Ozone pollution is a serious problem now prevailing in China. Ozone and its secondary pollutants generated indoor severely threaten human health. The catalytic decomposition of ozone at room temperature is an effective way to prevent the pollution. This review firstly summarizes the catalytic activity of carbon, zeolite, noble metal, transitional metal oxides and other materials; then focuses on manganese oxides, compares the activity of manganese oxides with different crystal structures and elaborates the research progress in mechanisms of ozone decomposition on manganese oxides. Currently, the major challenge in this field is the catalyst deactivation caused by water vapor. The comprehensive understanding in mechanisms of ozone decomposition and catalyst deactivation is the essential key to synthesize efficient catalysts.
Contents
1 Introduction
2 Catalysts for ozone decomposition
2.1 Carbon
2.2 Noble metal
2.3 Transitional metal oxides
2.4 Other materials
3 Manganese oxides for ozone decomposition
3.1α-MnO2
3.2γ-MnO2,ε-MnO2,ramsdellite and todorokite
3.3δ-MnO2
3.4 Amorphous and low-valent manganese oxides
4 Mechanisms of catalytic decomposition of ozone on manganese oxides
Ozone (O3) is an allotrope of oxygen. Its molecule contains three oxygen atoms and has a V-shaped configuration with a bond angle of 116.8 ° and a bond length of 0.1278 nm. At normal temperature and pressure, ozone is a light blue gas with pungent odor, which can be smelled at a very low concentration (0.01 ~ 0.05 ppm)[1]. Ozone plays an important role in the chemistry of the Earth's atmosphere. Stratospheric ozone absorbs a large amount of short-wave ultraviolet radiation, which is biologically destructive, and plays a protective role for surface organisms[2]. However, in the troposphere near the ground, ozone is a harmful atmospheric pollutant. It can reduce crop yields, damage buildings, pose a serious threat to human health, and contribute to global warming as a greenhouse gas[3,4]. It is reported that long-term exposure to low concentrations of ozone will cause symptoms and diseases of human respiratory and cardiovascular systems[5]. Trace ozone (8.7 ppb) can significantly affect children's health by increasing their heart rate and reducing their autonomic nervous function[6]. Statistics show that in 2010 alone, more than 1 million deaths worldwide were related to long-term exposure to ozone pollution[7]. In the Air Quality Guidelines issued by who, the recommended limit of 8-hour average concentration of ozone is 0.05 ppm. According to the current Ambient Air Quality Standard (GB 3095-2012) in China, the first level limit of the daily maximum 8-hour average concentration of ozone is 100μg·m-3(0.05 ppm), and the second level limit is 160μg·m-3(0.08 ppm). In addition, China's "Indoor Air Quality Standard" (GB/T 18883-2002) also stipulates the concentration of ozone in the indoor environment, and its one-hour average concentration limit is 18883.
The main source of tropospheric ozone pollution is the photochemical reaction of nitrogen oxides (Volatile organic compounds) and Volatile organic compounds (VOCs) under sunlight, so ozone pollution shows an annual cycle, which is most significant in summer[8]. Tropospheric ozone pollution is a common problem in the world, and the situation of ozone pollution in China has become more and more serious in recent years. It is reported that the daily maximum 8 H concentration of a typical monitoring station in North China was 50 ppb in 2004, and the value had risen to 70 ppb in 2015[9]. During the same period, the annual maximum ozone concentration at the station also increased from 122 ppb to 165 ppb. According to the statistics of China Environmental Monitoring Center, in August 2018, ozone was the primary pollutant in 95% of the cities in China[10]. In July and August 2019, the proportion rose to 99%[11,12].
The increase of outdoor ozone concentration will inevitably lead to indoor ozone pollution. There are two sources of ozone in the indoor environment. The first is the atmospheric ozone entering the room through air exchange, and the outdoor atmospheric ozone penetration rate can reach 0.73 ± 0.13[13]. Secondly, indoor ozone release sources, such as laser printers and photocopiers, air purification equipment (fresh air system and electrostatic, negative ion and plasma air purifiers), ozone disinfection equipment, etc[14][15]. Ozone concentrations as high as 132 ppb were reported near the operating printer[16]. In special environments such as aircraft cabins, the concentration of ozone can also exceed 150 ppb, which is much higher than the prescribed limit[17,18]. In addition, ozone can also react with VOCs in indoor environment to produce secondary pollutants such as formaldehyde, acetaldehyde, ketovaleraldehyde, organic aerosols and ultrafine particles, which further endanger human health[19][20][21][22].
Because people spend 80% to 90% of their time indoors, it is very important to effectively control indoor ozone pollution to protect human health[23]. At present, the reported methods of ozone removal include liquid absorption, thermal decomposition, activated carbon adsorption and catalytic decomposition[24][25][26]. Among them, the cost of liquid absorption method is high and the waste liquid is difficult to treat, the energy consumption of thermal decomposition method is high, and the activated carbon adsorption method is easy to saturate and difficult to regenerate, so the related research of these methods is less. The catalytic decomposition of ozone can achieve efficient decomposition of ozone at room temperature without any secondary pollution, which is the most valuable and potential technology for ozone removal. The key of this technology lies in the preparation of efficient catalytic materials.
In this paper, we systematically review and summarize the research on catalytic ozone decomposition in recent years. Carbon, zeolites, Metal-organic frameworks (MOFs), noble metals and transition metal oxides have been extensively studied for ozone catalysis. Among them, manganese oxides, which are relatively economical, have shown excellent properties and are the most widely studied and reported ozone catalytic materials recently. In this paper, the manganese oxide catalytic materials and the catalytic mechanism of ozone are introduced, and the challenges and opportunities in the field of catalytic ozone decomposition are put forward.
2 Research Status of Ozone Catalytic Decomposition Material
2.1 Carbon Material
Carbon material is a kind of ozone removal material which has been studied more in the early stage, and the related reports are mainly based on activated carbon materials, such as granular activated carbon and activated carbon fiber materials. Although it is generally believed that ozone is mainly adsorbed and directly reacted on the surface of carbon materials, many studies have pointed out that carbon materials also have catalytic decomposition of ozone, and appropriate modification can improve their catalytic activity. However, compared with the catalytic materials such as noble metals and transition metal oxides, the catalytic ability of carbon materials is very limited. Subrahmanyam et al. tested the ozone catalytic performance of 0.3 G of activated carbon fibers and observed two distinct stages in the ozone removal process at room temperature with an initial ozone concentration of 1600 ppm, a gas flow rate of 1 L·min-1, and humidity[26]. In the first stage, the ozone removal rate reached 100%, while in the second stage, the ozone removal rate dropped sharply to 20%, which they believed clearly corresponded to two processes. In the initial "high activity" stage, ozone reacts with activated carbon, and a large number of oxygen-containing groups are formed on the surface of activated carbon and gradually reach saturation. In the subsequent "low activity" stage, activated carbon catalyzes the decomposition of ozone into oxygen, thus possessing the activity of ozone removal.
