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Stability improvement of ZrO2-doped MnCeOx catalyst in ethanol oxidation
Catalysis Communications 12 (2011) 1361–1365
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Stability improvement of ZrO2-doped MnCeOx catalyst in ethanol oxidation Huaju Li, Tana, Xiaojing Zhang, Xiumin Huang, Wenjie Shen ⁎ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e
i n f o
Article history: Received 6 April 2011 Received in revised form 13 May 2011 Accepted 15 May 2011 Available online 23 May 2011 Keywords: Manganese–cerium oxide ZrO2-doping Solid solution Ethanol oxidation Stability
a b s t r a c t ZrO2-doped manganese–cerium oxide catalyzed ethanol oxidation effectively and ethanol was fully oxidized to CO2 at 453 K. The catalyst also showed quite promising stability for 120 h on-stream without obvious loss in ethanol conversion. Structural analyses have revealed that ZrO2-doping enhanced the oxygen storage capacity of ceria by generating more oxygen vacancies, and at the same time promoted the thermal stability through the formation of solid solution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing use of ethanol blended gasoline allows for cleaner and more complete combustion of fuels, but at the same time it increases the emissions of ethanol, acetaldehyde, and formaldehyde [1]. Catalytic abatement of these oxygenates presents a new challenge in the emission control of gasoline-fueled engines. Improving the oxidation ability of the currently used three-way-catalysts (TWCs) towards these toxic oxygenates is of great importance in practical applications [2]. MnOx–CeO2 oxides have been proved to be highly active for the oxidation of light oxygenates [3,4], but they tend to sinter at high temperatures, resulting in severe phase segregation. Doping zirconia to ceria materials have been typically used for promoting the oxygen storage capacity (OSC) and the thermal stability by forming solid solution [5–7]. The incorporation of zirconium into ceria lattice alters the local oxygen environment and creates more surface defects, and the formation of homogeneous CeO2–ZrO2 solid solution has greatly enhanced the thermal stability as well [5]. Similarly, ZrO2-doping to MnOx–CeO2 oxides has also been demonstrated to possess higher oxygen storage capacity and thermal stability than the binary oxides [8,9]. For instance, high-temperature aged Mn0.1Ce0.6Zr0.3Ox catalyzed ethanol oxidation more easily than the Mn0.1Ce0.9Ox system [8], although the overall reaction efficiency was not so pronounced due to the lower manganese content. We have recently reported that complete oxidation of formaldehyde [10] and ethanol [3] over MnOx–CeO2 oxides could be achieved at 373 and
⁎ Corresponding author. Tel.: + 86 411 84379085; fax: + 86 411 84694447. E-mail address: [email protected] (W. Shen). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.05.016
463 K, respectively. In both cases, the activation of molecular oxygen is the essential step and it necessitates the intimate interaction between MnO2 and CeO2 in the solid solution. In this work, we intended to examine the promoting effect of ZrO2-doping on the structural and catalytic properties of MnOx–CeO2 oxides with relatively higher manganese content for ethanol oxidation with respect to the activity and the stability. 2. Experimental 2.1. Catalyst preparation The Mn0.6Ce0.2Zr0.2Ox oxide was prepared by a co-precipitation method. 50 mL of 0.36 M Mn(NO3)2 aqueous solution was added to 200 mL of aqueous solution containing 1.92 g KMnO4, 5.54 g (NH4)2Ce (NO3)6 and 4.34 g Zr(NO3)4·5H2O. Then, 80 mL of 2 M KOH aqueous solution was added to the mixture at 353 K and maintained at that temperature for 2 h. After filtration and washing with water, the resultant solid was dried at 383 K for 12 h and calcined at 723 K for 6 h in air. For comparison, Mn0.6Ce0.4Ox was also prepared by coprecipitation as described elsewhere [3]. The samples were calcined at 823 and 873 K for thermal stability tests. ICP analyses have revealed that the actual compositions were Mn0.59Ce0.41Ox and Mn0.61Ce0.21Zr0.18Ox, respectively. 2.2. Characterization N2 adsorption–desorption isotherms were recorded at 77 K using an ASAP 2000 instrument (Micromeritics). The specific surface area was calculated by using the BET equation from the adsorption isotherm. Power X-ray diffraction (XRD) patterns were recorded on
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H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
MnOx
a D/Max-2500/PC diffractometer (Rigaku) using Cu Kα radiation operated at 40 kV and 200 mA. X-ray photoelectron spectra (XPS) were taken on an ESCALAB MK-II spectrometer (VG Scientific Ltd. UK) with Al Kα radiation. The spectra were deconvoluted using XPSPEAK program by curve fitting with the mixed Gaussian–Lorentzian peaks. High resolution transmission electron microscopy (HRTEM) images were recorded on a FEI Tecnai G 2 F30S-Twin microscope operated at an acceleration voltage of 300 kV. Hydrogen temperature-programmed reduction (H2-TPR) was conducted with a U-type quartz tubular reactor connected to a thermal conductivity detector (TCD). 50 mg samples were pretreated with N2 at 573 K for 0.5 h. After cooling room temperature and introducing a 5% H2/N2 mixture (40 mL min − 1), the temperature was programmed to rise at a ramp of 10 K min − 1. OSC was measured with an Autochem II 2920 instrument (Micromeritics) analyzed with a TCD. 70 mg samples were pre-reduced with a 10% H2/Ar mixture (40 mL min − 1) at 673 K. After cooling down to 473 K or 423 K under Ar flow, pulse of oxygen was injected periodically and the OSC was calculated from the total quantity of oxygen consumed.
