Highly Ordered Mesoporous Cobalt-Copper Composite Oxides for Preferential CO Oxidation

Transcription

1 Catal Surv Asia (2017) 21:45 52 DOI /s Highly Ordered Mesoporous Cobalt-Copper Composite Oxides for Preferential CO Oxidation Mingshi Jin 1 Zhenghua Li 2 Wenxiang Piao 1 Jing Chen 1 Long Yi Jin 1 Ji Man Kim 2 Springer Science+Business Media New York 2017 Abstract Highly ordered mesoporous cobalt-copper composite oxides were prepared by the nanocasting method with various Co and Cu ratios. The catalysts obtained were characterized by X-ray diffraction, N 2 adsorption desorption, H 2 -temperature programmed reduction, CO-temperature programmed desorption and X-ray photoelectron spectroscopy. All of the catalysts had uniform mesopores and high surface areas. The distinct catalytic properties of these well-characterized mesoporous materials were demonstrated for preferential CO oxidation. It is found that the mesoporous cobalt-copper composite oxides, exhibited the higher catalytic activity for CO conversion and selectivity compared with the mesoporous Co 3 O 4 and mesoporous CuO. Among these catalysts the mesoporous cobalt-copper catalyst with Co:Cu molar ratio of 70:30, shows the best catalytic activity and the broadest operating temperature window for the high CO conversion in the range of o C. The higher catalytic activity was attributed to the higher CO adsorption and oxygen vacancies. Keywords Mesoporous Cobalt-copper composite oxides Oxygen vacancies PROX * Long Yi Jin lyjin@ybu.edu.cn * Ji Man Kim jimankim@skku.edu 1 Key Laboratory for Organism Resources of the Changbai Mountain and Functional Molecules, and Department of Chemistry, College of Science, Yanbian University, Yanji, Jilin, China 2 Department of Chemistry, Sungkyunkwan University, Suwon , Republic of Korea 1 Introduction Proton exchange membrane fuel cell (PEMFC) is an attractive technology for power generation mainly due to its high efficiency and environmental friendship. The hydrogen produced by steam reforming or methanol or partial oxidation of hydrocarbon containing % CO of in an excess of H 2 (40 75%) and 20 25% CO 2 [1, 2]. Since the CO molecule present in stream is poison the Pt electrodes anode used in fuel cells, it is important to reduce its level below 10 ppm [3]. There have the different methods to remove CO from the hydrogen stream such as pressure swing adsorption, methanation, membrane separation and preferential oxidation (PROX) process and so on. Among these methods the selective oxidation of CO with molecular oxygen process seems to be the most suitable and simple. Therefore, the development of selective CO oxidation catalysts has stimulated considerable interest worldwide [4 6]. The key point of this method is to find catalysts having good activity and selectivity for CO oxidation in the presence of H 2 at a low temperature, wide-temperature window and high resistance towards CO 2 and H 2 O those exist in the reformed fuel, however, low activity for the unwished side reactions like H 2 oxidation, reverse WGS, methanization, etc [7, 8]. The catalysts reported for PROX include supported noble metal catalysts such as Pt, Rh, Ru, and Au catalysts and transition metal oxide catalysts, such as Co 3 O 4, MnO 2, CuO CeO 2, Co 3 O 4 CeO 2 has been recognized as one of the promising candidates [9 11]. Among these catalysts, the Co 3 O 4 was known to exhibit good catalytic performance and high selectivity for the CO oxidation at lower temperatures [12, 13]. However, the Co 3 O 4 catalyst involves deactivation by common gases like water and by solid reaction. It would be favorable to use a promoter and/ or support in Co 3 O 4 catalysts for CO oxidation. The Kang s Vol.:( )

