Two-dimensional nitrides as highly efficient potential candidates for CO2 capture and activation

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

MXenes à la Carte: Tailoring the Epitaxial Growth Alternating Nitrogen and Transition Metal Layers

A high-throughput analysis based on density functional simulations underscores the viable epitaxial growth of MXenes by alternating nitrogen and metal adlayers. This is supported by an exhaustive analysis of a number of thermodynamic and kinetic thresholds belonging to different critical key steps in the course of the epitaxial growth. The results on 18 pristine N- and C-based MXenes with M₂X stoichiometry reveal an easy initial N₂ fixation and dissociation, where N₂ adsorption is controlled by the MXene surface charge and metal d-band center and its dissociation controlled by the reaction energy change. Furthermore, formation energies indicate the plausible formation of N-terminated M₂XN₂ MXenes. Moreover, the further covering with metal adlayers is found to be thermodynamically driven and stable, especially when using early transition metal atoms. The most restrictive analyzed criterion is the N₂ adsorption and dissociation at nearly full N-covered adlayers, which is yet achievable for almost half of the explored M₂X seeds. The present results unfold the possibility of expanding, controlling, and tuning the composition, width, and structure of the MXene family.

Holdups in Nitride MXene's Development and Limitations in Advancing the Field of MXene

As nanomaterials are becoming a key component in various electronics, 2D nanomaterials are emerging and attracting tremendous attention in the scientific community due to their unique physical, chemical, and structural properties. In recent years, a new family of 2D carbides and nitrides, known as MXenes, has become the center of attention for many electrochemical energy storage and conversion systems. While nitride MXenes have some publications centered around them, the overwhelming majority revolve around carbide and their direct application to systems without understanding the underlying mechanism behind their performance. The lack of publications in both of these fields, nitrides and mechanistic understanding, causes a major stopgap in MXene research and needs to be remedied in order to truly utilize their potential for future electronics and energy conversion systems. In this work, the limited works on nitride MXenes and the applications of in situ/operando characterization techniques in understanding the underlying mechanisms of energy storage and conversion in MXenes are reviewed, major progress and remaining challenges in both fields are identified, recommendations on how to circumvent the challenges and limitations are provided, and finally, new research directions that must be performed to advance the field of 2D carbide and nitride MXenes are proposed.

Ti2N nitride MXene evokes the Mars-van Krevelen mechanism to achieve high selectivity for nitrogen reduction reaction

We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene. NRR technology is a viable alternative to reducing the energy and greenhouse gas emission footprint from NH3 production. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. The MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes at the sites for NH3 synthesis. We show that the new class of 2D materials, nitride MXenes, evoke the MvK mechanism to achieve the highest Faradaic efficiency (FE) towards NH3 reported for any pristine transition metal-based catalyst-19.85% with a yield of 11.33 μg/cm2/hr at an applied potential of - 250 mV versus RHE. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

Mechanistic insights for electrochemical reduction of CO2 into hydrocarbon fuels over O-terminated MXenes

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) has attracted intensive attention in electrochemical reduction of CO2 into renewable fuels and chemical feedstock. Although encouraging progress has been made so far, but...

Adsorption and Activation of CO2 on Nitride MXenes: Composition, Temperature, and Pressure effects

The interaction of CO₂ with nitride MXenes of different thickness is investigated using periodic density functional theory‐based calculations and kinetic simulations carried out in the framework of transition state theory, the ultimate goal being predicting their possible use in Carbon Capture and Storage (CCS). We consider the basal (0001) surface plane of nitride MXenes with M_n+1N_n (n=1–3; M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometry and also compare to equivalent results for extended (001) and (111) surfaces of the bulk rock‐salt transition metal nitride compounds. The present results show that the composition of MXenes has a marked influence on the CO₂‐philicity of these substrates, whereas the thickness effect is, in general, small, but not negligible. The largest exothermic activation is predicted for Ti‐, Hf‐, and Zr‐derived MXenes, making them feasible substrates for CO₂ trapping. From an applied point of view, Cr‐, Mo‐, and W‐derived MXenes are especially well suited for CCS as the interaction with CO₂ is strong enough but molecular dissociation is not favored. Newly developed kinetic phase diagrams are introduced supporting that Cr‐, Mo‐, and W‐derived MXenes are appropriate CCS substrates as they are predicted to exhibit easy capture at mild conditions and easy release by heating below 500 K.

