Chapter 1: Heterogeneous Catalytic Ozonation over Metal Oxides and Mechanism Discussion Free

Nowadays, the catalytic ozonation process, which includes homogeneous and heterogeneous catalytic ozonation, is often regarded as a potential technology in water treatment due to its notable performance. Compared to single ozonation and homogeneous catalytic ozonation, heterogeneous catalytic ozonation could increase oxidation rate as well as mineralization degree and decrease the utilization efficiency of ozone, and it is characteristic of reclamation and lack of secondary pollution. Thus it is considered to be a promising wastewater treatment process. Various metal oxides, including typical transition metal oxides, other single-metal oxides and mixed metal oxides, have been widely adopted as heterogeneous catalysts in catalytic ozonation because of their excellent catalytic capability. A comprehensive summary of the application and mechanism in heterogeneous catalytic ozonation can provide theoretical support and guidance for designing and choosing suitable catalysts to obtain higher removal efficiency in different kinds of wastewater. Therefore, the application and recent advances of various metal oxides in catalytic ozonation of organic contaminants in wastewater are summarized in this chapter. Moreover, their mechanism of heterogeneous catalytic ozonation by metal oxides, the reactive oxygen species and surface reaction process will also be discussed.

Several typical transition metal oxides that are promising as alternative catalysts have been intensively exploited during the heterogeneous catalytic ozonation process, such as manganese oxides, iron oxides and cobalt oxides.^1–4  Advances in their possible application and relevant ozonation mechanisms are described here in detail.

Manganese oxides possess different crystal structures (α-, β-, γ- etc.) and oxidation states (Mn²⁺, Mn³⁺, Mn⁴⁺ etc.); hence various manganese oxide catalysts including MnO₂, Mn₂O₃ and Mn₃O₄ have been applied for water and wastewater treatment during heterogeneous catalytic ozonation.^5–7  Numerous studies have been conducted on catalytic ozonation by manganese oxides in aqueous solution during the past decades. For instance, a manganese catalyst has been studied in the heterogeneous catalytic ozonation of phenol, indicating that hydroxyl radicals (˙OH)generated from the manganese catalyst play a significant role in the oxidation of phenol.⁵  Manganese catalysts in this study can react with ozone (O₃) and then undergo several steps to completely mineralize the phenol. The catalytic activity of Mn₂O₃ nanoparticles also has been investigated in the presence of ozone for the removal and decomposition of humic acids (HAs).⁸  The results show that Mn₂O₃ with a higher point of zero charge exhibits effectively catalytic performance for ozone decomposition and HAs removal. Besides, Nawaz et al. have investigated several manganese oxides with controlled morphologies (including MnO₂, Mn₂O₃ and Mn₃O₄) for catalytic ozonation of phenolic compounds; it was ascertained that MnO₂ has greater catalytic activity than Mn₂O₃ and Mn₃O₄ since it possesses considerable electron transferability and a larger number of oxygen vacancies and hydroxyl groups on the surface.⁹ 

Among the various manganese oxides, MnO₂ has been proved to be the most efficient catalyst for the decomposition of ozone and degradation of organic pollutants in many studies. Apart from the oxidation states of manganese oxides, crystal structure is an important factor determining the activity of manganese oxides. The degradation of 4-nitrophenol (4-NP) by catalytic ozonation over various MnO₂ with six crystal phases (α-, β-, γ-, δ-, ε- and λ-MnO₂) have been studied, and the results indicate that there are various removal efficiencies of 4-NP in MnO₂ with different crystal structures.¹⁰  Among them, α-MnO₂ is the most efficient catalyst for the catalytic ozonation of 4-NP, in which the generated superoxide radicals (˙O₂⁻, 60.2%) are the primary reactive oxygen species (ROS), while singlet oxygen (¹O₂, 27.7%) contributes a little bit to 4-NP removal. In addition, three kinds of MnO₂ with tunneling structures, including α-MnO₂, β-MnO₂ and γ-MnO₂, have been synthesized and applied to catalytic ozonation in order to explore the effect of the crystal structure of MnO₂, indicating that the order of the catalytic capability by MnO₂ in heterogeneous catalytic ozonation is α-MnO₂ > γ-MnO₂ > β-MnO₂.¹¹  It is obvious that α-MnO₂ exhibits superior activity during catalytic ozonation due to its larger specific surface area, the lower average oxidation state of Mn and the higher density of oxygen vacancies. To investigate the effect of the crystal phase in the catalytic ozonation process, three types of MnO₂ (α-, β- and γ-MnO₂) have been developed by a uniform hydrothermal process and further investigated for the catalytic ozonation of phenol.¹²  Compared with single ozonation, ROS with strong oxidation can be formed by the reaction between ozone and Mn–O bonds during the catalytic ozonation processes, which contributes to better oxidation efficiency. In addition, the increasing catalytic ozonation activity has been observed in the α-MnO₂/O₃ system with its enhanced surface active oxygen. The analysis confirmed that there were two critical factors for the enhanced catalytic activity of α-MnO₂: active surface oxygen and lattice oxygen. Also, the catalytic activities of β-MnO₂ and γ-MnO₂ are affected by the lattice oxygen and the bonded manganese of MnO₂.

According to previous studies, moderately acidic pH is of great benefit to the heterogeneous catalytic ozonation for organic contaminants removal by MnO₂ in an aqueous solution. The obvious indication is that the catalytic ozonation efficiency of oxalic acid significantly increases with the decreasing pH of the aqueous solution and that the appropriate range of pH is from 4.1 to 6.0 for the improvement of ozonation capability.¹³  Therefore, the pH of the solution substantially influences the catalytic efficiency of manganese oxides, and the results acquired by heterogeneous catalytic ozonation over MnO₂ catalysts with different pH_pzc values are consistent with the reaction mechanism involving the formation of a surface manganese–oxalic acid complex. Similarly, a lack of catalytic activity occurred in different types of MnO₂ with a lower pH_pzc than that of the solution pH.

In addition, it has been concluded that the capabilities of manganese oxides in catalytic ozonation are dependent on morphology. In previous research, an α-MnO₂ nanotube and β-MnO₂ nanowires have been developed as ozonation catalysts and have revealed remarkable stability and catalysis for phenol degradation.^14,15  Compared to that of ozonation alone, the α-MnO₂ nanotube and β-MnO₂ nanowires remarkably accelerate the removal of phenol and chemical oxygen demand (COD). The degradation efficiency of phenol is up to approximately 94.9% in the presence of an α-MnO₂ nanotube, while there are increases of 38% and 27.1% on the degradation and mineralization efficiencies of phenol in the β-MnO₂ nanowires/ozone system, respectively. Petal-like δ-MnO₂ microspheres have been successfully synthesized and evaluated for efficiencies of catalytic ozonation for the degradation of bisphenol A (BPA) and ibuprofen (IBP).¹⁶  The degradation efficiencies of BPA and IBP in 20 min are 68.2% and 68.5%, respectively. Clearly, petal-like δ-MnO₂ microspheres have greater catalytic efficiencies than those of ozone alone and other heterogeneous catalytic commercial MnO₂/O₃ systems. In addition to the generation of ˙OH, the strong interaction between ozone and organic matters on the surface of metal oxide catalysts contributes to the rapid catalytic ozonation of organic compounds in the system of δ-MnO₂ microspheres and ozone.

