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Photocatalytic CO2 reduction in metal–organic frameworks: A mini review
Journal of Molecular Structure 1083 (2015) 127–136
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Photocatalytic CO2 reduction in metal–organic frameworks: A mini review Chong-Chen Wang a,b,⇑, Yan-Qiu Zhang a, Jin Li a, Peng Wang a a b
Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing University of Civil Engineering and Architecture, Beijing 100044, China Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Metal–organic frameworks are very
effective to conduct photocatalytic CO2 reduction. Mechanisms for photocatalytic CO2 reduction with MOFs were summarized. MOFs are surely believed to have a bright prospect in the field of photocatalytic reduction of CO2.
a r t i c l e
i n f o
Article history: Received 7 August 2014 Received in revised form 29 October 2014 Accepted 17 November 2014 Available online 22 November 2014 Keywords: Metal–organic frameworks Carbon dioxide Photocatalytic Reduction
a b s t r a c t Photocatalytic reduction of CO2 for value-added chemicals is an attractive process to address both energy and environmental issues. This mini review paper presents two different conversion processes, namely conversion to organic chemicals (like CH4, CH3OH, HCOOH and so on) and being split into CO, in metal–organic frameworks (MOFs). The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the photocatalytic performance, the involved challenges, and the prospects are discussed and estimated. It is clear that MOFs have a bright prospect in the field of photocatalytic reduction of CO2. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Global warming resulting from the emission of greenhouse gases has become a widespread concern in the recent years. Among the greenhouse gases, CO2 contributes more than 60% to global warming because of its huge emission amount [1]. Therefore, the capture and efficient use of CO2 is an important issue. One of the best solutions is to photocatalytically convert CO2 into ⇑ Corresponding author at: Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing University of Civil Engineering and Architecture, Beijing 100044, China. http://dx.doi.org/10.1016/j.molstruc.2014.11.036 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
valuable chemicals, like CO, CH4, CH3OH, CH3CH2OH, HCOOH and so on, by means of solar energy. Many photocatalysts, including TiO2 [2–4], BiVO4 [5], BiWO6 [6], Zn2GeO4 [7] and other composites [8] have been investigated for their performance in photocatalytic CO2 reduction. CO2 is highly stable, in which the C@O bond energy is 750 kJ/mol, much higher than CAC (336 kJ/mol), CAO (327 kJ/ mol) and CAH (411 kJ/mol) [9], leading to low reduction efficiency. Furthermore, most of the photocatalysts already investigated are only active in the UV region. It is necessary to develop highly efficient photocatalysts that can reduce CO2 under visible light. Metal–organic frameworks (MOFs), a class of newly-developed inorganic-organic hybrid porous materials, has generated a rapid
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development due to their diverse and easily tailored structures [10–15] and various potential applications, such as in catalysis [16–23], separation [24–29], gas storage [18,30–33], carbon dioxide capture [34–37], and so on [10–15,24–27,38–41]. Recent research indeed not only demonstrated porous MOFs materials to be a new class of photocatalysts usable in catalytic degradation of organic pollutants under UV/visible/UV–visible irradiation [20], but also triggered an intense interest in exploring MOF applications as photocatalysts in other aspects [42–48]. Based on the richness of metal-containing nodes and organic bridging linkers, as well as the controllability of synthesis, it is easy to construct MOFs with tailorable capacity to absorb light, thereby initiating desirable photocatalytic properties for specific application in CO2 reduction. The study of MOF applications in this topic thus has a bright future even if not being so widely explored to date. Herein, we highlight the research progress of MOF applications in photocatalytic CO2 reduction. The reported examples are collected and analyzed; the reaction mechanism and the influence of various factors on the catalytic performance are discussed; as well as the involved challenges and the prospects are estimated.
Photocatalytic CO2 reduction into valuable organic chemicals in MOFs Green plants can harvest solar energy and use the harvested energy to convert CO2 and H2O into carbohydrates via photosynthesis. Some artificial systems and devices using inorganic and organic materials to simulate photochemical reactions have been developed recently [49–54]. Three fundamental steps are needed to convert solar energy to chemical energy: (1) sunlight absorption by photosensitizers to create charge-separated excited states; (2) generation of redox equivalents and their migration to reactive centers, and (3) reduction and oxidation half reactions with the redox equivalents (electrons and holes) at the catalytic centers [55]. As a new family of inorganic–organic hybrid materials, MOFs serve as an interesting platform to design and study artificial photosynthetic systems. In general, MOFs can contain photosensitizers and catalytic centers in a single solid, which provide the structural organization to integrate the three fundamental steps of artificial photosynthesis into a single material [55]. A number of recent papers have demonstrated that MOFs can be used to achieve light harvesting and to drive photocatalytic reduction of CO2 into small organic molecules, like HCOOH, CH3OH, and so on. MIL-125(Ti) (Ti8O8(OH)4(BDC)6, BDC = benzene-1,4-dicarboxylate) can not only introduce high density of the immobilized Ti sites into porous MOFs, but also lead to various isostructural MOFs, whose photocatalytic properties can be tuned simply via incorporation of BDC derivatives. Taking 2-aminoterephthalate as organic linker, NH2-MIL-125 (Ti) was synthesized to be used to perform photocatalytic reduction of CO2 into HCOOH [56]. A significant change in NH2-MIL-125 (Ti) is its optical absorption, which shows an extra absorption band edge to around 550 nm, falling in the visible region, significant red shift from absorption band edge to 350 nm of MIL-125 (Ti). The introduction of ANH2 group influences O to Ti charge transfer (LMCT) in TiO5(OH) inorganic cluster, which results in the formation of NH2-MIL-125 (Ti) with absorption in the visible light region. Another noticeable change resulting from the introduction of ANH2 functionality is the increase of its adsorption capability toward CO2, because aromatic molecules functionalized with some polar substituent groups like AOH, ACOOH or ANH2 enhance the interactions of CO2 with the functionalized aromatic molecules [57,58]. The photocatalytic reduction of CO2 with NH2-MIL-125 (Ti) as photocatalyst was carried out in acetonitrile (MeCN) with triethanolamine (TEOA) as sacrificial agent under visible light irradiation
[56]. After 10 h, ca. 8.14 lmol HCOO was detected by ion chromatograph. No HCOO was generated in the dark, implying it is a photocatalytic process. The inactivity of the MIL-125 (Ti) under completely similar conditions indicated that the photocatalytic performance for CO2 reduction over NH2-MIL-125 (Ti) actually originates from the amino functionality. Labeled 13CO2 was used for the photocatalytic process confirming the HCOO originated from CO2. NH2-MIL-125 (Ti) is stable during the photocatalytic reaction, which was confirmed by XRD, BET, TGA, IR and Raman spectra. In order to deduce the mechanism of the reduction of CO2 over NH2-MIL-125 (Ti), the suspension of NH2-MIL-125 (Ti) and TEOA in MeCN was irradiated under visible light in the presence of N2, CO2 and O2. In the presence of N2, the color of the suspension changed from the original bright yellow to green. While, when CO2 or O2 was introduced into the system, the green color of the suspension changed gradually back to the original bright yellow, as shown in Fig. 1(a). The UV/Vis spectrum of the green solids exhibit broad intense absorption in the visible light region corresponding to Ti3+, which can be attributed to the Ti3+–Ti4+ intervalence charge transfer in TiAO cluster. Based on these experimental observations, the mechanism of the reduction of CO2 in NH2-MIL-125 (Ti) can be proposed, as illustrated in Fig. 1(b). Upon light absorption in the LMCT band, a long-lived charge separated excited state occurs by transferring an electron from an organic ligand to Ti4+ which can further be reduced to Ti3+ with TEOA as electron donor. The CO2 can be reduced into HCOO by Ti3+. In this system, TEOA not only acts as electron donor, but also facilitate the photocatalytic reduction process due to its more basic nature. For semiconductor-based photocatalysts, the separation efficiency of the photogenerated charge carriers is a crucial factor in determining their photocatalytic performance. The introduction of noble metals (Pt, Pd, Au) into semiconductor photocatalysts can decrease the recombination of the photogenerated electrons and holes [59–61]. The formation of a Schottky barrier at the junction between the semiconductor and the noble metal results in an efficient separation of the photogenerated charge carriers, which ultimately enhances the photocatalytic activities [62,63]. In addition, the different height of the Schottky barrier formed at the metal/semiconductor junction can influence the electron flow from semiconductor to the noble metal, leading to different photocatalytic performance [64]. Li and coworkers prepared the M-doped NH2-MIL-125(Ti) (M = Pt and Au), and studied the effects of the noble metals (Pt and Au) on the photocatalytic activity of NH2MIL-125(Ti) [65]. All the photocatalytic reactions were carried out in saturated CO2 with TEOA as a sacrificial agent under visible-light irradiation. The results revealed that only formate can be formed over pure NH2-MIL-125(Ti), while both formate and hydrogen were produced over M-doped NH2-MIL-125(Ti) (M = Pt and Au). Pt/NH2-MIL-125(Ti) showed more efficient activities for photocatalytic formation than pure NH2-MIL-125(Ti), while Au has a negative effect on the formation of formate, as shown in Fig. 2(a). The introduction of noble metals (like Pt or Au) into NH2-MIL-125(Ti) can initiate the photocatalytic hydrogen evolution, as illustrated in Fig. 2(b), since the noble metals are good electron traps and can lower its overpotential. In order to elucidate the role of hydrogen in the photocatalytic CO2 reduction, electron spin resonance (ESR) was applied to study the effect of hydrogen on the production of Ti3+, which is the active species for the photocatalytic CO2 reduction over M/NH2-MIL125(Ti). As depicted in Fig. 3 (b), the introduction of hydrogen into Pt/NH2-MIL-125(Ti) under visible light irradiation can not only increase the intensity of three ESR signals in the dark (g = 2.003, being ascribed to the oxygen defect in TiAO cluster; g = 2.015 and 2.023, being assigned to adsorbed Ti4+AO 2 species in M/ NH2-MIL-125(Ti)), but also present a new signal at g = 1.988, which
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Fig. 1. (a) Photos and corresponding ESR spectra of NH2-MIL-125(Ti) under different conditions: (i) fresh NH2-MIL-125(Ti), (ii) TEOA, visible light, and N2 and (iii) after the introduction of CO2 (or O2); (b) proposed mechanism for the photocatalytic CO2 reduction over NH2-MIL-125(Ti) under visible light irradiation [56].
