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The rate-limiting step for ammonia (NH 3 ) production via the Haber-Bosch process is known to be the dissociation of molecular nitrogen (N 2 ), which requires quite harsh working conditions, even when using appropriate heterogeneous catalysts. Here, motivated by the demonstrated enhanced chemical activity of MXenes a new class of two-dimensional inorganic materials towards the adsorption of quite stable molecules such as CO 2 and H 2 O, we use density functional theory including dispersion to investigate the suitability of such MXene materials to catalyze the N 2 dissociation. Results show that MXenes exothermically adsorb N 2 , with rather large adsorption energies ranging from -1.11 to -3.45 eV and elongation of the N 2 bond length by ~20%, greatly facilitating its dissociation with energy barriers below 1 eV, reaching 0.28 eV in the most favorable studied case of W 2 N. Microkinetic simulations indicate that the first hydrogenation of adsorbed atomic nitrogen is feasible at low pressures and moderate temperatures, and that the production of NH 3 may occur above 800 K on most studied MXenes, in particular in W 2 N. These results reinforce the promising capabilities of MXenes to dissociate nitrogen and suggest combining them co-catalytically with Ru nanoparticles to further improve the efficiency of ammonia synthesis.

Two-dimensional (2D) transition-metal nitrides and carbides (MXenes), containing a few atomic layers only, are novel materials which have become a hub of research in many applied technological fields, ranging from catalysis, to environmental scrubber materials, up to batteries. MXenes are obtained by removing the A element from precursor MAX phases, and it is for this reason that it is often assumed that the resulting 2D material displays the MAX atomic layer stacking -an ABC sequence with trigonal (D 3d ) symmetry. By means of density functional theory based calculations, including dispersion, the present work thoroughly explores the stability of alternative ABA stacking, with D 3h hexagonal symmetry, for a total of 54 MXene materials with M

We present a method of construction of exact localized many-body eigenstates of the Hubbard model in decorated lattices, both for U = 0 and U → ∞. These states are localized in what concerns both hole and particle movement. The starting point of the method is the construction of a plaquette or a set of plaquettes with a higher symmetry than that of the whole lattice. Using a simple set of rules, the tight-binding localized state in such a plaquette can be divided, folded and unfolded to new plaquette geometries. This set of rules is also valid for the construction of a localized state for one hole in the U → ∞ limit of the same plaquette, assuming a spin configuration which is a uniform linear combination of all possible permutations of the set of spins in the plaquette.

Two-dimensional pristine M 2 X MXenes are proposed as highly active catalytic materials for carbon dioxide (CO 2 ) greenhouse gas conversion into carbon monoxide (CO) on the basis of a multiscale modeling approach, coupling calculations carried out in the framework of density functional theory and newly developed kinetic phase diagrams. The extremely facile CO 2 conversion into CO leaves the MXene surfaces partially covered by atomic oxygen, recovering its pristine nature by a posterior catalyst regeneration by hydrogen (H 2 ) treatment at high temperatures, with MXenes effectively working as two-step catalysts for the reverse water−gas shift reaction.

We report a molecular modeling paradigm to describe silica polymerization reactions in aqueous solutions at conditions that are representative of realistic experimental processes like biosilicification or porous silica synthesis – i.e. at close to ambient temperatures and over a wide range of pH. The key point is to describe the Si-O-Si chemical bond formation and breakage processes through a continuous potential with a balance between attractive and repulsive interactions between suitably placed virtual sites and sticky particles. The simplicity of the model, its applicability in standard parallelized molecular dynamics codes, and its compatibility with the widely used MARTINI coarse-grained force-field allows for the study of systems containing millions of atoms over microsecond time scales. The model is calibrated to match experimental results for the temporal evolution of silica polymerization in aqueous solution close to the isoelectric point, and can describe silica polymerization and self-assembly processes during encapsulation of a surfactant micelle.

Due to their vast range of promising biomedical and electronic applications, there is a growing interest in bioinorganic lamellar nanomaterials. MXenes are one such class of materials, which stand out by virtue of their demonstrated biocompatibility, pharmacological applicability, energy storage performance, and feasibility as single-molecule sensors. Here, we report on first-principles predictions, based on density functional theory, of the binding energies and ground-state configurations of six selected amino acids (AAs) adsorbed on O-terminated two-dimensional titanium carbide, Ti2CO2. We find that most AAs (aspartic acid, cysteine, glycine, and phenylalanine) prefer to adsorb via their nitrogen atom, which forms a weak bond with a surface Ti atom, with bond lengths of around 2.35 Å. In contrast, histidine and serine tend to adsorb parallel to the MXene surface, with their α carbon about 3 Å away from it. In both adsorption configurations, the adsorption energies are on the order of the tenths of an electronvolt. In addition, we find a positive, nearly linear correlation between the binding energy of each studied AA and its van der Waals volume, which suggests an adsorption dominated by van der Waals forces. This relationship allowed us to predict the adsorption energies for all of the proteinogenic AAs on the same Ti2CO2 MXene. Our analysis additionally shows that in the parallel adsorption mode there is a negligible transfer of charge density from the AA to the surface but noticeable in the N-bonded adsorption mode. In the latter, the isosurfaces of charge density differences show accumulation of shared electrons in the region between N and Ti, confirming the predicted N–Ti bond. The moderate adsorption energy values calculated, as well as the preservation of the integrity of both the AAs and the surface upon adsorption, reinforce the capability of Ti2CO2 as a promising reusable biosensor for amino acids.

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