Catalytic Ozonation of Formaldehyde with an Oxygen-Vacancy-Rich MnO_x/γ-Al₂O₃ Catalyst at Room Temperature

Abstract

Formaldehyde (HCHO) is known as one of the important indoor organic pollutants. How to remove and decompose the low concentration of formaldehyde at room temperature is important for indoor environments. Catalytic ozonation is an efficient method to thoroughly remove HCHO at room temperature, with high efficiency and few byproducts. A series of MnO_x/γ-Al₂O₃ catalysts were prepared in this work via the impregnation method and treated with different reagents (acid, alkali, and H₂O₂) to evaluate their catalytic activity for HCHO removal. The results showed that MnAl-II (acid treatment) performed well in activity tests, reaching a nearly 100% HCHO conversion at an O₃/HCHO of 2.0 and attaining a CO₂ selectivity of above 95% at an O₃/HCHO of 3.0 at 30 °C, with almost no ozone residual existing. The larger specific surface area, abundant oxygen vacancies, and higher number of acid sites contributed to the excellent performance of MnAl-II. Stability and H₂O resistance tests of MnAl-II were also conducted. To reveal the intermediate product formation and further investigate the reaction mechanism of HCHO ozonation, in-situ DRIFTS measurement was carried out combined with DFT calculations.

1. Introduction

With the heavy use of building and decorative materials, formaldehyde (HCHO), which commonly exists in paints, adhesives, furniture, electrical appliances, etc., has become a major pollutant of indoor air worldwide [1,2,3]. As a colorless but irritating gas, HCHO does great harm to human health, with quite strong teratogenicity and carcinogenicity. Long-term exposure to even low concentrations of HCHO will cause respiratory problems and permanently damage the lungs [4]. High concentrations of HCHO can cause breathing difficulties and even death [4]. Thus, it is of great significance to develop methods to efficiently remove HCHO and improve indoor air quality.

Many methods have been applied for the elimination of HCHO, such as ventilation, adsorption, botanical purification, photocatalytic oxidation, biological degradation, catalytic oxidation, and catalytic ozonation [5,6]. Among them, the catalytic ozonation method, which can achieve the complete oxidation of VOCs (volatile organic compounds) into harmless CO₂ and H₂O without secondary pollution at low temperatures, is the most attractive and effective in HCHO treatment. The strong oxidizability of ozone (O₃) is due to the free radicals (·O₂⁻, ·O²⁻, ·OH) generated after activation, which can efficiently break C–O, C–C, and C–H bonds for the deep oxidation of VOCs [7]. Catalysts can reduce the activation energy and accelerate the reaction rate, thus the selection of catalyst materials is vital for the reaction process [8]. Noble metal catalysts (Pt, Pd, Rh, Au, etc.) always exhibit a good catalytic performance; however, their rarity and high cost severely impede their large-scale application [9,10,11,12]. Consequently, it is very necessary to find alternatives which have a low price and high activity.

The cost-effective transition metals have attracted extensive attention from researchers, due to their diverse oxidation states, favorable stability, and excellent performance in catalytic reactions [13]. Among them, manganese oxides (MnO_x), which have multiple valence states, a diverse structure, and superior redox properties, have been widely explored in pollutant degradation. Typically, Sekine et al. [14] tested the HCHO removal efficiencies of different metal oxides (Pd, Co, Ce, Fe, Mn, etc.), and MnO₂ appeared to offer the best catalytic performance suitable for the catalytic oxidation of HCHO, with no release of hazardous byproducts. Zhang et al. [15] synthesized a series of rod-like MnO₂ samples with various crystal structures (α-, β-, γ-MnO₂) via a hydrothermal process, and α-MnO₂ exhibited the highest HCHO conversion and CO₂ yield with ozone assistance at room temperature. The increase in O₃ can significantly facilitate the mineralization of HCHO, attributed to the reactive oxygen species produced by O₃ decomposition.

