Highly Dispersive Cobalt Oxide Constructed in Confined Space for

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Highly dispersive cobalt oxide constructed in confined space for oxygen evolution reaction Meng-Xuan Gu, Yu Kou, Shi-Chao Qi, Ming-Qi Shao, Mingbo Yue, Xiao-Qin Liu, and Lin-Bing Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06214 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Highly dispersive cobalt oxide constructed in confined space for oxygen evolution reaction Meng-Xuan Gu,†,1 Yu Kou,†,1 Shi-Chao Qi,1 Ming-Qi Shao,1 Ming Bo Yue, 2 Xiao-Qin Liu,*,1 and Lin-Bing Sun*,1 1. State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, 30 Puzhu South Road, Nanjing 211816, China. 2. Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal University, 57 Jingxuan West Road, Qufu 273165, China.

†

These authors contribute equally to this work.

*Email: [email protected]; [email protected]

ABSTRACT Cobalt-based catalysts are highly promising for the oxygen evolution reaction (OER) in terms of both cost and performance. The dispersion state of Co3O4 impacts the catalyst performances directly, while to develop an efficient method for Co3O4 dispersion remains a pronounced challenge. In this study, it is the first time that the confined space strategy (CSS) is employed to make highly dispersive Co3O4 on the mesoporous silica (MS) SBA-15. The precursor of Co3O4 is successfully introduced into the confined space inherent in as-prepared SBA-15 (between the silica walls and the template). The CSS facilitates the formation of Co3O4 with extremely high dispersion after the decomposition of Co precursor. Up to 4 mmol of Co3O4 can be dispersed in per gram of MS without any X-ray diffraction lines (the obtained sample is denoted as 4CoAS), while obvious diffraction lines are detected in the counterpart prepared from calcined SBA-15 (the sample is denoted as 4CoCS). Further calculation indicates that the size of Co3O4 nanoparticles in 4CoCS is 9.8 nm, which is much larger than that in 4CoAS by using the CSS (below detection limits). Our results also show 1

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that high dispersion of Co3O4 corresponds to high activity in OER. The typical catalyst 4CoAS exhibits a potential of 0.73 V at the current density of 10 mA∙cm‒2. It is superior to that of 4CoCS (0.78 V) and of the reference catalyst 4Co3O4/CS (0.90 V) with the same Co content. Furthermore, the catalyst 4CoAS presents quite good stability in OER and obviously better than 4CoCS. KEYWORDS: SBA-15, Confined space, Co3O4, Dispersion, Oxygen evolution reaction

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INTRODUCTION The production of renewable clean energy has become one of the most profound challenges for the coming several decades.1-3 Electrocatalytic water splitting that converts electric energy into molecular hydrogen is one of the prevailing ways for producing clean fuels, which can meet the future need for environmentally friendly and renewable energy sources.4-6 It is of great significance to fabricate stable and efficient catalysts for water oxidation into oxygen before evolving hydrogen efficiently. The study of electrocatalysts for oxygen evolution reaction (OER) has drawn much attention.7-12 Until now a series of catalysts have been applied to the OER, including some complexes and oxides of noble metals such as ruthenium (Ru), iridium (Ir) with high cost.13,14 Recently, oxides of non-noble transition metals like Fe, Co, Ni, and Mn have been studied as promising candidates of OER catalysts,8,15 of which Co3O4-based catalysts are widely explored because of their relatively low cost and high performance. However, owing to the relatively low surface area and poor conductivity, bulk transition metal oxides are inert for the OER. There have been three approaches reported to enhance the performance of Co3O4 catalysts. The first one was to increase the surface area through the adjustment of nanostructures, including the nanoparticles, nanowires, or nanosheets. For example, Tüysüz et al. used the template-directed method to fabricate the Co3O4 of highly-ordered mesoporous, and the resultant catalyst showed enhanced OER activity.16 The second one was to prepare metal−carbon nanohybrids or to support the catalysts onto the electrically conductive materials. For instance, with the carbonization of metal-organic frameworks, Ma et al. made the hybrid Co3O4−carbon nanowires at the presence of Cu foil to enhance the OER performance.17 The third one is to change the chemical environment by doping/controlling facets. These interesting approaches are helpful to the preparation of Co3O4−based OER catalysts with enhanced activity. However, the dispersion state of Co3O4 in the catalysts has not been valued. It is discovered that the dispersion state of active species impacts the activity of catalysts directly.18,19 The catalysts with high dispersion of Co3O4 gives better catalytic activity. Nevertheless, the development of an efficient method to disperse Co3O4 remains a pronounced challenge. Because of the high surface area and large pore volume, mesoporous silica (MS) is an ideal 3



