Investigation Of Quantum Size Effects In Precisely-Synthesized Pd-based Catalysts

Supported metal catalysts are an important class of heterogenous catalysts,which are extensively used in industry for fine chemicals,biomass conversion and automotive exhaust treatments.Modulating the particle size,composition and morphology of metal nanoparticles can significantly affect their catalytic reactivity.Among them,alteration of metal particle size has been the most extensive and effective way to optimize catalytic performance and has been a hot topic of research in recent years.However,there are still limited studies on the influence of supporte effect over metal particle size effect,which also has a significant effect on the particle size effect of the active metal.In addition,catalytic performance can also be improved through optimization of synergistic effect between the components of bimetals by regulating structure and component.However,the exploration of particle size effect of bimetallic catalysts,especially on the regulation of bimetallic particle size alone while maintaining a similar core-shell structure,has rarely reported in early literatures.Atomic layer deposition（ALD）,which relies on alternative self-limiting molecular surface reactions,provides a new promising method to construct nanostructured catalysts precisely with atomic fine-control from the bottom up.In this doctoral dissertation,we focused on the development of strategies for atomically-precise synthesis of Pd-based catalysts by combining wet-chemistry and selective Pd ALD strategy,and further achieved the well-controlled monometallic Pd nanoparticles,Pd@Au bimetallic and GaOx@Pd@Au catalysts.Through the optimization of the particle size effect,bimetallic synergetic effect and oxide overcoating,the performance in the catalytic hydrogenation of pare-chloronitrobenzene（p-CNB）and solvent-free benzyl alcohol oxidation were greatly improved.Further exploration of the structureactivity relationships was also discussed.The main results achieved are as follows:（1）Support-induced unsual size dependence of Pd nanocatalysts in chemoselective hydrogenation ofp-CNB.We have demonstrated that Pd NPs supported on carbon black and oxygen-functionalized carbon black both showed prominent particle size effect in chemoselective hydrogenation of p-CNB,where the activity showed a similar volcanolike trend on particle size with the maximized activity at about 2 nm size on these two supports,while the chemoselectivity showed an unexpected inverse trend on Pd particle size.In-situ XPS revealed that the Pd NPs in Pd/O-C650 were more positively charged through enhanced electronic metal-support interactions with phenol functionalized anchoring sites than those Pd NPs in Pd/C with similar sizes.FTIR measurements further unveiled that the electronic properties of Pd NPs govern the chemoselectivity decisively,where only a moderate positive charge of Pd sites(eg.Pd²⁺)was found to favor preferential chemisorption of p-CNB via the-NO₂ group,thus exhibiting the high p-CAN selectivity.In contrast,neither Pd species in too high valence(eg.Pd⁴⁺)or in too low valence（eg.Pd⁰）can lead to the adsorption of less electronegativity-Cl groups,thus dehalogenating and producing the byproduct of dehalogened aniline.（2）Atomically precise synthesis of supported Au@Pd core-shell bimetallic catalysts with different Pd overlayers and optimization of their catalytic activity in chemoselective oxidation of benzyl alcohol.By combining wet chemical methods and selective Pd atomic layer deposition（ALD）strategy,a set of SiO₂ supported Au@Pd core shell catalysts with precisely regulated Pd shell thicknesses were designed and synthesized.ICP results clearly demonstrated that the selective deposition of Pd on Au/SiO₂.HAADF-STEM,CO DRIFTS and in-situ XAFS confirmed the atomicallypresice control of Pd shell thickness and demonstrated the evolution of Pd species from tiny aggregations or even isolated islands to the continuous shells as increase of Pd coverage.XPS indicated the electronic properties of suface Pd overlayer were accurately regulated by Pd coverage.In the reaction of solvent-free Benzyl alcohol（BzOH）oxidation,Au@yML-Pd core-shell catalysts clearly demonstrated a volcanolike trend change of the activity as a function of Pd shell thickness,which together renders an unprecedentedly high activity of 6.86×104 h-1 to be achieved on an Au@Pd catalyst with a Pd shell thickness of 2-3 MLs.Theoretical calculations unveiled that the synergy between lattice strains and ligand effects have remarkable impact on the adsoption of BzOH,thus yielding a cooperative enhancement in the high activity.（3）Atomically precise synthesis of supported Au@Pd core-shell bimetallic catalysts with varied Au core sizes.By combining wet chemical methods and selective Pd ALD strategy,we further synthesized a big set of SiO₂ supported Au@Pd core shell catalysts with precisely regulated Au core sizes and Pd shell thicknesses at the atomic level.ICP results clearly demonstrated that Pd nucleation sites are similar at Au NPs with varied particle sizes.HAADDF-STEM,CO DRIFTS and in-situ XAFS confirmed the evolution of surface Pd morphology and XPS indicated the electronic modification of the surface Pd becomes more pronounced as Au core size increased.In the reaction of BzOH oxidation,these Au@Pd core-shell catalysts clearly demonstrated a dual quantum size effect of remarkable change of the activity in a volcano-like trend as a function of Pd shell thickness and in a monotonic increase on the Au core size.Combined experimental adsorpsion tests with theoretical calculations unveiled that variation of these two sizes have remarkable impact on modulation of lattice strains and ligand effects,thus yielding a cooperative enhancement the high activity to the largest extent so far.Such dual quantum size effect in core-shell bimetallic catalysis could be general and has been successfully demonstrated in Au@Pt catalyzed selective hydrogenations.（4）Precisely-cotrolled fabrication of GaOx coating over Au@Pd/SiO₂ bimetallic catalysts and improvment of catalytic performance in BzOH oxidation.TEM,XRD,CO DRIFTS were employed to characterize the geometric and electronic structure of Au@Pd core-shell catalyst before and after the GaOx coating.The Pd low-coordinated sites（LCSs）were selectively blocked by GaOx thus altering the type and morphology of the exposed Pd active sites.In the solvent-free BzOH oxidation,GaOx coating over Au@Pd significantly improved the selectivity of the target product benzaldehyde and achieved 86%selectivity at 99%BzOH conversion at 120℃ on 3GaOx@Pd@Au,which was more significant than that of GaOx coating over monometallic Pd catalysts.In addition,GaOx overcoating effectively relieved the metal agglomeration and component loss in liquid phase reactions.Since the synergy between Pd@Au bimetals enhances the activity in catalytic oxidation,GaOx@Pd@Au catalyst integrates the synergistic effect in AuPd bimetals and the oxide coating protection and achieves a high activity along with high selectivity and stability.