In addition, the activity of Mn-based catalysts is determined by many factors, such as doped metals, support types, acid-alkali treatment, and the preparation methods used [16]. Different types of support always greatly influence the properties of catalysts, including the surface area, lattice defects, and metal dispersion. Oyama et al. [17] loaded MnO_x onto Al₂O₃ and SiO₂ for acetone catalytic ozonation, and the Al₂O₃-supported catalyst acquired higher activity and turnover frequencies (TOFs) than SiO₂. Shao et al. [6] prepared supported Mn-based catalysts over γ-Al₂O₃, SiO₂, and TiO₂ for toluene catalytic ozonation. They found that Mn³⁺ and surface oxygen species played a positive role in catalytic activity and 5% Mn/γ-Al₂O₃ achieved the highest toluene conversion and ozone decomposition efficiency. M. Ghavami et al. [18] synthesized a MnO_x/Al₂O₃-P catalyst through the polyol method, which showed higher toluene conversion and CO₂ selectivity than the impregnated catalyst, due to its smaller manganese particle size, better dispersion, and greater surface area. Yang et al. [19] treated a copper-manganese spinel with acid and alkali for toluene catalytic combustion. They discovered that both acid and alkali treatment could produce a large number of surface oxygen vacancies and appropriate acidity, which greatly improved the catalytic activity and stability of catalysts.

Hence, a series of MnO_x/γ-Al₂O₃ catalysts were prepared and treated with CH₃COOH, NH₃·H₂O, and H₂O₂ solutions to research the catalytic ozonation of HCHO at room temperature. The HCHO conversion efficiency and CO₂ selectivity of the catalysts were evaluated during the activity tests. The properties of the catalysts, such as their crystal structures, textual properties, oxygen species, and acidity, were investigated by various characterization methods. The H₂O resistance experiment was also tested to explore the effect of humidity. Moreover, to study the reaction mechanism of HCHO catalytic ozonation, the process of intermediate formation was conducted through in situ DRIFTs analysis. The DFT calculations were carried out as well for further discussion of the catalysts’ properties.

2. Results and Discussion

2.1. Catalytic Ozonation of HCHO

To study the catalytic ozonation performance of the prepared MnAl catalysts, activity tests were conducted and the results are shown in Figure 1a. The ratio of O₃/HCHO was set from 0.5 to 3.0, and the theoretical equivalent of O₃/HCHO for complete oxidation is 2, as presented in Equation (1). Apparently, the conversion of HCHO improved with the increase of the O₃/HCHO ratio, suggesting that more O₃ input could promote the process of catalytic reaction. MnAl-II showed the best HCHO catalytic activity, which achieved nearly 100% HCHO conversion at an O₃/HCHO ratio of 2.0 and reached above 95% HCHO conversion at an O₃/HCHO ratio of 1.5. In general, the O₃/HCHO ratio corresponding to 90% HCHO conversion (O₉₀) increased in the following order: MnAl-II (1.3) < MnAl-IV (1.5) < MnAl-I (1.7) < MnAl-III (2.0). Across the whole range of the ozone input, there was almost no O₃ residual for all the catalysts. A comparison of the catalyst researched in this work with previously published catalysts for HCHO ozonation is summarized in

Table 1. Comparison of catalysts for HCHO catalytic ozonation.

Catalyst	Reaction Mixture	GHSV (h−1)	Temperature (°C)	Conversion (%)	Reference
MnOx	HCHO: 82 ppm	200,000	25	~100	[20]
	Ozone: 720 ppm
α-MnO2	HCHO: 60 ppm	/	Ambient temperature	80	[15]
	Ozone: 230 ppm
MnO@C	HCHO: 60 ppm	/	30	100	[21]
	Ozone: 180 ppm
Mn0.5Ce0.5O2	HCHO: 61 ppm	10,000	25	~83	[22]
	Ozone: 203 ppm
MnAl-II	HCHO: 30 ppm	50,000	30	100	This work
	Ozone: 60 ppm

.

