Specific Adsorption Sites Research Articles

Diffusion Monte Carlo study of O2 adsorption on single layer graphene

Diffusion Monte Carlo (DMC) calculations were performed for an accurate description of the nature of the ${\mathrm{O}}_{2}$ adsorption on single layer graphene. We investigated the stable orientation of ${\mathrm{O}}_{2}$ at a specific adsorption site as well as its equilibrium adsorption energy. At equilibrium adsorption distances, an ${\mathrm{O}}_{2}$ molecule was found to prefer a horizontal orientation, where the O-O bond is parallel to the graphene surface, to the vertical orientation. However, the vertical orientation is favored at ${\mathrm{O}}_{2}$-graphene distances shorter than the equilibrium distance, which could be understood by the steric repulsion between O and C atoms. Contrary to previous DFT calculations, our DMC calculations show that the midpoint of a C-C bond (a bridge site) is energetically preferred for the ${\mathrm{O}}_{2}$ adsorption to a center of a hexagonal ring (a hollow site). The lowest DMC adsorption energy was found at an intermediate point between a hollow site and a bridge site, where the ${\mathrm{O}}_{2}$ adsorption energy was estimated to be $\ensuremath{-}0.142(4)$ eV, which is in very good agreement with the recently reported experimental value. Finally, we have found that ${\mathrm{O}}_{2}$ is very diffusive on the surface of graphene with the diffusion barrier along a bridge-hollow-bridge path being as small as $\ensuremath{\sim}11$ meV.

STRUCTURE STABILITY AND ELECTRONIC PROPERTIES OF PtmIrn(m+n=8−10) AND ADSORPTION SITES OF NO ON Pt–Ir CLUSTERS AT THE DFT LEVEL

The geometrical structures, stability and electronic properties of PtmIrn and PtmIrnNO ([Formula: see text]) clusters have been studied by using density functional theory (DFT). Simple cube evolution pattern is revealed for Ir[Formula: see text] and Ir-rich clusters. The curve of the second-order energy difference of PtnIr[Formula: see text] clusters shows the evident even–odd oscillations, indicating that PtIr8, Pt3Ir6, Pt5Ir4 and Pt7Ir2 clusters are more stable than their neighbors. The minimum excess energies values are seen for Pt7Ir, PtIr8 and Pt2Ir8, which means that these clusters show the stronger mixing tendency. Analysis of magnetic properties shows that the total magnetic moment contributed from Ir and Pt atoms, and the magnetic moment mainly comes from localization of the d orbit electron. All ground-state structures show an adsorption of NO at the top site of the bare cluster via the [Formula: see text] atom. In all of the alloy clusters, NO molecule prefers to be adsorbed near the Ir atom site. Fukui function is an accurate predictor of the reactivity on a specific adsorption site in Pt–Ir alloy clusters.

Carboxylic acid induced near-surface restructuring of a magnetite surface

A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.

Engineering the Pore Size of Pillared-Layer Coordination Polymers Enables Highly Efficient Adsorption Separation of Acetylene from Ethylene.

The pore size of adsorbents plays a vital role in determining the overall separation performance of gas separation and purification by adsorption. In this work, the pore apertures of the coordination pillared layer (CPL) was systematically controlled by adjusting the length of pillared ligands. We used pyrazine, 4,4'-bipyridine, and 1,2-di(4-pyridyl)-ethylene with increased length to synthesize CPL-1 (L = pyrazine), CPL-2 (L = 4,4'-bipyridine), and CPL-5 [L = 1,2-di(4-pyridyl)-ethylene], respectively. The aperture size of these CPLs varies from 4 to 11 Å: CPL-1 (4 × 6 Å2), CPL-2 (9 × 6 Å2), and CPL-5 (11 × 6 Å2). Among the three frameworks, CPL-2 exhibits the highest C2H2 uptake at ambient conditions as it has moderate pore size and porosity. However, CPL-1 has the best separation performance in the breakthrough experiments with binary gas mixture of C2H2/C2H4, thanks to the optimal pore size nearly excluding C2H4, which is only observed in the state-of-the-art UTSA-300a so far. The DFT calculations were carried out to elucidate the specific adsorption sites for both acetylene and ethylene among these frameworks. The modeling results suggest that binding strength is highly related to aperture size and that CPL-1 shows the highest adsorption selectivity owing to the optimal pore size. This work demonstrates that engineering pore size enables us to fabricate the highly efficient metal-organic framework (MOF)-based adsorbents for specific gas separation on the basis of the isoreticular chemistry.