2 46 M. Jin et al. group reported the CoO x -CeO 2 catalysts are very active for CO oxidation, because of the facile Ce 4+ /Ce 3+ redox cycle and high oxygen storage capacity as well as the dispersion of cobalt oxides can be greatly promoted by the addition of a small amount of ceria [14, 15].On the other hand, CuO is also known to be one of the main active components for the PROX reaction, and a few studies have been contributed to CuO-CeO 2 catalysts [16, 17].Combine with the main active metal oxides of Co 3 O 4 and CuO could be bring to synergistic effect for the PROX reaction.co 3 O 4 CuO composite oxides with the spinel structure wereeffective for the CO oxidation and the oxidative elimination of volatile organic compounds (e.g.,hexane) [18]. Recently, mesoporous materials display high specific surface areas provided by an ordered arrangement of mesopores which might enhance adsorption/desorption performance. In particular, the high specific surface area involves an increase in the number of active sites [19]. In this present work, mesoporous cobalt-copper composite oxides (denoted as meso-cocu) were designed and facilely synthesized via nano-replication method. The received meso-cocu possessed excellent textural properties. Fig. 1 a XRD patterns and b N 2 adsorption desorption isotherm of the KIT-6 template Fig. 2 XRD patterns a small angle, b wide angle of meso-co 3 O 4, meso-cuo and meso-cocu composite oxides

3 Highly Ordered Mesoporous Cobalt-Copper Composite Oxides for Preferential CO Oxidation 47 Hereby, this series catalyst would simultaneously display high catalytic activity. More details about these materials as catalysts for PROX reaction would be described specifically in the following text. 2 Experimental 2.1 Synthesis of KIT 6 Silica Template Mesoporous silica template (KIT-6) was synthesized following previously reported methods [20]. A triblock copolymer (Pluronic P123, EO 20 PO 70 EO 20, Aldirch) was dissolved in an distilled water and BuOH, then the HCl and tetraethyl orthosilicate (TEOS) was added at 35 o C.The solution was stirred for 24h by a magnetic stirrer at the same temperature. The molar composition of the mixture was P123:BuOH:TEOS:HCl:H 2 O = 0.017:1.01:1.33:0.45: 198. The mixture put in an oven at an elevated temperature of 100 C for 24 h. After this hydrothermal aging, the solid product formed was recovered by filtration and dried at 80 C. Finally, the template was removed by extraction in an ethanol-hcl mixture (addition of 1 g of product in the mixture of 20 ml ethanol and 2 ml concentrated HCl) for 1 h, followed by calcination at 550 C for 3 h. 2.2 Synthesis of Meso CoCu Composite Oxides Catalysts The meso-cocu materials were synthesized by nano-replication method. A mixed solution of Co(II) nitrate hexahydrate and Cu(II) nitrate hexahydrate were dissolving in Table 1 Physicochemical properties of all the catalysts Materials S BET (m 2 /g) a V total (cm 3 /g) b Pore size (nm) c Crystalline size (nm) d meso-co 3 O , meso-cocu (90:10) , meso-cocu (70:30) , meso-cocu (50:50) , meso-cocu (30:70) , meso-cocu (10:90) , meso-cuo , a BET surface areas calculated in the range of p/p 0 = b Total pore volumes measured at p/p 0 = 0.99 c Average pore diameter calculated by BJH method d Crystalline size caluculated by Scheerer equation Fig. 3 a N 2 -isotherms, b pore size distributions of meso- Co 3 O 4, meso-cuo and meso- CoCu composite oxides

