English

• Volatile organic compounds (VOCs) have detrimental effects on environment and human health, not only due to they are important precursors of fine particulate matter (PM_2.5) and ozone (O₃), but also because of their toxicity, mutagenesis and carcinogenicity [1, 2]. Catalytic oxidation is considered to be an efficient technique for VOCs abatement due to its advantages of high efficiency, low reaction temperature, low energy consumption and no secondary pollution, etc. [3]. Development of high efficiency and cost-effective non-noble metal catalyst for VOCs oxidation at low temperature is important.

  Moreover, in practical applications, powder catalysts have the problem of high pressure drop, difficult separation and easy to lose, which are difficult to be applied to real industrial production. Monolithic catalysts such as ceramic monoliths (mostly made of cordierite and currently the most used) can avoid these problems to a large extent [4]. Unfortunately, poor attachment stability between the honeycomb ceramic carrier and powder lead to the loss of surface-active species during the reaction process, which reduces the catalytic efficiency and limits the application. Besides, the ceramic supports also have some drawback such as low mechanical strength, uneven internal heat distribution and inhomogeneous distribution of surface catalyst powder.

  In order to overcome those shortcomings, metallic monoliths catalysts with high mechanical strength and plasticity, good thermal conductivity and mass transfer ability, appropriate roughness and porosity were developed [5]. At present, the application of metal-supported monoliths catalysts mainly includes Ni foam, alloys, aluminum foil, and wire mesh, etc. Compared with other metallic monoliths catalysts, iron mesh (IM)-based catalysts have connected three-dimensional channel structure, which cannot only serve as catalyst support, but also provide iron atoms. In our previous work, IM-supported vertically aligned Co-Fe layered double oxides was prepared and compared with traditional cordierite-based catalysts. The IM based catalysts showed good catalytic activity and water resistance [6], suggesting IM can be a promising carrier for monolithic catalysts.

  In this work, in order to further improve the low temperature catalytic performance, manganese oxide was in-situ growth on the surface of IM by using one-step hydrothermal method, which can not only achieve the stable decoration of manganese precursor on the surface of IM, but also ensure the uniform dispersion of active components. The hydrothermal conditions, catalytic activity, thermal stability, water resistance, cycle performance and the toluene reaction process of the MnO₂/IM monolithic catalyst were investigated systematically.

  The crystalline phase of the catalysts was illustrated by X-ray diffraction (XRD) analyses (Fig. S1 in Supporting information). After growing MnO₂ on IM surface, the obtained MnO₂/IM-T monolithic catalysts with different hydrothermal temperature (Fig. S1a) depicted diffraction peaks at 2θ = 28.68°, 37.33°, 42.82° and 56.65° were corresponded to (110), (101), (111) and (211) lattice planes, respectively, which were consistent with β-MnO₂ (pyrolusite) crystalline phase (JCPDS No. 24-0735), suggesting that MnO₂ successfully grown on IM. Simultaneously, a part of the iron element in the IM were transformed into Fe₃O₄ through high temperature hydrothermal and calcination (JCPDS No. 75-0033). Similarly, MnO₂/IM-80-t and n-MnO₂/IM-80-12 catalysts were similar with those of MnO₂/IM-T (Figs. S1b and c), indicating that the hydrothermal conditions within the experimental range showed no significant effect on crystal structure of the monoliths samples.

  Scanning electron microscope (SEM) showed that the pure IM presented a rough surface (Fig. 1a), which is conducive to the growth of other substances. After hydrothermal reaction, the IM was coated with in situ grown iron and manganese oxides as shown in Figs. 1b–h. The morphologies of Fe and Mn oxides covered on the monolithic catalyst surface was mainly cubic structure and flake. The morphologies of the Fe and Mn oxides on the surface of IM were obviously different with different hydrothermal temperature conditions. Similarly, suitable hydrothermal time (t = 12 h) can make crystal phase develop completely, make the reaction more thorough and produce more cubic structure (Fig. S2 in Supporting information). Fig. S3 (Supporting information) also showed that the concentration of manganese ion in the hydrothermal solution play an important role in the development of cubic structure, and the best precursor concentration was 0.03 mol/L.

