Catalytic Acetone Oxidation over MnO_x Catalysts: Regulating Their Crystal Structures and Surface Properties

Abstract

This study investigates the catalytic oxidation of acetone by different crystal phases of MnO₂ prepared via different methods. Compared with β-MnO₂ and γ-MnO₂, α-MnO₂ exhibited superior catalytic activity. Moreover, as replacements for traditional hydrothermal methods and air calcination, the use of microwave hydrothermal methods and N₂ calcination significantly enhanced the catalytic performance of the MnO₂ catalyst. The optimal catalyst, MnO₂-WN (α-MnO₂ synthesized via microwave hydrothermal method and N₂ calcination), converted 100% of 100 ppm acetone below 150 °C, with the CO₂ yields reaching 100%. Further, the stability of the catalyst and its potential for other volatile organic compounds (VOCs) were also determined. The experimental data demonstrated that its outstanding activity primarily stemmed from the improved preparation method, enhancing the specific surface area of the catalyst, optimizing the pore structure, improving the redox performance, and generating more acidic sites and active oxygen species, thereby creating a synergistic effect. Finally, the reaction pathway of acetone oxidation on the catalyst surface has been explored. This work provides a new perspective for developing economically efficient MnO_x catalysts for removing VOCs.

1. Introduction

Amidst a myriad of atmospheric pollutants, volatile organic compounds (VOCs) have garnered considerable scholarly attention owing to their profound negative impact on the environment and human health [1]. Moreover, some VOCs exhibit potent toxicity and other carcinogenic properties. They also contribute to secondary pollution by serving as pivotal precursors to O₃ and haze [2]. Within the realm of VOCs, oxygenated volatile organic compounds (OVOCs) demonstrate heightened reactivity and are predisposed to serve as precursors to ozone when juxtaposed with other VOCs by virtue of their inherent oxygen atoms [3]. Notably, acetone, a ubiquitous OVOC in the atmosphere, ranks among the most prevalent VOCs in confined indoor settings like laboratories and offices [4]. Acetone vapor emanates primarily from acetone solvents, which are extensively utilized in industries encompassing pharmaceuticals, printing, and plastics manufacturing [5].

Commonly used techniques for acetone elimination include condensation [6], absorption [7], combustion [8], biodegradation [9], low-temperature plasma [10], and catalytic oxidation [11]. In particular, catalytic oxidation has garnered widespread attention amongst researchers due to its cost-effectiveness and high efficiency, resulting in the complete conversion of acetone into benign byproducts. Consequently, the pursuit of developing a cost-effective and high-efficiency catalyst stands as the focal point of future research on the catalytic oxidation of acetone.

Catalysts employed for catalytic oxidation can be classified into two distinct kinds: supported noble metal catalysts and transition metal oxide-based catalysts [12]. Noble-metal catalysts typically exhibit superior catalytic activity at low temperatures [13]. The main drawback of utilizing precious metals lies in their exorbitant cost. Moreover, the propensity for these catalysts to undergo sintering at elevated temperatures represents an acute issue necessitating immediate attention [14]. Due to their exceptional oxygen mobility, transitional metal oxides (Ce, Mn, Co, Fe, Cu, etc.) with different valence states harbor considerable potential for catalytic oxidation [15]. Sun et al. synthesized a NiOOH-CuO nano-heterostructure anchored on a three-dimensional conducting copper foam. The performance of the prepared catalyst exceeded that of the noble metal catalysts, showing excellent ethanol oxidation activity [16]. Manganese oxide, in particular, stands out as a highly active oxide endowed with a plethora of coordination numbers and oxidation states. Moreover, its surface properties create a pronounced affinity for the adsorption and activation of VOCs. Because of these properties, MnO_x can effectively remove acetone even at low temperatures [17]. Cheng et al. studied the catalytic oxidation of acetone over MnO_x with divergent crystal structures. Among these, α-MnO₂ in its α, β, γ, and δ phases showed the best performances [18]. The distinctive crystal phase of α-MnO₂ resulted in a synergistic effect, which manifested in larger crystal tunnel dimensions. A higher proportion of Mn⁴⁺ cations and weaker Mn-O bond strengths enhanced oxygen mobility and redox properties. Lu et al. investigated the effect of calcination temperature on manganese oxide properties. Many unsaturated manganese vacancies and oxygen vacancies are produced on the α-MnO₂ at the optimum calcination temperature (600 °C), especially the surface oxygen vacancy clusters [19]. Hence, the modulation of crystal phases, coupled with alterations in the preparation conditions, significantly modifies the properties of manganese oxide. However, previous studies have seldom investigated the impact of the calcination atmosphere.