Charge controlled capture/release of CH4 on Nb2CTx MXene: A first-principles calculation

Methane is not only the main cause of coal mine accidents but also a contributor to global warming, meanwhile, it is clean energy. It is necessary to find an advanced material which can capture methane efficiently for its utilization. In this paper, the adsorption of CH₄ gas molecules on Nb₂CT_x(T = O, F, Cl, OH) is studied by first-principles calculation. The results indicate that the adsorption of CH₄ on Nb₂CT_x(T = O, F, Cl, OH) is weak, and the adsorption of CH₄ on Nb₂C(OH)₂ is the best. The calculation results of binding energy and cohesive energy show that Nb₂CO₂ has the best stability. The adsorption behavior of CH₄ on Nb₂CO₂ under charge control is further studied. With the increase of negative charge state in the system, the adsorption of CH₄ on Nb₂CO₂ is significantly enhanced, from physical adsorption to chemical adsorption; when the charge state of the system is greater than or equal to -2, Nb₂CO₂ can capture CH₄ effectively, and the charges transferred from Nb₂CO₂ to CH₄ mainly come from Nb atom. After the removal of the extra charge, the adsorption of CH₄ on Nb₂CO₂ becomes weak and returns to physical adsorption state; CH₄ gas molecules are easy to desorb. Therefore, Nb₂CO₂ can capture and release CH₄ molecules by regulating the charge state of Nb₂CO₂, and Nb₂CO₂ is expected to become an excellent candidate material for CH₄ capture/release.

Two-dimensional molybdenum carbide 2D-Mo2C as a superior catalyst for CO2 hydrogenation

Early transitional metal carbides are promising catalysts for hydrogenation of CO₂. Here, a two-dimensional (2D) multilayered 2D-Mo₂C material is prepared from Mo₂CT_x of the MXene family. Surface termination groups T_x (O, OH, and F) are reductively de-functionalized in Mo₂CT_x (500 °C, pure H₂) avoiding the formation of a 3D carbide structure. CO₂ hydrogenation studies show that the activity and product selectivity (CO, CH₄, C₂–C₅ alkanes, methanol, and dimethyl ether) of Mo₂CT_x and 2D-Mo₂C are controlled by the surface coverage of T_x groups that are tunable by the H₂ pretreatment conditions. 2D-Mo₂C contains no T_x groups and outperforms Mo₂CT_x, β-Mo₂C, or the industrial Cu-ZnO-Al₂O₃ catalyst in CO₂ hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO₂ hydrogenation.

The development of robust and efficient catalysts for CO₂ hydrogenation to value-added chemicals is an urgent task. Here the authors report two-dimensional carbide catalyst based on earth-abundant molybdenum that hydrogenates CO₂ with high activity, stable performance and tunable selectivity.

Exfoliation Energy as a Descriptor of MXenes Synthesizability and Surface Chemical Activity

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component-a p-block element. The MAX exfoliation energy, Eexf, is considered a chemical descriptor of the MXene synthesizability. Here, we show, by density functional theory (DFT) estimations of Eexf values for 486 different MAX phases, that Eexf decreases (i) when MAX is a nitride, (ii) when going along a metal M component d series, (iii) when going down a p-block A element group, and (iv) when having thicker MXenes. Furthermore, Eexf is found to bias, even to govern, the surface chemical activity, evaluated here on the CO2 adsorption strength, so that more unstable MXenes, displaying larger Eexf values, display a stronger attachment of species upon.

Impact of structure, doping and defect-engineering in 2D materials on CO2 capture and conversion

2D material based strategies for adsorption and conversion of CO2 to value-added products.

