Current Developments in Ozone Catalyst Preparation Techniques and Their Catalytic Oxidation Performance

Abstract

Through the use of heterogeneous catalysts, catalytic ozone oxidation technology, an effective and eco-friendly advanced oxidation process (AOP), facilitates the breakdown of ozone into reactive oxygen species (like ·OH) and greatly increases the mineralization efficiency of pollutants. This study examines the development of heterogeneous ozone catalysts through a critical evaluation of the five primary preparation techniques: ion exchange, sol–gel, coprecipitation, impregnation, and hydrothermal synthesis. Each preparation method’s inherent qualities, benefits, drawbacks, and performance variations are methodically investigated, with an emphasis on how they affect the breakdown of different resistant organic compounds. Even though heterogeneous catalysts are more stable and reusable than homogeneous catalysts, they continue to face issues like active component leaching, restricted mass transfer, and ambiguous mechanisms. In order to determine the key paths for catalyst selection in catalytic ozone treatment going forward, the main goal of this review is to provide an overview of the accomplishments in the field of the heterogeneous ozone catalyst treatment of wastewater that is difficult to degrade.

1. Introduction

More than 420 billion tons of organic wastewater are released into rivers, lakes, and oceans annually worldwide due to the rapidly expanding pharmaceutical, chemical, printing and dyeing, and metallurgical-related industries [1]. These wastewaters contain organic pollutants like dyes, detergents, pharmaceuticals, personal care items, pesticides, carboxylic acids, and aromatic compounds [2]. Long-term exposure to these organic wastewaters will harm the water body’s ecology and reduce its dissolved oxygen content, but it will also endanger human health due to the food chain and biological deposition enrichment effect [3]. Physical (adsorption, precipitation), biological (biofilm, activated sludge), and chemical (coagulation, UV disinfection) methods are currently the conventional treatment methods for these organic wastewaters [4], but they typically have clear limitations and are unable to effectively remove organics that are difficult to degrade in wastewater [5]. Due to its high efficiency and environmentally friendly qualities, ozone oxidation, an advanced oxidation process (AOP), exhibits significant promise for use in the treatment of wastewater that is challenging for biodegradation.

With a redox potential of 2.07 V in alkaline solutions [6], ozone is a potent oxidizing agent that can either directly oxidize pollutants or react with them indirectly through the production of -OH during ozone decomposition [7]. Ozone oxidation by itself, however, has drawbacks, including low ozone utilization, high energy consumption, low pollutant mineralization, a high pH effect, and certain organic byproducts that could pollute water bodies more severely [8]. Because of its quick rate of organic pollutant degradation, low operating costs, stable and dependable operation, and capacity to function across a broad pH range, the catalytic ozone oxidation process can significantly offset the drawbacks of individual ozone oxidation. A higher pollutant removal efficiency can be attained by adding ozone catalysts, which can also encourage the breakdown of ozone and the production of reactive free radicals, increasing the degree of organic pollutant mineralization [9]. Depending on the kind of catalyst, catalytic ozone oxidation is typically divided into two categories: homogeneous and non-homogeneous. Common homogeneous catalysts include transition metals Fe²⁺, Mn²⁺, Cu²⁺, Ni²⁺, Ag⁺, and others. In homogeneous catalytic ozone oxidation, the catalyst typically exists in a dissolved state [10], which can be evenly distributed in the aqueous solution and has a higher reaction efficiency; however, the catalysts and wastewater share common characteristics, making it difficult to recycle the catalysts. Additionally, the high concentration of metal ion residue in the wastewater can easily lead to the secondary pollution of the water body, which restricts its use in real-world applications [11].

In order to overcome the drawbacks of homogeneous catalysts, which are hard to recycle and prone to secondary pollution, heterogeneous catalytic ozone oxidation typically uses a stable solid catalyst to catalyze the ozone oxidation of organic matter. It also offers the benefits of superior catalytic performance, reusability, and large-scale preparation and production, and is frequently used to treat wastewater that is hard to degrade [12]. Non-homogeneous catalysts can be broadly divided into three categories based on their active ingredients: loaded catalysts, metal-based catalysts, and carbon-based catalysts. Non-homogeneous catalysts can significantly accelerate the breakdown of ozone to create reactive oxygen species (ROS), including ·OH, ·O²⁻, etc. Consequently, organic matter in wastewater is degraded, and the rate of ozone utilization is somewhat increased. Nowadays, impregnation, coprecipitation, sol–gel, hydrothermal, and ion exchange techniques are the primary techniques used to prepare non-homogeneous catalysts. Additionally, various catalysts have a significant impact on the cost of treatment and the efficiency of catalytic oxidation during the reaction process. Therefore, the key to implementing catalytic ozone oxidation technology to lower treatment costs and increase ozone utilization efficiency is to find a heterogeneous ozone catalyst that is both affordable and environmentally friendly. The preparation techniques of widely used catalysts in heterogeneous ozone oxidation in recent years, as well as their uses in the mineralization and degradation of organic pollutants in water that are difficult for biodegradation, are reviewed in this paper. The difficulties and potential future directions of this technology are also suggested, which will serve as a foundation for the creation of new high-efficiency ozone catalysts.

2. Heterogeneous Catalytic Ozone Oxidation Mechanism

The process of heterogeneous catalytic ozone oxidation is extremely complicated and involves three-phase reactions in gas, liquid, and solid phases. These reactions include the mass transfer of O₃ to the liquid phase, adsorption of O₃ and pollutants on the catalyst, diffusion of active species on the catalyst surface to the liquid phase, and interaction between O₃ and active species and the pollutants. In Figure 1, the heterogeneous catalytic ozone oxidation mechanism is depicted. Currently, the main mechanisms for the degradation of pollutants by heterogeneous catalytic ozone oxidation are as follows:

(1) Free radical theory. This theory holds that ozone adsorbed on the catalyst catalytic activity center decomposes to produce stronger oxidizing hydroxyl radicals (·OH), superoxide anion radicals (·O²⁻), and other active substances, which in turn triggers a free-radical chain reaction on the catalyst surface and in the solution of the decomposition of organic matter [13]. The catalytic activity center is the catalyst surface of the active species in the catalyst surface and diffusion to the liquid phase, as well as active species attacks on the pollutants and other processes. Where ozone splits and transforms into radicals on the catalyst surface is the epicenter of catalytic activity. Ozone breaks down more quickly to produce ·OH when it is adsorbed on the catalyst surface with H₂O, creating surface hydroxyl groups. Water-soluble O₃ adsorbs on the surface of a semiconductor-type heterogeneous catalyst in a heterogeneous reaction system, where it breaks down to release free radicals. Following their attack on organic compounds, these free radicals result in ring-opening and chain-breaking reactions. Numerous small organic molecules are produced by the destroyed organic compounds, and free radicals then cause these small organic molecules to mineralize into inorganic ones. As a result, free radicals in the reaction system are crucial for increasing catalytic oxidation’s efficiency.