The incomplete oxidation of VOCs usually generates undesirable intermediates which are environmentally hazardous. To achieve green emission, CO₂ and H₂O are expected to be the final products at the outlet. As shown in Figure 1b, it was obvious that the CO₂ selectivity increased almost linearly with the molar ratio of O₃/HCHO. With the optimal catalytic activity, MnAl-II also attained the highest CO₂ selectivity. When the ratio of O₃/HCHO was 3, MnAl-II obtained above 95% CO₂ selectivity, and the order for the CO₂ selectivity decreased as follows: MnAl-II > MnAl-I > MnAl-IV > MnAl-III.

HCHO + (2 − x)O₃ → (1 − x)CO₂ + xCO + (2 − x)O₂ + H₂O

(1)

2.2. Catalyst Characterization

2.2.1. Crystalline Structures

As shown in Figure 2, the XRD patterns presented the crystalline structures of the synthesized catalysts, which had similar characteristic diffraction peaks. The peaks at 37.6°, 45.8°, and 66.8° were ascribed to the (3 1 1), (4 0 0), and (4 4 0) planes of γ-Al₂O₃ (PDF#29-0063), while the peaks appearing at 37.3° and 42.8° should be ascribed to the (1 0 1) and (1 1 1) planes of α-MnO₂ (PDF#24-0735) [6]. Obviously, the intensity of the MnAl-II catalyst was weaker than the other three catalysts, indicating its better metal dispersion and smaller metal particle size.

The SEM images of these MnAl catalysts exhibit different morphologies and microstructures, as depicted in Figure 3. MnAl-I was composed of oval particles, with a relatively larger size, while the other three catalysts exhibited bulk-like structures with a rough surface and an irregular structure. The surface micropore structure of the catalysts might facilitate the formation of defects, promoting the adsorption of VOCs and the further ozonation process [23,24].

2.2.2. Textual Properties

The textural properties of the prepared catalysts were characterized by the N₂ adsorption–desorption isotherms and the distribution curves of pore size (Figure S1). Referring to the IUPAC classification, the catalysts exhibited type IV isotherms with an H3 hysteresis loop, revealing irregular mesoporous structures [25]. As tabulated in

Table 2. Textual properties of the prepared catalysts.

Catalyst	BET Surface Area /m2·g−1	Pore Volume a /cm3·g−1	Average Pore Diameter b/nm
MnAl-I	183.5	0.35	6.0
MnAl-II	188.3	0.34	5.9
MnAl-III	169.4	0.32	5.9
MnAl-IV	174.5	0.33	5.9

^a BJH desorption cumulative volume of pores. ^b BJH desorption average pore diameter.

, the surface area, pore volume, and pore diameter were summarized by BET and BJH methods. The catalysts had a similar pore size and their BET surface area decreased in the following order: MnAl-II (188.3 m²·g⁻¹) > MnAl-I (183.5 m²·g⁻¹) > MnAl-IV (174.5 m²·g⁻¹) > MnAl-III (169.4 m²·g⁻¹), corresponding to the sequence of CO₂ selectivity. According to the reports, a higher BET surface area could provide more effective active sites and promote the diffusion effect of gaseous reactants [26]. With full contact between the VOCs and the catalytic active sites, the deep oxidation was further enhanced, therefore increasing the selectivity of CO₂ [27].

2.2.3. Surface Properties

To evaluate the surface properties of the catalysts, the deconvoluted results of XPS spectra are presented in Figure 4, and the contents of the Mn and O species with different oxidation states are tabulated in

Table 3. XPS results for the synthesized catalysts.

