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Hollow and Core-Shell Nano-Structured Co3O4 Derived from Metal Formate Framework towards High Catalytic Activity of CO Oxidation Chi Zhang, Li Zhang, Guancheng Xu, Xin Ma, Jinling Xu, Lu Zhang, Chunling Qi, Yangyang Xie, Zhipeng Sun, and Dianzeng Jia ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00246 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Nano Materials

Hollow and Core-Shell Nano-Structured Co3O4 Derived from Metal Formate Framework towards High Catalytic Activity of CO Oxidation Chi Zhang,a,b,d Li Zhang,* a,b,c Guancheng Xu, a,b,d Xin Ma,a,b,d Jinling Xu,a,b,d Lu Zhang,a,b,d Chunling Qi,a,b,d Yangyang Xie,a,b,d Zhipeng Suna,b,d and Dianzeng Jia*a,b,d

a Key Laboratory of Energy Materials Chemistry (Xinjiang University), Ministry of Education, Urumqi, Xinjiang, 830046, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-991-8580586; Tel: +86-991-8580586 b Key Laboratory of Advanced Functional Materials, Autonomous Region, Urumqi, Xinjiang, 830046, P. R. China c Physics and Chemistry Detecting Center, Xinjiang University, Urumqi, Xinjiang, 830046, P. R. China d Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang, 830046, P. R. China

*Corresponding author. +86-991-8580586

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[email protected],

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Abstract Hollow and core-shell particles are currently attracting global attention due to special properties. We herein proposed a facile self-sacrificial template strategy for the synthesis of Co3O4 with hollow and core-shell nano-structures. Starting with metal formate framework (MFF) template [CH3NH3][Co(HCOO)3], series of core-shell nano-structure MFF@Co(OH)2 microboxes and hollow Co(OH)2-H were successfully obtained through the reaction between MFF template and NaOH combined with different washing processes. Finally, hollow and core-shell nano-structure Co3O4 products were obtained after calcination, which inherited the structures from corresponding precursors. In addition, the formation processes of hollow and core-shell nano-structures were studied. Moreover, the catalytic activity of the as-obtained Co3O4 for CO oxidation was investigated. Such hollow and core-shell nano-structures gave different physiochemical properties. Hollow structure Co3O4 exhibited higher catalytic activity than that of core-shell nano-structure Co3O4, which reached 100% CO conversion at 90 °C. Furthermore, core-shell nano-structure Co3O4 had higher long-term catalytic stability than that of hollow structure Co3O4. We explored the relationships between different structures and catalytic properties. KEYWORDS: Metal formate framework, CO oxidation, Catalysis, Co3O4, Hollow structure, Core-shell nano-structure 1. Introduction Catalytic oxidation of CO has attracted considerable attention because of serious health issues associated with CO exposure.1 Hollow and core-shell structure catalysts have been highlighted recently due to excellent catalytic activities for CO oxidation. Such materials have large surface areas, low densities and special structural characteristics,2 so considerable efforts have been devoted to their fabrication over the past decades. For example, Zheng et al. synthesized hollow Co3O4 polyhedrons through direct decomposition of ZnO@ZIF-67 polyhedrons in air, which reached 100% CO conversion at 120 °C.3 Yoo et al. synthesized hollow manganite-coated silica microspheres, which exhibited excellent catalytic performance for CO oxidation.4 These well-defined hollow and core-shell structures are potentially applicable to various fields. Although catalysts with different structures have been studied, it is still complicated to prepare materials with precisely controlled hollow and core-shell structures. Among a variety of synthetic methods, the template method has been most widely used because of clear steps.5 The template method for fabricating hollow and core-shell structures is based on the Kirkendall effect,6 chemical etching,7 ionic exchange,8 self-assembly9 or thermal decomposition.10 Several materials, such as zeolites,11 mesoporous silicates,12 colloidal particles13 and metal oxides,14 have been used as flexible templates. Recently, metal organic frameworks (MOFs) have been investigated as new sacrificial templates for fabricating various hollow and core-shell particles. Due to highly ordered structures with abundant organic ligands and regular arrangement of metal nodes in MOFs, they can be used as templates and precursors to prepare metals, metal oxides, metal sulfides and other nanostructured metal-based materials.15 However, selecting an appropriate MOF template is a prerequisite for the synthesis of a desired structure, which should be easily prepared to simplify the whole preparation process and easily removed while retaining the original stereoscopic shape. More importantly, the high reactivity of some MOFs allows the fabrication of new materials. In this study, we used metal formate framework (MFF), a relatively small but crucial class of MOFs, as the self-sacrificial template.16 Perovskite porous MFFs with high reactivity can be easily