Considering their unique structures with large specific surface areas and high porous properties, manganese oxides with hollow structures have been designated as an efficient ozonation catalyst with superior ozonation capability owing to the synergetic effect of adsorption and degradation for organic contaminant removal.¹⁷  Three-dimensional α-MnO₂ porous hollow microspheres have been applied for the degradation of BPA in catalytic ozonation, and more than 90% degradation efficiency can be achieved within 30 min. This is higher than that of β-MnO₂ porous hollow microspheres and is attributed to the acceleration of the ROS generated rate (such as ˙O₂⁻ and ˙OH), as well as to the more abundant lattice oxygen on the surface of α-MnO₂ porous hollow microspheres. Recently, α-MnO₂ with its mesoporous structure possesses a higher surface area and also shows the highest catalytic activity compared to those with lower surface areas. This indicates that the critical factors affecting the catalytic efficiencies of catalysts are the porous structure and specific surface areas in catalytic ozonation. For example, the mesoporous α-MnO₂ has been synthesized by using cetyltrimethylammonium bromide (CTAB), and studies on its catalytic ozonation activities for 4-NP degradation, compared with the MnO₂ synthesized with sodium dodecyl benzene sulfate (SDBS) as well as commercial MnO₂, have also been carried out.¹⁸  Although 4-NP is completely degraded after 90 min during these catalytic ozonation processes, the removal efficiency of total organic carbon (TOC) is the highest in the mesoporous α-MnO₂/O₃ system. In this study, superoxide radicals are verified to have made a great contribution to the catalytic ozonation process, whereas hydroxyl radicals are not attributed to the removal of 4-NP. An ordered mesoporous β-MnO₂ has also been prepared by the nanocasting method and investigated for the catalytic degradation efficiency of phenol in the catalytic ozonation process.¹⁹  There was an obviously great increase in phenol removal compared with the control MnO₂. In addition, mesoporous Mn₂O₃ has shown higher activity in benzene oxidation with ozone than commercial bulk Mn₂O₃ because of its higher surface area as well as its large number of oxygen vacancies and lattice oxygen.²⁰ 

Owing to the abundant active sites and hydroxyl groups on their surface, iron oxides, including Fe₂O₃, Fe₃O₄ and FeOOH, with diverse structures and properties have been widely investigated in the catalytic ozonation process.^21–23  The outstanding performances in catalytic ozonation are attributed to the differences in their composition and crystal structures. For instance, FeOOH possesses abundant hydroxyl groups on its surface, while Fe₃O₄ offers a superparamagnetic property. Furthermore, many Lewis acidic sites deposited on the surface of iron oxides can promote the decomposition of ozone into ROS. In previous studies, Trapido et al. have investigated and compared their catalytic ozonation of m-dinitrobenzene by different metal oxides as heterogeneous catalysts.²⁴  Their results have revealed that the catalytic capability of Fe₂O₃ is the highest of all others due to the additional ozone decomposition accomplished by accumulating the formation of ROS; these results indicate that iron oxides could be the most potent catalysts in m-dinitrobenzene ozonation. Recently, the transformation process of ozone on various iron oxides, including α-Fe₂O₃, α-FeOOH and Fe₃O₄, with different acid sites and hydroxyl groups has been investigated.²³  It has been shown that Lewis acid sites located on α-FeOOH and Fe₃O₄ are the active centers of catalytic ozonation, in which ozone molecules could substitute for the surface ˙OH on the Lewis acid sites of iron oxides and directly interact with the iron ions in the surface of iron oxides, effectively decomposing into ROS and initiating the redox of iron ions.

Compared with various iron oxides, FeOOH has received significant attention and has become a promising heterogeneous catalyst in catalytic ozonation because of their extremely low solubility in water and more Lewis acid sites on their surface.^25–27  In addition, they are also efficient adsorbents for the removal of organic matter and inorganic ions in water. FeOOH, with a variety of crystal phases, is used to catalyze ozone for the degradation of contaminants, including goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH)and feroxyhyte (δ-FeOOH). The comparison of catalytic activities of α-FeOOH in acidic and neutral conditions for the ozonation of oxalic acid (OA)has been carried out in Sui's work.²⁶  It was revealed that α-FeOOH could efficiently boost the formation of ROS (˙OH)under acidic and neutral conditions, contributing to the improvement of the ozonation efficiency of OA by accelerating ozone decomposition. In this case, ˙OH has been generated from ozone decomposition both in a neutral state (Fe–OH)and a positive charge state (Fe–OH₂⁺) and performs as the main ROS in heterogeneous catalytic ozonation. In addition, different FeOOH catalysts, including SO₄²⁻FeOOH, Cl⁻–FeOOH and NO₃⁻–FeOOH, have been proposed and used for IBP removal in catalytic ozonation. Among them, the degradation efficiencies of IBP follow the order of SO₄²⁻–FeOOH (40.2%) > NO₃⁻–FeOOH (35.7%) > Cl⁻–FeOOH (34.6%). The results reveal that the pH of the aqueous solution is a critical factor for the charge properties of surface hydroxyl groups on the interface of metal oxides. Because of the closeness of the pH_pzc value of SO₄²⁻–FeOOH (7.12) to the pH of IBP solution (7.05), SO₄²⁻–FeOOH presents a higher capacity than that of the other two FeOOH in catalytic ozonation. Moreover, SO₄²⁻–FeOOH has possessed more hydroxyl groups than that of other catalysts, which can enhance its catalytic activity by the generation of ˙OH. Furthermore, ultra-small β-FeOOH nanorods are used as an effective catalyst to decompose ozone for the degradation of 4-chlorophenol (4-CP) in water.²⁷  Compared with ozonation alone, the degradation efficiency of 4-CP in heterogeneous catalytic ozonation has been significantly improved by adding β-FeOOH catalyst. The removal efficiencies in the ozonation alone and heterogeneous catalytic ozonation system within 40 min are 67% and 99%, respectively. The high degradation efficiency of 4-CP in the β-FeOOH/O₃ system can be attributed to direct ozonation by ozone, the heterogeneous catalytic ozonation and homogeneous catalysis owing to β-FeOOH dissolution.

Although the redox transformation between Fe²⁺ and Fe³⁺ ions accelerates the generation of ˙OH by directly decomposing ozone, iron ions dissolved and released from iron oxides could lead to secondary pollution in an aqueous environment. The recovery of the catalyst after catalytic ozonation and the release of iron ions restricts the reuse of the iron oxides in heterogeneous catalytic ozonation. Therefore, several studies have attempted to suppress the release of metal ions by improving the stability of the catalysts. Ferrocene, which is used as an efficient and recyclable heterogeneous catalyst, has been developed for catalytic ozonation since it is nontoxic and highly stable.²⁸  Once ferrocene is introduced to the ozonation system, amaranth can be almost completely degraded in 120 min, and TOC decreases significantly during the degradation. It has been demonstrated that the degradation of amaranth has been considerably improved in the ferrocene/O₃ system. Furthermore, there is little change in the degradation efficiencies of amaranth after several cycles, and ferrocene remains intact and undecomposed without any regeneration treatments, indicating that ferrocene as an alternative catalyst can be recyclable and remain highly efficient for catalyzing ozonation.

