The determining role of Tx species in the catalytic potential of MXenes: Water adsorption and dissociation on Mo2CTx

Density functional theory is used to investigate the origins of the excellent catalytic activity of the Mo2CTx MXene for the water gas shift reaction. By considering different possibilities for the MXene surface termination (Tx = none, O, F, or a mixture of O and F), we conclude that its ideal composition should contain both F and O adatoms, essential for controlling the exothermicity of the reaction and avoiding saturation by oxygenated species. More precisely, while Mo2CO2 and Mo2CF2 are too inert towards water adsorption and dissociation and the bare Mo2C MXene is inactivated upon coverage by oxygenated species, our calculations predict that regions near one or two O adatoms in the midst of F surface terminations should be the active catalytic sites. Indeed, in the vicinity of the O adatoms, water adsorbs with moderate strength, dissociates with a very low energy barrier (0.14–0.20 eV), and the dissociation is moderately exothermic.publishe

ENTRE O PÚBLICO E O ÍNTIMO: TENSÕES PERANTE AS FRONTEIRAS INCERTAS DO ÍNTIMO E DO PÚBLICO EM CONFRONTO NA EDUCAÇÃO SEXUAL ESCOLAR

Entre o íntimo: tensões perante as fronteiras incertas do íntimo e o público em confronto na educação sexual escolar?

O MANDATO DOCENTE EM FACE DAS RECONFIGURAÇÕES DO SISTEMA DE ENSINO: TENSÕES, MAL-ESTAR E DILEMAS NO QUADRO DE UMA INSTITUIÇÃO ESCOLAR COMPÓSITA

O mandato docente à prova das reconfigurações do sistema de ensino: tensões, mal-estar e dilemas no quadro de uma instituição escolar compósita?

Transition metal atom adsorption on the titanium carbide MXene: trends across the periodic table for the bare and O-terminated surfaces

MXenes are a family of two-dimensional materials with great interest due to their unique properties, e.g., adjustability based on changes in their composition, structure, and surface functionality, which grant MXenes a variety of applications. One way of changing the catalytic effect of MXenes consists in adsorbing isolated metallic elements, such as transition metals (TMs), onto their surface, leading to the formation of single-atom catalysts (SAC). Herewith, the adsorption behavior of 31 TMs on the surface of two titanium carbide MXenes, viz. Ti2C and Ti2CO2, is analyzed by means of density-functional theory (DFT) calculations. We find that the oxygen surface termination causes most of the TM atoms to adsorb on a hollow site above a carbon atom, whereas on bare Ti2C, the adsorption preference follows a pattern related to groups of the Periodic Table. The interaction between the TM atoms and the surface of both Ti2C and Ti2CO2 is strong, as demonstrated by the calculated adsorption energies, which range between about -1 and -9 eV on either surface. Upon adsorption on Ti2CO2, electrons are transferred from the adatom to the MXene surface, whereas on Ti2C, the only TM atoms for which this happens are the ones in group 3 of the Periodic Table. All the other transition metal atoms become negatively charged after adsorption on Ti2C. On the oxygen-covered MXene, stronger adsorptions are accompanied by higher charge transfers. The energy barriers for TM adatom diffusion on Ti2C are very small, meaning that the adatoms can move rather freely along it. On Ti2CO2, however, higher diffusion barriers were found, many being above 1 eV, which suggests that the oxygen termination layer blocks the diffusion. On both surfaces, the highest diffusion barriers were found to correspond to the TM elements which adsorb most strongly.publishe

Catalytic reactions for H2 production on multimetallic surfaces: a review

Herewith, an overview is provided on the recent developments in the utilization of multimetallic catalysts to produce large amounts of molecular hydrogen, especially via the steam reforming of hydrocarbons and the water–gas shift reaction. Emphasis is given on the explanation of the problems affecting the currently used catalysts and how the addition/incorporation of other metals in available or new catalysts may lead to improved catalyst activity, selectivity and stability. We compare results from selected key examples taken from the literature where multimetallic catalysts are used for the aforementioned reactions. The methanol and ammonia decompositions are also critically analyzed, with focus on Earth-abundant metal elements.publishe

MXenes atomic layer stacking phase transitions and their chemical activity consequences