Typically, the hydrothermal method is used to synthesize the manganese oxide catalysts. However, the conventional hydrothermal approach is time-consuming, and there are risks associated with the experimental conditions, which involve elevated pressure. In contrast, the microwave hydrothermal technique harnesses microwave energy to heat the sample, offering the advantages of a shortened preparation timeframe, homogeneous heating, and ease of temporal regulation [20]. Despite the burgeoning research into catalyst fabrication via the microwave hydrothermal method in recent years, there is a lack of reports concerning their application to the catalysis of VOCs.

This study evaluated the catalytic activity and CO₂ yield of different crystal forms of MnO₂ towards acetone oxidation. Additionally, the surface properties of MnO₂ were modified by varying the hydrothermal method as well as the calcination atmosphere. This study was primarily aimed at screening the optimal MnO_x catalyst and applying it to the catalytic oxidation of acetone. Notably, α-MnO₂, calcined in an N₂ atmosphere via the microwave hydrothermal method (MnO₂-WN), exhibited the best catalytic performance. A 90% conversion as well as a mineralization rate could be achieved at 132 °C. Impressively, the prepared catalyst also had good stability and excellent removal effects on other VOCs. Finally, the structure-activity relationship between the catalytic activities and the surface properties of the catalysts was explored via comprehensive characterization tests, demonstrating that the excellent performance primarily stemmed from the improved preparation method, enhancing the specific surface area of the catalyst, optimizing the pore structure, improving the redox performance, and generating more acidic sites and active oxygen species, thereby creating a synergistic effect. Finally, the reaction pathway of acetone oxidation on the catalyst surface has been explored. This work provides a new perspective for developing economically efficient MnO_x catalysts for removing VOCs.

2. Materials and Methods

2.1. Preparation of Catalysts

All reagents used for synthesis were analytical-grade and did not require further purification.

Synthesis of different crystal forms of MnO₂.

α-MnO₂: 1.58 g potassium permanganate (KMnO₄) and 0.71 g ammonium oxalate monohydrate ((NH₄)₂C₂O₄·H₂O) were dissolved in 30 mL and 20 mL deionized water, respectively. An ammonium oxalate solution was prepared, added dropwise to the potassium permanganate solution, and thoroughly mixed. The mixture was then transferred to a stainless-steel autoclave lined with 100 mL of Teflon (Guohua Electric Co., Changzhou, China), and heated at 180 °C for 24 h. Next, the solution was cooled to room temperature, centrifuged to collect precipitates, washed with deionized water 3–5 times, and dried overnight at 105 °C. Finally, the dried solid was calcinated in a static air atmosphere at 350 °C for 2 h.

β-MnO₂: 3.38 g manganese sulfate monohydrate (MnSO₄·H₂O) and 4.56 g ammonium thiosulfate ((NH₄)₂S₂O₈) were completely dissolved in 80 mL deionized water. The solution was heated at 140 °C for 12 h.

γ-MnO₂: 6.75 g manganese sulfate monohydrate (MnSO₄·H₂O) and 7.40 g ammonium thiosulfate ((NH₄)₂S₂O₈) were completely dissolved in 80 mL deionized water. The solution was heated at 90 °C for 24 h.

Synthesis of MnO₂-WK (W: microwave; K: calcinating in the air atmosphere): MnO₂-WK replaced the traditional hydrothermal with microwave hydrothermal (the reaction conditions for the microwave hydrothermal method are the following: 180 °C, 2 h, 700 w, 4 MPa). The other conditions were the same as those for the preparation of α-MnO₂.

Synthesis of MnO₂-WN (W: microwave; N: calcinating in an N₂ atmosphere): The MnO₂-WN catalyst was prepared using the same procedure as MnO₂-WK, with the exception of changing the calcination atmosphere from air to N₂.

2.2. Characterization

The specific surface area and pore structure of the catalysts were measured by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods from N₂ adsorption-desorption isotherms tested on a Micromeritics ASAP 2460 nitrogen adsorption apparatus (USA). X-ray diffraction (XRD) was carried out to identify the crystal structure using an X’Pert Pro MPD instrument. The morphology of catalysts was examined by scanning electron microscopy (SEM) using a ZEISS Sigma (Oberkochen, Germany) 500. X-ray photoelectron spectroscopy (XPS) analysis was taken by a Thermo Scientific ESCALAB 250xi (Thermo Scientific, Waltham, MA, USA) K-alpha analyzer to analyze the valence distribution of different elements on the surface of the catalyst. The binding energies were calibrated by C 1 s of 284.8 eV.

