In situ alternative switching between Ti⁴⁺ and Ti³⁺ driven by H₂O₂ in TiO₂ nanostructures: mechanism of pseudo-Fenton reaction

Abstract

Here, we report the discovery that Ti⁴⁺ in TiO₂ nanostructures can be reduced in situ to Ti³⁺ in the presence of H₂O₂ to generate hydroxyl radicals for the oxidation of organic pollutants. The Ti⁴⁺ in the TiO₂ nanostructure and the reduced Ti³⁺ driven by H₂O₂ can alternatively switch in situ, which endows the catalyst with a long life and excellent cyclic ability.

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The ever-increasing issue of polluted water resources is currently one of the greatest concerns for science and technology.^1,2 Among the various types of pollutants, organic chemicals drained into water resources are one of the most serious due to the fast growth of the manufacturing industry. Many water treatment methods and processes have been applied to eliminate organic chemicals and purify waste water, such as precipitation, bio-degradation, and film filtration.^3–6 However, waste water treated by traditional water treatment methods still maintains a high chemical oxygen demand (COD), which cannot match the standard for industrial waste water disposal. Therefore, some techniques based on the oxidation or degradation of organic molecules in water have been used for the advanced treatment of waste water to further decrease COD, such as Fenton,^7–10 micro-electrolysis,¹¹ electrochemical catalytic oxidation,¹² photocatalytic degradation,^13–19 and photoelectrocatalytic methods.^20,21 Among these oxidation methods, an advanced oxidation process called the Fenton reaction²² has strikingly contributed to purify dye waste water by applying the hydroxyl radical to destroy the C–C, C–H, C–N, C–P or C–S bonds in the organic components and turn them into inorganic micromolecular compounds. The classical Fenton oxidation process involves adding Fenton reagent (mostly a solution with +2 valence iron ions and H₂O₂) into polluted water and adjusting the pH of the system to acidic conditions for the oxidation of organic molecules. The usual Fenton mechanism is the oxidation of ferrous to ferric ions to decompose H₂O₂ into hydroxyl radicals, which is represented by eqn (1).

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH (1)

The Fenton reaction needs a narrow pH range (∼3) to finish the water treatment, which requires large amounts of acid. In addition, this homogeneous Fenton system with the aqueous ferrous reagent produces masses of iron sludge that inevitably cause secondary pollution. To overcome the disadvantages of the classic Fenton methods, versatile efforts have been made to ameliorate the Fenton catalyst. Heterogeneous Fenton catalysis based on solid-state Fenton catalysts has been proposed and has attracted much attention. Compared with homogeneous Fenton reactions, heterogeneous Fenton reactions are differentiated by the use of nanostructured oxides, such as Fe₂O₃,²³ FeCo₂O₄,²⁴ MnFe₂O₄,^25,26 or CuFe₂O₄,²⁷ to substitute soluble ferrous reagents. During heterogeneous Fenton reactions, the oxidation reaction mainly occurs at the solid–liquid interface. The proposed mechanism of heterogeneous Fenton reactions is as follows: H₂O₂ reacts with the surface metal ions with a low valence, such as Fe²⁺, to produce hydroxyl radicals, and oxidizes organic pollutants to inorganic molecules. A simplified view of this mechanism is given in eqn (2)

Fe_surf²⁺ + H₂O₂ → Fe_surf³⁺ + OH⁻ + ˙OH (2)

