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Electronic Supplementary Material
Nano Research Nano Res DOI 10.1007/s01274-015-0704-3 Mass production of Co3O4@CeO2 core@shell nanowires for catalytic CO oxidation Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (), Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b, Yinghui Wanga and Hongjie Zhanga () Nano Res., Just Accepted Manuscript • DOI: 1 10.1007/s01274-015-0704-3 http://www.thenanoresearch.com on December 24 2014 © Tsinghua University Press 2014 Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. 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To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication. 1 TABLE OF CONTENTS (TOC) Mass Production of Co3O4@CeO2 Core@Shell Nanowires for Catalytic CO Oxidation Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (), Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b, Yinghui Wanga and Hongjie Zhanga () aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, China. bSchool of the Chinese Academy of Sciences, Beijing Co3O4@CeO2 core@shell nanowires have been successfully prepared 100039 (China) in # The two authors contribute equally to this work. Co(CO3)0.5(OH)·0.11H2O@CeO2 mass production by thermal precursors. decomposition The of successful fabrication of the core@shell structures leads to remarkably improved catalytic activity and stability of Co3O4. The best sample can catalyze 100 % CO conversion at a temperature as low as 160 oC. Detailed study reveals that CO oxidation possibly takes place at the interface of Co3O4 and CeO2, demonstrating obvious synergistic effects between the two components. Provide the authors’ webside if possible. Hongjie Zhang, http://lab.datatang.com/2007DA173041/AreaIndex.aspx?ItemID=68553 Mass Production of Co3O4@CeO2 Nanowires for Catalytic CO Oxidation Core@Shell Jiangman Zhena,b #, Xiao Wanga #, Dapeng Liua (), Zhuo Wanga,b, Junqi Lia,b, Fan Wanga,b, Yinghui Wanga and Hongjie Zhanga () Received: day month year ABSTRACT Revised: day month year In this paper, Co3O4@CeO2 core@shell nanowires were successfully prepared via thermal decomposition of Co(CO3)0.5(OH)·0.11H2O@CeO2 core@shell nanowire precursors. As the CO oxidation catalyst, Co3O4@CeO2 shows remarkably enhanced catalytic performance compared to Co3O4 nanowires and CeO2 NPs, demonstrating obvious synergistic effects between the two components. It also suggests that the CeO2 shell coating can effectively keep Co 3O4 nanowires from agglomeration, and hence remarkably improve the structure stability of Co3O4 catalyst. And the fabrication of the well dispersed core@shell structure results in a maximized interface area between Co3O4 and CeO2 as well as a smaller Co3O4 size, which might be responsible for the enhanced catalytic activity of Co3O4@CeO2. Further study reveals that CO oxidation possibly takes place at the interface of Co3O4 and CeO2. The influence of calcination temperatures and component ratio between Co3O4 and CeO2 have been then investigated in detail on the catalytic performance of Co3O4@CeO2 core@shell nanowires, the best of which obtained by calcination at 250 oC for 3 h with a Ce molar content about 38.5 % can catalyze 100 % CO conversion at a lower temperature of 160 oC. More importantly more than 2.5 g of the Co3O4@CeO2 core@shell nanowires can be produced in one pot by this simple process, which would be beneficial to their practical applications as automobile exhaust gas treatment catalysts. Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS Co3O4@CeO2, core@shell, nanowires, CO oxidation, synergistic effects Nano Res. 2 1 Introduction Catalytic oxidation of carbon monoxide (CO) has components. drawn continuous attention because of the serious exhibited health effects associated with exposure to CO. stability as oxidation reaction catalysts[12,22-25]. Hence Co3O4, a typical spinel-structure transition metal it is reasonably considered that the activity as well as oxide, has been subjected to intense interest recently the stability of Co3O4 catalyst could be optimized due to its excellent ability for catalytic CO oxidation, through and has been regarded as an alternative to noble core@shell structures. metal catalysts For instance, Ag@CeO2, Pt@CeO2, Au@CeO2 and Pd@CeO2 core@shell catalysts have good the activity facile and high-temperature fabrication of Co 3O4@CeO2 . Co3O4 nanorods