  Figure 1

  

  Figure 1.  (a) SEM images of pure iron mesh; SEM images of MnO₂/IM monolithic catalysts at hydrothermal temperature of (b) 60 ℃, (c) 80 ℃, (d) 100 ℃, (e) 120 ℃, (f) 140 ℃, (g) 160 ℃, (h) 180 ℃, (i) elemental mappings results of MnO₂/IM monolithic catalyst with hydrothermal temperature of 140 ℃.

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  For a clearer observation, the MnO₂/IM monolithic catalyst prepared at 140 ℃ was selected for Energy dispersive spectrometer (EDS) mapping as shown in Fig. 1i. The cubic structure on IM surface was composed of rich Mn and O elements, and the vertical flake shape was composed of rich Fe and O elements, all the elementals were highly dispersed. Therefore, a layer of flake Fe₃O₄ was formed on the surface of the iron mesh, and then cubic structure MnO₂ were grown on top of it.

  Fig. S4 (Supporting information) shows the hydrogen temperature programmed reduction experiments (H₂-TPR) curves of the catalysts prepared under different hydrothermal conditions. In general, MnO₂ was reduced in steps of Mn⁴⁺ → Mn³⁺ → Mn²⁺ [7]. For all samples, the reduction peak observed at lower temperature (280~400 ℃) was mainly the reduction process of Mn⁴⁺ to Mn³⁺and Mn³⁺ was further reduced to Mn²⁺. Significantly, compared with the catalysts prepared at other hydrothermal temperatures, the temperature required for Mn⁴⁺ reduction to Mn³⁺ moved to a lower direction for MnO₂/IM-80 monolithic catalyst (Fig. S4a). Also, among the samples prepared at different time, the reduction temperature for Mn⁴⁺ to Mn³⁺ of MnO₂/IM-12 and MnO₂/IM-24 samples were basically the same, but the intensity of the reduction peak of MnO₂/IM-12 samples was higher (Fig. S4b). At the same, the Mn⁴⁺ could be reduced to Mn³⁺ at ~280 ℃ for MnO₂/IM-0.03 sample, while the reduction temperature of others was approximately at 400 ℃. The reduction peaks observed at higher temperature (about 530 ℃) correspond to the co-reduction processes of Mn³⁺ → Mn²⁺ and Fe³⁺ → Fe²⁺ → Fe. And the reduction peak at about 600 ℃ was further reduction of Fe²⁺ → Fe.

  The surface Mn⁴⁺/(Mn³⁺ + Mn²⁺), Fe³⁺/Fe²⁺ and O_ads/O_latt were summarized in Table 1. The full X-ray photoelectron spectroscopy (XPS) spectrum of the samples indicated the presence of Mn, Fe and O elements. MnO₂/IM-80 showed the strongest Mn 2p peak and the weakest Fe 2p peak (Fig. S5a in Supporting information). The Mn 2p_3/2 spectra of the catalyst could be decomposed into three spin orbital lines with binding energies of 641.3, 641.7 and 643.4 eV, attributed to Mn²⁺, Mn³⁺, and Mn⁴⁺ species, respectively (Fig. S5b in Supporting information) [8]. It is well known that the coexistence of Mn⁴⁺ and Mn³⁺ facilitates the migration and conversion of electrons, which is beneficial to the redox properties of the samples, thus improving their catalytic activity [9]. The Mn⁴⁺/(Mn³⁺ + Mn²⁺) molar ratio decreased with the trend: MnO₂/IM-80 (0.76) > MnO₂/IM-100 (0.74) > MnO₂/IM-120 (0.57) > MnO₂/IM-60 (0.53). This sequence suggests that different hydrothermal temperatures affect the Mn⁴⁺/(Mn³⁺ + Mn²⁺) molar ratio on IM surface, further suggesting that the MnO₂/IM-80 catalyst has the best redox properties, which is consistent with the H₂-TPR results.

  Table 1

  

  Table 1.  Surface Mn, Fe, and O elements molar ratios for MnO₂/IM monolithic catalyst with different hydrothermal temperature.

