MnOx Catalysts Modified By Nonthermal Plasma For NO Catalytic

Article pubs.acs.org/JPCC

MnOx Catalysts Modified By Nonthermal Plasma For NO Catalytic Oxidation Xiaolong Tang, Kai Li, Honghong Yi,* Ping Ning, Ying Xiang, Jiangen Wang, and Chi Wang Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, China

ABSTRACT: Nonthermal plasma modification (NTPM) has been applied to MnOx catalysts for the catalytic oxidation of nitrogen oxide (NO) at low temperature (50−250 °C), and much higher NO conversion was obtained over the nonthermal plasma-treated catalysts. These modified catalysts were characterized in different waysscanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer−Emmett−Teller (BET), and Fourier transform infrared (FT-IR)and the results can help us to understand the effect of the plasma treatment. The results showed that the surface active sites of these catalysts were enhanced with the treatment of plasma, which improved the dispersion of catalysts. In addition, after NTPM, a considerable amount of deposition with limited depths appeared on the MnOx surface, the average pore size was slightly enlarged, and the pore size distributions of the MnOx became wider. Also, XPS results revealed that the NTPM could increase the oxygen, nitrogen, and kalium functional groups on the catalyst’s surface which contributed to a higher activity for NO catalytic oxidation at low temperature. Our investigation results indicate that nonthermal plasma treatment is an effective way to manipulate catalyst surface properties for NO catalytic oxidation.

1. INTRODUCTION To improve the performance of catalytic materials, much more effort has to been done in the catalyst’s surface properties modification. The surface modification is accomplished via various processing modes: thermal treatment (in O2, H2, or inactive gas), wet impregnation, electrochemical treatment, plasma treatment, ion or cluster bombardment, supercritical fluids, and so forth. Because of its multifold dominances (fast-reacting, solventfree, accurate control, etc.), nonthermal plasma (NTP) technique has been widely introduced in the use of surface properties modification of materials (particularly for polymers) to adapt them for specific application.1−3 NTP can be produced by electrical discharge in which a majority of the electrical energy goes into the generating of energetic electrons rather than into heating the gas. Simultaneously, it is also easy to create any ambiance (i.e., ions, radicals, electrons, photons, and excited molecules) for oxidative, reductive, or inactive reactions, and it can also produce chemically active species on the surface of materials to affect their functions without changing their bulk properties significantly.4−8 On the © 2012 American Chemical Society

other hand, much more attention has been given to catalyst design and optimizing with NTP-based techniques.9−12 Considering the appropriate ratio of NO and NO2, the NOx in flue gas can be easily removed by absorption. The studies show that the catalytic oxidation of NO to NO2 at low temperature is promising for NO reduction.13 Many transition metal oxide catalysts, especially manganese oxides (MnOx), are effective in oxidation reaction processes.14−16 However, a previous investigation in modifying MnOx catalyst preparation was only focused on thermal treatment.17 Comparatively little research has been done on electric treatment especially NTP treatment. In addition, the mechanism of the improved performance of the catalysts, which had been modified with plasma, is still unclear. Received: January 19, 2012 Revised: March 14, 2012 Published: March 16, 2012 10017

