Carbon Capture and Usage by MXenes

Contrasting Metallic (Rh0) and Carbidic (2D-Mo2C MXene) Surfaces in Olefin Hydrogenation Provides Insights on the Origin of the Pairwise Hydrogen Addition

Kinetic studies are vital for gathering mechanistic insights into heterogeneously catalyzed hydrogenation of unsaturated organic compounds (olefins), where the Horiuti-Polanyi mechanism is ubiquitous on metal catalysts. While this mechanism envisions nonpairwise H2 addition due to the rapid scrambling of surface hydride (H*) species, a pairwise H2 addition is experimentally encountered, rationalized here based on density functional theory (DFT) simulations for the ethene (C2H4) hydrogenation catalyzed by two-dimensional (2D) MXene Mo2C(0001) surface and compared to Rh(111) surface. Results show that ethyl (C2H5*) hydrogenation is the rate-determining step (RDS) on Mo2C(0001), yet C2H5* formation is the RDS on Rh(111), which features a higher reaction rate and contribution from pairwise H2 addition compared to 2D-Mo2C(0001). This qualitatively agrees with the experimental results for propene hydrogenation with parahydrogen over 2D-Mo2C1-x MXene and Rh/TiO2. However, DFT results imply that pairwise selectivity should be negligible owing to the facile H* diffusion on both surfaces, not affected by H* nor C2H4* coverages. DFT results also rule out the Eley-Rideal mechanism appreciably contributing to pairwise addition. The measurable contribution of the pairwise hydrogenation pathway operating concurrently with the dominant nonpairwise one is proposed to be due to the dynamic site blocking at higher adsorbate coverages or another mechanism that would drastically limit the diffusion of H* adatoms.

Carbon-based Flame Retardants for Polymers: A Bottom-up Review

This state-of-the-art review is geared toward elucidating the molecular understanding of the carbon-based flame-retardant mechanisms for polymers via holistic characterization combining detailed analytical assessments and computational material science. The use of carbon-based flame retardants, which include graphite, graphene, carbon nanotubes (CNTs), carbon dots (CDs), and fullerenes, in their pure and functionalized forms are initially reviewed to evaluate their flame retardancy performance and to determine their elevation of the flammability resistance on various types of polymers. The early transition metal carbides such as MXenes, regarded as next-generation carbon-based flame retardants, are discussed with respect to their superior flame retardancy and multifunctional applications. At the core of this review is the utilization of cutting-edge molecular dynamics (MD) simulations which sets a precedence of an alternative bottom-up approach to fill the knowledge gap through insights into the thermal resisting process of the carbon-based flame retardants, such as the formation of carbonaceous char and intermediate chemical reactions offered by the unique carbon bonding arrangements and microscopic in-situ architectures. Combining MD simulations with detailed experimental assessments and characterization, a more targeted development as well as a systematic material synthesis framework can be realized for the future development of advanced flame-retardant polymers.

Defect-Driven Efficient Selective CO2 Hydrogenation with Mo-Based Clusters

Synthetic fuels produced from CO2 show promise in combating climate change. The reverse water gas shift (RWGS) reaction is the key to opening the CO2 molecule, and CO serves as a versatile intermediate for creating various hydrocarbons. Mo-based catalysts are of great interest for RWGS reactions featured for their stability and strong metal-oxygen interactions. Our study identified Mo defects as the intrinsic origin of the high activity of cluster Mo2C for CO2-selective hydrogenation. Specifically, we found that defected Mo2C clusters supported on nitrogen-doped graphene exhibited exceptional catalytic performance, attaining a reaction rate of 6.3 gCO/gcat/h at 400 °C with over 99% CO selectivity and good stability. Such a catalyst outperformed other Mo-based catalysts and noble metal-based catalysts in terms of facile dissociation of CO2, highly selective hydrogenation, and nonbarrier liberation of CO. Our study revealed that as a potential descriptor, the atomic magnetism linearly correlates to the liberation capacity of CO, and Mo defects facilitated product desorption by reducing the magnetization of the adsorption site. On the other hand, the defects were effective in neutralizing the negative charges of surface hydrogen, which is crucial for selective hydrogenation. Finally, we have successfully demonstrated that the combination of a carbon support and the carbonization process synergistically serves as a feasible strategy for creating rich Mo defects, and biochar can be a low-cost alternative option for large-scale applications.

