Reaction Mechanism and Strategy for Optimizing the Hydrogen Evolution Reaction on Single-Layer 1T' WSe2 and WTe2 Based on Grand Canonical Potential Kinetics

Reaction Mechanism of Rapid CO Electroreduction to Propylene and Cyclopropane (C3+) over Triple Atom Catalysts

The carbon monoxide reduction reaction (CORR) toward C2+ and C3+ products such as propylene and cyclopropane can not only reduce anthropogenic emissions of CO and CO2 but also produce value-added organic chemicals for polymer and pharmaceutical industries. Here, we introduce the concept of triple atom catalysts (TACs) that have three intrinsically strained and active metal centers for reducing CO to C3+ products. We applied grand canonical potential kinetics (GCP-K) to screen 12 transition metals (M) supported by nitrogen-doped graphene denoted as M3N7, where M stands for Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au. We sought catalysts with favorable CO binding, hydrogen binding, and C-C dimerization energetics, identifying Fe3N7 and Ir3N7 as the best candidates. We then studied the entire reaction mechanism from CO to C3H6 and C2H4 as a function of applied potential via, respectively, 12-electron and 8-electron transfer pathways on Fe3N7 and Ir3N7. Density functional theory (DFT) predicts an overpotential of 0.17 VRHE for Fe3N7 toward propylene and an overpotential of 0.42 VRHE toward cyclopropane at 298.15 K and pH = 7. Also, DFT predicts an overpotential of 0.15 VRHE for Ir3N7 toward ethylene. This work provides fundamental insights into the design of advanced catalysts for C2+ and C3+ synthesis at room temperature.

Rational Design of Phase-Engineered WS2/WSe2 Heterostructures by Low-Temperature Plasma-Assisted Sulfurization and Selenization toward Enhanced HER Performance

Efficient hydrogen generation from water splitting underpins chemistry to realize hydrogen economy. The electrocatalytic activity can be effectively modified by two-dimensional (2D) heterostructures, which offer great flexibility. Furthermore, they are useful in enhancing the exposure of the active sites for the hydrogen evolution reaction. Although the 1T-metallic phase of the transition metal dichalcogenides (TMDs) is important for the hydrogen evolution reaction (HER) catalyst, its practical application has not yet been much utilized because of the lack of stability of the 1T phase. Here, we introduce a novel approach to create a 1T-WS2/1T-WSe2 heterostructure using a low-temperature plasma-assisted chemical vapor reaction (PACVR), namely plasma-assisted sulfurization and plasma-assisted selenization processes. This heterostructure exhibits superior electrocatalytic performance due to the presence of the metallic 1T phase and the beneficial synergistic effect at the interface, which is attributed to the transfer of electrons from the underlying WS2 layer to the overlying WSe2 layer. The WS2/WSe2 heterostructure catalyst demonstrates remarkable performance in the HER as evidenced by its small Tafel slope of 57 mV dec-1 and exceptional durability. The usage of plasma helps in replacing the top S atoms with Se atoms, and this ion bombardment also increases the roughness of the thin film, thus adding another factor to enhance the HER performance. This plasma-synthesized low-temperature metallic-phase heterostructure brings out a novel method for the discovery of other catalysts.

Vertical graphene nanowalls supported hybrid W2C/WOx composite material as an efficient non-noble metal electrocatalyst for hydrogen evolution

