Brenda Torres

HRTEM and molecular modeling of the MoS2–Co9S8 interface: understanding the promotion effect in bulk HDS catalysts

As environmental regulations increase, more selective transition metal sulfide (TMS) catalytic materials for hydrotreating applications are needed. Highly active TMS catalysts become more and more desirable triggering new interest for unsupported Co-promoted MoS2-based systems that have high volumetric activity as reported here. Contrary to the common observation for alumina-supported MoS2-based catalysts, we found in our previous studies with dibenzothiophene (DBT) hydrodesulfurization (HDS) that the catalytic activity is directly proportional to the increase of surface area of the sulfide phases (Co9S8 and MoS2) present in Co-promoted MoS2 unsupported catalysts. This suggests that activity is directly connected with an increase of the contact surface area between the two sulfide phases. Understanding of the nature of the possible interaction between MoS2 and Co9S8 in unsupported catalytic systems is therefore critical in order to get a more generalized overview of the causes for synergy. This has been achieved herein through the detailed characterization by XRD, XPS, and HRTEM of the highly active Co9S8/MoS2 catalyst resulting in a proposed model for a Co9S8/MoS2 interface. This model was then subjected to a DFT analysis to determine a reasonable description of the surface contact region between the two bulk phases. Modelling of the interface shows the creation of open latent vacancy sites on Mo atoms interacting with Co and formation of direct Co–Mo bonds. Strong electron donation from Co to Mo also occurs through the intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found. These changes in coordination and electronic properties are expected to favor a synergetic effect between Co and Mo at the proposed localized interface region between the two bulk MoS2 and Co9S8 phases.

Hardness and Elastic Modulus on Six-Fold Symmetry Gold Nanoparticles

The chemical synthesis of gold nanoparticles (NP) by using gold (III) chloride trihydrate (HAuCl∙3H2O) and sodium citrate as a reducing agent in aqueous conditions at 100 °C is presented here. Gold nanoparticles areformed by a galvanic replacement mechanism as described by Lee and Messiel. Morphology of gold-NP was analyzed by way of high-resolution transmission electron microscopy; results indicate a six-fold icosahedral symmetry with an average size distribution of 22 nm. In order to understand the mechanical behaviors, like hardness and elastic moduli, gold-NP were subjected to nanoindentation measurements—obtaining a hardness value of 1.72 GPa and elastic modulus of 100 GPa in a 3–5 nm of displacement at the nanoparticle’s surface.

In-situ HRTEM study of the reactive carbide phase of Co/MoS2 catalyst

Hydrotreatment catalytic operations are commonly performed industrially by layered molybdenum sulfide promoted by cobalt or nickel in order to remove heteroelements (S, N, O) from fossil fuels and biofuels. Indeed, these heteroelements are responsible of the emission of pollutants when these fuels are used in vehicles. In this respect, previous studies made by our research group have shown that the active phase under steady state conditions is partially carbided while strong bending effects of MoS2 slabs were also observed. However, up to now, the morphology of the resulting Co/MoSxCy carbided catalyst has not been fully characterized. In the present study, for the first time, a chemical reaction between the carbon content of a TEM Cu/C grid and a freshly sulfide Co/MoS2 catalyst was in situ observed at 300 °C and 450 °C by HRTEM experimental techniques at ~10 nm of resolution. Results indicate that bending of MoS2 layers occurred due to carbon addition on MoS2 edge sites, as observed in stabilized catalysts after HDS reaction. Using a silicon grid, only cracks of MoS2 slabs were observed without bending effect confirming the role of structural-carbon in this change of morphology.

Bimetallic CoMoS Composite Anchored to Biocarbon Fibers as a High-Capacity Anode for Li-Ion Batteries

Our work reports the hydrothermal synthesis of a bimetallic composite CoMoS, followed by the addition of cellulose fibers and its subsequent carbonization under Ar atmosphere (CoMoS@C). For comparison, CoMoS was heat-treated under the same conditions and referred as bare-CoMoS. X-ray diffraction analysis indicates that CoMoS@C composite matches with the CoMoS4 phase with additional peaks corresponding to MoO3 and CoMoO4 phases, which probably arise from air exposure during the carbonization process. Scanning electron microscopy images of CoMoS@C exhibit how the CoMoS material is anchored to the surface of carbonized cellulose fibers. As anode material, CoMoS@C shows a superior performance than bare-CoMoS. The CoMoS@C composite presents an initial high discharge capacity of ∼1164 mA h/g and retains a high specific discharge capacity of ∼715 mA h/g after 200 cycles at a current density of 500 mA/g compared to that of bare-CoMoS of 102 mA h/g. The high specific capacity and good cycling stability could be attributed to the synergistic effects of CoMoS and carbonized cellulose fibers. The use of biomass in the anode material represents a very easy and cost-effective way to improve the electrochemical Li-ion battery performance.