Machine Learning-Driven Discovery of Key Descriptors for CO2 Activation over Two-Dimensional Transition Metal Carbides and Nitrides

Fusing high-throughput quantum mechanical screening techniques with modern artificial intelligence strategies is among the most fundamental ─yet revolutionary─ science activities, capable of opening new horizons in catalyst discovery. Here, we apply this strategy to the process of finding appropriate key descriptors for CO₂ activation over two-dimensional transition metal (TM) carbides/nitrides (MXenes). Various machine learning (ML) models are developed to screen over 114 pure and defective MXenes, where the random forest regressor (RFR) ML scheme exhibits the best predictive performance for the CO₂ adsorption energy, with a mean absolute error ± standard deviation of 0.16 ± 0.01 and 0.42 ± 0.06 eV for training and test data sets, respectively. Feature importance analysis revealed d-band center (ε_d), surface metal electronegativity (χ_M), and valence electron number of metal atoms (M_V) as key descriptors for CO₂ activation. These findings furnish a fundamental basis for designing novel MXene-based catalysts through the prediction of potential indicators for CO₂ activation and their posterior usage.

Theoretical study of the catalytic performance of Fe and Cu single-atom catalysts supported on Mo2C toward the reverse water-gas shift reaction

The reverse water-gas shift (RWGS) is an attractive process using CO₂ as a chemical feedstock. Single-atom catalysts (SACs) exhibit high catalytic activity in several reactions, maximizing the metal use and enabling easier tuning by rational design than heterogeneous catalysts based on metal nanoparticles. In this study, we evaluate, using DFT calculations, the RWGS mechanism catalyzed by SACs based on Cu and Fe supported on Mo₂C, which is also an active RWGS catalyst on its own. While Cu/Mo₂C showed more feasible energy barriers toward CO formation, Fe/Mo₂C presented lower energy barriers for H₂O formation. Overall, the study showcases the difference in reactivity between both metals, evaluating the impact of oxygen coverage and suggesting Fe/Mo₂C as a potentially active RWGS catalyst based on theoretical calculations.

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.

Efficient CO2 Capture and Activation on Novel Two-Dimensional Transition Metal Borides

The large-scale production of CO₂ in the atmosphere has triggered global warming, the greenhouse effect, and ocean acidification. The CO₂ conversion to valuable chemical products or its capture and storage are of fundamental importance to mitigate the greenhouse effect on the environment. Therefore, exploring new two-dimensional (2D) materials is indispensable due to their potential intriguing properties. Here, we report a new family of 2D transition metal borides (M₂B₂, M = Sc, Ti, V, Cr, Mn, and Fe; known as MBenes) and demonstrate their static and dynamic stability. These MBenes show a metallic nature and exhibit excellent electrical conductivity. The CO₂ adsorption energy on MBenes ranges from -1.04 to -3.95 eV and exhibits the decreasing order as Sc₂B₂ > Ti₂B₂ > V₂B₂ > Cr₂B₂ > Mn₂B₂ > Fe₂B₂. The spin-polarization calculation shows a reduction in the adsorption energy for magnetic systems. Bader charge transfer indicates the formation of CO₂^δ- moiety on the MBene surface, so-called activated CO₂, which is essential for its reaction with other surface chemicals. Differential charge density plots reveal a significant charge accumulation around the CO₂ molecule. Our theoretical results predict the usage of new MBenes as a cost-effective catalyst for CO₂ capture and activation.

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...

Identifying the Atomic Layer Stacking of Mo2C MXene by Probe Molecule Adsorption

A density functional theory study is presented here aimed at investigating whether the atomic stacking on the new family of two-dimensional MXene materials has an influence on their adsorption properties and whether these properties can provide information about this structural feature. To this end, the Mo₂C MXene, exhibiting two nearly degenerate crystal structures with either ABC or ABA atomic stacking, is chosen as a case study. The study of the adsorption of CO, CO₂, and H₂O on both polymorphs of Mo₂C reveals substantial differences that could be used in experiments to provide information about the atomic stacking of a given sample. Particularly, we show that the asymmetric and symmetric stretching modes of the adsorbed CO₂ and the CO stretching mode are clear features that allow one to identify the stacking of atomic layers of the Mo₂C MXene. The present finding is likely to apply to other MXenes as well.