Catalytic Oxidation of NO by Ozone over Mn-Ce/Al₂O₃/TiO₂ Catalyst

Abstract

In this study, Mn-Ce/Al₂O₃/TiO₂ catalyst prepared by impregnation method was used for synergistic O₃ oxidation NO. The catalyst prepared by impregnating Al₂O₃/TiO₂ at a Mn:Ce molar ratio of 4:1 showed the best catalytic activity. The catalyst performance showed that when the molar ratio of Mn:Ce was 4:1 and the volume ratio of O₃:NO was 1:4, the removal rate of NO could reach 63%, which could increase the removal rate by 40% compared with that of NO oxidized by O₃ alone. BET, XRD, and TEM characterization results showed that when the molar ratio of Mn:Ce was 4:1, the catalyst specific surface area, and pore capacity were the largest. A large amount of MnOx and CeOx were distributed on the catalyst surface. The XPS analysis showed that the oxidation-reduction and oxygen vacancy of Mn (IV)/Mn (III)/Mn (II) and Ce (IV)/Ce (III), had a synergistic effect on the decomposition of O₃ into reactive oxygen species(O*), thus improving the catalytic capacity of Mn-Ce/Al₂O₃/TiO₂ catalyst for O₃. The O₂-TPD analysis showed that the oxygen vacancies and oxygen species in the catalyst could be used as the active point of decomposition of O₃ into O*. The experimental results show that the prepared catalyst can significantly improve the efficiency of ozone oxidation of NO and reduce the amount of ozone. The catalyst can be applied to ozone oxidation denitrification technology.

1. Introduction

With the development of society, the energy demand is increasing. Clean energy, renewable energy, etc., are constantly proposed, and traditional energy sources are mainly non-renewable resources such as coal and oil. Biomass is a new environmentally friendly fuel that is gradually being used. Biomass energy has the advantages of easy storage, stable combustion, and low pollutant emissions compared to traditional energy sources [1,2]. Nowadays, the use of biomass energy is increasing [3,4,5], such as oil palm, coconut shells, grains, livestock manure, green waste, etc. [6,7]. At present, the main application scenario of biomass in China is mainly boiler combustion, and the mixture of biomass and traditional coal combustion can reduce NO and SO₂ emissions [8,9]. Although the concentration of air pollutants emitted from biomass boilers is lower compared to traditional coal-fired boilers, existing studies show that NOx emissions from biomass boilers are between 200–500 mg/m³, and meeting the emission standards still requires treatment. At present, most biomass boilers in China have not carried out flue gas denitrification. A small number of enterprises use traditional SNCR denitrification technology, but there are a series of problems such as corrosion of the boiler. A small number of enterprises use SCR denitrification technology, but there are problems such as easy clogging and catalyst poisoning [10]. Some enterprises also use ozone (O₃) oxidation denitrification technology, but there are problems such as high O₃ consumption and high operating costs. At present, China lacks economical and efficient treatment technologies for the removal of flue gas nitrogen oxides from biomass boilers.

Given this aforementioned problem, relevant researches have been carried out at home and abroad. One of the directions is to develop suitable catalysts to improve the oxidation performance of O₃ and reduce the usage of O₃. O₃ oxidation for the removal of pollutants generally works in conjunction with associated catalysts to achieve high removal efficiencies. O₃ oxidation efficiency is mainly affected by O₃ concentration and flue gas temperature [11]. For common active components of catalysts, such as Mn-based catalysts, Mn mainly contributes to the decomposition of O₃ into reactive oxygen ions to participate in oxidation [12]. Low oxidation state Mn compounds in Mn/γ-Al₂O₃ catalyst contribute to the decomposition of O₃ to promote the oxidation of pollutants [13]. Under the co-treatment of NO by TiO₂ catalyst and a high concentration of O₃, the TiO₂ catalyst contributes to the transformation of NO₂ into N₂O₅ [14]. When the volume ratio of O₃:NOx > 1.57, the oxidation rate of NO can reach 95% under the synergistic action of MnOx/Al₂O₃ catalyst and O₃ [15], but too high a concentration of O₃ can produce leaks. V₂O₅-(NH₄)₂V₆O [16] catalyst was used to synergistically oxidize NO with O₃, when the volume ratio of NO: O₃ = 2:1, the oxidation efficiency of NO could be the highest, and the O₃ escape was less than 1 ppm 16. In terms of flue gas treatment, after flue gas was treated by wet flue gas desulfurization (WFGD), Mn-Ce/TiO₂ catalyst was used to oxidize NOx with O₃, and the removal rate of NOx could reach 77.1% [17]. The synergy of O₃ and catalyst has a good effect on the treatment of gas pollutants. In practical application, it is risky to use a high concentration of O₃ in pollutant treatment, and it costs a lot to the economy. Therefore, it is necessary to prepare catalysts that can adapt to lower O₃ concentrations and have higher treatment efficiency.

