Cordierite Catalysts for

Mar 7, 2018 - Cooking oil fumes (COFs) are considered to be one of the important pollution sources in urban areas, and nonmethane hydrocarbon (NMHC) ...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Improving the Efficiency of Mn-CeOx/Cordierite Catalysts for Nonmethane Hydrocarbon Oxidation in Cooking Oil Fumes Honghong Yi,†,‡ Yonghai Huang,† Xiaolong Tang,*,†,‡ Shunzheng Zhao,†,‡ Fengyu Gao,† Jiangen Wang,† and Zhongyu Yang† †

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Department of Environmental Engineering, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China ‡ Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing 100083, P. R. China ABSTRACT: Cooking oil fumes (COFs) are considered to be one of the important pollution sources in urban areas, and nonmethane hydrocarbon (NMHC) can be an index of volatile organic compounds (VOCs) from COFs. In this paper, cordierite supported Mn-CeOx catalysts were used for NMHC oxidation in COFs. The results showed that the catalytic oxidation activity of Mn-CeOx/cordierite was obviously improved by adding an appropriate proportion of Ce. The catalytic activity of Mn4Ce1/C was obviously better than catalysts with other molar ratios of Mn/Ce and Mn/C because of the extra adsorbed oxygen species and high ratio of Ce4+/Ce3+ according to the XPS analysis. The physisorption of N2 and SEM-EDS revealed that the structure of catalysts prepared by a sol−gel method presented a higher BET specific surface area, more abundant pore structure, and better dispersion of catalysts’ active components which were favorable for catalytic oxidation reaction of NMHC.

1. INTRODUCTION The degradation of air quality has become an important environmental problem in recent years in China, especially in the metropolitan regions.1−3 In urban areas, cooking oil fumes (COFs) have been considered to be one of the important domestic pollution sources for particulate matter (PM) and volatile organic compounds (VOCs). There are many kinds of VOCs, such as hydrocarbons, aldehydes, ketones, acids, etc., in COFs.4−7 Nonmethane hydrocarbon (NMHC) is an important component of VOCs which can be used as a standard of the VOCs concentration and can result in the photochemical reactions with NOx, the generation of tropospheric ozone and secondary organic aerosol (SOA),8−10 and also are harmful to both human health and the environment with the components of benzene, toluene, xylenes, etc.11−13 The commonly used COFs purification technologies including mechanical cleaning, electrostatic deposition, and filtration adsorption have some disadvantages, such as large size, high operating costs, and relatively low removal efficiency of VOCs.6,8 Thus, it is necessary to develop a more environmentally friendly and effective COFs purification technology. Catalytic combustion has been proved to be an environmentally friendly method for the removal of COFs due to its low power consumption, avoiding of NOx generation, and high purification efficiency of VOCs,14,15 in which the catalyst is the technological core. Manganese oxide has been recognized as a kind of excellent catalyst in the transition metal oxides attributed to its variety oxidation states, abundant lattice oxygen and surface adsorption oxygen species and oxygen vacancies provided by lattice defects.16−19 © 2018 American Chemical Society

As a catalytic promoter, ceria (CeO2) can not only improve the high ability of oxygen storage/release capacity and good capacity for oxygen delivery but also promote the oxidation−reduction properties between Ce4+ and Ce3+.20−22 In order to simulate the real situation of COFs, the most commonly used soybean oil in China was chosen to generate COFs by heating at 200 °C. The commercial cordierites before and after nitric acid treatment were used as the catalyst supports. Catalysts with different molar ratios of Mn/Ce were prepared by incipient wetness impregnation method and different Mn4Ce1 catalysts were prepared by sol−gel method. The catalytic activity of all of the catalysts and catalyst supports using for NMHC oxidation in COFs was investigated. In order to analyze how the Mn/Ce molar ratios of catalysts and different catalyst preparation method affect the catalysts’ catalytic performance, all of the samples were characterized by physisorption of N2, SEM, EDS, XRD, and XPS. In addition, this paper was expected to promote the efficiency of COFs purification so as to improve the air quality in the metropolitan areas.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The commercial cordierite support (TENNECO, 64 cells per square centimeter, 0.18 mm wall thickness) was cut into a cylinder of 20 mm radius and 10 mm Received: Revised: Accepted: Published: 4186

November 26, 2017 February 22, 2018 March 7, 2018 March 7, 2018 DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