(2) Oxygen vacancy theory. One particular instance of radical theory is the oxygen vacancy theory. The decomposition pathway of ozone on catalysts is significantly impacted by the numerous lattice defects that are frequently present on the surface of oxides. Using CeO₂ as an example, it has a high capacity for both storing and releasing oxygen. Ce easily changes from Ce⁴⁺ to Ce³⁺. Oxygen vacancies are created following high-temperature reduction. A portion of the Ce⁴⁺ changes into Ce³⁺ when oxygen levels are low, and vice versa when oxygen levels are high. Oxygen vacancies on the catalyst surface are crucial for the catalytic breakdown of organic compounds by ozone, which is more effective when the Ce³⁺/Ce⁴⁺ ratio on the CeO₂ surface rises [14]. Through a heavy metallization reaction that submerges zero-valent iron particles in Cu(NO₃)₂ to achieve the controlled decomposition and formation of ·OH, ref. [15] indirectly confirmed that O₃ can use CFO NPs in aqueous solutions to synthesize CFO NPs. They also suggested that the Cu(II)/Cu(I) and Fe (III)/Fe(II) redox pairs which activated O₃ are the source of the high catalytic activity of CFO NPs. O₃ interacts with hydroxyl radicals to form Cu(I)₂O–OH–O₃ precursors. Cu(I) oxidizes to Cu(II) while reducing the lattice’s oxygen to O₂, resulting in surface oxygen vacancies. The aforementioned statement makes clear that the amount of oxygen vacancies in metal oxides has a direct impact on their catalytic activity. Therefore, the restoration of oxygen vacancies depends critically on the timely breakdown of the peroxides on the catalyst surface. The catalyst will gradually deactivate if they are unable to be restored because they will not be able to take part in the subsequent reaction process. The average oxidation state of the metal oxide will rise as a result of O₂²⁻, which cannot be broken down into other free radicals, being transformed into lattice oxygen [16].

(3) The complexation theory of surface coordination. According to the surface coordination complexation theory, transition metals with empty electron orbitals form metal–organic complexes with organic matter. These complexes are subsequently adsorbed on the catalyst surface, where ozone molecules oxidize and decompose them. There are two general categories into which this mechanism can be divided. The first is that organic matter is adsorbed onto the catalyst’s surface via chemical bonding, and the second is that coordination complexation is the sole mechanism involved. The other is that organic matter and ozone are adsorbed on the catalyst’s surface, and the redox reaction is propelled by electron transfer [17]. Zhang and associates [18] examined the outcomes of ozone-catalyzed oxidation and used Ce-doped TiO₂ nanoparticles to break down humic acid. They suggested that ozone can react directly with complex products formed by organic compounds adhering to the surface of heterogeneous catalysts. The radical mechanism pathway is not dominant when the pH is acidic, which is necessary for the complexation pathway to function. Ozone has a redox potential of 2.07 eV, making it a potent oxidizing agent that can directly react with organic matter to break it down into aldehydes or carboxylic acids through redox reactions. However, the full mineralization of organic pollutants is severely limited because neither of these pathways reacts with ozone oxygen. Furthermore, the degree of mineralization of organic matter is further decreased by the selective and comparatively slow reaction between ozone and organic matter [19].

(4) The theory of the surface oxygen atom. Bulanin was the first to propose the function of surface oxygen atoms in heterogeneous catalytic oxidation. On the surface of n-type transition metal oxides, O₃ adsorbs and produces *O, an intermediate between O₃ and ·OH with an oxidation potential of 2.42 eV. As a result, *O has a greater oxidizing effect than direct ozone oxidation; however, it is less capable of mineralizing than ·OH. In heterogeneous catalytic ozone oxidation technology, in situ Raman spectroscopy is thought to be a useful technique for clarifying the production and transformation of *O [20]. *O is created when O₃ combines with the Pd surface’s active sites on the PdO/CeO₂ bimetallic catalyst. The organic matter adsorbed on Ce(IV) is oxidized and degraded by *O after some *O reacts with some of the O₃ to produce intermediate products, surface peroxides (*O₂), and the active sites on the Pb surface are reduced once more. Most people think that *O is not a single product in the reaction; instead, it frequently collaborates with ·OH to finish the oxidation process. O₃ molecules have a tendency to catalyze dissociation into *O and free peroxides (*O₂) in investigations involving surface oxygen atoms. Quenching tests and in situ EPR spectroscopy demonstrate that *O either directly targets adsorbed organic matter or produces ·OH free radicals by reacting with nearby water and oxygen molecules.

3. Preparation and Application of Heterogeneous Catalyst

3.1. Impregnation

The method of impregnation is to introduce the carrier into a liquid or gas containing the active substances, which are gradually adsorbed on the surface of the carrier, and then drying, cooking, activation, etc. This method of impregnation has the following advantages: catalytic carriers of different shapes and sizes are commercially available, which eliminates the need for catalyst molding steps; suitable carriers can provide a more suitable specific surface area, pore size, mechanical strength, etc., for the physical structure characteristics required for catalytic converters; the active substance is not only loaded on the surface, which is important for rare metal catalysts; and it is highly used and low cost. The dormant method is particularly suited for the manufacture of rare earths catalysts, catalysts with a low active ingredient content, and catalysts with high mechanical strength.

During the impregnation process, the metal was modified by physically filling the pores on the surface of the carrier, which resulted in an increase in pore volume and pore diameter, significantly increasing the specific surface area necessary for adsorption [21]. Among these, iron-based catalysts and their composites demonstrated exceptional efficacy in treating a range of pollutants, including Fe₃O₄-MnO₂ magnetic composites (

Table 1. Catalyst prepared by impregnation method and its performance.

