Transition Metal Sulfide Catalysts Research Articles

Three-dimensional self-supporting micro-nanostructured MoS2/CoS2/CC heterojunction derived from ZIF-67 for high efficiency electrocatalytic hydrogen evolution in both acid and alkali electrolytes

In this paper, a novel three-dimensional (3D) MoS2/CoS2 micro-nano heterostructured electrocatalyst has been successfully derived from ZIF-67 on carbon cloth (CC) by a simple method of low temperature vulcanization. The metal-organic framework (MOFs) of ZIF-67 on CC was obtained at room temperature and used as both the precursor of Co and structure template to construct the 3D self-supporting spindle CoS2 nanodiscs on CC. Large amounts of 2D MoS2 nanosheets were subsequently grown on the surface of spindle-shaped CoS2 nanodiscs to form 3D self-supporting MoS2/CoS2/CC heterostructural electrocatalyst. This 3D self-supporting heterostructural electrocatalyst showed good hydrogen evolution reaction (HER) performance in both acid and alkali electrolytes. At 10 mA/cm2, the HER overpotentials of the MoS2/CoS2/CC heterostructural electrocatalyst in the alkali and acid electrolyte were 147 mV and 119 mV, and their corresponding Tafel slopes were 90.5 mV dec−1 and 111.4 mV dec−1, respectively, indicating that this MoS2/CoS2/CC heterostructural electrocatalyst had a good application prospect in the electrolysis of water to produce hydrogen. Moreover, this work provides a new idea for low-cost, scale fabrication of 3D self-supporting heterostructured transition metal sulfide catalysts.

Enhancing bifunctional oxygen electrocatalysis with in-situ etching of oxide overlayers on sulfur-vacancy enriched CuCo2O4/CuCo2S4

Transition metal sulfide (TMS) are considered promising alternatives to noble metals for high-efficiency oxygen reduction and oxygen evolution (ORR/OER). However, traditional preparation methods or post-treatment processes can lead to the formation of oxide overlayers on the TMS catalysts, which can hinder their electrochemical performance. Herein, a sulfur-vacancy enriched CuCo2O4/CuCo2S4 heterostructure was prepared using cold plasma. Plasma also achieved the in-situ etching of oxide overlayers on the surface of catalysts. There is a strong correlation between the oxide overlayers and ORR/OER activity. Specifically, the number of ORR electrons transferred increased (n: from 2.8 to 3.7), while the OER overpotential decreased (η: from 417 to 330 mV) in 0.1 M KOH with the decrease of the oxide overlayer concentration (SO42−/SM−S: from 1.795 to 0.798). The theoretical calculation reveals the influence of oxide overlayers and S vacancy on the binding energy of the intermediate. The findings suggest that this work provides a new idea for designing metal sulfide electrochemical catalyst.

Facile Construction of Three-Dimensional Heterostructured CuCo2S4 Bifunctional Catalyst for Alkaline Water Electrolysis

Developing an efficient multi-functional electrocatalyst with high efficiency and low cost to replace noble metals is significantly crucial for the industrial water electrolysis process and for producing sustainable green hydrogen (H2) fuel. Herein, ultrathin CuCo2S4 nanosheets assembled into highly open three-dimensional (3D) nanospheres of CuCo2S4 (Cu/Co = 33:67) were prepared by a facile one-pot solvothermal approach and utilized as a bifunctional electrocatalyst for efficient overall water splitting. The as-prepared CuCo2S4 is characterized structurally and morphologically; the BET surface area of the CuCo2S4 (Cu/Co = 33:67) catalyst was found to have a larger specific surface area (21.783 m2g−1) than that of other catalysts with a Cu/Co ratio of 67:33, 50:50, and 20:80. Benefiting from a highly open structure and ultrathin nanosheets with excellent exposure to catalytically active sites, CuCo2S4 (Cu/Co = 33:67) is identified as an efficient catalyst for the proton reduction and oxygen evolution reactions in 1 M KOH with an overpotential of 182 and 274 mV at 10 mA cm−2, respectively. As expected, a low cell voltage of 1.68 V delivers a current density of 10 mA cm−2. Stability and durability are also greatly enhanced under harsh alkaline conditions. Therefore, this work provides a simple strategy for the rational design of spinel-based transition metal sulfide catalysts for electrocatalysis.

