Removal of VOCs by Ozone: n -Alkane Oxidation under Mild Conditions

: Volatile organic compounds (VOCs) have a negative effect on both humans and the environment; therefore, it is crucial to minimize their emission. The conventional solution is the catalytic oxidation of VOCs by air; however, in some cases this method requires relatively high temperatures. Thus, the oxidation of short-chain alkanes, which demonstrate the lowest reactivity among VOCs, starts at 250–350 ◦ C. This research deals with the ozone catalytic oxidation (OZCO) of alkanes at temperatures as low as 25–200 ◦ C using an alumina-supported manganese oxide catalyst. Our data demonstrate that oxidation can be signiﬁcantly accelerated in the presence of a small amount of O 3 . In particular, it was found that n -C 4 H 10 can be readily oxidized by an air/O 3 mixture over the Mn/Al 2 O 3 catalyst at temperatures as low as 25 ◦ C. According to the characterization data (SEM-EDX, XRD, H 2 -TPR, and XPS) the superior catalytic performance of the Mn/Al 2 O 3 catalyst in OZCO stems from a high concentration of Mn 2 O 3 species and oxygen vacancies.


Introduction
Volatile organic compounds (VOCs) are a large group of chemical compounds with boiling points below 260 • C; they include alkanes, alkenes, aromatic hydrocarbons, aldehydes, ketones, alcohols, etc. [1]. VOCs are considered major air pollutants because of their significant contribution to the formation of photochemical smog, tropospheric O 3 and secondary aerosols [2][3][4][5]. VOCs negatively affect human health because of their toxic, malodorous, mutagenic and carcinogenic nature [6]. The main anthropogenic sources of VOCs are vehicle exhaust fumes and emissions from chemical and power plants, petroleum refining, food and textile manufacturing, etc. [7]. Rapid urbanization and industrialization contribute to the growing emissions of VOCs into the environment. Therefore, the development of effective methods and materials for the abatement of VOCs is of significant importance.
Among various OZCO catalysts, supported MnO x demonstrates excellent VOC oxidation performance [22][23][24]. Einaga et al. compared M/Al 2 O 3 and M/SiO 2 (M = Mn, Fe, Co, Ni, Cu) according to their catalytic performances in OZCO [25,26]. It has been observed that Mn-containing catalysts are the most active for benzene and cyclohexane oxidation. Huang et al. investigated ozone catalytic oxidation of benzene over Mn, Co, Cu, Ni, Zn and Ce supported on ZSM-5 [27]. It has been shown that MnO 2 /ZSM-5 exhibits the best activity and CO 2 selectivity (100% and 84.7%, respectively) at room temperature. Gopi et al. studied the OZCO of toluene over Mn, Ce, Cu, Ag and Co metal oxides supported on 13X zeolite [28]. It was revealed that the activity of the catalysts decreases in the following order: Mn/13X > Ce/13X > Cu/13X > Ag/13X > Co/13X. Chen et al. studied the ozonation of chlorobenzene over manganese supported on different carriers (Al 2 O 3 , TiO 2 , SiO 2 , CeO 2 , and ZrO 2 ) to reveal the effect of carrier on the catalytic performance of MnO x in OZCO [29]. The authors found that Mn/Al 2 O 3 exhibited the highest efficiency in chlorbenzene abatement.
The high efficiency of supported MnO x catalysts in the OZCO of VOCs can be attributed to their ability in terms of O 3 decomposition [30]. It has been reported that highly reactive oxygen species such as atomic oxygen, peroxide, and OH radicals are formed (Equations (1)-(4)) on the surface of metal oxides as a result of O 3 decomposition [30][31][32][33]. These species can completely oxidize adsorbed VOCs to CO 2 , even at room temperature (RT).
Recently, it was shown that OZCO may be considered a promising solution for the abatement of methane, which is difficult to destroy compared to alkenes and aromatics [34,35]. Thus, the conversion of methane achieved by catalytic oxidation using an air/O 3 mixture is almost four times higher than that using air only [35]. However, to the best of our knowledge, there are no available data on ozone catalytic oxidation of alkanes, which are the dominant VOCs generated by the petroleum industry [36,37]. Alkanes are also one of the major components among VOCs detected in vehicle exhaust [38]. Therefore, the aim of the present work was to investigate the possibility of ozone catalytic oxidation of n-butane (selected as a representative short-chain VOC molecule) at temperatures as low as 25-200 • C using an alumina-supported manganese oxide catalyst. Figure 1 shows the results of scanning electron microscopy (SEM) with energydispersive X-ray (EDX) spectroscopy analysis performed for the parent Al 2 O 3 and the Mn/Al 2 O 3 catalyst. The alumina ( Figure 1a) has a relatively smooth surface. After loading 10 wt% Mn, the surface becomes rough with a number of defects ( Figure 1b). The metal content is 9.4 wt% (Figure 1c), which is in a good agreement with data for inductively coupled plasma optical emission spectroscopy (ICP-OES) (see Section 4.1). The EDX mapping ( Figure 1d) shows that Mn species are uniformly distributed on the catalyst surface.