Catalysts	Mn 2p3/2		O 1s		Mn3+/Mn4+	Oad/Ola
	Mn4+ (%)	Mn3+ (%)	Ola (%)	Oad (%)
MnAl-I	37.9	62.1	19.3	80.7	1.64	4.18
MnAl-II	37.7	62.3	13.7	86.3	1.65	6.30
MnAl-III	42.3	57.7	21.4	78.6	1.36	3.67
MnAl-IV	38.9	61.1	17.7	82.3	1.57	4.65

. As shown in Figure 4a, the Mn 2p_3/2 spectrum consisted of two peaks at ~642 eV and 643 eV, ascribed to the Mn³⁺ and Mn⁴⁺ species, respectively. Clearly, Mn³⁺ was the main Mn species, and the order of the Mn³⁺/Mn⁴⁺ molar ratio was as follows: MnAl-II (1.65) > MnAl-I (1.64) > MnAl-IV (1.57) > MnAl-III (1.36). In general, Mn³⁺ usually triggers the Jahn–Teller distortion, and to preserve the electrostatic balance, oxygen vacancies are accordingly generated in the process (Equation (2), V_o refers to a oxygen vacancy). Moreover, as reactive sites, more oxygen vacancies are beneficial for O₃ decomposition and HCHO ozonation [28,29].

4Mn⁴⁺ + O²⁻ → 4Mn⁴⁺ + 2e/V_o + 1/2O₂ → 2Mn⁴⁺ + 2Mn³⁺ + V_o + 1/2O₂

(2)

The O 1s spectra of the prepared catalysts are shown in Figure 4b, and the curves exhibited two characteristic peaks at ~530 and 531 eV, corresponding to lattice oxygen species (O_la) and surface-adsorbed oxygen species (O_ad), respectively. The O_ad/O_la molar ratio decreased in the following sequence: MnAl-II (6.30) > MnAl-IV (4.65) > MnAl-I (4.18) > MnAl-III (3.67). With a higher mobility, O_ad facilitated the formation of oxygen vacancies, providing efficient active sites for oxygen adsorption, ozone decomposition, and HCHO ozonation [30].

Among all the catalysts, MnAl-II possessed the highest proportion of Mn³⁺ and O_ad, generating abundant oxygen vacancies and promoting the catalytic activity. As for MnAl-III, its Mn³⁺ and O_ad peaks slightly shifted to a higher binding energy, indicating its higher oxygen-escaping energy, which was bad for oxygen vacancies formation and oxygen species mobility, fitting with its HCHO conversion, which was the lowest [16]. Overall, the XPS results were roughly consistent with the catalytic efficiency, illustrating that oxygen vacancies may be an important factor for the catalytic ozonation of HCHO.

2.2.4. Temperature-Programmed Studies

The redox ability of the synthesized catalysts was revealed by the H₂-TPR profiles, as shown in Figure 5a. The catalysts exhibited typical reduction peaks, which could be assigned to the successive reduction process of MnO₂ (IV) → Mn₂O₃ (III) → Mn₃O₄ (II and III) → MnO (II) [31]. By contrast, MnAl-II appeared at a lower H₂ reduction temperature, at 236 °C, showing an excellent reducibility at a low temperature [7]. As the report goes, a higher H₂ uptake amount always contributes to a better redox ability [32]. However, the amount of H₂ uptake (

Table 4. Amount of O₂, NH₃ desorption, and H₂ uptake in the synthesized catalysts.

Catalyst	H2 Uptake /(mmol·gcat−1)	O2 Desorption /(a.u.·gcat−1)	NH3 Desorption /(mmol·gcat−1)
MnAl-I	4.03	3.60	3.26
MnAl-II	4.05	3.75	3.29
MnAl-III	5.28	3.36	3.28
MnAl-IV	5.15	3.68	3.22

) increased in the following order: MnAl-I (4.03 mmol·g_cat⁻¹) < MnAl-II (4.05 mmol·g_cat⁻¹) < MnAl-IV (5.15 mmol·g_cat⁻¹) < MnAl-III (5.28 mmol·g_cat⁻¹), inconsistent with the results of the removal efficiency. It demonstrated that there may be no positive correlation between the H₂ uptake amount and catalytic ozonation activity.