Mn0.6Ce0.2Zr0.2Ox-873 Mn0.6Ce0.2Zr0.2Ox-823
Intensity / a.u.
Mn0.6Ce0.2Zr0.2Ox-120 h Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.4Ox-823 Mn0.6Ce0.4Ox-120 h Mn0.6Ce0.4Ox
20
30
40
50
60
2 Theta / degree Fig. 1. XRD patterns of the oxides. Table 1 The surface areas, OSCs, and crystalline sizes of the oxides. Sample
Mn0.6Ce0.4Ox Mn0.6Ce0.4Ox-823 Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.2Zr0.2Ox-823 Mn0.6Ce0.2Zr0.2Ox-873 a
Surface area (m2 g−1)
177 134 201 171 163
Crystalline size (nm)a
3.9 5.1 3.6 4.3 4.7
OSC (mmol g− 1) 423 K
473 K
1.23 1.12 1.33 1.29 1.13
1.32 1.20 1.53 1.51 1.44
2.3. Ethanol oxidation Ethanol oxidation was conducted in a fixed-bed quartz tubular reactor under atmospheric pressure. 60 mg catalysts (40–60 mesh) were sandwiched by two quartz wool layers. Gaseous ethanol was generated by flowing N2 over ethanol in an incubator maintained at 308 K. The typical feed gas contained 0.23% ethanol and 20% O2 balanced with N2. The gas hourly space velocity (GHSV) was 100,000 mL g− 1 h− 1. The
Calculated from the (111) plane of ceria.
Fig. 2. HRTEM images of (a) fresh and (b) used Mn0.6Ce0.4Ox; (c) fresh and (d) used Mn0.6Ce0.2Zr0.2Ox.
H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
a
Mn0.6Ce0.2Zr0.2Ox 4+
Intensity / a.u.
Ce (91.6 %)
Mn0.6Ce0.4Ox
4+
Ce (90.3 %)
920
910
900
880
890
BE / eV
b
Mn0.6Ce0.2Zr0.2Ox 4+
Mn0.6Ce0.2Zr0.2Ox Oα (86.3 %)
c
Intensity / a.u.
Mn (60.4 %)
Mn0.6Ce0.4Ox
Mn0.6Ce0.4Ox
Oα (87.4 %)
4+
Mn (73.8 %) 3+
Mn
Oβ
656
650
644
638
533
531
529
527
BE / eV Fig. 3. XP spectra of Ce 3d (a), Mn 2p (b) and O 1s (c) of the oxides.
effluent from the reactor was analyzed by an on-line HP 6890 gas chromatograph equipped with a TCD and a flame ionization detector (FID). To determine the exact concentration of carbon oxides, a nickel catalyst converter was placed before the FID and used for converting CO and CO2 quantitatively into methane in the presence of hydrogen. Typically, the reaction data were obtained by operating the reaction at desired temperatures for 3 h. The conversion of ethanol and the selectivity of the product were calculated on the basis of carbon-atom number balance in the product as described elsewhere [3].