Understanding Electrocatalytic Pathways in Complex Organic and Inorganic Composites in Aqueous and Non Aqueous Environments

There are very few techniques that lend themselves to rigorous requirements, where the individual role of different transition metal components of nano-phasic materials can be investigated in both amorphous and crystalline forms. We have validated a new analytical technique along with theoretical calculations, which enables the amount and specific adsorption site of species such as H, OH, O, CO, other C1 moieties and even anions such as bisulfate to be determined in situ on a metal surfaces using synchrotron XANES data. This technique, commonly called the ‘Delta Mu (∆μ) Technique’ can be taken in situ in both electrochemical cells and operating fuel cells, and can be obtained from a wide variety of transition metal surfaces and alloys,[3] including non-Pt based metal electrocatalysts with element specificity. EXAFS data taken concurrently provide information on the changes in short range atomic order around the absorber atom thereby providing structural information such as bond distances and coordination numbers (thereby information on average cluster size, homogeneity and surface segregation etc.). In this part of the presentation among other things we will show in-situ XAS study of group of (Metal-Nitrogen-Carbon) MNC catalysts from perspective of metal center. We will demonstrate the presence of ORR active bio-mimetic porphyrin-like FeNx functionalities, as well as other forms of the metal which indicate no catalytic activity. The XAS in-situ studies will be combined with electroanalytical evaluations using cyclic voltammetry (CV) and square wave voltammetry (SWV). Further the factors governing the ORR activity at MNC centers, such as the Lewis basicity of the carbon support and the accompanying changes in the redox potential of the metal center will be discussed. This part of the presentation will bring together electrochemistry and XAS spectroscopy (including ∆μ) on effect of pH (inner and outer sphere charge transfer) and aqueous vs. non-aqueous electrolyte (concerted vs. non-concerted charge transfer).

Direct Observations of the Rotation and Translation of Anisotropic Nanoparticles Adsorbed at a Liquid-Solid Interface.

We can learn about the interactions between nanoparticles (NPs) in solution and solid surfaces by tracking how they move. Here, we use liquid cell transmission electron microscopy (TEM) to follow directly the translation and rotation of Au nanobipyramids (NBPs) and nanorods (NRs) adsorbed onto a SiN x surface at a rate of 300 frames per second. This study is motivated by the enduring need for a detailed description of NP motion on this common surface in liquid cell TEM. We will show that NPs move intermittently on the time scales of milliseconds. First, they rotate in two ways: (1) rotation around the center of mass and (2) pivoted rotation at the tips. These rotations also lead to different modes of translation. A NP can move through small displacements in the direction roughly parallel to its body axis (shuffling) or with larger steps via multiple tip-pivoted rotations. Analysis of the trajectories indicates that both displacements and rotation angles follow heavy-tailed power law distributions, implying anomalous diffusion. The spatial and temporal resolution afforded by our approach also revealed differences between the different NPs. The 50 nm NRs and 100 nm NBPs moved with a combination of shuffles and rotation-mediated displacements after illumination by the electron beam. With increasing electron fluence, 50 nm NRs also started to move via desorption-mediated jumps. The 70 nm NRs did not exhibit translational motion and only made small rotations. These results describe how NP dynamics evolve under the electron beam and how intermittent pinning and release at specific adsorption sites on the solid surface control NP motion at the liquid-solid interface. We also discuss the effect of SiN x surface treatment on NP motion, demonstrating how our approach can provide broader insights into interfacial transport.

Hierarchical Fe3O4‐derived organic/inorganic hybrids constructed by modified bio‐inspired functionalization: efficient adsorbents for water‐soluble methylene blue and mechanism

AbstractBackgroundThe development of efficient and sustainable adsorbents is still a challenge for dye removal in practical applications. Here a method to prepare hierarchical Fe3O4‐derived organic/inorganic hybrid adsorbents for methylene blue removal is demonstrated. Typically, two different structured magnetic nanoparticles were synthesized through solvothermal route and chemical co‐precipitation (named as S‐Fe3O4 and C‐Fe3O4 respectively), which were further functionalized by a novel modified mussel‐inspired method with dopamine (DA) and (3‐aminopropyl)triethoxysilane (KH550), thus obtaining core‐shell Fe3O4/poly(DA + KH550) adsorbents.ResultsThe effects of microstructure, morphology and surface functionalization on the adsorption performance of core‐shell Fe3O4 adsorbents were systematically investigated, thus achieving target structurally optimized core‐shell Fe3O4 adsorbents. Especially, the maximum adsorption capacity of core‐shell S‐Fe3O4/poly(DA + KH550) adsorbent reached 400.00 mg g−1 because of high specific surface area and abundant active adsorption sites. In addition, the adsorption kinetics and adsorption isotherm fitted a pseudo‐second‐order model and Langmuir model respectively.ConclusionThe recycling process of adsorbent is sustainable in economy and energy by applying an external permanent magnet. In summary, the high adsorption capacity, easy separation and excellent reusability of S‐Fe3O4/poly(DA + KH550) core‐shell adsorbents make them promising for dye effluent removal. © 2019 Society of Chemical Industry