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prepared and conveniently removed by using water.17 Recently, our group has successfully synthesized hollow materials using MFFs as templates. For example, uniform hierarchical hollow NiS cubes were facilely fabricated through the anion exchange reaction of a nickel formate framework template [CH3NH3][Ni(HCOO)3] with S2-.18 Hollow Mn3O4 microcubes can also be synthesized from a manganese formate framework [CH3NH3][Mn(HCOO)3] through the ion exchange reaction in alkaline solution under hydrothermal condition.19 Therefore, MFFs may be appropriate templates for fabricating new materials with novel morphologies. Herein, we employed a general approach for the facile synthesis of hollow and core-shell Co3O4 products. Hollow and core-shell structure precursors were initially prepared by accurately controlling the reaction between MFF template [CH3NH3][Co(HCOO)3] and NaOH, and then washing processes. Afterwards, hollow and core-shell Co3O4 products were obtained through the pyrolysis of corresponding precursors. The formation mechanism for hollow and core-shell structures and the catalytic performance of Co3O4 products were studied. The results provide a simple route for the synthesis of hollow and core-shell structures and shed some light on the fabrication of other hollow structures. 2. Experimental details All chemical reagents were purchased commercially and used without further purification. 2.1 Synthesis of [CH3NH3][Co(HCOO)3] Co(NO3)2•6H2O (0.2910 g) and PVP-K30 (0.5 g) were dissolved in 25 mL of ethanol in a beaker. Meanwhile, 0.3623 g of methylamine in methanol (CH3NH2, 30%), 0.2337 g of anhydrous formic acid (HCOOH, 98.5%) and 0.5 g of PVP-K30 were dissolved in another 25 mL of ethanol. Then the cobalt nitrate solution was dropped into the anhydrous formic acid solution. The mixture was stirred at room temperature for 1 h and aged for 24 h. The resulting pink precipitates were collected by centrifugation-redispersion cycles to remove any possible residual reactants, and finally dried under vacuum at 40°C for 5 h. 2.2 Synthesis of hollow and core-shell precursors [CH3NH3][Co(HCOO)3] (0.1808 g) and PEG 2000 (0.5 g) were dispersed in 10 mL of ethanol in a beaker. Meanwhile, 0.032 g of NaOH and 0.5 g PEG 2000 were dispersed in 10 mL of ethanol in another beaker. Then the [CH3NH3][Co(HCOO)3] suspension was dropped into the NaOH suspension. The mixture was stirred at room temperature for 5 h, and the as-prepared products were treated by four washing methods. After washing with ethanol several times, core-shell structure MFF@Co(OH)2 was obtained and dried under vacuum at 40°C for 5 h. MFF@Co(OH)2-2 and MFF@Co(OH)2-4 were obtained by washing MFF@Co(OH)2 with H2O (2 mL)-C2H5OH (6 mL) solution twice and four times, respectively. Hollow structure Co(OH)2-H was obtained by washing MFF@Co(OH)2 with H2O several times. 2.3 Synthesis of hollow and core-shell Co3O4 Calcining MFF@Co(OH)2, MFF@Co(OH)2-2, MFF@Co(OH)2-4 and Co(OH)2-H at 350°C for 2 h with a heating rate of 1 K min-1 gave core-shell Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and hollow Co3O4-H respectively. 2.4 Material characterizations



