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Preparation and catalytic activities of the novel double perovskite-type oxide La2CuNiO6 for methane combustion
Catalysis Communications 21 (2012) 38–41
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation and catalytic activities of the novel double perovskite-type oxide La2CuNiO6 for methane combustion Ruisheng Hu a,⁎, Ranran Ding a, Jie Chen a, Jianan Hu a, Yulong Zhang b a b
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China Western Research Institute, 365 N 9th St, Laramie, WY, USA
a r t i c l e
i n f o
Article history: Received 17 November 2011 Received in revised form 11 January 2012 Accepted 12 January 2012 Available online 21 January 2012 Keywords: Rare earth Double perovskite-type oxide Methane catalytic combustion
a b s t r a c t Rare earth perovskite-type oxides La2CuNiO6 and LaNiO3 were prepared by a sol–gel method and investigated as catalysts for the combustion of methane. The obtained powders were characterized by XRD, H2-TPR, XPS and M techniques. XRD data shows that both La2CuNiO6 and LaNiO3 formed single-phase perovskitetype oxides. The experimental results show that the catalytic activity of La2CuNiO6 was much higher than that of LaNiO3 because of the different structure and surface properties. This study points out a potential way to develop more active double perovskite-type catalysts for the combustion of methane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Methane is not only the main component of natural gas as a highly economical energy source, but also present at low concentrations in many fugitive emissions requiring further treatment to prevent pollution [1]. As a heat source for various chemical processes, conventional flame combustion has several problems such as emission of NOx and unburned hydrocarbons, as well as low energy efficiency [2]. Catalytic combustion has very attractive characteristics compared with flame combustion, due to its higher energy conversion efficiency and ultra-low emissions of environmental pollutants [3]. With a catalyst, the combustion temperature is lower and the combustion is performed at a concentration range outside of flammability limits [4]. A variety of catalyst systems have been developed for catalytic combustion to pursue suitable catalysts with high thermal stability and activity [5]. At present, methane catalytic combustion materials are mainly concentrated on three kinds of catalysts including noble metal, supported metal oxide and composite metal oxide. In methane catalytic combustion, although noble metal-based catalysts show very high specific activity [6,7], their utilization in combustors is limited by their high costs, high volatility of pure metals and their oxides, and the tendency toward to sintering at moderate temperature. Compared to noble metal catalysts, perovskite-type catalytic materials have attracted considerable interests for many years due to their high activity and thermal stability in the catalytic total oxidation of hydrocarbons. Generally, perovskite-type catalysts were divided
⁎ Corresponding author. Tel.: + 86 471 4992982; fax: + 86 471 4992981. E-mail address: [email protected] (R. Hu). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.01.008
into single perovskite compounds (ABO3) and double perovskite compounds (A2B′B″O6). The double perovskite-type oxides have more variations than the single perovskite-type oxides. The greater variation may promote catalytic chemistry for double perovskitetype catalysts. Kobayashi [8] reported that double perovskite-type oxide Sr2FeMoO6 had colossal magnetoresistance at room temperature, which had attracted great attention. The physics community carried out a large number of extensive researches about the electrical, magnetic and transport properties of such materials [9,10]. In contrast, the chemical sector did little research about double perovskite-type oxides. In 2004, Falcón et al. [11] first reported that Sr2FeMoO6 could be used as a catalyst in methane combustion synthesized with a citric acid decomposition method. At 527 °C the methane conversion rate was as high as 80%. However, little work had been published on the double perovskite-type oxides as the catalysts of methane combustion since then. In this work, the rare earth double perovskite-type oxide La2CuNiO6 was prepared to examine its catalytic behavior for the combustion of methane, an attempt for correlating between activity, structure and surface properties. 2. Experimental 2.1. Catalysts preparation La2CuNiO6 and LaNiO3 were prepared by the sol–gel method. Stoichiometric amounts of La(NO3)3·6H2O Cu(NO3)2·3H2O and Ni(NO3)2·6H2O were dissolved in de-ionized water, then a certain amount of citric acid was added to form citric acid chelate compounds. The solution was thickened at 70 °C under vigorous stirring to form a viscous gel and then was dried at 100 °C for 24 hours. The