To research the properties of the oxygen species, the O₂-TPD profiles are shown in Figure 5b. As the desorption temperature goes from low to high, the oxygen species are classified as chemical-adsorbed oxygen (<250 °C), surface lattice oxygen (250~650 °C), and bulk lattice oxygen (>650 °C) [33]. Compared to the other three catalysts, the desorption peak of MnAl-II at 426 °C was relatively lower, indicating its superior surface lattice oxygen mobility. As tabulated in Table 4, the total O₂ desorption amount decreased in the following order: MnAl-II (3.75 a.u.·g_cat⁻¹) > MnAl-IV (3.68 a.u.·g_cat⁻¹) > MnAl-I (3.60 a.u.·g_cat⁻¹) > MnAl-III (3.36 a.u.·g_cat⁻¹). This is fitting with the results of O 1s. With the highest HCHO conversion efficiency, MnAl-II also possessed better oxygen mobility and a higher oxygen desorption amount, confirming that abundant active oxygen species were crucial for the catalytic ozonation of HCHO [34].

Surface acidity is important for the catalytic ozonation of VOCs, affecting adsorption, degradation, and desorption processes [35]. Herein, the NH₃-TPD experiments were conducted and the obtained profiles are shown in Figure 5c. According to differences in the acidity strength, the desorption peaks below 280 °C were assigned to weak acidity, while the peaks at 280~450 °C and 450~600 °C were ascribed to moderate and strong acidity, respectively [6]. All the catalysts showed only weak and strong acid sites, corresponding to Lewis and Brönsted acid sites, respectively. Generally, Lewis acid sites are conducive to the cleavage of C–C bonds and the deep oxidation of VOCs, while Brönsted acid sites favor the adsorption of VOCs and the desorption of products [36,37]. With the highest total acid sites (3.29 mmol·g_cat⁻¹), MnAl-II performed well in the activity tests. Furthermore, the amount of MnAl-III in the total acid sites was 3.28 mmol·g_cat⁻¹, which was more than the MnAl-I catalyst. It suggested that both acid treatment and alkaline treatment contribute to the formation of acid sites, as report [19] recorded. However, the catalytic efficiency of MnAl-III was the worst; this could be because the activity was determined by many factors, not only the acidity.

2.3. Effect of H₂O

As water vapor commonly exists in indoor air, it may influence catalytic performance and activity. Thus, a long-term H₂O resistance test was carried out on MnAl-II to study the effect of H₂O on its excellent performance in the activity tests. The reaction temperature was 30 °C, and the molar ratio of O₃/HCHO was set as 2.5. As depicted in Figure 6, in the stability test, the conversion of HCHO was maintained at ~99% for 7 h, demonstrating the favorable stability of MnAl-II. When RH = 50%, a slight drop in HCHO conversion was caused, falling from ~99% to ~98% after 2 h. When the H₂O was stopped, the conversion of HCHO gradually recovered to ~99%. Subsequently, when RH = 100%, the HCHO conversion decreased to ~97% and then recovered to ~98% after stopping H₂O. Obviously, the effect of H₂O was negligible, and the slight deactivation was fortunately reversible. Moreover, the presence of H₂O promoted the selectivity of CO₂; this may be because the generated OH active radicals were beneficial to the complete oxidation of HCHO to CO₂ [38].

2.4. In Situ DRIFTS Measurement

To study the transformation of intermediate species, in situ DRIFTS experiments were performed to reveal the possible reaction mechanism of HCHO ozonation over the MnAl-II catalyst. Initially, the process of HCHO adsorption at 30 °C (with N₂ as a background gas) was investigated, as recorded in Figure 7a. Notably, the peak located at 2896 cm⁻¹ could be ascribed to the C–H stretching of adsorbed HCHO, while the bands at 1431, 1136, and 1097 cm⁻¹ corresponded to dioxymethylene (DOM) species [39,40]. The absorbance bands centered at 1266, 1360, 1575, and 1593 cm⁻¹ were related to the symmetric and asymmetric stretching vibration of formate species [41]. Additionally, the peaks at 1663 and 3703 cm⁻¹, which could be designated to adsorbed water and hydroxyl groups on the catalyst surface, decreased to negative peaks, possibly due to the consumption of OH species during the adsorption process [42,43].