Catalyst Characterization
X-ray powder diffraction (XRD) was used to identify the main MnO x phases of the Mn/Al 2 O 3 catalyst. Figure 2 shows XRD patterns of the parent alumina and the catalyst. The characteristic reflections of γ-Al 2 O 3 (2θ = 37 • , 46 • and 67 • ) are detected in both spectra, which is in a good agreement with the published literature [23,29]. In the Mn/γ-Al 2 O 3 pattern, typical reflexes of MnO 2 at 28.7 • , 37.3 • , 42.7 • and 56.6 • and Mn 2 O 3 at 32.7 • are observed, indicating that the two phases coexist [22,29]. The average particle sizes calculated from the diffraction peaks at 37.3 • (MnO 2 ) and 32.7 • (Mn 2 O 3 ) using the Debye-Scherrer equation are 19.0 nm and 9.0 nm, respectively.
The redox properties of Mn/Al 2 O 3 were investigated using temperature programmed reduction (H 2 -TPR). A reduction profile composed of two peaks was observed (Figure 3a). The first peak, with a higher intensity at approximately 220-400 • C (T max = 325 • C), corresponds to the reduction of MnO 2 to Mn 2 O 3 . The second peak, with T max = 431 • C, is attributed to the reduction of Mn 2 O 3 to MnO [39].   The redox properties of Mn/Al2O3 were investigated using temperature programmed reduction (H2-TPR). A reduction profile composed of two peaks was observed ( Figure 3a). The first peak, with a higher intensity at approximately 220-400 °С (Tmax = 325 °С), corresponds to the reduction of MnO2 to Mn2O3. The second peak, with Тмах = 431 °С, is attributed to the reduction of Mn2O3 to MnO [39].
To determine the valence states of the Mn atoms, the Mn/Al2O3 catalyst was examined by X-ray photoelectron spectroscopy (XPS). In the Mn 2p spectra (Figure 3b) of Mn/Al2O3, the peaks at 642.4, 641.8 and 640.8 eV were assigned to Mn 4+ , Mn 3+ and traces of Mn 2+ , respectively [40][41][42]. The contents of Mn 3+ and Mn 4+ in the Mn/Al2O3 catalyst were approximately 69% and 31%, respectively.   The redox properties of Mn/Al2O3 were investigated using temperature programmed reduction (H2-TPR). A reduction profile composed of two peaks was observed ( Figure 3a). The first peak, with a higher intensity at approximately 220-400 °С (Tmax = 325 °С), corresponds to the reduction of MnO2 to Mn2O3. The second peak, with Тмах = 431 °С, is attributed to the reduction of Mn2O3 to MnO [39].
To determine the valence states of the Mn atoms, the Mn/Al2O3 catalyst was examined by X-ray photoelectron spectroscopy (XPS). In the Mn 2p spectra (Figure 3b) of Mn/Al2O3, the peaks at 642.4, 641.8 and 640.8 eV were assigned to Mn 4+ , Mn 3+ and traces of Mn 2+ , respectively [40][41][42]. The contents of Mn 3+ and Mn 4+ in the Mn/Al2O3 catalyst were approximately 69% and 31%, respectively.

Catalytic Activity
Three series of experiments were performed on the Mn/Al2O3 catalyst and the parent alumina: (a) n-C4H10 oxidation by air, (b) n-C4H10 oxidation by an air/O3 mixture (OZCO), and (c) O3 decomposition. In order to estimate the possible contribution of the gas-phase To determine the valence states of the Mn atoms, the Mn/Al 2 O 3 catalyst was examined by X-ray photoelectron spectroscopy (XPS). In the Mn 2p spectra (Figure 3b) of Mn/Al 2 O 3 , the peaks at 642.4, 641.8 and 640.8 eV were assigned to Mn 4+ , Mn 3+ and traces of Mn 2+ , respectively [40][41][42]. The contents of Mn 3+ and Mn 4+ in the Mn/Al 2 O 3 catalyst were approximately 69% and 31%, respectively.