The development of magnetic iron oxides also can resolve the issue of catalyst recovery because it can be easily separated from the aqueous solution for further reuse. In addition to its magnetic property, Fe₃O₄ generally contains both Fe²⁺ and Fe³⁺ in the octahedral sites, which can remarkably accelerate electron transfer during the heterogeneous catalytic ozonation process. This unique property can be employed to facilitate the formation of ROS and further enhance catalytic ozonation efficiency. Thus in recent years, magnetic Fe₃O₄ has been fabricated by various methods and successfully applied as an efficient catalyst for the mineralization of organic contaminants in heterogeneous catalytic ozonation. In the research of Yin et al., Fe₃O₄ nanoparticles have been prepared by a low-cost and green route and utilized in the catalytic ozonation of sulfamethoxazole (SMX).²⁹  It has been demonstrated that Fe₃O₄ nanoparticles can significantly boost the ozonation efficiency of SMX, which has been increased by 51% compared with ozonation alone. The increasing number of Lewis acid sites on the interface of the catalyst can be formed to interact with ozone for SMX degradation by adding Fe₃O₄ into the catalytic ozonation process, which is the easiest way to attack the targeted contaminants for greater catalytic efficiency in SMX removal. Furthermore, Fe₃O₄, with its ordered mesoporous structure, has been successfully synthesized and intensively studied for its catalytic capability in the heterogeneous ozonation process.³⁰  Compared to Fe₃O₄ nanoparticles synthesized by the conventional method, the ordered mesoporous Fe₃O₄ exhibits a superior capability for atrazine (ATZ) degradation in heterogeneous catalytic ozonation. Homogeneous catalytic ozonation using the leached iron ions does not contribute to the degradation of ATX, which reveals that the heterogeneous catalytic ozonation promotes the generation of ROS and dominates the degradation of organic contaminants in the ordered mesoporous Fe₃O₄/O₃ system. In addition, the redox cycles between Fe²⁺ and Fe³⁺ that occurred in Fe₃O₄ contribute to the generation of ˙OH which is the dominant ROS in ATZ degradation during the heterogeneous catalytic ozonation process.

Transition metal cobalt oxides also are the prevalent catalysts that have been proposed for heterogeneous catalytic ozonation. Co₃O₄ is a common form in cobalt oxide, and the improvement of organic contaminants mineralization has been achieved in the Co₃O₄/O₃ system owing to the formation of ˙OH from ozone decomposition.^31–33  Many researchers have investigated the catalytic capability of Co₃O₄ as an alternative heterogeneous ozonation catalyst for the mineralization of organic contaminants; this indicates that the morphology and structure of cobalt oxide significantly affect its catalytic ozonation performance. Initially, Co₃O₄ nanoparticles with different average diameters have been synthesized, and their properties of catalytic ozonation have been investigated in the degradation of phenol.³⁴  The degradation efficiencies of phenol in aqueous solution have been studied in the absence and presence of Co₃O₄ nanoparticles during the ozonation process. The results indicate their dramatically catalytic capabilities for phenol mineralization, which denotes an alternative application as a heterogeneous ozonation catalyst in wastewater treatment. In a comparison of various Co₃O₄ nanoparticles with different average diameters, there is a slight increase in catalytic capabilities with decreasing average diameter, which is attributed to the better dispersion and higher surface area of Co₃O₄ with smaller sizes. It is further explained that these are important for catalysis efficiency.

Subsequently, two kinds of cobalt oxides, bulky Co₃O₄ and Co₃O₄ nanoparticles, were investigated to assess their catalytic activity, which indicated that Co₃O₄ nanoparticles exhibited superior ozonation efficiency in the mineralization of phenol compared with bulky Co₃O₄ and ozone alone.³⁵  There is a small negative effect on catalytic ozonation in the presence of tert-butyl alcohol, revealing that ˙OH is the main ROS and that the catalytic reaction significantly depends on the surface properties of Co₃O₄ nanoparticles in heterogeneous catalytic ozonation for phenol degradation. In particular, the specific surface area, the number of hydroxyl groups and the good dispersibility of Co₃O₄ nanoparticles are beneficial for their higher catalytic efficiency than that of bulky Co₃O₄.

Compared with other metal oxides, Co₃O₄ not only exhibits an excellent catalytic activity but also possesses a high selectivity for catalytic products during heterogeneous catalytic ozonation. Ichikawa et al. have elucidated the catalytic ozonation activities of various metal oxide catalysts for the ozonation decomposition of ammonia nitrogen in water.³⁶  Among these metal oxide catalysts, MgO and NiO have exhibited excellent catalytic activities, but they possess low selectivity to catalytic ozonation products, which necessitates producing a large amount of NO₃⁻. Nevertheless, Co₃O₄, which is slightly less efficient than MgO and NiO, has a higher selectivity for gaseous products in all of those metal oxides. Thus Co₃O₄ is regarded as the optimal catalyst in catalytic ozonation under the comprehensive consideration of catalytic activity, selectivity and stability. Similarly, they have further investigated and found that the catalytic activity of Co₃O₄ can be greatly improved by repeated catalysis for the catalytic ozonation of ammonia nitrogen, which is primarily the reaction between the ammonium ion with hydroxyl groups and the formation of Co–NH_x groups on the Co₃O₄ surface.³⁷  Recently, the catalytic ozonation mechanism has been the subject of an intensive investigation involving the heterogeneous catalytic ozonation of ammonia nitrogen in the presence of Co₃O₄.³⁸  The results demonstrate that Co₃O₄ effectively promotes the formation of chloramines as ozonation products during the heterogeneous catalytic ozonation process.

Currently, there is a decreasing trend in catalytic ozonation by cobalt oxide alone. In recent decades, much investigation has been done on cobalt oxides with excellent behavior being supported on some catalysts, including other metal oxides and carbon materials and/or being combined with other oxides for catalytic ozonation.

Heterogeneous catalytic ozonation involves the decomposition of ozone by various transition metal oxides; thus the reaction process and mechanisms are much more complicated. To a large extent, the efficiencies of heterogeneous catalytic ozonation are heavily dependent on the properties of the catalyst as well as on their surface structure. On the active sites of the catalyst's surface, organic pollutants can be adsorbed; these then form surface complexes with hydroxyl groups or ozone that can be decomposed to all kinds of ROS, including surface atomic oxygen (*O), ˙O₂⁻ and ˙OH, on the surface of the catalysts. The decisive factor regarding the ozonation rate of metal oxides species and the removal efficiency of organic contaminants depends on the generated ROS involved in different reaction mechanisms. Several corresponding reaction mechanisms of heterogeneous catalytic ozonation have been carried out in previous investigations, including the radical mechanism, surface complexes theory, oxygen vacancies theory and the surface atomic oxygen mechanism.

According to previous studies performed on several transition metal oxides, catalytic ozonation mainly follows the radical oxidation pathways in some cases in which ˙O₂⁻ and ˙OH are the main ROS involved in the heterogeneous catalytic ozonation. The surface-bond and/or isolated hydroxyl groups on the surface of metal oxides can accelerate the decomposition of ozone, subsequently initiating the generation of strong oxidative ROS, including ˙O₂⁻, ˙OH and even ¹O₂. Specifically, this is greatly prone to the hydroxylation that occurred on the surface of metal oxides in the aqueous solution; the Brønsted acid sites can then be observed in these situations by delivering the protons from the surface hydroxyl groups on metal oxides. Meanwhile, diverse metal cations, as well as coordinately unsaturated oxygen, can be formed, which may be used at the Lewis acid and base centers for heterogeneous catalytic ozonation. Surface hydroxyl groups and these active centers deposited on the surface of metal oxides not only affect the adsorption of organic contaminants but also influence ozone decomposition during catalytic ozonation. Therefore, the Brønsted acid sites, as well as the Lewis acid and base sites, are regarded as the reactive centers of metal oxide catalysts for the degradation of organic contaminants in heterogeneous catalytic ozonation, which always reacts with ozone according to its electrophilic or nucleophilic property. Generally, ozone can combine primarily with the active centers on the surface of metal oxides and can then be decomposed into different radicals through a series of catalytic reactions (eqn (1.1)–(1.5)). The corresponding catalytic ozonation mechanisms by typical transition metal oxides are shown in Figure 1.1.

