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Semiconductor–redox catalysis promoted by metal–organic frameworks for CO2 reduction† Sibo Wang, Jinliang Lin and Xinchen Wang*

Received 19th May 2014, Accepted 29th May 2014 DOI: 10.1039/c4cp02173h www.rsc.org/pccp

A noble-metal-free system for photochemical reduction of CO2 has been developed by integrating graphitic carbon nitride (g-C3N4) with a cobalt-containing zeolitic imidazolate framework (Co-ZIF-9). g-C3N4 acts as a semiconductor photocatalyst, whereas Co-ZIF-9 is a cocatalyst that facilitates the capture/concentration of CO2 and promotes light-induced charge separation. The two materials cooperate efficiently to catalyze CO2-to-CO conversion upon visible light illumination under mild reaction conditions. A 13C-labelled isotropic experiment proved that CO2 is the carbon source of the produced CO. Even without noble metals, the system still achieved an apparent quantum yield of 0.9 percent. The system displayed high photocatalytic stability, without noticeable alterations in the chemical and crystal structures of g-C3N4 and Co-ZIF-9 after the reaction.

The development of semiconductor–redox systems with high efficiency for the conversion of CO2 into C1 building blocks or fuels is not only of scientific interest but could also offer a sustainable pathway to solve energy and environmental problems.1 Tremendous research effort has been focused on the photocatalytic reduction of CO2 to energized molecules (i.e. CO, CH4, and CH3OH)2 as stimulated by natural photosynthesis by which solar energy, CO2 and water are converted into bio-compounds and dioxygen. However, it is a great challenge to activate a linear CO2 molecule by artificial materials due to its high thermodynamic stability.3 To fulfill the crucial mission of solar-driven CO2 conversion, one often needs to integrate light harvesters, charge mediators, and cocatalysts into a cascade catalytic system to achieve efficient CO2 reduction.4 Many catalytic systems that contain noble metal catalysts5 and/or cocatalysts6 have been designed for the conversion of CO2. But, the scarcity of these noble metals limits their longterm and large-scale development. Exploring noble-metal-free State Key Laboratory of Photocatalysis on Energy and Environment, and College of Chemistry, Fuzhou University, Fuzhou, 350002, People’s Republic of China. E-mail: [email protected]; Fax: +86-591-83920097; Tel: +86-591-83920097 † Electronic supplementary information (ESI) available: Experimental details for synthesis, characterization, and the photocatalytic test. See DOI: 10.1039/ c4cp02173h

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systems for efficient CO2 photofixation is therefore actively perused nowadays. Several types of transition-metal-based semiconductor catalysts such as TiO2,7 ZrO2,8 MgO,9 Ga2O3,10 ZnGa2O411 and ZnGe2O412 have been reported for photocatalytic reduction of CO2, but the activity of the reaction systems is very low, mainly due to the fast recombination of photogenerated electron–hole pairs and surface kinetic limitations. Thus, to obtain high photocatalytic efficiency, cocatalysts that are capable of developing Schottky diodes for quickly transferring the excited electrons to react with activated CO2 are essential in photocatalytic CO2 reduction. Transition-metal ions with multiple redox states and organic ligands can serve as excellent cocatalysts to rapidly transfer the excited electrons for the subsequent CO2 reduction reaction, inhibiting the recombination of photoinduced electrons and holes. As a result, the multi-electron reduction of CO2 is accelerated, especially when the process is coupled with protons. Then again, to accomplish CO2 conversion more effectively, a suitable CO2 activator should be involved. Recently, intensive exploration of CO2 activators has focused on metal-free organocatalysts.13 Particularly, imidazolate-based ionic liquids14 have been demonstrated to exhibit high adsorptive capacities for CO2, while still being capable of stabilizing CO2 anions.15 The stabilizing effect of the organic imidazolate motifs on the CO2 anion greatly reduces the overpotential of the CO2 reduction reaction, because one-electron reduction of CO2 requires a high energy input and it is identified as a rate-limiting step.16 Therefore, we believe that the integration of imidazolate-based organic ligands for CO2 activation with the redox-active transition metals can facilitate CO2 conversion. This inspires us to rationally develop a material consisting of transition metal ions with imidazolate-based linkers in a defined metal–organic framework. This will create a synergistic effect of the organic CO2-activating ligands and redox functions of the transition metal on the capture, activation and conversion of CO2. A model case of such hybrid materials is microporous metal–organic frameworks (MOFs),17 which was recently introduced as a dual-function cocatalyst to support CO2 reduction by Ru-based photosensitizers.18 It was