Catalyst	Pollutants	Experimental Condition	Catalytic Performance	Reusability	References
Iron-based catalyst	Industrial wastewater	[Cat] = 200 g/L [O3] = 5 mg/min	TOC removal rate 78.7%	After 50 uses, the TOC above 60%	[21]
		[Pull]0 = 69.3 mg/L pH = 6.8
Fe3O4-MnO2 magnetic composite	Bisphenol A	[Cat] = 0.1 g/L [O3] = 0.1 L/min	BPA removal rate was 97%	After five cycles, the degradation efficiency of BPA was 88.8%.	[22]
	(BPA)	[Pull]0 = 0.05 mg/L pH = 7.0
Fe3Ce2/NaY	Quinoline	[Cat] = 0.5 g/L [O3] = 2.19 g/h [Pull]0 = 0.05 mg/L pH = 7.0	Quinoline removal rate was 99.14%	The COD removal efficiencies were nearly same during four cycles	[23]
Cu-Fe-O nanoparticles	Dimethyl phthalate (DMP)	[Cat] = 75 mg/L [O3] = 1.8 mg/min	DMP degradation rate close to 100%	After three cycles, the degradation rate of DMP was close to 100%.	[15]
		[Pull]0 = 50 mg/L pH = 5.7
Mn-Cu/Al2O3	4-chloro-3-methylphenol (PCMC)	[Cat] = 15 mg/L [O3] = 4.0 mg/min	PCMC removal rate near 100%	After five reuses, the PCMC removal rate decreased from 100% to 90.8%.	[24]
		[Pull]0 = 100 mg/L pH = 7.0
Co/Al2O3	Pyruvic acid	[Cat] = 5 g/L [O3] = 40 mg/min	PA removal rate was 94.4%	/	[25]
	(PA)	[Pull]0 = 0.1 g/L pH = 4.0
Mn-CeOx/γ-Al2O3	Bromaminic acid	[Cat] = 1 g/L [O3] = 20 mg/L	BAA almost completely degraded	After three reuses, the TOC removal rate dropped to 57.2%.	[26]
	(BAA)	[Pull]0 = 50 mg/L pH = 6.8
C/Cu-Al2O3	High-salt petrochemical wastewater	[Cat] = 400 g/L [O3] = 12 mg/L	COD removal of pollutants was 62.5%	After 20 reuses, its COD removal rate for pollutants exceeds 53%.	[27]
		[Pull]0 = 100 mg/L pH = 7.8
Fe3O4/MWCNTs	Sulfamethizole (SMT)	[Cat] = 0.5 g/L [O3] = 9 mg/min	TOC removal rate of SMT was 39.1%	/	[28]
		[Pull]0 = 20 mg/L pH = 4.0
Cu-Ce@Az	Purified terephthalic acid	[Cat] = 86 g/L [O3] = 2 g/h	COD removal rate of PAT was 84.2%	After 30 reuses, the COD removal rate of PTA wastewater was 68.2%.	[29]
	(PTA)	[Pull]0 = 178.6 mg/L pH = 8.0
Mn-Cu/γ-Al2O3	Tannery wastewater	[Cat] = 2 g/L [O3] = 0.3 g/h	COD removal rate of tannery wastewater was 88%	/	[30]
		[Pull]0 = 5200 mg/L pH = 7.0

). The impregnation method was used to prepare the materials: KMnO₄ solution was added dropwise at a 4:1 Fe/Mn ratio to a mixture of FeSO₄·7H₂O/PVP/NaOH, reacted for two hours at 70 °C, and then processed to yield the end product. Characterization using Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), and X-ray diffraction (XRD) techniques verified its mesoporous structure, high specific surface area (127.26 m²/g), amorphous MnO₂ shell rich in Mn⁴⁺ active sites, and superparamagnetic properties (26.82 emu/g). This material significantly outperforms single-component catalysts in the catalytic ozone degradation of bisphenol A (BPA): at 100 mg/L catalyst, 100 mL/min O₃, and pH 7.0, the removal rate reaches 97.0% within 30 min and maintains 88.8% high efficiency and stability after five cycles [22], while Fe₃Ce₂/NaY catalysts achieved 99.14% of 90 mg/L quinoline in 30 min [23], demonstrating effective removal of nitrogen-containing heterocyclic pollutants. These authors list the catalysts produced by the impregnation method and their catalytic properties, which include transition metal composite catalysts such as Cu-Fe-O nanoparticles [15] and Mn-Cu/Al₂O₃ [24]. Cu-Fe-O nanocatalysts (CFO NPs) are made by forming a composite structure through a displacement reaction with zero-valent iron (ZVI) and Cu (NO₃)₂ as precursors (Cu/Fe molar ratio = 1:1). In order to create a (Cu₂O)₀ composite, hydrothermal synthesis was conducted at 120 °C, precipitation at pH 8, and calcination at 600 °C for four hours. For the composite of CuO-α-FeO₃, its spherical nanostructure (SEM), surface rich in Cu⁺ active sites (Cu(I) make up 50–5% of the XPS-Cu 2p), and low metal leaching (Fe < 0.03 mg/L, Cu = 0.49 mg/L) were all confirmed by characterization using XRD/SEM/XPS/BET. With a TOC removal rate of 50.1% (as opposed to 18.6% with ozone alone), DMP was fully broken down in 20 min at 100 mg/L catalyst, 1.8 mg/min O₃, and pH 5.7. The activity stayed above 95% after five reuses. Although the degradation rate of DMP by the Cu-Fe-O nanocatalyst is close to 100%, its active metal Cu²⁺ leaching is 0.49 mg/L, which is 10 times higher than the drinking water standard. The synthesis of Mn-Cu/Al₂O₃ was accomplished by ultrasonic impregnation. The oxidation states of the metal oxides (Mn³⁺/Mn⁴⁺ and Cu⁺/Cu²⁺) changed in tandem with the formation of oxygen vacancies, according to XPS analysis, which greatly accelerated the breakdown of ozone and the production of ROS. The degradation rate of PCMC reached 100% in 30 min under ideal experimental conditions, which included an initial pH of 7, a catalyst dosage of 15 g/L, and an ozone dosage of 4–0 mg/min. Mn-Cu/AlO₃ has outstanding stability, reusability, and industrial application potential, according to reusability tests. According to mechanistic research, the ozone catalytic system greatly benefits from the contributions of O₂ and 1O₂. Furthermore, possible degradation pathways were deduced from the intermediate products found using LC-MS. In conclusion, ozone catalyzed by Mn-Cu/AlO₃ is a successful technique for breaking down PCMC in wastewater. Although the degradation rates of DMP and PCMC by Cu-Fe-O and Mn-Cu/Al₂O₃ catalysts are close to 100%, under long-term operation or changes in environmental conditions (such as pH fluctuations), the leaching of supported metals from the catalyst surface poses a serious risk of secondary pollution and potential environmental toxicity, which is a matter of concern.