dx.doi.org/10.1021/jp300664f | J. Phys. Chem. C 2012, 116, 10017−10028

The Journal of Physical Chemistry C

Article

was CTP-2000P. The plasma was generated at a discharge output voltage (KV) of 2.4−6.4 KV. All the samples (1.0 g catalyst every time) were placed in the chamber in the same way, and the treatment environment was at atmospheric pressure. The detailed experimental parameters of the different samples are listed in Table 1. 2.3. Catalyst Characterization. X-ray diffraction (D/ MAX-2200) patterns were recorded with a Rigaku diffractometer operated at 36 kV and 30 mA using Ni filtered Cu Kα radiation (λ = 0.15406 nm) at a rate of 5°/min with 2θ = 20− 80°. XL30ESEM-TMP (Poland) scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectrometry (EDS) was used for microstructure observation of the interface and element distribution. XPS (PHI 5500) analysis used Al Kα radiation with energy of Al target and power 200 W. Surface area and pore size distribution for the samples were measured using a NOVA2000e (Quantachrome instruments) surface area analyzer which used the nitrogen adsorption−desorption method. The samples were initially outgassed at 393 K for 24 h before adsorption isotherms were generated by dosing nitrogen (at 77 K). The result was analyzed by the density functional theory (DFT) model. The BET equation was used to obtain the specific surface areas (SBET) and the adsorption average pore width (Wp, 4 V/A). The amount of N2 adsorbed at relative pressures near unity (P/P0 = 0.99) was employed to determine the total pore volume (vt). The t-plot theory was employed to calculate the micropore (∼2 nm) surface area (Smi), the external surface area (Se), and micropore volumes (vmi). Density function theory (DFT) was used to analyze the successive pore size distribution (PSD) curves from micropores to macropores (>50 nm). Fourier transform infrared spectroscopy (FT-IR) made by the American Varian company was used to determine the species formed on the catalyst surface. 2.4. Catalytic Activity Measurement. Catalytic activity tests were performed in a Pyrex tube reactor of 8 mm internal diameter. NO and NO2 concentrations were simultaneously measured by Antaris IGS Analyzer made by Thermo Scientific. O2 was measured by Flue Gas Analyzer (TH-990). At steady state, a gas N2 mixture containing 500 ppm NO, 3% O2 was introduced into the reactor. In all the tests, the total flow rate was fixed at 200 mL/min, which corresponded to a GHSV (gas hourly space velocity) of 35 000 h−1. A 0.15 g catalyst was used. O2 was infused directly into the reactor to avoid possible reaction with NO before the catalyst bed. The conversion of NO (%) is defined as

Herein, we attempt to extend the nonthermal plasma treatment method for the preparation of MnOx catalysts to NO catalytic oxidation. The variations of surface active groups, chemical composition, and topography of specimens under optimal NTP treatment conditions were examined by X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The purposes of this work are to explain the effect of NTP on the textural characteristics of catalysts, to investigate how the functional groups and the hetero atoms change after NTP treatment at different conditions, and to evaluate the MnOx catalysts catalytic oxidation capability for NO, which were treated by NTP.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. MnOx catalysts were prepared by the coprecipitation (CP) method. Three solutions were prepared in a water bath at 40 °C and were stirred well to make sure that the solute was dissolved completely. Solution I: [Mn(CH3COO)2·4H2O] (18.4 g), distilled water 500 mL. Solution II: [HO(CH2CH2O)nH] (2.5 g), distilled water 100 mL. Solution III: (KMnO4) (7.9 g), distilled water 300 mL. Solution II was added to solution I, and then solution III was gradually added with thorough stirring after mixing them well. The mixtures were stirred steadily for 6 h at room temperature and then were filtered and were washed with deionized water three to four times. The solid product was dried in a vacuum drier at 100 °C for 12−15 h. Then, the catalysts were treated by NTP at different conditions. All the catalysts above were sized to 40−60 mesh for testing. 2.2. Nonthermal Plasma Treatment. The plasma was produced by dielectric barrier discharge under atmospheric pressure, and the treatment was at atmospheric pressure. It was obtained in a coaxial cylinder-type dielectric barrier discharge reactor by using a corundum tube (inner diameter 20 mm, outer diameter 25 mm) as a dielectric barrier between the inner high voltage electrode (stainless steel tube, outer diameter 10 mm) and a grounded electrode (stainless steel mesh) on the outer wall (as shown in Figure 1). The plasma electrical source was produced by Nanjing Suman electron LTD, and the model

[conv. ]NO = {([NO]inlet − [NO]outlet )/[NO]inlet } × 100%

(NO and NO2 are the main products; we did not detect other products from the outlet. The sum of the outlet NO and NO2 concentrations is consistent with the inlet NO concentration.)

Figure 1. The configuration of dielectric barrier discharge reactor.

Table 1. Experimental Parameters of Different Samples sample MnOx/untreated MnOx/2.4 KV/40 MnOx/5.0 KV/40 MnOx/5.8 KV/40 MnOx/6.0 KV/40

min min min min

output voltage (KV)

treatment time (min)

0 2.4 5.0 5.8 6.0

0 40 40 40 40

sample MnOx/6.4 MnOx/5.8 MnOx/5.8 MnOx/5.8 MnOx/5.8

10018

KV/40 min KV/5 min KV/20 min KV/60 min KV/300 min

output voltage (KV)

treatment time (min)

6.4 5.8 5.8 5.8 5.8

40 5 20 60 300



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3. RESULTS AND DISCUSSION 3.1. XRD Results. The XRD patterns of the MnOx/ untreated catalysts and the MnOx/6.0 KV/40 min are shown in Figure 2. The XRD pattern of MnOx/untreated is quite similar