Article

Industrial & Engineering Chemistry Research

the samples were coated with a thin layer of Pt. X-ray diffraction (XRD) was used to analyze the crystalline phases presented in the catalysts. The diffraction patterns were recorded with a Rigaku D/MAX-2200 powder diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 5°/min from 2θ equal to 10° up to 90°. The diffractometer was operated at 36 KV and 30 mA. The JCPDS files were used for the samples’ phase identification. The X-ray photoelectron spectroscopy (XPS) was performed on a VG Scientific ESCALab220i-XL electron spectrometer to analyze the chemical composition and functional groups on the surface of the catalysts. The Al Kα radiation as excitation source was operated at 300 W. Wide scans were performed from 1100 to 0 eV with a dwell time of 100 ms and steps of 1 eV. Narrow scans were performed with steps of 0.05 eV if the range is 20 eV with a dwell time of 100−400 ms. The binding energy (BE) was corrected by the C 1s peak at 284.6 eV.

height. Some catalyst supports were washed by deionized water and dried at 110 °C for 10 h (named “C”), and others were washed in an ultrasonic bath with nitric acid (20%) for 2 h and then washed by deionized water until the pH of the cordierite was neutral and dried at 110 °C for 10 h (named “NC”). The weight of each prepared support was about 6 g. The catalysts were prepared by incipient wetness impregnation method using the nitrate precursors of Ce (Ce(NO3)3·6H2O) and Mn (Mn(NO3)2 (50% solution)). The metal nitrates were dissolved in deionized water with different molar ratios. After the cordierites were impregnated in the solution, the obtained samples were dried at 110 °C for 10 h and calcined at 400 °C for 3 h subsequently. The prepared catalysts were named “MnxCey/C” where x and y were the molar ratios of Mn and Ce, respectively. The catalysts were prepared by sol−gel method using the same raw materials as the catalysts prepared by incipient wetness impregnation method. The nitrate precursors and citric acid were dissolved in the deionized water to form a mixed solution, and the molar ratio of Mn and Ce was 4:1. The solution was heated at 60 °C under continually stirring until it turned to a sol. The supports were immersed in the formed sol, and then the blocked channels of the cordierites were purged under a weak air flow. The pretreated samples were dried at 110 °C for 10 h and then calcined at 400 °C for 3 h subsequently. The catalysts were named “g-Mn4Ce1/C” and “g-Mn4Ce1/NC” when using the “C” and “NC” as the catalyst supports prepared by sol−gel method. The total loading of metals in all of the prepared catalysts were 5 wt %. 2.2. Catalytic Performance Evaluation. The catalytic activity of the catalysts using for NMHC oxidation in cooking oil fumes was measured in a fixed-bed reactor under atmospheric pressure at 200 to 400 °C. In order to simulate the real situation, soybean oil was heated at 200 °C to generate cooking oil fumes, and the concentration of NMHC from cooking oil fumes was 5000 ppm at a flow rate of 200 mL/min which was controlled by rotameter. The concentration of NMHC in feed gas and outlet gas was measured using a gas chromatograph (GC7900, Techcomp Co., Ltd. China) equipped with two flame ionization detectors (FID), a glass microspheres stainless steel column (0.5 m × 3 mm), and a GDX-502 stainless steel column (5 m × 3 mm) using N2 (99.999%) as the carried gas. The conversion of NMHC was calculated using the following formula conversion =

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity of Mn-CeOx/Cordierite for NMHC Oxidation. Figure 1 shows the conversion of NMHC over

Figure 1. Conversion curves of NMHC over MnxCey/C catalysts with different molar ratios of Mn and Ce.

C in − Cout × 100% Cout

where Cin (ppm) and Cout (ppm) are the concentrations of NMHC in the feed gas and outlet gas, respectively. 2.3. Catalysts’ Characterization. The specific surface area, pore volume, and average pore diameter of the catalysts were measured by physisorption of N2 using a Micromeritics ASAP 2460 instrument at 77 K after the samples were degassed for 2 h at 298 K. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was calculated by the method of Barrett−Joyner− Hallender (BJH). The surface microstructure of the catalysts was obtained from high-resolution images performed on a Hitachi SU8010 type scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDS) of the samples was tested on the same SEM instrument equipped a HORIBA silicon drift X-ray detector to analyze the element distributions on the surface. In order to improve the quality of the images,

Figure 2. Conversion curves of NMHC over cordierite support and all Mn4Ce1 catalysts. 4187

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Industrial & Engineering Chemistry Research

Figure 3. XRD patterns of the catalysts with different Mn/Ce molar ratios (a: wild angle patterns, b: 2θ equal to 31°−35°, c: 2θ equal to 46°−48°, d: 2θ equal to 36°−39.5°, and e: 2θ equal to 56°−57.5°).

