Challenges and opportunities of catalytic decomposition of ozone at room temperature

Abstract

Catalytic decomposition is a key technology for the removal of ozone (O₃). Practical application scenarios require catalysts with superior O₃ decomposition performance at ambient temperatures, high humidity, and high space velocities. In this review, the strategies for the design of catalysts with efficient and stable O₃ decomposition performance are presented, and the three factors that limit the catalytic decomposition of O₃ at ambient temperature (insufficient active sites, competitive adsorption of H₂O and O₃ molecules, and difficult desorption of intermediate oxygen species) are systematically summarized for the first time. Subsequently, the research progress in recent years is categorized and summarized in terms of addressing the three factors limiting O₃ decomposition, which in turn suggests the shortcomings of current research and the focus of future research.

Graphical abstract

1. Introduction

Ozone (O₃), as a strong oxidizing agent, affects the oxidizing capacity of the atmosphere and the formation of secondary particulate matter [1]. In addition, O₃ is also harmful to ecosystems and human health [[2], [3], [4]]. In addition to the guideline value of 100 µg/m³ (∼51 ppb) for the 8-h average O₃ concentration, the World Health Organization introduced a peak seasonal average O₃ concentration of 60 µg/m³ (∼31 ppb) in 2021 based on the effects of long-term O₃ exposure on the total mortality rate and the mortality rate from respiratory diseases. The standard heat, standard free energy and entropy of the O₃ decomposition reaction are −142 kJ/mol, −163 kJ/mol and 239 J/(K·mol), respectively, indicating that O₃ is thermodynamically unstable and can spontaneously decompose to oxygen [5]. At atmospheric pressure, the half-life of O₃ in oxygen at 25 °C, 100 °C and 250 °C is 160 h, 210 s and 0.03 s, respectively [5]. Thus, O₃ is stable at room temperature. Catalysts can lower the energy barrier of O₃ decomposition and serve to accelerate the decomposition of O₃.

Highly efficient catalytic decomposition of O₃ is urgently needed for treating industrial exhaust and purifying air in confined spaces and atmospheric environments. During the treatment of flue gas, wastewater and drinking water by the catalytic ozonation technology, excess O₃ is injected to achieve higher removal efficiencies, and practical application conditions are variable, which can lead to O₃ residues (80 ppb–70 ppm) in the exhaust [[5], [6], [7]]. Catalysts with efficient and stable activity at ambient temperature (20−70 °C), high humidity and medium space velocity need to be developed for the removal of residual O₃ in the exhaust. In addition, the exhausts contain other pollutants such as NO_x, SO₂ and VOCs, so it is necessary to develop catalysts with high resistance to poisoning. O₃ pollution (50–300 ppb) is also present in the indoor environments due to the operation of disinfection equipment, negative ion generators, electrostatic precipitators, photocopiers and laser-jet printers [8,9]. Catalysts with efficient and stable activity at room temperature, moderate humidity, and high space velocity need to be developed for the removal of O₃ in indoor environments. The cabins of aircraft flying at high altitudes also face O₃ pollution (1800 ppb) [10,11]. Catalysts with efficient and stable activity at low temperature, low humidity and ultra-high space velocity must be developed to remove O₃ in the cabins [[11], [12], [13], [14], [15]].

In the atmosphere, O₃ is a typical secondary pollutant. The relationship between O₃ concentration and its precursors (NO_x and VOCs) is non-linear [1,3,16]. In an urban environment, O₃ is usually more sensitive to VOCs, which are difficult to control due to their highly complex and dispersed emissions [17]. Moreover, O₃ pollution may even be exacerbated by the rapid reduction of NO_x [17]. As a result, effectively solving the O₃ pollution by reducing the precursors from the emission sources remains a great challenge in many countries. In this case, catalytic decomposition of O₃ has become a potential ancillary means of solving O₃ pollution in the atmospheric environment. Catalysts with efficient and stable activity at room/low temperature, medium to high humidity, and ultra-high space velocity need to be developed for the removal of O₃ in atmospheric environments. In addition, complex atmospheric environments, such as co-existing pollutants, place even greater demands on the catalyst. Therefore, the development of catalysts with efficient and stable activity for O₃ decomposition in various scenarios is the key to practical applications.