Similarly, Fe-Mn-O loading on granular activated carbon (GAC) by impregnation [31] significantly enhanced the mineralization efficiency of Reactive Black 5 in water by catalytic ozone oxidation, with an 18% increase in TOC removal compared to unmodified GAC [32], suggesting that the synergistic effect of the bimetal significantly enhances the hydroxyl radical generation efficiency. Al₂O₃ has become a widely used carrier material by virtue of its porous structure, stable mechanical properties, and ability to provide more active sites for the reactants [33]. The 94.4% removal of pyruvic acid by Co/Al₂O₃ under acidic conditions (pH = 4) [25], and the Mn-CeO_x/γ-Al₂O₃ Ce³⁺/Ce⁴⁺ redox pair was introduced, which enhanced the concentration of oxygen vacancies on the catalyst surface, promoted ozone adsorption and dissociation on the catalyst surface, and shifted the degradation pathway of bromoacetic acid (BAA) from direct ozone oxidation to a free radical-dominated reaction [26], which revealed the synergistic effect between the acidic sites on the carrier surface and the active revealing the synergistic mechanism between the acidic sites on the carrier surface and the active components. In our earlier research, we used the impregnation method to create a Cu-Ce@γ-Al₂O₃ catalyst that effectively treated chemically concentrated wastewater from reverse osmosis (RO) membranes. Fourier transform infrared spectroscopy (FTIR) and SEM/BET/XRD/XPS/X-ray fluorescence (XRF) are used to thoroughly characterize the catalyst, exposing its microstructure, elemental composition, and crystal structure. The following were found to be the ideal reaction conditions: a reactor aspect ratio of 5:1, an initial pH of 9.0, a catalyst loading rate of 10%, and an ozone dosage of 8 mg/L/min. The catalytic ozone treatment’s chemical oxygen demand (COD) removal efficiency under these circumstances was 63.04% [34]. Although Al₂O₃ is widely used as a carrier due to its porous structure and stability, and studies on Mn-CeO_x/γ-Al₂O₃ show that there is a synergistic effect between the acidic sites of the carrier and the active components (such as Ce³⁺/Ce⁴⁺ pair), or that Cu-Ce@γ-Al₂O₃ can achieve 63.04% COD removal under specific conditions, these synergistic mechanisms are usually poorly described, and the carrier itself may introduce new restrictions or costs. The degradation process of the above catalysts is presented in Figure 2. Although the active components of the impregnation method are distributed on the carrier surface, which is conducive to the reaction, and the production method is simple, with a high production capacity and low cost, which is suitable for mass production, the leaching toxicity of the metal elements on the surface of the loaded metal catalysts will be harmful to the environment, resulting in secondary pollution, and the agglomeration of the metal particles in the process of calcination will lead to the deterioration of the performance of the catalytic carriers, which, together with the surface of the solid–liquid phase between the diffusion between solid and liquid phases, results in poor mass transfer efficiency and other problems that need to be solved.

3.2. Coprecipitation

When creating an ultrafine powder of composite oxides with two or more metal elements, coprecipitation is a crucial technique. Coprecipitation has the following benefits: first, it creates nanopowder materials with a consistent chemical composition directly through a variety of chemical reactions in solution; second, it makes it easier to prepare materials with uniform distribution and small particle sizes. One of the primary techniques for creating nanoparticles, nanocomposites, and catalysts is coprecipitation, which works well for creating multi-component catalysts.

Table 2. Catalyst prepared by coprecipitation method and its performance.

Catalyst	Pollutants	Experimental Condition	Catalytic Performance	Reusability	References
Ce0.1Fe0.9OOH	Sulfamethazine (SMT)	[Cat] = 0.4 g/L [O3] = 15 mg/L	SMT removal rate was 42%	/	[35]
		[Pull]0 = 40 mg/L pH = 3.0
α-Fe0.9Mn0.1OOH	Iohexol	[Cat] = 100 mg/L [O3] = 0.8 mg/L	Iohexol removal rate was 87.6%	After three cycles, the mineralization rate of TOC still reached 71.7%.	[36]
		[Pull]0 = 1 mg/L pH = 7.0
MnxFeyOz/AC	Sulfadiazine (SMZ)	[Cat] = 0.05 g/L [O3] = 50 mL/min [Pull]0 = 10 mg/L pH = 6.1	SMZ removal rate was 90.5%	After five reuses, the removal rate of SMZ decreased from 90.5% to 77.5%.	[37]
CuMn2O4/g-C3N	Benzophenone (BP-4)	[Cat] = 0.25 g/L [O3] = 20 mg/L	BP-4 removal rate was 87%	/	[38]
		[Pull]0 = 0.084 mmol/L pH = 6.4
Ag-La-Co nano-metal oxide	Reactive Black 5 (RB5)	[Cat] = 1 g/L [O3] = 30 L/h	TOC removal rate of RB5 was 95%	After three reuses, the catalytic activity does not decrease significantly.	[39]
		[Pull]0 = 100 mg/L pH = 7.0
CeO2@HSZSM-5	Sulfamethoxazole	[Cat] = 0.4 g/L [O3] = 9.3 mg/min	TOC removal rate of SMX was 80.4%	After five cycles, the TOC removal rate decreased by only 7.3%.	[40]
	(SMX)	[Pull]0 = 20 mg/L pH = 7.0
Fe3O4/GO nanohybrid material	Acid Red 88	[Cat] = 0.25 g/L [O3] = 6 mg/min	COD removal rate of AR88 was 69%	After five cycles of reuse, the degradation efficiency of AR88 remains high.	[41]
	(AR88)	[Pull]0 = 150 mg/L pH = 4.5
Fe3O4/ZnO	1-Hexyl-3-methylimidazole bromide (HMIMBr)	[Cat] = 0.25 g/L [O3] = 36 mL/min	HMIMBr removal rate was 90.5%	After five cycles of reuse, the degradation efficiency of [HMIM]Br can still reach 90%.	[42]
		[Pull]0 = 150 mg/L pH = 9.0
Cu-Al LDHs	Ciprofloxacin (CIP)	[Cat] = 0.79 g/L [O3] = 62 mg/min	CIP removal rate was 96%	After five cycles, the TOC removal rate remained at approximately 70%.	[43]
		[Pull]0 = 10 mg/L pH = 9.0
Zn-Cu-Ni composite silicate	Ciprofloxacin	[Cat] = 0.5 g/L [O3] = 1.5 mg/L	COD removal of PAT was 84.2%	/	[44]
	(CIP)	[Pull]0 = 3 mg/L pH = 7.0

