Efficient Photocatalytic Hydrogen Peroxide Production over TiO₂ Passivated by SnO₂

Abstract

Photocatalysis provides an attractive strategy for synthesizing H₂O₂ at ambient condition. However, the photocatalytic synthesis of H₂O₂ is still limited due to the inefficiency of photocatalysts and decomposition of H₂O₂ during formation. Here, we report SnO₂-TiO₂ heterojunction photocatalysts for synthesizing H₂O₂ directly in aqueous solution. The SnO₂ passivation suppresses the complexation and decomposition of H₂O₂ on TiO₂. In addition, loading of Au cocatalyst on SnO₂-TiO₂ heterojunction further improves the production of H₂O₂. The in situ electron spin resonance study revealed that the formation of H₂O₂ is a stepwise single electron oxygen reduction reaction (ORR) for Au and SnO₂ modified TiO₂ photocatalysts. We demonstrate that it is feasible to enhance H₂O₂ formation and suppress H₂O₂ decomposition by surface passivation of the H₂O₂-decomposition-sensitive photocatalysts.

1. Introduction

Hydrogen peroxide (H₂O₂) is a clean oxidant [1] that has wide applications in environmental purification and organic synthesis [2]. At present, H₂O₂ is industrially produced by the multi-step anthraquinone method, which consumes a lot of energy and organic solvent [3]. As an alternative to the anthraquinone method, direct photocatalytic synthesis of H₂O₂ has attracted widespread attention due to its mild reaction conditions, such as normal temperature and pressure under light irradiation. Many representative UV-light and visible-light active semiconductors (e.g., ZnO [4,5], C₃N₄ [6,7,8,9], BiVO₄ [10], etc.) have been verified as active materials for H₂O₂ production, particularly when proper cocatalysts are loaded on these semiconductors. An important feature of photocatalytic H₂O₂ formation is that it is accompanied by a decomposition process. TiO₂ is a widely used photocatalyst for synthesizing H₂O₂ directly from O₂ reduction in liquid phase under the irradiation of ultraviolet (UV) light [3,11,12,13,14,15,16,17], without using hydrogen. However, photocatalytic H₂O₂ synthesis with TiO₂ has a low formation reaction rate due to the catalytic decomposition [17,18]. Some studies have revealed the decomposition mechanism of H₂O₂ in aqueous TiO₂ suspension under UV irradiation [18,19,20]. Zhao et al. [18] reported that adsorption of H₂O₂ on TiO₂ will readily form surface peroxide complexes, which can be easily photodegraded with a zero-order kinetic process over TiO₂ even with the irradiation of visible light, thus leading to the decrease of H₂O₂ production. Therefore, in order to produce concentrated H₂O₂, on one hand, the formation rate of H₂O₂ should be increased, and on the other hand, the decomposition of H₂O₂ should be controlled.

Au is an active cocatalyst and has shown great potential in improving the performance of photocatalytic H₂O₂ production [15,16]. Tada et al. [15] reported enhancement of the photocatalytic activity of TiO₂ in producing H₂O₂ by Au cocatalyst loading. They also found that the formation of H₂O₂ was generally accompanied by decomposition. The concentration of H₂O₂ produced from the photocatalytic reduction of O₂ can also be increased by restricting its thermocatalytic decomposition through controlling the temperature and pH of the reaction solution [16]. Indeed, these studies improved H₂O₂ formation activity by suppressing the decomposition process by controlling the reaction condition. However, there is still no effective means of enhancing the intrinsic activity of photocatalysts for H₂O₂ production. It is well known that both the structures and band gaps of TiO₂ and SnO₂ are similar. They all consist of MO₆ octahedrons as the structural unit. The conduction band (CB) of SnO₂ (0 V vs. SHE) is more positive than that of the TiO₂ (−0.3 V vs. SHE) and it is possible to improve charge separation in a photocatalytic reaction when heterojunctions are formed by both oxides [21,22,23]. In this study, we prepared SnO₂-modified anatase TiO₂ (SnO₂-TiO₂) heterojunction using the molten salt method [24]. SnO₂ passivation suppressed H₂O₂ complexation and decomposition on the surface of TiO₂. Loading of Au nanococatalysts further remarkably enhanced the H₂O₂ formation rate.

