Lee–Yang–Parr correlation function. While a double-zeta LanL2DZ basis set which takes the relativistic effects into account³⁵ was adopted for the heavy atom Bi, a 6-31G* basis set was used for the other elements. Vibration frequencies were calculated to confirm if all of the structures optimized were at stationary points or true minima on the potential energy surfaces. Additional calculations were performed using the intrinsic reaction coordinate (IRC) method to confirm if each transition state was uniquely connected with the reactant and product under investigation. Moreover, re-optimization and frequency calculations were carried out for the optimized structures of the reactants, products, intermediates, and transition state structures using a polarizable continuum model (PCM) as the solvent model.^34,36

3. Experimental

The procedures for sample analysis are detailed in the ESI.† Briefly, the photogenerated intermediates in the BPA solution were separated by high performance liquid chromatography (HPLC, LC-10AT, Shimadzu, Kyoto, Japan) equipped with a UV detector and a C18 reversed phase column (5 μm, 4.6 mm × 250 mm) at room temperature with an injection volume of 20 μl. The mobile phase composition was acetonitrile–water (40/60, v/v) at a flow rate of 1 ml min⁻¹. To identify the intermediates from the photocatalytic oxidation of BPA, coupled liquid chromatography/mass spectrometry (LC-MS) in negative ion mode was applied. Selective ion monitoring (SIM) mode with a dwell time of 200 ms was used to acquire the mass spectra of BPA and its intermediates with a scan range of m/z 70–400.

4. Results and discussion

4.1 Analysis of degradation intermediates

To identify the photodegradation products of BPA, reaction intermediates from the photocatalytic process were analyzed with the HPLC and LC-MS techniques. BPA (m/z = 227, Fig. 1) was eluted at the retention time (RT) of 9.25 min and decreased to a minimum after 1 h, Besides BPA, three other intermediates were detected at respective RT values of 3.57 min (m/z = 151, m/z = 133) and 2.35 min (m/z = 137). The two intermediate products of m/z 151 and m/z 133 were always observed at the same time (Fig. 2a), which were identified as methyl p-hydroxybenzoate and 4-isopropenylphenol, which is in agreement with the literatures.^16,21,37 Then, the abundances of these two intermediate products increased to a maximum after 40 min and decreased gradually after subsequent photoreaction (Fig. 1b). The intermediate product of m/z 137 was identified as p-hydroxybenzoic acid, as shown in Fig. 2b. We detected it for the first time. With the decrease of the abundance of methyl p-hydroxybenzoate, p-hydroxybenzoic acid was hardly observed after 40 min of irradiation.

Fig. 1 (a) HPLC chromatograms of the BPA degradation over the Bi₂O₃ nanosheets under visible-light irradiation; (b) evolution of the intermediates of the BPA degradation during the photocatalysis process calculated from the peak areas (in the extracted ion chromatogram (EIC)) of the product ions.

Fig. 2 Mass spectra and the proposed corresponding structures at the retention times of (a) 3.57 min; and (b) 2.35 min, respectively.

4.2 Mechanism of photodegradation

Wang et al.³⁸ reported the photodegradation of BPA under simulated solar light irradiation by the photocatalyst Bi_3.84W_0.16O_6.24 and found no hydroxylated compound as determined by LC-MS or GC–MS or even by the ESR spin-trapping technique used with DMPO to monitor the reactive oxygen species generated in the reaction system. The reason for this finding is that the redox potential of Bi^V/Bi^III (E⁰ = +1.59 eV at pH 0)³⁷ is smaller than that of ˙OH/OH⁻ (1.99 eV).³⁸ As for Bi₂O₃, the photogenerated holes on its surface cannot react with OH⁻/H₂O to form any ˙OH. This implies that in the Bi₂O₃ photocatalytic reaction system, after the absorption of light with energy greater than 2.4 eV (wavelengths below 516 nm), the photocatalyst generates electron–hole pairs to yield conduction band electrons and valence band holes, as described in [eqn (1)] below. Then, the photogenerated holes, in the form of positive radical ions, directly act on the BPA molecules. The electrons trapped by the ubiquitous oxygen molecules at the particle surface to produce the superoxide radical anion, which is easily protonated to form hydroperoxy radicals in acidic solution [(eqn (2))].^12,39 The formation of O₂˙⁻ was also observed by using a chemiluminescent probe luminol in SMPD, as suggested in the literature,⁴⁰ which revealed that dissolved O₂ can trap the injected electrons, or the unpaired electrons of the organic radicals formed during the degradation reaction, or the unpaired electron of a hydrogen atom to form O₂˙⁻.³⁹ Experimental evidence for the formation of superoxide radicals has been observed in many degradation systems by the spin-trapping ESR technique,⁴¹ where the intermediates can be oxidized by it during the degradation.

