Transition Metal Carbides as Novel Materials for CO2 Capture, Storage, and Activation

The capture and activation of the greenhouse gas carbon dioxide (CO2) is a prerequisite to its catalytic reforming or breakdown. Here we report, by means of density functional theory calculations including dispersive forces, that transition metal carbides (TMC; TM = Ti, Zr, Hf, Nb, Ta, Mo) are able to uptake and activate CO2 on their most-stable (001) surfaces with considerable adsorption strength. Estimations of adsorption and desorption rates predict a capture of CO2 at ambient temperature and even low partial pressures, suggesting TMCs as potential materials for CO2 abatemen

Biogas upgrading by transition metal carbides

The separation of carbon dioxide (CO2) from methane (CH4) is critical in biogas upgrading, requiring materials with high selectivity toward one of the two gas components. Hereby we show, by means of density functional theory based calculations including dispersive forces description, the distinct interaction of CO2 and CH4 with the most stable (001) surfaces of seven transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, and Mo). Transition state theory derived ad-/desorption rates suggest a very high CO2 uptake and selectivity over CH4 even at ambient temperature and low partial gas pressures

Transition metal adatoms on graphene: A systematic density functional study

Transition Metal (TM) atoms adsorption on graphene results in a tuning of their electronic, magnetic, storage, sensing, and catalytic properties. Herein we provide a thorough density functional theory study, including dispersion, of the structural, energetic, diffusivity, magnetic, and doping properties for all 3d, 4d, and 5d TM atoms adsorbed on graphene. TMs prefer to sit on hollow sites when chemisorbed, but on bridge or top sites when physisorbed; which is the case of atoms with d5 and d10 configurations. Diffusion energy barriers follow the adsorption energy trends. Dispersive forces simply increase the adsorption strength by ∼0.35 eV. Adatom height seems to be governed by the bond strength. All TMs are found to n-dope graphene, except Au, which p-dopes. The electron transfer decays along the d series due to the electronegativity increase. Early TMs infer noticeable magnetism to graphene, yet for elements with more than five electrons in the d shell the local magnetic moments abruptly decay to low or zero values. Experimental observations on adatom position, height, temperature clustering and Ostwald ripening, p- or n-doping, or the electronic configuration can be rationalized by present calculations, which deliver a solid theoretical ground from which experimental features can be interpreted and discussed

Approaching the quantitative description of enantioselective adsorption by the density functional theory means

The applications of enantiopure organic compounds range from medicine to green agrochemistry. Their racemic or enantioselective synthesis permits their acquisition beyond the extraction from life forms. These procedures need chiral resolution steps to achieve the required degrees of enantiomeric purity, though. Many research endeavours are addressed at finding chiral materials able to separate the enantiomers by their selective adsorption upon. Transition metal chiral surfaces have been found to reach enantiomeric excess degrees of purity outperforming surfaces of naturally existing chiral materials. Future research can be driven by high-throughput computational screening, given the employed methodology is able to discern the subtle enantiomeric differences of free energies of adsorption. The capabilities of density functional theory methods are here evaluated on the textbook case of D/L-aspartic acid adsorption on chiral Cu(3,1,17)R&S metal surfaces. Results show that dispersive forces are a prerequisite to properly describe the enantioselective adsorption, whereas the inclusion of fundamental vibrational energy and adsorbate vibrational free energies are key ingredients to approach a quantitative description. Simulated X-ray photoemission and infrared spectra indicate that the adsorption conformations can be qualitatively recognized

Matildite Contact with Media: First-Principles Study of AgBiS2 Surfaces and Nanoparticle Morphology

Motivated by the interest in AgBiS2 material for solar light harvesting applications, a detailed bulk first-principles quantum mechanical study of its surface properties is presented. Density functional theory based calculations with the Perdew-Burke-Ernzerhof functional have been carried out for different surface orientations and terminations of the matildite polymorph. From the results, two particularly stable facets are predicted to dominate Wulff shaped AgBiS2 nanoparticles. These are the (001) type nonpolar surface and the (111) polar terminations where facets are exposed containing solely Ag or S atoms. The Wulff equilibrium shape is predicted to be a cube with only two edges capped. This particular shape explains a previously reported surface enrichment of Ag with respect to Bi of ∼1.5. The (001) surfaces display an ionic character similar to bulk AgBiS2, with a low work function of 4.31 eV, although the inspection of the density of states (DOS) reveals a bandgap increased by 0.3 eV compared to bulk. This surface effect could explain the bulk wavelength overestimation of the absorption coefficient decay, as previously determined. Last but not least, the DOS of the (111) polar termination reveals a metallic character, where Fermi level is located below that on the (001) surfaces. Possible implications of the different electronic structure of these surfaces in the reported photoactivity are discussed

Surface activity of early transition-metal oxycarbides: CO2 adsorption case study

