Understanding selectivity changes during hydrodesulfurization of dibenzothiophene on Mo2C/carbon catalysts

Comparative DFT study of methanol decomposition on Mo2C(001) and Mo2C(101) surfaces

CONTEXT

In this study, the complete reaction mechanism of methanol decomposition on metallic Mo₂C(001) and Mo/C-mixed Mo₂C(101) hexagonal Mo₂C crystalline phases was systematically investigated using plane-wave-based periodic density functional theory (DFT). The main reaction route for Mo₂C(001) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CHO + 3H → CO + 4H → C + O + 4H. Hence, C, O, and H are the main products. It was found that the energy barrier for CO dissociation was low. Therefore, it was concluded that the Mo₂C(001) surface was too active to be easily oxidized or carburized. The optimal reaction pathway for Mo₂C(101) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH₃ + O + H → CH₄ + O. Therefore, CH₄ is the major product. The hydrogenation of CH₃ leading to CH₄ showed the highest energy barrier and the lowest rate constant and should be the rate-determining step. In addition, the formation of CO + 2H₂ was competitive on Mo₂C(101), and the optimal path was CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH + O + 3H → C + O + 4H → CO + 2H₂. The computed energy barrier and rate constant indicate that the rate-determining step is the last step in CO formation. In agreement with the experimental observations, the results provide insights into the Mo₂C-catalyzed decomposition of methanol and other side reactions.

METHODS

All calculations were performed by using the plane-wave based periodic method implemented in Vienna ab initio simulation package (VASP, version 5.3.5), where the ionic cores are described by the projector augmented wave (PAW) method. The exchange and correlation energies were computed using the Perdew, Burke and Ernzerhof functional with the latest dispersion correction (PBE-D3).

Molybdenum Carbide/Ni Nanoparticles Embedded into Carbon Nanofibers as an Effective Non-Precious Catalyst for Green Hydrogen Production from Methanol Electrooxidation

Molybdenum carbide co-catalyst and carbon nanofiber matrix are suggested to improve the nickel activity toward methanol electrooxidation process. The proposed electrocatalyst has been synthesized by calcination electrospun nanofiber mats composed of molybdenum chloride, nickel acetate, and poly (vinyl alcohol) under vacuum at elevated temperatures. The fabricated catalyst has been characterized using XRD, SEM, and TEM analysis. The electrochemical measurements demonstrated that the fabricated composite acquired specific activity for methanol electrooxidation when molybdenum content and calcination temperature were tuned. In terms of the current density, the highest performance is attributed to the nanofibers obtained from electrospun solution having 5% molybdenum precursor compared to nickel acetate as a current density of 107 mA/cm² was generated. The process operating parameters have been optimized and expressed mathematically using the Taguchi robust design method. Experimental design has been employed in investigating the key operating parameters of methanol electrooxidation reaction to obtain the highest oxidation current density peak. The main effective operating parameters of the methanol oxidation reaction are Mo content in the electrocatalyst, methanol concentration, and reaction temperature. Employing Taguchi's robust design helped to capture the optimum conditions yielding the maximum current density. The calculations revealed that the optimum parameters are as follows: Mo content, 5 wt.%; methanol concentration, 2.65 M; and reaction temperature, 50 °C. A mathematical model has been statistically derived to describe the experimental data adequately with an R² value of 0. 979. The optimization process indicated that the maximum current density can be identified statistically at 5% Mo, 2.0 M methanol concentration, and 45 °C operating temperature.

Controllable synthesis of Mo2C with different morphology and application to electrocatalytic hydrogen evolution reaction

In order to evaluate the effect of precursors and synthesis strategies on catalytic ability of Mo₂C in the hydrogen evolution reaction (HER), four kinds of Mo₂C were synthesized using two kinds of MoO₃by two strategies. Compared with the one-step direct carbonization strategy, Mo₂C with a large special surface area and a better performance could be synthesized by the two-step strategy composed of a nitridation reaction and a carbonization reaction. Additionally, the as-prepared porous Mo₂C nanobelts (NBs) exhibit good electrocatalytic performance with a small overpotential of 165 mV (0.5 M H₂SO₄) and 124 mV (1 M KOH) at 10 mA cm^-2, as well as a Tafel slope of 58 mV dec^-1(0.5 M H₂SO₄) and 59 mV dec^-1(1 M KOH). The excellent catalytic activity is ascribed to the nano crystallites and porous structure. What's more, the belt structure also facilitates the charge transport in the materials during the electrocatalytic HER process. Therefore, the two-step strategy provides a new insight into the structural design with superior performance for electrocatalytic HER.