lists the catalysts prepared by the coprecipitation method and their catalytic performance. Among them, iron-based catalysts exhibit good catalytic activity under acidic conditions. For example, the Ce_0.1Fe_0.9OOH catalyst was synthesized using Fe(NO₃)₃ and Ce(NO₃)₃ as precursors (Fe:Ce = 9:1). The mixed solution (total metal concentration 0.53 M) was reacted with 2 M NaOH, followed by aging at 60 °C for 15 days to form needle-like structures (SEM showed lengths of 1–2 μm and widths of 0.1–0.4 μm). Characterization via XRD/SEM/EDS confirmed that the synthesized catalyst exhibits a single needle hematite phase without CeO₂ impurities, with uniform elemental doping (Fe:Ce atomic ratio ≈ 9:1, as determined by EDS). Under conditions of 0.2 g/L catalyst, 15 mg/L O₃, and pH 7.0, the TOC removal rate of sulfamethoxazole (SMT) reached 42% after 120 min (compared to 27.8% with ozone alone) [35]. α-Fe_0.9Mn_0.1OOH achieves an 87.6% removal rate for iopamidol under neutral conditions [36], indicating that Mn doping optimizes the surface electronic structure of the catalyst and enhances its ability to generate reactive oxygen species (ROS) from ozone decomposition. Similarly, FeCl₃/FeSO₄/MnCl₂ was coprecipitated with activated carbon (AC) (Fe:Mn = 1:11.2) to create the Mn_xFeᵧO₂/AC catalyst. This was then aged for two hours at 70 °C and calcined at 600 °C. According to BET/SEM/XPS characterization, the synthesized catalyst has a uniform distribution of Mn/Fe/O, an ultra-high specific surface area (819.3 m²/g), and outstanding recovery performance. The degradation rate of Sulfadiazine (SMZ) reached 90.5% (64.8% with ozone alone) in 8 min at a catalyst concentration of 0.05 g/L, 50 mL/min O₃, and pH 6.1. After five cycles, the activity stayed at 77.5%. Transition metal oxides and composite catalysts have a synergistic effect on the catalytic ozone oxidation process [37]. CuMn₂O₄/g-C₃N₄ increased the degradation rate constant of benzophenone-4 (BP-4) to three times that of ozone oxidation alone at pH = 6.4 [38], indicating that the g-C₃N₄ carrier promotes electron transfer and enhances ozone activation. Additionally, Ag-La-Co nano-metal oxides achieve a TOC removal rate of up to 95% for Active Black 5 (RB5) under neutral conditions [39], indicating that the synergistic effect of noble metals (Ag) and rare earth metals (La) significantly enhances the ozone mineralization capacity. Although the introduction of supports (such as AC, g-C₃N₄) or specific doping (such as Mn optimized electronic structure, Ce/La enhanced stability) has been reported to improve the catalytic performance, the synergistic mechanism (such as charge transfer, active site formation) is usually inadequate or too simplified. The limitations of the carrier itself have not been fully discussed and may bring additional costs or complexity.

Compared to carbon-based materials, mesoporous molecular sieves, although possessing advantages such as a large adsorption capacity and adjustable pore size, lack protonic acid and Lewis acid centers, and thus generally lack catalytic activity [45]. Therefore, introducing rare earth dopant elements such as Ce and La can significantly enhance ozone utilization efficiency and catalyst stability. The CeO₂@HSZSM-5 catalyst was prepared using cerium nitrate as the precursor, which was mixed with high-silica ZSM-5 (SiO₂/Al₂O₃ = 500), followed by ultrasonic aging, ammonia precipitation (pH = 10), and calcination at 500 °C. Characterization using SEM/High Resolution Transmission Electron Microscope (HRTEM)/XPS/BET/Hydrogen Temperature-Programmed Reduction (H₂-TPR) confirmed that it possesses a high specific surface area (348–395 m²/g), small grain size (8.2–33.5 nm), and abundant surface oxygen vacancies (Ce³⁺ accounts for 68.59%). This material exhibits excellent performance in the catalytic ozone degradation of sulfamethoxazole (SMX): under conditions of 0.4 g/L catalyst, 9.3 mg/(min·L) O₃, and pH 7.0, the TOC removal rate of sulfamethoxazole (SMX) reached 80.4% after 180 min, significantly outperforming ozonation alone (40%). After five cycles, the efficiency decreased by only 7.3%, and the cerium leaching amount was negligible (<0.016 mg/L) [40]. In contrast, Fe₃O₄/GO achieved a COD removal rate of 69% for acidic red dye (AR88) at pH = 4.5 [41], indicating that while the introduction of graphene (GO) enhances electron conductivity, acidic conditions may limit the decomposition efficiency of ozone. Additionally, magnesium–aluminum layered double hydroxide (MgAl-LDH) nanoparticles were prepared using a simple and rapid coprecipitation method and used as a catalyst for the ozone oxidation of PNA. The structure of the synthesized MgAl-LDH was investigated using X-ray diffraction patterns and field emission scanning electron microscopy–energy dispersive spectroscopy. Under optimal conditions of an initial PNA concentration of 162.5 mg/L, pH of 8.25, LDH loading of 750 mg/L, and reaction time of 70 min, the PNA removal rate reached 91.5% [46]. Some catalysts exhibit excellent performance under alkaline conditions (pH = 9), such as Fe₃O₄/ZnO, which achieves a removal rate of 96% for the ionic liquid HMIMBr [42], and Cu-Al LDHs, which achieve a removal rate of 72% for ciprofloxacin (CIP) [43], indicating that alkaline conditions may promote the generation of hydroxyl radicals (·OH), thereby enhancing the oxidative capacity. The performance of different catalysts is highly pH dependent (e.g., Fe₃O₄/ZnO and Cu-Al LDHs are better in alkaline solution). This not only increases the complexity of process control, but also limits the applicability of catalysts in the treatment of practical wastewater with large pH fluctuations. Additionally, Zn-Cu-Ni composite silicates were prepared using the chemical coprecipitation method. The prepared catalyst was characterized using SEM, XRD, XPS, nitrogen adsorption–desorption, and Fourier transform infrared analysis (FTIR). The catalyst exhibits a high specific surface area (308.137 m²/g) and an irregular morphology, with a particle size of 7.6 µm, and contains Si-O-Si, Ni-O-Si, and Zn-O-Si. Under optimal conditions (pH 7.0, CIP 3.0 mg/L, ozone 1.5 mg/L), the removal rates of CIP and total organic carbon (TOC) were increased by 51.09% and 18.72%, respectively, compared to using ozone alone [44]. Figure 3 illustrates the degradation process of the aforementioned catalysts. Although the coprecipitation method has advantages such as simple operation, cost-effectiveness, practical preparation process, uniform component distribution, relatively low reaction temperature, controllable particle size and composition of the product, and typically no involvement of organic solvents, it also has limitations such as a high sensitivity to reaction preparation conditions, and the prepared catalysts generally have small specific surface areas and a low probability of contact with organic matter [47].

3.3. Sol–Gel Method

Using solutions, sols, and gels to solidify metal–organic or inorganic compounds, followed by heat treatment to create oxides or other solid compounds, is known as the sol–gel method. The sol–gel method has the following benefits: first, it can quickly achieve molecular-level uniformity; second, trace elements can be easily and quantitatively incorporated through the solution reaction step, achieving molecular-level uniform doping; and third, chemical reactions are simpler to perform and only require lower synthesis temperatures than solid-phase reactions.