Álvarez et al. investigated the ozone removal performance of various granular activated carbons, and considered that the removal of ozone by activated carbons mainly included two ways: direct reaction and catalytic decomposition, and carried out kinetic calculation for the above processes[27]. The results show that the reaction rate constant of ozone removal by granular activated carbon is k1, and the k2 is related to the specific surface area, pore volume, surface oxygen content, ash content and metal content. Ondarts et al. Used nitric acid and sodium hypochlorite treated granular activated carbon and activated carbon fibers for ozonolysis[28]. Under the conditions of 20 ℃, initial ozone concentration of 10 ppm, face velocity of 1.2 m·s-1, and relative humidity of 50%, the ozone removal rate of the original sample was maintained at about 20% for 2 H, while the performance of the sodium hypochlorite treated sample was improved by about 10%.
In addition to activated carbon, carbon nanotubes and other emerging carbon materials have also been used for ozone removal. Yang et al. Obtained carbon nanotube/quartz fiber films by growing carbon nanotubes on pure quartz fibers by chemical vapor deposition[29]. When 0.3 G of the above carbon nanotube/quartz fiber film was continuously tested for 10 H at 25 ℃, an initial ozone concentration of 300 ppb, a gas flow rate of 2.6 L·min-1, and a relative humidity of 10% ± 5%, the ozone removal rate of the material could be maintained at 96%.
2.2 Precious metal material
Noble metals are a kind of excellent catalytic materials for ozone decomposition. The reported noble metals are mainly Pd, and also include Pt, Au, Ru, Ag and so on. Noble metals are usually supported on carriers in the form of particles for ozone catalysis, and some materials have excellent catalytic properties. Compared with transition metal oxides, noble metal catalytic materials have good moisture resistance and slow deactivation under high humidity conditions. On the other hand, the high cost of precious metals limits their wide application in practical production.
Yu et al. Synthesized a series of Pd-Mn/SiO2-Al2O3 catalysts by impregnation method[30]. Under the conditions of 27 ℃, initial ozone concentration of 0.6 ppm, space velocity of 635 000 h-1 and relative humidity of 55% ~ 65%, the conversion rate of ozone can reach 88.3%. Under the conditions of 25 ℃, initial ozone concentration of 1500 ppm, space velocity of 48 L·g-1·h-1 and relative humidity of 70%, the Pd-Co-MnOx/Al2O3 catalyst prepared by Tao et al with 0. 1 wt% Pd supported on the carrier can maintain 100% ozone conversion within 4 H, and the noble metal Pd greatly improves the moisture resistance of the catalyst[31]. In addition, Yu et al. And Ren et al. Also investigated the catalytic performance of supported Pd-MnOx, and tested the effects of calcination temperature and Pd doping amount[32][33]. Kameya et al calculated the kinetics of ozone decomposition catalyzed by Pd-MnO2[34]. Chang et al. compared the ozone degradation performance of Pt/Rh and Pd/Rh catalysts at 25 ℃, initial ozone concentration of 588 ppm and space velocity of 15 000 h-1, and found that the performance of Pd/Rh was much better than that of Pt/Rh[35]. They believe that Pt catalysts are more prone to deactivation than Pd catalysts because the intermediate species in the ozone decomposition process are relatively difficult to desorb on the surface of Pt catalysts.
Zhang et al. supported highly dispersed Au nanoparticles on coal-based activated carbon by sol immobilization method to prepare Au/AC ozone catalytic materials[36]. Under the conditions of room temperature, initial ozone concentration of 25 ppm, space velocity of 120 000 h-1 and relative humidity of 45%, the ozone conversion rate of the 0. 5 wt% Au/AC material still maintained at 90. 7% after 40 H, indicating that Au had good catalytic performance for ozone decomposition. Lejeune et al. loaded zero-valent metal Ru nanoparticles on glass foam by impregnation and used them for ozone removal[37]. Under the conditions of room temperature, initial ozone concentration of 4500 ppm and space velocity of 336 h-1, the conversion rate of ozone can reach 52%.
Ag is also a kind of noble metal material which has been studied and reported more. It can participate in or affect the catalytic decomposition of ozone in the form of Ag0 and Ag+. Oyama et al. Tested the ozone catalytic performance of various metal oxides including Ag2O[38]. The results show that the Ag2O has a certain ability to catalyze ozone, but its activity is lower than that of MnO2, Co3O4, NiO and Fe2O3. Nikolov et al. Synthesized Ag-loaded silica (Ag/SiO2) materials by incipient wetness impregnation, in which Ag mainly exists in the form of elemental Ag0[39]. Under the test conditions of 23 ℃, initial ozone concentration of 4390 ppm, and space velocity of 133 333 h-1, the Ag/SiO2 can maintain 100% conversion of ozone in the first 2 ~ 3 H, and maintain about 90% conversion in the following 120 H. In the above reaction process, almost all the elemental Ag0 were converted into oxides such as AgO and Ag2O3. Li et al. Prepared Ag-modified manganese oxide AgMnOx by co-precipitation method[40]. The results showed that all the silver existed in the form of Ag+, and the Ag1.8Mn8O6 crystal was the key component to catalyze ozone decomposition efficiently. Li et al. Also prepared Ag-modified manganese oxide materials by impregnation method[41]. Among them, the 6%Ag/α-Mn2O3-I prepared by the impregnation method showed excellent ozone catalytic performance, which was higher than that of the Ag1.8Mn8O6: prepared by the coprecipitation method. The ozone removal rate of 99% was still maintained after 6 H test at an initial ozone concentration of 40 ppm, a space velocity of 840 000·h-1, and a relative humidity of 65%. The test results show that Ag on the 6%Ag/α-Mn2O3-I exists in the form of simple substance Ag0. The deactivation of the catalyst was due to the oxidation of elemental silver, and the activity of the deactivated catalyst could be completely regenerated by high temperature treatment at 350 ℃ in nitrogen atmosphere.