Fig. 2. The amount of the product formed as a function of irradiation time over the as-prepared samples: (a) HCOO; (b) hydrogen. The solutions were irradiated by using a Xe lamp with filters producing light in the range of 420–800 nm. Photocatalysts: 50 mg, MeCN/TEOA (5:1), solution volume: 60 ml [65].
can be assigned to Ti3+ [66,67]. While, ESR spectra revealed that no Ti3+ can be formed over Au/NH2-MIL-125(Ti) and pure NH2-MIL125(Ti), as illustrated in Fig. 3, which was supported by theoretical density functional theory (DFT) calculations [68–70]. The mechanism of photocatalytic CO2 reduction over M/NH2-MIL-125(Ti) (M = Pt and Au) can be proposed as the following. In Pt/NH2-MIL125(Ti) and Au/NH2-MIL-125(Ti), the noble metals, as good electron traps, can accept the electrons from the excited ATA (H2ATA = 2-aminoterephthalate). Photocatalytic hydrogen evolution can occur with TEOA as proton source, since noble metals can provide redox reaction sites, and further lower the over potential for hydrogen evolution. The transfer of the photogenerated electron from excited ATA to the noble metals resulted in less Ti3+, which make lower photocatalytic activity for formate forma-
tion over M/NH2-MIL-125(Ti). But, for Pt/NH2-MIL-125(Ti), there are another pathway to form Ti3+ which results from hydrogen dissociation from Pt nanoparticles, increasing the enhancement of photocatalytic performance to generate formate. Compared to TiIV/TiIII (0.10 V) [71], ZrIV/ZrIII (1.06 V) [72] possesses more negative redox potential. Therefore, NH2–UiO– 66(Zr) was proposed to perform photocatalytic reduction of CO2 with TEAO as sacrificial agent under visible-light irradiation [73]. The amount of change of HCOO produced over NH2–UiO– 66(Zr), UiO–66(Zr) and H2ATA ligand with the reaction time under visible light is illustrated in Fig. 4(a). The results demonstrated that the amount of HCOO reached 13.2 lmol over NH2–UiO–66(Zr) under visible light irradiation in 10 h. While, no HCOO formed over UiO–66(Zr) and H2ATA, implying that
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Fig. 3. The ESR spectra of: (a) NH2-MIL-125(Ti), Au/NH2-MIL-125(Ti) and Pt/NH2-MIL-125(Ti) in the dark; (b) Pt/NH2-MIL-125(Ti); (c) Au/NH2-MIL-125(Ti) and (d) NH2-MIL125(Ti) in different atmosphere (Ar and H2) under visible light irradiation [65].
Fig. 4. (a) Amount of HCOO- produced as a function of irradiation time over NH2–UiO–66(Zr); UiO–66(Zr), and H2ATA. The solutions were irradiated with an Xe lamp and filters producing light in the range of 420–800 nm. Photocatalyst: 50 mg, MeCN/TEOA (5/1), solution volume: 60 ml. (b) ESR spectra of NH2–UiO–66(Zr), UiO–66(Zr), and H2ATA in different conditions under visible-light irradiation. The solid was just immersed in MeCN/TEOA (5/1). (c) Proposed mechanism for photocatalytic CO2 reduction over NH2–UiO–66(Zr) under visible-light irradiation [73].
the photocatalytic performance originated from the ATA ligand and the metal clusters. In addition, no HCOO- was produced in the dark, indicating NH2–UiO–66(Zr) was truly photocatalytic. ESR spectroscopy was applied to determine the active species during visible-light irradiation over the NH2–UiO–66(Zr), as shown in Fig. 4(b). Under visible light irradiation, an ESR signal with g = 2.004 was observed over H2ATA, which can be assigned to the spatially confined amino groups [74]. While a new signal
with g = 2.002, which can be ascribed to ZrIII [74–77] emerged over NH2–UiO–66(Zr). No ESR signal was observed over UiO– 66(Zr) with visible light irradiation, implying that ZrIII can only be formed upon visible light irradiation of NH2–UiO–66(Zr). After CO2 was introduced into the irradiated NH2–UiO–66(Zr) system, the signal of g = 2.002 was quenched, which implied that photogenerated ZrIII ions were the active species controlling the photocatalytic CO2 reduction.
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Since amine groups are highly effective for CO2 adsorption [57,58] and visible-light response [78], the increase of ANH2 groups may result in an increase in the photocatalytic performance of CO2 reduction. ATA was partly substituted by 2,5-diaminoterephthalic acid (DTA) to obtain mixed NH2–UiO–66(Zr). Under the same conditions, the amount of HCOO formed was about 20.7 lmol in 10 h, 50% higher than over pure NH2–UiO–66(Zr). In addition, even under irradiation with wavelengths longer than 515 nm, there was also 7.28 lmol HCOO that could detected, while, no HCOO was formed over pure NH2–UiO–66(Zr) upon irradiation in this spectral region, implying the more amino groups can enhance the photocatalytic activity for CO2 reduction. As illustrated in Fig. 4 (c), the ATA in NH2–UiO–66(Zr) acts as an antenna to absorb visible light. Once irradiated, the excited ATA can transfer electrons to the Zr oxo-clusters, and ZrIV in the ZrAO clusters is reduced to ZrIII, which further reduce CO2 to HCOO- with TEOA as hydrogen source. In this reaction system, TEOA play the role of both electron donor and providing a basic environment to facilitating the photocatalytic CO2 reduction. The substitution of ATA with benzyl alcohol, ethylenediaminetetraacetic acid, or methanol as electron donors failed to produce HCOO. Owing to broad visible-light absorption from singlet-triplet transitions and relatively long lifetime of the excited states, Ir complexes are candidates as water oxidation and CO2 reduction catalysts. Luo and coworkers reported that Y[Ir(ppy)2(dcbpy)]2[OH] (ppy = 2-phenylpyridine, dcbpy = 2,20 -bipyridine-4,40 -dicarboxylate) acts as both photosentisizer to absorb the visible light and photocatalyst active for CO2 reduction [79]. The photoreduction of CO2 was performed by using a certain amount of Y[Ir(ppy)2(dcbpy)]2[OH] as the photocatalyst in a mixture of MeCN and TEOA under irradiation with visible light. As shown in Fig. 5(a), the amount of HCOO formed can reach 38.0 lmol in 6 h. The turnover frequency of Y[Ir(ppy)2(dcbpy)]2[OH] for photocatalytic reduction of CO2 can be calculated to be 118.8 lmol (g of Cat.)1 h1, which is higher than that of other reported MOF-based photocatalysts, like NH2-MIL-125(Ti) (16.3 lmol (g of Cat.)1 h1) and NH2–UiO–66(Zr) (26.4 lmol (g of Cat.)1 h1). The higher performance of Y[Ir(ppy)2(dcbpy)]2[OH] can be attributed to the Ir(ppy)2(dcbpy) unit, as illustrated in Fig. 5(a). It was worthy to noting that the amount of HCOO produced over Y[Ir(ppy)2(dcbpy)]2[OH] was larger than that over Ir(ppy)2(hdcbpy) after 4 h, due to that an inefficient light sensitizer, [Ir(ppy)2(CH3CN)2]+, formed in homogeneous system and then limited the catalytic activity. The possible mechanism for
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the photocatalytic CO2 reduction to HCOO over Y[Ir(ppy)2 (dcbpy)]2[OH] is shown in Fig. 5(b). Under visible light irradiation, the Ir(ppy)2(dcbpy) unit in its excited state can be reductively quenched by TEOA, which acts as sacrificial reagent to donate electrons to the excited photosensitizer. Then, the CO2 molecules receives two electrons from the two adjacent one-electron reduced Ir(ppy)2(dcbpy)2 units to be reduced into HCOO. As is well known, TiO2 can be used as a semiconductor to carry out photocatalytic CO2 reduction into CH4 with the aid of H2O, while HKUST-1 (Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate) has been proven as good material for CO2 storage [80–82]. Xiong and coworkers reported that they prepared a hybrid material, Cu3(BTC)2@TiO2, with good photocatalytic performance for conversion of CO2 into CH4 [83]. The structural illustration, TEM image, and SEM image of Cu3(BTC)2@TiO2 are illustrated in Fig. 6(a)–(c), which indicate that the core-shell structures inherit the octahedral profile from the Cu3(BTC)2 cores and have a rough surface. The results of XRD, EDS and ICP-MS revealed that Cu3(BTC)2@TiO2 is a hybrid structure between TiO2 and Cu3(BTC)2 with the ratio of 1:1. The Cu3(BTC)2@TiO2 possess the same ability as Cu3(BTC)2 for CO2 capture as Cu3(BTC)2, which was confirmed by CO2-sorption measurements. The production yields of CH4 and H2 from CO2 using Cu3(BTC)2@TiO2 core-shell structures as photocatalysts under UV irradiation for 4 h, with TiO2 and Cu3(BTC)2 as references, respectively, are shown in Fig. 6(d). When bare TiO2 was used as photocatalyst, the production rate of CH4 and H2 were 1 1 1 1 0.52 lmol g and 2.64 lmol g , respectively. catalyst h catalyst h No CH4 and H2 can be detected when bare Cu3(BTC)2 was used as photocatalyst. The production rate of CH4 and H2 were 1 1 1 1 2.64 lmol g and 0 lmol g when Cu3(BTC)2@TiO2 catalyst h catalyst h core–shell structures were utilized as photocatalyst. These results revealed that excellent efficiency and selectivity for conversion of CO2 to CH4 can be achieved in Cu3(BTC)2@TiO2 core-shell structures, implying that CO2 molecules have easy access through the macroporous TiO2 shells and are absorbed on the microporous Cu3(BTC)2 cores. And Cu3(BTC)2@TiO2 core-shell structures demonstrated good stability due to excellent performance for photocatalytic reduction of CO2 in the three circles’ test. Upon irradiation with visible light, the TiO2 in the Cu3(BTC)2@TiO2 core-shell structures is photoexcited to produce electron–hole pairs, and the produced electrons are transferred to the Cu3(BTC)2, which was confirmed by ultrafast spectroscopy. During the photocatalysis, the CO2 reduction occurs on the Cu sites
Fig. 5. (a) The amount of HCOO- produced as a function of the time of irradiation over microcrystals of Y[Ir(ppy)2(dcbpy)]2[OH] (j), visible-light irradiation without a sample (d) and Ir(ppy)2(hdcbpy) in homogeneous photocatalytic system (N). The solutions were irradiated with a Xe lamp and filters producing light in the range of 420–800 nm. Photocatalyst: 40 mg, MeCN/TEOA (20/1), solution volume: 60 ml. (b) Possible mechanism of photocatalytic CO2 reduction to HCOO over Y[Ir(ppy)2(dcbpy)]2[OH] under visible light irradiation [79].
