Enhancement effect of acid treatment on Mn2O3 catalyst for toluene oxidation

Catalysis Today xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Enhancement effect of acid treatment on Mn2O3 catalyst for toluene oxidation ⁎

Xueqin Yanga,b, Xiaolin Yua,b, , Mengya Lina,b, Xiuyun Maa,b, Maofa Gea,b,c,

⁎

a State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, PR China b University of Chinese Academy of Sciences, Beijing, 100049, PR China c Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Acid treatment Enhancement effect Vacancy defects Toluene oxidation

The effect of acid treatment on manganese oxide samples has been investigated for catalytic oxidation of toluene. The acid treatment had no obvious influence on the textural properties of catalysts, but could remarkably improve the catalytic performance of catalysts, especially the catalysts acid-treated with low acid concentration. The acid-treated catalysts exhibited the relatively high amount of surface Mn4+ along with the probable formation of vacancy defects via the disproportionation of Mn3+ by acid leaching. The structural defects could further contribute to the good surficial oxygen mobility and the low-temperature reducibility, which is indispensable for the significant enhancement of toluene oxidation. As a result, the specific toluene reaction rate increased and the activation energy decreased with the increase of acid concentration. In situ DRIFTS results indicated that the intermediates such as alkoxide, benzaldehyde, benzoate, and maleic anhydride species were produced during the toluene oxidation process.

1. Introduction In recent years, atmospheric pollution has attracted strong public concerns due to the emission of gaseous contaminants to the environment. Volatile organic compounds (VOCs), such as formaldehyde, benzene and toluene, which are the key precursor for the formation of haze, are the major air pollutants to the environment and human health [1–3]. Catalytic oxidation is considered as a promising technique for VOCs abatement among the various treatment methods (adsorption, plasma catalysis, photocatalytic oxidation, etc.), because of its energy saving, high efficiency and environmental friendliness [4,5]. In spite of the excellent low-temperature catalytic oxidation performance of noble metal catalysts, some drawbacks including high cost, low thermal stability and easy sintering have still restricted their industrial application [6]. Alternatively, the transition metal oxides (e.g. Co3O4, Fe2O3, CeO2, and MnOx) with low cost, high thermal stability and chemical composition diversity exhibit the potential advantages and are becoming the very promising and versatile catalysts in VOCs oxidation reactions [7]. Among these transition metal oxides, manganese oxides (MnOx) have been proven to be highly active, durable and low-cost catalysts [8]. Much work so far has focused on the modification of MnOx to further enhance its catalytic performance in consideration of practical

application. For example, Sun et al. reported that the modification of MnOx catalyst by Eu could significantly improve the NH3-SCR activity and broaden its operating temperature window [9]. Zhou et al. constructed the novel core-shell α-MnO2@L-MnO2 heteroepitaxy by oriented growth and found that the special structure of manganese oxides enhanced room-temperature HCHO oxidation activity greatly [10]. Xu et al. investigated the promotion effect of isolated potassium atoms anchored on surfaces of Hollandite manganese oxide and confirmed that the alkali metal could improve its low-temperature catalytic activity for the HCHO and ethyl acetate oxidation [11]. Recent studies have proved that acid treatment is an effective means to change the surficial chemical properties of catalysts such as metal oxidation state, active oxygen species and structural defects, which is indispensable for high-efficiency catalysts [12–15]. Lee et al. leached Na0.44MnO2 nanowires in acid and successfully introduced the controllable defects in catalysts [13]. Quiroz et al. reported that the acid treatment could alter the oxidation state of manganese and greatly improve the formaldehyde conversion and the intrinsic reaction [14]. Si et al. selectively removed the La cations in MnO2/LaMnO3 catalyst and found that the acidtreated catalyst exhibited an excellent catalytic activity for toluene oxidation [15]. Herein, we prepared a series of acid-treated manganese oxides with

⁎ Corresponding authors at: State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail addresses: [email protected] (X. Yu), [email protected] (M. Ge).

https://doi.org/10.1016/j.cattod.2018.04.041 Received 17 January 2018; Received in revised form 28 March 2018; Accepted 19 April 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Yang, X., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.04.041

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

toluene in the inlet and outlet were measured by an online gas chromatograph of Agilent 6820 equipped with a flame ionization detector (FID). The toluene conversion (xtoluene) and the specific toluene reaction rate (Rs) of the A-x catalysts were calculated according to the following equations [17–19]:

different concentrations and investigated the effect of acid treatment on toluene catalytic behavior. The activity tests showed that the acid-leached catalysts not only exhibited excellent catalytic performance, but also displayed the low activation energy and high specific toluene reaction rate. The reasons of the acid enhancement effects on Mn2O3 samples were studied profoundly by various characterizations, and the possible mechanism was also proposed.

