Surface-dependent sulfidation and orientation of MoS2 slabs on alumina-supported model hydrodesulfurization catalysts

Friend or Foe? Revising the Role of Oxygen in the Tribological Performance of Solid Lubricant MoS2

Molybdenum disulfide (MoS₂) is a solid lubricant used in various forms, such as a dry lubricant by itself or as a component of a more complex coating. In both these forms, the effect of oxygen contamination on the sliding properties of the MoS₂ coatings is traditionally considered detrimental, resulting in expensive technological processes to produce pure MoS₂. Here, it is shown that the high oxygen content does not necessarily hinder the solid lubricant properties and may even result in a lower friction and wear when compared to pure MoS₂. Mo-S-O coatings were fabricated by unbalanced magnetron sputtering and tribologically tested under vacuum conditions. Oxygen caused amorphization of the as-deposited coatings but did not prevent the triboactivated formation of an ultra-thin crystalline MoS₂ tribolayer with the incorporated oxygen. Such an imperfect tribolayer was found to reduce the coefficient of friction to 0.02, a value lower than that of pure MoS₂. Moreover, owing to the higher density and hardness of oxygen-containing films, the wear rate was also found to be lower. Molecular dynamics simulations performed using a newly developed Mo-S-O force field confirmed that such an imperfect tribolayer can mitigate friction in a manner comparable to that of MoS₂.

Surface-Dependent Activation of Model α-Al2 O3 -Supported P-Doped Hydrotreating Catalysts Prepared by Spin Coating

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS₂ hydrotreating catalysts supported on α-Al₂ O₃ single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al₂ O₃ crystal orientations, C(0001), A(11 2 ‾ 0), M(10 1 ‾ 0) and R(1 1 ‾ 02) that exhibit different speciations of surface -OH. ³¹ P and ⁹⁵ Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS₂ phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 1 ‾ 0)>A(11 2 ‾ 0), R(1 1 ‾ 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al₂ O₃ surface orientations from the C(0001) to the R(1 1 ‾ 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 2 ‾ 0), R(1 1 ‾ 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS₂ . The surface-dependent phosphorus doping on model α-Al₂ O₃ supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al₂ O₃ carrier.

Properties and Composition of Products from Hydrotreating of Straight-Run Gas Oil and Its Mixtures with Light Cycle Oil Over Sulfidic Ni-Mo/Al2O3 Catalyst

Straight-run gas oil (SRGO) and its mixtures with 5, 10, 15, and 20 wt % light cycle oil (LCO) from fluid catalytic cracking (FCC) were hydrotreated on a commercial NiMo/Al₂O₃ catalyst in a laboratory tubular reactor with the cocurrent flow of the raw material and hydrogen. The hydrotreating of the raw material was undertaken at a temperature of 350 °C, a pressure of 4 MPa, a weight hourly space velocity of ca 1.0 h^-1, and a hydrogen-to-raw-material ratio of 240 m³·m^-3. The LCO had a high density due to the high content of bicyclic aromatics and the high content of sulfur species, which are difficult to desulfurize. Therefore, increasing the content of the LCO in the raw material resulted in increasing the density and increasing the content of the sulfur and polycyclic aromatics in the hydrotreated products. Only the products prepared from the raw material with LCO content up to 10 wt % fulfilled the density requirement of EN 590. To improve the product density, the products prepared from the raw material containing 15 wt % LCO were blended with refined kerosene. The addition of the kerosene decreased the density of the mixtures prepared, but the cold filter plugging point (CFPP) of the mixtures was only lowered by about 1-2 °C. It was necessary to add a depressant in an amount of 600 mg·kg^-1 to achieve a cold filter plugging point of -20 °C. Some refined products were blended with desulfurized heavy naphtha from the FCC. The addition of the heavy naphtha was mainly limited by its high density. Up to 10 wt % heavy naphtha could be added to the product obtained by hydrotreating the raw material containing 10 wt % LCO. More than 15 wt % heavy naphtha could be added to the mixture of the hydrotreated product and 20 wt % kerosene.

Management of γ-Alumina with High-Efficient {111} External Surfaces for HDS Reactions

A series of γ-alumina samples with different exposure ratio of {111} facet were synthesized by an efficient hydrothermal method via adjusting the pH value of the gel precursor. The nanorod alumina supported catalyst with the highest exposure of {111} facet exhibited the best hydrodesulfurization (HDS) activities of both thiophene and dibenzothiophene (DBT). Characterization of the sulfided NiMo/Al2O3 catalyst with preferential exposure of {111} facet showed that the MoS2 nano slabs were inclined to distribute in the direction along the edges of alumina nanocrystal in reduced stacking layers. The selective exposure of {111} facet played a decisive role in obtaining alumina-supported HDS catalysts with improved intrinsic activity. This work helps to better understand the relationship between catalytic properties and varied support surfaces, which demonstrate a proper design of the catalyst support morphology on the facet-level.

Hierarchical Structure of NiMo Hydrodesulfurization Catalysts Determined by Ptychographic X-Ray Computed Tomography

Hydrodesulphurization, the removal of sulphur from crude oils, is an essential catalytic process in the petroleum industry safeguarding the production of clean hydrocarbons. Sulphur removal is critical for the functionality of downstream processes and vital to the elimination of environmental pollutants. The effectiveness of such an endeavour is among other factors determined by the structural arrangement of the heterogeneous catalyst. Namely, the accessibility of the catalytically active molybdenum disulphide (MoS₂ ) slabs located on the surfaces of a porous alumina carrier. Here, we examined a series of pristine sulfided Mo and NiMo hydrodesulphurization catalysts of increasing metal loading prepared on commercial alumina carriers using ptychographic X-ray computed nanotomography. Structural analysis revealed a build consisting of two interwoven support matrix elements differing in nanoporosity. With increasing metal loading, approaching that of industrial catalysts, these matrix elements exhibit a progressively dissimilar MoS₂ surface coverage as well as MoS₂ cluster formation at the matrix element boundaries. This is suggestive of metal deposition limitations and/ or catalyst activation and following prohibitive of optimal catalytic utilization. These results will allow for diffusivity calculations, a better rationale of current generation catalyst performance as well as a better distribution of the active phase in next-generation hydrodesulphurization catalysts.

Recent Insights in Transition Metal Sulfide Hydrodesulfurization Catalysts for the Production of Ultra Low Sulfur Diesel: A Short Review

The literature from the past few years dealing with hydrodesulfurization catalysts to deeply remove the sulfur-containing compounds in fuels is reviewed in this communication. We focus on the typical transition metal sulfides (TMS) Ni/Co-promoted Mo, W-based bi- and tri-metallic catalysts for selective removal of sulfur from typical refractory compounds. This review is separated into three very specific topics of the catalysts to produce ultra-low sulfur diesel. The first issue is the supported catalysts; the second, the self-supported or unsupported catalysts and finally, a brief discussion about the theoretical studies. We also inspect some details about the effect of support, the use of organic and inorganic additives and aspects related to the preparation of unsupported catalysts. We discuss some hot topics and details of the unsupported catalyst preparation that could influence the sulfur removal capacity of specific systems. Parameters such as surface acidity, dispersion, morphological changes of the active phases, and the promotion effect are the common factors discussed in the vast majority of present-day research. We conclude from this review that hydrodesulfurization performance of TMS catalysts supported or unsupported may be improved by using new methodologies, both experimental and theoretical, to fulfill the societal needs of ultra-low sulfur fuels, which more stringent future regulations will require.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.