Effect of Precipitation Method and Ce Doping on the Catalytic Activity of Copper Manganese Oxide Catalysts for CO Oxidation

Transcription

1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 24, NUMBER 1 FEBRUARY 27, 2011 ARTICLE Effect of Precipitation Method and Ce Doping on the Catalytic Activity of Copper Manganese Oxide Catalysts for CO Oxidation Xue-bin Zhang a, Kuo-yan Ma b, Ling-hui Zhang a, Guo-ping Yong a, Ya Dai b, Shao-min Liu a a. Department of Chemistry, University of Science and Technology of China, Hefei , China b. Technology Center, China Tobacco Chuanyu Industrial Corporation, Chengdu , China (Dated: Received on October 25, 2010; Accepted on December 30, 2010) The influence of Ce doping and the precipitation method on structural properties and the catalytic activity of copper manganese oxides for CO oxidation at ambient temperature have been investigated. The catalysts were characterized by means of the powder X-ray diffraction and N 2 adsorption-desorption, the inductively coupled plasma atomic emission spectrometry, the temperature programmed reduction, diffuse reflectance UV-Vis spectra, and the X-ray photoelectron spectroscopy. It was found that after doping little amount of Ce in copper manganese oxide, CeO 2 phase was highly dispersed and could prevent sintering and aggregating of the catalyst, the size of the catalytic material was decreased, the reducibility was enhanced, the specific surface area was increased and the formation of the active sites for the oxidation of CO was improved significantly. Therefore, the activity of the rare earth promoted catalyst was enhanced remarkably. Key words: CO oxidation, Ce-doped, Copper manganese oxide, Catalytic activity, Reverse co-precipitation I. INTRODUCTION Catalytic oxidation of carbon monoxide (CO) has been of considerable interest due to its importance in many industrial applications [1]. In the past years, low temperature CO oxidation has received increasing attentions, since Haruta et al. demonstrated that supported gold catalysts are highly active [2, 3]. However, the most widely used commercial catalyst is the mixed copper manganese oxide, CuMn 2 O 4 (Hopcalite) [4]. Therefore, many attempts have been made to improve the performance of Hopcalite catalyst, in particular by optimizing the preparation conditions [5 7] and exploring new preparation method [8 10]. Meanwhile, a number of researchers focus on doping the Hopcalite catalyst with different metal dopants to improve its catalytic activity. Taylor et al. found that after addition of 3%Au to the copper manganese oxide catalyst, the rate of CO oxidation was increased, and the deactivation of the catalyst was reduced [11]. However, due to the high cost of precious metal catalysts, using low-cost metals, such as transition metals as dopants is proven to be beneficial to Hopcalite catalyst. Hutchings et al. reported that catalytic activity for CO oxidation of copper manganese oxide could be enhanced by doping low levels of cobalt [12]. As indicated by Stephanopoulos [13, 14], Author to whom correspondence should be addressed. liusm@ustc.edu.cn, Tel.: , FAX: when cerium was doped to transition metal oxides, in situ forming cerium oxide could promote oxygen storage and release, enhance oxygen mobility, and improve the redox properties of the oxides. In this work, a series of copper manganese and Cedoped copper manganese oxides were prepared using reverse co-precipitation method, and their catalytic performance for CO oxidation at ambient temperature was studied. The results show that the catalytic activity of copper manganese oxide catalyst can be improved for CO oxidation at ambient temperature prepared by inverse co-precipitation method and doping a small amount of cerium can further enhance its activity. In addition, the structural, morphological, and catalytic property of copper manganese oxide and Ce-doped copper manganese oxide were completely investigated. II. EXPERIMENTS All chemicals obtained from Shanghai Chemical Reagent Ltd. Co. of China are analytical grade and used as received without further purification. A. Preparation of catalysts A series of copper manganese oxide catalysts with Cu/Mn=1/1, 1/2, 1/3, 1/4, 1/5, and 1/6 were prepared using inverse co-precipitation method. Aqueous solutions of Cu(NO 3 ) 2 and Mn(NO 3 ) 2 were premixed. After drop-wise addition of the mixture into 97