support to load metal oxides.20-22 Many attempts have been made to promote the dispersion of metal oxides, for example, by grafting functional groups onto the surface to interact with guest oxides.23,24 Post-modification provides useful method for the dispersion of metal oxides, however, the inherent characteristic of as-prepared MS is usually ignored.25,26 There is a special confined space between the silica walls and template in as-prepared MS that allows the introduction of guest species. For example, amines were incorporated into as-prepared MS MCM-41 and an efficient adsorbent for CO2 capture was fabricated.27 Hence, the confined space might be beneficial to the dispersion of Co3O4 with enhanced OER activity, while such work has never been reported until now. In this study, the confined space strategy (CSS) is employed to design the highly dispersive Co3O4 on a typical MS, SBA-15, for the first time. With employing the as-prepared SBA-15 as the host, the Co-containing precursor is introduced into the confined space (Scheme 1). The decomposition of Co-containing precursor occurred within the confined space facilitates the formation of Co3O4 with high dispersion. By use of the CSS, the amount of Co3O4 dispersed in SBA-15 can reach 4 mmol·g−1 and the size of nanoparticles is below detection limits. In contrast, the counterpart prepared from the calcined SBA-15 (template-free one) with the same Co loading shows serious aggregation of Co3O4 with the particle size of 9.8 nm. More importantly, the materials prepared by using the CSS show better catalytic activity and stability in OER than that prepared without using the CSS. EXPERIMENT SECTION Synthesis of SBA-15.28-31 In a typical process, Pluronic P123 (2.0 g) was dissolved into the aqueous HCl solution (1.60 M, 75.0 g). Tetraethylorthosilicate (TEOS, 4.25 g) was added and stirred at 40 oC for 24 h, followed by the hydrothermal treatment at 100 oC for 24 h. After filtration and then dried under ambient conditions, the template-occluded SBA-15 (denoted as AS) was recovered. Calcined in a flowing air at 550 oC for 5 h, the template P123 was removed to get the template-free SBA-15 (denoted as CS). Synthesis of Co-Containing Catalysts. Under ambient conditions, Co(NO3)2·6H2O of prescribed amount was introduced into the AS by grinding for 30 min, and then underwent the calcination at 400 oC under an air atmosphere, to obtain the samples denoted as nCoAS (n in 4


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mmol·g−1, the amount of Co3O4 obtained). For example, 105 mg of Co(NO3)2·6H2O was introduced to 200 mg of AS, followed by calcination to obtain the sample denoted as 4CoAS. For comparison, Co(NO3)2·6H2O was also introduced into CS via the process described above to get nCoCS. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded with a Bruker D8 Avance diffractometer and with Cu Kα radiation at 40 kV and 40 mA. The N2 adsorption-desorption isotherms at 77 K were measured on an ASAP 2020. The samples were degassed at 423 K for 3 h before the measurement. Transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDX) were performed with a Philips Analytical FEI Tecnai 30 electron microscope. Fourier transform infrared (FTIR) spectra were tested with a Nicolet Nexus 470 spectrometer at the spectra resolution of 2 cm‒1. Thermogravimetric (TG) curves and their derivatives (DTG) were obtained on a thermobalance (STA-499C, NETZSCH). About 5 mg of sample was heated from the room temperature to 800 °C in a ﬂow

of

air

(20

mL·min-1).

