Solvent-Free Chemical Approach to Synthesize Various

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A Solvent-Free Chemical Approach to Synthesize Various Morphological Co3O4 for CO Oxidation Kun Wang, Yali Cao, Jindou Hu, Yizhao Li, Jing Xie, and Dianzeng Jia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Applied Materials & Interfaces

A Solvent-Free Chemical Approach to Synthesize Various Morphological Co3O4 for CO Oxidation Kun Wang, Yali Cao†, Jindou Hu, Yizhao Li, Jing Xie and Dianzeng Jia† Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China.

†Corresponding author. Tel: +86-991-8583083; Fax: +86-991-8588883; Emails:

[email protected]; [email protected].

Abstract The synthesis of Co3O4 nanomaterials with diverse morphologies were usually synthesized in liquid phase accompanied by the template or surfactant, even harsh conditions, which further restricted their practical application. Herein, we reported an extremely simple and practical solid-state chemical method to synthesize Co3O4octahedrons, -plates and -rods. Among these, the shape control of Co3O4-octahedrons and Co3O4-plates involve the variation of the amount of reactant, and the formation of Co3O4-rods with {110} facet can be achieved by replacing the reactant. The formation of the Co3O4 nanomaterials with different morphology originated from the different microenvironment of reaction and the structure of reactants. The catalytic activity of Co3O4 samples for CO oxidation was evaluated in normal feed gas. The as-prepared Co3O4-rods exposed {110} facet exhibited superior catalytic activity for CO oxidation, which can be attributed to more oxygen defects on Co3O4-rods surface. Additionally, Co3O4-rods exhibited excellent durablility (without pretreatment) in normal feed gas and even in the presence of moisture, comparable or better than that in reported literatures. The practical and environmental friendly solvent-free strategy provided a new promising route to large-scale prepare (metal) oxide with remarkable CO oxidation performance for practical application. Keywords: Oxides; Co3O4; Solid-state synthesis; Catalyst; CO oxidation

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1. Introduction As global consumption of fossil fuels continue to ramp up, environment pollution and energy shortage issues are emerging as the challenges for humanity.1-3 As promising semiconductor materials, Co3O4 with the favorable features of low-cost, rich reserves and remarkable performance has attracted considerable attentions in catalysis,4-6 Li-ion batteries,7-9 supercapacitor,10-12 quantum dots13 and sensors.14-16 Notably, Co3O4 exhibit high catalytic activity for CO oxidation, which is considered as a substitute for precious metals catalysts.17-18 As we all know, the size and shape play crucial roles in determining the catalytic performance of nanomaterials.19-23 Rational design of Co3O4 nanomaterials with preferred morphologies and sizes would be have better application prospects in environmental remediation and energy conversion. Considerable works have been devoted to design and synthesize Co3O4 with preferential facet exposure by tuning the morphology. Hu et al.24 have synthesized Co3O4 nanosheets, nanobelts and nanocubes for methane combustion by solvothermal method and the Co3O4 nanosheets with the exposure of (112) planes exhibited higher catalytic activity for methane combustion. Xiao et al.25 utilized hydrothermal method to prepare Co3O4 cubes, truncated octahedrons and octahedrons, their results of electrochemical test showed that Co3O4 octahedrons with the exposure of (111) planes exhibited higher electrochemical performance. However, for Co3O4 nanomaterials with tunable size and morphologies, the template or surfactant, even harsh conditions are generally required, which restricted large-scale preparation and practical application of such catalyst. From the practical and environmental viewpoint, it remains a challenge to develop a facile, low energy consumption and surfactant-free


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reliable process for synthesizing Co3O4 nanomaterials with different sizes and morphologies. In this work, we report a facile solvent-free reaction process to prepare Co3O4octahedrons, -plates and -rods. The fabrication process presented the features of simple, low energy consumption, template-free and convenient operation at mild condition, which is applicable to industrial mass production.26-27 Among obtained Co3O4 samples, Co3O4-octahedrons and Co3O4-plates can be controlled synthesized by changing the molar ratios of reactants, while Co3O4-rods can be obtained by replacing the reactants. Meanwhile, the possible growth mechanism of Co3O4 nanomaterials with different morphologies was proposed. The as-obtained Co3O4-rods with the exposed (110) planes exhibited the best catalytic activity and excellent durability for CO oxidation. The simple and environmental friendly solid-sate chemical synthesis approach provided a new promising route for the convenient preparation of high-activity catalysts for CO oxidation.