The purpose of this experiment is to study the effect and process mechanism of catalysts promoting O₃ oxidation of NO. Catalysts with different ratios of active components (Mn:Ce = 1:2, Mn:Ce = 2:1, Mn:Ce = 4:1, Mn:Ce = 5:1) were prepared by impregnation method. The performance of catalysts was investigated by different characterization methods, and the mechanism of catalysts to enhance ozone oxidation of NO was researched. The study can provide theoretical data support for the research on ozone oxidation denitrification technology of biomass boiler flue gas.

2. Materials and Methods

2.1. Preparation of Catalysts

Mn-Ce was selected as the main active component of the catalyst and prepared by the impregnation method. Mn(NO₃)₂ and CeN₃O₉·6H₂O were used as reagents, and nano titanium dioxide and Al₂O₃ were used as carriers. The carrier molar ratio was m (Al₂O₃):m (TiO₂) = 1:1, the mess ratios of m (active component):m (carrier) = 1:1. The active component molar ratios were n (Mn:Ce) = 1:2, n (Mn:Ce) = 2:1, n (Mn:Ce) = 4:1, n (Mn:Ce) = 5:1, and the relevant information of the reagents are shown in

Table 1. Reagents used in the experiments and purity.

Name of the Reagent	Source of Reagent	Purity
Mn(NO3)2	Maclean Biochemical Technology Co., Ltd., Shanghai, China	AR
CeN3O9·6H2O	Aladdin Biochemical Technology Co., Ltd., Shanghai, China	99.95%
Nano titanium dioxide	Hechan Trading Co., Ltd., Guangzhou, China	AR
Al2O3 (α-crystalline about 90%, γ-crystalline about 10%)	Damao Chemical Reagent Factory., Tianjing, China	AR
C₂H₆O	Lingfeng Chemical Reagent Co., Ltd., Shanghai, China	AR

. The weighed Mn(NO₃)₂ and CeN₃O₉·6H₂O were fully mixed in 40 mL deionized water, and then the carrier was added, fully mixed, and stirred for 4 h at 700 RPM. After mixing, the water was filtered and washed alternately with C₂H₆O and deionized water. Then, it was placed in a 100 °C oven and dried for 2 h, then calcined for 2 h in a muffle oven at 300 °C, cooled, removed, and finally ground and screened. Mn-Ce/Al₂O₃/TiO₂ catalysts with different proportions of active components were prepared (Mn:Ce = 1:2, Mn:Ce = 2:1, Mn:Ce = 4:1, Mn:Ce = 5:1 referred to as catalysts).

2.2. Experimental Setup

The experimental setup consists of O₃ generator and catalytic oxidation adsorption platform, as shown in Figure 1.

The O₃ required for the experiment was generated from an ozone generator (Feige Environmental Protection Technology Co., Ltd., Guangzhou, China). After the concentration of O₃ was measured by the ozone analyzer (Zhipu Automation Technology Co., Ltd., Zibo, China), it was passed into the catalytic oxidation adsorption platform. The gas pipeline in the experimental platform was mainly stainless steel, and the gas inflow rate was controlled by a flow meter. N₂ (99.9% N₂, Yuejia Gas Co., Ltd., Guangzhou, China), NO (4.01% NO + nitrogen balance, Yuejia Gas Co., Ltd., Guangzhou, China), and O₂ (99.9% O₂, Yuejia Gas Co., Ltd., Guangzhou, China) were mixed in the mixing zone. The mixer can be heated and kept warm as needed. The simulated flue gas was mixed with O₃ before entering the reactor. A quartz tube was placed in the middle of the reactor, and quartz wool was arranged in the middle of the quartz tube as support. The catalyst was evenly placed on the upper part of the quartz wool. During the experiment, the gas passed through the catalyst from top to bottom, and after the gas reacted, the change in the concentration of the exhaust gas was detected by the TESTO 350.