Coral-shaped Mn-CuS with hierarchical pores and crystalline defects for high-efficiency H2O2 production via electrocatalytic two-electron reduction

We report the development of non-noble transition metal sulfide catalysts Mn-CuS-x for high-efficiency and selective H2O2 production. In this study, we synthesized Mn-CuS-x with coral-shaped hierarchical porous structure through one-pot hydrothermal reaction. The Mn-CuS-x possesses large specific surface area with plenty of crystalline defects via heteroatom doping of Mn. The Mn-CuS-x kinetically favored 2e− pathway over 4e− pathway, resulting in high selectivity (>92%) to H2O2 production and high H2O2 concentration (∼24.5 mM at 0.6 V vs. RHE) within 3 h for the optimized Mn-CuS-2 catalysts (synthesized with a molar ratio of 1:3:8 (MnCl2·4 H2O: CuCl2·2 H2O: C2H5NS)). The Mn-CuS-2 showed excellent stability, indicating that the Mn-CuS electrocatalysts can simultaneously achieve both high stability and enhanced H2O2 production. Our first-principles calculations further confirmed that the heteroatom Mn doping to CuS thermodynamically makes the 2e–-ORR pathway more favorable. This study provides a green, low-cost, and efficient route to produce H2O2, which is very promising for generating chemicals and liquid fuels.

Active-site-enriched dendritic crystal Co/Fe-doped Ni3S2 electrocatalysts for efficient oxygen evolution reaction.

The electrochemical decomposition of water plays a critical role in green and sustainable energy. However, the development of efficient and low-cost non-noble metal catalysts to overcome the high potential of the anodic oxygen evolution reaction (OER) is still challenging. In this work, electrocatalysts with high OER activity were obtained by doping Co/Fe bimetals into Ni3S2 (CF-NS) via a simple single-step hydrothermal method by adjusting the doping ratio of bimetals. A series of characterization studies revealed that the introduction of a Co/Fe co-dopant increased the number of active sites and improved the electroconductibility, while optimizing the electronic structure of Ni3S2. Meanwhile, Fe-induced high valence Ni contributed to the production of an OER active phase NiOOH. The unique dendritic crystal morphology facilitated the disclosure of the active sites and the expansion of mass transfer channels. The optimized sample required a low overpotential of 146 mV to obtain a current density of 10 mA cm-2 in 1.0 M KOH solution. The optimized sample also operated stably for at least 86 h. In sum, the proposed method looks very promising for designing efficient, stable, and low-cost non-precious metal catalysts with high conductivity and multiple active sites, useful for future synthesis of transition metal sulfide catalysts.

Porous Ni3S2-Co9S8 Carbon Aerogels Derived from Carrageenan/NiCo-MOF Hydrogels as an Efficient Electrocatalyst for Oxygen Evolution in Rechargeable Zn-Air Batteries.

Herein, we fabricate N-doped porous Ni3S2-Co9S8/carbon aerogels (Ni3S2-Co9S8/NCAs) using carrageenan/NiCo-metal-organic framework (MOF) hydrogels as the precursor via the high-temperature carbonization route with excellent electrocatalytic properties for the oxygen evolution reaction (OER). The electrochemical measurements indicate that the Ni3S2-Co9S8/NCA as a quintessential electrocatalyst exhibits excellent OER performance, which has outperformed most transition metal sulfide (TMS) catalysts in alkaline environments, as attested with a lower overpotential of 337 mV at 10 mA cm-2 and a smaller Tafel slope of 77 mV dec-1. Meanwhile, a Zn-air battery based on Ni3S2-Co9S8/NCA + Pt/C achieves a large power density of up to 256 mW cm-2 (and 193 mW cm-2), small charge/discharge voltage gap, and good cycling stability, notably better than the conventional RuO2 + Pt/C-based Zn-air batteries. These excellent electrocatalytic properties are mainly attributed to the distinct hierarchical porous structure and interfacial synergy between the Ni3S2 and Co9S8 nanoparticle structure with rich defects, facilitating the mass transport and high graphitization degree beneficial for electron mobility. It is envisioned that the research provides a novel approach for the exploration of marine biomass as an electrocatalyst.

