Preparation of CuO/CeO2 Hollow Spheres and Study on the Performance of CO Oxidation

SHAO Xia, QU Zhenyang, SONG Qingzheng,Qi Fangzheng, YANG Zhengjun

(School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China)

Abstract: A simple and convenient method for preparing CuO-doped CeO2 hollow spheres via PS templates was presented. The structural characteristics of CuO/CeO2 hollow spheres were characterized by scanning electron microscopy (SEM), Energy Dispersive Spectrometer (EDS), X-ray diffraction (XRD), N2 adsorption-desorption isotherm and X-ray photoelectron spectroscopy (XPS). To evaluate the differences of catalytic properties among the CuO/CeO2 hollow spheres, industrial CeO2 and CeO2 hollow spheres, H2-temperature programmed reduction (H2-TPR) and CO oxidation tests were carried out. The results show that the synthesized samples have good morphology, large specific surface area and high purity. The H2-TPR analysis reveals that the reducibility of CuO/CeO2 hollow spheres has diverse surface oxygen species. CuO doped CeO2 hollow spheres exhibit better performance than CeO2 hollow spheres and commercial CeO2 in CO oxidation in that the copper can effectively enhance the catalytic activity of the samples for CO oxidation through the increase of peroxide defects and synergistic effect.

Key words: Cu; CeO2; hollow spheres; doped; CO oxidation

The increasingly serious air pollution problems of CO caused by vehicle exhaust and fuel combustion, which brings a long-term environmental toxicity and a short-term public health damage, therefore an effective method of degrading CO plays an important role in our daily life[1]. CO oxidation is one of the methods to solve the problem. The kind of catalyst[2]for CO oxidation usually contained noble metals catalyst, such as Au, Pt, and Pd and non-precious metals catalyst, such as Cu, Mn, Fe. With the increasing price of noble metals and the improving research of nanotechnology, an increasing number of scholars have paid attention to the preparation and performance of nanoparticles.

As one of the rare-earth metal oxides, CeO2has been widely used in many fields such as high-storage capacitors, absorbent for water treatment, catalyst, UV-shielding and fuel cell[3], which is due to its remarkable thermal stability, optical properties, fast oxygen ion transportation ability and oxygen storage capability. In addition, Ce3+and Ce4+could switch easily[4]. So CeO2plays a significant role in CO oxidation. He had prepared CeO2of nanostructures with various morphologies successfully through a hydrothermal method without any surfactants and templates, and concluded that CeO2nanowires represented superior activity for CO oxidation due to the large surface area and average pore size. In recent years, some experts have tried many methods to enhance the surface area of CeO2, such as compound different particle size and morphology. However, the catalytic performance of pure CeO2is still not satisfied. So, some researchers spend their time doping metals to improve CeO2catalytic activity of CO oxidation[5]. The metal of Au, Pt and other noble metals had been reported that the effect of catalysis is superior. But the cost of noble metal is high, some new non-precise metals were studied. The valence electron structure and cluster of metal Cu and Au are the same, so the study of Cu doping CeO2wasvery popular. Many researchers prepared CuO/CeO2nanoparticles and studied their properties. The preparation of CeO2hollow spheres have been studied[6]. Few reports were involved the preparation of CuO/CeO2hollow spheres, and its application in the catalytic oxidation of CO is also rarely studied.

Based on the above facts, PS spheres were used as template to prepare CeO2hollow spheres and CuO/CeO2hollow spheres. The performance of CO oxidation is studied. In addition, SEM&EDS, XRD, BET, H2-TPR and XPS are used to characterize the basic properties of the obtained samples.

