Catalytic combustion of ethyl acetate over CeMnOx and CeMnZrOx compounds synthesized by coprecipitation method

Journal of Natural Gas Chemistry 20(2011)623–628

Catalytic combustion of ethyl acetate over CeMnOx and CeMnZrOx compounds synthesized by coprecipitation method Xiaoshuang Li, Jianli Wang, Chuanwen Liao, Hongyan Cao,

Yaoqiang Chen,

Maochu Gong∗

Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China [ Manuscript received April 18, 2011; revised July 9, 2011 ]

Abstract Ce0.6 Mn0.4 O2 catalysts with different sources of manganese and Ce0.6−x Zrx Mn0.4 O2 mixed oxide catalysts were prepared by coprecipitation method and were characterized by N2 adsorption-desorption, TPR, XRD, and XPS techniques. The activities of the prepared catalysts for ethyl acetate combustion, and the effects of calcination temperature and space velocity on catalytic activity were investigated. The results showed that partial replacement of Mn(NO3 )2 with KMnO4 as sources of manganese could improve activities of catalysts. Ce0.45 Zr0.15 Mn0.4 O2 catalyst exhibited the best catalytic activity and high thermal stability, e.g., T90 could be still below 210 ◦ C even if space velocity was up to 20000 h−1 . Key words ethyl acetate; catalytic combustion; volatile organic compound; CeMnZrOx catalysts

1. Introduction Volatile organic compounds (VOCs), which are regarded as major pollutants to air, are emitted from a variety of industrial and commercial processes, such as printing, chemical production, transportation and so on. Most of them are harmful to the environment and are identified as carcinogenic and teratogenic chemicals by the World Health Organization [1]. Therefore, the removal of VOCs remains one of important research hot topics for environmental treatment [2,3]. By now, direct thermal combustion and catalytic combustion have been used to remove VOCs. Direct thermal combustion, which is the most common method for the removal of VOCs, usually performes at above 1000 ◦ C under high reactant concentration and results in the generation of toxic by-products such as NOx [4]. Compared with the conventional thermal process, catalytic combustion can convert contaminant into carbon dioxide (CO2 ) and water at much lower temperature and with lower reactant concentration, which inhibits the formation of nitrogen oxides and saves the cost. Hence catalytic combustion is considered as a promising method for the removal of VOCs. Among VOCs, ethyl acetate often appears in various gas exhaust streams and leads to a lot of harms to environment and human health. A lot of VOCs can be completely oxidized

to CO2 by Pd, Pt-catalysts, such as Pd/Mg-Al hydrotalcite [5], Pd/ZrO2 [6], Pd/Al2 O3 [7] and Pt/Al2 O3 [8]. Because noble metal catalysts are expensive and easy to be poisoned, their industrial application is greatly limited, and therefor the development of non-noble metal oxide catalysts with high catalytic activity is badly in need. According to the literatures, Mn-containing mixed oxide catalysts are effective for catalytic combustion of VOCs [9]. Delimaris and Ioannides found that the activities of MnOx CeO2 catalysts were higher than those of Mg/Mn/Al prepared from LDH (layered double hydroxide) precursors due to the presence of more Mn4+ species and richer lattice oxygen [10−13], and complete conversion temperature of toluene over MnOx -CeO2 catalysts is 260 ◦ C (GHSV = 50000 h−1 ), but MnOx -CeO2 catalysts have poor thermal stability. Some literatures report that the incorporation of zirconium into the ceria oxide lattice could improve oxygen storage capacity, redox property, thermal resistance, and enhance catalytic activity [14−18]. In the present work, the mixed oxide catalysts, CeMnOx and CeMnZrOx , were prepared by a co-precipitation method for catalytic combustion of ethyl acetate. The influences of different sources of manganese, Zr/Ce molar ratio, calcination temperature and space velocity on the ethyl acetate combustion were investigated in the paper.