Scanning electron microscopy (SEM) images were captured using a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images were obtained with a Hitachi H600 microscope. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 advance diffractometer at 40 kV and 40 mA using Cu Kα radiation (λ = 0.15405 nm), with a step size of 0.02° in 2θ. FT-IR spectrum was recorded on a Bruker Vertex 70 spectrophotometer. Raman spectrum were measured by a Bruker Senterra R200-L spectrometer (532 nm, Ar+ ion laser ). The surface area and N2 adsorption-desorption isotherm were measured on a Micromeritics ASAP 2020 analyzer at 77 K. Prior to measurement, the sample was degassed at 120°C for 6 h in the vacuum line. X-ray photoelectron spectroscopy (XPS) was performed on Escalab 250 Xi (Thermo Fisher Scientific). H2 temperature-programmed reduction (H2-TPR) analysis was performed by using a Micromeritics Chemisorb 2920 apparatus. For each analysis, an accurate amount of calcined sample (60~65 mg) was purged in a flow of pure argon at 200°C for 120 min to remove trace water (heating rate 10 °C/min). After cooling to room temperature, H2-TPR experiments were performed using a 10 vol% H2/Ar mixture at a flow rate of 50 mL/min. The sample was heated from ambient temperature to 800°C at a heating rate of 10 °C/min and H2 consumption was detected by a thermal conductivity detector. 2.5 Catalytic performance test Catalysts (50 mg) were put in a quartz glass reaction tube without any pre-treatment. The reaction temperature was monitored by a thermocouple placed in the middle of the catalyst bed. A mixture of 1 vol% CO and 16 vol% O2 balanced by N2 was introduced as the reactants. The total flow rate was 50 mL min−1, corresponding to a space velocity of 60000 mL g−1 h−1. Online analysis of CO and CO2 was performed at each temperature after a stabilization time of about 20 min with Agilent GC7890 gas chromatograph. CO conversion was calculated based on the change in CO concentrations of the inlet and outlet gases as follow:

 CO OUT RCO = 1 −  CO IN 

  ×100%  

3. Results and discussion The template method for the synthesis of hollow and core-shell Co3O4 products is illustrated in Scheme 1. Cubic [CH3NH3][Co(HCOO)3] was employed as the template. Well-defined hollow and core-shell precursors were synthesized through room-temperature reaction of MFF template with NaOH in ethanol and subsequent washing treatment. Then hollow and core-shell structure Co3O4 products were easily obtained by calcinating corresponding precursors.


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Scheme 1 Schematic illustration for the formation of hollow and core-shell Co3O4 products. Firstly, uniform microcubes of [CH3NH3][Co(HCOO)3] were synthesized by a modified method using ethanol as the solvent and reducing the concentration of reactants.20 SEM image (Fig. 1a) reveals that [CH3NH3][Co(HCOO)3] microcubes are sized 2-4 µm, with smooth facets and high dispersibility. PXRD analysis (Fig. 1b) illustrates that all the diffraction peaks of microcubes can be assigned to crystalline [CH3NH3][Co(HCOO)3], confirming high crystallinity and phase purity.

Fig. 1 (a) SEM image of [CH3NH3][Co(HCOO)3]. (b) Experimental and simulated PXRD patterns of [CH3NH3][Co(HCOO)3]. TEM images of (c) MFF@Co(OH)2, (d) Co(OH)2-H (inset shows the details of Co(OH)2-H at high magnification). Afterwards, [CH3NH3][Co(HCOO)3] microcubes with high morphological uniformity and phase purity were applied as the template. Utilizing the chemical reactivity of [CH3NH3][Co(HCOO)3] and NaOH, core-shell structure MFF@Co(OH)2 were prepared by accurately controlling the [CH3NH3][Co(HCOO)3]/NaOH ratio. The reaction can be described below: [CH3NH3][Co(HCOO)3](s) + 3NaOH(aq.) → Co(OH)2(s) + CH3NH2(aq.) + 3NaCOOH(aq.) + H2O In the above equation, the theoretical ratio of [CH3NH3][Co(HCOO)3]:NaOH is 1:3, while the actual ratio was selected as 1:1, aiming to fabricate hollow and core-shell structure as well as maintain the cubic shape. TEM image (Fig. 1c) presents that MFF@Co(OH)2 turns to a core-shell structure cubic, indicating the reaction firstly occurred on the surface of microcubes and



