Edge Surface Sites Research Articles

Influence of Fe(II), Fe(III), and Al(III) isomorphic substitutions on acid-base properties of edge surfaces of cis-vacant montmorillonite: Insights from first-principles molecular dynamics simulations and surface complexation modeling

Abstract Knowing the influence of isomorphic substitutions on the acid-base properties of smectite edge surfaces is an important aspect of the detailed understanding of clay minerals’ interfacial properties with implications in the modeling of adsorption processes. We investigated the intrinsic acidity constants of Fe(II)/Fe(III) and Al(III) substituted edge surface sites of montmorillonite with a cis-vacant structure, which includes four crystallographic orientations perpendicular to [010], [010], [110], and [110], using the first-principles molecular dynamics (FPMD) based vertical energy gap method. Fe(II) and Fe(III) substitutions resulted, respectively, in a significant increase and decrease in pKa values of amphoteric groups directly associated with Fe octahedra. In addition, Fe(II) substitution increased the pKa values of the neighboring silanol sites, while Fe(III) substitution had a weak influence on these sites. The Al-substituted tetrahedra had amphoteric sites with higher pKa values than the non-substituted Si tetrahedra, and they increased significantly the pKa values of the sites bridging the tetrahedral and octahedral sheets on surfaces perpendicular to [010] and [110]. The acid-base properties of substituted and non-substituted surface sites of cis-vacant montmorillonite were used to build a state-of-the-art surface complexation model, which successfully reproduced the best available experimental acid-base titration data. This model was further used to predict acid-base properties of dioctahedral smectites (montmorillonite, beidellite, and nontronite) according to their cis- or trans-vacant structures and their layer chemistry. According to these predictions, these smectites exhibit very similar overall pH buffering properties despite significant differences in structure and chemistry. A detailed analysis of the acid-base properties as a function of crystallographic directions demonstrated, however, that these differences should have a large influence on the adsorption of ionic species.

Near-Edge Ligand Stripping and Robust Radiative Exciton Recombination in CdSe/CdS Core/Crown Nanoplatelets.

We address the relation between surface chemistry and optoelectronic properties in semiconductor nanocrystals using core/crown CdSe/CdS nanoplatelets passivated by cadmium oleate (Cd(Ol)2) as model systems. We show that addition of butylamine to a nanoplatelet (NPL) dispersion maximally displaces ∼40% of the original Cd(Ol)2 capping. On the basis of density functional theory simulations, we argue that this behavior reflects the preferential displacement of Cd(Ol)2 from (near)-edge surface sites. Opposite from CdSe core NPLs, core/crown NPL dispersions can retain 45% of their initial photoluminescence efficiency after ligand displacement, while radiative exciton recombination keeps dominating the luminescent decay. Using electron microscopy observations, we assign this robust photoluminescence to NPLs with a complete CdS crown, which prevents charge carrier trapping in the near-edge surface sites created by ligand displacement. We conclude that Z-type ligands such as cadmium carboxylates can provide full electronic passivation of (100) facets yet are prone to displacement from (near)-edge surface sites.

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACTWe present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu12 and Fe6Cu49 ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys.This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Template synthesis and characteristics of nanoparticle MgO

Oxide nanoparticles can exhibit unique physical and chemical properties due to their limited size and a high density of corner or edge surface sites. In this study, MgO nanoparticle was synthesized using Mg(CH3COO)2 and hexamethylenetetramine as starting materials. The structure and optical properties of these particles are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Visible absorption. The XRD analysis discloses that MgO nanoparticle is successfully synthesized. Dispersive analysis of X-RAY (EDAX) was used to characterize the size and morphology of the MgO nanoparticle on the template. The morphology of MgO was nanospheres.

Distinctive Reactivities at Biotite Edge and Basal Planes in the Presence of Organic Ligands: Implications for Organic-Rich Geologic CO2 Sequestration.

