Cobalt salophen functionalized SBA-15 as an active catalyst for photocatalytic water oxidation

Abstract

Design and synthesis of highly efficient and cost-effective catalysts for water oxidation is one of the biggest challenges that chemists are facing today. The low-cost salophen cobalt complex represents a promising water oxidation visible-light-photocatalyst under neutral conditions. To avoid degradation and improve the photocatalytic activity of cobalt salophen, we have synthesized a cobalt salophen-based organic–inorganic hybrid mesoporous material (MC) via grafting cobalt salophen into mesoporous SBA-15 for water oxidation under mild conditions. MC is found to be more active and stable than the neat salophen cobalt. Under similar conditions, MC shows a significantly higher activities (turnover frequency: 59.64 mol of O₂ h⁻¹ mol Co⁻¹) than the corresponding cobalt complex (turnover frequency: 2.520 mol of O₂ h⁻¹ mol Co⁻¹). After being used four times, the reactivity and the yield of oxygen for MC is still 10 times higher than pure salophen cobalt. MC could be considered as a promising catalyst for water oxidation.

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Introduction

The water oxidation reaction has been regarded as a promising strategy for solar or electrical energy storage.¹ In the past few years, a lot of efforts have been devoted to the design and synthesis of efficient and cost-effective catalysts for water oxidation.² Inspired by other oxidation reactions, first-row transition metal–Schiff base complex have been reported for water oxidation applications, such as manganese and cobalt.³ Particularly, the cobalt salophen complex represented a promising water oxidation visible-light-photocatalyst under neutral conditions. However, the Co macrocyclic complex undergoes dimerization, and this will severely shorten the lifetime of the Co–Schiff base catalytic system.⁴ So it is important to improve the photocatalytic activity of the cobalt salophen complex by avoiding dimerization and degradation of the Co–Schiff base complexes.

In general, there are some methods to improve the stability of the Schiff base complexes. For example, immobilizing the Schiff base complexes onto/into mesoporous materials such as zeolites, silica and carbon is one of the most efficient approaches.⁵ Besides enhancing stability, the periodicity of nanoscale pores in these mesoporous materials facilitate mass transfer and allow a high concentration of active sites per mass of material, which favor to improve the activity of the reaction.⁶ Among the various mesoporous materials, mesoporous silica SBA-15 with two-dimensional hexagonal structure of uniform cylindrical mesopores has been used as an excellent support for many photocatalyst applications due to its higher surface area, larger pore volume and better-defined and tunable pore size than the others.⁷

Herein, we designed and synthesized a new cobalt salophen functionalized SBA-15 (MC) via 2-acetylpyridine and cobalt nitrate reacting with aminopropyl-functionalized SBA-15. Under similar conditions, MC shows a significantly higher activities (turnover frequency: 59.64 mol of O₂ h⁻¹ mol Co⁻¹) than the corresponding Schiff–base (turnover frequency: 2.520 mol of O₂ h⁻¹ mol Co⁻¹). After being used four times of MC, the reactivity and the yield of oxygen are still 10 times higher than pure salophen cobalt. The compound could be used for photocatalytic water oxidation with high activity and good stability. To the best of our knowledge, this is the first report of the synthesis, characterization and catalytic applications of MC as an efficient water oxidation under visible-light.

Experimental

Materials and physical measurements

All reagents and solvents are purchased from commercial sources and used without further purification. FT-IR spectra are performed on a Nicolet Nexus 470 FT-IR spectrophotometer. Elemental analyses of C, H, and N are performed on a Perkin-Elmer 240C elemental analyzer. Atomic Absorption Spectroscopy (AAS) is used to determine the content of cobalt on a Perkin-Elmer 41000 ZL spectrophotometer. X-ray powder diffraction (XRD) measurements are performed on a Bruker D8 diffractometer operated at 40 kV and 40 mA using a Cu-K_α radiation (λ = 1.54056 Å). The N₂ adsorption isotherms and BJH pore size distributions are measured at 77 K by using a Micromeritics ASAP 2020M volumetric adsorption analyzer. Transmission electron microscopy (TEM) images are recorded on a JEOL 2010 microscope operating at 200 kV. Scanning electron microscopy (SEM) images are obtained with a JSM-6700F field emission scanning electron microscope. The electronic absorption spectrum is recorded using a UV-2450 UV-vis spectrophotometer at room temperature. Oxygen evolution is monitored by gas chromatography using a thermal conductivity detector (GC-TCD).