One of the advantages of solid-state Fenton catalysis is that the heterogeneous Fenton reaction can occur in the reaction system with a broader pH range, which can reduce the necessity for acidic conditions. Another important advantage is that the solid-state catalyst can be used repeatedly in theory. However, the ions that dissolve during the Fenton reaction limit the cyclic application of solid-state Fenton catalysts. Zhang et al. developed Mn₃O₄ as a Fenton-like catalyst that was used at a temperature of 75 °C and achieved a fast organic molecule degradation. This finding indicated that the reaction temperature facilitates the kinetics of the decomposition of H₂O₂.²⁸ Nevertheless, defects still remain for solid-state catalyst-based heterogeneous Fenton reactions, such as the low recycling capability of catalysts and that iron ions dissolve into the waste water, which requires a second treatment. In addition, heterogeneous Fenton reactions should be conducted at over 50 °C to facilitate the reaction process. The higher operation temperature of these reactions could limit their practical applications. To the best of our knowledge, the acknowledged solid-state catalysts are oxides with the highest catalytic activity based on Mn⁴⁺ and Fe²⁺ as the active elements. However, these catalysts have poor cyclic properties and a low catalytic activity. Finding a new Fenton agent with novel catalytic properties is the most important task for achieving a breakthrough that uses a new type of solid-state catalysts for water treatment.

Titanium dioxide (TiO₂), one of the most popular semiconductors, has been applied in many fields, such as electric ceramics, gas sensors, lithium-ion batteries, solar cells, supercapacitors, and photochromic materials,^29–36 because of its excellent electronic band structure, abundance in the world, very good physical and chemical stability, and environmentally friendly properties.^37–41 Because variable valences can exist in TiO₂ nanostructures and because the valence of TiO₂ can be changed by environmental conditions in the presence of H₂O₂, TiO₂ is supposed to act as a Fenton catalyst in heterogeneous Fenton reactions.

Here, we report the discovery of the catalytic properties of TiO₂ nanostructures for the degradation of organic pollutant molecules with H₂O₂ present in the reaction system. The catalytic ability of TiO₂ with a high valence should originate from Ti³⁺ formed in situ by the H₂O₂ present in the system at a certain temperature, which differs from the heterogeneous Fenton process. Therefore, we defined this process as a pseudo-Fenton method. Compared with other Fenton catalysts, TiO₂ nanomaterials possess a very high catalytic activity, perfect cyclic properties, and a high stability, and they do not consume the catalyst or form sludge during water treatment. This new concept of pseudo-Fenton catalysts and methods will have great applications in practical waste water treatment.

The material characteristics of phase composition and morphology were studied via X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), as shown in Fig. 1. As shown in Fig. 1a, all the diffraction peaks in the pattern can be unambiguously indexed to the tetragonal anatase type (JCPDS card no. 99-0008). The Raman spectrum shows that there are six characteristic Raman active modes (3E_g + 2B_1g + A_1g), with a Raman shift of E_g = 146.1 cm⁻¹, which can be assigned to the anatase structure with the tetragonal space group I4₁/amd (Fig. 1b).^42,43 The product powder is composed of aggregates of microspheres approximately 1–3 μm in diameter (Fig. 1c). The microspheres are assembled by TiO₂ nanowires (Fig. 1d). The nanowires are approximately 15–20 nm in diameter and up to several hundred nanometers in length (Fig. 1e). The as-assembled TiO₂ microstructure is hollow, similar to an urchin; therefore, it is called an urchin-like microstructure in this study. Moreover, the SAED pattern in Fig. 1f shows that the as-synthesized TiO₂ has a polycrystalline structure, with a series of diffraction rings corresponding to the diffractions from the (101), (004), (200), (105), (204), (220), (215) and (224) planes, elucidating the formation of anatase TiO₂, which is consistent with the XRD result. The lattice structure image of the assembled nanowires clearly shows excellent, successive 2D atomic lattices with a spacing of 0.35 nm, corresponding to (101) planes of anatase TiO₂. The TiO₂ nanowire grew along [010], which is the common growth direction of 1D TiO₂ nanostructures (Fig. 1g).²⁹

Fig. 1 (a) XRD pattern of TiO₂ microspheres; (b) Raman spectrum of TiO₂ microspheres; (c and d) SEM images of TiO₂ microspheres; (e) TEM images of the TiO₂ microspheres and (f) the corresponding SAED pattern; (g) HRTEM image of TiO₂ nanowires on the surface of the TiO₂ microspheres.