2 98 Chin. J. Chem. Phys., Vol. 24, No. 1 Xue-bin Zhang et al. 300 ml of aqueous Na 2 CO 3 under stirring at 25 C, the ph was controlled at 8.3 by the addition of aqueous Na 2 CO 3, aged for 12 h. The precipitate was recovered by filtration, washed with hot deionized water, dried at 80 C overnight, and then calcined in air at 400 C for 12 h, named as Cu/Mn=1/1, 1/2, 1/3, 1/4, 1/5, and 1/6, respectively. Ce-doped copper manganese oxide catalysts with different content of cerium (x%=0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%) were prepared by the same procedures, except for Ce(NO 3 ) 3 aqueous solution was added into the cooper manganese mixed solutions, named as x% Ce-doped Cu/Mn=1/4 oxide catalyst. Copper manganese oxide catalyst with Cu/Mn=1/2 was prepared by co-precipitation method [6], named as co-cu/mn=1/2. B. Characterization of catalysts The powder X-ray diffraction (XRD) patterns were obtained on a Philips X Pert PRO SUPER diffractometer with nickel-filtered Cu-Kα radiation at 40 kv and 200 ma. N 2 adsorption-desorption isotherms were obtained at 196 C on an Omnisorp 100CX instrument. SEM analysis of the catalysts were performed using a JEOL JSM-6700F scanning electron microscope (SEM) with a 15 kv electron beam at a magnification of X-ray photoelectron spectroscopy (XPS) measurements were recorded by ESCALAB 250 high performance electron spectrometer using monochromatized Al Kα excitation source (hν= ev). Diffuse reflectance UV-Vis spectra of the catalysts were recorded using a PerkinElmer Lambda35 UV- Vis spectrophotometer with MgO disk as reference, and all spectra were recorded under ambient conditions. The content of Ce loaded in copper manganese oxide was determined by the inductively coupled plasma atomic emission spectrometry (ICP-AES). Temperature-programmed reduction (TPR) measurements were carried out at atmospheric pressure in a fixed-bed reactor. 50 mg catalyst was loaded in a fixedbed micro-reactor and the catalyst was heated to 600 C (heating rate of 10 C/min) in a flowing hydrogen mixture (30 ml/min, 5%H 2 in N 2 ). Hydrogen consumption was monitored by a thermal conductivity detector. C. CO oxidation The catalytic tests were performed in a fixed-bed flow-type reactor (diameter of 9 mm) with 100 mg of the catalyst at ambient temperature. The reaction was started at the total flow rate of 50 ml/min (2.53%CO, 10.8%O 2, and balance N 2, the gases used were purchased from Nanjing special gas Co., Ltd.) controlled by mass flow meters, the gas hourly space velocity was h 1. The products were analyzed using on-line gas chromatography with a 3 m packed Carbosieve column. FIG. 1 Composition dependency of the catalytic activity for oxidation of CO after 1 h. 1/1 is copper/manganese atomic ratio prepared by reverse co-precipitation method, co-1/2 is copper/manganese atomic ratio prepared by coprecipitation method. III. RESULTS AND DISCUSSION A. Catalytic oxidation of CO It is well known that the preparation method can highly influence the catalytic activity of copper manganese oxide catalysts [5, 8, 9, 15 17]. Precipitation method which was a traditional method for producing copper manganese oxide catalysts had been widely investigated by many researchers. But during the produce, almost