weakened the reflection intensity of cubic fluorite structure and caused the Bragg angle shift to a slightly higher value. This indicates that MnOx and ZrO2 have incorporated into CeO2 lattice, forming a solid solution [9,11,12]. Because of this, the lattice parameter decreased from 0.534 nm (Mn0.6Ce0.4Ox) to 0.526 nm (Mn0.6Ce0.2Zr0.2Ox). The mean crystalline size slightly decreased from 3.9 to 3.6 nm and accordingly the surface areas increased from 177 to 201 m 2 g− 1 (Table 1). Fig. 2 shows the HRTEM images of the oxides. In both cases, the particles had an average size of about 4 nm and they were crystallined poorly and oriented randomly. MnOx–CeO2 oxides usually tend to sinter at high temperatures, and manganese oxides were readily segregated from Mn0.5Ce0.5Ox when calcined at 773 K [13]. In order to verify the thermal stability, the samples were then calcined at high temperatures. The diffraction lines of manganese oxides appeared at 873 K in the ZrO2doped sample whereas they appeared at 823 K in the Mn0.6Ce0.4Ox sample (Fig. 1). This result confirms that the strong Mn–Ce–Zr interaction in the solid solution have effectively inhibited the crystalline growth and in turn improved the thermal stability of Mn0.6Ce0.2Zr0.2Ox. Fig. 3 illustrates the XP spectra of the oxides. Quantitative analyses on Ce 3d profiles revealed that Ce 4+ / (Ce 4+ + Ce 3+) was 91.6% on Mn0.6Ce0.2Zr0.2Ox while 90.3% on Mn0.6Ce0.4Ox (Table 2), following the procedure of Larachi et al. [14]. The co-existence of Mn 4+ and Mn 3+ species have been identified by the binding energies at 642.6 and 641.3 eV [3,15], respectively. Quantitative discrimination of the oxidation states and the exact amounts of manganese species are particularly problematic owing to their multiple chemical states and overlapped binding energies [16]. Santos et al. [17] have determined the average oxidation state of manganese based on a correlation between the binding energies of the doublet separation of Mn 3s (ΔEs). By deconvoluting the Mn 2p spectra [18], the amount of surface Mn 4+ in the ZrO2-doped oxide was estimated to be 60.4% that is evidently lower than that of Mn0.6Ce0.4Ox (73.8%). Accordingly, the oxidation states of manganese were determined to be 3.6 and 3.7, respectively. The O 1s spectra contained two types of oxygen species, the binding energy at 529.1–529.6 eV was assigned to the lattice oxygen (Oα) while the binding energy at 531–533 eV was attributed to oxygen species in the defects or hydroxyl-like groups (denoted as Oβ) [15,16]. However, there was no obvious difference between the relative abundance of surface lattice oxygen (Oα) in both cases (Table 2). Fig. 4 shows the H2-TPR profiles of the oxides. There were two distinct reduction peaks at about 560 and 631 K, respectively. The lower temperature reduction peak represented to the reduction of MnO2/ Mn2O3 to Mn3O4 while the higher temperature reduction referred to the combined reduction of Mn3O4 to MnO and surface Ce4+ to Ce3+ species [3,16,17]. The total amounts of hydrogen consumed for Mn0.6Ce0.2Zr0.2Ox
3. Results and discussion 3.1. Structural properties of the catalysts Fig. 1 shows the XRD patterns of the oxides. The Mn0.6Ce0.4Ox sample showed broad diffraction lines of cubic fluorite ceria (PDF# 43–1002) but without diffraction lines of manganese oxides [3]. ZrO2-doping Table 2 Surface compositions of the oxides. Catalyst
Mn0.6Ce0.4Ox Mn0.6Ce0.2Zr0.2Ox
Surface molar ratio (%)a
Surface concentration (%) Mn
Ce
Zr
O
Mn4+
Ce4+
Oα
20.3 20.1
13.4 7.4
– 8.5
66.3 64.0
73.8 60.4
90.3 91.6
87.4 86.3
a Mn4+, Ce4+, and Oα referred to as Mn4+/(Mn4+ + Mn3+), Ce4+/(Ce4+ + Ce3+), and Oα/(Oα + Oβ), respectively.
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Fig. 4. H2-TPR profiles of the oxides.
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H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
Conversion or Selectivity / %
80
a
100
Mn0.6Ce0.2Zr0.2Ox
60
Mn0.6Ce0.4Ox
100
60 CH3CHO selectivity
20 100
60
b
C2H5OH conversion
C2H5OH conversion
95
Conversion or Selectivity / %
100
Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.4Ox
16 CH3CHO selectivity
8
0 100
CO2 selectivity
80 CO selectivity 2
20 413
423
433
443
453
463
0
20
Temperature / K
40
60
80
100
120
Time on stream / h
Fig. 5. (a) Ethanol oxidation over the oxides (100,000 mL g− 1 h− 1) and (b) stability test at 463 K (150,000 mL g− 1 h− 1). The reaction gas contained 0.23% C2H5OH and 20% O2 balanced with N2.
(4.1 mmol g− 1) was less than that of Mn0.6Ce0.4Ox (4.6 mmol g− 1) due to the lower content of cerium oxide. By assuming that the hydrogen is stoichiometrically consumed for the reduction of Mn4+/Mn3+ reduction to Mn2+ and Ce4+ to Ce3+ based on the XPS analysis, 76.1% of CeO2 was reduced in the Zr-doped oxide, being much higher than that in Mn0.6Ce0.4Ox (35.7%). This reaffirms that ZrO2-doping has considerably improved the reduction of ceria in the mixed oxide. As a result, the Mn0.6Ce0.2Zr0.2Ox had higher OSCs of 1.33 mmol O2 g− 1 at 423 K and 1.53 mmol O2 g− 1 at 473 K (Table 1). The discrepancy between the H2TPR and OSC results is likely caused by the fact that the hydrogen reduction occurs both on the surface and in bulk in the TPR process, whereas the subsequent oxygen titration to measure the OSC mainly involves refilling the surface oxygen vacancies. It seems that ZrO2doping has promoted the oxygen storage capacity and the thermal stability of the oxides.
