Single-Atomic Co-N4 Sites with CrCo Nanoparticles for Metal-Air Battery-Driven Hydrogen Evolution.

Designing highly active and robust earth abundant trifunctional electrocatalysts for energy storage and conversion applications remain an enormous challenge. Herein, we report a trifunctional electrocatalyst (CrCo/CoN4@CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co-N4 single atom and CrCo alloy nanoparticles and exhibits outstanding electrocatalytic performance for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The catalyst is able to deliver a current density of 10 mA cm-2 in an alkaline electrolytic cell at a very low cell voltage of ∼1.60 V. When the catalyst is equipped in a liquid rechargeable Zn-air battery, it endowed a high open-circuit voltage with excellent cycling durability and outperformed the commercial Pt/C+IrO2 catalytic system. Furthermore, the Zn-air battery powered self-driven water splitting system is displayed using CrCo/CoN4@CNT-5 as sole trifunctional catalyst, delivering a high H2 evolution rate of 168 µmol h-1. Theoretical calculations reveal synergistic interaction between Co-N4 active sites and CrCo nanoparticles, favoring the Gibbs free energy for H2 evolution. The presence of Cr not only enhances the H2O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co-N4 sites, thus giving rise to accelerated H2 evolution kinetics. This work highlights the importance of the presence of small quantity of Cr in enhancing the electrocatalytic activity as well as robustness of single-atom catalyst and suggests the design of the multifunctional robust electrocatalysts for long-term H2 evolution application.

Dual Single-Atomic Co-Mn Sites in Metal-Organic-Framework-Derived N-Doped Nanoporous Carbon for Electrochemical Oxygen Reduction.

Synthesizing dual single-atom catalysts (DSACs) with atomically isolated metal pairs is a challenging task but can be an effective way to enhance the performance for electrochemical oxygen reduction reaction (ORR). Herein, well-defined DSACs of Co-Mn, stabilized in N-doped porous carbon polyhedra (named CoMn/NC), are synthesized using high-temperature pyrolysis of a Co/Mn-doped zeolitic imidazolate framework. The atomically isolated Co-Mn site in CoMn/NC is recognized by combining microscopic as well as spectroscopic techniques. CoMn/NC exhibited excellent ORR activities in alkaline (E1/2 = 0.89 V) as well as in acidic (E1/2 = 0.82 V) electrolytes with long-term durability and enhanced methanol tolerance. Density functional theory (DFT) suggests that the Co-Mn site is efficiently activating the O-O bond via bridging adsorption, decisive for the 4e- oxygen reduction process. Though the Co-Mn sites favor O2 activation via the dissociative ORR mechanism, stronger adsorption of the intermediates in the dissociative path degrades the overall ORR activity. Our DFT studies conclude that the ORR on an Co-Mn site mainly occurs via bridging side-on O2 adsorption following thermodynamically and kinetically favorable associative mechanistic pathways with a lower overpotential and activation barrier. CoMn/NC performed excellently as a cathode in a proton exchange membrane (PEM) fuel cell and rechargeable Zn-air battery with high peak power densities of 970 and 176 mW cm-2, respectively. This work provides the guidelines for the rational design and synthesis of nonprecious DSACs for enhancing the ORR activity as well as the robustness of DSACs and suggests a design of multifunctional robust electrocatalysts for energy storage and conversion devices.

Coupling Single-Ni-Atom with Ni-Co Alloy Nanoparticle for Synergistically Enhanced Oxygen Reduction Reaction.

Developing nonprecious metal-based oxygen reduction reaction (ORR) electrocatalysts with superior activity and durability is crucial for commercializing proton-exchange membrane (PEM) fuel cells. Herein, we report a metal-organic framework (MOF)-derived unique N-doped hollow carbon structure (NiCo/hNC), comprising of atomically dispersed single-Ni-atom (NiN4) and small NiCo alloy nanoparticles (NPs), for highly efficient and durable ORR catalysis in both alkaline and acidic electrolytes. Density functional theory (DFT) calculations reveal the strong coupling between NiN4 and NiCo NPs, favoring the direct 4e- transfer ORR process by lengthening the adsorbed O-O bond. Moreover, NiCo/hNC as a cathode electrode in PEM fuel cells delivered a stable performance. Our findings not only furnish the fundamental understanding of the structure-activity relationship but also shed light on designing advanced ORR catalysts.

