Ultrafine Pt,Pd Clusters And AgPd Bimetallic Catalysts:Atomically-Precise Sythesis And Their Catlytic Performance

In past decades,as the increasing demand for novel functional materials,atomic layer deposition(ALD)as a new and important technique to fabricate nano-materials has attracted rapidly increasing attention.Compared with CVD,the self-limiting character of ALD makes it possible to achieve uniform deposits on high-surface-area support.It has been widely applied in many areas,such as nanoelectronic devices,2D materials and battery.Moreover,ALD has deomstrated the possible to grow uniform catalytic mateirals with near atomically-precise control,and it appears to be a promising new method of supported catlysts synthesis,as an alatanative to the traditional wet-chemistry based methods.In this project,we took advantage of ALD,atomically precise control,to explore the possibility to "bottom-up" synthesize ultrafine metal clusters,including isolated Pt(or Pd)single atoms and dimers,and Ag-Pd bimetallic catalysts,their catalytic performances were also investigated.In chapter one,we will first introduce the background of ALD,and arelated catalytic systems.After that synthsis of Pd single-atom catalyst on graphene and its catalytic performance in selective hydrogenation of 1,3-butadiene is dicussed in chapter two.By precisely controlling the type and amount of oxygen species on graphene,Pd single-atom catalyst(Pd1/graphene)was successfully fabricated using the ALD technique.HAADF-STEM and XFAS both confirmed that individual Pd atoms were dominiate on the graphene support without presence of any Pd clusters or nanoparticles.In selective hydrogenation of 1,3-butadiene,Pd1/graphene showed superior catalytic performance:(1)achieving 100%butenes selectivity at almost 100%conversion at about 50 ?.More importantly,the selectivity to the most desired product(1-butene)is 70%,which is the highest one compared to previous work.(2)Showing excellent durability against deactivation via either aggregation of metal atoms or carbonaceous deposits formation during a total 100 h of reaction time on stream.(3)In the presence of propene,the propene stream could be greatly preserved by sufficiently suppressing its conversion to only 0.1%,showing the great opportunity for practical applications.Further DFT calculations suggest that the change of 1,3?butadiene adsorption mode and the enhanced steric effect induced by 1,3-butadiene adsorption on isolated Pd atoms both play important roles in the improvement of butenes selectivity,especially 1-butene.Supported ultrafine metal clusters with only a few atoms are of great interest.Removing or adding one atom might largely alter the electronic structure thus significantly changing their physical and chemical properties.It is a long-term goal to synthesize size-selected clusters on high surface area support.In chapter three,based on our previous work of Pd single atom catalyst synthesis,we further successfully synthesized Pt(Pd)single atom and Pt(Pd)dimer catalysts on graphene by taking advantage of the unique feature of self-limiting surface reactions of ALD.HAADF-STEM and XFAS both confirmed that Pt(Pd)single atoms and dimers were uniformally dispersed on the graphene support without presence of any Pt(Pd)clusters or nanoparticles.In hydrolytic dehydrogenation of ammonia borane for hydrogen generation,we surprisingly found that the graphene supported Pt dimers catalyst exhibited an extremely high activity with a turnover frequency of 2800 molH2 molpt-1 min-1 at room temperature,which is near 20 times higher than both the graphene supported Pt single atoms and metal nanoparticles.Recycling test showed that the Pt dimeric catalyst is stable under current conditions.Our work opens a new route to "bottom-up" fabrication of "size-controlled" metal clusters on high-surface-area supports for practical applications.In chapter four,ALD was utilized for precise synthesis of Ag-Pd/SiO2 bimetallic catalyst.By controlling the ALD condition,the Pd was selectively deposited on Ag nanoparticle surface,but not on the SiO2 support to exclusively form Ag-Pd bimetallic nanoparticles,while avoiding monometallic nanoparticle formation;the composition of Ag-Pd bimetallic can be tuned by varying the Pd ALD cycles.ICP-AES,EDS-mapping confirmed the selective Pd deposition.DRIFTS CO chemisorption,and UV-vis and XPS measurements revealed the evolution of the ensemble of Pd atoms on Ag nanoparticle surface and the electronic properties of the bimetallic nanoparticles as a function of Pd ALD cycles.In the formic acid decomposition,we found that the catalytic activities of Ag-Pd/SiO2 bimetallic catalysts demonstrate a volcano-like trend as a function of Pd ALD cycles;therein,the Ag-Pd/SiO2 bimetallic catalyst with 10 cycles of Pd ALD showed the maximum activity,which was about 12 times higher than the corresponding Pd/SiO2 catalyst.Based on the DRFIFTS and XPS results,the remarkablly improved activity is likely due to the synergistic effect via both ensemble and electronic promotion.