R. Hu et al. / Catalysis Communications 21 (2012) 38–41
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xerogel was decomposed at 500 °C for 3 hours and finally calcined in a muffle furnace at 1100 °C in air for 3 hours to form the double perovskite La2CuNiO6 catalyst designated as sample 1 #. For comparison, a single perovskite LaNiO3 was synthesized with the same procedure, and was named as sample 2 #. 2.2. Catalyst characterizations The X-ray diffraction patterns were recorded with a D8 ADVANCE diffractometer (Bruker, Germany) using CuKα radiation (λ = 0.1541 nm, 36 KV, 20 mA, scanning step = 2°/min). TPR was conducted with a TP-5000 multiple adsorption instrument (Xianquan, China). It was performed at a rate of 10 °C min − 1 from room temperature up to 900 °C in a flow of 5% H2/N2. XPS was performed on an AMICUS spectrometer (KRATOS, England). Binding energies were referenced to C1s at 284.6 eV. Magnetization was conducted with a LakeShore 7407 vibrating sample magnetometer (LakeShore, America) at 295 ± 0.1 K, with a precision 10 − 7 Am 2. 2.3. Catalyst evaluation The reaction of methane was carried out in a conventional flow system under atmospheric pressure. 200 mg catalyst (40–60 mesh) was loaded in a fixed bed quartz reactor (i.d. 10 mm). The reaction gas composition is CH4:O2:N2 = 2:18:80, controlled by mass flow controllers at space velocity of 48,000 l/h. The inlet and outlet gas compositions were analyzed by an online gas chromatography (GC-2014C, Japan) with a packed column of carbon molecular sieve and a thermal conductivity detector. 3. Results and discussion 3.1. X-ray diffraction analysis The XRD patterns of the two catalysts are shown in Fig. 1. It can be seen that the sample 1 # has diffraction peaks at 2θ = 23°, 32°, 40°, 46°, 58° and 68°, which are typical perovskite diffraction peaks. No other peaks were observed. Compared with the sample 2 #, some shift of the diffraction peaks occurred. It can be assured from the diffraction peaks and literature data [12] that the sample 1 # formed single-phase La2CuNiO6. In addition, narrow and intensive high diffraction peaks of the sample 1 # indicated a well crystallized structure. The XRD patterns of sample 2 # have good agreement with the standard LaNiO3. No additional peaks were observed. It indicates that
Fig. 2. H2-TPR profiles of (a) La2CuNiO6 and (b) LaNiO3.
sample 2 # is a single phase LaNiO3. In perovskite structure, when the structure is non-ideal, t (Goldschmidt tolerant factor) is used to represent the deviation. When t = 1, the corresponding structure is an ideal perovskite structure. When 0.75 b t b 1, ABO3 stable perovskites could be formed with hexagonal, cubic, or orthorhombic structures according to different ion radius [13]. The tolerance factor, crystallite and lattice constants of catalysts are shown in Table 1. From the crystallite and lattice constants we can see that both catalysts have hexagonal crystal structures, deviated from the ideal cubic crystal. 3.2. H2-TPR analysis Temperature-programmed reduction profiles were performed with the aim to study the reducibility of the prepared perovskites (Fig. 2). Typically, the LaNiO3 has two peaks at 420 °C and 580 °C, corresponding to Ni3 + → Ni2 + and Ni2 + → Ni0 reduction respectively [14]. The CuO has two peaks at 350 °C and 500 °C which can be assigned to the reduction of Cu 2 + → Cu+ and Cu+ → Cu 0 respectively [15]. H2-TPR profiles of La2CuNiO6 catalyst exhibits three hydrogen consumption peaks. The first H2 consumption peak, centered at 410 °C, is due to the overlapping reduction of Cu2 + to Cu + and Ni3 + to Ni2 +. The peaks at 512 °C and 588 °C are probably to the further reduction of Cu+ to Cu0 and Ni2 + to Ni0 respectively. It can be seen from Fig. 2 that the reduction temperature of sample 1# is lower than that of sample 2 # and peak area for sample 1 # is larger than that for sample 2#, suggesting that sample 1 # can provide more active oxygen species available for the oxidation reaction, which is in accord with the methane catalytic combustion activity. 3.3. XPS analysis The surface chemical species of the catalysts were characterized by XPS and the results are summarized in Table 2. The O1s spectra in Table 1 Tolerance factor, crystallite and lattice constants of catalysts. Catalysts Phasea
1# 2# a
Fig. 1. XRD patterns of (a) La2CuNiO6 and (b) LaNiO3.
