Visible-Light-Induced Photocatalytic CO₂ Reduction Over TpPa-1 And TpBpy Covalent Organic Frameworks

The capture and efficient utilization of CO₂ is an important issue since CO₂released by burning of fossil fuels is a primary cause of global warming.One of the best solutions is to convert CO₂ into valuable organic products by means of solar energy.The key to the effcient utilization of CO₂ is the development of highly efficient photocatalysts for CO₂ reduction.Covalent organic frameworks?COF?represent a new class of hybrid porous materials with:?a?good thermal and chemical stability;?b?high surface area and free pore volume,low crystal density;?c?easy to design,diverse structure,modify and modulate;?d?abundant conjugation?electric system;?e?excellent adsorption selectivity for CO₂ due to the imine linkage,in which make them attractive in photocatalysis.However,the great potential of COFs as photocatalysts for CO₂ reduction is still a few appreciated.In this thesis,two imine-linkage COFs?TpPa-1 and TpBpy?were employed in the photocatalytic CO₂ reduction under visible light irradiations.TpPa-1 and TpBpy with highly ordered pore structure were synthesized via the amine-aldehyde condensation,and characterized by some techniques such as XRD,XPS,UV-vis DRS,photoluminescence,photocurrent and so on.Furthermore,their photocatalytic performance for CO₂ reduction was also studied to elucidate how the composition and the structure of these imine-linkage COF materials can influence their photocatalytic activity and the photocatalytic mechanism of COF-based photocatalysts.The main results are summarized as bellow:?1?TpPa-1 and TpBpy were successfully synthesized by a combination of reversible Schiff-base reaction and irreversible enol-keto tautomerism using p-phenylenediamine or 2,2'-bipyridine-5,5'-diamine as amino monomer,and triformylphloroglucinol as aldehyde monomer.Both of them are good in crystallinity,stability and visible-light response.?2?TpPa-1 and TpBpy exhibit photocatalytic activity for CO₂ reduction to give HCOOH under visible light irradiations with triethanolamine?TEOA?as sacrificial agent,while TpBpy shows better photocatalytic performance than that of TpPa-1 due to the more conjugation?electric system.?3?The Ru nanoparticles?NPs?modified photocatalysts have an enhanced activity in the photocatalytic reduction of CO₂.In combination with extensive characterizations by various techniques,it is found that the doping of the Ru NPs encapsulated in TpBpy can enhance the visible-light harvesting and electron transfer,leading to a higher photocatalytic activity.Photocatalytic reduction of CO₂ to HCOOH under visible light irradiations is for the first time realized over the imine-linkage COF materials in the thesis.This work not only can expand the application of the COF-based materials,but also can enrich the theory of the photocatalysis and provide us with some useful information in the photocatalytic CO₂ fixation.