Shape-Control Synthesis, Surface Decoration And Catalytic Performance Of Low Dimensional Metal Sulfides

Fabrication of catalysts with improved activity has been a hot topic in the catalysis area. Metal sulfide catalysts have been playing key roles in current and future energy sources such as fossil oil, biomass and hydrogen gas, because they are catalytically active, inexpensive and earth abundant, compared to noble metal catalysts,In order to prepare highly active and selective sulfide catalysts, we need to study their surface chemistry and to build up the correlation between surface structure and catalytic performance, which requires a well-defined surface model. With the development of nanotechnology and characterization techniques, qualitative leaps has been made on the shape control synthesis and fine structure analysis of low dimensional metal sulfides, especially single layer MoS2. This provides the strongest foundation for the deep study on the mechanism on surface reactions.This thesis aims at synthesizing metal sulfides with a well-defined structure, at improving catalytic performance via surface decoration with other multi-components as cocatalysts and at further studying the relationships between catalytic activity and interfacial geometric or electronic structure, which may guide us on the design strategy for catalysts with improved performance in the future. Here, this work mainly focuses on single-or few-layer MoS2 nanosheets for catalytic hydrodeoxygenation (HDO) of 4-methylphenol and partially involves CdS for photo-catalytic hydrogen production from water splitting. Four chapters of work are included as follows:(1) We present a facile and effective method to exfoliate MoS2 and other two dimensional materials in a wide range of solvent systems to prepare few-layers with the assistance of N2H4. The exfoliated MoS2 with more active sites shows high activity towards the HDO of 4-methylphenol, even higher when Co promoters are loaded.(2) High yield single-layer MoS2 (S-MoS2) sheets are achieved through Li intercalation method by exfoliating MoS2 few-layers from the last work. In stark contrast to physical exfoliation, chemical insertion helps to create more sulphur vacancy sites on the basal plane of S-MoS2, thus activating inert basal plane to be active for the HDO reaction. It is found that a large number of sulphur vacancy sites are distributed on the basal plane, which accounts for the improved HDO activity when layer thickness is reduced from bulk to monolayer.(3) Commercial bulk MoS2 based catalysts are unable to find applications in hydro-deoxygenation reactions due to poor activity, sulphide leaching and rapid deactivation. Here, we report, jfor the first time the synthesis of single Co atom decorated MoS2 monolayer sheets (Co-S-MoS2), via covalent bonding of Co to sulphur vacancies on the basal plane. This new bottom-up designed catalyst exhibits remarkable activity, selectivity and stability for the model reaction:HDO of 4-methylphenol to toluene with superior intrinsic activity than that of conventional counterparts. Thus the reaction temperature can be greatly reduced from the typically used 300? to 180?. The higher activity at lower operation temperature allows the catalysis to proceed without sulphide leaching into solution. High-resolution STEM, aberration-corrected TEM, and DFT calculations indicate that the superior catalytic performance is the result of significantly enhanced number of activated interfacial sites in contact with Co-Mo atoms on MoS2 basal surface. This offers an exciting possibility of using this novel type of atomic dispersed catalysts for hydro-deoxygenation of biomass molecules.(4) We present a fabrication of well-defined CdS nanocrystals and surface decoration with PdS nanoclusters for photo-induced hydrogen evolution reaction (HER) from water splitting. Three CdS nanocrystals with different morphologies were tested for HER and it shows a rate order of long rods> short rods> triangular plates. The long rod sample possesses a higher degree of surface defects which serve as trapped sites for photo-generated electrons and holes, thus facilitating effective charge separations, compared to short rods and triangular plates. A small amount of PdS is loaded on the surface of CdS nanocrystals to greatly improve its intrinsic HER activity. Specifically, The PdS promoted CdS short rods shows the best activity with an HER rate of 7884 ?mol h-1 g-1,43 folds higher than that of unpromoted counterpart. The great promotion can be explained by the formation of hetero-junction structure and synergy effect between multi-components.