Effect of preparation method and Synergistic reaction of ozone

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Kinetics, Catalysis, and Reaction Engineering

Plasma-catalytic removal of hexanal over Co-Mn solid solution: Effect of preparation method and Synergistic reaction of ozone Xin Yao, Yizhuo Li, Zeyun Fan, Zhixiang Zhang, Mingxia Chen, and Wenfeng Shangguan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00191 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Industrial & Engineering Chemistry Research

Plasma-catalytic removal of hexanal over Co-Mn solid solution: Effect of preparation method and Synergistic reaction of ozone Xin Yao†, Yizhuo Li‡, Zeyun Fan†, Zhixiang Zhang†, Mingxia Chen†, Wenfeng Shangguan*† †Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P. R. China ‡Shenyang Academy of Environmental Sciences, 98 Quanyun No.3 Road, Shenyang 110167, P. R. China ABSTRACT: Removal of hexanal via a post-plasma catalysis system over Co-Mn solid solution at ambient temperature and pressure was investigated in this study. Results showed that CoMn (9/1) prepared by citric acid method exhibited the best catalytic activity, which could be ascribed to the higher redox property. Moreover, co-precipitation method was applied and improved CO2 selectivity significantly, which could due to smaller grain size, larger surface area and higher oxygen storage capacity. The reaction pathway and intermediates were analyzed by in-situ FT-IR. Also, results indicated that the removal of hexanal included direct decomposition by plasma and further oxidation of intermediates on catalyst surface. Furthermore, it could be inferred that the intermediates were further oxidized by the synergistic effect between active oxygen species and catalyst, the utilization of ozone was the key point in the process. KEYWORDS: Plasma, Catalysis, CoMn solid solution, hexanal, Ozone

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1. INTRODUCTION Volatile organic compounds (VOCs) are major indoor and atmospheric contaminants1. Most VOCs are harmful to human health and the environment. Among them, hexanal (C6H12O) is a typical pollution component released from cooking fume2, which is regarded as an indoor air pollutant and the most important precursors for the photochemical oxidants and organic aerosols 3. Long-term exposure to cooking fume can lead to lung cancer4. The removal of such indoor pollutants has become a hot topic in atmospheric environmental protection. Conventional methods to control VOCs emissions are well-established technologies such as catalytic oxidation, membrane separation, bioreaction and photocatalysis5. However, these methods have technical or economic disadvantages, such as high energy cost and low conversion efficiency6. VOCs can be adsorbed effectively by activated carbon, but it has to be regenerated by a thermal process or replaced because of its deactivation7, 8. Recently, non-thermal plasma (NTP) technology has become increasingly crucial for VOCs removal5. Compared with the high temperature-based technologies such as combustion and catalytic oxidation, NTP can be ignited and works at room temperature9. However, NTP alone also has some disadvantages, such as low carbon dioxide (CO2) selectivity and toxic by-products emissions (such as NOx and O3) 10. Catalytic oxidation is considered as one of the most effective and economical technology for VOCs removal. It usually requires lower energy cost due to the use of catalysts. Therefore, researchers combined the advantages of NTP and catalysis in a technique called plasma-catalysis. This innovative method has become a hot topic over the last ten years11.

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Many works demonstrated that plasma-catalysis exhibited high activity at room temperature for the abatement of VOCs, such as benzene and toluene. Cal and Schluep 12 investigated the decomposition of benzene in a dielectric barrier discharge (DBD) reactor, nearly complete destruction (>99.9%) of benzene was achieved at the SIE of 2000J/L. In a study of Jiang et al.13, a DC micro hollow cathode glow discharge was applied to remove 300 ppm benzene from dry air, and 90% of benzene was removed with SIE of 3000J/L. S. Ognier et al. 14 applied a DBD tube discharge reactor for toluene removal, 21% removal efficiency of toluene was achieved with SIE of 240J/L. However, high energy cost, low removal efficiency, the formation of NOx and O3 and other disadvantages limited their further application. Meanwhile, many efforts have been performed to improve hexanal removal efficiency by plasma-catalysis. Xiang et al.15 prepared MnOx/SBA-15 and tested for hexanal removal in a DBD system. Chen et al.16 reported a combination of non-thermal plasma and natural mordenite for hexanal removal. However, the hexanal removal and COx selectivity were not satisfactory, reaction pathway and stability of the catalysts were not discussed. Many works showed that transition metal oxides have been widely applied in abatement of VOCs and exhibited high activity. Shi et al.

17

prepared a series of

MnxCo3-xO4 solid solution which was highly active in thermal oxidation of formaldehyde at low temperature. Tang et al. 18 applied Mn-Co mixed oxide Nanorods for benzene removal by thermal oxidation. However, the performance of Co-Mn catalysts for the degradation of hexanal has not been investigated in any plasma-catalysis system. In our previous works19, 20, the combination of DBD and catalysts was found to be a