The Restructuring-Induced CoO x Catalyst for Electrochemical Water Splitting.

Restructuring is an important yet less understood phenomenon in the catalysis community. Recent studies have shown that a group of transition metal sulfide catalysts can completely or partially restructure during electrochemical reactions which then exhibit high activity even better than the best commercial standards. However, such restructuring processes and the final structures of the new catalysts are elusive, mainly due to the difficulty from the reaction-induced changes that cannot be captured by ex situ characterizations. To establish the true structure–property relationship in these in situ generated catalysts, we use multimodel operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity to investigate the restructuring of a representative catalyst, Co9S8, that shows better activity compared to the commercial standard RuO2 during the oxygen evolution reaction (OER), a key half reaction in water-splitting for hydrogen generation. We find that Co9S8 ultimately converts to oxide cluster (CoOx) containing six oxygen coordinated Co octahedra as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory calculations verify the in situ generated CoOx consisting of edge-sharing CoO6 octahedral clusters as the actual active sites. Our results also provide insights to design other transition-metal-based materials as efficient electrocatalysts that experience a similar restructuring in OER.

Cover Feature: Vacancy Engineering in Transition Metal Sulfide and Oxide Catalysts for Hydrodeoxygenation of Lignin‐Derived Oxygenates (ChemSusChem 20/2021)

Cover Feature: Vacancy Engineering in Transition Metal Sulfide and Oxide Catalysts for Hydrodeoxygenation of Lignin‐Derived Oxygenates (ChemSusChem 20/2021)

Vacancy Engineering in Transition Metal Sulfide and Oxide Catalysts for Hydrodeoxygenation of Lignin-Derived Oxygenates.

The catalytic hydrodeoxygenation (HDO) of lignin has long been a hot research topic and vacancy engineering is a new means to develop more efficient catalysts for this process. Oxygen vacancies and sulfur vacancies are both widely used in HDO. Based on the current research status of vacancies in the field of lignin-derived oxygenates, this Minireview discusses in detail design methods for vacancy engineering, including surface activation, synergistic modification, and morphology control. Moreover, it is clarified that in the HDO reaction, vacancies can act as acidic sites, promote substrate adsorption, and regulate product distribution, whereas for the catalysts, vacancies can enhance stability and reducibility, improve metal dispersion, and improve redox capacity. Finally, the characterization of vacancies is summarized and strategies are proposed to address the current deficiencies in this field.

Fe and Co dual-doped Ni3S4 nanosheet with enriched high-valence Ni sites for efficient oxygen evolution reaction

Transition metal compound catalysts with high efficient and stable performance have attracted extensive attention of researchers. In this study, dual cation doped Ni3S4 nanosheets loaded on nickel foam were prepared by two step hydrothermal method. Co and Fe dual doping can induce the internal electronic interaction of the catalyst, thus exposing more active sites. Meanwhile, high Ni3+ content enhances the chemisorption of OH−, which is conducive to the transfer of electrons in the reaction process. The electrochemical test results show that Co and Fe doped Ni3S4 have better electrocatalytic performance for oxygen evolution than single cation doped and pure Ni3S4. In 1.0 M KOH solution, the FeCo-Ni3S4 electrode exhibits a remarkable oxygen evolution reaction activity with low overpotentials of 230 mV at 20 mA cm−2 and 279 mV at 100 mA cm−2. The water-alkali electrolyzer using FeCo-Ni3S4 as anode achieves stable overall water splitting with a low potential of 1.47 V at 10 mA cm−2. This work will provide a new direction and guidance for the synthesis and application of transition metal sulfide catalysts.

