Preferred Adsorption Sites Research Articles

Effects of Silica Modification (Mg, Al, Ca, Ti, and Zr) on Supported Cobalt Catalysts for H2-Dependent CO2 Reduction to Metabolic Intermediates.

Serpentinizing hydrothermal systems generate H2 as a reductant and harbor catalysts conducive to geochemical CO2 conversion into reduced carbon compounds that form the core of microbial autotrophic metabolism. This study characterizes mineral catalysts at hydrothermal vents by investigating the interactions between catalytically active cobalt sites and silica-based support materials on H2-dependent CO2 reduction. Heteroatom incorporated (Mg, Al, Ca, Ti, and Zr), ordered mesoporous silicas are applied as model support systems for the cobalt-based catalysts. It is demonstrated that all catalysts surveyed convert CO2 to methane, methanol, carbon monoxide, and low-molecular-weight hydrocarbons at 180 °C and 20 bar, but with different activity and selectivity depending on the support modification. The additional analysis of the condensed product phase reveals the formation of oxygenates such as formate and acetate, which are key intermediates in the ancient acetyl-coenzyme A pathway of carbon metabolism. The Ti-incorporated catalyst yielded the highest concentrations of formate (3.6 mM) and acetate (1.2 mM) in the liquid phase. Chemisorption experiments including H2 temperature-programmed reduction (TPR) and CO2 temperature-programmed desorption (TPD) in agreement with density functional theory (DFT) calculations of the adsorption energy of CO2 suggest metallic cobalt as the preferential adsorption site for CO2 compared to hardly reducible cobalt-metal oxide interface species. The ratios of the respective cobalt species vary depending on the interaction strength with the support materials. The findings reveal robust and biologically relevant catalytic activities of silica-based transition metal minerals in H2-rich CO2 fixation, in line with the idea that autotrophic metabolism emerged at hydrothermal vents.

Accurate Nonempirical Range-Separated Hybrid van der Waals Density Functional for Complex Molecular Problems, Solids, and Surfaces

We introduce a new, general-purpose, range-separated hybrid van der Waals density \ph{functional, termed vdW-DF-ahbr,} within the non-empirical vdW-DF method [JPCM 32, 393001 (2020)]. It combines correlation from vdW-DF2 with a screened Fock exchange that is fixed by \ph{a new model of exchange effects} in the density-explicit vdW-DF2-b86r functional [PRB 89, 121103(R) (2014)]. The new vdW-DF2-ahbr prevents spurious exchange binding and has a small-density-gradient form set from many-body perturbation analysis. It is accurate for \ph{bulk as well as layered materials} and it systematically and significantly improves the performance of present vdW-DFs for molecular problems. Importantly, vdW-DF2-ahbr also outperforms present-standard (dispersion-corrected) range-separated hybrids on a broad collection of noncovalent-interaction benchmark sets, while at the same time successfully mitigating the density-driven errors that often affect the description of molecular transition states and isomerization calculations. vdW-DF2-ahbr furthermore improves on state of the art density functional theory approaches by 1) correctly predicting both the substrate structure and the site preference for CO adsorption on Pt(111), 2) outperforming existing non-empirical vdW-DFs for the description of CO$_2$ adsorption in both a functionalized and in a simple metal-organic framework, and 3) being highly accurate \ph{for the} set of base-pair interactions in a model of DNA assembly.

Molecular Diffusion and Self-Assembly: Quantifying the Influence of Substrate hcp and fcc Atomic Stacking.

Molecular diffusion is a fundamental process underpinning surface-confined molecular self-assembly and synthesis. Substrate topography influences molecular assembly, alignment, and reactions with the relationship between topography and diffusion linked to the thermodynamic evolution of such processes. Here, we observe preferential adsorption sites for tetraphenylporphyrin (2H-TPP) on Au(111) and interpret nucleation and growth of molecular islands at these sites in terms of spatial variation in diffusion barrier driven by local atomic arrangements of the Au(111) surface (the 22× √3 “herringbone” reconstruction). Variable-temperature scanning tunnelling microscopy facilitates characterization of molecular diffusion, and Arrhenius analysis allows quantitative characterization of diffusion barriers within fcc and hcp regions of the surface reconstruction (where the in-plane arrangement of the surface atoms is identical but the vertical stacking differs). The higher barrier for diffusion within fcc locations underpins the ubiquitous observation of preferential island growth within fcc regions, demonstrating the relationship between substrate-structure, diffusion, and molecular self-assembly.

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues.