Catalytic Activity
Three series of experiments were performed on the Mn/Al 2 O 3 catalyst and the parent alumina: (a) n-C 4 H 10 oxidation by air, (b) n-C 4 H 10 oxidation by an air/O 3 mixture (OZCO), and (c) O 3 decomposition. In order to estimate the possible contribution of the gas-phase reactions, blank tests using an empty reactor were carried out.

n-C 4 H 10 Oxidation
The blank oxidation reactions were carried out by air and air/O 3 mixture. The results obtained are summarized in Figure 4a. Evidently, the gas-phase oxidation of n-C 4 H 10 by air does not proceed, even at temperatures as high as 350 • C. Contrarily, OZCO starts at 125 • C, and 80-90% conversion of n-C 4 H 10 is achieved at 290-350 • C.
Over the Mn/Al 2 O 3 catalyst, n-C 4 H 10 can be oxidized by air; however, oxidation starts only at 225-250 • C, and at 350 • C the conversion does not exceed 20% ( Figure 4a). In the course of OZCO, n-C 4 H 10 is oxidized on the Mn/Al 2 O 3 catalyst at 25 • C. Initially the conversion increases to 70% with the temperature and levels off at 85-200 • C (i.e., a plateau). At 200-250 • C, the conversion gradually decreases to~55%; however, at 250-350 • C, it rises again to 85%.
It is known that the oxidation reaction may proceed not only on Mn-containing sites but also on the carrier surface. For this reason it was informative to evaluate the contribution of the parent Al 2 O 3 (Figure 4a). The reaction between n-C 4 H 10 and air does not proceed even at 350 • C. In the presence of O 3 the alkane is oxidized at 100 • C; however, the conversion is much lower than on Mn/Al 2 O 3 . This observation allows us to conclude that the activity of alumina surface in OZCO is negligible compared to MnO x species.

O3 Decomposition
Since the enhancement of the oxidation efficiency in the presence of O3 is attributable to a formation of highly reactive O atoms [29,32,33], catalytic O3 decomposition (over Mn/Al2O3 or Al2O3) as well as gas-phase decomposition (in the empty reactor) was also studied in detail. The results obtained are shown in Figure 5. The gas-phase O3 decomposition proceeds at relatively high temperatures (150-350 °C). In contrast, the reaction over the Mn/Al2O3 catalyst occurs at 25 °C, and the complete conversion of O3 is achieved at 250 °C. On the parent alumina, O3 decomposition starts only at temperatures above 100 °C. Analysis of the reaction products reveals that n-C 4 H 10 is completely oxidized by air over Mn/Al 2 O 3 ; CO 2 selectivity is almost 100% (Figure 4b). When n-C 4 H 10 reacts with both air and O 3 simultaneously, significant amounts of CO are formed. Thus, in the course of the gas-phase process and OZCO over the parent alumina, CO 2 selectivity gradually decreases with the increase in temperature, and at 350 • C it becomes 66% and 44%, respectively. In contrast, when n-C 4 H 10 is oxidized by the air/O 3 mixture over the Mn/Al 2 O 3 catalyst, CO mostly forms at relatively low temperatures (150-250 • C), and CO 2 selectivity is ca. 70%. At 350 • C the selectivity rises to ca. 85%. Other byproducts as well as a carbon deposition on the catalyst surface were not detected, which is in good agreement with the high (>99%) carbon balance.

O 3 Decomposition
Since the enhancement of the oxidation efficiency in the presence of O 3 is attributable to a formation of highly reactive O atoms [29,32,33], catalytic O 3 decomposition (over Mn/Al 2 O 3 or Al 2 O 3 ) as well as gas-phase decomposition (in the empty reactor) was also studied in detail. The results obtained are shown in Figure 5. The gas-phase O 3 decomposition proceeds at relatively high temperatures (150-350 • C). In contrast, the reaction over the Mn/Al 2 O 3 catalyst occurs at 25 • C, and the complete conversion of O 3 is achieved at 250 • C. On the parent alumina, O 3 decomposition starts only at temperatures above 100 • C.

