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Zinc metal–organic frameworks (Zn-MOFs) of phthalic acid (ZnPA) and nitro-phthalic acid (ZnNPA) microcrystalline particles were fabricated and utilized for the removal of organic cationic dyes (crystal violet (CV), methylene blue (MB) and rhodamine B (RhB)) from water. PXRD studies confirmed the formation of crystalline materials, and BET indicated the mesoporous structure. HR-SEM studies showed the rod and plate morphologies for ZnPA and ZnNPA, respectively. ZnPA revealed low adsorption of dyes (CV, 20%) whereas ZnNPA showed significantly enhanced dye adsorption (CV, 99%). In particular, ZnNPA exhibited higher adsorption of CV (99%) and MB dye (83%) compared to RhB (54%). Nevertheless, ZnNPA could not be reused due to strong adsorption of dyes. Hence, a natural binder was introduced by dipping ZnNPA into tea extract, which also showed high adsorption of CV and MB dyes. Kinetics of adsorption exhibited best fitting with the pseudo-second-order model, indicating strong interactions between the dyes and adsorbent. Adsorption isotherm fitting suggested a good correlation coefficient (R2) with the Freundlich model, suggesting multi-layer adsorption. Interestingly, ZnNPA showed high adsorption of CV and MB dyes across the pH range between 3.0 and 11.0. The binder-introduced ZnNPA adsorbent was easily regenerated and demonstrated three cycles of reusability without significant change in the dye adsorption. Thus, the present work indicated the role of functional groups in the ligand and binder for improving the dye adsorption characteristics of MOFs. [ABSTRACT FROM AUTHOR]

Cobalt hydroxide Co(OH)2 is considered to be a potential material for electrocatalyst, especially for oxygen evolution reaction (OER), owing to the earth's abundance, environmentally benign nature, and redox-active properties of cobalt. Herein, we report the fabrication of pristine Co(OH)2 and AgNP-integrated Co(OH)2 (Ag–Co(OH)2) by simple wet chemical methods and explore the electrocatalytic OER activity in the alkaline medium. High-resolution transmission electro-microscopic (HR-TEM) analysis revealed featureless nanostructures for Co(OH)2 with coexisting amorphous and crystalline phases, as well as the inclusion of crystalline AgNPs in Ag–Co(OH)2. X-ray photoelectron spectroscopic (XPS) analysis confirmed the inclusion of metallic AgNPs and the presence of Co at a mixed oxidation state. Electrocatalytic OER studies indicated that pristine Co(OH)2 required the overpotential of 299 mV to achieve a geometric current density of 10 mA cm−2. The integration of AgNPs in Co(OH)2 (Ag–Co(OH)2) showed a gradual improvement in the OER activity. The optimized sample, Ag–Co(OH)2-5, required the overpotential of 253 mV to produce 10 mA cm−2 current density. Tafel slope analysis revealed a lower value upon AgNP integration and electrochemical impedance showed lower charge transfer resistance. The lower Tafel value and charge transfer resistance of Ag–Co(OH)2-5 indicated good chemical coupling and faster reaction kinetics at the electrode surface. AgNPs incorporation with Co(OH)2 also showed enhanced turn-over frequency, electrochemical active surface area, and double-layer capacitance. The fabricated hybrid Ag–Co(OH)2-5 catalyst also exhibited good stability over 60 h. Thus, the electrocatalytic activity of low-cost Co(OH)2 was improved by fabricating coexisting amorphous and crystalline phases and integrating noble silver nanoparticles. [ABSTRACT FROM AUTHOR]

