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This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

The nature of the electronic ground state of the Ti2C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti2C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

Surface composition on Ti3C2 MXene (0001) surface and its effect on hydrogen evolution reaction are analyzed based on systematic density functional calculations.

Establishing links between the structure and physical properties of solid-state ionic conductors contributes not only to a rationale of their fundamental nature, but also provides design principles to accelerate the discovery of new materials. Lithium ion conduction in complex halides is not well-elucidated and so the interplay between lattice dynamics, electronic structure and electrochemical properties in such halides has been explored in the isostructural family of lithium tetrahaloaluminates LiAlX4 (X = Cl, Br, I). Using a combination of experimental methods (Diffuse reflectance UV-Vis spectroscopy, Pulse-Echo Speed of Sound measurements, Raman spectroscopy, Inelastic Neutron Scattering) and periodic density functional theory (DFT) based calculations, we demonstrate that softer lattices (quantified in terms of Debye frequencies or Li-phonon band centres as a function of X) provide lower activation energies for Li+ migration. However, the relationship between polarisability and Li+ conductivity is not straightforward. In line with expectations emergent from the Meyer-Neldel rule, the activation energy for Li+ hopping, Ea, and the pre-exponential terms collated as σ0 in the Arrenhius equation for activated conductivity, correlate. It is also evident that the electrochemical oxidative potential limit correlates with the X– phonon band centre in the vibrational density of states (VDOS) and that the electrochemical stability window (EW) and optical band gap are interlinked, as expected.

Transition metal carbides are increasingly used as catalysts for the transformation of CO2 into useful chemicals. Recently, the effect of nanostructuring of such carbides has started to gain relevance in tailoring their catalytic capabilities. Catalytic materials based on molybdenum carbide nanoparticles (MoCy) have shown a remarkable ability to bind CO2 at room temperature and to hydrogenate it into oxygenates or light alkanes. However, the involved chemistry is largely unknown. In the present work, a systematic computational study is presented aiming to elucidate the chemistry behind the bonding of CO2 with a representative set of MoCy nanoparticles of increasing size, including stoichiometric and non-stoichiometric cases. The obtained results provide clear trends to tune the catalytic activity of these systems and to move towards more efficient CO2 transformation processes.

Nanoscale titanium carbide (TiC) is widely used in composites and energy applications. In order to design and optimize these systems and to gain a fundamental understanding of these nanomaterials, it is important to understand the atomistic structure of nano-TiC. Cluster beam experiments have provided detailed infrared vibrational spectra of numerous Ti

MXenes compounds, a recently discovered family of 2D materials, have been found to become semiconductors and photoactive when their pristine surfaces are functionalized with an electronegative termination. MXenes may present...

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 Mn+1 Nn (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.

The interaction of molecular hydrogen with a series of 28 two-dimensional (2D) carbides and nitrides, known as MXenes, has been studied by means of periodic density functional calculations. This study shows that trends in atomic and molecular adsorption energies can be rationalized in terms of the electrostatic potential above the surface site and the Bader charge on the surface metal atoms. For all systems, molecular hydrogen is found to dissociate with almost negligible barriers, meaning that at low temperature the MXene surface will be passivated by adsorbed atomic hydrogen. The conditions at which the MXene surface is partly covered and, hence, able to participate in hydrogenation reactions are investigated by means of ab initio thermodynamics and phase diagrams derived from microkinetic simulations. The first provide the equilibrium conditions for a given H coverage on the MXene of interest, whereas the second provides the conditions at which a given configuration is reachable at the working conditions. For fast enough processes, both approaches necessarily lead to the same result, but this may differ when high energy barriers are involved, as it the case here for the H adatoms recombination step. With this suite, we show that Fe2C, W2N, and Mo2C are promising hydrogenation catalysts. This work serves as a first step toward the rational design and implementation of MXene-based hydrogenation catalysts.

The adsorption and dissociation of carbon monoxide (CO) have been studied on 81 Transition Metal (TM) surfaces, with TMs having body centred cubic (bcc), face centred cubic (fcc), or hexagonal close-packed (hcp) crystalline structures. For each surface, CO, C, and O adsorptions, and C+O co-adsorptions were studied by density functional theory calculations on suited slab models, using Perdew-Burke-Ernzerhof functional with Grimme’s D3 correction for dispersive forces. CO dissociation activation and reaction energies, ΔE, were determined. The values, including zero point energy, were used to capture chemical trends along groups, d-series, and crystallographic phases concerning CO adsorption and dissociation, while simulated infrared (IR) spectroscopies and thermodynamic phase diagrams are provided. Late fcc TMs are found to adsorb CO weakly, perpendicularly, and are IR-visible, opposite to early bcc TMs, while hcp cases are distributed along these two extremes. The d-band centre, εd, is found to be the best descriptor for CO adsorptions and C+O co-adsorptions, ΔE, and activation energies. The implications of the found trends and descriptors are discussed on processes requiring CO dissociation, such as Fischer-Tropsch.

