CoMoS Phase Research Articles

N-octyltrimethylammonium bromide-assisted self-assembly of CoMo precursors to prepare CoMoS/Al2O3 catalysts for highly selective hydrodesulfurization of thiophene

A novel (C11H26N)2Co(MoS4)2 complex was prepared via the n-octyltrimethylammonium bromide-assisted self-assembly of CoMo precursors for the selective hydrodesulfurization (HDS) of thiophene. The complex was generated by the interaction between the negatively charged CoMo compound and the positively charged n-octyltrimethylammonium ions. Then positively charged bimolecular layers were formed owing to van der Waals forces between the C11H26N+ and the complex, and they were anchored on the negatively charged γ-Al2O3. Compared with CoMoS/γ-Al2O3 catalysts prepared by impregnation, the optimal CoMoN-0.3/γ-Al2O3 catalyst exhibited better performance for selective HDS due to the enhanced Mo dispersion, the close contact between Co and Mo, and the increased amount of CoMoS phases on γ-Al2O3. Among these catalysts, the optimal CoMoN-0.3/γ-Al2O3 catalyst with the highest concentration of CoMoS phases exhibited the highest HDS reaction rate constant of 6.95 × 10-6 mol·g−1·s−1 and the best selective factor of 26.1 for the selective HDS of thiophene with 1-hexene.

A novel ligand protection strategy based on lacunary polyoxometalate to precisely construct CoMoS active sites for hydrodesulfurization

Efficient CoMoS catalysts with intimate bimetallic synergy are derived from cobalt-substituted phosphomolybdate and exhibited excellent hydrodesulfurization (HDS) activity. Based on ligand protection strategy, the introduction of pyridine ligands onto the vacancies of the trivacant polyoxometalates (POMs) successfully overcame their low stability and isomerization behavior in solution. Subsequently, Co2+ was anchored in the molecular skeleton of [A-α-PMo9O31(py)3]3− (PMo9-py) to form CoxPMo9-py precursors with high Co/Mo ratios, which enhanced the reducibility of Mo species and transformed them to abundant CoMoS active phase during sulfidation process. The quenching effect of pyridine molecules on the acidic sites of support and carbon deposition generated in the high temperature sulfidation weakened the strong metal-support interactions (SMSI) while improving the dispersion and sulfidation of active metals. Due to the unique structure of CoxPMo9-py, the CoMoS phase with high dispersion, efficient synergy and high concentration was obtained. The combination of POMs precursors and pyridine ligands enabled the S-CoxPMo9-py/Al2O3 catalyst to achieve up to 99.5% conversion of dibenzothiophene (DBT).

Selective hydrodesulfurization of FCC naphtha over carbon coated alumina supported CoMoS catalysts

Herein, carbon coated alumina (C@Al2O3) samples obtained by chemical vapor deposition of ethylene have been proposed as supports for CoMoS hydrodesulfurization (HDS) catalysts of full-range FCC naphtha. Various degree of support graphitization (from 30 to 74%) was achieved by changing the treatment time of alumina in ethylene from 15 to 120 min. The influence of the alumina graphitization on the state and morphology of the sulfide component and catalytic performance of CoMoS catalysts has been studied in detail. The dispersion of sulfide particles and the CoMoS phase content is found to increase with the growth of the graphitization degree. This is accompanied by an increase in the selectivity of the CoMoS/C@Al2O3 catalysts in the HDS of FCC naphtha, whereas the reverse trend is observed for the activity. Thus, by changing the support graphitization degree, the selectivity of CoMoS catalysts in FCC naphtha HDS can be tuned. An increase in the support graphitization degree to 74% made it possible to reduce the sulfur content in FCC naphtha from 238 to 10 ppm, with a total loss in octane number of less than 1.5 units, which demonstrates the great potential of CoMoS/C@Al2O3 catalysts for commercial application in selective HDS of FCC naphtha.