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promising approach for the removal of acetaldehyde. To investigate the performance of hexanal removal and lower the energy consumption, herein in this study, a DBD reactor coupled with Co-Mn solid solution was applied for C6H12O oxidation. The effects of Co/Mn ratios and preparation method were studied, physicochemical properties were discussed to investigate the relationship between characteristics and catalytic activities of the samples. The reaction pathway and removal process were investigated. Moreover, the synergistic effect between ozone and catalyst was discussed, especially the utilization of ozone on different samples. 2. EXPERIMENTAL SECTION 2.1. Materials All chemical reagents (analytical purity) were purchased from Sinopharm Chemical Reagent Co., Ltd, China, and employed without further purification. Catalysts preparation procedures are described as follow: (a) Citric acid (CA) method: Totally 0.05 mol of cobaltous acetate (Co (CH3COO)2·4H2O) and manganese nitrate (Mn (NO3)2) with different mole ratio (11:1, 9:1, 7:3, 5:5, 3:7 and 1:9) and 20 g citric acid were dissolved in 80 ml distilled water in an open corundum crucible. The solution was stirred at 80 °C until a Sol-gel formed. The as-made samples were dried at 110 °C for 12 h and then calcined at 500 °C for 6 h. The final sample is denoted as CoMn (X)-CA, where X stands for Co/Mn mole ratio. (b) Co-precipitation (CP) method: An aqueous solution of 1 mol/L precipitant made from ammonium hydrogen carbonate (NH4HCO3) was added dropwise (10 ml/min) to 250 mL solution containing 0.15 mol/L of cobaltous acetate (Co(CH3COO)2·4H2O) and manganese nitrate (Mn (NO3)2) under magnetic stirring at ambient temperature. After 4


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aging for 1 h, the resultant precipitate was filtered and washed by deionized water until the filtrate was neutral, then dried at 80 °C for 12 h, followed by calcination at 400°C for 2 h. 2.2. Characterization The structure and phase of catalysts were determined by powder X-ray diffraction (XRD) on a D8 ADVANCE (Bruker Ltd., Germany) by Cu-Kα radiation (scan rate 6° /min at 40 kV and 20 mA). Scanning electron microscopy (SEM) images were recorded by an S-4800 microscope (HITACHI Ltd., Japan). Hydrogen temperature-programmed reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) were determined by ASAP 2720 (Micromeritics Ltd., USA). The surface area and N2 adsorption-desorption were determined by Brunauer-Emmett-Teller (BET) measurement, and the pore size distribution was derived from the desorption branch of the N2 isotherm using the Barrett-Joyner-Halenda (BJH) method on ASAP 3020 (Micromeritics Ltd., USA). X-ray photoelectron spectroscopy (XPS) was carried out on 5400 ESCA (Physical Electronics, INC, USA). Fourier Transform infrared spectroscopy (FT-IR) was recorded in the range of 700-4000 cm-1 with a resolution of 2cm-1 by a spectrometer (Nicolet-6700, Thermo Fisher Scientific, USA). 2.3. Experimental setup The experimental setup of C6H12O removal by post plasma-catalysis is described in Fig.1, which included sections of gas piping, reactor, and detection. The system was operated at ambient temperature and pressure. Dry air supplied by a gas cylinder was divided into two air flows, the flow rate was controlled by a mass flow controller (MFC). One stream passed through the pure C6H12O liquid kept in a water bath (25 ± 1°C), then 5



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mixed with another stream for dilution. The concentration of C6H12O was fixed at 100ppm and fed into a DBD reactor by Teflon pipe (total flow rate 100ml/min). The DBD reactor used in this study was the same as that described in the previous paper19. The catalytic section was made up of a glass tube loaded with 200 mg of catalyst, crushed, 40~60 mesh particle size. The outlet of the reactor was connected to gas chromatography (GC9560, Shanghai Huaai, China) equipped with a flame ionization detector (FID). Meanwhile, the O3 concentration was measured on-line by an ozone analyzer (Model 49i, Thermo Fisher Scientific, USA). 2.4. Post-plasma catalytic performance evaluation C6H12O removal efficiency (η, %), CO2 selectivity (SCO2, %) and specific input energy (J/L) were defined as follows:

η=

Cin − Cout × 100% Cin

SCO2 =

SIE =

CCO2 ,out − CCO2 ,ini 6 × Cin

(1)

× 100%

(2)

P × 60 Q

(3)

Where, Cin and Cout are the initial and final C6H12O concentrations, respectively; CCO2,

out

represents the CO2 concentration at the outlet, while CCO2,

ini

is the CO2

concentration of the background. In equation (3), P stands for the power of the DBD with a unit of watt (W), while Q is the air flow with a unit of L/min. 3