Xtoluene =

2. Experimental

Rs =

2.1. Catalysts preparation

Cinlet − Coutlet × 100% Cinlet

(1)

Xtoluene QCf WSBET

(2)

Where Cinlet and Coutlet are the toluene concentration in the inlet and outlet gas. Q is the volumetric flow rate (mL h−1) and Cf is the inlet concentration of toluene (mol mL−1). W is the mass of the catalyst employed (g) and SBET is the BET surface area of the catalyst (m2 g−1). The activation energy (Ea) was calculated from the slope of the Arrhenius-type plot of the toluene oxidation rate.

The precursor of manganese oxide was prepared by a hydrothermal method as reported by Joel Henzie et al [16]. In detail, 1.648 g PVP (polyvinylpyrrolidone) was dissolved in a 35 mL DMF (N, N-dimethylformamide), and then 50 wt % aqueous Mn(NO3)2 solution was added under the magnetic stirring to form a homogeneous solution. After that, the solution was kept in a 100 mL autoclave with a Teflon liner at 180 °C for 6 h. When cooled to room temperature, the black precipitate was washed with DMF, dried in a vacuum oven at 60 °C, and finally calcined at 500 °C for 4 h, denoted as A-0. The calcined samples were treated with an aqueous solution of H2SO4 (2.5, 5, 10, 15 mol/L) by stirring in a beaker for 1 h. The final product was centrifuged, washed and dried at 105 °C. The obtained samples were denoted as A-x, in which the x represents the concentration of H2SO4.

3. Results 3.1. Catalyst activity The catalytic performances of A-x samples were evaluated by gaseous toluene oxidation tests. Fig. 1a shows the changes in the conversion of toluene over catalysts with varying reaction temperature. As presented, acid treatment had a significant influence on the catalytic performance of A-x samples, especially for the samples acid-treated with the low concentration. The low acid concentration (< 5 mol/L) resulted in the large increase of toluene catalytic activity, which can be seen from the reaction temperature T50 and T90 in Table 1. Compared with A-5 sample, further increasing the acid-treated concentration did not promote the improvement of the toluene catalytic activity. Therefore, we will discuss the effect of low acid concentration on the A-x catalysts for toluene abatement in the following sections. In order to investigate the effect of acid treatment on the catalytic performance deeply, the reaction kinetics of A-x samples were measured. The specific toluene reaction rates (per unit surface area of catalyst) and activation energies (Ea, calculated by the Arrhenius plots in Fig. 1b) are summarized in Table 1. As presented, the specific toluene reaction rate increased from 3.33 × 10−3 to 8.96 × 10−3 mmol h−1 m−2 and the activation energy decreased from 253.7 to 161.8 kJ/mol in the sequence of A-0, A-2.5 and A-5. Apparently, such an increasing tendency of catalytic performance of A-x samples had a closely relationship with the acid-treated concentration (Fig. 1c). It is generally agreed that the specific reaction rate and activation energy are related to the intrinsic catalytic activity [18]. Therefore, we speculated that acid treatment could enhance intrinsic catalytic efficiency significantly. As can be seen in Fig. 1d, A-5 sample exhibited an excellent stability and no significant deactivation was observed even over a 140 h long test under the same test conditions.

2.2. Catalyst characterization X-ray diffraction (XRD) data were collected using a D/max 2500 Xray powder diffractometer with Cu Ka radiation at a scan rate (2θ) of 2° min−1. The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size distribution of the samples were obtained by an autosorbiQ instrument at 77 K. Prior to the measurement, all samples were degassed at 473 K for 2.5 h. Raman spectra were measured under ambient conditions on a LabRAM Aramis Raman Spectrometer (HORIBA Jobin Yvon S. A. S.) equipped with an excitation laser of 532 nm. The morphology of samples were analyzed using field emission scanning electron microscopy (FE-SEM, S-4800 and SU-8020). The transmission electron microscopy (TEM) was conducted using a HT7700. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB250XI with Al Kα radiation. H2-TPR and O2-TPD tests were carried out on an AutoChem 2720. Prior to H2-TPR, the samples (50 mg) were heated at 200 °C for 30 min in a He flow (20 cm3 min−1). Then, the samples were reduced in a flow of 2% H2/He (50 cm3 min−1) and the temperature was increased from 100 to 630 °C at a heating rate of 10 °C min−1. For O2-TPD, the samples (50 mg) were pretreated at 300 °C for 30 min in a He flow (50 cm3 min−1) and then cooled to 50 °C with a purge of 5% O2/He (50 cm3 min−1) for 30 min. After that, a He flow (30 cm3 min−1) was introduced and kept for 60 min. Then, the temperature was increased from 100 to 830 °C at a heating rate of 10 °C min−1. The in situ diffuse reflectance FTIR spectra (DRIFTS) were collected on a Nicolet 6700. Prior to the measurement, the catalyst was heated at 240 °C in a flow of N2 for 2 h. The background spectrum was subtracted from each spectrum, and the sample was tested at 240 °C under the following conditions: 1000 ppm toluene, 20% O2 and 62% N2 (balance).