Immobilizing a homogeneous manganese catalyst into MOF pores for α-alkylation of methylene ketones with alcohols.

Herein, for the first time, a metal-organic framework (MOF) is reported as a catalyst for α-alkylation of ketones with alcohols. Using an encapsulation strategy via nano-confinement of a homogeneous Mn-phenanthroline complex into MOF pores, functionalized branched ketones were selectively produced. Mechanistic investigations and deuterium labelling experiments validated the utilization of the borrowing hydrogen strategy. The formation of extra Lewis acid sites, defects, and pore enhancement during catalysis helped in achieving higher activity and selectivity.

Heterogenizing a Homogeneous Nickel Catalyst Using Nanoconfined Strategy for Selective Synthesis of Mono- and 1,2-Disubstituted Benzimidazoles.

A homogeneous Ni-phenanthroline catalyst was successfully immobilized into the cavities of a metal-organic framework, ZIF-8. The as-synthesized heterogeneous catalyst, Ni-Phen@ZIF, represents the first MOF based catalyst that enables dehydrogenative coupling of alcohols with aromatic diamines for selective synthesis of both mono- and 1,2-disubstituted benzimidazoles. The catalyst survived under harsh basic conditions, characterized by SEM, TEM, BET, PXRD, and EDX elemental mappings. The presence of the nanoconfined Ni-phenanthroline complex and the formation of extra Lewis acid sites during catalysis in the Ni-Phen@ZIF structure, confirmed by TPD analysis and kinetic experiments, might be responsible for higher activity and selectivity.

MOF-Templated Assembly Approach for Fe3 C 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.

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.

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.

From a metal-organic framework to hierarchical high surface-area hollow octahedral carbon cages.

For the first time, high surface-area hierarchical hollow octahedral carbon cages have been successfully fabricated by carbonization of a metal-organic framework, MIL-100(Al), followed by subsequent controlled acid etching, which exhibit significant CO2 and H2 adsorption capacities.

Surfactant-free Pd nanoparticles immobilized to a metal-organic framework with size- and location-dependent catalytic selectivity.

Surfactant-free Pd nanoparticles, immobilized to a metal-organic framework (MIL-101), have been used for the first time as highly active and durable catalysts in water for biomass refining (hydrodeoxygenation of vanillin, a typical compound of lignin) with metal nanoparticle size- and location-dependent catalytic activity and selectivity.

Construction of non-interpenetrated charged metal-organic frameworks with doubly pillared layers: pore modification and selective gas adsorption.

The rigid and angular tetracarboxylic acid 1,3-bis(3,5-dicarboxyphenyl)imidazolium (H4L(+)), incorporating an imidazolium group, has been used with different pyridine-based linkers to construct a series of non-interpenetrated cationic frameworks, {[Zn2(L)(bpy)2]·(NO3)·(DMF)6·(H2O)9}n (1), {[Zn2(L)(dpe)2]·(NO3)·(DMF)3·(H2O)2}n (2), and {[Zn2(L)(bpb)2]·(NO3)·(DMF)3·(H2O)4}n (3) [L = L(3-), DMF = N,N'-dimethylformamide, bpy = 4,4'-bipyridine, dpe = 1,2-di(4-pyridyl) ethylene, bpb = 1,4-bis(4-pyridyl)benzene]. The frameworks consist of {[Zn2(L)](+)}n two-dimensional layers that are further pillared by the linker ligands to form three-dimensional bipillared-layer porous structures. While the choice of the bent carboxylic acid ligand and formation of double pillars are major factors in achieving charged non-interpenetrated frameworks, lengths of the pillar linkers direct the pore modulation. Accordingly, the N2 gas adsorption capacity of the activated frameworks (1a-3a) increases with increasing pillar length. Moreover, variation in the electronic environment and marked difference in the pore sizes of frameworks permit selective CO2 adsorption over N2, where 3a exhibits the highest selectivity. In contrast, the selectivity of CO2 over CH4 is reversed and follows the order 1a > 2a > 3a. These results demonstrate that even though the pore sizes of the frameworks are large enough compared to the kinetic diameters of the excluded gas molecules, the electronic environment is crucial for the selective sorption of CO2.

