Enhanced photocatalytic activity of a B₁₂-based catalyst co-photosensitized by TiO₂ and Ru(II) towards dechlorination

Abstract

A novel hybrid photocatalyst denoted as B₁₂–TiO₂–Ru(II) was prepared by co-immobilizing a B₁₂ derivative and trisbipyridine ruthenium (Ru(bpy)₃²⁺) on the surface of a mesoporous anatase TiO₂ microspheres and was characterized by DRS, XRD, SEM and BET et al. By using the hybrid photocatalyst, DDT was completely didechlorinated and a small part of tridechlorinated product was also detected in the presence of TEOA only after 30 min of visible light irradiation. Under simulated sunlight, the hybrid exhibited a significantly enhanced photocatalytic activity for dechlorination compared with B₁₂–TiO₂ under the same condition or itself under visible light irradiation due to the additivity in the contribution of UV and visible part of the sunlight to the electron transfer. In addition, this hybrid catalyst can be easily reused without loss of catalytic efficiency. This is the first report on a B₁₂-based photocatalyst co-sensitized by two photosensitizers with wide spectral response.

---

Introduction

Photocatalysis has played a special role in both pollutant degradation and organic synthesis from the viewpoint of eco-friendliness in recent decades.^1–5 As photocatalysts, vitamin B₁₂ and its derivatives have exhibited excellent photocatalytic activity for decomposing halogenated organic compounds due to the supernucleophilic reactivity of Co(I) species, which could be produced from the Co(III) or Co(II) center of B₁₂ and its derivatives under ultraviolet light irradiation in the presence of electron donor precursors.^6–11 In 2004, Hisaeda et al.¹² firstly reported the photocatalytic activity of a heptamethyl cobyrinate perchlorate, [Co(II)7C₁ester]ClO₄, in methanol for DDT dechlorination under visible light irradiation in the presence of Ru(bpy)₃²⁺ as photosensitizer and proved that the Co(I) species could be formed through accepting electrons from the photosensitizer under visible light irradiation. In order to establish a more efficient, economical and environmental friendly visible-light photocatalytic system, heterogenous catalysis has been performed through immobilizing the B₁₂ catalyst and a photosensitizer onto catalyst supports and some recycled hybrid catalysts based on B₁₂ were obtained.^13,14 In 2011, Hisaeda group¹⁴ immobilized a B₁₂ derivative and trisbipyridyl ruthenium proportionally on the same polymer backbone through radical polymerization reaction, and the obtained copolymer catalyst showed high catalytic activity for dehalogenation reaction. They proved that the electron transfer rate from the photosensitizer to the Co center of the B₁₂ catalyst was significantly accelerated attributing to the short distance between the catalyst and the photosensitizer in the copolymer.

TiO₂ as a semiconductor has been widely applied in various fields such as dye-sensitized solar cell, organic pollution degradation, organic synthesis and so on for its strong oxidation and reduction abilities generated by UV light irradiation.^15–18 Hiseada group has successfully immobilized B₁₂ on P25 TiO₂ or AMT600 TiO₂ nanoparticles and the obtained hybrid catalysts showed high catalytic activity for dehalogenation reactions under UV light irradiation.^19–22 In the hybrid systems, TiO₂ served not only as a catalyst support but also an effective photosensitizer. In 2011, Reisner group²³ reported a self-assembled system in which a cobalt catalyst (Et₃NH)[Co^IIICl(dmgH)₂ (pyridyl-4-hydrophosphonate)] was attached on a ruthenium dye [Ru(bpy)₃]²⁺-sensitized P25 TiO₂ nanoparticles. It was proved that TiO₂ played an important role as an electron mediator in the hybrid and promoted the electron transfer from the excited [Ru(bpy)₃]²⁺ moieties to the cobalt catalyst under visible light irradiation. In addition, in recent years, many efforts have been focused on the photocatalytic applications of mesoporous TiO₂ microspheres for their high surface areas, suitable pore structures, and uniform pore size distributions, which could supply more active sites for adsorption and photocatalytic reactions.^24–27 Aiming to develop more efficient photocatalyst for dechlorination, in this paper, a novel B₁₂-based hybrid catalyst co-sensitized by TiO₂ and Ru(bpy)₃²⁺, B₁₂–TiO₂–Ru(II), was prepared through immobilizing a vitamin B₁₂ derivative, cobyrinic acid, and a derivative of trisbipyridine ruthenium, Ru(dcb)(bpy)₂(PF₆)₂, on the surface of mesoporous anatase TiO₂ microspheres as shown in Fig. 1. The catalytic properties of this hybrid catalyst for 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) dechlorination under visible light irradiation or simulated sunlight irradiation were reported. This is the first report on B₁₂-based photocatalyst co-sensitized by two photosensitizers with wide spectral response.

Fig. 1 Illustration of the preparation of the hybrid B₁₂–TiO₂–Ru(II).