RESULTS AND DISCUSSION

3.1. Effect of Co/Mn mole ratio on catalytic activity 3.1.1

Physicochemical characterization

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Figure 2 displays the XRD patterns of Co-Mn oxides with different Co/Mn molar ratios prepared by CA method, and average lattice constant results were given in Table S1. The pure MnOx showed complex diffraction peaks which can be recognized as a mixture of Mn2O3 (PDF card No. 41-1442) and Mn5O8 (PDF card No. 39-1218). This could be partly explained by the different crystal structure and valence of Mn ion, which has multivalence of +2, +3 and +4 that lead to the formation of various manganese oxides18. After addition 10% cobalt species into manganese oxide, the CoMn (1/9) showed the XRD patterns of Mn3O4 (PDF card No. 24-0734), but all diffraction peaks shifted to higher 2θ angle. For the CoMn (3/7) and (5/5), the diffraction peaks showed XRD patterns of (Co, Mn) (Co, Mn) 2O4 (PDF card No.18-0408). With further increase of the cobalt amount, the reflection peaks corresponding to Co3O4 were clearly observed. For the Co3O4, the diffraction peaks matched well with the standard patterns of Co3O4 (PDF card No.43-1003), and 2θ values of 19.00°, 31.27°, 36.86°, 44.81°, 59.35° and 65.23°were observed, which corresponded to (111), (220), (311), (400), (511) and (440) plane reflections of the spinel Co3O4 structure respectively. It is worth noticing that for the CoMn (9/1) and CoMn (11/1), all diffraction peaks shifted to lower 2θ angle. The results indicate that the sample is not a mixture of MnOx or CoOx but a new mixed oxide with homogeneous crystal phase. The Mn species were incorporated into the spinel structure of Co3O4, and a cubic crystal phase was formed. The specific surface areas, pore volume and pore size of the samples are summarized in Table 1. The specific surface area of MnOx and Co3O4 were 26.8 m2/g and 14.8 m2/g, respectively, while the Co-Mn oxides were in the range of 14.8-26.8 m2/g, the differences in the trend of as-prepared samples reveal that the incorporation of cobalt

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species decreased the surface area. The total pore volume of samples was in the range of 0.04-0.18 cm3/g. The pore volume decreased with the increase of Co content, which is in good agreement with the trend of surface area. Among the Co-Mn oxides, CoMn (5/5) had the largest average pore diameter of 5.62nm, indicating that the samples are mesoporous materials.

Table 1. Structural and textural properties of CoMn(X)-CA Sample(Co/Mn) a

Co3O4

11/1

9/1

7/3

5/5

3/7

1/9

MnOx

2

14.8

15.2

15.5

16.8

18.7

22.2

22.7

26.8

3

0.04

0.06

0.06

0.08

0.16

0.16

0.17

0.18

2.39

2.89

3.06

3.07

5.62

4.77

4.75

3.59

12.76

10.83

14.29

8.04

7.81

6.83

8.55

8.56

SBET (m /g) b

Vtotal (cm /g) c

dBJH (nm) H2 (mmol/gCat) a

d

Specific surface area

b

Total pore volume

c

Mean pore size

d

H2 consumption

The redox properties of as-prepared samples were investigated, the profiles are given in Fig.3 and H2 consumption results are listed in Table 1. MnOx showed two reduction steps in the temperature around 315°C and 438°C, which can be ascribed to the reduction of Mn2O3 to Mn3O4 and Mn3O4 to MnO, as reported by Tang et al21. In terms of Co3O4, two reduction peaks were observed at around 350°C and 400°C, which can be ascribed to the reduction of Co3O4 to CoO and CoO to Co0, respectively17. For Co-Mn oxides, H2 consumption increased with the content of cobalt species, indicating that the redox properties of Co-Mn solid solutions were promoted. Among the samples, CoMn (9/1) exhibited the lowest reduction temperature below 350°C. It reveals that the formation of Co-Mn solid solution is able to produce more easily reducible species and over that the catalytic oxidation reactions will be enhanced. 3.1.2 Catalytic performance Figure.4 shows the effect of Co/Mn ratio on catalytic activity for C6H12O removal at 8


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the SIE range of 314-628J/L. It is observed that the introduction of catalysts improved removal efficiency. The C6H12O removal followed the order: CoMn (9/1)> CoMn (7/3)> CoMn (3/7)> MnOx> CoMn (5/5)> CoMn (1/9)> CoMn (11/1) = Co3O4 > plasma only. Among those catalysts, the CoMn (9/1) catalyst exhibited the best C6H12O removal over 99% at the SIE of 494J/L, that was in accordance with its higher redox property (as shown in Fig.3). The catalytic activity obtained in this work is greater than others, especially at relative low SIE than the previous research16. 3.2. Effect of synthesis method on catalytic activity Among the series of catalysts, CoMn (9/1)-CA performs excellently in hexanal removal reaction. However, CO2 selectivity is only 38.7% at the SIE of 1032J/L, which is necessary to further improve the catalytic activity. It was reported22 that synthesis by co-precipitation method is an effective way to achieve smaller particle size and larger surface area, which is often related to the reduction properties, thus enhance the VOCs removal towards total oxidation. For this purpose, the best-performed catalyst CoMn (9/1) was synthesized by co-precipitation method and applied for hexanal removal. 3.2.1. Physicochemical characterization Figure.5a shows the XRD patterns of as-prepared samples. The Co3O4-CP and Mn3O4-CP display diffraction peaks attributed to cubic Co3O4 phase and tetragonal Mn3O4 phase, respectively, whereas the CoMn (9/1)-CP sample showed XRD pattern of Co3O4, with no diffraction peak ascribed to manganese oxides. Detailed information concerning the variation of the peak (311) is presented in Fig.5b. It is noted that in the case of CoMn (9/1)-CP, the diffraction peak (311) at 2θ=36.86° shifts to 2θ=36.77°, shows an apparent shift towards lower 2θ angle compared with Co3O4-CP, revealing the 9