b
Crystallite Tolerance Lattice parameteras (nm)a factorb a/Å b/Å c/Å α
La2CuNiO6 48.1 LaNiO3 65.8
0.825 0.833
β
γ
5.50 5.50 13.33 90° 90° 120° 5.47 5.47 6.60 90° 90° 120°
Phase, crystallite, lattice parameters obtained by XRD analysis. Tolerance factor obtained by calculation.
40
R. Hu et al. / Catalysis Communications 21 (2012) 38–41
Fig. 4. The hysteresis loops at room temperature of (a) La2CuNiO6 and (b) LaNiO3.
(854.5 eV) agrees with that of Ni2O3 (854.0 eV) in handbook, so Ni in La2CuMnO6 mainly exists as Ni3 +. 3.4. Magnetics analysis Hysteresis curve of Fig. 4 confirms the ferromagnetic behavior of La2CuNiO6 and LaNiO3 catalysts. Usually, magnetization intensity (M) of ferromagnetic materials increases rapidly with external magnetic field (H) until saturation. As shown in Fig. 4, the hysteresis loop of sample 2 # complies with the rule, showing S curve with a small hysteresis loop and a small hysteresis loss. However, the hysteresis loop of sample 1 # is unique. The saturation magnetization intensity of sample 1 # is lower than that of sample 2 #. In this case, M is undoubtedly a superposition of an antiferromagnetic behavior and ferromagnetism, where the presence of Cu 2 + triggers antiferromagnetic interactions with Ni 3 +. In addition, the saturation magnetization of the sample is highly affected by the occurrence of oxygen vacancies. Because the presence of oxygen vacancies undermined the super-exchange of B′–O–B ″, so saturation magnetization intensity reduced [20]. Oxygen vacancy concentration is closely related to its
Fig. 3. O1s XPS spectra of (a) La2CuNiO6 and (b) LaNiO3.
Fig. 3 show doublet peak profiles which is consistent with the reported data on the perovskite-type oxides [16]. The peak at the lowest binding energy (527.5–530.0 eV) corresponds to lattice oxygen (Olat) and the one at the highest binding energy (530.0–531.0 eV) corresponds to adsorbed oxygen (Oads) on catalyst surface [16]. It is observed from Table 2 that the ratios of [Oads]/[Olat] of sample 1# was much higher than that of sample 2#, therefore oxygen vacancies of sample 1 # are more than that of sample 2# [17]. La3d peaks of both perovskite catalysts show double peaks at 851.6 eV and 834.8 eV, indicating that La exists as La 3 + [18]. In Table 2, the peak at 933.5 eV for Cu2p3/2 and the shake-up contribution at about 942.3 eV suggest the Cu in La2CuNiO6 mainly exists as Cu2 + [19]. In addition, binding energy of Ni2p Table 2 The XPS result of catalysts.a Catalysts
BE of O1s/eV
Area percentage
BE of Ni2p/eV
BE of Cu2p/eV
BE of La3d/eV
La2CuNiO6
531.4 529.0 531.1 528.7
80.2% 19.8% 63.6% 36.4%
854.5
943.0 934.1 –
852.2 835.6 834.1 851.5
LaNiO3 a
855.1
The C1s peak at 284.6 eV was selected as an inner standard calibration peak.
Fig. 5. Temperature dependence of CH4 combustion over (a) La2CuNiO6, (b) LaNiO3 and (c) mechanical mixing of LaNiO3 and LaCuO3 at 1:1 mole ratio catalysts.
