MoS2-supported gold nanoparticle for CO hydrogenation

Which Is Better for Hydrogen Evolution on Metal@MoS2 Heterostructures from a Theoretical Perspective: Single Atom or Monolayer?

Single atom (SA)- and monolayer (ML)-supported catalysts are two main technical routines to increase electrochemical catalytic performance and reduce cost. To date, it is still a debate which one is better for catalysis in experiments as both routines face a puzzling problem of searching for balance between stability and catalytic activity. Here, hydrogen evolution on two-dimensional 2H-MoS₂ with SA- and ML-adsorbed metal atoms (23 kinds in total) is taken as an example to solve this question by first-principles calculations. The thermodynamic stability during synthesis, in vacuum, and in electrochemical reaction conditions is determined to access the stability of MoS₂ loaded with single (M_S@MoS₂) and monolayer metal atoms (M_M@MoS₂). The realistic catalytic surfaces determined by surface Pourbaix diagrams, the free energy changes of hydrogen atoms at different coverages, and the exchange current densities are applied to determine hydrogen evolution reaction (HER) activity. The results show that all M_M@MoS₂ are much more stable than the corresponding M_S@MoS₂ as the metal-metal interaction in MLs could make the former structures more stable. In general, M_M@MoS₂ show higher hydrogen evolution activities than those of M_S@MoS₂. In detail, the exchange current densities of MoS₂ loaded by Pd ML and Au ML are 6.208, and 1.109 mA/cm^-2, respectively, which are comparable to Pt(111). Combining with small binding energies, the Pd and Au MLs are the most promising catalysts for hydrogen evolution. The purpose of this work is to highlight the advantages and disadvantages of SA- and ML-supported surfaces as HER catalysts and provide a fundamental standard for studying them.

Methanol carbonylation to acetaldehyde on Au particles supported by single-layer MoS2grown on silica

Homogenous single-layer MoS₂films coated with sub-single layer amounts of gold are found to isolate the reaction of methanol with carbon monoxide, the fundamental step toward higher alcohols, from an array of possible surface reactions. Active surfaces were prepared from homogenous single-layer MoS₂films coated with sub-single layer amounts of gold. These gold atoms formed clusters on the MoS₂surface. A gas mixture of carbon monoxide (CO) and methanol (CH₃OH) was partially converted to acetaldehyde (CH₃CHO) under mild process conditions (308 kPa and 393 K). This carbonylation of methanol to a C₂species is a critical step toward the formation of higher alcohols. Density functional theory modeling of critical steps of the catalytic process identify a viable reaction pathway. Imaging and spectroscopic methods revealed that the single layer of MoS₂facilitated formation of nanoscale gold islands, which appear to sinter through Ostwald ripening. The formation of acetaldehyde by the catalytic carbonylation of methanol over supported gold clusters is an important step toward realizing controlled production of useful molecules from low carbon-count precursors.

The band shifts in MoS2(0001) and WSe2(0001) induced by palladium adsorption

The band structures of the transition metal dichalcogenides (TMD's) 2H-MoS2(0001) and 2H-WSe2(0001), before and after palladium adsorption, were investigated through angle-resolved photoemission. Palladium adsorption on 2H-MoS2(0001) is seen to result in very different band shifts than seen for palladium on 2H-WSe2(0001). The angle resolved photoemission results of palladium adsorbed on WSe2(0001) indicate that palladium accepts electron density from substrate. The resulting band shift will lead to a decrease in the barriers to the hole injection. The opposite band shifts occur upon palladium adsorption between 2H-MoS2(0001). The overall trend is consistent with the deposition of other metals deposited on TMD's, except that for palladium adsorption on MoS2(0001), there is an increase in the MoS2(0001) substrate band gap with palladium adsorption, as is evident from the combination of photoemission and inverse photoemission.

