NiCo2O4 heterostructures

Journal of Colloid and Interface Science 526 (2018) 295–301

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Hollow urchin-like NiO/NiCo2O4 heterostructures as highly efficient catalysts for selective oxidation of styrene Jiangyong Liu a,⇑, Tingting Chen a, Panming Jian a, Lixia Wang a, Xiaodong Yan b a b

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 24 February 2018 Revised 27 April 2018 Accepted 1 May 2018 Available online 3 May 2018 Keywords: Hierarchical Hollow NiO NiCo2O4 Styrene oxidation

a b s t r a c t Three-dimensional (3D) hierarchical hollow urchin-like NiO/NiCo2O4 heterostructures have been prepared via a facile one-pot hydrothermal method. The 3D urchin-like structure brings about high specific surface area of 40.2 m2 g 1. The NiO/NiCo2O4 heterostructures are composed of 59 wt% of NiO and 41 wt% of NiCo2O4 and enriched with NiO-NiCo2O4 phase boundaries. When used as catalysts for styrene oxidation reaction (SOR), the NiO/NiCo2O4 heterostructures present a markedly high selectivity of 90.8% to styrene oxide (SO) and a high SO yield of 81.4%. The high catalytic performance of the NiO/NiCo2O4 heterostructures can be attributed to the high specific surface area and the abundant NiO-NiCo2O4 phase boundaries, both of which contribute to the numerous active sites. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction Transition metal oxides such as Co3O4 [1,2], NiO [3–5], CoO [6], Fe2O3 [7] and NiCo2O4 [8,9] have gained extensive research interests due to their wide applications in the fields of energy-storage, electrics, optics and catalysis [10]. Size, morphology and microstructure of these solid materials have remarkable effect on their properties and ultimate performances in applications, as the ⇑ Corresponding author. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.jcis.2018.05.001 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

differences in size, morphology and microstructure may result in different electronic and surface properties [1,11–15]. In heterogeneous catalysis, the reaction kinetics is highly dependent on the electronic properties and surface area and porosity of the catalysts [1,13–18]. Therefore, catalyst materials with controlled size, morphology and microstructure have been intensively studied [19– 23]. For instance, cerium-doped cobalt ferrite nanocrystals exhibited markedly enhanced conversion and selectivity toward SOR [21]. Architectural design offers another option to tune the catalytic performance. For example, 3D dandelion-like NiCo2O4 presents larger specific surface area than 3D flower-like NiCo2O4,