Experimental

Materials

1,1-Bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT), Ru(bpy)₂Cl₂, Tetrabutyl titanate, and Ru(bpy)₃Cl₂ was purchased from J & K Scientific Ltd. Cyanoaqua cobyrinic acid [(CN)(H₂O)Cob(III)7COOH]Cl,²⁸ [(CN)(H₂O)Cob(III)7C₁ester]Cl,¹⁹ mesoporous anatase TiO₂ microspheres,²⁵ 4,4′-diethyl ester-2,2′-bipyridine (deep) and Ru(dcb)(bpy)₂(PF₆)₂ (ref. 29) were synthesized according to the literatures. Other reagents were purchased from Sinopharm Chemical Reagent Beijing Co. (SCRC). All the reagents were of analytical grade and were used as received without further purification.

Characterization

The SEM images and EDS analysis were acquired using a Hitachi SU-8010 equipped with an EDX analyzer operated at an accelerating voltage of 5 kV. The TEM images were obtained on a JEM 2100 operating at 200 kV. The UV-vis diffuse reflectance spectroscopy (DRS) measurements were obtained on a UV-vis spectrometer (Shimadzu UV-2550) using BaSO₄ as a reference standard. The specific surface area of the samples was measured by the Brunauer–Emmett–Teller (BET) method using nitrogen adsorption and desorption isotherms on a Micrometrics ASAP 2020 system. The UV-vis spectra were obtained on a Perkin Elmer Lamda 25 spectrophotometer. The NMR spectra were recorded on a Mercury Vx-300 MHz NMR spectrometer. XRD patterns were obtained on a Bruker D8-Advance. The luminescence spectra were measured using a Hitachi F-4500 spectrophotometer in MeOH at room temperature. A 500 W xenon lamp (CHFXQ 500 W, Global xenon lamp power) with a λ ≥ 420 nm optical filter, AM 1.5 optical filter and a heat cut-off filter provided visible light or simulated sunlight illumination.

Synthesis of Ru(dcb)(bpy)₂(PF₆)₂

Ru(dcb)(bpy)₂(PF₆)₂ was synthesized according to the literature²⁹ with slight modification. Ru(bpy)₂Cl₂·2H₂O (75 mg, 0.15 mmol) and 4,4′-diethyl ester-2,2′-bipyridine (44 mg, 0.15 mmol) were dissolved in ethanol (15 mL) and the mixture refluxed for 8 h. The solution was allowed to cool, filtered, and taken to dryness. The residue was dissolved in NaOH solution (0.3 mol L⁻¹, 15 mL) and refluxed for 1 h and cooled to room temperature. A large excess of NH₄PF₆ was added to the solution under vigorously stirring and then HCl (5 mol L⁻¹) was added to the mixture dropwise until much precipitate appeared. Then the precipitate was filtered and washed with a minimal amount of water, CH₂Cl₂, Et₂O respectively. After being dried in vacuum for 6 h, a dark red crystal was obtained. ¹H NMR (CD₃CN), δ: 7.39 (q, 4H), 7.68 (dd, 4H), 7.80 (d, 2H), 7.87 (d, 2H), 8.06 (q, 4H), 8.50 (dd, 4H), 9.19 (s, 2H).

Preparation of B₁₂–TiO₂–Ru(II)

Mesoporous anatase TiO₂ microspheres (30 mg) was added into the methanol solution (6 mL) containing [(CN)(H₂O)Cob(III)7COOH]Cl (8 × 10⁻⁴ mol L⁻¹) and Ru(dcb)(bpy)₂(PF₆)₂ (1.6 × 10⁻³ mol L⁻¹). After the mixture was stirred at room temperature for 4 h, the hybrid B₁₂–TiO₂–Ru(II) was obtained by centrifugation and washed with methanol for three times.

General catalytic procedure

A methanol solution (5 mL) containing B₁₂–TiO₂–Ru(II) (3 mg), DDT (2.4 × 10⁻³ mol L⁻¹) and triethanolamine (0.2 mol L⁻¹) was degassed after three of freeze–pump–thaw circles. After the solution was irradiated for 0.5 h using a xenon lamp with a λ ≥ 420 nm optical filter and a heat cut-off filter or a AM 1.5 optical filter, the catalyst was separated by centrifugation. Methanol was evaporated under reduced pressure and the residue was dissolved in CHCl₃ (5 mL). The CHCl₃ layer was extracted with water (10 mL) to remove TEOA, and then the solvent was evaporated under reduced pressure to dryness. The residues were analyzed by ¹H NMR using 1,4-dioxane as the internal standard. The hybrid catalyst B₁₂–TiO₂–Ru(II) was washed with methanol and then reused after adding fresh triethanolamine and the substrate.

