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Effect of microwave absorption properties and morphology of manganese dioxide on catalytic oxidation of toluene under microwave irradiation
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Effect of microwave absorption properties and morphology of manganese dioxide on catalytic oxidation of toluene under microwave irradiation Honghong Yia,b, Lingling Songa, Xiaolong Tanga,b,∗, Shunzheng Zhaoa,b, Zhongyu Yanga, Xizhou Xiea, Chuanbo Maa, Yuanyuan Zhanga, Xiaodong Zhanga a b
Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing, 100083, PR China
ARTICLE INFO
ABSTRACT
Keywords: Toluene MnO2 Microwave radiation Catalytic oxidation Microwave absorption properties
A large number of studies had shown that the morphology of the sample had a significant effect on the microwave absorption properties and catalytic activity of the sample. Manganese dioxide with different morphologies was synthesized by hydrothermal method through different precursors. The effects of sample morphology and microwave absorption properties on the catalytic activity of the sample in conventional thermal and microwave fields were studied. The results indicated that compared with the conventional thermal field, the catalytic activity of the samples in microwave field were obviously improved, and the activation energy of the reaction were decreased. Compared with the conventional thermal field, the conversion of toluene in microwave thermal field of MnO2(Ac), MnO2(S) and MnO2(N) increased by 59%, 42% and 12%, and the mineralization rate increased by 36%,11% and 2%, respectively, when the catalytic temperature was 150 °C. Compared with the traditional thermal field, the activation energy of the sample MnO2(Ac) in the microwave field was reduced by 88.3 KJ. A series of characterization results showed that the sample MnO2(Ac) had good catalytic activity in the microwave field was due to: MnO2(Ac) had proper microwave absorption properties, large amount of surface functional groups, large specific surface area and rich pore structure. The analysis results of electromagnetic parameters showed that: the reason that the sample MnO2(Ac) had good microwave absorption performance was that the MnO2(Ac) had proper impedance matching, high attenuation constant and Debye dipole relaxation effect.
1. Introduction With the development of industrialization, air pollution has become a problem that cannot be ignored. Most VOCs are irritating, toxic and carcinogenic, which could destroy the ozone layer, acid rain and producing photochemical smog [1–3]. Thus, VOCs has become one of the main sources of atmospheric pollutants. At present, the most economical and effective method for removing VOCs is catalytic oxidation. Compared with other methods, catalytic oxidation has the advantages of low auxiliary cost, simple operation, high removal rate and no secondary pollution. Most of the current catalytic oxidation technologies are carried out in conventional thermal fields. However, compared with the conventional heating, microwave heating has the advantages of rapid and
efficient heating, no secondary pollution, energy-saving, cleaning, simple operation and so on [4,5]. Microwave heating is the heating of ultra-high frequency electromagnetic waves generated by a magnetron in a microwave oven [2,6]. Since the 21st century, microwave catalysis has attracted more and more researchers' attention and achieved a series of achievements [7–11]. Zhou et al. used CuO-CeO2@AC for microwave degradation of 100 mg/L crystal violet wastewater, the degradation rate reached 99.5% at 400 W for 5 min [7]. Costa et al. used Fe/Ac catalyst for microwave radiation to degrade phenol in wastewater. The removal efficiency of microwave irradiation for 15 min can reach 100% [8]. Zhang et al. used carbon nanotubes to degrade the aqueous solution containing organic pollutants and completely degradation at 450 W for 3 min [12]. Some studies used mussel-excited chemistry to prepare samples for efficient removal of organic dyes
∗ Corresponding author. Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail addresses: [email protected] (H. Yi), [email protected] (L. Song), [email protected] (X. Tang), [email protected] (S. Zhao), [email protected] (Z. Yang), [email protected] (X. Xie), [email protected] (C. Ma), [email protected] (Y. Zhang), [email protected] (X. Zhang).
https://doi.org/10.1016/j.ceramint.2019.10.020 Received 1 September 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Honghong Yi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.020
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[13–15]. Zeng et al. prepared MoS2-PDA-Ag nanocomposites via combination of mussel inspired chemistry and microwave irradiation. Compared with pure MoS2 and MoS2-PDA, the obtained composite material had higher catalytic activity for the reduction of 4-nitrophenol and great antibacterial activity [16]. Wang et al. combined microwave radiation and photocatalysis to degrade tetracycline, the results showed that microwave irradiation enhanced the separation of photogenerated charge carriers and catalytic performance [17]. It can be seen that the method of using microwave for catalysis of organic pollutants was feasible. The catalytic activity of the sample in the microwave field is related to the microwave absorption performance of the sample, which refers to the ability of the sample to convert microwave radiation energy into heat energy. Manganese oxide can form different morphologies by different bonding methods and MnO2 has good catalytic activity for benzene series [18,19]. A series of studies have shown that the morphology has a great effect on the microwave absorption properties of the materials [20–24]. Zhou's research showed that the tetragonal nanorods had stronger microwave absorption properties than the columnar nanorods and the tetragonal nanotubular MnO2 [25]. Wang's research showed that the reflection loss of manganese dioxide hollow spheres was 2.5 times higher than that of manganese dioxide nanoribbons [23]. Guan's research showed that the absorption performance of one-dimensional a-MnO2 nanorods was two times higher than that of MnO2 without obvious morphology [19]. However, the influence of the morphology and microwave absorption properties of MnO2 on the catalytic oxidation of toluene has not been studied. In this paper, manganese dioxide with different morphology was prepared by hydrothermal method with manganese acetate, manganese sulfate and manganese nitrate as precursors. The toluene catalytic oxidation activity of the samples in conventional thermal and microwave fields was investigated. The effects of the morphology of the samples on the microwave absorption properties and the relationship between the microwave absorption properties and the catalytic activity were investigated by a series of characterizations. This study proposed the relationship between microwave absorption properties and catalytic activity of materials, and provides a theoretical basis for explaining the promotion of catalytic reactions by microwave radiation.