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and thus the former exhibits a much higher catalytic activity for the oxygen reduction reaction [24]. SOR is playing an important role in the manufacturing of fine chemicals and pharmaceuticals since its products SO and benzaldehyde are important raw materials for the synthesis of medicines and perfumes [25]. However, corrosive, expensive and hazardous organic peracids as the oxidants are traditionally used in SOR in industry, and the poor selectivity to SO generates many byproducts [26]. Therefore, it is desirable to develop more efficient reaction processes that use green oxidants and offer high selectivity to SO. Recently, the selective oxidation of styrene with tertbutyl hydrogen peroxide (TBHP) as the oxidant has attracted much interest [27,28]. Various catalysts including homogeneous ones and heterogeneous ones have been explored for the selective oxidation of styrene. Homogeneous catalysis suffers from separation and recycling issues [29], while heterogeneous catalysis can avoid them owing to the solid nature of the catalyst materials such as metal oxides [30], precious metals [31] and metal doped zeolites [32]. Among metal oxides (e.g. NiO, CoO, Fe2O3, Co3O4, Mn3O4 and MnO2), Ni- and Co-based nanocatalysts show relatively high catalytic activity toward selective oxidation of styrene to SO with TBHP as the oxidant [28,30,33]. However, the catalytic performances of these Ni- and Co-based catalysts are still unsatisfactory for practical applications in industry. Developing novel catalyst systems with high catalytic activity and selectivity are thus the urgent need to exploit the advantages of the SOR through heterogeneous catalysis. In this study, we develop novel 3D hierarchical hollow urchin-like NiO/NiCo2O4 heterostructures that are synthesized by a facile hydrothermal reaction, followed by calcination in air. When used as catalysts for selective oxidation of styrene to SO, the NiO/NiCo2O4 heterostructures exhibit superior catalytic activity and selectivity compared to those reported in the literature. A high SO yield of 81.4% and a high selectivity of 90.8% to SO were obtained. 2. Experimental section 2.1. Preparation of the NiO/NiCo2O4, NiO and Co3O4 catalysts The NiO/NiCo2O4 heterostructures were prepared by a hydrothermal method. In a typical synthesis, 0.73 g of Co(NO3)26H2O and 2.18 g of Ni(NO3)26H2O were dissolved in 50 mL of deionized (DI) water under magnetic stirring to obtain a homogeneous solution. While stirring, 0.8 g of polyvinylpyrrolidone (PVP, K30) was added into the solution. The suspension was stirred on a magnetic stirrer for at least 1 h. Then 1.20 g of CO(NH2)2 was added to the solution. After stirred for 1 h, the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave for hydrothermal reaction at 180 °C for 7 h. After cooled down to room temperature, the product was collected by filtration and washed thoroughly with DI water and ethanol for several times. Finally, the NiO/NiCo2O4 was dried at 50 °C for 12 h and then calcined at 500 °C for 2 h. For comparison, NiO and Co3O4 were prepared under similar conditions with either Co(NO3)26H2O or Ni(NO3)26H2O added in the reaction solution.

high-resolution TEM (HRTEM) images were obtained on a JEM2100 microscope with an energy-diffusive X-ray spectroscopy (EDS) attachment. The N2 adsorption and desorption isotherms were collected at 77 K on a Quantachrome Autosorb-iQ3. The Xray photoelectron spectroscopy (XPS) data were collected on a Thermo, Fisher Scientific ESCALAB 250Xi spectrometer. 2.3. Catalyst test The SOR over the NiO/NiCo2O4 was carried out in a 100 mL round bottom flask equipped with a reflux condenser. Typically, 0.1 g of catalyst, 15 mmol of styrene, 16 mL of acetonitrile, and 75 mmol of TBHP were added into the round bottom flask while stirring. Then the flask was immersed in a water bath at 80 °C for reaction. The reaction mixture was sampled once an hour. The products of the reaction were analyzed by a gas chromatography (GC) with a KB-1 column. To test the catalyst stability, the used catalyst was separated from the reaction mixture by magnetic separation, washed with ethanol, dried at 50 °C for 12 h, and applied for the next run. 3. Results and discussion Fig. 1 shows the XRD patterns of the NiO/NiCo2O4, Co3O4 and NiO. The XRD pattern of the NiO exhibits five distinct diffraction peaks at 37.2°, 43.3°, 62.9°, 75.4° and 79.4° that are indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes, respectively, of the cubic NiO (JCPDS 47–1049). The XRD pattern of the Co3O4 presents the characteristic peaks at 19.0°, 31.3°, 36.9°, 44.8°, 59.4° and 65.2°, which can be assigned to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of the spinel Co3O4 (JCPDS 42–1467). According to diffraction peaks at 37.2°, 43.3°, and 62.9°, the NiO phase is the main component in the NiO/NiCo2O4. The XRF analysis further confirms that the NiO component accounts for 59 wt% of the NiO/NiCo2O4. Fig. 2 presents the representative SEM images of the NiO, Co3O4 and NiO/NiCo2O4. The Co3O4 shows a sheet-like structure with high porosity (Fig. 2a), while the NiO exists in the form of porous microspheres (Fig. 2b). The NiO/NiCo2O4 present a hollow urchin-like structure (Fig. 2c), which is different from those of the Co3O4 and NiO. The magnified SEM image shown in Fig. 2d clearly displays the hierarchical hollow structure of the NiO/NiCo2O4 and the well-defined metal oxide nanoneedles. To further reveal the phase composition of the mixed metal oxide, HR-TEM analysis was performed. Fig. 3a shows the HR-

2.2. Catalyst characterization The powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance diffractometer using Cu Ka radiation (k = 0.154056 n m). The composition of the samples were determined by a ARL Quant’X XRF analyzer. The SEM images of the samples were obtained on a field-emission scanning electron microscope (SEM, Hitachi S-4800). The transmission electron microscopy (TEM) and

Fig. 1. XRD patterns of the NiO, Co3O4 and NiO/NiCo2O4.