Design of improved CoMo hydrotreating catalyst via engineering of carbon nanotubes@alumina composite support

An efficient strategy for the synthesis of nanostructured composite supports for CoMoS hydrotreating catalysts has been proposed. The synthesis concept is based on the growth of multi-walled carbon nanotubes (MWCNTs) on alumina crystallites modified with Fe2Co nanoparticles. The morphology and structural characteristics of the obtained MWCNT@Al2O3 supports were tuned by varying Fe2Co content. A comprehensive analysis using advanced techniques revealed the optimal content and structural characteristics of MWCNTs in composite supports that positively affect the morphology of the sulfide component. It has been established that for the best CoMoS/MWCNT@Al2O3 catalyst, which is characterized by the highest dispersion of sulfide component and CoMoS phase content, the activity in dibenzothiophene hydrodesulfurization and quinoline hydrodenitrogenation significantly exceeds the activity of CoMoS/Al2O3 and CoMoS/MWCNT catalysts. The proposed approach can be applied to improve the activity of both conventionally used hydrotreating catalysts and catalysts for other practically important processes by carefully tuning metal-support interaction.

Synergistic conversion of acid gases (H2S and CO2) to valuable chemicals: Carbonyl sulfide synthesis over vacancy-defective CoMo sulfide catalysts

Synergistically convert acid gases to valuable chemicals is meaningful. Herein, unsupported CoMo sulfide catalysts with abundant sulfur vacancies were fabricated to attain synergistical conversion of H2S and CO2 to COS. Various metal sulfides were obtained, such as Co3S4, Co9S8 and vacancy-defective MoS2. Besides, simultaneous sulfidation of Mo with Co prompted formation of CoMoS phase, which enhanced catalytic performance. Among them, CoAlMo-S catalyst exhibited superior activity, allowing for equilibrium COS yield (6%, 300 ℃). DFT calculations indicate that chemisorbed H2S induced CO2 activation via van der Waals interaction between C atom in CO2 and S atom in H2S, contributed to efficient C–S bond formation. Sulfur vacancy significantly promoted dissociative adsorption of H2S while Co atom in CoMoS phase was beneficial for CO2 adsorption and facilitated C–S bond construction. This work not only achieves H2S pollution control and carbon emission reduction, but also provides an alternative approach to synthesis COS.

A Facile Synthesis of Highly Efficient MXene Supported CoMoS Catalysts for Hydrodesulfurization

AbstractIn this study, ammonium tetrathiomolybdate (ATM) and 2D‐layer MXene support were used to prepare more efficient MoS2 catalysts with cobalt promoter for the elimination of sulfur atoms in thiophene. A series of well‐dispersed and highly active MoS2 and Co−Mo−S phase on MXene catalysts were derived by thermal activation of precursors varying promoter cobalt to molybdenum molar ratio up to 0.6. The physicochemical properties of synthesized catalysts were characterized by XRD, TPR, SEM/EDS, XPS, HRTEM, and Raman spectroscopy. The as‐prepared MoS2/MXene and Co−MoS2/MXene catalysts were examined for thiophene hydrodesulfurization (HDS) at 300–400 °C. The synergetic effect between Co and few‐layer MoS2 on MXene, enhanced their HDS activity to ∼ 20.6 mmol g−1 h−1 with an excellent turnover frequency of about 55 h−1 achieved on Co−MoS2/MXene catalysts, which were 8‐fold higher than the conventional CoMoS/alumina catalyst. Co−MoS2/MXene catalysts produce high butenes selectivity ∼73% at low temperature, whereas, at high‐temperature alkane levels were increased ∼42% in CoMoS phase through hydrogen activation. Hence, the few‐layer MoS2 obtained from ATM and promoter Co intercalated MXene catalysts enhances the rate of C−S bond cleavage in HDS, and the result is scrubbing a large volume of sulfur atoms in clean fuel processes.