Results and discussion

Fig. 1 illustrates the preparation procedure of the hybrid B₁₂–TiO₂–Ru(II). The mesoporous anatase TiO₂ microspheres were firstly synthesized by hydrothermal reaction and then the catalyst [(CN)(H₂O)Cob(III)7COOH]Cl and the photosensitizer Ru(dcb)(bpy)₂(PF₆)₂ were grafted on the surface of mesoporous anatase TiO₂ microspheres through their carboxylic groups only at room temperature. After the decoration, the colour of TiO₂ microspheres changed from white to saffron yellow as shown in Fig. 1. The Co and Ru contents in the hybrid B₁₂–TiO₂–Ru(II) were 6.83 × 10⁻⁵ mol g⁻¹ and 1.5 × 10⁻⁴ mol g⁻¹, which were determined through detecting the absorbance change of the characteristic peaks of [(CN)(H₂O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)₂(PF₆)₂ at 523 nm and 480 nm respectively in the supernatant by UV-vis spectra. The Co content in this hybrid was obviously increased and was 5 times higher than that of the hybrid B₁₂–TiO₂ (AMT600) reported in the literature,²² which should be attributed to the large surface area and pores of the mesoporous anatase TiO₂ microspheres. In addition, it is worth to mention that the ratio of Co and Ru in the hybrid was about 1 : 2.2 which was similar to their ratio 1 : 2 in the raw solution before adding TiO₂. This result indicated that the proportional co-immobilization of these two different molecules, [(CN)(H₂O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)₂(PF₆)₂, on the surface of TiO₂ microspheres could be easily realized by one pot reaction. In this hybrid, the Ru(II) complexes and the TiO₂ microspheres as photosensitizer are expected to be electron donors for B₁₂ activation to form reactive Co(I) species.

Fig. 2 shows the UV-vis diffuse reflectance spectra of the hybrid B₁₂–TiO₂–Ru(II) and mesoporous anatase TiO₂ microsphere and the UV-vis spectra of [(CN)(H₂O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)₂(PF₆)₂ in methanol. The mesoporous anatase TiO₂ microspheres strongly absorbed ultraviolet light and had very weak absorption after 400 nm.²⁵ The catalyst [(CN)(H₂O)Cob(III)7COOH]Cl exhibited a dominant absorption peak located at λ_max = 490 nm accompanied by a prominent shoulder located at 523 nm and a small absorption peak at 404 nm.²⁸ Ru(dcb)(bpy)₂(PF₆)₂ exhibits a broad characteristic absorptions at λ_max = 480 nm accompanied a small shoulder at 423 nm, which coincided with the literature.²⁹ Comparing with the UV-vis spectra of the above reference compounds, the broad absorption from 400 nm to 600 nm shown by the hybrid B₁₂–TiO₂–Ru(II) fully indicated that the B₁₂ catalyst and the Ru(II) photosensitizer had been co-immobilized on the surface of mesoporous anatase TiO₂ microspheres successfully. The hybrid could absorb not merely ultraviolet but also visible light.

Fig. 2 UV-vis diffuse reflectance spectra of B₁₂–TiO₂–Ru(II) and TiO₂, and UV-vis spectra of [(CN)(H₂O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)₂(PF₆)₂ in methanol.

Fig. 3 gives the XRD patterns of the hybrid B₁₂–TiO₂–Ru(II) and the raw material mesoporous anatase TiO₂ microspheres. The as-prepared TiO₂ exhibited seven obviously diffraction peaks at 2θ = 25.4°, 37.9°, 48.1°, 55.0°, 62.8°, 68.8° and 75.0°, which corresponding to the (101), (004), (200), (211), (204), (116) and (215) crystal faces of anatase TiO₂ according to the JCPDS No. 21-1272.²⁵ The sharp diffraction peaks indicated that the crystallinity of the anatase TiO₂ was relatively high. After the modification with the two functional molecules, the shape and the intensity of the diffraction peaks did not change significantly, confirming that the crystallinity of the mesoporous anatase TiO₂ microspheres was well kept.

Fig. 3 XRD patterns of the hybrid B₁₂–TiO₂–Ru(II) and the raw material mesoporous anatase TiO₂ microspheres.

The morphologies and structures of the mesoporous anatase TiO₂ microspheres and the hybrid B₁₂–TiO₂–Ru(II) were characterized by SEM and TEM as shown in Fig. 4. Before or after the modification, the support TiO₂ exhibited good spherical morphology with an uniform size of about 500 nm in diameter and each TiO₂ microsphere was stacked by innumerable interconnected nanocrystals. Compared with TiO₂ nanocrystals (P25 or AMT600), the larger particle diameter could make them more easily separated from the dispersion and recycled.

Fig. 4 SEM images of TiO₂ (A) and B₁₂–TiO₂–Ru(II) (B); TEM images of TiO₂ (C) and B₁₂–TiO₂–Ru(II) (D).