1 Experimental

1.1 Catalysts preparation

First of all, PS microspheres via soap-free emulsion polymerization were prepared, and then the CeO2hollow spheres were synthesized by PS templates. Next, the CuO/CeO2hollow spheres were prepared, referring to the preparation of CeO2hollow spheres. Weighing 4 g PS templates in a three-neck flask, adding 40 mL water and ultrasonic dispersing for 10 min, the mixture solution was heated to 65 ℃ and stirred for 60 min with a fixed rate. And then adding 20 mL 0.1 mol·L-1Ce(NO3)·6H2O aqueous solution, and continuing to stir for 60 min. After adding 5 mL Cu(NO3)2solution (1×10-4mol Cu(NO3)2in 5 mL water), go on stirring for 120 min.Then injecting 20 mL NaOH solution (0.23 g NaOH in 20 mL water, the injection rate is 20 mL·h-1), keeping the temperature of water bath constant and stirring for 2 hin same rate, stopping heat and go on stirring for another 2 h. In the end, the suspension was washed by centrifugation (washing twice with deionized water and alcohol, respectively), and dried at 70 ℃, then calcinated 2 h at 500 ℃ to gain the samples.

1.2 Catalyst characterization

The morphology and microstructure of the samples were characterized by a scanning electron microscope and matching EDS at an accelerating voltage of 15 kV (SEM&EDS, HITACHI S-3400N).The crystal and purity of the materials were analyzed by a powder X-Ray diffraction (XRD, Bruker D8-Advance) with Cu Ka incident radiation (λ=0.154 18 nm), The X-ray diffraction (XRD) patterns were obtained for 2θangles from 20° to 90° at a scan rate of 0.02°·min-1.Nitrogen adsorption and desorption experiments were performed at 77 K on a NOVA4000 gas adsorption analyzer (Quantachrome Corp.). The X-Ray photoelectron spectra was obtained using a ESCA 3400 X-Ray photoelectron spectrometer with Al Ka radiation, the binding energy reference is taken at 284.1 eV for the C 1speak.

1.3 Catalyst activity measurement

H2-temperature program reduction (H2-TPR) through mass spectroscopy (MS, Pfeiffer vacuum QMS 200) was used for the reduction properties of samples. The reactor was made up of a quartz U-shape tube (I.D. 12 mm) on densely packed quartz wool. Each sample (100 mg) was pretreated at 300 ℃ for 1 h under Ar flow (100 cm3min-1) and then cooled down to room temperature (RT). Then increase the temperature to 800 ℃ at a ramp rate of 10 ℃·min-1in stream of 10% H2/Ar (100 cm3·min-1). The mass signals of m/z=2 (·H2) were detected by MS.

The activity measurement was carried out in a fixed bed reactor (sample consumption 0.1 g). The catalyst was loaded in quartz wool plugs in the middle of the reactor. The composition of feed gas is 4%CO, 10% O2and 86%N2and the flow rate is 30 mL·min-1.The catalyst was heated in N2(50 mL·min-1) at a rate of 5 ℃ min-1from RT to 200 ℃ and held at this temperature for 40 min in order to remove some possible impurities. After cooling to room temperature in N2atmosphere, the feed gas was introduced into the system. To allow for the detection of CO and CO2with flame ionization detector (FID), a methanator was inserted between a GC column and a FID. The composition of the gas after the reaction was analyzed by an on-line gas chromatography with a FID, connected with a computer integrator system.

2 Results and discussion

2.1 SEM and EDS result

The morphology and average size of thesamples aredetected by SEM, as shown in Fig.1.

Fig.1a and Fig.1b shows that the CeO2hollow spheres and CuO/CeO2hollow spheres are obtained successfully with a diameter of 300 nm, and the interval of PS templates on the surface of hollow spheres caused some holes which had non-uniform holes. Fig.1c briefly displays that the morphology of commercial CeO2is irregular granular and the average size is about 300～500 nm. Meanwhile, we analyze the doping element of Cu in CeO2hollow microspheres with EDS spectrum, it is shown in Fig.1d and Tab.1.

Fig.1 SEM images of (a) CeO2 hollow spheres, (b) Cu/CeO2 hollow spheres and (c) commercial CeO2 and (d) EDS of Cu/CeO2 hollow spheres

Tab.1 Mass ratio of elements in Cu/CeO2 hollow spheres %

From the element lines in Fig.1d, it is identified that the element of Cu exists in CuO/CeO2hollow spheres and whose detail data are listed in Tab.1, it is found that the mass ratio of Cu element is about 0.26% which is coincided with the Cu molar ratio of 5% within the error range from the Tab.1. In a word, SEM&EDS reveals that the morphology of samples is uniform and Cu element is successfully doped in the CeO2hollow microspheres.