Surface-dependent activity of model CoMoS hydrotreating catalysts

The current environmental and industrial contexts require constant revision and improvement of hydrotreating processes for obtaining cleaner (bio)fuels through the development of supported transition metal sulfide (TMS) catalysts. The present work proposes a surface-science approach to describe the surface-dependent genesis of the cobalt-promoted active phase (CoMoS) supported on α-Al2O3 single crystals used as surrogates of the industrial γ-Al2O3 support. Co-Mo precursor solutions are deposited by aqueous-phase impregnation onto four α-Al2O3 single crystals of C(0001), A(112¯0), M(101¯0), and R(11¯02) orientations, which display specific OH sorption sites. The model catalysts are characterized in the oxide and sulfide states by XPS, AFM, and TEM revealing a surface dependent sulfidation and dispersion as a result of metal-support interactions of diverse nature. The surfaces can be ranked following metal-support interactions of increasing strength C(0001) < A(112¯0), M(101¯0) < R(11¯02) in good agreement with model non-promoted catalysts. However, AFM and TEM imaging also reveal that cobalt incorporation leads to an increase in the stacking of CoMoS particles with respect to non-promoted catalysts. The activity of the model catalysts in thiophene hydrodesulfurization cannot be simply related to the common features of the active phase (sulfidation, dispersion, promotion extent) but a volcano-shape curve is obtained when plotting the activity as a function of metal-support interactions. These results suggest that high activity is obtained on surfaces with intermediate metal-support interactions that lead to an optimal metal-sulfur bond energy in agreement with the Sabatier principle. Transposing these results to the traditional γ-Al2O3 support suggests an enhanced activity of the (100) facet, which can lead to the development of improved supports for hydrotreating catalysts.

Plasma-assisted catalytic reduction of SO2 to elemental sulfur: Influence of nonthermal plasma and temperature on iron sulfide catalyst

Herein, we report on low-temperature sulfur dioxide (SO2) reduction to elemental sulfur via the coupling of nonthermal plasma (NTP) with supported transition metal sulfide catalysts in a dielectric barrier discharge (DBD) reactor. The transition metals include Mo, Fe, Co, Ni, Cu, and Zn. Combining NTP with supported metal sulfide catalysts significantly promotes low-temperature reduction of SO2 by 148–200%, with over 98% selectivity to elemental sulfur. Over FeS2/Al2O3 catalyst at low temperatures (<250 °C), temperature does not influence SO2 conversion in the plasma catalytic process, whereas at higher temperatures, the reaction follows a similar trend as thermal catalysis. Strong synergistic effects are attained, as the sulfur yield is about 47–82% higher than the sum of the yield when using DBD-plasma and thermal catalysis separately. The physico−chemical properties of fresh and spent FeS2/Al2O3 catalysts after thermal and plasma reactions are evaluated by N2 physisorption, FESEM, XPS, XRD, HRTEM, STEM/EDS, and EELS to understand plasma and thermal effects on the catalyst. Our investigations reveal that running the reaction under plasma preserves the surface FeS2 active phase, preventing its oxidation that otherwise occurs in the thermal catalysis process. Furthermore, plasma inhibits the thermal agglomeration of iron sulfide nanoparticles under reaction conditions.

Mo-Doped Zn, Co Zeolitic Imidazolate Framework-Derived Co9S8 Quantum Dots and MoS2 Embedded in Three-Dimensional Nitrogen-Doped Carbon Nanoflake Arrays as an Efficient Trifunctional Electrocatalysts for the Oxygen Reduction Reaction, Oxygen Evolution Reaction, and Hydrogen Evolution Reaction.

