Study On The Photocatalytic Hydrogenation Of Nitrobenzenes And Their Reaction Mechanisms

Photocatalysis, which is a green technology, has promising applications in the fields of environment and energy. It is expected to become one of the most effective ways to solve environmental pollution and energy crisis which are two issues in today’s society. For now, photocatalysis has been applied in the degradation of organic pollutants and the evolution of H2 from water. Since photocatalysis satisfies with almost all of the proposed requirements for green chemistry, it has received considerable attention in the field of organic synthesis.This dissertation mainly studies the photocatalytic hydrogenation of nitrobenzenes in water and their reaction mechanisms. In this study, the crystal structures, optical absorption properties, surface states, morphologies and band structures of photocatalysts were characterized in detail by using X-ray powder diffraction, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and electrochemistry techniques, respectively. Further, electron spin resonance and GC-MS were introduced to investigate the reaction mechanisms for the photocatalytic hydrogenation of nitrobenzenes.The main conclusions are as follows:(1) SrBi2Nb2O9 photocatalyst (band gap 3.5 eV) prepared by the sol-gel method showed high photocatalytic activity and good stability for the hydrogenation of 4-nitroaniline under UV light irradiation. The high photocatalytic activity of SrBi2Nt2O9 could be ascribed to the strong reductive power of its photoinduced electrons. (2) PbBi2Nb2O9 photocatalyst (band gap 2.8 eV) were successfully synthesized by the sol-gel method. The as-prepared samples showed photocatalytic activities for the hydrogenation of 4-nitroaniline under visible light irradiation. Photoinduced holes of PbBi2Nb2O9 could react with C2O42- to produce ·CO2- radicals. The ·CO2- radicals and photoinduced electrons of PbBi2Nb2O9 were identified as the main active species for the hydrogenation of 4-nitroaniline. (3) CdS photocatalyst (band gap 2.4 eV) showed high photocatalytic activity and good stability for the hydrogenation of 4-nitroaniline under visible light irradiation. HCO2NH4 not only served as a hole scavenger to suppress the photocorrosion of CdS, but also could produce the active species (·CO2- radicals and photoinduced electrons). (4) 4-Nitroaniline could be hydrogenated to p-phenylenediamine highly effective and stably over In2S3 photocatalyst (band gap 2.0 eV) under visible light irradiation in the presence of triethanolamine as a hole scavenger. Further experimental results revealed that the photoinduced electrons of In2S3 were the main active species. (5)·OH radicals obtained by irradiated photocatalysts (TiO2 and ZnO) could react with alcohols (methanol, ethanol and isopropanol) to produce corresponding alcohol radicals. These radicals had strong reductive abilities, and therefore could hydrogenate 4-nitroaniline to p-phenylenediamine. (6) The photocatalytic hydrogenation of other nitrobenzenes (such as nitrobenzene) could be achieved over Bi2MoO6 photocatalyst (band gap 2.7 eV) under visible light irradiation. The photoinduced electrons of Bi2MoO6 were identified as the main active species.In this dissertation, the hydrogenation of nitrobenzenes has been achieved in water by using photocatalysis, and their reaction mechanisms have also been investigated in detail. The results may allow us to provide mechanistic and experimental guidances for understanding the nature of photocatalysis, developing highly efficient visible-light-induced photocatalysts and expanding the applications of photocatalysis in the field of organic synthesis.