The

pore

size

distribution

was

calculated

by

Barrett-Joyner-Halenda (BJH) method according to the adsorption branch. Catalytic Reactions. The OER electrocatalytic activities were evaluated by a ring disk electrode (RDE) using a rotating disk electrode made of glassy carbon (GC, Φ = 4.0 mm) with the same amount of catalyst loading under identical conditions.32-34 The GC electrode was pre-polished with α-Al2O3 slurries (average particle diameter of 50 nm) loaded on a polishing cloth, sonicated for 30 s and rinsed with deionized water. For the preparation of catalyst inks, firstly, 10.0 mg of the catalysts and 10.0 mg of carbon black (Super P Li) were mixed in a solution containing 1.0 mL of ethanol and 0.1 mL of Nafion (5.0%, wt) under sonication for 2 h to obtain a homogenous ink. Then, 5 mL of the homogenous ink was pipetted onto the glassy carbon, which resulted in a catalyst loading of 0.361 mg∙cm-2 disk, and the ink was left to dry before the OER test. With the Pt wire as the counter electrode and the Ag/AgCl in 3.5 M KCl solution as the reference one, electrochemical measurements were performed at the room temperature. The electrolyte was made of 0.1 M KOH aqueous solution, saturated with pure O2 before and during each test to maintain the O2/H2O equilibrium. Linear sweep voltammogram (LSV) plots were recoreded at a scan rate of 5 mV∙s-1 from 0.2 V to 1.0 V at stirring rate of 1600 rpm. The long-term stability (LTS) test was 5



performed as the cycle voltammograms (CVs) of the RDE with a scan rate of 10 mV∙s-1 for 100 times. The LTS was also evaluated by chronopotentiometry (CP) at a constant current density of 10 mA∙cm-2 for 3 h. RESULTS AND DISCUSSION Structural and Surface Properties All samples show the similar intense diffraction peak and two relatively weak ones in low-angle XRD patterns (Fig. 1A), respectively indexing to (100), (110), and (200) reflections and corresponding to a two-dimensional hexagonal pore symmetry.35,36 This reflects that the ordered mesoporous structure of all samples is well kept after the loading of Co. Comparing with CoCS, the CoAS samples show stronger peak of (110) and (200) reflections, which indicates better mesoporous structure of CoAS samples. It is obvious that all samples show a broad XRD peak that is centered at 23o (Fig. 1B), which is attributed to the amorphous silica walls.37,38 No new diffraction lines appear on the samples 2CoAS and 4CoAS. However, some new characteristic peaks located at 31.2o, 36.7o, 44.7o, 59.5o, and 65.3o derived from the Co3O4 emerge on all of the CoCS samples and CoAS with a high Co loading (i.e. 6CoAS); this shows the aggregation of Co3O4 in these samples.39 With the increase of Co loading in the CoCS samples, the peaks become stronger. As compared with CoCS, the CoAS samples exhibit no or broader diffraction peaks of Co3O4, indicating the better dispersion and smaller size of Co3O4 in CoAS. The size of Co3O4, that is obtained by Scherrer equation, can be calculated based on the Co3O4 (111) diffraction peak. Because no diffraction lines are detected on 4CoAS, the particle size is below detection limits. However, the counterpart 4CoCS has the particle size of 9.8 nm, which is much larger than that in 4CoAS (Table 1). The increase of Co loading to 6 mmol·g−1 leads to the enhancement of particle size, and the particle size in 6CoCS (11.4 nm) is still obviously larger than that in 6CoAS (7.8 nm). Based on the above results, it is reasonable to say that the use of CSS promotes the dispersion of Co3O4 in MS. As seen in Fig. 2 and Table 1, the isotherm of the Co-containing sample in possession of IV shape and H1 hysteresis loop is similar to that of the CS, which indicates the presence of 6