Bi₂O₃ + hv → Bi₂O₃˙⁺_vb + e⁻_cb (1)

O₂ + e⁻_cb → O₂˙⁻ + H⁺ → ˙OOH (2)

The frontier electron density (ESI†) of BPA was calculated to predict the attack sites of the electrons and radicals, and the results are listed in Table 1. Owing to the direct hole oxidation that predominates at the beginning of the reaction, the first step is defined as an electrophilic reaction, where C4 or C10 with a maximum density value could be attacked more easily by the hole Bi₂O₃˙⁺, as shown in Fig. 3.

Fig. 3 The active sites of BPA for Bi₂O₃ attacks (grey for carbon, red for oxygen, and white for hydrogen).

Table 1 Frontier electron densities on atoms of BPA calculated at the B3LYP/6-31G* level

Atom	fe
C1	0.00988
C2	0.0327
C3	0.0326
C4	0.165
C5	0.0614
C6	0.0253
C7	0.0346
C8	0.0908
C9	0.114
C10	0.165
C11	0.0253
C12	0.0614
C13	0.0908
C14	0.0346
C15	0.114
O16	0.162
O17	0.162

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4.3 Calculation results

Fig. 4a illustrates the mechanism of the photogenerated holes Bi₂O₃˙⁺ attacking the BPA molecule, defined as step 1. First, the Bi₂O₃˙⁺ approaches and attacks the C10 of BPA until a C–O is formed, while C–C is broken (IM1) via the TS1 with a free energy barrier of 11.8 kcal mol⁻¹ (Fig. 4b). The C–O bond length decreases from 4.029 Å to 1.864 Å, the C–C distance increases from 1.676 Å to 3.324 Å, experiencing a transition state TS2, and its free energy barrier is 1.1 kcal mol⁻¹. By now, the Bi₂O₃ photocatalyst has split BPA into two molecular fragments, denoted as IM2, and its energy decreases by 15.4 kcal mol⁻¹ compared with reactant 1. The energy of this step in aqueous solution is higher than in the gaseous phase.

Fig. 4 (a) The proposed mechanism for step 1; (b) the optimized structures of the reactant, intermediates, transition state structures, and energy profile for the reaction of step 1-1 (grey for carbon, red for oxygen, white for hydrogen, and purple for bismuth).

Next, we analyzed the Mulliken atomic charges and Mulliken atomic spin densities of IM2. The results are listed in Table 2. Here, we denoted the left part of IM2 as A and right part as B. The total Mulliken atomic charge is 1 and the multiplicity is 2 at the beginning of this step, the positive charge and single electron being distributed on the photocatalyst Bi₂O₃. After the reaction, as we can see from Table 2, the positive charge transfers to A and the single electron is retained on the Bi₂O₃ (B). The mechanism of this step is shown in Fig. 5a. In this step, the charge and the single electron are respectively transferred to both parts of BPA (A or B). The Bi₂O₃ acts as a carrier of the charges and single electron deliverer, which presents another crucial effect of the photocatalyst Bi₂O₃.

Fig. 5 (a) The energy changes and optimized structures of the reactant, intermediates, and transition structures for step 1-2; (b) the energy changes and optimized structures of the reactant, intermediates, and transition structures for step 1-3 (grey for carbon, red for oxygen, white for hydrogen, and purple for bismuth).