3.2. Catalyst characterization Fig. 2a shows the XRD patterns of A-x samples. It was seen that all the diffraction peaks could be ascribed to the typical bixbyite α-Mn2O3 (JCPDS PDF no. 41–1442), and no diffraction peak related to other manganese oxide was observed [8,20]. As shown in Table 1, the crystallite size calculated from the Debye-Scherrer formula based on the reflection peaks at (222) around 33° was ca. 30 nm for the all samples. Obviously, the acid treatment did not change the crystal structure and crystallite size, which implied a neglect impact on the textural properties. However, the intensities of the diffraction peaks for the samples after acid treatment were strengthened, probably resulting from the partial dissolution of amorphous α-Mn2O3 region. Nitrogen adsorptiondesorption isotherm analysis was conducted to determine the BET surface area and pore size distribution of samples. As displayed in

2.3. Activity test The activity tests for toluene oxidation over the catalysts (50 mg) were evaluated in a continuous flow fixed-bed quartz tubular reactor (i.d. = 4 mm). Gaseous toluene was generated by flowing nitrogen through a container with the pure toluene in ice-water bath. The experimental conditions were 1000 ppm toluene and 20% O2 balanced by N2, and the total flow rate was 50 mL min−1. The weight hourly space velocity (WHSV) was 60 000 mL gcat−1 h−1. The concentrations of 2

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

Fig. 1. (a) Toluene conversion over A-x catalysts. Reaction conditions: 1000 ppm toluene, 20% O2, WHSV = 60 000 mL gcat−1 h−1; (b) Arrhenius plots of toluene reaction rates over A-x catalysts; (c) the relationship between the specific rates and activation energies of samples and the H2SO4 concentration used; (d) the stability test of the A-5 catalyst at 228 °C. Reaction conditions: 1000 ppm toluene, 20% O2, WHSV = 60 000 mL gcat−1 h−1.

the increase of acid-treated concentration to 5 mol/L, a flower-like morphology with more obvious rough surface appeared (Fig. 3c), and many small particles aggregated on the margins of sample in Fig. 3f. Thus, the acid etch may induce the exposure of the defective surfaces with high energy, such as corners, vertexes, edges and steps, which is vital for the high-efficiency catalyst [13,22,23]. Raman spectroscopy is an effective technique to gain insight into the structural defects such as lattice disorder and oxygen vacancies, herein we employed this technique to investigate the effect of acid treatment on the structure of samples [18]. The Raman spectra of A-x catalysts are displayed in Fig. 4. For all samples, the distinct bands at 346, 454–571 and 632 cm−1 were observed, which are assigned to the out-of-plane bending modes, the asymmetric stretching of bridge oxygen species and symmetric stretching of Mn2O3, respectively [24,25]. Unlike the Raman spectrum of A-0 sample, the bands of the acid-treated samples became broader and the peak intensity was dramatically decreased. In addition, as shown in the inset of Fig. 4, the band around 632 cm−1 shifted to a higher frequency (641 cm−1) with

Fig. 2b and Table 1, A-0 sample had the lowest specific surface areas 6.4 m2/g and total pore volume 0.033 cm3/g. After acid treatment, the specific surface areas and total pore volume for A-2.5 and A-5 samples increased to 8.7, 9.9 m2/g and 0.049, 0.051 cm3/g, respectively. Although the average pore diameter (3.8 nm) almost had no change, the amount of this pore increased with increasing the acid concentration, which presumably was created by acid-leached role. The changes of pore structure are beneficial to the adsorption and transport of guest species to catalytic active sites [21]. As presented in Fig. 3, the morphologies and structural features of A-x samples were investigated by SEM and TEM. Evidently, A-0 sample presented a pretty smooth and uniform surface, as shown in Fig. 3a and d. After treating with 2.5 mol/L acid, some thin flakes appeared on the surface of A-2.5 sample due to the acid corrosion (Fig. 3b). The corrosion was slight for the sample surface, but the sample microstructure had an obvious change. From TEM of Fig. 3e, the margins of sample became thin and many nanopores were formed on the marginal region, which resulted from the partial dissolution of amorphous region. With Table 1 Physical-chemical and toluene catalytic properties of A-x catalysts. Samples

Surface area (m2/g)

Total pore volume (cm3/g)

Crystallite sizea (nm)

T50 (°C)

T90 (°C)

Rsb at 200 °C (mmol h−1 m−2)

Eac (kJ/mol)

A-0 A-2.5 A-5

6.4 8.7 9.9

0.033 0.049 0.051

30.9 31.2 30.7

241 235 231

248 243 239

3.33 × 10−3 8.17 × 10−3 8.96 × 10−3

253.7 223.5 161.8

a

The crystallite size calculated from the Debye-Scherrer formula based on the reflection peaks at (222) around 33°. Specific toluene reaction rate of samples estimated at 200 °C under a kinetically controlled regime. Reaction conditions: 1000 ppm toluene, 20% O2, WHSV = 60,000 mL gcat−1 h−1. c The activation energy calculated from the slope of the Arrhenius-type plot of the toluene oxidation rate. b