possible replacement of Co by Mn ions in the cubic spinel structure. This is reasonable because the ionic radii of manganese ions (Mn3+: 0.065 nm; Mn2+: 0.083 nm) are bigger in size than cobalt ions (Co3+: 0.061 nm; Co2+:0.065 nm). Moreover, the peak (311) is broadened, and the intensity decreased, demonstrating that the sample has a much smaller crystal size than the pure Co3O4. According to the Scherrer equation, the grain size of as-prepared catalysts can be calculated by using the full width at half maximum of (311) (2θ around 36.5°), the results are summarized in Table 2. Figure.6 presents the morphology of as-prepared samples. It reveals that the surface of CoMn (9/1)-CA is composed of sheet-like nanoclusters with inhomogeneous size, most of the particles with average diameter of 100-150 nm, while Mn3O4-CP has a much smaller grain size as shown in Fig.6b. The morphology of Co3O4-CP (Fig.6c), which has a smooth surface and shows a hexagonal shape, most of the particles are homogeneous size with an average diameter of 50 nm. Shown in Fig.6d is the image of CoMn (9/1)-CP, most of the particles with average diameter of 30-40 nm, and grain size was apparently smaller than CoMn (9/1)-CA, suggesting the formation of smaller particles by CP method. These results infer that synthesis by co-precipitation is an effective way to achieve better homogeneity in composition and narrower particle size distribution for the catalysts. Meanwhile, under such doping, smaller particle size and the porous surface will be achieved which bring out excellent catalytic ability18. The BET surface area and porous structure of the samples were characterized by N2 adsorption-desorption isotherms and results are summarized in Table 2. As seen from Fig.7a, all samples present type Ⅱ isotherms (IUPAC classification), exhibiting a sharp characteristic of capillary condensation in the relative pressure (p/p0) range of 0.7-1.0, 10


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which indicated that the samples are typical porous materials. The CoMn (9/1)-CP sample had a surface area of 105.2 m2/g and a pore volume of 0.286 cm3/g, both of which were larger than single oxides. It is clear that by doping 10% Mn into Co3O4 lattice, the formation of solid solution oxides tends to produce smaller pores and larger surface area, the total pore volume increased, and the crystallite size decreased (from XRD results). The results of BET surface areas are in good agreement with the SEM results. Therefore, the change in BET surface area, pore volume, pore size and crystallite size indicated that the rich porosity would expose more active sites, thus would be beneficial for C6H12O removal. It is generally believed that the decomposition of the hydro carbonate precipitant, structural water and the release of gaseous COx and H2O would leave behind large numbers of voids and porous structures were formed. As reported by Zhang et al.23, such uniformed pores will create numerous active sites, which will facilitate the adsorption and diffusion of organic molecules, improving mass transfer efficiency over interfaces and promoting their catalytic activities. Table 2. Physicochemical characterization of the samples BET Surface

BJH Pore Volume

BJH Pore

Crystallite

H2 consumptionb

Area (m2/g)

(cm3/g)

Size (nm)

Size a (nm)

(mmol/gCat)

Co3O4-CP

49.7

0.123

10.259

45.1

12.81

Mn3O4-CP

65.8

0.194

9.008

35.5

8.88

CoMn(9/1)-CP

105.2

0.286

11.090

15.2

18.08

CoMn(9/1)-CA

15.5

0.06

3.057

49.2

14.29

Sample

a

determined by Scherrer equation from XRD results

b

calculated by peak area from H2-TPR results

The redox properties of the samples were investigated, and the results are presented Fig. 8a, H2 consumption results were given in Table 2. Two H2 reduction peaks at 282°C 11



and 376°C were observed over Co3O4-CP, suggesting the stepwise reduction of Co3O4→CoO→Co0 24, 25. Over Mn3O4-CP, H2 reduction peaks at 282 °C and 392 °C corresponds to the reduction of MnOx to Mn3O4 and Mn3O4 to MnO respectively 26. After addition of Mn into Co3O4, reduction peaks at 300 °C and 380 °C were observed. While in the case of CoMn (9/1)-CA, reduction peaks at 372 °C and 406 °C were observed, both are higher temperature than CoMn (9/1)-CP. It is noted that the one at around 280 °C could be observed over all samples prepared by CP method, but not over CoMn (9/1)-CA. The peak at 280 °C is attributed to the reduction of surface oxygen species generated by the presence of oxygen vacancies on pore surfaces27. It is observed that H2 consumption over CoMn (9/1)-CP is much higher than other samples, which may result in much higher redox capability. Figure 8b shows the O2-TPD profiles of as-prepared samples. The results show that 10% Mn-doped into Co3O4 lattice may be an ideal candidate for catalytic oxidation because maximum oxygen storage capacity is observed. It was reported that the interaction of adsorbed toluene with adsorbed oxygen determined the oxidation of toluene, and adsorbed oxygen species (such as O−, O2−, and O22−) played the dominant role in the total oxidation of toluene28. In other words, Mn-substitution for Co on the catalyst led to higher active oxygen which will relate to the better catalytic performance. The CoMn (9/1) samples prepared by CA and CP methods were investigated by XPS to examine the surface chemical composition and the oxidation state. The XPS spectra of O 1s, Co 2p, and Mn 2p are displayed in Fig. 9a, 9b and 9c, respectively, detailed information is listed in Table 3. As shown in Fig. 9a, by means of deconvolution, the distribution of oxygen species can be recognized as two kinds of oxygen species, the one