Table 2. Pore Structure Parameters

sample MnOx/ untreatment MnOx/5.0 KV/40 min MnOx/6.0 KV/40 min MnOx/5.8 KV/5 min MnOx/5.8 KV/20 min MnOx/5.8 KV/40 min

Smi (m2 g−1) SBET 2 −1 t-plot (m g )

Se 2 −1

(m g ) t-plot

vt (cm3 −1 g )

vmi (cm3 g−1) t-plot

Wp (nm) BET (4 V/A)

499

94

405

0.56

0.04

4.49

427

89

337

0.65

0.04

6.08

290

32

259

0.67

0.01

9.17

484

85

399

0.72

0.03

5.94

475

77

378

0.74

0.04

6.24

401

65

336

0.73

0.03

7.23

Table 3. Surface Elemental Analysis of MnOx Catalysts Using XPS sample MnOx/untreated MnOx/5.8 KV/5 min MnOx/5.8 KV/ 10 min MnOx/5.8 KV/ 20 min MnOx/5.8 KV/ 40 min MnOx/5.8 KV/ 300 min MnOx/2.4KV/ 40 min MnOx/5.0 KV/ 40 min MnOx/6.0KV/ 40 min MnOx/6.4 KV/ 40 min

Figure 2. XRD patterns of MnOx/untreated and MnOx/6.0 KV/40 min catalysts.

to that of the MnOx/6.0 KV/40 min. Their diffraction peaks are very broad, and the XRD patterns presented here do not show intense or sharp peaks with manganese oxides. Therefore, it is difficult to identify the phases and oxidation states from these curves, and it is clear that the crystallinity of these catalysts must be very poor and that the particles turn into amorphous phase. Meanwhile, the XRD results of the other MnOx catalyst samples, which were modified on the different conditions by NTP, show the same characterization. Their diffraction peaks are very broad too (not shown). From the XRD results, it can be implied that the crystallinity of catalysts was hardly affected by NTP treatment. Even if the output voltage is high enough, the effect of NTP treatment is not completely similar to the effect of calcination treatment. Lower temperature (the temperature of NTP is low) cannot change the structure of crystallinity, and so even though there are weak changes, the XRD patterns can not be differentiated. 3.2. SEM Results. Figure 3 shows the SEM micrographs of three typical samples on which the amorphous phase can be observed. Plasma treatment does not greatly affect the external MnOx catalyst’s surface on the micrometer when the treatment is not intense enough. From Figure 3a and b, because the treatment voltage was not high enough, the change of MnOx catalyst’s surface morphology was not obvious. Only the aggregation phenomenon was weakened after treatment. However, dramatic variations can be seen in Figure 3c and d. With the increase of treatment voltage as high as 6.4 KV, the catalyst’s surfaces turned to acicular structure because of the etching effect of plasma, and the aggregation phenomenon was weakened. It indicated that NTP is modified in favor of dispersivity of catalysts. 3.3. BET Results. Figure 4a and Figure 5a show the nitrogen adsorption isotherms of some typical samples. Detailed information on the textural properties of these samples is listed in Table 2. According to the IUPAC classification, the adsorption isotherms of all samples are of type II, which indicates that these samples are materials with large pores (mesopores and macropores). The significant increase at the end (P/P0 = 1) can

C1s (at.%)

O1s (at.%)

Mn (at.%)

K (at.%)

N1s (at.%)