morphology of the sample. The nitrogen adsorption and desorption curve was used to analysis nitrogen adsorption and desorption, the specific surface area and pore size. A Nicolet IS 50 Fourier transform infrared (FTIR) was used to investigate functional groups of samples surface ranging the wavenumber from 400 to 4000 cm−1. The vector network analysis (VNA) was used to determine the electromagnetic parameters of the sample. The sample and paraffin were mixed in a certain ratio and pressed into a ring of a certain thickness ( in = 3mm , out = 7mm) , and the measured values of the real permittivity (ε’), imaginary permittivity (ε’‘), real permeability (μ’), and imaginary permeability (μ’’) were recorded on a vector network analyzer. According to the transmission line theory, the expression of reflection loss (RL) was calculated as follows:
µr = µ
µ i
(1)
=
i
(2)
r
Zin = Z0
µr
tanh j
2 fd µr
r
RL = 20 log (Zin
r
c
Z0)/(Zin + Z0 )
(3) (4)
Where μr is complex permeability of the absorber, εr is complex permittivity of the absorber, f is the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of the absorber. 2.4. Catalyst activity evaluation The experimental flow chart was shown in Fig. 1. The catalytic oxidation of toluene was carried out in a quartz reactor at atmospheric pressure. A certain amount of catalyst was put in the quartz reaction tube (0.7 cm in internal diameter). In conventional thermal field catalysis, the quartz reaction tube was placed on a tubular heating furnace. The temperature of the catalyst was controlled by electric furnace heating. In microwave field catalysis, the quartz reactor was placed in microwave oven, and the catalyst was heated by adjusting the microwave power, and then the catalyst in the reactor was connected to microwave oven by thermocouple to monitor the temperature of catalyst in real time. Toluene vapor with the concentration of 300 ppm was generated by gas distribution system and ice bath toluene bubble device. The total flow was 190 mL/min with the gas hourly space velocity (GHSV) of 30000 h−1. The tail gas after reaction entered the gas chromatograph to analyze the concentrations of toluene, carbon monoxide and carbon dioxide. The catalytic activity of the sample was evaluated by toluene removal rate and mineralization rate, respectively. The expression of toluene conversion rate ( ) was as follows,
2. Experimental 2.1. Raw materials Manganese(II) acetate tetrahydrate (Mn(Ac)2·4H2O, 99%), manganese sulfate monohydrate (MnSO4·H2O, 99%), manganese nitrate solution (Mn(NO3)2, 50%) potassium hypermanganate (KMnO4, 99%), toluene solution (99.5%), absolute ethyl alcohol, were used without further purification. 2.2. Materials synthesis 1.4171 g KMnO4, 0.53 g MnSO4·H2O (0.8217 g Mn(Ac)2·4H2O, 7 ml Mn(NO3)2) were separately added into 80 ml deionized water for dissolution and kept stirring for 0.5 h. The solution mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave for crystallization at 180 °C for 12 h (CH). The black resultants were collected by filtration, washed with DI water and dried in an oven at 80 °C for 12 h. The black powder was placed in a muffle furnace at 500 °C for 5 h. The samples obtained were marked as: MnO2(Ac), MnO2(S) and MnO2(N), respectively.
=
C0
Ci C0
× 100%
(5)
Where C0(ppm) is the import toluene concentration and Ci(ppm) is the toluene outlet concentration(i = 1, 2, 3, … …). The mineralization rate (W) was calculated by the following formula,
2.3. Catalyst characterization
W=
The crystallinity of the samples was recorded by X-ray diffraction (XRD). The scanning electron microscope (SEM) was used to observe the
CI +CII × 100% C0 × 7
(6)
Where CI (ppm) is carbon monoxide concentration in tail gas and CII (ppm) is carbon dioxide concentration in tail gas. 2
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Fig. 1. Experimental flow chart.
3. Results and discussion
Fig. 3 was the morphology of manganese dioxide prepared by different precursors. The morphology of MnO2 (Ac) was shown in Fig. 3(a) and (b). It was found from the SEM diagram that the sample was a network structure composed of nanorods, and nanospheres were attached to the nanorods. The formation of this special morphology was due to the short hydrothermal time. The research of Guan et al. showed that when hydrothermal time was 1 h, the manganese dioxide was spherical. When the time was extended to 4 h, the sample was nanorod and nanosphere interlaced, the sample exhibited a clear rod-like structure after prolonging the time to 12 h [19]. However, it was precisely because of the low crystallinity of the sample that the sample formed this particular morphology, which provided a rich pore structure for the sample. The shape of MnO2(S) was shown in Fig. 3(c) and (d). It can be seen from the figure that the sample was spherical, and it can be seen that the sphere was composed of a plurality of nanosheet clusters. The morphology of MnO2(N) was shown in Fig. 3(e) and (f). It can be seen from the figure that the sample was block-shaped and the size was between 50 and 200 nm. Fig. 4(a) indicated that the pore of MnO2(AC) was mainly mesoporous, and the number of pores was large, so it had a large pore volume. Fig. 4(b) showed that the sample MnO2(S) had a small number of mesoporous and macropores and was dominated by macropores, but the number of pores was small. Table 1 was the specific surface area
3.1. Effect of sample morphology on microwave absorption properties The microwave absorption properties of the materials were related to the properties of the materials, such as crystal structure, morphology and pore structure. In order to study how they affect the microwave absorption performance and catalytic activity of the samples, SEM, BET and XRD were used to characterize them. Fig. 2 showed the XRD patterns of different samples. The results showed that the manganese oxide synthesized by using manganese acetate, manganese sulfate and manganese nitrate as the precursor was manganese dioxide. Fig. 2(a) was a diffraction peak of MnO2(Ac) and MnO2(S), directed to the pure tetragonal phase of α-MnO2(PDF #721982). Fig. 2(b) showed the diffraction peak of the sample MnO2(N), which was attributed to pure tetragonal β-MnO2 (PDF#81-2216) [26]. It can be seen from Fig. 2 that the diffraction peak of the sample MnO2(Ac) was wider, indicating that the crystallization degree of the sample was lower; the diffraction peak of the sample MnO2(S) was narrower than that of the sample MnO2(Ac), indicating that the crystallization degree of the sample MnO2(S) was higher than that of the MnO2(Ac), The peak of MnO2(N) was high and narrow, indicating that it has higher crystallinity.
Fig. 2. XRD patterns of (a)MnO2(Ac),MnO2(S)(b)MnO2(N). 3
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Fig. 3. SEM images of sample:(a,b)MnO2(Ac)(c,d)MnO2(S)(e,f)MnO2(N).