The novel ZrIV-based perfluorinated metal–organic framework (PF-MOF) [Zr6O4(OH)4(TFS)6] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H2TFS) as a bridging ditopic linker. Since H2TFS can be seen as the fully aliphatic and perfluorinated C4 analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate (MOF-801). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H2TFS aqueous solutions were carried out. Despite the different H2TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr6O4(μ3-OH)4(μ1-OH)2.08(H2O)2.08(FUM)4.04(HTFS)1.84] (PF-MOF1) and [Zr6O4(μ3-OH)4(μ1-OH)1.83(H2O)1.83(FUM)4.04(HTFS)2.09] (PF-MOF2) (FUM2– = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO2 and N2 adsorption isotherms collected on all samples reveal that both CO2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Qst) and CO2/N2 adsorption selectivity increase with the amount of incorporated TFS2–, reaching the maximum values of 30 kJ mol–1 and 41 (IAST), respectively, in PF-MOF2. This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO2 adsorption in MOF-801 and ZrTFS. Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO2 adsorption site is the tetrahedral pore in the “UiO-66-like” structure. The extra energy stabilization stems from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster.

High-throughput screening of metal–organic frameworks for hydrogen purification

In industry, the separation/purification of hydrogen from methane impurity is of significant importance, especially under the background of carbon dioxide mitigation towards carbon neutrality. Adsorption separation is an effective route for this application regarding energy cost and separation efficiency, for which the selection of adsorbents is the key. This work proposes a computer-aided high-throughput screening strategy for the selection of metal–organic frameworks (MOFs) for CH4/H2 separation, striving to accelerate the practice of discovering better adsorbents for this particular application. Firstly, a prescreening was conducted on 12,020 computation-ready three-dimensional MOFs based on intuitive structure and chemical analysis, shortlisting 5,096 candidates. Secondly, a molecular simulation-assisted structure–property relationship analysis was performed based on small sampled MOFs, from which two structure criteria were derived for high-performing MOFs. Based on the criteria, a screening procedure was then carried out, narrowing the candidate MOFs down to 2,408. The rigorous molecular simulation was again applied to assess the separation performance metrics of these MOFs, covering adsorption selectivity, working capacity, adsorbent performance score, regenerability and separation potential. By analysing the simulated performance indicators, top-performing MOFs were identified and a more detailed analysis of the structure-performance relationship was carried out. Additionally, the impact of several practical factors, covering humidity, operation temperature and inlet gas composition, on the performance of the selected adsorbents in practice was explored. It was observed that the robustness of MOFs at humidity depends on their hydrophilicity and preferential adsorption sites. Meanwhile, operation temperature can be manipulated to accommodate flexible production scenarios.

Adsorption Site Preference Determined by Triangular Topology: Application of the Method of Moments to Transition Metal Surfaces

The adsorption sites of the top and hollow on the close-packed surfaces of transition metals are well known. In this paper, which site is more preferred for the adsorption of atoms and molecular fragments on the metal surfaces is discussed based on the topology of the adsorption geometry. For this purpose, the method of moments for the electronic density of states is applied to the surface. Adsorption at the hollow site generates a triangular topology, leading to a more negative value of the third moment (μ3) than that at the top site, which generates no triangular topology. When the difference in energy between the two adsorption sites is plotted against the band filling of the metal surface, a characteristic node at around the intermediate band filling can be found. This is a signature that the energy difference curve is controlled by μ3. Roughly speaking, the hollow-site adsorption, which has a more negative μ3 value, takes precedence at low band fillings, while the top site adsorption, which has a less negative μ3 value, takes precedence at high band fillings. One can conclude that an adsorption structure with more three-membered rings on a surface is more stable at low electron counts whereas that with less three-membered rings is more stable at high electron counts. However, if the strength of the metal–adsorbate bond is significantly greater than that of the metal–metal bond, the effect of the second moment (μ2) on the energy difference curve cannot be neglected. The hollow-site adsorption leads to a larger value of μ2 due to the topological feature of a larger coordination number around the adsorbate atom. As a result, the hollow-site adsorption is preferred over the top site at any band filling.

