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Kinetic studies are vital for gathering mechanistic insights into heterogeneously catalyzed hydrogenation of unsaturated organic compounds (olefins), where the Horiuti-Polanyi mechanism is ubiquitous on metal catalysts. While this mechanism envisions nonpairwise H2 addition due to the rapid scrambling of surface hydride (H*) species, a pairwise H2 addition is experimentally encountered, rationalized here based on density functional theory (DFT) simulations for the ethene (C2H4) hydrogenation catalyzed by two-dimensional (2D) MXene Mo2C(0001) surface and compared to Rh(111) surface. Results show that ethyl (C2H5*) hydrogenation is the rate-determining step (RDS) on Mo2C(0001), yet C2H5* formation is the RDS on Rh(111), which features a higher reaction rate and contribution from pairwise H2 addition compared to 2D-Mo2C(0001). This qualitatively agrees with the experimental results for propene hydrogenation with parahydrogen over 2D-Mo2C1-x MXene and Rh/TiO2. However, DFT results imply that pairwise selectivity should be negligible owing to the facile H* diffusion on both surfaces, not affected by H* nor C2H4* coverages. DFT results also rule out the Eley-Rideal mechanism appreciably contributing to pairwise addition. The measurable contribution of the pairwise hydrogenation pathway operating concurrently with the dominant nonpairwise one is proposed to be due to the dynamic site blocking at higher adsorbate coverages or another mechanism that would drastically limit the diffusion of H* adatoms.
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Ab initio atomistic thermodynamics (AIAT) has become an indispensable tool to estimate Gibbs free energy changes for solid surfaces interacting with gaseous species relative to pressure (p) and temperature (T). For such systems, AIAT assumes that solid vibrational contributions to Gibbs free energy differences cancel out. However, the validity of this assumption is unclear for nanoscale systems. Using hydrated titania nanoparticles (NPs) as an example, we estimate the vibrational contributions to the Gibbs free energy of hydration (ΔGhyd(T,p)) for arbitrary NP size and degree of hydration. Comparing ΔGhyd(T,p) phase diagrams for NPs when considering these contributions (AIATnano) relative to a standard AIAT approach reveals significant qualitative and quantitative differences, which only become negligible for large systems. By constructing a size-dependent ΔGhyd(T,p) phase diagram, we illustrate how our approach can provide deeper insights into how nanosytems interact with their environments, with many potential applications (e.g., catalytic nanoparticles, biological colloids, nanoparticulate pollutants).
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Correction for 'The nature of the electronic ground state of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes' by Néstor García-Romeral et al., Phys. Chem. Chem. Phys., 2023, 25, 31153-31164, https://doi.org/10.1039/D3CP04402E.
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First-principles calculations based on density functional theory are performed to investigate the formation of titania/MXene composites taking (TiO2)5/M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) as cases of study. The present systematic analysis confirms a favorable, high exothermic interaction, which promotes important structural reconstructions of the (TiO2)5 cluster along with charge transfer from the MXene to titania. MXenes composed of d3 transition metals promote the strongest interaction, deformation energy, and charge transfer, followed by d4 and d5 M2C MXenes. We provide evidence that the formation of these (TiO2)5/M2C composites is governed by charge transfer, leading to scaling relationships. By using the electronegativity of the metal composing MXene and the MXene d-band center, we also establish linear correlations that can be used to predict the interaction strength of (TiO2)5/M2C composites just from the knowledge of the MXene composition. It is likely that the present trends hold for other TiO2/MXene composites.
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The synthesis and properties of stoichiometric, reduced, and Co-doped In2O3 are described in the light of several experimental techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-visible spectroscopy, porosimetry, and density functional theory (DFT) methods on appropriate models. DFT-based calculations provide an accurate prediction of the atomic and electronic structure of these systems. The computed lattice parameter is linearly correlated with the experimental result in the Co concentration ranging from 1.0 to 5.0%. For higher Co concentrations, the theoretical-experimental analysis of the results indicates that the dopant is likely to be preferentially present at surface sites. The analysis of the electronic structure supports the experimental assignment of Co2+ for the doped material. Experiments and theory find that the presence of Co has a limited effect on the material band gap.