R. Hu et al. / Catalysis Communications 21 (2012) 38–41
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catalytic activity for methane combustion, the greater of oxygen vacancy concentration, the better catalytic activity for methane combustion. The hysteresis loop of sample 1 # exhibits a rather complex behavior: when the external magnetic field reached 1.5 T, it followed the original trend of return and formed inverted V-shaped hysteresis loops at both ends, which may be depending on the nature of the components, their ratio and the structure and surface properties.
performance. It indicated that the double perovskite is a better catalyst than the single perovskite for methane combustion. This could be correlated well with the catalyst reducibility and the surface adsorbed oxygen species which play a predominant role in methane combustion.
3.5. Catalytic methane combustion
We thank the National Natural Science Foundation of China for financial support (no. 20763003).
The catalytic behaviors for methane combustion over catalysts are shown in Fig. 5. All the catalysts, tested under identical experimental conditions, exhibit the S-shaped profiles for CH4 conversion as a function of reaction temperature. The only products detected were carbon dioxide and scarcely carbon monoxide during oxidation process. Usually, the activity of methane catalytic combustion was characterized by T10, T50 and T90 representing the reaction temperature at methane conversion of 10, 50 and 90% respectively. The lower T10 value (444.1 °C) was attained by sample 1 #, which also showed relatively high activities at high conversion level (655.7 °C). Especially, when compared with the sample 2 #, the light-off temperature T10 decreases by 21.5 °C and total conversion temperature T90 decreases by 63.2 °C, respectively. Catalytic activity of La2CuNiO6 is close to the Mn substituted La-hexaaluminate catalysts prepared by Wang et al. [21] with a hydrolysis supercritical drying method, on which T10 and T90 are 450 °C and 650 °C respectively. It indicated that the double perovskite-type oxides have potential catalytic advantages for methane combustion. In order to understand the excellent methane catalytic combustion activity of the double perovskite La2CuNiO6, a catalyst with the same composition was prepared by mechanical mixing of LaNiO3 and LaCuO3 at 1:1 mole ratio. We can see that the double perovskite-type catalyst is more active than the mechanical mixture of the two single perovskite-type oxides. It indicated that synergetic interaction occurred between Cu and Ni in double perovskite La2CuNiO6 which is beneficial to methane combustion. 4. Conclusions Sample 1 # and sample 2 # could form single-phase oxide after calcination at 1100 °C for 3 h in air. Compared with the single perovskite-type oxide LaNiO3 and the mechanical mixture of LaNiO3 and LaCuO3 at 1:1 mole ratio, La2CuNiO6 exhibited better catalytic
Acknowledgments
References [1] T.F. Tian, M.C. Zhan, W.D. Wang, C.S. Chen, Catalysis Communications 10 (2009) 513. [2] S. Kim, D.W. Lee, J.Y. Lee, H.J. Eom, H.J. Lee, I.H. Cho, K.Y. Lee, Journal of Molecular Catalysis A: Chemical 335 (2011) 60. [3] Z. Jiang, J.J. Yu, J. Cheng, T.C. Xiao, M.O. Jones, Z.P. Hao, P.P. Edwards, Fuel Processing Technology 91 (2010) 97. [4] G.Q. Guan, K. Kusakabe, M. Taneda, M. Uehara, H. Maeda, Journal of Chemical Engineering 144 (2008) 270. [5] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Applied Catalysis A: General 234 (2002) 1. [6] F. Arosio, S. Colussi, A. Trovarelli, G. Groppi, Applied Catalysis B: Environmental 80 (2008) 335. [7] L.H. Xiao, K.P. Sun, X.L. Xu, X.N. Li, Catalysis Communications 6 (2005) 796. [8] K.L. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [9] H.M. Han, J.H. Wang, W. Zhong, Y.W. Du, Journal of Functional Materials 32 (2001) 568. [10] Y.H. Huang, R.I. Dass, Z.L. Xing, J.B. Goodenough, Science 312 (2006) 254. [11] H. Falcón, J.A. Barbero, G. Araujo, M.T. Casais, M.J. Martínez-Lope, J.A. Alonso, J.L.G. Fierro, Applied Catalysis B: Environmental 53 (2004) 37. [12] R. Shaheen, J. Bashir, Solid State Sciences 12 (2010) 1496. [13] G. Pecchi, C. Campos, O. Peña, L.E. Cadus, Journal of Molecular Catalysis A: Chemical 282 (2008) 158. [14] H.Q. Chen, H. Yu, F. Peng, G.X. Yang, H.J. Wang, J. Yang, Y. Tang, Journal of Chemical Engineering 3 (2010) 54. [15] C. Torre-Abreu, M.F. Ribeiro, C. Henriques, G. Delahay, Applied Catalysis B: Environmental 14 (1997) 261. [16] Z.X. Wei, Y.Q. Xu, H.Y. Liu, C.W. Hu, Journal of Hazardous Materials 165 (2009) 1056. [17] Q. Ye, R.P. Wang, B.Q. Xu, Acta Physico-Chimica Sinica 22 (2006) 33. [18] L.B. Yang, L.Q. Jing, S.D. Li, B.J. Jiang, W. Fu, H.G. Fu, Chemical Journal of Chinese University 28 (2007) 415. [19] X. Li, J. Bian, X.D. Zhu, L.S. Qiang, Chemical Journal of Chinese University 28 (2007) 1115. [20] D. Stoeffler, S. Colis, Journal of Materials Science and Engineering: B 126 (2006) 133. [21] J.W. Wang, Z.J. Tian, J.G. Xu, Y.P. Xu, Z.S. Xu, L.W. Lin, Catalysis Today 83 (2003) 213.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation and catalytic activities of the novel double perovskite-type oxide La2CuNiO6 for methane combustion Ruisheng Hu a,⁎, Ranran Ding a, Jie Chen a, Jianan Hu a, Yulong Zhang b a b
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China Western Research Institute, 365 N 9th St, Laramie, WY, USA
a r t i c l e
i n f o
Article history: Received 17 November 2011 Received in revised form 11 January 2012 Accepted 12 January 2012 Available online 21 January 2012 Keywords: Rare earth Double perovskite-type oxide Methane catalytic combustion
a b s t r a c t Rare earth perovskite-type oxides La2CuNiO6 and LaNiO3 were prepared by a sol–gel method and investigated as catalysts for the combustion of methane. The obtained powders were characterized by XRD, H2-TPR, XPS and M techniques. XRD data shows that both La2CuNiO6 and LaNiO3 formed single-phase perovskitetype oxides. The experimental results show that the catalytic activity of La2CuNiO6 was much higher than that of LaNiO3 because of the different structure and surface properties. This study points out a potential way to develop more active double perovskite-type catalysts for the combustion of methane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Methane is not only the main component of natural gas as a highly economical energy source, but also present at low concentrations in many fugitive emissions requiring further treatment to prevent pollution [1]. As a heat source for various chemical processes, conventional flame combustion has several problems such as emission of NOx and unburned hydrocarbons, as well as low energy efficiency [2]. Catalytic combustion has very attractive characteristics compared with flame combustion, due to its higher energy conversion efficiency and ultra-low emissions of environmental pollutants [3]. With a catalyst, the combustion temperature is lower and the combustion is performed at a concentration range outside of flammability limits [4]. A variety of catalyst systems have been developed for catalytic combustion to pursue suitable catalysts with high thermal stability and activity [5]. At present, methane catalytic combustion materials are mainly concentrated on three kinds of catalysts including noble metal, supported metal oxide and composite metal oxide. In methane catalytic combustion, although noble metal-based catalysts show very high specific activity [6,7], their utilization in combustors is limited by their high costs, high volatility of pure metals and their oxides, and the tendency toward to sintering at moderate temperature. Compared to noble metal catalysts, perovskite-type catalytic materials have attracted considerable interests for many years due to their high activity and thermal stability in the catalytic total oxidation of hydrocarbons. Generally, perovskite-type catalysts were divided
⁎ Corresponding author. Tel.: + 86 471 4992982; fax: + 86 471 4992981. E-mail address: [email protected] (R. Hu). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.01.008