2.3 Transition metal oxide material
Transition metal oxides are often used as catalytic materials for redox reactions because of their semiconductor properties, in which the metal cations often have empty d orbitals or lone pair electrons, which are prone to electron abstraction or electron loss reactions. Compared with noble metals, transition metal oxides are a kind of more economical ozone catalytic materials. Oyama et al. Summarized the literature reports on ozone catalytic materials before 1997, and found that p-type semiconductor oxides had higher activity, among which MnO2 had the best activity[38]. In the past ten years, there have been a lot of reports on manganese oxide ozone catalytic materials, which will be described in detail in the next section.
In addition to manganese oxides, other transition metal oxide ozone catalytic materials have been reported, including oxides of Fe, Ti, Cu, Ni and other metals. Among them, the heterostructured Ni/NiO nanocrystals showed good catalytic activity for ozone.
Mathew et al. synthesized a mesoporous ferrihydrite ozone catalyst, which showed 95% ozone removal efficiency at an initial ozone concentration of 600 ppm and a space velocity of 1500 L·g-1·h-1[42]. Through X-ray absorption characteristics and DFT calculations, they revealed the mechanism of adsorption and decomposition of ozone by unsaturated Fe sites in the catalyst as active sites, and also found that mesoporous structure can expose more surface active sites. Patzsch et al. prepared anatase TiO2 and further modified it with Cu, Fe and Mn ions. The catalytic performance of the obtained material for ozone decomposition under UVA irradiation was improved by 3 to 4 times[43].
Chen et al. carried out a series of studies on the catalytic decomposition of ozone by transition metal oxides in recent years, and successively investigated four materials of Cu2O-CuO-Cu(OH)2, Cu2O/, graphene, LaFeO3 and Ni/NiO and their ozone catalytic processes, among which the heterostructured Ni/NiO showed the best catalytic activity[44⇓⇓~47]. The heterostructured Cu2O-CuO-Cu(OH)2 prepared by Gong et al. Through the solution method was used for ozone removal, and the ozone conversion rate was maintained at 80% during the 16 H test at 25 ℃, initial ozone concentration of 20 ppm, space velocity of 240 L·g-1·h-1 and high humidity (RH 90%)[44]. It is reported that the delocalized holes facilitate the transfer of electrons from the intermediate peroxy species to the catalyst surface, thus facilitating the catalytic reaction. Gong et al. Also synthesized a Ni-modified perovskite-type LaFeO3 by a simple citric acid sol-gel method, and the ozone conversion rate of the material was increased to 93% in a 4 H test at 25 ℃, an initial ozone concentration of 1000 ppm, and the same space velocity and humidity[46]. Ni doping improves the reducibility and redox ability of the catalyst, and promotes the desorption of intermediate products, thus improving the catalytic activity. Based on this, the researchers further synthesized heterostructured Ni/NiO by citric acid sol-gel method, and the ozone conversion rate was always above 98% in the 8 H test at 25 ℃, initial ozone concentration of 1000 ppm, the same space velocity and humidity[45]. The results show that the good activity of Ni/NiO nanocrystals is related to the weak adsorption of water on the surface of the material and the rapid desorption of intermediate products due to the rapid transfer of surface electrons.
2.4 Other ozone catalytic material
In addition to the above materials, researchers have also studied the ozone catalytic properties of zeolites, metal-organic frameworks (MOFs) and other materials. Vald Valdés et al. And Brodu et al. Have carried out a number of studies on zeolite-catalyzed ozone decomposition[48⇓⇓~51]. Vald Valdés et al. Immersed natural zeolite in ammonium sulfate solution for ion exchange, and prepared modified zeolite after vacuum treatment at 550 ℃[48]. The ozone removal rate of 0.15 G of the material was about 20% after 3 H test at 20 ℃, initial ozone concentration of 0.345μmol·L-1, and dry gas flow rate of 740 mL·min-1. Brodu et Al. investigated the ozone removal ability of zeolites with different Si/Al contents[49]. The results show that the total amount of ozone removal is proportional to the amount of Lewis acid sites in the zeolite. In the initial stage of ozone removal by zeolite, the removal rate can reach 100%, and then slowly decreases to 0. The above initial stage of 100% removal corresponds to a large amount of adsorption of ozone molecules on the zeolite surface; After that, the lower removal rate corresponds to the catalytic degradation of ozone on the acidic surface. As a result, they proposed a three-step decomposition mechanism of ozone. In addition, Brodu et al. Also explored the influence of pore width and micropore size of zeolite materials on ozone catalytic performance[50]. The results show that the Lewis acid sites are located in the pore walls of zeolites, and the narrow channels and cage-like structures slightly larger than the size of ozone molecules are most conducive to the interaction between ozone and acid sites. Recently, Vald Valdés et al studied the process of ozone decomposition catalyzed by Co-modified zeolites by in situ infrared spectroscopy, and found that ozone was adsorbed and decomposed on strong Lewis acid sites, and the adsorbed ozone reacted with the hydroxyl groups on the surface of zeolites in this process[51]. The ozone removal capacity of the zeolite modified by transition metal Co was significantly improved.In the dry gas test at 20 ℃, initial ozone concentration of 9 000 ppm and space velocity of 30 L·g-1·h-1, the ozone removal rate of the material was stable at about 63% after 5 H catalytic reaction. From the above reports, it can be seen that the ozone removal capacity of zeolites is limited compared to noble metal and transition metal oxide materials.
Metal-organic frameworks (MOFs), as a new type of materials, have also been used in the research of catalytic decomposition of ozone. Wang et al. synthesized an iron metal-organic framework (MIL-100) for ozone removal, which showed 100% ozone removal efficiency at room temperature, ozone concentration of 45 ppm, space velocity of 190 000 h-1 and relative humidity of 45%, and no deactivation occurred for 100 H[52]. In addition, with the increase of humidity, the catalytic activity of the metal organic framework increases. They used density functional calculations to show that water molecules participate in the catalytic reaction and play a catalytic role, in which the active site of the metal-organic framework is the unsaturated central metal atom Fe.
