Arshad Aijaz

Co@Co3O4 Encapsulated in Carbon Nanotube‐Grafted Nitrogen‐Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode

Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2 , and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes.

From Metal–Organic Framework to Nitrogen-Decorated Nanoporous Carbons: High CO2 Uptake and Efficient Catalytic Oxygen Reduction

High-surface-area N-decorated nanoporous carbons have been successfully synthesized using the N-rich metal-organic framework ZIF-8 as a template and precursor along with furfuryl alcohol and NH4OH as the secondary carbon and nitrogen sources, respectively. These carbons exhibited remarkable CO2 adsorption capacities and CO2/N2 and CO2/CH4 selectivities. The N-decoration in these carbons resulted in excellent activity for the oxygen reduction reaction. Samples NC900 and NC1000 having moderate N contents, high surface areas, and large numbers of mesopores favored the four-electron reduction pathway, while sample NC800 having a high N content, a moderate surface area, and a large number of micropores favored the two-electron reduction process.

Catalysis with Metal Nanoparticles Immobilized within the Pores of Metal–Organic Frameworks

Metal-organic frameworks (MOFs) are highly ordered crystalline porous materials prepared by the self-assembly of metal ions and organic linkers having low-density framework structures of diversified topologies with tunable pore sizes and exceptionally large surface areas. Other than outstanding gas/molecule storage properties, loading of metal nanoparticles (MNPs) into the pores of MOFs could afford heterogeneous catalysts having advantages of controlling the particle growth to a nanosize region, resulting in highly active sites and enhanced catalytic performances, and these entrapped MNPs within MOF pores could be accessed by reactants for chemical transformations. This is a rapidly developing research area, and this Perspective addresses current achievements and future challenges for diverse MOF-immobilized MNPs within their pores, focusing especially on their preparation, characterization, and application as heterogeneous catalysts.

Two-Dimensional Coordination Polymer with a Non-interpenetrated (4,4) Net Showing Anion Exchange and Structural Transformation in Single-Crystal-to-Single-Crystal Fashion

A new non-interpenetrated two-dimensional (2D) rectangular-grid coordination polymer, {[Co(L)(2)(H(2)O)(2)] x (BF(4))(2) x 4 DMF}(n) (1), has been synthesized using a new rod-like ligand, 3,5-bis(4-imidazol-1-ylphenyl)-[1,2,4]triazol-4-ylamine (L). Weakly H-bonded BF(4)(-) anions present within the voids can be exchanged by ClO(4)(-) and NO(3)(-) anions to generate {[Co(L)(2)(H(2)O)(2)] x (ClO(4))(2) x 2 DMF x 2 H(2)O}(n) (2) and {[Co(L)(2)(H(2)O)(2)] x (NO(3))(2) x 2 DMF x 2 H(2)O}(n) (3) in single-crystal-to-single-crystal (SC-SC) manner. In the case of exchange by Cl(-) ion, the crystallinity is not maintained, and so it is proven by IR spectroscopy, PXRD, and elemental analysis. In addition, 3 shows an interesting structural transformation (2D --> 1D) with bond rupture/formation leading to the formation of a new coordination polymer, {[Co(L)(2)(H(2)O)(2)] x (NO(3))(2) x 2 DMF x H(2)O}(n), (5), again in SC-SC fashion.

Metal-organic framework-immobilized polyhedral metal nanocrystals: reduction at solid-gas interface, metal segregation, core-shell structure, and high catalytic activity.

For the first time, this work presents surfactant-free monometallic and bimetallic polyhedral metal nanocrystals (MNCs) immobilized to a metal-organic framework (MIL-101) by CO-directed reduction of metal precursors at the solid-gas interface. With this novel method, Pt cubes and Pd tetrahedra were formed by CO preferential bindings on their (100) and (111) facets, respectively. PtPd bimetallic nanocrystals showed metal segregation, leading to Pd-rich core and Pt-rich shell. Core-shell Pt@Pd nanocrystals were immobilized to MIL-101 by seed-mediated two-step reduction, representing the first example of core-shell MNCs formed using only gas-phase reducing agents. These MOF-supported MNCs exhibited high catalytic activities for CO oxidation.

Hollow Zn/Co Zeolitic Imidazolate Framework (ZIF) and Yolk-Shell Metal@Zn/Co ZIF Nanostructures.

Metal-organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well-defined hollow Zn/Co-based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn-MOF (ZIF-8) on preformed Co-MOF (ZIF-67) nanocrystals that involve in situ self-sacrifice/excavation of the Co-MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co-ZIF shells to generate yolk-shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co-ZIF with dominance of the Zn-MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs.

Coordination polymers of various architectures built with mixed imidazole/benzimidazole and carboxylate donor ligands and different metal ions: syntheses, structural features and magnetic properties

Ten coordination polymers {[Cd(L1)2]}n (1), {[Cu(L1)2]}n (2), {[Cd2(L1)2(HCOO)2(H2O)]}n (3), {[Cd2(L1)4]·3H2O}n (4), {[Cd(L1)(CH3COO)]}n (5), {[Cd(L1)2(C5H5N)]·2.5H2O}n (6), {[Zn(L2)2]}n (7), {[Cd2(L2)4]·H2O}n (8), {[Cd3(L2)6(H2O)]·2EtOH·H2O}n (9) and {[Cd(L3)2]·2DMF·4H2O}n (10), where HL1 = 4-imidazole-1-yl-benzoic acid, HL2 = 3-imidazole-1-yl-benzoic acid and HL3 = 4-benzmidazole-1-yl-benzoic acid, have been synthesized under different experimental conditions. Their structures are determined by single-crystal X-ray diffraction analyses and further characterized by IR spectra, thermogravimetric (TG) and elemental analyses. The structure of 1 is a 2D → 2D (2D = two dimensional) interpenetrating network with (4,4) grid topology while 2 has an interesting Kagome structure. Compounds 4 and 6 crystallize in 4- and 3-fold interpenetrating diamondoid frameworks respectively. Compound 7 is a layered non-interpenetrating (4,4) grid network whereas 8 has an unprecedented self-penetrating structure with 2D framework. Compound 9 is a self-penetrating 3D structure while 10 crystallizes in a non-interpenetrating (4,4) square-grid with one-dimensional (1D) channels. From these results, it is demonstrated that the structures of the coordination polymers are strongly dependent on the geometry of L1, L2 and reaction conditions. Variable temperature magnetic susceptibility study of 2 has also been performed.

MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions

Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe3 C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1 m KOH (0.89 V at -1 mA cm-2 ) and in 0.5 m H2 SO4 (0.73 V at -1 mA cm-2 ) with a hydrogen peroxide yield of below 1 % in both electrolytes. Due to encapsulation of the Fe3 C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70 hours in both alkaline and acidic media.