2. Optimization strategies of catalysts based on the main factors limiting O₃ decomposition

Among various catalysts for the decomposition of O₃, manganese oxides (MnO_x) have attracted much attention due to their excellent activity and low cost. The active site for the decomposition of O₃ on MnO_x catalysts is oxygen vacancies (V_O) [18,19]. The detailed mechanism of O₃ decomposition is described below (Eqs. (1)–(3)) [20,21]:

$${\mathrm{O}}_3+{\mathrm{V}}_{\mathrm{O}}-\mathrm{M}{\mathrm{n}}^{\mathrm{n}+}\to {\mathrm{O}}_2+{\mathrm{O}}^{2-}-\mathrm{M}{\mathrm{n}}^{\left(\mathrm{n}+2\right)+}\left(\mathrm{n}\text{represents}2,3\text{or}4\right)$$ (1)

$${\mathrm{O}}_3+{\mathrm{O}}^{2-}-\mathrm{M}{\mathrm{n}}^{\left(\mathrm{n}+2\right)+}\to {\mathrm{O}}_2+{{\mathrm{O}}_2}^{2-}-\mathrm{M}{\mathrm{n}}^{\left(\mathrm{n}+2\right)+}$$ (2)

$${{\mathrm{O}}_2}^{2-}-\mathrm{M}{\mathrm{n}}^{\left(\mathrm{n}+2\right)+}\to {\mathrm{O}}_2+{\mathrm{V}}_{\mathrm{O}}-\mathrm{M}{\mathrm{n}}^{\mathrm{n}+}$$ (3)

Oxygen vacancies act as electron donors and acceptors for redox reactions during the adsorption of O₃ and the desorption of intermediate oxygen species, respectively. By applying the methodology of classifying and summarizing the limiting factors of performance in the preliminary study [22,23], the limiting factors about the activity of MnO_x catalysts for the decomposition of O₃ were identified. Firstly, the insufficient number of oxygen vacancies will limit the activity of MnO_x catalysts for the decomposition of O₃ (Fig. 1) [24,25]. In addition, the decomposition of O₃ is inevitably confronted with the competitive adsorption of H₂O and O₃ molecules (Fig. 1) [25]. Finally, in terms of the O₃ decomposition mechanism, the desorption of peroxide species in the final step is a rate-limiting step, so the difficult desorption of peroxide species is another important factor limiting the decomposition of O₃ (Fig. 1) [19,20,26]. In summary, the key factors limiting the decomposition of O₃ are the insufficient number of active sites, the competitive adsorption of H₂O and O₃ molecules, and the difficult desorption of intermediate oxygen species. Among them, the competitive adsorption of H₂O and O₃ molecules and the difficult desorption of intermediate oxygen species are the difficulties of O₃ decomposition. The competitive adsorption of H₂O and O₃ molecules has a more rapid but reversible effect on the O₃ decomposition activity of catalysts. The O₃ decomposition activity of the catalyst usually decreases rapidly under transition from dry to humid conditions, indicating that H₂O molecules have a significant effect on its activity. However, the O₃ decomposition activity of the catalyst recovers rapidly when switching to dry conditions, indicating that the effect of H₂O molecules on the O₃ decomposition activity is reversible. Notably, H₂O molecules are inevitably present in the actual environment, so the competitive adsorption of H₂O and O₃ molecules has a great influence on O₃ decomposition activity. The effect of accumulated intermediate oxygen species on the O₃ decomposition activity of catalysts is slow but difficult to recover. During O₃ decomposition, intermediate oxygen species slowly accumulate on the catalysts, resulting in a gradual decline in their activity. To restore the activity after deactivation caused by the accumulation of intermediate oxygen species, high-temperature calcination is often necessary. In summary, the difficult desorption of intermediate oxygen species has a significant impact on O₃ decomposition activity. Corresponding solutions to the above problems include increasing the number of active sites, weakening the adsorption of H₂O molecules or activating H₂O molecules to promote the participation of H₂O molecules in the decomposition of O₃, accelerating the desorption of intermediate oxygen species or modulating the pathway of O₃ decomposition, respectively.

