Adsorption and dissociation of H2 and H2S on MoS2 and NiMoS catalysts

Theoretical study of the catalytic hydrodeoxygenation of furan, methylfuran and benzofurane on MoS2

Herein, we have studied the direct deoxygenation (DDO) (without prior hydrogenation) of furan, 2-methylfuran and benzofuran on the metal edge of MoS2 with a vacancy created under pressure of dihydrogen. For the three molecules, we found that the desorption of the water molecule for the regeneration of the vacancy is the most endothermic. Based on the thermodynamic and kinetic aspects, the reactivity order of the oxygenated compounds is furan ≈ 2-methylfuran > benzofuran, which is in agreement with literature. We present the key stages of the mechanisms and highlight the effects of substituents.

Unraveling Hydrogen Adsorption on Transition Metal-Doped [Mo3S13]2- Clusters: Insights from Density Functional Theory Calculations

Molecular and dissociative hydrogen adsorption of transition metal (TM)-doped [Mo3S13]2- atomic clusters were investigated using density functional theory calculations. The introduced TM dopants form stable bonds with S atoms, preserving the geometric structure. The S-TM-S bridging bond emerges as the most stable configuration. The preferred adsorption sites were found to be influenced by various factors, such as the relative electronegativity, coordination number, and charge of the TM atom. Notably, the presence of these TM atoms remarkably improved the hydrogen adsorption activity. The dissociation of a single hydrogen molecule on TM[Mo3S13]2- clusters (TM = Sc, Cr, Mn, Fe, Co, and Ni) is thermodynamically and kinetically favorable compared to their bare counterparts. The extent of favorability monotonically depends on the TM impurity, with a maximum activation barrier energy ranging from 0.62 to 1.58 eV, lower than that of the bare cluster (1.69 eV). Findings provide insights for experimental research on hydrogen adsorption using TM-doped molybdenum sulfide nanoclusters, with potential applications in the field of hydrogen energy.

The influence of activation atmosphere on the active phase and hydrotreating activity of LDH-based NiW pre-sulfurized catalysts

Activation atmosphere and temperature determined active phases of NiW pre-sulfurized hydrotreating catalysts prepared via a tetrathiotungstate intercalated NiAl layered double hydroxide.

Edge engineering on layered WS2 toward the electrocatalytic reduction of CO2: a first principles study

Transition-metal dichalcogenides (TMDCs) have been modified to show excellent electrocatalytic performance for the CO₂ reduction reaction (CO₂RR). However, little research has been reported on the edge modification of WS₂ and its electrocatalytic CO₂RR. In this work, the edge structure of WS₂ with W atoms exposed in the top layer was established by density functional theory calculations. Through using WS₂-xTM-y (x = 1, 2 or 3; y = 1 or 2; TM = Zn, Fe, Co or Ni) models by doping TM atoms on the top layer of WS₂, the effects of dopant species, doping concentration and adsorption sites on their electrocatalytic activity were investigated. Among the models, the active site for the CO₂RR is the W atoms. The doping of TM atoms would affect the bond strength between W and S atoms. After the doping of TM atoms in WS₂-2TM-1 ones, the electrical conduction of S atoms and the underlying W atoms can greatly be improved. Thus the catalytic activities can be significantly increased, in which the WS₂-2Zn-1 model shows the best catalytic activity. The limiting potential (U_L) of the CO₂RR to CO on the WS₂-2Zn-1 model is -0.51 V and the Gibbs energy change (ΔG) for the adsorption of intermediates on the WS₂-2Zn-1 model is ΔG(COOH*) = -0.37 and ΔG(CO*) = -0.51 eV, respectively. Solvation correction showed that WS₂-2Zn-1 could maintain good catalytic performance in a wide range of pH values. The present results may provide a theoretical basis for the design and synthesis of novel electrocatalysts with high performance for the CO₂RR.