synthesized by Generally, core@shell structures are synthesized Xie’s group have shown good catalytic performance, through hydrolysis of the precursors to deposit the which can catalyze CO oxidation at a low shell component onto a preformed core[25]. However, temperature of –77 oC in a trace moist stream of it is necessary to do some surface modification on the normal feed gas[2]. They attributed this to the core in advance so as to avoid independent abundance of active Co3+ species on {110} planes of nucleation the Co3O4 nanorods. Besides, the size of Co3O4 layer-by-layer technology is a multistep process that nanostructure is also thought to be important in requires determining its catalytic activity . However, there modification, so it is not conducive to large-scale is still few reports concerning optimizing the synthesis and has seriously limited the practical stability of Co3O4 catalysts, because for practical applications of such catalysts. Besides, the reverse needs, catalysts are often required to be working at micelle method can be used to prepare core@shell relatively high temperatures without removing a structures[6,9]. However, the synthetic procedure is mass conditions, also multistep and consumes considerable time and nanomaterials are apt to aggregate or deform, energy. Meanwhile, to get the specific core@shell resulting in heavy loss of catalytic active centers structures, organic species such as surfactants have and even been largely used[25,26], some of which are hard to be inactivation. Therefore, the synthesis of Co3O4 removed completely, and hence the catalytic active catalysts with high activity as well as stability has centers of nanocatalysts might be contaminated, become an area of great focus in material science. resulting [1-5] of streams. serious Under catalytic [4,5] such deterioration and of the precise in shell control and unsatisfactory component. complex catalytic This surface activity. Fabrication of core@shell structures has been Consequently, it seems more meaningful to develop identified as an efficient way to inhibit agglomeration an effective way to realize the facile, clean and mass so as to improve the stability of nanomaterials[6-15]. In production of Co3O4@CeO2 core@shell structures. this consideration numerous kinds of oxides, such as Here, we report the synthesis of high-quality CeO2, SiO2, and ZrO2, have been adopted as the Co3O4@CeO2 core@shell structures in gram level. stable shell components[6,7,8,12,15]. In particular, CeO2, a First, typical kind of multifunctional rare earth oxide, prepared as precursors receives intense attention due to its wide applications coated by a CeO2 shell followed by the previously in catalysis[16-25]. It possesses strong oxygen storage reported strategy[28]. After calcination in air, the capacity that makes it highly active in oxidation as-obtained reaction. More importantly, it can also show excellent core@shell nanowires can be thermally decomposed synergistic and transformed into the final monodisperse effects with other catalytic active Co(CO3)0.5(OH)·0.11H2O [27] nanowires were , and then they were Co(CO3)0.5(OH)·0.11H2O@CeO2 Address correspondence to Dapeng Liu, liudp@ciac.ac.cn; Hongjie Zhang, hongjie@ciac.ac.cn www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 3Nano Res. Co3O4@CeO2 core@shell nanowires built up by were purified by centrifugation and washed with Co3O4 and CeO2 nanoparticles (NPs). In order to deionized water and ethanol for three times, and investigate the transformation process, thermal then gravimetric of Co(CO3)0.5(OH)·0.11H2O@CeO2-1. By tuning the Co(CO3)0.5(OH)·0.11H2O@CeO2 has been done in amount of Ce(NO3)3 and HMT, another three combination with the CO catalytic test, X-ray Co(CO3)0.5(OH)·0.11H2O@CeO2 diffraction (XRD), scanning electron microscope synthesized followed by the above procedure, and (SEM) and transmission electron microscopic (TEM) the analyses. Co(CO3)0.5(OH)·0.11H2O@CeO2-2 analysis Then the (TGA) influence of calcination dried at 60 as-obtained o C and precursors products (1.3 Co(CO3)0.5(OH)·0.11H2O@CeO2-3 (0.325 systematically investigated to study the optimal Ce(NO3)3, solution) condition for catalytic CO oxidation. Co(CO3)0.5(OH)·0.11H2O@CeO2-4 10 mL of HMT HMT as mmol Co3O4@CeO2 was of named Ce(NO3)3, nanowires mL were as were temperatures on the catalytic performance of core@shell 30 named solution), mmol (0.16 and mmol Ce(NO3)3, 5 mL of HMT solution), respectively. 