stabilizing the particle size. Ethanol oxidation generally follows the Mars–van Krevelen mechanism, and both the oxidation state of manganese and the mobility of surface oxygen species affect the reaction performance [16,19]. Ethanol is initially oxidized by MnO2, and the restoration of manganese oxidation state is realized using the oxygen species generated from CeO2 and the partially reduced CeO2 − x is replenished by molecular oxygen in the feed gas [3,10]. In other words, MnO2 acts as the active site while CeO2 serves as an oxygen reservoir and thus the mobility of surface oxygen species plays a vital role. The superior performance of the Mn0.6Ce0.2Zr0.2Ox oxide is apparently due to the rapid mobility of oxygen species caused by ZrO2-doping. Therefore, it can be concluded that ZrO2-dopping not only chemically promoted the oxygen storage capacity but also enhanced the thermal stability texturally. 4. Conclusion
3.2. Ethanol oxidation Ethanol was completely oxidized to CO2 at 453 K on Mn0.6Ce0.2Zr0.2Ox and at 463 K over Mn0.6Ce0.4Ox (Fig. 5a). In order to identify the possible deactivation, stability tests were then conducted at 463 K under a higher GHSV of 150,000 mL g− 1 h− 1. As shown in Fig. 5b, there was an induction period of about 20 h, especially in the case of the Mn0.6Ce0.4Ox catalyst. This is caused by the restructuring of the catalyst under reaction conditions that lowered the mobility of oxygen species for further oxidizing acetaldehyde, resulting in decreased selectivity of CO2 but increased selectivity towards acetaldehyde [3]. Thereafter, the conversion of ethanol and the selectivities of acetaldehyde and CO2 remained stable for the rest 100 h. On the Mn0.6Ce0.2Zr0.2Ox oxide, the conversion of ethanol kept at almost 100%, and the selectivity of CO2 only slightly decreased from 99.7% to 97.8% while the selectivity of acetaldehyde increased from practically zero to about 1% (with about 1% selectivity of CO). On the Mn0.6Ce0.4Ox catalyst, however, the conversion of ethanol decreased from 99.8% to 95.5%, and the selectivity of CO2 decreased significantly from 98.4% to 80.7% whereas the selectivity of acetaldehyde remarkably increased from 0.1% to 13.5%. XRD and HRTEM measurements of the used samples indicated that the particle size increased from 4 to about 6 nm in the Mn0.6Ce0.4Ox catalyst but it still maintained at about 4 nm in the ZrO2-doped catalyst (Figs. 1 and 2), demonstrating the promotional role of ZrO2-doping in
ZrO2-doping to MnOx–CeO2 oxide has enhanced the oxygen storage capacity and the thermal stability owing to the formation of solid solution. As a result, Mn0.6Ce0.2Zr0.2Ox catalyzed ethanol oxidation efficiently; ethanol was totally oxidized to CO2 at 453 K, and the conversion of ethanol and the selectivity of CO2 maintained almost unchanged at 463 K for 120 h on-stream. Acknowledgement Financial support is acknowledged from the National Natural Science Foundation of China (20923001 and 21025312). References [1] A.K. Agarwal, Prog. Energ. Combust. 33 (2007) 233–271. [2] S.G. Poulopoulos, D.P. Samaras, C.J. Philippopoulos, Atmos. Environ. 35 (2001) 4399–4406. [3] H. Li, G. Qi, Tana, X. Zhang, X. Huang, W. Li, W. Shen, Appl. Catal. B 103 (2011) 54–61. [4] D. Delimaris, T. Ioannides, Appl. Catal. B 84 (2008) 303–312. [5] Y. Nagai, T. Yamamoto, T. Tanaka, S. Yoshida, T. Nonaka, T. Okamoto, A. Suda, M. Sugiura, Top. Catal. 47 (2008) 137–147. [6] M.P. Yeste, J.C. Hernandez, S. Trasobares, S. Bernal, G. Blanco, J.J. Calvino, J.A. PerezOmil, J.M. Pintado, Chem. Mater. 20 (2008) 5107–5113. [7] I. Atribak, N. Guillen-Hurtado, A. Bueno-Lopez, A. Garcia-Garcia, Appl. Surf. Sci. 256 (2010) 7706–7712.