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Fig. 6. (a) Structural illustration; (b) TEM, and (c) SEM images of the synthesized Cu3(BTC)2@TiO2 core–shell structures. (d) Production yields of CH4 and H2 from CO2 using Cu3(BTC)2@TiO2 core–shell structures as photocatalysts under UV irradiation for 4 h, in reference to bare TiO2 nanocrystals calculated by the weight of photoactive TiO2. Bare Cu3(BTC)2 microcrystals were also used as a reference. 100 mg bare TiO2, 200 mg bare Cu3(BTC)2, and 300 mg Cu3(BTC)2@TiO2 hybrid structures were used in the measurements. (e) Production yields of CH4 with Cu3(BTC)2@TiO2 photocatalyst in recycling tests [83].
of Cu3(BTC)2, while oxidation takes place on the TiO2. This work will open a door to implementing MOF structures in photocatalyst design for gaseous reactions. Another semiconductor-MOF composite for photocatalytic reduction of CO2 to methanol is ZIF-8 (zinc-containing zeolitic imidazolate framework)/Zn2GeO4, which inherits both high CO2 adsorption capacity of ZIF-8 nanoparticles and high crystallinity of Zn2GeO4 nanorods [84]. The high-magnification FE-SEM image of ZIF-8/Zn2GeO4 revealed the presence of many regular nanoparticles (ZIF-8) on the surfaces of Zn2GeO4 nanorods, as drawn in
Fig. 7(a)–(b). The success for formation of ZIF-8/Zn2GeO4 can further be confirmed by X-ray photoelectron spectroscopy (XPS) and FTIR. The comparison of CO2 adsorption isotherms of ZIF-8, Zn2GeO4 and ZIF-8/Zn2GeO4 demonstrated that ZIF-8/Zn2GeO4 has a higher adsorption capability of CO2 than Zn2GeO4, resulting from the high adsorption of ZIF-8, shown in Fig. 7(c). The adsorption capacity of ZIF-8, Zn2GeO4 and ZIF-8/Zn2GeO4 of CO2 in aqueous solution with low CO2 concentration under ambient conditions indicated that ZIF-8/Zn2GeO4 containing 25 wt% ZIF-8 exhibited 3.8 times higher adsorption capacity than bare Zn2GeO4 nanorods.
Fig. 7. (a) and (b) FE-SEM images. (c) CO2 adsorption isotherms (273 K) of the as-prepared samples. (d) CH3OH generation over (i) 1 wt% Pt-loaded Zn2GeO4/ZIF-8 nanorods, (ii) Zn2GeO4/ZIF-8 nanorods, and (iii) Zn2GeO4 nanorods as a function of light irradiation time [84].
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Compared with the Zn2GeO4 nanorods as photocatalyst, the yield of CH3OH over ZIF-8/Zn2GeO4 was enhanced by 62% under irradiation for 10 h, as illustrated in Fig. 7(d). The rate of CH3OH generation over the Zn2GeO4/ZIF-8 nanorods could be further enhanced by loading Pt as a cocatalyst to improve the separation of the photogenerated electron–hole pairs, as demonstrated in photocatalytic reduction of CO2 and water splitting [85,86].
Photocatalytic CO2 reduction into small inorganic molecules in MOFs An important pathway for the consumption of CO2 is its reduction to CO by the enzyme acetyl-CoA synthase/CO dehydrogenase (ACS-CODH) [87]. Due to the large energy input required to generate it from CO2, CO is produced industrially from fossil fuels [88]. Even with strong reducing agents, however, overcoming the O@CO bond enthalpy of 532 kJ/mol often presents kinetic difficulties [89,90]. Lin and coworkers obtained a MOF photocatalyst Zr6(l3-O)4 (l3-OH)4(bpdc)5.83(Re(CO)3(dcbpy)Cl)0.17 (1) by doping Re(CO)3 (5,50 -dcbpy)Cl (5,50 -dcbpy = dcbpy = 2,20 -bipyridine-5,50 -dicarboxylic acid) into UiO-67 (Zr6O4(OH)4(bpdc), bpdc = biphenyldicarboxylate) [91], which can reduce CO2 into CO in CO2-saturated acetonitrile (MeCN), using Triethylamine (TEA) as sacrificial agent under visible-light irradiation. The total CO-TON (CO evolution turnover number) of 1 was estimated to be 10.9 after 20 h, almost three times higher than that of Re(CO)3(5,50 -dcbpy)Cl as catalyst under the same conditions. No CO generation was observed in absence of CO2 under the same reaction conditions, implying that the detected CO was not produced from the decomposition of organic ligands. The photocatlytic nature of the reaction was tested by the fact that no CO was produced in the dark. The parent UiO-67 showed no photocatalytic performance on CO2 reduction, demonstrating that Re(CO)3(5,50 -dcbpy)Cl was responsible for the photocatalytic CO2 reduction. But 1 became inactive for CO generation after two six-hour reaction runs. After 20 h reaction, 43.6% of the Re leached into the aqueous solution, while only 3.5% Zr was detected in the solution. From this point, MOF 1 faces the challenge of water-stability. Wang and coworkers conducted CO2 reduction reactions using [Ru(bpy)3]Cl26H2O (bpy = 2,20 = 2,20 -bipyridine) and Co-ZIF-9 (cobalt-containing zeolitic imidazolate framework) as photosensitizer and cocatalyst, respectively, along with TEOA as an electron donor under mild reaction conditions (20 °C, 1 atm, visible-light irradiation) [92]. CO2 can be split into CO at a rate of 41.8 lmol/ 30 min, along with the rate of H2 evolution of 29.9 lmol/30 min. In the absence of [Ru(bpy)3]Cl26H2O, no CO and H2 were produced. And when Co-ZIF-9 was removed from the system, CO and H2 production decreased dramatically to 1.2 lmol/30 min and 1.8 lmol/ 30 min, respectively. 13CO2 was used to confirm the source of the produced CO, and after 30 min, the GC-MS peak at 3.57 min with m/z = 29 can be assigned to 13CO, implying Co-ZIF-9 indeed promotes photocatalytic CO2 reduction into CO. The amount of Co-ZIF-9 influences its photocatalytic performance. When a small amount of Co-ZIF-9 (like 0.1 mg) was added (region I in Fig. 8), the amount of CO and H2 formed increase substantially with the increase of Co-ZIF-9 added. As illustrated in region II of Fig. 8, when the amount of Co-ZIF-9 increased to 1 mg, the maximum amount of CO and H2 was obtained. But, further increases in the amount of Co-ZIF-9 result in a slight increase of H2 and a slight decrease of CO. In region I (the amount of Co-ZIF9 being no more than 0.2 mg), the reaction rate is mainly controlled by the number of the photocatalyst centers, while in region II (Co-ZIF-9 > 0.2 mg), the reaction rate is largely limited by electron-transfer kinetics. Excessive Co-ZIF-9 would facilitate electron
Fig. 8. The effect of the amount of Co-ZIF-9 on the evolution of CO and H2 from the CO2 photoreduction system [92].
transfers to reduce proton to H2. In addition, the degradation of Ru(bpy)2+ 3 formed Ru speices that is cocatalyst for H2 evolution. So, the selectivity of CO and H2 changed when excessive Co-ZIF-9 was added in the system, which resulted in the increase of H2 and the decrease of CO when the values of log10 (quality) > 0.5. To compare the photocatalytic function of Co-ZIF-9, some other MOFs were adopted as photocatalysts to promote CO2 reduction. As listed in Table 1, when Co-MOF-74 (2,5-dihydroxyterephthalic acid as ligand) was applied as photocatalyst, the CO and H2 amount produced decreased sharply, implying that imidazolate-based ligand can enhance CO2 capture. When Mn-MOF-74 was used as cocatalyst, no noticeable improvement was found over the cocatalyst free system, indicating that electron-mediating functions of cobalt species are indispensable for supporting CO2 conversion. Therefore, tiny quantities of CO and H2 can be detected when ZnZIF-8 (2-methylimidazole as ligand) or Zr–UiO–66–NH2 (NH2-1,4benzenedicarboxylic acid as ligand) were used as cocatalysts. When cobaltocene was also used as homogeneous cocatalyst in the reaction system, only a tiny enhancement in production of H2 and CO was seen, further confirming the synergetic effect of cobalt and benzimidazolate for CO2 reduction. Since Co-ZIF-9 is a good cocatalyst that facilitates the capture of CO2 and promotes the CO2 reduction, and g-C3N4 commonly acts as semiconductor photocatalyst, the combination of these two materials achieved efficient photocatalytic CO2 reduction with bipyridine (bpy) as auxilliary and TEOA as electron donor under visible light irradiation, as illustrated in Fig. 9 [93]. The absence of gC3N4 or light inactivity of the reaction system. Upon irradiation with visible light for 2 h, the reaction system gave 20.8 lmol CO and 3.3 lmol H2, indicating that g-C3N4 undergoes photoexcitation upon visible light illumination. Once Co-ZIF-9 was removed from
Table 1 Comparison of cocatalytic functions of Co-ZIF-9 with other MOFs and cobalt complex [92]. MOFs a
Co-ZIF-9 Co-MOF-74b Mn-MOF-74b Zn-ZIF-8b Zr–UiO–66-NH2b Cobaltocenceb
CO (lmol)
H2 (lmol)
41.8 11.7 1.5 2.1 1.2 5.0
29.9 7.3 2.9 2.4 2.2 2.7
a Reaction conditions: [Ru(bpy)3]Cl26 H2O (10.0 mmol), Co-ZIF-9 (0.8 mmol, activated), solvent (5 ml, acetonitrile/H2O = 4:1), TEOA (1 ml), CO2 (1 atm), k P 420 nm, 20 °C, 30 min. b Reaction conditions are the same as those to Co-ZIF-9.