into single perovskite compounds (ABO3) and double perovskite compounds (A2B′B″O6). The double perovskite-type oxides have more variations than the single perovskite-type oxides. The greater variation may promote catalytic chemistry for double perovskitetype catalysts. Kobayashi [8] reported that double perovskite-type oxide Sr2FeMoO6 had colossal magnetoresistance at room temperature, which had attracted great attention. The physics community carried out a large number of extensive researches about the electrical, magnetic and transport properties of such materials [9,10]. In contrast, the chemical sector did little research about double perovskite-type oxides. In 2004, Falcón et al. [11] first reported that Sr2FeMoO6 could be used as a catalyst in methane combustion synthesized with a citric acid decomposition method. At 527 °C the methane conversion rate was as high as 80%. However, little work had been published on the double perovskite-type oxides as the catalysts of methane combustion since then. In this work, the rare earth double perovskite-type oxide La2CuNiO6 was prepared to examine its catalytic behavior for the combustion of methane, an attempt for correlating between activity, structure and surface properties. 2. Experimental 2.1. Catalysts preparation La2CuNiO6 and LaNiO3 were prepared by the sol–gel method. Stoichiometric amounts of La(NO3)3·6H2O Cu(NO3)2·3H2O and Ni(NO3)2·6H2O were dissolved in de-ionized water, then a certain amount of citric acid was added to form citric acid chelate compounds. The solution was thickened at 70 °C under vigorous stirring to form a viscous gel and then was dried at 100 °C for 24 hours. The
R. Hu et al. / Catalysis Communications 21 (2012) 38–41
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xerogel was decomposed at 500 °C for 3 hours and finally calcined in a muffle furnace at 1100 °C in air for 3 hours to form the double perovskite La2CuNiO6 catalyst designated as sample 1 #. For comparison, a single perovskite LaNiO3 was synthesized with the same procedure, and was named as sample 2 #. 2.2. Catalyst characterizations The X-ray diffraction patterns were recorded with a D8 ADVANCE diffractometer (Bruker, Germany) using CuKα radiation (λ = 0.1541 nm, 36 KV, 20 mA, scanning step = 2°/min). TPR was conducted with a TP-5000 multiple adsorption instrument (Xianquan, China). It was performed at a rate of 10 °C min − 1 from room temperature up to 900 °C in a flow of 5% H2/N2. XPS was performed on an AMICUS spectrometer (KRATOS, England). Binding energies were referenced to C1s at 284.6 eV. Magnetization was conducted with a LakeShore 7407 vibrating sample magnetometer (LakeShore, America) at 295 ± 0.1 K, with a precision 10 − 7 Am 2. 2.3. Catalyst evaluation The reaction of methane was carried out in a conventional flow system under atmospheric pressure. 200 mg catalyst (40–60 mesh) was loaded in a fixed bed quartz reactor (i.d. 10 mm). The reaction gas composition is CH4:O2:N2 = 2:18:80, controlled by mass flow controllers at space velocity of 48,000 l/h. The inlet and outlet gas compositions were analyzed by an online gas chromatography (GC-2014C, Japan) with a packed column of carbon molecular sieve and a thermal conductivity detector. 3. Results and discussion 3.1. X-ray diffraction analysis The XRD patterns of the two catalysts are shown in Fig. 1. It can be seen that the sample 1 # has diffraction peaks at 2θ = 23°, 32°, 40°, 46°, 58° and 68°, which are typical perovskite diffraction peaks. No other peaks were observed. Compared with the sample 2 #, some shift of the diffraction peaks occurred. It can be assured from the diffraction peaks and literature data [12] that the sample 1 # formed single-phase La2CuNiO6. In addition, narrow and intensive high diffraction peaks of the sample 1 # indicated a well crystallized structure. The XRD patterns of sample 2 # have good agreement with the standard LaNiO3. No additional peaks were observed. It indicates that
Fig. 2. H2-TPR profiles of (a) La2CuNiO6 and (b) LaNiO3.
