Modification Of Pd-loaded TiO₂ Nanosheets And Their Excellent Photocatalytic Hydrogen Production Activity

Hydrogen production by photolysis of water is a promising way to obtain hydrogen.TiO₂ nanomaterials have received a lot of attention from researchers as excellent photocatalysts.In this thesis,TiO₂ nanosheets were synthesized by hydrothermal method,and the series of TiO₂-based photocatalysts were prepared by modifying TiO₂ nanosheets using metal ion doping,noble metal Pd loading and carbon quantum dots（CQDs）composite.The photocatalytic activity of the catalysts was evaluated by studying their photocatalytic hydrogen production（HER）efficiency,and the photocatalytic reaction mechanism was proposed.The main contents are as follows:1.The main group metal ions(Na⁺,K⁺,Mg²⁺,Al³⁺)doped TiO₂ nanosheets were prepared by hydrothermal method,and Pd nanoparticles（NPs）with small particle size were loaded on TiO₂ by photodeposition method.The samples were characterized by means of XRD,SEM,TEM,BET and XPS spectra,and their photocatalytic hydrogen production activities were tested in a methanol aqueous solution.The effects of doped cationic charge,radius and ion concentration on the photocatalytic activities of the catalysts were investigated systematically.The photocatalytic reaction mechanism was discussed by comprehensively considering the three aspects:specific surface area,light absorption and charge transfer/separation.The results show that the cation dopings significantly increased the photocatalytic activities of the TiO₂ nanosheets,which may be attributed to the enhanced UV-vis light absorption and accelerated charge transfer/separation of the catalysts after cation doping.Particularly,the Pd/0.2%K⁺-TiO₂ possesses the highest photocatalytic H₂ production activity(76.6μmol h^-1),which is more than twofold higher than that of the undoped Pd/TiO₂.The apparent quantum efficiency of hydrogen evolution system reaches 3.0%at 365 nm.The high activity of the Pd/K⁺-TiO₂ may be attributed to the lower electronegativity of K⁺,caused by the lower cationic charge or the larger cationic radius,compared to Na⁺,Mg²⁺and Al³⁺.The doping metal cations with higher electronegativity may compete electrons with H⁺,which eventually partly depressed the reduction of H⁺to H₂.2.The Pd-CQDs/TiO₂ composite nanomaterials were prepared by two-step hydrothermal method and photodeposition method.The samples were characterized by means of XRD,SEM,TEM and XPS spectra.The optimal experimental conditions for the highest efficiency of hydrogen production were explored by changing the conditions of sacrificial agents,temperature of the photocatalytic system,amount of Pd loading,amount of CQDs and optical power,and the mechanism of photocatalytic reaction was proposed.The results showed that the highest photocatalytic hydrogen production efficiency of 13.4 mmol g^-1 h^-1 was achieved when the sacrificial agent was lactic acid,the temperature of the reaction system was 25℃,the amount of Pd loading was 1%,the amount of CQDs was 0.5 m L,and the optical power was 540 m W cm^-2.CQDs played an important role in the photocatalytic system.We found that the presence of CQDs could promote the reduction of Pd²⁺,resulting in all Pd²⁺was reduced to Pd⁰ in the Pd-CQDs/TiO₂(only part of Pd²⁺was reduced in the catalyst without CQDs),which could form more Pd active centers,beneficial to improvement of the photocatalytic activity.In addition,CQDs can capture photogenerated electrons from the conduction band of TiO₂,promote the effective separation of photogenerated electron-hole pairs,and hinder their recombination,which is conducive to the improvement of photocatalytic activity.