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Fig. 9. Representation of the cooperation of Co-ZIF-9 and g-C3N4 for the photocatalytic reduction of CO2 under visible light irradiation [93].
the reaction system, CO and H2 formation would stop, which confirmed that Co-ZIF-9 remarkably promoted the CO2 reduction by acting as CO2 activator and redox promotor. In the absence of bpy, the catalytic activity was reduced significantly, implying that the bpy plays the role of cooperatively transferring the excited electrons. No CO (0.9 lmol H2 formed only) can be found upon replacing CO2 with Ar in the system, indicating the participation of CO2 in the system. 13CO2 was also used to validate the source of the produced CO, and the result revealed that the GC-MS peak at 3.54 min with m/z 29 was assigned to 13CO [92]. Electron donors played crucial roles in the catalytic performance. As listed in Table 2, some different tertiary amines were applied in the system to assess the effects on CO2 reduction. The results showed that once TEOA was replaced by TEA, TPA and TBA, the amount of CO and H2 decreased sharply, but the selectivity for CO obviously increased with some b-hydroxylated amines as electron donors. It was worthy to pointing out that, under the same conditions, TEOA presented superior catalytic activity in the formation of CO, and TIPOA displayed the highest selectivity towards CO formation. The effects of different reaction mediums on the photocatalytic activity were also investigated. Some aprotic solvents, such as MeCN, DMF, THF and DMSO were favorable to achieve good performance of the reaction system. Nitrogen and/or oxygen atoms are believed to promote solubilizing CO2 via Lewis acid – base interactions [96]. When the reaction was operated in DCM, neither CO nor H2 was detected, due to weak chemical affinity towards CO2 mol-
Table 2 Effects of various tertiary amines on the yield of CO and H2 from the CO2 photoreduction systema [93]. Entry 1 2 3 4 5 6 7
Amineb TEOA (E(TEOA+/TEOA) = 0.82 V [94]) TEA (E(TEA+/TEA) = 0.93 V [95]) TPA TBA DEAIPO DEAEO TIPOA
CO/ lmol
lmol
Sel.COc/ %
20.8
3.3
86.3
2.0 1.4 1.0 19.4 15.2 10.9
4.6 1.9 0.6 2.3 0.8 0.4
30.3 42.4 62.5 89.4 95.0 96.5
H2/
a Reaction condition: C3N4 (20 mg), Co-ZIF-9 (1 mg), bpy (10 mg), solvent (5 ml, MeCN:H2O = 3:2), amine (1 ml), k > 420 nm, 30 °C, 2 h. b TEOA = triethanolamine; TEA = Triethylamine; TPA = Tri-n-propylamine; TBA = Tri-n-butylamine; DEAIPO = 1-(diethylamino)-2-propanol; DEAEO = 2(Diethylamino)ethanol; TIPOA = Tri-2-propanolamine. c Sel.CO = mol(CO)/mol(CO + H2).
ecules. In pure water, the photocatalytic CO2 reduction was completely inhibited. While the production of CO and H2 and the selectivity of CO achieved maximum values when the system contained 40% H2O. After the reaction, no noticeable changes of the chemical and crystal structures of Co-ZIF-9 and g-C3N4 can be found, reflecting the stable characteristics of the coupling and working of the MOF and g-C3N4 in photocatalytic CO2 reduction. Compared with many catalytic systems containing noble metal catalysts, this noble metal free system for efficient CO2 reduction possesses many advantages.
Conclusion and outlook Metal–organic frameworks, a new class of crystalline molecular solids built from linking organic ligands with metal-cluster connecting points, have recently emerged as a versatile platform to conduct photocatalytic reactions [20]. Compared to conventional photocatalytic CO2 reduction systems, photoactive MOFs have some advantages: (i) the intrinsic porosity can facilitate the adsorption of the CO2, which is essential for a high photocatalytic efficiency; (ii) versatile synthetic strategies, including solvothermal, vapor diffusion, emulsion-assistant precipitation, ultrasonication and even post-synthesis modification, allow a high degree of crystalline quality and morphologies of MOF photocatalysts. (iii) harnessing solar energy more efficiently, as their structural features of tunable active sites (i.e., metal-oxoclusters and organic linkers). Visible light photocatalytic activity can be introduced via linker substitution with an amino group [56,97–100]. (iv) The well-defined crystalline structure of MOFs is beneficial to the characterization and study of structureproperty relationship of these solid photocatalysts. Up to now, it has been difficult to perform high-throughput synthesis with amounts of kilogram quantities in a matter of hours at ambient pressure, which is an impediment in the study of wide-spread applications for MOFs. The solvothermal (including hydrothermal) methods, involving the use of an autoclave, and slow-diffusion processes will take days or weeks to complete, eliminating the possibility of an industrially relevant process [101,102]. Although photocatalytic CO2 reduction has been achieved with a number of MOFs, all of these systems require the use of a sacrificial agent, such as TEOA, which is not economic and environmentally friendly. Most MOFs tend to have modest hydrolytic and thermal stabilities, which make them impractical in solar energy applications, especially for photocatalytic CO2 reduction. The development of stable MOFs, like UiOs, MIL-140,
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125, 101 and the ZIF series, can overcome the instability issue that can plague other MOFs in many applications and, in particular, will further spur the interest in exploring photocatalysis with MOFs. Additionally, most MOFs also do not possess strong mechanical properties, good processability, and high electric conductivity, all of which will hinder the integration of MOFs into functional solar devices [103]. Therefore, a way to synthesize achieve inexpensive, stable, and efficient MOFs photocatalysts still presents a big challenge. Overall, we believe that the present studies will stimulate intensive research in MOF-based photocatalysis and will open new perspectives for the development of photocatalytic CO2 reduction. Acknowledgements We thank the financial support from the Beijing Natural Science Foundation & Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201410016018), the Training Program Foundation for the Beijing Municipal Excellent Talents (2013D005017000004), the Importation & Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&CD201404076), the China Postdoctoral Science of Foundation (2013M540831) and Open Research Fund Program of Key Laboratory of Urban Stormwater System and Water Environment (Beijing University of Civil Engineering and Architecture) of Ministry of Education. References [1] J. Albo, P. Luis, A. Irabien, Ind. Eng. Chem. Res. 49 (2010) 11045–11051. [2] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731– 737. [3] A. Ahmad Beigi, S. Fatemi, Z. Salehi, J. CO2 Utiliz. 7 (2014) 23–29. [4] M. Anpo, J. CO2 Utiliz. 1 (2013) 8–17. [5] Y. Liu, B. Huang, Y. Dai, X. Zhang, X. Qin, M. Jiang, M.-H. Whangbo, Catal. Commun. 11 (2009) 210–213. [6] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, Chem. Commun. 48 (2012) 9729–9731. [7] Q. Liu, Y. Zhou, Z. Tian, X. Chen, J. Gao, Z. Zou, J. Mater. Chem. 22 (2012) 2033– 2038. [8] T. Ohno, N. Murakami, T. Koyanagi, Y. Yang, J. CO2 Utiliz. 6 (2014) 17–25. [9] Y. Liu, Y. Yang, Q. Sun, Z. Wang, B. Huang, Y. Dai, X. Qin, X. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 7654–7658. [10] C.-C. Wang, H.-Y. Li, G.-L. Guo, P. Wang, Transition Met. Chem. 38 (2013) 275– 282. [11] C.-C. Wang, G.-L. Guo, P. Wang, Transition Met. Chem. (2013) 1–8. [12] C.-C. Wang, G. Guo, P. Wang, J. Mol. Struct. 1032 (2012) 93–99. [13] C.-C. Wang, Z. Wang, F. Gu, G. Guo, J. Mol. Struct. 1004 (2011) 39–44. [14] C.-C. Wang, P. Wang, G.-S. Guo, Transition Met. Chem. 35 (2010) 721–729. [15] C.-C. Wang, Z. Wang, F. Gu, G. Guo, J. Mol. Struct. 979 (2010) 92–100. [16] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450–1459. [17] C.-Y. Sun, S.-X. Liu, D.-D. Liang, K.-Z. Shao, Y.-H. Ren, Z.-M. Su, J. Am. Chem. Soc. 131 (2009) 1883–1888. [18] R.J. Kuppler, D.J. Timmons, Q.-R. Fang, J.-R. Li, T.A. Makal, M.D. Young, D. Yuan, D. Zhao, W. Zhuang, H.-C. Zhou, Coord. Chem. Rev. 253 (2009) 3042–3066. [19] X.-S. Wang, S. Ma, D. Sun, S. Parkin, H.-C. Zhou, J. Am. Chem. Soc. 128 (2006) 16474–16475. [20] C.-C. Wang, J.-R. Li, X.-L. Lv, Y.-Q. Zhang, G. Guo, Energy Environ. Sci. 7 (2014) 2831–2867. [21] C.-C. Wang, H.-P. Jing, P. Wang, S.-J. Gao, J. Mol. Struct. 1080 (2015) 44–51. [22] C.-C. Wang, H.-P. Jing, P. Wang, J. Mol. Struct. 1074 (2014) 92–99. [23] C.-C.W.H.-P. Jing, Y.-W. Zhang, P. Wang, R. Li, RSC Adv. 4 (97) (2014) 54454– 54462. [24] J.-R. Li, J. Yu, W. Lu, L.-B. Sun, J. Sculley, P.B. Balbuena, H.-C. Zhou, Nat. Commun. 4 (2013) 1538. [25] J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 112 (2011) 869–932. [26] J.-R. Li, H.-C. Zhou, Nat. Chem. 2 (2010) 893–898. [27] J.-R. Li, D.J. Timmons, H.-C. Zhou, J. Am. Chem. Soc. 131 (2009) 6368–6369. [28] J.-R. Li, R.J. Kuppler, H.-C. Zhou, Chem. Soc. Rev. 38 (2009) 1477–1504. [29] L. Pan, D.H. Olson, L.R. Ciemnolonski, R. Heddy, J. Li, Angew. Chem. 118 (2006) 632–635. [30] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127–1129. [31] J.L. Rowsell, O.M. Yaghi, Angew. Chem. Int. Ed. 44 (2005) 4670–4679. [32] D.J. Collins, H.-C. Zhou, J. Mater. Chem. 17 (2007) 3154–3160. [33] S. Ma, H.-C. Zhou, Chem. Commun. 46 (2010) 44–53.