and pore volume of the sample. It can be seen from Table 1 that the specific surface area and pore volume of the sample MnO2(S) were reduced by 11.9 times and 6 times, respectively, compared with MnO2(Ac). Fig. 4(c) showed that the pores of the sample MnO2(N) were mainly mesoporous, but the number of pores was so small, and it can be seen from Table 1 that the specific surface area and pore volume of the sample MnO2(N) were reduced by 28 times and 30 times, respectively, compared with MnO2(Ac). The VNA results of the sample were shown in Fig. 5. Fig. 5 showed the electromagnetic parameters of the catalyst, and Fig. 5(a), (b) and (e) represented the dielectric parameters of the sample, which represented the ability of the sample to absorbing microwave radiation, the ability of samples to convert microwave radiation energy into heat energy and the comprehensive ability of the sample to use microwave radiation energy. It can be seen from Fig. 5 that the ability of the sample to absorb microwave was: MnO2(S) > MnO2(Ac) > MnO2(N)(Fig. 5(a)), and the ability to convert microwave radiation into heat energy was: MnO2(Ac) > MnO2(S) > MnO2(N) (Fig. 5(b)), the comprehensive ability of the sample to use microwave radiation energy was: MnO2(Ac) > MnO2(S) > MnO2(N) (Fig. 5(e)). It can be seen that the order of ε′′ and the tangent of the dielectric loss was the same, but it was not consistent with the ε′. This showed that the comprehensive ability of the sample to use microwave radiation mainly depends on the ability of the sample to convert the microwave radiation energy into heat energy. This was due to the fact that "ε" not only represented the ability of the sample to exchange microwave radiation energy into thermal energy, but also represented the internal electric field generated when the microwave passes through the dielectric material,
causing free charge motion and even polarization effects [27–29]. And, the depolarization effect of the sample was an important factor in the microwave absorption performance of the sample [30]. Fig. 5(c), (d) and (f) were the magnetic loss parameters of the sample, which were the μ’, μ’’ and the tangent of the magnetic loss, respectively. It can be seen that the order of the three parameters was MnO2(N) > MnO2(S) > MnO2(Ac), and the values of the three parameters were very small, which was contrary to the order of catalytic activity and dielectric loss tangent. This also proved that manganese oxide was a dielectric loss type material [26]. The results of several researchers showed that the microwave absorption properties of the samples were related to the thickness [31–33]. Therefore, the absorption characteristics of samples with different thicknesses were studied. Fig. 6 showed the reflection loss value of the sample, which showed the absorption characteristics of the sample. The results showed that the sample MnO2(Ac) had the best absorption performance at a frequency of 5.1 GHz and a thickness of 5 mm, RL = −25 dB (Fig. 6(a) and (b)). MnO2(S) had the best absorbing performance at a frequency of 11.6 GHz and a thickness of 2 mm, RL = −18 dB(Fig. 6(c) and (d)). MnO2(N) had the best absorbing performance at a frequency of 18 GHz and a thickness of 5 mm, RL = −17 dB(Fig. 6(e) and (f)). It can be seen that the order of the absorbing properties of the samples was: MnO2(Ac) > MnO2(S) > MnO2(N). XRD results showed that the sample MnO2(Ac) and sample MnO2(S) directed to the pure tetragonal phase of α-MnO2. The SEM image showed that the sample MnO2(Ac) was in a network structure with nanospheres attached to it, and the sample MnO2(S) was a spherical shape composed of nanosheets. The sample MnO2(N) was 4
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Fig. 4. Pore size distribution map of sample:(a)MnO2(Ac)(b)MnO2(S)(c)MnO2(N).
conventional thermal field and microwave thermal field was: MnO2(Ac) > MnO2(S) > MnO2(N). In the conventional thermal field, the temperatures required for the mineralization rate of samples MnO2(Ac), MnO2(S) and MnO2(N) to reach 90% were: 250 °C, 275 °C and 275 °C, respectively; in the microwave thermal field were 200 °C, 250 °C and 275 °C, respectively. Compared with the conventional thermal field, the conversion of toluene in microwave thermal field of MnO2(Ac), MnO2(S) and MnO2(N) increased by 59%, 42% and 12%, and the mineralization rate increased by 36%,11% and 2%, respectively, when the catalytic temperature was 150 °C. It can be seen that the catalytic activity of the sample in the microwave field was better than conventional thermal field. It can be seen that in the microwave field, the catalytic activity of the sample MnO2(Ac) was increased the most, followed by MnO2(S) and MnO2(N), respectively. It was also found that the ability of sample to utilize microwave radiation was proportional to the catalytic activity of the sample, which indicated that the catalytic activity of the sample in microwave field was related to the ability of sample itself to utilize microwave radiation. It can be seen from Fig. 5(b) that the ability of the sample to generate dielectric loss by free charge shift and even polarization effect was MnO2(Ac) > MnO2(S) > MnO2(N) [27]. This positive correlation between microwave absorption performance and microwave radiation catalytic activity may be due to the good microwave absorption characteristics not only make the material temperature up rapidly, but also produce a stronger “dipolar polarization effect” [35]. Fig. 7(e) showed the catalytic oxidation of toluene in a microwave field without catalyst. The results showed that pure microwave radiation have no ability for catalytic oxidation of toluene, and the increase of the catalytic activity of the sample in the microwave field was the result of the interaction of the catalyst and microwave radiation.
Table 1 Specific surface area and total Pore Volume of different catalysts. Sample
Surface Area (m2/g)
Total Pore Volume (cm3/g)
MnO2(Ac) MnO2(S) MnO2(N)
198.9 16.7 7.1
1.2 0.2 0.04
directed to β-MnO2 and was in the form of a block. It can be seen that αMnO2 had better microwave absorption characteristics than β-MnO2. The results of BET showed that the order of pore volume was MnO2(Ac)-1.2 cm3/g > MnO2(S)-0.2 cm3/g > MnO2(N)-0.04 cm3/g, which was consistent with the microwave absorption property of the sample. Some studies had shown that the samples with large specific surface area and porous structure had better microwave absorption properties because of their interfacial polarization and multiple reflection [27,29,34]. Wang et al.'s research results showed that the rich pore structure will cause multiple reflections of the microwaves injected into the sample, thereby reducing the reflection loss and increasing the microwave utilization rate, and apparently improving the microwave absorption performance of the sample [23]. The combination of morphology and pore structure made the samples obtained by different precursors had different microwave absorption properties. 3.2. Relationship between catalytic activity and microwave absorption performance of samples Fig. 7 showed the catalytic activity of MnO2 prepared by different precursors in conventional thermal and microwave fields. It can be seen from Fig. 7 that the order of catalytic activity of different samples in the 5
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Fig. 5. (a) ε′(b) ε′′(c) μ′(d) μ′′(e) dielectric loss tangent (f)magnetic loss tangent of samples at different frequencies.
The surface groups of the catalyst were analyzed by FTIR and the results were shown in Fig. 8. The bands at 612 and 493 cm−1 attributed to Mn-O or/and Mn=O [36]. The peaks at around 542, 715 were ascribed to the vibrations of Mn-O of MnO6 octahedral. The absorption bands at around 1633, 1383 and 1108 cm−1 were assigned to the O-H bending vibrations combined with Mn atoms. The broad band at around 3421 cm−1 was attributed to the O-H stretching vibration [37]. It can be seen from the diagram that the peak intensities of Mn-O, Mn-OH and OH of sample MnO2(Ac) were higher than those of sample MnO2(S) and MnO2(N). This further explained the good catalytic activity of MnO2(Ac). In order to study the polarization effect of samples and the mechanism of microwave absorption, the Debye relaxation phenomenon in the system was studied. Dipole relaxation process was an important mechanism to investigate the microwave absorption properties of the sample. According to the Debye theory, the dielectric constant of the sample can be expressed by the following equation [38]:
r
=
i=
+
s
1+i
0
(7)
where s , and 0 are the static dielectric constant, the dielectric constant at infinite frequency and the relaxation time, respectively. Therefore, and can be deduced as: s
=
+
=
0( s 1+(
1+(
0)
(8)
2
) 0)
(9)
2
According to formula (8) and (9), the relationship between can be expressed as:
(
2
)2 + ( ) = (
s
)2
and (10)
Based on formula (10), it was clear that the plot of versus was a single semicircle, which was denoted as a Cole-Cole semicircle. Each 6
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Fig. 6. Three-dimensional plot of RL and Reflection loss values of different samples:(a,b)MnO2(Ac)(c,d)MnO2(S)(e,f)MnO2(N).
semicircle was related to one Debye dipolar relation process. Fig. 9 showed the plots of versus for the MnO2(Ac), MnO2(S) and MnO2(N). It can be seen from the diagram that there were multiple semicircles in all three samples, indicating that there was dipolar polarization relaxation in all three samples. However, it can be seen from the diagram that the semicircle in MnO2(Ac) and MnO2(S) had different degrees of distortion, which was due to the existence of interfacial polarization, atomic polarization and electron polarization in the two samples [28,33,39–41]. According to Maxwell-Wagner effect, the morphology, size difference and surface defects of the samples were also the causes of interfacial polarization. It can be seen from the SEM image that the sample MnO2(Ac) had a network structure with nanospheres attached thereto, this superimposed morphology caused interfacial polarization on the surface of the sample. It can be found that the sample MnO2(S) was a flower ball composed of multi-layer nanoparticles, and this kind of multi-layer folding morphology also caused the interface polarization of the sample [42,43].
