Communication pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO2‑to-CO Conversion with Visible Light Claudio Cometto,† Ryo Kuriki,‡,§ Lingjing Chen,∥ Kazuhiko Maeda,*,‡ Tai-Chu Lau,*,⊥ Osamu Ishitani,*,‡ and Marc Robert*,† †
Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, UMR 7591 CNRS, 15 rue Jean-Antoine de Baïf, F-75205 Paris, Cedex 13, France ‡ Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-1 Okayama, Meguro-ku, Tokyo 152-8550, Japan § Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan ∥ School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, Guangdong 523808, P. R. China ⊥ Department of Chemistry and Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China S Supporting Information *
of very abundant elements, it can absorb visible light, and it is highly stable under photochemical conditions and in many solvents, including water.11 Moreover, the estimated conduction band energy minimum (ca. −1.35 V vs SCE) is negative enough to promote the reduction of several molecular catalysts to their active states.12,13 Such a hybrid system may thus exploit the high oxidation power of the semiconductor together with selectivity for CO2 reduction against side reactions, typically H2 evolution. Photochemical CO2 reduction by mpg-C3N4 and a ruthenium complex, trans(Cl)-[Ru{4,4′(CH2PO3H2)2-2,2′-bipyridine}(CO)2Cl2], has been reported, and HCOOH was found as the main product (selectivity >80%) under visible light irradiation, upon using triethanolamine (TEOA) as a sacrificial electron donor.13 The radiative excitation of the organic semiconductor promotes electrons from the valence to the conduction band. The holes produced in the valence band of mpg-C3N4 are then consumed by an electron donor, TEOA, while the electrons from the conduction band are used to reduce the chemically adsorbed metal complex that catalyzes the CO2 reduction.14−17 The same authors reported that the catalytic activity detected upon using bulk g-C3N4 as a photosensitizer is negligible, despite the adsorption of the same Ru-complex catalyst.17 Remarkably, a large enhancement in the HCOOH formation was observed with the introduction of controlled mesoporosity into g-C3N4, which was obtained by tuning the SiO2/cyanamide ratio during the synthesis of the semiconductor. A shorter distance for migration of electron−hole pairs to the mpg-C3N4 surface was proposed to explain this positive effect. Conversely, excessive mesoporosity led to a decrease of the catalytic activity, as a consequence of increasing density of defect sites acting as traps for the photogenerated charge carriers.17 An optimized system including a RuRu′/Ag/mpg-C3N4 hybrid system, of which the dinuclear RuRu′ moiety is a supramolecular photocatalyst, led to a very high turnover number (TON) for HCOOH (>33000)
ABSTRACT: Efficient and selective photostimulated CO2-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C3N4) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH3CN/triethanolamine (4:1, v/v) solution containing [Fe(qpy)(H 2 O) 2 ] 2+ (qpy = 2,2′:6′,2′′:6′′,2′′-quaterpyridine) and mpg-C3N4 resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO2 as a renewable feedstock.
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n the quest for producing solar fuels from CO2 so as to store renewable energy into chemical bonds,1−3 one approach consists of directly using sunlight and activating metal-based complexes as catalysts. Molecular catalysts based on earth abundant metals have been used to do so, the two main reaction products being then carbon monoxide or formate. Ni, Co, Mn, and Fe based complexes including polypyridines, carbonyl, and porphyrin ligands have been shown to be the most efficient compounds.4−6 In rare cases, the reduction of CO2 with more than two electrons was achieved.7 In most of these examples, the catalysts were used jointly with metal-based redox photosensitizers, typically employing expensive metals such as Ru or Ir. Few systems involve cheaper organic dyes or abundant transition metal complexes as the photosensitizers.4,8−10 If the use of noble-metal based photosensitizers is useful for preliminary characterization and mechanistic studies, the invention of suitable cheaper ones associated with an earth abundant catalyst is a mandatory step toward future larger-scale device applications, and it remains a high challenge. In this context, organic semiconductor mesoporous graphitic carbon nitride (noted mpg-C3N4 in the following) is one of the most promising.11 This semiconductive material is only constituted © XXXX American Chemical Society
Received: April 15, 2018
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DOI: 10.1021/jacs.8b04007 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