3 Manganese oxide ozone catalytic material
From 1995 to 2001, Oyama et al carried out kinetic calculation, in-situ Raman measurement and isotope measurement of ozone decomposition process for supported manganese oxide MnO2/Al2O3, MnO2/ZrO2 and MnO2/TiO2, and proposed the mechanism of ozone catalytic reaction[53⇓⇓⇓~57]. The results show that the catalytic reaction rate of ozone can be expressed as r(O3)=k· , where X is related to the properties of the supported materials. The activation energy for the conversion of (intermediate products of ozone decomposition reaction) on the surface of MnO2 to O2 and subsequent desorption is different due to the different supports, which results in the difference of ozone reaction rate among the materials. Oyama et al. Also studied the ozone catalytic performance of carbon-supported composite metal oxides, and found that MnO2-Fe2O3 had the best catalytic effect, believing that the introduction of binders enhanced the stability and catalytic ability of the material[58]. Researchers such as Jiang and Wang prepared activated carbon-supported manganese oxide ozone catalytic materials in situ by the reaction of potassium permanganate and activated carbon. When the content of manganese is low, the manganese oxide on the surface of activated carbon presents lichen-like porous morphology and has certain ozone catalytic activity[59,60]. However, the catalytic performance of these manganese oxides is limited, and they only show good performance in dry gas flow.
From the above early studies, it can be seen that the catalytic reaction sites on the surface of manganese oxide materials are still unclear, and the methods of material preparation and modification are limited. Therefore, how to modify manganese oxides to prepare high efficiency materials under high humidity conditions, and further explore the mechanism of manganese oxides catalysis and deactivation are the hot spots of manganese oxides ozone catalysis research in recent years.
3.1 α-MnO2 catalytic material
α-MnO2 is a kind of widely studied material for catalytic decomposition of ozone. It is composed of two octahedral double chains connected by a common vertex, and has two tunnel structures of 1 × 1 and 2 × 2. Among them, the larger 2 × 2 tunnel center can accommodate water molecules and various cations. Therefore, based on the ideal α-MnO2structure, there are a variety of manganese oxide species that accommodate different cations, such as arsonite (Manjiroite,Na+), cryptomelane (Cryptomelane,K+) and manganite (Hollandite,Ba2+). The existence of these cations can keep the structure of the α-MnO2stable and maintain the tunnel structure at the high temperature of 300 to 400 deg C; The α-MnO2 without cation support will collapse into pyrolusite (1 × 1), ramsdellite (1 × 2) or γ-MnO2(1×2) with smaller tunnel size. In general, the chemical formula of the α-MnO2 can be expressed as A2-yB8-xO16, where A is the cation in the tunnel,And B represents Mn4+, Mn3+, Fe3+, V+, or other transition metal ions of similar diameter in the octahedral structure[61]. Thanks to these cations in the α-MnO2 tunnel, the MnO6 octahedron can contain abundant Mn3+ species or other metals, while maintaining the relative stability of the structure. In the field of ozone catalysis, all the reported α-MnO2 materials are cryptomelane (OMS).
Zhang et al. Investigated the effects of crystal form, morphology and high-valence metal doping modification on its ozone catalytic performance[62⇓⇓~65]. Jia et al. First tested the ozone catalytic performance of α-MnO2, β-MnO2 and γ-MnO2, among which α-MnO2 showed the best ozone catalytic activity[62]. The results show that the α-MnO2 has larger specific surface area, more abundant Mn3+ species and more active adsorbed oxygen species. In situ Raman spectroscopy was used to observe the formation of peroxide intermediates in the process of ozone catalytic decomposition, and the peroxide on the α-MnO2 was most easily desorbed. Based on these results, a mechanism of ozone decomposition using oxygen defects as active sites was proposed. In addition, they found that the nanowire-like α-MnO2 with exposed (211) crystal face had the best ozone catalytic activity[63]. The results show that the oxygen vacancies on different crystal planes are different, which is related to the dispersion of oxygen vacancies on specific crystal planes and the different chemical environments. On this basis, Yang et al also prepared α-MnO2 doped with tungsten (W) and vanadium (V), and found that both W-MnO2 and V-MnO2 had better activity than α-MnO2[64,65]. The doping of the W6+ and the V5+ restrains the growth of the manganese dioxide, reduces the particle size of the manganese dioxide, enriches the crystal defects and improves the specific surface area of the material; On the other hand, doping makes the α-MnO2 expose high-index crystal planes, which have more unsaturated coordination atoms, so they have more active sites. In addition, the acidity of the high-valence metal ions also contributes to the improvement of the catalytic performance.
He et al. Have also carried out a series of studies on cryptomelane manganese dioxide[66⇓⇓⇓~70]. When Mn(Ac)2, MnCl2 and Mn(NO3)2 were used as precursors to synthesize OMS-2, the nanorod OMS-2-Ac prepared with MnAc2 as reducing agent showed the best catalytic activity for ozone[66]. They believe that this is due to the combination of the carbonyl group in the Ac group and Mn-OH, which inhibits the agglomeration of manganese oxides, resulting in the formation of OMS-2-Ac with larger specific surface area and more defects. Further studies showed that the specific surface area and oxygen defects of OMS-2 could be further improved by increasing the amount of reducing agent Mn(Ac)2, which enhanced the catalytic performance of the material[69]. On the basis of OMS-2, they also prepared Ce-OMS-2 materials with excellent properties by doping transition metals[67]. The results show that Ce replaces the K+ in cryptomelane tunnel and part of the Mn4+ in MnO6 octahedron at the same time, which inhibits the growth of manganese oxides, and then forms Ce-OMS-2 materials with fine honeycomb structure, large specific surface area and abundant defects. The doping of metal Co3+ and Fe3+ replaced the Mn3+ in the MnO6, and the morphologies of Co-OMS-2 and Fe-OMS materials after doping changed to some extent, but the honeycomb-like morphology with large specific surface area in Ce-OMS-2 did not appear. This indicates that the size of the doping ions and the doping sites have an important influence on the crystal growth and material morphology[67]. Further study shows that only under the hydrothermal condition with high material filling ratio, Ce can replace the K+ in the tunnel and finally form Ce-OMS-2 with good ozone catalytic activity[70].
In addition to the above reports, Zhu et al. treated cryptomelane manganese dioxide at high temperature and vacuum, and found that the oxygen vacancies introduced by the treatment promoted the catalytic decomposition of ozone. The calculation showed that the oxygen vacancies were the active sites of ozone adsorption reaction, and proposed a mechanism of ozone catalytic decomposition involving water and oxygen vacancies[71]. Zhu et al. Also prepared catalysts with a large number of oxygen defects and good activity by controlling the K+ content in cryptomelane tunnels[72]. On the basis of cryptomelane manganese dioxide, Hong et al. Prepared Na-OMS-2 material with Na+ in the tunnel. The introduction of Na+ increased the content of oxygen defects, enhanced the adsorption performance of the material for O3 molecules, and reduced the apparent activation energy of ozone reaction catalyzed by Na-OMS-2[73].