For example, Sun et al. have investigated the catalytic efficiency of OA in the MnO_x/O₃ system, in which ˙OH is the primary ROS followed by the radical mechanism.³⁹  The protonated surface hydroxyl groups (Mn–OH₂⁺) significantly promote the adsorption of OA and the generation of ˙OH on the surface of MnO_x in catalytic ozonation, which can play a critical role in the catalytic capability of metal oxides catalysis. The superior degradation efficiency of OA can be achieved by adding the MnO_x because of an increasing number of surface hydroxyl groups generated on the surface of MnO_x. Furthermore, Sui et al. also have demonstrated that FeOOH as an efficient heterogeneous ozonation catalyst can effectively accelerate the generation of ˙OH under acidic and neutral pH conditions and further improve the removal capability of OA by ozonation alone.²⁶  It is clear that all the protonated and neutral hydroxyl groups on the surface of typical metal oxides can be used as the active sites for decomposing ozone into ˙OH.

In addition, previous studies have shown that oxygen vacancy (OV), which is a 2-electron donor and is commonly distributed on the surface of transition metal oxides, is the major factor for organic contaminants removal during catalytic ozonation. Jia et al. have proposed the possible mechanism for ozone decomposition according to the involvement and recycling of oxygen vacancy deposited on the MnO₂.¹¹  First of all, ozone molecules combine with oxygen vacancy existing on the metal oxides’ surface by withdrawing two electrons from oxygen vacancy to the oxygen atom of ozone when ozone contacts the ozonation catalyst. Thus an oxygen species (O²⁻) in the oxygen vacancy site and an oxygen molecule that desorbs into the air can be generated, as described in eqn (1.6). Subsequently, the oxygen molecule and peroxide species (O₂²⁻) are produced by the reaction of another ozone molecule with oxygen species (eqn (1.7)). As shown in eqn (1.8), O₂²⁻ finally decomposes to release an oxygen molecule and consequently recovers the oxygen vacancy, a reaction that can occur circularly in ozone decomposition.

Although the various catalytic reaction mechanisms have been introduced specifically, the process of catalytic ozonation by transition metal oxides is much more complicated and involves many reactions and reactive species. Generally, in many cases the outcome depends on the coexistence of various catalytic mechanisms during the catalytic ozonation processes.

In addition to transition metal oxides, some other metal oxides are used as catalysts during the ozonation process. Aluminum, titanium, magnesium and calcium oxides have been the common ozonation catalysts in previous research. Their performances and possible mechanisms for heterogeneous catalytic ozonation of organic contaminants in water are briefly discussed next.

Aluminum oxides can be regarded as suitable and alternative catalysts for the mineralization of organic contaminants in heterogeneous catalytic ozonation, including Al₂O₃ and AlOOH.^40,41  Apart from their exceptional catalytic capabilities, some study has been conducted on the ability of γ-AlOOH and γ-Al₂O₃ to exhibit excellent stabilities due to the small amount of leaching of aluminum ions in the ozonation degradation of 2-isopropyl-3-methoxypyrazine (IPMP) from water.⁴²  The catalytic efficiency of IPMP by catalytic ozonation in the presence of γ-AlOOH and γ-Al₂O₃ is 94.2% and 90.0%, respectively. In the γ-AlOOH/O₃ system, the surface hydroxyl group on aluminum oxides also is a dominantly active center for ozone decomposition in the formation of ˙OH during the heterogeneous catalytic ozonation of IPMP. While the surface complexes can be easily produced through physical adsorption between the ozone molecule and the catalyst surface in the heterogeneous catalytic ozonation over γ-Al₂O₃, they can further affect the capability of IPMP degradation. Based on these results, γ-AlOOH and γ-Al₂O₃ have exhibited different reaction mechanisms (shown in Figure 1.3). Therefore, a detailed introduction on the application of these two kinds of aluminum oxides in heterogeneous catalytic ozonation is presented here.

As we all know, there are both Lewis acid and basic sites (AlOH–H⁺ and Al–OH) on the surface of γ-Al₂O₃ since it is an amphoteric solid; thus the number and structure of hydroxyl groups existing on the alumina surface determine its acidity and basicity. Similarly, these hydroxyl groups are of critical importance in ozone decomposition and ROS formation.⁴³  Generally, the number of basic sites decreases to some extent due to the adsorption of carboxylates after catalytic ozonation, while several investigations give evidence that the Lewis acid sites remain constant. Indeed, it has been recently reported that Al₂O₃ can decompose ozone by the heterogeneous catalytic ozonation and improve the generation of various ROS by the interaction between their hydroxyl groups and ozone. Ikhlaq et al. have demonstrated the mechanisms of catalytic ozonation for coumarin degradation on γ-alumina, and the results show that alumina is an efficient catalyst for coumarin removal by catalytic ozonation.⁴⁴  Catalytic ozonation for coumarin degradation follows a corresponding radical mechanism in the presence of alumina, and ˙OH can be produced from the reaction between ozone and surface hydroxyl groups and is almost certainly involved in the heterogeneous catalytic ozonation process.

Some other studies have demonstrated that γ-Al₂O₃ reacts with O₃ to elevate the harvest of ˙OH through various radical reactions involving ˙O₂⁻ and/or ozonide radicals (˙O₃⁻), which contributes to the more efficient removal of carboxylic acids and 2,4-dimethylphenol (2,4-DMP) than ozonation alone.⁴⁵  There was clear observation of the degradation of 2,4-DMP within 25 min by the heterogeneous catalytic ozonation over γ-Al₂O₃. No adsorption of 2,4-DMP has occurred on γ-Al₂O₃, while an increase of 2,4-DMP oxidation can be observed after adding γ-Al₂O₃ in the ozonation process. Indeed, the removal efficiencies of TOC in single ozonation and in catalytic ozonation by γ-Al₂O₃ is 14% and 46%, respectively. Similarly, the removal of COD has increased from 35% to 75% in the γ-Al₂O₃/O₃ system. Moreover, the catalytic effect of α-Al₂O₃ has been studied by several ozonation systems, indicating a crucial role of the active sites in catalytic ozonation. Although alumina is regarded as an inefficient adsorbent for BPA removal without ozone, there is an obvious increase in BPA mineralization, which can reach above 90% when alumina is joined in the ozonation system.⁴⁶  In another investigation, the results demonstrate that alumina remarkably promotes the degradation efficiency of NOM in the ozonation process.⁴⁷  In addition to the prominent adsorption property of alumina, the NOM removal efficiency in heterogeneous catalytic ozonation is twice as high as that of ozonation alone. In addition, there is no significant decline in the catalytic capability of alumina during the catalytic ozonation process with 62 cycles, which is attributed to the definite surface property of alumina.