2. Experimental Section 2.1. Synthesis of catalysts. 2.1.1. Preparation of Co3O4-octahedrons and Co3O4-plates. 10 mmol solid cobalt acetate (Co(CH3COO)2·4H2O) was ground into powder in a agate mortar, then 20 mmol ammonium fluoride (NH4F) was added into the agate mortar for solid-state chemical reaction by grinding to promote the contact of reactants at ambient condition. After an hour, the obtained mixture was calcined at 400oC for 2 h in air atmosphere by raising the temperature at rate of 5oC/min to produce Co3O4 octahedrons. The prepare procedure of Co3O4-plates is similar to octahedrons, only changing the amount of NH4F to 60 mmol.



2.1.2. Preparation of Co3O4-rods. 10 mmol cobalt sulfate (CoSO4·7H2O) was ground into powder in a agate mortar, then 20 mmol oxalic acid (H2C2O4·2H2O) was added into the agate mortar for solidstate chemical reaction by grinding to promote the contact of reactants. After an hour, the obtained mixture was rinsed with a large amount of distilled water to obtain the precursor. The precursor was dried at ambient temperature for 12 h, and then were calcined at 300oC for 2 h in air atmosphere by raising the temperature at rate of 2oC/min to produce Co3O4-rods. 2.1.3. Optimized processing of Co3O4 nanomaterials. The experiment of morphological optimization for Co3O4 nanomaterials was operated. The aforementioned mixture for the synthesis of Co3O4-octahedrons was transferred to 60oC water bath for 12 h, 24 h and 48 h, respectively. After that, the mixture was calcined at 400oC for 2 h in air atmosphere by raising the temperature at rate of 5oC/min and then the typical Co3O4 octahedrons were obtained. The experimental procedure of morphological optimization for Co3O4-rods was similar to Co3O4-octahedrons. The final products were calcined at 300oC for 2 h in air atmosphere by raising the temperature at rate of 2oC/min to produce Co3O4-rods. 2.2. Materials Characterization. The size and morphology of samples were obtained by field emission scanning electron microscope (FESEM, Hitachi S-4800H) with an accelerating voltage of 5 kV. High resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL JEM-2010F electron microscope with an accelerating voltage of 200 kV. The structure of the samples was characterized by X-ray powder diffraction (XRD) on


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Bruker D8 employing Cu-Kα radiation (1.54056 Å) with an operating voltage of 40 kV and a beam current of 40 mA. The Fourier transform infrared (FT-IR) spectrum was recorded by a Bruker VERTEX 70 spectrometer in the range of 400-4000 cm-1. The element composition of samples was analysized by energy disperse X-ray (EDS) spectrum (EDAXTLS) with an operating voltage of 30 kV. The surface component and structure of samples was characterized by X-ray photoelectron spectra (XPS, Thermo Fisher Scientific ESCALAB250Xi) employing Al K Alpha 1486.6 eV. The specific surface area, pore size and pore volume of the samples were obtained using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods on a surface area and porosity analyzer (Quantachrome, Autosorb-iQ2). The samples were degassed at 110oC for 6 h to remove the moisture and physical adsorbed gases. The N2 isothermal adsorption obtained at relative pressures (P/P0) from 0.05 to 0.995 and then desorption from 0.995 to 0.05. 2.3. Measurement of catalytic activity. 100 mg as-prepared Co3O4 catalysts were evaluated in a continuous-flow quartz reactor (30 cm in length, 8 mm in i.d.) at atmospheric pressure with 1 vol% CO and 20 vol% O2 balanced with N2. The flow rate of mixture gas was 50 ml/min (space velocity = 30000 ml·gcat-1·h-1) and all the gases were controlled by mass flow controllers (Horiba S49 32/MT). The concentrations of CO, O2, and N2 in the inlet and outlet streams were analyzed by an online gas chromatography system (Agilent 7890B) with a thermal conductivity detector (TCD). The catalytic activity was evaluated by conversion of CO, which could be calculated on the basis of the content change of CO in feed gas and an effluent stream: CO conversion =

