Performance Optimization And Mechanism Exploration Of Co-catalyst Modified G-C₃N₄ In CO₂ Photocatalytic Reduction

Utilizing the solar light and semiconductor photocatalysts to convert CO₂ into valuable chemical feedstocks,can simultaneously realize the resource utilization of CO₂ and alleviate the global greenhouse effect,which is currently one of the hot topics concerned by many researchers.Graphitic carbon nitride?CN?,as a new type of non-metallic semiconductor photocatalyst,has attracted extensive attention.However,its CO₂ photocatalytic reduction efficiency is limited by severe charge recombination,limited light absorption and reaction sites.To overcome the above drawbacks,this thesis developed co-catalysts with high efficiency,great stability and low cost that can effectively improve the CO₂ reduction performance performance of CN,making an instructive exploration for the design and development of efficient CO₂ photocatalytic reduction photocatalysts.First,non-noble metal cocatalyst Bi nanospheres were applied to improve the CO₂photocatalytic reduction performance of CN and the influence of the modification of Bi nanospheres was investigated in detail.It was found that Bi deposition could not only promote the light harvesting of CN,but also form a Schottky junction with CN,thus facilitating the separation and transfer of photogenerated charges.Besides,hydrothermal treatment caused the upshift of conduction band?CB?of the prestine CN,improved the mobility of photogenerated electrons,therefore enhanced the reduction ability of photogenerated electrons.Under the combined influence of Bi decoration and the property optimization of CN,the yields of CO and CH₄ of the best modified sample?30%Bi/CN?,under 8 h visible light irradiation,were about 3 times and 9 times those of pristine CN,respectively.However,the generation selectivity of the multi-electron reduction product CH₄of the modified samples remains to be improved.Secondly,in order to promote its CH₄ production and selectivity,carbon quantum dots?CQDs?,kind of cocatalyst serving as electron traps and facilitating the adsorption and activation of CO₂ molecules,were developed.The CQDs modified samples were verified to not only facilitate the CO₂ adsorption and activation,but also improve the transfer of photogenerated electrons as expected.Meanwhile,the N atoms in CN were partly substituted by O atoms during the hydrothermal treatment process.As a result,abundant vacancies were introduced and the valence band position positively shifted,thus facilitating the dissociation and oxidation of water molecules and serving as the proton reservoir for CO₂ reduction.After CQDs modification,the CH₄ selectivity of CN was significantly enhanced,being as high as more than 70%.A gas-solid synthesis method was adopted to optimize the phosphating degree,achieving the co-mofidication of CN by reductive Ni₂P and oxidative NiO cocatalysts.Charaterization results showed that the co-decoration of Ni₂P and NiO could synergistically improve the transfer and separation of photogenerated charges,forming plenty of effective photogenerated charges.The CO and CH₄ production of the optimal sample Ni₂P/NiO/CN?0.25?were about 4 times and 29 times higher than that of CN.Based on the results of characterization analysis,the transport path of photogenerated charges and relative promotion mechanism of Ni₂P/NiO/CN for CO₂ photocatalytic reduction were proposed.Palladium?Pd?was chosen as the representative and the CO₂ photocatalytic reduction performance of CN modified with different chemical states?including single atomic Pd,Pd O_x and elemental Pd?was systematically studied.It was found that all these Pd species could promote the CO₂ photocatalytic activity of CN,but their promotion effect presented significant difference.Pd/CN-NP possessed excellent light absorption ability,abundant H⁺production and high·H utilization efficiency,but its formation of·H was frustrated.Pd O_x/CN showed poor·H utilization efficiency and limited redox ability.Pd/CN-SA possessed excellent·H utilization efficiency and the reduction ability of its photogenerated electrons was strong,thus showing the best CO₂ photocatalytic reduction activity.Besides,based on the performance evaluation and cost analysis,it was concluded that Ni₂P/NiO dual cocatalysts possesed the application potential to substitute the noble-metal Pd cocatalysts in the field of CO₂ photocataltic reduction,due to its advantages of high CO₂conversion efficiency,feasible large-scale production,controllable potential danger and pollution as well as low cost.