At present, there are many studies on the ozone catalysis of α-MnO2. The synthesis methods are mainly hydrothermal, and the modification methods are mainly doping and tunneling ion control. Due to the special 2 × 2 tunnel structure in the α-MnO2, doping can occur either in the tunnel cation or in the 2 × 2 octahedron, which have distinct effects on the catalytic properties of the material. The properties of α-MnO2 ozone catalytic materials are shown in Table 1, among which Ce-OMS series is the α-MnO2 material with the best ozone performance reported so far.
3.2 Other tunnel structured manganese oxide catalytic material
In addition to α-MnO2, other tunnel structured manganese dioxide materials for ozone catalytic decomposition have been reported, including γ-MnO2 and Ramsdellite with 1 × 2 tunnel structure, Todorokite with 3 × 3 tunnel structure, and ε-MnO2 with irregular tunnels[74⇓⇓⇓⇓-79]. Wherein the 1 * 2 tunnel of the γ-MnO2 and the ramsdellite can only accommodate a H+ or a Li+ of a smaller size,The larger 3 × 3 tunnels in baryerite can accommodate water molecules and cations such as Mg2+, Ca2+, Na+ and K+. The manganese in the ε-MnO2 is highly disordered and randomly distributed in the octahedral holes formed by the close arrangement of six oxygen atoms, so the tunnel structure formed is also irregular[61]. These tunnels of different sizes can also accommodate cations such as H+, Li+, Na+, K+, etc.
Jia et al. Introduced iron based on the preparation method of α-MnO2, and prepared iron-doped γ-MnO2[74]. Doping can increase the specific surface area of the material, increase the content of oxygen defects, and significantly improve the catalytic performance of ozone. Li et al. Prepared cerium-doped Ce-γ-MnO2 by hydrothermal reaction and calcination using MnSO4 and (NH4)2S2O8 as raw materials, which is one of the best non-noble metal-modified manganese oxide materials reported so far[75]. The Ce-γ-MnO2 is mainly composed of γ-MnO2, and also contains α-MnO2 and amorphous components. Cerium doping inhibits the growth of the crystals, and at the same time, a large number of grain boundaries are formed between the crystals of the different species, which promotes the formation of oxygen defects and improves the activity of oxygen species.
Chen et al. Prepared cerium-doped manganese-iron composite oxide by citric acid sol-gel method, in which the crystal form of manganese oxide is rhombohedral manganese dioxide[76]. The lattice defects caused by the doping of Ce and Fe atoms make the composite material have high specific surface area and a large number of oxygen defects, thereby promoting the ozone catalytic performance of the material.
Liu et al. studied the catalytic decomposition of ozone by manganite MnO2, and found that cerium doping and subsequent calcination could improve its catalytic activity for ozone. Cerium doping made the crystallization of manganese dioxide worse, the specific surface area increased, and the oxygen defects increased[77,78]. Calcination removes the Mn-OH on the surface of the material and slows down the deactivation caused by the reaction of O3 with Mn-OH to form H2O.
Hong et al. Prepared ε-MnO2 ozone catalytic materials containing Li+ and a large number of Mn defects by selective dissolution of Mn-Li precursors[79]. The calculation and characterization results show that the appropriate amount of Li+ and manganese defects promote the formation of oxygen defects on the surface of the material, and improve its ability to selectively adsorb O3. The catalyst with large specific surface area, weak crystallinity, strong reducibility and higher catalytic activity can be prepared by dissolving the precursor with acid of appropriate concentration. On this basis, the mechanism of ozone catalytic decomposition involving Li+, manganese defects and oxygen defects was proposed.
In general, compared with α-MnO2 and γ-MnO2, the catalytic properties of baryerite, rhombohedral manganese dioxide and ε-MnO2 for ozone still have a certain gap, and the synthesis methods are relatively complex, so the potential of research and application is relatively limited. The ozone catalytic performance of the tunnel structure manganese dioxide materials is shown in Table 2, and the ozone catalytic decomposition performance of the Ce-γ-MnO2 is the best.
3.3 δ-MnO2 catalytic material
Birnessite-type manganese dioxide (δ-MnO2) is a kind of layered manganese oxide widely found in nature, with a layer spacing of about 0.7 nm, which can accommodate water molecules and various cations such as K+, H+ and N. The replacement of these interlayer ions can significantly affect the morphology, chemical properties and catalytic performance of δ-MnO2, which is a common modification method to improve the ozone catalytic activity of δ-MnO2.
Gopi et al. Synthesized δ-MnO2 by hydrothermal method, and replaced the K+ between the layers with H+ by acid treatment. The specific surface area and pore volume of the material increased, the content of oxygen defects increased, and the catalytic performance of ozone in dry gas flow was significantly enhanced[80].
Since then, Zhang et al. Have carried out in-depth research on the catalytic decomposition of ozone by birnessite-type manganese dioxide[81,82]. Liu et al. Treated the δ-MnO2 in the 0.5 M HNO3 solution at 50 ℃ for 6 H to prepare the H-MnO2, and the catalytic performance of the material in the wet gas flow was significantly improved compared with the untreated δ-MnO2[81]. In addition to the improvement of morphology and the increase of oxygen defects, they also found that the acid-treated H-MnO2 was more likely to form oxygen defects, more hydrophobic and more likely to adsorb ozone molecules through in-situ infrared testing and DFT calculation of ammonia adsorption, so it showed good ozone catalytic activity and stability under wet conditions. On this basis, a mechanism of ozone catalytic decomposition involving oxygen defects and acidic sites was proposed. Cao et al. Treated the δ-MnO2 with NH4Cl solution to replace the K+ between the layers with N, and prepared N-MnO2 materials with excellent ozone catalytic decomposition performance[82]. The results show that N exist in the interlayer and surface of N-MnO2, which disperse the original δ-MnO2 into smaller and thinner particles, and produce abundant oxygen defects and acid sites.
It can be seen that the replacement of interlayer ions significantly improves the ozone catalytic performance of the δ-MnO2. Unlike the α-MnO2, the reported methods for the treatment of δ-MnO2 are carried out at atmospheric pressure, and the types of ions used for replacement are relatively small. The ozone catalytic performance of the above δ-MnO2 materials is shown in Table 3, and there is a certain gap between their performance and the Ce-OMS series and Ce-γ-MnO2 mentioned above.