[CO ]in − [CO ]out × 100% [CO ]in


(1)


2.4. Hydrogen temperature-programmed reduction (H2-TPR). H2-TPR (Thremo TPDRO 1100 series) test was carried out in a U-shape quartz reactor with the sample (30 mg) kept under a H2-N2 mixture gas (VH 2 = 5.25%). Before switching the feed gas to H2-N2 stream, the sample was pretreated in N2 stream at 200°C for 30min, and then cooled to room temperature. The TPR profile was recorded with temperature programming from 50 to 750oC at a rate of 10oC/min with the flow rate of 20 ccm/min. H2 consumption was monitored by a thermo-conductive detector.

3. Results and discussion 3.1. Morphological analysis.

Figure 1 SEM and TEM images of (a, b, c) Co3O4-octahedrons, (d, e, f) Co3O4-plates and (g, h, i) Co3O4-rods.


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Co3O4-octahedron, -plate and -rod nanomaterials were successfully synthesized by a extremely facile solvent-free process. The micromorphology of Co3O4 nanomaterials was characterized by SEM and TEM. As shown in Figure 1a-c, the shape of as-obtained Co3O4 samples presented typical octahedron-like shape with a size of 100-200 nm. Co3O4-plate nanomaterials were composed of closely packed irregular particles with size of 70-80 nm, and the well-defined pore structures in platelike Co3O4 nanomaterials were clearly observed (in Figure 1d-f). Figure 1g-i showed that Co3O4-rods with a diameter about 50 nm and a length about several micrometers were obtained by the simple thermal decomposition of the precursor (CoC2O4·2H2O) (Figure S1 in supporting information), which was obtained under mild condition. The surface pore structures of rod-like Co3O4 nanomaterials can be clearly observed from the images in Figure 1g-i. The surface pore structure can be ascribed to the successive release of the gas originated from the decomposition of precursor during the calcination.

Figure 2 SEM images of as-obtained Co3O4 octahedrons with different optimized processing time at 60oC: (a) 0 h, (b) 12 h, (c) 24 h, and (d) 48 h.



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Figure 3 SEM images of the Co3O4 nanomaterials prepared by the solid-state chemical reaction under the variation of the molar ratio of reactants (Co(CH3COO)2·4H2O and NH4F): (a) 1:2, (b) 1:4, (c) 1:6 and (d) 1:8.

Among of as-prepared Co3O4 nanomaterials, Co3O4-octahedrons and Co3O4plates can be controlled synthesized by changing the molar ratio of reactants (Co(CH3COO)2·4H2O and NH4F), while Co3O4-rods can be obtained by replacing the reactant NH4F using H2C2O4·2H2O. The morphological evolution process of Co3O4 nanomaterials

was

shown

in

Figure

2-4.

When

the

molar

ratio

of

Co(CH3COO)2·4H2O and NH4F was 1:2, the Co3O4 nanomaterials presented octahedron-like shape and the size of octahedrons was gradually increased with prolonging the reaction time at 60oC (show in Figure 2a-d), indicating that the reaction time at 60oC can only change the size of Co3O4-octahedrons, rather than the morphology in solid-state chemical synthesis. When the molar ratio of Co(CH3COO)2·4H2O and NH4F were altered to 1:4, the Co3O4-octahedrons was transformed into plate-like nanomaterials (show in Figure 3b-d). The molar ratio was further adjusted to 1:6 and 1:8, the shape of plates has no significant difference except that the thickness of the plates was decreased, indicating that the amount of NH4F has


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an important effect on the evolution of morphology from octahedron-like to plate-like nanomaterials. In our study, the different reactants have also a great influence on the morphology of the product. When H2C2O4·2H2O was selected as reactant to replace NH4F, and reacted with cobalt salt, the morphology of products was changed to rodlike structures, as shown in Figure 4. The diameter of as-synthesized Co3O4-rod nanomaterials was gradually increased by prolonging the reaction time at 60oC, which is similar to Co3O4-octahedrons.