Research for the development of noble metal-free electrodes for hydrogen evolution has blossomed in recent years. Transition metal carbides compounds, such as W2C, have been considered as a promising alternative to replace Pt-family metals as electrocatalysts towards hydrogen evolution reaction (HER). Moreover, hybridization of TMCs with graphene nanostructures has emerged as a reliable strategy for the preparation of compounds with high surface to volume ratio and abundant active sites. The present study focuses in the preparation of tungsten carbide/oxide compounds deposited in a three-dimensional vertical graphene nanowalls (VGNW) substrate via chemical vapor deposition, magnetron sputtering and thermal annealing processes. Structural and chemical characterization reveals the partial carburization and oxidation of the W film sputtered on the VGNWs, due to C and O migration from VGNWs towards W during the high temperature annealing process. Electrochemical characterization shows the enhanced performance of the nanostructured hybrid W2C/WOx on VGNW compound towards HER, when compared with planar W2C/WOx films. The W2C/WOx nanoparticles on VGNWs require an overpotential of -252 mV for the generation of 10 mA cm-2. Chronoamperometry tests in high overpotentials reveal the compounds stability while sustaining high currents, in the order of hundreds of mA. Post-chronoamperometry test XPS characterization unveils the formation of a W hydroxide layer which favours hydrogen evolution in acidic electrolytes. We aspire that the presented insights can be valuable for those working on the preparation of hybrid electrodes for electrochemical processes.

The concerted proton-electron transfer mechanism of proton migration in the electrochemical interface

The proton migration in the electrochemical interface is a fundamental electrochemical processes in proton involved reactions. We find fractional electron transfer, which is inversely proportional to the distance between the proton and electrode, during the proton migration under constant potential. The electrical energy carried by the transferred charge facilitates the proton to overcome the chemical barrier in the migration pathway, which is accounting for more than half electrical energy in the proton involved reactions. Consequently, less charge transfer and energy exchange take place in the reduction process. Therefore, the proton migration in the electrochemical interface is an essential component of the electrochemical reaction in terms of electron transfer and energy conversation, and are worthy of more attention in the rational design and optimization of electrochemical systems.

CO2 Reduction to Methane and Ethylene on a Single-Atom Catalyst: A Grand Canonical Quantum Mechanics Study

In recent years, two-dimensional metal-organic frameworks (2D MOF) have attracted great interest for their ease of synthesis and for their catalytic activities and semiconducting properties. The appeal of these materials is that they are layered and easily exfoliated to obtain a monolayer (or few layer) material with interesting optoelectronic properties. Moreover, they have great potential for CO2 reduction to obtain solar fuels with more than one carbon atom, such as ethylene and ethanol, in addition to methane and methanol. In this paper, we explore how a particular class of 2D MOF based on a phthalocyanine core provides the reactive center for the production of ethylene and ethanol. We examine the reaction mechanism using the new grand canonical potential kinetics (GCP-K) or grand canonical quantum mechanics (GC-QM) computational methodology, which obtains reaction rates at constant applied potential to compare directly with experimental results (rather than at constant electrons as in standard QM). We explain the reaction mechanism underlying the formation of methane and ethylene. Here, the key reaction step is direct coupling of CO into CHO, without the usual rate-determining CO-CO dimerization step observed on Cu metal surfaces. Indeed, the 2D MOF behaves like a single-atom catalyst.

Electrocatalytic Reduction of N2 to NH3 Over Defective 1T'-WX2 (X=S, Se, Te) Monolayers

Defects in transition metal dichalcogenides (TMDs) can serve as active sites in catalytic reactions. In this work, by means of first-principles calculations, the catalytic activities of WX₂ (X=S, Se, Te) monolayers in the 1T' phase with both vacancy defects (missing chalcogen atoms, ^X V_d ) and antisite defects (replacing chalcogen atoms with W atoms, ^X A_d ) were evaluated for the nitrogen reduction reaction (NRR). Results showed that all these defective catalysts had great potential toward electrocatalytic ammonia synthesis by exhibiting low limiting potentials (U_L ). Over 1T'-WTe₂ @^Te V_d , 1T'-WS₂ @^S A_d , 1T'-WSe₂ @^Se A_d , and 1T'-WTe₂ @^Te A_d , the corresponding U_L values were -0.49, -0.21, -0.19, and -0.15 V, much smaller than that of the benchmark catalyst, the Ru (0001) surface (U_L =-0.98 V). Furthermore, the hydrogen evolution reaction (HER) was inhibited. 1T'-WX₂ monolayers with the antisite defects showed better NRR activity than those with the vacancy defects because of the smaller steric hindrance at the former. Results suggest that the steric effect at the active surface sites should be utilized to develop better catalysts.