The chemical composition of the hybrid B₁₂–TiO₂–Ru(II) was also determined by EDS as shown in Fig. 5. The element Ti was derived from TiO₂, while the Ru, P and F peaks arose from the photosensitizer Ru(dcb)(bpy)₂(PF₆)₂, Co peak originated from [(CN)(H₂O)Cob(III)7COOH]Cl and the C and N peaks originated from both Ru(dcb)(bpy)₂(PF₆)₂ and [(CN)(H₂O)Cob(III)7COOH]Cl. The element O was originated from Ru(dcb)(bpy)₂(PF₆)₂, TiO₂ and [(CN)(H₂O)Cob(III)7COOH]Cl.

Fig. 5 EDS spectrum of the hybrid B₁₂–TiO₂–Ru(II).

High dispersity of a heterocatalyst in the reaction medium could significantly increase the effective collision probability between the catalyst and the substrate and thus promote its catalytic activity in a heterogeneous catalytic system. Therefore, the dispersity of the mesoporous TiO₂ microspheres and the hybrid B₁₂–TiO₂–Ru(II) in methanol was evaluated and the pictures were taken after standing for 1 h at room temperature. As shown in Fig. 6, both the mesoporous anatase TiO₂ microspheres and the hybrid B₁₂–TiO₂–Ru(II) dispersed well in methanol though their diameter were larger than that of the P25 or AMT600.

Fig. 6 Dispersions of TiO₂ (a) and B₁₂–TiO₂–Ru(II) (b) in methanol after standing for 1 h at room temperature.

Nitrogen adsorption–desorption isotherms and the Barrett–Joyner–Halenda (BJH) pore-size distribution analysis are usually employed to detect structural information for porous materials. The nitrogen adsorption–desorption isotherms at 77 K of the mesoporous anatase TiO₂ microspheres and the hybrid B₁₂–TiO₂–Ru(II) are shown in Fig. 7. The as-prepared TiO₂ microspheres showed an apparent capillary condensation step at a relative pressure (P/P₀) of 0.5–0.85, which was corresponded to a pore size of 8 nm obtained by the BJH method. Their BET specific surface area was 172 m² g⁻¹, which is three times that of P25 (ref. 19) and AMT600,²¹ and the total pore volume was 0.64 cm³ g⁻¹. From the isotherms of the hybrid B₁₂–TiO₂–Ru(II), it can be confirmed that the pore size of the hybrid reduced to 7 nm and the BET specific surface area and the total pore volume decreased to 159 m² g⁻¹ and 0.55 cm³ g⁻¹ respectively, which should be attributed to the modification of the B₁₂ catalyst [(CN)(H₂O)Cob(III)7COOH]Cl and the photosensitizer Ru(dcb)(bpy)₂(PF₆)₂. These data indicated that the two functional molecules were immobilized not only onto the surface but also inside the pore of the TiO₂ microspheres.

Fig. 7 Nitrogen adsorption–desorption isotherms measured at 77 K and the pore-size distribution (inset) of the mesoporous anatase TiO₂ microspheres (left) and the hybrid B₁₂–TiO₂–Ru(II) (right).

Formation of the Co(I) species of the B₁₂ complex in the presence of TiO₂ was monitored by the UV-vis spectral change in methanol containing a sacrificial reductant triethanolamine (TEOA) under visible light irradiation (λ ≥ 420 nm). In order to avoid that the UV-vis spectral change of B₁₂ in solution can't be detected sensitively after it was fixed on the TiO₂ microspheres, the compounds [(CN)(H₂O)Cob(III)7C₁ester]Cl and Ru(bpy)₃Cl₂ without carboxylic groups (see Fig. S1, ESI†), were used instead of [(CN)(H₂O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)₂(PF₆)₂ respectively. As shown in Fig. 8a, the mixture of [(CN)(H₂O)Cob(III)7C₁ester]Cl, Ru(bpy)₃Cl₂ and TiO₂ produced three characteristic peaks at 352 nm, 463 nm and 542 nm respectively. Comparing to the UV-vis absorption spectra of [(CN)(H₂O)Cob(III)7C₁ester]Cl and Ru(bpy)₃Cl₂ (see Fig. S2, ESI†), the absorbance at 352 nm and 542 nm was assigned to the Co(III) species of the B₁₂ complex and the peak at 463 nm was originated from both Co(III) and Ru(II) before irradiation. After irradiation for 25 minutes, one new peak at 392 nm was clearly observed, which was typical for the Co(I) state of the B₁₂ complex,¹³ and simultaneously the peaks at 352 nm and 542 nm disappeared and the absorbance at 463 nm decreased. In contrast, a slow Co(I) formation was observed in the mixture of the B₁₂ catalyst and the photosensitizer without TiO₂ (Fig. 8b). Through the comparison, it was proved that the introduction of TiO₂ can efficiently promote the formation of Co(I) species, attributing to the semiconductor property of TiO₂ as an electron mediator, not only accelerating the electron transfer from the Ru(II) photosensitizer to the Co center of the catalyst, but also effectively suppressing the back electron-transfer process. When both the B₁₂ catalyst and the Ru(II) photosensitizer were grafted on the TiO₂ microspheres to give a new hybrid catalyst, the distance among B₁₂, TiO₂ and Ru(II) would be sharply shortened and the electron transfer from the photosensitizer to the Co center of the B₁₂ catalyst via TiO₂ is expected to be further accelerated under visible light irradiation.