Figure 4 (a, b) SEM images of rod-like Co3O4 nanomaterials without optimized processing, (c, d) SEM images of rod-like Co3O4 nanomaterials with optimized processing in water bath at 60oC for 24 h.

3.2. Compositional analysis. The XRD pattern of three samples was shown in Figure 5a, revealed that the Co3O4 with spinel structure was synthesized and the diffraction peaks of all samples could be well fitted with the data of (111), (220), (311), (222), (400), (422), (511) and (440) planes of the cubic spinel Co3O4 (JPCDS NO. 42-1467), respectively. No peaks from other phase or impurity are observed in patterns of Co3O4 nanomaterials. Figure



5b showed the FT-IR spectrum of three samples, two characteristic peaks at around 568 cm-1 and 664 cm-1 can be clearly observed, which are assigned to the intrinsic stretching vibrations of Co-O bond and the peak at around 3400 cm-1 can be assigned to the O-H group.28 Figure 5c showed the EDS pattern of as-synthesized plate-like samples, only Co and O elements were detected, which confirmed that fluorine atoms are easily removed completely after calcination. The above results further revealed that Co3O4 nanomaterials with high purity were successfully obtained. The BET specific surface areas of three samples were listed in Table 1. The specific surface areas of Co3O4-rods and -plates are 37 and 35 m2·g-1, higher than as-prepared Co3O4octahedrons of 29 m2·g-1. It can be ascribed to the well-defined pore structures that were created on surface of Co3O4-rods and Co3O4-plates. The surface composition and structure of three samples were investigated by XPS. Figure 5d showed the full XPS survey scan spectra of the three samples, only Co, C and O elements were detected and the peak of fluorine element was not detected at binding energy of 684.8 eV,29-30 further confirmed that fluorine ions have been removed completely from the surface of samples. Figure 5e showed the Co (2p) XPS spectra of three Co3O4 nanomaterials. The main peak at binding energy about 780 eV was characteristic of Co (2p3/2) with Co3+ (779.3 eV) and Co2+ (781.4 eV). While the shoulder peak at binding energy of 795 eV was attributed to Co (2p1/2). Figure 5f showed O (1s) XPS spectra of three samples, the major peak of O 1s of Co3O4-rods at about 529.78 eV was assigned to lattice oxygen OL (O2-), lower than Co3O4-octahedrons of 530.68 eV and Co3O4-plates of 530.38 eV. The major peak of Co 2p and O 1s moving in the direction of high binding energy can be attributed to the different combination manner of O2- bonded with Co3+ and Co2+ on the surface of catalysts (the three-coordinated O bonded with three Co3+, O bonded with one Co2+


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and two Co3+, and two-coordinated O bonded with one Co2+ and one Co3+),31-32 leading to the binding energy of O2- bonded with Co ions has considerable difference. Besides, a shoulder peak of Co3O4-rods centered at 531.68 eV was clearly observed, it was assigned to low coordination oxygen species (O2, O2- and O-) or oxygen adsorbate residual on oxygen vacancies (OV), which was in agreement with the reported literatures. 33-34 Namely, oxygen vacancies were formed on the surface of Co3O4-rods.35 The broad peak centered at 533.18 eV from Co3O4-rods can be assigned to carbonates.33,36-38

Figure 5 (a) XRD patterns and (b) FT-IR spectrum of Co3O4 -octahedrons, -plates and -rods nanomaterials; (c) EDS pattern of Co3O4-plates; XPS spectra of Co3O4-octahedron, -plate and -rod nanomaterials: (d) survey, (e) Co 2p and (f) O 1s.



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Table 1 The specific surface area obtained from BET, area percentages of different elemental components obtained from quantitative analysis of XPS. Co3O4

SBET (m2g-1)

(Co3+/Co2+)

OL

Ov

Ow

Rods

37

1.99

47.9

39.4

12.7

Octahedrons

29

2.07

80.4

19.6

--

Plates

35

2.28

80.3

19.7

--

3.3. Microstructural analysis.

Figure 6 HRTEM images of (a) Co3O4-octahedrons; (b) Co3O4-plates and (c, d) Co3O4-rods; The insets are enlarge HRTEM and FFT images of three samples.

