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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.