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

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 to 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 <i>and</i> atomic and/or molecular orbitals

Jacob’s Ladder as Sketched by Escher: Assessing the Performance of Broadly Used Density Functionals on Transition Metal Surface Properties

The present work surveys the performance of various widely used density functional theory exchange–correlation (xc) functionals in describing observable surface properties of a total of 27 transition metals with face-centered cubic (fcc), body-centered cubic (bcc), or hexagonal close-packed (hcp) crystallographic structures. A total of 81 low Miller index surfaces were considered employing slab models. Exemplary xc functionals within the three first rungs of Jacob’s ladder were considered, including the Vosko–Wilk–Nusair xc functional within the local density approximation, the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA), and the Tao–Perdew–Staroverov–Scuseria functional as a meta-GGA functional. Hybrids were excluded in the survey because they are known to fail in properly describing metallic systems. In addition, two variants of PBE were considered, PBE adapted for solids (PBEsol) and revised PBE (RPBE), aimed at improving adsorption energies. Interlayer atomic distances, surface energies, and surface work functions were chosen as the scrutinized properties. A comparison with available experimental data, including single-crystal and polycrystalline values, shows that no xc functional is best at describing all of the surface properties. However, in statistical mean terms the PBEsol xc functional is advised, while PBE is recommended when considering both bulk and surface properties. On the basis of the present results, a discussion of adapting GGA functionals to the treatment of metallic surfaces in an alternative way to meta-GGA or hybrids is provided

Molecular Mechanism and Microkinetic Analysis of the Reverse Water Gas Shift Reaction Heterogeneously Catalyzed by the Mo₂C MXene

The potential of the Mo2C MXene to catalyze the reverse water gas shift (RWGS) reaction has been investigated by a combination of density functional theory (DFT)-based calculations, atomistic thermodynamics, and microkinetic simulations. Different catalytic routes are explored including redox and associative (carboxyl and formate) mechanisms at a high temperature at which the RWGS reaction is exothermic. The present study predicts that, on the Mo2C MXene, the RWGS reaction proceeds preferentially through the redox and formate catalytic routes, the rate-limiting step being the formation of the OH intermediate followed by the H2O formation, whereas the carboxyl route to form the carboxyl intermediate is hindered by a large energy barrier. Microkinetic simulations confirm the formation of carbon monoxide (CO) under relatively mild conditions (i.e., ∼400 °C and 1 bar). The CO formation is not affected either by the total pressure or by the CO2/H2 ratio. However, water formation requires high temperatures of ∼700 °C and pressures above 5 bar. In addition, an excess of hydrogen in the CO2/H2 ratio favors water formation. Shortly, the present study confirms that the Mo2C MXene emerges as a heterogeneous catalyst candidate for generating a CO feedstock that can be used for subsequent transformation into methanol through the Fischer–Tropsch process

Assessing <i>GW</i> Approaches for Predicting Core Level Binding Energies

Here we present a systematic study on the performance of different <i>GW</i> approaches: <i>G</i>₀<i>W</i>₀, <i>G</i>₀<i>W</i>₀ with linearized quasiparticle equation (lin-<i>G</i>₀<i>W</i>₀), and quasiparticle self-consistent <i>GW</i> (qs<i>GW</i>), in predicting core level binding energies (CLBEs) on a series of representative molecules comparing to Kohn–Sham (KS) orbital energy-based results. KS orbital energies obtained using the PBE functional are 20–30 eV lower in energy than experimental values obtained from X-ray photoemission spectroscopy (XPS), showing that any Koopmans-like interpretation of KS core level orbitals fails dramatically. Results from qs<i>GW</i> lead to CLBEs that are closer to experimental values from XPS, yet too large. For the qs<i>GW</i> method, the mean absolute error is about 2 eV, an order of magnitude better than plain KS PBE orbital energies and quite close to predictions from Δ<i>S</i>CF calculations with the same functional, which are accurate within ∼1 eV. Smaller errors of ∼0.6 eV are found for qs<i>GW</i> CLBE shifts, again similar to those obtained using Δ<i>S</i>CF PBE. The computationally more affordable <i>G</i>₀<i>W</i>₀ approximation leads to results less accurate than qs<i>GW</i>, with an error of ∼9 eV for CLBEs and ∼0.9 eV for their shifts. Interestingly, starting <i>G</i>₀<i>W</i>₀ from PBE0 reduces this error to ∼4 eV with a slight improvement on the shifts as well (∼0.4 eV). The validity of the <i>G</i>₀<i>W</i>₀ results is however questionable since only linearized quasiparticle equation results can be obtained. The present results pave the way to estimate CLBEs in periodic systems where ΔSCF calculations are not straightforward although further improvement is clearly needed