Preparation of catalyst (MC)

First, according to the previously reported procedure,⁸ the NH₂ group-functionalized SBA-15 (NH₂-SBA-15) was prepared by co-condensation of (3-aminopropyl)triethoxysilane (APTS) and tetraethyl orthosilicate (TEOS) with the molar ratio of 1 : 9 in the presence of surfactant P123 as a template. The surfactant and other organic residues were removed by refluxing in ethanol at 353 K for 24 h.

Then, in a 100 mL beaker, 250 mg of the as-prepared NH₂-SBA-15 was suspended in acetonitrile, and then 0.5 mL 2-acetylpyridine was added to the suspension, the resulting suspension was stirring for 5 h at 318 K. Subsequently, 5 mL of cobalt nitrate aqueous solution was added dropwise to the mixture. The reaction temperature was kept at 318 K for 4 h. The products were isolated by filtration, washed with water/ethanol (volume ratio of 1 : 1) for several times and dried. The prepared catalyst is designated as MC (Scheme 1A). Atomic absorption spectrometric result showed cobalt content of MC is ca. 3.8% (wt). The molecular structures of MC complexes are shown in Scheme 1.

Scheme 1 Preparation of MC catalyst.

Catalytic reaction

Cyclic voltammograms are recorded using a CHI-730 chemical workstation and a platinum working electrode (PE), a platinum-wire auxiliary electrode, and a saturated calomel reference electrode (SCE). KCl is used as the supporting electrolyte at a concentration of 0.1 M. Potentials are reported relative to K₃[Fe(CN)₆] (0.5 mM)/in KCl (0.1 M); the potential E_1/2 of K₃[Fe(CN)₆] is 0.190 V against SCE.

Light-driven water-oxidation reactions are performed in a glass photolysis vessel containing 0.05 mol catalyst, 0.45 μmol of [Ru(bpy)₃]Cl₂ (sensitizer), 0.08 mM Na₂S₂O₈ (electron acceptor) and 15 mL water at pH = 8.5 (Na₂HPO₄/NaH₂PO₄). After purging with N₂, the vessel is illuminated by the light source using a LED lamp (450–550 nm, 4 W). The aqueous suspension is maintained under stirring and illumination for 120 min. Oxygen is analyzed with a gas chromatograph equipped with a thermal conductivity detector.

Results and discussion

FT-IR spectrum

Fig. 1 shows the FT-IR spectrum of MC and recovered MC after catalysis (RMC). In the hydroxyl region (3600–3200 cm⁻¹) a broad band is seen, assigned to the silanol groups inside the channels of MC and RMC. The band at 1060 cm⁻¹ is assigned to the asymmetric vibrations of Si–O–Si and the band at 933 cm⁻¹ is attributed to vibrations of Si–OH. Further, cobalt salophen is grafted to the silica successfully by the presence of the characteristic C=N band (1635 cm⁻¹), the vibrations of pyridine (794 cm⁻¹) and the absorption of Co–N band (460 cm⁻¹).

Fig. 1 FT-IR spectrum of MC and recovered MC after catalysis (RMC).

X-ray diffraction (XRD)

The small-angle XRD (SXRD) patterns of NH₂-SBA-15 and MC are presented in Fig. 2. NH₂-SBA-15 shows three well-resolved peaks that can be indexed as (100), (110) and (200) diffractions associated with the characteristic pattern of the typical 2D hexagonal (P6mm) mesoporous structure. Meanwhile, the SXRD pattern of MC shows the reduced peak intensity (100) and the disappearance of two small peaks indicative of (110) and (200) diffractions. These imply that mesoporous structure of MC is remained almost unchanged but the long range order is significantly decreased due to Co–Schiff base incorporated inside the SBA-15 pores.⁹ Comparing with NH₂-SBA-15, the peak (100) position of MC shifts to higher angle which is indicative of smaller pore size due to the constriction of unit cell. These results are entirely consistent with the corresponding pore diameter analyses (Table 1).

