Introduction

Carbon dioxide is the major greenhouse gas (GHG) that result in global warming, utilization of carbon dioxide is an effective and direct way that deals with the global warming problem—the high emission of carbon dioxide which keeps increasing relentlessly. Converting carbon dioxide into fuels, other useful stock and fine chemicals by photocatalytically has become one of the best options and been considered as "the holy grail" to reduce green-house gas emission, counter the energy shortage and ameliorate the environment pollution, it can be seen as the solution to "kill three birds with one stone". Many photocatalysts such as TiO₂, BiVO₄, BiWO₆, Zn₂GeO₄ have been investigated in photocatalytic carbon dioxide reduction, but suffering the disadvantage of 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 carbon dioxide under visible light. 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 and COFs with tailorable capacity to absorb light, thereby initiating desirable photocatalytic properties for specific application in carbon dioxide reduction.

Conversion Ways of Photoreduction of CO₂ by MOFs and COFs

Photocatalytic reduction of carbon dioxide by MOFs and COFs presents two different conversion processes, namely conversion to organic chemicals (like CH₄, CH₃OH, HCOOH and so on) and being split into carbonic oxide.

• Conversion to organic chemical: Green plants can use the harvested energy to convert carbon dioxide and water into carbohydrates via photosynthesis. By being inspired by that, some artificial systems using inorganic and organic materials to simulate photochemical reactions have been developed. MOFs and COFs can contain photosensitizers and catalytic centers in a single solid, which can provide the structural organization to integrate the artificial photosynthesis into a single material. A number of recent papers have demonstrated that MOFs can be used to achieve light harvesting and to drive photocatalytic reduction of carbon dioxide into small organic chemicals, like HCOOH, HCOH, CH₃OH, CH₄ and so on. For examples, the NH₂-UiO-66(Zr) can absorb visible light, once irradiated, the excited MOFs can transfer electrons to the Zr oxo-clusters, and Zr^IV in the Zr-O clusters is reduced to Zr^III, which further reduce CO₂ to HCOO^- with triethanolamine as hydrogen source^[1]. Lan et al developed the 2D-based DQTP-COF for use in reduction of CO₂ into HCOOH, the HCOOH was generated with an excellent selectivity up to 90% and a formation rate of 152.5 µmol/g/h^[2].

Fig.1 Proposed mechanism for the photocatalytic reduction of carbon dioxide into HCOOH with DQTP-COF

• Split into carbon monoxide: An important pathway for the consumption of carbon dioxide is its reduction to carbon monoxide. For examples, Co-ZIF-9 combine the g-C₃N₄ achieved efficient photocatalytic carbon dioxide reduction into carbon monoxide, upon irradiation with visible light for 2 h, the reaction system gave 20.8 µmol carbon monoxide, once Co-ZIF-9 was removed from the reaction system, carbon monoxide would stop, which confirmed that Co-ZIF-9 remarkably promoted the carbon dioxide reduction^[3]. Cooper and co-workers synthesized Re-Bpy-sp²-c-COF photocatalyst, which is capable of reducing carbon dioxide to carbon monoxide, producing 81% carbon monoxide selectivity at a rate of 1040 µmol/g/h over 17.5 h under visible-light irradiation, showing a satisfactory result^[4].

Fig.2 Representation of the cooperation of Co-ZIF-9 and g-C₃N₄ for the photocatalytic reduction of carbon dioxide into carbon monoxide under visible light irradiation

What Can Alfa Chemistry Do

Alfa Chemistry provides various MOFs and COFs with high-activity for use in photocatalytic carbon dioxide reduction. And our professional technology team also provide customers with professional, high-quality MOFs and COFs design and customization services, no matter what design ideas you have, we will implement them together with you. In addition, Alfa Chemistry is committed to supporting customers a series of solutions in photocatalytic carbon dioxide reduction by using MOFs and COFs. Please contact us immediately to order or cooperate in research and development with high quality and reasonable price.