The conversion of NMHC over pure cordierite support and those with Mn4Ce1 catalysts were shown in Figure 2. In the case of C and NC, the conversion of NMHC was quite low even at a very high reaction temperature which indicated that there was no active component on the surface of the cordierites. After loading Mn4Ce1 on the supports, the conversion of NMHC was greatly improved. Meanwhile, different Mn4Ce1 catalysts for NMHC oxidation were ordered as follow: g-Mn4Ce1/NC > g-Mn4Ce1/C > Mn4Ce1/C. 3.2. Catalyst Characterization. Figure 3 presents the XRD patterns of the catalysts with different Mn/Ce molar ratios. From Figure 3a, it is observed that all the samples showed the relatively strong characteristic peaks at 2θ = 10.4°, 18.0°, 21.6°, 26.3°, 28.4°, 33.9°, and 54.2° which correspond to the crystallite structures of cordierite (JCPDS 12-235) because of the high content of cordierite. Meanwhile, the relatively weak diffraction peaks of active species at 2θ = 33.0° and 47.4° and 2θ = 37.3° and 56.7° ascribed to cubic fluorite-type CeO2

Mn-CeOx/cordierite with different molar ratios of Mn/Ce at the reaction temperature from 200 to 400 °C. The experimental results show that activity of MnxCey/C was observed to follow the sequence below: Mn4Ce1/C > Mn20Ce1/C > Mn/C > Mn16Ce1/C > Mn2Ce1/C> Mn8Ce1/C> Mn12Ce1/C. It can be shown that, with the increase of the Ce content in the catalysts, the activity of the catalysts decreased first and then increased until the molar ratios of Mn/Ce was 4. The activity of the catalyst decreased again when the molar ratio of Mn/Ce was greater than 4 and Mn4Ce1/C exhibited the highest catalytic activity for NMHC oxidation with conversion of 90% at 400 °C. The results indicated that adding the appropriate proportion of Ce can obviously improve the activity of catalysts for NMHC oxidation in cooking oil fumes. The possible reason for why the addition of Ce promoting the catalytic activity of catalysts is because of it enhanced not only the oxygen storage and release capacity of the catalysts but also oxygen mobility in the surface and vacancies. 4188

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Industrial & Engineering Chemistry Research

(JCPDS 81-0792) and pyrolusite-type MnO2 (JCPDS 24-735) respectively. According to Figure 3b,c, it is clear that the characteristic peak of CeO2 was not obvious as the Mn/Ce molar ratio was higher than 4. It is speculated that the crystal structure of CeO2 had been distorted by the incorporation of the Mn4+ and the high dispersion of CeO2. Compared with Mn/C, no samples showed any diffraction peaks of MnO2 from Figure 3d,e which indicated that the Mn4+ had incorporated into the lattice of CeO2 to form a homogeneous Mn−Ce−O solid solution since the ionic radius of Mn4+ (0.053 nm) is smaller than that of the Ce4+ (0.087 nm).23−25 It can also be deduced that the crystallinity of the MnO2 became lower and the crystallites size of the MnO2 became smaller after the addition of CeO2.26−28 From Figure 4, it can be seen clearly that the XRD curves of nitic acid treated and Mn4Ce1 supported cordierites has no difference from the virgin one, which illustrated the formation of the Me−Ce−O solid solution and the high dispersion of the active components. The SEM images of the catalysts prepared by incipient wetness impregnation method with different Mn/Ce molar ratios

Figure 4. XRD patterns of cordierite support and all of the Mn4Ce1 catalysts.