When the adsorption of CH₂O was saturated, O₃ was introduced to the reaction gas (Figure 7b). Meanwhile, the absorption peak of monodentate formate species (1593 cm⁻¹) was converted to bidentate formate species (1575 cm⁻¹) and declined with time. Moreover, the peaks corresponding to DOM species (1431, 1136, 1097 cm⁻¹) and HCHO (2896 cm⁻¹) disappeared, while a peak related to bicarbonate species (1421 cm⁻¹) emerged [44]. A negative peak at 1663 cm⁻¹ was not observed, indicating the generation of H₂O during the mineralization of formate species. Additionally, the peaks at 2876 and 2396 cm⁻¹ could be assigned to the hydrocarbon vibrations of formate species and carbon oxides, respectively [41].

2.5. Theoretical Calculation and Reaction Mechanism

Vital to the O₃ decomposition and ozonation process, the effect of oxygen vacancies on the catalytic reaction was further researched through theoretical calculations. As the main reaction sites for O₃, the calculations were carried out on a α-MnO₂ (1 0 0) surface_. As depicted in Figure 8, the adsorption energy of an O₃ molecule on α-MnO₂ with an oxygen vacancy (E_ads = −1.59 eV) was lower than the original one (E_ads = −1.38 eV). Generally, a lower adsorption energy illustrated a stronger interaction between O₃ and the α-MnO₂ surface, meaning that oxygen vacancies facilitated the adsorption of O₃ [45]. Next, with the elongation of the O–O bond, O₃ was decomposed into *O and O₂. For α-MnO₂ with an oxygen vacancy, the energy change for O₃ decomposition was −2.20 eV, proving to be an exothermic reaction, which was thermodynamically favorable and spontaneously occurred. As for the original α-MnO₂, the required energy change to overcome the energy barrier was 0.50 eV, limiting the reaction rate. Therefore, the generation of oxygen vacancies greatly enhanced the adsorption and decomposition of O₃, accelerating the speed of catalytic ozonation.

To summarize, a possible reaction mechanism of HCHO catalytic ozonation was proposed, as presented in Figure 9. Firstly, HCHO was adsorbed on OH species through hydrogen bonds, while O₃ was adsorbed on the α-MnO₂ surface. With the promotion of oxygen vacancies, O₃ was activated and broken down into oxygen and active oxygen radicals (O*). Next, the adsorbed HCHO was oxidized to DOM species and subsequently transformed into formate species under the action of O* radicals. Then, formate species were deeply oxidized into labile carbonic acid and rapidly resolved into CO₂ and H₂O, which are harmless to the environment. Simultaneously, the active sites and free radicals (O*, OH, etc.) were regenerated. Overall, the reaction pathway followed the decomposition route as follows: HCHO→DOM→formate→carbonic acid→CO₂ and H₂O.

3. Materials and Methods

3.1. Catalyst Preparation

The Mn-based catalysts were prepared by the equivalent-volume impregnation method, supported by activated γ-Al₂O₃. Typically, a 1.1 mL Mn(CH₃COO)₂·4H₂O (Aladdin, Shanghai, China) solution with a concentration of 139.8 g/L was evenly impregnated onto 1 g γ-Al₂O₃ (Aladdin, China, 40~60 meshes). After being treated by an ultrasonic wave for 1 h and placed for 23 h, the mixtures were dried at 100 °C for 8 h and calcined at 500 °C for 3 h in air. Then a 5%Mn/γ-Al₂O₃ catalyst was obtained, sieved to 40–60 meshes again, and labeled as MnAl-I. Analogously, MnAl-II, MnAl-III, and MnAl-IV were synthesized by the same method. The difference was that the γ-Al₂O₃ samples were pretreated by 0.5 mol/L CH₃COOH (Sinopharm, Shanghai, China), NH₃·H₂O (Sinopharm, China), and H₂O₂ (30 wt%, Sinopharm, China) solutions, respectively. The Mn content of the above MnAl catalysts were measured by ICP-OES, and the results were similar to the theoretical mass fraction (~5%).