Catalytic C2H2 synthesis via low temperature CO hydrogenation on defect-rich 2D-MoS2 and 2D-MoS2 decorated with Mo clusters

Rational design of novel catalytic materials used to synthesize storable fuels via the CO hydrogenation reaction has recently received considerable attention. In this work, defect poor and defect rich 2D-MoS₂ as well as 2D-MoS₂ decorated with Mo clusters are employed as catalysts for the generation of acetylene (C₂H₂) via the CO hydrogenation reaction. Temperature programmed desorption is used to study the interaction of CO and H₂ molecules with the MoS₂ surface as well as the formation of reaction products. The experiments indicate the presence of four CO adsorption sites below room temperature and a competitive adsorption between the CO and H₂ molecules. The investigations show that CO hydrogenation is not possible on defect poor MoS₂ at low temperatures. However, on defect rich 2D-MoS₂, small amounts of C₂H₂ are produced, which desorb from the surface at temperatures between 170 K and 250 K. A similar C₂H₂ signal is detected from defect poor 2D-MoS₂ decorated with Mo clusters, which indicates that low coordinated Mo atoms on 2D-MoS₂ are responsible for the formation of C₂H₂. Density functional theory investigations are performed to explore possible adsorption sites of CO and understand the formation mechanism of C₂H₂ on MoS₂ and Mo₇/MoS₂. The theoretical investigation indicates a strong binding of C₂H₂ on the Mo sites of MoS₂ preventing the direct desorption of C₂H₂ at low temperatures as observed experimentally. Instead, the theoretical results suggest that the experimental data are consistent with a mechanism in which CHO radical dimers lead to the formation of C₂H₂ that presents an exothermic desorption.

Role of H Transfer in the Gas-Phase Sulfidation Process of MoO3: A Quantum Molecular Dynamics Study

Layered transition metal dichalcogenide (TMDC) materials have received great attention because of their remarkable electronic, optical, and chemical properties. Among typical TMDC family members, monolayer MoS₂ has been considered a next-generation semiconducting material, primarily due to a higher carrier mobility and larger band gap. The key enabler to bring such a promising MoS₂ layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO₃ is a key elementary reaction, forming MoS₂ layers on a target substrate. Recent studies have proposed the use of gas-phase H₂S precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS₂ crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO₃ sulfidation by H₂S. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO₃ flake using a H₂S precursor. Our QMD results reveal that gas-phase H₂S molecules efficiently reduce and sulfidize MoO₃ through the following reaction steps: Initially, H transfer occurs from the H₂S molecule to low molecular weight Mo _xO _y clusters, sublimated from the MoO₃ flake, leading to the formation of molybdenum oxyhydride clusters as reaction intermediates. Next, two neighboring hydroxyl groups on the oxyhydride cluster preferentially react with each other, forming water molecules. The oxygen vacancy formed on the Mo-O-H cluster as a result of this dehydration reaction becomes the reaction site for subsequent sulfidation by H₂S that results in the formation of stable Mo-S bonds. The identification of this reaction pathway and Mo-O and Mo-O-H reaction intermediates from unbiased QMD simulations may be utilized to construct reactive force fields (ReaxFF) for multimillion-atom reactive MD simulations.

Hierarchical Heterostructures of Ultrasmall Fe2O3-Encapsulated MoS2/N-Graphene as an Effective Catalyst for Oxygen Reduction Reaction

In this study, a facile approach has been successfully applied to synthesize a hierarchical three-dimensional architecture of ultrasmall hematite nanoparticles homogeneously encapsulated in MoS₂/nitrogen-doped graphene nanosheets, as a novel non-Pt cathodic catalyst for oxygen reduction reaction in fuel cell applications. The intrinsic topological characteristics along with unique physicochemical properties allowed this catalyst to facilitate oxygen adsorption and sped up the reduction kinetics through fast heterogeneous decomposition of oxygen to final products. As a result, the catalyst exhibited outstanding catalytic performance with a high electron-transfer number of 3.91-3.96, which was comparable to that of the Pt/C product. Furthermore, its working stability with a retention of 96.1% after 30 000 s and excellent alcohol tolerance were found to be significantly better than those for the Pt/C product. This hybrid can be considered as a highly potential non-Pt catalyst for practical oxygen reduction reaction application in requirement of low cost, facile production, high catalytic behavior, and excellent stability.