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 (D3d) symmetry. By means of density functional theory calculations, including dispersion, this work thoroughly explores the stability of alternative ABA stacking, with D3h hexagonal symmetry, for a total of 54 MXene materials with M2X, M3X2, and M4X3 stoichiometries (M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and X=C or N), revealing that for clean MXenes, the ABA stacking is fostered (i) by the number of d electrons in M, (ii) when X=N rather than X=C, and (iii) when the surface is terminated by oxygen adatoms. The results suggest that stacking phase transitions are likely to take place under working operando conditions, surmounting affordable layer sliding energy barriers, in accordance with the experimentally observed layer distortions in Mo2N. Finally, we tackled the adsorptive and catalytic capabilities implications of such layer phase transition by considering N2 adsorption, dissociation, and hydrogenation on selected ABC and ABA stacked MXenes. Results highlight changes in adsorption energies of up to ∼1 eV, and in N2 dissociation energy barriers of up to ∼0.3 eV, which can critically change the reaction step rate constant by three to four orders of magnitude for working temperatures in the 400–700 K range. Consequently, it is mandatory to carefully determine the atomic structure of MXenes and to use models with the most stable stacking when inspecting their chemical or physical properties.publishe

Facile heterogeneously catalyzed nitrogen fixation by MXenes

The rate-limiting step for ammonia (NH3) production via the Haber–Bosch process is the dissociation of molecular nitrogen (N2), which requires quite harsh working conditions, even when using appropriate heterogeneous catalysts. Here, motivated by the demonstrated enhanced chemical activity of MXenes— a class of two-dimensional inorganic materials— toward the adsorption of quite stable molecules such as CO2 and H2O, we use density functional theory including dispersion, to investigate the suitability of such MXene materials to catalyze N2 dissociation. Results show that MXenes exothermically adsorb N2, with rather large adsorption energies ranging from −1.11 to −3.45 eV and elongation of the N2 bond length by ∼20%, greatly facilitating their dissociation with energy barriers below 1 eV, reaching 0.28 eV in the most favorable studied case of W2N. Microkinetic simulations indicate that the first hydrogenation of adsorbed atomic nitrogen is feasible at low pressures and moderate temperatures, and that the production of NH3 may occur above 800 K on most studied MXenes, in particular, in W2N. 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.publishe

Carbon capture and usage by MXenes

Two-dimensional pristine M2X MXenes are proposed as highly active catalytic materials for carbon dioxide (CO2) 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 CO2 conversion into CO leaves the MXene surfaces partially covered by atomic oxygen, recovering its pristine nature by a posterior catalyst regeneration by hydrogen (H2) treatment at high temperatures, with MXenes effectively working as two-step catalysts for the reverse water–gas shift reaction.publishe

Surface and trapping energies as predictors for the photocatalytic degradation of aromatic organic pollutants

In this study, anatase samples enclosed by the majority of three different crystal facets {0 0 1}, {1 0 0}, and {1 0 1} were successfully synthesized. These materials were further studied toward photocatalytic degradation of phenol and toluene as model organic pollutants in water and gas phases. The obtained results were analyzed concerning their surface structure, reaction type, and surface development. Moreover, the regression model was created to find the correlation between the possible predictors and the photodegradation rate constants (k). From the studied factors, the trapping energy of charge carriers at the surface was found to be the most significant one, exponentially affecting the observed k. This resulted in the overall per-surface activity between the samples being in the order {1 0 1} > {1 0 0} > {0 0 1}. Further introduction of the surface energy (Esurf) to the regression model and the number of possible trapping centers per number of pollutant’s molecules (ntrap·n–1) improved the model accuracy, simultaneously showing the dependence on the reaction type. In the case of phenol photocatalytic degradation, the best accuracy was observed for the model including Esurf ·(ntrap·n–1)1/2 relation, while for the toluene degradation, it included Esurf2 and the S·n–1 ratio, where S is the simple surface area. Concerning different surface features which influence photocatalytic performance and are commonly discussed in the literature, the results presented in this study suggest that trapping is of particular importance.publishe

First-principles calculations on the adsorption behavior of amino acids on a titanium carbide MXene

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.publishe