4 48 M. Jin et al. Table 2 XPS results for meso-cocu composite oxides along with meso-co 3 O 4 and meso-cuo Samples Binding Energy (ev) O H V /O L area ratio Co 2p 3/2 Cu 2p 3/2 O1s meso-co 3 O meso-cocu (90:10) meso-cocu (70:30) meso-cocu (50:50) meso-cocu (30:70) meso-cocu (10:90) meso-cuo distilled water. Then, the solution impregnated into 5 g of mesoporous silica templates by simple incipient wetness method, and the composites were dried at 353 K for 24 h. The materials were heated to 823 K for 3 h under ambient conditions. Finally, the silica template was removed by using 2 M NaOH aqueous solution. The precursors with nominal molar ratios were varied Co:Cu = 100:0, 90:10, 70:30, 50:50, 30:70, 10:90 and 0: Catalytic Activity The reactions were performed in a packed bed reactor at atmospheric pressure. Typically, 0.06 g of catalyst was used. The feed mixtures were prepared using mass flow controllers (MKS Instruments, Inc.). A mixture gas consist of 1.0% CO, 0.8% O 2 and balanced with He at a total flow rate of 40 cm 3 min 1. The space velocity (GHSV) is 18,000 cm 3 g cat 1 h 1. Empty reactor (without sample) shows no activity under such conditions. The effluent gas stream from the reactor was analyzed online by thermal conductivity detector (TCD) of parallel gas chromatography (GC, Younglin, Inc.) with carboxen 1000 column. To determine conversion the products were collected during 30 min of steady-state operation at each temperature. 2.4 Characterization X-ray diffraction patterns(xrd) were obtained from a Rigaku D/MAX 2200 ultima equipped with Cu Kα at 30 kv and 40 ma. N 2 adsorption desorption isotherms were collected on Micromeritics ASAP 2000 at liquid N 2 temperature. Transmission electron microscope (TEM) images were obtained using a JEOL JEM-2100F at an accelerating voltage of 200 kv. X-ray photoelectron spectroscopy (XPS) was performed using K-Alpha (Thermo) with Mg K radiation. H 2 -temperature programmed reduction (H 2 -TPR) was carried out with 0.01 g of calcined catalyst, from room temperature to 700, at a heating rate of 10 min 1 with a 40 ml min 1 flow rate of 10 wt% H 2 / He mixed gas, which is performed in a fixed bed reactor system connected with thermal conductivity detector (TCD). The hydrogen consumption was analysed via a TCD. CO-temperature programmed desorption (CO-TPD) was carried out with 0.05 g of the catalyst. First, performed sample cleaning with He(30 ml mim 1 ) from RT to 200 (30 min), and cooling to room temperature, then the adsorption of CO at this temperature for 1h. He flow was next passed to eliminate the physically absorbed CO. Fig. 4 TEM images of a meso-co 3 O 4, b meso-cocu (90:10), c meso-cocu (70:30), d meso-cocu (50:50), e meso-cocu (30:70), f meso- CoCu (10:90), and g meso-cuo

5 Highly Ordered Mesoporous Cobalt-Copper Composite Oxides for Preferential CO Oxidation 49 Temperature-programmed desorption was then carried out by heating the samples from to 700, at a heating rate of 10 o Cmin 1. 3 Results and Discussion The characteristics of the KIT-6 template are shown in Fig. 1. The mesosturcture of the KIT-6 template is determined 3D cubic, as shown in XRD pattern (Fig. 1a) and the N 2 sorption isotherms (Fig. 1b) of the KIT-6 is typical type IV isotherm with hysteresis loops and narrow pore size distribution. The specific surface area of KIT-6 by the BET equation is 702 m 2 /g and the total volume is 0.79 cm 3 /g. The result of the pore size distribution calculated by the BJH equation indicates that the KIT-6 is around 7.2 nm. The XRD diffraction patterns of mesoporous Co-Cu composite oxides catalysts, which well replica with KIT-6 template present in Fig. 2. All of the catalysts show three well resolved XRD peaks in the region of 2θ = , which correspond to (100), (211), (220), and (332) reflections based on the cubic Ia3d system [21]. From the wide angle XRD pattern, it can be concluded that the all of the samples are well crystallized. The meso-co 3 O 4 and meso- CuO assigned to single Co 3 O 4 and CuO phase. For the meso-cocu composite oxides, when the Co:Cu ratio was 90:10 showed Co 3 O 4 diffraction peak, whereas the 30:70 and 10:90 showed CuO diffraction peaks. The meso- CoCu (70:30) and meso-cocu (50:50) exhibited Co 3 O 4 and small CuO diffraction peaks and the Co 3 O 4 diffraction peaks became broader compared with meso-co 3 O 4 and meso-cocu (90:10). The broadening of diffraction peaks indicates that the crystalline size become smaller. Hence, we have estimated the crystalline size of all the catalysts. The result shows that the CuO crystalline size for meso- CoCu (30:70), meso-cocu (10:90), meso-cuo and Co 3 O 4 crystalline size for meso-co 3 O 4, meso-cocu (90:10) was approximately ~11 nm. However, the Co 3 O 4 crystalline size of the meso-cocu (70:30) and meso-cocu (50:50) was 8.2 and 7.6 nm, respectively (Table 1). This result suggested presence of CuO may be resistance of Co 3 O 4 crystal growth [22, 23]. Figure 3 shows the N 2 adsorption and desorption isotherms are of type IV classification, which have typical hysteresis loops of mesoporous materials [24, 25]. On the other hand, the surface area, the total pore volume and distribution of the pore diameters, calculated according to the BJH model applied on the adsorption branch summarized in Table 1. Transmission electron microscopy (TEM) investigations also reveal that the mesostructure regularity is well preserved after the nanocasting replication, as shown in Fig. 4. The TEM images suggest that the all the samples has ordered mesostructure throughout it whole particle domain. No large nonporous impurity particles were observed. Figure 5 shows the H 2 -TPR profiles of the samples. Meso-Co 3 O 4 exhibited two-step reduction, and the reduction peaks were observed at 321 C and Fig. 5 H 2 -TPR profiles of a meso-co 3 O 4, b meso-cocu (90:10), c meso-cocu (70:30), d meso-cocu (50:50), e meso-cocu (30:70), f meso-cocu (10:90), and g meso-cuo Fig. 6 CO-TPD profiles of a meso-co 3 O 4, b meso-cocu (90:10), c meso-cocu (70:30), d meso-cocu (50:50), e meso-cocu (30:70), f meso-cocu (10:90), and g meso-cuo