2.2 XRD result

The crystal phase and purity of the samplesare illustrated by the XRD patternin Fig.2.

Fig.2 XRD pattern of the commercial CeO2, CeO2 hollow spheres and CuO/CeO2 hollow spheres

Fig.2 shows the XRD patterns of the commercial CeO2, CeO2hollow spheres and CuO/CeO2hollow spheres. The strong and sharp diffraction peaks indicate the good crystal structure of the three kinds of samples. All of these characteristic peaks are existed in commercial CeO2, CeO2hollow spheres and CuO/CeO2hollow spheres, and these peaks reveal that the CeO2crystal belong to fluorite cubic phase according to JCPDS card NO.34-1349. But, we could not find the characteristic peaks of Cu in the XRD pattern of CuO/CeO2hollow spheres, which suggests that the doping amount of Cu is too little or copper oxide is exist as small crystals or highly dispersed on the surface of CeO2hollow spheres.

2.3 N2 adsorption-desorption result

Fig.3 displays the N2adsorption-desorption isotherm of commercial CeO2nanoparticles, CeO2hollow spheres and CuO/CeO2hollow spheres.

Fig.3 N2 adsorption-desorption isotherm of commercial CeO2, CeO2 hollow spheres and CuO/CeO2 hollow spheres

According to the IPUAC classification, the isotherm plot reveals that CeO2hollow spheres belong to the porosity materials. The surface area of commerical CeO2, CeO2hollow spheres and CuO/CeO2nanoparticles are 12.95 m2·g-1, 54.3 m2·g-1and 55.39 m2·g-1, respectively. It is found that the structure of hollow spheres is good for surface area and the dopping of Cu almost have no effect on the surface area of CeO2hollow spheres.

2.4 XPS result

In order to analyze the each elements’ chemical valence of as-prepared samples, XPS is used.

Fig.4(a) displays the spectrum of wide XPS scan of commercial CeO2nanoparticles, CeO2hollow spheres and CuO/CeO2hollow spheres.The elements of Ce, O and C are all find in three samples. The peak at 284.8 eV is caused by contaminant carbon. No other elements are observed, which suggests that there are no impurity in samples and the PS template is completely removed. According to the Ce3d spectrum [Fig.4(b)], the peak at 882.8 eV reveals that cerium existed as Ce(Ⅳ) oxidation state in the three samples. But from the Ce 3dspectrum of CuO/CeO2hollow spheres, significant changes are observed in binding energy with Cu element incorporation, the main signal of Ce(Ⅲ) is found at the peak of 885.9 eV[7], which shows there are more Ce(Ⅲ) in the samples of CuO/CeO2hollow spheres than the other two samples. Fig.4c shows the O 1sspectrum and displays significant changes upon Cu incorporation. The main peak and shoulder peak of commercial CeO2nanoparticles and CeO2hollow spheres are at around 529.4 eV and 532.5 eV respectively, however the peaks of CeO2hollow spheres are 531.8 eV and 529.4 eV. The lower binding energy is due to the surface lattice oxygen in crystalline CeO2and the higher binding energy is corresponded to the O2-ions in oxygen deficiency or the adsorption oxygen molecules. Comparing the area of O 1speaks, it is easy to find that the doping of Cu element could increase oxygen deficiencies. From the Fig.4(d) we can see the XPS spectrum of Cu 2p, there is much peaks between 920 eV to 970 eV, but the characteristic peaks of Cu 2p(around 931 eV and 953 eV) are not obvious, which may be related with the doping amount. All in all, copper incorporation plays an important impact on the the amout of Ce(Ⅲ) and oxygen deficiencies in CuO/CeO2hollow spheres.

Fig.4 XPS spectrum of the-prepared samples of commercial CeO2, CeO2 hollow spheres and CuO/CeO2 hollow spheres (a) survey spectrum, (b) Ce 3d spectra, (c) O 1s spectra and (d) Cu 2p spectrum

2.5 H2-TPR result

In order to study the reducibility which determines the catalytic properties of catalyst, H2-TPR is used to research the characteristic of commerical CeO2, CeO2hollow spheres and CuO/CeO2nanoparticles.