Figure 5. SEM images of the catalysts prepared by incipient wetness impregnation method with different Mn/Ce molar ratios (a: Mn2Ce1/C, b: Mn4Ce1/C, c: Mn8Ce1/C, d: Mn12Ce1/C, e: Mn16Ce1/C, and f: Mn20Ce1/C). 4189

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Industrial & Engineering Chemistry Research

Figure 6. SEM images of the cordierite support and all Mn4Ce1 catalysts (a: C, b: NC, c: Mn4Ce1/C, d: g-Mn4Ce1/C, and e: g-Mn4Ce1/NC).

are shown in Figure 5. From Figure 5, it can be observed that all of the catalysts were covered with a homogeneous layer of active component and the active particles of all of the samples have a certain degree of agglomeration which might be caused by the preparation method. As shown in Figure 5a−f, among all of the samples, the catalyst with Mn/Ce molar ratio = 4 (Figure 5b) had the most abundant pore structure which leads to a relatively high catalytic activity. Figure 6 shows the SEM images of cordierite support and all Mn4Ce1 catalysts. It was found that the surface of pure cordierite (Figure 6a) was very smooth so that its specific surface area was very low. After treating by nitric acid, there are more pores that appeared on the surface of cordierite and made the surface rougher (Figure 6b), which was consistent with BET data from Table 1. The SEM images of catalysts prepared by the incipient wetness impregnation method and sol−gel method are shown in Figure 6c,d, respectively. As can be seen in the figure, the active phase of those samples made by incipient wetness was obviously agglomerated while those samples made by sol− gel method had better dispersion, which might be due to the fact that the catalysts prepared by sol−gel method have larger specific surface area, smaller total pore volume, and smaller

Table 1. Textural Properties of the Catalyst Supports and Mn4Ce1 Catalysts materials

SBETa (m2/g)

DVb (10−8m3/g)

DPc (nm)

C NC Mn4Ce1/C g-Mn4Ce1/C g-Mn4Ce1/NC

1.76 3.02 8.18 12.14 15.06

0.33 0.36 2.95 2.58 2.95

10.2 8.1 14.9 7.1 8.2

a BET specific surface area. bBJH desorption cumulative volume of pores. cBJH desorption average pore width.

average pore size than the catalysts prepared by the incipient wetness impregnation method. Figure 6e shows the surface image of the catalyst supported on the cordierite after nitric acid treatment by sol−gel method. Compared with other samples, its active phase had the best dispersion and pore structure which is in agreement with BET data. In order to analyze the distribution of catalysts’ surface elements, an EDS energy spectrum was carried out. As shown in Figure 7, some elements composition such as Si, Al, and Mg accounted for a large proportion attributed to the components of cordierite, and the Mn/Ce molar ratios were similar to the theoretical 4190

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Industrial & Engineering Chemistry Research

Figure 7. EDS patterns of catalysts prepared by sol−gel method and incipient wetness impregnation method (a: g- Mn4Ce1/C and b: Mn4Ce1/C).

The specific surface area (SBET), total pore volume (DV), and average pore size (DP) of cordierite support and all of the Mn4Ce1 catalysts are shown in Table 1. It is clear that the specific surface area and total pore volume of the cordierite increased after nitric acid treatment, but the average pore size showed the opposite effect. That might be because the cordierite was corroded by nitric acid on the surface and generated many little pores. The specific surface area and total pore volume of all of the Mn4Ce1 catalysts were much higher than the cordierite. Comparing Mn4Ce1/C with g-Mn4Ce1/C in Table 1, it can be seen that the catalysts prepared by the sol−gel method have a larger specific surface area, a smaller total pore volume,

value which indicated that the active phase had been well attached onto the surface of cordierite. The EDS patterns of catalysts prepared by the sol−gel method and the incipient wetness impregnation method are shown in Figure 7a,b, respectively. It is clear that the active phase of catalyst prepared by the sol−gel method had better dispersion and those prepared by the incipient wetness impregnation method were obviously agglomerated as seen in EDS-mapping images from Figure 7. From the catalytic activity and EDS-mapping results, it can be speculated that better dispersion of catalysts’ active component is one of the reasons for the good catalytic performance. 4191

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Industrial & Engineering Chemistry Research and a smaller average pore size than those prepared by the incipient wetness impregnation method, which indicated that the catalysts can obtain a richer pore structure through sol−gel method. Among all of the catalysts, g-Mn4Ce1/NC has the largest specific surface area (15.0639 m2/g) and best pore structure which resulted in a high catalytic activity in the oxidation of NMHC in cooking oil fumes, as shown in Figure 2. Figure 8 shows the XPS spectra for Mn 2p3/2, Ce 3d, and O 1s of various Mn−Ce/cordierite catalysts. The surface element

compositions calculated from the ratios of peak areas of XPS spectra are displayed in Table 2. Table 2. Surface Element Compositions of the Catalysts with Different Mn/Ce Molar Ratios materials