2 Experimental Preparation of Co(CO3)0.5(OH)·0.11H2O nanowires nanowires: (Co precursor): Co(CO3)0.5(OH)·0.11H2O nanowires Co(CO3)0.5(OH)·0.11H2O@CeO2-1 were calcined at were 250, 350 and 500 synthesized by hydrothermal procedure a [27] previously reported . Typically, 0.56 g of Preparation corresponding of Co3O4@CeO2 The o core@shell precursor of C for 3 h in air, and the products are named as CoSO4·7H2O was dissolved in 40 mL of a mixture Co3O4@CeO2-1-250, containing 7 mL of glycerol and 33 mL of deionized Co3O4@CeO2-1-500, water. After stirred for about 10 min, a transparent Co(CO3)0.5(OH)·0.11H2O@CeO2-2, -3, and –4 were all solution was obtained, into which 0.10 g of urea calcined at 250 oC for 3 h in air as well, and the was then added. 30 min later, the solution was corresponding transferred into a 50 mL Teflon-lined stainless steel Co3O4@CeO2-2-250, autoclave, followed by heating at 170 Co3O4@CeO2-4-250, respectively. o C for a period of 24 h in an electric oven. Afterwards the autoclave was cooled naturally to room temperature. The products were collected and Co3O4@CeO2-1-350 respectively. products Preparation were For control, named Co3O4@CeO2-3-250, of Co3O4 and as and nanowires: Co(CO3)0.5(OH)·0.11H2O nanowires were directly calcined at 250 oC for 3 h in air. washed with deionized water and ethanol for three Preparation of Co3O4-CeO2 hybrids: 0.03 g of the times by centrifugation, and then dried at 60 oC as-prepared Co3O4 nanowires were ultrasonically overnight. dispersed in a mixed solution of 12 mL of water and Preparation of Co(CO3)0.5(OH)·0.11H2O@CeO2 12 mL of ethanol, and then 0.24 mmol Ce(NO3)3 and core@shell nanowires (Co precursor@CeO2): 0.1 g of 10 mL of 0.02 g/mL HMT aqueous solution were Co(CO3)0.5(OH)·0.11H2O were added in turn. Then the temperature of the solution ultrasonically dispersed in a mixed solution of 50 was increased to 70 oC and refluxed for 2 h before mL of water and 50 mL of ethanol, and then 0.65 being cooled to room temperature. The products mmol g/mL were purified by centrifugation and washed with hexamethylenetetramine (HMT) aqueous solution deionized water and ethanol for three times, and were added in turn. Then the temperature of the then dried at 60 oC. Ce(NO3)3 and nanowires 20 mL of 0.02 solution was increased to 70 oC and refluxed for 2 h Preparation of pure CeO2 NPs: 1 mmol Ce(NO3)3 before cooled to room temperature. The products was dissolved in a mixed solution of 20 mL of www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 4 deionized water and 20 mL of ethanol. Then 25 mL CO2. For each time, 30 mg of the sample was heated of 0.02 g/mL HMT aqueous solution was added. from room temperature to 900 °C at a rate of Then the temperature of the mixture was increased 10 °C/min. A gaseous mixture of 5 vol. % H 2 in N2 to 70 C and refluxed for 2 h before being cooled to was used as reductant at a flow rate of 20 mL/min. o room temperature. The products were purified by Catalytic tests: 25 mg of catalysts were put in a centrifugation and washed with deionized water stainless steel reaction tube. The CO oxidation and ethanol for three times, and then dried at 60 oC. catalytic tests were performed under an atmosphere Finally, the products were calcined in air at 250 oC of 1 % CO and 20 % O2 in N2 at a fixed space for 3 h in air. velocity of 50 mL/min. The composition of the gas Preparation of Co3O4-CeO2 mixtures: 0.058 g of was monitored on-line by gas chromatography. the above mentioned Co3O4 nanowires and 0.042 g of CeO2 NPs were physical mixed by grinding in an 3 Results and discussion agate mortar for half an hour. Characterization: The XRD data of the products were collected on a Rigaku-D/max 2500 V X-ray diffractometer with Cu-K radiation ( = 1.5418 Å ), with an operation voltage and current maintained at 40 kV and 40 mA. TEM images were obtained with a TECNAI G2 high-resolution transmission electron microscope operating at 200 kV. A HITACHI S-4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphology of the samples. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB-MKII 250 photoelectron spectrometer (VG Co.) with Al-K X-ray radiation Figure 1. (A) SEM and (B and C) TEM images of Co precursor; (D) SEM and (E and F) TEM images of Co precursor@CeO2 (Inset: HRTEM of CeO2). as the X-ray source for excitation. TGA curves of the sample was acquired by using a SDT 2960 thermal analyzer at a heating rate of 10 oC min-1 in air atmosphere within a temperature range between 20 and 700 oC. A GC 9800 gas chromatography tester was employed to obtain the CO conversion curves of the samples. N2 sorption isotherms were obtained at 77 K on an Auto-sorb-1 apparatus. Scheme 1. Schematic process for preparation of Co3O4@CeO2 Inductively coupled plasma (ICP) analyses were core@shell nanowires. performed with a Varian Liberty 200 spectrophotometer to determine the Ce content. H2-temperature-programmed reduction (TPR) was conducted on a TPDRO 1100 apparatus supplied by Thermo-Finnigan Company. Before detection by the TCD, the gas was purified by a trap containing CaO + NaOH materials in order to remove the H2O and The as-obtained samples were characterized by SEM and TEM. From the SEM and TEM images (Figure 1A to 1C), it can be clearly seen that Co precursor is composed by uniform and well dispersed nanowires with several micrometres in length and tens of nanometers in width. After www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 5Nano Res. coated with a CeO2 shell, the smooth surface of Co of precursor nanowire becomes obviously rough Co(CO3)0.5(OH)·0.11H2O@CeO2-1. Figure 3 shows (Figure 1D to 1F), indicating the success of CeO2 that the major weight loss (about 10 %) takes place shell coating process. The as-prepared products in the temperature range of 230 to 300 °C, which is well keep the wire-like morphology as Co precursor in consistence with its corresponding DSC analysis and each nanowire is completely wrapped by a (Figure S4). This part of loss should be attributed to shell built up by hundreds of 6 nm sized CeO2 NPs the decomposition of carbonates and hydroxide self-assembled together. The inset in Figure 1F groups shows the lattice spacing of 0.31 nm which Coincidentally, the first cycling curve of CO corresponds well to the characteristic (111) plane of conversion fully supports the TGA-DSC results that fluorite-phase CeO2. Combining with the XRD above 230 °C the sample can totally catalyze CO results (Figure S1) it firmly demonstrates the oxidation core@shell Co(CO3)0.5(OH)·0.11H2O into Co3O4. structure formation of the decomposition of process of Co(CO3)0.5(OH)·0.11H2O[27]. due to the transformation of Co(CO3)0.5(OH)·0.11H2O@CeO2. More than 3 g of Co(CO3)0.5(OH)·0.11H2O@CeO2 can be obtained in 250 one pot (see Figure S2), and its schematic o process as described in Scheme 1. Catalytic oxidation of CO is chosen here as the model reaction to evaluate 200 T100 ( C) fabrication has been summarized to a two-step the about the transformation Decrease Stable 100 catalytic performance of the samples. In order to study the details 150 50 of Co(CO3)0.5(OH)·0.11H2O as well as its influence on 0 1 the catalytic performance, the CO oxidation cycling 2 3 4 5 6 7 8 9 10 Cycle tests of Co(CO3)0.5(OH)·0.11H2O@CeO2-1 have been done in the temperature range from 50 to 250 oC. As Figure 2. Cycling tests of Co(CO3)0.5(OH)·0.11H2O@CeO2-1 shown in Figure 2 and S3, it can be found that for CO conversion. during the tests the value of T100 (the temperature o C in the following cycles. In general, catalysts often degrade more or less under long-term and high-temperature catalytic conditions due to aggregation, growth or some other reasons like poisoning. The abnormal enhancement of the catalytic activity aroused our great interests to investigate this phenomenon in depth. 100 120 80 110 60 100 40 90 20 80 0 70 50 As reported by Lou, et al[27], the transformation of 100 150 200 250 300 Weight Loss (%) fifth cycle to 160 oC and then remained stable at 160 CO Conversion (%) for 100 % CO conversion) kept decreasing until the 350 Temperature ( C) o Co(CO3)0.5(OH)·0.11H2O to Co3O4 starts at about 200 o C, so it is considered that such transformation would proceed during the catalytic process. Firstly, Figure 3. TGA curve and the first cycling curve of CO conversion of Co(CO3)0.5(OH)·0.11H2O@CeO2-1. TGA was employed to get the detailed information www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. Further 6 insight into the transformation Co(CO3)0.5(OH)·0.11H2O@CeO2-1 requires of XRD analysis. After ten cycling tests for CO oxidation, the sample was collected and labeled as Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after 