Different nano-engineered grazynes have been studied as possible membranes to separate methane (CH4) from carbon dioxide (CO2) by computational simulations based on density functional theory (DFT) and molecular dynamics (MD). The study focuses on the thermodynamics, kinetics, and dynamical aspects associated to the diffusion rates and selectivities in the context of biogas upgrading. Small adsorption energy values have been obtained for three semi-permeable grazynes, with low diffusion energy barriers below 1 eV and getting lower values as the grazyne pore increases. Selectivity estimates are found to decrease with temperature with [1],[1,2]{0,1}-grazyne obtaining the highest CO2 selectivity over CH4 at room temperature, of ca. 17, while, for the rest of grazynes studied, the selectivity falls below 5. MD simulations reveal that selectivities can be larger, up to 39 at high pressures on [1],[2]{2}-grazyne, closely to the value obtained for [1],[2]{(00),2}-grazyne, suiting grazynes membranes for biogas upgrading.

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. published

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 M

MXenes, a rather new family of 2D carbides and nitrides, have shown to be promising materials in many technological applications, particularly in electrocatalysis. The as-synthesized MXenes exhibit a variety of surface terminations involving mixtures of O, OH, H, or F surface groups. These terminations play a crucial role in the electrocatalytic performance of these materials as these may change depending on the reaction conditions. The Pourbaix diagrams have long being used to provide the thermodynamically stable surface under certain conditions of pH and potential, U. However, experimental determination of Pourbaix diagrams may be quite challenging while first-principles studies, considering the most likely terminations, allow deriving reliable insights. Here, Pourbaix diagrams for a series of representative MXenes are provided; the Ti2C, Ti3C2, V2C, and Mo2C MXenes, with the novelty of considering single and several double mixed terminations. The possible implications of the obtained results are discussed, especially for a proper choice of models in theoretical electrocatalysis studies, including the water splitting related hydrogen evolution reaction (HER), or the oxygen reduction reaction (ORR), but also serving as a guide to any further computational studies and also to electrocatalytic experiments.

Molybdenum carbides are promising alternative catalysts to Pt-group metals for the hydrogenation of unsaturated hydrocarbons. Nanostructuring has been shown to be an efficient way to boost the cata...

Deoxygenation processes are key in the transformation of bio-sourced molecules to sustainable fuels, a field where heterogeneous catalysis plays a central role. A recent study suggested a link between deoxygenation intermediates and metathesis initiator sites on Mo2C surfaces through alkylidene formation from ketones and aldehydes. There, in a density functional theory (DFT) analysis of experimental data, isolated Mo atoms were found to be active sites for carbonyl bond scission, and surface carbon was found to bond to the alkylidene intermediate. By extension, the present DFT study explores the single-atom alloy (SAA) approach for carbonyl bond dissociation steps when Mo is embedded in a Cu(111) surface. The study finds that the doped-Mo Cu(111) SAA breaks the Bronsted–Evans–Polanyi linear relation, displaying both a low activation energy for carbonyl bond scission and moderate adsorption energy of the resulting alkylidene. The findings point towards new horizons in SAA design through using highly reactive early transition metals as single-atom dopants.

Transition metal carbides have been long proposed as replacements for expensive Pt-group transition metals as heterogeneous catalysts for hydrogenation reactions, featuring similar or superior activities and selectivities. Combining experimental observations and theoretical calculations, we show that the hydrogenating capabilities of molybdenum carbide can be further improved by nanostructuring, as seen on MoCy nanoclusters anchored on an inert Au(111) support, revealing a more prominent role of Mo active sites in the easier H2 adsorption, dissociation, H adatom diffusion, and elongated chemisorbed H2 Kubas moieties formation when compared to the bulk δ-MoC(001) surface, thus explaining the observed stronger H2 interaction and the larger formation of CHx species, making these systems ideal to catalyze hydrogenation reactions.