The oxygen vacancies of the catalysts were determined by electron paramagnetic resonance (EPR), which was carried out on an EMXPlus instrument (Bruker, Billerica, MA, USA). Raman spectra were measured on the HORIBA LabRAM HR Evolution (Horiba, Kyoto, Japan) using an excitation wavelength of 514 nm. H₂ temperature-programmed reduction (H₂-TPR), NH₃ temperature-programmed desorption (NH₃-TPD), O₂ temperature-programmed desorption (O₂-TPD), and CO₂ temperature-programmed desorption (CO₂-TPD) were conducted on a chemical adsorption analyzer (PCA-1200) to analyze various surface chemical properties. Prior to the experiments, N₂ was ventilated at room temperature for 10 min to remove the residual gas. Next, the tube loaded with the catalysts was heated to 573 K at a rate of 10 °C per minute in an N₂ atmosphere for 10 min to dehydrate the catalysts. For H₂-TPR, ~50 mg of the purged catalysts was heated up to 923 K (10 K/min) under 30 mL/min of 10 vol.% H₂/He, and a TCD detector recorded the signal intensity of H2 residual. For the NH₃-TPD, O₂-TPD, and CO₂-TPD analyses, ~100 mg of purged catalysts were treated with 30 mL/min of 5 vol.% NH₃/N₂, 5 vol.% O₂/He, and pure CO₂, respectively, to reach saturated adsorption. Subsequently, the mixtures were heated up to 1173 K (10 K/min) under 30 mL/min N₂ to collect the desorbed NH₃/O₂/CO₂ signals.

In situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) analysis of acetone oxidation was performed using a Tensor II FTIR spectrometer (Bruker). Initially, the catalyst was loaded on the light absorption cell and purged in N₂ for 30 min at 300 °C. After background collection, 300 ppm acetone was injected for 30 min to evaluate the single adsorption of acetone. Subsequently, catalytic oxidation of acetone began after the addition of 10% oxygen, and the data was recorded for another 30 min. The catalyst was successively heated to 125, 150, 175, and 200 °C and held at each temperature period for 30 min to investigate the surface variation of the catalyst during reactions at different temperatures.

The details of the characterization procedures have been reported in our previous work [21].

2.3. Performance Evaluation of Catalytic Oxidation

Catalytic oxidation of acetone was carried out in a fixed-bed reactor, as reported previously [22]. The mixed gases containing 100 ppm VOCs (acetone, dichloromethane, methyl mercaptan, and toluene), 10% O_2, and N₂ balanced with a total flow rate of 100 mL/min were injected into the reactor. Moreover, 100 mg catalysts (40–60 mesh) were loaded on the central section of the quartz tube, corresponding to a weight hourly space velocity (WHSV) of 60,000 mL g_cat⁻¹ h⁻¹. A gas chromatogram (GC, GC9790II, Zhejiang Fuli Co., Ltd., Wenling, China) equipped with FID detectors at the outlet detected the concentration of acetone and CO/CO₂. CO and CO₂ were converted into CH₄ in a catalytic reactor for measurement. The following equations calculate acetone conversion and CO₂ yield:

$$\mathrm{V}\mathrm{O}\mathrm{C} \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}=({\left[\mathrm{V}\mathrm{O}\mathrm{C}\right]}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}-{\left[\mathrm{V}\mathrm{O}\mathrm{C}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}})/{\left[\mathrm{V}\mathrm{O}\mathrm{C}\right]}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}\times 100\mathrm{\%}$$

(1)

$${\mathrm{C}\mathrm{O}}_2 \mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=({\left[{\mathrm{C}\mathrm{O}}_2\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}/{\left[\mathrm{V}\mathrm{O}\mathrm{C}\right]}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}\times \mathrm{N})\times 100\mathrm{\%}$$

(2)

where [VOC]_initial and [VOC]_outlet are the initial and outlet VOC concentrations (ppm), respectively, [CO₂] _outlet is the outlet CO₂ concentration (ppm), and N is the number of carbon atoms contained in a VOC molecule.

3. Results and Discussion

3.1. Catalytic Performance

The differences in catalytic behaviors of acetone oxidation over the synthesized catalysts are shown in Figure 1 and

Table 1. The conversion temperature of acetone oxidation over all catalysts.