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Photocatalytic CO2 reduction in metal–organic frameworks: A mini review Chong-Chen Wang a,b,⇑, Yan-Qiu Zhang a, Jin Li a, Peng Wang a a b
Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing University of Civil Engineering and Architecture, Beijing 100044, China Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Metal–organic frameworks are very
effective to conduct photocatalytic CO2 reduction. Mechanisms for photocatalytic CO2 reduction with MOFs were summarized. MOFs are surely believed to have a bright prospect in the field of photocatalytic reduction of CO2.
a r t i c l e
i n f o
Article history: Received 7 August 2014 Received in revised form 29 October 2014 Accepted 17 November 2014 Available online 22 November 2014 Keywords: Metal–organic frameworks Carbon dioxide Photocatalytic Reduction
a b s t r a c t Photocatalytic reduction of CO2 for value-added chemicals is an attractive process to address both energy and environmental issues. This mini review paper presents two different conversion processes, namely conversion to organic chemicals (like CH4, CH3OH, HCOOH and so on) and being split into CO, in metal–organic frameworks (MOFs). The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the photocatalytic performance, the involved challenges, and the prospects are discussed and estimated. It is clear that MOFs have a bright prospect in the field of photocatalytic reduction of CO2. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Global warming resulting from the emission of greenhouse gases has become a widespread concern in the recent years. Among the greenhouse gases, CO2 contributes more than 60% to global warming because of its huge emission amount [1]. Therefore, the capture and efficient use of CO2 is an important issue. One of the best solutions is to photocatalytically convert CO2 into ⇑ Corresponding author at: Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing University of Civil Engineering and Architecture, Beijing 100044, China. http://dx.doi.org/10.1016/j.molstruc.2014.11.036 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
valuable chemicals, like CO, CH4, CH3OH, CH3CH2OH, HCOOH and so on, by means of solar energy. Many photocatalysts, including TiO2 [2–4], BiVO4 [5], BiWO6 [6], Zn2GeO4 [7] and other composites [8] have been investigated for their performance in photocatalytic CO2 reduction. CO2 is highly stable, in which the C@O bond energy is 750 kJ/mol, much higher than CAC (336 kJ/mol), CAO (327 kJ/ mol) and CAH (411 kJ/mol) [9], leading to low reduction efficiency. Furthermore, most of the photocatalysts already investigated are only active in the UV region. It is necessary to develop highly efficient photocatalysts that can reduce CO2 under visible light. Metal–organic frameworks (MOFs), a class of newly-developed inorganic-organic hybrid porous materials, has generated a rapid
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development due to their diverse and easily tailored structures [10–15] and various potential applications, such as in catalysis [16–23], separation [24–29], gas storage [18,30–33], carbon dioxide capture [34–37], and so on [10–15,24–27,38–41]. Recent research indeed not only demonstrated porous MOFs materials to be a new class of photocatalysts usable in catalytic degradation of organic pollutants under UV/visible/UV–visible irradiation [20], but also triggered an intense interest in exploring MOF applications as photocatalysts in other aspects [42–48]. Based on the richness of metal-containing nodes and organic bridging linkers, as well as the controllability of synthesis, it is easy to construct MOFs with tailorable capacity to absorb light, thereby initiating desirable photocatalytic properties for specific application in CO2 reduction. The study of MOF applications in this topic thus has a bright future even if not being so widely explored to date. Herein, we highlight the research progress of MOF applications in photocatalytic CO2 reduction. The reported examples are collected and analyzed; the reaction mechanism and the influence of various factors on the catalytic performance are discussed; as well as the involved challenges and the prospects are estimated.
Photocatalytic CO2 reduction into valuable organic chemicals in MOFs Green plants can harvest solar energy and use the harvested energy to convert CO2 and H2O into carbohydrates via photosynthesis. Some artificial systems and devices using inorganic and organic materials to simulate photochemical reactions have been developed recently [49–54]. Three fundamental steps are needed to convert solar energy to chemical energy: (1) sunlight absorption by photosensitizers to create charge-separated excited states; (2) generation of redox equivalents and their migration to reactive centers, and (3) reduction and oxidation half reactions with the redox equivalents (electrons and holes) at the catalytic centers [55]. As a new family of inorganic–organic hybrid materials, MOFs serve as an interesting platform to design and study artificial photosynthetic systems. In general, MOFs can contain photosensitizers and catalytic centers in a single solid, which provide the structural organization to integrate the three fundamental steps of artificial photosynthesis into a single material [55]. A number of recent papers have demonstrated that MOFs can be used to achieve light harvesting and to drive photocatalytic reduction of CO2 into small organic molecules, like HCOOH, CH3OH, and so on. MIL-125(Ti) (Ti8O8(OH)4(BDC)6, BDC = benzene-1,4-dicarboxylate) can not only introduce high density of the immobilized Ti sites into porous MOFs, but also lead to various isostructural MOFs, whose photocatalytic properties can be tuned simply via incorporation of BDC derivatives. Taking 2-aminoterephthalate as organic linker, NH2-MIL-125 (Ti) was synthesized to be used to perform photocatalytic reduction of CO2 into HCOOH [56]. A significant change in NH2-MIL-125 (Ti) is its optical absorption, which shows an extra absorption band edge to around 550 nm, falling in the visible region, significant red shift from absorption band edge to 350 nm of MIL-125 (Ti). The introduction of ANH2 group influences O to Ti charge transfer (LMCT) in TiO5(OH) inorganic cluster, which results in the formation of NH2-MIL-125 (Ti) with absorption in the visible light region. Another noticeable change resulting from the introduction of ANH2 functionality is the increase of its adsorption capability toward CO2, because aromatic molecules functionalized with some polar substituent groups like AOH, ACOOH or ANH2 enhance the interactions of CO2 with the functionalized aromatic molecules [57,58]. The photocatalytic reduction of CO2 with NH2-MIL-125 (Ti) as photocatalyst was carried out in acetonitrile (MeCN) with triethanolamine (TEOA) as sacrificial agent under visible light irradiation
[56]. After 10 h, ca. 8.14 lmol HCOO was detected by ion chromatograph. No HCOO was generated in the dark, implying it is a photocatalytic process. The inactivity of the MIL-125 (Ti) under completely similar conditions indicated that the photocatalytic performance for CO2 reduction over NH2-MIL-125 (Ti) actually originates from the amino functionality. Labeled 13CO2 was used for the photocatalytic process confirming the HCOO originated from CO2. NH2-MIL-125 (Ti) is stable during the photocatalytic reaction, which was confirmed by XRD, BET, TGA, IR and Raman spectra. In order to deduce the mechanism of the reduction of CO2 over NH2-MIL-125 (Ti), the suspension of NH2-MIL-125 (Ti) and TEOA in MeCN was irradiated under visible light in the presence of N2, CO2 and O2. In the presence of N2, the color of the suspension changed from the original bright yellow to green. While, when CO2 or O2 was introduced into the system, the green color of the suspension changed gradually back to the original bright yellow, as shown in Fig. 1(a). The UV/Vis spectrum of the green solids exhibit broad intense absorption in the visible light region corresponding to Ti3+, which can be attributed to the Ti3+–Ti4+ intervalence charge transfer in TiAO cluster. Based on these experimental observations, the mechanism of the reduction of CO2 in NH2-MIL-125 (Ti) can be proposed, as illustrated in Fig. 1(b). Upon light absorption in the LMCT band, a long-lived charge separated excited state occurs by transferring an electron from an organic ligand to Ti4+ which can further be reduced to Ti3+ with TEOA as electron donor. The CO2 can be reduced into HCOO by Ti3+. In this system, TEOA not only acts as electron donor, but also facilitate the photocatalytic reduction process due to its more basic nature. For semiconductor-based photocatalysts, the separation efficiency of the photogenerated charge carriers is a crucial factor in determining their photocatalytic performance. The introduction of noble metals (Pt, Pd, Au) into semiconductor photocatalysts can decrease the recombination of the photogenerated electrons and holes [59–61]. The formation of a Schottky barrier at the junction between the semiconductor and the noble metal results in an efficient separation of the photogenerated charge carriers, which ultimately enhances the photocatalytic activities [62,63]. In addition, the different height of the Schottky barrier formed at the metal/semiconductor junction can influence the electron flow from semiconductor to the noble metal, leading to different photocatalytic performance [64]. Li and coworkers prepared the M-doped NH2-MIL-125(Ti) (M = Pt and Au), and studied the effects of the noble metals (Pt and Au) on the photocatalytic activity of NH2MIL-125(Ti) [65]. All the photocatalytic reactions were carried out in saturated CO2 with TEOA as a sacrificial agent under visible-light irradiation. The results revealed that only formate can be formed over pure NH2-MIL-125(Ti), while both formate and hydrogen were produced over M-doped NH2-MIL-125(Ti) (M = Pt and Au). Pt/NH2-MIL-125(Ti) showed more efficient activities for photocatalytic formation than pure NH2-MIL-125(Ti), while Au has a negative effect on the formation of formate, as shown in Fig. 2(a). The introduction of noble metals (like Pt or Au) into NH2-MIL-125(Ti) can initiate the photocatalytic hydrogen evolution, as illustrated in Fig. 2(b), since the noble metals are good electron traps and can lower its overpotential. In order to elucidate the role of hydrogen in the photocatalytic CO2 reduction, electron spin resonance (ESR) was applied to study the effect of hydrogen on the production of Ti3+, which is the active species for the photocatalytic CO2 reduction over M/NH2-MIL125(Ti). As depicted in Fig. 3 (b), the introduction of hydrogen into Pt/NH2-MIL-125(Ti) under visible light irradiation can not only increase the intensity of three ESR signals in the dark (g = 2.003, being ascribed to the oxygen defect in TiAO cluster; g = 2.015 and 2.023, being assigned to adsorbed Ti4+AO 2 species in M/ NH2-MIL-125(Ti)), but also present a new signal at g = 1.988, which
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Fig. 1. (a) Photos and corresponding ESR spectra of NH2-MIL-125(Ti) under different conditions: (i) fresh NH2-MIL-125(Ti), (ii) TEOA, visible light, and N2 and (iii) after the introduction of CO2 (or O2); (b) proposed mechanism for the photocatalytic CO2 reduction over NH2-MIL-125(Ti) under visible light irradiation [56].