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The search for cheap and active materials for the capture and activation of CO2 has led to many efforts aimed at developing new catalysts. In this context, earth-abundant transition metal carbides (TMCs) have emerged as promising candidates, garnering increased attention in recent decades due to their exceptional refractory properties and resistance to sintering, coking, and sulfur poisoning. In this work, we assess the use of Group 5 TMCs (VC, NbC, and TaC) as potential materials for carbon capture and sequestration/utilization technologies by combining experimental characterization techniques, first-principles-based multiscale modeling, vibrational analysis, and catalytic experiments. Our findings reveal that the stoichiometric phase of VC exhibits weak interactions with CO2, displaying an inability to adsorb or dissociate it. However, VC often exhibits the presence of surface carbon vacancies, leading to significant activation of CO2 at room temperature and facilitating its catalytic hydrogenation. In contrast, stoichiometric NbC and TaC phases exhibit stronger interactions with CO2, capable of adsorbing and even breaking of CO2 at low temperatures, particularly notable in the case of TaC. Nevertheless, NbC and TaC demonstrate poor catalytic performance for CO2 hydrogenation. This work suggests Group 5 TMCs as potential materials for CO2 abatement, emphasizes the importance of surface vacancies in enhancing catalytic activity and adsorption capability, and provides a reference for using the infrared spectra as a unique identifier to detect oxy-carbide phases or surface C vacancies within Group 5 TMCs.
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CONTEXT: The present work provides a systematic theoretical analysis of the nature of the chemical bond in Al2O3, Ga2O3, and In2O3 group 13 cubic crystal structure metal oxides. The influence of the functional in the resulting band gap is assessed. The topological analysis of the electron density provides unambiguous information about the degree of ionicity along the group which is linearly correlated with the band gap values and with the cost of forming a single oxygen vacancy. Overall, this study offers a comprehensive insight into the electronic structure of metal oxides and their interrelations. This will help researchers to harness information effectively, boosting the development of novel metal oxide catalysts or innovative methodologies for their preparation. METHODS: Periodic density functional theory was used to predict the atomic structure of the materials of interest. Structure optimization was carried out using the PBE functional, using a plane wave basis set and the PAW representation of the atomic cores, using the VASP code. Next, the electronic properties were computed by carrying out single point calculations employing PBE, PBE + U functionals using VASP and also with PBE and the hybrid HSE06 functionals using the FHI-AIMS software. For the hybrid HSE06, the impact of the screening parameter, ω, and mixing parameter, α, on the calculated band gap has also been assessed.
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Reducing the size of titania (TiO2) to the nanoscale promotes the photoactive anatase phase for use in a range of applications from industrial catalysis to environment remediation. The nanoscale dimensions of these systems affect the magnitude of the electronic energy gap by quantum confinement. Upon interaction with aqueous environments or water vapour, the surfaces of these systems will also be hydroxylated to some degree. In turn, this affects the electronic energy levels due to the cumulative electrostatic effect of the dipolar hydroxyl (-OH) ligands (i.e. the ligand dipole effect). Using accurate density functional calculations, we investigate the combined effects of quantum confinement and the hydration-induced ligand dipole effect on a set of realistic titania nanosystems over a wide range of hydroxylation. Our detailed investigation reveals that, contrary to previous models, the ligand dipole effect does not-linearly depend on the ligand coverage due to the formation of inter-ligand OHâ¯OH hydrogen bonds. To account for the resulting effects, we propose a refined model, which describes the ligand dipole effect more accurately in our systems. We show that both hydroxylation (by the ligand dipole effect) and size (by quantum confinement) have significant but distinct impacts on the electronic energy levels in nanotitania. As an example, we discuss how variations in these effects can be used to tune the highest unoccupied energy level in nanotitania for enhancing the efficiency of the hydrogen evolution reaction. Overall, we show that any specific energy shift can be achieved by a range of different combinations of nanosystem size and degree of hydroxylation, thus providing options for energy-level tuning while also allowing consideration of practical constraints (e.g. synthetic limitations, operating conditions) for photochemical applications.