The microwave attenuation characteristics and impedance matching (Z) were also the key factors affecting the absorbing properties of the samples. The microwave attenuation characteristics of the samples were determined by the attenuation constant(α) of the samples, which was defined as follows:
=
Z=
2 f × c
µr r
(µ
µ )+
(µ
2
µ ) +( µ +
µ)
2
(11) (12)
Where f is the frequency of the electromagnetic wave and C is the velocity of light. Fig. 10(a) showed the impedance matching of the samples, and the results indicated that the order of the impedance matching of the sample was as follows: MnO2(N) > MnO2(Ac) > MnO2(S). Fig. 10(b) showed the frequency dependence of the attenuation constant. The results showed that the order of the values of the samples 7
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Fig. 7. Catalytic activity of samples in different thermal fields: (a,b)conventional thermal field(c,d)microwave heat field; (e)Microwave radiation catalyzed toluene without sample.
was: MnO2(Ac) > MnO2(S) > MnO2(N). It indicated that the sample MnO2(Ac) had good attenuation and microwave absorption characteristics. Although sample MnO2(N) had good impedance matching, because of its small attenuation constant, the absorbing performance of sample MnO2(N) was poor [22,27,38,42]. The analysis results of electromagnetic parameters showed that the good microwave absorption performance of MnO2(Ac) was due to the proper impedance matching, high attenuation constant and Debye dipole relaxation.
3.3. Kinetic parameters In order to study the difference of toluene catalystic-oxidation by manganese oxides prepared by different precursor systems in microwave radiation (WR) and conventional thermal field (CH), the kinetic parameters of catalytic reaction of manganese oxides in different thermal fields were calculated. According to the related literature, the kinetic analysis of catalytic degradation of toluene gas under oxygenrich conditions can be described by first-order reaction [44–46]. Therefore, the first order reaction was also used to analyze the kinetics 8
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rA
b Rn
K c CAb CWP =
rA (obs) c R2 De CAs
(14)
<1
(15)
The kinetic parameters of the sample were showed in Fig. 11 and Table 2. The order of activation energy required for catalystic the oxidation of toluene in the microwave field and the conventional thermal field was MnO2(AC) < MnO2(S) < MnO2(N). Compared with the conventional thermal field, the activation energy of the reaction in the microwave field was reduced by 2.77 times, 2.14 times and 1.98 times, respectively, which indicates that the catalytic degradation of toluene gas was more likely to occur under microwave irradiation. The order of activation energy required for the reaction of the sample in microwave field was MnO2(Ac)-50.1 kJ/mol < MnO2(S)-90.8 kJ/ mol < MnO2(N)-116.3 kJ/mol. It can be found that the better the microwave absorption performance of the sample, the stronger the dipole effect of the sample in the microwave field reaction, the lower the activation energy of the reaction. From the results of FTIR, it can be seen that the content of functional groups on the surface of MnO2(Ac) was obviously higher than that of MnO2(S) and MnO2(N). Some studies had shown that OH on the surface of the sample promoted the degradation of toluene. And the reaction of the catalyst on the surface of the sample depended on the adsorption of toluene on the surface of the sample and the strength of the Mn-O bond [47]. According to the results of BET, the sample MnO2(Ac) had a large specific surface area, so that the sample has more adsorption sites. The toluene molecules adsorbed on the surface of the sample react with the –OH and Mn-O groups on the surface to form CO2 and water [48]. The better catalytic activity of the sample in the microwave field was due to:
Fig. 8. The FTIR spectrum of the sample.
of toluene catalytic oxidation under microwave thermal field and conventional thermal field. The equations were as follows: r = -kc = [-Aexp(-Ea/RT)]c
< 0.15
(13)
where A, r, Ea and k are pre-exponential factor (s−1), reaction rate (molcm−3 s−1), apparent activation energy (kJ/mol) and rate constant (s−1). The k value was calculated according to the experimental space velocity and the conversion of toluene. Mass transfer calculation of the highest reaction rate were carried out for catalytic oxidation of toluene. The Weisz–Prater criterion,
Fig. 9. The relation between (ε′) and ε’’ (Cole-Cole plot) of (a) MnO2(Ac),(b) MnO2(S),(c)MnO2(N). 9
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Fig. 10. (a)Impedance matching, (b) attenuation constant of samples.
manganese nitrate as precursors. The toluene catalytic oxidation activity of the samples in conventional thermal and microwave fields was investigated. The effects of sample morphology and microwave absorption properties on the catalytic activity of the samples were analyzed by XRD, SEM, BET and VNA. The results showed that the catalytic activity of the sample in the microwave field was significantly improved. The toluene conversion of the sample MnO2(Ac) was increased by 59% at a catalytic temperature of 150 °C, which was due to the best microwave absorption properties of the sample MnO2(Ac). MnO2 (Ac) had good impedance matching, high attenuation constant and Debye dipole relaxation effect were the reasons for its good microwave absorption performance. The relationship between the catalytic activity of the samples and the microwave absorption performance was studied, and the apparent activation energy of the catalytic reactions in different thermal fields was calculated. It was found that the ability of the sample to utilize microwave radiation was directly proportional to the catalytic activity of the sample. The positive correlation between microwave absorption performance and microwave radiation catalytic activity may be due to the sample's good microwave absorption performance, which not only caused the material temperature to rise rapidly, but also produced a stronger “dipole polarization effect”. This allowed the polar compound molecules to vibrate at high speed, thereby reducing the activation energy required for the reaction to promote oxidation.
Fig. 11. Arrhenius plots for degradation of toluene over materials under microwave radiation (WR) and conventional thermal field(CH). Table 2 Reaction kinetics fitting data. sample
catalytic thermal field
Ea(KJ/mol)
MnO2(Ac) MnO2(S) MnO2(N) MnO2(Ac) MnO2(S) MnO2(N)
WR WR WR CH CH CH
50.1 90.8 116.3 138.4 194.3 220.1
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
on the one hand, microwave radiation created a “hot spot effect” on the surface of the sample, which means that the local temperature of the sample was much higher than the temperature of other parts, thereby promoting the oxidation reaction [49]. On the other hand, microwave radiation produced polarization effect and high speed vibration of molecules, which reduces the activation energy of the reaction and makes the reaction easier, thus promoting the deep degradation of toluene and showing a higher mineralization rate [35].