Theoretical studies and experiments have suggested that transition metal carbides (TMCs) can be useful materials for carbon capture and storage or use technologies from air sources. However, TMCs are known to become easily oxidized in the presence of molecular oxygen, and their properties jeopardized while being transformed into transition metal oxycarbides (TMOCs), which can affect the TMCs chemical activity, e.g. towards CO2. Here, by means of density functional theory (DFT) based calculations including dispersion we address the possible effect of oxycarbide formation in the CO2 capture course. A careful analysis of different models show that for group 4 TMCs (TM = Ti, Zr, Hf), their oxidation into TMOCs involves a negligible structural distortion of the outermost oxide surface layer, whereas severe rumplings are predicted for group 5 and 6 TMOCs (TM = V, Nb, Ta, Mo). The large surface distortion in the latter TMOCs results in a weak interaction with CO2 with adsorption energies below -0.27 eV. On the contrary, on group 4 TMOCs surfaces CO2 adsorption becomes stronger, with the adsorption values strengthening by 0.44-1.2 eV, a fact that, according to adsorption/desorption rates estimates, increments the air CO2 capture temperature window by 175-400 K. The present DFT results point to group 4 TMCs, TiC in particular, as promising materials for air CO2 capture and storage/conversion, even in the presence of oxygen and the possible formation of transition metal oxycarbides

Bandgap engineering by cationic disorder: case study on AgBiS2

The influence of cationic disorder on the electronic structure of ternary compounds, here exemplified on AgBiS2 material, is studied by means of accurate first principles periodic density functional theory based calculations. For AgBiS2 cationic disorder in going from semiconducting matildite to a metallic arrangement crystal structure is found to induce a significant decrease in the band gap, as a result of cation-disorder conduction band tail states penetrating into the matildite bandgap. Properly aligned conduction band minimum and valence band maximum show that cationic disorders lead to a noticeable drop of the former and a slight increase of the latter. The present results indicate that temperature effects triggering cationic disorder will have a beneficial effect on the photoactivity of AgBiS2 samples provided that the metallic limit is not reached

Performance of the TPSS functional on predicting core level binding energies of main group elements containing molecules: a good choice for molecules adsorbed on metal surfaces

Here we explored the performance of Hartree-Fock (HF), Perdew-Burke-Ernzerhof (PBE), and Tao-Perdew-Staroverov-Scuseria (TPSS) functionals in predicting core level 1s Binding Energies (BEs) and BE shifts (ΔBEs) for a large set of 68 molecules containing a wide variety of functional groups for main group elements B→F and considering up to 185 core levels. A statistical analysis comparing with X-Ray Photoelectron Spectroscopy (XPS) experiments shows that BEs estimations are very accurate, TPSS exhibiting the best performance. Considering ΔBEs, the three methods yield very similar and excellent results, with mean absolute deviations of ~0.25 eV. When considering relativistic effects, BEs deviations drop approaching experimental values. So, the largest mean percentage deviation is of 0.25% only. Linear trends among experimental and estimated values have been found, gaining offsets with respect ideality. By adding relativistic effects to offsets, HF and TPSS methods underestimate experimental values by solely 0.11 and 0.05 eV, respectively, well within XPS chemical precision. TPSS is posed as an excellent choice for the characterization, by XPS, of molecules on metal solid substrates, given its suitability in describing metal substrates bonds and atomic and/or molecular orbitals

On the prediction of core level binding energies in molecules, surfaces and solids

Core level binding energies, directly measured by X-ray photoelectron spectroscopy (XPS), provide unique information regarding the chemical environment of atoms in a given system. However, interpretation of XPS in extended systems may not be straightforward and requires assistance from theory. The different state-of-the-art theoretical methods commonly used to approach core level binding energies and their shifts with respect to a given reference are reviewed and critically assessed with special emphasis on recently developed theoretical methods and with a focus on future applications in materials and surface sciences

Electronic structure of stoichiometric and reduced ZnO from periodic relativistic all electron hybrid density functional calculations using numeric atom-centered orbitals

The atomic and electronic structure of stoichiometric and reduced ZnO wurtzite has been studied using a periodic relativistic all electron hybrid density functional (PBE0) approach and numeric atom-centered orbital basis set with quality equivalent to aug-cc-pVDZ. To assess the importance of relativistic effects, calculations were carried out without and with explicit inclusion of relativistic effects through the zero order regular approximation. The calculated band gap is ∼0.2 eV smaller than experiment, close to previous PBE0 results including relativistic calculation through the pseudopotential and ∼0.25 eV smaller than equivalent nonrelativistic all electron PBE0 calculations indicating possible sources of error in nonrelativistic all electron density functional calculations for systems containing elements with relatively high atomic number. The oxygen vacancy formation energy converges rather fast with the supercell size, the predicted value agrees with previously hybrid density functional calculations and analysis of the electronic structure evidences the presence of localized electrons at the vacancy site with a concomitant well localized peak in the density of states ∼0.5 eV above the top of the valence band and a significant relaxation of the Zn atoms near to the oxygen vacancy. Finally, present work shows that accurate results can be obtained in systems involving large supercells containing up to ∼450 atoms using a numeric atomic-centered orbital basis set within a full all electron description including scalar relativistic effects at an affordable cost