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regular mesoporous structures.40,41 Compared with CoAS, the CoCS sample shows a tail at desorption stage, which is gradually highlighted with the increased Co content. Correspondingly, the hysteresis loop of CoCS moves towards the low relative pressure area in contrast to that of parent SBA-15 and CoAS. According to further calculation, all CoAS samples prepared by the CSS show higher surface areas and pore volumes than their analogues CoCS. These results confirm that the Co3O4 prepared by the CSS is well dispersed in the MS, while aggregation is observed in the samples derived from template-free SBA-15. TEM provides other important information for the mesoporous structure and the dispersion of Co3O4. Periodic mesopores of the support SBA-15 can be seen from Fig. S1. After loading Co, both CoAS and CoCS give periodic mesoporous structure as shown in the bright-field TEM images (Fig. 3). This indicates that the structure of the raw material, SBA-15, is not destroyed after the introduction of Co. What’s more, no aggregated particles can be observed from the TEM images of 4CoAS, which means that Co3O4 is highly dispersive on the MS. The TEM images of 4CoCS show many nanoparticles, which implies serious aggregation of Co3O4. The presence of Co can be evidenced from the EDX spectra of the samples 4CoAS and 4CoCS. The elemental mapping images (Fig. 4) also show that Co is introduced into the MS successfully. Moreover, the Co species of 4CoAS exhibit better dispersion than that of 4CoCS. It is worthy to note that the color of samples derived from different supports is quite different (Fig. 5). It is known that the MS is white for both AS and CS. After introducing Co precursor, the samples turn to pink, while the one derived from CS is a little darker. Noteworthily, after the decomposition of precursor and formation of Co3O4, the sample 4CoAS becomes white, while 4CoCS is brown. The big difference in color is obviously caused by the different dispersion state of Co3O4 in the MS, and the sample prepared by using the CSS with better dispersion shows lighter color. Based on the above-mentioned results, it is safe to say that Co3O4 is well dispersed on the MS by use the CSS, while severe aggregation takes place in the counterparts derived from MS without confined space (the template-free one). The higher dispersion of Co3O4 leads to better catalytic performance in OER as shown below. Catalytic Performance 7



The catalytic activities of the materials for OER were evaluated and the results were depicted in Fig. 6. All of the potential values were iR-corrected during each LSV measurement to offset the resistance of the solution. The results reveal that sample 2CoAS exhibits an onset potential at about 0.58 V, which is lower than that of 2CoCS (0.60 V). With the increase of potential, 2CoAS shows the higher current density, which indicates a better OER catalytic activity than that of 2CoCS. With the increase of the Co loading, the activities of catalysts increase. The sample 4CoAS shows a similar onset potential to 2CoAS. When the current density is 10 mA∙cm-2, the overpotential of 4CoAS is 0.73 V, which is obviously lower than that of 4CoCS (0.78 V). These results prove that good dispersion of Co3O4 leads to better catalytic activities in OER. Similarly, 6CoAS shows better activity in OER than 6CoCS. To fully evaluate the catalytic performance, a reference sample 4Co3O4/CS was prepared by grinding Co3O4 with the support CS. With the same current density, the potential is 0.90 V for 4Co3O4/CS, which is much higher than that for 4CoAS (0.73 V). This suggests that the sample prepared by the CSS possesses better catalytic OER activity than that prepared by the conventional methods. What’s more, the stability of catalysts 4CoAS and 4CoCS is measured. In the case of 4CoAS, there is no obvious decrease in catalytic activity after 100 cycles, while the drop in activity is observed under the same conditions (Figs. 6 and S2). This reflects the better stability of 4CoAS than that of 4CoCS. These results thus demonstrate that the samples prepared by the CSS exhibit good catalytic performance in terms of both activity and stability. Proposed Mechanism Based on the results aforementioned, it is clear that the Co3O4 can be well dispersed on the MS SBA-15 by using the CSS and that the obtained materials exhibit better catalytic performance in OER as compared with their counterparts prepared through the conventional methods. It is believable that the confined space plays an important role in the dispersion of Co3O4. The introduction of Co precursor to the confined space is the first step for the preparation of materials, which can be evidenced by TG results. As shown in Fig. 7, an obvious weight loss of the sample AS is observed, which corresponds to a DTG peak at 180 oC.

