Dispersion-corrected Density Functional Theory Calculations Research Articles

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr2O72- and CrO42-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K+ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K+ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K+ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO42- rather than Cr2O72- is dominant at clay interfaces, and HCrO4- can co-exist under acidic conditions. Cr2O72- transformation to CrO42- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Intermediates for catalytic reduction of CO2 on p-block element surfaces

When used as electrocatalysts for the reduction of CO2, p-block metals (e.g., Bi, Sb, Tl, Pb, In, and Sn) are known to exhibit high selectivity toward the production of formic acid (HCOOH). Nonetheless, the current knowledge on the two key intermediates, namely COOH and OCHO, is lacking, which hinders the mechanistic understanding and development of efficient p-block metal catalysts. In this study, the molecular adsorption phenomena related to the electrocatalytic reduction of CO2 (ERC) on p-block metal surfaces are investigated using dispersion-corrected density functional theory (DFT-D) calculations. It is demonstrated that the surfaces of p-block metals display low chemical reactivity toward CO2. The adsorbates of OCHO with Ometal bonds are consistently more stable than those of COOH with Cmetal bonds. Consequently, the adsorption of OCHO is favored over COOH adsorption, leading to the selective formation of HCOOH instead of CO. Structural, vibrational, and electronic properties of these two adsorbates are discussed to facilitate future experimental mechanistic studies.

Activating the FeS (001) Surface for CO2 Adsorption and Reduction through the Formation of Sulfur Vacancies: A DFT-D3 Study

As a promising material for heterogeneous catalytic applications, layered iron (II) monosulfide (FeS) contains active edges and an inert basal (001) plane. Activating the basal (001) plane could improve the catalytic performance of the FeS material towards CO2 activation and reduction reactions. Herein, we report dispersion-corrected density functional theory (DFT-D3) calculations of the adsorption of CO2 and the elementary steps involved in its reduction through the reverse water-gas shift reaction on a defective FeS (001) surface containing sulfur vacancies. The exposed Fe sites resulting from the creation of sulfur vacancies are shown to act as highly active sites for CO2 activation and reduction. Based on the calculated adsorption energies, we show that the CO2 molecules will outcompete H2O and H2 molecules for the exposed active Fe sites if all three molecules are present on or near the surface. The CO2 molecule is found to weakly physisorb (−0.20 eV) compared to the sulfur-deficient (001) surface where it adsorbs much strongly, releasing adsorption energy of −1.78 and −1.83 eV at the defective FeS (001) surface containing a single and double sulfur vacancy, respectively. The CO2 molecule gained significant charge from the interacting surface Fe ions at the defective surface upon adsorption, which resulted in activation of the C–O bonds confirmed via vibrational frequency analyses. The reaction and activation energy barriers of the elementary steps involved in the CO2 hydrogenation reactions to form CO and H2O species are also unraveled.

Stability and NMR Chemical Shift of Amorphous Precursors of Methane Hydrate: Insights from Dispersion-Corrected Density Functional Theory Calculations Combined with Machine Learning.

Clathrate hydrates of natural gases are important backup energy sources. It is thus of great significance to explore the nucleation process of hydrates. Hydrate clusters are building blocks of crystalline hydrates and represent the initial stage of hydrate nucleation. Using dispersion-corrected density functional theory (DFT-D) combined with machine learning, herein, we systematically investigate the evolution of stabilities and nuclear magnetic resonance (NMR) chemical shifts of amorphous precursors from monocage clusters CH4(H2O)n (n = 16-24) to decacage clusters (CH4)10(H2O)n (n = 121-125). Compared with planelike configurations, the close-packed structures formed by the water-cage clusters are energetically favorable. The 512 cages are dominant, and the emerging amorphous precursors may be part of sII hydrates at the initial stage of nucleation. Based on our data set, the possible initial fusion pathways for water-cage clusters are proposed. In addition, the 13C NMR chemical shifts for encapsulated methane molecules also showed regular changes during the fusion of water-cage clusters. Machine learning can reproduce the DFT-D results well, providing a structure-energy-property landscape that could be used to predict the energy and NMR chemical shifts of such multicages with more water molecules. These theoretical results present vital insights into the hydrate nucleation from a unique perspective.