43.12 40.47 36.72

36.36 37.16 38.47

19.31 20.56 22.82

0.10 0.60 0.68

1.11 1.21 1.31

33.33

39.63

24.20

0.94

1.44

42.64

34.44

17.30

1.39

4.23

41.69

37.56

19.52

1.23

0.00

50.70

31.15

16.13

0.94

1.09

48.22

32.34

16.80

1.29

1.35

35.61

40.42

21.61

0.90

1.43

27.18

45.85

23.24

1.90

1.82

be attributed to the presence of macropores. The increase of mesopores and macropores might be ascribed to the increasing activation sites or pore widening caused by NTP treatment or possibly because of enhanced polarity of the samples.18 As shown in Figure 4b, Figure 5b, and Table 2, it can be concluded that NTP treatment mainly affects the SBET, Smi, and Se. After NTP treatment, all of them decreased greatly. Smi and SBET decreased more significantly at high voltages and long treatment times. The low values of the specific surface area reveal that some kind of blockage was produced after NTP treatment, but a small quantity of micropores still remains available. Thus, NTP treatment can change the pore structure of the samples in different degrees. As shown in Figure 4b, Figure 5b, and Table 2 for the MnOx catalysts, after NTP treatment, vt and vmi decreased slightly while the Wp increased significantly. The decreasing of the SBET, vt, and vmi might be due to the following reasons, namely, the pores are blocked by the new functional group or species and NTP can destroy the chemical bond and generate new ones. The increase of the mesopores or macropores may mainly be caused by other reasons, that is, the etching effect during the NTP treatment. 3.4. XPS Results. The XPS survey spectra exhibited prominent peaks which were C1s, K2p, Mn2p, O1s, and N1s; consequently, the high-resolution XPS measurements were configured to include these elements. The high-resolution 10019



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Figure 3. The morphologies under different treatment voltages by plasma (a) 0 KV, (b) 5.8 KV, (c) 6.4 KV, (d) enlarged view of c (treatment time: 40 min).

The high-resolution XPS spectra of the C1s region are illustrated in Figure 6a. Deconvolution of the C1s spectra gives three individual component groups that represent the graphitic, aromatic, or aliphatic carbon (C−C, 284.6 eV); the ether, phenol, or CN groups (C−O, 286.4 eV); and the carbonyl, carboxyl, lactone, or esters groups (CO; OC−O, 288.6 eV). Table 4 shows that after NTPM the graphitic, aromatic, or aliphatic carbon and the phenol, ethers, or CN groups decrease dramatically especially in the condition of high voltage and long treatment time while the carbonyl, carboxyl, lactone, or esters groups increase after NTPM. The K2p spectra of NTP treated MnOx catalysts were classified into two kinds of K species: a K functional group of KNO3, KN2, or KNO2 at 292.9 eV and the peak at 295.3 eV which is attributed to K intercalation compounds on the surface of catalysts. After the plasma treatments on the MnOx catalysts, the two kinds of K atomic portions dramatically increased (Table 4, Figure 6a). This is obvious with the increase of

spectra of C1s, K2p, Mn2p, O1s, and N1s were acquired over 275−290, 291−300, 638−647, 521−541, and 388−408 eV with the same step size of 0.1 eV. Table 3 summarizes the elemental compositions (at.%) of the samples over the sampling depth of several atomic layers from the surface. As shown in Table 3, the relative percentages of C atoms are decreased after NTPM except for MnOx/2.4 KV/40 min and MnOx/5.0 KV/40 min, and the O atoms increased slightly except for MnOx/2.4 KV/ 40 min, MnOx/5.0 KV/40 min, and MnOx/5.8 KV/40 min. This might suggest that a larger amount of free radicals is formed in such conditions compared with other samples. These free radicals might react with air, and thus, the concentration of oxygen atoms is increased. Meanwhile, the relative percentages of K and N atoms increase remarkably on the surface for MnOx after NTP treatment. With increasing treatment voltage, the C1s (at.%) continuously decreases. The reason is that the interior temperature of NTP increases with increasing voltage, and a small quantity of C will be eliminated. 10020



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Figure 4. (a) Nitrogen adsorption isotherms and (b) corresponding PSDs obtained by the DFT of the samples (effect of different treatment voltages).

Figure 5. (a) Nitrogen adsorption isotherms and (b) corresponding PSDs obtained by the DFT of the samples (effect of different treatment times).

voltage and time. From this change of K, it is considered that inner K atoms of catalysts are excited to the catalyst’s surface. During this process, K intercalation compounds are generated. Meanwhile, high voltage and long treatment time can contribute to more K atomic excitation from inside to surface. The K atoms on the surface will react with reactive radicals to produce the KNO3, KN2, or KNO2. The high-resolution XPS spectra of the O1s region are illustrated in Figure 6b. Three different O functional groups as well as a contribution of chemisorbed water were identified as reported in the studies of Kodama et al.5 The peak at 530.4 eV corresponds to oxygen atoms in keto and quinone groups (CO); the peak at 531.7 eV corresponds to the carbonyl oxygen atoms in esters, amides, and anhydrides as well as to oxygen atoms in hydroxyls or ethers (carbonyl) which include adsorption oxygen; and the peak at 533.4 eV corresponds to the oxygen atoms in the carboxyl groups (carboxyl). As shown in Table 4, the CO and carboxyl groups that have CO double bond or −OH in the surfaces of catalysts remarkably decrease after NTPM compared with the MnOx/untreated in spite of the fact that the total oxygen concentration has increased or decreased. On the contrary, the carbonyl oxygen atoms in esters, amides, and anhydrides as well as the oxygen atoms in hydroxyls or ethers (carbonyl, 531.7 eV) increase significantly after NTPM. Therefore, it can be assumed that the CO double bonds and the −OH of the catalyst’s surface are more active and are easier to be bombarded with atmosphere plasma and then to be