H. Li et al. / Catalysis Communications 12 (2011) 1361–1365 [8] T. Rao, M. Shen, L. Jia, J. Hao, J. Wang, Catal. Commun. 8 (2007) 1743–1747. [9] L. Jia, M. Shen, J. Hao, T. Rao, J. Wang, J. Alloy. Compd. 454 (2008) 321–326. [10] X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang, W. Shen, Appl. Catal. B 62 (2006) 265–273. [11] E. Moretti, L. Storaro, A. Talon, M. Lenarda, P. Riello, R. Frattini, M.d.V.M. de Yuso, A. Jiménez-López, E. Rodríguez-Castellón, F. Ternero, A. Caballero, J.P. Holgado, Appl. Catal. B 102 (2011) 627–637. [12] Z. Yang, Z. Fu, Y. Zhang, R. Wu, Catal. Lett. 141 (2011) 78–82. [13] J.A. Perez-Omil, J.J. Delgado, W. Ouahbi, A.B. Hungria, N. Browning, M.A. Cauqui, J.M. Rodriguez-Izquierdo, J.J. Calvino, J. Phys. Chem. C 114 (2010) 8981–8991.
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[14] F. Larachia, J. Pierrea, A. Adnota, A. Bernis, Appl. Surf. Sci. 195 (2002) 236–250. [15] X. Wang, Q. Kang, D. Li, Appl. Catal. B 86 (2009) 166–175. [16] V.P. Santos, M.F.R. Pereira, J.J.M. Órfão, J.L. Figueiredo, Appl. Catal. B 99 (2010) 353–363. [17] V.P. Santos, M.F.R. Pereira, J.J.M. Órfão, J.L. Figueiredo, Top. Catal. 52 (2009) 470–481. [18] V.D. Castro, G. Polzonetti, J. Electron Spectrosc, Relat. Phenom. 48 (1989) 117–123. [19] J. Luo, Q. Zhang, J. Garcia-Martinez, S.L. Suib, J. Am. Chem. Soc. 130 (2008) 3198–3207.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Stability improvement of ZrO2-doped MnCeOx catalyst in ethanol oxidation Huaju Li, Tana, Xiaojing Zhang, Xiumin Huang, Wenjie Shen ⁎ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e
i n f o
Article history: Received 6 April 2011 Received in revised form 13 May 2011 Accepted 15 May 2011 Available online 23 May 2011 Keywords: Manganese–cerium oxide ZrO2-doping Solid solution Ethanol oxidation Stability
a b s t r a c t ZrO2-doped manganese–cerium oxide catalyzed ethanol oxidation effectively and ethanol was fully oxidized to CO2 at 453 K. The catalyst also showed quite promising stability for 120 h on-stream without obvious loss in ethanol conversion. Structural analyses have revealed that ZrO2-doping enhanced the oxygen storage capacity of ceria by generating more oxygen vacancies, and at the same time promoted the thermal stability through the formation of solid solution. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing use of ethanol blended gasoline allows for cleaner and more complete combustion of fuels, but at the same time it increases the emissions of ethanol, acetaldehyde, and formaldehyde [1]. Catalytic abatement of these oxygenates presents a new challenge in the emission control of gasoline-fueled engines. Improving the oxidation ability of the currently used three-way-catalysts (TWCs) towards these toxic oxygenates is of great importance in practical applications [2]. MnOx–CeO2 oxides have been proved to be highly active for the oxidation of light oxygenates [3,4], but they tend to sinter at high temperatures, resulting in severe phase segregation. Doping zirconia to ceria materials have been typically used for promoting the oxygen storage capacity (OSC) and the thermal stability by forming solid solution [5–7]. The incorporation of zirconium into ceria lattice alters the local oxygen environment and creates more surface defects, and the formation of homogeneous CeO2–ZrO2 solid solution has greatly enhanced the thermal stability as well [5]. Similarly, ZrO2-doping to MnOx–CeO2 oxides has also been demonstrated to possess higher oxygen storage capacity and thermal stability than the binary oxides [8,9]. For instance, high-temperature aged Mn0.1Ce0.6Zr0.3Ox catalyzed ethanol oxidation more easily than the Mn0.1Ce0.9Ox system [8], although the overall reaction efficiency was not so pronounced due to the lower manganese content. We have recently reported that complete oxidation of formaldehyde [10] and ethanol [3] over MnOx–CeO2 oxides could be achieved at 373 and
⁎ Corresponding author. Tel.: + 86 411 84379085; fax: + 86 411 84694447. E-mail address: [email protected] (W. Shen). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.05.016