A high specific surface area, homogeneous composition, controllable pore structure, and composite or doped components—such as metal oxides, composite oxides, noble metal-supported catalysts, porous materials, and doped catalysts—can all be prepared using the sol–gel method. Because the sol–gel process is performed in solution, specific structural elements can be customized. The sol–gel catalysts and their catalytic performance are listed in

Table 3. Catalyst prepared by sol–gel method and its performance.

Catalyst	Pollutants	Experimental Condition	Catalytic Performance	Reusability	References
Fe-Mn/PAC	phenolic compounds	[Cat] = 4 g/L [O3] = 50 mg/L [Pull]0 = 700 mg/L pH = 7.5	Phenol removal rate was 95%	Over 50 operating days, the COD removal rate remained stable at around 74%.	[48]
Fe-Cu@SiO2	Salicylic acid (SA)	[Cat] = 0.1 g/L [O3] = 4.6 mg/L	TOC removal rate of SA was 88%	/	[49]
		[Pull]0 = 11.2 mg/L pH = 7.0
CuFe2O4	N,N-Dimethylacetamide (DMAC)	[Cat] = 30 g/L [O3] = 0.06 L/min [Pull]0 = 200 mg/L pH = 6.8	DMAC removal rate was 95.4%	After five reuses, the removal rate of DMAC did not decrease.	[50]
Ag/MnFe2O4	Butyl phthalate (DBP)	[Cat] = 10 mg/L [O3] = 20 mg/L	DBP removal rate was 75.3%	/	[51]
		[Pull]0 = 0.5 mg/L pH = 7.3
Nano TiO2	Nitrobenzene (NB)	[Cat] = 0.1 g/L [O3] = 15 mg/L	NB removal rate was 75.3%	/	[52]
		[Pull]0 = 20 mg/L pH = 7.0
CeO2-T	Phenol	[Cat] = 0.1 g/L [O3] = 172.8 mg/L	Phenol removal rate was 91.7%	/	[53]
		[Pull]0 = 100 mg/L pH = 7.8
CeO2	4-Nitrophenol	[Cat] = 0.4 g/L [O3] = 1.6 mg/min	TOC removal rate of PNP was 86%	After three reuses, the TOC removal rate decreased from 86.14% to 75.38%.	[54]
	(PNP)	[Pull]0 = 25 mg/L pH = 5.7

. Bimetallic/polymeric metal catalysts have been thoroughly investigated in heterogeneous catalytic ozone systems due to their high stability, diverse oxidation states, and high catalytic activity. The Fe-Mn/PAC catalyst was prepared using the sol–gel method: PAC was added to a precursor solution of iron nitrate/manganese nitrate/citric acid at a Fe/Mn molar ratio of 1:1, followed by ultrasonic dispersion, ethylene glycol cross-linking, drying, and calcination (400 °C). The catalyst was then pressed into shape using PTFE as a binder (diameter 1 cm). Characterization by XPS/FT-IR/XRD/BET confirmed the successful loading of the bimetallic catalyst (Fe 11.9 wt%, Mn 9.3 wt%), rich in C=O/C-OH surface functional groups, and retention of a mesoporous structure (specific surface area 682.14 m²/g, pore size 3–5 nm). Under conditions of 50 mg/L ozone and 1 g/L catalyst, the removal efficiencies for COD and phenol reached 79% and 95%, respectively, and the COD removal efficiency remained at 74% after continuous operation for 100 days [48]. By encouraging ozone adsorption and dissociation through surface hydroxyl groups and efficiently preventing the leaching of active components (Fe-Cu alloy), the SiO₂ shell in Fe-Cu@SiO₂ core–shell catalysts raises the TOC removal rate of salicylic acid to 88% (as opposed to 35% with ozone alone) [49]. Although the core–shell structure can reduce leaching, the SiO₂ shell may hinder mass transfer and increase the complexity of synthesis; further, it has poor reproducibility and is difficult to scale up. Similarly, the ozone treatment of N,N-dimethylacetamide (DMAC) was studied by introducing Cu-Fe bimetallic elements to create CuFe₂O₄ magnetic nanoparticles. The TOC removal rate was only 22.3%, despite the CuFeO₄/O₃ process achieving a degradation rate of 95.4% for DMAC. This suggests that intermediate products accumulated, requiring bimetallic site design to optimize the mineralization pathway [50]. Furthermore, doping the catalyst with certain noble metals can improve its stability and ozone activation effectiveness. A series of well-crystallized silver-doped MnFe₂O₄ catalysts with a spinel structure were synthesized using the sol–gel method. The substitution of Ag for Mn in the structure was confirmed by SEM, EDS, and XRD, and the alteration of the oxygen coordination environment was determined by XPS. This stable and reusable magnetic porous material efficiently catalyzes the ozone treatment of DBP, with 0.5% Ag doping (relative to Mn) increasing the apparent rate constant by a factor of three [51]. Although the introduction of noble metals (such as Ag doping) can improve the apparent activity, the cost of precious metals is high and resources are scarce, and it is doubtful whether the activity improvement of trace doping (0.5%) can offset its economic and environmental costs.

A Nobel Prize winning nano-tio-oxide prepared by the solvent method was used to catalyze the oxidation of nitrobenzene to ozone. Studies show that TiO₂ increases the formation of ·OH radicals and thus significantly increases the removal efficiency of nitrobenzene [52]. In addition, certain mesoporous materials may increase the mineralization capacity of pollutants by virtue of catalytic synergies. A large specific surface area and an ordered pore structure of the mesoporous CeO₂-T significantly increases the adsorption capacity of the phenol, while the oxygen vacancies on the surface promote the oxidation of the ozone to OH, with a phenol removal rate of 91.7% [53]. Likewise, CeO₂-OCNT composite materials were made by the sol–gel method, which involved using carbon nanotubes (OCNT) as the carrier and hydrothermally reacting the precursor Ce(NO₃)₃·6H₂O in a NaOH solution for 24 h at 100 °C, then calcining the mixture at 300 °C. Its mesoporous structure (average pore size 12.6 nm), high dispersion, high specific surface area, and effective Ce³⁺/Ce⁴⁺ redox cycling were all confirmed by characterization using XRD/SEM/XPS/TPR-H₂/BET. Within 60 min, the phenol mineralization rate (TOC removal) reached 96% under the conditions of 0–10 g/L catalyst, 12 mg/L O₃, and pH 6.2. After five cycles, the rate remained at a 94% high stability [55]. Figure 4 illustrates the degradation process of the aforementioned catalysts. For the time being, the solvent–gel preparation process for supported nanocatalysts and the theories of reaction kinetics and thermodynamics in the system need to be clarified.