Salla et al. have investigated and compared the performance of α-Al₂O₃ and MnO₂ for HAs removal in the catalytic ozonation process, indicating that α-Al₂O₃ exhibits better activity than MnO₂ for the catalytic ozonation of HAs.⁸  Moreover, α-Al₂O₃ in a few amounts has been used as an alternative catalyst to degrade HAs in the ozonation process, and the catalytic reaction has taken place on the interface of α-Al₂O₃ by decomposing ozone into various ROS to further mineralize diverse organic contaminants. Also, γ-AlOOH is widely employed as an ozonation catalyst in the degradation of organic contaminants. For instance, the degradation of 2,4,6-trichloroanisole (TCA) has been investigated in the heterogeneous catalytic ozonation process with γ-AlOOH. This study demonstrated that the catalytic efficiency can achieve 80% within 10 min in catalytic ozonation when the initial concentration of TCA is 200 mg L⁻¹, whereas it is only 37.3% in ozonation alone.⁴³  In addition, γ-AlOOH as an effective catalyst has been further explored to remove 2-methylisoborneol (MIB), while the effect of adsorption occurring at the interface of γ-AlOOH also can be discussed for explaining the role of surface-active centers in γ-AlOOH (including acid and base sites) during heterogeneous catalytic ozonation.⁴⁸  The authors also discuss the mechanism of catalytic ozonation, in which surface hydroxyl groups of γ-AlOOH play a vital role in the adsorption and ozonation of MIB. The results of radical scavenger experiments have indicated that the surface hydroxyl groups determine the catalytic capability of γ-AlOOH in the ozonation process and that they are active sites of catalysts. In addition, the radical mechanism may be involved by exploring the pathway of catalytic ozonation by γ-AlOOH, in which the chemical adsorption occurs at the interface of γ-AlOOH by the interaction between surface hydroxyl groups and MIB. Owing to the competitive reaction on the interface, there are significant effects on ozone decomposition and ROS generation. Therefore, the adsorption occurring in the active centers can restrain the formation of ROS and inhibit the catalytic ozonation of MIB. An intensively study of the surface structure and property of ozonation catalysts would be beneficial for the in-depth understanding of the reaction mechanism in heterogeneous catalytic ozonation.

As an inexpensive, efficient and notable catalyst, titanium dioxide (TiO₂) is used in a variety of technological fields, especially those having to do with energy and environment.⁴⁹  Generally, TiO₂ comes in two types, anatase and rutile, and has been intensely applied in photocatalysis due to its semiconductor characteristics and other properties. According to previous studies, TiO₂ effectively accelerates ozone decomposition into ˙OH for the improvement of catalytic efficiency in organic contaminants removal in the heterogeneous catalytic ozonation, especially the rutile phase. In fact, TiO₂ exhibits superior capability in the catalytic ozonation of ATZ compared to ozonation alone.⁵⁰  There is a remarkable increase in ATZ degradation efficiency with the increase of the amount of TiO₂ and ozone, but it is clear that there is no influence on catalytic ozonation by the different initial concentrations of ATZ. Ninety-three percent degradation efficiency of ATZ can be achieved within 30 min in heterogeneous catalytic ozonation, whereas mineralization achieves 56% in the same conditions.

Another study compared the heterogeneous catalytic ozonation over TiO₂ catalyst with ozonation alone for the degradation of naproxen and carbamazepine, and the results reveal that two kinds of organic contaminants are completely removed within several minutes in catalytic ozonation.⁵¹  Nevertheless, the degradation of naproxen and carbamazepine in ozone alone without TiO₂ catalyst takes some time, and only 50% mineralization can be reached in 20 min. In addition, the results show that the adsorption capacity of the catalyst significantly affects the ozonation efficiency, which is consistent with that of alumina previously mentioned. More hydroxyl radicals are generated from ozone decomposition for improving the mineralization degree in catalytic ozonation since the adsorption can take place on acid catalytic sites, especially in slightly acidic conditions. Moreover, the influences of ozonation and heterogeneous catalytic ozonation over TiO₂ on the structure and the amount of NOM have been studied and compared.⁵²  In the heterogeneous catalytic ozonation process, producing radicals is favorable in NOM degradation, which follows the radical mechanism. And the DOC removal efficiency in heterogeneous catalytic ozonation (up to 18%) is higher than that of ozonation alone (only 6%).

As we all know, there are abundant oxygen vacancies with Ti³⁺ atoms in the lattice of both anatase- and rutile-phase TiO₂. According to the oxygen vacancy theory, ozone molecules easily combine with oxygen vacancy on the TiO₂ surface by withdrawing two electrons from oxygen vacancy to the oxygen atom of ozone, which contributes to the generation of strong oxidative ROS for organic contaminants removal during catalytic ozonation. Also, the morphology and crystal phase of TiO₂ can be regarded as the primary influences on their catalytic capabilities in heterogeneous catalytic ozonation. Therefore, some research has been conducted on the effect of the morphology and crystal structure of TiO₂ on the catalytic ozonation of phenol.⁵³  It has been indicated that TiO₂, with its high specific surface area and abundant surface hydroxyl groups, shows excellent catalytic activity for phenol removal in catalytic ozonation. Apart from the surface property, the crystal phase of TiO₂ exerts a great influence on its capability since TiO₂ with different crystal phases possesses various amounts of oxygen vacancy, which contributes to the performance of the heterogeneous catalytic ozonation. For instance, many surface hydroxyl groups are deposited on the rutile rather than other crystalline phases, and the degradation of phenol strongly relies on the concentration of surface hydroxyl groups. In conclusion, the mechanism of catalytic ozonation over TiO₂ catalyst is related to the adsorption capacity of the reaction intermediates on the Lewis acid sites of the TiO₂ surface and does not follow the pathway of radicals.

Furthermore, the crystal facet of TiO₂ also can severely influence their catalytic efficiencies of ozonation. For anatase TiO₂ catalyst, the thermodynamically stable (101) facet, rather than the more reactive (001) facet, dominates catalytic efficiency due to the rapid decrease of the surface with high reactivity during the crystal growth process as a result of the minimization of surface energy.⁵⁴  However, the catalytic activity of TiO₂ with exposed (001) facets could be enhanced through surface fluorination, and the ozonation efficiency of OA clearly can be accelerated by adding fluorinated TiO₂ catalyst.⁵⁵  Thus it can be deduced that two aspects, the high surface energy of (001) facets and the increased oxygen vacancies, contribute to the improved activity of fluorinated TiO₂ in the catalytic ozonation.

Magnesium oxides and calcium oxides have always gathered much attention for catalytic ozonation as a result of their superior catalytic performances in organic contaminants degradation. In addition, magnesium and calcium oxides have a variety of advantages, including abundant surface basic sites, high insolubility, low toxicity, environmental friendliness and large specific surface area, which promote their application in heterogeneous catalytic ozonation.^56–60  First, MgO has been applied especially to the removal of COD and phenol from wastewater. The influence of certain factors on the efficiency of the catalytic ozonation also is evaluated, including MgO dosage, solution pH and the coexisting substance in the wastewater.⁶¹  The experiment results reveal that 70% of the COD and 96% of the phenol are respectively removed at neutral pH in the MgO/O₃ system and that a synergistic efficiency for phenol degradation and COD removal can reach 39%. Meanwhile, the organic azo dye (Red 198) is degraded by the heterogeneous catalytic ozonation over MgO.⁵⁶  The catalytic efficiency of Red 198 has been greatly enhanced by adding MgO catalyst; thus the superior activity of heterogeneous catalytic ozonation leads to a shortened reaction time compared to single ozonation. It takes only 9 min to completely remove Red 198 in the heterogeneous catalytic ozonation, while the reaction time is about 30 min in ozonation alone. In the previous study, various metal oxides as the catalyst (such as Co, Ni, Fe, Sn, Mn, Cu, Mg and Al oxides) are evaluated for the ozonation of ammonia nitrogen in the presence of Cl⁻, indicating that MgO possesses the optimum catalytic activity for the decomposition of ammonia nitrogen.³⁶ 

Due to the notable effects of surface area on its catalytic activity, the MgO catalyst synthesized by the conventional approach possesses a relatively large particle size and low specific surface area. Therefore, an increasing number of nano-sized MgO have been developed recently and applied for catalytic ozonation due to their controlled particle size and crystallinity. The use of a nano-sized MgO catalyst to degrade quinoline in wastewater has been investigated, and the results show that about 90.7% of quinolone can be effectively removed after 15 min.⁶²  It is also evidenced that the ˙OH produced from ozone decomposition by the nano-sized MgO catalyst contributes to quinoline mineralization and that the partial adsorption occurring on the surface of nano-sized MgO can be ignored. In addition, mesoporous nanocrystalline MgO nanoparticles have been prepared by the hydrothermal or sol–gel approach and applied to improve the ozonation efficiency of acetaminophen, which indicates this MgO has superior catalytic activity in catalytic ozonation.⁶³  The functional groups on the MgO surface are crucial for boosting the ozone decomposition to ˙OH, in which the reaction between O₃ molecules and the hydroxyl groups can occur on the interface. The reaction between hydroxyl radical with acetaminophen molecules can also take place in the solution. In addition, MgO catalysts can adsorb the water molecules and dissociate them into hydroxyl radicals and hydrogen ions, thus enhancing the amount of ˙OH in the heterogeneous catalytic ozonation process.