Fig. 1.

The key factors limiting O₃ decomposition on MnO_x catalysts.

2.1. Increasing the number of active sites

In previous studies, it has been determined that MnO₂ catalysts have higher O₃ decomposition activity than other MnO_x (e.g., Mn₂O₃, Mn₃O₄ or MnO). Therefore, researchers have focused on regulating MnO₂ catalysts, resulting in a series of highly efficient O₃ decomposition catalysts. The O₃ decomposition activity of manganese dioxides (MnO₂) can be successfully improved by increasing the number of oxygen vacancies. As shown in Fig. 2, the number of oxygen vacancies in the modulated MnO₂ is significantly increased compared to pristine MnO₂, with a corresponding significant improvement in O₃ decomposition activity. The main methods to increase the number of oxygen vacancies over MnO₂ include modulation of the content and type of ions (H⁺, K⁺, NH₄⁺ or Na⁺) in the tunnels of MnO₂ [[27], [28], [29], [30], [31]], in situ doping of other metal elements (Ce, Fe, W or V) during synthesis [24,25,[32], [33], [34], [35]], vacuum deoxidation to remove oxygen atoms from MnO₂ [36], or optimization of raw materials, conditions and methods of preparation [[37], [38], [39]]. Our group found that acetate groups can prevent the aggregation of MnO₂ particles, thereby increasing the specific surface area and the number of oxygen vacancies on cryptomelane-type manganese oxide (OMS-2) catalysts [18]. The number of oxygen vacancies was further increased by in situ doping with Ce during the hydrothermal synthesis. Cerium-doped OMS-2 (Ce-OMS-2) catalyst showed O₃ conversion of 90% at RH of 90% and space velocity of 600,000 h⁻¹, which is a promising catalyst for the decomposition of O₃ under high humidity [24]. Then, the hydrothermal conditions (fill percentage, hydrothermal temperature, and hydrothermal time) of the Ce-OMS-2 catalysts are further optimized to increase the number of oxygen vacancies and the O₃ decomposition activity of the catalyst, and the Ce-OMS-2 catalysts that prepared under 80% of fill percentage, 95–100 °C of hydrothermal temperature and 8–24 h of hydrothermal time showed higher O₃ conversion than other Ce-OMS-2 catalysts [38,39].

Fig. 2.

The relationship between O₃ decomposition rate and the number of Mn³⁺ (oxygen vacancies) in MnO₂ catalysts [28,30–34,39].

In response to the insufficient number of oxygen vacancies on MnO_x catalysts, Ag active sites have been constructed to improve the O₃ decomposition performance of MnO_x catalysts [40]. Under a relative humidity of 65% and a high space velocity of 840,000 h⁻¹, the 6%Ag/MnO_x-I catalyst prepared by the impregnation method showed a 99% conversion of 40 ppm O₃ after 6 h, which was far superior to the Ag-Mn catalysts prepared by other methods and the pure MnO_x support (Fig. 3) [40]. The excellent O₃ decomposition performance of the 6%Ag/MnO_x-I catalyst is attributed to the formation of metallic Ag nanoparticles (Ag_n⁰) on it. Ag_n⁰ was determined to be a better active site for the decomposition of O₃ than Ag_1.8Mn₈O₁₆ and oxidized Ag clusters (Ag_n^δ+) [40,41], which is attributed to the weaker competitive adsorption of H₂O and O₃ molecules, faster activation of O₃ molecules, and easier desorption of O₂^2- [42]. More profoundly, the excellent activity of Ag_n⁰ stems from the highly efficient electron transfer between Ag_n⁰ and O₃ molecules. The decomposition of O₃ on Ag_n⁰ is a redox reaction (Eqs. (4)–(6)):

$${\mathrm{O}}_3+\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^0\to {\mathrm{O}}_2+{\mathrm{O}}^{2-}\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^{2+}$$ (4)

$${\mathrm{O}}_3+{\mathrm{O}}^{2-}\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^{2+}\to {\mathrm{O}}_2+{{\mathrm{O}}_2}^{2-}\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^{2+}$$ (5)

$${{\mathrm{O}}_2}^{2-}\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^{2+}\to {\mathrm{O}}_2+\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^0$$ (6)

$${{\mathrm{O}}_2}^{2-}\mathrm{A}{{\mathrm{g}}_{\mathrm{n}}}^{2+}\to \mathrm{A}{\mathrm{g}}_2{\mathrm{O}}_2+\mathrm{A}{{\mathrm{g}}_{\mathrm{n}-2}}^0$$ (7)

Fig. 3.