To acquire a deep understanding of the structure of as-obtained Co3O4 nanomaterials, the HRTEM analysis of three samples was performed (in Figure 6). The enlarged HRTEM images of three samples are shown in Figue 6 (insets), only the normal (311) plane with a lattice spacing of 0.242 nm was observed, meaning that the {111} facets was exposed on the surface of Co3O4-octahedrons. The (111) plane with a lattice space of 0.456 nm and the distinct fringe spacing of 0.284 nm corresponds to the (220) plane were observed (show in Figure 6b), suggest that {001} ACS Paragon Plus Environment

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planes were partly exposed on the surface of Co3O4-plates. Figure 6c-d showed the lattice fringe space of 0.241 nm and 0.463 nm, corresponding to the normal (311) and (111) planes of the cubic Co3O4 spinel structure, respectively. Moreover, (220) plane with a lattice space of 0.280 nm was also clearly observed, indicating that {110} planes were exposed on the surface of Co3O4-rods.39 These results indicate that the octahedrons, nanoplates and nanorods are highly crystalline. Moreover, only one lattice orientation was presented in single particle and each nanoparticle has a different orientation. The bright dots arrays can be clearly observed from the FFT images(in Figure 6 - insets), which was recorded as a single crystal. In addition, the Ball-and-stick model for Co3O4 with {110}, {111}, {001} crystal planes along [011], [111], [001] orientation were shown in Figure 7. Apparently, the computer modeling has proved that {001} and {111} plane only contain Co2+ cations, and Co3+ cations are solely present in {110} planes, which agrees well with above results from Co (2p) XPS spectra.

Figure 7 Ball-and-stick model for Co3O4 with different planes: (a) {110}, (b) {111} and (c) {001} planes;The surface atomic configurations in {110}, {111} and {001} planes corresponded to (d), (e) and (f).



To better understand the possible evolution process of morphology from Co3O4octahedrons to -plates, the changes of precursors during thermal treatment were studied, untill to obtain Co3O4 nanomaterials (the detailed discussion can be seen in supporting information). For spinel metal oxide, the final morphology of products is dominated by the competitive growth of {111} and {100} facets.40 Furthermore, simulation calculation for Co3O4 show that the {111} facets have the lowest surface energy, which result in the growth of Co3O4 along [001] orientation. Finally the stable {111} facets was preferentially exposed and then the formation of octahedrons after calcination.41 Therefore, the possible growth mechanism of Co3O4-octahedrons and Co3O4-plates nanomaterials is proposed in solid-state chemical reaction based on that F- was an effective agent in changing the surface energy of metal (oxide) facets.42 When a small amount of NH4F was added into the aforementioned reaction system, firstly, Co2+ ions preferentially interact with F- to form (NH4)2CoF4 complexes, which was confirmed by XRD pattern (Figure S1b). Generally, the tendency for minimization of the surface energy, which determined the final morphology of products during the annealing process, is favorable to the growth of crystallite.43-44 Therefore, the low-index {111} facets with the lowest surface energy were preferentially exposed, which result in the formation of octahedron-like Co3O4 nanomaterials during calcination (in Figure S3a-c in supporting information). When further increased the amount of NH4F to a certain level, a large amount of fluorine ions were able to adsorb onto facets with high surface energy, leading to the surface energy of {100} facet decreased sharply and make it lower than {111} facets. The presence of CoF2 (JPCDS card No. 33-0417, in Figure S2 in supporting information) obtained at 300oC can be attributed to a large amount of fluorion adsorbed on metal ions surface, which inhibited the formation of Co-O bond. Namely, the {100} facets


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were blocked by F- in some ways, which restrained the growth of {100} facets along [001] orientation and finally lead to the formation of plate-like Co3O4 nanomaterials by following calcination (in Figure S3d-f in supporting information).42,

45

The

presence of (220) and (111) planes on surface of Co3O4-plate nanomaterials was in good agreement with the supposition that we proposed. The formation mechanism of Co3O4-rod nanomaterials is different from the above supposition. In order to explore its growth mechanism, a series of control experiments were performed based on the standard synthetic procedure. The CoC2O4·2H2O nanorods precursors were prepared by the solid-state chemical reaction between CoSO4·7H2O and H2C2O4·2H2O, then the Co3O4-rod nanomaterials were obtained by the calcination of precursor at different temperature (The detailed discussion was shown in supporting information). As shown in Figure S6 in supporting information, when CoSO4·7H2O and Co(NO3)2·6H2O were introduced, the morphology of products was still rod-like. Surprisingly, when cobalt salt switched to Co(CH3COO)2·4H2O, the morphology of products was changed to nanoparticles, which may be caused by the much slower reaction speed than the other two cobalt salts. The above results indicate that the cobalt salts also affect the morphology of products.