10. In Figure 4, it can be clearly seen that the heat treatment during the cycling tests transformation results of in obvious orthorhombic-phase Co(CO3)0.5(OH)·0.11H2O into spinel-phase Co3O4, as the intensity of the corresponding peak of Co(CO3)0.5(OH)·0.11H2O decreased quickly, but that of Co3O4 was gradually enhanced. Based on the above analysis of CO catalysis associated with TGA and XRD curves, it confirms that the transformation of Co(CO3)0.5(OH)·0.11H2O into Co3O4 happened Figure 4. XRD patterns Co(CO3)0.5(OH)·0.11H2O@CeO2-1, Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after 10 Co3O4@CeO2-1-250. of and (A) (B) (C) during the whole cycling tests process, which might start from the surface parts of Co(CO3)0.5(OH)·0.11H2O nanowires to the inner. As the cycling tests continued, more and more Co(CO3)0.5(OH)·0.11H2O component was decomposed, and after five cycles, its surface parts that do work for catalytic CO oxidation transformed completely into Co3O4. That is why at this stage their catalytic activity became better and the T100 was decreased continuously. For the last five cycles, the conversion possibly proceeded in the inner parts of the Co(CO3)0.5(OH)·0.11H2O nanowires, therefore the T100 remained unchanged any more. These results suggest that only surface Co3O4 Figure 5. (A to C) TEM images of Co3O4@CeO2-1-250, (D and E) the corresponding EDX mapping analysis. components adjacent to CeO2 worked well for while the peak intensity of Co3O4 becomes more catalytic CO oxidation. That is to say CO oxidation intense. TEM (Figure 5) and SEM images (Figure S5) possibly takes place at the interface of Co 3O4 and show that the wire-like structure maintained well CeO2 components after either CO catalytic test or calcination. [29-30] . To confirm the above judgement on the catalytic CO oxidation process, constant exists in the core position of the nanowires, and calcination temperature of 250 oC and prolonged element Ce distributes wider which is a typical shell the calcination time to 3 h to realize the complete feature. The absence of Co peaks and the presence transformation of of we kept a Elemental mapping indicates element Co only Co(CO3)0.5(OH)·0.11H2O into Ce peaks in the XPS spectrum of Co3O4. The as-obtained product is labeled as Co3O4@CeO2-1-250 (see Figure S6) further verifies Co3O4@CeO2-1-250. Its corresponding XRD pattern the thick shell coating of CeO2. The catalytic test (Figure shows that Co3O4@CeO2-1-250 can also catalyze 4) shows that the peaks of Co(CO3)0.5(OH)·0.11H2O completely disappear, www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 7Nano Res. 100 % CO conversion at 160 oC (see Figure 6) which reveals that the CeO2 shell coating efficiently is prevented these nanowires from aggregation when the same with that of Co(CO3)0.5(OH)·0.11H2O@CeO2-1-after 10. All these proofs point to the fact calcined. that Co(CO3)0.5(OH)·0.11H2O@CeO2 should go through a intermediate core@shell@shell state of Co(CO3)0.5(OH)·0.11H2O@Co3O4@CeO2, and finally turns into Co3O4@CeO2, as described in Scheme 1. Despite the difference of the core components between Co(CO3)0.5(OH)·0.11H2O@Co3O4-1-after 10 and Co3O4@CeO2-1-250, they show much similar catalytic activities, firmly indicating that only those surface Co3O4 components interfaced with CeO2 do work well for catalytic CO oxidation. Conversion of CO (%) 100 80 Co3O4 @CeO2-1-250 60 Figure 7. (A and B) TEM images of Co3O4 nanowires, and (C 40 and D) Co3O4-CeO2 hybrids. 20 Co3O4 nanowires 50 100 150 200 250 o 300 Temperature ( C) 350 Figure 6. CO conversion curve of Co3O4@CeO2-1-250. However, until now it is still inconclusive whether coating a CeO2 shell could improve the Conversion of CO (%) 100 0 Co3O4-CeO2 mixtures Co3O4-CeO2 hybrids 80 CeO2 NPs 60 40 20 0 catalytic activity and stability against calcination of the Co3O4 nanowires or not, so comparative studies 50 100 150 200 250 o 300 350 Temperature ( C) have been performed towards the following four samples of Co3O4 nanowires, Co3O4-CeO2 mixtures, Co3O4-CeO2 hybrids and pure CeO2 NPs (details see Figure 8. CO conversion curves of Co3O4 nanowires, Experimental Section). As shown in Figure 7 and S7, Co3O4-CeO2 mixtures, Co3O4-CeO2 hybrids and pure CeO2 all of the as-obtained Co3O4 nanowire samples lost NPs. their original wire-like morphology, and most of them aggregated comparison, together into