The DTG peak should be ascribed to the decomposition of template P123. The 8


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temperature is obviously lower than that of the pure P123 (210 oC) because that silica walls can catalyze the decomposition of the template.38 For the CoCS samples, a DTG peak at around 200 oC is observed, which is due to the conversion of Co precursor to oxide (Fig. S3). In the case of the CoAS samples, an evident DTG peak at around 300 oC appears in addition to the one at 200 oC derived from Co precursor conversion. For instance, the sample 4CoAS shows an additional DTG peak at 310 oC, which is obviously higher that of the P123 decomposition in AS (180 oC). This indicates that the Co precursor has been successfully introduced to the confined space, which separates P123 from the silica walls, so that silica walls would not catalyze the decomposition of P123. Actually, such a decomposition temperature at 310 oC is also higher than unsupported P123 (210 oC). Hence, the Co precursor located in confined space enhances the temperature for P123 decomposition. In addition, the abundant Si−OH groups existing in AS favors the dispersion of Co3O4 as well. FTIR spectra were employed to monitor the hydroxyl and other groups of different samples before and after calcination (Figs. 8, S4-S6). The sample AS shows the FTIR bands in the range of 2850–3000 cm-1 and 1350–1500 cm-1, which are caused by the C–H stretching and bending vibrations of the P123 (Fig. S4).37 The band located at 960 cm-1 is attributed to the stretching vibration of Si–OH.36 It is clear that the sample AS has more hydroxyl groups than CS, and the same trend is also observed for CoAS and CoCS after loading Co. However, the band located at 960 cm-1 of CoAS is lower than that of CoCS, which means that more silanol groups are consumed during calcination for CoAS than that for CoCS. (Fig. 8) The hydroxyl groups on silica walls can interact with Co species during calcination, which benefits the dispersion of Co3O4. The strong interaction of metal with hydroxyl groups has been proven through computer simulation.42 As a result, higher density of hydroxyl groups shows better resistance to the aggregation of the loaded Co3O4. Based on the above analysis, it is conclusive that the confined space as well as strong interaction caused by abundant hydroxyl groups are responsible for the good dispersion of Co3O4 by using the CSS. CONCLUSIONS A facile and efficient CSS is developed to disperse Co3O4 in the typical MS SBA-15. 9



Highly dispersive metal oxides are obtained by using as-prepared SBA-15, AS, as the support. Up to 4 mmol∙g‒1 of Co3O4 can be well dispersed in SBA-15 without any X-ray diffraction lines (4CoAS), whereas noticeable diffraction lines are observed in the counterpart prepared at the presence of template-free SBA-15 (4CoCS). The nanoparticle size of aggregated Co3O4 is estimated to be 9.8 nm in 4CoCS, while that in 4CoAS is below detection limits. The high dispersion of Co3O4 is attributed to the confined effect and the strong interaction caused by template-containing MS, which is absent in template-free MS. It is also demonstrated that the materials with higher Co3O4 dispersion exhibit higher catalytic stability in OER. The typical catalyst 4CoAS shows a potential of 0.73 V at the current density of 10 mA cm‒2. This is superior to that of 4CoCS (0.78 V) and the reference catalyst 4Co3O4/CS (0.90 V). Such catalytic activity of 4CoAS can be remained without any loss even after 100 cycles. Our strategy should enable us to fabricate functional materials decorated with various metal oxides, bimetal oxides, and even composites with high dispersion for applications in catalysis, adsorption, sensing, etc. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors. *X.-Q. L.: E-mail: [email protected]; *L.-B. S.: E-mail: [email protected].

ACKNOWLEDGMENT We acknowledge financial support of this work by the National Natural Science Foundation of China (21722606, 21576137, and 21676138) and Six Talent Plan of Jiangsu Province 10


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(2016XCL031).

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Scheme 1. Formation of (A) highly dispersed Co3O4 by using the confined space in AS and (B) aggregated Co3O4 via the conventional method.