2.3. Catalyst Performance Test

The simulated total flue gas flow rate was 1 L/h, O₂ concentration was 13%, and the volume ratio of O₃:NO = 1:2, 1:3, 1:4, the O₃ flow rate was 1 L/h, a total of 2 L gas passes through the catalyst, and gas hourly space velocity (GHSV) was 60,000 h⁻¹. The initial incoming NO concentration was 300 ppm, and the O₃ concentration was 100 ppm. By adjusting different O₃:NO ratios, the main oxidation efficiency was expressed as NO_[removal].

$${\mathrm{NO}}_{\left[\mathrm{removal}\right]}=\frac{{\mathrm{NO}}_{\left[\mathrm{a}\right]}-{\mathrm{NO}}_{\left[\mathrm{b}\right]}}{{\mathrm{NO}}_{\left[\mathrm{a}\right]}}\times 100\%,$$

(1)

where NO_[a] was the concentration of NO at the inlet; NO_[b] was the concentration at the exit of O₃ (with catalyst).

The main mechanism of the reaction between O₃ and NO was as follows [14]:

$$\mathrm{NO}+{\mathrm{O}}_3\to {\mathrm{NO}}_2+{\mathrm{O}}_2,$$

(2)

$${\mathrm{NO}}_2+{\mathrm{O}}_3\to {\mathrm{NO}}_3+{\mathrm{O}}_2,$$

(3)

$${\mathrm{NO}}_2+{\mathrm{NO}}_3\to {\mathrm{N}}_2{\mathrm{O}}_5,$$

(4)

$${\mathrm{N}}_2{\mathrm{O}}_5\to {\mathrm{NO}}_3+{\mathrm{O}}_2,$$

(5)

$$\mathrm{NO}+{\mathrm{NO}}_3\to 2{\mathrm{NO}}_2,$$

(6)

2.4. Characterization of Catalysts

Specific surface area and pore size were determined by Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. XRD using D8 ADVANCE X-ray diffractometer from Bruker, Germany. TEM was measured using a JEM 2100F instrument with an accelerating voltage of 200 KV and an electron wavelength of 0.0251 Å. XPS was determined using Thermo Scientific Escalab 250Xi. O₂-TPD was determined using MicroActive for AutoChem II 2920 Version 6.01.

3. Results

3.1. Morphology Analysis of Catalysts

Table 2. Catalyst pore structure data for different activity group distribution ratios.

Sample	SBET (m²/g)	Vp (cm³/g)	Dp (nm)
Mn:Ce = 1:2	27.564	0.230	43.87
Mn:Ce = 2:1	30.131	0.269	45.25
Mn:Ce = 4:1	52.080	0.346	34.00
Mn:Ce = 5:1	26.134	0.229	41.61