Preparation and performance of aminated polyacrylonitrile nanofibers for highly efficient copper ion removal

As promising sorbent, nanofibers with high specific surface area and adsorption site loading show excellent adsorption capacity. Polyacrylonitrile (PAN) is an industrial by-product with abundant nitrile groups (CN). However, raw polyacrylonitrile shows almost no binding ability toward metal ions. Herein, aminated polyacrylonitrile (APAN) nanofibers are synthesized by electrospinning followed by grafting branched polyethyleneimine (bPEI). The microstructures of these nanofibers show a uniform fiber diameter of 595 nm and a high surface area of 2.81 m2 g−1. In addition, the > N/NH group loading is as high as 5.7 wt.%. Fourier transform infrared spectroscopy and elemental analysis results confirm the formation of covalent bonds (C(O)NH) between PAN and bPEI, resulting in a stable structure and hence excellent reproducibility. And the original adsorption capacity can be maintained by 61.6% after 5 cycles. The influence of solution pH, adsorption isotherm, initial concentration, and contact time on Cu2+ adsorption is systematically investigated. The maximum adsorption capacity of APAN nanofibers is 149.8 mg g-1, which is difficult to achieve by most low density adsorbents. Furthermore, sorption-isotherm experiments reveal that the adsorption behavior obeys Langmuir model, and the adsorption kinetic agrees second-order dynamics model. Cu2+ ions are chelated with > N/NH groups in five-membered rings. Therefore, these APAN nanofibers open up delighted opportunities for the efficient removal of Cu2+ from industrial wastewater.

Selective Removal of Acid Fuchsin from Aqueous Solutions by Rapid Adsorption onto Polypyrrole Crosslinked Cellulose/Gelatin Hydrogels

The dispersion and macroization of nanomaterials have great potential in the development of high-efficiency adsorbents. To address organic dye pollution, two inexpensive and renewable biological resources, gelatin (Gel) and carboxymethyl cellulose (CMC), were crosslinked by nanostructured polypyrrole (PPy) to achieve a novel PPy-hydrogel. The morphology and composition of the CMC/Gel/PPy hydrogels were characterized. The result shows that the CMC/Gel solution can provide an excellent condition for the dispersion of polypyrrole (PPy). Based on the combination of PPy and the biomass matrix, specific adsorption sites can be formed by the synergistic effect of the functional groups derived from CMC, Gel and PPy. The CMC/Gel/PPy hydrogel has an exclusive adsorption of acid fuchsin (AF). Furthermore, the adsorption process of AF on the CMC/Gel/PPy hydrogel was investigated, and influencing factors, such as adsorbent dosage, temperature, initial dye concentration and adsorbent recyclability, were discussed in detail. Under optimal conditions, the CMC/Gel/PPy hydrogel can reach an adsorption equilibrium within 50 minutes in a 60 mg/L AF solution at 24 °C, and the AF removal efficiency was approximately 94%. It was also observed that the AF removal efficiencies of CMC/Gel/PPy remained almost unchanged after at least seven consecutive adsorption–desorption cycles.

Irreversible adsorption of acidic, basic, and water gas molecules on calcium-deficient hydroxyapatite.