Fig. 8 UV-vis spectral change of [(CN)(H₂O)Cob(III)7C₁ester]Cl (5 × 10⁻⁶ mol L⁻¹) with TiO₂ (a) and without TiO₂ (b) in the presence of Ru(bpy)₃Cl₂ (5 × 10⁻⁶ mol L⁻¹) and TEOA (0.2 mol L⁻¹) by visible light irradiation (λ ≥ 420 nm) under N₂ atmosphere in methanol.

The visible light photocatalytic activity of the hybrid B₁₂–TiO₂–Ru(II) for DDT dechlorination was investigated firstly using triethanolamine as the electron sacrificer in methanol. The results are summarized in Table 1. When using the hybrid B₁₂–TiO₂–Ru(II) as catalyst, DDT conversion was 65% only after 10 min of visible light irradiation and four kinds of dechlorinated products were obtained including one mono-dechlorinated product 1,1-bis(4-chlorophenyl)-2,2-dichloroethane (DDD, 35% yield), three di-dechlorinated products 1,1-bis(4-chlorophenyl)-2-chloroethylene (DDMU, 4% yield) and 1,1,4,4-tetrakis(4-chlorophenyl)-2,3-dichloro-2-butene (TTDB (E/Z), 9% yield), one tri-dechlorinated product methyl 2,2-bis(4-chlorophenyl) acetate (DDA methyl ester, 3% yield) (entry 1 in Table 1). When the irradiation time was increased to 30 min, not only DDT was totally converted, but also the mono-dechlorinated product DDD disappeared completely. Accordingly, the yields of the di- and tri-dechlorinated products rose and one more di-dechlorinated product 1,1-bis(4-chlorophenyl)-2-chloroethane (DDMS) was also found (entry 2 in Table 1). TTDB (E/Z) were the products of coupling reaction between two didechlorinated carbene intermediates,¹² therefore di-dechlorinated products obtained with a high yield of 93% after 30 min of irradiation, which was the highest among the B₁₂-based photocatalytic systems reported in the literatures. In addition, from the data it can be concluded that DDD could be further degraded as soon as it was produced in such a hybrid catalytic system. The reaction did not proceed under dark conditions or using TiO₂ instead of the hybrid B₁₂–TiO₂–Ru(II) (entries 3 and 4 in Table 1). When we used the hybrid B₁₂–TiO₂ or Ru(II)–TiO₂ (see Fig. S3–S5, ESI†) alone (entries 5 and 6 in Table 1), the DDT conversion decreased to 24% and 13% respectively and DDD was detected as the single product after 30 min of irradiation. In contrast to the result in the hybrid system (entry 2 in Table 1), the DDT conversion was only 11% with DDD as the single product when a 1 : 2.2 mixture of the B₁₂ catalyst [(CN)(H₂O)Cob(III)7C₁ester]Cl and the photosensitizer Ru(bpy)₃²⁺ was used under the same experimental conditions (entry 7 in Table 1). While adding a certain amount of mesoporous TiO₂ microspheres into the simple mixture system (entry 8 in Table 1), the DDT conversion was increased to 42% and the di-dechlorinated products TTDB (E/Z) were found with 10% yield except DDD. The results shown here indicated that after the introduction of TiO₂ to the simple mixture, especially after the co-immobilization of the B₁₂ catalyst and the Ru(II) photosensitizer on TiO₂ microspheres, reaction efficiency was significantly enhanced compared to that of the simple mixture system, which should be attributed to the very important role of the mesoporous anatase TiO₂ microspheres not only as an adsorption reagent for the B₁₂ catalyst, the photosensitizer and the substrate DDT or DDD etc. but also as an electron mediator.

Table 1 Photocatalytic dechlorination of DDT catalyzed by B₁₂–TiO₂–Ru(II) under visible light irradiation^a