High-Performance Room-Temperature Conductometric Gas Sensors: Materials and Strategies

Chemiresistive sensors have gained increasing interest in recent years due to the necessity of low-cost, effective, high-performance gas sensors to detect volatile organic compounds (VOC) and other harmful pollutants. While most of the gas sensing technologies rely on the use of high operation temperatures, which increase usage cost and decrease efficiency due to high power consumption, a particular subset of gas sensors can operate at room temperature (RT). Current approaches are aimed at the development of high-sensitivity and multiple-selectivity room-temperature sensors, where substantial research efforts have been conducted. However, fewer studies presents the specific mechanism of action on why those particular materials can work at room temperature and how to both enhance and optimize their RT performance. Herein, we present strategies to achieve RT gas sensing for various materials, such as metals and metal oxides (MOs), as well as some of the most promising candidates, such as polymers and hybrid composites. Finally, the future promising outlook on this technology is discussed.

DFT insights into the hydrodenitrogenation mechanism of quinoline catalyzed by different Ni-promoted MoS2 edge sites: Effect of the active phase morphology

Density functional theory calculations are performed to investigate the hydrodenitrogenation (HDN) mechanism of quinoline over different Ni-promoted MoS₂ edges. Based on the calculations, the hydrogenation and ring-opening reaction pathways are explored systematically, and the structure-activity relationship of different active sites is discussed in detail. In the hydrogenation reaction process, the 100% Ni-promoted M-edge and 50% Ni-promoted S-edge are favorable for the formations of 5,6,7,8-tetrahydroquinoline and 1,2,3,4-tetrahydroquinoline, respectively. Furthermore, the 100% Ni-promoted M-edge is more preferable for the generation of decahydroquinoline rather than the 50% Ni-promoted S-edge. In the denitrogenation reaction step, the 100% Ni-promoted M-edge is beneficial for the formation of ortho-propylaniline and 2-propylcyclohexylamine, while 50% Ni-promoted S-edge is only conducive to the formation of 2-propylcyclohexylamine. Therefore, it can be concluded that both hydrogenation derivatives and denitrogenation products exhibit strong dependence on the active phase morphology, meaning that multiple active sites can be involved in one catalytic HDN cycle.

Computational Screening toward Hydrogen Evolution Reaction by the Introduction of Point Defects at the Edges of Group IVA Monochalcogenides: A First-Principles Study

Exploring materials with high hydrogen evolution reaction (HER) performance is of importance for the development of clean hydrogen energy, and the defects on the surfaces of catalysts are essential. In this work, we evaluate the HER performance among group IVA monochalcogenides MXs (M = Ge/Sn, X = S/Se) with M/X point defects on the edges. Compared with basal planes and bare edges, the GeS edge with Ge vacancy (ΔG_H* = 0.016 eV), GeSe edge with Se vacancy (ΔG_H* = 0.073 eV), and SnSe edge with Sn vacancy (ΔG_H* = -0.037 eV) hold the best HER performances, which are comparable to or even better than the value for Pt (-0.07 eV). Furthermore, the relationships between ΔG_H* and p-band centers of considered models are summarized. The stability of proposed electrocatalysts are analyzed by vacancy-formation energy and strain engineering. In summary, the HER performance of MXs is greatly improved by introduction of point defects at the edges, which is promising for their use as electrocatalysts for the conversion and storage of energy in the future.

Hydrogen Sensors Based on MoS2 Hollow Architectures Assembled by Pickering Emulsion