Catalyst	T10 (°C)	T50 (°C)	T90 (°C)
α-MnO2	44	154	175
β-MnO2	101	169	209
γ-MnO2	49	179	222
MnO2-WK	40	126	147
MnO2-WN	35	117	132

. The control experiment confirmed that acetone did not undergo self-oxidation within the experimental temperature range. As depicted in Figure 1a, the acetone conversion rates of the three distinct crystal forms of the MnO_x catalysts progressively increased with increasing temperatures, stabilizing at 100% conversion upon reaching a threshold temperature of 240 °C. A detailed comparative analysis revealed that α-MnO₂ exhibited superior catalytic activity across low and high-temperature regimes, with a T₉₀ (temperature corresponding to 90% acetone conversion) of approximately 175 °C (see Table 1). Although β-MnO₂ displayed a lower acetone conversion relative to γ-MnO₂ at low temperatures, its activity increased significantly at elevated temperatures, resulting in a complete conversion of acetone with a T₉₀ of around 210 °C. The T₉₀ of γ-MnO₂ was approximately 220 °C, indicating the weakest catalytic activity at high temperatures. Figure 1b displays the CO₂ yield rate as a function of temperature, exhibiting a trend similar to that displayed by the acetone conversion curve. The CO₂ yield increased with rising temperature, ultimately reaching 100%, suggesting that acetone was completely converted to CO₂ and H₂O via catalytic oxidation. Notably, the CO₂ yields for α-MnO₂ were consistently higher than the other two crystal phases at all temperatures. Consequently, due to its enhanced activity and mineralization rate, α-MnO₂ appeared to be particularly well-suited for removing acetone. Varying crystal phases resulted in disparate crystal pore structures and pore sizes, thereby altering the contact area and frequency between the acetone molecules and the catalyst, influencing the catalytic activity [18]. This result was also consistent with BET tests (vide infra).

The preparation methodology was refined to augment the catalytic activity further. Figure 1c shows the catalytic behaviors of acetone oxidation over α-MnO₂ prepared via different methods. Compared with the α-MnO₂ synthesized via conventional hydrothermal means, the catalytic efficacy of the MnO₂ produced via microwave hydrothermal synthesis was markedly increased. The T₉₀ of MnO₂-WK was approximately 150 °C, substantially inferior to that of the standard α-MnO₂. Subsequently, the calcined atmosphere was modulated, building upon the foundation of MnO₂-WK. MnO₂-WN was procured via calcination in a nitrogen atmosphere. The catalytic activity of MnO₂-WN was higher than MnO₂-WK, with T₉₀ reaching 132 °C. In fact, MnO₂-WN displayed higher activity than many other reported precious metal catalysts (Table S1). Figure 1d evaluates the mineralization rates of the three catalysts. The overall trend is consistent with the activity curve. Notably, MnO₂-WN achieved an impressive mineralization rate at relatively low temperatures, a phenomenon rarely reported in previous studies. Given its low cost, high activity, and mineralization rate, MnO₂-WN holds significant application potential. Subsequent characterization revealed the underlying rationale for its exceptional performance.

3.2. Stability and Catalytic Oxidation of Other Typical VOCs

The deactivation of catalysts is an issue that must be considered for practical applications. Less robust catalysts are susceptible to deactivation over prolonged use, exhibiting subpar performance at high velocities. Thus, protracted stability assessments and high-velocity tests were conducted to assess the potential application of the optimal catalyst comprehensively. Figure 2a shows the influence of weight hourly space velocity on the catalytic prowess of MnO₂-WN. Notably, MnO₂-WN exhibited commendable catalytic efficacy, with 90% conversion attained at a temperature <140 °C even as the WHSV was elevated to 120,000 gcat⁻¹ h⁻¹. Nevertheless, there was a slight decline in activity at lower temperatures as WHSV was further augmented to 180,000 gcat⁻¹ h⁻¹. This was attributed to the decreased interaction time of acetone with the catalytic interface. However, with rising temperatures, there was less deactivation, enabling a 90% conversion rate at 140 °C. Figure 2b shows that MnO₂-WN was still able to reach a mineralization rate of 100% even at high WHSV. Figure 2c, 24-h shows the stability trials for the catalytic oxidation of acetone were performed at 150 °C for α-MnO₂ and MnO₂-WN. Evidently, MnO₂-WN remained admirably stabilized, with the conversion rate maintained at approximately 99%. In contrast, there was a gradual decline in the α-MnO₂ conversion rate after 12 h, resulting in a significant difference in the conversion rate compared with MnO₂-WN, with the difference increasing from an initial value of 48% to 52%. Hence, it can be concluded that MnO₂-WN exhibits exceptional stability in the catalytic conversion of acetone.

Furthermore, the catalytic efficacy of MnO₂-WN toward several other prototypical VOCs was investigated. Specifically, three emblematic gases comprising sulfur (methyl mercaptan), chlorine (dichloromethane), and aromatics (toluene) were chosen. Figure 2d showed that MnO₂-WN exhibited commendable catalytic performance across all three VOCs, achieving complete conversion. The temperatures at which the three VOCs were completely transformed were among the lower temperatures in previous studies, indicating that MnO₂-WN has the potential for the catalytic oxidation of diverse VOCs.