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4. Conclusion Manganese dioxide with different morphology was prepared by hydrothermal method with manganese acetate, manganese sulfate and 10
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Effect of microwave absorption properties and morphology of manganese dioxide on catalytic oxidation of toluene under microwave irradiation Honghong Yia,b, Lingling Songa, Xiaolong Tanga,b,∗, Shunzheng Zhaoa,b, Zhongyu Yanga, Xizhou Xiea, Chuanbo Maa, Yuanyuan Zhanga, Xiaodong Zhanga a b
Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing, 100083, PR China
ARTICLE INFO
ABSTRACT
Keywords: Toluene MnO2 Microwave radiation Catalytic oxidation Microwave absorption properties
A large number of studies had shown that the morphology of the sample had a significant effect on the microwave absorption properties and catalytic activity of the sample. Manganese dioxide with different morphologies was synthesized by hydrothermal method through different precursors. The effects of sample morphology and microwave absorption properties on the catalytic activity of the sample in conventional thermal and microwave fields were studied. The results indicated that compared with the conventional thermal field, the catalytic activity of the samples in microwave field were obviously improved, and the activation energy of the reaction were decreased. Compared with the conventional thermal field, the conversion of toluene in microwave thermal field of MnO2(Ac), MnO2(S) and MnO2(N) increased by 59%, 42% and 12%, and the mineralization rate increased by 36%,11% and 2%, respectively, when the catalytic temperature was 150 °C. Compared with the traditional thermal field, the activation energy of the sample MnO2(Ac) in the microwave field was reduced by 88.3 KJ. A series of characterization results showed that the sample MnO2(Ac) had good catalytic activity in the microwave field was due to: MnO2(Ac) had proper microwave absorption properties, large amount of surface functional groups, large specific surface area and rich pore structure. The analysis results of electromagnetic parameters showed that: the reason that the sample MnO2(Ac) had good microwave absorption performance was that the MnO2(Ac) had proper impedance matching, high attenuation constant and Debye dipole relaxation effect.
1. Introduction With the development of industrialization, air pollution has become a problem that cannot be ignored. Most VOCs are irritating, toxic and carcinogenic, which could destroy the ozone layer, acid rain and producing photochemical smog [1–3]. Thus, VOCs has become one of the main sources of atmospheric pollutants. At present, the most economical and effective method for removing VOCs is catalytic oxidation. Compared with other methods, catalytic oxidation has the advantages of low auxiliary cost, simple operation, high removal rate and no secondary pollution. Most of the current catalytic oxidation technologies are carried out in conventional thermal fields. However, compared with the conventional heating, microwave heating has the advantages of rapid and
efficient heating, no secondary pollution, energy-saving, cleaning, simple operation and so on [4,5]. Microwave heating is the heating of ultra-high frequency electromagnetic waves generated by a magnetron in a microwave oven [2,6]. Since the 21st century, microwave catalysis has attracted more and more researchers' attention and achieved a series of achievements [7–11]. Zhou et al. used CuO-CeO2@AC for microwave degradation of 100 mg/L crystal violet wastewater, the degradation rate reached 99.5% at 400 W for 5 min [7]. Costa et al. used Fe/Ac catalyst for microwave radiation to degrade phenol in wastewater. The removal efficiency of microwave irradiation for 15 min can reach 100% [8]. Zhang et al. used carbon nanotubes to degrade the aqueous solution containing organic pollutants and completely degradation at 450 W for 3 min [12]. Some studies used mussel-excited chemistry to prepare samples for efficient removal of organic dyes
∗ Corresponding author. Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail addresses: [email protected] (H. Yi), [email protected] (L. Song), [email protected] (X. Tang), [email protected] (S. Zhao), [email protected] (Z. Yang), [email protected] (X. Xie), [email protected] (C. Ma), [email protected] (Y. Zhang), [email protected] (X. Zhang).
https://doi.org/10.1016/j.ceramint.2019.10.020 Received 1 September 2019; Received in revised form 29 September 2019; Accepted 2 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Honghong Yi, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.020
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[13–15]. Zeng et al. prepared MoS2-PDA-Ag nanocomposites via combination of mussel inspired chemistry and microwave irradiation. Compared with pure MoS2 and MoS2-PDA, the obtained composite material had higher catalytic activity for the reduction of 4-nitrophenol and great antibacterial activity [16]. Wang et al. combined microwave radiation and photocatalysis to degrade tetracycline, the results showed that microwave irradiation enhanced the separation of photogenerated charge carriers and catalytic performance [17]. It can be seen that the method of using microwave for catalysis of organic pollutants was feasible. The catalytic activity of the sample in the microwave field is related to the microwave absorption performance of the sample, which refers to the ability of the sample to convert microwave radiation energy into heat energy. Manganese oxide can form different morphologies by different bonding methods and MnO2 has good catalytic activity for benzene series [18,19]. A series of studies have shown that the morphology has a great effect on the microwave absorption properties of the materials [20–24]. Zhou's research showed that the tetragonal nanorods had stronger microwave absorption properties than the columnar nanorods and the tetragonal nanotubular MnO2 [25]. Wang's research showed that the reflection loss of manganese dioxide hollow spheres was 2.5 times higher than that of manganese dioxide nanoribbons [23]. Guan's research showed that the absorption performance of one-dimensional a-MnO2 nanorods was two times higher than that of MnO2 without obvious morphology [19]. However, the influence of the morphology and microwave absorption properties of MnO2 on the catalytic oxidation of toluene has not been studied. In this paper, manganese dioxide with different morphology was prepared by hydrothermal method with manganese acetate, manganese sulfate and manganese nitrate as precursors. The toluene catalytic oxidation activity of the samples in conventional thermal and microwave fields was investigated. The effects of the morphology of the samples on the microwave absorption properties and the relationship between the microwave absorption properties and the catalytic activity were investigated by a series of characterizations. This study proposed the relationship between microwave absorption properties and catalytic activity of materials, and provides a theoretical basis for explaining the promotion of catalytic reactions by microwave radiation.