For rapid hydrogen gas (H₂) sensing, we propose the facile synthesis of the hollow structure of Pt-decorated molybdenum disulfide (h-MoS₂/Pt) using ultrathin (mono- or few-layer) two-dimensional nanosheets. The controlled amphiphilic nature of MoS₂ surface produces ultrathin MoS₂ NS-covered polystyrene particles via one-step Pickering emulsification. The incorporation of Pt nanoparticles (NPs) on the MoS₂, followed by pyrolysis, generates the highly porous h-MoS₂/Pt. This hollow hybrid structure produces sufficiently permeable pathways for H₂ and maximizes the active sites of MoS₂, while the Pt NPs on the hollow MoS₂ induce catalytic H₂ spillover during H₂ sensing. The h-MoS₂/Pt-based chemiresistors show sensitive H₂ sensing performances with fast sensing speed (response, 8.1 s for 1% of H₂ and 2.7 s for 4%; and recovery, 16.0 s for both 1% and 4% H₂ at room temperature in the air). These results mark the highest H₂ sensing speed among 2D material-based H₂ sensors operated at room temperature in air. Our fabrication method of h-MoS₂/Pt structure through Pickering emulsion provides a versatile platform applicable to various 2D material-based hollow structures and facilitates their use in other applications involving surface reactions.

Design of high-performance MoS2 edge supported single-metal atom bifunctional catalysts for overall water splitting via a simple equation

MoS₂ edges exhibit good hydrogen evolution reaction (HER) activity but poor oxygen evolution reaction (OER) activity. The development of MoS₂ edge supported single-atom catalysts (SACs) for both the HER and OER is critical for overall water splitting. In this work, for the purpose of triggering OER performance and maintaining HER performance, 28 single transition-metal (TM) SACs supported on MoS₂ edges as bifunctional electrocatalysts for overall water splitting have been screened by using density functional theory (DFT) calculations. In order to design and achieve high OER performance, a simple equation derived from the chemical environment and local structure of the active center is used as a structure descriptor to predict the OER activities of MoS₂-based SACs. Among these candidates, the T1-vacancy termination modified using a Pt single atom shows the lowest theoretical overpotential for the hydrogen/oxygen evolution reaction being just -0.10/0.46 V, respectively, which is comparable to those of the precious-metal-group benchmark catalysts for overall water splitting. It is expected that our results can offer a theoretical basis for simplifying and steering the design of efficient electrocatalytic materials.

Quantum mechanistic study of furan and 2-methylfuran hydrodeoxygenation on molybdenum and tungsten sulfide clusters

One of the possibilities of limiting carbon dioxide emissions is to use pyrolysis oils from biomass. However, their very high oxygen content confers to these oils a chemical instability and a high viscosity. Among the oxygen-containing compounds present in bio-oils, furanic compounds derived from the decomposition of cellulosic and hemi-cellulosic biomass are the most refractory to deoxygenation. The major products of their hydrodeoxygenation are alkanes and secondly alkenes, but the intermediates are still subject to controversy. In this work, we performed a DFT simulation of the hydrodeoxygenation of furan (C₄H₄O) and 2-methylfuran in the presence of molybdenum and tungsten sulphide Mo(W)S_2. The aim of this work is to elucidate the reaction intermediates and to compare the activities of the two catalytic sites used in our reaction conditions. Our calculations show that the partial hydrogenation of the two molecules occurs preferentially in position (2,5). The hydrogenolysis reactions of the C-O bonds occur in two steps. The molybdenum sulphide exhibits higher catalytic activity.

Effect of 2,6-Bis-(1-hydroxy-1,1-diphenyl-methyl) Pyridine as Organic Additive in Sulfide NiMoP/γ-Al₂O₃ Catalyst for Hydrodesulfurization of Straight-Run Gas Oil

The effect of 2,6-bis-(1-hydroxy-1,1-diphenyl-methyl) pyridine (BDPHP) in the preparation of NiMoP/γ-Al₂O₃ catalysts have been investigated in the hydrodesulfurization (HDS) of straight-run gas oil. The γ-Al₂O₃ support was modified by surface impregnation of a solution of BDPHP to afford BDPHP/Ni molar ratios (0.5 and 1.0) in the final composition. The highest activity for NiMoP materials was found when the molar ratio of BDPHP/Ni was of 0.5. X-ray diffraction (XRD) results revealed that NiMoP (0.5) showed better dispersion of MoO₃ than the NiMoP (1.0). Fourier transform infrared spectroscopy (FT-IR) results indicated that the organic additive interacts with the γ-Al₂O₃ surface and therefore discards the presence of Mo or Ni complexes. Raman spectroscopy suggested a high Raman ratio for the NiMoP (0.5) sample. The increment of the Mo=O species is related to a major availability of Mo species in the formation of MoS₂. The temperature programmed reduction (TPR) results showed that the NiMoP (0.5) displayed moderate metal–support interaction. Likewise, X-ray photoelectron spectroscopy (XPS) exhibited higher sulfurization degree for NiMoP (0.5) compared with NiMoP (1.0). The increment of the MoO₃ dispersion, the moderate metal–support interaction, the increase of sulfurization degree and the increment of Mo=O species provoked by the BDPHP incorporation resulted in a higher gas oil HDS activity.

