Preparation Of Pd-modified GC₃N₄ Nanostructures And Its Photocatalytic Performance

Graphitic carbon nitride(g-C3N4)is regarded as a promising metal-free and visible-light-responsive photocatalyst in the field of solar energy conversion and environmental remediation.Simultaneous,g-C3N4 has tunable electronic structure,good thermal and chemical stability,facile synthesis,and high element abundance.However,the pristine g-C3N4 suffers from several shortcomings,such as small specific surface area,insufficient visible light utilization,and rapid recombination rate of photogenerated electron-hole,leading to low quantum efficiency and unsatisfactory catalytic activity in the actual requirements.To address these issues,researchers employ numerous methods to improve the photocatalytic performance of g-C3N4,like modifing its morphology,doping exotic atoms,and compositing with other semiconductors.The tri-s-triazine unit is generally recognized as the building block for g-C3N4.Therefore,there are numerous six-fold cavities formed by pyridinic nitrogen atoms in g-C3N4.Such structure facilitates the incorporation and intercalation of exotic atoms into the g-C3N4 matrix,which provide a means of tuning the structure and reactivity of g-C3N4.In this dissertation,I utilized palladium to modify g-C3N4 in order to improve the photocatalytic performance of g-C3N4 for hydrogen production and Suzuki coupling reaction.The specific contents of this dissertation are as follows:1.Pd-doped graphitic carbon nitride(named as g-C3N4-Pd)was successfully synthesized by thermal polymerization,and the mechanism for improvement of photocatalytic water splitting for hydrogen evolution was studied.The doped Pd is uniformly distributed on g-C3N4 in the form of Pd2+.The doped Pd brings negligible change of the g-C3N4 morphology.The in-plane periodicity is slightly increased by the Pd-doping,while the interlayer distance are not changed,indicating that the Pd atoms locate at the six-fold cavities of g-C3N4.The Pd doping extends the absorption of g-C3N4 toward longer wavelength.The highest photocatalytic activity of Pd-doped g-C3N4 is 15.3 times that of g-C3N4.Density functional theory(DFT)calculation shows that the Pd doping results in electrons excited directly from g-C3N4 to the doped Pd atoms,which facilitates the separation of electron-hole.And the Gibbs free energy of hydrogen adsorbed on Pd is greatly reduced compared with hydrogen adsorbed on g-C3N4,indicates that Pd doping improves the hydrogen evolution kinetics on g-C3N4.2.The UCN-Pd was synthesized for photocatalytic Suzuki coupling reaction by direct growth of Pd nanoparticle on pre-synthesized g-C3N4 from urea.Pd nanoparticles are evenly dispersed on the surface of UCN with much low amount of Pd as 2 wt.%.The Suzuki coupling reaction of iodobenzene and phenylboronic acid is chosen as a model reaction to evaluate the catalytic activities of UCN-Pd photocatalysts.The catalytic activity of UCN-Pd for Suzuki coupling reaction strongly depends on the light absorption.The phenomenon indicates that UCN-Pd efficiently use photonic energy to drive the Suzuki coupling reaction.In addition,UCN-Pd catalyst has good cycle stability.UCN-Pd catalyst is suitable for a wide scope of reactions with variously substituted aryl iodide and aryl boronic acid.More importantly,UCN-Pd exhibits higher catalytic performance than homogeneous catalyst Pd(PPh3)4 and Pd nanocube with the same amount of Pd.