Hydroxyapatite [Ca10(PO4)6(OH)2, HAP] has P-OH Brønsted acidic sites, Ca2+ Lewis acidic sites, and OH- and O2- basic sites on which acidic and basic gas molecules can be selectively adsorbed, and has no micropore onto which various molecules adsorb regardless of the chemical properties of gas molecules. The interaction between the surface sites and acidic and basic gas and water molecules has been investigated by evaluating the adsorption properties of various molecules on the surfaces of calcium-deficient HAP. The specific adsorption sites were assessed by examining the reversible and irreversible adsorption of NH3, CO2, aldehydes, and water vapor on HAP at the temperature of 298 K, using two HAP samples with different Ca/P ratios, but similar structures and surface areas: Ca-deficient HAP with an extreme lower Ca/P ratio (named P-HAP) and one with a higher Ca/P ratio (named C-HAP). Irreversible adsorption of NH3 on C-HAP is attributed to the adsorption on both Ca2+ Lewis acidic and P-OH Brønsted acidic sites. Irreversible adsorption on P-HAP is attributed to the adsorption on P-OH Brønsted acidic sites only. Irreversible adsorption of CO2 occurred on C-HAP only, and preferentially on OH- basic sites. Acetaldehyde undergoes a catalytic reaction over both OH- basic sites and surface P-OH Brønsted acidic sites at 298 K. Water irreversible adsorption was extensively observed for P-HAP, and water was barely desorbed at low pressures. In situ powder X-ray diffraction showed an asymmetric expansion of the lattice in the [100] direction, indicating that water was incorporated into P-HAP crystals, especially on structural OH- sites. Irreversible adsorption of acidic and basic molecules was therefore less observed on P-HAP than on C-HAP, but P-HAP had considerable irreversible adsorption of water vapor with associated asymmetric lattice expansion. The incorporation of water vapor was first observed and could be useful to improve adsorption or catalytic performance with the mediation of water vapor and/or hydration.

A spatially varying charge model for regulating site-selective protein adsorption and cell behaviors.

Implanted materials that enter the body first interact with proteins in body fluids, and cells then perceive and respond to the foreign implant through this layer of adsorbed proteins. Thus, spatially specific regulation of protein adsorption on an implant surface is pivotal for mediating subsequent cellular behaviors. Unlike the surface modulation strategy for traditional biomaterials, in this research, materials with a nonuniform spatial distribution of surface charges were designed to achieve site-selective protein adsorption and further influence cell behavior by charge regulation. Spatially varying microdomains with different levels of piezoelectricity were generated via a focus laser beam-induced phase transition. In addition, after polarization, the zones with different levels of piezoelectricity showed significant differences in surface charge density. The results of scanning Kelvin probe force microscopy (SKPM) showed that the surface charge on the material exhibits a nonuniform spatial distribution after laser irradiation and polarization. Site-specific charge-mediated selective protein adsorption was demonstrated through a protein adsorption experiment. Cell behavior analysis showed that the increase in charge density was conducive to promoting cell adhesion and the formation of filopodia while the nonuniform spatial distribution of charge promoted an oriented arrangement of cells; both features accelerated cell migration. This study provides a new method for spatially regulating protein adsorption through surface charges to further influence cell behaviors.

Site-Specific Adsorption of CO2 in Zeolite NaK-A

Zeolite |Na12|-A is a commercial adsorbent, and its CO2-over-N2(CH4) selectivity can be further enhanced kinetically by replacing Na+ in the 8-ring windows that control gas diffusion with large cations. In this study, samples of zeolite |Na12–xKx|-A with x = 0.0, 0.8, 2.0, and 3.0 were prepared, and the positions of adsorbed CO2 molecules were determined using in situ neutron powder diffraction through profile refinement. Adsorbed CO2 molecules were located at three different sites within the large α-cavities in the zeolite structure, revealing the interaction between the adsorbed CO2 and the host framework. The number of CO2 molecules at each site depends on CO2 pressure and follows site-specific CO2 isotherms described with a Langmuir model. Most of the CO2 molecules in zeolite |Na12–xKx|-A bridge two cations at neighboring 8-ring sites. These are relatively weakly physisorbed, and therefore, most of the working capacity of CO2 adsorption is related to this site. The CO2 molecules at the second most pop...

In Silico Screening of MOFs with open copper sites for C2H2/CO2 separation

Selective separation of acetylene (C2H2) from carbon dioxide (CO2) remains one of the most challenging industrial processes because of their very similar molecular sizes and physical properties. Considering the low compression limit of C2H2, it is highly desirable to seek porous materials with specific adsorption sites for efficiently discriminating the two gases under ambient conditions. With accurate force fields derived at the ab‐initio level, a large‐scale computational screening was performed in this work to explore the performances of a library of MOFs with open Cu sites for C2H2/CO2 separation. Out of about 800 existing materials retrieved in our database, 3 MOFs were identified that outperform the current adsorbents in terms of C2H2/CO2 selectivity and C2H2 uptake. This prediction paves the ways towards the revisit of such MOFs for this targeted application. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4089–4096, 2018