The coordination environment and networks of metal–organic frameworks (MOFs) play a significant role in their functional properties, especially catalysis. In recent years, the use of MOFs for electrocatalytic water-splitting reactions to produce green hydrogen fuel has gained increasing attention. Herein, we report the preparation of Co-benzene dicarboxylic acid-based MOFs (Co-MOF-1 and Co-MOF-2) with different coordination environments and correlated their structure-controlled electrocatalytic OER activity in an alkaline medium. Both Co-MOF-1 and Co-MOF-2 showed 3D network structures with different coordination environments around the Co metal centre. In Co-MOF-1, the Co metal centre is coordinated with DMF and pyridine along with a benzene dicarboxylic acid (BDC) ligand, whereas the Co metal centre is coordinated only with BDC ligands in Co-MOF-2. The Co metal centres in Co-MOF-1 contain labile solvent coordination but have completely saturated coordination with the BDC ligand alone in Co-MOF-2. Interestingly, Co-MOF-1 showed relatively strong OER activity and required the overpotential (η) of 294 mV to produce 10 mA cm−2 current density. However, Co-MOF-2 required the overpotential of 343 mV to achieve the 10 mA cm−2 current density. Further Co-MOF-1 showed comparatively low Tafel slope and charge transfer resistance and high electrochemical surface area than Co-MOF-2 and supported the enhanced OER activity. XPS analysis of Co-MOF-1 before and after catalysis suggested the conversion of Co-MOF-1 to CoOOH during the electrocatalysis that boosted the OER activity. The presence of a labile coordination site in Co-MOF-1 might have facilitated the interaction of hydroxyl electrolyte with the metal centre and active site generation by forming CoOOH easily which enhanced the OER activity. Co-MOF-1 also showed excellent stability for over 48 h. Hence, the present work provides structural insight for designing new MOFs for electrocatalytic water-splitting reactions. [ABSTRACT FROM AUTHOR]

The versatility of metal coordination geometry and organic linkers is used to derive metal–organic frameworks (MOFs) with an intriguing network structure in the solid-state, exerting significant control over its functional application. In this study, we prepared Co-benzene dicarboxylic acid based MOFs with 1D and 2D network structures with similar coordination environment and studied their electrocatalytic OER activity in alkaline medium. The use of bidentate amines (2,2′-bipyridine and DABCO) controlled the dimensionality and produced water coordinated 1D (Co-MOF-1D) and 2D (Co-MOF-2D) network structures. Although both Co-MOFs showed strong electrocatalytic OER activity, Co-MOF-2D exhibited relatively strong OER activity (required overpotential (η) of 211 mV to produce 10 mA cm−2 current density) compared to Co-MOF-1D (η10 = 237 mV). Co-MOF-2D exhibited excellent stability with a low Tafel slope, charge transfer resistance, and high electrochemical surface area that further supported the enhanced OER activity. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) with their tunable topology, functionality and coordination environment have been considered as potential materials for various applications including electrocatalysis. Herein, we have synthesised a water coordinated nickel based 2D metal–organic framework (Ni-MOF) and a coordination complex (Ni-C) and investigated their electrocatalytic OER activity. The Ni-MOF showed a 2D sheet structure with one water coordination whereas a four water molecule coordinated charged complex was formed in the Ni-C. Thermogravimetric analysis (TGA) confirmed their water coordination and good thermal stability. Interestingly, electrocatalytic OER studies showed strongly enhanced activity for the Ni-MOF and that it required a low overpotential (194 mV) to produce a geometric current density of 10 mA cm−2. The Ni-C required 225 mV to produce 10 mA cm−2. The post-catalytic analysis suggested that the Ni-MOF and Ni-C are converted to nickel hydroxides/oxyhydroxides during electrocatalysis and acted as the catalytic centre. The low Tafel slope and charge transfer resistance further supported the higher activity of Ni-MOF based nickel hydroxides/oxyhydroxides. Chronoamperometric studies revealed the excellent stability of the Ni-MOF based catalyst over 72 h. The present study revealed the potential of developing highly active electrocatalysts based on Ni-MOFs by optimizing the topology and coordination environment. [ABSTRACT FROM AUTHOR]

Water pollution by organic dyes poses great challenge to the environment and living organism. Hence effective removal of organic dyes by cost effective methods have received significant attention in recent years. Herein, we report the complete removal of organic dyes (rhodamine B), methylene blue) and eosin yellow) from water via effective adsorption by MoO 3 catalyst. Hydrothermally synthesised MoO 2 (1) and amorphous MoS x (2) using ammonium molybdate without and with thiourea exhibited low dye adsorption. In contrast, crystalline micro/nanoplates of MoO 3 (3 and 4) obtained from calcination of 1 and 2 showed highly enhanced dye adsorption. Particularly 4 showed higher dye adsorption compared to 3. UV-Visible absorption studies confirmed complete removal of organic dyes upon stirring with MoO 3 catalyst. Dye removal studies further revealed that cationic dyes are adsorbed faster than anionic dye that could be attributed to the surface charge of MoO 3 . Interestingly, the adsorbed dyes were not released from MoO 3 for more than 50 days. The exhausted MoO 3 catalyst can be recovered by annealing at 400 °C. MoO 3 catalyst has also been used as packing materials in dropper column and demonstrated effective removal of dyes by passing through dyes separately as well as mixture., (Copyright © 2021 Elsevier Ltd. All rights reserved.)