A

B

Co3O4 6CoCS

6CoCS

4CoCS 4CoAS 2CoCS 2CoAS

6CoAS

Intensity (a.u.)

6CoAS

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4CoCS 4CoAS 2CoCS 2CoAS

CS 1

2

3

2 (degrees)

CS 20

4

40

60

2 (degrees)

80

Fig. 1 (A) Low-angle and (B) wide-angle XRD patterns of CS and Co-containing samples.

17



1000

6CoAS 4CoCS

-1

1500

B 6CoCS

4CoAS

3

2000

3

-1

2500

Pore volume (cm g ,STP)

A Volume adsorbed (cm g ,STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2CoCS 2CoAS

500 0 0.0

Page 18 of 23

CS

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

1.4

6CoCS

1.2

6CoAS

1.0

4CoCS

0.8

4CoAS

0.6

2CoCS

0.4

2CoAS

0.2

CS

0.0 4

8

12

16

Pore diameter (nm)

20

Fig. 2 (A) N2 adsorption-desorption isotherms and (B) pore size distributions of CS, CoCS, and CoAS samples. Curves are plotted offset for clarity.

Fig. 3 Bright-field TEM images of (A) 4CoAS and (B) 4CoCS as well as EDX spectra of (C) 4CoAS and (D) 4CoCS.

18


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Fig. 4 Dark-field TEM images of (A) 4CoAS and (B) 4CoCS; EDX elemental mapping of Si on (C) 4CoAS and (D) 4CoCS as well as Co on (E) 4CoAS and (F) 4CoCS.

Fig. 5 Photographs of the samples (A) 4CoAS and (B) 4CoCS before calcination as well as (C) 4CoAS and (D) 4CoCS after calcination.

19



Fig. 6 LSV curves of the samples (A) 2CoAS and 2CoCS, (B) 6CoAS and 6CoCS, as well as (C) 4CoAS, 4CoCS, and 4Co3O4/CS (the reference sample) for the OER on the RDE (1600 rpm) in O2-saturated 0.1 M KOH solution at a scan rate of 5 mV∙s-1; (D) LSV curves of 4CoAS before and after 100th CV scans of 4CoAS with an oxide catalysts loading of 0.361 mg∙cm-2disk supported on a RDE in O2-saturated 0.1 M KOH electrode at a scan rate of 10 mV∙s-1 (1600 rpm).

20


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B

100

20 AS 2CoAS 4CoAS 6CoAS

80 70

o

90

DTG (wt % / C)

A

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

6CoAS 4CoAS

10

2CoAS

0

AS

-10

50 200

400

600 Temperature ( C) o

800

200

400

600 Temperature ( C) o

Fig. 7 (A) TG and (B) DTG curves of the samples AS, 2CoAS, 4CoAS, and 6CoAS.

Fig. 8 IR spectra of 4CoAS and 4CoCS before calcination (a) and after calcination (b).

21


800


Page 22 of 23

Table 1. Physicochemical properties and catalytic performance of different samples SBET

Vp

Dp

dCo3O4a

Eb

(m2·g−1)

(cm3·g −1)

(nm)

(nm)

(V)

CS

794

0.862

8.1

/

/

2CoAS

652

0.848

8.1

NDc

0.77

2CoCS

611

0.695

7.1

9.0

0.83

4CoAS

552

0.698

8.1

NDc

0.73

4CoCS

530

0.679

7.1

9.8

0.78

6CoAS

464

0.600

8.1

7.8

0.78

6CoCS

428

0.579

7.1

11.4

0.83

Sample

a Estimated

from XRD using the Scherrer equation; b The potential of the current density at 10

mA·cm2; c Below detection limits.

22


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For Table of Contents Use Only

Formation of highly dispersed Co3O4 for oxygen evolution reaction was realized by using the confined space inherent in as-prepared mesoporous silica.

23


Highly Dispersive Cobalt Oxide Constructed in Confined Space for

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