mainly shows the BET surface area, total pore volume, and average pore size of Mn-Ce/Al₂O₃/TiO₂ catalyst samples with different active component ratios. The specific surface area and pore volume of the catalyst reached the maximum when the molar ratio of Mn:Ce =4:1, but when Mn:Ce =5:1, the specific surface area and pore size of the catalyst began to decrease. The results indicated that when the molar ratio of Mn:Ce =4:1, the carrier has reached the maximum load, and the active component cannot be fully loaded on the carrier when Mn is excessive, and the loading effect is poor. Figure 2a,b shows the N₂ adsorption-desorption isotherms and corresponding pore size distribution curves of Mn-Ce/Al₂O₃/TiO₂. It can be seen from Figure 2a that the N₂ adsorption and desorption isotherms of Mn-Ce/Al₂O₃/TiO₂ with different ratios of active components showed similarly. The first half of the isotherm was similar to the type II isotherm, and the second half was the type IV (a) isotherm, which was the combination of type II and type IV (a), which indicated that there were mesopores (2~50 nm) in the sample. In the relative pressure (P/P₀) range of 0.8~1.0, the shape of the hysteresis loop on the IV (a) isotherm was H3 type. The H3-type hysteresis loop was formed by the stacking of nanoblocks, which indicated the presence of fracture holes, and this analysis was consistent with the results of the TEM (Figure 3). In addition, Mn-Ce/Al₂O₃/TiO₂ has higher adsorption at the relative pressure (P/P₀) of 1, indicated that the sample contains macropores. Figure 2b shows the pore size distribution of the sample. It can be seen from the figure that Mn-Ce/Al₂O₃/TiO₂ has a wide peak range, which was concentrated at 20~50 nm and 60~90 nm, and the results showed that the layered mesoporous/macroporous structure was formed. Table 1 shows the distribution of specific surface area, pore volume, and pore size of the sample. The specific surface area and pore volume increased with the increase of Mn:Ce molar ratios from 1:2 to 4:1. However, the specific surface area and pore volume of Mn-Ce/Al₂O₃/TiO₂ decreased with further increasing of Mn:Ce molar ratios, because the agglomeration of the excess MnOx nanoparticles caused the blockage of some mesopores. It has been generally acknowledged that the high specific surface areas were beneficial to promote the catalytic performance by offering more active sites.

XRD and TEM were used to analyze the structure of oxide and carrier components in the catalyst. Figure 2c shows the XRD patterns of catalysts prepared under the different molar ratios of Mn:Ce. XRD peaks at 2θ = 25.584, 37.784, 43.362, 57.581, 70.351, 76.880, 84.375, 86.375, 89.018 are attributed to α-Al₂O₃ (JCPDS 10-0173). XRD peaks at 2θ = 31.988, 66.761 are attributed to the γ-Al₂O₃ (JCPDFs 29-0063). There is no γ-Al₂O₃ peak in the XRD results of Mn:Ce = 1:2, Mn:Ce = 2:1, Mn:Ce = 4:1, the main reason may be that Mn ions promote the conversion of γ-alumina [18]. However, γ-Al₂O₃ was detected at Mn:Ce = 5:1 and γ-Al₂O₃ appeared at 2θ = 31.988, 66.761. The possible reason is that the interaction between Mn-Ce is strengthened [19], and the effect of Mn ions is weakened, so γ-Al₂O₃ was preserved. XRD peaks at 2θ = 48.049, 53.890, 62.119, 62.688, 75.029, 82.659, are attributed to the anatase TiO₂ (JCPDS 21-1272). XRD peaks at 2θ = 31.015, 36.085, 53.859, corresponding to 200, 211, 312 of Mn₃O₄ (JCPDFs 24-0734). When Mn:Ce = 5:1, the peaks of Mn₃O₄ began to increase (a diffraction peak of Mn₃O₄ appeared at 32.441). With the increase of the amount of Mn ion recombination, the Mn₃O₄ phase increased, mainly due to the coexistence of MnO₂ and Mn₃O₄ phases [20]. XRD peaks at 2θ = 37.685, 66.815, 68.594, corresponding to 011, 310, 130 of MnO₂ (JCPDFs 50-0866) [21]. According to the calculation results of the Scherrer equation, it was found that the crystalline size was 17.8 nm at Mn:Ce = 1:2, 15.7 nm at Mn:Ce = 2:1, 16.3 nm at Mn:Ce = 4:1, and 13.8 nm at Mn:Ce = 5:1. It can be seen that in the case of Mn:Ce = 5:1, its crystalline size was the smallest. From the crystallinity and crystalline size of MnO₂, it can be seen that the crystallinity of MnO₂ decreased and the grain size decreased, indicating that the increase of Mn:Ce ratio inhibited the growth of MnO₂ crystalline [22]. At the same time, combined with the removal efficiency of NO under different Mn:Ce, it can be seen that the change of crystallinity of MnO₂ in the catalyst has a certain influence on the catalytic performance of the catalyst. In addition, XRD peaks at 2θ = 37.868, corresponding to 332 of Ce₂O₃ (JCPDFs 49-1458). XRD peaks at 2θ = 27.334, 52.649, and 70.478, corresponding to 112, 006, and 226 of CeO₂ (JCPDFs 44-1001). With the increase of Mn content, the peak intensity of CeO₂ changed slightly. It is most obvious at 52.649, possibly because the crystallinity of CeO₂ decreased due to the limitation of the crystallite growth of CeO₂ with the increase of the recombination amount of Mn ions [13].