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with binding energy in the range of 530.4-531.3 eV is assigned to surface adsorbed oxygen (Oads), while the one in the range of 529.1-530.0 eV is assigned to surface lattice oxygen (Olatt)29. Results show that CoMn (9/1)-CP is significantly enriched with surface adsorbed oxygen compared with CoMn (9/1)-CA. It is believed that surface adsorbed oxygen is the critical point in C6H12O oxidation. The sample with higher amount of surface adsorbed oxygen would exhibit higher catalytic activity, which is in good agreement with the O2-TPD results. By curve-fitting, the Co 2p and Mn 2p peaks (Fig. 10b and 10c), Co3+/Co2+ and Mn3+/Mn2+ atomic ratios were calculated. Compared with CoMn (9/1)-CA, the best catalyst CoMn (9/1)-CP exhibits the higher content of Co3+ and Mn3+. It is reported that when the content of high valence metal ions rises, the chemical potential and reactivity of oxygen adjacent to the metal ions will be promoted30. According to the H2-TPR results, the higher valance of metal ions tends to produce higher oxidation states, in other words, will introduce more active oxygen which will relate to the catalytic performance. Table 3. XPS results collected over CoMn (9/1) CA and CP Samples

Olatt(eV)

Oads(eV)

Oads/ Olatt

Co3 +/Co2 +

Mn3 +/Mn2 +

CoMn(9/1)-CA

530.0

531.3

0.113

0.620

0.294

CoMn(9/1)-CP

529.1

530.4

0.386

1.565

1.205

3.2.2 Catalytic activity The effect of synthesis method on C6H12O removal was studied and presented in Fig. 10a. It is noted that the catalytic activity of C6H12O oxidation increases with the rise of SIE in all situations. Furthermore, the removal efficiency is significantly improved when 13



catalysts were introduced. The removal efficiency is observed over the CoMn (9/1) prepared by CA and CP methods, which are 100% at the SIE of 538J/L. Over Mn3O4-CP and Co3O4-CP, complete oxidation occurred at 628J/L. It is worth noting that as a reference object, C6H12O removal by plasma only is 17.6% at 494J/L and reaches 65.3% at 538J/L, suggesting that electron-impact reactions play an important role for its decomposition because of the characteristic of higher electron energy31. Figure.10b presents the CO2 selectivity results of the samples. It is evident that the CO2 selectivity increases with the rise of SIE in all situations, while only 11% at the SIE of 1032J/L was achieved in the case of plasma alone. The introduction of catalysts improved CO2 selectivity remarkably, which means that complete oxidation of C6H12O was enhanced by the synergistic effect of plasma and catalysis, thus more intermediates were oxidized to CO2. Among the samples, CoMn (9/1)-CP shows 77.8% which is the best CO2 selectivity at the SIE of 1032J/L. As a contrast, the CO2 selectivity of CoMn (9/1)-CA is 38.7% under the same SIE, suggesting that the CP method is more beneficial to the catalytic activity than CA. The best activity measured in the presence of CoMn (9/1)-CP can be explained by the oxidation sites on the surface of catalysts, as shown in H2-TPR and XPS results. The durability test of CoMn (9/1)-CP was conducted at the SIE of 1032J/L. As shown in Fig. S1, the catalytic performance remained 100% removal within a test period of 36h. With regards to CO2 selectivity, only a slight drop (from 77.8% to 76.1%) was observed. The catalyst prepared by CP method has reliable durability in at least 36 h for hexanal removal. 3.2.3 In situ FT-IR study

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Fourier transform infrared (FT-IR) spectroscopy was used to determine the reaction on the surfaces of sample32,

33

, yielding information on the overall functional-group

composition. Fig.11a shows in situ FT-IR spectra recorded during C6H12O adsorption over CoMn (9/1)-CP (100ppm C6H12O/21%O2/N2 balance). A prominent band at 2965cm-1 was observed which can be assigned to the antisymmetric stretch of CH3 group. Meanwhile, the band at 2938 and 2879cm-1 can be assigned to the antisymmetric stretch of CH2 group and the symmetric stretch of CH3 group, respectively, and the intensities of which increased with the time of C6H12O exposure. Similarly, the band at 1712cm-1 was observed which is a characteristic band for an aldehyde, which can be assigned to the stretch of C=O. The band at 1425cm-1 can be assigned to CH2 scissoring bend, which shifted by carbonyl group that lowers the bending force constant for the CH2 group adjacent to it. The bands of 1110 and 1261cm-1 were observed, which was assigned to the symmetric stretch of C-C. These bands could be associated with C6H12O adsorption species. The oxidation of C6H12O occurs after adsorption on the catalyst, spectra are shown in Fig.11b. With the increase of SIE to 898.2J/L, the FT-IR spectrum changed significantly. The bands at 2923, 2877 and 2861cm−1 attributed to an antisymmetric stretch of CH3 group, an antisymmetric stretch of CH2 group and a symmetric stretch of CH3 group were observed, which shifted to lower wavenumbers compared with adsorption state. This can be explained by the conjugative effect of an unsaturated bond and carbonyl group34. Furthermore, the band at 1731cm-1 overlapped with a new band at 1650cm-1, which can be assigned to the stretch of C=O in COOH group and C=C respectively, indicated that carboxylic acid and alkene species were generated. While the