463 K, respectively. In both cases, the activation of molecular oxygen is the essential step and it necessitates the intimate interaction between MnO2 and CeO2 in the solid solution. In this work, we intended to examine the promoting effect of ZrO2-doping on the structural and catalytic properties of MnOx–CeO2 oxides with relatively higher manganese content for ethanol oxidation with respect to the activity and the stability. 2. Experimental 2.1. Catalyst preparation The Mn0.6Ce0.2Zr0.2Ox oxide was prepared by a co-precipitation method. 50 mL of 0.36 M Mn(NO3)2 aqueous solution was added to 200 mL of aqueous solution containing 1.92 g KMnO4, 5.54 g (NH4)2Ce (NO3)6 and 4.34 g Zr(NO3)4·5H2O. Then, 80 mL of 2 M KOH aqueous solution was added to the mixture at 353 K and maintained at that temperature for 2 h. After filtration and washing with water, the resultant solid was dried at 383 K for 12 h and calcined at 723 K for 6 h in air. For comparison, Mn0.6Ce0.4Ox was also prepared by coprecipitation as described elsewhere [3]. The samples were calcined at 823 and 873 K for thermal stability tests. ICP analyses have revealed that the actual compositions were Mn0.59Ce0.41Ox and Mn0.61Ce0.21Zr0.18Ox, respectively. 2.2. Characterization N2 adsorption–desorption isotherms were recorded at 77 K using an ASAP 2000 instrument (Micromeritics). The specific surface area was calculated by using the BET equation from the adsorption isotherm. Power X-ray diffraction (XRD) patterns were recorded on
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H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
MnOx
a D/Max-2500/PC diffractometer (Rigaku) using Cu Kα radiation operated at 40 kV and 200 mA. X-ray photoelectron spectra (XPS) were taken on an ESCALAB MK-II spectrometer (VG Scientific Ltd. UK) with Al Kα radiation. The spectra were deconvoluted using XPSPEAK program by curve fitting with the mixed Gaussian–Lorentzian peaks. High resolution transmission electron microscopy (HRTEM) images were recorded on a FEI Tecnai G 2 F30S-Twin microscope operated at an acceleration voltage of 300 kV. Hydrogen temperature-programmed reduction (H2-TPR) was conducted with a U-type quartz tubular reactor connected to a thermal conductivity detector (TCD). 50 mg samples were pretreated with N2 at 573 K for 0.5 h. After cooling room temperature and introducing a 5% H2/N2 mixture (40 mL min − 1), the temperature was programmed to rise at a ramp of 10 K min − 1. OSC was measured with an Autochem II 2920 instrument (Micromeritics) analyzed with a TCD. 70 mg samples were pre-reduced with a 10% H2/Ar mixture (40 mL min − 1) at 673 K. After cooling down to 473 K or 423 K under Ar flow, pulse of oxygen was injected periodically and the OSC was calculated from the total quantity of oxygen consumed.
Mn0.6Ce0.2Zr0.2Ox-873 Mn0.6Ce0.2Zr0.2Ox-823
Intensity / a.u.
Mn0.6Ce0.2Zr0.2Ox-120 h Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.4Ox-823 Mn0.6Ce0.4Ox-120 h Mn0.6Ce0.4Ox
20
30
40
50
60
2 Theta / degree Fig. 1. XRD patterns of the oxides. Table 1 The surface areas, OSCs, and crystalline sizes of the oxides. Sample
Mn0.6Ce0.4Ox Mn0.6Ce0.4Ox-823 Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.2Zr0.2Ox-823 Mn0.6Ce0.2Zr0.2Ox-873 a
Surface area (m2 g−1)
177 134 201 171 163
Crystalline size (nm)a
3.9 5.1 3.6 4.3 4.7
OSC (mmol g− 1) 423 K
473 K
1.23 1.12 1.33 1.29 1.13
1.32 1.20 1.53 1.51 1.44
2.3. Ethanol oxidation Ethanol oxidation was conducted in a fixed-bed quartz tubular reactor under atmospheric pressure. 60 mg catalysts (40–60 mesh) were sandwiched by two quartz wool layers. Gaseous ethanol was generated by flowing N2 over ethanol in an incubator maintained at 308 K. The typical feed gas contained 0.23% ethanol and 20% O2 balanced with N2. The gas hourly space velocity (GHSV) was 100,000 mL g− 1 h− 1. The
Calculated from the (111) plane of ceria.
Fig. 2. HRTEM images of (a) fresh and (b) used Mn0.6Ce0.4Ox; (c) fresh and (d) used Mn0.6Ce0.2Zr0.2Ox.
H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
a
Mn0.6Ce0.2Zr0.2Ox 4+
Intensity / a.u.
Ce (91.6 %)
Mn0.6Ce0.4Ox
4+
Ce (90.3 %)
920
910
900
880
890
BE / eV
b
Mn0.6Ce0.2Zr0.2Ox 4+
Mn0.6Ce0.2Zr0.2Ox Oα (86.3 %)
c
Intensity / a.u.