2. Results and Discussion

2.1. Characterization of the Photocatalysts

Figure 1a shows the XRD patterns of 0.1% Au loaded SnO₂-TiO₂ obtained at different sintering temperatures. When the sintering temperature is 300 °C, almost no SnO₂ can be detected. It has been well documented that the melting point of LiCl-KCl eutectic of composition 61:39 is 352 °C [24]. Hence, the Sn precursor is impossible to dissolve by molten salt and SnO₂ cannot be formed on the TiO₂ at this temperature. The presence of SnO₂ can be identified when the sintering temperature reaches 400 °C. At a sintering temperature of 500 °C, the peak of SnO₂ becomes more obvious, implying higher modification of SnO₂. At the sintering temperature of 600 °C, anatase was partially transformed into a less active rutile phase [25]. The diffraction peaks of SnO₂ can be clearly identified with the increase in the Sn/Ti ratio (Figure 1b). The presence of SnO₂ and Au are certified by the EDX (Figure S1). STEM mapping images (Figure 1c–f) display that SnO₂ and Au are finely dispersed on the surface of TiO₂. Formation of such a SnO₂-TiO₂ heterojunction structure will improve the charge separation and photocatalytic activity [21,22,23].

As shown in Figure 1g, there are two prominent absorption bands in the UV-Vis spectra of Au/SnO₂-TiO₂. Besides the characteristic absorption bands of TiO₂ at 370 nm, the broad absorption bands in the visible region between 500 nm and 650 nm is due to the excitation of gold surface plasmon. In addition, the nanometal cocatalyst exhibit lattice fringes of 0.235 nm, which matches the interplanar spacing of Au (1 1 1) plane (Figure S2) [26]. The actual content of Au in 0.1% Au/SnO₂-TiO₂ was 0.106% by inductively coupled plasma mass spectrometry (ICP-MS).

2.2. Catalytic Activity of the Photocatalysts

The photocatalytic activity of H₂O₂ synthesis on Au/SnO₂-TiO₂ hybrids was tested under UV light and the concentration of H₂O₂ was quantified by spectrophotometry with copper ions and 2,9-dimethyl-1,10-phenanthroline (DMP) [27]. The standard curve indicates that the absorbance and concentration of H₂O₂ exhibits a good linear relationship with R square of 0.9996 (Figure S3). Figure 2a shows time courses for the H₂O₂ generation of photocatalysts obtained at different sintering temperatures. Noticeably, 0.1% Au loaded SnO₂-TiO₂ obtained at 500 °C exhibits the highest photocatalytic activity compared to that obtained at the other sintering temperatures. In combination with XRD data, when the sintering temperature is 300 °C, the content of SnO₂ in the samples is minimal, and the activity is not obviously improved. The photocatalytic activity of the samples obtained at 600 °C is the lowest, possibly because of the formation of a less active rutile phase.

Compared with pristine Au/TiO₂, Au/SnO₂-TiO₂ hybrids exhibited enhanced photocatalytic activity (Figure 2b). The highest photocatalytic activity was obtained over the sample with a Sn/Ti ratio of 4%, which also showed stable H₂O₂ production rate. The improved activity is mainly due to the suppression of H₂O₂ decomposition by SnO₂ passivation as discussed below. The excessive SnO₂ modification will possibly shield the photo absorption of TiO_2, and thus reduce the photocatalytic activity. It should be noted that we also did a control experiment, which is shown in Figure S4. It was found that there is a synergistic effect between Au and SnO₂ in H₂O₂ synthesis over TiO₂.

2.3. ESR Analysis

In general, H₂O₂ from 2e^– ORR by CB electrons can be produced through stepwise coupled electron and proton transfers (Equations (1)–(3)) [28,29]. In order to further study the formation mechanism of H₂O₂, in situ ESR measurements were carried out using DMPO as the trapper of radical species. The radical signals of DMPO-^•OOH consisting of six characteristic peaks were detected for all of Au and/or SnO₂ modified TiO₂, while there is no obvious signals for pure TiO₂ (Figure 3a,b), showing that H₂O₂ formation over Au loaded and/or SnO₂ modified TiO₂ photocatalysts is indeed a stepwise single electron reduction process, and both Au and SnO₂ could promote the formation of HO₂• via Equation (2).