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  The surface molar ratio of Mn⁴⁺/Mn³⁺ also further affects the lattice oxygen (O_latt) and adsorbed oxygen (O_ads) species on the MnO₂ surface, such as O⁻, O²⁻ and O₂²⁻. In Fig. S5c (Supporting information), the O 1s XPS spectrum of the samples can be divided into three peaks corresponding to lattice oxygen (O_latt, 530 ± 0.2 eV), surface adsorbed oxygen (O_ads, 531.3 ± 0.2 eV) and adsorbed OH groups and water molecule (O_OH, 533 eV) [10]. The O_ads/O_latt ration decreased in the order of MnO₂/IM-80 (0.52) > MnO₂/IM-120 (0.45) > MnO₂/IM-100 (0.43) > MnO₂/IM-60 (0.42). According to the relevant literature, surface adsorbed oxygen has higher mobility than lattice oxygen, which makes MnO₂/IM-80 more active in the reaction [11, 12].

  In addition, the Fe 2p peaks in Fig. S5d (Supporting information) could be split into four peaks and two satellite peaks. The peaks of Fe 2p_3/2 could be divided into two peaks of Fe³⁺ (712.9 eV) and Fe²⁺ (710.7 eV) [6]. This indicated that the iron in IM was oxidized to Fe₃O₄, which was consistent with the XRD results. Fe³⁺/Fe²⁺ molar ratio of the MnO₂/IM-80 (0.88) was higher than that of MnO₂/IM-60 (0.44), MnO₂/IM-100 (0.83), MnO₂/IM-120 (0.75). In conclusion, MnO₂/IM-80 has the highest number of O_ads and high-valent Mn⁴⁺ and Fe³⁺ elements, which can be presumed to have the best catalytic performance.

  The toluene catalytic activity showed a trend of increasing and then decreasing with the increasing of hydrothermal temperatures (Fig. 2), which was consistent with the XPS analysis described above. Different hydrothermal time and precursor concentration also had some effect on the toluene oxidation. The results can be concluded that the MnO₂/IM monolithic catalyst prepared at a hydrothermal temperature of 80 ℃, hydrothermal time of 12 h, and precursor manganese ion concentration of 0.03 mol/L had the best catalytic activity for toluene oxidation, which T_50% and T_90% (the temperature at 50% and 90% toluene conversion) were 252 ℃ and 265 ℃, respectively. Furthermore, combined with SEM-EDS analysis, it can be concluded that MnO₂ with a cubic morphology on the surface of IM is the main active component for toluene oxidation.

  Figure 2

  

  Figure 2.  Catalytic oxidation of toluene over MnO₂/IM monolithic catalysts at different (a) hydrothermal temperatures, (b) hydrothermal times and (c) precursor concentrations.

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  Additionally, Table S1 (Supporting information) summarizes the catalytic performance for toluene oxidation over different monolithic catalysts. Compared with the previous results, the MnO₂/IM monolithic catalyst prepared in this paper presents better catalytic activity of toluene when taking into account the different experimental conditions. The three-dimensional porous network of the IM matrix is beneficial to the formation of the active component of MnO₂, which not only shows good mass transfer performance for the reactant molecule, but also increases the contact between the reactant and the surface site [5]. Beside, it is well known that due to the exposure of active interfaces and enrichment of oxygen vacancies, MnO₂ can enhance the toluene oxidation activity [13]. In addition, the shedding rate of MnO₂/IM monolithic catalyst was only 0.14%, suggesting that this in situ growth technique makes the metal oxide bond firmly to the substrate, which is more conducive to practical industrial applications.

  The thermal stability of the best performance 0.03-MnO₂/IM-80-12 monolithic catalysts calcined at 400~700 ℃ was investigated (Fig. 3a). T_50% and T_90% of MnO₂/IM-400 was 252 ℃ and 265 ℃. When the calcination temperature was 500 ℃, 600 ℃ and 700 ℃, the T_50% increased to 305 ℃, 313 ℃ and 340 ℃ while T_90% increased to 332 ℃, 333 ℃ and 368 ℃, respectively, which probably due to partial sintering of the active component or loss on the surface of the catalyst [14].