Fig. 2. The amount of the product formed as a function of irradiation time over the as-prepared samples: (a) HCOO; (b) hydrogen. The solutions were irradiated by using a Xe lamp with filters producing light in the range of 420–800 nm. Photocatalysts: 50 mg, MeCN/TEOA (5:1), solution volume: 60 ml [65].
can be assigned to Ti3+ [66,67]. While, ESR spectra revealed that no Ti3+ can be formed over Au/NH2-MIL-125(Ti) and pure NH2-MIL125(Ti), as illustrated in Fig. 3, which was supported by theoretical density functional theory (DFT) calculations [68–70]. The mechanism of photocatalytic CO2 reduction over M/NH2-MIL-125(Ti) (M = Pt and Au) can be proposed as the following. In Pt/NH2-MIL125(Ti) and Au/NH2-MIL-125(Ti), the noble metals, as good electron traps, can accept the electrons from the excited ATA (H2ATA = 2-aminoterephthalate). Photocatalytic hydrogen evolution can occur with TEOA as proton source, since noble metals can provide redox reaction sites, and further lower the over potential for hydrogen evolution. The transfer of the photogenerated electron from excited ATA to the noble metals resulted in less Ti3+, which make lower photocatalytic activity for formate forma-
tion over M/NH2-MIL-125(Ti). But, for Pt/NH2-MIL-125(Ti), there are another pathway to form Ti3+ which results from hydrogen dissociation from Pt nanoparticles, increasing the enhancement of photocatalytic performance to generate formate. Compared to TiIV/TiIII (0.10 V) [71], ZrIV/ZrIII (1.06 V) [72] possesses more negative redox potential. Therefore, NH2–UiO– 66(Zr) was proposed to perform photocatalytic reduction of CO2 with TEAO as sacrificial agent under visible-light irradiation [73]. The amount of change of HCOO produced over NH2–UiO– 66(Zr), UiO–66(Zr) and H2ATA ligand with the reaction time under visible light is illustrated in Fig. 4(a). The results demonstrated that the amount of HCOO reached 13.2 lmol over NH2–UiO–66(Zr) under visible light irradiation in 10 h. While, no HCOO formed over UiO–66(Zr) and H2ATA, implying that
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Fig. 3. The ESR spectra of: (a) NH2-MIL-125(Ti), Au/NH2-MIL-125(Ti) and Pt/NH2-MIL-125(Ti) in the dark; (b) Pt/NH2-MIL-125(Ti); (c) Au/NH2-MIL-125(Ti) and (d) NH2-MIL125(Ti) in different atmosphere (Ar and H2) under visible light irradiation [65].
Fig. 4. (a) Amount of HCOO- produced as a function of irradiation time over NH2–UiO–66(Zr); UiO–66(Zr), and H2ATA. The solutions were irradiated with an Xe lamp and filters producing light in the range of 420–800 nm. Photocatalyst: 50 mg, MeCN/TEOA (5/1), solution volume: 60 ml. (b) ESR spectra of NH2–UiO–66(Zr), UiO–66(Zr), and H2ATA in different conditions under visible-light irradiation. The solid was just immersed in MeCN/TEOA (5/1). (c) Proposed mechanism for photocatalytic CO2 reduction over NH2–UiO–66(Zr) under visible-light irradiation [73].
the photocatalytic performance originated from the ATA ligand and the metal clusters. In addition, no HCOO- was produced in the dark, indicating NH2–UiO–66(Zr) was truly photocatalytic. ESR spectroscopy was applied to determine the active species during visible-light irradiation over the NH2–UiO–66(Zr), as shown in Fig. 4(b). Under visible light irradiation, an ESR signal with g = 2.004 was observed over H2ATA, which can be assigned to the spatially confined amino groups [74]. While a new signal
with g = 2.002, which can be ascribed to ZrIII [74–77] emerged over NH2–UiO–66(Zr). No ESR signal was observed over UiO– 66(Zr) with visible light irradiation, implying that ZrIII can only be formed upon visible light irradiation of NH2–UiO–66(Zr). After CO2 was introduced into the irradiated NH2–UiO–66(Zr) system, the signal of g = 2.002 was quenched, which implied that photogenerated ZrIII ions were the active species controlling the photocatalytic CO2 reduction.
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Since amine groups are highly effective for CO2 adsorption [57,58] and visible-light response [78], the increase of ANH2 groups may result in an increase in the photocatalytic performance of CO2 reduction. ATA was partly substituted by 2,5-diaminoterephthalic acid (DTA) to obtain mixed NH2–UiO–66(Zr). Under the same conditions, the amount of HCOO formed was about 20.7 lmol in 10 h, 50% higher than over pure NH2–UiO–66(Zr). In addition, even under irradiation with wavelengths longer than 515 nm, there was also 7.28 lmol HCOO that could detected, while, no HCOO was formed over pure NH2–UiO–66(Zr) upon irradiation in this spectral region, implying the more amino groups can enhance the photocatalytic activity for CO2 reduction. As illustrated in Fig. 4 (c), the ATA in NH2–UiO–66(Zr) acts as an antenna to absorb visible light. Once irradiated, the excited ATA can transfer electrons to the Zr oxo-clusters, and ZrIV in the ZrAO clusters is reduced to ZrIII, which further reduce CO2 to HCOO- with TEOA as hydrogen source. In this reaction system, TEOA play the role of both electron donor and providing a basic environment to facilitating the photocatalytic CO2 reduction. The substitution of ATA with benzyl alcohol, ethylenediaminetetraacetic acid, or methanol as electron donors failed to produce HCOO. Owing to broad visible-light absorption from singlet-triplet transitions and relatively long lifetime of the excited states, Ir complexes are candidates as water oxidation and CO2 reduction catalysts. Luo and coworkers reported that Y[Ir(ppy)2(dcbpy)]2[OH] (ppy = 2-phenylpyridine, dcbpy = 2,20 -bipyridine-4,40 -dicarboxylate) acts as both photosentisizer to absorb the visible light and photocatalyst active for CO2 reduction [79]. The photoreduction of CO2 was performed by using a certain amount of Y[Ir(ppy)2(dcbpy)]2[OH] as the photocatalyst in a mixture of MeCN and TEOA under irradiation with visible light. As shown in Fig. 5(a), the amount of HCOO formed can reach 38.0 lmol in 6 h. The turnover frequency of Y[Ir(ppy)2(dcbpy)]2[OH] for photocatalytic reduction of CO2 can be calculated to be 118.8 lmol (g of Cat.)1 h1, which is higher than that of other reported MOF-based photocatalysts, like NH2-MIL-125(Ti) (16.3 lmol (g of Cat.)1 h1) and NH2–UiO–66(Zr) (26.4 lmol (g of Cat.)1 h1). The higher performance of Y[Ir(ppy)2(dcbpy)]2[OH] can be attributed to the Ir(ppy)2(dcbpy) unit, as illustrated in Fig. 5(a). It was worthy to noting that the amount of HCOO produced over Y[Ir(ppy)2(dcbpy)]2[OH] was larger than that over Ir(ppy)2(hdcbpy) after 4 h, due to that an inefficient light sensitizer, [Ir(ppy)2(CH3CN)2]+, formed in homogeneous system and then limited the catalytic activity. The possible mechanism for
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the photocatalytic CO2 reduction to HCOO over Y[Ir(ppy)2 (dcbpy)]2[OH] is shown in Fig. 5(b). Under visible light irradiation, the Ir(ppy)2(dcbpy) unit in its excited state can be reductively quenched by TEOA, which acts as sacrificial reagent to donate electrons to the excited photosensitizer. Then, the CO2 molecules receives two electrons from the two adjacent one-electron reduced Ir(ppy)2(dcbpy)2 units to be reduced into HCOO. As is well known, TiO2 can be used as a semiconductor to carry out photocatalytic CO2 reduction into CH4 with the aid of H2O, while HKUST-1 (Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate) has been proven as good material for CO2 storage [80–82]. Xiong and coworkers reported that they prepared a hybrid material, Cu3(BTC)2@TiO2, with good photocatalytic performance for conversion of CO2 into CH4 [83]. The structural illustration, TEM image, and SEM image of Cu3(BTC)2@TiO2 are illustrated in Fig. 6(a)–(c), which indicate that the core-shell structures inherit the octahedral profile from the Cu3(BTC)2 cores and have a rough surface. The results of XRD, EDS and ICP-MS revealed that Cu3(BTC)2@TiO2 is a hybrid structure between TiO2 and Cu3(BTC)2 with the ratio of 1:1. The Cu3(BTC)2@TiO2 possess the same ability as Cu3(BTC)2 for CO2 capture as Cu3(BTC)2, which was confirmed by CO2-sorption measurements. The production yields of CH4 and H2 from CO2 using Cu3(BTC)2@TiO2 core-shell structures as photocatalysts under UV irradiation for 4 h, with TiO2 and Cu3(BTC)2 as references, respectively, are shown in Fig. 6(d). When bare TiO2 was used as photocatalyst, the production rate of CH4 and H2 were 1 1 1 1 0.52 lmol g and 2.64 lmol g , respectively. catalyst h catalyst h No CH4 and H2 can be detected when bare Cu3(BTC)2 was used as photocatalyst. The production rate of CH4 and H2 were 1 1 1 1 2.64 lmol g and 0 lmol g when Cu3(BTC)2@TiO2 catalyst h catalyst h core–shell structures were utilized as photocatalyst. These results revealed that excellent efficiency and selectivity for conversion of CO2 to CH4 can be achieved in Cu3(BTC)2@TiO2 core-shell structures, implying that CO2 molecules have easy access through the macroporous TiO2 shells and are absorbed on the microporous Cu3(BTC)2 cores. And Cu3(BTC)2@TiO2 core-shell structures demonstrated good stability due to excellent performance for photocatalytic reduction of CO2 in the three circles’ test. Upon irradiation with visible light, the TiO2 in the Cu3(BTC)2@TiO2 core-shell structures is photoexcited to produce electron–hole pairs, and the produced electrons are transferred to the Cu3(BTC)2, which was confirmed by ultrafast spectroscopy. During the photocatalysis, the CO2 reduction occurs on the Cu sites
Fig. 5. (a) The amount of HCOO- produced as a function of the time of irradiation over microcrystals of Y[Ir(ppy)2(dcbpy)]2[OH] (j), visible-light irradiation without a sample (d) and Ir(ppy)2(hdcbpy) in homogeneous photocatalytic system (N). The solutions were irradiated with a Xe lamp and filters producing light in the range of 420–800 nm. Photocatalyst: 40 mg, MeCN/TEOA (20/1), solution volume: 60 ml. (b) Possible mechanism of photocatalytic CO2 reduction to HCOO over Y[Ir(ppy)2(dcbpy)]2[OH] under visible light irradiation [79].