O3 Decomposition
Since the enhancement of the oxidation efficiency in the presence of O3 is attributable to a formation of highly reactive O atoms [29,32,33], catalytic O3 decomposition (over Mn/Al2O3 or Al2O3) as well as gas-phase decomposition (in the empty reactor) was also studied in detail. The results obtained are shown in Figure 5. The gas-phase O3 decomposition proceeds at relatively high temperatures (150-350 °C). In contrast, the reaction over the Mn/Al2O3 catalyst occurs at 25 °C, and the complete conversion of O3 is achieved at 250 °C. On the parent alumina, O3 decomposition starts only at temperatures above 100 °C.

The Effect of O3/n-C4H10 Ratio on the Oxidation Efficiency
The effect of O3 concentration on the n-C4H10 conversion on the Mn/Al2O3 catalyst was studied using feed gas containing different concentrations of O3 (0-800 ppm) while keeping the n-butane concentration constant (100 ppm). Thus, the O3/n-C4H10 ratio varies from 0 to 8. The results are summarized in Figure 6. The increase of n-butane conversion with O3 concentration is observed. The complete conversion is obtained at 125 °C using an O3/n-C4H10 ratio of eight.

The Effect of O 3 /n-C 4 H 10 Ratio on the Oxidation Efficiency
The effect of O 3 concentration on the n-C 4 H 10 conversion on the Mn/Al 2 O 3 catalyst was studied using feed gas containing different concentrations of O 3 (0-800 ppm) while keeping the n-butane concentration constant (100 ppm). Thus, the O 3 /n-C 4 H 10 ratio varies from 0 to 8. The results are summarized in Figure 6. The increase of n-butane conversion with O 3 concentration is observed. The complete conversion is obtained at 125 • C using an O 3 /n-C 4 H 10 ratio of eight.

Structure of Mn/Al2O3 Catalyst
The characterization data suggest that the Mn/Al2O3 catalyst contains a mixture of manganese oxides. The XRD pattern (Figure 2) shows pronounced peaks of MnO2 along with a weak peak of Mn2O3, indicating that both oxide phases coexist in the catalyst

Structure of Mn/Al 2 O 3 Catalyst
The characterization data suggest that the Mn/Al 2 O 3 catalyst contains a mixture of manganese oxides. The XRD pattern (Figure 2) shows pronounced peaks of MnO 2 along with a weak peak of Mn 2 O 3 , indicating that both oxide phases coexist in the catalyst [22,29]. The broadness of the MnO 2 peaks allows us to suggest that the oxide particles have poor crystalline structure and may contain structural defects, including oxygen vacancies. Low intensity of the Mn 2 O 3 peak indicates that the oxide phase is well dispersed on alumina surface.
It is known that the structural defects of MnO 2 , such as oxygen vacancies, also favor OZCO due to their activity in O 3 decomposition. According to the mechanism proposed in the literature [30,[44][45][46][47], O 3 molecules are adsorbed on the oxygen vacancies to form highly reactive oxygen species (O 2− , O 2 2− , O − ), which readily react with VOC molecules. Thus, it has been reported that the high catalytic activity of the Mn-Ag/HZSM-5 for toluene oxidation by ozone is attributed mainly to the abundant oxygen vacancies presented on the catalyst surface [48].