catalyst deactivation occurred via the formation of the stable 18-electrons Fe0(qpy)(CO) species. Herein we report the selective photocatalytic reduction of CO2 to CO by the compound [Fe(qpy)(H2O)2]2+ in organic solvent (acetonitrile, CH3CN), under irradiation in the visible region and in the presence of mpg-C3N4 as the redox photosensitizer. A detailed preparation method for the carbon nitride material is provided in the Supporting Information. The successful synthesis of mpg-C3N4 having a ca. 12 nm pore was confirmed by means of XRD analysis (Figure S1) and a nitrogen adsorption/desorption experiment at 77 K (Figure S2). The pore volume and specific surface area of mpg-C3N4 were 0.55 cm3 g−1 and 186 m2 g−1, respectively. mpg-C3N4 used in this work exhibits a steep absorption edge at ca. 460 nm (Figure S3). For comparison, the XRD pattern and DRS spectrum of nonporous C3N4 (g-C3N4) are also shown in Figures S1 and S3 respectively. Cyclic voltammetry of Fe(qpy) in the presence of TEOA (Figure S4a) showed a positive shift of the first reduction wave, indicating an association reaction between the amine and FeI(qpy). When the solution was purged with CO2, the anodic shift of the first peak could be assigned to binding between CO2 and the one-electron-reduced-species while the second peak at −1.20 V vs SCE is due to the FeI(qpy)CO2 adduct reduction (Figure S4b), just positive to a large catalytic wave, showing that CO2 could be activated at a sufficiently positive potential as compared to the conduction band level of the carbon nitride (see discussion below). Photocatalytic experiments were performed using a suspension of mpg-C3N4 (8 mg/4 mL of solvent) in an CH3CN/ TEOA (4:1 v/v) mixed solution containing the Fe(qpy) complex (20 μM). A 400 W high-pressure Hg lamp with a NaNO2 solution filter (λ ≥ 400 nm) was employed as a light source, and the temperature was controlled during each experiment by a water bath at 298 K. As shown in Figure S5, the iron-based catalyst does not show any significant transition bands in the visible region under our experimental conditions, indicating that most of the incident light is absorbed by mpgC3N4, as further shown in Figure S3. After 17 h of irradiation, CO was the major product in the gas phase (ca. 12 μmol), with 97% selectivity and a TON of 155, while only traces of H2 were detected (TON of 1); see Table 1. A typical time course of visible-light CO2 reduction using Fe(qpy) (20 μM)/mpg-C3N4 is shown in Figure 1. Analysis of the liquid phase by capillary electrophoresis revealed only a small amount of HCOOH, confirming the excellent selectivity of this system. When a solution with a 10-times larger concentration of catalyst (0.2
in organic solvent, with good selectivity (87−99%), while keeping such good selectivity (70−86%) in aqueous conditions as well.18 Regarding the CO2-to-CO conversion, examples are much rarer. The coupling between a RuII−ReI binuclear complex and carbon nitride via silica resulted in the CO evolution under visible light with 90% selectivity. This Zscheme system produced a TON(CO) of 29 after 5 h of irradiation.19 By focusing on catalysts that contain abundant metals, a [Co(bpy)3 ]2+ (where bpy = 2,2′-bipyridine) complex20 has been described as well as a cobalt porphyrin/ C3N4 hybrid.21 In the former case, the selectivity and TON(CO) were respectively 86% and ca. 4, while in the latter a 80% selectivity was reported, with an overall TON(CO) inferior to 1 (with respect to the number of cobalt porphyrin units). A very recent report using an iron porphyrin/C3N4 hybrid led to a maximum TON of 5.7.22 Despite these early examples that gave encouraging results, a catalytic system that simultaneously would be only based on earth abundant elements, efficient, and fully selective for the CO production is still missing. Recently, the catalytic activity of iron quaterpyridine for the CO2-to-CO conversion under photochemical conditions was described.9 With [Ru(bpy)3]2+ as the photosensitizer and 1,3dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial donor in an CH3CN/triethanolamine (TEOA) solution under visible-light excitation, [Fe(qpy)(H2O)2]2+ (Fe(qpy), Scheme 1) produced CO in high yields with 97% Scheme 1. Schematic Illustration of Photocatalytic CO2 Reduction with the Fe(qpy)/C3N4 System under Visible Light Illumination
selectivity. It was shown by electrochemistry that the oneelectron-reduced-species FeI(qpy) produced upon the first reduction wave binds to CO2 and the FeI(qpy)(CO2) adduct is then reduced at ca. −1.2 V vs SCE, with consequent formation of CO by C−O bond cleavage.23 Carbon monoxide was observed as the only product with 99% selectivity, before
Table 1. Photochemical Reduction of CO2 from the Fe(qpy)/mpg-C3N4 Photocatalytic System Product (μmol) entry
catalysta
H2
HCOOHc
CO
TON (CO)b
CO selectivity (%)
1 2 3 4 5 6
Fe/mpg-C3N4 mpg-C3N4 Fe/mpg-C3N4d Fe/Al2O3 Fe/mpg-C3N4e Fe/g-C3N4