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Fig. 6. (a) Structural illustration; (b) TEM, and (c) SEM images of the synthesized Cu3(BTC)2@TiO2 core–shell structures. (d) Production yields of CH4 and H2 from CO2 using Cu3(BTC)2@TiO2 core–shell structures as photocatalysts under UV irradiation for 4 h, in reference to bare TiO2 nanocrystals calculated by the weight of photoactive TiO2. Bare Cu3(BTC)2 microcrystals were also used as a reference. 100 mg bare TiO2, 200 mg bare Cu3(BTC)2, and 300 mg Cu3(BTC)2@TiO2 hybrid structures were used in the measurements. (e) Production yields of CH4 with Cu3(BTC)2@TiO2 photocatalyst in recycling tests [83].
of Cu3(BTC)2, while oxidation takes place on the TiO2. This work will open a door to implementing MOF structures in photocatalyst design for gaseous reactions. Another semiconductor-MOF composite for photocatalytic reduction of CO2 to methanol is ZIF-8 (zinc-containing zeolitic imidazolate framework)/Zn2GeO4, which inherits both high CO2 adsorption capacity of ZIF-8 nanoparticles and high crystallinity of Zn2GeO4 nanorods [84]. The high-magnification FE-SEM image of ZIF-8/Zn2GeO4 revealed the presence of many regular nanoparticles (ZIF-8) on the surfaces of Zn2GeO4 nanorods, as drawn in
Fig. 7(a)–(b). The success for formation of ZIF-8/Zn2GeO4 can further be confirmed by X-ray photoelectron spectroscopy (XPS) and FTIR. The comparison of CO2 adsorption isotherms of ZIF-8, Zn2GeO4 and ZIF-8/Zn2GeO4 demonstrated that ZIF-8/Zn2GeO4 has a higher adsorption capability of CO2 than Zn2GeO4, resulting from the high adsorption of ZIF-8, shown in Fig. 7(c). The adsorption capacity of ZIF-8, Zn2GeO4 and ZIF-8/Zn2GeO4 of CO2 in aqueous solution with low CO2 concentration under ambient conditions indicated that ZIF-8/Zn2GeO4 containing 25 wt% ZIF-8 exhibited 3.8 times higher adsorption capacity than bare Zn2GeO4 nanorods.
Fig. 7. (a) and (b) FE-SEM images. (c) CO2 adsorption isotherms (273 K) of the as-prepared samples. (d) CH3OH generation over (i) 1 wt% Pt-loaded Zn2GeO4/ZIF-8 nanorods, (ii) Zn2GeO4/ZIF-8 nanorods, and (iii) Zn2GeO4 nanorods as a function of light irradiation time [84].
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Compared with the Zn2GeO4 nanorods as photocatalyst, the yield of CH3OH over ZIF-8/Zn2GeO4 was enhanced by 62% under irradiation for 10 h, as illustrated in Fig. 7(d). The rate of CH3OH generation over the Zn2GeO4/ZIF-8 nanorods could be further enhanced by loading Pt as a cocatalyst to improve the separation of the photogenerated electron–hole pairs, as demonstrated in photocatalytic reduction of CO2 and water splitting [85,86].
Photocatalytic CO2 reduction into small inorganic molecules in MOFs An important pathway for the consumption of CO2 is its reduction to CO by the enzyme acetyl-CoA synthase/CO dehydrogenase (ACS-CODH) [87]. Due to the large energy input required to generate it from CO2, CO is produced industrially from fossil fuels [88]. Even with strong reducing agents, however, overcoming the O@CO bond enthalpy of 532 kJ/mol often presents kinetic difficulties [89,90]. Lin and coworkers obtained a MOF photocatalyst Zr6(l3-O)4 (l3-OH)4(bpdc)5.83(Re(CO)3(dcbpy)Cl)0.17 (1) by doping Re(CO)3 (5,50 -dcbpy)Cl (5,50 -dcbpy = dcbpy = 2,20 -bipyridine-5,50 -dicarboxylic acid) into UiO-67 (Zr6O4(OH)4(bpdc), bpdc = biphenyldicarboxylate) [91], which can reduce CO2 into CO in CO2-saturated acetonitrile (MeCN), using Triethylamine (TEA) as sacrificial agent under visible-light irradiation. The total CO-TON (CO evolution turnover number) of 1 was estimated to be 10.9 after 20 h, almost three times higher than that of Re(CO)3(5,50 -dcbpy)Cl as catalyst under the same conditions. No CO generation was observed in absence of CO2 under the same reaction conditions, implying that the detected CO was not produced from the decomposition of organic ligands. The photocatlytic nature of the reaction was tested by the fact that no CO was produced in the dark. The parent UiO-67 showed no photocatalytic performance on CO2 reduction, demonstrating that Re(CO)3(5,50 -dcbpy)Cl was responsible for the photocatalytic CO2 reduction. But 1 became inactive for CO generation after two six-hour reaction runs. After 20 h reaction, 43.6% of the Re leached into the aqueous solution, while only 3.5% Zr was detected in the solution. From this point, MOF 1 faces the challenge of water-stability. Wang and coworkers conducted CO2 reduction reactions using [Ru(bpy)3]Cl26H2O (bpy = 2,20 = 2,20 -bipyridine) and Co-ZIF-9 (cobalt-containing zeolitic imidazolate framework) as photosensitizer and cocatalyst, respectively, along with TEOA as an electron donor under mild reaction conditions (20 °C, 1 atm, visible-light irradiation) [92]. CO2 can be split into CO at a rate of 41.8 lmol/ 30 min, along with the rate of H2 evolution of 29.9 lmol/30 min. In the absence of [Ru(bpy)3]Cl26H2O, no CO and H2 were produced. And when Co-ZIF-9 was removed from the system, CO and H2 production decreased dramatically to 1.2 lmol/30 min and 1.8 lmol/ 30 min, respectively. 13CO2 was used to confirm the source of the produced CO, and after 30 min, the GC-MS peak at 3.57 min with m/z = 29 can be assigned to 13CO, implying Co-ZIF-9 indeed promotes photocatalytic CO2 reduction into CO. The amount of Co-ZIF-9 influences its photocatalytic performance. When a small amount of Co-ZIF-9 (like 0.1 mg) was added (region I in Fig. 8), the amount of CO and H2 formed increase substantially with the increase of Co-ZIF-9 added. As illustrated in region II of Fig. 8, when the amount of Co-ZIF-9 increased to 1 mg, the maximum amount of CO and H2 was obtained. But, further increases in the amount of Co-ZIF-9 result in a slight increase of H2 and a slight decrease of CO. In region I (the amount of Co-ZIF9 being no more than 0.2 mg), the reaction rate is mainly controlled by the number of the photocatalyst centers, while in region II (Co-ZIF-9 > 0.2 mg), the reaction rate is largely limited by electron-transfer kinetics. Excessive Co-ZIF-9 would facilitate electron
Fig. 8. The effect of the amount of Co-ZIF-9 on the evolution of CO and H2 from the CO2 photoreduction system [92].
transfers to reduce proton to H2. In addition, the degradation of Ru(bpy)2+ 3 formed Ru speices that is cocatalyst for H2 evolution. So, the selectivity of CO and H2 changed when excessive Co-ZIF-9 was added in the system, which resulted in the increase of H2 and the decrease of CO when the values of log10 (quality) > 0.5. To compare the photocatalytic function of Co-ZIF-9, some other MOFs were adopted as photocatalysts to promote CO2 reduction. As listed in Table 1, when Co-MOF-74 (2,5-dihydroxyterephthalic acid as ligand) was applied as photocatalyst, the CO and H2 amount produced decreased sharply, implying that imidazolate-based ligand can enhance CO2 capture. When Mn-MOF-74 was used as cocatalyst, no noticeable improvement was found over the cocatalyst free system, indicating that electron-mediating functions of cobalt species are indispensable for supporting CO2 conversion. Therefore, tiny quantities of CO and H2 can be detected when ZnZIF-8 (2-methylimidazole as ligand) or Zr–UiO–66–NH2 (NH2-1,4benzenedicarboxylic acid as ligand) were used as cocatalysts. When cobaltocene was also used as homogeneous cocatalyst in the reaction system, only a tiny enhancement in production of H2 and CO was seen, further confirming the synergetic effect of cobalt and benzimidazolate for CO2 reduction. Since Co-ZIF-9 is a good cocatalyst that facilitates the capture of CO2 and promotes the CO2 reduction, and g-C3N4 commonly acts as semiconductor photocatalyst, the combination of these two materials achieved efficient photocatalytic CO2 reduction with bipyridine (bpy) as auxilliary and TEOA as electron donor under visible light irradiation, as illustrated in Fig. 9 [93]. The absence of gC3N4 or light inactivity of the reaction system. Upon irradiation with visible light for 2 h, the reaction system gave 20.8 lmol CO and 3.3 lmol H2, indicating that g-C3N4 undergoes photoexcitation upon visible light illumination. Once Co-ZIF-9 was removed from
Table 1 Comparison of cocatalytic functions of Co-ZIF-9 with other MOFs and cobalt complex [92]. MOFs a
Co-ZIF-9 Co-MOF-74b Mn-MOF-74b Zn-ZIF-8b Zr–UiO–66-NH2b Cobaltocenceb
CO (lmol)
H2 (lmol)
41.8 11.7 1.5 2.1 1.2 5.0
29.9 7.3 2.9 2.4 2.2 2.7
a Reaction conditions: [Ru(bpy)3]Cl26 H2O (10.0 mmol), Co-ZIF-9 (0.8 mmol, activated), solvent (5 ml, acetonitrile/H2O = 4:1), TEOA (1 ml), CO2 (1 atm), k P 420 nm, 20 °C, 30 min. b Reaction conditions are the same as those to Co-ZIF-9.