Establishing the Accuracy of Broadly Used Density Functionals in Describing Bulk Properties of Transition Metals

The performance of various commonly used density functionals is established by comparing calculated values of atomic structure data, cohesive energies, and bulk moduli of all transition metals to available experimental data. The functionals explored are the Ceperley–Alder (CA), Vosko–Wilk–Nussair (VWN) implementation of the Local Density Approximation (LDA); the Perdew–Wang (PW91) and Perdew–Burke–Ernzerhof (PBE) forms of the Generalized Gradient Approximation (GGA), and the RPBE and PBEsol modifications of PBE, aimed at better describing adsorption energies and bulk solid lattice properties, respectively. The present systematic study shows that PW91 and PBE consistently provide the smallest differences between the calculated and experimental values. Additional calculations of the (111) surface energy of several face centered cubic (<i>fcc</i>) transition metals reveal that LDA produces the most accurate results, while all other functionals significantly underestimate the experimental values. RPBE severely underestimates surface energy, which may be the origin for the reduced surface chemical activity and the better performance of RPBE describing adsorption energies

Ionic Liquid Chiral Resolution: Methyl 2‑Ammonium Chloride Propanoate on Al(854)^<i>S</i> Surface

The adsorption of the chiral methyl 2-ammonium chloride propanoate ionic liquid on the chiral Al(854)^<i>S</i> surface has been investigated by density functional calculations on periodic slab models. The results show that the molecule features an enantioselective dissociative adsorption at the surface chiral center. The low coordination of Al atoms at kinked steps and the strong attractive forces toward molecular O atoms are the main causes of the dissociation. At the surface, 2-ammonium propanal, methoxy groups, and Cl atoms are generated, attached at different sites depending on the precursor enantiomer. The adsorption strengths reveal that the bonding of the <i>R</i>-enantiomer is more favorable than <i>S</i>-enantiomer by 0.20 eV, enough for a chiral resolution process with an enantiomeric excess of >99%, whereas adsorption on achiral Al(111) surface reveals no enantiomeric discrimination with a weak molecular adsorption and no dissociation. Enantiomeric discrimination on chiral Al(854)^<i>S</i> surface is possible due to different semicore molecular levels binding energies and to distinct infrared vibrational fingerprints. The present results open the possibility for a rather simple way to separate these enantiomers