3.2. Catalytic Activity

The experimental setup schematic of formaldehyde (HCHO) catalytic ozonation is depicted in Figure S2, containing a gas mixing part, a catalytic reaction part, and measuring instruments. The simulated flue gas, containing HCHO (60 ppm/balance N₂), N₂ (99.999%) and O₂ (99.999%), was fixed at 200 mL/min and supplied by cylinder gases (Jingong Gas Co., Ltd., Hangzhou, China. Each gas was regulated by an individual mass flow controller (MFC S500, HIRIBA METRON Co., Ltd., Irvine, CA, USA) and the initial concentration of HCHO and O₂ were 30 ppm and 10 vol%, respectively. Ozone was generated by a dielectric barrier discharge reactor (VMUS-1S, AZCO Industries., Ltd., Vancouver, BC, Canada) and the produced O₃/O₂ mixtures were divided into two streams. One was injected into the mixing tank and the other flowed into a wide-range ozone analyzer (BMT-964BT, OSTI, Inc., Monterey, CA, USA, 0~20 g/Nm³, ±0.1 g/Nm³) to control the concentration of ozone. The water vapor was carried by N₂ through the bubbling method. About 0.15 g of the catalyst was mixed with the appropriate amount of SiO₂ (Sinopharm, 40~60 mesh), corresponding to a gas hour space velocity (GHSV) of 50,000 h⁻¹, and then placed in the middle of a quartz tube, with a thermal couple to monitor the reaction temperature in the furnace. After the catalytic reaction, the exhaust gas flowed into a FTIR gas analyzer (Gasmet FTIR DX4000, Vantaa, Finland) and a gas chromatograph (GC9790II, Zhejiang Fuli Co., Ltd., Taizhou, China) to measure the concentration of HCHO and CO/CO₂, while the residual O₃ was detected by a low-range ozone analyzer (BMT-932-1, OSTI Inc.; 0~100 ppm, ±0.1 ppm).

The catalytic activity was evaluated by the conversion efficiencies of HCHO and O₃ and the selectivity of CO and CO₂, which were calculated using the following equations

$$[\mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}]\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}.={\displaystyle \frac{[\mathrm{C}\mathrm{O}]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}+[{\mathrm{C}\mathrm{O}}_2]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}{[\mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}]\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}\times 100\%$$

(3)

$$[{\mathrm{O}}_3]\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}.={\displaystyle \frac{[{\mathrm{O}}_3]\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}-[{\mathrm{O}}_3]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}{[{\mathrm{O}}_3]\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}\times 100\%$$

(4)

$$[{\mathrm{C}\mathrm{O}}_2]\mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}.={\displaystyle \frac{[{\mathrm{C}\mathrm{O}}_2]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}{[\mathrm{C}\mathrm{O}]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}+[{\mathrm{C}\mathrm{O}}_2]\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}\times 100\%$$

(5)

where [HCHO]_initial and [O₃]_initial are the initial concentrations of HCHO and O₃ in ppm, respectively. [CO]_outlet, [CO₂]_outlet, and [O₃]_outlet are the outlet concentrations of CO, CO₂, and O₃ in ppm, respectively.