15



band 1712cm-1 was no longer observed, indicating that aldehyde group was oxidized. The bands at 1434 and 1380cm-1 were the dominant species in the spectrum after the reaction, which can be assigned to the umbrella bend of CH3 group and CH2 bend in alkane species. Meanwhile, the band at 1189cm-1 increased which means aldehyde species converts to alkane species since the wavenumber can be attributed to the stretch of C-C bones in alkane molecules. Moreover, the intensity of the peaks was continuously increased with the advance of time, which meant the organic by-products were accumulated on the catalyst surface. It can be inferred from the above results that intermediates such as alkene and carboxylic acid compounds were detected, and then carboxylic acid can be oxidized to form formic species, which are further oxidized to form CO2 and H2O via various oxidation reactions. 3.3. Synergistic effect of ozone and catalyst Ozone is a long-lived component, and for post-plasma catalytic reactions, it plays an important role in the oxidation of VOCs35. Generally, ozone decomposition on catalyst surface may lead to the formation of strong oxidant and atomic oxygen, which could improve the CO2 selectivity

28, 36-39

. Herein, the role of the typical by-product O3

produced by oxygen molecules ionization29, 40-42 was studied. As seen from Fig. 12, the removal of C6H12O increased with the rise of SIE by plasma catalysis over CoMn (9/1)-CP, and complete removal occurs at the SIE of 538J/L, whereas the CO2 selectivity is only 22%, which means that intermediates account for a large portion in the total products. With the SIE further rises to 1032J/L, the CO2 selectivity was improved notably from 22% to 78%. Meanwhile, the concentration of O3 increased along with the improvement of CO2 selectivity. The above results inferred that 16


Page 16 of 38

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the removal of hexanal depends on two mechanisms including 1. direct removal caused by the collision of electrons; 2. reactions between intermediates and gas-phase active species. The improved performance was thought due to in-situ decomposition of ozone and the formation of atomic oxygen led to higher CO2 selectivity43. In order to investigate the synergistic effect of O3 and catalysts, three samples were applied: Mn3O4, Co3O4, CoMn (9/1) (all by CP method), respectively. The experiment was conducted in two conditions, without C6H12O and 100 ppm C6H12O, and the O3 concentration at the outlet was observed. As shown in Table 4, in the first condition, the SIE was fixed at 673J/L (C6H12O complete removal), the O3 concentration was 180.0 ppm by plasma alone and decreased to 40.1ppm, 59.4ppm and 54.2ppm on Mn3O4, Co3O4, CoMn (9/1), respectively. Afterward, 100ppm C6H12O was added into the system, it was observed that the O3 concentration dropped slightly. The decrease may be due to decomposition of O3 on catalysts and lead to the formation of active oxygen species like peroxide species (O22-), superoxide anion radical (O2-) and atomic oxygen species (O)

44

that react with the hexanal. As shown in Fig.13, the O3 decomposition followed the order: Mn3O4>CoMn (9/1)>Co3O4. Results show that Mn3O4 showed the best O3 removal efficiency which was 79.9%, but only 3.01% of O3 decomposition contributed to the C6H12O oxidation. Co3O4 showed 71.5% of O3 removal in which 6.73% participated in the C6H12O oxidation, whereas on CoMn (9/1), it showed O3 removal of 76.5% and 9.00% O3 utilization, respectively. Therefore, O3 utilization followed the order: CoMn (9/1) > Co3O4 > Mn3O4. It is in agreement with the excellent performance of CO2 selectivity over CoMn (9/1), which facilitates the intermediates toward further oxidation45.

Table 4. O3 concentration collected over Co-Mn catalysts (by CP method) 17



Catalysts

Page 18 of 38

Without

100ppm

Subtraction

O3 removed

Utilization

C6H12O

C6H12O

(ppm)

(ppm)

(%)

Plasma alone 180.0

178.5

1.5

--

--

Mn3O4

40.1

35.8

4.3

142.7

3.01

Co3O4

59.4

50.8

8.6

127.7

6.73

CoMn(9/1)

54.2

41.9

12.3

136.6

9.00

In fact, many plasma-catalytic oxidation process begin with the generation of active oxygen. In the presence of catalyst, the electric field close to its surface is enhanced, the oxidation process is initiated by the dissociation of adsorbed ozone on the catalyst surface, generating O atoms, which in turn involved in surface reactions46. Note that the oxidation potential of atomic oxygen is far higher than that of ozone. It has been reported that ozone decomposed to O2 on catalysts as expressed by equations (4)-(6), where * denotes active sites on the catalyst47-49. O3 + * → O2 + O*

(4)

O* + O3 → O2 + O2*

(5)

O2*→ O2 + *

(6)

The adsorption of oxygen species, as well as VOCs on a catalyst, generally depends on the nature of its surface and its porosity. Besides, the valence of metal ions, as well as the existence of adsorbed oxygen, also contributed to hexanal removal. Further reaction of these carbonyl intermediates may produce total oxidation products, CO2 and H2O.