The effect of cobalt promoter on the CO methanation reaction over MoS2 catalyst: a density functional study

The potential mechanism of sulfur-resistant CO methanation reaction over Co-MoS₂ catalyst was investigated via density functional theory (DFT + D) calculations, and the effect of Co-promoter was studied.

Insight into the effect of non-stoichiometric sulfur on a NiMoS hydrodesulfurization catalyst

A sulfur dynamic equilibrium between the NiMoS edge and the gas phase which determines the number of CUS, SH groups and HDS activity is elucidated.

Band structure engineering of monolayer MoS2: a charge compensated codoping strategy

The monolayer MoS₂, possessing an advantage over graphene in that it exhibits a band gap whose magnitude is appropriate for solar applications, has attracted increasing attention because of its possible use as a photocatalyst.

Rational design of MoS2 catalysts: tuning the structure and activity via transition metal doping

Density functional theory is used to elucidate and understand the trends in hydrogen evolution activity of transition-metal doped MoS₂ catalysts.

Initial hydrogenations of pyridine on MoP(001): a density functional study

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(β),C(β),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(β) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.

Recent density functional studies of hydrodesulfurization catalysts: insight into structure and mechanism

The present article will highlight some recent density functional theory (DFT) studies of hydrodesulfurization (HDS) catalysts. It will be summarized how DFT in combination with experimental studies can give a detailed picture of the structure of the active phase. Furthermore, we have used DFT to investigate the reaction pathway for thiophene HDS, and we find that the reaction entails a complex interplay of different active sites, depending on reaction conditions. An investigation of pyridine inhibition confirmed some of these results. These fundamental insights constitute a basis for rational improvement of HDS catalysts, as they have provided important structure-activity relationships.

Catalytic Hydrodeoxygenation of Methyl-Substituted Phenols: Correlations of Kinetic Parameters with Molecular Properties

The hydrodeoxygenation of methyl-substituted phenols was carried out in a flow microreactor at 300 degrees C and 2.85 MPa hydrogen pressure over a sulfided CoMo/Al(2)O(3) catalyst. The primary reaction products were methyl-substituted benzene, cyclohexene, cyclohexane, and H(2)O. Analysis of the results suggests that two independent reaction paths are operative, one leading to aromatics and the other to partially or completely hydrogenated cyclohexanes. The reaction data were analyzed using Langmuir-Hinshelwood kinetics to extract the values of the reactant-to-catalyst adsorption constant and of the rate constants characterizing the two reaction paths. The adsorption constant was found to be the same for both reactions, suggesting that a single catalytic site center is operative in both reactions. Ab initio electronic structure calculations were used to evaluate the electrostatic potentials and valence orbital ionization potentials for all of the substituted phenol reactants. Correlations were observed between (a) the adsorption constant and the two reaction rate constants measured for various methyl-substitutions and (b) certain moments of the electrostatic potentials and certain orbitals' ionization potentials of the isolated phenol molecules. On the basis of these correlations to intrinsic reactant-molecule properties, a reaction mechanism is proposed for each pathway, and it is suggested that the dependencies of adsorption and reaction rates upon methyl-group substitution are a result of the substituents' effects on the electrostatic potential and orbitals rather than geometric (steric) effects.