[CH3NH3][Co(HCOO)3] was retained at the center. The shell thickness was about 60 nm. Moreover, there were voids between shells and cores at corners, because the chemical reaction at corners consumed more reactants than that on planes did. FT-IR spectrum (Fig. 2a) of MFF@Co(OH)2 demonstrate the chemical reaction between [CH3NH3][Co(HCOO)3] and NaOH. There is a broad band from 3600 to 2500 cm-1, corresponding to the O–H stretching of OH groups. The characteristic peaks of [CH3NH3][Co(HCOO)3] at 1770-800 cm-1 can also be found in MFF@Co(OH)2, suggesting that MFF template [CH3NH3][Co(HCOO)3] still existed after reacting with NaOH. When the ratio of [CH3NH3][Co(HCOO)3]:NaOH was 1:3, the final product after washing with water turns to solid structure (Fig. S1). So the strategy for preparing core-shell structure with MFF template was successful. Interestingly, as [CH3NH3][Co(HCOO)3] is soluble in water, the core of MFF@Co(OH)2 can be thoroughly removed using water, leaving hollow Co(OH)2-H shell alone, as evidenced by the TEM (Fig. 1d) and SEM images (Fig. S2). SEM image of Co(OH)2-H (Fig. S2a) shows that most particles maintain the cube shape, and the hollow structure can also be observed from the cracked microboxes (Fig. S2b). The shell thickness was about 60 nm (inset of Fig. 1d). In addition, the shell thickness can be controlled by adjusting the ratio of [CH3NH3][Co(HCOO)3]:NaOH. As shown in Fig. S3, the shell thickness of the hollow structure product increased to 200 nm when the ratio of [CH3NH3][Co(HCOO)3] to NaOH was 1:2. To explore the chemical composition of Co(OH)2-H, PXRD analysis was conducted. Since the characteristic peaks of Co(OH)2 do not exist in the PXRD pattern of the as-synthesized sample (Fig. S4), the obtained Co(OH)2-H had an amorphous nature. FT-IR and Raman spectra were carried out to further determine the composition of Co(OH)2-H. From FT-IR spectroscopy (Fig. 2a) we can see a new band at 526 cm-1 represented Co-O stretching vibrations and a broad peak centered at 3441 cm-1 corresponded to the O–H vibration of hydroxyl groups, conforming the formation of Co(OH)2. Moreover, Raman spectra of the Co(OH)2-H (Fig. 2b) exhibits features typical of Co(OH)2, such as the strong band at 523 cm−1 due to vibrations of the Co−O (Ag) symmetric stretching mode, the intense band at 457 cm-1 assigned to an OCoO bending mode and a band in low intensity at 1042 cm-1 attributed to the OH deformation modes.21 The composition of Co(OH)2-H was also verified by XPS. Co 2p peaks of Co(OH)2-H (Fig. 2c) are centered at 780.9, 785.7, 796.8 and 802.5 eV, which match well with those reported before.21,22

Fig. 2 (a) FT-IR spectra of [CH3NH3][Co(HCOO)3], MFF@Co(OH)2, MFF@Co(OH)2-2, MFF@Co(OH)2-4 and Co(OH)2-H. (b) Raman spectrum of Co(OH)2-H and (c) XPS spectrum of Co 2p. On the basis of the above experimental results, we prepared MFF@Co(OH)2-2 and MFF@Co(OH)2-4 by controlling the dissolution of the core with mixed solvent H2O-C2H5OH. By controlling the number of washing times, differently dissolved MFF core yielded core-shell


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structure MFF@Co(OH)2-2 and MFF@Co(OH)2-4 with different sizes of cores. As presented in Fig. 3a and b, MFF@Co(OH)2-2 and MFF@Co(OH)2-4 have core/shell structures comprising black cores and grey shells. Compared with the TEM image of MFF@Co(OH)2, the core size decreased with increasing number of washing times. FT-IR spectra of MFF@Co(OH)2-2 and MFF@Co(OH)2-4 (Fig. 2a) are similar to that of MFF@Co(OH), which also confirm the coexistence of Co(OH)2 and [CH3NH3][Co(HCOO)3]. The results verified the feasibility of this facile approach.