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found that Co-ZIF-9 effectively captured and concentrated CO2 in its microporous network and thereafter cooperated with the sensitizer to reduce CO2. Although Co-ZIF-9 was rather stable in cocatalyzing CO2 reduction,18 the system still suffered from limitation of the involvement of noble metal Ru in the dye that is typically unstable during photochemical operations.19 It is therefore desirable to find metal-free and photochemically stable light harvesters to couple with Co-ZIF-9 to construct a stable semiconductor–redox system made of sustainable substances for promoting the CO2 reduction reaction under visible light. To this end, we are interested in coupling Co-ZIF-9 with a metal-free conjugated carbon nitride polymer semiconductor20 that was introduced in 2009 as a new family of solar energy transducers for water photosplitting21 and organic photosynthesis.22 Conjugated g-C3N4 polymers were demonstrated to have a high thermal stability up to 550 1C in air and have extreme chemical stability, with suitable semiconductor–redox energetics for CO2 reduction and water splitting. The combination of the two emerging functional materials will open new opportunities in the development of cost-affordable chemical systems for artificial photosynthesis because of wide tunability of the chemical/physical properties of both materials by modification of their electronic, textural and crystal structures. The CO2 reductions were carried out in a visible-light-driven catalytic system employing mesoporous g-C3N4 as a photocatalyst, Co-ZIF-9 as a microporous crystalline cocatalyst, bipyridine (bpy) as an auxiliary electron mediator, and triethanolamine (TEOA) as an electron donor, for the deoxygenative reduction of CO2 to CO under mild reaction conditions at 30 1C and 1 bar CO2 (Scheme 1). Firstly, a series of reference experiments were conducted to demonstrate the essential of all components for the photoreduction of CO2 (Table S1, ESI†). In the dark, no gas was detected in the reaction system. Exposure to visible light (4420 nm) for 2 h resulted in the production of 20.8 mmol CO from the photocatalytic reduction of CO2, together with generation of 3.3 mmol H2. The absence of g-C3N4 led to the inactivity of the reaction system. These observations imply that g-C3N4 undergoes photoexcitation upon visible light illumination. We also analyzed the potential formation of products in the liquid phase, and our results obtained by ion chromatograph analysis, NMR and GC-MS revealed that no detectable formic acid, methanol and C–C

Scheme 1 Representation of the cooperation of Co-ZIF-9 and g-C3N4 for the photocatalytic reduction of CO2 under visible light irradiation.

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coupling products were formed in the reaction system. The function of bpy of cooperatively transferring the excited electrons was demonstrated by the fact that the catalytic activity of the reaction system was reduced significantly in the absence of bpy. When Co-ZIF-9 was removed from the reaction system, CO and H2 productions were consistently stopped. The examination of the cocatalytic effect of Co2+ or Co-ZIF-9 precursors (benzimidazole and Co(NO3)26H2O) showed poorer catalytic activities than that of Co-ZIF-9. The results strongly support that the Co-ZIF-9 cocatalyst remarkably promotes the photoreduction of CO2 by serving as a CO2 activator and a redox promoter that are combined in a microporous framework as a heterogeneous cocatalyst. Such a rational design of cocatalyst architectures could produce extra benefits such as adsorption and concentration of CO2, which can be estimated by the CO2 adsorption measurement of Co-ZIF-9 (Fig. 1). As is shown, Co-ZIF-9 has a high CO2 adsorption loading of 2.7 mmol g 1 and affords a high micropore surface area of 1607 m2 g 1. Actually, Co-ZIF-9 exhibits very interesting "breathe" behaviour for CO2 adsorption, that is, it exhibits a low uptake of CO2 at low pressures but then opens up significantly at higher pressures, but still adsorbs a negligible amount of other gases (e.g. N2, CH4).23 These intriguing properties render Co-ZIF-9 a very promising candidate for CO2 conversions. No CO was detected upon replacing CO2 with Ar in the system (0.9 mmol H2 produced only), which however implied the participation of CO2 in the reaction indirectly. To further validate the source of the produced CO, an isotopic experiment using 13CO2 was conducted under the identical photoreaction conditions. The evolution of CO was analyzed by GC-MS.

Fig. 1

CO2 adsorption isotherm of Co-ZIF-9 measured at 0 1C.