Insights into the intrinsic kinetics for efficient hydrodesulfurization of 4,6-dimethyldibenzothiophene over mesoporous CoMoS2/ZSM-5

The intrinsic kinetics of hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) over mesoporous CoMoS2/ZSM-5 (CoMoS2/MZSM-5) was investigated by comparing the commercial CoMoS2/γ-Al2O3. A pseudo-first-order kinetic model considering the direct desulfurization (DDS) and two detailed partial and full hydrogenation pathways was formulated with simple power law rate equations. The apparent rate constants of five lumped reactions on CoMoS2/MZSM-5 are 2–4 times larger than those of CoMoS2/γ-Al2O3. With both catalyst system, the pre-exponential factor plays a dominant role in the reaction rate of DDS pathway showing equally high activation energies. The apparent activation energies show a critical effect on the reaction rate of two hydrogenation and subsequent desulfurization reactions. Furthermore, the Delplot analysis was applied to identify the reaction sequence of product formation and verify the reaction network. The HDS performance and reaction rates of individual reaction steps can be rationally linked to the morphology and available content of CoMoS phase as identified by HRTEM and XPS characterization. Thus, multi-stacked MoS2 nanocrystallites on CoMoS2/MZSM-5 provide more available sulfur vacancies to facilitate the DDS pathway, and catalyze efficiently the hydrogenation and subsequent desulfurization with the synergy of brim sites and sulfur vacancies.

Role of Phosphorus and Triethylene Glycol Incorporation on the Activity of Model Alumina‐Supported CoMoS Hydrotreating Catalysts

AbstractEnvironmental regulations concerning fuel‐related sulfur emissions are currently experiencing worldwide expansion. This is motivating the research and development of improved hydrotreating (HDT) catalysts with carefully engineered synthesis methods that maximize activity as ultimate goal. In this contribution, the effect of phosphorus doping and triethylene glycol (TEG) incorporation on the genesis of the CoMoS phase of model HDT catalysts is assessed. Following a surface‐science approach, α‐Al2O3 single crystals with four orientations: C(0001), A(11 0), M(10 0), and R(1 02) are used as model supports of the traditional γ‐Al2O3 support for the study of active phase‐support interactions at the molecular scale. The model catalysts are prepared by traditional impregnation of a solution containing metal and phosphorus precursors and TEG. The catalysts are characterized in the sulfide phase by XPS, XAS, TEM, and AFM, revealing that the calcination step is key in the emergence of distinctive metal‐support interactions, which are directly related to the nature of alumina surface sorption sites. In addition, TEG incorporation on dried catalysts increases metal sulfidation and enhances promotion with respect to non‐additivated systems, especially at mild sulfidation temperatures. The catalytic activity of the model catalysts is tested in a thiophene hydrodesulfurization reaction, revealing the following activity trend with respect to alumina surface orientations: A ≫ M >R , C(0001). This trend is identical to the one observed for non‐phosphorus doped non‐TEG additivated model systems, confirming the predominance of surface effects on the catalytic activity over those exerted by P and TEG. By transposing these results to the industrial γ‐Al2O3 support, the (100) facet would provide surface sites that lead to better performing catalysts. This could result in the development of novel supports, engineered to expose surface sites that maximize hydrotreating activity.

Ultrafine Co-MoS2 monolayer catalyst derived from oil-soluble single-molecule polyoxometalates for slurry phase hydrocracking

Herein, a novel single-molecule confinement-sulfurization strategy was proposed to construct ultrafine Co-MoS2 monolayer catalysts by using oil-soluble single-molecule polyoxometalates (POMs), (DODA)6Co2Mo10, as precursors. Well-defined Co2Mo10-POM, acting as molecular pre-assembling platform, is introduced for the atomically precise construction of bimetallic sites. Highly efficient conversion of VR under severe condition is realized with (DODA)6Co2Mo10. The origin of prominent catalytic performance can be attributed to the in-situ formation of ultrafine single-layered Mo-based nanosheets with abundant exposed CoMoS active sites. By combining synchrotron radiation-based X-ray absorption spectroscopy and density functional theory calculations, we further discover that there is clearly intimate interaction between Mo and Co atoms and the dissociation of molecular hydrogen at CoMoS phases is energetically preferred, leading to remarkable hydrogenation and coke suppression activity. This work highlights an idea on the rational design and targeted synthesis of ultrafine bimetallic catalysts for slurry phase HCK with dramatically synergistic catalysis.