The discovery of oxide-derived copper catalysts for CO2 electroreduction is an outstanding advance in the field of electrocatalysis. Their low overpotentials and improved selectivity towards C2 products make them unique. However, the structure of the active sites responsible for these improvements remains unclear. Herein, by means of a computational model including thermodynamic, kinetic, solvent and cation effects, we outline the atomic structure of the active sites responsible for ethanol evolution in OD-Cu catalysts. We also point out the specific features that determine selectivity and pinpoint the design criteria that should be fulfilled to enhance the catalysts’ selectivity toward ethanol. Specifically, we propose that square, four-atom Cu islands are the active sites of OD-Cu for CORR to ethanol, as they display favorable *CO dimerization and ethanol selectivity by virtue of their square, undercoordinated structure. This work has been supported by Spanish MICIUN’s RTI2018-095460-B-I00 and Marı́a de Maeztu MDM-2017-0767 grants and, in part, by Generalitat de Catalunya 2017SGR13 grant and by COST Action 18234, supported by the COST (the European Cooperation in Science and Technology). F.C.-V. thanks the Spanish MICIUN for a Ramón y Cajal research contract (RYC-2015-18996), and F.I. acknowledges additional support from the 2015 ICREA Academia Award for Excellence in University Research. O.P. thanks the Spanish MICIUN for an FPI PhD grant (PRE2018-083811). F.V. thanks the Spanish MICIUN for a Ramón y Cajal research contract (RYC-2012-10129). We are thankful to Red Española de Supercomputación (RES) for supercomputing time at SCAYLE (projects QS-2019-3-0018, QS-2019-2-0023, and QCM-2019-1-0034). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences. F.C.-V. thanks Dr. Ioannis Katsounaros for insightful discussions. Patricia Verdugo is gratefully acknowledged for her help with the design of the table of contents figure.

Previous work has shown that doping the TiC(001) surface with early transition metals significantly affects CO2 adsorption and activation which opens a possible way to control this interesting chem...

A density functional theory based study is presented with the aim of addressing the surface energy stabilization mechanisms of transition metal carbide and nitride surfaces from a crystal structure different from that of the most stable polymorph. To this end, we consider the MoC(001), MoN(001), WC(001), and WN(001) surface of rocksalt structures, which, for these compounds, is not the most stable one. The geometry optimization of suitable slab models shows that all these surfaces undergo a sensible reconstruction. The energy difference per formula unit between the rock salt and the most stable polymorph seems to be the driving force behind the observed reconstruction. A note of caution is given in that certain small periodic boundary conditions can artificially restrain such reconstructions, for which at least (2×2) supercells are needed. Also, it is shown that neglecting such a surface reconstruction can lead to artifacts in the prediction of the chemical activity and/or reactivity of these surfaces.

We present a computational study of the activity and selectivity of early transition-metal carbides as carbon dioxide reduction catalysts. We analyze the effects of the adsorption of CO2 and H2 on the (001), (011), and metal-terminated (111) surfaces of TiC and ZrC, as carbon dioxide undergoes either dissociation to CO or hydrogenation to COOH or HCOO. The relative stabilities of the three reduction intermediates and the activation energies for their formation allow the identification of favored pathways on each surface, which are examined as they lead to the release of CO, HCOOH, CH3OH, and CH4, thereby also characterizing the activity and selectivity of the two materials. Reaction energetics implicate HCO as the key common intermediate on all surfaces studied and rule out the release of formaldehyde. Surface hydroxylation is shown to be highly selective toward methane production as the formation of methanol is hindered on all surfaces by its barrierless conversion to CO.

Here we show, combining CO stripping voltammograms on different PtCu nanoparticle (NP) low-temperature fuel cell electrocatalysts and density functional calculations, that surface chemical ordering and the presence of certain defects explain the CO tolerance vs. poisoning of such systems. The CO withdrawal for these duelling CO-slingers depends on whether they are well-shaped core@shell Cu@Pt NPs, more CO-tolerant, or having Cu-surrounded surface Pt atoms or adatoms/vacancies surface defects, less CO-tolerant. The latter sites are critical on nm-sized PtCu NPs, displaying stronger CO adsorption compared to pure Pt NPs. Avoiding such sites is key when designing less expensive and CO-poisoned Cu@Pt NP-based electrocatalysts.

