Summary and Perspectives

Summary and Perspectives

DOI: 10.4018/978-1-5225-2274-4.ch005
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1. The Summary

We have seen increasing efforts in developing new synthetic strategies for the growth of 2D Mo(W) dichalcogenides. Thermal CVD process have been evidenced to be effective. And the improved MOCVD can grow high quality of 2D layer Mo(W) dichalcogenides. Obtained materials are shown to be active in water splitting to produce hydrogen and CO2 electric reduction. However, these preparing techniques are now limited for initial proof-of-concept studies of model catalysts owing to only small quantity available. On the contrary, the solvent or surfactant assisted exfoliation and wet chemical methods can prepare 2D Mo(W) dichalcogenides in larger quantity.

Electrochemical intercalation can effectively shift the chemical potentials of 2D Mo(W) dichalcogenides materials to the optimized position for efficient catalysis. Sometimes the intercalation process introduces a phase transition of the host matrix, which change catalytic activity and selectivity. Heterostructures of 2D Mo(W) dichalcogenides have been evidenced multi-catalytic functions and adjustable properties However, the synthetic integration of multiple Mo(W) dichalcogenides to create desired heterostructure or superlattice is an uneasy job. And it is difficult to integrate of these 2D layered materials with other materials or support without damaging their lattice structure or altering their intrinsic electronic properties. 2D Mo(W) dichalcogenides and their derivatives have been active in the water splitting and CO2 electrochemical reduction.

The 2D Mo(W)S2 based catalysts are extensively used in the hydrotreating process for removal of heteroatoms and enhancement of oil quality. Insight into structure, mechanism and reactivity of these catalysts have recently gained using the combination of novel experimental and theoretical techniques, such as STM, DFT and HAADF. It is suggested that the hydrogenation reactions may take place on the brim sites, whereas the sulfur removal can take place at both Mo and S edges. And the STM results reveal the detailed structures of Ni–Mo–S and Co–Mo–S active phase and DFT reveal the Co-Mo-C active structure. And the relation of morphology and the electronic structure with Co and Ni promoters have also been resolved.

It has long been debated about the promoting mechanism of Co and Ni ions. Although the Co(Ni)-Mo-S model is accepted by most researchers, the Remote Control mechanism is still referred in some studies. In fact, these two models all have their sayings. Which one plays more important role depends on the preparation method and components of hydrotreating catalysts. And competition or synergy may exist in real catalyst system.

Meeting the stringent specifications represent one of the major challenges for the petroleum refining industry. Dropping of sulfur content of diesel fuel to very low levels, (e.g. 10 ppm) requires the removal of refractory sulfur species such as 4,6-DMDBT from the diesel stream. This issue is exacerbated by the inhibiting effect of polyaromatics, nitrogen compounds and the formed H2S gas. Hydrogenation of aromatic cycle and isomerization of substituted groups can decrease the steric hindrance of 4,6-DMDBT molecule and facilitates the desulfurization. And hydrodenitrogenation (HDN) is also preferred for HDS of 4,6-DMDBT derivatives.

To CoMo, NiMo and NiW catalysts aiming to diesel deep HDS, the improvement can be achieved by increasing loading of active metal (Mo, W, etc.); by adding one more transient metal (e.g. Ni to CoMo or Co to NiMo); and by incorporating a noble metal (Pt, Pd, Ru, etc.). However, the favored strategies are fine control of the active structures and the interaction with support. The catalytic properties can be improved by using different supports (carbon, TiO2, TiO2-Al2O3, HY, MCM-41, etc.) which adjust the acidity and the interaction with active phases. Usage of chelating agent or P additives are also effective.

Another problem faced by the refiners is deep HDS of gasoline without the apparent drop of octane value. Contrary to the diesel HDS, Mo(W)S2 based catalysts for gasoline HDS should be effective desulfurization of thiophene derivatives with least hydrogenation to olefins (HYDO). And some isomerization activity is preferred which can restore the octane number in some extent. Improved FCC naphtha HDS has been achieved over NiW catalyst where W-based hybrid nanocrystals are supported and promoted with Ni. It is reported that Co increase both HDS activity and HDS/HYDO selectivity. Meanwhile K increase HDS/HYDO selectivity accompanying the decrease of HDS activity.

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