Fig. 3 TEM images of (a) MFF@Co(OH)2-2 and (b) MFF@Co(OH)2-4. Finally, a series of hollow and core-shell structure Co3O4 were easily prepared by calcining corresponding hollow and core-shell strucuture precursors. The PXRD results of calcined products are shown in Fig. 4a. All the diffraction peaks of the four samples can be assigned to the standard card (JCPDS No. 43-1003), proving the formation of Co3O4. Moreover, the core of core-shell structure products come from direct calcination of the [CH3NH3][Co(HCOO)3]. In our previous work, we have obtained Co3O4 by one-step pyrolysis of [CH3NH3][Co(HCOO)3]. 20 The result shows all the diffraction peaks of the obtained Co3O4 also can be assigned to the Co3O4 standard card (JCPDS No. 43-1003). Therefore, these results prove the core material and the shell material have the same phase. N2 adsorption-desorption isotherm curves (Fig. 4b) exhibit type-IV adsorption isotherms with clear hysteresis loops, indicating the presence of mesoporous structure. The BET surface areas and pore volumes of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H are listed in Table 1. With decreasing core size, the BET surface areas of Co3O4-CS, Co3O4-CS-2 and Co3O4-CS-4 increased from 15.9 to 40.6 m2 g-1. When the cores disappeared, Co3O4-H had the largest specific surface area of 56.1 m2 g-1. The BJH pore size distribution curves of Co3O4-CS, Co3O4-CS-2 and Co3O4-CS-4 (Fig. S5a-c) have wide size distributions, but Co3O4-H (Fig. S5d) shows an aggregate pore size distribution at around 5 nm.



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Fig. 4 (a) PXRD patterns and (b) nitrogen adsorption and desorption isotherms of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H. Table 1 Specific surface areas and pore volumes of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H. Sample

BET area (m2 g-1)

Pore volume (cm3 g-1)

Co3O4-H

56.1

0.11

Co3O4-CS-4

40.6

0.10

Co3O4-CS-2

39.1

0.11

Co3O4-CS

15.9

0.04

SEM images of Co3O4-CS, Co3O4-CS-2 and Co3O4-CS-4 (Fig. 5a-c) exhibit that all Co3O4 products inherit the morphologies from corresponding precursors. All the cores of core/shell Co3O4 were porous and composed of small particles sized around 50 nm. TEM images (Fig. 5e-g) further show porous and hollow structures in which there are obvious gaps between inner cores and outer shells. SEM image of a fragmented microbox of Co3O4-H (Fig. 5d) presents a well-defined cubic shape with hollow structure. TEM image (Fig. 5h) shows the shell of Co3O4-H is porous and composed of numerous small nanoparticles sized about 10 nm (inset of Fig. 5h). High-resolution transmission electron microscopy (HR-TEM) images are shown in Fig. S6. The obtained cobalt oxides with crystalline walls exhibited well defined lattice fringes. The d-spacing values are measured to be 0.28 and 0.24 nm, corresponding to {220} and {311} planes of the Co3O4 spinel structure, respectively.

Fig. 5 (a, b, c, d) SEM images and (e, f, g, h) TEM images of Co3O4-CS (a, e), Co3O4-CS-2 (b, f), Co3O4-CS-4 (c, g) and Co3O4-H (d, h). Higher magnification TEM image of Co3O4-H is shown in the inset. The catalytic activities of the as-obtained Co3O4 catalysts for CO oxidation were investigated. The reactions were carried out in a fixed-bed flow reactor with 50 mg of catalyst and a mixed gas flow consisting of 1% CO, 16% O2 and 83% N2. T50 and T100 (CO conversions reached 50% and