Besides,

sodium

oxalate

(Na2C2O4)

and

ammonium

oxalate

((NH4)2C2O4·H2O) were chosen to react with CoSO4·7H2O. When Na2C2O4 and (NH4)2C2O4·H2O were introduced, nanorods were also gradually formed. Due to the existence of bidentate ligand C2O42-, two bidentate ligands and two H2O molecules will coordinate with a Co2+ cation to form complexes of six coordinate bond. In this process, due to the formation of the H bond in water molecules and the interaction of the coordination bond, the CoC2O4·2H2O particles that have a suitable speed of formation will grow along a certain direction, and finally present the nanorods.



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3.4. Catalytic activity of Co3O4 nanomaterials. The catalytic activity of as-prepared Co3O4 nanomaterials (without pretreated) was evaluated in normal feed gas. As shown in Figure 8a, the Co3O4-rods exhibited the best catalytic performance and the 100% CO conversion was achieved at 80oC, lower than Co3O4-octahedrons of 160 oC and Co3O4-plates of 180oC. Namely, Co3O4rods exhibited the highest catalytic activity of CO oxidation. Here, the catalytic performance of Co3O4 nanomaterials was comparable or better than their composites and Au nanoparticles supported on metal oxide in reported literatures, such as 3D porous hierarchical Co3O4 nanostructures (T100=140oC),46 Cu-doped Co3O4 nanowires (T100=125oC),47

Co3O4-COP

composites

(T100=100oC

and

110oC),48

Co3O4

nanomaterials with pretreatment (T100=120oC) and Au/h-, c- and t- Co3O4 (T100=80 oC, 90oC

and

110oC),49

MnO2/CeO2-MnO2

(T100=206oC),50

Au144(SR)60/CeO2

(T100=80oC),51 and even better than gold supported on metal oxides (CuO, NiO, Y2O3 and La2O3)52 (in Table 2). In addition, the stability test of CO oxidation over Co3O4rods was carried out at 80oC (Figure 8b). After running 5 h at 80oC, the conversion of CO oxidation drops rapidly, after running 50 h at 80oC, only 45% conversion was maintained over Co3O4-rods. The deactived Co3O4-rods catalyst was regenerated in 20% O2/N2 atmosphere at 300 oC for 1 h. The catalytic activity of deactived Co3O4rods catalyst was fully recovered and CO conversion reached to 100% again (in Figure S7). When the temperature was further raised to 90oC, the conversion of CO oxidation returned to 100% and 100% CO conversion over Co3O4-rods was still maintained after running 50 h at 90oC. Furthermore, the intact structure of Co3O4-rods was maintained and has not been damaged after durability test (in Figure S8). These results revealed that Co3O4-rods exhibited favorable durability at 90oC in normal feed gas.


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Table 2 The catalytic activity of CO oxidation and the reaction set-ups of cobalt oxide, their composites and gold supported on metal oxide reported in the open literatures. Samples

Amount of catalyst (mg)

Pretreatment

♦3D hierarchical Co3O4

50

non

pre

♦ Cu doping Co3O4 nanowires ♦ Co3O4-COP composites

100

pre

T100 (oC)

Ref

140

46

125

47

CO/O2/N2=4:10:86

100

48

(ml·gcat-1·h-1)

