Design,synthesis,modification And Photocatalytic CO₂ Reduction Of Two-dimensional Non-metallic Porous Materials

Energy shortages and environmental issues have become critical and vicious issues which restricting human development,posing a serious threat to human survival and activities.Therefore,mankind urgently need to search a cheap,safe,clean and renewable way to recycle carbon sources for the sustainable development.Photocatalytic CO₂ reduction is one of the most promising strategies to resolve the energy crisis and achieve the global carbon cycle.Two-dimensional non-metal porous materials have numerous advantages such as low cost,high stability,high specific surface area and rapid separation of electron-hole pairs,which are widely used in photocatalytic CO₂ reduction.Moreover,the properties of this material can be easily modified through strategies such as morphology control,external electric field,element doping,heterostructure and metal loading,thus providing new opportunities for photocatalytic CO₂ reduction.In this paper,a variety of modified two-dimensional porous materials were prepared and applied to photocatalytic CO₂ reduction.The main research contents are as follows:1.At present,a major factor restricting CO₂ reduction is the rapid self-recombination of electron-hole pairs?e^--h⁺?and high catalyst prices.So for this issue,noble-metal-free polyoxometalates?Co4?with oxidative ability was first introduced into g-C₃N₄ resulted in inexpensive hybrid materials?Co4@g-C₃N₄?with staggered band alignment.The staggered composited materials show a higher activity of CO₂ reduction than bare g-C₃N₄.Mechanistic studies revealed the introduction of Co4 not only facilitate the charge transfer of g-C₃N₄ but greatly increased the surface catalytic oxidative ability.This work creatively combined g-C₃N₄ with oxidative polyoxometalates which provide novel insights into the design of low-cost photocatalytic materials for CO₂ reduction.2.Based on reducing e^--h⁺ self-recombination,most of the current photocatalytic CO₂ reduction is only concerned with the reduction of the redox reaction without discussing the oxidation half-reaction,and there are few work to combine the two parties.So for this issue,we propose a threecomponent photocatalyst design strategy for reducing CO₂ to CO coupled with water oxidation via a two-electron/two-step pathway.Employing polyoxotitanium([Ti₁₇O₂₄?OPri?₂₀])as a titanium source,ultrasmall TiO_2-xnanoparticles coated with ultrathin carbon layers(C-TiO_2-x)were fabricated and loaded on to a g-C₃N₄ matrix through chemical bonding(C-TiO_2-x@g-C₃N₄)for the first time.The optimized photocatalyst showed a very high activity under visible-light irradiation,which represents the highest CO production rate to date among the reported TiO₂-based materials under similar conditions.The excellent adsorption capability of photocatalyst for photons,H⁺ protons,and CO₂ molecules together with efficient charge separation and the two-electron/two-step oxidative pathway lead to the high reactivity.3.Based on reducing e^--h⁺ self-recombination and combining reduction with oxidation,most of the current photocatalytic CO₂ reduction occurs in pure CO₂ atmosphere and does not have a good antitoxic effect in industrial waste gas.And the mechanism of photocatalytic CO₂ reduction is not clear.So for this issue,we designed and fabricated a metal free C₃N₄-COF covalent composite photocatalyst with Schiff-base COF as light absorption center and g-C₃N₄ as reductive site.This composite presents enhanced visible-light absorption,facilitated photo-induced charge separation and accelerated electron transfer ability.By using simulated industrial flue gas as raw material,composite material produced syngas with the catalytic high reactivity,revealing its good anti-poisoning ability.Dedicated experiments and calculations highlight that efficient electron transfer from COF1 to g-C₃N₄ together with CO₂-concentration-switched interaction between TEOA and the hydrazine bond on COF,leading to extension of light absorption to red and infrared region,may mainly account for the high activity in low CO₂ content.This work not only presents an efficient and durable photocatalyst for syngas production under flue gas but provides new insight in design efficient catalysts via control catalyst-liquid interface.4.At present,the reduction sites,mechanisms and pathways of photocatalytic CO₂ reduction are controversial,and the order and sites of CO₂ and H⁺ adsorption have not been investigated.So for this issue,we designed and prepared a new type of Schiff base COF material with synergistic effect of triazine ring and C=C double bond,together with can be a photocatalyst to reduct CO₂.This kind of COF can directly perform photocatalytic reduction of CO₂ to CO in the atmosphere of pure CO₂ without adding any cocatalyst,and the selectivity is almost 100 %?99.9 %?.Quantum-chemical calculation results show the possible reaction mechanism and catalytic pathway,and reveal the following conclusions: The first adsorption on COF is H⁺ instead of CO₂;and then the reduction reaction occurs because the synergy between triazine ring and C=C bonds.This work not only prepared a brand-new non-metallic COF material for photocatalytic CO₂ reduction,but also provided new insights for the reaction mechanism and reduction site of the photocatalytic CO₂ reduction reaction of COF materials through quantum chemical calculations.