To better understand how scCO2-saturated brine-mineral interactions can affect safe and efficient geologic CO2 sequestration (GCS), we studied the effects of organic ligands (acetate and oxalate) on biotite dissolution and surface morphological changes. The experimental conditions were chosen to be relevant to GCS sites (95 °C and 102 atm CO2). Quantitative analyses of dissolution differences between biotite edge and basal planes were made. Acetate slightly inhibited biotite dissolution and promoted secondary precipitation. The effect of acetate was mainly pH-induced aqueous acetate speciation and the subsequent surface adsorption. Under the experimental conditions, most of acetate exists as acetic acid and adsorbs to biotite surface Si and Al sites, thereby reducing their release. However, oxalate strongly enhanced biotite dissolution and induced faster and more significant surface morphology changes by forming bidentate mononuclear surface complexes. For the first time, we show that oxalate selectively attacks edge surface sites and enhances biotite dissolution. Thus, oxalate increases the relative reactivity ratio of biotite edge surfaces to basal surfaces, while acetate does not impact this relative reactivity. This study provides new information on reactivity differences at biotite edge and basal planes in the presence of organic ligands, which has implications for safe CO2 storage in organic-rich sites.

Natural attenuation of TCE, As, Hg linked to the heterogeneous oxidation of Fe(II): an AFM study

Hydrous ferric oxide (HFO) colloids formed, in strictly anoxic conditions upon oxidation of Fe 2+ ions adsorbed on mineral surface, were investigated under in situ conditions by contact mode atomic force microscopy (AFM). Freshly cleaved and acid-etched large single crystals of near endmember phlogopite were pre-equilibrated with dissolved Fe(II) and then reacted with Hg(II), As(V) and trichlorethene (TCE)-bearing solutions at 25 °C and 1 atm. HFO structures are found to be of nanometer scale. The As(V)–Fe(II) and Hg(II)–Fe(II) reaction products are round (25 nm) microcrystallites located predominantly on the layer edges and are indicative of an accelerated Fe(II) oxidation rate upon formation of Fe(II) inner sphere surface complexes with the phyllosilicate edge surface sites. On the other hand, TCE–Fe(II)–phlogopite reaction products are needle-shaped (45 nm long) particles located on the basal plane along the Periodic Bond Chains (PCBs) directions. Experiments with additions of sodium chloride confirm the importance of the Fe(II) adsorption step in the control of the overall heterogeneous Fe(II)–TCE electron transfer reaction. Kinetic measurements at the nanomolar level of Hg° formed upon reduction of Hg(II) by Fe(II) in presence of phlogopite particles provide further convincing evidence for reduction of Hg(II) aq coupled to the oxidation of Fe(II) adsorbed at the phlogopite–fluid interface, and indicate that sorption of Fe(II) to mineral surfaces enhances the reduction rate of Hg(II) species. The Hg(II) reduction reaction follows a first-order kinetic law. Under our experimental conditions, which were representative of many natural systems, 80% of the mercury is transferred to the atmosphere as Hg° in less than 2 h. The reduction of a heavy metal (Hg), a toxic oxyanion (arsenate ion) and a chlorinated solvent (TCE) thus appear to be driven by the high reactivity of adsorbed Fe(II). This is of environmental relevance since these three priority pollutants are that way reductively transformed to a volatile, an immobilizable and a biodegradable species, respectively. Such kinetic data and reaction pathways are important in the evaluation of natural evaluation scenarios, in the optimization of Fe(II)/mineral mixtures as reductants in technical systems, and in general, in predicting the fate and transport of pollutants in natural systems.

Characteristics of carbon-coated graphite prepared from mixture of graphite and polyvinylchloride as anode materials for lithium ion batteries

A carbon-coated graphite is investigated as the negative electrode for Li-ion batteries. The carbon-coated graphite particles are prepared by simple heat-treatment of mixtures of graphite and poly(vinyl chloride), PVC, at 800–1000°C in an argon flow. The carbon coating reduces significantly the initial irreversible capacity of the graphite in a propylene carbonate-based electrolyte, by suppressing the solvated lithium ion intercalation, and also improves the initial charge–discharge coulombic efficiency. By carbon coating, the specific surface area of graphite particles is greatly increased. These findings can be explained by assuming that a turbostratic structure of PVC-carbon resists irreversible side-reactions which are controlled predominantly by active, edge surface sites.