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100%) were used to evaluate the catalytic activity. Fig. 6a shows the typical CO conversion profiles as a function of temperature. The catalytic activities of the samples followed a descending sequence: Co3O4-H (T50: 70°C) > Co3O4-CS-4 (T50: 106°C) > Co3O4-CS-2 (T50: 112°C) > Co3O4-CS (T50: 157°C). T100 values of Co3O4-H, Co3O4-CS-4, Co3O4-CS-2 and Co3O4-CS were 90°C, 130°C, 130°C and 180°C. We calculated the reaction rate of Co3O4-H, Co3O4-CS， Co3O4-CS-2，Co3O4-CS-4 according to the CO conversion at 90 oC and BET areas, which are 0.0080, 0.0017, 0.0019, 0.0032 mmol min-1 m-2, respectively. Co3O4-H showed the highest catalytic activity. Moreover, compared to other high-activity Co-based catalysts reported previously, Co3O4-H catalyst had a faster catalytic reaction rate under the same temperature (Table S1). 23-27 The outstanding catalytic performance of Co3O4-H can be attributed to high surface area, tiny particles and aggregate pore size distribution.

Fig. 6 (a) CO conversion versus temperature curves of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H. (b) Variation of CO conversion with time over Co3O4-H and Co3O4-CS-4. (c-f) SEM images of Co3O4-H (c, d) and Co3O4-CS-4 (e, f) after long-time cycling. As Co3O4 catalysts may suffer from deactivation after long-time reaction, the catalytic stabilities of Co3O4-H and Co3O4-CS-4 were evaluated. In order to observe the stability of the catalysts under the corresponding extreme conditions, we choose to investigate the stability of Co3O4-H and Co3O4-CS-4 at different temperatures 90 and 130 oC, respectively. As shown in Fig. 6b, Co3O4-H stably realizes 100% CO conversion during the first 16 h at 90°C. However, the catalytic performance decreased in the next three hours. In contrast, the CO conversion of Co3O4-CS-4 hardly dropped even after 30 h at 130°C. The differences may be attributed to the different stabilities of hollow and core/shell structures. To assess the effect of structural stability on catalytic stability, SEM was used to observe the structural changes of the two catalysts after stability test. As displayed in Fig. 6c and d, after long-time cycling, Co3O4-H collapses into sheets with a loose structure. However, Co3O4-CS-4 (Fig. 6e-f) retained the original core structure even



after 30 h of cycling. The decreased activity of Co3O4-H catalyst may be ascribed to structure collapse. To validate this postulation, we tested the catalytic CO oxidation of milled Co3O4-H which was prepared by grinding fresh Co3O4-H in agate mortar. As evidenced by SEM images (Fig. S7a-b), various sizes of milled Co3O4-H granules accumulate irregularly, with T100 of 130°C (Fig. S7c). Thus, the catalytic activity of milled Co3O4-H was lower than that of fresh Co3O4-H, suggesting that structure collapse indeed attenuated the catalytic activity of the former. The cores in core/shell structure may protect shells from collapse through buffering, thereby maintaining the catalytic performance of such structure. Catalysts with higher activity against oxidation reaction generally have high reducibility. Therefore, H2-TPR measurements were performed to investigate the difference between the redox properties of core/shell and hollow Co3O4 products (Fig. 7). All the four Co3O4 products present two reduction signals of which the signal at lower temperature is attributed to reduction from Co3O4 to CoO and the other one at higher temperature belongs to reduction from CoO to Co0.28 Given that Co3O4-H shows the lowest reduction temperature, the oxygen therein is most active, being associated with a more open structure surface. In addition, the curve of Co3O4-H presents two non-overlapping reduction peaks, whereas those of other three core-shell Co3O4 products present peak overlapping, because the former consisted of tiny particles but the latter three comprised larger particles.29