Herein, we first propose a facile strategy to synthesize Co9S8 and MoS2 nanocrystals embedded in porous carbon nanoflake arrays supported on carbon nanofibers (Co9S8-MoS2/N-CNAs@CNFs) by the pyrolysis of Mo-doped Zn, Co zeolitic imidazolate framework grown on carbon nanofibers and subsequent sulfuration. The electrocatalyst shows high and stable electrocatalytic performance, with a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential at 10 mA cm-2 for oxygen evolution reaction (0.34 V) and hydrogen evolution reaction (0.163 V), which outperform the metal-organic framework-derived transition metal sulfide catalysts reported so far. Furthermore, the Co9S8-MoS2@N-CNAs@CNFs are employed as an air cathode in a liquid-state and all-solid-state zinc-air battery, presenting high power densities of 222 and 96 mW cm-2, respectively. Such excellent catalytic activities are mainly owing to the unique three-dimensional structure and chemical compositions, optimal electronic conductivity, adequate surface area, and the abundance of active sites. Thus, this work provides an important method for designing other metal-organic framework-derived three-dimensional structural sulfide quantum dot multifunctional electrocatalysts for wider application in highly efficient catalysis and energy storage.

Clean Syn-Fuels via Hydrogenation Processes: Acidity–Activity Relationship in O-Xylene Hydrotreating

Transition metal sulfide catalysts are actually the most performing catalytic materials in crude oil hydrotreating (HDT), for energetic purposes. However, these systems suffer from several drawbacks that limit their exploitation. Aiming to meet the even more stringent environmental requirement, through a remarkable improvement of HDT performance in the presence of refractory feedstock (i.e., in terms of activity, selectivity, and stability), a deeper knowledge of the structure–activity relationship of catalysts must be achieved. Therefore, in this study, CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts were characterized and tested in the o-xylene hydrogenation model reaction, assessing the influence of both support acidity and catalyst acid strength on reaction pathway by employing γ-Al2O3 and Y-Type zeolite as acid reference materials. A clear relationship between concentration and strength of acid sites and the performance of the catalytic materials was established. Cobalt based catalyst (CoMoSx) proves a higher acidic character with respect to Nickel (NiMoSx), prompting isomerization reactions preferentially, also reflecting a greater o-xylene conversion. The different chemical properties of metals also affect the catalytic pathway, leading on the CoMoSx system to the preferential formation of p-xylene isomer with respect to m-xylene.

The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO2 Reduction Conditions: An Experimental and Theoretical Study

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO2 reduction (CO2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO2R. On Ni-MoS2, a limitation is the kinetic barrier for the proton–electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO2R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.

Multifunctional (Fe0.5Ni0.5)S2 nanocrystal catalysts with high catalytic activities for reduction of I3− and electrochemical water splitting

It is important to synthesize environmentally friendly transition metal sulfide catalysts with superior performance for electrochemical water splitting and reduction of I3− in dye-sensitized solar cells (DSSCs). In this work, cubic (Fe0.5Ni0.5)S2 nanocrysals with a monodispersed size of ca. 10 nm were successfully prepared via a hot-injection reaction with a Schlenk line system. Because of the FeNi-oleate complex as transition metal source, no other by-products were produced. The (Fe0.5Ni0.5)S2 nanocrystal/carbon black composite with different nanocrystal contents was control fabricated. Because the composite possessed two merits, more catalytically active sites of nanocrystals and fast electron transfer of carbon black, it is a promising catalyst for electrochemical water splitting and reduction of I3− in DSSCs. As the counter electrode catalysts for reduction of I3−, DSSCs based on the composite with 57% (Fe0.5Ni0.5)S2 nanocrystal contents have a high power conversion efficiency of 6.71%, which was comparable to Pt-based DSSCs (7.05%). The electrochemical measurement showed that (Fe0.5Ni0.5)S2 nanocrystals had a good catalytic activity for reduction of I3−. As the catalytic electrode for hydrogen evolution reaction (HER), the composite electrode with 57% (Fe0.5Ni0.5)S2 nanocrystal contents displayed an overpotential of 250 mV to reach the current density of 10 mA cm−2 in alkaline solution. It retained good HER activity for 1000-cycle measurements. The density functional theory showed the free energy of hydrogen adsorbed on the Ni site near S defects was − 0.12 eV, which was smaller than that of the Fe site near S defects. So, the Ni site near S defects of (Fe0.5Ni0.5)S2 was the main catalytically active site for HER. Also, the (Fe0.5Ni0.5)S2 nanocrystals displayed good electrocatalytic activity for oxygen evolution reaction. This type of double metal sulfide with monodispersed size paves the way for new insight into earth-abundance catalysts for water splitting and I3− reduction.