To evaluate the catalytic properties of TiO₂ urchin-like nanostructures for H₂O₂ transformation, a 10 mL reaction system with methyl blue (MB) as a model pollutant was used.⁴⁴ To eliminate the photocatalytic oxidation effect on TiO₂ nanostructures during the pseudo-Fenton reaction process, the reaction was performed in a dark room. The MB degradation and cyclic MB degradation properties were recorded with various TiO₂ catalyst amounts, H₂O₂ dosages, and reaction temperatures (Fig. 2). In the initial reaction system, the concentration of MB was 100 mg L⁻¹, with 1.5 mL of H₂O₂. Different amounts of TiO₂ urchin-like nanostructures were added in the reaction system to optimize the catalyst dosage. As shown in Fig. 2a, without TiO₂ nanostructures, less than 5% of the MB is degraded in the system with 1.5 mL H₂O₂ and a temperature of 60 °C, even after a 50 min reaction. This finding indicates that the H₂O₂ is decomposed into hydroxyl radicals at a low rate at a certain temperature. Surprisingly, the degradation ratio of MB reaches 60% after a 50 min reaction by using only 1 mg of TiO₂ nanostructures as the catalyst in the same reaction system. When the amount of TiO₂ nanostructures increases, the MB degradation rate accelerates. It takes 30 min and 20 min to achieve the complete degradation of MB with 5 mg and 10 mg of TiO₂ microspheres, respectively. This result indicates that TiO₂ urchin-like nanostructures are the key factor in the oxidation reaction. Similarly, the effect of the amount of H₂O₂ on MB degradation was assessed by fixing the amount of TiO₂ nanostructures at 5 mg and varying the amount of H₂O₂ in the system. Fig. 2b illustrates the degradation of MB in the reaction system when the volume of H₂O₂ varies from 0 to 2.0 mL. Without H₂O₂, TiO₂ nanostructures cannot degrade MB in the dark despite a 5% physical adsorption. The degradation rate of MB can reach 92% in 50 min with only 0.5 mL of H₂O₂ added to the system. This result indicates that a small amount of H₂O₂ can endow the system with a very strong degradation rate of MB. When an increasing amount of H₂O₂ is added, the degradation rate of MB in the system increases. With over 1 mL of H₂O₂, MB is mostly removed in 50 min. Therefore, in the following experiments, we fixed the amounts of TiO₂ urchin-like nanostructures at 5 mg and of H₂O₂ at 1.5 mL.

Fig. 2 MB removal under various conditions. (a) Effect of catalyst dosage on MB degradation (10 mL of the initial MB concentration, 100 mg L⁻¹; H₂O₂ 1.5 mL, at 60 °C). Error bars represent standard deviation of the mean; (b) effect of H₂O₂ dosage on MB degradation (initial MB concentration, 100 mg L⁻¹; TiO₂, 5 mg, at 60 °C). Error bars represent standard deviation of the mean; (c) effect of temperature on MB degradation (initial MB concentration, 100 mg L⁻¹; catalyst, 5 mg; H₂O₂, 1.5 mL). Error bars represent standard deviation of the mean; (d) recycling/degradation of MB in presence of TiO₂ and H₂O₂ (10 mL of the initial MB concentration, 100 mg L⁻¹; H₂O₂, 1.5 mL, at 60 °C; TiO₂, 5 mg).