Core-shell catalysts with CoMoS phase embedded in clay nanotubes for dibenzothiophene hydrodesulfurization

Nanoarchitectural strategy for design of highly active catalysts with CoMoS encased in 50-nm diameter aluminosilicate tubes (halloysite nanoclay) serving as an abundantly available natural support was proposed. This encapsulation conception implies the protection of active metal sulfides from aggregation, coking and leaching. Synthesized nanocatalytic system was investigated by TEM, XPS, N2 adsorption, hydrogen temperature programmed reduction and desorption of ammonia techniques. Using of Mo-heteropolyacid anionic precursor complex (ca. 10 wt%) possess selective loading of molybdenum oxide into 15-nm diameter inner cavity of the nanotubes (covered with positively charged alumina) by an electrostatic attraction. Subsequent sulfidation results in formation of CoMoS particulate core inside the aluminosilicate shell. These tubular nanoreactors were pelletized using alumina binder to produce a mechanically strong mesoporous catalyst efficient for hydrotreatment of diesel petroleum fraction to clean fuels.

On the contribution of the cobalt sulfide phase to the global activity of industrial-type CoMo/Al2O3 catalysts in the HDS of DBT

To analyze if the HDS activity is the result of contributions of both CoMoS and cobalt sulfides phases, a series of CoMo/Al2O3 catalysts with increasing Co/(Co+Mo) ratios (r = 0.1, 0.2, 0.3 and 0.4) prepared by simultaneous impregnation, and composites formed with the CoMo/Al2O3 catalysts and Co/Al2O3, were tested in the hydrodesulfurization of dibenzothiophene. The catalysts were characterized by UV–Visible–NIR and infrared of adsorbed CO spectroscopies. The results showed that the amount of CoMoS adsorbing sites remains basically the same in the series of promoted catalysts, while an increase of the precursor of cobalt sulfide is observed. The activity experiments showed that the addition of Co/Al2O3 to Mo/Al2O3 or CoMo/Al2O3 catalysts promotes both the MoS2 and CoMoS phases, and that the rate constants of CoMo/Al2O3 r = 0.2 and CoMo/Al2O3 r = 0.3 can be closely reproduced with composites of CoMo/Al2O3 r = 0.1 with different amounts of Co/Al2O3, indicating that the increased activity in the series of Co-promoted catalysts is related to the presence of cobalt sulfide. It was evidenced that Co-promoted catalysts are complex systems in which several surface species contribute to the global hydrodesulfurization activity and that cobalt sulfide, far from being a spectator in HDS catalysis, has in fact a secondary but significant role in the global performance of industrial-type CoMo/Al2O3 hydrodesulfurization catalysts. All these contributions must be optimized to obtain highly active and selective hydrodesulfurization catalysts.

Activating CoMoS with CoP3 Phase for High‐efficient Hydrogen Evolution Reaction in Acidic Condition

AbstractCurrently, the construction of heterostructures to optimize electrocatalytic activities is a hot research topic, since there is still a certain gap between the electrocatalytic properties of these heterostructures and that of Pt/C. With this in mind, we utilized in‐situ vapor‐phase phosphorization‐sulfurization to obtain the CoP3/MoS2−Co0.75Mo3S3.75 (CoP/CoMoS, for short) heterostructure on carbon cloth (CC), which exhibits Pt‐like electrocatalytic performance and excellent stability in acid electrolyte. Activated by CoP3 active phase, the electronic configuration of CoMoS and the adsorption energy of H* (▵GH*) are greatly optimized. Meanwhile, the rapid oxidation of CoMoS phase can be effectively avoided under the protection of CoP3 layer. This work provides a reference for the design of heterostructures and their mechanism of action in electrocatalysis.

Influence of arsenic on light cycle oil hydrodesulfurization over a CoMo catalyst

Samples of a commercial hydrotreating catalyst with different initial arsenic content were used in the study light cycle oil (LCO) feed in a hydrotreating unit operating at high pressure (350 °C, 3 MPa) to evaluate the conversion and selectivity of the catalyst in the presence of arsenic. It was determined that arsenic in the catalyst modifies the textural properties and has the strongest influence on the conversion of the HDS reaction. For the first time, evidence is provided that the arsenic replaces cobalt in CoMoS phase. The incorporation of As is proposed to occur in “edge” sites of the molybdenum sulfide disfavoring the C–S cleavage during the HDS reaction, while hydrogenation sites would be favored.