Insights into diverse performance for the electroanalysis of Pb(II) on Fe2O3 nanorods and hollow nanocubes: Toward analysis of adsorption sites

The surface areas and the exposure of adsorption sites of nanomaterials can both affect the adsorption of heavy metal ions, and then lead to a diverse electrochemical detection response. However, it is an unsolved mystery that which one is the key factor for detection. Here, Fe2O3 hollow nanocubes with larger surface areas and Fe2O3 nanorods with more active adsorption sites are successfully synthesized derived from Prussian blue. The Fe2O3 with two different morphologies are applied to modified bare electrodes that can serve as sensing interface for the electrochemical analysis of Pb(II). Interestingly, the Fe2O3 nanorods with (010) plane, as an electrochemical sensor, are observed to be more advantageous than hollow nanocubes with (006) plane. The underlying mechanism about the diverse electrochemical performance between the two materials is sufficiently proved by Brunauer-Emmett-Teller and X-ray photoelectron spectroscopy. Remarkably, the evidences clearly suggest that a large surface area is not dominant to obtain a good analysis ability; while the exposed specific adsorption site on the surface of nanomaterials shows more determinant.

A molecular overlayer with the Fibonacci square grid structure

Quasicrystals differ from conventional crystals and amorphous materials in that they possess long-range order without periodicity. They exhibit orders of rotational symmetry which are forbidden in periodic crystals, such as five-, ten-, and twelve-fold, and their structures can be described with complex aperiodic tilings such as Penrose tilings and Stampfli–Gaehler tilings. Previous theoretical work explored the structure and properties of a hypothetical four-fold symmetric quasicrystal—the so-called Fibonacci square grid. Here, we show an experimental realisation of the Fibonacci square grid structure in a molecular overlayer. Scanning tunnelling microscopy reveals that fullerenes (C60) deposited on the two-fold surface of an icosahedral Al–Pd–Mn quasicrystal selectively adsorb atop Mn atoms, forming a Fibonacci square grid. The site-specific adsorption behaviour offers the potential to generate relatively simple quasicrystalline overlayer structures with tunable physical properties and demonstrates the use of molecules as a surface chemical probe to identify atomic species on similar metallic alloy surfaces.

Site-Specific Adsorption of Aromatic Molecules on a Metal/Metal Oxide Phase Boundary.

Nanostructured surfaces are ideal templates to control the self-assembly of molecular structures toward well-defined functional materials. To understand the initial adsorption process, we have investigated the arrangement and configuration of aromatic hydrocarbon molecules on nanostructured substrates composed of an alternating arrangement of Cu(110) and oxygen-reconstructed stripes. Scanning tunneling microscopy reveals a preferential adsorption of molecules at oxide phase boundaries. Noncontact atomic force microscopy experiments provide a detailed insight into the preferred adsorption site. By combining submolecular resolution imaging with density functional theory calculations, the interaction of the molecule with the phase boundary was elucidated excluding a classical hydrogen bonding. Instead, a complex balance of different interactions is revealed. Our results provide an atomistic picture for the driving forces of the adsorption process. This comprehensive understanding enables developing strategies for the bottom-up growth of functional molecular systems using nanotemplates.

Strain relaxation and epitaxial relationship of perylene overlayer on Ag(110).

We present a room temperature STM study of perylene self-assembly on Ag(110) beyond the monolayer coverage regime. Coupling of the perylene aromatic boards yields π-π bonded stacks. The perylene stacks self-assemble into a continuous three-dimensional epitaxial overlayer of (3 × 5) symmetry. The self-assembly is driven by thermodynamic balance established under coupling of the intra- and intermolecular interactions and the molecule-substrate interaction all accommodating the short-range thermal motion of the constituent molecules. The balance bestows to the overlayer the unique ability to accommodate the underlying substrate morphology and to spread over the surface steps as a single structure preserving its lateral order and keeping epitaxial relationship with every surface terrace. The complete epitaxy is driven by (i) anchoring of half of the perylene stacks into specific adsorption sites on each terrace, (ii) interlacing of the perylene stacks across the steps within the entire H-bonded network, and (iii) relaxation of the overlayer strain via enhancement of the overlayer-specific vibrational modes and short-range thermal motion of the constituent molecules. This complete epitaxy phenomenon is described via (i) structural and statistical analysis of the molecularly resolved STM topographies, (ii) monitoring of the short-range molecular displacements under the strain relaxation, (iii) highlighting of specific intra-molecular and inter-molecular vibration modes through detailed analysis of HREELS spectra, and (iv) parametrization of the intermolecular interaction via pair potential calculation.