(a) Schematic diagram and relative ratio of Ag species on Ag-Mn catalysts. (b) The relationship between O₃ decomposition activity (Conditions: ozone inlet concentration 40 ppm, temperature 30 °C, relative humidity 65%, and space velocity 840 L g⁻¹ h⁻¹) and the state of Ag species [40].

Then, to address the low utilization of Ag due to large Ag nanoparticles and uneven distribution of Ag nanoparticles on the current catalysts, abundant manganese vacancies are constructed by modulating the morphology of MnO_x supports to enhance the interaction between Ag and MnO_x supports; finally Ag_n⁰ is uniformly dispersed and more metallic Ag sites are exposed [43]. The excellent O₃ decomposition performance was achieved with a lower Ag loading (2%) on the Ag/MnO_x-C catalysts (catalysts with Ag supported on porous cubic MnO_x), which was only one third of the Ag loading of the previously reported Ag/MnO_x catalyst [43].

2.2. Weakening the adsorption of H₂O molecules or activating H₂O molecules to participate in the decomposition of O₃

The adsorption of H₂O molecules on manganese oxides can be alleviated to some extent by modulating the type and physicochemical properties of oxygen vacancies [[44], [45], [46]], designing non-oxygen vacancy sites for the adsorption of H₂O molecules [[47], [48], [49]], constructing porous manganese oxides with abundant macropores and mesopores [50,51], constructing isolated hydrophilic sites (N-Mn-N) over TpBpy-COF catalyst [52], and combining hydrophobic graphene shells [53], functionalized activated carbon [49,54] and hydrophobic carbon layer [55] with manganese oxides (Fig. 4). Besides, DFT calculations revealed that the adsorption of H₂O molecules on the metallic Ag sites was much weaker than their adsorption on the oxygen vacancies of MnO_x (Fig. 5a), so the competitive adsorption of H₂O and O₃ molecules can be attenuated by constructing metallic Ag sites on MnO_x. When the relative humidity increased to 30%, the MnO_x catalyst had no O₃ decomposition activity due to the competitive adsorption of H₂O and O₃ molecules (Fig. 5b). However, when the relative humidity increased to 80%, the 6%Ag/MnO_x catalyst still retained 97% of the O₃ conversion (Fig. 5b), indicating that the H₂O resistance of the catalyst was significantly improved by the construction of metallic Ag sites [40].

Fig. 4.

Measures to alleviate the adsorption of H₂O molecules on manganese oxides [44–55].

Fig. 5.

(a) Adsorption energy of H₂O on Ag_n⁰, Ag₂O and oxygen vacancy of the MnO_x. (b) O₃ decomposition activity of Ag_n⁰ and oxygen vacancy under dry gas, relative humidity of 30%, 65% and 80%. (c) O₃ decomposition activity of K-Ag/MnO_x catalyst under dry gas, relative humidity of 30% and 65%. (d) The formation mechanism of hydroxylated Ag active sites [40,44,56]. Conditions: ozone inlet concentration 40 ppm, temperature 30 °C, and space velocity 840 L g⁻¹ h⁻¹.