Metal–organic framework-derived MoSx composites as efficient electrocatalysts for hydrogen evolution reaction

Metal–organic frameworks (MOFs) have emerged as a class of crystalline porous material for energy-related applications. Many MOF-based materials are efficient catalysts for hydrogen evolution reactions (HERs). Herein, we illustrate a strategy to modify Co-based MOFs into amorphous molybdenum sulfide (MoSx) via a facial solvothermal method. The modification gives rise to CoMoS phases that reduce hydrogen adsorption energy of catalysts. As a result, MoSx substantially improves the catalytic activity of Co-based MOF for HERs. An optimal sample with 40% MoSx delivered the best HER performance with a low onset potential of −147 mV and a Tafel slope of ∼68 mV decade−1. Furthermore, the composite catalyst was stable for up to 1000 cycles without any changes in performance. These results suggest that the MoSx/Co–MOF–74 composite is a viable candidate for replacing noble metals as a high-performance catalyst for HER in the future.

In Situ Formation of CoMoS Interfaces for Selective Hydrodeoxygenation of p-Cresol to Toluene

Catalytic hydrodeoxygenation (HDO) of phenolics to aromatics is one of the most important reaction routes in bio-oil upgrading. Sulfided CoMo catalysts are good candidates for this reaction, but their activities are strongly associated with the number of exposed surface sites and Co–Mo synergy. Herein, we employ zeolitic imidazolate framework-67 as a cobalt precursor and template for inverse deposition of MoS2 ultrathin sheets inside the framework to form CoMoS interfaces by using a simple hydrothermal method. The size-confined MoS2 possesses a few-layered structure and abundant surface defects. It is found that in situ generation of a CoMoS active phase would take place to reach the best activity of our catalysts under the HDO reaction condition or by H2 prereduction treatment. With varying different Co–Mo molar ratios, the optimized CoMoS-0.18 catalyst exhibits the highest HDO activity, with a p-cresol conversion of 92.4% and a toluene selectivity of 95.5% at 250 °C. The high performance is ascribed to the formation of an accessible surface CoMoS phase after surface restructuring.

Role of pore structure on the activity and stability of sulfide catalyst

Effect of pore structure on the activity and stability of sulfide phase in hydrodesulfurization (HDS) of diesel were systematically investigated. A series of alumina with different pore structure were prepared by calcining pseudo boehmite at different temperatures. Various characterizations suggested that the surface concentration of OH groups and Lewis acid sites generally decreased with calcination temperature. The properties of sulfide phase in fresh and spent catalysts were analysis by X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). The results showed that the concentration of CoMoS species was greatly decreased after the reaction while the average length of MoS2 slabs was substantially increased. The concentration of preserved CoMoS species was proportional to the average pore size of support indicating that the strip-off of Co from CoMoS phase can be reduced when the average pore size is larger. Furthermore, the activity evaluation showed that larger pore structure of support is beneficial for the intrinsic activity and stability of CoMoS phase in the HDS of diesel. The reason was partially attributed to that the pore with larger diameter can reduce the coke deposition on the catalyst so that increase the intrinsic activity and stability of CoMoS sites.

Synthesis of unsupported Co-Mo hydrodesulfurization catalysts with ethanol-water mixed solvent: Effects of the ethanol/water ratio on active phase composition, morphology and activity

Co-Mo sulfide catalysts with different particle size and structures were prepared in ethanol-water mixed solvent followed by a low-temperature sulfurization process, and were tested in dibenzothiophene hydrodesulfurization. The effects of the ethanol/water ratio on the particle size of precursors, active phase composition, active site number, morphology, and hydrodesulfurization activity of the final catalysts were investigated. The atomic ratio of Co/Mo and active phase morphology of catalysts closely relate to the dielectric constant of ethanol-water mixed solvent. Appropriate dielectric constant favors the formation of (Co)MoS2 slabs with shorter length (∼ 5.0 nm) and higher stacking number (∼ 4.3 layers), resulting in more coordinatively unsaturated sites, especially in the corner sites. The catalytic activity not only depends on the active sites of CoMoS phase, but also on that of Co9S8 and MoS2 phases because of synergistic effect. For MoS2 and CoMoS phases, the corner sites show higher direct desulfurization activity than the edge sites.