∗

Corresponding author. Tel: +86-28-85418451; Fax: +86-28-85418451; E-mail: nic7501@ scu.edu.cn This work was supported by the National Natural Science Foundation of China (No. 20773090), the National High Technology Research and Development Program of China (863 Program, No. 2006AA06Z347) and the Youth Fund of Sichuan University (No. 2008119). Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60249-6

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2. Experimental

2.3. Catalyst activity measurement

2.1. Catalyst preparation

The catalysts were tested in a conventional fixed-bed reactor made up of a vertical tubular electric furnace (30 cm in length) and a quartz tube (10.7 mm in diameter). The catalysts were placed in the quartz tube. The stream of ethyl acetate was generated by bubbling a flow of air through a saturator, and it was diluted with another air flow before reaching the catalysts. The concentration of ethyl acetate was 1000 ppm in air. All gas lines of the apparatus were kept at 120 ◦ C to minimize adsorption and condensation of ethyl acetate on the walls. The reactants or product gases were analyzed with an online gas chromatograph (GC-2000, Shanghai Institute of Computer Techniques, China) using a Porapak T column equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD).

Ce0.6 Mn0.4 O2 catalyst was prepared by the coprecipitation method. Aqueous solutions of Ce(NO3 )3 and Mn(NO3 )2 were mixed together and precipitated by the buffer solution of (NH4 )2 CO3 and NH3 ·H2 O under vigorous stirring, and the value of pH was kept at 10 consistently. The obtained precipitate was dried at 110 ◦ C overnight and calcined at 400 ◦ C for 5 h in flowing air to gain Ce0.6 Mn0.4O2 catalyst, which was designed as CeMna. Ce0.6−x Zrx Mn0.4O2 (x = 0, 0.05, 0.1, 0.15, 0.2) catalysts were prepared in the same way, and ZrOCO3 was used as the precursor of ZrO2 . However, the difference is that Mn(NO3 )2 was partially replaced by KMnO4 with the ratio of Mn(NO3 )2 to KMnO4 of 3 : 2 for purpose of obtaining a stoichiometric valence of Mn of 4. The catalysts were noted as CeMnZr0, CeMnZr0.05, CeMnZr0.1, CeMnZr0.15, and CeMnZr0.2. By mixing the catalyst powder and appropriate amount of water, a slurry was obtained, and the slurry was coated on a monolithic substrate with 400 pore/inch2. The monolithic catalysts were dried at 110 ◦ C and calcined at different temperatures (400 ◦ C, 550 ◦ C, 750 ◦ C) in static air for 5 h. The coating amount was 160 g/L. 2.2. Catalyst characterization X-ray diffraction (XRD) patterns were performed on a DX-1000 X-ray diffractometer using Cu Kα (λ = 0.15406 nm) radiation equipped with a graphite monochromator. The working voltage of the X-ray tube was 45 kV and the current was 25 mA. Samples were scanned for 2θ values from 10o up to 90o and the X-ray diffraction line positions were determined with a step size of 0.03o and a slit of 1o . H2 -temperature-programmed reduction (H2 -TPR) experiments were performed on a conventional flow apparatus equipped with a thermal conductivity detector (TCD). All the samples (0.05 g) were pre-treated in a quartz U-tube in a flow of pure N2 stream at 400 ◦ C for 1 h, and then cooled to room temperature. The reduction was carried out in a flow of 5 vol% H2 /N2 mixture (30 mL/min) from room temperature to 800 ◦ C with a heating rate of 8 ◦ C/min. The consumption of H2 was detected by TCD. The textural properties were measured by N2 adsorptiondesorption with Brunauer-Emmett-Teller (BET) model at −196 ◦ C using Autosorb-ZXF-06 instrument (Xibei Chemical Institute, China). The samples were previously pre-treated at 350 ◦ C for 1 h under high vacuum before N2 adsorptiondesorption. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM-800 high performance electron spectrometer using Mg Kα (hν = 1253.6 eV) radiation as the excitation source. The C 1s peak at 284.6 eV was taken as the reference for BE (binding energy) calibration.