Table 2 Mulliken atomic charges and Mulliken atomic spin densities for IM2 (B3LYP/C H O 6-31G* Bi LanL2DZ)

	Atomic charges		Atomic spin densities
	Gas	Aqueous	Gas	Aqueous
A	0.782	0.873	0.043	0.009
B	0.218	0.127	0.957	0.991

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Then, the B reacts with water molecules, which undergo a hydrolysis reaction, denoted as step 1-2. There are two structures of the transition state in this step, respectively formed by a four-membered ring TS3-1 with a free energy barrier of 10.9 kcal mol⁻¹ and a five-membered ring TS3-2 with a free energy barrier of 15.4 kcal mol⁻¹ (Fig. 5a). The difference between these structures is the reaction site on the reactant by the water molecule. In the TS3-1, with the approach of the water molecule, a Bi1–O2 bond formed, and the O2–H bond of H₂O is stretched to O1, to form a hydrogen bond O1⋯H⋯O2. Meanwhile, in the TS3-2, O2 is bonded with Bi2 forming a new Bi2–O2 bond. Then, the O2–H bond is broken with the retreat of the Bi₂O₃H, forming the first product hydroquinone, which might be detected as a dimer because its monomer is too active in aqueous solution. The energy of the system in aqueous solution is lower than that in the gaseous phase. Apparently, the photocatalyst Bi₂O₃ plays a crucial role in the BPA degradation, which segments the BPA and initiates the subsequent reactions. The photocatalyst Bi₂O₃ exists in aqueous solution in the form of Bi₂O₃H.⁴² As for A, since it has a positive ion on the carbon atom, water molecules can take an H atom from A and form a next-step product, 4-isopropenylphenol, by passing the TS4 (Fig. 5b) with a free energy barrier of 7.1 kcal mol⁻¹. This intermediate product corresponds to m/z 133 detected by LC-MS.

4-Isopropenylphenol is oxidized by O₂˙⁻ (Fig. 6) through step 2. The oxidized product 4-hydroxyacetophenone was detected previously.^43,44 It has been shown that dissolved oxygen and the amount of superoxide can efficiently enhance the degradation rate,^45–47 so we treated O₂˙⁻, adding a water molecule as the active species. Firstly, O1 moves close to C1, through a transition state TS5, and then IM3 is formed. With the approach of O1, the bond length of C1–O1 decreases to 1.44 Å (IM3) from 1.68 Å (TS5). The free energy barrier of TS5 is 21.7 kcal mol⁻¹ in the aqueous phase. Then, another oxygen atom O2 gets close to C2, and the bond length of C2–O2 declines from 3.08 Å (IM3) to 1.73 Å (TS6), where the free energy barrier of TS6 is 27.3 kcal mol⁻¹ in the aqueous phase. Finally, the product 4-hydroxyacetophenone is formed in the gaseous phase. Meanwhile, in the aqueous phase the product 4 is formed too, through an intermediate structure IM4, which could not exist stably in the gaseous phase. The total energy in aqueous solution presents a rising trend.

Fig. 6 (a) The proposed mechanism for step 2; (b) the energy changes and optimized structure of the reactant, intermediates, and transition structures for step 2 (grey for carbon, red for oxygen, and white for hydrogen).

Then, 4-hydroxyacetophenone is further oxidized by ˙OOH and finally forms methyl p-hydroxybenzoate through step 3 (Fig. 7), which is similar to a Baeyer–Villiger reaction.^48–51 Owing to the existence of hydroquinone, the solution is acidic and the 4-hydroxyacetophenone protonates easily. Then, with attack by ˙OOH, the bond length of C1–O1 decreases to 1.423 Å (IM5) via TS7 with a free energy of 17.0 kcal mol⁻¹ in aqueous solution, the original C1=O3 changes into C1–O3, and the bond length of C1–C2 stretches to 3.196 Å (TS8) from 1.538 Å (IM5). At the same time, the bond length of O1–O2 increases to 1.664 Å from 1.464 Å, and finally, the ˙OH leaves with H in the form of H₂O. The –CH₃ moves to O1 and bonds with it, resulting in the formation of methyl p-hydroxybenzoate. The total energy in aqueous solution decreases relatively compared with the gaseous phase. The free energy barrier of TS8 is about 24 kcal mol⁻¹, but the energy of product 4 decreases to 52.8 kcal mol⁻¹ compared with reactant 5. The total energy of the system drops significantly. This product corresponds to the m/z 151 detected in the experiment.