Fig. 2 Small-angle XRD patterns of (A) (NH₂-SBA-15) and (B) (MC) samples.

Table 1 The relevant parameters of N₂ adsorption–desorption

Catalysts	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
MC	283	4.133	4.64
NH2-SBA-15	519	5.890	6.15

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Nitrogen sorption

The surface area, pore volume and pore size distribution of NH₂-SBA-15 and MC is analyzed by nitrogen adsorption–desorption isotherms. As shown in Fig. 3, both of them exhibit a typical type IV isotherm characteristic for mesoporous materials.^7a The BET specific surface areas of MC and NH₂-SBA-15 are calculated to be 283 and 519 m² g⁻¹, respectively (Table 1). MC has lower specific surfaces than NH₂-SBA-15 due to the grafting of Schiff–base (Co). The amount of adsorbed nitrogen decreases and the inflection point of the step shifts to a lower value of relative pressure for MC. These indicate that pore size become smaller after the grafting process as expected. The results are entirely consistent with those from the small-angle XRD analysis and pore size distributions (inset in Fig. 3) of NH₂-SBA-15 and MC.

Fig. 3 N₂ adsorption–desorption isotherms at 77 K and pore size distribution profiles (inset) of (A) (NH₂-SBA-15) and (B) (MC).

SEM-TEM

The morphology and microstructure of NH₂-SBA-15 and MC are characterized by SEM (Fig. 4) and TEM (Fig. 5) respectively. TEM result is consistent with that of SXRD measurement. It reveals that both NH₂-SBA-15 and MC exhibit 2D hexagonal pore arrangement, but that the structural ordering is lower after the grafting process. The estimated pore size for NH₂-SBA-15 and MC is 7 and 4 nm, respectively. Certainly, MC's morphology has been altered to some extent due to the grafting process. NH₂-SBA-15 has a rod-like hexagonal mesoporous morphology. While, MC has defined rope like aggregates in its SEM images due to Co complex which are partly deposited on the external surface of the NH₂-SBA-15 rods.

Fig. 4 SEM images of NH₂-SBA-15 (a and b) and MC (c and d).

Fig. 5 TEM images of NH₂-SBA-15 (a) and MC (b).

UV-vis spectrum

UV-vis diffuse reflectance spectra of MC and NH₂-SBA-15 are shown in Fig. 6. Comparing with NH₂-SBA-15, MC exhibits excellent absorption in the ultraviolet and visible regions. For MC, the strong absorption at ∼276 nm could be assigned to a Π–Π* electron transition of pyridine. And the absorption peak at 500–550 nm characteristically belongs to d–d electron transition of tetrahedrally coordinated Co with a d⁷ configuration.¹⁰ The spectrum clearly indicates that MC has been successfully synthesized.

Fig. 6 UV-vis absorption spectra of MC and NH₂-SBA-15.

Electrochemical properties of MC

To further elucidate the water oxidation activity of NH₂-SBA-15 & MC, the electro-catalytic measurements are performed in phosphate buffer (pH 8.5) using cyclic voltammetry (CV) at a scan rate of 100 mV s⁻¹. As shown in Fig. 7, SBA-15 catalytic current is not apparent until a potential of 1.50 V, while the intense anodic wave beginning at 1.20 V (vs. SCE), which indicate that bare SBA-15 is inactive for water oxidation. For MC, a cathodic wave appeared in −0.63 V, assigning to the reduction of dioxygen formed at the working electrode, while the cathodic wave is lower than −0.90 V for NH₂-SBA-15 in the same condition. The results demonstrate clearly that SBA-15 is the only support to enhance water oxidation and does not enhance water oxidation itself. Moreover, LED light enhanced catalytic current was observed when MC coated FTO was used as working electrode confirming that the water oxidation catalyzed by MC is driven by LED light (Fig. 7, black line).

Fig. 7 Cyclic voltammograms of MC coated FTO working electrode (blue line, without irradiation; black line, irradiated under LED light (4 W)) and NH₂-SBA-15 coated FTO working electrode (red line) in 1 mM KCl solution. Scan rate 100 mV s⁻¹.

