Studies On The Synthesis And Photocatalytic Properties Of Nanometer Niobium Oxide Catalysts

Nb₂O₅is a new type of semiconductor photocatalyst with a band gap of 3.4-5.3 eV,which can absorb ultraviolet light.Nb₂O₅ is also a bi-functional solid acid catalyst with aqueous stable Lewis and Bronsted acidity.To further improve the acid-and photo-catalytic performances of Nb₂O₅,the main research contents and results obtained in this paper are as follows:First,the mesoporous Nb₂O₅ was prepared using hexadecyl trimethyl ammonium bromide as a template.The impregnation method was used to modify the mesoporous Nb₂O₅with boric acid.The results show that the prepared Nb₂O₅is a pseudo-hexagonal crystal with ordered mesoporous pores with uniform pore size,and the doped boric acid is uniformly loaded on the surface of porous Nb₂O₅.The inner layer of tetragonal boron and the outer layer of tetragonal boron are formed.The surface acidity of the solid catalyst was adjusted by changing the boron doping amount,and it was applied to catalyze the conversion of glucose to HMF,which improved the catalytic activity and HMF（5-hydroxymethylfurfural）yield.Second,to better understand the relationship between BA and LA acidity and catalytic activity,mesoporous Nb₂O₅ was prepared using hexadecyl trimethyl ammonium bromide as a template.The mesoporous Nb₂O₅ was modified by impregnation with inorganic acids（megalomanic acid,silicic acid,phosphoric acid,and sulfuric acid）of different strengths.The surface acidity of the solid catalyst is modulated systematically by varying the strength and concentration of the impregnating acid.It was used to catalyze the conversion of glucose to HMF,which improved the catalytic activity and HMF yield.The results show that when Nb₂O₅is modified with an inorganic acid,the stronger B acid is conducive to the improvement of glucose conversion,while the more L acid is conducive to the improvement of HMF selectivity.Third,Nb₂O₅ short nanorods were synthesized by hydrothermal method using ammonium niobate oxalate hydrate as a precursor.The hydrothermal synthesis of short nanorods showed much higher photocatalytic activity in the decomposition of methylene blue than the precipitation and solvothermal nanorods.By XRD,N₂ adsorption and desorption,UV-vis diffuse reflection spectroscopy,PL spectroscopy,¹H MAS NMR,and trimethyl phosphine probe molecules,the hydrothermal synthesis Nb₂O₅ short nanorods have the highest BET specific surface area and accessible total acid sites.However,its amorphous structure leads to wider E_g and shorter photoelectron lifetime,which limits the absorption and utilization of light,resulting in lower photocatalytic activity than calcinated hydrothermally synthesized Nb₂O₅short nanorods.Although Nb₂O₅ nanoparticles prepared by the precipitation method and nanorods by solvothermal method have the same crystal structure and higher crystallinity as short Nb₂O₅ nanorods prepared by the hydrothermal method,their specific surface area and the number of accessible BA and LA acid sites on the surface are lower than those of the latter.Considered togather,a large number of accessible acid sites and the ordered crystal structure of the hydrothermal synthesis of Nb₂O₅ short nanorods are the reasons for the photocatalytic activity.Fourth,The g-C₃N₄/Nb₂O₅ composite catalyst was prepared by ultrasonic-assisted impregnation and characterized by XRD,SEM,TEM,UV-Vis DRS,and SSNMR taking advantage of the B acid,L acid,and photocatalytic properties of Nb₂O₅.The results showed that Nb₂O₅ was successfully loaded onto the two-dimensional layered g-C₃N₄ surface.When the loading amount of ammonium niobium oxalate was small or the calcination time was short,Nb₂O₅showed non-crystalline dispersion.When the loading amount of ammonium niobium oxalate is high and the calcination time is long,the pseudo-hexagonal phase crystal Nb₂O₅nanoparticles are formed.After loading Nb₂O₅,g-C₃N₄ still retains the basic structural unit,and the band gap is decreased compared with that of pure Nb₂O₅.The prepared catalyst was used for hot catalytic conversion of glucose to 5-hydroxymethylfurfural,and the sample preparation conditions were optimized.The composite photocatalyst g-C₃N₄/Nb₂O₅ with a calcination time of 3 h and impregnation concentration of niobium oxalate of 0.05 g/m L was determined as the best.The catalyst sample was used to catalyze the conversion of 5-hydroxymethylfurfural to 5-hydroxymethyl-furan-2-carboxylic acid acid by visible light,and the optimum illumination time was determined to be 5 h.Furthermore,the g-C₃N₄/Nb₂O₅composite catalyst was used to catalyze the conversion of glucose to 5-hydroxymethyl-furan-2-carboxylic acid by one-pot method,and the thermo-optical cascade reaction of glucose to 5-hydroxymethyl-furan-2-carboxylic acid was realized by using the same catalyst.