Zr-Based Metal–Organic Framework Nanocrystals for Water Remediation

A series of derivatives of the Zr-based MOF UiO-66 were synthesized, fully characterized, and tested for their efficiency in the adsorption and removal of the highly toxic elements lead (Pb(II)), cadmium (Cd(II)), arsenate As(V), and selenite Se(IV). Different structural engineering techniques were employed to alter the properties of the Zr-MOFs, such as linker functionalization and defect creation to increase the density of adsorption sites. The functionalization of the UiO-66-based structures with carboxylate groups increased the removal efficiency of Pb(II) cations by more than 70% and that of Cd(II) cations by more than 60% to reach 125.2 and 69.1 mg/g, respectively. Moreover, the induced cluster defects were found to be the preferred adsorption sites of the anion pollutants. The adsorption of As(V) and Se(IV) by the high-defect UiO-66-200FA was 132.5 and 30.8 mg/g, respectively, which was higher than that of the less defect UiO-66, while almost no adsorption was achieved by UiO-66(COOH)2. Furthermore, a multivariate (MTV-MOF) approach was employed to investigate the effect of the functional groups on the diffusion of the adsorbates within the porous networks. Interestingly, adsorption equilibrium was reached within minutes for the best-performing MOFs, highlighting their potential to be used in practical water treatment applications. Finally, chemical adsorption seemed to be dominant among binding mechanisms for all the pollutants, which were analyzed in detail to determine the characteristics that govern the adsorption efficiency of UiO-66-based adsorbents. The conclusions provided by this comprehensive study help serve as a guideline for future researchers aiming to employ UiO-66-based adsorbents by providing them with a deep understanding of the design strategies required to maximize the performance of this robust MOF.

Finite-Size Error Cancellation in Diffusion Monte Carlo Calculations of Surface Chemistry.

The accurate prediction of reaction mechanisms in heterogeneous (surface) catalysis is one of the central challenges in computational chemistry. Quantum Monte Carlo methods─Diffusion Monte Carlo (DMC) in particular─are being recognized as higher-accuracy, albeit more computationally expensive, alternatives to Density Functional Theory (DFT) for energy predictions of catalytic systems. A major computational bottleneck in the broader adoption of DMC for catalysis is the need to perform finite-size extrapolations by simulating increasingly large periodic cells (supercells) to eliminate many-body finite-size effects and obtain energies in the thermodynamic limit. Here, we show that it is possible to significantly reduce this computational cost by leveraging the cancellation of many-body finite-size errors that accompanies the evaluation of energy differences when calculating quantities like adsorption (binding) energies and mapping potential energy surfaces. We analyze the cancellation and convergence of many-body finite-size errors in two well-known adsorbate/slab systems, H2O/LiH(001) and CO/Pt(111). Based on this analysis, we identify strategies for obtaining binding energies in the thermodynamic limit that optimally utilize error cancellation to balance accuracy and computational efficiency. Using one such strategy, we then predict the correct order of adsorption site preference on CO/Pt(111), a challenging problem for a wide range of density functionals. Our accurate and inexpensive DMC calculations are found to unambiguously recover the top > bridge > hollow site order, in agreement with experimental observations. We proceed to use this DMC method to map the potential energy surface of CO hopping between Pt(111) adsorption sites. This reveals the existence of an L-shaped top-bridge-hollow diffusion trajectory characterized by energy barriers that provide an additional kinetic justification for experimental observations of CO/Pt(111) adsorption. Overall, this work demonstrates that it is routinely possible to achieve order-of-magnitude speedups and memory savings in DMC calculations by taking advantage of error cancellation in the calculation of energy differences that are ubiquitous in heterogeneous catalysis and surface chemistry more broadly.

The preparation of ultrastable Al3+ doped CeO2 supported Au catalysts: Strong metal-support interaction for superior catalytic activity towards CO oxidation

The classical strong metal-support interaction (SMSI) plays a key role in improving thermal stability for supported Au catalysts. However, it always decreases the catalytic activity because of the encapsulation of Au species by support. Herein, we demonstrate that Al3+ is a functional additive which could effectively improve both catalytic activity and sintering resistant property for H2 pretreated Al3+ doped CeO2 supported Au (AuCeAl) catalyst at high temperature. The physical characterization and in-situ DRIFTS results provide insight that more oxygen vacancies generated by Al3+ doping could be as preferential adsorption sites for CO molecules when the encapsulation of Au species occurred, which is certificated by an accelerated formation of bicarbonate species. In the meantime, smaller Au nanoparticles with higher dispersion (2.8 nm, 85.63%) is achieved in AuCeAl catalysts, compared with that in CeO2 supported Au (AuCe) catalysts (5.1 nm, 36.17%). Additionally, the as-prepared AuCeAl catalysts also have superior catalytic performance even after calcination at 800 °C in air.

