Monthly Archives: September 2023

Hydrogen‐induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations

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Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations

Electronic states on the MoS2 basal plane due to the formation of sulfur vacancies while annealing in hydrogen are revealed using ambient pressure XPS and DFT calculations. The XPS spectra shows the development of new components in Mo 3d due to the formation of sulfur vacancies with increasing temperature. The DFT calculations with appropriate vacancy models reproduce the core-level shifts.


Abstract

Sulfur vacancy on an MoS2 basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS2 basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.

Research Progress on Direct Contact Evaporation Heat Transfer between Two Immiscible Working Fluids

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Research Progress on Direct Contact Evaporation Heat Transfer between Two Immiscible Working Fluids

Simulation methods, experimental results, and enhancement pathways for direct contact vaporization heat transfer of organic workpieces in the past fifteen years are summarized, focusing on the dispersed phase of organic working fluids and the continuous phase of immiscible inorganic fluids. The changes in droplet population state and methods of enhanced heat transfer are discussed.


Abstract

The direct contact evaporation heat transfer of two immiscible fluids can realize low-temperature differential heat transfer, which is an important method to improve the heat transfer coefficient. This article focuses on the dispersed phase of organic working fluids and the continuous phase of immiscible inorganic fluids and summarizes the research on direct contact evaporation heat transfer in the past 15 years. The main research methods include theoretical analysis, numerical models, and experimental research. The evaporation process of a single droplet mainly involves bubble droplet growth, droplet configuration and discriminant, droplet shape change, rising speed and path, etc. In practice, dispersed phases mostly exist in populations, and the research mainly focuses on collisions, coalescence, and rupture between droplets and bubbles, as well as on the number and size distribution. The volumetric heat transfer coefficient is used to reflect the heat transfer capacity of the heat exchanger. The factors affecting evaporation heat transfer performance are complex, and increasing the uniform mixing of bubbles is an important method to improve the heat transfer coefficient. In the future, direct contact evaporation heat transfer is expected to be promoted to multiple fields.

Flow‐Integrated Preparation of Norbornadiene Precursors for Solar Thermal Energy Storage

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Molecular solar thermal (MOST) energy storage systems are getting increased attention related to renewable energy storage applications. Particularly, 2,3-difunctionalized norbornadiene-quadricyclane (NBD-QC) switches bearing a nitrile (CN) group as one of the two substituents are investigated as promising MOST candidates thanks to their high energy storage densities and their red-shifted absorbance. Moreover, such NBD systems can be prepared in large quantities (a requirement for MOST-device applications) in flow through Diels-Alder reaction between cyclopentadiene and appropriately functionalized propynenitriles. However, these acetylene precursors are traditionally prepared in batch from their corresponding acetophenones using reactive chemicals potentially leading to health and physical hazards, especially when working on a several-grams scale. Here, we develop a multistep flow-chemistry route to enhance the production of these crucial precursors. Furthermore, we assess the atom economy (AE) and the E-factor showing improved green metrics compared to classical batch methods. Our results pave the way for a complete flow synthesis of NBDs with a positive impact on green chemistry aspects.

Mechanisms of CH4 activation over oxygen‐preadsorbed transition metals by ReaxFF and AIMD simulations

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Mechanisms of CH4 activation over oxygen-preadsorbed transition metals by ReaxFF and AIMD simulations

Methane catalytic oxidation has been extensively studied for its huge potential for energy and industrial synthesis. Oxygen atoms adsorbed on transition metal surfaces often change the catalytic activity to activate methane. The dynamic processes of methane oxidation are vital for the understanding of the effects of pre-adsorbed oxygen atoms on transition metal surfaces. This article reports the CH4 dissociation pathways on Au, Cu, Ni, Pt, and Pd surfaces using reactive force field molecular dynamic simulations.


Abstract

The chemisorbed oxygen usually promotes the CH bond activation over less active metals like IB group metals but has no effect or even an inhibition effect over more active metals like Pd based on the static electronic structure study. However, the understanding in terms of dynamics knowledge is far from complete. In the present work, methane dissociation on the oxygen-preadsorbed transition metals including Au, Cu, Ni, Pt, and Pd is systemically studied by reactive force field (ReaxFF). The ReaxFF simulation results indicate that CH4 molecules mainly undergo the direct dissociation on Ni, Pt, and Pd surfaces, while undergo the oxygen-assisted dissociation on Au and Cu surfaces. Additionally, the ab initio molecular dynamics (AIMD) simulations with the umbrella sampling are employed to study the free-energy changes of CH4 dissociation, and the results further support the CH4 dissociation pathway during the ReaxFF simulations. The present results based on ReaxFF and AIMD will provide a deeper dynamic understanding of the effects of pre-adsorbed oxygen species on the CH bond activation compared to that of static DFT.

Synthesis of Chrysanthemum‐Like BiOBr Microspheres by Structure Induction of Chitosan for Enhancing Photocatalytic Activity

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Synthesis of Chrysanthemum-Like BiOBr Microspheres by Structure Induction of Chitosan for Enhancing Photocatalytic Activity

Chrysanthemum-like BiOBr microspheres (CLB0.45) were designed and prepared by using chitosan (CS) as a soft template/structure inducer. CLB0.45 demonstrated efficient performance for the degradation of organic pollutants (dyes and antibiotics) due to suitable lattice plane exposure and band gap. The material also exhibited good cyclic stability.