Co3O4@CeO2-1-250 bundles. In showed well-dispersed wire-like core@shell structure. It For Co3O4-CeO2 hybrids, the CeO2 shell coating was fabricated after the calcination process, resulting in irregular CeO2 coated Co3O4 bundles. In www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 8 other words, the CeO2 shell coating should proceed physical and chemical properties of materials[3,33]. before calcined, so Co3O4 nanowires could be Calcination time, calcination atmosphere, especially efficiently prevented from aggregation during the calcination temperature could greatly affect the calcination process, resulting in the remarkably catalytic performance of the catalysts[12,26]. So the improved structure stability of Co3O4 catalysts. The effects of calcination temperature need to be further sizes of Co3O4 NPs in Co3O4 nanowires, Co3O4-CeO2 investigated towards our core@shell catalysts. mixtures and Co3O4-CeO2 hybrids are 9.9, 10.0 and Co(CO3)0.5(OH)·0.11H2O@CeO2-1 precursors were 10.1 nm, respectively (XRD patterns see Figure S8), then calcined at 350 and 500 oC, and thus obtained calculated by the Scherrer equation. Whereas the products are named as Co3O4@CeO2-1-350 and Co3O4 NPs in Co3O4@CeO2-1-250 show a much Co3O4@CeO2-1-500, respectively. smaller size of about 5.9 nm. Obviously, the coating Figure of CeO2 shell leads to much smaller Co3O4 NPs, Co3O4@CeO2-1-500 which might be responsible for the optimization of core@shell the catalytic activity. Co3O4@CeO2-1-250. S9 and S10, As shown in Co3O4@CeO2-1-350 are in structures similar and wire-like compared with In the following, the influence of the CeO2 shell was discussed on the catalytic activity of Co 3O4 catalysts. As shown in Figure 8, Co3O4 nanowires can catalyze 100 % CO conversion at 360 oC. More worse, the CO conversion for pure CeO2 NPs was only 40 % at 350 oC. While Co3O4-CeO2 mixtures and Co3O4-CeO2 hybrids can catalyze 100 % CO conversion at lower temperatures of about 320 and 300 o C, respectively. The enhancement of their catalytic activities could be ascribed to the synergistic effects between Co3O4 and CeO . 2 29-31 However, Co3O4@CeO2-1-250 can catalyze 100 % CO conversion at a much lower temperature of 160 oC. Figure 9. XRD patterns of (A) Co3O4@CeO2-1-250, (B) The optimal catalytic activity of Co 3O4@CeO2-1-250 Co3O4@CeO2-1-350, and (C) Co3O4@CeO2-1-500. compared to Co3O4-CeO2 mixtures and Co3O4-CeO2 hybrids could be ascribed to the fabrication of the XRD patterns in Figure 9 present that the peaks of well-dispersed core@shell structures explained by CeO2 show no difference among Co3O4@CeO2-1-250, the following points: (1) the maximized interface Co3O4@CeO2-1-350 area resulting from the well-dispersed core@shell However, there are some obvious differences of the structure, which is beneficial for CO oxidation; (2) Co3O4 peaks among the three samples. As the the smaller Co3O4 size resulted from the effective calcination temperature was increased from 250 oC CeO2 shell coating of the core@shell structure. This to 350 oC to 500 oC, the intensity of the Co3O4 peaks comparative become stronger, and the peaks become sharper test well supports the above indicating core@shell structures is efficient to optimize the Co3O4@CeO2-1-500 than Co3O4@CeO2-1-350 than catalytic activity and stability of Co3O4 catalysts. Co3O4@CeO2-1-250. The size of the Co3O4 NPs are important to exert a significant impact on the better Co3O4@CeO2-1-500. hypothesis that the fabrication of Co3O4@CeO2 As known, calcination process is fundamentally its and crystallinity of 5.9, 10.1 and 12.7 nm for Co3O4@CeO2-1-250, Co3O4@CeO2-1-350 and Co3O4@CeO2-1-500, www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 9Nano Res. respectively. The XPS spectra of Co3O4@CeO2-1-350 and Co3O4@CeO2-1-500 in Figure S11 both show five Ce peaks and no obvious Co peaks, which are o 403 C similar to Co3O4@CeO2-1-250. It suggests both similar shell coating of CeO2 compared to Co3O4@CeO2-1-250. The N2 adsorption–desorption isotherm of the three samples is depicted in Figure S12, indicating Type IV behavior of nanoporous Co3O4@CeO2-1-250, Co3O4@CeO2-1-350 Intensity (a.u.) Co3O4@CeO2-1-350 and Co3O4@CeO2-1-500 have the o 308 C o 410 C E o × 3 750 C o 316 C o 420 C o ×3 D o 357 C 750 C o 487 C o ×3 C 750 C o 374 C and B Co3O4@CeO2-1-500 with high surface area of 144.9, A o o 380 C 121.4, 64.0 m2g-1 and average pore