Single Ru Sites on Covalent Organic Framework-Coated Carbon Nanotubes for Highly Efficient Electrocatalytic Hydrogen Evolution.

Covalent organic frameworks (COFs) with precisely controllable structures and highly ordered porosity possess great potential as electrocatalysts for hydrogen evolution reaction (HER). However, the catalytic performance of pristine COFs is limited by the poor active sites and low electron transfer. Herein, to address these issues, the conductive carbon nanotubes (CNTs) are coated by a defined structure RuBpy(H2 O)(OH)Cl2 in bipyridine-based COF (TpBpy). And this composite with single site Ru incorporated can be used as HER electrocatalyst in alkaline conditions. A series of crucial issues are carefully discussed through experiments and density functional theory (DFT) calculations, such as the coordination structure of the atomically dispersion Ru ions, the catalytic mechanism of the embedded catalytic site, and the effect of COF and CNTs on the electrocatalytic properties. According to DFT calculations, the embedded single sites Ru act as catalytic sites for H2 generation. Benefitting from increasing the catalyst conductivity and the charge transfer, the as-prepared c-CNT-0.68@TpBpy-Ru shows an excellent HER overpotential of 112 mV at 10 mA cm-2 under alkaline conditions as well as an excellent durability up to 12 h, which is superior to that of most of the reported COFs electrocatalysts in alkaline solution.

Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine.

The electrocatalytic reduction of nitrate (NO3-) to ammonia (NH3) not only offers an effective solution to environmental problems caused by the accumulation of NO3- but also provides a sustainable alternative to the Haber-Bosch process. However, the conversion of NO3- to NH3 is a complicated process involving multiple steps, leading to a low Faradaic efficiency (FE) for NH3 production. The structural designability of covalent organic frameworks (COFs) renders feasible and precise modulation at the molecular level, facilitating the incorporation of multiple well-defined catalytic sites with different reactivities into a cohesive entity. This promotes the efficiency of the overall reaction through the coupling of multistep reactions. Herein, heterobimetallic CuP-CoBpy was prepared by postmodification, involving the anchoring of cobalt ions to the CuP-Bpy structure. As a result of the cascade effect of the bimetallic sites, CuP-CoBpy achieved an outstanding NH3 yield of 13.9 mg h-1 mgcat.-1 with a high FE of 96.7% at -0.70 V versus the reversible hydrogen electrode and exhibited excellent stability during catalysis. A series of experimental and theoretical studies revealed that the CuP unit facilitates the conversion of NO3- to NO2-, while the CoBpy moiety significantly prompts the reduction of NO2- to NH3. This study demonstrates that tailoring the structural units for the construction of COFs based on each step in the multistep reaction can enhance both the catalytic activity and product selectivity of the overall process.

Selective Integrating Molecular Catalytic Units into Bipyridine-Based Covalent Organic Frameworks for Specific Photocatalytic Fuel Production.

Molecular metal compounds have demonstrated excellent catalytic activity and product selectivity in the H2 evolution reaction (HER) and the CO2 reduction reaction (CO2RR). The heterogenization of molecular catalysts is regarded as an effective approach to improve their applicability. In this work, the molecular catalytic units [Cp*Ir(Bpy)Cl]+ and [Ru(Bpy)(CO)2Cl2] are constructed in situ on the bipyridine sites of the covalent organic framework for photocatalytic HER and CO2RR, respectively. Inheriting the impressive performance of molecular catalysts, the functionalized TpBpy-M exhibits excellent catalytic activity and product selectivity. Under visible light irradiation, the H2 production rate of TpBpy-Ir is about 760 µmol g-1 h-1, which is 6.7 times higher than that of TpBpy without built-in catalytic sites. Also, the HCOOH production rate of TpBpy-Ru is 271 µmol g-1 h-1, with an impressive selectivity of 88%. Control experiments validated that this improvement is attributed to the incorporation of molecular catalytic units into the framework. Photoluminescence spectroscopy measurements and theoretical calculation consistently demonstrate that, under illumination, the photosensitizer [Ru(Bpy)3]Cl2 is excited and transfers electrons to the catalytic sites in TpBpy-M, which then catalyzes the reduction of H+ and CO2.

Covalent Metalloporphyrin Polymer Coated on Carbon Nanotubes as Bifunctional Electrocatalysts for Water Splitting.