morphology of the sample. The nitrogen adsorption and desorption curve was used to analysis nitrogen adsorption and desorption, the specific surface area and pore size. A Nicolet IS 50 Fourier transform infrared (FTIR) was used to investigate functional groups of samples surface ranging the wavenumber from 400 to 4000 cm−1. The vector network analysis (VNA) was used to determine the electromagnetic parameters of the sample. The sample and paraffin were mixed in a certain ratio and pressed into a ring of a certain thickness ( in = 3mm , out = 7mm) , and the measured values of the real permittivity (ε’), imaginary permittivity (ε’‘), real permeability (μ’), and imaginary permeability (μ’’) were recorded on a vector network analyzer. According to the transmission line theory, the expression of reflection loss (RL) was calculated as follows:
µr = µ
µ i
(1)
=
i
(2)
r
Zin = Z0
µr
tanh j
2 fd µr
r
RL = 20 log (Zin
r
c
Z0)/(Zin + Z0 )
(3) (4)
Where μr is complex permeability of the absorber, εr is complex permittivity of the absorber, f is the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of the absorber. 2.4. Catalyst activity evaluation The experimental flow chart was shown in Fig. 1. The catalytic oxidation of toluene was carried out in a quartz reactor at atmospheric pressure. A certain amount of catalyst was put in the quartz reaction tube (0.7 cm in internal diameter). In conventional thermal field catalysis, the quartz reaction tube was placed on a tubular heating furnace. The temperature of the catalyst was controlled by electric furnace heating. In microwave field catalysis, the quartz reactor was placed in microwave oven, and the catalyst was heated by adjusting the microwave power, and then the catalyst in the reactor was connected to microwave oven by thermocouple to monitor the temperature of catalyst in real time. Toluene vapor with the concentration of 300 ppm was generated by gas distribution system and ice bath toluene bubble device. The total flow was 190 mL/min with the gas hourly space velocity (GHSV) of 30000 h−1. The tail gas after reaction entered the gas chromatograph to analyze the concentrations of toluene, carbon monoxide and carbon dioxide. The catalytic activity of the sample was evaluated by toluene removal rate and mineralization rate, respectively. The expression of toluene conversion rate ( ) was as follows,
2. Experimental 2.1. Raw materials Manganese(II) acetate tetrahydrate (Mn(Ac)2·4H2O, 99%), manganese sulfate monohydrate (MnSO4·H2O, 99%), manganese nitrate solution (Mn(NO3)2, 50%) potassium hypermanganate (KMnO4, 99%), toluene solution (99.5%), absolute ethyl alcohol, were used without further purification. 2.2. Materials synthesis 1.4171 g KMnO4, 0.53 g MnSO4·H2O (0.8217 g Mn(Ac)2·4H2O, 7 ml Mn(NO3)2) were separately added into 80 ml deionized water for dissolution and kept stirring for 0.5 h. The solution mixture was then transferred into a 100 mL Teflon-lined stainless steel autoclave for crystallization at 180 °C for 12 h (CH). The black resultants were collected by filtration, washed with DI water and dried in an oven at 80 °C for 12 h. The black powder was placed in a muffle furnace at 500 °C for 5 h. The samples obtained were marked as: MnO2(Ac), MnO2(S) and MnO2(N), respectively.
=
C0
Ci C0
× 100%
(5)
Where C0(ppm) is the import toluene concentration and Ci(ppm) is the toluene outlet concentration(i = 1, 2, 3, … …). The mineralization rate (W) was calculated by the following formula,
2.3. Catalyst characterization
W=
The crystallinity of the samples was recorded by X-ray diffraction (XRD). The scanning electron microscope (SEM) was used to observe the
CI +CII × 100% C0 × 7
(6)
Where CI (ppm) is carbon monoxide concentration in tail gas and CII (ppm) is carbon dioxide concentration in tail gas. 2
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Fig. 1. Experimental flow chart.
3. Results and discussion
Fig. 3 was the morphology of manganese dioxide prepared by different precursors. The morphology of MnO2 (Ac) was shown in Fig. 3(a) and (b). It was found from the SEM diagram that the sample was a network structure composed of nanorods, and nanospheres were attached to the nanorods. The formation of this special morphology was due to the short hydrothermal time. The research of Guan et al. showed that when hydrothermal time was 1 h, the manganese dioxide was spherical. When the time was extended to 4 h, the sample was nanorod and nanosphere interlaced, the sample exhibited a clear rod-like structure after prolonging the time to 12 h [19]. However, it was precisely because of the low crystallinity of the sample that the sample formed this particular morphology, which provided a rich pore structure for the sample. The shape of MnO2(S) was shown in Fig. 3(c) and (d). It can be seen from the figure that the sample was spherical, and it can be seen that the sphere was composed of a plurality of nanosheet clusters. The morphology of MnO2(N) was shown in Fig. 3(e) and (f). It can be seen from the figure that the sample was block-shaped and the size was between 50 and 200 nm. Fig. 4(a) indicated that the pore of MnO2(AC) was mainly mesoporous, and the number of pores was large, so it had a large pore volume. Fig. 4(b) showed that the sample MnO2(S) had a small number of mesoporous and macropores and was dominated by macropores, but the number of pores was small. Table 1 was the specific surface area
3.1. Effect of sample morphology on microwave absorption properties The microwave absorption properties of the materials were related to the properties of the materials, such as crystal structure, morphology and pore structure. In order to study how they affect the microwave absorption performance and catalytic activity of the samples, SEM, BET and XRD were used to characterize them. Fig. 2 showed the XRD patterns of different samples. The results showed that the manganese oxide synthesized by using manganese acetate, manganese sulfate and manganese nitrate as the precursor was manganese dioxide. Fig. 2(a) was a diffraction peak of MnO2(Ac) and MnO2(S), directed to the pure tetragonal phase of α-MnO2(PDF #721982). Fig. 2(b) showed the diffraction peak of the sample MnO2(N), which was attributed to pure tetragonal β-MnO2 (PDF#81-2216) [26]. It can be seen from Fig. 2 that the diffraction peak of the sample MnO2(Ac) was wider, indicating that the crystallization degree of the sample was lower; the diffraction peak of the sample MnO2(S) was narrower than that of the sample MnO2(Ac), indicating that the crystallization degree of the sample MnO2(S) was higher than that of the MnO2(Ac), The peak of MnO2(N) was high and narrow, indicating that it has higher crystallinity.
Fig. 2. XRD patterns of (a)MnO2(Ac),MnO2(S)(b)MnO2(N). 3
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Fig. 3. SEM images of sample:(a,b)MnO2(Ac)(c,d)MnO2(S)(e,f)MnO2(N).