110

---

Feed gas CO/O2N2=1:19.8:79.2 30000 (ml·gcat-1·h-1) CO/O2/He=2:3:95 120 sccm

30000

---

♦ Au/t-Co3O4

50

non

Au/c-Co3O4

---

---

Au/h-Co3O4

---

---

♦ MnO2/CeO2-MnO2

100

pre

♦ Au144(SR)60/CeO2

50

pre

♦ Au/metal oxide

200

non

♦ Co3O4-rod NCs

100

non

CO/O2/N2=1.6:21:77.4 30000 (ml·gcat-1·h-1) CO/O2/N2=0.8:20:79.2 60000 (ml·gcat-1·h-1) CO/O2/He=1.67:3.37:96.96 15000 (ml·gcat-1·h-1) CO/O2/He=5:10:85 30000

90

49

80 206

50

80

51

120

52

90

This study

(ml·gcat-1·h-1)

CO/O2/N2=1.07:19.98:78.53 30000 (ml·gcat-1·h-1),


110


Figure 8 (a) Conversion as a function of reaction temperature for CO oxidation over different-morphology Co3O4 samples; (b) Long term stability of CO oxidation over Co3O4rods; (c) CO light off plot of Co3O4-rods; (d) Stability test of Co3O4-rods under moisture rich conditions (~3.5% H2O); (e) XRD patterns and (f) FT-IR spectra of Co3O4-rods before and

after long-term stability tests under normal feed gas with moisture for 35 h. Generally, H2O molecule will compete with the CO molecule adsorbed on the surface of catalyst, which occupied the active sites on the surface and reduced the catalytic performance of catalyst. The effect of moisture on catalytic activity over Co3O4-rods was also studied. The catalytic activity test of Co3O4-rods was performed in feed gas with moisture. As shown in Figure. 8c, the catalytic activity over Co3O4rods was declined in moisture condition and the temperature of 100% CO conversion shifted to higher temperature (100oC). In addition, the long-term stability of CO


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oxidation was performed in feed gas with moisture. As shown in Figure 8d, 100% CO conversion over Co3O4-rods can be achieved at 100oC for consecutive 26 h, then CO conversion started to decrease, but still maintained over 70% conversion after 35 h. These results indicated that H2O molecule existed in feed gas will lead to the deactivation of Co3O4-rods catalyst. Figure 8e and 8f show the XRD patterns and IR spectrum of Co3O4-rods before and after long-term stability test under moisture conditions for 35 h. The XRD diffraction patterns have no change before and after durability test. The characteristic peaks of Co-O bond at 568 cm-1 and 664 cm-1 were still well remained, indicating that the spinel structure was still maintained after durability test at 100oC.36 After durability test, the stretching vibration peak of adsorbed H2O was obviously observed at 3430 cm-1.53 Moreover, The intensity of peaks at 1521 cm-1 and 1612 cm-1 was increased after durability test, and the peaks at 1521 cm-1 and 1612 cm-1 were assigned to the bending vibration of carbonates and the vibration of adsorbed H2O molecular, respectively.16, 54 The formation of carbonates and the accumulation of adsorbed water content on catalyst might be responsible for the activity decay after long-term reaction at 100oC for 35 h.36 3.5. H2-TPR analysis. The reducibility of three Co3O4 samples was investigated by H2-TPR experiment. The H2-TPR profiles of the samples were shown in Figure 9, the broad reduction peak of Co3O4-rods centered at 304oC was assigned to the reduction process from Co3+ to Co2+, lower than Co3O4-octahedrons at 375oC and Co3O4-plates at 378oC, the latter reduction peak at 375oC can be attribute to Co2+ to Co0, lower than Co3O4octahedrons at 428oC and Co3O4-plates at 433oC. Besides, the weak reduction peak of Co3O4-rods can be observed at 153oC, which was assigned to the reduction of surface oxygen adsorbed on oxygen defects (vacancies). The integration areas of the



reduction peaks in different temperature regions was shown in Table S1, indicating the ratio of the two peaks was close to the stoichiometric ratio of 1:3 (3.17 of Co3O4rods, 3.06 of Co3O4-octahedron and 2.66 of Co3O4-plates). Moreover, the XRD pattern of Co3O4-rods reduced at different reduction stages further confirmed initial reduction of Co3O4 to CoO and the subsequent CoO to Co (in Figure S9). Generally, the catalytic activity was intimately associated with the low-temperature reducibility.31,

35

Co3O4-rod nanomaterials were easily reduced by CO at low

temperature, which was in accordance with the results that Co3O4-rods show the highest catalytic activity for CO oxidation. The redox reaction was easily occurred on Co3O4-rods that can be attributed to more active oxygen species on the surface of Co3O4-rods. Notably, the more easy to reduce an oxide is, the more liable to generate oxygen species.55-56 The active oxygen species are more easily generated on the surface of Co3O4-rods, which is consistent with the O 1s spectra of three samples.

