Photocatalytic reduction of CO2 in water using a single integrated system utilizing sunlight is the ultimate goal for artificial photosynthesis. Here, we report the design and multistep synthesis of Zr-MBA-Ru/Re-MOF...

Achieving more than a two-electron photochemical CO2 reduction process using a metal-free system is quite exciting and challenging, as it needs proper channeling of electrons. In the present study, we report the rational design and synthesis of a redox-active conjugated microporous polymer (CMP), TPA-PQ, by assimilating an electron donor, tris­(4-ethynylphenyl)­amine (TPA), with an acceptor, phenanthraquinone (PQ). The TPA-PQ shows intramolecular charge-transfer (ICT)-assisted catalytic activity for visible-light-driven photoreduction of CO2 to CH4 (yield = 32.2 mmol g–1) with an impressive rate (2.15 mmol h–1 g–1) and high selectivity (>97%). Mechanistic analysis based on experimental results, in situ DRIFTS, and computational studies reveals that the potential of TPA-PQ for catalyzing photoreduction of CO2 to CH4 was energetically driven by photoactivated ICT upon surface adsorption of CO2, wherein adjacent keto groups of PQ unit play a pivotal role. The critical role of ICT for stimulating photocatalysis is further illustrated by synthesizing another redox-active CMP (TEB-PQ), bearing triethynylbenzene (TEB) and PQ, that shows 8-fold lesser activity for photoreduction toward CO2 to CH4 (yield = 4.4 mmol g–1) as compared to TPA-PQ. The results demonstrate a novel concept for CO2 photoreduction to CH4 using an efficient, sustainable, and recyclable metal-free robust organic photocatalyst.

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with Ru II ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO 2 photoreduction to CO (3.5 mmol g À 1 h À 1 ) with > 99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e À /8H + photoreduction of CO 2 to CH 4 is realized with > 95 % selectivity (6.7 mmol g À 1 h À 1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY) 2 ] 2 + acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO 2 reduction to CO and CH 4 was outlined.

The much-needed renewable alternatives to fossil fuel can be achieved efficiently and sustainably by converting solar energy to fuels via hydrogen generation from water or CO2 reduction. Herein, a soft processable metal-organic hybrid material is developed and studied for photocatalytic activity towards H2 production and CO2 reduction to CO and CH4 under visible light as well as direct sunlight irradiation. A tetrapodal low molecular weight gelator (LMWG) is synthesized by integrating tetrathiafulvalene (TTF) and terpyridine (TPY) derivatives through amide linkages and results in TPY-TTF LMWG. The TPY-TTF LMWG acts as a linker, and self-assembly of this gelator molecules with ZnII ions results in a coordination polymer gel (CPG); Zn-TPY-TTF. The Zn-TPY-TTF CPG shows high photocatalytic activity towards H2 production (530 μmol g−1h−1) and CO2 reduction to CO (438 μmol g−1h−1, selectivity > 99%) regulated by charge-transfer interactions. Furthermore, in situ stabilization of Pt nanoparticles on CPG (Pt@Zn-TPY-TTF) enhances H2 evolution (14727 μmol g−1h−1). Importantly, Pt@Zn-TPY-TTF CPG produces CH4 (292 μmol g−1h−1, selectivity > 97%) as CO2 reduction product instead of CO. The real-time CO2 reduction reaction is monitored by in situ DRIFT study, and the plausible mechanism is derived computationally.