3

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

Fig. 2. XRD patterns (a) and nitrogen adsorption-desorption isotherms and corresponding pore-size distribution curves (b) of A-x catalysts.

migration of oxygen species [10,31]. Therefore, it is further verified that acid treatment could induce the sample to expose abundant defects on the surface, in good accordance with the above characterization results. As shown in Fig. 5b, the O 1s spectra exhibited three peaks centered at 529.5–529.7, 531.4–531.7 and 533.2–533.3 eV, corresponding to the lattice oxygen (Olatt), surface-adsorbed oxygen (Oads) and oxygen-containing (hydro)carbons, respectively [32–34]. As presented in Table 2, the ratio of Oads/Olatt in sample increased from 0.43 to 1.30 with the increase of the acid concentration, probably due to the appearance of abundant defects created by acid etching. It is universally agreed that the VOCs oxidation over transition metal oxide catalysts follows the Mars-van Krevelen mechanism and the surface lattice oxygen species play an important role in the oxidation reactions [12,17]. The gaseous O2 molecules can be rapidly adsorbed and activated over vacancy defects in acid-treated samples, and then the active oxygen species can be readily transferred to replenish the consumed surface lattice oxygen species during VOCs oxidation [35,36]. The easy activation and rapid replenishment of oxygen species is beneficial for the enhancement of toluene oxidation. To further clarify the effect of acid treatment on oxygen species, the

the increase of acid concentration. These results confirmed the presence of more defects in acid-leached samples, probably resulting from the exposure of high-energy surfaces, which is consistent with the SEM and TEM results [18,26]. X-ray photoelectron spectroscopy (XPS) was carried out to investigate the surface chemical states of the O and Mn elements over A-x catalysts. The XPS spectra are displayed in Fig. 5, and the Oads/Olatt and Mn3+/Mn4+ ratios calculated from the corresponding peak areas are summarized in Table 2. As shown in Fig. 5a, the Mn 2p3/2 XPS signal displayed two components at binding energy around 641.3 and 642.8 eV, which are ascribed to Mn3+ and Mn4+ species, respectively [27–29]. From Table 2, it was apparent that the predominant oxidation state of Mn in all A-x samples is + 3, but the surface Mn3+/Mn4+ ratio exhibited an obvious decrease tendency after acid leaching. The A-5 catalyst possessed the highest relative content of surface Mn4+. It is well-known that the acid leaching will accelerate the disproportionation of Mn3+, accompanying a redox driven extraction of Mn2+ and an increase in the oxidation state in the form of Mn4+, along with the probable formation of oxygen vacancies [13,30]. The increase of surface Mn4+ suggested the existence of more vacancy defects on the surface of catalyst, which is beneficial to the adsorption, activation and

Fig. 3. SEM images and corresponding TEM images of A-x catalysts: A-0 (a, d), A-2.5 (b, e) and A-5 (c, f). 4

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

Fig. 4. Raman spectra of as-synthesized A-x catalysts.

structural defects of high-energy surfaces; the Oβ species was corresponded to the surficial lattice oxygen; the Oγ species was ascribed to the lattice oxygen in the bulk [12,28,37]. Interestingly, as the acidleached concentration increased, the Oα and Oβ desorption peaks shifted to a lower temperature, suggesting that the Oα and Oβ species in

O2-TPD profiles of A-0, A-2.5 and A-5 catalysts were carried out. As displayed in Fig. 6a, all A-x samples exhibited three O2-desorption peaks centred at 359–430, 408–531 and 729–752 °C, which were referred as Oα, Oβ and Oγ, respectively. As reported, the Oα species represented the surface chemisorbed oxygen, probably generating from

Fig. 5. XPS spectra of A-x catalysts: (a) Mn 2p and (b) O1s. 5

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

1176 cm−1) and benzoate species (1490 and 1397 cm−1) were also formed and dominant, indicating the further oxidation of toluene [43,45]. The bands also appeared at 1917, 1816, 1301 and 1233 cm−1, which were ascribed to maleic anhydrides [43,46]. With an increase in the purging time, the peaks at 1559 and 1440 cm−1 (assigned to the C]C on an aromatic ring) quickly decreased, implying that the aromatic ring in toluene was cleaved once in contact with A-5 sample [27]. Also, the bands around 3600 cm−1 (ascribed to the hydroxyl group) were identified, probably caused by the reaction between catalyst surface and toluene [27].