3.4 Amorphous and low valence manganese oxide catalytic material
Amorphous manganese oxides (Amorphous MnOx) refer to manganese oxides with very poor crystallinity, which can be prepared by the reaction of potassium permanganate with a strong reducing agent at room temperature or under mild conditions[83,84]. In addition, amorphous manganese oxides with very weak crystallinity can also be prepared by doping other metal species with high content under hydrothermal conditions[85]. Compared with the conventional preparation method, the crystal growth of the manganese oxide in the above reaction is extremely inhibited, so that the manganese oxide has poorer crystallinity, more crystal defects and larger specific surface area, thereby contributing to the catalytic decomposition of ozone. On the other hand, the strong or excessive reducing agent and a large number of crystal defects also make the valence state of manganese in amorphous manganese oxides often significantly lower than that in MnO2 prepared by traditional methods. Some researchers believe that in addition to the common Mn3+-Mn4+ redox pairs in MnO2 materials, Mn2+-Mn3+ in these low-valent manganese oxides also participate in the catalytic decomposition of ozone[85].
Liu et al. Prepared amorphous manganese oxide MnOx-I rich in mesopores by oxidation-reduction reaction of Mn(Ac)2 and KMnO4 at room temperature[83]. The calculation results of apparent reaction rate constants show that the abundant low valence Mn2+ and Mn3+ species are more conducive to the activation and decomposition of ozone than Mn4+. The grain boundaries around the tiny crystals are also favorable for the formation of oxygen defect active sites. Ji et al. Synthesized supported amorphous MnO2/CNTs in situ by reacting carbon nanotubes (CNTs) directly with KMnO4 at 70 ℃ and atmospheric pressure. The loose structure formed by the winding of CNTs is conducive to gas mass transfer, and the electron transfer between graphitic carbon and manganese oxide makes manganese oxide more active[84]. Lian et al. Prepared amorphous MnFe0.5Ox by doping a large number of Fe3+ under hydrothermal conditions, and proposed that the existence of Mn2+-Mn3+, Mn3+-Mn4+ and Fe2+-Fe3+ redox couples is conducive to the catalytic decomposition of ozone[85]. Li et al. Prepared amorphous cerium-doped manganese oxide Ce-MnOx by rapid reaction with KMnO4 and Ce(NO3)3 at room temperature using organic dopamine as reducing agent[86]. Extensive atomic doping introduces a large number of defect sites into the material. The results showed that the coordination of dopamine with manganese inhibited the growth of manganese oxides and controlled the morphology of the materials, and the resulting structure with abundant mesopores and macropores slowed down the inactivation caused by competitive adsorption of water on active sites in a certain humidity range. The morphologies of the amorphous manganese oxides are similar, and the amorphous manganese oxides are mainly mesoporous particles with secondary packing[83,85,87]. The structure enables the material to have high specific surface area and large pore volume at the same time, which is not only beneficial to exposing more active sites and gas mass transfer, but also can slow down the condensation of water to a certain extent.
These amorphous and low-valent manganese oxides with very weak crystallinity and high specific surface area show good catalytic performance for ozone, but most of them have poor stability and rapid deactivation. Liu et al.'S study on the ozonation of MnOx-I showed that the instability was related to the gradual increase of the oxidation state of manganese species in the ozonation reaction and the competitive adsorption of water[83]. On the other hand, the mild reaction conditions also make the preparation of manganese oxides vulnerable to interference: slight changes in the order and speed of addition, room temperature and stirring hydraulic gradient may cause significant changes in the product. Therefore, on the basis of mild reaction, some researchers have further modified manganese oxide materials by high temperature calcination to make them play a more stable ozone catalytic performance.
Yu et al. calcined the aforementioned amorphous MnOx-I at 300 ° C in air for 3 H to obtain a MnO2-300 material with improved crystallinity[87]. Calcination reduced the number of hydroxyl groups on the surface of the manganese oxide and weakened its hydrophilicity, thus slowing down the deactivation caused by the reaction of Mn-OH with O3 and the adsorption of H2O, and enhancing the catalytic stability and moisture resistance of the material. On the basis of carbon nanotubes supported amorphous manganese oxide, Ji et al. Calcined the MnO2/CNT material in nitrogen at 300 ℃, and prepared Mn3O4/CNT with low valence state through the oxidation-reduction reaction of amorphous MnO2 and carbon at high temperature, which improved the ozone catalytic activity of the material[84]. Li et al. Obtained a series of manganese oxide materials with ozone catalytic properties by roasting the co-precipitation products of MnSO4 and AgNO3 at room temperature[40]. Among them, the 8%AgMnOx treated at 600 ℃ showed the best catalytic performance. Studies have shown that the Ag1.8Mn8O16 species in the 8%AgMnOx have excellent catalytic activity. Ma et al. Synthesized the mixture of MnCO3 and Ce2O(CO3)2 by co-precipitation method using the reaction of Mn(Ac)2 and Ce(NO3)3 at 90 ℃ and normal pressure.After calcination at 600 ℃ for 3 H in air, the CeMn10Ox material with α-Mn2O3 as the main active component was obtained[88]. Under the conditions of an initial ozone concentration of 40 ppm, a mass space velocity of 840 L·g-1·h-1, a relative humidity of 65%, and a test time of 6 H, the material exhibits excellent ozone catalytic activity and still maintains an ozone removal rate of 96%, which is one of the best non-noble metal modified manganese oxide ozone catalytic materials reported so far. According to the experiment and DFT calculation, the (111) crystal face of cerium-doped α-Mn2O3 has a strong adsorption capacity for ozone molecules, and is the key active component in the ozone catalytic process.
The catalytic properties of the above materials and the low-valence manganese oxides are shown in Table 4. In the past two years, there have been more and more reports on amorphous and low-valence manganese oxides, which show good catalytic performance for ozone decomposition, and some of them are more suitable for practical production because of their mild preparation conditions. However, amorphous materials synthesized directly under mild conditions often suffer from rapid deactivation, and subsequent calcination treatment is a potential solution to this problem. On the other hand, some low valence manganese oxides have been found to have excellent ozone catalytic activity, such as α-Mn2O3 and Ag1.8Mn8O16. Compared with the more reported MnO2, there is still a lack of systematic research on the catalytic decomposition of ozone by low-valent manganese oxides.