Molecular mechanisms of methane dry reforming on Co3Mo3N catalyst with dual sites

With density functional theory and microkinetic modeling, mechanisms responsible for the promoted dry reforming of methane (DRM) reactivity and coke resistance on the dual-site Co₃Mo₃N(111) surface are explained.

Thickness biased capture of CO2 on carbide MXenes

The synthesis of two-dimensional transition metal carbides (MXenes) with a predefined number of atomic layers offers a possible way to tune these nanomaterials chemical activity. MXenes have been theoretically predicted to be able to store CO2 even at high temperatures and low CO2 partial pressures, a prediction which has been experimentally confirmed afterwards. In the present work, the influence of the number of atomic layers on CO2 adsorption is systematically investigated by means of density functional theory based calculations, using suitable periodic models representing the (0001) surface of a series of these materials with formula Mn+1Cn (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W) and n = 1-3. The interaction of CO2 with the MXene surfaces is always favorable with the adsorption energy decreasing as the transition metal electronic configuration goes from d2 through d3 to d4, in agreement with previous work for n = 1. The influence of the thickness is found to be rather small, yet noticeable, although somewhat erratic. Nevertheless, the adsorption energy seems to converge to a defined clear limit for sufficiently thick MXenes. Interestingly, this value is close to that corresponding to the (111) surface of bulk Transition Metal Carbides (TMCs). The close structural similarity between the MXene (0001) and TMC (111) surfaces strongly suggests that the former provide a practical way to approach this otherwise unstable surface. The possibility to tune the CO2 interaction based on the MXene thickness is further investigated by means of kinetic phase diagrams. These provide additional evidence that carbide MXene surfaces are promising materials for CO2 capture even at low CO2 partial pressures, and that the MXene thickness can be used to fine tune this appealing behavior.

Computational Discovery and Design of MXenes for Energy Applications: Status, Successes, and Opportunities

MXenes (M_n+1X_n, e.g., Ti₃C₂) are the largest 2D material family developed in recent years. They exhibit significant potential in the energy sciences, particularly for energy storage. In this review, we summarize the progress of the computational work regarding the theoretical design of new MXene structures and predictions for energy applications including their fundamental, energy storage, and catalytic properties. We also outline how high-throughput computation, big data, and machine-learning techniques can help broaden the MXene family. Finally, we present some of the major remaining challenges and future research directions needed to mature this novel materials family.

The Counterion Effect of Imidazolium-Type Poly(ionic liquid) Brushes on Carbon Dioxide Adsorption

Imidazolium-based poly(ionic liquid) brushes were attached to spherical silica nanoparticles bearing various functionalities by using a surface-initiated atom transfer radical polymerization ("grafting from" technique). A temperature-programmed desorption process was applied to evaluate and analyze the carbon dioxide adsorption performance of the synthesized polymer brushes. The confined structure of the surface-attached polymer chains facilitates gas transport and adsorption, leading to an enhanced adsorption capacity of carbon dioxide molecules compared with pure polymer powders. Temperature-programmed desorption profiles of the synthesized polymer brushes after carbon dioxide adsorption reveal that the substituent groups on the nitrogen atom at the 3-position of the imidazole ring, as well as the associated anions significantly affect the adsorption capacity of functionalized poly(ionic liquid) brushes. Of the tested samples, amine-functionalized poly(ionic liquid) brushes associated with hexafluorophosphate ions exhibit the highest carbon dioxide adsorption capacity of 2.56 mmol g^-1 (112.64 mg g^-1 ) at 25 °C under a carbon dioxide partial pressure of 0.2 bar.

Applications of 2D MXenes in energy conversion and storage systems

This article provides a comprehensive review of MXene materials and their energy-related applications.

CO2 interaction with violarite (FeNi2S4) surfaces: a dispersion-corrected DFT study

The interaction between the CO₂ molecule and the violarite FeNi₂S₄{001} and {111} surfaces is studied using different exchange–correlation functionals and long-range dispersion correction approximations.