X‐Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances. Figure 1

Fundamental Properties of Transition-Metals-Adsorbed Germanene: A DFT Study

The transition metal (TM)-absorbed germanene systems enriched by strong chemical bonding are investigated using first-principles calculations. Dedicated calculations include the geometry, preferable adsorption sites, atom-dominated band structure, spin–density distributions, spatial charge distribution, and the projected density of states (DOS). The strong multi-orbital chemical bonds between TMs and Ge atoms can create seriously buckled structures and a non-uniform chemical environment, which are responsible for the unusual electronic properties. Of the three chosen systems, the Fe–Ge and Co–Ge ones possess magnetic properties, while the Ni–Ge system exhibits non-magnetic behavior. The orbital-hybridization-induced characteristics are revealed in van Hove singularities of the DOS.

Experimental and DFT investigations on enhanced stability found on Re-, Rh-, and Nb-promoted Pt/WOx/γ-Al2O3 catalyst during aqueous-phase glycerol hydrogenolysis

The aqueous-phase glycerol hydrogenolysis is one of the catalytic methods used to convert the glycerol by-product from biodiesel production to highly valuable chemicals, especially 1,3–propanediol (1,3PDO). The density functional theory (DFT) and experiments were used to study the promotional effects of third metals promoters: Re, Rh, and Nb in WOx doped Pt-based catalysts that withstand deactivation via metal leaching, sintering, and coking. The third metal promoter improves catalytic activity and 1,3PDO selectivity due to increased electron density at the Pt active site confirmed by DFT. However, improved stability was found only on re- and Rh-promoted systems, evident from sustained activity and selectivity of spent catalysts. The deactivation study revealed that Pt leaching was negligible in all cases. Still, W leaching is more concerned and can be reduced only by Re and Nb promoters, whereas Pt sintering can be prevented only in the case of Re promoter. Mitigation of coking can be realized from DFT that only the Re promoter has comparable electron donor property to Pt and can prevent active site blockage by shifting the preferable coke adsorption site from Pt to a promoter atom, as also confirmed by the lowest amount of type I coke measured by the thermal gravimetric analysis leading to a high product yield.

Enhanced removal of Cd2+ from water by AHP-pretreated biochar: Adsorption performance and mechanism

The sesame straw-derived biochar was successfully prepared via alkaline hydrogen peroxide (AHP) pretreatment in this study. Systematic experimental characterizations, 15 relevant batch and column adsorption models, combined with density functional theory (DFT) calculation were used to investigate the performances and micro-mechanisms of Cd2+ adsorption onto biochar. We found AHP-pretreatment could greatly improve the adsorption performance of biochar for Cd2+. The maximum Cd2+ adsorption capacity of AHP-pretreated biochar (87.13 mg g−1) was much larger than that of unpretreated biochar. Cd2+ adsorption was mainly dominated by the chemisorption of the homogeneous surface monolayer. The hydroxyl and carboxyl groups on the surface of biochar provided preferential adsorption sites, and liquid film diffusion and intra-particle diffusion were two dominant rate-controlling steps. Our results showed that ion exchange, co-precipitation, surface complexation, and Cd2+-π interaction were the dominant adsorption mechanisms. Especially, DFT calculations well-identified that lone-pair electrons during complexation and π electrons during coordination were provided by oxygen-containing functional groups and aromatic rings, respectively. The experimental breakthrough curves fitted better with the theoretical value of the BJP model, compared to Thomas, Yoon-Nelson, and EXY models. Overall, our study provides a promising method for Cd2+ removal from wastewater and resource utilization of agricultural wastes.

Integrating tri-mural nanotraps into a microporous metal-organic framework for C2H2/CO2 and C2H2/C2H4 separation

Considering the tremendous significance of purification and capture for acetylene (C2H2) in the fields of manufacturing and the petrochemical industry, seeking appropriate materials with prominent performance is a crucial task and also remains an enduring challenge. To pursue this target, we report a robust microporous metal-organic framework (NUM-15) featuring two types of elaborate tri-mural nanotraps, in which fruitful nano-space confinement for accommodating gas molecules and multiple preferential adsorption sites providing multiple specific interactions with C2H2 were supplied to form cooperative effect for efficient separation property. The NUM-15a (activated NUM-15) exhibits high loading for C2H2 (3.5 mmol g−1) over carbon dioxide (CO2) and ethylene (C2H4) at 298 K and 1.0 bar and shows efficient separation performance for binary C2H2/CO2 and C2H2/C2H4 mixtures. The GCMC calculation revealed the crucial role of the multiple interactions in nanotraps for the selective capture of C2H2. Under ambient conditions, dynamic breakthrough experiments revealed that NUM-15a demonstrates the enormous potential for actual industrial gas separation and is expected to be applied to the correlative industrial process. Both experiments and simulation calculations distinctly demonstrated that fabricating tri-mural nanotraps in MOFs is a feasible strategy for efficient C2H2 capture and separation and provided a new route for exploiting high-performance materials for separation and purification technology.