Fig. 7 (a) The proposed mechanism for step 3; (b) the energy changes and optimized structure of the reactant, intermediates, and transition state structures for step 3 (grey for carbon, red for oxygen, and white for hydrogen).

The methyl p-hydroxybenzoate further undergoes a hydrolysis reaction in acidic solution during step 4 (Fig. 8). First, we regarded the protonated methyl p-hydroxybenzoate and the water trimer as the reactants.⁵² During the reaction, an oxonium ion is interacting with two oxygen atoms through a hydrogen bond, the water molecules act as an internal proton donor and transfer,⁵³ and the protonated methyl p-hydroxybenzoate approaches the water molecule via a cyclic structure shown as TS9. The energy barrier is calculated to be 7.9 kcal mol⁻¹ in aqueous solution. The H3 transfers to and bonds with O3, the –O1H1 group of molecules bonds with C1, and a six-membered ring is formed. Then the C1–O3 bond is broken, CH₃OH is formed, and the remaining p-hydroxybenzoic acid exists in protonated form. The total energy decreases by 26.1 kcal mol⁻¹, compared with that in reactant 6. The p-hydroxybenzoic acid was detected by LC-MS, which corresponds to m/z 137.

Fig. 8 (a) The proposed mechanism for step 4; (b) the energy changes and optimized structures of the reactant, transition state structure, and product for step 4 (grey for carbon, red for oxygen, and white for hydrogen).

Finally, combining the theoretical calculations and experimental results, we put forward the proposed degradation mechanism of BPA by the Bi₂O₃ photocatalyst (Fig. 9). First, the Bi₂O₃ catalyst receives visible-light irradiation and generates e⁻–h⁺ pairs. The BPA molecules react with the photogenerated holes (Bi₂O₃˙⁺) directly to form the intermediates of hydroquinone and 4-isopropenylphenol. Then, the 4-isopropenylphenol is oxidized by O₂˙⁻ to give 4-hydroxyacetophenone, which is further oxidized to methyl p-hydroxybenzoate by ˙OOH. The methyl p-hydroxybenzoate undergoes a hydrolysis reaction in acidic solution and forms p-hydroxybenzoic acid. Finally, they are degraded to simple organic acid, which are further transferred to CO₂ and H₂O. Certainly, this is just one of the reasonable reaction pathways.

Fig. 9 The suggested degradation pathway of BPA by Bi₂O₃.

5. Conclusions

In summary, DFT calculations have been carried out to investigate the photocatalytic mechanism of bisphenol-A (BPA) on Bi₂O₃ nanosheets under visible-light irradiation. It is attempted to investigate the mechanism of degradation with DFT methods. Most of all, the photogenerated hole is taken into account in the calculation for the first time and it is assumed that it participates in the early stage of the photocatalytic reaction, which can efficiently reduce the recombination of holes and electrons and thus overcome the deadly shortcoming of photocatalysis. According to this assumption, five intermediate products were determined by DFT calculations. They are hydroquinone, 4-isopropenylphenol, 4-hydroxyacetophenone, methyl p-hydroxybenzoate and p-hydroxybenzoic acid. Moreover, to verify our calculation results, nano-Bi₂O₃ was prepared and the photodegradation experiment of BPA was carried out. By analyzing the intermediate products collected, three of the calculated intermediates were detected. We also explore the function of the catalyst Bi₂O₃. It illustrates that the catalyst Bi₂O₃ does play an important role in the direct oxidation of holes with bismuth-based semiconductors in the process of visible-light photocatalysis, which initiates a series of photocatalytic reactions and acts as a carrier of the charges and single electrons. Combining the calculations and experiments, a proposed pathway is presented. Our results will be helpful for understanding the process of visible-light photodegradation.

Acknowledgements

The support of the National Natural Science Foundation of China (21273081) is greatly appreciated. Financial support from the Project supported by Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2011) is also acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization of Bi₂O₃, materials and reagents, preparation and characterization of Bi₂O₃, frontier orbital analysis. See DOI: 10.1039/c3ra46783j

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