TiO₂ → e⁻ + h⁺

(1)

e⁻ + O₂ + H⁺ → HO₂•

(2)

HO₂• + H⁺ + e⁻ → H₂O₂

(3)

2.4. The Photodecomposition of H₂O₂

Figure 4 shows the decomposition of H₂O₂ under different conditions. Pure H₂O₂ remains stable under dark conditions in the presence of 0.1% Au/SnO₂-TiO₂ (4% Sn/Ti ratio) (Figure 4a). Under UV irradiation without catalyst, the decomposition of H₂O₂ has a linear relationship with time. In the presence of TiO₂, 0.1% Au/TiO₂ or 0.1% Au/SnO₂-TiO₂, the H₂O₂ decomposition was accelerated by UV light, but the decomposition rate of H₂O₂ over 0.1% Au/SnO₂-TiO₂ is lower than TiO₂ and 0.1% Au/TiO₂. The relationship between hydrogen peroxide decomposition and time is nonlinear under UV irradiation, and the decomposition of H₂O₂ is similar to a one-order kinetics [30]. In order to investigate the decomposition solely induced by peroxide complexes, we tested the decomposition of H₂O₂ in an aqueous photocatalyst suspension with similar initial H₂O₂ concentrations under visible light irradiation (Figure 4b). H₂O₂ barely decomposed under dark conditions in an aqueous SnO₂-TiO₂ suspension or under visible light irradiation without any photocatalysts. In the presence of photocatalysts, the H₂O₂ decomposition was accelerated, but again, SnO₂ passivated TiO₂ had the lowest decomposition activity. All the decomposition processes under visible light irradiation can be roughly fitted with a near zero-order equation (linear relationship) [18]. It can be inferred that there is a synergistic effect between the photocatalysts and light irradiation in catalyzing H₂O₂ decomposition. However, SnO₂ passivated TiO₂ consistently showed suppressed decomposition activity in any condition (UV and visible light). It is important for SnO₂-TiO₂ heterojunction photocatalyst to maintain high and stable H₂O₂ production rate during the photocatalytic reaction. The generation and decomposition mechanism of H₂O₂ is shown in Figure 5. The electrons excited by UV light are transferred to the Au and SnO₂ to promote reduction of O₂ for H₂O₂ formation. Compared with easy H₂O₂ decomposition on pure TiO₂ via forming peroxide complexes, SnO₂ passivated TiO₂ suppressed H₂O₂ decomposition. Hence, this study provides a useful method for promoting H₂O₂ production over TiO₂ photocatalysts.

3. Materials and Methods

3.1. Materials Preparation

To produce a SnO₂-TiO₂ heterojunction structure, anatase TiO₂ powder (with a grain size of 5 nm and a specific surface area of 280 m²/g, Yifu Industrial Co., Ltd, Shanghai, China) was used as a photocatalyst. SnCl₄•4H₂O (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was used to modify TiO₂ by a molten salt method. In general, 3.4 g anatase TiO₂ and 0.6 g SnCl₄•4H₂O (4% molar ratio of Sn:Ti) were mixed with 3.77 g LiCl (Yongda Chemical Reagent Co., Ltd, Tianjin, China) and 4.23 g KCl (Yongda Chemical Reagent Co., Ltd, Tianjin, China) in an alumina crucible. The powder mixtures were calcined at 500 °C, then the mixtures were transferred into the deionized water for grinding, centrifuged, washed and dried. The Au with 0.1 wt % was loaded on photocatalysts by the deposition-precipitation method reported previously [31]. The decomposition of H₂O₂ can be suppressed to a certain extent by sodium fluoride. SnO₂-TiO₂ can produce valence band electrons and holes by UV irradiation.