added the precipitator into metal ions solution. Since the ph for Cu 2+ and Mn 2+ to precipitate is not the same with CO 2 3 as the precipitator at 25 C, the two metal ions would precipitate at different phase, and thus influence its catalytic activity for CO. Herein, we added the metal ions solution into the precipitator in order to ensure the two metal ions can precipitate simultaneously. Figure 1 shows the catalytic activity of copper manganese oxide catalysts with different Cu/Mn ratio prepared by inverse co-precipitation method, and that of copper manganese oxide catalyst with Cu/Mn=1/2 prepared by co-precipitation method for oxidation of CO at ambient temperature. Interestingly, all copper manganese oxide catalysts (Cu/Mn=1/1, 1/2, 1/3, 1/4, 1/5, 1/6) prepared by inverse co-precipitation exhibit higher catalytic activity than commercial hopcalite and co- Cu/Mn=1/2 catalyst. We also found that the copper manganese oxide catalyst possessed of high catalytic activity with the Cu/Mn ratio in the range of 1/2 1/6, and Cu/Mn=1/4 has the highest catalytic activity. However, the reasons for this phenomenon are unclear. As for this, we speculate that the difference in Cu/Mn ratio of the catalysts may ascribe to different produce method. Hutchings et al. reported that cooper manganese oxides catalyst with Cu/Mn=1/2 possessed of high activity by co-precipitation method [5], Njagi et al. used redox method to produce the catalyst and

3 Chin. J. Chem. Phys., Vol. 24, No. 1 Catalytic Activity of Copper Manganese Oxide Catalysts 99 FIG. 2 The catalytic oxidation of CO with various copper manganese oxides. (a) 1.5%Ce-doped Cu/Mn=1/4, (b) 3%Ce-doped Cu/Mn=1/4, (c) 0.5%Ce-doped Cu/Mn=1/4, (d) Cu/Mn=1/4, (e) co-cu/mn=1/2, (f) hopcalite. found Cu/Mn=1/9 to be the best ratio [9], furthermore, Krämer et al. prepared the catalyst with Cu/Mn=1/4 exhibiting more activity by sol-gel method [15], which indicate that the catalytic activity of copper manganese oxide catalyst may have rather weak dependence on the copper content. As follows, the Cu/Mn=1/4 oxide catalyst was chosen to be doped with Ce. Figure 2 shows the promoting effects of Ce-doped copper manganese oxides for CO oxidation at ambient temperature compared with copper manganese oxides and commercial hopcalite. It is obviously found that Ce-doped copper manganese oxide exhibits the highest initial and overall activity for oxidize high concentration of CO, compared to other copper manganese oxide catalysts in the catalysts investigated. B. Effect of structure and morphological on the catalytic properties Figure 3 presents XRD patterns of the catalysts, It s obvious to find that the phase of CuMn 2 O 4, Mn 2 O 3, and CuO exists in the catalyst prepared by co-precipitation and inverse co-precipitation method with Cu/Mn=1/2. But the intensity of CuMn 2 O 4 and CuO in Cu/Mn=1/2 catalyst is lower than that of co-cu/mn=1/2. The amorphous CuMn 2 O 4 phase is active for oxidize CO at ambient temperature, so the Cu/Mn=1/2 exhibits higher activity than co- Cu/Mn=1/2 catalyst, which is in accordance with the activity profiles in Fig.1. When the ratio of Cu/Mn is increased to 1/4, the major phases are CuMn 2 O 4, CuMnO x, and Mn 2 O 3 and the catalytic activity is improved. After Ce doped, the intensity of CuMn 2 O 4 phase in Ce-doped Cu/Mn=1/4 is lower than Cu/Mn=1/4, with its diffraction peak broaden, indicating Ce-doped Cu/Mn=1/4 has a lower crystallization, which is beneficial to the catalytic oxidation of CO [16]. Because crystalline CuMn 2 O 4 phases have FIG. 3 XRD patterns of various copper manganese oxides. A. CuMn 2O 4, B. Mn 2O 3, C. CuMnO x, and D. CuO. FIG. 4 Diffuse reflectance UV-Vis spectra of different catalysts. (a) Cu/Mn=1/4 oxide mixed with CeO 2, (b) Cu/Mn=1/4 oxide, and (c) Ce-doped Cu/Mn=1/4 oxide. less activity