Solvent-Kaolinite Interactions Investigated using the 3D-Rism-Kh Molecular Theory of Solvation

AbstractThe oil sands of western Canada represent the third largest hydrocarbon deposit in the world. Bitumen, a very heavy petroleum, is recovered from mined oil sands using warm water extraction followed by separation treatments to isolate the bitumen product. The high energy, water use, as well as tailings remediation challenges associated with the warm water extraction process raise major environmental concerns. Non-aqueous extraction using organic solvents at room temperature has been investigated extensively as an alternative to the warm water extraction process. The main challenge to the large-scale implementation of non-aqueous extraction is the retention of solvent in the tailings. The objective of this work was to present and validate a computational model for the interaction of solvents used in non-aqueous extraction with minerals, such as the abundant and adsorbent clay mineral kaolinite. The model system contained a periodically extended kaolinite platelet immersed in a solvent and all were treated at the atomic level using the 3D Reference Interaction Site Model with the Kovalenko-Hirata closure approximation (3D-RISM-KH) molecular theory of solvation. The solvent solvation free energy of interaction with kaolinite as well as site-specific adsorption energies and kinetic barriers for desorption were computed based on the solvent site density distribution functions. Moreover, the lateral and integrated density distributions were computed to analyze the organization of solvent at kaolinite surfaces. The integrated density distribution profiles were correlated with experimental adsorption isotherms. The results showed very strong adsorption of ethanol and weak adsorption of hydrocarbon solvents on kaolinite, which were in qualitative agreement with experimental solvent extraction reports. The model and these findings are valuable in understanding the mechanism of solvent retention in tailings after non-aqueous extraction and highlight the action of hydroxylated cosolvent additives to enhance extraction using nonpolar solvents.

Guanidinium ionic liquid-controlled synthesis of zeolitic imidazolate framework for improving its adsorption property

The massive release of rhodamine B (RhB) to water system is an emerging problem, which dramatically threaten environment and human health. The development of an adsorbent with enhanced removal efficiency for RhB is urgently needed. Herein, we report an environment-friendly synthesis of high quality zeolitic imidazolate framework-8 (ZIF-8) and functional ionic liquid@ZIF-8 in water-based system without heat treatment for improving its adsorption property. Guanidinium ionic liquids (ILs) could not only act as greener agents instead of volatile bases and toxic surfactants to efficiently control the nucleation and growth rate of ZIF-8, but also were incorporated as shell material to add specific adsorption sites. The relationship between nanoparticle structure and adsorption performance for RhB was systematically investigated. Due to high surface area (1167 m2 g−1), high porosity (0.79 cm3 g−1), high crystallinity, nano size (about 100 nm) and monodispersity, the as-obtained ZIF-8 showed improved adsorption capacity toward RhB (80% removal efficiency). Heteropolyanion-based guanidinium IL@meso-ZIF-8 (HPAIL@meso-ZIF-8) exhibited the high RhB uptake capacity of 278 mg g−1 (higher than most of the reported adsorbents) and effectively removed 99% of RhB within 15 min. The results showed that the adsorption process of prepared materials fitted well with pseudo-second-order kinetics and Langmuir isotherm model. The existence of mesopores in ZIF-8 facilitated the diffusion of RhB and the incorporated guanidinium IL played a significant role in enhancing the adsorption affinity. Moreover, the reusability results revealed the HPAIL@meso-ZIF-8 as a highly efficient adsorbent for RhB removal with satisfactory performance and structural stability. Therefore, HPAIL@meso-ZIF-8 is one of the most promising adsorbents for organic dye removal from water.

Structure and Dynamics of Water Confined in Imogolite Nanotubes.

We have studied the properties of water adsorbed inside nanotubes of hydrophilic imogolite, an aluminum silicate clay mineral, by means of molecular simulations. We used a classical force field to describe the water and the flexible imogolite nanotube and validated it against the data obtained from first-principles molecular dynamics. With it, we observe a strong structuration of the water confined in the nanotube, with specific adsorption sites and a distribution of hydrogen bond patterns. The combination of number of adsorption sites, their geometry, and the preferential tetrahedral hydrogen bonding pattern of water leads to frustration and disorder. We further characterize the dynamics of the water, as well as the hydrogen bonds formed between water molecules and the nanotube, which is found to be more than 1 order of magnitude longer than water-water hydrogen bonds.