3.2. Material Characterization

The powder X-ray diffraction (XRD) data were collected on an X-ray diffractometer (D/Ma-2500, Rigaku, Tokyo, Japan) operating with Cu Kα radiation (λ = 0.15406 nm). UV-VIS spectra were recorded with a Spectrum Lambda 750 S (Perkin-Elmer, Waltham, MA, USA). High-resolution transmission electron microscopy (TEM) characterization was performed with 8000EX microscope (JEOL, Tokyo, Japan) operating at 200 kV.

3.3. Photocatalytic Reactivity Test

A photocatalytic reaction kettle (200 mL) was used as a photocatalytic reaction device. NaF (0.1 M, Shentai Chemical Reagent Co., Ltd, Tianjin, China) was added as part of reaction medium together with 0.2 g Au/SnO₂-TiO₂ into the reaction solution of alcohol (4 wt %) and deionized water. The suspension solution was under ultrasonic treatment for 2 to 3 min. Then the mixed solution was poured into the reaction kettle and the oxygen was passed for 30 min. Then, a 300 W Xe arc lamp (PLS-SXE300, Bofeilai Technology Co., Ltd, Beijing, China) was used as a light source and was turned on the solution. The light was emitted by the xenon lamp and reflected by the UV light reflector. Magnetic stirring of the suspension was maintained throughout the reaction. The condensate water continued to pass the reaction kettle. Then, the concentrations of H₂O₂ generated were determined by using the DMP (2,9-dimethyl-1,10-phenanthroline, Wengjiang Chemical Reagent Co., Ltd, Guangdong, China) method [27].

3.4. Quantification of H₂O₂ (DMP Method)

One mL of DMP (0.1 g/L), 1 mL of cupper (II) sulfate (0.1 M), and 1 mL of phosphate buffer (pH 7.0) solution, and 1 mL of reaction solution were added to a 10 mL volumetric flask and was mixed, and then deionized water was added to the volumetric flask to the tick mark. After mixing, the absorbance of the sample at 454 nm was measured. The blank solution was prepared in the same manner but without H₂O₂.

The concentrations of H₂O₂ were calculated by the following formula:

A₄₅₄ = ζ [H₂O₂] × 1/10

where A₄₅₄ is the difference of the absorbance between sample and blank solutions at 454 nm, ζ is the slope of the calibration curve, and [H₂O₂] is the H₂O₂ concentration (μM).

3.5. Photocatalytic H₂O₂ Decomposition

Basically, photocatalytic H₂O₂ decomposition was carried out in the same reaction medium for photocatalytic H₂O₂ synthesis. Photocatalyst (0.2 g) was dispersed in water (200 mL) containing NaF (0.1 M), 4% C₂H₅OH (99.7%, Yongda Chemical Reagent Co., Ltd, Tianjin, China) and H₂O₂ (30%, Yongda Chemical Reagent Co., Ltd, Tianjin, China) with a fixed concentration. The suspension was stirred in the dark conditions for 30 min under constant N₂ bubbling to remove the air before light irradiation.

3.6. In Situ ESR Test

In situ electron spin resonance (ESR) analysis was performed to confirm the reduction pathway of O₂ over different catalysts, which uses 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping reagent. ESR signals of radicals trapped by DMPO were detected with a ESP 300E spectrometer (Brucker, Switzerland). Typically, catalyst (1 mg) was added to a mixture containing 1 mL alcohol/water (4 wt %) and 0.125 mmol DMPO. After passing the O₂ for 3 min, the sample was irradiated under UV light for 5 min before testing.

4. Conclusions

Au-modified SnO₂-TiO₂ was successfully prepared for enhanced photocatalytic activity for H₂O₂ production. The SnO₂-TiO₂ (4% SnO₂ modified anatase TiO₂) heterojunction prepared at 500 °C showed the best performance. H₂O₂ formed through a stepwise single electron reduction process over Au and/or SnO₂ modified TiO₂ via the formation of HO₂• intermediate. Under the band-gap excitation with UV light, decomposition of H₂O₂ seems to conform to a one-order kinetics process. SnO₂ passivation suppressed the decomposition of H₂O₂ over TiO₂ under both UV and visible light. This study provides a useful strategy to improve the performance of TiO₂ by modifying H₂O₂-inert oxide to decrease the decomposition of H₂O₂ during its photocatalytic synthesis.