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Photoreduction of CO 2 to methanol with hexanuclear molybdenum [Mo 6 Br 14 ] 2À cluster units under visible light irradiation † RSC Advances COMMUNICATION

Abstract : Octahedral molybdenum clusters were found to be efficient visible light homogeneous photocatalysts for the reduction of carbon dioxide (CO 2) to methanol. Photoreduction was carried out by using 20 watt white cold LED flood light in dimethyl formamide/water or acetoni-trile/water solutions containing triethylamine as a reductive quencher. Among the two cluster-based compounds, Cs 2 [Mo 6 Br 14 ] exhibited higher photocatalytic efficiency and afforded higher yield of methanol than (TBA) 2 [Mo 6 Br 14 ] (TBA ¼ tetrabutylammonium). After 24 h illumination , the yield of methanol was 6679.45 and 5550.53 mmol g À1 cat. using Cs 2 [Mo 6 Br 14 ] and (TBA) 2 [Mo 6 Br 14 ] cluster compounds as photocatalysts, respectively. The continuously increasing concentration of carbon dioxide (CO 2) in the atmosphere has become one of the most serious problems with regard to the greenhouse effect. 1 Photocatalytic reduction of CO 2 with water for the production of hydrocarbons could be one of the viable solutions to address the issues related to climate change as well as energy shortage. 2 Over the past few decades, extensive efforts have been devoted to develop visible light active photocatalytic materials as visible light is ubiquitous , renewable and clean source of energy. 3 Although semiconductors such as TiO 2 , ZnS, and CdS have widely been used as photocatalysts for CO 2 reduction, their quantum yields and selectivities of products are low. 4 Homogeneous photocatalysts, including transition metal complexes such as ruthenium(II) polypyridine carbonyl complex, 5 cobalt(II) trisbipyridine, 6 and cobalt(III) macrocycles 7 have been widely investigated for this reaction. Enzyme catalysts have also been used for such photoreduction processes. 8 Transition metal based molecular complexes are advantageous due to their high quantum effi-ciencies and high selectivity of products. However, in some instances, a weak absorption in the visible region makes the utility of these complexes limited. This limitation can be overcome by using high nuclearity transition metal cluster complexes having the basic formula, [M 6 (m 3-X) 8 L 6 ] 2À , which is also written as [Mo 6 X i 8 L a 6 ] 2À (i ¼ inner; a ¼ apical; X ¼ halogen, L ¼ halogen or functional organic ligand). 9 In such face-capped metal atom clusters, the metallic electrons are delocalized on all the metal centers leading to particular intrinsic properties (magnetic, optical, redox processes). These nanosized molecular units exhibit a large absorption window from UV to visible as well as a large emission window from red to infrared due to the delocalization of valence electrons on all metal centers. 10 Molybdenum halide based cluster complexes have been known since a long time and have been used for photochemical processes for energy storage, as singlet oxygen photocatalysts and as bio-imaging agents. 11 However, to the best of our knowledge, these cluster complexes have not been investigated so far for the photocatalytic reduction of CO 2. In the present paper, we wish to report on the use of octa-hedral molybdenum cluster-based compounds Cs 2 [Mo 6 Br 14 ] (i.e. Cs 2 [Mo 6 Br i 8 Br a 6 ]) and (TBA) 2 [Mo 6 Br 14 ] (i.e. (TBA) 2 [Mo 6-Br i 8 Br a 6 ], TBA ¼ tetrabutyl ammonium) as efficient visible light active photocatalysts for the photoreduction of CO 2 to methanol by using water as the reaction medium and triethylamine as a sacricial donor. The synthesis and characterizations of Cs 2 [Mo 6 Br 14 ] and (TBA) 2 [Mo 6 Br] 14 have been fully described in the literature. 12 The structures are based on [Mo 6 X i 8 L a 6 ] 2À cluster units depicted in Fig. 1 as anionic building blocks and Cs + and (TBA) + as counter cations. Initially, Mo 6 Br 12 (Mo 6 Br i 8 Br a–a 4/2 Br a 2) was prepared by the reaction of Br 2 with Mo at 750 C. Then an excision reaction was performed between a stoichiometric amount of MoBr 2 and CsBr at 850 C for two days. Aerwards, Cs 2 Mo 6 Br 14 was dissolved in dry acetone, ltrated and recrystallized in pure form.
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Pawan Kumar, Subodh Kumar, Stéphane Cordier, Serge Paofai, Rabah Boukherroub, et al.. Photoreduction of CO 2 to methanol with hexanuclear molybdenum [Mo 6 Br 14 ] 2À cluster units under visible light irradiation † RSC Advances COMMUNICATION. RSC Advances, Royal Society of Chemistry, 2014, 4, pp.10420 - 10423. ⟨10.1039/c3ra47255h⟩. ⟨hal-01456464⟩



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