Photocatalytic dioxygen evolution

For comparison, photocatalytic activity of MC (Fig. 8, line A), salophen cobalt (Fig. 8, line B) and NH₂-SBA-15 are investigated under identical conditions. During our experiment, we find NH₂-SBA-15 produce only negligible O₂, indicating that the host is inactive for oxidation. As shown in Fig. 8, the activity of MC is higher than salophen cobalt under our experiment condition. Normalized to the same amount of Co, the O₂ yield for MC is drastically increased at least 70 times as compared to salophen cobalt. Based on the surface structure analysis of MC and salophen cobalt, the results indicate that porous surface structure and increased surface area are responsible for the high visible light photocatalytic activity of MC.

Fig. 8 Photocatalytic oxygen evolution of (A) (MC Co 1 μmol) and (B) (salophen cobalt Co 1 μmol).

The overall cycle for light-driven water oxidation is shown in Scheme 2. It is known that Ru(bpy)₃²⁺ is first excited to form an excited state, Ru(bpy)₃²⁺*. Subsequently the Ru(bpy)₃²⁺* is oxidized by S₂O₈²⁻ to Ru(bpy)₃³⁺, which is subject to degradation due to the nucleophilic attack of water and OH⁻ before reaching the surface of the catalyst, where the electron transfer from the catalyst to Ru(bpy)₃³⁺ happens.¹¹ Hence, the high surface area of salophen cobalt in MC enables more facile access of Ru(bpy)₃³⁺ to catalytic sites, which contributes to a high activity.

Scheme 2 Light-driven water oxidation by S₂O₈²⁻ catalyzed by MC and Ru(bpy)₃²⁺ as photosensitizer.

On the other hand, in order to check reusability of MC, a series of four consecutive runs of the water oxidation reaction are carried out. After each run, the used catalyst is recovered by filtration, washed thoroughly with ethanol and dried. As it is seen (Fig. 9), MC has the potential for efficient recycling for a further 3 additional reaction cycles with the much higher yield of oxygen than pure salophen cobalt. In general, MC can be reusable due to the following reasons: (1) cobalt complex is immobilized in the pores of SBA-15, (2) reduced formation of other species in the pores due to the steric effects of SBA-15 framework and (3) the interaction of encapsulated cobalt complex with the SBA-15 lattice.¹² In order to ascertain the stability, the structure of the recovered MC (labelled as RMC) is analyzed by FT-IR spectrometry (Fig. 1). The characteristic peaks of C=N, Co–N and pyridine are still observed which show that Schiff–base are still immobilize into the SBA-15 after the catalytic action. However, compared with MC, RMC shows the characteristic Co–O stretching (1635 cm⁻¹) and disappears the weak-coordination NO₃⁻ (1388 cm⁻¹), which suggest that the coordination between Co and N (NO₃⁻) has been taken place by Co–O (H₂O). Based on these findings, we have succeed to avoid the degradation of salophen cobalt although the coordination environment of Co has been changed.

Fig. 9 Photocatalytic oxygen evolution of MC (Co 1 μmol) in three consecutive runs.

Conclusion

In summary, for the first time, we have successfully synthesized a Schiff-based organic–inorganic hybrid mesoporous material MC and investigated its catalytic performance for water oxidation. MC exhibits excellent photocatalytic water oxidation, which may be related to its large surface area and tuneable porous structures. Under similar conditions, the O₂ yield for MC exceeds that of bare Schiff base by a factor of 70. In addition, after being used four times of MC, the reactivity and the yield of oxygen is still higher 10 times than pure salophen cobalt, which indicate that the good reusability of MC. MC holds promise a cheap photocatalyst for water oxidation. A further study on different metal center of Schiff-based mesoporous catalyst for water oxidation is in progress.

Acknowledgements

We are grateful for the financial support from the Natural Science Foundation of Jiangsu Province (BK20130485), Highly Qualified Professional Initial Funding of Jiangsu University (12JDG052) and the National Science Foundation of China (21271090 and 21571085).

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