Metalloporphyrins have exhibited excellent electrocatalytic activities for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In order to improve the efficiency and conductivity, these molecular catalysts need to be immobilized on conductive electrode materials. Herein, a facile "one-pot" strategy was developed to coat a covalent metalloporphyrin polymer on a carbon nanotube (CNT) as bifunctional catalysts [denoted as MTIPP@CNTs, H2TIPP = 5,10,15,20-tetra(4-(imidazole-1-yl)phenyl)porphyrin)] for water splitting in alkaline solution. MTIPP@CNTs have shown excellent electrocatalytic activities for both the HER and OER when metalloporphyrin's central metal is optimized as well as the amount of catalysts that is loaded on the CNT. The overpotential (η10) of NiTIPP@CNT-2 for the OER is only 320 mV at a current density of 10 mA cm-2 in 1.0 M KOH, and CoTIPP@CNT-1 exhibited an excellent electrocatalytic activity for the HER (η10 = 450 mV for 10 mA cm-2). Furthermore, the remarkable bifunctional electrocatalytic performance (a cell voltage of 2.04 V with a current density of 10 mA cm-2) was also explored in the overall water splitting test.

Fine tuning dual active sites in modulating cascade electrocatalytic nitrate reduction over covalent organic framework.

Conversion of NO3- to NH3 proceeds stepwise in natural system under two different enzymes involving intermediate NO2-. Artificial electro-driven NO3- reduction also faces the obstacle of low faradaic efficiency due to insufficient utilization of this intermediate. Herein, we demonstrate a bimetallic COF-based electrocatalyst for the cascade catalysis of NO3--to-NO2--to-NH3 for the first time. TpBpy-Cu2Co4 exhibits a significantly improved performance, with an enhancement factor of 1.4-2 compared to monometallic TpBpy-M. The NH3 yield rate achieves 25.6 mg h-1 mgcat.-1 at -0.55 V vs RHE over TpBpy-Cu2Co4, together with excellent faradaic efficiency (93.4 %). This achievement demonstrates cascade catalysis between Co and Cu units, and their distinct roles are investigated through electrochemical experiments and theory calculations. In electrocatalytic process, Cu site facilities *NO3-to-*NO3H step, while the Co site significantly decreases the energy barrier of *NHOH-to-*NH. The present work provides a valuable inspiration in designing efficient catalysts for cascade reaction.

Orthogonal Supramolecular Assembly Triggered by Inclusion and Exclusion Interactions with Cucurbit[7]uril for Photocatalytic H2 Evolution.

The fabrication of efficient and convenient photocatalytic H2 evolution systems is a fascinating research topic in the field of solar energy conversion. A ternary self-assembled photocatalytic H2 evolution system was fabricated through supramolecular host-guest chemistry. The system consisted of the H2 evolution catalyst [Co(dmgH)2 (4-ppy)2 ]NO3 (1; dmgH2 =dimethylglyoxime, 4-ppy=4-phenylpyridine) and the photosensitizer Eosin Y (EY) assembled with the macrocyclic compound cucurbit[7]uril (CB[7]) to form the 1@CB[7]/EY complex through inclusion and exclusion interactions, respectively. The synchronous self-assembly drives an orthogonal arrangement of the 1@CB[7]/EY system. The inclusion complex 1@CB[7] was successfully characterized by 1 H NMR spectroscopy and single-crystal XRD. The exclusion process of CB[7] with EY was identified by NMR titration and the optimized geometry of the exclusion structure was determined by DFT calculations. The use of CB[7] resulted in a 6-fold increase in turnover number, a 3-fold increase in turnover frequency, and a 3-fold extension of lifetime for photocatalytic H2 evolution as compared with the system in the absence of CB[7]. The improvement of the light-driven H2 evolution activity was ascribed to the ability of CB[7] to link the photosensitizer and catalyst.

Ultrasmall Ru Nanoparticles Highly Dispersed on Sulfur-Doped Graphene for HER with High Electrocatalytic Performance.

Nanostructuring and metal-support interactions have been explored as effective methods to improve the electrocatalytic activity in heterogeneous catalysis. In this study, we have fabricated ultrasmall Ru nanoparticles (NPs) dispersed on S-doped graphene (denoted as Ru/S-rGO) by a facile "one-pot" procedure. The experimental results indicated that both the S doping and moderate degree of oxidization of GO can induce the formation and high dispersion of the ultrasmall Ru NPs with larger electrochemically active surface areas for exposing more active sites. Metal-support interaction between S-doped graphene and Ru NPs was observed from the X-ray photoelectron spectroscopy and electronic charge-difference studies. It resulted in the decrease in the electron density of Ru, which facilitated electron release from H2O and H-OH bond breakage. The results of density functional theory calculation confirmed that the S-dopants could reduce the energy barrier for breaking the H-OH bond to accelerate water dissociation during the alkaline hydrogen evolution reaction (HER). At a current density 20 mA cm-2, the lowest overpotential of 14 mV, superior to that of Pt/C in alkaline solution, was observed for Ru/S-rGO-24. The observed lowest value of overpotential was because of the ultrasmall size, high dispersion, and metal-support interaction. This work provides a simple and effective method in designing advanced electrocatalysts for the HER in an alkaline electrolyte.