Zhendong Pan

Facile hydrothermal synthesis of MoS2 nano-sheets with controllable structures and enhanced catalytic performance for anthracene hydrogenation

MoS2 nano-sheets with controllable structures were prepared using a hydrothermal method. Effects of crystallization time, temperature and pH of the precursor solution on the structures, components and morphologies of MoS2 nano-sheets were investigated. MoS2 nano-sheets were characterized using XRD, Raman, elemental analysis, TG/DTG, SEM, and HRTEM. The characterization results show that intercalated MoS2, intermediate MoS2 with intercalated and semi-crystallized structures, and well crystallized hexagonal MoS2 (denoted as 2H-MoS2) were prepared by varying crystallization conditions, especially crystallization time. Intercalated MoS2 possesses rich active sites and poor crystallinity. 2H-MoS2 demonstrates high crystallinity but few active sites. As a transition structure during the crystallization process from intercalated MoS2 to 2H-MoS2, intermediate MoS2 simultaneously possesses rich active sites and good crystallinity. The possible growth mechanism of MoS2 nano-sheets in the hydrothermal process was proposed. The catalytic activities of MoS2 nano-sheets with various structures were evaluated by anthracene hydrogenation. The evaluation results show that intermediate MoS2 exhibits optimized catalytic activities of anthracene hydrogenation, which can be ascribed to the abundant active sites and desired stability of intermediate MoS2. This work may provide theoretic guidance for creating active sites and tuning catalyst structure for the controllable synthesis of layered transition metal sulfides with high catalytic activities.

Surfactant-assisted hydrothermally synthesized MoS 2 samples with controllable morphologies and structures for anthracene hydrogenation

MoS 2 samples with controllable morphologies and structures were synthesized using surfactant-assisted hydrothermal processes. The effects of surfactants (PEG, PVP, P123, SDS, AOT, and CTAB) on the morphologies and structures of MoS 2 samples were investigated. The results revealed that spherical, bulk-like, and flower-like MoS 2 particles assembled by NH 4 + -intercalated MoS 2 nano-sheets were synthesized. The morphologies of the MoS 2 samples and their structures (including the slab length and the number of stacked layers) of MoS 2 nano-sheets in these samples could be controlled by adjusting the surfactants. Mono-dispersed spherical MoS 2 particles could be synthesized with PEG via the creation of MoS 2 nano-sheets with slab lengths shorter than 15 nm and fewer than six stacked layers. Possible formation mechanisms of these MoS 2 samples created via surfactant-assisted hydrothermal processes are proposed. Further, the catalytic activities of MoS 2 samples for anthracene hydrogenation were evaluated in a slurry-bed reactor. The catalyst synthesized with the surfactant PEG exhibited the highest catalytic hydrogenation activity. Compared with the other catalysts, it had a smaller particle size, mono-dispersed spherical morphology, shorter slab length, and fewer stacked layers; these were all beneficial to exposing its active edges. This work provides an efficient approach to synthesize transition metal sulfides with controllable morphologies and structures.

Designing MoS2 nanocatalysts with increased exposure of active edge sites for anthracene hydrogenation reaction

Designing MoS2 nanocatalysts rich with active edge sites by engineering of the nanostructures is an effective strategy to enhance their catalytic activity. A series of MoS2 nanoflowers with self-assembled nanosheets was successfully synthesized by engineering of nanostructures. The compositions and structures of MoS2 nanoflowers were characterized by elemental analysis, XPS, TG, XRD, Raman, SEM, and HRTEM. The growth mechanism for MoS2 samples was proposed. MoS2 nanoflowers with a short slab of 5–10 nm, 3–5 stacking layers and expanded basal spacing of 0.98 nm were synthesized via a one-pot solvothermal synthesis method using high boiling point and viscosity ethylene glycol as solvent, maximizing the exposure of active edge sites. In the catalytic anthracene hydrogenation reaction in a slurry-phase reactor, the hydrogenation percentage and selectivity to deep hydrogenation products of the optimized MoS2 nanoflowers are respectively 3.2 times and 31.2 times as high as those of commercial MoS2. The structure–activity relationship of MoS2 catalysts suggests that the engineering of nanostructures to increase the exposure of active edge sites can dramatically improve the catalytic hydrogenation performance of MoS2 catalysts. This study provides the theoretical instructions for designing MoS2 catalysts with improved activity. The increased understanding and research on MoS2 catalysts will drive the industrialization of heavy oil/residue conversion into clean fuels.

Microemulsion-mediated hydrothermal synthesis of flower-like MoS 2 nanomaterials with enhanced catalytic activities for anthracene hydrogenation

Flower-like intercalated MoS2 nanomaterials have been successfully synthesized via a microemulsionmediated hydrothermal (MMH) method, and characterized by X-ray diffraction, Raman spectroscopy, element analysis, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy in detail. Their catalytic performance for anthracene hydrogenation was evaluated using a slurry-bed batch reactor with an initial hydrogen pressure of 80 bar at 350 °C for 4 h. The intercalated MoS2 nanoflowers synthesized from Na2MoO4 (MoS2-S) and H2MoO4 (MoS2-A) as molybdenum precursors have diameters of about 150 and 50 nm, respectively. MoS2 nanosheets on MoS2-S and MoS2-A possess stacking layer numbers of 5–10 and 2–5, and slab lengths of about 15 and 10 nm, respectively. The interlayer distances of MoS2-S and MoS2-A are both enlarged from 0.62 nm to about 0.95 nm due to the intercalation of NH4 + and surfactant molecules. The MoS2 nanoflowers have high catalytic activities for anthracene hydrogenation. The selectivity for octahydroanthracene, a deeply hydrogenated product, over MoS2-A is 89.8%, which is 31.0 times higher than that over commercial bulk MoS2. Fully hydrogenated product (perhydroanthracene) was also detected over MoS2 nanoflowers with a selectivity of 3.7%. The enhanced hydrogenation activities of MoS2 nanoflowers can be ascribed to the high exposure of catalytic active sites, resulting from the smaller particle size, fewer stacking layer, shorter slab length and enlarged interlayer distance of MoS2 nanoflowers compared with commercial bulk MoS2. In addition, a possible growth mechanism of MoS2 nanoflowers synthesized via the MMH method was proposed.

Ionic liquid assisted hydrothermal synthesis of MoS2 double-shell polyhedral cages with enhanced catalytic hydrogenation activities

MoS2 double-shell polyhedral cages are synthesized via an ionic liquid assisted hydrothermal process, in which the polyhedral cages of organic–inorganic hybrid phosphomolybdic acid-ionic liquids (PMA-ILs) are formed in situ and serve as a sacrificial template. The as-synthesized MoS2 hierarchical polyhedral cages have a lateral length of 0.5–1.5 μm and average shell thickness of about 150 nm. The surfaces of MoS2 double-shell polyhedral cages are very rough and composed of small nanosheets. The synthesis parameters including ionic liquid dosage, crystallization time and sulfur source are investigated to clarify the growth mechanism. Polycyclic aromatic hydrocarbons including naphthalene and anthracene were used as model compounds to evaluate the catalytic hydrogenation performance of the as-synthesized MoS2 sample. It turns out that MoS2 double-shell polyhedral cages manifest better catalytic hydrogenation activities than MoS2 nanoparticles and commercial bulk MoS2. The double-shell hollow structure and the vertical-alignment of nanosheets in the polyhedral shell are responsible for the enhanced catalytic activities of MoS2 double-shell polyhedral cages.