New Salt and Cocrystal of Mequitazine: Impact of Coformer Flexibility and Hydrogen Bond Donors on Polymorphism

New multicomponent crystals of mequitazine (MQZ) with succinic acid (SUC), fumaric acid (FUM), glycolic acid (GLC), and glycolamide (GLA) were obtained after liquid-assisted grinding. The hydrate/anhydrate availability in the crystals differed owing to slight differences in the coformer structure. Therefore, the causes of these differences in the crystal structure were investigated. The structures of crystals, for which single crystals were difficult to obtain, were obtained using powder diffraction crystal structure analyses. The crystal structures, including the disordered structure and the structure in which the H atom position could not be identified, were validated via dispersion-corrected density functional theory calculations using the Quantum ESPRESSO code. MQZ-SUC and MQZ-FUM formed isostructural anhydrates, but only MQZ-SUC formed a hydrate by twisting the alkyl straight chain; hence, the difference between the dihedral angles that the coformer can form led to a small change in polymorphism. MQZ-GLC hydrate and MQZ-GLA anhydrate had no isomorphic crystals, MQZ-GLC formed a hydrate, and MQZ-GLA formed an anhydrate. Differences in the number of hydrogen bond donors in the coformer led to major changes in polymorphism. The results of this study might be useful for coformer selection and controlling multicomponent crystal polymorphism.

Asymmetric Spirocyclization Enabled by Iridium and Brønsted Acid-Catalyzed Formal Reductive Cycloaddition

A catalytic, enantioselective spirocyclization of formanilides or formylindolines and enamides has been developed herein. The reaction proceeds through a sequential iridium-catalyzed hydrosilylatio...

Scavenging properties of yttrium nitride monolayer towards toxic sulfur gases

We employ first-principles calculations based on density functional theory (DFT) to investigate the adsorption characteristics of a novel 2D material, hexagonal yttrium nitride (h-YN) monolayer, towards sulfur-containing gases (SCG) such as H2S and SO2. Dispersion corrected DFT calculations were carried out to explore the adsorption mechanism, structural and electronic properties of pristine and SCG-adsorbed h-YN (with and without the presence of O2). Our calculations reveal that both H2S and SO2 are strongly adsorbed on pristine h-YN with adsorption energies of –3.24 and –4.21 eV, respectively. However, the presence of molecular oxygen plays an important role in reducing the adsorption energies to –2.46 and –1.75 eV for H2S and SO2, respectively. Strong chemisorption, even in the presence of O2, makes h-YN suitable for non-reversible capturing of H2S and SO2. In case of SO2, molecular adsorption coupled with significant variations in the electronic properties and charge transfer indicates the suitability of h-YN for SO2 capture and a disposable sensing material.

Boosting Selective Adsorption of Xe over Kr by Double-Accessible Open-Metal Site in Metal-Organic Framework: Experimental and Theoretical Research.

Obtaining highly valuable Xe from air or other sources is highly important but still seriously restricted by its inherent inert nature and the great difficulty in separation from other inert gases, especially for Xe and Kr that show comparable size. In this work, we show both experimental and theoretical research of how to boost the selective adsorption of Xe over Kr by double-accessible open-metal site in metal-organic framework (MOF). The MOF, namely, UTSA-74, shows a high Xe uptake up to 2.7 mmol/g and a lower Kr uptake of 0.58 mmol/g at 298 K and 1 bar, leading to a high selectivity of 8.4. The effective Xe/Kr separation was further confirmed by both transient breakthrough simulation and experimental breakthrough. The separation mechanism, as unveiled by the grand canonical Monte Carlo simulation and dispersion-corrected density functional theory calculation, is due to the unique double-accessible open-metal site in UTSA-74 that affords stronger interaction toward Xe than Kr.

Energetics of Hybrid Structures between Cycloparaphenylene and Carbon Nanotubes: A Dispersion-Corrected Density Functional Theory Study

Dispersion-corrected density functional theory (DFT) calculations examined energetically preferable hybrid structures between [13]cycloparaphenylene ([13]CPP) and an armchair (m,m) tube (5 ≤ m ≤ 9)...

2D 1T′-MoS2-WS2 van der Waals heterostructure for hydrogen evolution reaction: dispersion-corrected density functional theory calculations