3.3. Structural Characterization

The catalysts were characterized in detail to explore the effects of microwave hydrothermal treatment and N₂ calcination on the exceptional catalytic properties of MnO_x. In particular, the textural properties, crystalline structures, morphologies, reducibility, acidity, elemental valence, oxygen species distribution, and oxygen defects of synthesized catalysts were studied.

3.3.1. Textural Properties, Crystalline Structures, and Morphologies of the Catalysts

Nitrogen adsorption-desorption tests were performed to investigate the relationship between the textural properties of the catalysts and their catalytic activities. Figure 3a–e shows the nitrogen adsorption-desorption isotherms and pore size distribution curves for five catalysts: α-MnO₂, β-MnO₂, γ-MnO₂, MnO₂-WN, and MnO₂-WK. All five catalysts exhibited typical Type IV isotherm characteristics, with an H3 hysteresis loop in the curves, indicating that they contained a significant number of mesopores [23]. Additionally, none of the five catalysts reached saturation adsorption up to p/p₀ = 1, indicating the presence of a certain amount of macropores [24,25]. The pore size distribution curves further confirmed these results.

Table 2. Textural properties of the synthesized catalysts.

Catalyst	Crystallite Size (nm)	BET Surface Area a (m2/g)	Total Pore Volume b (cm3/g)	Avg. Pore Diameter b (nm)
α-MnO2	15	45.68	0.15	13.7
β-MnO2,	27	13.29	0.05	14.8
γ-MnO2	21	36.11	0.25	18.1
MnO2-WN	19	90.03	0.22	10.1
MnO2-WK	17	126.37	0.25	8.0

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

shows the specific surface area, pore volume, and average pore diameter of the catalysts. Clearly, α-MnO₂ had a larger specific surface area and a smaller average pore diameter compared to the other two crystalline forms of MnO₂. A larger specific surface area is beneficial for the increased interaction of gas molecules with the active sites on the catalyst surface, which is favorable for the catalytic reaction. Meanwhile, the adsorption and activation of gas molecules on the catalyst surface are facilitated by a smaller average pore diameter, which is closer to the kinetic diameter of gas molecules [26]. For these reasons, α-MnO₂ exhibits superior catalytic oxidation activity. Notably, the specific surface area and pore volume of the two catalysts prepared by the microwave hydrothermal method, MnO₂-WN and MnO₂-WK, were significantly higher than those of MnO_x prepared by the conventional hydrothermal method. However, the average pore diameter was smaller. Therefore, the microwave hydrothermal method could effectively improve the specific surface areas and pore structures of the MnO_x, enhancing their catalytic activity [27].

Figure 3f illustrates the XRD patterns of all five catalysts. α-MnO₂, β-MnO₂, and γ-MnO₂ corresponded respectively to standard PDF cards, PDF#44-0141, PDF#24-0735, and PDF#14-0644. Corresponding to the (310), (211), and (541) crystal planes, the diffraction peaks of α-MnO₂ were at 28.8°, 37.5°, and 69.7°—with (211) being the predominant crystal plane. Further, corresponding to (110), (101), (111), (211), (220), and (301) crystal planes, the diffraction peaks of β-MnO₂ primarily occurred at 28.7°, 37.3°, 42.8°, 56.7°, 59.4°, and 72.3°. Herein, (110) represents the principal crystal plane. In the case of γ-MnO₂, the diffraction peaks of the (120), (131), (300), (160), (421), and (003) crystal planes were mainly located at 22.4°, 37.1°, 42.6°, 57.4°, 65.6°, and 68.9°. Among these, (131) was the predominant crystal plane [28,29]. No impurity peaks were observed in the XRD patterns of the three catalysts, indicating they were present in well-defined single phases. Utilizing Scherrer’s equation, we have computed the individual crystallite size of α-MnO₂ based on its primary (211) peak, β-MnO₂ according to the (110) peak, and γ-MnO₂ from the (131) peak, which are detailed in Table 2. Evidently, the crystallite size of β-MnO₂ surpassed that of the other two crystal phases, signifying its superior degree of crystallinity. Similarly, α-MnO₂ exhibited the smallest crystallite size, suggesting a lower level of crystallinity. This phenomenon was attributed to the specific preparation conditions. The XRD patterns of MnO₂-WN and MnO₂-WK aligned with the standard PDF#44-0141 card, affirming their identity as α-MnO₂. The diffraction peaks of these samples were located at 28.8°, 37.5°, 42.0°, 49.9°, 60.3°, 65.1°, and 69.7°, corresponding to (310), (211), (301), (411), (521), (002), and (541) crystal planes, with (310) representing the principal crystal plane. Notably, the diffraction peak intensities of MnO₂-WN and MnO₂-WK were distinctly greater than those of typical α-MnO₂. Furthermore, in accordance with Table 2, their crystallite size (based on their primary (211) peak) exhibited a gradual augmentation in comparison to the conventional α-MnO₂. In fact, MnO₂-WN exhibited more intense and narrower diffraction peaks than MnO₂-WK. The crystallite dimensions of MnO₂-WN exhibited a slightly larger size than MnO₂-WK. Thus, it could be safely concluded that the microwave hydrothermal synthesis method effectively enhanced the crystallinity of manganese oxide, while calcination under the N₂ atmosphere similarly augmented the crystallinity of MnO₂.

