Unraveling the oxide layer on Mo2C as the active center for hydrogen evolution reaction

First-Principles Insights into Tungsten Semicarbide-Based Single-Atom Catalysts: Single-Atom Migration and Mechanisms in Oxygen Reduction

Understanding the structural evolution of single-atom catalysts (SACs) in catalytic reactions is crucial for unraveling their catalytic mechanisms. In this study, we utilize density functional theory calculations to delve into the active phase evolution and the oxygen reduction reaction (ORR) mechanism of tungsten semicarbide-based transition metal SACs (TM1/W2C). The stable crystal phases and optimal surface exposures of W2C are identified by using ab initio atomistic thermodynamics simulations. Focusing on the W-terminated (001) surface, we screen 13 stable TM1/W2C variants, ultimately selecting Pt1/W2C(001) as our primary model. The surface Pourbaix diagram, mapped for this model under ORR conditions, reveals dynamic Pt1 migration on the surface, triggered by surface oxidation. This discovery suggests a novel single-atom evolution pathway. Remarkably, this single-atom migration behavior is also discerned in seven other group VIII SACs, enhancing both their catalytic activity and their stability. Our findings offer insights into the evolution of active phases in SACs, considering substrate structural arrangement, single-atom incorporation, and self-optimization of catalysts under various conditions.

Current Progress of Mo-Based Metal Organic Frameworks Derived Electrocatalysts for Hydrogen Evolution Reaction

As an important half-reaction for electrochemical water splitting, electrocatalytic hydrogen evolution reaction suffers from sluggish kinetics, and it is still urgent to search high efficiency non-platinum-based electrocatalysts. Mo-based catalysts such as Mo2 C, MoO2 , MoP, MoS2 , and MoNx have emerged as promising alternatives to Pt/C owing to their similar electronic structure with Pt and abundant reserve of Mo. On the other hand, due to the adjustable topology, porosity, and nanostructure of metal organic frameworks (MOFs), MOFs are extensively used as precursors to prepare nano-electrocatalysts. In this review, for the first time, the progress of Mo-MOFs-derived electrocatalysts for hydrogen evolution reaction is summarized. The preparation method, structures, and catalytic performance of the catalysts are illustrated based on the types of the derived electrocatalysts including Mo2 C, MoO2 , MoP, MoS2 , and MoNx . Especially, the commonly used strategies to improve catalytic performance such as heteroatoms doping, constructing heterogeneous structure, and composited with noble metal are discussed. Moreover, the opportunities and challenges in this area are proposed to guide the designment and development of Mo-based MOF derived electrocatalysts.

A Ni-doped Mo2C/NCF composite for efficient electrocatalytic hydrogen evolution

Ni-Mo₂C nano catalysts dispersed on N-doped carbon flowers: a composite with nitrogen-containing carbon flowers carrying nickel-modified molybdenum carbide exhibits enhanced HER catalytic activity in alkaline electrolyte.

Soft-template derived Ni/Mo2C hetero-sheet arrays for large current density hydrogen evolution reaction

Practical structural design and electronic regulation are significant for synthesising efficient electrocatalysts. Therefore, a facile soft-template approach has been applied to successfully grow Ni/Mo₂C heterojunction nanosheet arrays on nickel foam (NF) skeleton (NS-Ni/Mo₂C@NF) using polyvinylpyrrolidone (PVP) as a soft template. The density functional theory (DFT) calculations reveal that abundant Ni/Mo₂C heterojunction in NS-Ni/Mo₂C@NF can provide many active sites with a moderate hydrogen adsorption free energy (ΔG_H*, 0.037 eV). Benefiting from this nanosheet array structure and abundant Ni/Mo₂C heterojunctions, the NS-Ni/Mo₂C@NF catalyst can efficiently catalyze HER, especially at large current densities. As a result, only 151 and 271 mV overpotentials are needed to deliver 100 and 1000 mA/cm² HER, respectively. More importantly, the hydrogen production testing with NS-Ni/Mo₂C@NF as the working electrode can run stably for 500 h without activity decay under the current density of 500 mA/cm² commonly used in industrial water electrolyzers, indicating that NS-Ni/Mo₂C@NF has broad application prospects.