3.4. Hydrothermal Method

The hydrothermal method refers to the process of preparing the material in a sealed pressure vessel with water as a solvent, in which the powder is dissolved and subsequently re-treated. Compared with other powder preparations, hydrothermal-produced powders show good crystalline development, small particle size, homogeneous distribution, minimal agglomeration, the possibility of using relatively cheap raw materials, and ease of achieving the appropriate stoichiometry and crystal morphology. The hydrothermal method can be used for the preparation of single component microcrystals and of specialized compound compounds of two or more components. It is used for preparing powdered materials such as metals, oxides, and powdered waxes. The size range of particles in the resulting powder materials usually ranges from 0.1 micrograms to several micrometers, some of them reaching up to several tens of nanometers.

Table 4. Catalyst prepared by hydrothermal method and its performance.

Catalyst	Pollutants	Experimental Condition	Catalytic Performance	Reusability	References
α-MnO2	Metoprolol (MET) Ibuprofen (IBU)	[Cat] = 0.1 g/L [O3] = 1 mg/min [Pull]0 = 10 mg/L pH = 7.0	MET removal rate was 99.62%; IBU removal rate was 99.51%	After four reuses, there was no significant decrease in pollutant removal efficiency.	[56]
rGO	4-Hydroxybenzoic acid (PHBA)	[Cat] = 0.05 g/L [O3] = 20 mg/L	PHBA removal rate close to 100%	After five reuses, the inactivation phenomenon continues.	[57]
		[Pull]0 = 20 mg/L pH = 3.5
CoFe2O4	Melanoidin	[Cat] = 0.1 g/L [O3] = 10 mL/min [Pull]0 = 300 mg/L pH = 6.85	TOC removal rate of Melanoidin was 80%	/	[60]
MnFe2O4@Co3S4	Methylene Blue (MB)	[Cat] = 0.6 g/L [O3] = 2.5 mg/L	MB removal rate was 93.55%	After five cycles of reuse, the degradation rate of MB dropped to 90.02%.	[61]
		[Pull]0 = 50 mg/L pH = 6.6
Fe-MCM-48	Diclofenac (DCF)	[Cat] = 0.15 g/L [O3] = 100 mg/h	TOC removal rate of DCF was 49.9%	/	[62]
		[Pull]0 = 15 mg/L pH = 7.0
Fe2O3/Al2O3@SBA-15	Ibuprofen (IBU)	[Cat] = 1.5 g/L [O3] = 30 mg/L	TOC removal rate of IBU was 49.9%	/	[63]
		[Pull]0 = 10 mg/L pH = 7.0
Ag/MCM-41	p-chlorobenzoic acid (p-CBA)	[Cat] = 1 g/L [O3] = 100 mg/h	TOC removal rate of p-CBA was 84%	The TOC removal rate decreased by only 4% after four reuses.	[64]
		[Pull]0 = 10 mg/L pH = 4.2
Cu2S/Ni3S2@NF	4-Nitrophenol (PNP)	[Cat] = 3 pcs [O3] = 2 mg/min	PNP removal rate was 99.9%	After five cycles of reuse, the removal rate of PNP decreased to only 90.1%.	[65]
		[Pull]0 = 100 mg/L pH = 7.8
MnO2-NH2-GO	Cefalexin (CLX)	[Cat] = 25 g/L [O3] = 0.12 mg/L [Pull]0 = 1 mg/L pH = 4.2	CLX removal rate was 55.6%	/	[66]
MgO	P-chlorophenol (4-CP)	[Cat] = 1 g/L [O3] = 5 mg/L [Pull]0 = 100 mg/L pH = 6.2	4-CP removal rate was 95.1%	During the 7.5 h of operation, the removal rate remained at around 99.0%.	[67]
Sludge carbon/TiO2	Bisphenol A (BPA)	[Cat] = 0.2 g/L [O3] = 40 mL/min [Pull]0 = 10 mg/L pH = 7.0	BPA removal rate was 75%	/	[68]
ZnAl2O4	5-Sulfosalicylic acid (SSal)	[Cat] = 0.2 g/L [O3] = 10 mg/min [Pull]0 = 500 mg/L pH = 7.0	SSal removal rate was 64.8%	After three reuses, the removal rate of SSal was 64.8–59.7%.	[69]

lists several catalysts synthesized using the hydrothermal method and their catalytic performance. Among these, transition metal oxides such as α-MnO₂ and CoFe₂O₄ exhibit excellent catalytic performance in ozone systems. For example, the combination of α-MnO₂ and ozone can achieve removal rates exceeding 99.5% for metoprolol (MET) and ibuprofen (IBU) within 30 min [56]. Similarly, rGO achieved a nearly 100% removal efficiency for p-hydroxybenzoic acid (PHBA) under acidic conditions, with its high efficiency potentially attributed to the high specific surface area and abundant active sites of graphene materials [57]. Similarly, Luo et al. [58] synthesized a petal-shaped δ-MnO₂ microsphere via the hydrothermal method and tested its performance in the catalytic ozonolysis degradation of bisphenol A (BPA) and IBU. The experimental results showed that the degradation efficiency reached over 68% within 20 min, outperforming both ozone treatment alone and commercial MnO₂ catalysts, demonstrating the broad-spectrum degradation capability of metal oxide catalysts for pharmaceutical pollutants. In a similar manner, the hydrothermal method was utilized to prepare the Cl/S-CN catalyst. Melamine and cyanuric acid were employed as precursors, which formed hexagonal prisms through hydrogen bonding. Following an 8 h hydrothermal reaction at 180 °C, the material was calcined for 4 h at 520 °C in a nitrogen atmosphere to yield carbon nanotubes (CN); HCl/H₂SO₄ was then added to achieve Cl/S co-doping. Its ordered nanotube structure (tube diameter 56 nm), high specific surface area (65.78 m²/g), mesoporous properties, broad visible light response (absorption edge at 494 nm), and optimized band structure (CB = −0.96 eV, VB = 1.46 eV) were all confirmed by XRD/SEM/XPS/BET/UV-Vis characterization. In the O₃/visible light/Cl/S-CN system, tetracycline (TC) attained a 90.1% degradation rate in 10 min and a 78% TOC removal rate in 30 min at a concentration of 100 mg/L. After five cycles, the TC degradation efficiency dropped by less than 5% [59]. Transition metal oxides such as CoFe₂O₄ [60] and MnFe₂O₄@Co₃S₄ [61] exhibit high TOC removal rates (80%) and color removal rates (93.55%) for black ink and methylene blue (MB), respectively, indicating that they enhance degradation efficiency by promoting ozone decomposition to generate strong oxidative free radicals (such as ·OH). However, some catalysts, such as Fe-MCM-48, exhibit low TOC removal efficiency for diclofenac (DCF) (49.9%), which may be attributed to the complexity of the pollutant structure or insufficient accessibility of the catalyst’s active sites [62].