Activating H₂O molecules to participate in the decomposition of O₃ is also an efficient means of solving the competitive adsorption of H₂O and O₃ molecules. H₂O molecules could enhance the O₃ decomposition activity of K-Ag/MnO_x catalysts (Fig. 5c), suggesting that the competitive adsorption of H₂O and O₃ molecules on Ag/MnO_x catalysts was further resolved by the addition of alkali metals [56]. Due to electron transfer by alkali metals, the O atom with a more negative valence in Ag₂O-K can take away H atoms of H₂O molecules, favoring the adsorption and dissociation of H₂O molecules to form the new hydroxylated Ag active sites (Ag-O(OH)_x-K) with high activity for O₃ decomposition (Fig. 5d). In summary, it is tentatively speculated that the common features of sites activating H₂O molecules are the more negative valence of the O atom in metal oxides. On the new materials such as iron-organic framework (MIL-100(Fe)) [57], manganese-organic framework (ZZU-281(Mn)) [58], bimetallic-organic framework (PCN-250(Fe₂Co)) [59] and monolithic MoO₃/graphdiyne [60], the H₂O molecules or OH groups activated during the humid O₃ decomposition or present in the structure of the catalysts are active sites for O₃ decomposition, and the key step is the transfer of the H atom from the active sites to the first O₃ molecule to form the OOOH radical. Taking ZZU-281(Mn) as an example, the specific reaction mechanism is as follows (Fig. 6) [58]. After the formation of the Mn-OH and OOOH radical, the second O₃ molecule adsorbs on the adjacent OH group, followed by the rearrangement of the intermediate. Accompanied by the release of an O₂ molecule, the coordinated H₂O molecule is restored, and a Mn-O-O-Mn intermediate is formed. Then, the Mn-O-O-Mn species react with the OOOH radical to form Mn-O-O(OH)-Mn and an O₂ molecule. Finally, Mn-O-O(OH)-Mn further releases another O₂ molecule and the adjacent OH group is regenerated. In summary, the highly efficient H₂O-activated O₃ decomposition mechanism on the above new materials ensures the good activity of the catalysts under humid conditions, thus solving the competitive adsorption of H₂O and O₃ molecules. However, there is a tendency for slow oxidation of the organic ligands of metal-organic frameworks (MOFs) during the decomposition of O₃. The preparation of the MOFs is complicated, and their yield is low. Therefore, it remains to be investigated whether the MOFs are good O₃ decomposition materials for practical applications.

Fig. 6.

(a) Calculated O₃ adsorption sites on ZZU-281. (b) Free energy diagrams and optimized structures intermediates along the ZZU-281-catalyzed O₃ decomposition pathway. (c) Proposed mechanism of O₃ degradation by ZZU-281 [58].

2.3. Accelerating the desorption of intermediate oxygen species or modulating the pathway of O₃ decomposition

To solve the difficult desorption of intermediate oxygen species on conventional MnO_x catalysts during O₃ decomposition, many measures have been taken to accelerate the capture of electron from intermediate oxygen species by the oxygen vacancies to rapidly release the active sites, such as the development of other Mn-based catalysts (mullite YMn₂O₅ [61], amorphous MnO_x [62], spinel (Mn, Co)₃O₄ [63]), modulating the type of oxygen vacancies on MnO_x [44,64], and modifying the physicochemical properties of oxygen vacancies by encapsulating MnO_x with graphene layers [53], loading MnO_x on functionalized activated carbon [49,54], coating hydrophobic carbon on mesocrystalline MnO [55], doping MnO_x with chlorine during the synthesis [45], or synthesizing MnCO₃/Mn₃O₄ heterostructure [65]. Although the competitive adsorption of H₂O and O₃ molecules has been solved by combining Ag and MnO_x, the O₃ decomposition on the metallic Ag sites is still a redox reaction. Metallic Ag sites are inevitably oxidized by intermediate oxygen species. To overcome the above problems, alkali metals were added to tune the electronic structure of the Ag active sites on the Ag/MnO_x catalysts, promoting the dissociation of H₂O molecules and forcing the Ag active sites to become hydroxylated [56]. The O₂^2- species on the new active sites (Ag-O(OH)_x-K) of the K-Ag/MnO_x catalyst can easily desorb, thus significantly improving the O₃ decomposition stability of the catalyst (Fig. 7) [56].

Fig. 7.

(a) O₃ conversion (Conditions: ozone inlet concentration 40 ppm, temperature 30 °C, relative humidity 65%, and space velocity 840 L g⁻¹ h⁻¹) and (b) the amount and desorption energy of O₂^2- on Ag/MnO_x and K-Ag/MnO_x catalysts [56].

