Atomically-Precise Synthesis Of Pd-Based Catalysts For Understanding Structure-Reactivity Relationships

Catalysis plays vital roles in chemicals production and environmental protection.Development of highly-efficient and selective catalysts is of great importance for booming national economy as well as solving the global energy and environmental issues.In particular,supported metal catalysts have received increasing attentions from both the industrial field and academic society due to their superior catalytic performance.However,the traditional wet-chemistry synthetic methods are generally lack of precise control of catalyst structures,thus rendering it challenging for optimizing the catalytic performance and hendering the understanding of stuctrure-activity relationships at atomic-scale.In recent years,it has been successfully demonstrated the achievements of atomic-level precise construction and modification of catalytically active sites on high specific surface area support using atomic layer deposition（ALD）by taking advantages of its unique feature of molecular-level "self-limiting" surface reactions.Combination of conventional wet-chemistry synthesis with the ALD technique opens a new avenue to facilitate new-generation advanced catalysts.Herein,during my PhD studies,I have focused on the development of strategies for atomically-precise synthesis and modification of Pd-based catalysts by combining wet-chemistry and ALD synthetic methods,and further explored the structure-reactivity relationships of the resulting such Pd-based catalysts in the reactions of benzyl alcohol oxidation and benzonitrile hydrogenation.The key results are as follows:1.Disentangling the size-dependent geometric and electronic effects of palladium nanocatalysts in Pd-catalyzed benzyl alcohol oxidation.We demonstrated that in Pd-catalyzed solvent-free aerobic oxidation of benzyl alcohol,the geometric and electronic effects interplay and compete so intensively that both activity and selectivity showed in volcano trends on the Pd particle size unprecedentedly.By developing "site-selective blocking" strategy,the Pd low-coordinated sites（LCSs）and high-coordinated sites（HCSs）could be selectively blocked by using Al2O3 and FeOx ALD,respectively.This site-selective blocking strategy endows us precisely manipulating the geometric structure of Pd nanoparticles without interfering their electronic structure.We unveiled that the geometric effect dominates the right side of the volcano with larger-size Pd particles,whereas the electronic effect directs the left of the volcano with smaller-size Pd particles substantially.Combining with density functional theory（DFT）calculations,we found that the competitive removal of hydrogen governs the reaction paths on HCSs and LCSs,which is the origin of geometric effect for larger Pd particle;for smaller Pd particles,the Pd work function decreases,thus enhances the electron transfer towards adsorbates,and strengthens the interaction of adsorbates with metal surface.This significantly affects the activity and selectivity.On the other hand,by selectively blocking the Pd LCSs via Al2O3 ALD on the 4-nm Pd particle catalysts,the optimal activity,selectivity and stability were achieved at the sme time.2.Precisely-controlled synthesis of supported Au@Pd core-shell bimetallic catalysts and their catalytic performance in the reaction of benzyl alcohol oxidation.Here we first synthesized a 4-nm Au/SiO₂ catalyst via wet chemistry method.By adpoting "selective metal ALD" strategy,we selectively deposited Pd only on Au nanoparticle surface to form Au@Pd core-shell bimetallic nanoparticles while avoiding monometallic nanoparticle formation on the SiO₂ support;therein,the Pd shell thickness can be tuned at the atomic level by varying the number of Pd ALD cycles.ICP-AES,HAADF-STEM,and DRIFTS CO chemisorption confirmed the atomically-precise controll of Pd shell thickness and demonstrated the evolution of Pd species from tiny aggregates or even isolated atoms to large islands then to continuous shells as increase of Pd coverage.In solvent-free aerobic oxidation of benzyl alcohol,the catalytic activities of the resulting Au@Pd/SiO₂ core shell bimetallic catalysts showed a clear volcano-like trend with the Pd shell thickness,reaching a maximum at a Pd shell thickness of 0.6-0.8 nm due to the optimized synergy of ensemble and electronic effects.3.Atomically precise synthesis of Ni@Pd core-shell bimetallic catalysts and their catalytic performance in benzonitrile（BN）hydrogenation.Based on "selective metal ALD" strategy,Pd was selectively deposited onto surface of Ni nanoparticles to form Ni@Pd core-shell bimetallic catalysts.At low Pd coverage,PdiNi single-atom surface alloy（SASA）catalyst was achieved due to the siginificant steric effect of large ligands in the Pd precursor,as confirmed by HAADF-STEM,DRIFTS CO chemisorption and XAFS measurements.In BN hydrogenation,the Pd1Ni SASA catalyst broke the strong metal-selectivity relations impressively,by prompting the yield of dibenzylamine（DBA）drastically from～5 to 97%under mild conditions,and boosting an activity to about eight and four times higher than Pd and Pt standard catalysts,respectively.More importantly,the undesired carcinogenic toluene byproduct was completely prohibited.Further investigation by DFT calculations revealed that relative differences of effective energy barriers of imine intermediate formation and its further hydrogenation govern the documented metal-dependent selectivity by regulating the resident time of the benzylimine（BI）intermediate on metal surfaces.