Ethylene Hydrogenation Molecular Mechanism on MoC<i>_y</i> Nanoparticles

Ethylene hydrogenation catalyzed by MoCy nanoparticles has been studied by means of density functional theory methods and several models. These include MetCar (Mo8C12), Nanocube (Mo14C13), and Mo12C12 nanoparticles as representatives of experimental MoCy nanostructures. The effect of hydrogen coverage has been studied in detail by considering low-, intermediate-, and high-hydrogen regimes. The calculated enthalpy and energy barriers show that ethylene hydrogenation is feasible on the MetCar, Mo12C12, and Nanocube but at low, medium, and high hydrogen coverages, respectively. An additional step, related to the H* migration from a Mo to a C site in the nanoparticle, has been found to be the key to establishing the best hydrogenation system. In most cases, the reactions are exothermic, featuring low hydrogenation energy barriers, especially for the Nanocube at high hydrogen coverage. In addition, the calculated adsorption Gibbs free energy shows that, for this system, the C2H4 adsorption is feasible in the 300–400 K temperature range and pressures from 10–10 to 2 atm. For the hydrogenation steps, calculated transition state theory rates show that the overall process is limited by the first hydrogenation step (C2H4 → C2H5) at temperatures of 330–400 K. However, at the lower temperatures of 300–320 K, the reaction rates are comparable for the two steps. The present results indicate that the Mo14C13 Nanocube models of MoCy nanoparticles exhibit appropriate thermodynamic and kinetic features to catalyze ethylene hydrogenation at a high-hydrogen-coverage regime. The present findings provide a basis for understanding the chemistry of active MoCy catalysts, suggest appropriate working conditions for the reaction to proceed, and provide a basis for future experimental studies

Combining Theory and Experiment for Multitechnique Characterization of Activated CO₂ on Transition Metal Carbide (001) Surfaces

Early transition metal carbides (TMC; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) with face-centered cubic crystallographic structure have emerged as promising materials for CO₂ capture and activation. Density functional theory (DFT) calculations using the Perdew–Burke–Ernzerhof exchange–correlation functional evidence charge transfer from the TMC surface to CO₂ on the two possible adsorption sites, namely, MMC and TopC, and the electronic structure and binding strength differences are discussed. Further, the suitability of multiple experimental techniques with respect to (1) adsorbed CO₂ recognition and (2) MMC/TopC adsorption distinction is assessed from extensive DFT simulations. Results show that ultraviolet photoemission spectroscopies (UPS), work function changes, core level X-ray photoemission spectroscopy (XPS), and changes in linear optical properties could well allow for adsorbed CO₂ detection. Only infrared (IR) spectra and scanning tunnelling microscopy (STM) seem to additionally allow for MMC/TopC adsorption site distinction. These findings are confirmed with experimental XPS measurements, demonstrating CO₂ binding on single crystal (001) surfaces of TiC, ZrC, and VC. The experiments also help resolving ambiguities for VC, where CO₂ activation was unexpected due to low adsorption energy, but could be related to kinetic trapping involving a desorption barrier. With a wealth of data reported and direct experimental evidence provided, this study aims to motivate further basic surface science experiments on an interesting case of CO₂ activating materials, allowing also for a benchmark of employed theoretical models

Effective and Highly Selective CO Generation from CO₂ Using a Polycrystalline α‑Mo₂C Catalyst

Present experiments show that synthesized polycrystalline hexagonal α-Mo₂C is a highly efficient and selective catalyst for CO₂ uptake and conversion to CO through the reverse water gas shift reaction. The CO₂ conversion is ∼16% at 673 K, with selectivity toward CO > 99%. CO₂ and CO adsorption is monitored by DRIFTS, TPD, and microcalorimetry, and a series of DFT based calculations including the contribution of dispersion terms. The DFT calculations on most stable model surfaces allow for identifying numerous binding sites present on the catalyst surface, leading to a high complexity in measured and interpreted IR- and TPD-spectra. The computational results also explain ambient temperature CO₂ dissociation toward CO as resulting from the presence of surface facets such as Mo₂C­(201)-Mo/Cdisplaying Mo and C surface atomsand Mo-terminated Mo₂C­(001)-Mo. An <i>ab initio</i> thermodynamics consideration of reaction conditions, however, demonstrates that these facets bind CO₂ and CO + O intermediates too strongly for a subsequent removal, whereas the Mo₂C­(101)-Mo/C exhibits balanced binding properties, serving as a possible explanation of the observed reactivity. In summary, results show that polycrystalline α-Mo₂C is an economically viable, highly efficient, and selective catalyst for CO generation using CO₂ as a feedstock