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Fig. 2. SEM images of (a) Co3O4, (b) NiO, and (c and d) NiO/NiCo2O4.

TEM image of the NiO/NiCo2O4. The nanoneedles are composed of nanocrystals, and these nanocrystals are highly crystallized as evidenced by the clear lattice fringes. The d-spacing value can be counted from the HR-TEM image and enables the identification of the corresponding crystal. An interplanar spacing of 0.24 nm can be indexed to the (1 1 1) plane of NiO, while the d-spacing of 0.47 and 0.20 nm can be assigned to the (1 1 1) and (4 0 0) planes of NiCo2O4, respectively. Therefore, the NiO/NiCo2O4 heterostructures mainly have two components (i.e. NiO and NiCo2O4) and are rich in NiO-NiCo2O4 phase boundaries. EDS analysis manifests that the NiO/NiCo2O4 are composed of Co, Ni and O (Fig. 3b). The signals from C and Cu are from the carbon-enhanced copper grid that holds the sample. The TEM image (Fig. 3c) confirms the hollow urchin-like structure of the NiO/NiCo2O4. Fig. 3d displays the EDS element mapping of the NiO/NiCo2O4, which shows the homogeneous element distribution. Fig. 4 shows the N2 adsorption-desorption isotherm of the NiO, Co3O4 and NiO/NiCo2O4. All the samples show a type IV isotherm, with a H2 hysteresis loop, indicating their mesoporous structure. The specific surface area of the NiO/NiCo2O4 determined by Brunauer-Emmett-Teller method is 40.2 m2 g 1, and the average pore size is about 35.5 nm. The NiO and Co3O4 have smaller specific surface area of 30.7 and 13.9 m2 g 1, respectively. The NiO/NiCo2O4 is anticipated to offer higher catalytic performance because of the higher specific surface area. To gain more insights into the surface chemical states and elemental composition, the NiO/NiCo2O4 was analyzed by the XPS technique [34]. XPS survey presents the presence of Ni, Co and O

in the NiO/NiCo2O4 (Fig. 5a). Fig. 5b shows the high-resolution XPS Co 2p spectrum. Signals corresponding to Co 2p3/2 and Co 2p1/2 can be observed at 780.1 and 795.6 eV, respectively. With a Gaussian fit, the Co 2p spectrum can be well fitted into two spinorbit peaks and two shake-up satellite (Sat.) peaks. The peaks at 779.8 and 795.6 eV can be attributed to Co3+ while the peaks at 781.6 and 795.7 eV correspond to Co2+. Fig. 5c shows the deconvolution of the XPS Ni 2p spectrum. Signals at 854.1 and 871.9 eV associated with Ni2+ are observed while peaks at 856.2 and 874.3 eV derived from Ni3+are identified [4,35,36]. In the O1s spectrum (Fig. 5d), the peaks at 529.6 and 531.2 eV can be ascribed to the metal-oxygen bond and the oxygen in hydroxyl groups, respectively [36]. These results demonstrate the coincidence of multiplevalence metal ions such as Ni2+, Ni3+, Co2+ and Co3+ in the NiO/ NiCo2O4. This indicates the incorporation of Ni ions into the Co3O4, replacing part of the Co ion sites, and possibly vice versa. This is consistent with the XRD and HR-TEM analyses. The selective oxidation of styrene using TBHP as the oxidant was carried out to evaluate the catalytic performance of the NiO/ NiCo2O4 under mild reaction conditions. For comparison, the Co3O4 and NiO catalysts were tested under the same conditions. GC analysis results showed that the catalytic oxidation of styrene resulted in two major products, i.e. SO and benzaldehyde, along with small amounts of phenylacetaldehyde and other products. Fig. 6 shows the catalytic performances of the Co3O4, NiO, and NiO/NiCo2O4. As expected, styrene conversion increases with the reaction time (Fig. 6a), and SO selectivity increases with reaction time and then decreases (Fig. 6b) if the reaction time is further