The interaction of CO

Catalytic-materials design requires predictive modeling of the interaction between catalyst and reactants. This is challenging due to the complexity and diversity of structure-property relationships across the chemical space. Here, we report a strategy for a rational design of catalytic materials using the artificial intelligence approach (AI) subgroup discovery. We identify catalyst genes (features) that correlate with mechanisms that trigger, facilitate, or hinder the activation of carbon dioxide (CO2) towards a chemical conversion. The AI model is trained on first-principles data for a broad family of oxides. We demonstrate that surfaces of experimentally identified good catalysts consistently exhibit combinations of genes resulting in a strong elongation of a C-O bond. The same combinations of genes also minimize the OCO-angle, the previously proposed indicator of activation, albeit under the constraint that the Sabatier principle is satisfied. Based on these findings, we propose a set of new promising catalyst materials for CO2 conversion., Here the authors demonstrate an artificial-intelligence based approach to identify catalytic materials features that correlate with mechanisms that trigger, facilitate, or hinder CO2 catalytic reactions.

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. published

Experiments under controlled conditions show that MoCx nanoclusters supported on an inert Au(111) support are efficient catalysts for CO2 conversion, although with a prominent role of stoichiometry. In particular, C-deficient nanoparticles directly dissociate CO2 and rapidly become deactivated. On the contrary, nearly stoichiometric nanoparticles reversibly adsorb/desorb CO2 and, after exposure to hydrogen, CO2 converts predominantly to CO with a significant amount of methanol and no methane or other alkanes as reaction products. The apparent activation energy for this process (14 kcal/mol) is smaller than that corresponding to bulk δ-MoC (17 kcal/mol) or a Cu(111) benchmark system (25 kcal/mol). This trend reflects the superior ability of MoC1.1/Au(111) to bind and dissociate CO2. Model calculations carried out in the framework of density functional theory provide insights into the underlying mechanism suggesting that CO2 hydrogenation on the hydrogen-covered stoichiometric MoCx nanoparticles supported on Au(111) proceeds mostly under an Eley-Rideal mechanism. The influence of the Au(111) is also analyzed and proven to have a role on the final reaction energy but almost no effect on the activation energy and transition state structure of the analyzed reaction pathways.

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble‐metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low‐coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low‐coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity.

Theoretical investigations and computational studies have notoriously contributed to the development of our understanding of heterogeneous catalysis during the last decades, when powerful computers have become generally available and efficient codes have been written that can make use of the new highly parallel architectures. The outcomes of these studies have shown not only a predictive character of theory but also provide inputs to experimentalists to rationalize their experimental observations and even to design new and improved catalysts. In this review, we critically describe the advances in computational heterogeneous catalysis from different viewpoints. We firstly focus on modeling because it constitutes the first key step in heterogenous catalysis where the systems involved are tremendously complex. A realistic description of the active sites needs to be accurately achieved to produce trustable results. Secondly, we review the techniques used to explore the potential energy landscape and how the information thus obtained can be used to bridge the gap between atomistic insight and macroscale experimental observations. This leads to the description of methods that can describe the kinetic aspects of catalysis, which essentially encompass microkinetic modeling and kinetic Monte Carlo simulations. The puissance of computer simulations in heterogeneous catalysis is further illustrated by choosing CO2 conversion catalyzed by different materials for most of which a comparison between computational information and experimental data is available. Finally, remaining challenges and a near future outlook of computational heterogeneous catalysis are provided. published

MXenes are a recently discovered class of two-dimensional materials, which have been attracting much interest by virtue of their promising biomedical and electronic applications. Here, we report on the results of first-principles calculations, based on density functional theory (DFT) including dispersion, of the adsorption energies and configurations of the five nucleobases, molecules conforming nucleotides in nucleic acids, such as DNA and RNA, on the oxygen-terminated titanium carbide MXene surface (Ti2CO2), chosen as a prototype MXene due to titanium being the most biocompatible transition metal. We find that physisorption is the most likely mechanism of adsorption on the Ti2CO2 (0001) basal surface, with the molecules sitting parallel to the MXene, about 2.5 A away. The calculated adsorption energies and Bader charge transfer values are moderate, as desired for sensing applications. We find a fair correlation between the adsorption energies and the van der Waals volumes of the nucleobases, hinting towards an adsorption dominated by van der Waals interactions. No structural deformation is observed on the molecules or on the surface. Thus, all of our conclusions support the potential applicability of the Ti2CO2 MXene as a suitable nucleobase sensor.

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component&mdash, 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.

