Anion Modulation of Ag-Imidazole Cuboctahedral Cage Microenvironments for Efficient Electrocatalytic CO2 Reduction.

How to achieve CO2 electroreduction in high efficiency is a current challenge with the mechanism not well understood yet. The metal-organic cages with multiple metal sites, tunable active centers, and well-defined microenvironments may provide a promising catalyst model. Here, we report self-assembly of Ag4L4 type cuboctahedral cages from coordination dynamic Ag+ ion and triangular imidazolyl ligand 1,3,5-tris(1-benzylbenzimidazol-2-yl) benzene (Ag-MOC-X, X= NO3, ClO4, BF4) via anion template effect. Notably, Ag-MOC-NO3 achieves the highest CO faradaic efficiency in pH-universal electrolytes of 86.1%(acidic), 94.1%(neutral) and 95.3% (alkaline), much higher than those of Ag-MOC-ClO4 and Ag-MOC-BF4 with just different counter anions. In situ attenuated total reflection Fourier transform infrared spectroscopy observes formation of vital intermediate *COOH for CO2-to-CO conversion. The density functional theory calculations suggest that the adsorption of CO2 on unsaturated Ag-site is stabilized by C-H⋅⋅⋅O hydrogen-bonding of CO2 in a microenvironment surrounded by three benzimidazole rings, and the activation of CO2 is dependent on the coordination dynamics of Ag-centers modulated by the hosted anions through Ag⋅⋅⋅X interactions. This work offers a supramolecular electrocatalytic strategy based on Ag-coordination geometry and host-guest interaction regulation of MOCs as high-efficient electrocatalysts for CO2 reduction to CO which is a key intermediate in chemical industry process.

Two-Dimensional Porphyrinic Metal-Organic Framework Composites as a Photocatalytic Platform for Chemoselective Hydrogenation.

Chemoselective hydrogenation with high efficiency under ambient conditions remains a great challenge. Herein, an efficient photocatalyst, the 2D porphyrin metal-organic framework composite AmPy/Pd-PPF-1(Cu), featuring AmPy (1-aminopyrene) sitting axially on a paddle-wheel unit, has been rationally fabricated. The 2D AmPy/Pd-PPF-1(Cu) composite acts as a photocatalytic platform, promoting the selective hydrogenation of quinolines to tetrahydroquinolines with a yield up to 99%, in which ammonia borane serves as the hydrogen donor. The AmPy molecules coordinated on a 2D MOF not only enhance the light absorption capacity but also adjust the layer spacing without affecting the network structure of 2D Pd-PPF-1(Cu) nanosheets. Through deuterium-labeling experiments, in situ X-ray photoelectron spectroscopy, electron paramagnetic resonance studies, and density functional theory calculations, it is disclosed that Cu paddle-wheel units in 2D AmPy/Pd-PPF-1(Cu) nanosheets behave as the active site for transfer hydrogenation, and metalloporphyrin ligand and axial aminopyrene molecules can enhance the light absorption capacity and excite photogenerated electrons to Cu paddle-wheel units, assisting in photocatalysis.

Cooperative Catalytic Alkyne Hydrosilylation by a Porphyrinic Metal-Organic Framework Composite.

Vinylsilanes are valuable building blocks and important structural units in organic chemistry. Herein, catalytic alkyne hydrosilylation was reported to be promoted by a porphyrin metal-organic framework with the incorporation of Pd nanoparticles (Pd@Ir-PCN-222). Catalytic results showed that Pd@Ir-PCN-222 displayed high catalytic efficiency, giving rise to the E isomer vinylsilane with an excellent turnover frequency (TOF) of 2564 h-1. The mechanism studies revealed that the enhancement of the catalytic activity originated from the cooperation between iridium porphyrin and the Pd nanoparticle in confined spaces. The iridium porphyrin was prone to absorb and condense the hydrosilane and alkyne in the inner cavities of Ir-PCN-222, not only accelerating the reaction but also promoting the Pd nanoparticle to activate the Si-H and C≡C bonds of hydrosilane and alkyne, respectively.

Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction.

As a type of heterogeneous catalyst expected for the maximum atom efficiency, a series of single-atom catalysts (SACs) containing spatially isolated metal single atoms (M-SAs) have been successfully prepared by confining M-SAs in the pore-nanospaces of porphyrinic metal-organic frameworks (MOFs). The prepared MOF composites of M-SAs@Pd-PCN-222-NH2 (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover number (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h-1 based on M-SAs under visible light irradiation (λ ≥ 420 nm). The photo-/electrochemical property studies and density functional theory calculations disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chemical binding could accelerate electron-hole separation and charge transfer in pore-nanospaces, thus promoting the catalytic H2 evolution reaction with lasting effectiveness.

Nickel sulfide-oxide heterostructured electrocatalysts: Bi-functionality for overall water splitting and in-situ reconstruction.

Bi-functional electrocatalysts are desired for both hydrogen and oxygen evolution reactions (HER and OER). Herein, facile O2-plasma activation is introduced to improve the bi-functionality via constructing nickel sulfide-oxide heterostructures. Ni3S2-NiOx supported by nickel foam delivers obviously elevated HER and OER activity in comparison with pristine Ni3S2 and recently reported NiSx-based electrocatalysts, featured by the low overpotentials for HER (104 mV) and OER (241 mV) at ±10 mA cm-2 in 1.0 M KOH, as well as a voltage of 1.52 V for overall water splitting. As revealed by in-situ Raman spectroscopy, on the one hand, Ni(OH)2 generated from Ni3S2 during alkaline HER accelerates water dissociation toward the gradually improved performance; on the other hand, this heterostructure undergoes extensive oxidation during OER, leading to excessive NiOOH covering on Ni3S2 and thereby declining activity. These changes are interpreted by the distinct thermodynamic relationship under specific electrochemical conditions via density functional theory calculations.