Deep insights into enhanced direct-desulfurization selectivity of thiourea-modified CoMoP/γ-Al2O3: An investigation of catalyst microstructures

Decreasing hydrogen consumption is the most challenging in industrial HDS technology. A series of CoMoP/γ-Al2O3 HDS catalysts were prepared with different complexing agents and studied by XRD, XPS, Py-FTIR, HRTEM, and H2-TPR. The adding of thiourea leads to weak interaction between metal and the supports, and shows small MoS2 slab with high stacking layers in its microstructure, leading to the increasing ratio of active CoMoS phase at the edge, finally results in high direct desulfurization selectivity; meanwhile, the highest Bronsted acid amount obtained from pyridine-FTIR results also facilitate the high DDS selectivity. These provide a sample method to modulate the direct desulfurization selectivity of HDS catalyst and correlate their microstructure and catalytic performance.

Solid-state synthesis of few-layer cobalt-doped MoS2 with CoMoS phase on nitrogen-doped graphene driven by microwave irradiation for hydrogen electrocatalysis.

The high catalytic activity of cobalt-doped MoS2 (Co–MoS2) observed in several chemical reactions such as hydrogen evolution and hydrodesulfurization, among others, is mainly attributed to the formation of the CoMoS phase, in which Co occupies the edge-sites of MoS2. Unfortunately, its production represents a challenge due to limited cobalt incorporation and considerable segregation into sulfides and sulfates. We, therefore, developed a fast and efficient solid-state microwave irradiation synthesis process suitable for producing thin Co–MoS2 flakes (∼3–8 layers) attached on nitrogen-doped reduced graphene oxide. The CoMoS phase is predominant in samples with up to 15 at% of cobalt, and only a slight segregation into cobalt sulfides/sulfates is noticed at larger Co content. The Co–MoS2 flakes exhibit a large number of defects resulting in wavy sheets with significant variations in interlayer distance. The catalytic performance was investigated by evaluating the activity towards the hydrogen evolution reaction (HER), and a gradual improvement with increased amount of Co was observed, reaching a maximum at 15 at% with an overpotential of 197 mV at −10 mA cm−2, and a Tafel slope of 61 mV dec−1. The Co doping had little effect on the HER mechanism, but a reduced onset potential and charge transfer resistance contributed to the improved activity. Our results demonstrate the feasibility of using a rapid microwave irradiation process to produce highly doped Co–MoS2 with predominant CoMoS phase, excellent HER activity, and operational stability.

Catalytic hydropyrolysis of biomass using supported CoMo catalysts – Effect of metal loading and support acidity

Catalytic hydropyrolysis of biomass to green fuels was performed using supported, sulfided CoMo catalysts. With MgAl2O4 as support material the CoMo loading was varied between 4.1 and 12.0 wt% at constant Co/Mo atomic ratio of 0.3. Increasing the metal loading decreased the amount of oxygen in the condensed organic phase from 9.0 to 4.7 wt% on dry basis (db) and the condensable organic yield decreased from 25.2 to 22.7 wt% on dry ash free (daf) basis, corresponding to a decrease in the carbon recovery from 39 to 37%. Using zeolite H-ZSM-5 mixed with alumina as support with a CoMo loading of 4.1 wt%, the condensed organics contained only 5.2 to 6.1 wt% db oxygen. The condensable organic yield was between 23.9 and 24.4 wt% db, and the carbon recovery was 39–40%. Thus using an acidic support can remove the oxygen without decreasing the carbon recovery. The latter was ascribed to alkylation of the aromatics when the zeolite support was used.Elemental maps of the spent catalysts were obtained using STEM-EDS, showing that the CoMo phase was mainly located as monolayer MoS2 slab structures (>93%) on the support and indicated a high dispersion of cobalt, consistent with incorporation of Co into the MoS2 structure in the CoMoS phase. Potassium was observed on all the spent catalysts, indicating transfer of alkali metal from the biomass to the catalyst. Potassium may decrease the acidity of the catalyst over time, thus reducing the positive effect of using a more acidic support.