Figure 3g depicts the SEM images of three catalysts: MnO₂-WN, MnO₂-WK, and α-MnO₂. Prepared by conventional hydrothermal synthesis, the α-MnO₂ catalyst showcased a cauliflower-like morphology consisting of nanosphere clusters with diameters less than 50 nm, along with a small number of nanorods exceeding 200 nm in diameter. The closely aggregated nanospheres diminished the contact area between the catalyst and the gas, thereby reducing the reaction space and impeding reaction progression. In contrast, the MnO₂-WN and MnO₂-WK catalysts synthesized via microwave hydrothermal methods presented a morphology characterized by a flocculated skeleton structure, supplemented by a sheet-like structure. This structure implied a larger specific surface area and a higher degree of crystallinity, facilitating optimal contact between gas molecules and the active centers on the catalyst surface, thereby promoting reaction activity, in line with the findings of nitrogen adsorption-desorption experiments and XRD experiments. However, there were no significant differences in the structures of MnO₂-WN and MnO₂-WK, indicating that N₂ calcination had a minimal impact on morphology.

3.3.2. Surface Properties of the Catalysts

The chemical valence state of Mn and the distribution of oxygen species in the catalysts were determined by XPS measurements (see Figure 4). Using Gaussian functions, the XPS spectra of the samples were deconvoluted to discern the Mn and O species. Subsequently, the proportions were calculated by integrating the peak areas (see

Table 3. Distribution of Mn ions and O species of α-MnO₂, MnO₂-WK, MnO₂-WN, and MnO₂-WN-S based on XPS results.

Catalyst	Mn3+ (%)	Mn4+ (%)	AOS	Oads. (%)	Olatt. (%)
α-MnO2	62.8	37.2	3.65	20.7	79.3
MnO2-WK	64.5	35.5	3.55	44.1	55.9
MnO2-WN	69.5	30.5	3.42	36.0	64.0
MnO2-WN-S	65.9	34.1	3.52	58.5	41.5

). Arranged in descending order of binding energy, Figure 4a shows the characteristic peaks at approximately 644.2 eV and 641.8 eV, corresponding to Mn⁴⁺ and Mn³⁺ species, respectively [30]. Table 3 indicated that MnO₂-WN exhibited the highest proportion of Mn³⁺ at approximately 69.5%, followed by MnO₂-WK at around 64.5% and α-MnO₂ at the lowest level of 62.8%. Moreover, the spectra of Mn 3s (Figure 4b) showed two peaks at 83.7–84.4 eV and 88.2–89.1 eV. Further, the average oxidation state (AOS) of Mn can be calculated from these peaks according to the equation: AOS = 8.956–1.126 ΔEs, where ΔEs is the difference in the binding energy between the two peaks. Among these, MnO₂-WN exhibited the lowest AOS value (3.42), indicating that MnO₂-WN possessed a lower-valent Mn than the other catalysts. The order of increase in the AOS of the catalysts was consistent with that of the Mn⁴⁺ content. A higher proportion of Mn³⁺ is indicative of a greater propensity to induce Jahn-Teller distortion, thereby facilitating charge transfer and forming defect sites [31]. Defect sites typically promote the formation of active oxygen species. Consequently, a higher proportion of Mn³⁺ implies a higher concentration of active sites, which is advantageous for catalytic reactions [32]. For this reason, MnO₂-WN displayed the best catalytic activity. However, the Mn valence distribution of the spent catalyst (MnO₂-WN-S) for the catalytic oxidation of acetone did not change much compared to the fresh catalyst, implying a stable electron exchange, which favored a continuous and efficient reaction.