It can be seen from the TEM of Figure 3a,b that the Al₂O₃/TiO₂ was a uniform size nanoblock. The Al₂O₃/TiO₂ nanoblocks ranged from 45–80 nm, and the average nanoblock size was about 60 nm, and it can be clearly seen that there was an obvious heterojunction between Al₂O₃ and TiO₂. It can be seen from Figure 3c,d that the active components of Mn-Ce were uniform nanospheres. The average diameter of the nanospheres was about 2 nm, and the active components of Mn-Ce were evenly distributed on the surface of the Al₂O₃/TiO₂ carrier. The Mn-Ce like particles and Al₂O₃/TiO₂ like blocks were observed, which indicated that the complex contained two morphologies: particle and bulk. Figure 3e,f shows the typical HRTEM image of Mn-Ce(4:1)/Al₂O₃/TiO₂. The observed spacing between the lattice planes of the sample was measured to be 0.2008 nm and 0.2941 nm, matched with (312) crystal plane of Mn₃O₄ and (310) crystal plane of MnO₂, respectively. Therefore, the morphology of Mn-Ce/Al₂O₃/TiO₂ prepared by impregnation method was uniform, the active components were evenly dispersed, and no agglomeration occurred, which indicated that the simple impregnation method can be used to prepare the nano-catalyst with uniform morphology. At the same time, combined with XRD analysis, it can be seen that there was a mutual reaction between the Mn-Ce active component and the carrier Al₂O₃/TiO₂, and there was an interfacial link between the components.

3.2. Catalyst Performance Analysis

Figure 4a shows the efficiency of catalysts with different Mn:Ce molar ratios for the catalytic ozonation of NO when the volume of O₃:NO = 1:4. As can be seen from Figure 4a, when the volume ratio of O₃:NO remained unchanged, the temperature had a certain influence on NO removal. With the increase of temperature, NO removal by different ratios of active component catalysts changed greatly. Overall, the Mn-Ce (4:1)/Al₂O₃/TiO₂ catalyst was more stable and efficient than other catalysts for NO removal. Different components had a great influence on NO removal, and when the molar ratio of Mn:Ce = 1:4, the catalyst had a good and stable performance [13]. Low-concentration ozone (the volume ratio of O₃:NO < 2, O₃ concentration < 150 ppm) was mainly used in this experiment. According to the experimental data, the sum of the volume concentrations of NO and NO₂ after the reaction was equivalent to the volume concentration of NO before the reaction. It can be seen that ozone mainly oxidized NO to NO₂, and there were no other types of N-containing oxides. The reaction of ozone oxidation of NO was shown in Equation (2) [11].