Ultrathin Two-Dimensional Metal-Organic Framework Nanosheets Based on a Halogen-Substituted Porphyrin Ligand: Synthesis and Catalytic Application in CO2 Reductive Amination.

Ultrathin two-dimensional metal-organic framework nanosheets have emerged as a promising kind of heterogeneous catalysts. Herein, we report a series of 2D porphyrinic metal-organic framework nanosheets (X-PMOF, X=F, Cl, Br), which was prepared from the self-assembly of a halogen-based porphyrin ligand X-TCPP (X-TCPP=5-(4-halogenatedphenyl)-10,15,20-tris(4-carboxyphenyl)-porphyrin) and ZrCl4 in the presence of trifluoroacetic acid as the modulating reagent. The framework of X-PMOF possessed the ftw topology as in MOF-525. The lamellar X-PMOF nanosheets with the thickness of down to 4.5 nm were assembled and aggregated into a flower-like morphology. With the introduction of iridium(III) atoms into the porphyrin rings, the resultant X-PMOF(Ir) nanosheets were prepared by a similar method. Catalytic results show that Br-PMOF(Ir) nanosheets were efficient for CO2 reduction and aminolysis, giving rise to formamides in high yields under room temperature and atmospheric pressure, and can be recycled and reused for 3 runs. The total turnover number of Br-PMOF(Ir) after 3 runs was 1644 based on Ir. Mechanistic studies disclose that the high efficiency of Br-PMOF(Ir) nanosheets was ascribed to three factors, including the superior activation capability of iridium(III) porphyrin for Si-H bonds, more active sites on the external surfaces of Br-PMOF(Ir) nanosheets, and the defects caused by unsymmetrical porphyrin ligand that increased the framework's affinity towards CO2 .

Molybdenum Carbide-Oxide Heterostructures: In Situ Surface Reconfiguration toward Efficient Electrocatalytic Hydrogen Evolution.

Heterostructured Mo2 C-MoOx on carbon cloth (Mo2 C-MoOx /CC), as a model of easily oxidized electrocatalysts under ambient conditions, is investigated to uncover surface reconfiguration during the hydrogen evolution reaction (HER). Raman spectroscopy combined with electrochemical tests demonstrates that the MoVI oxides on the surface are in situ reduced to MoIV , accomplishing promoted HER in acidic condition. As indicated by density functional theoretical calculations, the in situ reduced surface with terminal Mo=O moieties can effectively bring the negative ΔGH* on bare Mo2 C close to a thermodynamic neutral value, addressing difficult H* desorption toward fast HER kinetics. The optimized Mo2 C-MoOx /CC only requires a low overpotential (η10 ) of 60 mV at -10 mA cm-2 in 1.0 m HClO4 , outperforming Mo2 C/CC and most non-precious electrocatalysts. In situ surface reconfiguration are shown on W2 C-WOx , highlighting the significance to boost various metal-carbides and to identify active sites.

Converting surface-oxidized cobalt phosphides into Co2(P2O7)-CoP heterostructures for efficient electrocatalytic hydrogen evolution.

Exploring noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) is a key issue in a hydrogen economy blueprint. As one of the promising candidates, transition metal phosphides unfortunately suffer from inevitable surface oxidation which obstructs active-site exposure. Herein, a facile reduction followed by a surface phosphorization is introduced to convert surface-oxidized cobalt phosphides to a Co2(P2O7)-CoP heterostructure embedded in N-doped carbon (Co2(P2O7)-CoP/NC), accomplishing an efficient HER in both acidic and alkaline electrolytes. It affords low overpotentials (η 10) of 88 and 97 mV to reach a current density of -10 mA cm-2, and small Tafel slopes of 51 and 61 mV dec-1 in 0.5 M H2SO4 and 1.0 M KOH, respectively, outperforming the parent surface-oxidized Co2P and most previously-reported Pt-free electrocatalysts. The remarkably improved electrocatalysis should be ascribed to the strong surface acidity of the Co2(P2O7) component and thereby the promoted HER kinetics on Co2(P2O7)-CoP interfaces. This work will encourage the development of cost-efficient electrocatalysts via surface engineering.

Metallic Cobalt@Nitrogen-Doped Carbon Nanocomposites: Carbon-Shell Regulation toward Efficient Bi-Functional Electrocatalysis.

To advance hydrogen economy, noble-metal-free electrocatalysts with good efficiency are urgently demanded. They can be developed from metal-organic frameworks (MOFs) with abundant structure-variety, in which a controlled pyrolysis is desired to rationalize nanostructure and maximize catalytic activity. Herein, the efficient regulation is proposed for the first time on the carbon-shell of MOFs-derived Co@NC nanocomposites via varying temperature and flow-rate during pyrolysis, enabling the good accessibility and the electronic optimization of active Co cores. With moderated temperature and flow-rate, the resulting ultrathin carbon-shell, on the one hand, renders Co cores easily accessible to electrolytes and, on the other hand, promotes the electronic penetration to optimize metallic Co active sites. As expected, the optimal Co@NC affords the benchmarking performance of noble-metal-free electrocatalysts in hydrogen evolution and oxygen reduction reactions, featured by the low overpotentials, the striking kinetic metrics, and the outstanding long-term stability. Elucidating the feasibility to design efficient electrocatalysts via controlled MOFs pyrolysis, this work will open up new opportunities for the development of cost-effective materials in the energy field.