Entry	Catalytic system	Irradiation time(min)	Conversion (%)	Product yield^b (%)					TON^c	TOF^c (h^−1)
				DDD	DDMU	DDMS	TTDB (E/Z)	DDA
a B12–TiO2–Ru(II) = 3 mg, [DDT] = 2.4 × 10^−3 mol L^−1, [TEOA] = 0.2 mol L^−1, MeOH: 5 mL, irradiation: λ ≥ 420 nm, 50 mW cm^−2, distance: 10 cm.b DDT conversion and the product yields were determined by ^1H HMR.c Turnover numbers (TON) and the turnover frequency (TOF) were based on the concentration of B12 and the product yields.d In the dark.e TiO2 = 3 mg.f B12–TiO2 = 3 mg.g Ru(II)–TiO2 = 3 mg.h [Ru(bpy)3Cl2] = 1.1 × 10^−4 mol L^−1, [(CN)(H2O)Cob(III)7C1ester]Cl = 4.9 × 10^−5 mol L^−1.
1	B12–TiO2–Ru(II)	10	65	35	4	Trace	9	3	52	312
2	B12–TiO2–Ru(II)	30	100	0	40	13	20	6	119	238
3^d	B12–TiO2–Ru(II)	30	Trace	—	—	—	—	—	0	0
4^e	TiO2	30	0	—	—	—	—	—	0	0
5^f	B12–TiO2	30	24	16	—	—	—	—	9	18
6^g	Ru(II)–TiO2	30	13	7	—	—	—	—	—	—
7^h	B12, Ru(bpy)3^2+	30	11	10	—	—	—	—	5	10
8^e^,^h	B12, Ru(bpy)3^2+, TiO2	30	42	21	Trace	Trace	10	Trace	30	60

---

TiO₂ and the Ru(II) moieties in the hybrid could be excited at the same time under sunlight irradiation because they absorb light in UV and visible region respectively. Therefore, the hybrid B₁₂–TiO₂–Ru(II) should exhibited enhanced photocatalytic activity for dechlorination under sunlight irradiation. To evaluate the co-photosensitized effect of TiO₂ and the Ru(II) complex on the B₁₂ catalyst, DDT dechlorination was subsequently carried out under simulated sunlight (in Table 2). After 5 min of simulated sunlight irradiation, the DDT conversion was 49% and five dechlorinated products including DDD, DDMU, TTDB (E/Z) and DDA were found when using the hybrid B₁₂–TiO₂ alone as catalyst (entry 2 in Table 2). While using the hybrid B₁₂–TiO₂–Ru(II) instead, the DDT conversion increased to 70% and one more product DDMS were detected (entry 1 in Table 2). When the irradiation time was prolonged to 10 min, the DDT conversion reached 100% in the hybrid B₁₂–TiO₂–Ru(II) system (entry 3 in Table 2), which was still 1.4 times that of the hybrid B₁₂–TiO₂ system (entry 4 in Table 2) and 1.5 times that of itself under visible light irradiation (entry 1 in Table 1). Comparing the data, the effect of co-photosensitization was clearly observed. TiO₂ was activated by the UV part of simulated sunlight and then transferred the electrons directly to the Co(I) center of the B₁₂ catalyst, while at the same time the Ru(bpy)₃²⁺ moieties in the hybrid were excited by the visible part and transferred its electrons to the Co(I) center through TiO₂. In other words, the hybrid B₁₂–TiO₂–Ru(II) realized the simultaneous utilization of both UV and visible part of the sunlight with high efficiency and the contribution of UV and visible part of the sunlight to the electron transfer has additivity.

Table 2 Photocatalytic dechlorination of DDT catalyzed by B₁₂–TiO₂–Ru(II) under simulated sunlight irradiation^a

Entry (%)	Catalyst	Irradiation time (min)	Conversion	Product yield^b (%)					TON^c	TOF^c (h^−1)
				DDD	DDMU	DDMS	TTDB (E/Z)	DDA
a B12–TiO2–Ru(II) = 3 mg, B12–TiO2 = 3 mg, [DDT] = 2.4 × 10^−3 mol L^−1, [TEOA] = 0.2 mol L^−1, MeOH: 5 mL, irradiation illuminant: simulated sunlight, 50 mW cm^−2, distance: 10 cm.b DDT conversion and the product yields were determined by ^1H HMR.c Turnover numbers (TON) and the turnover frequency (TOF) were based on the concentration of B12 and the product yields.
1	B12–TiO2–Ru(II)	5	70	22	4	2	17	4	67	804
2	B12–TiO2	5	49	13	2	Trace	12	2	41	492
3	B12–TiO2–Ru(II)	10	100	29	15	5	19	5	173	1038
4	B12–TiO2	10	74	25	2	Trace	20	4	70	42

---

Furthermore, the recycled catalysis of this hybrid catalyst B₁₂–TiO₂–Ru(II) under visible light irradiation and simulated sunlight irradiation was performed respectively since it can be easily separated from the reaction system by centrifugation and the relevant data were shown in Table 3. According to the data, this hybrid catalyst still kept high catalytic activity after 3 recyclings under 30 min of visible-light irradiation (entry 1 in Table 3). The recyclability of the hybrid B₁₂–TiO₂–Ru(II) at lower conversion has also been investigated under 5 min of simulated sunlight irradiation (entry 2 in Table 3). These data indicated that this hybrid catalyst had good stability and recyclability, and kept its catalytic activity after 3 recyclings even at lower conversion. The recovered B₁₂–TiO₂–Ru(II) after 3 recyclings irradiated by visible light was characterized by UV-vis spectrum and SEM respectively in Fig. 9, showing that the morphology and the optical property of the hybrid had no distinct change compared with its original.