Mn (60.4 %)
Mn0.6Ce0.4Ox
Mn0.6Ce0.4Ox
Oα (87.4 %)
4+
Mn (73.8 %) 3+
Mn
Oβ
656
650
644
638
533
531
529
527
BE / eV Fig. 3. XP spectra of Ce 3d (a), Mn 2p (b) and O 1s (c) of the oxides.
effluent from the reactor was analyzed by an on-line HP 6890 gas chromatograph equipped with a TCD and a flame ionization detector (FID). To determine the exact concentration of carbon oxides, a nickel catalyst converter was placed before the FID and used for converting CO and CO2 quantitatively into methane in the presence of hydrogen. Typically, the reaction data were obtained by operating the reaction at desired temperatures for 3 h. The conversion of ethanol and the selectivity of the product were calculated on the basis of carbon-atom number balance in the product as described elsewhere [3].
weakened the reflection intensity of cubic fluorite structure and caused the Bragg angle shift to a slightly higher value. This indicates that MnOx and ZrO2 have incorporated into CeO2 lattice, forming a solid solution [9,11,12]. Because of this, the lattice parameter decreased from 0.534 nm (Mn0.6Ce0.4Ox) to 0.526 nm (Mn0.6Ce0.2Zr0.2Ox). The mean crystalline size slightly decreased from 3.9 to 3.6 nm and accordingly the surface areas increased from 177 to 201 m 2 g− 1 (Table 1). Fig. 2 shows the HRTEM images of the oxides. In both cases, the particles had an average size of about 4 nm and they were crystallined poorly and oriented randomly. MnOx–CeO2 oxides usually tend to sinter at high temperatures, and manganese oxides were readily segregated from Mn0.5Ce0.5Ox when calcined at 773 K [13]. In order to verify the thermal stability, the samples were then calcined at high temperatures. The diffraction lines of manganese oxides appeared at 873 K in the ZrO2doped sample whereas they appeared at 823 K in the Mn0.6Ce0.4Ox sample (Fig. 1). This result confirms that the strong Mn–Ce–Zr interaction in the solid solution have effectively inhibited the crystalline growth and in turn improved the thermal stability of Mn0.6Ce0.2Zr0.2Ox. Fig. 3 illustrates the XP spectra of the oxides. Quantitative analyses on Ce 3d profiles revealed that Ce 4+ / (Ce 4+ + Ce 3+) was 91.6% on Mn0.6Ce0.2Zr0.2Ox while 90.3% on Mn0.6Ce0.4Ox (Table 2), following the procedure of Larachi et al. [14]. The co-existence of Mn 4+ and Mn 3+ species have been identified by the binding energies at 642.6 and 641.3 eV [3,15], respectively. Quantitative discrimination of the oxidation states and the exact amounts of manganese species are particularly problematic owing to their multiple chemical states and overlapped binding energies [16]. Santos et al. [17] have determined the average oxidation state of manganese based on a correlation between the binding energies of the doublet separation of Mn 3s (ΔEs). By deconvoluting the Mn 2p spectra [18], the amount of surface Mn 4+ in the ZrO2-doped oxide was estimated to be 60.4% that is evidently lower than that of Mn0.6Ce0.4Ox (73.8%). Accordingly, the oxidation states of manganese were determined to be 3.6 and 3.7, respectively. The O 1s spectra contained two types of oxygen species, the binding energy at 529.1–529.6 eV was assigned to the lattice oxygen (Oα) while the binding energy at 531–533 eV was attributed to oxygen species in the defects or hydroxyl-like groups (denoted as Oβ) [15,16]. However, there was no obvious difference between the relative abundance of surface lattice oxygen (Oα) in both cases (Table 2). Fig. 4 shows the H2-TPR profiles of the oxides. There were two distinct reduction peaks at about 560 and 631 K, respectively. The lower temperature reduction peak represented to the reduction of MnO2/ Mn2O3 to Mn3O4 while the higher temperature reduction referred to the combined reduction of Mn3O4 to MnO and surface Ce4+ to Ce3+ species [3,16,17]. The total amounts of hydrogen consumed for Mn0.6Ce0.2Zr0.2Ox
3. Results and discussion 3.1. Structural properties of the catalysts Fig. 1 shows the XRD patterns of the oxides. The Mn0.6Ce0.4Ox sample showed broad diffraction lines of cubic fluorite ceria (PDF# 43–1002) but without diffraction lines of manganese oxides [3]. ZrO2-doping Table 2 Surface compositions of the oxides. Catalyst
Mn0.6Ce0.4Ox Mn0.6Ce0.2Zr0.2Ox
Surface molar ratio (%)a
Surface concentration (%) Mn
Ce
Zr
O
Mn4+
Ce4+
Oα
20.3 20.1
13.4 7.4
– 8.5
66.3 64.0
73.8 60.4
90.3 91.6
87.4 86.3
a Mn4+, Ce4+, and Oα referred to as Mn4+/(Mn4+ + Mn3+), Ce4+/(Ce4+ + Ce3+), and Oα/(Oα + Oβ), respectively.