width of 6.14, 200 750 C 400 600 800 o Temperature ( C) 8.11, 9.79 nm, respectively. The test of catalytic CO oxidation (Figure 10) was then conducted performance to of evaluate the Co3O4@CeO2-1-350 catalytic Figure 11. H2-TPR profiles: (A) pure CeO2; (B) Co3O4 and nanowires; (C) Co3O4@CeO2-1-500; (D) Co3O4@CeO2-1-350 Co3O4@CeO2-1-500 compared with Co3O4@CeO2-250. and (E) Co3O4@CeO2-1-250. The T100 of the three Co3O4@CeO2 samples follows such an order: Co3O4@CeO2-1-250 (160 °C) < for CeO2 can be attributed to the reduction of Co3O4@CeO2-1-350 (250 °C) < Co3O4@CeO2-1-500 (> surface capping oxygen and bulk oxygen of CeO2, 380 °C). Co3O4@CeO2-1-250, which is obtained by respectively[12,32]. The two peaks at around 374 °C calcination at the lowest temperature shows the and 487 °C in Figure 11B could be attributed to the highest catalytic activity. In order to study the two reduction steps of Co3O4 species[1]. It can be synergetic effects of Co3O4 and CeO2, the catalysts seen from Figure 11C-E that the Co3O4 reduction were investigated by H2-TPR. Two broad TPR peaks of Co3O4@CeO2-1-250, Co3O4@CeO2-1-350 and peaks (Figure 11A) observed at 380 °C and 750 °C Co3O4@CeO2-1-500 all shifted towards lower temperature to about 308, 316, 357 oC for the first peak and 403, 410, 420 oC for the second peak, Conversion of CO (%) 100 80 Co3O4@CeO2-1-350 respectively, indicating a typical synergistic effect Co3O4@CeO2-1-500 between Co3O4 and CeO2. The previous work reported that the a lower calcination temperature 60 favors reducing the degree of Co3O4@CeO2 interface breakage that improves the oxidizability of Co3O4.26 40 That is why the oxidizability of Co3O4 in these 20 samples 0 follows Co3O4@CeO2-1-250 50 100 150 200 250o 300 Temperature ( C) 350 such > a sequence Co3O4@CeO2-1-350 that > Co3O4@CeO2-1-500, which is in agreement with the changing trends of their catalytic activities. If we Figure 10. CO conversion curves of Co3O4@CeO2-1-350 and enlarge the curve in the temperature range of Co3O4@CeO2-1-500. 650 °C to 900 °C for three times, the signal at 750 °C for CeO2 could be still clearly seen, indicating the existence of bulk oxygen of CeO2. Based on the www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 10 above results and discussions, it is concluded that Co3O4@CeO2-1-250, whereas the intensity ratio of the optimal catalytic activity of Co3O4@CeO2-1-250 CeO2 to Co3O4 peaks is decreased with the decreased can be ascribed to the following reasons. (1) better thickness of CeO2 shell. oxidizability of Co3O4, which might be caused by the lower degree of Co3O4@CeO2 interface breakage 100 Conversion of CO (%) resulting from the lower calcination temperature[26]; (2) the smaller sized Co3O4 NPs than the other two samples; (3) the bigger BET surface area than the other two samples[34]; (4) worse crystallinity of Co3O4@CeO2-1-250 that might bring more surface defects and thus higher surface energy, which are in 80 Co3O4@CeO2-2-250 60 Co3O4@CeO2-3-250 Co3O4@CeO2-4-250 40 20 favor of the CO adsorption, resulting in the optimal 0 catalytic activity for CO oxidation . [3] 50 100 150 Besides calcination temperatures, the component ratio of hetero-catalysts also play a significant role in the catalytic performance[32]. So by simply varying 200 250 300 o 350 Temperature ( C) Figure 12. CO conversion curves of Co3O4@CeO2-2-250, Co3O4@CeO2-3-250, and Co3O4@CeO2-4-250. the amount of Ce(NO3)3, a series of Co3O4@CeO2 core@shell nanowires have been synthesized to investigate the effects of component ratio between 270 Co3O4 and CeO2 on their catalytic activities. The Co3O4@CeO2-2-250, Co3O4@CeO2-4-250 are named Co3O4@CeO2-3-250, (experimental details as 240 and o samples T ( C) corresponding see Experimental Section). As shown in Figure S13 and 210 180 S14, the three comparative samples show similar core@shell wire-like structure to Co3O4@CeO2-1-250, 150 except for the CeO2 shell thickness. The average 0 10 diameters of Co3O4@CeO2 core@shell nanowires, estimated by the size distribution data, are 120, 95, 65, and 53 nm for Co3O4@CeO2-2-250, Co3O4@CeO2-1-250, Co3O4@CeO2-3-250, respectively, and indicating that the ICP-MS. As shown in Table S1, the Ce molar contents are 50.3 %, 38.5 %, 18.2 % and 8.9 % for Co3O4@CeO2-2-250, Co3O4@CeO2-1-250, and Co3O4@CeO2-4-250, respectively. Figure S15 presents the XRD patterns of Co3O4@CeO2-3-250, and Co3O4@CeO2-4-250 