transformed into ether or other groups. During this process, the adsorption oxygen is easy to generate. Thus, it can be seen that the NTPM can remarkably change the distribution of oxygen functional groups on the catalyst’s surface. During the NTPM, surface-oxygenated complexes may cause the MnOx surface to become more hydrophilic with a relatively large number of oxygen containing surface groups and, consequently, to become more accessible to the impregnating solution. These groups are located at the edges or defects of the basal planes or in the micropores.19 Accordingly, the manganese ions will be anchored on these sites. Furthermore, the groups created after the NTPM may help to disperse the manganese evenly. In Figure 6c, we present the Mn2p core-level XPS spectra obtained from a sample treated by NTP. Deconvolution of the Mn2p spectra gives two individual component groups that represent the Mn3+ (641.6 eV) and the Mn4+ (643.2 eV). As shown in Table 4, after NTPM, Mn3+ groups decrease and Mn4+ groups increase correspondingly, but the changes are slight. Though NTP have high energy and produce large amounts of activity radicals, the effect is different from thermal treatment. The catalysts are in the amorphous phase. Thus, NTPM cannot change the Mn functional group obviously. The slight changes indicate that the predominant species is MnO2 oxides. It can be concluded that the MnOx catalysts modified by NTP stabilizes Mn4+ species because of Mn3+ oxidation to Mn4+. 10021



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Figure 6. XPS results of MnOx before and after NTPM: (a) representative C1s and K2p spectra of MnOx/untreated and MnOx/5.8 KV/40 min; (b) representative O1s spectra of MnOx/untreated and MnOx/5.8 KV/40 min; (c) representative Mn2p spectra of MnOx/untreated and MnOx/5.8 KV/40 min. 10022



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Table 4. Relative Area (%) of C1s, K2p, O1s, Mn2p, and N1s XPS for the Untreated and Plasma-Treated MnOx Catalysts sample

MnOx/untreated MnOx/5.8 KV/5 min MnOx/5.8 KV/ 10 min MnOx/5.8 KV/ 20 min MnOx/5.8 KV/ 40 min MnOx/5.8 KV/ 300 min MnOx/2.4 KV/ 40 min MnOx/5.0 KV/ 40 min MnOx/6.0 KV/ 40 min MnOx/6.4 KV/ 40 min

C1s

O1s

Mn2p

K2p

N1s

284.6 (eV)

286.4 (eV)

288.4 (eV)

530.4 (eV)

531.7 (eV)

533.4 (eV)

641.6 (eV)

643.2 (eV)

292.9 (eV)

295.3 (eV)

400.2 (eV)

407.2 (eV)

