Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod (CTR)) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ~240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (≥5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Design of 3D Carbon Nanotube Monoliths for Potential-Controlled Adsorption

The design of 3D monoliths provides a promising opportunity to scale the unique properties of singular carbon nanotubes to a macroscopic level. However, the synthesis of carbon nanotube monoliths is often characterized by complex procedures and additives impairing the later macroscopic properties. Here, we present a simple and efficient synthesis protocol leading to the formation of free-standing, stable, and highly conductive 3D carbon nanotube monoliths for later application in potential-controlled adsorption in aqueous systems. We synthesized monoliths displaying high tensile strength, excellent conductivity (up to 140 S m−1), and a large specific surface area (up to 177 m2 g−1). The resulting monoliths were studied as novel electrode materials for the reversible electrosorption of maleic acid. The process principle was investigated using chronoamperometry and cyclic voltammetry in a two-electrode setup. A stable electrochemical behavior was observed, and the synthesized monoliths displayed capacitive and faradaic current responses. At moderate applied overpotentials (± 500 mV vs. open circuit potential), the monolithic electrodes showed a high loading capacity (~20 µmol g−1) and reversible potential-triggered release of the analyte. Our results demonstrate that carbon nanotube monoliths can be used as novel electrode material to control the adsorption of small organic molecules onto charged surfaces.

In Search of Factors Determining Activity of Co3O4 Nanoparticles Dispersed in Partially Exfoliated Montmorillonite Structure

The paper discusses a formation of Mt–PAA composite containing a natural montmorillonite structure partially exfoliated by poly(acrylic acid) introduced through intercalation polymerization of acrylic acid. Mt–PAA was subsequently modified by controlled adsorption of Co2+ ions. The presence of aluminosilicate packets (clay) and carboxyl groups (hydrogel) led to the deposition of significant amounts of Co2+ ions, which after calcination formed the Co3O4 spinel particles. The conditions of the Co2+ ions’ deposition (pH, volume and concentration of Co(NO3)2 solution, as well as a type of pH-controlling agent) were widely varied. Physicochemical characterization of the prepared materials (including X-ray fluorescence (XRF), X-ray powder diffraction (XRD), low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (H2-TPR)) revealed that the modification conditions strongly influenced the content as well as the distribution of the Co3O4 active phase, tuning its reducibility. The latter parameter was, in turn, very important from the point of view of catalytic activity in the combustion of aromatic volatile organic compounds (VOCs) following the Mars–van Krevelen mechanism.

ADSORPTION STUDIES ON THE REMOVAL OF NAPHTOL-AS DYE USING IRON FILINGS AS ADSORBENT

This study investigated the potential of iron filings as low cost and environmentally friendly adsorbent for removing Naphthol AS dye from aqueous solutions. The batch-adsorption experiments were performed as a function of pH, temperature, concentration, contact time as well as dynamics of the process. The Langmuir, Freundlich and Dubinin-Raduskevich isotherm models for the adsorption processes were tested but Freundlich isotherm provided the best description for adsorption of the dye solution onto the iron filings, suggesting a physisorption controlled adsorption. The adsorption process was highly pH-dependent and the result indicates that the optimum pH for adsorption of Naphthol AS onto the adsorbent occurred at 4.0 having a maximum removal efficiency of 88 % respectively. Pseudo first-order and second-order as well as the intra-particle diffusivity kinetic models were applied to the experimental data and results showed that the pseudo second-order provided best fit for the experimental data. Kinetic studies also showed that the adsorption transport mechanism was particle-diffusion controlled.

Precise regulating synergistic effect in metal–organic framework for stepwise-controlled adsorption

MAC-20 shows a unique two-step pore-shape change and executes a stepwise-controlled adsorption of dyes mixture in order of their sizes.