and pore volume of the sample. It can be seen from Table 1 that the specific surface area and pore volume of the sample MnO2(S) were reduced by 11.9 times and 6 times, respectively, compared with MnO2(Ac). Fig. 4(c) showed that the pores of the sample MnO2(N) were mainly mesoporous, but the number of pores was so small, and it can be seen from Table 1 that the specific surface area and pore volume of the sample MnO2(N) were reduced by 28 times and 30 times, respectively, compared with MnO2(Ac). The VNA results of the sample were shown in Fig. 5. Fig. 5 showed the electromagnetic parameters of the catalyst, and Fig. 5(a), (b) and (e) represented the dielectric parameters of the sample, which represented the ability of the sample to absorbing microwave radiation, the ability of samples to convert microwave radiation energy into heat energy and the comprehensive ability of the sample to use microwave radiation energy. It can be seen from Fig. 5 that the ability of the sample to absorb microwave was: MnO2(S) > MnO2(Ac) > MnO2(N)(Fig. 5(a)), and the ability to convert microwave radiation into heat energy was: MnO2(Ac) > MnO2(S) > MnO2(N) (Fig. 5(b)), the comprehensive ability of the sample to use microwave radiation energy was: MnO2(Ac) > MnO2(S) > MnO2(N) (Fig. 5(e)). It can be seen that the order of ε′′ and the tangent of the dielectric loss was the same, but it was not consistent with the ε′. This showed that the comprehensive ability of the sample to use microwave radiation mainly depends on the ability of the sample to convert the microwave radiation energy into heat energy. This was due to the fact that "ε" not only represented the ability of the sample to exchange microwave radiation energy into thermal energy, but also represented the internal electric field generated when the microwave passes through the dielectric material,
causing free charge motion and even polarization effects [27–29]. And, the depolarization effect of the sample was an important factor in the microwave absorption performance of the sample [30]. Fig. 5(c), (d) and (f) were the magnetic loss parameters of the sample, which were the μ’, μ’’ and the tangent of the magnetic loss, respectively. It can be seen that the order of the three parameters was MnO2(N) > MnO2(S) > MnO2(Ac), and the values of the three parameters were very small, which was contrary to the order of catalytic activity and dielectric loss tangent. This also proved that manganese oxide was a dielectric loss type material [26]. The results of several researchers showed that the microwave absorption properties of the samples were related to the thickness [31–33]. Therefore, the absorption characteristics of samples with different thicknesses were studied. Fig. 6 showed the reflection loss value of the sample, which showed the absorption characteristics of the sample. The results showed that the sample MnO2(Ac) had the best absorption performance at a frequency of 5.1 GHz and a thickness of 5 mm, RL = −25 dB (Fig. 6(a) and (b)). MnO2(S) had the best absorbing performance at a frequency of 11.6 GHz and a thickness of 2 mm, RL = −18 dB(Fig. 6(c) and (d)). MnO2(N) had the best absorbing performance at a frequency of 18 GHz and a thickness of 5 mm, RL = −17 dB(Fig. 6(e) and (f)). It can be seen that the order of the absorbing properties of the samples was: MnO2(Ac) > MnO2(S) > MnO2(N). XRD results showed that the sample MnO2(Ac) and sample MnO2(S) directed to the pure tetragonal phase of α-MnO2. The SEM image showed that the sample MnO2(Ac) was in a network structure with nanospheres attached to it, and the sample MnO2(S) was a spherical shape composed of nanosheets. The sample MnO2(N) was 4
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Fig. 4. Pore size distribution map of sample:(a)MnO2(Ac)(b)MnO2(S)(c)MnO2(N).
conventional thermal field and microwave thermal field was: MnO2(Ac) > MnO2(S) > MnO2(N). In the conventional thermal field, the temperatures required for the mineralization rate of samples MnO2(Ac), MnO2(S) and MnO2(N) to reach 90% were: 250 °C, 275 °C and 275 °C, respectively; in the microwave thermal field were 200 °C, 250 °C and 275 °C, respectively. Compared with the conventional thermal field, the conversion of toluene in microwave thermal field of MnO2(Ac), MnO2(S) and MnO2(N) increased by 59%, 42% and 12%, and the mineralization rate increased by 36%,11% and 2%, respectively, when the catalytic temperature was 150 °C. It can be seen that the catalytic activity of the sample in the microwave field was better than conventional thermal field. It can be seen that in the microwave field, the catalytic activity of the sample MnO2(Ac) was increased the most, followed by MnO2(S) and MnO2(N), respectively. It was also found that the ability of sample to utilize microwave radiation was proportional to the catalytic activity of the sample, which indicated that the catalytic activity of the sample in microwave field was related to the ability of sample itself to utilize microwave radiation. It can be seen from Fig. 5(b) that the ability of the sample to generate dielectric loss by free charge shift and even polarization effect was MnO2(Ac) > MnO2(S) > MnO2(N) [27]. This positive correlation between microwave absorption performance and microwave radiation catalytic activity may be due to the good microwave absorption characteristics not only make the material temperature up rapidly, but also produce a stronger “dipolar polarization effect” [35]. Fig. 7(e) showed the catalytic oxidation of toluene in a microwave field without catalyst. The results showed that pure microwave radiation have no ability for catalytic oxidation of toluene, and the increase of the catalytic activity of the sample in the microwave field was the result of the interaction of the catalyst and microwave radiation.
Table 1 Specific surface area and total Pore Volume of different catalysts. Sample
Surface Area (m2/g)
Total Pore Volume (cm3/g)
MnO2(Ac) MnO2(S) MnO2(N)
198.9 16.7 7.1
1.2 0.2 0.04
directed to β-MnO2 and was in the form of a block. It can be seen that αMnO2 had better microwave absorption characteristics than β-MnO2. The results of BET showed that the order of pore volume was MnO2(Ac)-1.2 cm3/g > MnO2(S)-0.2 cm3/g > MnO2(N)-0.04 cm3/g, which was consistent with the microwave absorption property of the sample. Some studies had shown that the samples with large specific surface area and porous structure had better microwave absorption properties because of their interfacial polarization and multiple reflection [27,29,34]. Wang et al.'s research results showed that the rich pore structure will cause multiple reflections of the microwaves injected into the sample, thereby reducing the reflection loss and increasing the microwave utilization rate, and apparently improving the microwave absorption performance of the sample [23]. The combination of morphology and pore structure made the samples obtained by different precursors had different microwave absorption properties. 3.2. Relationship between catalytic activity and microwave absorption performance of samples Fig. 7 showed the catalytic activity of MnO2 prepared by different precursors in conventional thermal and microwave fields. It can be seen from Fig. 7 that the order of catalytic activity of different samples in the 5
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Fig. 5. (a) ε′(b) ε′′(c) μ′(d) μ′′(e) dielectric loss tangent (f)magnetic loss tangent of samples at different frequencies.
The surface groups of the catalyst were analyzed by FTIR and the results were shown in Fig. 8. The bands at 612 and 493 cm−1 attributed to Mn-O or/and Mn=O [36]. The peaks at around 542, 715 were ascribed to the vibrations of Mn-O of MnO6 octahedral. The absorption bands at around 1633, 1383 and 1108 cm−1 were assigned to the O-H bending vibrations combined with Mn atoms. The broad band at around 3421 cm−1 was attributed to the O-H stretching vibration [37]. It can be seen from the diagram that the peak intensities of Mn-O, Mn-OH and OH of sample MnO2(Ac) were higher than those of sample MnO2(S) and MnO2(N). This further explained the good catalytic activity of MnO2(Ac). In order to study the polarization effect of samples and the mechanism of microwave absorption, the Debye relaxation phenomenon in the system was studied. Dipole relaxation process was an important mechanism to investigate the microwave absorption properties of the sample. According to the Debye theory, the dielectric constant of the sample can be expressed by the following equation [38]:
r
=
i=
+
s
1+i
0
(7)
where s , and 0 are the static dielectric constant, the dielectric constant at infinite frequency and the relaxation time, respectively. Therefore, and can be deduced as: s
=
+
=
0( s 1+(
1+(
0)
(8)
2
) 0)
(9)
2
According to formula (8) and (9), the relationship between can be expressed as:
(
2
)2 + ( ) = (
s
)2
and (10)
Based on formula (10), it was clear that the plot of versus was a single semicircle, which was denoted as a Cole-Cole semicircle. Each 6
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Fig. 6. Three-dimensional plot of RL and Reflection loss values of different samples:(a,b)MnO2(Ac)(c,d)MnO2(S)(e,f)MnO2(N).
semicircle was related to one Debye dipolar relation process. Fig. 9 showed the plots of versus for the MnO2(Ac), MnO2(S) and MnO2(N). It can be seen from the diagram that there were multiple semicircles in all three samples, indicating that there was dipolar polarization relaxation in all three samples. However, it can be seen from the diagram that the semicircle in MnO2(Ac) and MnO2(S) had different degrees of distortion, which was due to the existence of interfacial polarization, atomic polarization and electron polarization in the two samples [28,33,39–41]. According to Maxwell-Wagner effect, the morphology, size difference and surface defects of the samples were also the causes of interfacial polarization. It can be seen from the SEM image that the sample MnO2(Ac) had a network structure with nanospheres attached thereto, this superimposed morphology caused interfacial polarization on the surface of the sample. It can be found that the sample MnO2(S) was a flower ball composed of multi-layer nanoparticles, and this kind of multi-layer folding morphology also caused the interface polarization of the sample [42,43].