As is well known, the solid-state catalyst-based Fenton reaction works at a certain temperature. The temperature of the system is a crucial factor in such a reaction.²⁸ Therefore, the effect of temperature on the TiO₂-based pseudo-Fenton reaction was assessed by varying the temperature (Fig. 2c). To our surprise, at room temperature the degradation rate of MB is over 70% after a 50 min reaction with 5 mg of TiO₂ and 1.5 mL of H₂O₂, which clearly differs from that of the temperature requirement in the normal solid-state catalyst-based heterogeneous Fenton reaction system. A higher temperature results in a higher MB degradation rate. The complete degradation of MB can occur in 40 min at 50 °C. With temperatures over 60 °C, the MB in the system can be eliminated within 30 min. This TiO₂-based pseudo-Fenton process is more efficient than the normal solid-state catalyst-based Fenton process.⁴³ The cyclic performance of TiO₂ urchin-like nanostructures was studied by repeating the pseudo-Fenton reaction with the same TiO₂ catalyst under the same reaction conditions (Fig. 2d). After 10 cycles, the TiO₂-based pseudo-Fenton reaction achieves a MB degradation rate of almost 100%. Superior to other Fenton systems, TiO₂ nanostructures can be used repeatedly many times with the degradation efficiency almost unchanged. XRD patterns, SEM images, and Raman spectra of TiO₂ microspheres obtained after 12 cycles show that the crystallinity and morphology hardly change even after several cycles in the TiO₂-based pseudo-Fenton process, which further illustrates the stability of the catalytic properties of TiO₂ (ESI,† Fig. S1). As is well known, the traditional Fenton reaction is a catalyst-consuming process, and the reaction stops if all the low valence ions change to high valence ions. Therefore, we think that TiO₂ in this pseudo-Fenton reaction is not a conventional Fenton catalyst because it is a long-life, so a non-consumptive catalyst during the pseudo-Fenton process. The cyclic ability of the catalyst is a pivotal factor for its practical application. The pseudo-Fenton properties of TiO₂ urchin-like nanostructures are much better than those of other solid-state catalysts such as Fe₃O₄, especially for their perfect cyclic activity (ESI,† Fig. S2 and Table S1). The UV-vis absorption spectra taken from the reaction system at different reaction times confirmed that MB was degraded by a mineralization process during the pseudo-Fenton process (ESI,† Fig. S3).

According to the mechanism of traditional Fenton and solid-state Fenton reactions, radical-involved oxidation of the organic pollutants should be initiated from the reaction between low valence metal ions and H₂O₂. In this system, TiO₂ with a full 4+ valence should not possess the ability to catalytically decompose H₂O₂. However, the possible reaction between Ti⁴⁺ and H₂O₂ to form Ti³⁺ and HO₂˙ makes the Fenton reaction become reasonable. Therefore, a pseudo-Fenton reaction mechanism in this TiO₂-based system is suggested (Scheme 1). It is known that H₂O₂ can act as both an oxidizer and reducer.⁴⁵ In this work, we take advantage of this special chemical behavior of H₂O₂ and the valence variation ability of the titanium element to realize Fenton reactions in this system. Inspired by the Fe³⁺-based Fenton process,⁴⁶ when TiO₂ is in contact with a H₂O₂ molecule, Ti⁴⁺ on the surface of TiO₂ can react with H₂O₂ to generate HO₂˙, with the reduction of Ti⁴⁺ to Ti³⁺. Then, the formed Ti³⁺ sequentially reacts with H₂O₂ to generate ˙OH, with the oxidation of Ti³⁺ to Ti⁴⁺ in TiO₂ nanostructures. Both H₂O˙ and ˙OH can act as oxidizers to degrade organic molecules in solution. Therefore, TiO₂ coordinated with H₂O₂ has a very strong ability to degrade MB. During the pseudo-Fenton process driven by H₂O₂, Ti⁴⁺ can be reduced in situ to Ti³⁺, and Ti³⁺ can be oxidized in situ to Ti⁴⁺. The alternative switching between Ti⁴⁺ and Ti³⁺ ions endows TiO₂ with good cyclic properties in the pseudo-Fenton process, which is the greatest difference between the TiO₂ based pseudo-Fenton reaction and the conventional solid-state Fenton reaction.⁴⁷ The commercial TiO₂ product P25 also has a pseudo-Fenton performance but with a lower catalytic activity than that based on the urchin-like TiO₂ nanostructures in this work (ESI,† Fig. S4). The enhanced catalytic activity of urchin-like TiO₂ nanostructures should originate from their large surface area, which was 158.8167 m² g⁻¹ and 47.0438 m² g⁻¹ for urchin-like TiO₂ nanostructures and P25, respectively (ESI,† Fig. S5).