3.3. Catalyst Characterization

The elemental contents of the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5800, Santa Clara, CA, USA). X-ray powder diffraction (XRD) patterns were obtained by an X-ray diffractometer (PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA, from 10 to 80° (2θ, diffraction angle). The surface morphology of the samples was observed by a field emission scanning electron microscope (FE-SEM, Zeiss Sigma 300, Jena, Germany). The surface area, pore volume, and pore diameter were measured by N₂ adsorption–desorption isotherms and the BJH method using an automatic specific surface area and porosity analyzer (Micromeritics ASAP 2460, Norcross, GA, USA) at 77 K. The valence states of surface elements on the catalysts were carried out by an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA), and the binding energies of all elements were referenced to the C 1s peak at 284.8 eV. The O₂ temperature-programmed desorption (O₂-TPD), NH₃ temperature-programmed desorption (NH₃-TPD), and H₂ temperature-programmed reduction (H₂-TPR) measurements were conducted on a chemical adsorption analyzer (AutoChem II 2920, Micromeritics, Norcross, GA, USA). The reaction mechanism of the catalytic ozonation process was studied using an in situ diffuse reflectance infrared Fourier transform spectrometer (in situ DRIFTS, Thermo Scientific Nicolet iS50, Waltham, MA, USA) equipped with an MCT/A detector and a reaction chamber (Harrick, Pleasantville, NY, USA). All the in situ spectra were recorded at a resolution of 4 cm⁻¹ and within a range of 600~4000 cm⁻¹.

3.4. DFT Calculation

All the calculations were implemented in the CASTEP package using Material Studio software v2019 on the basis of density functional theory (DFT) [46]. The exchange–correlation interaction was processed with the generalized gradient approximation (GGA) using the Perdew–Burke–Enzerhof (PBE) function. Based on plane-wave theory, a cutoff energy of 650 eV was adopted. To further improve the accuracy, a DFT+U correction was applied to Mn 3d and the optimized Hubbard U = 1.6 eV was used in this work [47,48]. The convergence criteria of the self-consistent field (SCF) and absolute energy were set as 1.0 × 10⁻⁶ eV/atom and 1.0 × 10⁻⁵ eV/atom, respectively. A 3 × 3 × 9 Brillouin zone of the unit cell was employed.

After geometry optimization, the lattice parameters of bulk α-MnO₂ (a = b = 9.81 Å and c = 2.89 Å) were close to the experimental data (a = b = 9.75 Å and c = 2.86 Å), validating the accuracy of the calculation method used in this work. For its stability and reactivity, the (1 0 0) surface of α-MnO₂ was cleaved and a 2 × 3 supercell was built, as shown in Figure S3 [49]. A vacuum layer of 16 Å was constructed along the z-axis to avoid the next periodic slab effects. The adsorption energy (E_ads) of adsorbates on the surface of the catalyst were calculated using the following equation

E_ads = E_{(adsorbate-catalyst)} − (E_catalyst + E_adsorbate)

(6)

where E_{(adsorbate-catalyst)} is the total energy of the catalyst–adsorbate system, E_catalyst is the total energy of the catalyst, and E_adsorbate is the total energy of the gas–phase molecule.

4. Conclusions

Herein, a series of MnO_x/γ-Al₂O₃ catalysts were synthesized by the equivalent-volume impregnation method to study the catalytic ozonation of HCHO at room temperature. The MnAl-II (treated by acid solution) exhibited the highest catalytic activity, reaching an HCHO conversion above 95% at an O₃/HCHO ratio of 1.5, and achieved nearly 100% HCHO conversion with above 95% CO₂ selectivity at an O₃/HCHO ratio of 3.0 at 30 °C. The better metal dispersion, smaller particle size, and surface micropore structure contributed to the larger surface area of MnAl-II, while the correspondingly generated active sites promoted the deep oxidation of HCHO. Furthermore, its higher Mn³⁺ and O_ad content facilitated the formation of oxygen vacancies, accelerating the process of O₃ decomposition and HCHO ozonation. Moreover, the XPS results were roughly consistent with the order of catalytic efficiency, suggesting that oxygen vacancies may be an important factor for catalytic ozonation progress. Also, the plentiful acid sites and active oxygen species were crucial to the excellent performance of MnAl-II. In the stability and H₂O resistance tests, MnAl-II also performed well, maintaining over 95% HCHO conversion when RH = 100%. A possible reaction mechanism of HCHO ozonation was proposed based on in-situ DRIFTS measurement (HCHO→DOM→formate→carbonic acid→CO₂ and H₂O), while the important role of oxygen vacancies in ozone adsorption and decomposition was confirmed by DFT calculations.