Fig. 2 Gas chromatography and mass spectral (m/z = 29) analyses of the carbon source of the generated CO in the photocatalytic reduction of 13 CO2 promoted by Co-ZIF-9.

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Table 1 Effects of various tertiary amines on the yield of CO and H2 from the CO2 photoreduction systema






1 2 3 4 5 6 7


20.8 2.0 1.4 1.0 19.4 15.2 10.9

3.3 4.6 1.9 0.6 2.3 0.8 0.4

86.3 30.3 42.4 62.5 89.4 95.0 96.5

a Reaction conditions: C3N4 (20 mg), Co-ZIF-9 (4 mmol, 1 mg), bpy (10 mg), solvent (5 ml, MeCN: H2O = 3 : 2), TEOA (1 ml), l 4 420 nm, 30 1C, 2 h. b Sel.CO = mol(CO)/mol(CO + H2). c Triethylamine. d Tri-n-propylamine. e Tri-n-butylamine. f 1-(Diethylamino)-2-propanol. g 2-(Diethylamino)ethanol. h Tri-2-propanolamine.

Fig. 3 CO production from the Co-ZIF-9 catalyzed CO2 conversion system as a function of the reaction time. Inset: the stability test of the CO2 photoreduction system for seven repeated operations.

As shown in Fig. 2, the peak at 3.54 min and m/z 29 was assigned to 13CO. This experiment confirmed that CO2 is indeed the carbon source of CO, eliminating any doubt that the detected CO might be derived from the decomposition effect of any organics in the system. Controlled experiment showed that no reaction occurred when TEOA was omitted, which reflected the crucial role of the electron donor in CO2 reduction. Thus, various tertiary amines were applied in the system to investigate the effects of electron donors on the catalytic performance (Table 1). Once TEA was replaced by longer alkylamines like tri-n-propylamine (TPA) or tri-n-butylamine (TBA), the generation of CO and H2 reduced, but the selectivity of CO obviously increased with the prolonged alkyl chains from ethyl to butyl (entries 2–4). A significant enhancement in both the production and the selectivity of CO were observed when some b-hydroxylated amines were chosen as electron donors (entries 1, 5–7). Particularly, under the same reaction conditions, TEOA exhibited superior catalytic activity in the formation of CO, while TIPOA displayed the highest selectivity towards CO formation. These observations indicated that, due to the steric hindrance effect, the different adsorption levels of the sacrificial electron donors have a remarkable influence on both the catalytic activity and CO selectivity. The effects of the amount of Co-ZIF-9 and g-C3N4 on the catalytic performance of the reaction system were investigated (Fig. S1 and S2, ESI,† respectively). The results further identified the critical role of their cooperative functions in effective CO2 photoreduction. The initial evolutions of CO and H2 showed a linear dependence on both Co-ZIF-9 (Fig. S1b, ESI†) and g-C3N4 (Fig. S2b, ESI†) concentrations. The results clearly demonstrated that both g-C3N4 and Co-ZIF-9 are absolutely indispensable for CO2 reduction in the reaction system.24 The catalytic activity of the chemical system was found to be largely affected by the reaction temperature. As is shown in Fig. S3 (ESI†), the generation of both CO and H2 increased at first and then decreased with an increase in the reaction temperature from 10 to 60 1C, reaching a maximum value at 40 1C. The reason for the lower catalytic activity at relatively high temperatures may be attributed to the release/desorption of CO2 from the reaction mixture. The apparent quantum yield of the system