75.47 72.88 70.46

12.77 8.91 9.04

2.12 3.27 4.00

62.07 60.28 51.39

23.70 25.93 34.86

14.23 13.78 13.76

18.61 12.22 15.38

48.97 56.34 49.84

6.04 11.17 11.69

3.6 3.77 4.81

67.47

9.31

4.83

60.60

28.99

10.41

12.94

53.60

12.89

5.50

69.64

12.09

5.21

60.05

27.03

12.92

16.50

50.43

8.46

4.60

25.43

74.57

72.17

9.58

3.59

60.86

25.59

13.55

16.97

51.55

9.18

5.48

74.78

8.90

4.13

60.97

27.25

11.78

18.07

49.74

6.99

5.30

72.08

10.23

4.10

60.45

28.50

8.79

17.44

49.41

8.64

4.95

65.68

9.07

5.55

60.69

27.59

11.46

16.05

50.10

12.51

7.39

33.75

66.25

61.25

8.74

5.1

65.38

17.60

17.01

16.07

49.60

16.50

8.50

25.04

74.96

3.6 Activity Tests. 3.6.1. Effect of Nonthermal Plasma Modified. Figure 10 shows the effect of NTP treatment on NO conversion in the temperature range of 50−250 °C. The NO conversions on catalysts increase at the low-temperature range (50−125 °C) and decrease with increasing temperature (125− 250 °C). The reason is that NO catalytic oxidation is an exothermic reaction; in the initial stage, the rate of chemical reactions is increasing with the temperature rising; however, when the reaction temperature keeps rising at a certain point, NO2 decomposition would be accelerated with the elevating of reaction temperature. Thus, NO conversion will decrease. However, it is obvious that the catalytic activity of MnOx/5.8 KV/40 min is much stronger than that of MnOx/untreated catalysts. The NO conversion was over 88% at 125 °C, and even at 50 and 100 °C it achieved 65% and 73%, respectively. We replicated the experiment several times, and the results indicated that the nonthermal plasma modification was stabilized. The results have shown that NTPM can introduce O atom functions (CO, C−O, etc.). In the process of NTPM, these groups will be bombarded to the surface. Plasma energy destroys many chemical bonds as well as the effect of hydrogen extraction, and the functions can promote NO catalytic oxidation. Besides, according to the XPS results, it can be found that N groups grow in number with the increase of treatment voltage and time after NTPM. The aromatic amines −NH2 aniline and imines CNH have the lone pair of electrons, which possess strong alkalinity, in favor of the removal of acidic gas as NO.26 Moreover, K element content at the surface increased after NTPM. The sputtering function of plasma is useful for NTP treatment catalysts. When an ion acts on catalysts, parts of metal ions can be bombarded to the surface of catalysts. We conclude that the increase of K is connected with this function, and the alkalinity of the metal K can enhance the NO catalytic oxidation. To confirm that it is the main factor to enhance catalytic activity, we used 10% K2CO3 solution to dip the MnOx/untreated catalysts for 1 h and studied the catalytic activity of this catalyst (MnOx/untreated/K2CO3). Figure 11 displays that the catalytic capability of MnOx/untreated/K2CO3 is greatly enhanced. This experiment can directly confirm the extrapolation that the introduction of K is a main factor in enhancing

Contrary to C1s and O1s spectra, the N1s spectra are relatively feeble except for MnOx/5.8 KV/40 min, MnOx/6.0 KV/40 min, and MnOx/6.4 KV/40 min (Table 3 and Figure 7). According to data from existing literature,20 the N1s spectra are decomposed into two identified components in this study: the peak at 400.2 eV is attributed to the nitrogen atoms in aromatic amines −NH2 aniline and imines CNH, and the peak at 407.2 eV is attributed to nitrogen-NO3−. As shown in Table 4, they are two species of important functional N atoms on the surface of the MnOx catalysts when the treatment voltage is high enough. After NTPM, all of the functional N atoms increase. However, they do not change in proportion to the treated voltage or treated time. The results indicate that the NTPM allows nitrogen to become incorporated into the carbonaceous structure. Meanwhile, higher voltage may destroy the chemical bond (−NH2 or CNH), and so the peak at 400.2 decreases at 6.4 KV. However, nitrogen-NO3− is generated from the reaction of N atoms and O atoms in the air when the voltage is high enough. As a result, the variation is irregular. 3.5. FTIR Results. It is known that the FT-IR technique is one of the most sensitive methods used for surface analysis.21 Figures 8 and 9 represent the FT-IR spectra to determine the change of surface functionalities with respect to plasma treatment voltages and times. When the MnOx catalyst was modified with NTP, the oxygen-related peaks were observed on the MnOx catalyst’s surface (Figure 8a, Figure 9a). The peak at 2280−2390 cm−1 was attributed to CO stretching vibration which is increased after NTPM in each case. In addition, the peaks at 1386 and 1400 cm−1 were assigned for carboxylate C−O stretching vibrations when the treatment voltages and times were high enough and when these vibrations were intense. As shown in Figure 9b, the series of peaks at the nearby 1522 cm−1 are attributed to carboxylate salt. In addition, the peaks at 1114 cm−1 and 1196 cm−1 are assigned for alcoholic C−O and phenolic O−H stretching vibration especially MnOx/5.8 KV/40 min.21−23 These representative peaks of plasma-treated MnOx catalysts are explained by the increment of oxygen originating from the introduced hydroxyl group on the MnOx surface. These bands of the MnOx catalysts increased conspicuously in proportion to plasma treatment voltage and time.24,25 10023



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Figure 7. Representative N1s spectra of MnOx before and after NTPM. (a) MnOx/5.8 KV/40 min, (b) MnOx/6.0 KV/40 min, and (c) MnOx/ 6.4 KV/40 min.