4. CONCLUSIONS In summary, a combination of DBD with catalysts increased hexanal removal and inhibited by-product formation of plasma significantly. The process of hexanal removal 18


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included the direct decomposition caused by the collision of electrons and the reaction between intermediates and gas-phase active species on the catalyst surface. The catalytic performance of the Co-Mn oxides depended significantly on the constituent metal cations, and CoMn (9/1) was the most excellent for hexanal removal. The catalytic activity was further improved by co-precipitation preparation due to the larger surface area, higher surface oxygen storage capacity and more oxidation sites. Furthermore, O3 formed in plasma process was catalytically dissociated into active oxygen on the surface of catalysts and contributed to further oxidation of intermediates, which benefited to improve CO2 selectivity. The synergistic reaction of O3 and catalyst was the key point in the process. The Co-Mn oxides with the excellent catalytic performance show a promising application in the field of VOCs removal by plasma-catalysis, especially for cooking fume.

■ ASSOCIATED CONTENT Supporting Information Results of lattice constant, durability test, comparison of DBD and ozone alone on hexanal removal. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *Tel

& Fax: +86 021 34206020. E-mail address: [email protected].

Present Address §

Wenfeng Shangguan: Research Center for Combustion and Environment Technology, 19



Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P. R. China Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21577088) and the National Key Research & Development Plan (2017YFC0211804).

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23



LIST OF FIGURE CAPTIONS Figure 1. Experimental setup of C6H12O removal by post plasma-catalysis Figure 2. XRD patterns of CoMn(X)-CA Figure 3. H2-TPR profiles of CoMn(X)-CA Figure 4. Hexanal removal by plasma-catalysis over CoMn(X)-CA Experimental condition: a flow rate of 100 ml/min, 100 ppm hexanal initial concentration, at 25°C Figure 5. (a) XRD patterns of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA (b) Detailed description of peak 311 reflection Figure 6. SEM images of (a)CoMn (9/1)-CA, (b)Mn3O4-CP, (c)Co3O4-CP and (d)CoMn (9/1)-CP Figure 7. (a) N2 adsorption-desorption isotherm and (b) Pore size distributions of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA Figure 8. (a) H2-TPR and (b) O2-TPD profiles of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA Figure 9. O 1s (a), Co 2p (b) and Mn 2p (c) XPS spectra of CoMn (9/1) by CA and CP Figure 10. (a) Removal of C6H12O by plasma-catalysis over CoMn(X)-CP; (b) CO2 selectivity by plasma-catalysis over CoMn(X)-CP Experimental condition: a flow rate of 100 ml/min, SIE range of 314-628 J/L, at 25°C Figure 11. In situ FT-IR spectra of C6H12O adsorption (a) and oxidation (b) over CoMn (9/1)-CP

SIE:1032J/L

Figure 12. C6H12O removal, CO2 selectivity and O3 concentration over CoMn (9/1)-CP in the SIE range of 314-1032J/L Figure 13. O3 removal and utilization over different catalysts (by CP method) SIE:673J/L


Page 24 of 38

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Figure 1. Experimental setup of C6H12O removal by post plasma-catalysis



(2 0 0)



*

 

* 



(4 4 0)

*

(1 1 1)

1 3) #(3 1 1) (2 0 2)(1 #

(4 0 4)

(1 1 1)

(2 0 2) (1 1 3) #(3 1 1)

(4 0 4)

#

(1 1 1)



(1 1 1)





(1 1 1)



20



#

 

(2 2 0)



(2 2 0)



30

CoMn5/5

(4 0 0)

(5 1 1) (4 4 0)

(3 1 1) (4 0 0)

(5 1 1) (4 4 0)

(3 1 1) (4 0 0)

(5 1 1) (4 4 0)





(2 2 0)

CoMn3/7

#

 (3 1 1) 

CoMn1/9

#

#

#

(2 2 0)



(1 1 1)

(2 2 4)













(3 1 1)

CoMn7/3







CoMn9/1 CoMn11/1

(5 1 1) (4 4 0)

(4 0 0)





40

MnOx

*

(1 0 1)

#

10

(2 2 2)

(1 0 3) (2 1 1) (1 1 2) 



Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

50

60

Co3O4



70

*: Mn2O3 : Mn5O8  : Mn3O4 : Co3O4 #: (Co,Mn)(Co,Mn) O 2 4

Figure 2. XRD patterns of CoMn(X)-CA


80 O 2/( )

Page 27 of 38

MnOx CoMn1/9 CoMn3/7 CoMn5/5

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CoMn7/3 CoMn9/1 CoMn11/1 Co3O4 100

200

300 400 500 Temperature(C)

600

Figure 3. H2-TPR profiles of CoMn(X)-CA


700


100

Hexanal removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

80 60 plasma only CoMn1/9 CoMn3/7 CoMn5/5 CoMn7/3 CoMn9/1 CoMn11/1 MnOx Co3O4

40 20 0 300

350

400 450 500 550 Secific Input Energy (J/L)

600

Figure 4. Hexanal removal by plasma-catalysis over CoMn(X)-CA


650

Page 29 of 38

"

(a)

（2 1 1）（1 0 3）（1 1 2）

（1 0 1）

（2 2 4）

Mn3O4-CP

（3 1 1）

Intensity(a.u.)