How MoS2 assisted sulfur vacancies featured Cu2S in hollow Cu2S@MoS2 nanoboxes to activate H2O2 for efficient sulfadiazine degradation?

This work synthesized a series of hollow Cu2S@MoS2-x nanoboxes by using Cu2O nanocubes as precursor. Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) analysis identified the existence of Sv in Cu2S@MoS2-x. Sulfadiazine (SDZ), a typical antibiotic, was selected as the target contaminant to evaluate the catalytic activity of Cu2S@MoS2-x via H2O2 activation. With adding 0.2 g/L Cu2S@MoS2-2 and 5 mM H2O2, 95.9% of SDZ (20 μM) was removed in 60 min and the reaction rate constant obtained by Cu2S@MoS2-2 (0.239 min−1) was improved by 4 times and 10 times in contrast with Cu2S (0.0622 min−1) and MoS2 (0.0219 min−1). EPR analysis and radical scavenger tests confirmed •OH and 1O2 as the dominated reactive oxygen species (ROS). In Cu2S@MoS2-2 activated H2O2 system, the yield of 1O2 was 6.12 × 10-14 mol/L, more than 8 times that in Cu2S/H2O2 system (0.73 × 10-14 mol/L). Density functional theory (DFT) calculation revealed that the Cu atoms exposed by the formation of Sv on Cu2S@MoS2-2 were the preferred adsorption sites for O2, which further achieved the conversion of O2 to •O2−. In the activation process, Cu2S mainly produced •OH through sulfur-enhanced Fenton process, while MoS2 shell accounted for the O2 production. Thereafter, the obtained O2 acquired electrons from the Cu atoms exposed by Sv to produce •O2−, followed by the generation of 1O2. The cooperation between MoS2 and Cu2S resulted in the superior catalytic activity. The excellent recyclability, stability, and adaptability demonstrated Cu2S@MoS2-2 as a reliable candidate for activating H2O2 toward refractory organics degradation.

A nickel-based metal-organic framework for efficient SF6/N2 separation with record SF6 uptake and SF6/N2 selectivity

Seeking top-performing adsorbents with salient structural stability and renewability to realize efficient adsorption and enrichment of sulfur hexafluoride (SF6) from the gaseous blend is of great importance. Here, an ultramicroporous nickel-based metal-organic framework (MOF), termed Ni(NDC)(TED)0.5 with multiple naphthalene rings-functionalized pore environment, was synthesized to efficiently separate SF6/N2 mixture. Equilibrium adsorption results show that Ni(NDC)(TED)0.5 possesses a record IAST SF6/N2 selectivity (750) and SF6 uptake (61.86 cm3 cm−3), higher than all adsorbents reported so far, and resolves the trade-off phenomenon between SF6 adsorption capacity and SF6/N2 selectivity. In particular, the selectivity is 2.3 times that of the current highest material (SBMOF-1). Furthermore, theoretical calculations reveal that the pore centers of material are the most energetically-favorable preferential adsorption sites. Meanwhile, the stronger affinity between SF6 and framework is attributed to the shorter interaction distance between F atom on SF6 and pore wall. Breakthrough tests confirm this MOF can completely separate SF6/N2 mixture. The breakthrough uptake (56.60 cm3 cm−3) and dimensionless time (τ: 281.8) of this material for SF6 are separately 1.6 and 2.5 times that of the currently reported benchmark (Cu-MOF-NH2). In combination of excellent structural stability and recyclability, this material may be an ideal material for the recovery of SF6 from SF6/N2 mixture.