Computational and experimental study of the second metal effect on the structure and properties of bi-metallic MeMoS-sites in transition metal sulfide catalysts

Bimetallic (Me)MoS2 (Me=Nb, Fe, Co, Ni) sulfide systems, both modified and non-modified by potassium, were investigated by the DFT method. Geometric and electronic structures of active sites were compared in terms of CO and hydride hydrogen adsorption. Most of the studied metals are electron density acceptors on the S-edge of the crystallite, which inhibits hydride hydrogen adsorption and deactivates the S-edge. The CO adsorption energy on the Me-edge decreased in the order Fe>Co>Ni. Computational data were verified by experimental tests in syngas conversion. Modification by potassium was shown to hinder σ-bonding of the adsorbed molecule and to stabilize hydride hydrogen.

Transformation of Lignin and Its Model Compounds into Value-Added Chemicals Using Sulfide Catalysts

Lignin is mainly composed of hydroxy-substituted or methoxylated phenyl propane structures and serves as the only renewable bulk feedstock in nature for producing aromatic chemicals. By using suitable catalysts, the long chain structures of lignin can be selectively broken down to obtain different target products. This has been regarded as an important approach for the comprehensive utilization of lignin. Due to the high activity of hydrodeoxygenation, transition metal sulfide catalysts have been used in lignin conversion in recent decades. In this review, the application of transition metal sulfide catalysts in the catalytic conversions of lignin and its model compounds are summarized. The active components, support materials, reaction conditions and reaction mechanism are presented in detail. The existing challenges of sulfide catalysts in the degradation of lignin are discussed. Finally, potential solutions and future trends of this field are presented.

Experimental and computational study of syngas and ethanol conversion mechanisms over K-modified transition metal sulfide catalysts

Catalysts with the common composition KCoMoS/C/Al2O3 were synthesized, characterized by physicochemical methods, and tested in the synthesis of alcohols from CO and H2. It was noted that carbon supported on alumina promoted alcohol synthesis by enhancing the CO conversion and the alcohols/hydrocarbons ratio in comparison to pure alumina. The addition of potassium to CoMoS/C/Al2O3 could substantially affect both the morphology of molybdenum disulfide crystallites and catalyst activity in the synthesis of alcohols. According to quantum chemical calculations, using the density functional theory approach, potassium donates electronic density onto the Co atoms of CoMoS phase active sites. Reduction of metal atoms of active sites leads to a decrease in Lewis acidity and in the CO and H2 adsorption energy as well as promoting the oxidative addition of H2. The influence of ethanol addition to synthesis gas on its conversion and the product composition was examined with a view to checking whether the alkyl intermediate interacted with CO in the presence of potassium. A sharp increase of CO conversion in the presence of ethanol was detected. A reaction network of CO conversion on KCoMoS catalyst active sites to various products depending on the catalyst composition and reaction conditions was suggested.

ChemInform Abstract: Recent Advances in Heterogeneous Catalysts for Bio‐Oil Upgrading via “ex situ Catalytic Fast Pyrolysis”: Catalyst Development Through the Study of Model Compounds

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