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Fig. 3. HRTEM image (a), EDS analysis (b), TEM image (c), and EDS element mapping (d) of the NiO/NiCo2O4.

Fig. 4. Nitrogen adsorption-desorption isotherms of the NiO/NiCo2O4. The inset shows the nitrogen adsorption-desorption isotherms of the NiO (blue) and Co3O4 (red).

prolonged possibly because of the over-oxidation of SO to benzaldehyde, 1-phenyl-1, 2-ethanediol, and so on [37,38]. Obviously, the NiO/NiCo2O4 catalyst exhibits the highest catalytic activity. Upon 4 h, the styrene conversion reached 80% over NiO/NiCo2O4, while it was below 45% over Co3O4 or NiO. As shown in Fig. 5b, the NiO/NiCo2O4 catalyst shows remarkably high selectivity to SO. The selectivity to SO over NiO/NiCo2O4 is greater than 75% within the reaction duration, and reaches the highest value of 90.8% upon 5

h. With a styrene conversion of 89.6% at 5 h, the highest SO yield of 81.4% is obtained over NiO/NiCo2O4 (Fig. 6c), which is more than 2 times those of Co3O4 (26.9%) and NiO (30.1%). Figs. S1 and S2 show the effect of reaction time and reaction temperature on the catalytic performance, respectively. The results suggest that the reaction time of 5 h and reaction temperature of 80 °C are the optimal reaction parameters for the SOR over NiO/NiCo2O4. Such a high SO yield is better than those of the catalysts reported in the literature (Table 1). The high catalytic performance of the NiO/NiCo2O4 heterostructures can be attributed to the high specific surface area and the abundant NiO-NiCo2O4 phase boundaries, both of which contribute to the numerous active sites. According to the previous studies, it can be speculated that both Ni2+ and Co2+ in the NiO/NiCo2O4 heterostructures could be active for the SOR [39–41]. During the SOR, the generation of M-peroxo species may be the active sites for the SOR [42,43]. Based on these understandings and the current results in this work, we proposed the possible reaction pathway for the SOR with the NiO/NiCo2O4 heterostructures as the catalyst. Firstly, the MII (M = Co, Ni) sites coordinate with TBHP to generate the MIII-oxo species, which are then converted into the active MIII-peroxo species with the help of TBHP. Afterwards, the C@C bonds of the styrene molecules interact with the MIII-peroxo species, producing the peroxo metallocycles. Finally, these peroxo metallocycles are broken with the formation of SO and the simultaneous regeneration of the MII sites. The formation of benzaldehyde as the main byproduct may be due to the direct cleavage of the C@C bond of styrene in the SOR. Furthermore, the Ni2+ and Co2+ in the NiO/NiCo2O4 heterostructures


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Fig. 5. XPS survey (a), high-resolution Co 2p spectrum (b), Ni 2p spectrum (c) and O 1 s spectrum (d) of the NiO/NiCo2O4.