Indeed, the surface properties of catalysts, such as the exposed crystal facets, determine their active sites and markedly affect their catalytic performance in heterogeneous catalytic ozonation. Several MgO nanocrystals with different crystal facets of (111), (110), (100) and (200) have been synthesized and applied to investigate their reactivity in the catalytic ozonation of Escherichia coli.⁶⁴  Their catalytic capabilities follow the order of MgO(111) > MgO(200) > MgO(110) > MgO(100), indicating that the crystal facets play a crucial role in catalytic activity. This is also evidenced by scavenger experiments in which ˙OH detected in the heterogeneous catalytic ozonation was the main ROS in the MgO(111)/O₃ system, while more direct ozonation and a portion of ˙O₂⁻ can be observed in other catalytic ozonation processes.

Calcium oxides are relatively inexpensive solids, which have a variety of properties and applications, certainly including wastewater treatment and remediation. Among them, CaO₂ can be regarded as a stable source of oxygen release depending on the pH of the solution since it can react with water molecules to generate hydrogen peroxide and calcium hydroxide.⁶⁵  In Izadifard's study, CaO₂ and CaO are used for catalyzing O₃ to degrade sulfolane, whose complete removal and TOC can be achieved within 40 and 150 min with CaO₂ and CaO, respectively.⁶⁶  The results demonstrate that both CaO₂ and CaO are available and alternative catalysts for heterogeneous catalytic ozonation treatment of wastewater contaminated with sulfolane. In addition, CaO exhibits greater activity than other metal oxides in the catalytic ozonation of cinnamaldehyde since it has Brønsted acid sites.⁵⁹  Compared with the surface structure in a series of metal oxides, it is already clear that CaO possesses only trace Lewis acid sites but has a strong base site. According to its superior cinnamaldehyde conversion and benzaldehyde selectivity, CaO, with its strong base sites, can boost the ozone decomposition to various ROS, including *O, ˙OH and ˙O₂⁻ etc. And the amount and property of different ROS generated from catalytic ozonation have strongly depended on reaction temperature.

The typical radical pathway is the primary mechanism of heterogeneous catalytic ozonation over different metal oxides for organic contaminants degradation. Generally, the free radicals, ˙OH and ˙O₂⁻, are regarded as the main ROS in the catalytic ozonation. Based on electron transfer, the catalytic ozonation can occur in the surface-active center of metal oxides by dissociating the O–O bond in ozone to generate various ROS for contaminants degradation. In Section 1.1.1.4, the mechanism of heterogeneous catalytic ozonation following the radical pathway has been discussed in detail.

In addition to radical oxidation, previous studies also have investigated the degradation pathway of carboxylates followed by non-radical oxidation in the heterogeneous catalytic ozonation process, which is attributed to intramolecular electron transfer.⁶⁷  Moreover, the pathway of organic degradation depends on the generated surface complex in the active sites deposited on the surface of metal oxides. Under certain conditions, organic matters in aqueous solution react with the adsorbed ozone molecules on the interface of metal oxides to generate the surface complexes that induce non-radical oxidation by intermolecular electron transfer instead of ozone decomposition. Thus the influence of solution pH (as well as pH_pzc of the catalyst and pK_a of the organics) on catalytic ozonation efficiency has been investigated in the non-radical pathways due to the significant effect on the adsorption of ozone and organics.

The possible ozonation pathways for the surface-complex reactions can be summarized in the following three cases (shown in Figure 1.2). First, organic matter can be chemisorbed on the surface of the metal oxide catalysts and can further react with ozone molecules leading to the generation of various ROS. Then ozone molecules are chemisorbed on the surface of the metal oxide catalysts, and the generated surface-ozone complexes can destroy the organic contaminants. Finally, organic matter and ozone molecules are chemisorbed on the surface of the metal oxide catalysts at the same time, and then the organic contaminants are degraded by intramolecular electron transfer.

For the degradation of organic contaminants via a surface complex mechanism, γ-Al₂O₃ has been investigated as a typical ozonation catalyst in the catalytic ozonation of 2-isopropyl-3-methoxypyrazine.⁴²  The results reveal that ozone molecules adsorbed on the γ-Al₂O₃ surface could either directly oxidize organic matter or decompose into free radicals for catalytic ozonation. However, the oxidation potential for direct ozone decomposition can be promoted by increasing the amount of the adsorbed ozone on the γ-Al₂O₃ surface. Clearly, it is especially difficult to achieve the complete mineralization of contaminants by this surface complex due to coexisting radical oxidation. Therefore, the surface complexes can acquire a mild oxidation potential for unsaturated organic matter degradation, but the mineralization of aliphatic acid cannot be realized.

Although different ozonation mechanisms have been involved in heterogeneous catalytic ozonation over various metal oxides catalysts, the ozonation efficiencies of catalysts depend to a large extent on their surface properties as well as on the solution pH, which dominate the surface active sites and ozone decomposition reactions in aqueous solutions. The physical and chemical properties of metal oxide catalysts must be preferentially considered when selecting the catalyst in the ozonation process, including the crystal size, surface area, crystal phase, surface groups and even surface active sites. Generally, hydroxyl groups situated on the metal oxide surface can behave as Brønsted acid sites, and Lewis acids and Lewis bases are special sites located on the metal cation and coordinatively unsaturated oxygen, respectively. Those active sites on the surface of metal oxide catalysts are responsible for catalytic efficiency in heterogeneous catalytic ozonation. The adsorption behavior of metal oxide catalysts is of great benefit to their activity of heterogeneous catalytic ozonation since the adsorption reaction is the preferential stage in the overall process; thus it must be taken into consideration to improve catalytic efficiency. All in all, the heterogeneous catalytic ozonation process certainly involves various reactions, including the adsorption, single ozonation, and catalytic ozonation of those metal oxides catalysts.

An increasing number of mixed polymetallic oxides are used as the heterogeneous catalysts since they commonly possess greater catalytic capability and stability than those of monometallic oxides. Two typical representatives, perovskite and spinel-like oxides, have received increasing attention and possess good catalytic abilities in catalytic ozonation because of their availability, persistent structure, abundant metal valence states, controlled morphologies and various exposed defects.