Dispersion-corrected density functional theory calculations were implemented to investigate structural characteristics, as well as the hydrogen evolution reaction (HER) capability of 2D 1T′ phase MoS2-WS2 van der Waals heterostructures. Two van der Waals corrections were utilized in the study, namely DFT-D3 (semi-empirical-based) and vdW-DF2-B86R (ab-initio-based) corrections. Results show that the DFT-D3 correction stabilized the binding of the monolayers consistent with experimental observations, with binding energy per unit cell of -0.54 eV/cell. The Gibbs free energy of hydrogen adsorption ΔGads,H, which is the lone descriptor of HER, were calculated for the two known adsorption sites in the 1T′ phase, termed S1 (sulfur site with elongated bonds, more active for HER) and S2 (sulfur site with compressed bonds, less active for HER). It is revealed that at the van der Waals region, the S1 and S2 sites, acting as a single adsorption site, become active for HER, with significantly lowered value of ΔGads,H at 0.20-0.24 eV. This is linked to the synergistic interaction of the two sites in adsorbing hydrogen. In terms of electronic structure, the enhanced states in the vicinity of the Fermi level for the interacting S1 and S2 sites at the van der Waals region resulted from orbital hybridization among 3p states of the sulfur sites from the inner top and bottom surfaces. The merging of the two sites at the van der Waals region would result to HER efficiency that is expected to be higher by a factor of 2 compared to that on the top and bottom surfaces. This work has showed that 2D heterostructures could be of importance in catalysis, particularly in HER. Furthermore, it is showed that building a 2D heterostructure could be a good alternative to the application of strain in improving HER capability of 1T′ 2D materials without compromising the adsorption properties of other sites.

Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study.

Due to their atomic thicknesses and semiconducting properties, two-dimensional transition metal dichalcogenides (TMDCs) are gaining increasing research interest. Among them, Hf- and Zr-based TMDCs demonstrate the unique advantage that their oxides (HfO2 and ZrO2) are excellent dielectric materials. One possible method to precisely tune the material properties of two-dimensional atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. In the present work, the molecular adsorption of NO2 and NH3 on the two-dimensional trigonal prismatic (1H) and octahedral (1T) phases of Hf and Zr dichalcogenides (S, Se, Te) is studied using dispersion-corrected periodic density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are investigated. In addition, the effects of the molecular dopants (NH3 and NO2) on the electronic structure of the materials are studied. It was observed that the adsorbed NH3 donates electrons to the conduction band of the Hf (Zr) dichalcogenides, while NO2 receives electrons from the valance band. Furthermore, the NO2 dopant affects than NH3 significantly. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This study explores, not only the properties of the two-dimensional 1H and 1T phases of Hf and Zr dichalcogenides (S, Se, Te), but also tunes their electronic properties by adsorbing non-bonded dopants.

Unexpected Sandwiched-Layer Structure of the Cocrystal Formed by Hexamethylbenzene with 1,3-Diiodotetrafluorobenzene: A Combined Theoretical and Crystallographic Study

The cocrystal formed by hexamethylbenzene (HMB) with 1,3-diiodotetrafluorobenzene (1,3-DITFB) was first synthesized and found to have an unexpected sandwiched-layer structure with alternating HMB layers and 1,3-DITFB layers. To better understand the formation of this special structure, all the noncovalent interactions between these molecules in the gas phase and the cocrystal structure have been investigated in detail by using the dispersion-corrected density functional theory calculations. In the cocrystal structure, the theoretically predicted π···π stacking interactions between HMB and the 1,3-DITFB molecules in the gas phase can be clearly seen, whereas there are no π···π stacking interactions between HMB molecules or between 1,3-DITFB molecules. The attractive interactions between HMB molecules in the corrugated HMB layers originate mainly in the dispersion forces. The 1,3-DITFB molecules form a 2D sheet structure via relatively weak C–I···F halogen bonds. The theoretically predicted much stronger C–I···π halogen bonds between HMB and 1,3-DITFB molecules in the gas phase are not found in the cocrystal structure. We concluded that it is the special geometry of 1,3-DITFB that leads to the formation of the sandwiched-layer structure of the cocrystal.

Computational Study of APTES Surface Functionalization of Diatom-like Amorphous SiO2 Surfaces for Heavy Metal Adsorption.

The amorphous silica (SiO2) shell on diatom frustules is a highly attractive biomaterial for removing pollutants from aquatic ecosystems. The surface activity of silica can be enhanced by modification with organosilanes. In this work, we present an atomic-level theoretical study based on molecular dynamics and dispersion-corrected density functional theory calculations on the surface stability and adsorption of heavy metal (HM) compounds on silane- and 3-aminopropyltriethoxysilane (APTES)-covered SiO2 surfaces. Our simulations show that at low APTES coverage, the molecular adsorption of Cd(OH)2 and HgCl2 is more favorable near the modifier, compared to As(OH)3 that binds at the hydroxylated region on silica. At higher coverages, the metallic compounds are preferentially adsorbed by the terminating amino group on the surface, whereas the adsorption in the region between APTES and the oxide surface is also spontaneous. The adsorption is strongly driven by van der Waals interactions at the highly covered surface, where the consideration of dispersion corrections reduces the modifier-adsorbate interatomic distances and increases the adsorption energy by ca. 0.4-0.7 eV. The adsorption of water is favorable, although it is generally weaker than for the HM compounds. Based on our results, we conclude that the addition of APTES modifiers on silica increases the adsorption strength and provides extra binding sites for the adsorption of HM pollutants. These outcomes can be used for the design of more efficient structures of biomaterials for depollution of HMs.