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A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO2 â CO â HCO â CH â CH2 â CH3 â CH4 mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO2 (111) surface as potential good catalysts for high selective CO2 methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.
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The time evolution of the exciton generated by light adsorption in a photocatalyst is an important feature that can be approached from full nonadiabatic molecular dynamics simulations. Here, a crucial parameter is the nonradiative recombination rate between the hole and the electron that form the exciton. In the present work, we explore the performance of a Fermi's golden rule-based approach on predicting the recombination rate in a set of photoactive titania nanostructures, relying solely on the coupling of the ground and first excited state. In this scheme the analysis of the first excited state is carried out by invoking Kasha's rule thus avoiding computationally expensive nonadiabatic molecular dynamics simulations and resulting in an affordable estimate of the recombination rate. Our results show that, compared to previous ones from nonadiabatic molecular dynamics simulations, semiquantitative recombination rates can be predicted for the smaller titania nanostructures, and qualitative values are obtained from the larger ones. The present scheme is expected to be useful in the field of computational heterogeneous photocatalysis whenever a complex and computationally expensive full nonadiabatic molecular dynamics cannot be carried out.
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The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.
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First-principles calculations on titania clusters (TiO2)n (n=5 and 10) supported on the pristine Ti2C (0001) surface were carried out to understand the properties of semiconductor/MXene composites with implications in (photo)-catalysis. The reported results reveal a high exothermic interaction accompanied by a substantial charge transfer with a concomitant, notorious, deformation of the titania nanoclusters. The analysis of the density of states analysis of the composite systems evidences a metallic character with titania related states crossing the Fermi level. The picture of the chemical bonds is completed by the analysis of X-Ray Photoelectron Spectra (XPS) features, evidencing clear shifts of the C(1s) and O(1s) related peaks relative to the isolated systems that have a quite complex origin. This detailed analysis provides insights to experimentalists interested in the design and synthesis of these systems with possible applications in catalysis.
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The analysis of the C(1s) and O(1s) core-level binding energies (CLBEs) of selected molecules computed by means of total energy Hartree-Fock (ΔSCF-HF) differences shows that in some cases, the calculated values for the C(1s) are larger than the experiment, which is unexpected. The origin of these unexpected errors of the Hartree-Fock ΔSCF BEs is shown to arise from static, nondynamical, electron correlation effects which are larger for the ion than for the neutral system. Once these static correlation effects are included by using complete active space self-consistent field (CASSCF) wave functions that include internal correlation terms, the resulting ΔSCF BEs are, as expected, smaller than measured values.
