Synthesis,characterization And Photocatalytic Reduction Of Carbon Dioxide Over Titanic Acid Nanosheet Composites

Photocatalytic reduction of carbon dioxide is a hot topic in both photocatalysis and environmental chemistry.The difficulties of the adsorption,activation and catalytic conversion of carbon dioxide molecules on catalysts limit the photoreduction efficiency of carbon dioxide.Due to the unique structure of titanic acid nanosheets,they have the large surface area,high concentration of active sites,quantum size effect and easy structural adjustment,and have been considered as a candidate photocatalyst for the highly efificient reduction of CO2.Thus,the loading of alkaline material on the surface of ultrathin titanic acid nanosheets would weaken the superficial acidity of titanic acid nanosheets and enhance the adsorption capacity of CO2.Besides that,the modification of the nonmetal material may change the surface chemical features of the nanosheets which may enhance the photocatalytic activity of CO2 reduction.In this thesis,ultrathin titanic acid nanosheets have been prepared successfully through acid-exchange and chemical exfoliation process using Cs2Ti6O13 prepared by a solid state reaction as precursor.The highly dispersed alkaline hydroxide Co(OH)2,Ni(OH)2 and nonmetal S have been uniformly loaded on the surface of titanic acid nanosheets by an electrostatic-adsorption,coprecipitaion and self-assembly process.The samples have been applied into the photocatalytic reduction of CO2.The main results and conclusions obtained from the present studies are as follows:(1)The Co(OH)2 were in-situ deposited on the surface of titanic acid nanosheet for the first time and the as-prepared composites were applied into the photocatalytic reduction of CO2.By using[Ru(bpy)3]C12·6H2O as photosensitizer,the samples could photocatalytically reduce CO2 under visible light irradiation.The optimal production of CO was up to 56.5 p.mol h-1 g-1 which was much higher than that of the Co(OH)2 modified parent layered titanic acid sample.In-situ FIIR results revealed that surface adsorbed CO2-species were the main active species and H2O molecules donated the protons during the reduction process.(2)It was the first time that Ni(OH)2 were in-situ modified on the surface of titanic acid nanosheet and applied into the photocatalytic reduction of CO2 under visible light irradiation.The optimal production of CO over the composite was about 52.43 μmol h-1 g-1.The wavelength-dependent experiment showed that the yield of carbon monoxide was consistent with the photoabsorption performance of[Ru(bpy)3]C12·6H2O.That is,the photogenerated electrons derived from the photoinduced[Ru(bpy)3]Cl2·6H2O.Ni(OH)2 and titanic acid nanosheet served as the platform of the activation and catalytic transformation of CO2 and promote the separation of photogenerated charge carriers.(3)Nonmetal S was uniformly deposited on the surface of titanic acid nanosheet by an in-situ precipitation.It was found that the composite exhibited the great activities of CO2 reduction under visible light irradiation.Interestingly,the products of CO2 reduction were CH4 but not CO in the gas-solid heterogeneous reaction.The optimal production rate of CH4 is up to 1.89 μmol h-1 g-1 in the absence of cocatalysts and sacrificial agents.Results showed that introduction of S varied the surface properties of titanic acid nanosheet and improve the separation ration of photogenerated electrons and holes,which facilitated the photocatalytic process.The original results of this study are as follows:(1)Alkaline materials Co(OH)2,Ni(OH)2 and nonmetal S have been in-situ deposited on the surface of titanic acid nanosheet.And then the photoabsorption performance of the titanic acid semiconductor was increased significantly.The as-prepared composites were applied to photoreduction of CO2 with high efficiency for the first time.Moreover,the probable mechanisms of the photocatalytic reduction of CO2 have also been proposed.(2)The photocatalytic activities of bulk layered titanic acid and titanic acid nanosheets for the reduction of CO2 were investigated systematically.The probable reasons for the different activities between bulk precursor and the nanosheets were clarified.The outstanding performance of the two-dimensional titanic acid nanosheets for photocatalytic reactions was revealed.