Figure 9 H2-TPR profiles of Co3O4 nanomaterials with different morphology.

Conventionally, the catalytic activities of catalysts is strongly related to their BET surface areas, the catalyst with higher BET surface areas exhibited higher catalytic performance.19 However, in this work, these samples have similar specific


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surface area but exhibited obvious differences in CO oxidation activity, indicating that there may be other influential factors responsible for the different catalytic activity of three samples in CO oxidation. For Co3O4, a DFT theoretical calculattion and a large number of experiments have proved that CO molecule preferentially adsorbed on Co3+ cation sites, and Co3+ cations were regarded as the active sites for CO oxidation.34,57-61 Then, the activated CO molecules on Co3+ cation site was oxidized by the lattice oxygen species, leading that active cobalt sites (Co3+) were partially reduced (Co2+) and neighboring oxygen vacancies were generated, and then the oxygen vacancies was replenished by the external oxygen molecules that result in the regeneration of active Co3+ sites.61-62 Therefore, it is widely believe that the active sites and the surface oxygen species play significant roles in catalytic oxidation reaction.31, 63-65 In addition, the atomic ratio of Co3+/Co2+ cations on surface of Co3O4 nanomaterials were further extracted from XPS spectrum. The ratios of Co3+/Co2+ on the surface of Co3O4-rods was about 1.99 lower than Co3O4-octahedrons of 2.07 and Co3O4-plates of 2.28 (in Table 1). The surface oxygen species were further analyzed from deconvoluted O (1s) XPS spectra. From quantitative analysis of O 1s spectra, it is clearly that the Co3O4-rods had the most abundant adsorbed oxygen (oxygen defects) on Co3O4-rods surface (in Table 1). These results were reasonable explanation that Co3O4-rod nanomaterials show the excellent catalytic activity for CO oxidation.

4. Conclusions In summary, we reported a facile solvent-free process to successfully synthesize Co3O4-octahedrons, -plates and -rods via a solid-state chemical reaction between cobalt salt and NH4F or H2C2O4·2H2O. Co3O4-octahedrons and Co3O4-plates can be controlled synthesized by simply adjusting the molar ratio of Co2+/NH4F based on F-



preferentially adsorbed onto specific facets, which can reduce their surface energy and lead to the formation of stable structure in solid-state chemical reaction. When oxalic acid is used as a substitute for NH4F to react with cobalt salts, because of the presence of bidentate ligand C2O42-, it can help the growth of product particles to form nanorods. As-prepared Co3O4-rods exhibited the superior catalytic activity and excellent durability at 90oC, which reach 100% conversion at 90oC (without pretreated). Moreover, Co3O4-rods also show excellent catalytic activity for CO oxidation in feed gas with moisture and maintained 100% conversion for 26 h at 100oC, comparable or even better than the reported results in literature. The excellent catalytic performance over Co3O4-rods can be attributed to the exposure of {110} facets and more oxygen defects on Co3O4-rods surface. The fast and simple solventfree synthesis method offers a promising route to prepare diverse metal (oxide) nanomaterials for energy conversion and environmental remediation. Supporting Information XRD patterns of (NH4)2CoF4 and CoC2O4·2H2O2, XRD patterns, FT-IR spectrum and TEM images of three samples during different temperatures, SEM images of Co3O4-rods obtained with different cobalt salts and reactant ligands. The durability test at 80 oC in normal feed gas after regeneration, XRD pattern of Co3O4-rods at different reduction stages, H2 consumption at different temperature stages in H2-TPR profiles.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21471127, 21361024 and U1503392) and the Natural Science Foundation of Xinjiang Province (Nos. 2014211A013 and 2014711004).


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Solvent-Free Chemical Approach to Synthesize Various

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