Catalytic Activity
It was shown that oxidation of n-C 4 H 10 is significantly accelerated in the presence of small amounts of O 3 (Figure 4a). The pronounced positive effect of O 3 addition is observed in the both gas-phase and catalytic reactions. Thus, the oxidation of n-C 4 H 10 by air in the gas-phase and on the parent alumina does not proceed, even at temperatures as high as 350 • C. On the Mn/Al 2 O 3 catalyst, only a minor conversion is observed. O 3 promotes oxidation in the gas-phase and on the alumina; the OZCO starts at 100-125 • C, and near-complete conversion is achieved at 300-350 • C. The effect of O 3 is significantly more pronounced when the reaction proceeds on the Mn/Al 2 O 3 catalyst. In this case, the light-off temperature of the n-C 4 H 10 oxidation decreases from 250 • C to 25 • C.
It was found that the behavior of n-butane oxidation is correlated with the O 3 decomposition process. Thus, it was observed that O 3 readily decomposes over the Mn/Al 2 O 3 catalyst at 25 • C, the same temperature the n-C 4 H 10 oxidation by air/O 3 mixture begins (Figures 4a and 5, respectively). Similar parallels of light-off temperatures have been observed when O 3 decomposition and n-C 4 H 10 oxidation proceed in the gas-phase (at 150 • C) or over the alumina surface (at 100 • C). The results obtained are consistent with the published data [49]. Thus, it was found that at low temperatures the activation energy of benzene oxidation by O 3 is similar to that of O 3 decomposition, which suggests that O 3 decomposition can be considered as the rate-determining step of the oxidation process.
Comparative analysis of the conversion profiles obtained during the n-C 4 H 10 OZCO and O 3 decomposition (Figures 4a and 5) allows us to suggest that the alkane is oxidized over Mn/Al 2 O 3 in two stages. In the first stage (25-250 • C) the reaction proceeds mainly over MnO x species, because the contribution of the alumina and the gas-phase reaction is negligible. The n-C 4 H 10 is readily oxidized by O 3 ; however, the maximum conversion does not exceed 70% at 85-200 • C. One of the possible reasons of the limitation is the consumption of active atomic oxygen species in the reaction with O 3 (Equation (2)), which results in O 2 formation [24,30,31]. Apparently, the contribution of this reaction becomes more pronounced at temperatures of 200-250 • C, and n-C 4 H 10 conversion decreases to~55%.
At 250-350 • C, n-C 4 H 10 conversion rises again and reaches 85%. Comparison of the conversion profiles obtained in the gas-phase and over the Mn/Al 2 O 3 catalyst allows us to suggest that at higher temperatures the gas-phase n-C 4 H 10 oxidation by air/O 3 mixture becomes the dominant catalytic pathway (Figure 4a). In addition, the n-C 4 H 10 oxidation by air over Mn/Al 2 O 3 may contribute at temperatures above 250 • C (Figure 4a). This fact is indicated by the higher CO 2 selectivity as compared to the selectivity in the course of the gas-phase process and OZCO over the parent alumina (Figure 4b).
It has been shown that higher O 3 concentration enhances the alkane oxidation efficiency ( Figure 6). Thus, as the O 3 /n-C 4 H 10 ratio increases from two to eight, the maximum n-C 4 H 10 conversion obtained at low temperature (25-220 • C) rises from 43% to 100%. At the same time the plateau becomes wider, the decrease of conversion becomes less significant. According to the literature [24,50], both results are due to the increased availability of the highly reactive oxygen species for the oxidation of n-C 4 H 10 over MnO x species.

Catalyst Preparation
The 10%Mn/Al 2 O 3 catalyst was prepared by incipient wetness impregnation of commercial Al 2 O 3 carrier (SASOL, Sandton, South Africa; S BET = 150 m 2 /g, calcined at 550 • C for 4 h) by an appropriate amount of an aqueous solution of manganese (II) nitrate tetrahydrate (≥97%, Sigma Aldrich, St. Louis, MO, USA). The catalyst was dried at room temperature overnight and calcined in air flow (550 • C, 3 h).
Mn content in the resultant Mn/Al 2 O 3 catalyst was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Perkin Elmer OPTIMA 2000 instrument (PerkinElmer Inc., Wellesley, MA, USA) and found to be 9.8 wt%.

Catalyst Characterization
The morphologies of the samples were studied using a Hitachi SU8000 field-emission scanning electron microscope (Hitachi, Tokyo, Japan). Before measurements the samples were mounted on a 25 mm aluminum specimen stub and fixed by conductive graphite adhesive tape. Images were acquired in secondary electron mode at a 2-20 kV accelerating voltage and at a working distance of 8-10 mm. The energy dispersive X-ray spectroscopy (EDX) of the samples was tested on the same SEM instrument equipped with an Oxford Instruments X-max EDX system (Oxford Instruments, Abingdon, OX, UK) to analyze the element distributions on the surface.
The crystallographic structure of the as-prepared catalysts was characterized by X-ray powder diffraction (XRD) using a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with Bragg-Brentano geometry, Ni-filtered CuKα radiation, and LYNXEYE detector. The XRD patterns were recorded in the 2θ range 5-75 • (scan rate 1.2 • /min). Crystallographic parameters were calculated using the Rietan-FT software, which uses the Rietveld method. The X-ray photoelectron spectroscopy (XPS) spectra of the samples were recorded on Axis Ultra DLD spectrometer (Kratos Analytical Limited, Manchester, UK) with a monochromatic Al Kα source (1486.6 eV, 150 W). The binding energies were calibrated with reference to the standard binding energy of contaminant carbon (C1s 284.5 eV).