Table 2 Surface chemical composition and element molar ratios of A-x catalysts. samples

Oads/Olatt

Mn3+/Mn4+

A-0 A-2.5 A-5

0.43 0.88 1.30

3.33 1.47 1.30

A-2.5 or A-5 samples could be released more readily. This was probably attributed to the appearance of abundant vacancy defects in acid-etched samples, which could make catalysts adsorb and activate oxygen species more readily and the surficial Mn-O bonds break more facilely [12]. In additions, the A-5 sample exhibited the lowest temperature of O2 desorption and had the largest amount of Oα and Oβ, which is consistent with the catalytic activity and the O 1s XPS results. It is generally accepted that the catalytic performance of catalysts was closely associated with the low-temperature reducibility [35,38]. Therefore, H2-TPR was employed to investigate the reducibility of A-0, A-2.5 and A-5 catalysts. As shown in Fig. 6b, three kinds of peaks were observed for all samples. The first relatively wide peak (362–427 °C) was attributed to the reduction of the labile Mn on the surfaces. In A-x samples, the surficial Mn species, especially located on structural defects of acid-treated samples, are not strongly stabilized by the oxygen and the Mn-O bond strength is weak [39,40]. As reported, the reduction of Mn2O3 is usually successive reduction process: Mn2O3 → Mn3O4 → MnO [40]. Accordingly, the peak centred at 431–484 °C represented the reduction of Mn2O3 to Mn3O4 and that at 551–587 °C was ascribed to the deep reduction of Mn3O4 to MnO [40,41]. It was apparent that all peaks of acid-treated samples shifted to a lower temperature and that the low-temperature reducibility decreased in the sequence of A5 > A-2.5 > A-0. There was an intrinsic relation between the lowtemperature reducibility and catalytic activity of the sample, indicating the acid treatment did improve the catalytic performance. Therefore, the enhancement of catalytic property for acid-treated samples can be attributed to the abundant defects, high mobility of surficial oxygen species and excellent low-temperature reducibility. In order to detect the intermediates and investigate the catalytic mechanism of toluene oxidation, the in situ DRIFTS spectrum of A-5 catalyst exposed to toluene/O2 was recorded. As shown in Fig. 7, the weak bands around 3071 cm−1 were observed, which was ascribed to the νC–H of the aromatic rings [17,42]. The bands located at 2959, 2826 and 1363 cm−1 were assigned to νas(C–H), νs(C–H) of methyl groups and a CH2 deformation mode, respectively, which is characteristic of a benzyl species [42,43]. The weak multiple bands appeared at 1146, 1094 and 1070 cm−1 were assigned to a C–O vibration mode, which may be resulted from the reaction between toluene and catalyst surface to form alkoxide species [44]. The aldehydic species (1593, 1475 and

3.3. Catalytic mechanism It is reported that the toluene oxidation is a successive oxidation process to produce the intermediates such as benzylic, aldehydic, benzoate and anhydride species [17,42,45]. According to the previous speculations and the in situ DRIFTS analysis in this work, a possible reaction mechanism for toluene catalytic oxidation over A-x sample was proposed. During this oxidation process, toluene was firstly adsorbed on the catalyst surface in the form of benzyl species, and then reacted with the active oxygen species to produce the aromatic alkoxide, benzaldehyde and benzoate species sequentially. Then, the oxidized products could be further oxidized by active oxygen species, accompanying with the cleavage of aromatic ring and the formation of maleic anhydrides, which was further oxidized into CO2. It should be mentioned that oxygen vacancies play a vital role in the whole process, because oxygen vacancies can rapidly adsorb and activate the O2 molecules, improve the oxygen storage capacity and lower the oxygen migration activation energy, thus accelerating the bond cleavage of reactants [31]. Previous theoretical calculations also validated the decisive role of oxygen vacancy in the catalytic reaction. For example, Wu et al. confirmed that the surface defects in Mn-doped ceria would enable O2 adsorption, generate more active oxygen atoms, and decrease the C–H cleavage barriers of formaldehyde using DFT+U calculations [47]. Ye et al. verified that oxygen vacancy on the defective In2O3 (110) surface not only assists CO2 activation and hydrogenation, but also stabilizes the key intermediates during methanol formation from CO2 hydrogenation using periodic density functional theory calculations [48]. The aforementioned characterization results and activity tests have revealed that the acid leaching could induce the appearance of abundant vacancy defects in acid-treated samples, which further improved the specific toluene reaction rate and lowered the activation energy of acidtreated samples. Thus, the possible catalytic mechanism for toluene oxidation over acid-treated samples was proposed. As shown in Fig. 8, the abundant vacancy defects are induced by acid treatment in acidleaded Mn2O3 samples. The generated oxygen vacancies are further beneficial to the rapid activation of O2 molecules, while the consumed

Fig. 6. O2-TPD (a) and H2-TPR profiles (b) of A-x catalysts. 6

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

Fig. 7. Dynamic changes of in situ DRIFTS of A-5 catalyst as a function of time in a flow of O2 + toluene + N2 at 240 °C. Reaction conditions: 1000 ppm toluene, 20% O2 and 62% N2 (balance). The total flow rate was 50 mL min−1.