Additionally, the design of the support material significantly influences catalytic efficiency. For example, the preparation of Fe₂O₃/Al₂O₃@SBA-15 composite materials was conducted via the hydrothermal method: using AlCl₃ and FeCl₃ as precursors, Al₂O₃ and Fe₂O₃ were sequentially loaded onto the mesoporous SBA-15 carrier, yielding an optimal catalyst with a 12 wt% Fe loading. Characterization via XRD/TEM/XPS/pyridine adsorption FTIR confirmed that the material retains the ordered mesoporous structure of SBA-15, with Al₂O₃ and Fe₂O₃ exhibiting excellent high dispersion; the bimetallic catalyst exhibits the highest Lewis acid sites (264.3 μmol/g) and a near-neutral zero charge point (pH_pzc = 7.25). Under conditions of 1.5 g/L catalyst, 30 mg/L ozone, and pH 7.0, the TOC removal rate for IBU reached 90% after 60 min (compared to 26% with ozone alone), with stable activity and no metal leaching after six cycles [63]. Ag/MCM-41 achieved the efficient mineralization of p-chlorobenzoic acid (p-CBA) (84%) by leveraging the synergistic effect of silver nanoparticles [64]. Notably, some studies have optimized reactor designs, such as Cu₂S/Ni₃S₂@NF using plate-shaped catalysts to achieve the rapid degradation of p-nitrophenol (PNP) (99.9%) [65], but high catalyst loading (e.g., MnO₂-NH₂-GO requires 25 g/L) may limit their practical application [66]. Figure 5 illustrates the degradation processes of the aforementioned catalysts. Although the hydrothermal method is a synthesis method with low pollution, simple operation, good product dispersion, and high performance, it requires raw materials with high purity, resulting in relatively higher costs. Since the reaction occurs in a sealed container, operators cannot observe the reaction process in real-time and must adjust the process based on the characterization results of the products, which introduces a certain degree of lag. Overall, compared to other preparation methods, the hydrothermal method is more suitable for laboratory research or the preparation of small quantities (trace amounts) of solid materials requiring high purity [70].

3.5. Ion Exchange Method

The ion exchange process makes use of the exchangeable ions on the carrier’s surface. Under certain conditions, these ions may exchange with metal ions, resulting in the carrier’s initial loading. After additional processing stages like filtration and drying, a catalyst with a corresponding reaction activity can be created. The ion exchange method is commonly applied to carriers that have exchangeable ions on their surfaces, such as molecular sieves and ion exchange resins. Two characteristics of catalysts produced using this method are high catalytic activity and a good dispersion of active ingredients. Among other things, this method is effective for producing acid–base catalysts, catalysts with a low component content, catalysts with high utilization rates, and catalysts made of precious metals [71].

When designing nanocatalytic materials, ion exchange offers special benefits as a highly effective method of controlling the shape and structure of catalytic active sites. Figure 6 illustrates its mechanism of degradation. Mass transfer efficiency and the degree of catalytic active center exposure can be greatly increased by carefully regulating the loading form and distribution state of metal species through ion exchange reactions. Using catalysts based on zeolite as an example, Rutkowska et al., through ion exchange, added iron species to ZSM-5 and Y-type zeolites [72]. They discovered that the Y-type zeolite’s larger pore size promotes the formation of oligomeric or single-nuclear iron species, which leads to a noticeably higher N₂O decomposition activity than the ZSM-5 system. Further demonstrating the crucial role of the carrier pore structure in the dispersion state of metals, the stability of iron species is only slightly affected by the calcination temperature. By contrasting Mn/ZSM-5 catalysts made by impregnation and ion exchange techniques, it was discovered that the former produces a molecular-level dispersion of MnOx species, greatly increasing the concentration of surface adsorbed oxygen. This confirms the technical benefits of the ion exchange technique in enhancing metal dispersion. Zhang and colleagues [73] increased the method’s applicability by successfully creating hierarchical porous Ag₂ZnGeO₄ microspheres at room temperature by stepwise exchanging GeO₃²⁻ colloids with Zn²⁺/Ag⁺. Their ordered crystalline structure and high specific surface area (>150 m²/g) enhanced Rhodamine B’s degradation efficiency by 53% when compared to the original germanate, demonstrating the method’s potential for creating intricate nanostructures. The ion exchange technique is noteworthy because it not only successfully prevents metal particle agglomeration (e.g., forming isolated Fe or Mn active sites) [74], but it also makes it possible to regulate the coordination environment to prepare unique morphologies like hollow and core–shell structures [75], offering fresh perspectives on creating sophisticated catalysts with robust and high activity.

4. Conclusions and Future Perspective

This review highlights the unique benefits and drawbacks of recent developments in heterogeneous ozone catalyst preparation methods, including impregnation, coprecipitation, sol–gel, hydrothermal synthesis, and ion exchange. Although impregnation is inexpensive and scalable, it has drawbacks such as pore blockage and metal leaching. Coprecipitation produces materials with a limited surface area but allows for uniform multi-component catalysts. While hydrothermal techniques produce high crystallinity but have trouble scaling up to the industrial level, the sol–gel method offers precise structural control at the cost of complex synthesis. Although it greatly relies on carrier compatibility, ion exchange produces remarkable active-site dispersion. Through catalysis, metal oxides (e.g., supported composites and FeO₄, MnO₂, and CeO₂) improve the breakdown of ozone into reactive oxygen species (e.g., ·OH), greatly increasing the mineralization efficiency of a variety of refractory pollutants, including industrial contaminants and pharmaceuticals.

Notwithstanding advancements, significant obstacles still exist, such as unclear reaction mechanisms, restrictions on mass transfer, metal leaching, and catalyst deactivation. Practical implementation is hampered by these problems, especially in intricate wastewater matrices with high salinity or organic loads. Advanced catalyst design and process innovation are two interrelated fronts that need to be given top priority in future research. Non-precious multi-metal oxides should be the main focus of material development (e.g., Fe-Mn-Ce), with engineered acid–base sites, oxygen vacancies, and hierarchical porosity to improve accessibility, inhibit leaching, and increase stability. Meanwhile, mechanistic research employing in situ methods is necessary to clarify degradation pathways and radical pathways (e.g., Raman, EPR). New reactor designs are necessary for process optimization (e.g., systems with high gravity) and hybrid technologies (e.g., ozonation–photocatalysis) to maximize synergy and move past obstacles to mass transfer. In the end, combining sustainable methods—like catalysts made from waste and standardized stability tests—will connect lab advancements with real-world uses. By addressing these issues, catalytic ozonation will be promoted as a reliable, environmentally friendly method of treating stubborn wastewater, supporting international objectives for environmental sustainability and water security.