The O 1s spectra of all three catalysts revealed two characteristic peaks at approximately 531.7 eV and 529.9 eV, corresponding to adsorbed oxygen species (O_ads) and lattice oxygen species (O_latt), respectively (see Figure 4c) [23]. As shown in Table 3, the proportion of chemisorbed oxygen species in MnO₂-WN and MnO₂-WK was significantly higher than that of α-MnO₂. Typically, the chemisorbed oxygen included active oxygen species such as superoxide radicals and hydroxyl radicals, which improve their catalytic activity [33]. Moreover, it was also surprising to observe that the binding energy of chemisorbed oxygen in the MnO₂-WN, MnO₂-WK, and α-MnO₂ catalysts increased successively, indicating that the methods of microwave hydrothermal and nitrogen annealing weaken the interaction between Mn and surface O atoms, reducing the escape energy of oxygen and eventually increasing oxygen mobility [34]. Therefore, the highest chemisorbed oxygen ratio and the lowest binding energy led to excellent catalytic activity of MnO₂-WN. Notably, the MnO₂-WN-S had a substantially higher content of O_ads compared to the fresh catalyst, which might be related to the oxygen radicals generated during acetone decomposition. A high O_ads content promotes the reaction, contributing to MnO₂-WN maintaining high activity for a long time during the catalytic process.

The H₂-TPR test was performed to examine the reductive performance of the catalysts (see Figure 5a). All the catalysts exhibited only one reduction peak in the temperature range of 200–400 °C. The peak originated from two processes: the reduction from Mn⁴⁺ to Mn³⁺ and from Mn³⁺ to Mn²⁺ [35]. Notably, compared to α-MnO₂, the adsorption peak of the MnO₂-WN and MnO₂-WK shifted significantly to the left, from 375 °C to 345 °C and 247 °C, respectively. The shift in the adsorption peak was accompanied by an increase in hydrogen consumption (see

Table 4. Uptake of H₂ and O₂ by synthesized catalysts (semi-quantitative, expressed by peak area).

Catalyst	H2	O2
α-MnO2	16674491	6596
MnO2-WK	17237837	8896
MnO2-WN	17549969	9119

). Thus, while the microwave hydrothermal method enhanced the low-temperature reductive performance of the catalysts, calcination in the nitrogen atmosphere further strengthened this effect. Typically, excellent redox ability has been reported to lead to excellent catalytic performance, which was consistent with the activity test results. CO₂ is the primary byproduct of the catalytic oxidation of most VOCs, and its desorption from the catalyst surface plays a pivotal role in advancing the reaction. Figure 5b shows the CO₂ desorption abilities of the catalysts at different temperatures, as tested by CO₂-TPD. At both low and high temperatures, MnO₂-WN and MnO₂-WK exhibited greater CO₂ desorption peak areas than α-MnO₂. Furthermore, MnO₂-WN demonstrated the lowest initial desorption peak temperature, implying that it had the highest desorption capacity at low temperatures. Thus, the results suggest that the synergistic effect of microwave hydrothermal synthesis and N₂ calcination bolstered the CO₂ desorption capacity, fostering a positive feedback loop in the reaction and enhancing the catalytic activity.

The acidity on the catalyst surface was investigated by NH₃-TPD experimentation (see Figure 5c). The NH₃-TPD curve can be partitioned into three segments based on their acidity: 1. <280 °C signifies weak acid sites; 2. 280–650 °C denotes moderate acid sites; and 3. >650 °C represents strong acid sites [36]. The initial distinctive peak of all three catalysts was at approximately 160 °C. The peak strengths of the two catalysts synthesized via the microwave hydrothermal method markedly surpassed those of α-MnO₂. Notably, the MnO₂-WN peak in the weak acid region shifted toward lower temperatures, indicating a stronger weak acidity. Typically, weak acid sites participate in chemical bond cleavage during the reaction. Therefore, a weaker acidity would aid the deep oxidation of acetone [37]. There were no strong signals in the moderate acid range for the three catalysts, indicating a weaker moderate acidity. At temperatures close to 800 °C, all three catalysts exhibited conspicuous desorption peaks. Analogous to the weak acidity range, catalysts fabricated through the microwave hydrothermal method showed heightened peak intensities, suggesting more robust, strong acidity. Strong acid sites would also facilitate acetone elimination due to their significant stability. Subsequently, the distribution of oxygen species in the catalyst was examined via O₂-TPD (see Figure 5d). Generally, the oxygen species depicted on the spectrum can be categorized as chemisorbed oxygen (O_ads, temperature < 400 °C), surface lattice oxygen (O_latt, 400 °C < temperature < 600 °C), and bulk lattice oxygen (bulk O_latt, temperature > 600 °C) [38]. Evidently, MnO₂ prepared by the microwave hydrothermal method exhibited a lower peak temperature and a larger peak area in the region below 400 °C. N₂ calcination further reduced the peak temperature, indicating a higher abundance and easier desorption of O_ads, consistent with the XPS data. O_ads have been reported to be the primary active species involved in the catalytic oxidation of acetone. Furthermore, the peak temperatures of O_latt in the spectra of MnO₂-WK and MnO₂-WN were lower than those of α-MnO₂, indicating an easier release of surface lattice oxygen, which generated more active species and promoted catalytic oxidation.

