Highly Efficient and Selective Visible‐Light Driven CO₂‐to‐CO Conversion by a Co(II) Homogeneous Catalyst in H₂O/CH₃CN Solution

Abstract

The photochemical reduction of CO 2 to chemical resources has displayed the promise to solve energy and environmental problems. To facilitate this reaction, a considerable challenge is to design not only highly efficient and selective, but also economic catalysts. In this study, we report a homogeneous catalyst, [CoL 1 (CH 3 CN)](ClO 4 ) 2 (1, L 1 = Tris[2-(iso-propylamino) ethyl]amine) which displays high activity and selectivity for CO 2 reduction to CO driven by visible light in a water-containing system, with turnover numbers (TON CO ) and turnover frequencies (TOF), and CO selectivity values of 44800, 1.24 s À1 and 97 %, respectively. The excellent performances of 1 for the photocatalytic CO 2 -to-CO conversion is confirmed by control experiments and its catalytic mechanism is corroborated by DFT calculations.Artificial photosynthesis using solar energy and abundant resources, such as CO 2 and H 2 O, to produce renewable fuels has attracted significant attention because of two serious problems: energy shortage and global warming. [1] Sunlightdriven CO 2 reduction provides a sustainable approach to convert the greenhouse gas into more valuable molecules, like CO, CH 3 OH, etc., whereby the solar energy can be stored. Simultaneously, both the energy and environmental issues can be addressed in a carbon-neutral fashion. [2] In particular, the formation of CO is appealing as it serves as a feedstock in the production of liquid fuels via Fischer-Tropsch process. [3] Due to the inherent structural stability of CO 2 , [4] a long-standing challenge is to design efficient and selective catalysts to accelerate the CO 2 reduction.So far, much research has been devoted to exploring visible-light-driven homogeneous [2c] and heterogeneous [5] systems for CO 2 reduction. Homogeneous molecule-based catalytic systems have advantages in mechanistic analysis, as well as optimization of photocatalytic properties by choosing from a range of metals and ligands. Thus, precious-metal complexes (Re, [6] Ru, [7] Ir [8] , etc.) have been widely studied as homogeneous catalysts for solar-fuel production by the reduction of CO 2 . Considering the scarcity and the price of these precious metals, [9] a number of CO 2 reduction photocatalysts based on earth-abundant metals, Co, [10] Ni, [11] Mn, [12] Fe, [13] and Cu, [14] have been reported. However, these catalysts always display relatively low efficiency and/or selectivity, especially in watercontaining systems. [7b,11c,15] Besides, an artificial photosynthesis cycle includes not only photocatalytic CO 2 reduction but also water oxidation. Therefore, it is very significative to design photocatalysts that can display efficient and selective for CO 2 conversion in a water-containing catalytic system. Up to now, the development of earth-abundant homogeneous catalysts for efficient and selective CO 2 -to-CO conversion in water-containing systems is still a great challenge.Tripodal ligands [16] have been especially of interest for developing metal-based homogeneous catalysts, [17] a...

Mechanistic Insights of Photocatalytic CO2 Reduction: Experimental versus Computational Studies

“…[78] It is also common to find complexes with a coordination position occupied by a labile solvent molecule that can be readily exchanged by the carbon dioxide substrate. [63,76,79,80] Another component of the catalytic systems for CO 2 reduction that have a great influence on the effectiveness of the reaction is the photosensitiser. Its absorption spectra, lifetime of excited states, photostability and redox properties directly affect the activity and selectivity of the process.…”

“…These ligands are also found in combination with carbonyl or halogen ligands [78] . It is also common to find complexes with a coordination position occupied by a labile solvent molecule that can be readily exchanged by the carbon dioxide substrate [63,76,79,80] …”

CoIICryptates Convert CO2into CO and CH4under Visible Light

“…[31] Furthermore, substitution of one Co II by Zn II enhanced the catalytic activity of the new heterobinuclear complex (Figure 1), although Zn II wasa poor catalyst towards CO 2 . [32] Additional studies were performed on Co II mononuclear tripodal complexes to evaluate how structuralm odificationsa ssociated with steric [33] or conjugation [34] factorsinfluenced CO 2 photoreduction.…”

“…Apart from cryptates, cyclams and their derivatives, [6,10,35] porphyrins and somea nalogues, [12,13,36,37] polypyridyl [23,38,39] and other [30,33,[40][41][42][43] ligandsh ave been associated with cobalt to photoreduceC O 2 .F or instance, Tinnemans et al [10] used Co II and Co III complexes bearing diamine macrocyclic ligands as catalysts, with [Ru(bipy) 3 ] 2 + as PS and ascorbic acid as SED, leadingt ot he observation of CO and H 2 with al ow CO selectivity of < 35 %a nd turnover number (TON) values below 350.…”

“…Apart from cryptates, cyclams and their derivatives, porphyrins and some analogues, polypyridyl and other ligands have been associated with cobalt to photoreduce CO 2 . For instance, Tinnemans et al .…”