As alternative catalysts, mixed metal oxides possessing the perovskite structure have been extensively utilized for catalytic ozonation of organic contaminants during the last two decades.^68–70  Generally, perovskites possess a formula of ABO₃, in which A usually represents the rare earth cation (the majority being La), and the B position has a transition metal cation. Perovskite catalysts can endure the replacement of both cation sites, in which the structural defects, including anionic and cationic vacancies, can be generated when the A position is substituted by lower-valence metal ions. Subsequently, the oxidation state of metal ions is altered to maintain the electrical neutrality of perovskites, and the extensively available oxygen also can be formed with the increasing oxidation state of the B cation. The generated oxygen vacancies are responsible for promoting the catalytic activity in the ozonation process owing to the enhanced mobility of lattice oxygen. Thus the use of these perovskite catalysts has especially promoted applications involving ozone decomposition and has enhanced the catalytic ozonation rate of organic contaminants, even improving the mineralization degree during the ozonation process. To date, a series of perovskites (LaMO₃, M = Fe, Mn, Co, Cu, Ti and Ni) have been investigated as an effective alternative in the heterogeneous catalytic ozonation process for organic contaminants degradation.

Initially, the perovskite LaTi_0.15Cu_0.85O₃ has been successfully synthesized and evaluated for the catalytic ozonation of pyruvic acid, resulting in the formation of a refractory substance after catalytic ozonation.⁷¹  The same perovskite was evaluated in the catalytic ozonation of different phenolic wastewaters, but no obvious changes were observed in this study according to the results regarding removal efficiency and mineralization degree.⁷²  There were no differences in single ozonation and heterogeneous catalytic ozonation for non-refractory wastewater. Thus the addition of perovskite can effectively dispose of those refractory organic contaminants in wastewater. It is evidenced that there was no improvement in the removal efficiency of pharmaceutical compounds when the catalysts were added into the single ozonation process in ultrapure water.⁶⁹  However, TOC removal is significantly increased in the presence of perovskite catalysts, especially for copper perovskite, where 90% of TOC removal could be achieved after 120 min. This is attributed to the formation of hydroxyl radicals resulting from the reaction of ozone molecules with hydrogen peroxide generated in the aromatic ring and unsaturated carbon bond-breaking. In addition, ozone decomposition occurred on the catalyst surface, especially copper perovskite, leading to efficient TOC removal.

Further investigation also has shown that a significant enhancement of catalytic capability is observed in the presence of perovskites with a different composition, which severely affects the catalytic property during the ozonation process. For instance, several lanthanum-based perovskites were carried out for the catalytic ozonation of organic contaminants.⁷³  LaCoO₃ has been supposed to be an efficient catalyst for organic contaminants as a result of the rapid ozonation rate, complete mineralization of organics and negligible metal leaching. It exhibits significant catalytic activity for benzotriazole (BZA) degradation, where complete degradation and about 71% of BrO₃⁻ inhibition can be achieved within 15 min.⁷⁰  While no catalytic activity for BZA degradation can be observed in the LaFeO₃/O₃ process, there is 73% of inhibition for the generation of BrO₃⁻. In these cases, oxygen vacancies play a crucial role in the heterogeneous catalytic ozonation as a result of the ability to activate adsorbed species. It has been shown that H₂O₂ has an important but varied effect in different ozonation systems and combines with the surface hydroxyl groups to accelerate the BZA degradation and to inhibit the generation of BrO₃⁻. For other perovskites, LaMnO₃ also certainly exhibits catalytic performance, but it facilely suffers from the leaching of metal cations. Moreover, LaMnO₃ and La_0.8Sr_0.2MnO₃ perovskites have shown greatly inferior catalytic activity for ozone utilization in benzene oxidation than manganese monoxide, and La sites in perovskite accelerate the accumulation of less reactive by-product compounds on the catalysts.⁷⁴  In the case of manganese-based perovskite, ozone decomposition and organic contaminant degradation are decided by the catalyst composition.

Therefore, suitable modifications can be carried out to improve the properties of catalysts in catalytic ozonation. In Afzal's study, mesoporous nanocast LaMnO₃ and LaFeO₃ perovskites have been synthesized and applied effectively for 2-chlorophenol degradation in the heterogeneous catalytic ozonation process.⁷⁵  It has been demonstrated that nanocast perovskites, with their unique structure, exhibit higher catalytic efficiency than their uncast counterparts, as well as highly active Mn₃O₄ and Fe₂O₃. Based on various detailed analyses, it is evidenced that a neutrally charged surface (MnOH), as well as a protonated surface (MnOH₂⁺), presented as more catalytically active than the deprotonated surface (MnO⁻) in promoting ozone decomposition into ˙OH. Hence the generated ˙OH rather than surface O₂²⁻, *O, ˙O₂⁻ and ¹O₂ is the primary ROS that contributes to the excellent activity in the heterogeneous catalytic ozonation process.

Recently, some spinel oxides, with a general formula of AB₂O₄ in which A and B are metal ions, also are being extensively investigated for organic contaminants removal in heterogeneous catalytic ozonation systems owing to their superior catalytic activity, various valence states, and high stability.^76–80  All kinds of spinel oxides have been synthesized and applied to degrade the organic contaminants in catalytic ozonation. For instance, several magnetic spinel ferrites, including CoFe₂O₄, CuFe₂O₄, NiFe₂O₄ and ZnFe₂O₄, have been applied to dispose of shale gas in the heterogeneous catalytic ozonation.⁸¹  It is obvious that these magnetic spinel oxides all offer attractive catalytic and recycling performance; their catalytic efficiencies follow the order of CuFe₂O₄ > NiFe₂O₄ > CoFe₂O₄ > ZnFe₂O₄. Their capacities of catalytic ozonation mainly rely on the surface property of these catalysts, and the proposed interaction pathway follows the typical radicals-involved process. Firstly, the ozone molecules can be adsorbed into the surface of the spinel ferrite and react with the surface functional groups to generate various radicals. In addition, the metal cations cycles in spinel oxides surface such as Fe²⁺/Fe³⁺, Cu²⁺/Cu⁺ and Co²⁺/Co³⁺ significantly affect the generation of radicals. As shown in eqn (1.1)–(1.5), hydroxyl radicals could be finally produced by the series of chain reactions and then interact with the targeted organic matters for contaminants removal.

Furthermore, the same kinds of spinel oxides have been synthesized and investigated in the catalytic ozonation of oxalic acid.⁸²  In this study, 68.3% of TOC could be removed within 120 min in the CoFe₂O₄/O₃ system, indicating CoFe₂O₄ has a more superior catalytic performance than others for oxalic acid mineralization. The reason for the excellent catalytic performance is that spinel oxides possess reducibility and electron-donating capacity; the order of catalytic activity is CoFe₂O₄, NiFe₂O₄, CuFe₂O₄ and ZnFe₂O₄. Similarly, the radical-based catalytic mechanisms and reaction pathways also can be proposed in the cases of spinel oxides, which include the interaction of ozone molecules with hydroxyl groups and metal ions on the surface of catalysts.

Due to the fascinating electromagnetic property, a large amount of research has been conducted on heterogeneous catalytic ozonation over magnetic spinel ferrite for easily recycling.^83–85  Magnetic spinel oxide crystals have a completely inverse spinel structure, where iron ions have occupied the tetrahedral and octahedral sites and all other metal ions (such as Ni, Cu, Co, Mn etc.) have located in the octahedral sites. For instance, Zhang et al. have prepared copper ferrite magnetic nanoparticles (CuFe₂O₄ MNPs) by the sol–gel approach for the catalytic ozonation of N,N-dimethylacetamide (DMAC).⁸⁶  The results have demonstrated that the removal efficiency of DMAC in the heterogeneous CuFe₂O₄/O₃ process (95.4%) is much higher than that of only CuFe₂O₄ catalyst (0%), ozonation alone (55.4%) and other heterogeneous Fe₂O₃/O₃ processes (32.1%). Hydroxyl groups on the surface of CuFe₂O₄ catalyst are the active sites and contribute to the ozone decomposition into ˙OH as the dominant ROS in the CuFe₂O₄/O₃ process. Finally, a possible reaction mechanism in the CuFe₂O₄/O₃ process has been proposed according to research results, which mainly consists of ozonation alone as well as homogeneous and heterogeneous catalytic ozonation.