Fig. 7 H2-TPR profiles of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H. Several possible mechanisms for CO oxidation reactions have been proposed hitherto. Presumably, the exposed plane and position of Co3+ cations predominantly determine the catalytic activity of nanocrystals. In the presence of CO and O2, CO is adsorbed on the surface-exposed Co3+ sites of Co3O4, then producing CO2 by binding the surface oxygen that may be coordinated with Co3+ cations. The resulting oxygen vacancy is subsequently filled with adsorbed O2.30, 31 Therefore, Co3+ and oxygen vacancy are active sites for CO oxidation reactions. To evaluate the effects of active sites on the catalytic performance, XPS was performed to determine the oxidation states of elements at surface of core-shell and hollow structure Co3O4 products. The Co 2p3/2 and Co 2p1/2 peak positions are located at around ~779.8 and ~794.7 eV respectively (Fig. 8a), being consistent with the spectra of previously reported Co3O4 satellite regions.32 The deconvolution of Co 2p provides additional evidence that there are two kinds of Co species present on the surface of the four Co3O4 products. Co3+ is characterized by Co 2p peaks at the binding energies of ∼779.9 and 795 eV, while the peaks at ∼781.3 and 796.6 eV correspond to Co2+.33,34 The Co2+ / Co3+ ratios of Co3O4-H Co3O4-CS, Co3O4-CS-2, and Co3O4-CS-4 were calculated to be 0.64, 0.45, 0.61 and 0.46, respectively. Different surface oxygen species have been identified by the deconvoluted O 1s spectrum (Fig. 8b). The OL component of O 1s spectra, which is centered at 530±0.2 eV, is


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lattice oxygen in the Co3O4 phase. The OV component centered at 531±0.1 eV represents oxygen vacancies, which is associated with O2- ions in oxygen-deficient regions within the Co3O4 matrix. OC component is usually attributed to chemisorbed and dissociated oxygen species (O2-, O2-, or O-) and OH- component, which is centered at around 532±0.2 eV.35, 36 The peak area percentages of the two kinds of cobalt ions and three oxygen components are listed in Table 2. Co3O4-H had an intermediate amount of Co3+ (44.6%) and maximum oxygen vacancies (35.6%) on the surface. The surface oxygen vacancies promoted the diffusion of lattice oxygen from the bulk to the surface and the adsorption−desorption process of gas-phase species,37 thereby enhancing the catalytic performance. To summarize, Co3O4-H with a large amount of oxygen vacancies on the surface was more effective for O2 adsorption and activation, and considerable Co3+ provided more reactive sites for CO oxidation. When the excellent chemisorption ability for oxygen species was combined with a high specific surface area, the catalytic capacity of Co3O4-H was inevitably augmented markedly.

Fig. 8 XPS spectra of (a) Co 2p and (b) O 1s for Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H. Table 2 Area percentages of different elemental components obtained from XPS. Relative percentage (%) Sample

Co 2p3/2 2+

Co3O4-H Co3O4-CS Co3O4-CS-2 Co3O4-CS-4

Co 28.4 22.1 26.7 22.3

O 1s 3+

Co 44.6 49.5 43.5 48.6

OL 40.5 38.3 45.1 51.4

OV 35.6 28.5 20.8 25.1

OC 23.9 33.2 34.1 23.4

Conclusions In summary, hollow and core-shell structure Co3O4 were synthesized successfully by a MFF-derived approach, following the mechanism of controlled reaction and dissolution of template. The hollow Co3O4 shell was composed of tiny particles, which had a high surface area which provided abundant active sites beneficial to the catalytic activity. However, core/shell Co3O4 exhibited high long-term stability because Co3O4 cores provided supporting points while Co3O4 shell underwent the reaction without collapse. Additionally, such MFF template-involved approach is potentially applicable to the fabrication of hollow and core-shell particles with various



shapes. This synthetic strategy can be extended to MFFs, as a feasible supplement to the sacrificial template method.

Supporting Information TEM images of the MFF reacted with NaOH with various ratio after water washing, SEM images and XRD pattern of Co(OH)2-H, the pore-size distribution and HR-TEM images of of Co3O4-CS, Co3O4-CS-2, Co3O4-CS-4 and Co3O4-H, SEM images and CO conversion versus temperature curve of milled Co3O4-H and catalytic activity of CO oxidation and the reaction setups of cobalt oxide reported in the open literature. Acknowledgements This work was financially supported by Key Laboratory Open Research Foundation of Xinjiang Autonomous Region (No. 2016D03008), National Natural Science Foundation of China (No. 21661029, 21663029) and Graduate Research Innovation Project of Xinjiang (XJGRI2017003). Notes and references

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Hollow and CoreâShell Nanostructure Co3O4 ... - ACS Publications

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