Fig. 8. Illustration of the toluene oxidation process on acid-treated Mn2O3 catalyst.

lattice oxygen during the reaction process are timely replenished by these activated oxygen species. Finally the bond cleavage of benzene ring can be achieved, leading to the easy oxidation of toluene and the improvement of catalytic performance.

Acknowledgements

4. Conclusions

References

This project was supported by the National Natural Science Foundation of China (21777166), and the National Key Research and Development Program of China (2016YFC0202202).

In this work, we prepared a series of acid-treated manganese oxides and investigated the effect of acid treatment on toluene catalytic behavior. The acid leaching could remarkably improve the catalytic performance of catalysts, especially the catalysts acid-treated with low acid concentration. XRD, BET, SEM, TEM and Raman results indicated that the textural properties of samples were essentially unchanged by acid treatment, but the abundant vacancy defects appeared in acid-leached samples. From the characterization results of XPS, O2-TPD and H2-TPR, the acid treatment can not only induce the formation of abundant active oxygen species, but also improve the mobility of surficial oxygen species and the low-temperature reducibility of catalysts. The in situ DRIFTS results indicated that the toluene was sequentially oxidized to alkoxide, benzaldehyde, benzoate, maleic anhydride species, and finally to CO2. Importantly, the abundant defects induced by acid leaching can rapidly adsorb, activate, transfer and replenish oxygen species, thus improving the catalytic performance of acid-treated samples fundamentally. Our findings shed light on the enhancement effect of acid treatment on toluene catalytic oxidation, which may open a new strategy for the design of high-efficiency catalyst.

[1] P. Sun, W. Wang, X. Dai, X. Weng, Z. Wu, Mechanism study on catalytic oxidation of chlorobenzene over MnxCe1-xO2/H-ZSM5 catalysts under dry and humid conditions, Appl. Catal. B 198 (2016) 389–397. [2] H. Huang, H. Huang, Q. Feng, G. Liu, Y. Zhan, M. Wu, H. Lu, Y. Shu, D.Y.C. Leung, Catalytic oxidation of benzene over Mn modified TiO2/ZSM-5 under vacuum UV irradiation, Appl. Catal. B 203 (2017) 870–878. [3] G. Liu, J. Ji, H. Huang, R. Xie, Q. Feng, Y. Shu, Y. Zhan, R. Fang, M. He, S. Liu, X. Ye, D.Y.C. Leung, UV/H2O2: an efficient aqueous advanced oxidation process for VOCs removal, Chem. Eng. J. 324 (2017) 44–50. [4] H.B. Huang, Y. Xu, Q.Y. Feng, D.Y.C. Leung, Low temperature catalytic oxidation of volatile organic compounds: a review, Catal. Sci. Technol. 5 (2015) 2649–2669. [5] X. Wang, Y. Liu, T. Zhang, Y. Luo, Z. Lan, K. Zhang, J. Zuo, L. Jiang, R. Wang, Geometrical-site-dependent catalytic activity of ordered mesoporous Co-based spinel for benzene oxidation: in situ DRIFTS study coupled with Raman and XAFS spectroscopy, ACS Catal. (2017) 1626–1636. [6] B.Y. Bai, Q. Qiao, J.H. Li, J.M. Hao, Progress in research on catalysts for catalytic oxidation of formaldehyde, Chin. J. Catal. 37 (2016) 102–122. [7] Y. Liu, H. Dai, J. Deng, Y. Du, X. Li, Z. Zhao, Y. Wang, B. Gao, H. Yang, G. Guo, In situ poly(methyl methacrylate)-templating generation and excellent catalytic performance of MnOx/3DOM LaMnO3 for the combustion of toluene and methanol, Appl. Catal. B 140–141 (2013) 493–505. [8] Y.-F. Han, K. Ramesh, L. Chen, E. Widjaja, S. Chilukoti, F. Chen, observation of the reversible phase-transformation of α-MN2o3 nanocrystals during the catalytic combustion of methane by in situ Raman spectroscopy, J. Phys. Chem. C 111 (2007)