Structural and bonding properties of small hydrocarbons inside Ca(squarate)-metal organic framework: ab-initio study

Ab−initio Molecular Dynamic (MD) and static Density Functional Theory (DFT) are used to study the structural and bonding properties of small hydrocarbon adsorbates inside Ca(squarate)−Metal Organic Framework (MOF). Car−Parrinello Molecular Dynamics (CPMD) simulations of a single−adsorbate−MOF structure are used to obtain the adsorbate most preferred site of adsorption. This site is used for further structural and bonding analyses using static DFT. Unlike many other MOFs; we found that the Ca(squarate)−MOF physisorbs and weakly binds small adsorbate molecules such as C2H2, C2H4, C2H6, and C3H8 with no observed charge transfer and minimal hybridization with the MOF orbitals. No covalent bonding is seen near the preferred site of adsorption. The calculated binding energies decreases as the H content in the adsorbate molecule increases and found to be −18.71 kJ/mol, −18.14 kJ/mol, −15.75 kJ/mol, and—4.47 kJ/mol for C2H2, C2H4, C2H6, and C3H8 molecules respectively. Density of State (DOS) and a Crystal Orbital Overlap Population (COOP) analyses show that the interactions between C and H atoms in the molecule and C and O atoms in the MOF have antibonding characteristics near the Fermi level. These antibonding states tend to destabilize the overall electronic structure of the combined adsorbate/MOF system and hence decrease the binding energies of these adsorbates inside the MOF.

Molecular self-diffusivity and separation of CH4/H2S in metal organic framework MIL-47(V)

The loading dependence of self-diffusion coefficient (Ds), diffusion selectivity (sdiff), and structural properties of pure CH4, H2S, and their mixtures in MIL-47(V) material were investigated using Molecular Dynamics (MD) simulations. The preferential adsorption sites of gas molecules in the lattice, as well as the molecular structure of gases were determined using Radial Distribution Functions (RDFs). The diffusion behavior of gas molecules diffusing in the lattice was further investigated utilizing the 2D probability density plots and free energy profiles derived by the Histogram Sampling method. Furthermore, the findings of our computational study are examined and compared to previous research.

First principles study of Li adsorption properties of a Borophene based hybrid 2D material B5Se

In this work we investigate the Li adsorption properties of a hybrid 2D material, namely monolayer B5Se with first principles calculations. The density functional theory (DFT) calculations are performed with the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhoff (PBE) exchange correlation functional, inclusive of van der Waals corrections with Grimme's DFT-D3 scheme. Structural optimization calculations show 2 dimensional B5Se to have a distorted hexagonal structure with five B atoms and one Se atom at the vertices of each hexagon, with a favourable cohesive energy of formation of 0.648 eV/atom. A metallic nature was observed for the hybrid material from the band structure and density of states calculations. Electrode performance metrics, as the most preferred adsorption sites and adsorption energies, open circuit anode potentials, charge density differences and specific capacities for varying adatom coverage were evaluated with the DFT calculations. The adatom diffusion barriers were evaluated with a nudged elastic band (NEB) method. The ab-initio calculations predict a maximum theoretical specific capacity of 1486.87 mAhg−1 for Li adsorption on 2D B5Se, which is over four times that of conventional Li ion battery anode materials. This coupled with an open circuit anode potential of 0.291–0.179 V for different degrees of Li coverage, and a small Li diffusion barrier of 0.15 eV, metallic nature of the sheet under pure and lithiated conditions and good charge density variations, make monolayer B5Se a potent anode material for Li-ion battery applications.

Highly Active and Stable Fe/Co/N Co-doped Carbon-Anchored Pd Nanoparticles for Oxygen Reduction Reaction.

A highly active and stable electrocatalyst based on Pd nanoparticles anchored on zeolitic imidazolate framework-derived Fe/Co/N co-doped carbon (Pd/FeCoNC) is prepared. FeCo alloy nanoparticles are uniformly dispersed and wrapped by graphene layers in Fe/Co/N co-doped carbon (FeCoNC). The influences of carbonization temperature on the structure and catalytic activity of FeCoNC toward oxygen reduction reaction (ORR) are investigated. The FeCoNC prepared at 800 °C (FeCoNC-800) has a favorable ORR catalytic activity as a consequence of the synergistic effect of Fe/Co/N co-doping and hierarchical pore structures of coexisting micropores and mesopores. Pyridinic N in FeCoNC is a preferential adsorption site for anchoring Pd nanoparticles. Pd/FeCoNC exhibits both superior activity and durability to 40 wt % Pt/C at the same level of metallic mass loading, which shows a 44 mV higher half-wave potential (0.88 V) than Pt/C and a 91% remaining current of the initial after 10,000 s. The Fe/Co/N co-doping and hierarchical pores of FeCoNC contribute a large diffusion current, and the introduction of Pd realizes more positive onset and half-wave potentials. This work provides an easy way for preparing low-cost and high-efficiency catalysts for ORR.