3. Results and discussion 3.1. Catalytic performance Figure 1 shows the catalytic performance of CeMna and CeMnZr0 catalysts calcined at 400 ◦ C for combustion of ethyl acetate under GHSV = 10000 h−1 . After partially replaced by KMnO4 in the preparation process, the conversion temperature of ethyl acetate over the prepared catalyst decreases. T90 (the temperature of 90% ethyl acetate conversion, which is regarded as the complete conversion temperature) over CeMnZr0 is 203 ◦ C, which decreases for 5 ◦ C compared with that over the CeMna catalyst (208 ◦ C). So the activity of CeMnZr0 keeps slightly higher than that of CeMna in the reaction temperature range from 170 to 210 ◦ C .

Figure 1. Activities of CeMna and CeMnZr0 calcined at 400 ◦ C for catalytic combustion of ethyl acetate at GHSV = 10000 h−1

The activity profiles of ethyl acetate over the CeMnZrOx catalysts calcined at 400 ◦ C under GHSV = 10000 h−1 are shown in Figure 2. It can be seen that all the CeMnZrOx catalysts, over which the complete conversion temperatures are all below 210 ◦ C, exhibit excellent catalytic abilities. From the profiles, it can be observed that the catalytic activities of catalysts are different with different ZrO2 contents. With the

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increase of ZrO2 content, the catalytic activity declines at first and then increases. CeMnZr0.15 catalyst exhibits the best activity in all the catalysts. It indicates that the catalytic activity of CeMnOx catalyst can be enhanced by the proper addition of ZrO2 .

Figure 2. Activities of CeMnZrOx catalysts calcined at 400 ◦ C for catalytic combustion of ethyl acetate at GHSV = 10000 h−1

Figure 3 displays the catalytic activities of CeMnZr0 and CeMnZr0.15 catalysts calcined at 400 ◦ C, 550 ◦ C and 750 ◦ C for oxidation of ethyl acetate. It shows that the activities of both catalysts decrease with calcination temperature increasing, however, the change of CeMnZr0 catalyst is more intensive. Compared with those of catalysts calcined at 400 ◦ C, the complete conversion temperature of ethyl acetate over CeMnZr0 calcined at 550 ◦ C (217 ◦ C ) increases by 14 ◦ C (shown in Figure 3a), while that over CeMnZr0.15 (208 ◦ C ) only increases by 5 ◦ C (shown in Figure 3b). When the calcination temperature increases to 750 ◦ C, ethyl acetate is converted completely at 254 ◦ C over CeMnZr0.15, while complete conversion temperature is 304 ◦ C over CeMnZr0, which is 50 ◦ C higher than that over CeMnZr0.15. These illustrate that the thermal stability of catalysts can be enhanced by the addition of ZrO2 into CeMnOx solid solution. In summary, ZrO2 not only improves activities of catalysts, but also enhances thermal stability of catalysts. CeMnZr0.15 sample has the best catalytic activity and high ability of restraining deactivation. Among the catalysts calcined at different temperatures, the catalyst calcined at 400 ◦ C shows the highest activity, so the catalyst calcined at 400 ◦ C is chosen to discuss. The effect of space velocity on the catalytic performance is investigated over CeMnZr0.15 catalyst. As shown in Figure 4, catalytic activity decreases with increasing GHSV, which may be attributed to the shorter retention time of ethyl acetate in catalyst bed under higher GHSV. However, when space velocity is up to 20000 h−1 , CeMnZr0.15 catalyst still keeps good catalytic activity, over which T90 is still below 210 ◦ C .

Figure 3. Activities of CeMnZr0 (a) and CeMnZr0.15 (b) calcined at different temperatures for catalytic combustion of ethyl acetate at GHSV = 10000 h−1

Figure 4. Effect of GHSV on the catalytic performance of the CeMnZr0.15 catalyst calcined at 400 ◦ C for catalytic combustion of ethyl acetate

3.2. Characterization of catalysts The textural properties of CeMna, CeMnZr0 and CeMnZr0.15 catalysts calcined at different calcination temperatures are summarized in Table 1. The surface area and pore volume as well as the mean pore diameter of CeMnZr0 are all bigger than those of CeMna, which illustrates that the addition of KMnO4 could improve textural properties of catalysts. Excellent textural property is a reason why CeMnZr0 possesses better activity. After CeMnZr0 and CeMnZr0.15 catalysts were calcined at 400 ◦ C, the surface area of CeMnZr0.15 (157 m2 /g) is much lower than that of CeMnZr0 (201 m2 /g), which is opposite to the sequence of activities. It seems that textural properties are not the only key