that he peak positions and shapes of the three samples 40 50 60 Figure 13. The relationship of Ce molar contents and the catalytic activities of the Co3O4@CeO2 samples. corresponding thinner. The Co and Ce contents were determined by Co3O4@CeO2-2-250, 30 Co3O4@CeO2-4-250 average CeO2 shell thicknesses become thinner and Co3O4@CeO2-3-250, 20 Ce Content (mol %) are the same with The catalytic performance on CO oxidation of the three comparative samples were evaluated and the results are shown in Figure 12. The catalytic activity of the samples follows Co3O4@CeO2-1-250 (160 (170 o C) > such a order: C) > Co3O4@CeO2-3-250 o Co3O4@CeO2-2-250 (240 o C) > Co3O4@CeO2-4-250 (270 C). Figure 13 presents the o relationship between T100 and the Ce molar content of the Co3O4@CeO2 core@shell samples. First, from Co3O4@CeO2-4-250 to Co3O4@CeO2-3-250 and then www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 11 to Co3O4@CeO2-1-250, the catalytic activities were Table 1. Characteristics of the as-obtained samples and their enhanced with increasing Ce contents. However the catalytic performance for CO oxidation. catalytic activity decreased in the case of Co3O4@CeO2-2-250 while further increasing the Ce Sample content. This suggests that the catalytic activities of T o ( C) Size [a] (nm) T100 [b] (oC) our Co3O4@CeO2 samples are highly dependent on Co3O4@CeO2-1-250 250 5.9 160 the Ce molar content, and tuning component ratio Co3O4@CeO2-1-350 350 10.1 250 of this kind of hetero-catalysts would be an efficient Co3O4@CeO2-1-500 500 12.7 >380 way to optimize the catalytic performance. Co3O4 nanowires 250 9.9 360 Co3O4-CeO2 mixtures 250 10.0 310 cubes were prepared by the similar self-assembly Co3O4-CeO2 hybrids 250 10.1 300 process, however, the utilization of Co2+ was as low Pure CeO2 NPs 250 - >350 as about 10 %, which limited their practical Co3O4@CeO2-2-250 250 5.9 240 applications. Here, the utilization of Co2+ in the Co3O4@CeO2-3-250 250 5.9 170 preparation process of Co3O4@CeO2 core@shell Co3O4@CeO2-4-250 250 5.9 270 In our previous work, Co3O4@CeO2 core@shell nanowires has been increased to about 80 %. About [a] Calcination temperature 2 g of Co(CO3)0.5(OH)·0.11H2O nanowires could be [b] Average size of Co3O4 NPs calculated from XRD patterns synthesized by a 500 mL Teflon-lined stainless steel autoclave. Then more than 3 g of Co(CO3)0.5(OH)·0.11H2O@CeO2-1 could be obtained 4 Conclusions by the self-assembly process by a 1 L flask. After calcination, more than 2.5 g of Co3O4@CeO2-1-250 In summary, we have successfully realized the were obtained. In Figure S16, it can be clearly seen facile, clean and mass production of Co3O4@CeO2 that produced core@shell nanowires as catalyst for catalytic CO Co3O4@CeO2-1-250 oxidation. The catalytic performance of the samples the above mentioned Co(CO3)0.5(OH)·0.11H2O mass and show no change in their wire-like structures. has Meanwhile Co3O4@CeO2-1-250 obtained by mass experiments suggest that CO oxidation is apt to production can catalyze 100 % CO conversion at the take place at the interface between Co3O4 and CeO2 same temperature of 160 oC as well, which is more components. The high catalytic activity and stability active Co3O4 of Co3O4@CeO2 core@shell nanowires should be CeO2@Cu2O possibly caused by the optimal synergistic effects of than nanowires, the previously CeO2 reported nanorods, been investigated systematically. Control hollow Co3O4 and CeO2 components resulting from the nanotubes, specific core@shell structure. It also suggests that Co3O4@CeO2 cubes and ZnCo2O4@CeO2 spheres due the catalytic activity of the Co3O4@CeO2 core@shell to the lower conversion temperature or with lower nanowires strongly depends on the calcination weight of effective catalysts (see Table S1) [32, 34–40]. temperatures and component ratio between Co3O4 Hence and nanocomposite, microsphere, CeO2-ZnO Ce-Mn composite binary it is reasonably oxide considered that our CeO2. Co3O4@CeO2-1-250 obtained by Co3O4@CeO2 core@shell nanowires with good calcination at 250 oC for 3 h with a Ce molar content catalytic activity and stability for CO oxidation about 38.5 % shows the best catalytic activity, might have great potential for practical application attaining 100 % CO conversion at a temperature as as automobile exhaust gas treatment catalysts. low as 160 oC. 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