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Figure 8. FT-IR spectra of plasma-treated MnOx with different treatment voltages (treatment time: 40 min).

Figure 9. FT-IR spectra of plasma-treated MnOx catalysts with different treatment times (treatment voltage: 5.8 KV).

Figure 10. Effect of nonthermal plasma modified.

Figure 11. Effect of K. 10025



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Figure 12. Effect of treatment output voltages.

Figure 13. Effect of treatment times.

catalytic activity. The result is consistent with the previous study.27,28 Other important aspects include, from XRD and SEM results, that it can be implied that the catalysts are at an amorphous phase, and after treatment, dispersion of catalyst beads is enhanced while the aggregation phenomenon is weakened. From the BET results, the BET surface area and the total pore volume are decreased after NTPM, but the average pore width increases with the increase of treatment time and voltage. High average pore width plays a critical role in the NO catalytic oxidation process. 3.6.2. Effect of Treatment Voltage and Treatment Time. Output voltage is an important factor for NTPM. Figure 12 shows activities as a function of the different treatment voltages. The results illustrate that the NO conversion improves with the increase of output voltage at the beginning as shown in Figure 12a. The catalytic activity of catalysts decreased in sequence as follows: MnOx/5.8 KV/40 min > MnOx/5.0 KV/40 min > MnOx/2.4 KV/40 min > MnOx/untreated, and when the treatment voltage is 5.8 KV, the NO conversion reaches 88% at 125 °C. The enhancement of activity at low temperature is much stronger than that at high temperatures. However, when the treatment voltage is excessively high (above 5.8 KV in this study), the NO conversion is decreased markedly (as shown in

Figure 12b). At 6.4 KV, the highest NO conversion is only 76% at 125 °C, but it is still higher than that of MnOx/untreated. The variations for treatment times is similar to that of voltages. At first, when the treatment time is only 5 min, the effect of NTPM is limited, but with the increase of treatment time, the NTPM enhances the catalyst’s activity proportionately. As shown in Figure 13a, MnOx/5.8 KV/40 min has the best catalytic oxidation activity which reaches 88% at 125 °C. Nevertheless, Figure 13b displays that a longer treatment time makes catalyst activity decrease notably. The reasons for these effects of treatment voltage and time are similar. First of all, as a whole, the carbonyl, carboxyl, lactone, and esters groups (CO; OC−O, 288.6 eV, Table 4) decrease with high treatment time and voltage, while the carbonyl oxygen atoms in esters, amides, and anhydrides as well as the oxygen atoms in hydroxyls and ethers (carbonyl, 531.7 eV) also decrease. The decrease of these groups is an important factor which results in a decrease of catalytic activity. However, these groups are more than that of MnOx/untreated. Higher voltages and longer times may destroy the new chemical bonds. Some double bonds are changed to single bond. From Figures 8 and 9, the significantly increased peaks at 1386 and 1400 cm−1, which are assigned for carboxylate C−O stretching 10026



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functional group has increased). In addition, the rearrangement/ introducing of a functional group is an important process for NTPM, which can be confirmed by the increase in O and N groups. There are six kinds of particles: electrons, positive ions, negative ions, ground-state atoms or molecules, excited-state atoms or molecules, and photons in the process of NTPM. In the nonthermal plasma, the ranges of particle energy parameters are electron energy 0−20 eV; ion energy 0−20 eV; excited-state atoms or molecule energy 0−20 eV; visible light and ultraviolet light energy 3−40 eV. The bond energy of substrate material is as follows: C−H, 4.3 eV; C−C, 3.4 eV; CC, 6.1 eV; CC, 8.4 eV; CO, 8.0 eV; C−N, 2.9 eV; C−CL, 3.4 eV. Therefore, the particle energy of NTP is higher than the chemical bond energy of a material, and it makes physical and chemical reactions possible. When the kinetic energy of an electron is high enough, the chemical bond of the material’s surface would be fractured and would produce new ones. Besides, the chemical component could be verified by the combining of O with the molecular chain (as follows eqs 1−4). The NTP surface oxidation reaction is a freeradical action which can introduce plenty of oxy-polar base groups: hydroxyl group or carboxyl group and so on.

