Influence of preparation method on the performance of Mn Ce O catalysts

Transcription

1 React Kinet Catal Lett (2009) 97: DOI /s Influence of preparation method on the performance of Mn Ce O catalysts Hua Zhang Æ Weiling Yang Æ Dao Li Æ Xingyi Wang Received: 10 November 2008 / Accepted: 13 February 2009 / Published online: 11 July 2009 Ó Akadémiai Kiadó, Budapest, Hungary 2009 Abstract Mn Ce O catalysts were prepared by the sol gel method with different citric acid amounts in preparation. The catalysts were characterized by using BET, XRD, TPR, XPS and their catalytic activities in methane combustion were also investigated. Results showed that the surface area, Mn 4? and O latt are responsible for the high catalytic activity of Mn Ce O catalysts. Keywords Sol gel Citric acid Mn Ce Methane combustion Introduction CeO 2 -containing materials are under intense scrutiny in recent years. Cerium oxide, associated with transitional metal oxides, has been shown to promote oxygen storage and release, enhance oxygen mobility, form surface and bulk vacancies, and improve catalyst redox properties for the oxidation process [1 3]. MnCeO x mixed oxides showed high catalytic activities in absorption and reaction of NO x [4, 5], selective catalytic reduction of NO x with NH 3 at low temperature [6] and catalytic wet oxidation of organic compounds [7, 8]. In methane combustion, MnCeO x mixed oxide catalysts were highly active at low temperature and the catalysts prepared by modified coprecipitation exhibited much higher activity than those prepared by coprecipitation and plasma methods [9]. In previous studies, it was noted that the preparation methods and conditions are crucial issues for improving the reactivity of Mn Ce O mixed oxide catalysts [9, 10]. The aim of this work is to make a structural and surface characterization of Mn Ce O catalysts prepared by sol gel technology, with special interest in the H. Zhang W. Yang D. Li (&) X. Wang Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai , People s Republic of China lidao@ecust.edu.cn

2 264 H. Zhang et al. effect of citric acid (CA) amount during the catalyst preparation. The effect of the ratio of CA/M on the catalyst structure and the catalytic activity for methane combustion was studied. The reason for the high catalytic combustion activity of methane over Mn Ce O mixed oxide catalysts was discussed. Experimental The catalysts were prepared by citric acid sol gel method. Citric acid (CA) was added into an aqueous solution of Ce(NO 3 ) 3 and Mn(NO 3 ) 2 at Mn:Ce molar ratio of 1, with different molar ratios of CA: (Mn? Ce) = x:1 (x = 0.3, 0.5, 1, 1.5). The mixture was stirred at 80 C until a transparent gel formed. Dried at 120 C for 12 h and calcined in air at 550 C for 4 h, the catalysts were obtained and noted as MnCe CAx (x refers to the molar ratio of CA:M (Mn? Ce)). Pure ceria and pure manganese oxide were also prepared by citric acid sol gel method with CA:M molar ratio of 0.3:1. The catalytic activity was measured by using a conventional quartz flow reactor with a passing gas mixture of CH 4 :O 2 :N 2 = 1:10:89 (mole) at atmospheric pressure. The sample weighed 200 mg and GHSV was 60,000 h -1. The outlet products were analyzed on-line by a gas chromatograph with a thermal conductivity detector (TCD). Powder X-ray diffraction (XRD) was performed on Rigaku D/Max2 550V B/PC diffractometer employing Cu K a radiation at 40 kv and 100 ma. Surface areas of the catalysts were estimated using the N 2 adsorption isotherm at -196 C by the BET method using Quantachrome NOVA instrument. X-ray photoelectron spectra (XPS) were obtained on the Thermo ESCALAB 250 spectrometer with an Al anode for K a radiation. Charging effects were corrected by adjusting the binding energy of C1 s peak to ev. Temperature-programmed reduction (TPR) measurements were carried out at atmospheric pressure in a fixed-bed reactor. 100 mg catalyst was loaded in a U-shaped quartz micro-reactor and the catalyst was heated to 700 C (heating rate of 8 C min -1 ) in a flowing hydrogen mixture (30 ml min -1, 6.5% H 2 in Ar). Hydrogen consumption was monitored by using a thermal conductivity detector. Results and discussion The X-ray patterns of the prepared Mn Ce O catalysts are shown in Fig. 1. Only the broad peaks of cubic fluorite structure CeO 2 are observed and manganese oxide phases are not detected for the catalysts prepared with the CA/M ratio lower than 1.0. Compared with pure CeO 2, the diffraction peaks of cubic fluorite structure of Mn Ce O catalysts are slightly shifted to higher values in the range of As reported in Ref. [11], the ionic radius of Mn 3? (0.066 nm) is smaller than that of Ce 4? (0.094 nm), and the incorporation of Mn 3? into the fluorite lattice would result in the decrease of the lattice parameter. Therefore, the shift indicates that part of manganese species enter into the fluorite lattice to form MnCeO x solid