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factor that affects activities of catalysts. Owing to the agglomeration of fine particles and the sintering of pores, the surface area and pore volume decrease with calcination temperature increasing to 550 ◦ C [19]. A dramatic decrease of surface area occurs when calcination temperature is up to 750 ◦ C, which may be due to phase transformations [19]. As the calcination

temperature is enhanced to 550 ◦ C or 750 ◦ C, textural properties of CeMnZrO decrease sharply, while CeMnZr0.15 exhibits higher textural stability than CeMnZr0. Based on these results, conclusion could be drawn that the addition of ZrO2 can improve the thermal stability of CeMnOx catalysts, which is consistent with the results of activity tests.

Table 1. Textural properties of CeMna, CeMnZr0 and CeMnZr0.15 catalysts Samples

400 ◦ C

CeMna CeMnZr0 CeMnZr0.15

183 201 157

SBET (m2 /g) 550 ◦ C 115 126

750 ◦ C

400 ◦ C

11 32

0.25 0.28 0.28

XRD patterns of the catalysts calcined at 400 ◦ C are shown in Figure 5. According to the patterns, only diffraction peaks corresponding to cubic CeO2 phase are observed, and no visible MnOx and ZrO2 phases are detected. The possible reasons are that MnOx and ZrO2 species are amorphous or incorporated into the CeO2 lattice, or they are well dispersed on the surface of CeO2 in the form of small clusters that can not be detected by XRD [20]. MnOx calcined at 400 ◦ C for 5 h appears in the form of MnOx crystalloid [21]. Thus, it is most likely that MnOx is incorporated into the lattice of CeO2 or highly dispersed on the surface of CeO2 [22]. The diffraction peaks of samples slightly shift to higher values of Bragg angles, and lattice parameter of CeMnZr0.15 is smaller than that of CeMnZr0 (see Table 2). These indicate that Zr4+ species with small radius are incorporated into the lattice to form solid solution [23].

Figure 5. XRD patterns of CeMnZrOx calcined at 400 ◦ C Table 2. Crystallite size and lattice parameter of CeMnZr0 and CeMnZr0.15 catalysts Samples CeMnZr0 CeMnZr0.15

Mean crystallite size of CeO2 (nm) 3.3 3.7

Lattice parameter of CeO2 (nm) 0.5411 0.5349

Vpore (cm3 /g) 550 ◦ C 0.20 0.24

750 ◦ C

400 ◦ C

dpore (nm) 550 ◦ C

750 ◦ C

0.04 0.11

4.9 5.0 5.9

6.2 6.4

16.3 13.1

TPR profiles of catalysts calcined at 400 ◦ C are shown in Figure 6. According to literature, pure MnOx shows two overlapped big reduction peaks with apical temperature at 380 ◦ C and 480 ◦ C [23]. The peak at 380 ◦ C is assigned to the reduction of MnO2 /Mn2 O3 to Mn3 O4 , and that at 480 ◦ C is related to the reduction of Mn3 O4 to MnO. With the addition of CeO2 , the two strong reduction peaks shown in Figure 6(a) shift to lower temperature (330 ◦ C, 415 ◦ C ) owing to the interaction of cerium and manganese species. Furthermore, two small shoulder peaks appear at about 220 ◦ C and 550 ◦ C in all patterns. The shoulder peaks at 220 ◦ C are related to the readily reducible small clusters of surface manganese oxide [24,25], and the peaks at 550 ◦ C correspond to the reduction of Ce4+ to Ce3+ species on the surface. According to Figure 6(a), the reduction peak of CeMnZr0 is higher and broader with larger area than that of CeMna, which indicates that CeMnZr0 catalyst contains more high-valence manganese species which are effective to activate molecular oxygen through the oxygen transfer mechanism [26]. It also proves that CeMnZr0 catalyst has better activity. Figure 6(b) shows the results of TPR analysis for CeMnZrOx catalysts. With the addition of ZrO2 , a new peak at the range of 250−500 ◦ C appears and grows bigger along with the increase of ZrO2 content. The new peak is ascribed to the reduction of “isolated” Mn4+ ions which are “embedded” into the surface defective position of ceria lattice [23], which indirectly illustrates that the addition of ZrO2 could cause the defect of CeO2 lattice. Incorporation of Zr into the CeO2 lattice can result in the distortion and defect of crystal lattice of ceria, which could improve the mobility of lattice oxygen and increase the activities of catalysts [21]. The reduction peaks at the range of 300−500 ◦ C shift to higher temperature as the content of ZrO2 is up to 20%. It may be the reason why the activity of CeMnZr0.2 is poorer than that of CeMnZr0.15. The XPS spectra of O 1s and Mn 2p of the catalysts calcined at 400 ◦ C are shown in Figure 7. In Figure 7(a), there is an unsymmetrical peak at BE = 530.0 eV, which could be decomposed into two peaks. The peak with BE at 529.6 eV is assigned to lattice oxygen, and the peak with BE at 531.5 eV can be attributed to adsorbed oxygen and weakly bonded oxygen species (active oxygen) [27]. The contents of active oxygen in CeMnZr0, CeMnZr0.05, and CeMnZr0.15 catalysts are