Fig.5 shows the H2-TPR profiles of samples.

Fig.5 H2-TPR of commercial CeO2, CeO2 hollow spheres and CuO/CeO2 hollow spheres

It is easy to find that the H2-TPR profiles of commerical CeO2and CeO2hollow spheresboth contain two reduction peaks. The first peak at 510 ℃ is ascribed to the surface reduction and the second peak at 780 ℃ is related to the bulk reduction of Ce(Ⅳ) into Ce(Ⅲ)[8]. The area under the two peaks are different, which reveal that CeO2hollow spheres have more oxygen available on the surface than commercial CeO2nanoparticles[9]. That displays the higher surface area is good for reducibility. In addition, in the curve of CuO/CeO2hollow spheres, there is a new peak at 310 ℃ which is caused by the reduction of Cu species[10]. The other two peaks are ascribed to the reduction of surface and bulk oxygen. Because of little amount Ce(Ⅲ) in CuO/CeO2hollow spheres could enhance the surface reduction, the temperature of the peak islower. The H2-TPR profiles as shown in Fig.5 displays that the reducibility of CuO/CeO2hollow spheres have more surface oxygen species which is good for the property of catalysis.

2.6 CO oxidation

In this paper, the catalytic performances for CO oxidation of commercial CeO2, CeO2hollow spheres and CuO/CeO2hollow spheresare evaluated and the results are shown in Fig.6.

Fig.6 The conversion chart CO oxidation activity of commercial CeO2, CeO2 hollow spheres and CuO/CeO2 hollow spheres

CuO/CeO2hollow spheres catalyst express excellent activity in CO oxidation and the CO conversion is up to 100% at 380 ℃, while the CO conversion approached 100% at the temperature of 420 ℃ for the CeO2hollow spheres. Obviously, to commercial CeO2, the temperature at which CO is totally oxidized CO2is the highest (420 ℃). Compared with curves of (a) and (b), curve (b) is higher than curve (a) in the rate of CO conversion, thus due to CeO2hollow spheres have a higher surface area which could enhance the reducibility than commercial CeO2. The curve of (c) is the most effective in the experiment of CO oxidation, and that is because of the strong interaction which could present more Ce(Ⅲ)/Ce(Ⅳ) and Cu(Ⅰ)/Cu(Ⅱ) redox couples between CuO and CeO2[11]. As we know, CeO2could provide oxygen vacancies which is important for activate the oxygen species and the existence of Ce(Ⅲ) is beneficial to form the oxygen vacancies and Cu(Ⅰ) could active CO molecules during the redox process. Based on the above research, we could conclude that the mainly factor which influence the catalytic activity of commercial CeO2and CeO2hollow spheres is the surface area, the bigger surface area could provide more active sites. In addition, compared with CeO2hollow spheres and CuO/CeO2hollow spheres, there is little difference in BET, so the mainly influence factor is due to the addition of CuO. Copper incorporation is helpful for the produce of oxygen vacancies and the synergistic effect of CuO and CeO2is also play a significant role[1 2]. That is the reason why CuO/CeO2hollow spheres plays a good performance in CO oxidation than the pure CeO2.

3 Conclusion

In summary, the CeO2hollow spheres and CuO/CeO2hollow spheres were prepared by a PS template method. The images of SEM&EDS showed that the as-obtained samples have a structure of hollow sphere and small holes existed on the surface of spheres. The spectrums of XRD and BET revealed that the samples have a unique structure of cubic and higher surface area. The XPS and H2-TPR results demonstrated that CuO/CeO2hollow spheres have more Ce(Ⅲ) on the surface than CeO2hollow spheres, thus make CuO/CeO2hollow spheres display higher reducibility. The activities of samples for CO oxidation in the order is CuO/CeO2hollow spheres>CeO2hollow spheres>commercial CeO2and which due to the high surface area and synergistic effect of CuO and CeO2. The CuO/CeO2hollow spheres may exhibit excellent potential for other fields due to its good redox property.