The microwave attenuation characteristics and impedance matching (Z) were also the key factors affecting the absorbing properties of the samples. The microwave attenuation characteristics of the samples were determined by the attenuation constant(α) of the samples, which was defined as follows:
=
Z=
2 f × c
µr r
(µ
µ )+
(µ
2
µ ) +( µ +
µ)
2
(11) (12)
Where f is the frequency of the electromagnetic wave and C is the velocity of light. Fig. 10(a) showed the impedance matching of the samples, and the results indicated that the order of the impedance matching of the sample was as follows: MnO2(N) > MnO2(Ac) > MnO2(S). Fig. 10(b) showed the frequency dependence of the attenuation constant. The results showed that the order of the values of the samples 7
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Fig. 7. Catalytic activity of samples in different thermal fields: (a,b)conventional thermal field(c,d)microwave heat field; (e)Microwave radiation catalyzed toluene without sample.
was: MnO2(Ac) > MnO2(S) > MnO2(N). It indicated that the sample MnO2(Ac) had good attenuation and microwave absorption characteristics. Although sample MnO2(N) had good impedance matching, because of its small attenuation constant, the absorbing performance of sample MnO2(N) was poor [22,27,38,42]. The analysis results of electromagnetic parameters showed that the good microwave absorption performance of MnO2(Ac) was due to the proper impedance matching, high attenuation constant and Debye dipole relaxation.
3.3. Kinetic parameters In order to study the difference of toluene catalystic-oxidation by manganese oxides prepared by different precursor systems in microwave radiation (WR) and conventional thermal field (CH), the kinetic parameters of catalytic reaction of manganese oxides in different thermal fields were calculated. According to the related literature, the kinetic analysis of catalytic degradation of toluene gas under oxygenrich conditions can be described by first-order reaction [44–46]. Therefore, the first order reaction was also used to analyze the kinetics 8
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rA
b Rn
K c CAb CWP =
rA (obs) c R2 De CAs
(14)
<1
(15)
The kinetic parameters of the sample were showed in Fig. 11 and Table 2. The order of activation energy required for catalystic the oxidation of toluene in the microwave field and the conventional thermal field was MnO2(AC) < MnO2(S) < MnO2(N). Compared with the conventional thermal field, the activation energy of the reaction in the microwave field was reduced by 2.77 times, 2.14 times and 1.98 times, respectively, which indicates that the catalytic degradation of toluene gas was more likely to occur under microwave irradiation. The order of activation energy required for the reaction of the sample in microwave field was MnO2(Ac)-50.1 kJ/mol < MnO2(S)-90.8 kJ/ mol < MnO2(N)-116.3 kJ/mol. It can be found that the better the microwave absorption performance of the sample, the stronger the dipole effect of the sample in the microwave field reaction, the lower the activation energy of the reaction. From the results of FTIR, it can be seen that the content of functional groups on the surface of MnO2(Ac) was obviously higher than that of MnO2(S) and MnO2(N). Some studies had shown that OH on the surface of the sample promoted the degradation of toluene. And the reaction of the catalyst on the surface of the sample depended on the adsorption of toluene on the surface of the sample and the strength of the Mn-O bond [47]. According to the results of BET, the sample MnO2(Ac) had a large specific surface area, so that the sample has more adsorption sites. The toluene molecules adsorbed on the surface of the sample react with the –OH and Mn-O groups on the surface to form CO2 and water [48]. The better catalytic activity of the sample in the microwave field was due to:
Fig. 8. The FTIR spectrum of the sample.
of toluene catalytic oxidation under microwave thermal field and conventional thermal field. The equations were as follows: r = -kc = [-Aexp(-Ea/RT)]c
< 0.15
(13)
where A, r, Ea and k are pre-exponential factor (s−1), reaction rate (molcm−3 s−1), apparent activation energy (kJ/mol) and rate constant (s−1). The k value was calculated according to the experimental space velocity and the conversion of toluene. Mass transfer calculation of the highest reaction rate were carried out for catalytic oxidation of toluene. The Weisz–Prater criterion,
Fig. 9. The relation between (ε′) and ε’’ (Cole-Cole plot) of (a) MnO2(Ac),(b) MnO2(S),(c)MnO2(N). 9
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Fig. 10. (a)Impedance matching, (b) attenuation constant of samples.
manganese nitrate as precursors. The toluene catalytic oxidation activity of the samples in conventional thermal and microwave fields was investigated. The effects of sample morphology and microwave absorption properties on the catalytic activity of the samples were analyzed by XRD, SEM, BET and VNA. The results showed that the catalytic activity of the sample in the microwave field was significantly improved. The toluene conversion of the sample MnO2(Ac) was increased by 59% at a catalytic temperature of 150 °C, which was due to the best microwave absorption properties of the sample MnO2(Ac). MnO2 (Ac) had good impedance matching, high attenuation constant and Debye dipole relaxation effect were the reasons for its good microwave absorption performance. The relationship between the catalytic activity of the samples and the microwave absorption performance was studied, and the apparent activation energy of the catalytic reactions in different thermal fields was calculated. It was found that the ability of the sample to utilize microwave radiation was directly proportional to the catalytic activity of the sample. The positive correlation between microwave absorption performance and microwave radiation catalytic activity may be due to the sample's good microwave absorption performance, which not only caused the material temperature to rise rapidly, but also produced a stronger “dipole polarization effect”. This allowed the polar compound molecules to vibrate at high speed, thereby reducing the activation energy required for the reaction to promote oxidation.
Fig. 11. Arrhenius plots for degradation of toluene over materials under microwave radiation (WR) and conventional thermal field(CH). Table 2 Reaction kinetics fitting data. sample
catalytic thermal field
Ea(KJ/mol)
MnO2(Ac) MnO2(S) MnO2(N) MnO2(Ac) MnO2(S) MnO2(N)
WR WR WR CH CH CH
50.1 90.8 116.3 138.4 194.3 220.1
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
on the one hand, microwave radiation created a “hot spot effect” on the surface of the sample, which means that the local temperature of the sample was much higher than the temperature of other parts, thereby promoting the oxidation reaction [49]. On the other hand, microwave radiation produced polarization effect and high speed vibration of molecules, which reduces the activation energy of the reaction and makes the reaction easier, thus promoting the deep degradation of toluene and showing a higher mineralization rate [35].
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