than amorphous ones [18], the high activity of Ce-doped Cu/Mn=1/4 mixed oxides catalyst for CO oxidation at ambient temperature should attribute to the active amorphous CuMn 2 O 4 phase. In addition, the phase CuMnO x and Mn 2 O 3 of Cu/Mn=1/4 oxide catalyst amalgamated into a mixed phase in Cedoped Cu/Mn=1/4 oxide catalyst, which is caused by the formation of mixed oxides with Ce doped into the Cu/Mn=1/4 mixed oxides framework. More importantly, Ce-doped Cu/Mn=1/4 oxide catalyst without CeO 2 phase could be clearly identified, which implied that CeO 2 may be highly dispersed in the materials or lattice replace occurred between manganese and cerium [12]. Diffuse reflectance UV-Vis spectra (shown in Fig.4(c)) also demonstrated that CeO 2 was dispersed in host materials, because CeO 2 characteristic peak (370 nm) was not observed in Ce-doped Cu/Mn=1/4 oxide catalyst. Whereas, the physical mixture of Cu/Mn=1/4 and CeO 2 exhibits two peaks, especially, characteristic peak at 370 nm (Fig.4(a)). So the syner-

4 100 Chin. J. Chem. Phys., Vol. 24, No. 1 1 Xue-bin Zhang et al. 2 µm (a) 2 µm (b) µm (c) FIG. 5 SEM pictures of the catalysts. (a) Co-Cu/Mn=1/2 oxide, (b) Cu/Mn=1/4 oxide, and (c) Ce-doped Cu/Mn=1/4 oxide. FIG. 6 XPS spectra of Mn2p3/2 (a), Cu2p3/2 (b), and O1s (c) for copper manganese and Ce-doped copper manganese oxides. getic effect between cooper manganese and cerium together promoted the oxidation of CO. To determine the existence and content of Ce, ICP-AES analysis was used. The content of Ce is 1.37% in the 1.5%Cedoped Cu/Mn=1/4 oxide catalyst, indicating that Ce was doped into the catalyst. SEM pictures of the catalysts shown in Fig.5 also confirm the effect of precipitation method and Ce doped on the crystallite size. As for co-cu/mn=1/2 catalyst, the morphology was mostly spherical, which is due to the less crystal seed at the first precipitation and benefit for the growth of crystallite. However, in the inverse co-precipitation, more crystal seed produced at the existence of high concentrations of precipitator and prevented the growth of crystallite. While for the Cedoped Cu/Mn=1/4, owing to its excellent properties of resistance to sintering of Ce, in the calcination process, CeO2 could prevent the sintering and aggregation of copper manganese oxide and enhance dispersity of the catalyst [19]. Table I shows the BET results of the catalysts, it s obvious to see that the surface area of Cu/Mn=1/2 is about twice as much as that of co-cu/mn=1/2, and an increase in Ce-doped Cu/Mn=1/4, which is consistent with the XRD and SEM results. It is generally accepted that the surface area can largely influence the catalytic activity of catalysts [6, 11, 12, 15]. As for this, we considered that the higher surface area of the catalyst possess of more active sites to react with CO, and then improved its catalytic activity. TABLE I BET surface area A, pore size d, and pore volume V of the catalysts calcined at 400 C for 12 h. Catalyst A/(m2 /g) V /(cm3 /g) 3%Ce-doped Cu/Mn=1/ %Ce-doped Cu/Mn=1/ %Ce-doped Cu/Mn=1/ Cu/Mn=1/ Cu/Mn=1/ co-cu/mn=1/ d/a C. Redox property of catalysts XPS spectra of Mn2p3/2 and Cu2p3/2 orbital are shown in Fig.6 (a) and (b). It is found that Mn2p3/2 peaks of co-cu/mn=1/2, Cu/Mn=1/4 and Ce-doped Cu/Mn=1/4 oxides were 642.4±0.3 ev. The binding energy difference between the Mn2p3/2 and Mn2p1/2 is 11.7±0.1 ev, which is characteristic of Mn4+ valence state. The binding energies of Cu2p3/2 of copper cations in CuO and Cu2 O are about and ev, respectively. The Cu2p3/2 peaks of co-cu/mn=1/2, Cu/Mn=1/4, and Ce-doped Cu/Mn=1/4 oxides were at c 2011 Chinese Physical Society