  Figure 3

  

  Figure 3.  (a) Catalytic oxidation of toluene and (b) XRD of 0.03-MnO₂/IM-80–12 monolithic catalysts at different calcination temperatures, (c) effect of 5 vol% H₂O on toluene combustion and (d) cyclic test of 0.03-MnO₂/IM-80–12 monolithic catalyst.

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  From XRD analysis (Fig. 3b), the main manganese product of 400 ℃ calcination was β-MnO₂. After the calcination temperature raised to 500~700 ℃, the product gradually transformed into Mn₂O₃ (JCPDS No. 41–1442). Moreover, Fe₃O₄ transformed into Fe₂O₃ crystal phase compared with MnO₂/IM-400 (JCPDS No. 39–1346). And with the increasing of calcination temperature, the intensity of Mn₂O₃ and Fe₂O₃ diffraction peaks gradually increased. Since MnO₂ showed remarkable toluene oxidation activity than Mn₂O₃, which mainly attribute to strong redox ability and active oxygen species storage [15]. Therefore, MnO₂ phase on the surface of iron mesh changed to Mn₂O₃ phase is one of the main reasons for the decrease of catalyst activity at high calcination temperatures.

  Water vapor could cover and block the active sites of the catalyst [16]. After introduced water vapor in the airflow, the conversion rate of toluene decreased slightly from 95% to 89% after continuing the reaction for 1 h (Fig. 3c), which may be related to the competitive adsorption of toluene with water molecules on the surface of catalyst [17]. Subsequently, the toluene conversion was immediately restored when water vapor was removed. As a result, water vapor can be released from the surface of the catalyst in time, that is, the deactivation was reversible. This result indicated that the MnO₂/IM catalyst present certain water resistance, which may be attributed to the poor water absorption of the IM substance and the active component MnO₂ [18-20].

  Fig. 3d shows the cyclic test of 0.03-MnO₂/IM-80–12 catalyst for toluene catalytic oxidation. During each cycle, the catalyst was slowly increased from room temperature to 360 ℃ and keep it at 3 h. In the 1^st, 2^nd, 3^rd and 4^th cycle, the T_90% value was 281, 301, 307 and 317^oC, respectively. The catalytic performance was only slightly reduced after 4 cycles. The active components on the surface of IM may fall off due to continuous scouring of the airflow, leading to the reduction of the active components, thus reducing the catalytic activity [21]. Thus, the as-prepared 0.03-MnO₂/IM-80–12 monolithic catalyst is promising for toluene degradation because of its low cost, high catalytic activity, low shedding rate and excellent water resistance.

  Fig. S6 (Supporting information) exhibits the time-resolved in-situ diffuse reflectance infrared fourier transform spectroscopy (in-situ DRIFTS) spectra of toluene adsorption and oxidation with time on 0.03-MnO₂/IM-80–12 sample at 260 ℃. The peak at around 1603 cm⁻¹ was attributed to the stretching vibration of aromatic ring, and the band at 3057, 3028, 1452 and 1496 cm⁻¹ were assigned to the phenylic C-H stretching vibration and the bending vibration peaks of asymmetric methyl groups, respectively. The peaks intensity increased with the increasing of reaction time, which indicated that toluene was adsorbed on the surface of the catalyst [22-24]. Simultaneously, the peak at 1405 and 1540 cm⁻¹, respectively corresponded to the symmetric C-O tensile vibration peak and the asymmetric C-O tensile vibration peak of benzoic acid, indicating that the benzoate species (C₆H₅-COOH) were the main intermediates product in the oxidation of toluene over 0.03-MnO₂/IM-80–12 monolithic catalyst [25, 26]. Moreover, all the in-situ DRIFTS showed the final product of CO₂ at around 2342 and 2360 cm⁻¹, and the intensities gradually increased with the increasing of detection time [27, 28]. The results showed that the lattice oxygen on the surface of 0.03-MnO₂/IM-80–12 monolithic catalyst could participate in the degradation of absorbed toluene and oxidation of benzoic acid and other intermediated, but the intermediates could not be fully oxidized due to the lack of sustainable reactive oxygen species [29].