Magnetic NiFe₂O₄ as a catalyst also has been used for catalytic ozonation of di-n-butyl phthalate (DBP), and it has also been concluded that the surface hydroxyl groups as the catalytic active centers could boost ˙OH generation.⁸⁷  Moreover, Ni²⁺ in the catalyst could facilitate electron transfer from the catalyst surface to induce ozone decomposition and enhance the formation of hydroxyl radicals, which lead to the improvement of DBP degradation. Additionally, magnetic NiFe₂O₄ could also be proposed for heterogeneous catalytic ozonation of phenol contaminants,⁸⁸  in which study, Lewis acid sites were indicated as catalytic reactive centers in the process. Therefore, the reasons for the superior catalytic activity of NiFe₂O₄ catalyst are not only the greater number of surface-active centers, including surface hydroxyl groups and Lewis acid sites, but also the enhanced interfacial electron transfer. Furthermore, the magnetic NiFe₂O₄ can be easily and efficiently separated from the heterogeneous catalytic system with an external magnet, making it promising for being an attractively separable catalyst in the heterogeneous catalytic ozonation of organic contaminants.

In addition, ZnAl₂O₄, another spinel-like ozonation catalyst, has been significantly reported to improve phenol degradation, in which the removal rate of phenol with a high concentration of 300 mg L⁻¹ can arrive at 73.4% within 60 min.⁸⁹  After recycling four times on the catalyst, there is a slight decrease (only 5.7%) in the removal efficiency of phenol, illustrating the excellent reusability of ZnAl₂O₄ in the heterogeneous catalytic ozonation process. In addition to high reusability, ZnAl₂O₄ possesses brilliant stability since it can be extensively utilized in a wide pH range from 3.3 to 9.3, becoming a promising ozonation catalyst for wastewater treatment. These studies also imply that surface hydroxyl groups are the active sites and that ˙OH is the main ROS in catalytic ozonation.⁹⁰ 

For the spinel oxides, various metal cations possess different valence states, which can facilitate electron transfer on the catalyst surface for further boosting ozone decomposition to enhance oxidation efficiency.⁹¹  To further study the catalytic mechanism, the degradation of dimethyl phthalate by CuFe-based spinel oxide as an ozonation catalyst has been evaluated, and the redox recycling of Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ at the surface of the catalyst can accelerate electron transfer and ˙OH generation. Similar results have been obtained in the degradation of organic dye by CuAl₂O₄ catalyzed ozonation, and almost 100% of the dye and 87.2% of the COD can be removed within 25 min at neutral pH in this heterogeneous catalytic ozonation process. It is worth mentioning that the catalytic reaction rate in heterogeneous ozonation is higher than that of other systems since the better textural properties and the higher density of active sites in CuAl₂O₄ (both Cu²⁺ and Al³⁺) are critical for the degradation of the contaminants.

The reaction mechanisms of catalytic ozonation over mixed metal oxides are more complex due to the distinctive structure of catalysts. For example, a large number of defective sites and active sites exist in mixed metal oxides, including oxygen vacancies, acid sites, and basic sites.⁹²  Also, metal cations that possessed diverse valence states on the catalyst surface are beneficial for redox cycling and electron transfer leading to the facile generation of ROS in catalytic ozonation.

Some studies have reported oxygen vacancies as the active centers of catalysis for radical-based oxidation in catalytic ozonation. In La-based perovskite, the lattice oxygen vacancies possess the capability to adsorb the ROS, which is crucial for catalytic efficiency in the ozonation process.⁷³  Similarly, it is observed that the oxygen vacancies and multivalence Mn of perovskite are at the Lewis acid sites, which are regarded as the active centers for ozone decomposition and the formation of ˙OH, ˙O₂⁻, and ¹O₂.⁹³  Generally, Lewis acid sites are covered by hydroxyl groups on the perovskite surface, which might mean that the reaction between ozone molecules with hydroxyl groups rather than with Lewis acid sites in the perovskite can preferentially occur.

According to previous studies, the overall heterogeneous mechanism of catalytic ozonation over mixed metal oxides is illustrated in Figure 1.4. Generally, the radical-based and non-radical mechanisms of catalytic ozonation have been proposed by the interaction of ozone with surface active sites on mixed metal oxides, which can accelerate the decomposition of ozone. The surface hydroxyl groups are primarily formed on account of the interaction between a water molecule and metal ions located in the A-site of mixed metal oxides. In some cases, the surface hydroxyl groups with positive charges are prone to be formed since the pH of the solution containing organic contaminants is basically lower than pH_pzc of spinel oxides. Then it can react with ozone molecules to generate the intermediate hydroozonide radicals (HO₃˙) by electrostatic force, while ˙OH is produced by the reaction of ozone molecules with hydrogen bonds. An electron transfer from the bivalent metal ion in the A site to ozone molecules can be observed, thus resulting in the generation of ˙O₃⁻ and the trivalent metal ion in the A site. Also, HO₃˙ as well as ˙OH are formed by the reaction between the intermediate ˙O₃⁻ and hydrogen ions in the solution. At the same time, the trivalent A site metal ion is reduced to its bivalent state by the lattice oxygen. It should be emphasized that the coexistence of iron ions with multiple valences and metal ions in the A site may accelerate the process of ozone decomposition.

On account of the ROS from catalytic activation of ozone molecules, heterogeneous catalytic ozonation processes have been evidenced to surpass single ozonation by the non-selective mineralization of contaminants with superior efficiency. In addition to ˙OH, other ROS such as ˙O₂⁻ and ¹O₂ can also be generated in catalytic ozonation, which contributes to the degradation of organic contaminants. Meanwhile, the pH dependence in ozone decomposition can be rationally regulated by the catalysts, adapting the effectiveness of ozonation to a wider pH range. For the heterogeneous catalytic ozonation over mixed metal oxides, the variable valence states of metal ions are responsible for the electron transfer, and the surface hydroxyl groups and oxygen vacancies of the catalysts can serve as Lewis acid sites for accepting electrons.

To sum up, a comprehensive understanding of the heterogeneous catalytic ozonation process over various metal oxides is provided in this chapter based on advanced research. The characteristics of heterogeneous ozonation catalysts have been proven to significantly affect their catalytic activities and efficiencies, such as their components, crystalline structure and morphology. Moreover, the heterogeneous catalytic ozonation mechanisms over various metal oxides have been summarized and analyzed in detail. In particular, active centers on the surface of different metal oxides are described respectively, including hydroxyl groups, metal cations, oxygen vacancies, acid sites and basic sites. Generally, the radical and/or non-radical degradation pathways including radical mechanism, surface complexes theory and oxygen vacancies mechanism are involved in the ozonation process.

Although numerous studies on heterogeneous catalytic ozonation have been investigated in the last decade, several limitations restrict the further applications of these metal oxide catalysts in water treatment, including the leaching of toxic metal ions and the reuse of solid catalysts separated from the aqueous solution. Hence developing environmental-friendly heterogeneous ozonation catalysts with excellent long-term stability is the key to accelerating the practical application of this technique. In this chapter, the metal oxides composed of natural abundant elements (Mg, Ca etc. instead of transition metals) have exhibited a satisfying ozonation performance, where the organic pollutants can also be completely degraded and mineralized. In addition, an in-depth understanding of the possible ozonation mechanism regarding metal oxides can provide theoretical support for properly designing and applying ozonation catalysts.