Investigation On How The Atomic Number And Atomic Order Regulate The Catalytic Mechanisms

Nano catalysts have been widely concerned in recent years because of their unique physical and chemical properties.The atomic number and degree of atomic order are two important parameters of these nanomaterials.The atomic number is determined by the size of the nanostrcure catalysts.The precise regulation of the size can effectively control the atomic number of nanocrystals.The degree of atomic order mainly refers to the way of the relative arrangement between different atoms in a bimetaf or multi-metal system,which is mainly divided into ordered and disordered arrangement.The atomic number and degree of atomic order have great effect on the geometric and electronic structres of nanocatalyts.The structural variation of nanocrystals can also lead to the change of catalytic activity,selectivity and stability.As present,we still need to furter study ationalize the dependence of catalytic petrformance on the atomic number and the degree of atomic order.Here,for-the-atomic-number,we continuously reduced the size of nanocatalysts from clusters about 3 nm to the limit of the size adjustment,namely single atoms.We delved into the differences in catalytic performance between these nanocrystals and commercial counterparts.For the degree of atomic order,we mainly focused on the bimetallic material system.Moreover,we further explored the catalytic mechanism at the atomic scale.The main research contents of this thesis include the following aspects.1.Commercial silicon carbide(SiC)powders larger than 100 nm were etched by nitric acid and hydrofluoric acid for the synthesis of SiC clusters.When the size of SiC decreased from the bulk to the level of quantum dots,the surface was converted from hydrophobic to hydrophilic one.As such,the size difference can largely affect the catalyst surface properties.We compared the activity of commercial SiC and SiC quantum dots towards CO2 hydrogenation.We found that the mass activity of hydrophilic SiC is three orders of magnitude higher than that of hydrophobic commercial SiC.Further mechanism research revealed that the activity enhancement of SiC quantum dots was closely related to hydrophilicity.Specifically,the hydrophilic surface of SiC quantum dots was rich in hydroxyl.The surface hydroxyl species on SiC QDs was directly involved in CO2 hydrogenation through the addition of H atoms in hydroxyl groups into CO2 to form HCOO*as the intermediate.The unique reaction path decreased the energy barrier for the formation of HCOO*,f-acilitating the activation of CO2,thus increasing the activity of CO2 hydrogenation.2.We developed highly active and selective catalysts in selective hydrogenation by embedding Pt single atoms in the surface of Ni nanocrystals(denoted as Pti/Ni nanocrystals).During the hydrogenation of 3-nitrostyrene,the turnover frequency(TOF)number based on surface Pt atoms of Pt1/Ni was one order of magnitude higher than that of commercial Pt/C and Pt single atoms supported on active carbon,TiO2,SiO2,and ZSM-5.Besides,only nitro groups were hydrogenated over Pt1/Ni nanocrystals and Pt single atoms supported on other supports with the selectivity of>99%for 3-vinylaniline.The selectivity of 3-vinylaniline for Pt/C under the same catalytic condition was only about 50%.Mechanistic studies revealed that the remarkable activity of Pti/Ni nanocrystals derived from sufficient hydrogen supply because of spontaneous dissociation of H2 on both Pt and Ni atoms as well as facile diffusion of H atoms on Pt1/Ni nanocrystals.The selectivity mainly derived from the influence of single atom structure on the adsorption configuration of substrate molecules.3.We synthesized a highly efficient catalyst composed of PdFe intermetallic nanocrystals which enabled CO2 hydrogenation under atmospheric pressure.During the hydrogenation of CO2 into methane,the TOF number of PdFe intermetallic nanocrystals reached 86 h-1 under 1 bar at 180?,being 7.4 and 11.0 times as high as that of PdFe random alloys and Pd/C,respectively.It indicated that the atomic arrangement in nanocatalysts significantly influenced the activity of CO2 hydrogenation.We can adjust the catalytic activity by changing the degree of atomic order in the bimetallic system.The specific reaction mechanism was further explored.