Modulating the pathways of O₃ decomposition is an effective strategy to overcome the problems of O₃ decomposition on metal oxides at room temperature. There are two pathways of O₃ decomposition on metal oxides. In addition to oxygen vacancies, which can decompose ozone via the redox pathway, surface hydroxyl groups can also serve as active sites for the decomposition of O₃ (Eq. (8)).

$$\text{HO}-\mathrm{M}-\text{OH}+{\mathrm{O}}_3\to 2{\mathrm{O}}_2+\mathrm{M}-{\mathrm{H}}_2\mathrm{O}$$ (8)

However, the number of hydroxyl groups on metal oxides is low, and the accumulation of reaction product H₂O molecules on active sites will cause the deactivation of metal oxides [66]. The key to overcoming these problems lies in the activation of H₂O molecules and the regeneration of hydroxyl groups. Layered Double Hydroxides (LDHs) are a two-dimensional layered material with suitable interlayer spacing, which facilitates the diffusion and reaction of O₃ and H₂O molecules (Fig. 8a) [67]. The abundant hydroxyl groups between the layers of LDHs can act as active sites for the decomposition of O₃ [[67], [68], [69]]. In addition, the interlayer structure of LDHs can restrict H₂O molecules, and the hydrogen bonding of hydroxyl groups between the interlayer structure of LDHs can activate H₂O molecules, which is conducive to the regeneration of hydroxyl groups. The pathway for the catalytic decomposition of O₃ on NiFe-LDH catalysts at room temperature is a novel H-transfer redox mechanism with the H₂O molecule as an intermediate (Fig. 8b, Eqs. (9)–(10)) [67]:

$${\mathrm{O}}_3+2\mathrm{M}-\text{OH}\to {\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+2\mathrm{M}-\mathrm{O}$$ (9)

$$2{\mathrm{H}}_2\mathrm{O}+4\mathrm{M}-\mathrm{O}\to {\mathrm{O}}_2+4\mathrm{M}-\text{OH}$$ (10)

Fig. 8.

(a) Schematic diagram, (b-c) reaction pathway of LDH structure; (d-e) O₃ decomposition activity and stability (Conditions: ozone inlet concentration 40 ppm, temperature 30 °C, relative humidity 65%, and space velocity 840 L g⁻¹ h⁻¹) of NiFe-LDH and other catalysts [56–58,60,67,–70,[72], [73]–74].

Compared to NiAl-LDH, NiMn-LDH, CoFe-LDH and MgFe-LDH materials, the synergistic effect of Ni and Fe on NiFe-LDH catalysts induced the transfer of electrons from O atoms to H atoms, thus weakening the interactions between O atoms and H atoms, which facilitated hydrogen transfer reactions at room temperature (Fig. 8c) [70]. Of all the O₃ decomposition catalysts reported to date, the NiFe-LDH catalyst has the best activity and stability (Fig. 8d–e) [67]. Under a relative humidity of 65% and a space velocity of 840 L⋅g^-1⋅h^-1 (600,000 h^-1), the NiFe-LDH catalyst exhibits an O₃ conversion of over 95% during 22 consecutive days of O₃ decomposition testing (Fig. 8e). In order to optimize the preparation of conventional monolithic catalysts and overcome the degradation of the catalytic performance due to the weak bond between the catalyst powders and the support, NiFe-LDH monolithic catalysts self-loaded on iron foam (IF) were developed by a simple one-step hydrothermal method [71]. The NiFe-LDH/IF monolithic catalyst showed an O₃ conversion of 85% after 168 h under harsh conditions, demonstrating great potential for the decomposition of O₃ in practical applications [71].

The catalytic decomposition of O₃ at room temperature has been extensively studied. Only a few studies have focused on catalytic decomposition of O₃ at low temperatures for O₃ purification in the cabin of aircraft flying at high altitudes. Tang’s group found that the sheet-shaped α-MnO₂ with reactive (001) facets exhibited 100% conversion for 60 ppm O₃ even at −10 °C under a space velocity of 510 L g⁻¹ h⁻¹, which is due to abundant surface oxygen vacancies [75]. Wang’s group found that the mullite YMn₂O₅ catalyst had 100% conversion for 120 ppm O₃ at −5 °C under a space velocity of 600 L g⁻¹ h⁻¹, which is attributed to the moderate Mn−O bonding strength [61].