3 Influence of preparation method on the performance of Mn Ce O catalysts 265 Fig. 1 XRD patterns of the catalysts solutions. For MnCe CA1.5 catalyst, weak diffraction peaks of MnO x are observed, revealing that MnO x particles became large with the increase of CA/M ratio in the preparation. The surface compositions of the catalysts were characterized by XPS measurements, shown in Table 1. It is interesting to find that the ratio of Mn/Ce on the surface increases with the increase of the CA/M ratio in the preparation. The Mn/Ce atomic ratios of MnCe CA0.3 and MnCe CA0.5 are lower than the theoretic values, which may be associated with the fact that Mn 3? incorporates into the ceria lattice to form MnCeO x solid solution. To obtain information on the oxidation states of the surface elements, XPS spectra of Mn 2p and O 1s were analyzed. It is interesting to note that the atomic ratio of Mn 4? increases with the increase of the CA molar ratio, whereas those of Table 1 Structural and surface property of the catalysts Catalysts BET (m 2 /g) Lattice parameters (Å) Surface Mn/Ce a Relative abundances of Mn 4?,Mn 3? and Mn 2? (%) Relative abundances of O latt and O ads (%) Mn 4? Mn 3? Mn 2? O latt O ads MnCe CA MnCe CA MnCe CA MnCe CA a Estimated by XPS

4 266 H. Zhang et al. Mn 2? and Mn 3? decrease (see Table 1). According to the spectra of O 1s, two peaks are displayed: one broad peak at higher bonding energy of ev can be attributed to oxygen species in OH surface groups, and another narrow peak at ev represents lattice oxygen (O latt ) in Mn Ce O catalysts [12]. By integrating peak areas (see Table 1), it is found that MnCe CA0.3 has the most oxygen species in OH surface groups (32%). Meanwhile, the relative abundance of lattice oxygen increases with the increase of the CA/M ratio in the preparation, similarly to the change of the relative abundance of Mn 4? species. Figure 2 shows TPR profiles of the catalysts. Two reduction peaks at c.a. 340 and 440 C are observed for pure MnO x, which correspond to a typical two-step reduction process: the reduction of MnO 2 or Mn 2 O 3 to Mn 3 O 4, and that of Mn 3 O 4 to MnO [13]. All Mn Ce O catalysts display similar TPR profiles, although the two peaks significantly shift towards lower temperatures with c.a. 70 and 40 C, respectively, lined with the results in Ref. [14]. The phenomena indicate that the reduction of manganese oxides is promoted and the mobility of oxygen species is enhanced due to the interaction between manganese and cerium oxides, probably through the formation of solid solution. Figure 3 shows methane conversion over Mn Ce O catalysts. Pure CeO 2 catalyst presents poor activity and pure MnO x catalyst is much more active than CeO 2 for methane combustion. For Mn Ce O mixed oxide catalysts, the temperatures needed to reach 90% methane conversion are 600, 705, 638, and 615 C. Fig. 2 TPR profiles of the catalysts

5 Influence of preparation method on the performance of Mn Ce O catalysts 267 Fig. 3 Activities for methane combustion on Mn Ce O catalysts Fig. 4 Methane combustion rate as a function of CA molar ratio at different reaction temperatures Figure 4 illustrates the specific reaction rates of the catalysts based on per unit of surface area at different temperatures. Among the four catalysts under study, the increase of rate with the CA/M ratio in the preparation is found. It is generally accepted that the Mn 4? sites possess stronger catalytic activity than the Mn 3? sites [1] and the lattice oxygen of MnO x catalysts is considered to be primary active species for activating the C H bond of hydrocarbons [3]. In our experiments, the change of the reaction rates is in good accordance with those of the observed relative abundance of Mn 4? and O latt species. MnCe CA1.5 catalyst possessing the most Mn 4? and richest lattice oxygen species obviously favors methane combustion.

6 268 H. Zhang et al. Conclusions The amount of citric acid used in the synthesis of Mn Ce O catalysts has a significant influence on the physicochemical properties of the catalysts and their catalytic activities. The large surface area, relative abundance of Mn 4? and lattice oxygen resulted in high catalytic combustion activity of methane over Mn Ce O catalysts. Acknowledgements The authors gratefully acknowledge the financial support by the National Basic Research Program of China (2004CB719500) and the Commission of Science and Technology of Shanghai Municipality (06 JC14020). References 1. Yisup N, Cao Y, Feng W, Dai W, Fan K (2005) Catal Lett 99: Liotta LF, Di Carlo G, Pantaleo G, Deganello G (2005) Catal Commun 6: Liotta LF, Di Carlo G (2006) Appl Catal B 64: Machida M, Kurogi D, Kijima T (2003) Catal Today 84: Machida M, Kurogi D, Kijima T (2003) J Phys Chem B 107: Qi G, Yang RT (2004) J Phys Chem B 108: Qi G, Yang RT (2003) J Catal 217: Qi G, Yang RT, Chang R (2004) Appl Catal B 51:93 9. Shi L, Chu W, Qu F, Luo S (2007) Catal Lett 113: Arena F, Trunfio G, Negro J, Fazio B, Spadaro L (2007) Chem Mater 19: Terribile D, Trovarelli A, De Leitenburg C, Primavera A, Dolcetti G (1999) Catal Today 47: Larachi F, Pierre J, Adnot A, Bernis A (2002) Appl Surf Sci 195: Tang X, Li Y, Huang X (2006) Appl Catal B 62: Álvarez-Galván MC, de la Pena O Shea VA, Fierro JLG, Arias PL (2003) Catal Commun 4:223