Abstract

Photocatalysis is a green technology with important application prospects in environmental fields such as wastewater treatment. Herein, we synthesized chrysanthemum-like BiOBr microspheres (CLB0.45) by introduction of structure guide and soft-template. Natural biomass chitosan (CS) in the precursor of BiOBr successfully suppressed the growth of both (001) and (102) lattice planes through the solvent effect, and promoted the exposure of (110) lattice planes that dominated the photocatalytic performance. Significantly, the polymer chains regulated the nanosheets of BiOBr to accurately self-assemble. The obtained CLB0.45 had narrower band gap and more active sites due to surface defects, and the photocatalysts have higher photo-electron hole pairs separation efficiency. Additionally, the degradation performance towards rhodamine B (RhB; 99.7 %, 30 min) of the CLB0.45 was increased, which was 4.6 times higher than that of pure BiOBr. The degradation rate of tetracycline (TC) was also excellent (85.3 %). The photodegradation mechanism of CLB0.45 was proposed and verified. In summary, the prepared CLB0.45, obtained through structure guiding and using a soft template, have a promising future in the photocatalytic purification of organic pollutants.

Ammonia Synthesis on Ternary LaSi‐based Electrides: Tuning the Catalytic Mechanism by the Third Metal

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Ammonia Synthesis on Ternary LaSi-based Electrides: Tuning the Catalytic Mechanism by the Third Metal

La−TM−Si electrides catalysts for ammonia synthesis were compared and different catalytic mechanisms were shown for LaFe/CoSi and LaMnSi. A dual-site relay catalytic mechanism was demonstrated for LaCoSi and LaFeSi, breaking the scaling relations. In contrast, all the elementary steps were confined to Mn sites on LaMnSi, which resulted in inferior catalytic activity.


Abstract

Intermetallic electrides have recently drawn considerable attention due to their unique electronic structure and high catalytic performance for the activation of inert chemical bonds under mild conditions. However, the relationship between electride (anionic) electron abundance and catalytic performance is undefined; the key deciding factor for the performance of intermetallic electride catalysts remains to be addressed. Here, the secret behind electride catalysts La−TM−Si (TM=Co, Fe and Mn) with the same crystal structure but different anionic electrons was studied. Unexpectedly, LaCoSi with the least anionic electrons showed the best catalytic activity. The experiments and first-principles calculations showed that the electride anions promote the N2 dissociation which alters the rate-determining step (RDS) for ammonia synthesis on the studied electrides. Different reaction mechanisms were found for La−TM−Si (TM=Fe, Co) and LaMnSi. A dual-site module was revealed for LaCoSi and LaFeSi, in which transition metals were available for the N2 dissociation and La accelerates the NH x formation, respectively, breaking the Sabatier scaling relation. For LaMnSi, which is the most efficient for the N2 activation, the activity for ammonia synthesis is limited and confined by the scaling relations. The findings provide new insight into the working mechanism of intermetallic electrides.

Rhodium‐Catalysed Selective C−H Allylation of 1H‐Indazoles with Vinylethylene Carbonate: Easily Introducing Allylic Alcohol

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Rhodium-Catalysed Selective C−H Allylation of 1H-Indazoles with Vinylethylene Carbonate: Easily Introducing Allylic Alcohol

An efficient rhodium(III)-catalyzed C−H bond activation/allylation reaction of 3-aryl-1H-indazoles with easily available vinylethylene carbonate has been reported. A series of allyl alcohol substituted 3-aryl-1H-indazoles were obtained.


Abstract

An efficient rhodium(III)-catalysed C−H activation of 3-aryl-1-H-indazoles with easily available vinylethylene carbonate has been reported. A series of allyl alcohol substituted 3-aryl-1-H-indazoles were obtained with broad functional groups tolerance and favourable stereoselectivity. Notably, C−H and C−O bonds were selectively activated in “one pot” manner, releasing CO2 as the sole by-product and avoiding external oxidant. This protocol provides a powerful approach for the post stage C−H allylation of indazole-based substrates.

Electrochemical C7‐Indole Alkenylation via Rhodium Catalysis

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Electrochemical C7-Indole Alkenylation via Rhodium Catalysis

The merger of rhodium catalysis and electrochemical synthesis enabled the exclusive access to the C7−H electro-alkenylation of indoles.


Abstract

Indole derivatives are fundamental structural units in many bioactive compounds and molecular materials. The site-selective C7-functionalization of these moieties has been proven to be extremely challenging due to the inherent reactivity of the C2- and C3-positions. Herein, we report the first electro-C7-alkenylation of indoles. This novel and sustainable methodology provides highly exclusive access to the C7-position devoid of often toxic and expensive chemical oxidants. Moreover, an array of substrates was successfully alkenylated at the C7-position, and versatile product diversification was achieved.

Structural Modification of the Natural Product Valerenic Acid Tunes RXR Homodimer Agonism

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Structural Modification of the Natural Product Valerenic Acid Tunes RXR Homodimer Agonism

Valerenic acid is an RXR agonist with unique subtype and homodimer preference. We have studied the impact of structural modification of the natural product on RXR modulation and identified an analogue exhibiting enhanced and selective RXR homodimer activation.


Abstract

Retinoid X receptors (RXR) are ligand-sensing transcription factors with a unique role in nuclear receptor signaling as universal heterodimer partners. RXR modulation holds potential in cancer, neurodegeneration and metabolic diseases but adverse effects of RXR activation and lack of selective modulators prevent further exploration as therapeutic target. The natural product valerenic acid has been discovered as RXR agonist with unprecedented preference for RXR subtype and homodimer activation. To capture structural determinants of this activity profile and identify potential for optimization, we have studied effects of structural modification of the natural product on RXR modulation and identified an analogue with enhanced RXR homodimer agonism.