（2 2 0）

（1 1 1）

（4 4 0）（5 1 1）

Co3O4-CP

（3 1 1）（4 4 0）（5 1 1） CoMn(9/1)-CP

（2 2 0）

（1 1 1）

（3 1 1）

10

（4 4 0）（5 1 1）

（2 2 0）

（1 1 1）

20

30

CoMn(9/1)-CA

40

50

60

70

80

2/(O)

(b)

Co3O4-CA

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CoMn(9/1)-CP

CoMn(9/1)-CA

36.0

36.4

36.8 37.2 2/(O)

37.6

Figure 5. (a) XRD patterns of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA; (b) Detailed description of peak 311 reflection



Figure 6. SEM images of (a)CoMn (9/1)-CA, (b)Mn3O4-CP, (c)Co3O4-CP and (d)CoMn (9/1)-CP


Page 30 of 38

Page 31 of 38

CoMn(9/1)-CP Mn3O4-CP

150

Co3O4-CP

3

Quantity Adsorbed (cm /g STP)

200 (a)

CoMn(9/1)-CA

100

50

0 0.0

0.10

3

dV/dlog(r) Pore Volume (cm /g•Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.4 0.6 0.8 Relative Pressure (P/Po)

1.0

(b) CoMn(9/1)-CP

0.08

Mn3O4-CP Co3O4-CP

0.06

CoMn(9/1)-CA

0.04

0.02 0

20

40 60 Pore Radius (Å)

80

100

Figure 7. (a) N2 adsorption-desorption isotherm and (b) Pore size distributions of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA



(a)

Mn3O4-CP

Intensity (a.u.)

Co3O4-CP

CoMn(9/1)-CP CoMn(9/1)-CA 100

200

300

400

500

600

700

Temperature(C)

(b)

Mn3O4-CP

Co3O4-CP

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CoMn(9/1)-CA

CoMn(9/1)-CP

100

200 Temperature(C)

300

400

Figure 8. (a) H2-TPR and (b) O2-TPD profiles of Co3O4, Mn3O4, CoMn (9/1)-CP and CoMn (9/1)-CA


Page 32 of 38

Page 33 of 38

(a) O 1s 530.0

531.3

Intensity (a.u.)

Lattice O

Adsorbed O

CoMn(9/1)-CA

529.1

Lattice O

530.4 Adsorbed O

CoMn(9/1)-CP 525

528

(b) Co 2p 779.8

Intensity (a.u.)

Co

531 534 537 Binding Energy (eV)

Co2p3/2 Co

3+

Co

770

775

794.9 2+

Co

780.2 Co

3+

794.0 Co

785

790

796.1 2+ Co

3+

2+

780

540

543

Co2p1/2

781.0

778.9

CoMn(9/1)-CA

795.3 Co

3+

795

CoMn(9/1)-CP

2+

800

805

810

815

Binding Energy (eV)

(C) Mn 2p 641.9 2+ Mn

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


641.6 Mn

630

635

2+

640

653.3

644.4 3+ Mn

Mn

643.2 Mn

2+

653.3

3+

Mn

2+

645 650 Binding Energy (eV)

655.6 3+ Mn

CoMn(9/1)-CA

654.4 Mn

3+

CoMn(9/1)-CP

655

660

665

Figure 9. O 1s (a), Co 2p (b) and Mn 2p (c) XPS spectra of CoMn (9/1) by CA and CP



100

(a)

Hexanal removal (%)

80 60 40 CP-Co3O4 CP-Mn3O4 CoMn(9/1)-CA CoMn(9/1)-CP plasma only

20 0 300

80

350

400

450 500 550 600 Secific Input Energy (J/L)

650

700

(b) CP-Co3O4 CP-Mn3O4 CoMn(9/1)-CA CoMn(9/1)-CP plasma only

70 60

CO2 selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

50 40 30 20 10 0

500

600

700 800 900 Specific input energy (J/L)

1000

Figure 10. (a) Removal of C6H12O by plasma-catalysis over CoMn(X)-CP; (b) CO2 selectivity by plasma-catalysis over CoMn(X)-CP


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(a) Adsorption 120 min

Adsorbance (a.u)

80 min 40 min 20 min 10 min

1110

1000

5 min

1425 1712 1261 1560

1500

2879 2965 2938

2000

2500

3000

1 min

3500

4000

Wavenumber (cm-1)

(b) Reaction, 898.2J/L

40 min

Adsorbance (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 min

10 min 5 min 1 min 2923 2877 2861

1251 1521 1731 1189 1434 1650 1110 1380

1000

1500

2000

2500

3000

3500

4000

Wavenumber (cm-1)

Figure 11. In situ FT-IR spectra of C6H12O adsorption (a) and oxidation (b) over CoMn (9/1)-CP


100

80

80

60

60

40

40

20 20

O3 concentration(ppm)

Page 36 of 38

100

CO2 selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hexanal Removal (%)


0 300

450

600 750 900 Specific input energy (J/L)

1050

0

Figure 12. C6H12O removal, CO2 selectivity and O3 concentration over CoMn (9/1)-CP in the SIE range of 314-1032J/L


90

90 79.9%

80

76.5%

71.5%

70 O3 utilization (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70

60

60

50

50

40

40

30

30

20 10 0

O3 Removal (%)

Page 37 of 38

20 6.73%

3.01% Mn3O4

Co3O4

9.00%

CoMn(9/1)

10 0

Figure 13. O3 removal and utilization over different catalysts (by CP method) SIE:673J/L



Abstract Graphics:

For Table of Contents Only Hexanal is cleaved by electron impact dissociation and intermediates were further oxidized to CO2 and H2O on catalyst surface.


Page 38 of 38

Effect of preparation method and Synergistic reaction of ozone

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