under this optimized reaction condition was estimated to be 0.9% under monochromatic irradiation of 420 nm. The CO production from the Co-ZIF-9 and g-C3N4 based hybrid system as a function of reaction time was studied (Fig. 3). In the initial 2 h reaction, the generation of CO increased almost linearly with the time, but thereafter the production enhanced slightly. We attribute this to the depletion/degradation of CO2 and bpy in the system, but Co-ZIF-9 and g-C3N4 are still stable to preserve their intrinsic reactivity. To check the stability of Co-ZIF-9 and g-C3N4, we introduced fresh bpy and CO2 into the reaction system after the 2 h reaction to re-start the reaction. Results revealed that the reaction system kept its original catalytic activity almost unaffected during seven repeated operations (Fig. 3, inset), which however supports the stability of Co-ZIF-9 and g-C3N4 under the reaction conditions. The catalytic turnover number with respect to Co-ZIF-9 reaches 35 during this stability test. After the reaction, the mixture of Co-ZIF-9 and g-C3N4 was removed from the reaction solution and characterized by FTIR (Fig. S4, ESI†) and XRD analyses (Fig. S5, ESI†). The results showed that there is no obvious change in the chemical and crystal structures of Co-ZIF-9 and g-C3N4, which reflects the stable characteristics of the coupling and working of the MOF and g-C3N4 in the CO2 photoredox system. The effects of the reaction medium on the photocatalytic performance were evaluated (Fig. S6, ESI†). Aprotic solvents such as MeCN, DMF, THF and DMSO were proved to be favourable for the reaction, because they possess nitrogen or/and oxygen atoms that can interact with CO2 molecule via Lewis acid–base interactions,25 beneficial for solubilizing CO2. In pure H2O, a typical protic solvent, the catalytic activity of the reaction system was significantly inhibited. Neither CO nor H2 was detected when the reaction was operated in DCM that holds only a weak chemical affinity toward CO2 molecules. The effect of water on the catalytic efficiency of the reaction system was explored (Fig. S7, ESI†). Results revealed that the amount of water contained in the reaction system has remarkable effects on both the catalytic activity of the reaction and the selectivity for CO production. The yield of CO/H2 was enhanced gradually accompanied with an increase in the ratio of H2O in the reaction system. The production of the products and the

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Fig. 4 The dependence of the wavelength of the incident light on the evolution of CO and H2 from the CO2 photoreduction system. The lines represent the UV-Vis diffuse reflectance spectrum of g-C3N4 (solid) and Co-ZIF-9 (dash).

selectivity of CO achieved maximum values when the system contained a H2O ratio of 40%, further increasing the amount of H2O resulted in a decrease in the yield of CO/H2 and the selectivity of CO. The wavelength dependence of the CO production demonstrated that the trend of CO generation matched well with the photon absorption characteristics of the polymeric carbon nitride semiconductor, instead of Co-ZIF-9 (Fig. 4). This investigation confirmed that the reaction was indeed induced by the light excitation of the g-C3N4 semiconductor as a solar energy transducer for the photoredox catalytic reactions. To elucidate the fact that Co-ZIF-9 and bpy can cooperatively improve the separation of light-triggered charge carriers, in situ photoluminescence (PL) measurements of the reaction systems were carried out. Fig. 5 clearly shows that either Co-ZIF-9 or bpy can indeed suppress the quenching of the photogenerated charges in the reaction system. However, in the co-presence of Co-ZIF-9 and bpy, their synergistic effects can significantly inhibit the recombination of light-induced electrons and holes. The photocurrent characterizations (Fig. S8, ESI†) revealed that the photocurrent was remarkably enhanced by the addition of Co-ZIF-9 to the reaction system. The findings reflected the promoted separation of the electron–hole pairs, thus further disclosing the function of Co-ZIF-9 in promoting electron transfers.

Fig. 5 PL spectra of the reaction systems in various components at 400 nm laser irradiation at room temperature.

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In summary, a noble-metal-free photocatalytic system for CO2 reduction was constructed and operated under mild reaction conditions by merging Co-ZIF-9 with g-C3N4 as a cocatalyst and a light harvester, respectively. Co-ZIF-9 exhibited multiple functions in the promotion of both CO2 adsorption and lightinduced charge separation, leading to a high photocatalytic activity for CO2 reduction when combined with g-C3N4, even without the involvement of noble metal species. PL measurements confirmed the synergistic charge-mediating effects of Co-ZIF-9 and bpy on carbon nitride photocatalysis, which significantly inhibited the quenching of the photo-induced electron–hole pairs in the g-C3N4 semiconductor. Our further investigation also revealed that such promotional effects of MOF materials are also extendable to a variety of heterogeneous photocatalysts, such as CdS and TiO2. It is anticipated that this study has great potential for exploring new and efficient CO2 photofixation systems by optimization of the chemistry, morphology, and interactions of the two materials made of inexpensive and sustainable substances.

Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB632405), the National Natural Science Foundation of China (21033003, 21173043 and 21273039), and the State Key Laboratory of NBC Protection for Civilian (SKLNBC2013-04K).

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Semiconductor-redox catalysis promoted by metal-organic frameworks for CO2 reduction.

A noble-metal-free system for photochemical reduction of CO2 has been developed by integrating graphitic carbon nitride (g-C3N4) with a cobalt-contain...
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