From ionic-liquid@metal-organic framework composites to heteroatom-decorated large-surface area carbons: superior CO2 and H2 uptake.

For the first time, high surface area uniformly nitrogen (N)- and boron-nitrogen (BN)-decorated nanoporous carbons have been successfully fabricated by impregnation of ionic liquids (ILs) within a metal-organic framework (MOF), MIL-100(Al), followed by carbonization, which exhibit remarkable CO2 and H2 adsorption capacities.

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.

Structural variation in Zn(II) coordination polymers built with a semi-rigid tetracarboxylate and different pyridine linkers: synthesis and selective CO2 adsorption studies.

In an effort towards the rational design of porous MOFs with a functionalized channel surface, 3,3',5,5'-tetracarboxydiphenylmethane (H4L1) has been used in combination with two different bipyridine ligands of similar lengths as linkers, and Zn(II) ions as nodes. Under solvothermal conditions, two Zn(II) coordination polymers, {[Zn(H2L1)(L2)] · DMF · 2H2O}n (1) and {[Zn2(L1)(L3)(DMF)2] · DMF · 4H2O}n (2) (DMF = dimethyl formamide, L2 = 3,6-di-pyridin-4-yl-[1,2,4,5]tetrazine, L3 = 4,4'-bispyridylphenyl) are formed in moderate yields. The obvious kink in the central methylene spacer of H4L1 induces either C2v or Cs symmetry in the ligand, allowing different architectures in the resulting frameworks. Single crystal X-ray analysis shows that compound 1 is a one-dimensional (1D) double chain architecture with rhombus voids, linked by Zn2(CO2)4 paddle-wheel secondary building units (SBUs). The tetrazine and pyridine moieties of the co-ligand and free carboxylic acid groups are lined along the voids of the framework. Compound 2, on the other hand, crystallizes as an infinite two-dimensional corrugated sheet structure, where individual sheets are stacked in--ABAB--patterns along the crystallographic b-axis. Thermogravimetric analysis (TGA) and variable temperature powder X-ray diffraction (VTPXRD) studies reveal high thermal stability for 1 but 2 collapses soon after desolvation. The desolvated framework 1' shows selective CO2 adsorption over N2, H2, and CH4 at 273 K, with an isosteric heat of CO2 adsorption of 21.3 kJ mol(-1), suggesting an interaction of the CO2 molecules with the channel walls.

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.

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.

Immobilizing highly catalytically active Pt nanoparticles inside the pores of metal-organic framework: a double solvents approach.

Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal-organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a "double solvents" method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation.

Direct crystallographic observation of catalytic reactions inside the pores of a flexible coordination polymer.

A new flexible porous coordination polymer (PCP), {[Gd(2)(L)(3)(dmf)(4)]·4DMF·3H(2)O}(n) (1), was synthesized under solvothermal condition by reacting [Gd(NO(3))(3)]·6H(2)O with the ligand 2,6,2',6'-tetranitro-biphenyl-4,4'-dicarboxylic acid (H(2)L). Compound 1 had a 3D coordination polymeric structure with two types of 1D channels (A and B) that were occupied by DMF and water molecules. When crystals of 1 were separately exposed to vapors of various aromatic aldehydes, either the lattice or both the lattice and metal-bound solvent molecules were replaced by aldehyde molecules. The aldehyde molecules inside the pores spontaneously underwent cyanosilylation and Knoevenagel condensation reactions upon exposure to vapors of trimethylsilyl cyanide and malononitrile, respectively. These reactions took place at ambient temperature and pressure. Moreover, both the reactants and the products translocated from one cavity to another. The products that occupied the cavity were expunged upon exposure to the vapors of an aldehyde. Because crystallinity was maintained during these chemical transformations, direct crystallographic observation was possible. Herein, we showed that confinement of the reactants inside the void spaces of the PCP led to the products; we also assessed catalytic activities of this PCP in bulk quantities.

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.