Table 3 Recycled catalysis of B₁₂–TiO₂–Ru(II)^a

Entry	Irradiation illuminant (irradiation time)	Cycle	Conversion (%)	Product yield (%)^b
				DDD	DDMU	DDMS	TTDB (E/Z)	DDA
a B12–TiO2–Ru(II) = 3 mg, [DDT] = 2.4 × 10^−3 mol L^−1, [TEOA] = 0.2 mol L^−1, MeOH: 5 mL, irradiation conditions: 50 W cm^−2, distance: 10 cm.b DDT conversion and the product yields were determined by ^1H HMR.
1	Visible light (30 min)	1	100	0	40	13	20	6
		2	100	0	38	14	19	7
		3	100	0	39	12	19	6
2	Simulated sunlight (5 min)	1	70	22	4	2	17	4
		2	69	23	3	1	17	3
		3	68	23	3	1	16	3

---

Fig. 9 UV-vis spectrum (left) and SEM (right) of B₁₂–TiO₂–Ru(II) after 3 recyclings irradiated by visible light.

Finally, we investigated the photosensitizing mechanism of the Ru(II) complex in the hybrid. In general, reductive and oxidative quenching mechanisms exist in the complex Ru(bpy)₃²⁺ when it is activated by visible light irradiation.³⁰ To elucidate the quenching mechanism, the steady-state emission from the excited state of the Ru(II) complex before and after adding the B₁₂ complex and TiO₂ was measured, as shown in Fig. 10. After exciting the metal to ligand charge transfer (MLCT) transition band of the Ru(II) complex at 450 nm, a strong emission at 600 nm was observed, while the complex [(CN)(H₂O)Cob(III)7C₁ester]Cl showed no emission at this position. After adding [(CN)(H₂O)Cob(III)7C₁ester]Cl to the solution of Ru(bpy)₃²⁺, the emission intensity of the Ru(II) complexes was only slightly decreased, but if further adding a small amount of TiO₂ microspheres, the excited state of the Ru(II) complex was quenched obviously, which indicated that electron transfer should proceed efficiently between the excited state Ru(bpy)₃²⁺* and TiO₂ microspheres, which is consistent with the literature.³¹ Therefore, the so-called oxidative quenching mechanism could be expected for the visible-light-driven catalytic reaction in this hybrid system.

Fig. 10 The steady-state emission of Ru(bpy)₃Cl₂ before and after adding [(CN)(H₂O)Cob(III)7C₁ester]Cl and TiO₂ in methanol. ([Ru(bpy)₃Cl₂] = [[(CN)(H₂O)Cob(III)7C₁ester]Cl] = 4.7 × 10⁻⁵ mol L⁻¹, TiO₂ = 0.2 g L⁻¹, λ_ex = 450 nm).

The photocatalytic mechanism of the hybrid system under sunlight was illustrated in Fig. 11. The photosensitizer Ru(bpy)₃²⁺ was excited by visible light irradiation and then injected one electron to the conduction band of the support TiO₂ microspheres, and simultaneously, some electrons in the valence band of TiO₂ moved up to the conduction band under UV light irradiation. Subsequently, the Co(III) center of the B₁₂ derivative accepted two electrons from the conduction band of TiO₂ and was reduced to the Co(I) species. The supernucleophilic Co(I) species had a high reactivity with organic halides to induce the oxidative addition of the alkylating agents to the metal center with dehalogenation.^19,32 The obtained Ru(bpy)₃³⁺ and the holes in the valence band of TiO₂ were rapidly reduced by the sacrificial reagent TEOA.

Fig. 11 Proposed mechanism for DDT dechlorination catalyzed by B₁₂–TiO₂–Ru(II) under simulated sunlight.

Conclusions

In conclusion, a B₁₂-based catalyst co-sensitized by two photosensitizers, TiO₂ and a Ru(II) complex, as a UV-vis light-driven photocatalyst was successfully synthesized. In the catalytic system composed of B₁₂–TiO₂–Ru(II) and TEOA, DDT could be completely converted into didechlorinated products and a small part was tridechlorinated only after 30 min of visible light irradiation. The catalytic activity was much higher than that of the other reported B₁₂-based photocatalytic systems. Especially, under simulated sunlight, the hybrid exhibited a significantly enhanced photocatalytic activity for dechlorination compared with B₁₂–TiO₂ at the same condition or itself under visible light irradiation attributing to the additivity in the contribution of UV and visible part of the sunlight to the electron transfer. In addition, this hybrid catalyst can be easily reused without loss of catalytic efficiency. Therefore, the presented design strategy for photocatalytic system is efficient in full utilization of solar energy and it would be readily applicable to the design of an eco-friendly catalyst.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from Science Foundation of Educational Department of Liaoning Province (LYB201604) and the National Natural Science Foundation of China (51773085, 21203082).