7

Catalysis Today xxx (xxxx) xxx–xxx

X. Yang et al.

formaldehyde at room temperature, Chem. Eng. J. 334 (2018) 2050–2057. [30] J.K. Ngala, S. Alia, A. Dobley, V.M.B. Crisostomo, S.L. Suib, Characterization and electrocatalytic behavior of layered Li2MnO3 and its acid-treated form, Chem. Mater. 19 (2007) 229–234. [31] B. Zhao, B. Yan, S. Yao, Z. Xie, Q. Wu, R. Ran, D. Weng, C. Zhang, J.G. Chen, LaFe0.9Ni0.1O3 perovskite catalyst with enhanced activity and coke-resistance for dry reforming of ethane, J. Catal. 358 (2018) 168–178. [32] Q. Meng, W. Wang, X. Weng, Y. Liu, H. Wang, Z. Wu, Active oxygen species in Lan +1NinO3n+1 layered perovskites for catalytic oxidation of toluene and methane, J. Phys. Chem. C 120 (2016) 3259–3266. [33] N. Liu, W. Huang, X. Zhang, L. Tang, L. Wang, Y. Wang, M. Wu, Ultrathin graphene oxide encapsulated in uniform MIL-88A(Fe) for enhanced visible light-driven photodegradation of RhB, Appl. Catal. B 221 (2018) 119–128. [34] X. Zhang, Y. Wang, F. Hou, H. Li, Y. Yang, X. Zhang, Y. Yang, Y. Wang, Effects of Ag loading on structural and photocatalytic properties of flower-like ZnO microspheres, Appl. Surf. Sci. 391 (2017) 476–483. [35] H. Pan, Y. Jian, C. Chen, C. He, Z. Hao, Z. Shen, H. Liu, Sphere-shaped Mn3O4 catalyst with remarkable low-temperature activity for methyl–ethyl–ketone combustion, Environ. Sci. Technol. (2017). [36] W. Tan, J.G. Deng, S.H. Xie, H.G. Yang, Y. Jiang, G.S. Guo, H.X. Dai, Ce0.6Zr0.3Y0.1O2 nanorod supported gold and palladium alloy nanoparticles: high-performance catalysts for toluene oxidation, Nanoscale 7 (2015) 8510–8523. [37] M. Lin, X. Yu, X. Yang, K. Li, M. Ge, J. Li, Highly active and stable interface derived from Pt supported on Ni/Fe layered double oxides for HCHO oxidation, Catal. Sci. Technol. 7 (2017) 1573–1580. [38] J.G. Deng, S.N. He, S.H. Xie, H.G. Yang, Y.X. Liu, G.S. Guo, H.X. Dai, Ultralow loading of silver nanoparticles on Mn2O3 nanowires derived with molten salts: a high-efficiency catalyst for the oxidative removal of toluene, Environ. Sci. Technol. 49 (2015) 11089–11095. [39] W. Gac, The influence of silver on the structural, redox and catalytic properties of the cryptomelane-type manganese oxides in the low-temperature CO oxidation reaction, Appl. Catal., B 75 (2007) 107–117. [40] H. Jiang, C. Wang, H. Wang, M. Zhang, Synthesis of highly efficient MnOx catalyst for low-temperature NH3-SCR prepared from Mn-MOF-74 template, Mater. Lett. 168 (2016) 17–19. [41] L. Zhu, J. Wang, S. Rong, H. Wang, P. Zhang, Cerium modified birnessite-type MnO2 for gaseous formaldehyde oxidation at low temperature, Appl. Catal. B 211 (2017) 212–221. [42] S. Zhao, K. Li, S. Jiang, J. Li, Pd–Co based spinel oxides derived from pd nanoparticles immobilized on layered double hydroxides for toluene combustion, Appl. Catal. B 181 (2016) 236–248. [43] G. Busca, Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation, Catal. Today 27 (1996) 457–496. [44] H. Sun, Z. Liu, S. Chen, X. Quan, The role of lattice oxygen on the activity and selectivity of the OMS-2 catalyst for the total oxidation of toluene, Chem. Eng. J. 270 (2015) 58–65. [45] Z. Rui, M. Tang, W. Ji, J. Ding, H. Ji, Insight into the enhanced performance of TiO2 nanotube supported Pt catalyst for toluene oxidation, Catal. Today 297 (2017) 159–166. [46] J. Li, H. Na, X. Zeng, T. Zhu, Z. Liu, In situ DRIFTS investigation for the oxidation of toluene by ozone over Mn/HZSM-5, Ag/HZSM-5 and Mn–Ag/HZSM-5 catalysts, Appl. Surf. Sci. 311 (2014) 690–696. [47] H. Wu, S. Ma, W. Song, E.J.M. Hensen, Density functional theory study of the mechanism of formaldehyde oxidation on Mn-doped ceria, J. Phys. Chem. C 120 (2016) 13071–13077. [48] J. Ye, C. Liu, D. Mei, Q. Ge, Active oxygen vacancy site for methanol synthesis from CO2 hydrogenation on In2o3(110): a DFTf study, ACS Catal. 3 (2013) 1296–1306.