Figure 4b shows the effect of increasing the ratio of O₃ on NO removal without a catalyst. As can be seen from Figure 4b, when O₃:NO = 1:4, the average NO removal rate was 24%, when O₃:NO = 1:3, the average NO removal rate was 35%, and when O₃:NO = 1:2, the average removal rate of NO was 47%. With the increased O₃ ratio, the removal rate of NO increased gradually. Figure 4c shows the improvement of NO removal rate of O₃ under the co-catalysis of Mn-Ce (4:1)/Al₂O₃/TiO₂ catalyst. It can be found that under the catalysis of Mn-Ce (4:1)/Al₂O₃/TiO₂ catalyst, when O₃:NO = 1:4, the average NO removal rate was 51%, and when O₃:NO = 1:3, the average NO removal rate was 45%. When O₃:NO = 1:2, the average NO removal efficiency was 51%. Compared with Figure 4b,c, it can be seen that under the action of the catalyst, with the increase of O₃ concentration, the NO removal rate decreased. This is due to the fact that under the conditions of higher concentration of ozone, the initial NO₂ concentration was relatively high, and then the catalyst is easier to catalyze ozone to promote the conversion of NO₂ to N₂O₅, which reduced the reaction between O₃ and NO, and reduced the removal rate of NO [15]. Therefore, the conversion of NO to NO₂ was more readily promoted at lower ozone concentrations in the presence of catalysts. This study provided an opportunity to improve NO removal using catalyst catalysis at lower ozone concentrations, both to improve ozone utilization and to reduce ozone leakage. The removal of NOx by ozone oxidation + alkali absorption in the presence of catalyst is shown in Figure 5. When the initial NO concentration was 200 ppm, the volume ratio of O₃:NO was 1:4, the molar ratio of Mn:Ce was 4:1, the concentration of NaOH was 5 wt%, the NO outlet concentration was 36 ppm, the NO₂ emission concentration was 7 ppm, and the overall NOx emission concentration was 43 ppm, which could meet the relevant flue gas emission requirements. The main reaction equations for NO and NO₂ in NaOH solution are shown in Equations (7) and (8). As time goes on, the removal efficiency of NO₂ gradually decreased, but remained above 85% overall, while the removal efficiency of NO increased gradually with time. Higher concentrations of NO₂ mainly occurs in Equation (8). With the progress of the reaction, the pH of the absorption solution gradually decreased, and Equation (7) gradually became the main reaction. So the removal rate of NO gradually increased and the amount of NO₂ removed decreased [11].

$$\mathrm{NO}+{\mathrm{NO}}_2+2\mathrm{NaOH}\to 2{\mathrm{NaNO}}_2+{\mathrm{H}}_2\mathrm{O},$$

(7)

$$2{\mathrm{NO}}_2+2\mathrm{NaOH}\to {\mathrm{NaNO}}_2+{\mathrm{NaNO}}_3+{\mathrm{H}}_2\mathrm{O},$$

(8)

The distribution of Mn, Ce, and O elements on the surface of the catalyst was analyzed by XPS. Figure 6a is the XPS spectrum of Mn 2p. The Mn 2p XPS spectra showed two characteristic peaks of Mn 2p1/2 and Mn 2p3/2. The Mn 2p3/2 peak at 640.3 ev, 642.8 ev, 643.4 ev are attributed to Mn²⁺, Mn³⁺, and Mn⁴⁺ species [23]. The Mn 2p1/2 peak at 652.2 ev, 653.1 ev, 653.9 ev are attributed to Mn Mn²⁺, Mn³⁺, and Mn⁴⁺ species [24]. Figure 6a,b showed the XPS spectra of the catalyst (Mn:Ce = 1:2). Mn⁴⁺ was detected only at 638.3 eV, so the Mn ion content of Mn:Ce = 4:1 catalyst was more, and mainly increased Mn³⁺. Related studies have shown that Mn⁴⁺ can promote the generation of reactive oxygen species (O*) from ozone, and the presence of Mn³⁺ lead to the generation of oxygen vacancies and promoted the generation of O* from ozone, both of which can enhance the performance of NO oxidation by ozone [25]. Therefore, Mn:Ce = 4:1 catalyst is more efficient than Mn:Ce = 1:2 catalyst in promoting ozonation of NO.

Table 3. Distribution of Mn ions, Ce ions, and O species of synthesis catalysts based on XPS results.