The proton promoted dissolution kinetics of K-montmorillonite

The kinetics of proton promoted dissolution of K-montmorillonite was investigated at 298 ± 1 K by batch experiments in the range 10 −1 ≥ [H +] ≥ 10 −5 M using solutions of the constant KCl concentrations of 0.03 M, 0.10 M, and 1.0 M, respectively. In addition the concentration of adsorbed H +-ions (H +, [mol/g]) was determined in the acidic range using both titration and batch equilibration experiments. The dissolution rates R Si and R Al [mol/(g · h)] were obtained from the observed increase with time of both dissolved Si(IV) and Al(III). R Si was found to be linearly dependent on H +: R Si = k · H +. The values of the first order rate constant k [h −1] increase with increasing KCl concentration: 6.02 × 10 −4 ± 2.4 × 10 −5 h −1 (0.03 M KCl), 9.98 × 10 −4 ± 7.1 × 10 −5 h −1 (0.10 M KCl), and 2.61 × 10 −3 ± 3.9 × 10 −4 h −1 (1.0 M KCl). Adsorption of H +-ions was interpreted in terms of the surface complexation model. Proton uptake by the solid phase were formally attributed to H-K exchange and to protonation of edge surface aluminol groups. The dissolution reaction does, however, not discriminate between these two adsorption modes. At 0.10 M and 1.0 M KCl the observed ratio Q = R Si/ R Al was found to be close to the Si:Al ratio in the solid phase, suggesting congruent dissolution. In 0.03 M KCl and [H +] > 3.2 × 10 −2 M, Q exceeded the Si:Al ratio of the solid. This deviation from congruency at low KCl concentration was attributed to adsorption of dissolved Al(III) by cation exchanger sites as reported by Charlet et al. ( 1993b). Added Al(III) was found to inhibit the dissolution reaction at [H +] ≤ 10 −3 M. Observations by Charlet et al. (1993b) suggest that this inhibition originates from adsorption of Al(III) on crystal edge surface sites. Both the observed dissolution stoichiometry and the inhibition by added Al(III) leads to the conclusion that dissolution occurs predominantely at the crystal edge surfaces.

On the nature of electroactive sites in clay-modified electrodes

The electrochemical properties of montmorillonite clay films deposited on pyrolytic graphite electrodes have been studied by cyclic voltammetry in order to elucidate the nature of the electroactive sites. Electrodes coated with pre-exchanged ML 2+ 3 -montmorillonite (M = Fe, Ru, Os and L = bpy; M = Fe and L = phen), and MV 2+-montmorillonite (MV 2+ = methylviologen) do not exhibit a voltammetric response when immersed in solutions containing only electrolyte. This result shows that the cations bound electrostatically to the exchange sites, whether intercalated within the galleries or held at the external surfaces, are rigorously electroinactive. However, these films incorporate ML 2+ 3 and MV 2+ ions from solution readily and exhibit voltammetric responses characteristic of diffusional processes. Os(bpy) 3 2+-exchanged montmorillonite films immersed in Fe(bpy) 3 2+ solutions show a voltammogram only for the Fe(bpy) 3 3+/2+ couple, and peak current values indicate that Fe(bpy) 3 3+ does not accept electrons from Os(bpy) 3 2+ gallery cations. The electroactivity for films immersed in ML 3 2+ or MV 2+ solutions is attributed in part to cations which are bound at the clay surface in excess of the cation exchange capacity by an ion pairing mechanism. Experiments using synthetic clays of different particle sizes suggest that binding of the ion pairs is related to the presence of edge surface sites.