6 50 M. Jin et al. Fig. 7 XPS results for meso-cocu composite oxides along with meso-co 3 O 4 and meso- a Cu 2p3/2 spectra, b Co 2p3/2 spectra, and c O1s spectra 369 C. These two H 2 consumption peaks could be ascribed to the two-step reduction of Co 3 O 4 to Co, via, Co 3+ Co 2+ and Co 2+ Co 0 [26]. Meso-CuO was reduced at lower temperatures than meso-co 3 O 4, and the main reduction peak was located at 250 C. The reduction of the meso-cocu composite oxides occurred at temperatures significantly lower than those for meso-co 3 O 4 and quite similar to those for meso-cuo. This suggested the presence of a small amount of copper in the composite oxide could remarkably promote the reduction of meso-co 3 O 4. CO TPD was performed to study the adsorptions of CO on the catalysts (Fig. 6). In general, CO adsorbed on support desorbs in the form of CO 2 by during the process of TPD [27]. The CO 2 desorption peaks appeared both lowertemperature region ( C) and higher-temperature region ( C). The meso-co 3 O 4 profiles shows CO 2 desorption peaks at from 115 to 220 C and the meso-cuo peaks shows at from 171 to 310 C. The CO 2 desorption peaks of meso-cocu composite oxides at lower-temperature region are shifted to lower temperatures compared with the single meso-co 3 O 4, suggesting that the presence of Cu influences the reaction between CO and lattice oxygen. The meso-cocu (90:10), meso-cocu (70:30) showed one desorption peak, whereas the meso-cocu (50:50), meso-cocu (30:70) meso-cocu (10:90) showed two desorption peaks. As shown in Fig. 6, meso-cocu (90:10) and meso-cocu (70:30) shows relatively higher amount peak at low temperature compared to other catalysts. The higher CO 2 desorption may enhance its catalytic performance in the PROX of CO. XPS line shapes of Cu2p, Co2p and O1s spectra are displayed in Fig. 7, while the photoemission energies for cobalt, copper, oxygen and O H V /O L area ratios are summarized in Table 2. The binding energy values of Co2p 3/2 for all the composite oxides were ev, which were close to that for meso-co 3 O 4 (780.2 ev) [28].The Co2p 3/2 shake-up satellites have low intensity, indicating the presence of Co 3+ species at the surface of the monometallic and bimetallic oxides (Fig. 7a). The binding energy values of Cu 2p 3/2 for all these meso-cocu composite oxides were ev, while the meso- CuO were ev. The binding energies of meso- CoCu composite oxides were shifted lower and higher compared to meso-cuo. Moreover, the satellites peaks of Cu2p 3/2 showed that Cu 2+ species were present at the surface on all oxide systems (Fig. 7b). The O1s spectra for all the samples in Fig. 7c exhibited an intense peak at ev and a shoulder at ev. However, the BE values of the intense peak of O1s for the meso-cocu composite oxides were lower and higher than that for the single meso-co 3 O 4 and meso-cuo. This Table 3 Light-off temperature on CO conversion and Windows Samples T 50 ( o C) a T 100 ( o C) b Windows T 100 ( o C) meso-co 3 O meso-cocu (90:10) meso-cocu (70:30) meso-cocu (50:50) (97%) meso-cocu (30:70) (97%) meso-cocu (10:90) (87%) meso-cuo (57%) a Temperature at CO conversion of 50% b Temperature at CO conversion of 100%