Structural and energetic investigation on the host/guest inclusion process of benzyl isothiocyanate into β-cyclodextrin using dispersion-corrected DFT calculations

The formation of host-guest complex between benzyl isothiocyanate (BITC) and β-cyclodextrin (β-CD) was studied using dispersion-corrected density functional theory calculations. The complexation process was monitored using molecular docking simulations, natural bond orbital (NBO) technique, nuclear magnetic resonance (1H NMR) chemical shift calculations and non-covalent interactions (NCI) analysis. All these approaches are consistent with experimental findings. The calculated complexation energy was negative indicating the formation of inclusion complex. The most stable complexation of BITC involves the inclusion of its aromatic moiety in β-CD cavity (model A) in accord with experimental NMR chemical shift data. The non-covalent interactions (NCI) based on the reduced density gradient (RDG) analysis reveal that mainly weak Van der Waals intermolecular interactions between BITC and β-CD provide and ensure stability for the complexation process.

Enhancing hydrogen storage by metal substitution in MIL-88A metal-organic framework

MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated to be a promising material for gas storage and capture. However, the hydrogen storage capacity of MIL-88A has to be improved to meet the practical level at the ambient conditions. In this research, we elucidated the effects of transition metal substitution on the hydrogen storage capability of MIL-88A. The trivalent transition metals including Sc, Ti, V, Cr, Mn, and Ni have been selected to substitute for Fe in MIL-88A. Using the van der Waals dispersion-corrected density functional theory calculations, we explored the most favorable adsorption configurations of the hydrogen molecule in M-MIL-88A (M = Sc, Ti, V, Cr, Mn, Ni). We found that the V-MIL-88A has the strongest binding energy of 17 kJ mol−1 with the hydrogen molecule in the side-on configuration on the metal site. Besides, the grand canonical Monte Carlo simulations showed that the metal substitution greatly influences not only the favorable adsorption configuration and energy but also the hydrogen uptake due to the modification of the H2@MIL-88A interaction. Sc-MIL-88A was found to offer the highest gravimetric H2 uptake compared to the other M-MIL-88A. The value is 5.13 wt% at (cryogenic temperature 77 K, 50 bar) and 0.72 wt% at (room temperature 298 K, 100 bar) for the absolute; 4.63 wt% at (77 K, 10 bar) and 0.29 wt% at (298 K, 100 bar) for the excess capacity. Furthermore, Sc-MIL-88A also exhibited the highest volumetric uptake up to 52 g L−1 at 77 K and 7.1 g L−1 at 298 K for the absolute; 46 g L−1 at 77 K and 2.8 g L−1 at 298 K for the excess loading.

First-Principles Mechanistic Insights into the Hydrogen Evolution Reaction on Ni2P Electrocatalyst in Alkaline Medium

Nickel phosphide (Ni2P) is a promising material for the electrocatalytic generation of hydrogen from water. Here, we present a chemical picture of the fundamental mechanism of Volmer–Tafel steps in hydrogen evolution reaction (HER) activity under alkaline conditions at the (0001) and (10 1 ¯ 0) surfaces of Ni2P using dispersion-corrected density functional theory calculations. Two terminations of each surface (Ni3P2- and Ni3P-terminated (0001); and Ni2P- and NiP-terminated (10 1 ¯ 0)), which have been shown to coexist in Ni2P samples depending on the experimental conditions, were studied. Water adsorption on the different terminations of the Ni2P (0001) and (10 1 ¯ 0) surfaces is shown to be exothermic (binding energy in the range of 0.33−0.68 eV) and characterized by negligible charge transfer to/from the catalyst surface (0.01−0.04 e−). High activation energy barriers (0.86−1.53 eV) were predicted for the dissociation of water on each termination of the Ni2P (0001) and (10 1 ¯ 0) surfaces, indicating sluggish kinetics for the initial Volmer step in the hydrogen evolution reaction over a Ni2P catalyst. Based on the predicted Gibbs free energy of hydrogen adsorption (ΔGH*) at different surface sites, we found that the presence of Ni3-hollow sites on the (0001) surface and bridge Ni-Ni sites on the (10 1 ¯ 0) surface bind the H atom too strongly. To achieve facile kinetics for both the Volmer and Heyrovsky–Tafel steps, modification of the surface structure and tuning of the electronic properties through transition metal doping is recommended as an important strategy.