Effect of Adding Chelating Ligands on the Catalytic Performance of Rh-Promoted MoS2 in the Hydrodesulfurization of Dibenzothiophene

Hydrodesulfurization (HDS) is a widely used process currently employed in petroleum refineries to eliminate organosulfur compounds in fuels. The current hydrotreating process struggles to remove organosulfur compounds with a steric hindrance due to the electronic nature of the current catalysts employed. In this work, the effects of adding chelating ligands such as ethylenediaminetetraacetic acid (EDTA), citric acid (CA) and acetic acid (AA) to rhodium (Rh) and active molybdenum (Mo) species for dibenzothiophene (DBT) HDS catalytic activity was evaluated. HDS activities followed the order of RhMo/ɣ-Al2O3 (88%) > RhMo-AA/ɣ-Al2O3 (73%) > RhMo-CA/ɣ-Al2O3 (72%) > RhMo-EDTA/ɣ-Al2O3 (68%). The observed trend was attributed to the different chelating ligands with varying electronic properties, thus influencing the metal–support interaction and the favorable reduction of the Mo species. RhMo/ɣ-Al2O3 offered the highest HDS activity due to its (i) lower metal–support interaction energy, as observed from the RhMo/ɣ-Al2O3 band gap of 3.779 eV and the slight shift toward the lower BE of Mo 3d, (ii) increased Mo-O-Mo species (NMo-O-Mo ~1.975) and (iii) better sulfidation of Rh and MoO in RhMo/ɣ-Al2O3 compared to the chelated catalysts. The obtained data provides that HDS catalytic activity was mainly driven by the structural nature of the RhMo-based catalyst, which influences the formation of more active sites that can enhance the HDS activity.

Improving the hydrodesulfurization performance of the sulfur-resistant intermetallic Ni2Si based on a MOF-derived route

Carbon-supported intermetallic nickel silicide (Ni₂Si/C) derived from Ni-MOF-74 as a non-sulfide catalyst presents high activity and sulphur-resistance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT).

Structural Screening and Design of Dendritic Micro-Mesoporous Composites for Efficient Hydrodesulfurization of Dibenzothiophene and 4,6-Dimethyldibenzothiophene

Novel dendritic micro-mesoporous TS-1/dendritic mesoporous silica nanoparticle (DMSN) composites (TD) were assembled by TS-1 nanocrystals with ultrasmall particle size and strong acidity. TS-1 seeds and DMSNs were composited via the Ti-O-Si chemical bond, which stimulate the generation of Brønsted (B) and Lewis (L) acids. The spillover d-electrons produced by the Ti element of TS-1 seeds produced a spillover of d-electrons, which could interact with the surface of MoS₂ phases, thereby reducing Mo-S interactions and create sulfur vacancies that are favorable for dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization (HDS) reactions. The increased amount of B&L acid of NiMo/TD-2.0 with cetyltrimethylammonium bromide/sodium salicylate molar ratio of 2.0 played an important role in facilitating the hydrogenation (HYD) route of DBT HDS and the isomerization (ISO) route of 4,6-DMDBT HDS, which is more favorable for the reduction of steric hindrance of DBT and 4,6-DMDBT reactants in the HDS reaction process. The NiMo/TD-2.0 catalyst exhibited the highest turnover frequency (TOF) value and HDS reaction rate constant (k_HDS) of DBT and 4,6-DMDBT due to its ultrasmall particle size, uniform spherical dendritic morphology, strong B&L acidity, and good stacking degree.