Scheme 1 The mechanism for the pseudo-Fenton degradation of MB by TiO₂ in the presence of H₂O₂.

The existence of Ti³⁺ ions in the TiO₂ urchin-like nanostructures in the presence of H₂O₂ should be fundamental to the proposed mechanism of the pseudo-Fenton process. Electron spin resonance (ESR) spectra of TiO₂ urchin-like nanostructures were recorded at 90 K before and after being dipped in the H₂O₂ solution, focusing on the detection of Ti³⁺ ions. As shown in Fig. 3a, no signal of the Ti³⁺ ion can be detected in the as-synthesized TiO₂ urchin-like nanostructures. However, the ESR spectrum of TiO₂ urchin-like nanostructures treated with H₂O₂ gives rise to a very strong Ti³⁺ signal, which demonstrates the presence of Ti³⁺ in the sample. According to previous reports, when crystalline TiO₂ is treated with a strong reductant such as N₂H₄, and H₂ at a high temperature,^42,48–51 the oxygen atoms in the lattice can be removed, and Ti³⁺ is naturally generated. In fact, the formation of Ti³⁺ in TiO₂ nanostructures normally occurs under complex and harsh experimental conditions.⁵² However, in this work, Ti⁴⁺ ions can transfer to Ti³⁺ ions through a facile in situ process. This finding will be of great impact for the chemistry of titanium dioxide.

Fig. 3 (a) The X-band ESR spectra of TiO₂ before and after the reaction (T = 90 K) with H₂O₂; (b) 100 μL of suspension containing 0.1 mg TiO₂ and 20 μL DMPO as the control; (c) all the samples were 50 μL volumes of mixtures of 7.5 μL H₂O₂ and 20 μL DMPO, with different quantities of TiO₂; (d) all the samples were 50 μL volumes of mixtures of 0.1 mg TiO₂ and 20 μL DMPO, with different concentrations of H₂O₂; (e) amperometric i–t curves (the initial potential is −0.35 V, and the volume of the electrolyte is 100 mL) and the corresponding open circuit potential–time curves; (f) AC impedance curves of TiO₂ in different electrolytes (Na₂SO₄ and Na₂SO₄/H₂O₂).