Mn4+/Mn3+

Ce4+/Ce3+

Oads/Olatt

Mn2Ce1/C Mn4Ce1/C Mn8Ce1/C

1.07 1.99 1.33

6.21 7.39 3.80

0.48 1.12 0.39

From Figure 8a, the spectra for Mn 2p3/2 could be divided into two components at BE = 641.6 and 642.9 eV attributed to Mn3+ and Mn4+,29−31 respectively. Combined with Table 2, it is clear that the surface Mn4+/Mn3+ molar ratios were greatly affected by the addition of Ce. The Mn4Ce1/C had the highest surface Mn4+/Mn3+ molar ratios (1.99), whereas the Mn2Ce1/C and Mn8Ce1/C had relatively low surface Mn4+/Mn3+ molar ratios (1.07 and 1.33) which indicated that the Mn4Ce1/C had a relatively high average manganese oxidation state. Generally, the catalyst with higher Mn4+/Mn3+ molar ratios on the surface has more adsorbed oxygen species as active oxygen because of electroneutrality32,33 which will be demonstrated by the XPS spectra for O 1s. Figure 8b shows the XPS spectra for Ce 3d which exhibited eight characteristic peaks assignable to the Ce3+ (BE = 884.7 and 902.0 eV) and Ce4+ (BE = 882.5, 888.1, 898.3, 900.7, 906.6, and 916.7 eV).21 As summarized in Table 2, the molar ratios of Ce4+/Ce3+ were approximately 6.21 for Mn2Ce1/C, 7.39 for Mn4Ce1/C, and 3.80 for Mn8Ce1/C according to the measurement of the peak areas. The result shows that the oxidation state of Ce was predominantly tetravalent for all of the catalysts. Besides, a higher surface Ce4+/Ce3+ molar ratio may result in greater oxygen storage and release capacity and can easily generate labile oxygen vacancies and highly mobile oxygen therefore have better catalytic oxidation performance.21,22,34,35 In Figure 8c, the O 1s XPS spectra of all of the samples can be deconvoluted to three characteristic peaks at BE = 529.6, 531.2, and 532.3 eV corresponding to the lattice oxygen (O2−, denoted as Olatt), surface adsorbed oxygen species (O2−, O22−, or O−, denoted as Oads), and hydroxyl or adsorbed water.36−38 It is believed that Oads species has more activity than Olatt in the oxidation reactions due to higher oxygen mobility.36,39 So the ratio of Oads/Olatt can represent the activity of the catalyst to some extent. In Table 2, the ratio of Oads/Olatt in Mn4Ce1/C (1.12) was clearly higher than that in Mn2Ce1/C and Mn8Ce1/C (0.48 and 0.39) which is consistent with the XPS spectra for Mn 2p3/2 and the sequence of catalysts’ activity.

4. CONCLUSIONS In summary, catalysts with different molar ratios of Mn/Ce and Mn4Ce1 catalysts prepared by different methods were used for NMHC oxidation in COFs. The Mn4Ce1 catalyst supported on the cordierite after nitric acid treatment using a sol−gel method presented better catalytic performance than Mn4Ce1/C attributed to higher BET specific surface area, more abundant pore structure, and better dispersion of catalysts’ active component. Meanwhile, the Mn4Ce1/C exhibited higher catalytic activity than the catalysts with other molar ratios of Mn/Ce showing the 90% conversion of NMHC at 400 °C due to more adsorbed oxygen species and higher ratio of Ce4+/Ce3+.

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Figure 8. XPS spectra of the catalysts with different Mn/Ce molar ratios for a: Mn 2p3/2, b: Ce 3d, and c: O 1s.

*Tel./Fax:+86 010 62332747. E-mail: [email protected]. 4192

DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194

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Honghong Yi: 0000-0002-0097-9007 Xiaolong Tang: 0000-0003-3130-3883 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by program for New Century Excellent Talents in University (NCET-12-0776).

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DOI: 10.1021/acs.iecr.7b04904 Ind. Eng. Chem. Res. 2018, 57, 4186−4194