Currently, there are few studies on the effect of other pollutants on the O₃ decomposition performance of catalysts. Abdallah et al. [76] found that HNO₃-treated Ce-modified MnO₂ catalyst exhibited stable O₃ conversion of 94% in the presence of N₂O₅/HNO₃, which is attributed to the high density of acid sites and oxygen vacancies. Tang et al. [77] studied the effect of SO₂/H₂O on the O₃ decomposition performance of MnO₂, discovering that cactus-like MnO₂ nanospheres had excellent tolerance to water vapor and SO₂/H₂O. These cactus-like MnO₂ nanospheres could maintain O₃ conversion of > 88% in high humidity and sulfur-containing conditions. In summary, catalysts with excellent resistance to nitrogen-containing co-pollutants and SO₂ can be obtained by tuning the physical-chemical structure and properties of MnO₂, which is an important area for future research. In order to improve the resistance of catalysts to poisoning and maintain their activity in complex gas mixtures, the sacrificial sites for other pollutants should be designed, or the oxidation degree of co-existing pollutants during the reaction should be regulated by constructing specific active sites.

The economic feasibility and scalability of the three main catalysts (MnO_x, Ag/MnO_x and LDHs) for O₃ decomposition are discussed below. In terms of economic feasibility, MnO_x and LDHs catalysts are more feasible than Ag/MnO_x catalysts because they use abundant and inexpensive transition metals as the main active component, avoiding the use of noble metals and significantly reducing the cost of raw materials. From the point of view of scalability, MnO_x catalysts are typically prepared by hydrothermal and coprecipitation methods. These methods offer a broad range of temperature, pH and reaction time, making them insensitive to slight fluctuations. The Ag/MnO_x catalysts are prepared by the simple impregnation method, making them easy to manufacture on an industrial scale. LDH catalysts are prepared by coprecipitation methods. The preparation parameters of LDH catalysts are insensitive to slight fluctuations. LDH catalysts can be scaled up by conventional equipment in a chemical plant without the need for special equipment.

3. Perspectives

Many studies have been carried out in recent years to address the problems of catalysts for the decomposition of O_3, and significant progress has been made. However, the competitive adsorption of H₂O and O₃ molecules is still a problem to be faced for most catalysts, including MnO_x catalysts, Ag-based catalysts, and LDH catalysts. The problem can be solved either by inhibiting the occupation of active sites by H₂O molecules to weaken the competitive adsorption of H₂O and O₃ molecules or by accelerating the activation of H₂O molecules to facilitate the efficient participation of H₂O molecules in the O₃ decomposition. In addition, the desorption of the intermediate oxygen species is the key rate-limiting step on MnO_x and Ag-based catalysts, so how to accelerate the capture of electrons from the intermediate oxygen species by the active sites is the key to improving the O₃ decomposition activity and stability of the catalysts. The rate-limiting step in O₃ decomposition on LDH catalysts is the activation of the intermediate product H₂O molecules to form hydroxyl groups, so how to accelerate the activation of H₂O molecules at the active sites is the key to improving the O₃ decomposition activity and stability of LDH catalysts.

The key to regulating reaction processes on a catalyst is to regulate the physicochemical structures and properties of the catalyst, and significant progress has been made in this area. However, there is still a need for more precise tuning for specific catalysts. For manganese oxides, current studies mainly focus on the regulation of the number of oxygen vacancies. However, there are fewer studies on the regulation of the types and physicochemical properties of oxygen vacancies on manganese oxides, and a reliable theoretical system has not yet been developed. Therefore, more attention should be devoted to the preparation and microstructural characterization of manganese oxides. For Ag-based catalysts, the Ag sites with efficient activity of O₃ decomposition still need to be investigated in detail, which first requires a more precise modulation of the Ag species. For LDH catalysts, the roles of each part of the LDH structure (interlayer hydroxyl groups, metal elements on the lamellae, and interlayer anions) during the O₃ decomposition and the influences of these factors on the O₃ decomposition performance have been initially clarified, but the precise regulation of the O—H bond strength of the active sites (hydroxyl groups) is lacking, which should be strengthened in the subsequent studies. In addition, new types of catalysts with superior O₃ decomposition activity under humid gas, such as MOFs and covalent organic frameworks (COFs), have been successfully developed. However, the roles of specific parts of the MOF or COF structure (organic ligands, metal sites and their spatial arrangement) during the O₃ decomposition and the influences of these factors on the O₃ decomposition performance have not been clarified and should be addressed in follow-up studies.