Bimetallic alloys are actively investigated as promising new materials for catalytic and other energy-related applications. However, the stable arrangements of the two metals in prevailing nanostructured systems, which define their structure and surface reactivity, are seldom addressed. The equilibrium chemical orderings of bimetallic nanoparticles are usually different from those in the corresponding bulk phases and hard to control experimentally, which hampers assessment of the relations between composition, structure, and reactivity. Herewith, we study mixtures of platinum an essential metal in catalysis alloyed with coinage metals gold, silver, and copper. These systems are interesting, for instance, for reducing the costly Pt content and designing improved multifunctional catalysts, but the chemical orderings in such mixtures at the nanoscale are still debated. We therefore explore chemical orderings and properties of Pt-containing nanoalloys by means of a topological method based on density functional calculations. We determine the lowest-energy chemical orderings in 1.4 to 4.4 nm large Pt-Au, Pt-Ag and Pt-Cu particles with different contents of metals. Chemical ordering, bonding, and charge distribution in the nanoparticles are analyzed, identifying how peculiar structural motifs relevant for catalysis and sensing applications, such as monometallic skins and surface single-atom sites, emerge. We compare these results with previous data for the corresponding Pd-based particles, identifying trends in chemical ordering, deepening understanding of the behaviour of catalytically relevant bimetallic compositions, and establishing appropriate models for studying the bimetallic nanoalloys.

The discovery of graphene has catalyzed the search for other 2D carbon allotropes, such as graphynes, graphdiynes, and 2D π-conjugated polymers, which have been theoretically predicted or experimentally synthesized during the past decade. These materials exhibit a conductive nature bound to their π-conjugated sp2 electronic system. Some cases include sp-hybridized moieties in their nanostructure, such as acetylenes in graphynes; however, these act merely as electronic couplers between the conducting π-orbitals of sp2 centers. Herein, via first-principles calculations and quantum transport simulations, we demonstrate the existence of an acetylene-meditated transport mechanism entirely hosted by sp-hybridized orbitals. For that we propose a series of nanostructured 2D materials featuring linear arrangements of closely packed acetylene units which function as sp-nanowires. Because of the very distinct nature of this unique transport mechanism, it appears to be highly complementary with π-conjugation, thus potentially becoming a key tool for future carbon nanoelectronics., I.A. is grateful for a Humboldt postdoctoral fellowship from the Alexander von Humboldt Foundation and a Juan de la Cierva postdoctoral grant (FJC2019-038971-I) from the Ministerio de Ciencia e Investigación. Financial support by Villum Fonden (00013340) is gratefully acknowledged. ICN2 is funded by the CERCA Programme from Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). The Center for Nanostructured Graphene (CNG) is sponsored by the Danish National Research Foundation (DNRF103). This work has also been supported by the Spanish MICIUN/FEDER RTI2018-095460-B-I00, RTI2018-094757-B-I00, and María de Maeztu MDM-2017-0767 grants and, in part, by Generalitat de Catalunya 2017SGR13 grant.

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. published

Two-dimensional porous MXenes with the formula M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) are proposed as very promising sorbent materials for carbon dioxide (CO2) separation from methane (CH4) in the critical step of biogas upgrading. Density functional theory calculations including dispersion show that MXenes present very high CO2 uptakes and selectivities even under the most adverse working conditions.

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 calculations, including dispersion, this work thoroughly explores the stability of alternative ABA stacking, with ${D}_{3h}$ hexagonal symmetry, for a total of 54 MXene materials with ${\mathrm{M}}_{2}\mathrm{X}, {\mathrm{M}}_{3}{\mathrm{X}}_{2}$, and ${\mathrm{M}}_{4}{\mathrm{X}}_{3}$ stoichiometries ($\mathrm{M}=\mathrm{Ti}$, Zr, Hf, V, Nb, Ta, Cr, Mo, or W; and $\mathrm{X}=\mathrm{C}$ or N), revealing that for clean MXenes, the ABA stacking is fostered (i) by the number of $d$ electrons in M, (ii) when $\mathrm{X}=\mathrm{N}$ rather than $\mathrm{X}=\mathrm{C}$, 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 $\mathrm{M}{\mathrm{o}}_{2}\mathrm{N}$. Finally, we tackled the adsorptive and catalytic capabilities implications of such layer phase transition by considering ${\mathrm{N}}_{2}$ adsorption, dissociation, and hydrogenation on selected ABC and ABA stacked MXenes. Results highlight changes in adsorption energies of up to \ensuremath{\sim}1 eV, and in ${\mathrm{N}}_{2}$ dissociation energy barriers of up to \ensuremath{\sim}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.