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Fig. 4. H2-TPR profiles of the oxides.
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H. Li et al. / Catalysis Communications 12 (2011) 1361–1365
Conversion or Selectivity / %
80
a
100
Mn0.6Ce0.2Zr0.2Ox
60
Mn0.6Ce0.4Ox
100
60 CH3CHO selectivity
20 100
60
b
C2H5OH conversion
C2H5OH conversion
95
Conversion or Selectivity / %
100
Mn0.6Ce0.2Zr0.2Ox Mn0.6Ce0.4Ox
16 CH3CHO selectivity
8
0 100
CO2 selectivity
80 CO selectivity 2
20 413
423
433
443
453
463
0
20
Temperature / K
40
60
80
100
120
Time on stream / h
Fig. 5. (a) Ethanol oxidation over the oxides (100,000 mL g− 1 h− 1) and (b) stability test at 463 K (150,000 mL g− 1 h− 1). The reaction gas contained 0.23% C2H5OH and 20% O2 balanced with N2.
(4.1 mmol g− 1) was less than that of Mn0.6Ce0.4Ox (4.6 mmol g− 1) due to the lower content of cerium oxide. By assuming that the hydrogen is stoichiometrically consumed for the reduction of Mn4+/Mn3+ reduction to Mn2+ and Ce4+ to Ce3+ based on the XPS analysis, 76.1% of CeO2 was reduced in the Zr-doped oxide, being much higher than that in Mn0.6Ce0.4Ox (35.7%). This reaffirms that ZrO2-doping has considerably improved the reduction of ceria in the mixed oxide. As a result, the Mn0.6Ce0.2Zr0.2Ox had higher OSCs of 1.33 mmol O2 g− 1 at 423 K and 1.53 mmol O2 g− 1 at 473 K (Table 1). The discrepancy between the H2TPR and OSC results is likely caused by the fact that the hydrogen reduction occurs both on the surface and in bulk in the TPR process, whereas the subsequent oxygen titration to measure the OSC mainly involves refilling the surface oxygen vacancies. It seems that ZrO2doping has promoted the oxygen storage capacity and the thermal stability of the oxides.
stabilizing the particle size. Ethanol oxidation generally follows the Mars–van Krevelen mechanism, and both the oxidation state of manganese and the mobility of surface oxygen species affect the reaction performance [16,19]. Ethanol is initially oxidized by MnO2, and the restoration of manganese oxidation state is realized using the oxygen species generated from CeO2 and the partially reduced CeO2 − x is replenished by molecular oxygen in the feed gas [3,10]. In other words, MnO2 acts as the active site while CeO2 serves as an oxygen reservoir and thus the mobility of surface oxygen species plays a vital role. The superior performance of the Mn0.6Ce0.2Zr0.2Ox oxide is apparently due to the rapid mobility of oxygen species caused by ZrO2-doping. Therefore, it can be concluded that ZrO2-dopping not only chemically promoted the oxygen storage capacity but also enhanced the thermal stability texturally. 4. Conclusion
3.2. Ethanol oxidation Ethanol was completely oxidized to CO2 at 453 K on Mn0.6Ce0.2Zr0.2Ox and at 463 K over Mn0.6Ce0.4Ox (Fig. 5a). In order to identify the possible deactivation, stability tests were then conducted at 463 K under a higher GHSV of 150,000 mL g− 1 h− 1. As shown in Fig. 5b, there was an induction period of about 20 h, especially in the case of the Mn0.6Ce0.4Ox catalyst. This is caused by the restructuring of the catalyst under reaction conditions that lowered the mobility of oxygen species for further oxidizing acetaldehyde, resulting in decreased selectivity of CO2 but increased selectivity towards acetaldehyde [3]. Thereafter, the conversion of ethanol and the selectivities of acetaldehyde and CO2 remained stable for the rest 100 h. On the Mn0.6Ce0.2Zr0.2Ox oxide, the conversion of ethanol kept at almost 100%, and the selectivity of CO2 only slightly decreased from 99.7% to 97.8% while the selectivity of acetaldehyde increased from practically zero to about 1% (with about 1% selectivity of CO). On the Mn0.6Ce0.4Ox catalyst, however, the conversion of ethanol decreased from 99.8% to 95.5%, and the selectivity of CO2 decreased significantly from 98.4% to 80.7% whereas the selectivity of acetaldehyde remarkably increased from 0.1% to 13.5%. XRD and HRTEM measurements of the used samples indicated that the particle size increased from 4 to about 6 nm in the Mn0.6Ce0.4Ox catalyst but it still maintained at about 4 nm in the ZrO2-doped catalyst (Figs. 1 and 2), demonstrating the promotional role of ZrO2-doping in
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