  In toluene oxidation process, toluene characteristic peaks (1591, 1508 and 3065 cm⁻¹) and benzoic acid species characteristic peaks (1413 and 1558 cm⁻¹) can be observed after 20 vol% oxygen was introduced for 60 min. In addition, the oxidation path of toluene on the surface of 0.03-MnO₂/IM-80–12 monolithic catalyst could be quickly converted to intermediate species benzoic acid, and eventual oxidation to CO₂ (2318 and 2365 cm⁻¹) and H₂O. No characteristic peak of benzaldehyde species was observed during toluene oxidation, which may be the reason for the rapid conversion of benzaldehyde species into benzoic acid species. The intensity of toluene and benzoic acid characteristic peaks did not decrease significantly with the increasing of time, even after introduced 20 vol% oxygen, indicating that toluene and benzoic intermediates species absorbed on the surface of the catalyst reached saturation. Therefore, the reaction path of toluene on the catalyst surface can be inferred: the rapid conversion of toluene to the intermediate product benzoic acid and eventual transformed to CO₂ and H₂O.

  In this work, one-step in situ hydrothermal method was used to make MnO₂/IM monolithic catalysts. The MnO₂ can grow evenly and firmly on the IM surface by adjusting the hydrothermal conditions, which shedding rate was only 0.14%. The obtained catalysts (prepared at a hydrothermal temperature of 80 ℃, hydrothermal time of 12 h, and precursor manganese ion concentration of 0.03 mol/L) presented excellent toluene oxidation performance with a T_90% value of 265 ℃ (GHSV = 60, 000 mL g⁻¹ h⁻¹), this can be attributed to the unique three-dimensional network structure of the iron mesh carrier. XPS also proved that MnO₂/IM-80 presented the highest O_ads and high-valent Mn⁴⁺ and Fe³⁺ elements, which is benefit for catalytic reaction. High temperature may cause MnO₂ phase on the surface of IM transformed to Mn₂O₃, thus leading to a relatively low catalytic activity. Beside, due to the hydrophobicity of IM, the MnO₂/IM monolithic catalyst presented certain water resistance under 5 vol% water vapor. And the catalytic performance was only slightly reduced after 4 cycles. Thus, the as-prepared MnO₂/IM monolithic catalyst in this work is promising for the practical oxidation of toluene.

  Declaration of competing interest

  The authors report no declarations of interest.

  Acknowledgments

  This work was supported by the Fundamental Research Funds for the Central Universities (No. 2021ZY79), Beijing Municipal Education Commission through the Innovative Transdisciplinary Program "Ecological Restoration Engineering" (No. GJJXK210102), National Natural Science Foundation of China (Nos. 42075169, U1810209), National Key R & D Program of China (No. 2021YFE0110800) and Chinese-Serbian collaboration project (No. 451-03-1205/2021-09).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.04.035.

  
  
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  Figure 1  (a) SEM images of pure iron mesh; SEM images of MnO₂/IM monolithic catalysts at hydrothermal temperature of (b) 60 ℃, (c) 80 ℃, (d) 100 ℃, (e) 120 ℃, (f) 140 ℃, (g) 160 ℃, (h) 180 ℃, (i) elemental mappings results of MnO₂/IM monolithic catalyst with hydrothermal temperature of 140 ℃.

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  Figure 2  Catalytic oxidation of toluene over MnO₂/IM monolithic catalysts at different (a) hydrothermal temperatures, (b) hydrothermal times and (c) precursor concentrations.

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  Figure 3  (a) Catalytic oxidation of toluene and (b) XRD of 0.03-MnO₂/IM-80–12 monolithic catalysts at different calcination temperatures, (c) effect of 5 vol% H₂O on toluene combustion and (d) cyclic test of 0.03-MnO₂/IM-80–12 monolithic catalyst.

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  Table 1.  Surface Mn, Fe, and O elements molar ratios for MnO₂/IM monolithic catalyst with different hydrothermal temperature.

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