Notes and references

1. D. Friedmann, A. Hakki, H. Kim, W. Choi and D. Bahnemann, Green Chem., 2016, 18, 5391.
2. S. Klementova and M. Zlamal, Photochem. Photobiol. Sci., 2013, 12, 660.
3. M. Y. Ye, Z. H. Zhao, Z. F. Hu, L. Q. Liu, H. M. Ji, Z. R. Shen and T. Y. Ma, Angew. Chem., Int. Ed., 2017, 56, 8047.
4. X. Fan, K. Lai, L. Wang, H. Qiu, J. Yin, P. Zhao, S. Pan, J. Xu and C. Wang, J. Mater. Chem. A, 2015, 3, 12179.
5. Y. Sun, W. Zhang, J. Tong, Y. Zhang, S. Wu, D. Liu, H. Shimakoshi, Y. Hisaeda and X. M. Song, RSC Adv., 2017, 7, 19197.
6. H. Tian, H. Shimakoshi, K. Imamura, Y. Shiota, K. Yoshizawa and Y. Hisaeda, Chem. Commun., 2017, 53, 9478.
7. M. Giedyk, K. Goliszewska and D. Gryko, Chem. Soc. Rev., 2015, 44, 3391.
8. K. Gruber, B. Puffer and B. Kräutler, Chem. Soc. Rev., 2011, 40, 4346.
9. H. Shimakoshi, L. Li, M. Nishi and Y. Hisaeda, Chem. Commun., 2011, 47, 10921.
10. Y. Hisaeda and H. Shimakoshi, in Handbook of Porphyrin Science, ed. K. M. Kadish, K. M. Smith and R. Guilard, World Scientific, 2010, vol. 10, pp. 313–370.
11. K. Tahara, H. Shimakoshi, A. Tanaka and Y. Hisaeda, Dalton Trans., 2010, 39, 3035.
12. H. Shimakoshi, M. Tokunaga, T. Baba and Y. Hisaeda, Chem. Commun., 2004, 16, 1806.
13. W. Zhang, H. Shimakoshi, N. Houfuku, X. M. Song and Y. Hisaeda, Dalton Trans., 2014, 43, 13972.
14. H. Shimakoshi, M. Nishi, A. Tanaka, K. Chikama and Y. Hisaeda, Chem. Commun., 2011, 47, 6548.
15. W. Ke, C. C. Stoumpos, J. L. Logsdon, M. R. Wasielewski, Y. Yan, G. Fang and M. G. Kanatzidis, J. Am. Chem. Soc., 2016, 138, 14998.
16. A. Romeiro, D. Freitas, M. Emilia Azenha, M. Canle and H. D. Burrows, Photochem. Photobiol. Sci., 2017, 16, 935.
17. T. Yui, A. Kan, C. Saitoh, K. Koike, T. Ibusuki and O. Ishitani, ACS Appl. Mater. Interfaces, 2011, 3, 2594.
18. N. Hoffmann, Aust. J. Chem., 2015, 68, 1621.
19. H. Shimakoshi, E. Sakumori, K. Kaneko and Y. Hisaeda, Chem. Lett., 2009, 38, 468.
20. H. Shimakoshi, M. Abiru, S. Izumi and Y. Hisaeda, Chem. Commun., 2009, 14, 6427.
21. H. Shimakoshi and Y. Hisaeda, Chempluschem, 2014, 79, 1250.
22. H. Shimakoshi and Y. Hisaeda, Angew. Chem., Int. Ed., 2015, 54, 15439.
23. F. Lakadamyali and E. Reisner, Chem. Commun., 2011, 47, 1695.
24. C. Guo, M. Ge, L. Liu, G. Gao, Y. Feng and Y. Wang, Environ. Sci. Technol., 2010, 44, 419.
25. Y. Zhang, S. Lin, W. Zhang, Y. Zhang, F. Qi, S. Wu, Q. Pei, T. Feng and X. M. Song, Electrochim. Acta, 2014, 150, 167.
26. Y. M. Wu, M. Y. Xing, B. Z. Tian, J. L. Zhang and F. Chen, Chem. Eng. J., 2010, 162, 710.
27. Y. C. Feng, L. Li, M. Ge, C. S. Guo, J. F. Wang and L. Liu, ACS Appl. Mater. Interfaces, 2010, 2, 3134.
28. K. Tahara, H. Shimakoshi, A. Tanaka and Y. Hisaeda, Dalton Trans., 2010, 39, 3035.
29. C. A. Kelly, F. Farzad, D. W. Thompson, J. M. Stipkala and G. J. Meyer, Langmuir, 1999, 15, 7047.
30. A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. V. Zelewsky, Coord. Chem. Rev., 1988, 19, 85.
31. E. E. Beauvilliers and G. J. Meyer, Inorg. Chem., 2016, 55, 7517.
32. H. Shimakoshi, Y. Nagami and Y. Hisaeda, Appl. Chem. Eng., 2015, 4, 9.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra13037f

---