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Fig. 9. Representation of the cooperation of Co-ZIF-9 and g-C3N4 for the photocatalytic reduction of CO2 under visible light irradiation [93].
the reaction system, CO and H2 formation would stop, which confirmed that Co-ZIF-9 remarkably promoted the CO2 reduction by acting as CO2 activator and redox promotor. In the absence of bpy, the catalytic activity was reduced significantly, implying that the bpy plays the role of cooperatively transferring the excited electrons. No CO (0.9 lmol H2 formed only) can be found upon replacing CO2 with Ar in the system, indicating the participation of CO2 in the system. 13CO2 was also used to validate the source of the produced CO, and the result revealed that the GC-MS peak at 3.54 min with m/z 29 was assigned to 13CO [92]. Electron donors played crucial roles in the catalytic performance. As listed in Table 2, some different tertiary amines were applied in the system to assess the effects on CO2 reduction. The results showed that once TEOA was replaced by TEA, TPA and TBA, the amount of CO and H2 decreased sharply, but the selectivity for CO obviously increased with some b-hydroxylated amines as electron donors. It was worthy to pointing out that, under the same conditions, TEOA presented superior catalytic activity in the formation of CO, and TIPOA displayed the highest selectivity towards CO formation. The effects of different reaction mediums on the photocatalytic activity were also investigated. Some aprotic solvents, such as MeCN, DMF, THF and DMSO were favorable to achieve good performance of the reaction system. Nitrogen and/or oxygen atoms are believed to promote solubilizing CO2 via Lewis acid – base interactions [96]. When the reaction was operated in DCM, neither CO nor H2 was detected, due to weak chemical affinity towards CO2 mol-
Table 2 Effects of various tertiary amines on the yield of CO and H2 from the CO2 photoreduction systema [93]. Entry 1 2 3 4 5 6 7
Amineb TEOA (E(TEOA+/TEOA) = 0.82 V [94]) TEA (E(TEA+/TEA) = 0.93 V [95]) TPA TBA DEAIPO DEAEO TIPOA
CO/ lmol
lmol
Sel.COc/ %
20.8
3.3
86.3
2.0 1.4 1.0 19.4 15.2 10.9
4.6 1.9 0.6 2.3 0.8 0.4
30.3 42.4 62.5 89.4 95.0 96.5
H2/
a Reaction condition: C3N4 (20 mg), Co-ZIF-9 (1 mg), bpy (10 mg), solvent (5 ml, MeCN:H2O = 3:2), amine (1 ml), k > 420 nm, 30 °C, 2 h. b TEOA = triethanolamine; TEA = Triethylamine; TPA = Tri-n-propylamine; TBA = Tri-n-butylamine; DEAIPO = 1-(diethylamino)-2-propanol; DEAEO = 2(Diethylamino)ethanol; TIPOA = Tri-2-propanolamine. c Sel.CO = mol(CO)/mol(CO + H2).
ecules. In pure water, the photocatalytic CO2 reduction was completely inhibited. While the production of CO and H2 and the selectivity of CO achieved maximum values when the system contained 40% H2O. After the reaction, no noticeable changes of the chemical and crystal structures of Co-ZIF-9 and g-C3N4 can be found, reflecting the stable characteristics of the coupling and working of the MOF and g-C3N4 in photocatalytic CO2 reduction. Compared with many catalytic systems containing noble metal catalysts, this noble metal free system for efficient CO2 reduction possesses many advantages.
Conclusion and outlook Metal–organic frameworks, a new class of crystalline molecular solids built from linking organic ligands with metal-cluster connecting points, have recently emerged as a versatile platform to conduct photocatalytic reactions [20]. Compared to conventional photocatalytic CO2 reduction systems, photoactive MOFs have some advantages: (i) the intrinsic porosity can facilitate the adsorption of the CO2, which is essential for a high photocatalytic efficiency; (ii) versatile synthetic strategies, including solvothermal, vapor diffusion, emulsion-assistant precipitation, ultrasonication and even post-synthesis modification, allow a high degree of crystalline quality and morphologies of MOF photocatalysts. (iii) harnessing solar energy more efficiently, as their structural features of tunable active sites (i.e., metal-oxoclusters and organic linkers). Visible light photocatalytic activity can be introduced via linker substitution with an amino group [56,97–100]. (iv) The well-defined crystalline structure of MOFs is beneficial to the characterization and study of structureproperty relationship of these solid photocatalysts. Up to now, it has been difficult to perform high-throughput synthesis with amounts of kilogram quantities in a matter of hours at ambient pressure, which is an impediment in the study of wide-spread applications for MOFs. The solvothermal (including hydrothermal) methods, involving the use of an autoclave, and slow-diffusion processes will take days or weeks to complete, eliminating the possibility of an industrially relevant process [101,102]. Although photocatalytic CO2 reduction has been achieved with a number of MOFs, all of these systems require the use of a sacrificial agent, such as TEOA, which is not economic and environmentally friendly. Most MOFs tend to have modest hydrolytic and thermal stabilities, which make them impractical in solar energy applications, especially for photocatalytic CO2 reduction. The development of stable MOFs, like UiOs, MIL-140,
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125, 101 and the ZIF series, can overcome the instability issue that can plague other MOFs in many applications and, in particular, will further spur the interest in exploring photocatalysis with MOFs. Additionally, most MOFs also do not possess strong mechanical properties, good processability, and high electric conductivity, all of which will hinder the integration of MOFs into functional solar devices [103]. Therefore, a way to synthesize achieve inexpensive, stable, and efficient MOFs photocatalysts still presents a big challenge. Overall, we believe that the present studies will stimulate intensive research in MOF-based photocatalysis and will open new perspectives for the development of photocatalytic CO2 reduction. Acknowledgements We thank the financial support from the Beijing Natural Science Foundation & Scientific Research Key Program of Beijing Municipal Commission of Education (KZ201410016018), the Training Program Foundation for the Beijing Municipal Excellent Talents (2013D005017000004), the Importation & Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&CD201404076), the China Postdoctoral Science of Foundation (2013M540831) and Open Research Fund Program of Key Laboratory of Urban Stormwater System and Water Environment (Beijing University of Civil Engineering and Architecture) of Ministry of Education. References [1] J. Albo, P. Luis, A. Irabien, Ind. Eng. Chem. Res. 49 (2010) 11045–11051. [2] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731– 737. [3] A. Ahmad Beigi, S. Fatemi, Z. Salehi, J. CO2 Utiliz. 7 (2014) 23–29. [4] M. Anpo, J. CO2 Utiliz. 1 (2013) 8–17. [5] Y. Liu, B. Huang, Y. Dai, X. Zhang, X. Qin, M. Jiang, M.-H. Whangbo, Catal. Commun. 11 (2009) 210–213. [6] H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang, Y. Dai, Chem. Commun. 48 (2012) 9729–9731. [7] Q. Liu, Y. Zhou, Z. Tian, X. Chen, J. Gao, Z. Zou, J. Mater. Chem. 22 (2012) 2033– 2038. [8] T. Ohno, N. Murakami, T. Koyanagi, Y. Yang, J. CO2 Utiliz. 6 (2014) 17–25. [9] Y. Liu, Y. Yang, Q. Sun, Z. Wang, B. Huang, Y. Dai, X. Qin, X. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 7654–7658. [10] C.-C. Wang, H.-Y. Li, G.-L. Guo, P. Wang, Transition Met. Chem. 38 (2013) 275– 282. [11] C.-C. Wang, G.-L. Guo, P. Wang, Transition Met. Chem. (2013) 1–8. [12] C.-C. Wang, G. Guo, P. Wang, J. Mol. Struct. 1032 (2012) 93–99. [13] C.-C. Wang, Z. Wang, F. Gu, G. Guo, J. Mol. Struct. 1004 (2011) 39–44. [14] C.-C. Wang, P. Wang, G.-S. Guo, Transition Met. Chem. 35 (2010) 721–729. [15] C.-C. Wang, Z. Wang, F. Gu, G. Guo, J. Mol. Struct. 979 (2010) 92–100. [16] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc. Rev. 38 (2009) 1450–1459. [17] C.-Y. Sun, S.-X. Liu, D.-D. Liang, K.-Z. Shao, Y.-H. Ren, Z.-M. Su, J. Am. Chem. Soc. 131 (2009) 1883–1888. [18] R.J. Kuppler, D.J. Timmons, Q.-R. Fang, J.-R. Li, T.A. Makal, M.D. Young, D. Yuan, D. Zhao, W. Zhuang, H.-C. Zhou, Coord. Chem. Rev. 253 (2009) 3042–3066. [19] X.-S. Wang, S. Ma, D. Sun, S. Parkin, H.-C. Zhou, J. Am. Chem. Soc. 128 (2006) 16474–16475. [20] C.-C. Wang, J.-R. Li, X.-L. Lv, Y.-Q. Zhang, G. Guo, Energy Environ. Sci. 7 (2014) 2831–2867. [21] C.-C. Wang, H.-P. Jing, P. Wang, S.-J. Gao, J. Mol. Struct. 1080 (2015) 44–51. [22] C.-C. Wang, H.-P. Jing, P. Wang, J. Mol. Struct. 1074 (2014) 92–99. [23] C.-C.W.H.-P. Jing, Y.-W. Zhang, P. Wang, R. Li, RSC Adv. 4 (97) (2014) 54454– 54462. [24] J.-R. Li, J. Yu, W. Lu, L.-B. Sun, J. Sculley, P.B. Balbuena, H.-C. Zhou, Nat. Commun. 4 (2013) 1538. [25] J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 112 (2011) 869–932. [26] J.-R. Li, H.-C. Zhou, Nat. Chem. 2 (2010) 893–898. [27] J.-R. Li, D.J. Timmons, H.-C. Zhou, J. Am. Chem. Soc. 131 (2009) 6368–6369. [28] J.-R. Li, R.J. Kuppler, H.-C. Zhou, Chem. Soc. Rev. 38 (2009) 1477–1504. [29] L. Pan, D.H. Olson, L.R. Ciemnolonski, R. Heddy, J. Li, Angew. Chem. 118 (2006) 632–635. [30] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 300 (2003) 1127–1129. [31] J.L. Rowsell, O.M. Yaghi, Angew. Chem. Int. Ed. 44 (2005) 4670–4679. [32] D.J. Collins, H.-C. Zhou, J. Mater. Chem. 17 (2007) 3154–3160. [33] S. Ma, H.-C. Zhou, Chem. Commun. 46 (2010) 44–53.
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