sample 2 # is a single phase LaNiO3. In perovskite structure, when the structure is non-ideal, t (Goldschmidt tolerant factor) is used to represent the deviation. When t = 1, the corresponding structure is an ideal perovskite structure. When 0.75 b t b 1, ABO3 stable perovskites could be formed with hexagonal, cubic, or orthorhombic structures according to different ion radius [13]. The tolerance factor, crystallite and lattice constants of catalysts are shown in Table 1. From the crystallite and lattice constants we can see that both catalysts have hexagonal crystal structures, deviated from the ideal cubic crystal. 3.2. H2-TPR analysis Temperature-programmed reduction profiles were performed with the aim to study the reducibility of the prepared perovskites (Fig. 2). Typically, the LaNiO3 has two peaks at 420 °C and 580 °C, corresponding to Ni3 + → Ni2 + and Ni2 + → Ni0 reduction respectively [14]. The CuO has two peaks at 350 °C and 500 °C which can be assigned to the reduction of Cu 2 + → Cu+ and Cu+ → Cu 0 respectively [15]. H2-TPR profiles of La2CuNiO6 catalyst exhibits three hydrogen consumption peaks. The first H2 consumption peak, centered at 410 °C, is due to the overlapping reduction of Cu2 + to Cu + and Ni3 + to Ni2 +. The peaks at 512 °C and 588 °C are probably to the further reduction of Cu+ to Cu0 and Ni2 + to Ni0 respectively. It can be seen from Fig. 2 that the reduction temperature of sample 1# is lower than that of sample 2 # and peak area for sample 1 # is larger than that for sample 2#, suggesting that sample 1 # can provide more active oxygen species available for the oxidation reaction, which is in accord with the methane catalytic combustion activity. 3.3. XPS analysis The surface chemical species of the catalysts were characterized by XPS and the results are summarized in Table 2. The O1s spectra in Table 1 Tolerance factor, crystallite and lattice constants of catalysts. Catalysts Phasea
1# 2# a
Fig. 1. XRD patterns of (a) La2CuNiO6 and (b) LaNiO3.
b
Crystallite Tolerance Lattice parameteras (nm)a factorb a/Å b/Å c/Å α
La2CuNiO6 48.1 LaNiO3 65.8
0.825 0.833
β
γ
5.50 5.50 13.33 90° 90° 120° 5.47 5.47 6.60 90° 90° 120°
Phase, crystallite, lattice parameters obtained by XRD analysis. Tolerance factor obtained by calculation.
40
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Fig. 4. The hysteresis loops at room temperature of (a) La2CuNiO6 and (b) LaNiO3.
(854.5 eV) agrees with that of Ni2O3 (854.0 eV) in handbook, so Ni in La2CuMnO6 mainly exists as Ni3 +. 3.4. Magnetics analysis Hysteresis curve of Fig. 4 confirms the ferromagnetic behavior of La2CuNiO6 and LaNiO3 catalysts. Usually, magnetization intensity (M) of ferromagnetic materials increases rapidly with external magnetic field (H) until saturation. As shown in Fig. 4, the hysteresis loop of sample 2 # complies with the rule, showing S curve with a small hysteresis loop and a small hysteresis loss. However, the hysteresis loop of sample 1 # is unique. The saturation magnetization intensity of sample 1 # is lower than that of sample 2 #. In this case, M is undoubtedly a superposition of an antiferromagnetic behavior and ferromagnetism, where the presence of Cu 2 + triggers antiferromagnetic interactions with Ni 3 +. In addition, the saturation magnetization of the sample is highly affected by the occurrence of oxygen vacancies. Because the presence of oxygen vacancies undermined the super-exchange of B′–O–B ″, so saturation magnetization intensity reduced [20]. Oxygen vacancy concentration is closely related to its
Fig. 3. O1s XPS spectra of (a) La2CuNiO6 and (b) LaNiO3.
