Construction Of Noble Metal Pd And Ru-Based Single Atom Catalysts And Their Catalytic Performance

Supported noble metal single atoms catalysts represent a frontier in heterogeneous catalysis because of their almost 100%atomic utilization,unique quantum size effect,tunable electronic properties,etc.,thus demonstrating outstanding catalytic performance in a variety of reactions.However,the development of facile and efficient synthetic strategies for creating metal single atoms catalysts is highly desired.In this thesis,a series of synthetic strategies are employed to modulate the coordination environment,optimize charge distribution of active sites and electronic metal-support interactions to construct highly active,selective,and thermally stable supported single atoms catalysts.Density functional theory（DFT）calculations further reveal the relationship between the high catalytic reactivity and electronic structure of the active sites at the atomic level.The main research contents are listed below:A simple defect engineering strategy was used to construct an atomically dispersed palladium catalyst by anchoring the palladium atoms on oxygen vacancies created in Ce O₂ and the catalytic performance was evaluated in the hydrogenation of cinnamaldehyde.The atomic dispersion and coordination structure were confirmed by spherical aberration correction electron microscopy and X-ray absorption spectroscopy（XAFS）.The results show that the Pd was atomically dispersed over the support and the valence state was between 0 and+2.The as-prepared catalyst showed exceptional catalytic performance in the hydrogenation of cinnamaldehyde(99%conversion,99%selectivity,turnover frequency of 968 h^-1)under mild conditions（1 atm H₂ and 80℃）,outperforming those of support and Pd NPs/Ce O_2-x.In addition,this catalyst showed excellent cyclic stability without leaching during 8 cycles and high stability at high temperatures.Most importantly,this synthetic method can be scaled up while maintaining catalytic performance.A coordination-assisted strategy was employed to access single ruthenium（Ru）atoms over Bi vacancy-containing assemblage,with each Ru atom coordinated with four neighboring oxygen atoms（Ru₁?O₄）over the support.This was confirmed by spherical aberration correction electron microscopy and XAFS.The epoxidation of trans-stilbene was selected as a probe reaction to evaluate the catalyst’s performance and theoretical calculations were used to investigate the relationship between the nature of active sites and reactants.With molecular oxygen as an oxidant（1 atm O₂,110℃,120 min）,this catalyst delivered superior catalytic efficiency in the epoxidation of trans-stilbene to yield trans-stilbene oxide（99%conversion,99%selectivity）,exceeding those of Ru _C/Bi_2-xWO₆ and commercial 5 wt%Ru/C.This catalyst showed no aggregation of metal species during the 8-cycle usage.DFT calculations reveal that the high catalytic activity originates from the unique Ru?O coordination and distribution of charge of ruthenium sites,which plays an important role in the adsorption（not too strong）and effective activation of trans-stilbene.A simple and efficient molten salt-induced strategy was demonstrated for the creation of Ru single atoms anchored onto geometrically deformed nitrogen-doped carbon（Ru₁/NC）support.The molten KCl and NH₄Cl can induce anisotropic thermal shrinkage,resulting in the rough,concave-shaped carbon layer during the pyrolysis process.This also increases the porosity of Ru₁/NC to achieve a higher exposure degree of active Ru sites with enhanced mass transport ability.Spherical aberration correction electron microscopy and XAFS support the presence of isolated Ru atoms over the NC support with Ru₁?N₄ coordination environment.This Ru catalyst has been demonstrated to show excellent activity(turnover frequency of 1213h^-1)and selectivity（99%）in selectively catalyzing the oxidation of benzyl alcohol to benzaldehyde under mild conditions（1 atm O₂,90℃,120 min）.Moreover,excellent recyclability（8 cycles）and substrate tolerance ability are validated.DFT calculations demonstrate that the high catalytic reactivity stems from the unique electronic structure of Ru sites in the catalyst and its d band center,which contributes to the effective activation of reactants.