Surface Microporosity Design Of Nanosized TiO₂ To Promote The Adsorption And Photocatalytic Reduction Of CO₂

Photocatalytic CO₂reduction has become one of the promising approaches to environment control and clean energy utilization because it uses clean and abundant sunlight to drive the reduction and transformation of"greenhouse gas"CO₂to realize carbon resource regeneration.The development of efficient photocatalyst is key to promoting the application of photocatalytic CO₂reduction.Ti O₂with excellent photostability,low cost and environmental friendliness,has been considered as one of the most popular photocatalysts.However,the low specific surface area and lack of matched pores with CO₂make the activities of photocatalytic CO₂reduction of Ti O₂as traditional inorganic semiconductor materials low.Especially,in complex exhaust gas environment and air conditions,the CO₂reduction reaction of Ti O₂was significantly inhibited due to the competitive adsorption of O₂.The introductions of microporous materials such as microporous organic polymers and metal-organic frameworks with high CO₂adsorption capacity into the Ti O₂photocatalytic system,could drive that CO₂molecules absorbed selectively and enriched on the surface of the photocatalyst,thus promoting the adsorption and photocatalytic transformation of low concentration of CO₂in complex environment.(1)A zirconium-based metal-organic framework(Ui O-66)as an effective CO₂adsorbent was coupled with Ti O₂photocatalyst by electrostatic self-assembly.The designed two-step strategy endowed Ti O₂/Ui O-66 composite with hierarchical porous structure,which ensured sufficient exposed catalytic sites and a high CO₂uptake of 78.9 cm³g^-1.In consequence,a high CH₄production rate of 17.9?mol g^-1h^-1with 90.4%selectivity was achieved in a mild gas-solid system using water as electron donors.More importantly,the photocatalytic efficiency reached the similar level to that of pure CO₂atmosphere even under diluted CO₂condition(?2%).(2)The Ti O₂/HUST-1-Pd composites were prepared by in situ hypercrosslinking porphyrin-based polymers on Ti O₂surface followed by coordination with Pd(II)sites to realize the selective adsorption of CO₂.The selectivity of CO₂/O₂for Ti O₂/HUST-1-Pd composite was as high as 23.9,which was 7.7 times higher than that of Pd/Ti O₂,thus avoiding the inhibition of CO₂adsorption and activation by the competitive adsorption of O₂molecules in the air environment.In gas-solid system,the CH₄yield of Ti O₂/HUST-1-Pd was as high as 11.7?mol g^-1h^-1under air condition.After irradiation for 2 h,Ti O₂/HUST-1-Pd could convert about 11.9%of CO₂in air into CH₄,while Pd/Ti O₂could only convert 2.7%.By discussing the photocatalytic mechanism,we had confirmed that the high selective adsorption of CO₂and the promotion of coordination metals had important effects on the photocatalytic reduction of CO₂in aerobic environment.(3)The photocatalytic conversion for the co-existence of CO₂,NO and O₂in the exhaust gas environment is of practical significance for achieving the goals of air pollution control and carbon resource recycling.We assembled ultrafine Ti O₂nanoparticles on the surface of porphyrin-based polymers and coordinated with Pd(?)sites to prepare composite photocatalyst.Due to high specific surface area and abundant micropores of Ti O₂-120/HUST-1-Pd composite,the yield of CH₄could reach 13.1?mol g^-1h^-1in co-existence of CO₂,NO and O₂gas.Finally,the results of DFT calculation and in-situ infrared spectroscopy demonstrated that the active sites of photocatalytic CO₂reduction and NO oxidation were on the surface of Pd and Ti O₂respectively.The photocatalytic CO₂reduction and NO oxidation could be carried out simultaneously in mixed atmosphere.The work provided a new idea for the resource utilization of"greenhouse gas"CO₂in industrial waste gas and the removal of NO.In conclusion,the introduction of microporous materials in photocatalytic system could achieve the selective adsorption and photocatalytic reduction of low concentration CO₂,which provides new insights for effectively reducing CO₂concentration in air or flue gas and producing valuable solar fuel.