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33.7%, 34.7%, and 35.1%, respectively. It has been reported that CeZrO2 solid solution has oxygen vacancy [28]. The increase of active oxygen is due to the accession of ZrO2 , which

Figure 6. TPR profiles of the CeMna and CeMnZr0 (a) and CeMnZrOx (b) catalysts

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could cause oxygen vacancy in CeMnZrOx solid solution. Although CeMnZr0.05 has more active oxygen affecting catalytic activity, the surface area and pore volume of CeMnZr0.05 (141 m2 /g, 0.22 cm3 /g) are much lower than those of CeMnZr0 (201 m2 /g, 0.28 cm3 /g), as main factors that determine the catalytic performance here, and therefore the activity of CeMnZr0.05 is poorer than that of CeMnZr0. The surface area and active oxygen increase with the increase of ZrO2 content, leading to the improvement of catalytic activity, so CeMnZr0.15 possesses better activity than CeMnZr0. Figure 7(b) shows the Mn 2p XPS spectra of the catalysts. The BEs of Mn 2p3/2 for the CeMnZr0, CeMnZr0.05, and CeMnZr0.15 samples are 642.1 eV, 642.3 eV, and 642.7 eV, respectively, which increases with the increase of ZrO2 content. According to the literature, the BEs for MnO2 and Mn2 O3 are in the range of 641.1−642.4 eV and 641.2−641.9 V, respectively [29]. So there are MnO2 species in CeMnZr0, CeMnZr0.05 and CeMnZr0.15 catalysts. The BE of CeMnZr0.15 is higher than 642.4 eV, suggesting that ZrO2 strengthens the interaction of manganese with cerium species [30]. The BE of Mn 2p3/2 increases with the addition of Zr, because Mn tends to deficiency of electron, which profits the occurrence of redox reaction and the dispersion of the manganese oxide phase [29,31]. The XPS peaks for Mn 2p3/2 are all broad and unsymmetrical, which suggests that Mn appears in various oxidation states. 4. Conclusions Based on the characterization results, it can be concluded that, partial replacement of Mn(NO3 )2 with KMnO4 as sources of manganese can improve the content of high-valence manganese species and textural properties of CeMnOx , and thereby enhance the activity of catalysts. Through investigation of calcination temperature, CeMnZrOx catalysts calcined at 400 ◦ C have the best catalytic activities, and the complete conversion temperatures of ethyl acetate over catalysts CeMnZrOx are all below 210 ◦ C . Despite the fact that the surface area decreases, incorporation of Zr into the CeO2 lattice results in the distortion and defect of crystal lattice, creates oxygen vacancy, improves the mobility of lattice oxygen, and thereby enhances the activity and thermal stability of catalysts. References

Figure 7. (a) O 1s and (b) Mn 2p XPS spectra of CeMnZr0, CeMnZr0.05, and CeMnZr0.15

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