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A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes. Electronic band structure calculations in the framework of density functional theory (DFT), carried out with different types of basis sets and employing the generalized gradient approach (GGA) and hybrid functionals, provide strong evidence that Ti2C, Zr2C, Hf2C, and Cr2C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V2C, Nb2C, Mo2C, Ta2C, and W2C MXenes exhibit a diamagnetic conducting ground state. For Ti2C, Zr2C, Hf2C, and Cr2C, the analysis of the low-lying spin polarized solutions with different spin orderings indicates that their ground states are antiferromagnetic (AFM), consisting of two ferromagnetic (FM) metal layers coupled antiferromagnetically. For the diamagnetic MXenes, the converged spin polarized solutions are significantly less stable than the closed shell solution except for the case of V2C and Mo2C where those excited open shell solutions can be thermally accessible (less than 300 meV per formula unit). The analysis of charge and spin density distributions of the ground state of the MXenes reveals that, in all cases, the metal atoms have a net charge close to +1 e and C atoms close to -2 e. In the case of diamagnetic MXenes, the electronic structure of V2C, Nb2C, and Ta2C is consistent with metal atoms exhibiting a closed-shell s2d2 configuration whereas for Mo2C, and W2C is consistent with a low-spin s1d4 configuration although the FM solution is close in energy for V2C and Mo2C suggesting that they may play a role in their chemistry at high temperature. For the open shell MXenes, the spin density primarily located at the metal atoms showing one unpaired electron per Ti+, Zr+, and Hf+ magnetic center, consistent with s2d1 configuration of the metal atom, and of â¼3.5 unpaired electrons per Cr+ magnetic center interpreted as a mixture of s2d3 and high-spin s1d4 configuration. Finally, the analysis of the density of states reveals the metallic character of all these bare MXenes, irrespective of the nature of the ground state, with significant covalent contributions for Mo2C and W2C.
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The effect of N-doping of titania (TiO2) nanoparticles (NPs) on their reduction through neutral O vacancy (Ovac) formation is investigated using all electron density functional theory-based calculations, including hybrid density functionals, and taking the bipyramidal anatase (TiO2)84 NP as a realistic model. The location of the N dopant is systematically analyzed, including O substitution in the (TiO2)84 structure and N occupying interstitial regions. Our computational study concludes that interstitial N doping is more favorable than N substituting O atoms and confirms that the presence of N reduces the energy gap. In the N-doped NP, Ovac formation is more favored than in undoped NP but less than in the N-doped bulk, which has important consequences.
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BACKGROUND: The nasal microbiota of the piglet is a reservoir for opportunistic pathogens that can cause polyserositis, such as Glaesserella parasuis, Mycoplasma hyorhinis or Streptococcus suis. Antibiotic treatment is a strategy to control these diseases, but it has a detrimental effect on the microbiota. We followed the piglets of 60 sows from birth to 8 weeks of age, to study the effect of ceftiofur on the nasal microbiota and the colonization by pathogens when the treatment was administered to sows or their litters. We also aimed to revert the effect of the antibiotic on the nasal microbiota by the inoculation at birth of nasal colonizers selected from healthy piglets. Nasal swabs were collected at birth, and at 7, 15, 21 and 49 days of age, and were used for pathogen detection by PCR and bacterial culture, 16S rRNA amplicon sequencing and whole shotgun metagenomics. Weights, clinical signs and production parameters were also recorded during the study. RESULTS: The composition of the nasal microbiota of piglets changed over time, with a clear increment of Clostridiales at the end of nursery. The administration of ceftiofur induced an unexpected temporary increase in alpha diversity at day 7 mainly due to colonization by environmental taxa. Ceftiofur had a longer impact on the nasal microbiota of piglets when administered to their sows before farrowing than directly to them. This effect was partially reverted by the inoculation of nasal colonizers to newborn piglets and was accompanied by a reduction in the number of animals showing clinical signs (mainly lameness). Both interventions altered the colonization pattern of different strains of the above pathogens. In addition, the prevalence of resistance genes increased over time in all the groups but was significantly higher at weaning when the antibiotic was administered to the sows. Also, ceftiofur treatment induced the selection of more beta-lactams resistance genes when it was administered directly to the piglets. CONCLUSIONS: This study shed light on the effect of the ceftiofur treatment on the piglet nasal microbiota over time and demonstrated for the first time the possibility of modifying the piglets' nasal microbiota by inoculating natural colonizers of the upper respiratory tract.