Catalytic Tests
A scheme of the experimental setup is shown in Figure 7. N-butane was supplied from a 0.98% n-C 4 H 10 /N 2 cylinder (Linde Gas Rus, Balashikha, Russia) balanced with N 2 . O 3 was generated from air using a Triozon OG-A0.5 ozone generator (TriOzon, Moscow, Russia). The concentrations of n-butane and O 3 were 100 and 450 ppm, respectively (unless otherwise stated). The total gas flow rate was 750 mL/min, with a corresponding gas hourly space velocity (GHSV) of 155.000 h −1 . The gas mixture passed through a reactor (a quartz tube with an inner diameter of 10 mm) with the catalyst inside (0.2 g, fraction 0.4-1.0 mm). Catalyst activity (conversion) was studied as a function of temperature ranging from 25 to 350 °C (temperature ramp of 5 °C/min). The conversions (X) of n-C4H10 and O3 were calculated using the following equation: where Сin and Сout are the inlet and outlet concentrations of n-C4H10 and O3, respectively. Since only COx (CO, CO2) were detected as the carbon-containing products of n-C4H10 oxidation, CO2 formation was selected as a criterion for evaluating the effectiveness of the reaction. The CO2 selectivity was calculated by the following equations: where Сin and Сout are the inlet and outlet concentrations of n-C4H10 and CO2, respectively; m is the number of carbon atoms in the hydrocarbon (for n-C4H10, m = 4). All carbon balances were in the range (100 ± 5)%.

Conclusions
The effect of O3 on the efficiency of alkane oxidation was studied in details. It was shown for the first time that it is possible to oxidize short-chain VOCs by air/O3 mixture at ambient temperatures. It was observed that n-C4H10 reacted readily with O3 over the Mn/Al2O3 catalyst at temperatures as low as 25 °C, and 80-100% conversion could be achieved in the temperature range 70-200 °C (O3/n-C4H10 = 8).
Comparison of the catalytic and characterization data (SEM-EDX, XRD, H2-TPR, and The ozone concentration before and after the reactor was measured with an UV-100 ozone analyzer (Eco Sensors Division of KWJ Engineering Inc., Newark, CA, USA) in the range of 0-999 ppm with ±2% accuracy. For the correct analysis of the carbon-containing reaction products, an ozone trap with a saturated aqueous solution of KI was placed at the reactor outlet. An FTIR spectrometer Gasmet DX 4000 (Temet Instruments Oy, Helsinki, Finland) was used to detect concentrations of CO, CO 2 , and n-C 4 H 10 .
Catalyst activity (conversion) was studied as a function of temperature ranging from 25 to 350 • C (temperature ramp of 5 • C/min). The conversions (X) of n-C 4 H 10 and O 3 were calculated using the following equation: where C in and C out are the inlet and outlet concentrations of n-C 4 H 10 and O 3 , respectively. Since only CO x (CO, CO 2 ) were detected as the carbon-containing products of n-C 4 H 10 oxidation, CO 2 formation was selected as a criterion for evaluating the effectiveness of the reaction. The CO 2 selectivity was calculated by the following equations: CO 2 selectivity(%) = C(CO 2 ) out (C(C 4 H 10 ) in − C(C 4 H 10 ) out ) × m × 100%, where C in and C out are the inlet and outlet concentrations of n-C 4 H 10 and CO 2 , respectively; m is the number of carbon atoms in the hydrocarbon (for n-C 4 H 10 , m = 4).

Conclusions
The effect of O 3 on the efficiency of alkane oxidation was studied in details. It was shown for the first time that it is possible to oxidize short-chain VOCs by air/O 3 mixture at ambient temperatures. It was observed that n-C 4 H 10 reacted readily with O 3 over the Mn/Al 2 O 3 catalyst at temperatures as low as 25 • C, and 80-100% conversion could be achieved in the temperature range 70-200 • C (O 3 /n-C 4 H 10 = 8).
Comparison of the catalytic and characterization data (SEM-EDX, XRD, H 2 -TPR, and XPS) revealed that superior catalytic performance of the Mn/Al 2 O 3 catalyst in OZCO was due to high concentration of Mn 2 O 3 species and oxygen vacancies.
The data obtained indicated that the oxidation pathway depends on the reaction temperature. Thus, at low temperatures n-C 4 H 10 was oxidized by active oxygen, which was formed via O 3 decomposition over MnO x species, while at 250-350 • C the contributions of the gas-phase oxidation by air/O 3 mixture and conventional oxidation by air over Mncontaining active centers became significant.
The findings of this study may contribute to the development of the efficient technologies for the low-temperature abatement of the short-chain hydrocarbons.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.