Experimental and Theoretical Investigation of the Elastic Properties of HfV2O7

We investigated the elastic properties of the HfV2O7 high-temperature phase, exhibiting negative thermal expansion, in a synergetic strategy of first-principle calculations and nanoindentation experiments performed on sputtered films. Self-consistent results were obtained for the measured elastic modulus (73 ± 14 GPa) and dispersion-corrected density functional theory calculations. The elastic properties of HfV2O7 are affected by long-range dispersion interaction, which may be induced by severe modification in the second-nearest neighbor O-O bond distance as obtained upon compression. HfV2O7 is composed of HfO6, VO4, and V2O7 building blocks, whereby the latter is characterized by an increasing V-O(-V) bond length upon compression.

High selective catalyst for ethylene epoxidation to ethylene oxide: A DFT investigation

Ethylene epoxidation to produce ethylene oxide is crucial in both fundamental knowledge and industrial chemical process. The reaction mechanism of ethylene epoxidation with N2O catalyzed by Mn-coordinated porphyrin-like graphene (Mn-N4GP) was investigated using dispersion-corrected density functional theory calculations. The reaction proceeds through two consecutive steps, (1) MnO active site (MnO-N4GP) formation, followed by (2) ethylene epoxidation on the MnO. The first step is a feasible process with the energy barrier of 0.77 eV and the MnO-N4GP is thermodynamically stable. The ethylene epoxidation in the second step competitively can undergo three possible intermediate-pathways; carboradical, alkoxide radical, and manganaoxetane intermediates. By systematic study of all possible pathways, we found that the pathway for alkoxide radical intermediate shows the most feasibility which it subsequently converts to three competitive products with the energy barriers of 0.25 eV, 0.56 eV, and 0.46 eV for the formation of ethylene oxide, acetaldehyde, and 5-membered ring (5MR) species, respectively. This catalyst is remarkably selective to ethylene oxide by 105 and 104 times compared with acetaldehyde and 5MR side products, respectively. Thus, the Mn-N4GP catalyst is suggested as a promising catalyst in terms of activity and selectivity for ethylene epoxidation using N2O as an oxidizing agent in mild condition.

Amino-Functionalized Water-Stable Metal-Organic Framework for Enhanced C2H2/CH4 Separation Performance.

Using a hydrothermal method, a water-stable metal-organic framework based on 8-connected Ni2 units with (4·62)2(47·613·83) topology, [Ni2(μ2-OH2)(ctpd)2(NH2-bdc)]·(EtOH)2·(H2O)2 (NbU-9-NH2), constructed by mixing a rigid tridentate ligand and an amino-p-carboxyl ligand, displays an enhanced ability for adsorbing C2H2 and adsorptive selectivity for C2H2/CH4. Dispersion-corrected density functional theory calculation confirmed that the enhenced acetylene adsorption is mainly derived from the weak hydrogen-bonding between a hydrogen atom of C2H2 and the nitrogen atom of the amino group.

Electronic-level insight into the weak interactions of ion pairs in acetate anion-based ionic liquids

The weak interactions in ion pairs of acetate anion-based ionic liquids (ILs) were investigated by dispersion-corrected density functional theory (DFT) calculations, where the structural parameters and vibrational frequency changes of ion pairs and isolated ions were systematically analyzed. Four kinds of stable configurations for ILs 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) and 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]) were determined by calculating the interaction energies of different ion pairs. The essence of weak interactions between ion pairs was qualitatively and quantitatively characterized by the electrostatic potential (ESP) of anions and cations, natural bond orbital (NBO), atoms in molecules (AIM), and Generalized Kohn-Sham energy decomposition analysis (GKS-EDA). The results indicated that the red-shifted hydrogen bond (H-bond) of C1-H2⋯O contributed the most to the stability of ion pairs. For the different stable ion pairs, the H-bond interactions formed by acetate anion ([OAc]−) with H protons on the imidazolium ring and side chain were separately divided into medium and weak H-bond, showing partly covalent and noncovalent nature. GKS-EDA results revealed that the interactions of ion pairs were mainly electrostatic dominant, charge transfer and orbital overlap contributions were also significant, which were in accordance with the ESP, NBO, and AIM analyses results. Hopefully, this study could offer an insight into weak interactions principle of ILs and provide a theoretical basis for predicting their physicochemical properties.