Electronic Structure Regulation Of Supported Palladium-Based Single Atom Catalysts And Mechanisms For Small Molecule Activation

Metal-support interactions are of great importance in determining catalyst’s activity in heterogeneous catalysis.In this thesis,the structure and coordination environment of the Pd active sites over boron nitride nanosheet and titanium dioxide catalysts were analyzed at the atomic scale.Combined with the advanced characterization techniques and theoretical calculation methods,the unique mechanisms of single Pd atom catalysts in the activation of small molecules were discussed.These works pave the way for the precise control in the synthesis of catalysts to endow high catalytic activity and thermal stability at the atomic level.The main contents are listed below:1.A low-temperature synthetic strategy was proposed to create atomically dispersed palladium atoms anchored on defective hexagonal boron nitride nanosheet（Pd₁/h-BN）.The merit of this strategy lies in the mild low-temperature conditions（0°C）that were used,totally different from the harsh conditions of-80°C that were previously reported in the literature.Density-functional theory（DFT）calculations suggest that the nitrogen-containing B vacancy can provide stable anchoring sites for palladium atoms.The presence of single palladium atoms was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement.This catalyst showed exceptional efficiency in chemoselective hydrogenation of cinnamaldehyde,delivering turnover frequency（TOF）up to1112 h^-1 and the conversion was 99%,along with the selectivity was 93%at 3 h,superior to previously reported other catalysts.In addition,the catalyst also possesses excellent re-cyclability and sintering-resistant ability.2.A Pd single atom catalyst(Pd₁/TiO_2-x)supported by defective rutile was constructed by thermal emitting strategy.DFT calculations show that palladium atoms have high activity when anchored on defective rutile.The singly dispersed Pd atoms were determined by high Angle dark-field scanning transmission electron microscopy and electron paramagnetic resonance,etc.The catalyst showed good catalytic activity and stability in the Sonogashira coupling reaction under mild conditions,with a conversion rate of 99%,a yield of 98%at 5 h,along with a TOF of 4286 h^-1.After 6 cycles of repeated use of catalyst,its catalytic performance has no obvious decay.