Fig. 3 show doublet peak profiles which is consistent with the reported data on the perovskite-type oxides [16]. The peak at the lowest binding energy (527.5–530.0 eV) corresponds to lattice oxygen (Olat) and the one at the highest binding energy (530.0–531.0 eV) corresponds to adsorbed oxygen (Oads) on catalyst surface [16]. It is observed from Table 2 that the ratios of [Oads]/[Olat] of sample 1# was much higher than that of sample 2#, therefore oxygen vacancies of sample 1 # are more than that of sample 2# [17]. La3d peaks of both perovskite catalysts show double peaks at 851.6 eV and 834.8 eV, indicating that La exists as La 3 + [18]. In Table 2, the peak at 933.5 eV for Cu2p3/2 and the shake-up contribution at about 942.3 eV suggest the Cu in La2CuNiO6 mainly exists as Cu2 + [19]. In addition, binding energy of Ni2p Table 2 The XPS result of catalysts.a Catalysts
BE of O1s/eV
Area percentage
BE of Ni2p/eV
BE of Cu2p/eV
BE of La3d/eV
La2CuNiO6
531.4 529.0 531.1 528.7
80.2% 19.8% 63.6% 36.4%
854.5
943.0 934.1 –
852.2 835.6 834.1 851.5
LaNiO3 a
855.1
The C1s peak at 284.6 eV was selected as an inner standard calibration peak.
Fig. 5. Temperature dependence of CH4 combustion over (a) La2CuNiO6, (b) LaNiO3 and (c) mechanical mixing of LaNiO3 and LaCuO3 at 1:1 mole ratio catalysts.
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catalytic activity for methane combustion, the greater of oxygen vacancy concentration, the better catalytic activity for methane combustion. The hysteresis loop of sample 1 # exhibits a rather complex behavior: when the external magnetic field reached 1.5 T, it followed the original trend of return and formed inverted V-shaped hysteresis loops at both ends, which may be depending on the nature of the components, their ratio and the structure and surface properties.
performance. It indicated that the double perovskite is a better catalyst than the single perovskite for methane combustion. This could be correlated well with the catalyst reducibility and the surface adsorbed oxygen species which play a predominant role in methane combustion.
3.5. Catalytic methane combustion
We thank the National Natural Science Foundation of China for financial support (no. 20763003).
The catalytic behaviors for methane combustion over catalysts are shown in Fig. 5. All the catalysts, tested under identical experimental conditions, exhibit the S-shaped profiles for CH4 conversion as a function of reaction temperature. The only products detected were carbon dioxide and scarcely carbon monoxide during oxidation process. Usually, the activity of methane catalytic combustion was characterized by T10, T50 and T90 representing the reaction temperature at methane conversion of 10, 50 and 90% respectively. The lower T10 value (444.1 °C) was attained by sample 1 #, which also showed relatively high activities at high conversion level (655.7 °C). Especially, when compared with the sample 2 #, the light-off temperature T10 decreases by 21.5 °C and total conversion temperature T90 decreases by 63.2 °C, respectively. Catalytic activity of La2CuNiO6 is close to the Mn substituted La-hexaaluminate catalysts prepared by Wang et al. [21] with a hydrolysis supercritical drying method, on which T10 and T90 are 450 °C and 650 °C respectively. It indicated that the double perovskite-type oxides have potential catalytic advantages for methane combustion. In order to understand the excellent methane catalytic combustion activity of the double perovskite La2CuNiO6, a catalyst with the same composition was prepared by mechanical mixing of LaNiO3 and LaCuO3 at 1:1 mole ratio. We can see that the double perovskite-type catalyst is more active than the mechanical mixture of the two single perovskite-type oxides. It indicated that synergetic interaction occurred between Cu and Ni in double perovskite La2CuNiO6 which is beneficial to methane combustion. 4. Conclusions Sample 1 # and sample 2 # could form single-phase oxide after calcination at 1100 °C for 3 h in air. Compared with the single perovskite-type oxide LaNiO3 and the mechanical mixture of LaNiO3 and LaCuO3 at 1:1 mole ratio, La2CuNiO6 exhibited better catalytic
Acknowledgments
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