2830–2833. [9] P., Sun, R.-t., Guo, S.-m., Liu, S.-x., Wang, W.-g., Pan, M.-y., Li, The enhanced performance of MnOx catalyst for NH3-SCR reaction by the modification with Eu, Appl. Catal. A. [10] J. Zhou, L. Qin, W. Xiao, C. Zeng, N. Li, T. Lv, H. Zhu, Oriented growth of layeredMnO2 nanosheets over α-MnO2 nanotubes for enhanced room-temperature HCHO oxidation, Appl. Catal. B 207 (2017) 233–243. [11] F. Xu, Z.W. Huang, P.P. Hu, Y.X. Chen, L. Zheng, J.Y. Gao, X.F. Tang, The promotion effect of isolated potassium atoms with hybridized orbitals in catalytic oxidation, Chem. Commun. 51 (2015) 9888–9891. [12] W. Si, Y. Wang, Y. Peng, X. Li, K. Li, J. Li, A high-efficiency [gamma]-MnO2-like catalyst in toluene combustion, Chem. Commun. 51 (2015) 14977–14980. [13] J.-H. Lee, R. Black, G. Popov, E. Pomerantseva, F. Nan, G.A. Botton, L.F. Nazar, The role of vacancies and defects in Na0.44MnO2 nanowire catalysts for lithium-oxygen batteries, Energy Environ. Sci. 5 (2012) 9558–9565. [14] J. Quiroz, J.M. Giraudon, A. Gervasini, C. Dujardin, C. Lancelot, M. Trentesaux, J.F. Lamonier, Total oxidation of formaldehyde over MnOx-CeO2 catalysts: the effect of acid treatment, ACS Catal. 5 (2015) 2260–2269. [15] W. Si, Y. Wang, S. Zhao, F. Hu, J. Li, A facile method for in situ preparation of the MnO2/LaMnO3 catalyst for the removal of toluene, Environ. Sci. Technol. 50 (2016) 4572–4578. [16] J. Henzie, V. Etacheri, M. Jahan, H. Rong, C.N. Hong, V.G. Pol, Biomineralizationinspired crystallization of monodisperse [small alpha]-Mn2O3 octahedra and assembly of high-capacity lithium-ion battery anodes, J. Mater. Chem. A (2017). [17] S. Zhao, F. Hu, J. Li, Hierarchical core–shell Al2O3@Pd-CoAlO microspheres for low-temperature toluene combustion, ACS Catal. 6 (2016) 3433–3441. [18] X. Yang, X. Yu, M. Lin, M. Ge, Y. Zhao, F. Wang, Interface effect of mixed phase Pt/ ZrO2 catalysts for HCHO oxidation at ambient temperature, J. Mater. Chem. A 5 (2017) 13799–13806. [19] D.Y. Yoon, E. Lim, Y.J. Kim, J.H. Kim, T. Ryu, S. Lee, B.K. Cho, I.S. Nam, J.W. Choung, S. Yoo, NO oxidation activity of Ag-doped perovskite catalysts, J. Catal. 319 (2014) 182–193. [20] X. Zhang, H. Li, F. Hou, Y. Yang, H. Dong, N. Liu, Y. Wang, L. Cui, Synthesis of highly efficient Mn2O3 catalysts for CO oxidation derived from Mn-MIL-100, Appl. Surf. Sci. 411 (2017) 27–33. [21] B. Bai, H. Arandiyan, J. Li, Comparison of the performance for oxidation of formaldehyde on nano-Co3O4, 2D-Co3O4, and 3D-Co3O4 catalysts, Appl. Catal. B 142–143 (2013) 677–683. [22] M. Trépanier, A. Tavasoli, A.K. Dalai, N. Abatzoglou, Fischer–tropsch synthesis over carbon nanotubes supported cobalt catalysts in a fixed bed reactor: Influence of acid treatment, Fuel Process. Technol. 90 (2009) 367–374. [23] A. Ruiz Puigdollers, P. Schlexer, S. Tosoni, G. Pacchioni, Increasing oxide reducibility: the role of metal/oxide interfaces in the formation of oxygen vacancies, ACS Catal. (2017) 6493–6513. [24] Y. Luo, Y.-Q. Deng, W. Mao, X.-J. Yang, K. Zhu, J. Xu, Y.-F. Han, Probing the surface structure of α-Mn2O3 nanocrystals during CO Oxidation by Operando Raman Spectroscopy, J. Phys. Chem. C 116 (2012) 20975–20981. [25] S.-H. Shim, D. LaBounty, T.S. Duffy, Raman spectra of bixbyite, Mn2O3, up to 40 GPa, Phys. Chem. Miner. 38 (2011) 685. [26] A. Sinhamahapatra, J.P. Jeon, J. Kang, B. Han, J.S. Yu, Oxygen-deficient zirconia (ZrO2-x): a new material for solar light absorption, Sci. Rep. 6 (2016) 8. [27] X. Weng, P. Sun, Y. Long, Q. Meng, Z. Wu, Catalytic oxidation of chlorobenzene over MnxCe1–xO2/HZSM-5 catalysts: a study with practical implications, Environ. Sci. Technol. 51 (2017) 8057–8066. [28] W. Wang, Q. Meng, X. Weng, Z. Wu, Rapid syntheses of ultrafine LaMnO3 nanocrystallites with superior activity for catalytic oxidation of toluene, Catal. Commun. 84 (2016) 167–170. [29] R. Fang, H. Huang, J. Ji, M. He, Q. Feng, Y. Zhan, D.Y.C. Leung, Efficient MnOx supported on coconut shell activated carbon for catalytic oxidation of indoor

8