7 Highly Ordered Mesoporous Cobalt-Copper Composite Oxides for Preferential CO Oxidation 51 Fig. 8 Catalytic activity of a CO conversion, b CO selectivity of meso-cocu composite oxides, meso-co 3 O 4 and meso-cuo suggested existence of high concentration cation vacancy in the meso-cocu composite oxides [29]. This further indicated the strong interaction between Co and Cu, and created the much oxygen vacancies. These results also shown in Table 2, which calculated of O H L /O L area ratio fitted from O1s spectra [30]. From the results, the meso-cocu (70:30) exhibited higher oxygen vacancies compared with other catalysts. Chen et al. reported that the oxygen vacancies contributed to the catalytic activity [31]. Figure 8a shows the catalytic performances of the meso- CoCu composite oxide catalysts with different Co/Cu ratios for PROX. Meso-Co 3 O 4 single oxide exhibited a better catalytic activity than meso-cuo. In general, the presence of Cu in catalysts for CO oxidation results in enhanced catalytic properties, promoting the redox properties of the material, as in Hopcallite catalysts [32] or facilitating the CO chemisorption process during the reaction [33]. The light-off temperatures of the catalysts (temperature to reach 50% and 100% conversion) are listed in Table 3. The meso-co 3 O 4, meso-cocu (90:10) and meso-cocu (70:30) shows completely 100% conversion at a certain temperature, whereas others catalysts not achieved in 100% conversion. The meso-cocu (70:30) exhibited the highest catalytic activity and the temperature for complete conversion of CO and exhibited a wide temperature window for 100% conversion in the range of 125 to 200 C. It is indicating that the CO could only be completely removed between 125 and 200 C and CH 4 was formed when the reaction temperature was higher than 200 C. The good catalytic activity can ascribed the higher CO adsorption amounts at low temperature and higher oxygen vacancies. For the meso-cocu (90:10), the complete CO conversion temperature was 200 C, however, the windows of 100% conversion was in the range of 200 to 300 C. Catalytic activity follows the general order: meso-cocu (70:30) > meso-cocu (50:50) > meso-cocu (90: 10) > meso-cocu (30:70) > meso-co 3 O 4 > meso- CoCu (10:90) > meso-cuo. The selectivity to CO oxidation of all the catalysts is shown in Fig. 8b. The meso- CoCu (90:10) and meso-cocu (70:30) catalyst exhibited the highest selectivity. The decrease in both CO conversion and CO 2 selectivity can be observed while the copper oxide increased. Generally, selectivity decrease with increase of the reaction temperature due to H 2 oxidation. Therefore, 70/30 of appropriate Co/Cu ratio is required for CO PROX reaction. 4 Conclusion The mesoporous cobalt-copper composite oxides were prepared by a nanocasting method, exhibited better catalytic performances than meso-co 3 O 4 and meso-cuo.the mesoporous cobalt-copper composite oxide with a Co/Cu ratio of 90:10, 70:30 could provide 100% CO conversion at 200 and 125 o C, respectively. Presence of CuO can significantly enhance the catalytic activities toward the CO oxidation. The high catalytic activity was attributed to a combination of factors including higher CO adsorption and the presence of readily available oxygen for CO oxidation. Acknowledgements This work was supported by the National Natural Science Foundation of China (grants and ), the Open Projects of the Key Laboratory of Natural Resource of the

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