5 Chin. J. Chem. Phys., Vol. 24, No. 1 Catalytic Activity of Copper Manganese Oxide Catalysts 101 TABLE II The content of lattice oxygen and adsorbed oxygen in catalysts calcined at 400 C for 12 h. Catalyst O latt /% O ads /% (O latt +O ads )/% O ads /(O latt +O ads ) Ce-doped Cu/Mn=1/ Cu/Mn=1/ co-cu/mn=1/ FIG. 7 TPR patterns of Cu/Mn=1/4 oxide and Ce-doped Cu/Mn=1/4 oxide ev, indicating copper exists as +2 oxidation state [20]. The valence state of Cu and Mn in these copper manganese oxides is consistent with other studies [21]. Fortunato et al. indicated that the redox process between Cu and Mn was responsible for the reactivation of the copper manganese oxide catalyst [22]. Moreover, the doped Ce probably provided or withdrawn oxygen to Mn ions, in order to maintain its redox valence state, and complete the catalytic cycle. Figure 6(c) presents the binding energy of O1s of oxygen species on the surface of catalysts, and the data are summarized in Table II. The binding energies of O1s of the catalysts were and ev, which may be attributed to the surface adsorbed oxygen ( ev) and the lattice oxygen ( ev), respectively [23]. Puckhaber et al. reported that the most active catalysts had more chemisorbed oxygen on the surface compared to oxide material [24]. So the improvement of the initial activity of the catalyst may ascribe to the high amount of surface absorbed oxygen which can be activated to O 2 anion radicals and directly reaction with the chemisorbed CO [25]. With the addition of Ce, the binding energy of O1s of lattice oxygen on the catalysts shifts to high values because of Mn-O electron-transfer processes [26], which enable the formation of very reactive electrophilic oxygen species that are active oxygen species. Besides, cerium oxide associated with copper manganese oxides, could promote oxygen storage and release, enhance oxygen mobility, and form surface and bulk vacancies to supply oxygen during the reaction. In order to investigate the redox property of Cu/Mn=1/4 and Ce-doped Cu/Mn=1/4 oxides catalyst, H 2 -TPR studies were performed. TPR profiles of the Cu/Mn= 1/4 and Ce-doped Cu/Mn=1/4 oxide catalysts showed three reduction features at C (broad shoulder), 273 and 330 C (Fig.7). The shoulder at lower temperatures ( C) ascribes to the reduction of CuO, which was easily reduced, compared to MnO 2 or Mn 2 O 3 in copper manganese oxide [27], although CuO crystallite size is too small to be detected by the XRD. The main reduction features at higher temperatures can be attributed to the reduction of CuMn 2 O 4 and Mn 2 O 3 phases. Interestingly, after doping with Ce, these reduction features shifted to lower temperature. The phenomena indicate that the reduction of copper manganese oxides is promoted and the mobility of lattice oxygen is enhanced due to the interaction between copper manganese and cerium oxides [28]. Consequently, Ce-doped Cu/Mn=1/4 oxide possesses higher mobility of oxygen species, and this also promotes the higher CO oxidation activity. By comparing study of these catalysts, it is proposed that surface absorbed oxygen and readily releasable lattice oxygen both play important roles in CO catalytic oxidation. IV. CONCLUSION We have successfully prepared a series of copper manganese oxides and Ce-doped copper manganese oxides by inverse co-precipitation method. The copper manganese oxide catalyst prepared by inverse coprecipitation method possesses of higher catalytic activity, and doping low levels of Ce can also improve its initial and overall activity. It is proposed that the higher activity of the catalyst attribute to amorphous structure, larger surface area, highly dispersed CeO 2 in the lattice which can promote the generation of surface adsorbed oxygen and enhance the mobility of lattice oxygen. V. ACKNOWLEDGMENT This work was supported by the China National Tobacco Corporation (No ). [1] S. H. Taylor, G. J. Hutchings, and A. A. Mirzaei, Chem. Commun (1999).

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