A new MoCN monolayer containing stable cyano structural units as a high-efficiency catalyst for the hydrogen evolution reaction

In the hydrogen evolution reaction (HER), it is essential to find a high-efficiency and nonprecious electrocatalyst comparable to Pt, which needs to have rich inherently active sites and good conductivity. By combining a global minimum structure search and first-principles calculations, a hitherto unknown 2D MoCN monolayer was found, which can be considered as a structure in which Mo atoms interact with the stable CN units through triple bonds. The resultant MoCN monolayer possesses superior thermodynamic, dynamic, thermal, and mechanical stabilities, as well as inherent metallicity. In particular, it can exhibit outstanding HER catalytic activity due to the presence of many active sites with near-zero ΔG_H* values, whose density totals 1.80 × 10¹⁵ sites per cm², even more than Pt. In addition, we also propose a series of other 2D monolayers containing stable CN units (i.e., MoC₂N, MoCN₂ and MoC₂N₂), all of which can uniformly show high stability and good HER catalytic activity. Applying strain can further effectively improve the activities of C-rich (MoC₂N) and N-rich (MoCN₂) monolayers, inducing considerably high HER catalytic performance. For the MoCN, MoC₂N and MoCN₂ monolayers, the most active sites are located at the Mo-C-N chain involved. All these fascinating findings can not only provide new excellent candidates but also new insights into the design of highly efficient and nonprecious HER electrocatalysts as an alternative to Pt in the near future.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Investigation of Deoxidation Process of MoO3 Using Environmental TEM

In situ environmental transmission electron microscope (ETEM) could provide intuitive and solid proof for the local structure and chemical evolution of materials under practical working conditions. In particular, coupled with atmosphere and thermal field, the behavior of nano catalysts could be directly observed during the catalytic reaction. Through the change of lattice structure, it can directly correlate the relationship between the structure, size and properties of materials in the nanoscale, and further directly and accurately, which is of great guiding value for the study of catalysis mechanism and the optimization of catalysts. As an outstanding catalytic material in the application of methane reforming, molybdenum oxide (MoO₃)-based materials and its deoxidation process were studied by in situ ETEM method. The corresponding microstructures and components evolution were analyzed by diffraction, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectrum (EELS) techniques. MoO₃ had a good directional deoxidation process accompanied with the process of nanoparticles crushing and regrowth in hydrogen (H₂) and thermal field. However, in the absence of H₂, the samples would exhibit different structural evolution.

Janus Mo2P3 Monolayer as an Electrocatalyst for Hydrogen Evolution

The rational design of low-cost electrocatalysts with the desired performance is the core of the large-scale hydrogen production from water. Two-dimensional materials with high specific surface area and excellent electron properties are ideal candidates for electrocatalytic water splitting. Herein, we identify a hitherto unknown Mo₂P₃ monolayer with a Janus structure (i.e., out-of-plane asymmetry) through first-principle structure search calculations. Its inherent metallicity ensures good electrical conductivity. Notably, its catalytic activity is comparable to that of Pt and the density of active sites is up to 2.65 × 10¹⁵ site/cm² owing to the Mo → P charge transfer enhancing the catalytic activity of P atoms and asymmetric structure exposing more active sites to the surface. The Mo₂P₃ monolayer can spontaneously produce hydrogen through the Volmer-Heyrovsky pathway. These excellent performances can be well maintained under strain. The coexistence of covalent and ionic bonds results in Mo₂P₃ having high stability. All these excellent properties make the Mo₂P₃ monolayer a promising candidate for electrocatalytic water splitting.

Steric effects in the hydrogen evolution reaction based on the TMX4 active center: Fe-BHT as a case study

In this work, Fe-BHT is identified as the most efficient catalyst for the hydrogen evolution reaction (HER) among the TM-BHTs (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni), with an overpotential as low as 0.09 V. It is found that Fe d_z² orbitals do not participate in the bonding with surrounding S/N atoms in the FeX₄ active center but are bonding states for hydrogen adsorption. In accordance with our results, a steric effect determined energy gap acts as an efficient descriptor for the HER activity, which has never been discussed in previous studies. In addition, strain engineering proves the proposed steric effects, which also highlights the importance of the point group symmetry of active centers.

Structural Evolution and Underlying Mechanism of Single-Atom Centers on Mo2C(100) Support during Oxygen Reduction Reaction

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using Mo₂C(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the Mo₂C(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the Mo₂C(100) surface (TM@Mo₂C(100)_O* term for SAC). While the intermediate in 2 e^- ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e^- ORR can damage Rh@ and Cu@Mo₂C(100)_O*. Furthermore, on the basis of these findings, we propose Pt@Mo₂C(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e^- ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.

Surface phase structures responsible for the activity and deactivation of the fcc MoC (111)-Mo surface in steam reforming: a systematic kinetic and thermodynamic investigation

Multiscale investigation on MoC surface phase evolution to clarify surface structures responsible for reactivity and deactivation in steam reforming.