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Co-electrolysis of carbon oxides and nitrogen oxides promise to simultaneously help restore the balance of the C and N cycles while producing valuable chemicals such as urea. However, co-electrolysis processes are still largely inefficient and numerous knowledge voids persist. Here, we provide a solid thermodynamic basis for modelling urea production via co-electrolysis. First, we determine the energetics of aqueous urea produced under electrochemical conditions based on experimental data, which enables an accurate assessment of equilibrium potentials and overpotentials. Next, we use density functional theory (DFT) calculations to model various co-electrolysis reactions producing urea. The calculated reaction free energies deviate significantly from experimental values for well-known GGA, meta-GGA and hybrid functionals. These deviations stem from errors in the DFT-calculated energies of molecular reactants and products. In particular, the error for urea is approximately -0.25 ± 0.10 eV. Finally, we show that all these errors introduce large inconsistencies in the calculated free-energy diagrams of urea production via co-electrolysis, such that gas-phase corrections are strongly advised.
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The magnetic nature of Ti2C, Ti3C2, and Ti4C3 MXenes is determined from periodic calculations within density functional theory and using the generalized gradient approximation based PBE functional, the PBE0 and HSE06 hybrids, and the on-site Hubbard corrected PBE+U one, in all cases using a very tight numerical setup. The results show that all functionals consistently predict a magnetic ground state for all MXenes, with spin densities mainly located at the Ti surface atoms. The analysis of solutions corresponding to different spin orderings consistently show that all functionals predict an antiferromagnetic conducting ground state with the two ferromagnetic outer (surface) Ti layers being antiferromagnetically coupled. A physically meaningful spin model is proposed, consistent with the analysis of the chemical bond, with closed shell, diamagnetic, Ti2+ like ions in inner layers and surface paramagnetic Ti+ like centers with one unpaired electron per magnetic center. From a Heisenberg spin model, the relevant isotropic magnetic coupling constants are extracted from an appropriate mapping of total energy differences per formula unit to the expected energy values of the spin Hamiltonian. While the numerical values of the magnetic coupling constants largely depend on the used functional, the nearest neighbor intralayer coupling is found to be always ferromagnetic, and constitutes the dominant interaction, although two other non-negligible interlayer antiferromagnetic terms are involved, implying that the spin description cannot be reduced to NN interaction only. The influence of the MXene thickness is noticeable for the dominant ferromagnetic interaction, increasing its value with the MXene width. However, the interlayer interactions are essentially due to the covalency effects observed in all metallic solutions which, as expected, decay with distance. Within the PBE+U approach, a U value of 5 eV is found to closely simulate the results from hybrid functionals for Ti2C and less accurately for Ti3C2 and Ti4C3.
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Incorporating solvent-adsorbate interactions is paramount in models of aqueous (electro)catalytic reactions. Although a number of techniques exist, they are either highly demanding in computational terms or inaccurate. Microsolvation offers a trade-off between accuracy and computational expenses. Here, we dissect a method to swiftly outline the first solvation shell of species adsorbed on transition-metal surfaces and assess their corresponding solvation energy. Interestingly, dispersion corrections are generally not needed in the model, but caution is to be exercised when water-water and water-adsorbate interactions are of similar magnitude.
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Fusing high-throughput quantum mechanical screening techniques with modern artificial intelligence strategies is among the most fundamental âyet revolutionaryâ science activities, capable of opening new horizons in catalyst discovery. Here, we apply this strategy to the process of finding appropriate key descriptors for CO2 activation over two-dimensional transition metal (TM) carbides/nitrides (MXenes). Various machine learning (ML) models are developed to screen over 114 pure and defective MXenes, where the random forest regressor (RFR) ML scheme exhibits the best predictive performance for the CO2 adsorption energy, with a mean absolute error ± standard deviation of 0.16 ± 0.01 and 0.42 ± 0.06 eV for training and test data sets, respectively. Feature importance analysis revealed d-band center (εd), surface metal electronegativity (χM), and valence electron number of metal atoms (MV) as key descriptors for CO2 activation. These findings furnish a fundamental basis for designing novel MXene-based catalysts through the prediction of potential indicators for CO2 activation and their posterior usage.