Samples	Mn2+/(Mn2+ + Mn3+ + Mn4+) (%)	Mn3+/(Mn2+ + Mn3+ + Mn4+) (%)	Mn4+/(Mn2+ + Mn3+ + Mn4+) (%)	Ce3+/Ce4+	O (lat)	O (lat)/O (abs)
Mn:Ce = 4:1	40.18	26.61	33.21	1.29	38.03	0.51
Mn:Ce = 1:2	/	/	/	0.57	37.65	0.95

shows the proportion of Mn ions, Ce ions, and O species in the catalyst calculated based on XPS results. According to its distribution, it can be seen that under Mn:Ce = 4:1, the content of Mn²⁺ is the largest. Under ozone conditions, high proportion of Mn²⁺ promoted the conversion to Mn³⁺ and Mn⁴⁺. Figure 6b is the XPS spectrum of Ce 3d showed two characteristic peaks of Ce 3d5/2 and Ce3d3/2. It can be seen that under Mn:Ce = 1:2, the peaks V′′ (887.8 ev), U (881.3 ev), V′ (903.7 ev), V′′ (907.1 ev), V′′′′ (908.3 ev) are attributed to Ce³⁺; the peaks U⁰ (882.3 ev), U′ (886.3 ev), U′′′ (888.4 ev), U′′′′ (888.6 ev), U′V′′′′ (898.3 ev), V (900.8 ev), V⁰ (900.9 ev), V′′′ (907.3 ev), V′V′′′′ (916.7 ev) are attributed to Ce⁴⁺. It can be seen that under Mn:Ce = 4:1, the peaks 880.7 ev, 884.0 ev, 885.5 ev, 886.0 ev, 900.7 ev, 903.9 ev, 907.1 ev are attributed to Ce³⁺; the peaks 881.1 ev, 900.8 ev, 900.9 ev, 916.0 ev, 916.7 ev are attributed to Ce⁴⁺. According to Table 3, when Mn:Ce = 4:1, Ce³⁺ accounted for a large proportion, and a large amount of Ce³⁺ helped to generate O²⁻, and O²⁻ helped to promote the mutual conversion between Mn ions, which in turn promoted the decomposition of ozone into O* [26], which indirectly indicated that the Mn:Ce = 4:1 catalyst has higher catalytic activity. In Figure 6c, the XPS patterns of O 1 s showed the presence of two types of surface oxygen in the Mn-Ce/Al₂O₃/TiO₂ catalysts. The fitted peaks could be attributed to lattice oxygen O (lat) (529.5 eV) and chemisorbed oxygen (abs) (532.17 ev, 532.5 ev). It was well recognized that O (abs) species were more active in promoting ozone generation of O* than O (lat) due to their higher mobility [27,28,29]. Figure 6c and Table 3 show that the surface concentration of O (abs) species over Mn-Ce (4:1)/Al₂O₃/TiO₂ was higher than Mn-Ce (1:2)/Al₂O₃/TiO₂, which was another reason for its higher catalytic activity compared with Mn-Ce (1:2)/Al₂O₃/TiO₂.

O₂-TPD results can be used to represent the type of oxygen species on the catalyst. O (abs) is more easily desorbed than O (lat) and will desorb at lower temperatures. O (abs) generally contains both physically adsorbed oxygen and chemisorbed oxygen, of which physically adsorbed oxygen is usually more easily desorbed than chemisorbed oxygen [30]. As depicted in Figure 6d, two peaks appeared at around 200 °C and 400 °C, which are the characteristic peaks of O (abs); the peak at 600 °C is mainly O (lat). When Mn:Ce = 4:1, more peaks appear when the temperature rises, indicating that there are more oxygen species on the catalyst surface. The desorption capacity of surface oxygen species contributes to improving the oxidation capacity of catalysts.

3.3. Reaction Mechanism

The process mechanism of catalysts promoting O₃ oxidation of NO can be summarized as follows: The catalyst contains large amounts of Mn (II), Mn (III), and Mn (IV). In the ozone atmosphere, Mn (II) and Mn (III) were oxidized and O* was produced at the same time. In combination with Section 3.2, catalyst and ozone have an excellent effect on NO removal, and the effect of O* can be inferred as Equations (9)–(11). NO was combined with O* to form NO•O* in the adsorption state. The adsorbed (NO•O*) will form (NO•O*)₂ and react with O₂ in the gas phase to form NO₂. The O²⁻ produced by the oxidation reaction between Ce (III) and Ce (IV) in the catalyst contributes to the transformation of Mn (IV) to Mn (II). It can be concluded that the oxidation reaction process of Mn (II) and Mn (III) in the ozone atmosphere is the main reaction promoting the oxidation of NO to NO₂. The oxidation-reduction reaction in Ce (III) and Ce (IV) and the oxygen species on the catalyst can promote the oxidation reaction of Mn (II) and Mn (III) in the ozone atmosphere. The reaction pathway is shown in Figure 7. The main reactions here are as follows:

$$\mathrm{NO}+\mathrm{O}*\to \mathrm{NO}·\mathrm{O}*,$$

(9)

$$\mathrm{NO}·\mathrm{O}*+\mathrm{NO}·\mathrm{O}*\to {\left(\mathrm{NO}·\mathrm{O}*\right)}_2,$$

(10)

$${\left(\mathrm{NO}·\mathrm{O}*\right)}_2+{\mathrm{O}}_2\to 2{\mathrm{NO}}_2+2\mathrm{O}*,$$

(11)

$${\mathrm{Mn}}^{3+}+{\mathrm{O}}_3\to {\mathrm{Mn}}^{4+}+\mathrm{O}*+{\mathrm{O}}_2,$$

(12)

$${\mathrm{Mn}}^{2+}+{\mathrm{O}}_3\to {\mathrm{Mn}}^{3+}+\mathrm{O}*+{\mathrm{O}}_2,$$

(13)

$${\mathrm{Mn}}^{2+}+{\mathrm{O}}_3\to {\mathrm{Mn}}^{4+}+{\mathrm{O}}^{2-}+{\mathrm{O}}_2,$$

(14)

$${\mathrm{Mn}}^{4+}+{\mathrm{O}}^{2-}+{\mathrm{O}}_3\to {\mathrm{Mn}}^{2+}+{\mathrm{O}}_2,$$

(15)

$${\mathrm{Ce}}^{4+}+{\mathrm{Mn}}^{3+}+{\mathrm{O}}^{2-}\to {\mathrm{Ce}}^{3+}+{\mathrm{Mn}}^{4+},$$

(16)

$${\mathrm{Ce}}^{3+}+{\mathrm{O}}_2\to {\mathrm{Ce}}^{4+}+{\mathrm{O}}^{2-},$$

(17)

4. Conclusions

The experimental results show that the catalyst can significantly improve the efficiency of ozone oxidation of NO and reduce the amount of ozone. Based on the experimental results, the main conclusions are as follows:

• The Mn-Ce/Al₂O₃/TiO₂ catalyst was prepared by the impregnation method. The catalysts were characterized by BET, XRD, TEM, XPS, and O₂-TPD. The results show that the catalysts have a large specific surface area (52.080 m²/g) and pore volume (0.346 cm³/g). High-specific surface areas are beneficial to promote the catalytic performance by offering more active sites. The catalyst mainly contains particle and bulk morphology, and the active components (MnOx, CeOx) are uniformly dispersed, and no agglomeration occurs. At the same time, the catalyst contains a large number of CeOx, O (lat), and O (abs) species. The content of MnOx, oxygen species on the surface of the catalyst, is the factor affecting the activity of the catalyst.
• The mechanism study shows that the MnOx content on the catalyst surface is the main factor affecting the catalyst activity. The presence of Mn²⁺ and Mn³⁺ on the surface of the catalyst can promote the decomposition of ozone to produce O*. O^2- contributes to the reduction reaction between Mn²⁺ and Mn⁴⁺, and O* and oxygen species on the catalyst surface significantly promote the oxidation of NO to NO₂.
• Under the experimental conditions of flue gas temperature 100–150 °C, the molar ratio of Mn:Ce = 4:1, the volume ratio of O₃:NO = 1:4, and the NO removal rate can reach 63%. Compared with the oxidation of NO by ozone alone, the oxidation efficiency of NO can be increased by 40% by adding a catalyst. The experimental results show that the prepared catalyst can significantly improve the efficiency of ozone oxidation of NO and reduce the amount of ozone. After NaOH absorption, the NOx removal efficiency achieves up to 79.6% for the O₃ + Mn-Ce (4:1)/Al₂O₃/TiO₂ method, which can well meet the NOx emission standard in China.