The surface defect sites of the catalysts were explored by collecting the EPR of MnO₂-WN and MnO₂-WK (see Figure 6a). The EPR spectra of the two catalysts exhibited similar profiles, with a sharp and robust EPR signal at g = 2.004, primarily associated with the Zeeman effect of unpaired electrons captured on oxygen vacancies [33]. Thus, the data indicated the presence of a certain quantity of oxygen vacancies in both catalysts [39]. However, because of a higher concentration of oxygen vacancies, the signal from MnO₂-WK had a stronger intensity. Thus, MnO₂-WK also contained the greatest amount of O_ads since the abundance of oxygen vacancies is proportional to the content of O_ads. Moreover, the reduced oxygen vacancies in MnO₂-WN can be attributed to the enhanced crystallinity resulting from N₂ calcination, which is consistent with the results of XRD.

Raman spectra were used to gain insights into the crystal structure of MnO₂-WK and MnO₂-WN (Figure 6b) [31]. Notably, the spectra of the two catalysts were very similar, both having an intense characteristic peak near 630 cm⁻¹, which could be attributed to the Mn-O stretching vibration. Meanwhile, both catalysts showed a weak characteristic peak near 580 cm⁻¹, corresponding to the deformation mode of the Mn-O-Mn chain. Both characteristic peaks were attributed to the stretching mode of the [MnO₆] octahedron unit [40]. According to the previous studies, the stretching mode of Mn-O corresponded to a higher peak intensity, implying a higher degree of crystal ordering, consistent with the XRD results [41]. Meanwhile, the high similarity between the spectra confirmed that N₂ calcination has little effect on the crystal structure.

3.4. Reaction Path

The intermediate products, as well as the acetone degradation pathway on MnO₂-WN, were tested by in situ DRIFTS (Figure 7). The acetone adsorption at 50 °C in an anaerobic environment and the catalytic oxidation of acetone at different temperatures in an aerobic environment were tested. Figure 7a illustrates the vibrational characteristics of various species during acetone adsorption in an anaerobic environment. The vibrations at 1702, 1652, and 1542 cm⁻¹ could be attributed to the stretching vibration of C=O, while the symmetric and asymmetric deformation vibrations of the methyl (-CH₃) groups in acetone bands caused the bands at 1365 cm⁻¹ and 1318 cm⁻¹. Additionally, the band at 1232 cm⁻¹ might be attributed to the stretching vibration of the carbon skeleton during acetone adsorption [42,43], confirming that acetone was well adsorbed on MnO₂-WN. Moreover, the absence of bands corresponding to the vibrations of other species implied that there was no acetone oxidation in the anaerobic environment. This inference underlined the significance of surface-adsorbed oxygen as the principal oxygen species involved in the reaction. Upon the introduction of oxygen, the spectrum shifted from that of acetone adsorption to catalytic oxidation of acetone (see Figure 7b). With increasing temperature, the spectral bands generated from acetone adsorption gradually diminished, giving rise to numerous new bands, indicative of the production of new intermediate products from acetone oxidation. The bands at 1575 cm⁻¹ and 1348 cm⁻¹ could be attributed to the stretching vibrations of formate species, while the band at 1441 cm⁻¹ could be ascribed to the accumulation of acetate species [43]. Thus, the data indicates that the intermediates engendered during the catalytic oxidation of acetone were formate and acetate species, ultimately transforming them into H₂O and CO₂. Figure 8 depicts the plausible reaction pathway.

4. Conclusions

This study evaluated the catalytic performance of different crystal forms of MnO₂ towards acetone degradation. The α-MnO₂ was regulated by different hydrothermal methods or different calcination atmospheres to further enhance the catalytic performance. Additionally, the relationship between surface properties, activity, and reaction pathways was thoroughly investigated. The primary findings are outlined as follows: Among the different MnO_x crystal forms, α-MnO₂ exhibited the most superior catalytic performance for acetone degradation. Through microwave hydrothermal synthesis and N₂ calcination, the catalytic activity of α-MnO₂ was significantly enhanced. Under the reaction conditions of 100 ppm acetone and 10% O₂ balanced with N₂, WHSV = 60,000 mLg⁻¹ h⁻¹—the T₉₀ of MnO₂-WN reached 132 °C. MnO₂-WN also exhibited excellent stability and catalytic potential towards other common VOCs. The outstanding catalytic performance of α-MnO₂ was attributed to a larger specific surface area, better pore structure, better redox ability, abundant acid sites, and generating reactive oxygen species. The reaction pathway of acetone on the surface of MnO₂-WN involved the initial conversion of acetone to formate and acetate, which were further converted to H₂O and CO₂.