4 Mechanism of catalytic decomposition of ozone by manganese oxides
4.1 Catalytic ozone decomposition mechanism
Oyama et al. firstly discovered the intermediate product of ozone catalytic decomposition, , through the in-situ Raman test of ozone catalyzed by manganese oxide, and first proposed the mechanism of ozone catalytic decomposition by manganese oxide as shown in formulas 1 to 3[55]. Where * represents the reactive site, O* represents the O species adsorbed on the reactive site, and represents the O2 species (i.e., ) adsorbed on the reactive site. The O3 molecule first binds to the reaction site and decomposes to form a gaseous O2 and O bound to the site. The O adsorbed on the reaction site further reacts with another O3 molecule to form a gas-phase O2 and a O2 species bound on the site. The adsorbed O2 species are eventually released from the reaction site as gas phase O2 molecules, thereby completing the regeneration of the reaction site. In which that first step of the O3 adsorption reaction occur rapidly and there are almost no more empty reaction sites on the surface of the material. The latter two reactions occur more slowly, and their rates together determine the content of the intermediate on the surface of the material and the overall reaction rate.
O 3 +* → O * + O 2
O 3 + O * → + O 2
→ * + O 2
On the basis of the above mechanism, Jia et al. First proposed the ozone catalytic decomposition mechanism with the participation of oxygen defect, and considered that the oxygen vacancy is the site of ozone catalytic reaction on the surface of manganese oxide (Formula 4-6)[62,63]. An oxygen atom in the ozone molecule first binds to an oxygen vacancy (Vo) on the surface of the MnO2, and at this time two electrons are transferred from the oxygen vacancy to the oxygen atom, so that a first-step reaction occurs to generate an atomic oxygen species O2- bound to the oxygen vacancy and a gas-phase O2 molecule. The other ozone molecule continues to react with the atomic oxygen species to form the of the peroxy species. Finally, it is converted into O2 and desorbed from the oxygen vacancy, thus realizing the regeneration of the catalytic active site of the oxygen vacancy. Therefore, manganese oxides rich in oxygen vacancies tend to have good ozone catalytic activity. The results of in-situ Raman spectroscopy show that the dissociation of peroxide species from the surface is the rate-limiting step of the reaction, and the formation of peroxide intermediates on different materials is different, which is related to the dispersion of active sites, chemical environment and coordination mode of oxygen vacancies. If the peroxy species are more easily desorbed from the surface, the catalytic activity of the material is better.
O 3 + V O → O 2- + O 2
O 3 + O 2- → + O 2
→ V O + O 2
Zhu et al. Prepared α-MnO2 rich in oxygen vacancies by high temperature vacuum treatment, and based on this study, proposed a mechanism of ozone catalytic decomposition involving water and oxygen vacancies[71]. In this mechanism, some water molecules occupy oxygen vacancies through competitive adsorption and form Mn-OH2(state 2). The Mn-OH2 and the O3 molecule form an unstable six-membered ring structure through hydrogen bonding, and then the six-membered ring is converted into a surface hydroxyl group (Mn-OH) and HO3·(state 2-state 3) is released. Mn-OH then combines with another O3 molecule to form a five-membered ring, which then converts to a peroxy hydroxyl group (Mn-H ) and releases a O2 molecule (state 4-state 5). The Mn-H further binds to the third O3 molecule and is converted to the Mn- and the HO3·(state 5-state 6) is released again. Finally, the · species is released in the form of O2 molecules, resulting in the regeneration of the oxygen vacancy (state 6-state 1). Due to the strong adsorption capacity of Mn-OH species for H2O, the adsorption and accumulation of water in high humidity environment will block the above catalytic reaction and occupy oxygen vacancies, thus leading to the deactivation of catalytic materials. The above mechanism can explain the rapid recovery of the material performance when the humidity is reduced from 50% to < 5%: at high humidity, water adsorbs and accumulates on the oxygen vacancies, which inhibits the catalytic reaction; When the airflow is converted to low humidity, the water originally adsorbed on the oxygen vacancy can be removed by participating in the catalytic reaction of ozone, and finally the oxygen vacancy is released, thus rapidly eliminating the deactivation caused by water adsorption at high humidity.
Liu et al. Prepared H-MnO2 with significantly improved performance after acid treatment of δ-MnO2[81]. The test shows that the acid sites on the surface of the H-MnO2 are increased, and the water resistance is enhanced; The calculation shows that the defects are easier to form on the surface of the H-MnO2 and the O3 molecules are easier to adsorb on the surface. Based on the research results, they proposed an ozone decomposition mechanism involving both acid sites and oxygen vacancies, as shown in Figure 1. The O3 molecule is firstly adsorbed on the surface of the material through a hydrogen bond formed by the combination of a terminal oxygen atom and an acidic site, and then the oxygen at the other end reacts with an oxygen vacancy to generate an atomic oxygen species O2- and release the acidic site; Then another O3 molecule is adsorbed on the acidic site, and then reacts with the O2- in the oxygen vacancy to form a O2 and a peroxide species , and the acidic site is released again; Finally, the is separated from the oxygen vacancy to release the active site of the oxygen vacancy, and the whole ozone catalytic decomposition reaction is completed. In the above process, the catalytic decomposition of ozone still occurs in the oxygen vacancies, but the existence of acidic sites makes the O3 molecules more easily adsorbed on the surface of the material, thus accelerating the whole reaction.
Tatibouet et al. Tested the ozone catalytic performance of the two δ-MnO2, and calculated the kinetics of the reaction process under dry gas flow[89]. It is proposed that when the ozone concentration is low (about 2 ppm), the reaction between the O2- of atomic oxygen species and the O2 of O3 and the of peroxygen species is the rate-determining step. When the ozone concentration is higher (more than 20 ppm), the desorption of the from the material surface is the decisive step.
4.2 Catalyst deactivation mechanism
In the ozonolysis reaction catalyzed by manganese oxides, there are two reasons for the deactivation of the catalyst: one is the deactivation of the reaction sites caused by the accumulation or transformation of oxygen species intermediates, which is the main reason for the slow deactivation of manganese oxides in both dry and wet atmospheres[62,72,73]; The second is the competitive adsorption of water molecules on the surface of the catalyst, which is the reason why the performance of manganese oxides decreases greatly and deactivation accelerates significantly in wet atmosphere[66,71].