vibration at higher voltages and longer times, confirm this viewpoint. In addition, N groups are the highest at MnOx/5.8 KV/40 min, while under other voltages and time conditions, N groups descend. Meanwhile, K groups have a similar variation trend, and the decrease of these alkalinity groups inhibits NO catalytic oxidation. When the voltages and the treatment times are high enough, the phenomenon of etching will be more serious (Figure 3c), and the sputtering efficiency decreases. By contrast, when the voltages and treatment times are low (lower than 5.8 KV or 40 min), the sputtering phenomenon is feeble (most of the sputtering thresholds are 2.4 KV). From Figures 12 and 13, it can be seen that the NO catalytic activity of MnOx/5.8 KV/40 min is better than others. The relative percentages of K and N atoms of this catalyst are higher than that of others. Although the K relative percentage of MnOx/6.4 KV/40 min is higher than that of MnOx/5.8 KV/40 min, the N relative percentage is lower. The 5.8 KV/40 min is the optimum condition. When the treatment voltage and time are too high or too low, the effect of NTPM does not achieve the optimum situation. 3.7. Mechanism Discussion. The exact physical and chemical processes between the plasma and the material are hard to estimate. Plasma is a kind of ionized gaseous substance, including electrons, ions, and neutral particles which possess the ability to distribute energy. They can transfer autologous energy to the molecules and the atoms at the surface of materials. When plasma acts on that material, a large number of particles will inject the material surface to induce the collision, excitation, etching, cross-linking, rearrangement, isomerism, decomposition, oxidation, reduction, sputtering, introducing functional groups, and so on. In our study, the sputtering, rearrangement/introducing functional groups, and oxidation are the critical processes (as shown in Figure 14).

RH + 2O∗ → R∗ + H∗ + O2

(1)

RH + O∗ → R∗ + OH∗

(2)

R∗ + O2 → ROO∗

(3)

ROO∗ + R′H → ROOH + R′

(4)

Nevertheless, the results above precisely improve the NO catalytic oxidation activity of MnOx catalysts at low temperature. The aromatic amines −NH2 aniline and imines CNH have the lone pair electrons which possess strong alkalinity, and the alkalinity metal K can enhance NO catalytic oxidation. In the process of NTPM, O groups will be bombarded to the surface, and plasma energy destroys many chemical bonds as well as the effect of hydrogen extraction, and these functions can promote NO catalytic oxidation.

4. CONCLUSIONS NTPM could enhance the active surface sites of catalysts and could improve the dispersion of MnOx catalyst beads. Meanwhile, NTPM makes the average micropore width enlarge slightly and the PSD of the catalysts wider. XPS revealed that the NTPM could change the distribution of O, N, Mn, and K functional groups on the MnOx surface. It was observed that the carbonyl and hydroxyl surface groups and the K and N functional groups increased during NTPM. These changes can enhance the NO catalytic oxidation activity of MnOx catalysts. The activity tests show (1) the NO conversion on the sample improves with increasing treatment voltage in the range of 2.4− 5.8 KV; (2) the NO conversion on the sample increases with increasing treatment time in the range of 5−40 min; (3) the NO conversion decreased when the treatment voltage (above 5.8 KV) and the treatment time (above 40 min) were high enough. NTPM made MnOx catalysts exhibit high activity, while NTPM provides an effective measure for enhancing the NO catalytic oxidation activity of MnOx catalysts. Further study is needed to optimize the parameters of NTPM.

Figure 14. Schematic diagram of NTPM.

First, when ions and neutral particles interact with a material’s surface, kinetic energy of the incident particle will be passed on to the surface atoms via collision cascades. The material’s atoms can be sputtered out because of the fact that the kinetic energy of the surface atom is higher than the binding energy after NTPM. An increase in K groups on the surface after NTPM is the evidence for this viewpoint. Second, if the system contains O2 in the NTPM process, the oxidation effect could be observed on the surface of materials, which can be verified by the XPS results (the O

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-871-5170905. 10027



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (20907018, 21177051) and the Doctoral and New Teachers Foundation of Education Department (20095314120008).

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REFERENCES

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MnOx Catalysts Modified By Nonthermal Plasma For NO Catalytic

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