Fig. 6. Evaluation of the catalytic performances of the NiO/NiCo2O4, NiO and Co3O4 catalysts.

may work synergistically to achieve the excellent catalytic performance including good reactivity and high product selectivity [44]. Catalyst reusability is of vital importance for practical applications in industry. One of the merits of the NiO/NiCo2O4 catalyst is that it is separable by a facile magnetic separation process. For the stability test, the NiO/NiCo2O4 catalyst was separated by mag-

netic separation and used for the next run. Fig. 6d shows the recycling performance of the NiO/NiCo2O4 catalyst. The selectivity to SO is well maintained over the five cycles. The slight decrease in the catalytic activity may be due to the loss of catalyst during the separation process. All these results demonstrate that the NiO/NiCo2O4 catalyst is a highly efficient and stable catalyst,

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Table 1 Comparison of the catalytic performances of various catalysts.

a b c d e

Catalyst

Oxidant

Temp.a (°C)

Timeb (h)

Conv.c (%)

Sel.d (%)

Yielde (%)

Ref.

NiO/NiCo2O4 NiO CoO ZrO2 NiO-Al2O3 NiO-HMS Au/CaO(HDP) Au/CaO(DP) Ag NPs/CNFs Ce0.9Zr0.1O2 TS-1 Fe-MCM-41-DHT TS-1 Cu2(OH)PO4 Mg-Co-Al hydrotalcite Fe3O4@SiO2-NH2-Co Co2+-MCM-41 Co-Y-ZrO2

TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP H2O2 H2O2 H2O2 H2O2 air air air air

80 82–83 82–83 80 80 80 92 92 60 80 60 73 40 45 95 80 100 120

5 3 3 12 6 10 3 3 8 12 10 2 12 3 4 8 4 3

89.6 51.7 47.3 32.7 60.2 67.0 53.9 31.5 43.4 78.1 53.6 16.8 51 44.6 93 90.8 45 37

90.8 86.2 73.1 82.6 86.3 94.0 61.1 52.7 38.9 79.3 61.7 66.5 82 67.8 36 63.7 62 90

81.4 44.6 34.6 27.0 52.0 63.0 32.9 16.6 16.9 61.9 33.1 11.2 41.8 30.2 33.5 57.8 27.9 33.3

Our work [30] [30] [45] [46] [28] [47] [47] [48] [45] [49] [50] [51] [52] [53] [54] [55] [56]

Reaction temperature Reaction time. Styrene conversion. SO selectivity. SO yield.

making it a promising candidate catalyst for the selective oxidation of styrene to SO. 4. Conclusions In conclusion, the hollow urchin-like NiO/NiCo2O4 heterostructures have been developed and used as a catalyst for selective synthesis of SO from the SOR. The magnetically separable NiO/NiCo2O4 catalysts exhibit good catalytic activity, high SO selectivity, and satisfactory catalyst stability. The superior catalytic performance of the NiO/NiCo2O4 heterostructures can be owing to the high surface area and the synergistic effect between NiO and NiCo2O4. The high catalytic performance, along with the environmental benignity and cost-effectiveness, make NiO/NiCo2O4 an attractive candidate for the selective oxidation of styrene and beyond. Acknowledgements This work was supported by the Natural Science Foundation for High Education of Jiangsu Province (17KJB530011), the Science and Technology Innovation Foundation of Yangzhou University (2017CXJ015), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.05.001. References [1] J. Liu, D. Wang, M. Wang, D. Kong, Y. Zhang, J.-F. Chen, L. Dai, Uniform twodimensional Co3O4 porous sheets: facile synthesis and enhanced photocatalytic performance, Chem. Eng. Technol. 39 (2016) 891–898. [2] G. Rajeshkhanna, E. Umeshbabu, G. Ranga Rao, Charge storage, electrocatalytic and sensing activities of nest-like nanostructured Co3O4, J. Colloid Interface Sci. 487 (2017) 20–30. [3] X. Yan, L. Tian, X. Chen, Crystalline/amorphous Ni/NiO core/shell nanosheets as highly active electrocatalysts for hydrogen evolution reaction, J. Power Sources 300 (2015) 336–343. [4] X. Liu, C. Hao, H. Jiang, M. Zeng, R. Yu, Hierarchical NiCo2O4/Co3O4/NiO porous composite: a lightweight electromagnetic wave absorber with tunable absorbing performance, J. Mater. Chem. C 5 (2017) 3770–3778.

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