In addition to humidity, the O₃ decomposition performance of the catalyst will be affected by co-existing pollutants (SO₂, NO_x, CO, VOCs, etc.) in the actual application scenarios. The physicochemical properties of the co-existing pollutants, together with the type of active sites and the reaction mechanism on the catalysts, determine the influence mechanism of the co-existing pollutants on the O₃ decomposition performance of the catalysts, which may be as follows: the competitive adsorption of the co-existing pollutants and O₃ molecules on the active sites; the accumulation of the reaction products of the co-existing pollutants on the active sites; the influence of the co-existing pollutants on the pathway of O₃ decomposition on the active sites. There are fewer studies on the effects of co-existing pollutants on the O₃ decomposition performance of the catalysts, and the O₃ decomposition performance of the catalysts should be further optimized in the future after determining the influence mechanisms of co-existing pollutants on the O₃ decomposition performance of the catalysts. Besides, the catalysts used in aircraft cabins must withstand low temperatures of around −40 °C. Although Pd-based monolithic catalysts have been used in aircraft, they are expensive [11,14]. Current studies on the decomposition of O₃ by MnO_x, Ag-based, and LDH catalysts mainly focus on room temperature. The O₃ decomposition performance of the MnO_x, Ag-based, and LDH catalysts at low temperature should be studied as potential alternatives to Pd-based catalysts. In the future, other catalysts with efficient and stable activity at low temperatures should also be developed. Furthermore, the monolithic catalysts for real application must have excellent O₃ decomposition performance at ultra-high space velocities [78,79], which can be achieved by regulating the active components, ceramic honeycomb supports, additives and binders.

CRediT authorship contribution statement

Xiaotong Li: Writing – original draft, Visualization, Investigation, Funding acquisition, Data curation, Conceptualization. Zhisheng Wang: Writing – review & editing. Biwu Chu: Writing – review & editing. Jinzhu Ma: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Hong He: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Xiaotong Li (BRID: 01198.00.73023) received her Ph.D. in 2022 from the group of Prof. Hong He at the Research Center for Eco-Environmental Sciences (RCEES), Chinese Academy of Sciences (CAS). She is currently a postdoctor at RCEES, CAS. Her research is concerned with the development of ambient catalysts for ozone decomposition. To date, she has published over 21 research papers, with 871 citations and an h-index of 13.

Jinzhu Ma (BRID: 08257.00.72215) received his Ph.D. degree in environmental science from Research Center for Eco-Environmental Science (RCEES), Chinese Academy of Sciences (CAS) in 2011. He is currently a professor at RCEES, CAS. His research is concerned with environmental catalysis and air pollution control, especially the development of ambient catalysts for air pollution control. He is a member of the Youth Innovation Promotion Association of CAS and is supported by the National Science Fund for Distinguished Young Scholars. He has published over 120 publications with over 6400 citations and an h-index of 44.

Hong He (BRID: 09005.00.83900) is the director of the Institute of Urban Environment (IUE, CAS), a professor at the Research Center for Eco-Environmental Sciences (RCEES, CAS), Chinese Academy of Sciences (CAS), and a member of the Chinese Academy of Engineering. His research focuses on fundamental and applied research in environmental catalysis, in the areas of diesel emission control, indoor air purification, and haze formation mechanisms. He has published over 690 publications with over 42,000 citations and an h-index of 102. He was awarded the National Technology Invention Award (China) in 2011, the National Science and Technology Progress Award (China) in 2014, the National Natural Science Award (China) in 2019, and the Outstanding Science and Technology Achievement Prize of the China Academy Award in 2024.