4.2.1 Accumulation and transformation of intermediates of oxygen specie
Jia et al. Conducted in-situ Raman tests on MnO2 of different crystal forms and found that materials with easy desorption of peroxy species intermediates had better activity[62]. Zhu et al studied the ozonation process of α-MnO2 and confirmed that the accumulation of intermediate oxygen species was the main factor causing catalyst deactivation[72]. The signal at the ~1380 cm-1 in the infrared test is believed to be attributed to the intermediate species. Hong et al. Also identified the infrared signature of this oxygen species intermediate in their study of Na-OMS-2[73]. The intermediate products increased with the time of ozonation. After the regeneration treatment of the 425℃N2, the content of the substance was reduced in different degrees, and the performance of the catalyst was also regenerated. And the more this species is reduced, the better the performance of the regenerated catalyst (fig. 2).
Liu et al. studied the cerium-doped barium forsterite Ce-MnO2 and found that the hydroxyl group (Mn-OH) on the surface of the material was removed by calcination, which made the material have better ozone catalytic performance and stability[78]. The results show that the Mn-OH on the surface of manganese oxides can react with O3 molecules and form water, which occupies the active sites of oxygen vacancies and leads to the deactivation of catalytic materials. High temperature treatment at 300 ℃ can remove this part of Mn-OH without significantly affecting the content of oxygen vacancies, so the treated Ce-MnO2-300 showed better catalytic activity and stability. On the other hand, Mn-OH may originate from the combination of H2O and its dissociated species with oxygen vacancies, which are occupied by surface hydroxyl groups and are not conducive to ozone catalytic reaction. Combined with the previous literature reports, it can be speculated that Mn-OH may be an intermediate product of ozone catalytic reaction, and its accumulation will gradually deactivate the material (Figure 3).
4.2.2 Competitive adsorption of water
In addition to the deactivation caused by intermediate products, a large number of studies have shown that the ozone catalytic performance of manganese oxides decreases significantly and rapidly with the increase of humidity. According to Wang et al., water molecules and ozone molecules compete for adsorption on the surface of materials, and the combination of water molecules and oxygen vacancies on active sites leads to the decrease of ozone catalytic activity of materials[66][66]. Zhu et al. Suggested that a water film would form on the surface of the catalyst under high humidity conditions, which could cover most of the active sites, thus greatly affecting the performance of the material[71]. On the other hand, by reducing the inlet air humidity, part of the water adsorbed on the surface of the material can be desorbed again, thus restoring part of the activity of the material. At present, the mechanism and influencing factors of deactivation of manganese oxides caused by water vapor are not clear, and further systematic research is needed.
4.3 Catalyst regeneration and heat treatment
For the manganese oxide material which is deactivated after ozone catalysis, the catalytic activity can be regenerated by desorbing the intermediate products of oxygen species or water originally occupying the active sites of oxygen vacancies from the surface of the material through high-temperature treatment. In the existing literature, there are relatively few studies on the regeneration process of manganese oxides, and the regeneration conditions and effects of various materials are different. As shown in Table 5, the manganese valence state of several materials prepared by calcination is restored after heat treatment, and the catalytic activity is regenerated or becomes better[90][73,76][78]; However, the α-MnO2(KOH-4h) without calcination can not be completely regenerated by heat treatment[72]. The above results show that the effects of heat treatment on different manganese oxides are different, which may be related to the different surface chemical properties of the materials. Liu et al. studied cerium-doped barium-manganite and proposed that heat treatment could remove the surface hydroxyl species (Mn-OH) of manganese oxides and inhibit the deactivation process caused by the water generated by the reaction between Mn-OH and O3 occupying the reactive sites[78]. Yang et al. Investigated the effect of heat treatment at 200 ~ 500 ℃ in three atmospheres on the W-MnO2(α-MnO2)[91]. The results show that the specific surface area and the number of oxygen vacancies of the samples decrease after heat treatment under different conditions.Heat treatment at 400 ℃ in air and 300 ℃ in helium changes the exposed crystal planes of manganese oxides and irreversibly removes a large number of surface hydroxyl groups, which reduces the hydrophilicity of the materials and significantly improves the ozone catalytic activity of W-MnO2.
5 Conclusion and prospect
At present, most of the reported ozone catalytic materials are manganese oxides, and most of these materials perform well at low humidity, but their performance and stability at high humidity still need to be improved. Although the loading of noble metals can significantly improve the moisture resistance of the catalyst, the cost of such materials is high. Therefore, the preparation of relatively inexpensive manganese oxide catalytic materials with excellent performance at high humidity is the focus and difficulty in the field of catalytic ozone decomposition.
Although the research of catalytic ozone decomposition is facing many challenges, there are also many new opportunities. At present, people's demand for clean air is increasing day by day. In order to reduce the impact of particulate matter on human body, the large-scale application of fresh air system and air purifier makes the problem of indoor ozone pollution increasingly prominent. On the other hand, with the increasing application of ozone gas phase disinfection and plasma disinfection, the demand for ozone tail gas treatment of disinfection equipment is also gradually increasing. At the same time, the performance of ozone catalytic materials reported in recent years has been gradually improved, and some economical and efficient manganese oxide catalytic materials have emerged, such as Ce-OMS, CeMn10Ox, etc. The emergence of these materials has created conditions for practical production and application. Summarizing the current research, we can see that there is room for further modification and optimization of α-MnO2, δ-MnO2, low-valent manganese oxides and heterostructured transition metal oxides. However, there is a lack of systematic and in-depth research on other types of materials except α-MnO2, which is a potential development direction in the future research of ozone catalysis.
In the study of catalytic mechanism, the catalytic and deactivation mechanism of manganese oxides under humidity conditions needs to be further studied. It is generally believed that water molecules will compete with ozone molecules for adsorption on the surface of the catalyst, which is not conducive to the activity of the catalyst, but some researchers have proposed that an appropriate amount of water can participate in the decomposition reaction of ozone. The reaction in wet gas involves three phases of gas, liquid and solid, which is very complex, and the related reaction mechanism needs to be further studied. In addition, ozone catalytic materials also have the problem of slow deactivation, which is due to the fact that the intermediate products of oxygen species generated in the reaction can not be desorbed and accumulated in time, eventually leading to the deactivation of reactive sites. For deactivated catalysts, the material can be regenerated by high temperature treatment. However, the regeneration conditions and effects of different materials are not the same, and there is still a lack of systematic research and reports. The study of the above mechanism and regeneration parameters will contribute to the development and practical application of high efficiency ozone catalytic materials.
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