As proposed above, hydroxyl radicals act as putative contributors to the degradation of MB, where TiO₂ was used as a catalyst to decompose H₂O₂.⁵³ After the formation of Ti³⁺ in TiO₂ urchin-like nanostructures was demonstrated, the identification of the catalytic properties of Ti³⁺ ions for the formation of hydroxyl radicals from H₂O₂ should be the final information needed for the proposed mechanism of the pseudo-Fenton process. The ESR technique was applied to monitor the formation of hydroxyl radicals in the presence of both H₂O₂ and urchin-like TiO₂ nanostructures. In this experiment, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used to capture short-lived ˙OH radicals.^54,55 To avoid the hydroxyl radicals generated from the photocatalytic effect of TiO₂, all the experiments were performed in dark conditions. As shown in Fig. 3b, without H₂O₂ in the system, pure TiO₂ in water hardly causes a signal in dark conditions. In addition, the pure H₂O₂ in water caused weak signals. With both H₂O₂ and TiO₂ in the system, the ˙OH related signals are distinctively strong. Therefore, we conclude that TiO₂ bolsters the catalytic activity by accelerating the formation of hydroxyl radicals from decomposing H₂O₂, which accounts with the reaction mechanism. Moreover, when MB is added to the TiO₂ and H₂O₂ co-existing solution, the signal of DPMO/˙OH becomes very weak. This finding demonstrates that the decomposition of MB has consumed hydroxyl radicals generated from the reaction between TiO₂ and H₂O₂. The above results are the most convincing evidence for our suggested pseudo-Fenton reaction mechanism. To further confirm the catalytic effect of TiO₂ urchin-like nanostructures on the generation of hydroxyl free radicals, ESR measurements of DPMO/˙OH in the TiO₂ and H₂O₂ co-existing solution system with different amounts of TiO₂ nanostructures were recorded (Fig. 3c). The signal intensity of DPMO/˙OH obviously becomes stronger with more TiO₂ added to the system. Furthermore, the effect of the dosage of H₂O₂ on the formation of hydroxyl radicals was studied (Fig. 3d). Obviously, more H₂O₂ in the reaction system can induce a higher DPMO/˙OH signal; however, this enhancement is not significant. Similarly, as discussed in Fig. 2b, a small amount of H₂O₂ can induce a striking enhancement in MB degradation. However, the increase in H₂O₂ added does not result in a noticeable enhancement. This finding implies that the conversion of H₂O₂ into hydroxyl radicals may possess a threshold. Moreover, H₂O₂ does not induce a higher DPMO/˙OH signal, despite more ˙OH overall.

To further study the valence switching of the element Ti in TiO₂ urchin-like nanostructures with H₂O₂, electrochemical measurements of TiO₂ with and without H₂O₂ present were carried out on a model TiO₂ electrode sample through a three-electrode system by using 1 M Na₂SO₄ as the electrolyte. The model TiO₂ electrode was synthesized by assembling a TiO₂ nano-tree on the surface of a transparent conductive fluorine-doped tin oxide (FTO) glass substrate (ESI,† Fig. S7). The voltage of the three-electrode system is almost zero, but the voltage remains at ∼0.325 V when H₂O₂ is added to the electrolyte. Correspondingly, the current displays a stable intensity after adding H₂O₂ under conditions where the voltage is fixed at −0.35 V, which is beyond the opposite open circuit potential. Moreover, the electrochemical impedance spectra in Fig. 3f reflect that after H₂O₂ is added to the electrolyte, electron transportation is promoted. The above results confirm that TiO₂ reacts with H₂O₂, which enhances the electrical properties and reasonably explains the mechanism. To further couple the catalytic performance of TiO₂ microspheres/H₂O₂ with their practical application in the treatment of industry waste water, we exploited the degradation of dye waste water (initial COD, 600 mg L⁻¹). As shown in Fig. S6 (ESI†), the COD removal remains at ∼70%, and the catalytic activity of the TiO₂ microspheres is considerable after 12 cycles.

Conclusions

In summary, a new pseudo-Fenton reaction has been realized by using TiO₂ urchin-like nanostructures as catalysts with the presence of H₂O₂ in the reaction system at a temperature below 60 °C. The proposed pseudo-Fenton process is initiated from the in situ alternative switching between Ti⁴⁺ and Ti³⁺ in TiO₂ nanostructures driven by H₂O₂, and the Fenton reaction based on the reaction between Ti³⁺ ions reduced in situ by H₂O₂ in TiO₂ nanostructures and H₂O₂ forms Ti⁴⁺ and ˙OH. This in situ switching of Ti⁴⁺ and Ti³⁺ in TiO₂ nanostructures endows the catalyst with non-consumptive properties, excellent cyclic ability and a long life. This new discovery is dramatic for TiO₂-based nanomaterials applied in environmental protection, and promotes their practical applications in the degradation of organic pollutants.

Acknowledgements

The authors are thankful for funding from the National Natural Science Foundation of China (Grant No. 51372142), the 2014 Innovative Jiaxing Elite Leading Talents Program (A), and the Science and Technology Project Foundation of Jiaxing City (2015BZ12004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00163k

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