Herein, an effective tandem catalysis strategy is developed to improve the selectivity of the CO2 RR towards C2 H4 by multiple distinct catalytic sites in local vicinity. An earth-abundant elements-based tandem electrocatalyst PTF(Ni)/Cu is constructed by uniformly dispersing Cu nanoparticles (NPs) on the porphyrinic triazine framework anchored with atomically isolated nickel-nitrogen sites (PTF(Ni)) for the enhanced CO2 RR to produce C2 H4 . The Faradaic efficiency of C2 H4 reaches 57.3 % at -1.1 V versus the reversible hydrogen electrode (RHE), which is about 6 times higher than the non-tandem catalyst PTF/Cu, which produces CH4 as the major carbon product. The operando infrared spectroscopy and theoretic density functional theory (DFT) calculations reveal that the local high concentration of CO generated by PTF(Ni) sites can facilitate the C-C coupling to form C2 H4 on the nearby Cu NP sites. The work offers an effective avenue to design electrocatalysts for the highly selective CO2 RR to produce multicarbon products via a tandem route.

Highly Selective Tandem Electroreduction of CO2 to Ethylene over Atomically Isolated Nickel–Nitrogen Site/Copper Nanoparticle Catalysts

Applications of phthalocyanines (Pcs) in electrocatalysis—including the oxygen reduction reaction (ORR), the carbon dioxide reduction reaction (CO2RR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER)—have attracted considerable attention recently. Pcs and their derivatives are more attractive than many other macrocycles as electrocatalysts since, although they are structurally related to natural porphyrin complexes, they offer the advantages of low cost, facile synthesis and good chemical stability. Moreover, their high tailorability and structural diversity mean Pcs have great potential for application in electrochemical devices. Here we review the structure and composition of Pcs, methods of synthesis of Pcs and their analogues, as well as applications of Pc-based heterogeneous electrocatalysts. Optimization strategies for Pc-based materials for electrocatalysis of ORR, CO2RR, OER and HER are proposed, based on the mechanisms of the different electrochemical reactions. We also discuss the structure/composition–catalytic activity relationships for different Pc materials and Pc-based electrocatalysts in order to identify future practical applications. Finally, future opportunities and challenges in the use of molecular Pcs and Pc derivatives as electrocatalysts are discussed.

Recent advances in electrocatalysis with phthalocyanines.

ConspectusThe continuing increase of the concentration of atmospheric CO2 has caused many environmental issues including climate change. Catalytic conversion of CO2 using thermochemical, electrochemical, and photochemical methods is a potential technique to decrease the CO2 concentration and simultaneously obtain value-added chemicals. Due to the high energy barrier of CO2 however, this method is still far from large-scale applications which requires high activity, selectivity, and stability. Therefore, development of efficient catalysts to convert CO2 to different products is urgent. With their well-engineered pores and chemical compositions, high surface area, elevated CO2 adsorption capability, and adjustable active sites, porous crystalline frameworks including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are potential materials for catalytic CO2 conversion. Here, we summarize our recent work on MOFs and COFs for thermocatalytic, electrocatalytic, and photocatalytic CO2 conversion and describe the structure-activity relationships that could guide the design of effective catalysts.The first section of this paper describes imidazolium-functionalized porous MOFs, including porous liquid and cationic MOFs with nucleophilic halogen ions, which can promote thermocatalytically CO2 cycloaddition reaction with epoxides toward cyclic carbonates at one bar pressure. A porous liquid MOF takes on the role of a CO2 reservoir to tackle the low local CO2 concentrations in gas-liquid-solid heterogeneous reactions. Imidazolium-functionalized MOFs with halogen ions for CO2 cycloaddition could avoid the use of cocatalysts, and this leads to milder and more facile experimental conditions and separation processes.In a section dealing with the electrocatalytic CO2 reduction reaction (CO2RR), we developed a series of conductive porous framework materials with fast electron transmission capabilities, which afford high current densities and outperform the traditional MOF and COF catalysts that have been reported. The intrinsically conductive two-dimensional 2D MOFs and COFs nanosheets based on the fully π-conjugated phthalocyanine motif with excellent electron transport capability were prepared, and strong electron transporters were also integrated into metalloporphyrin-based COFs for CO2RR. Cu2O quantum dots and Cu nanoparticles (NPs) can be uniformly dispersed on porous conductive MOFs/COFs to afford synergistic and/or tandem electrocatalysts, which can achieve highly selective production of CH4 or C2H4 in CO2RR.A third section describes our efforts to facilitate electron-hole separation in CO2 photocatalysis. Our focus is on regulation of coordination spheres in MOFs, fabrication of the architecture of MOF heterojunctions, and engineering MOF films to facilitate photocatalytic CO2 reduction.Finally, we discuss several problems associated with the studies of MOFs and COFs for CO2 conversion and consider some prospects of the fabrication of effective porous frameworks for CO2 adsorption and conversion.

Thermo-, Electro-, and Photocatalytic CO2 Conversion to Value-Added Products over Porous Metal/Covalent Organic Frameworks.

Synthesis of functional 3D COFs with irreversible bond is challenging. Herein, 3D imide-bonded COFs were constructed via the imidization reaction between phthalocyanine-based tetraanhydride and 1,3,5,7-tetra(4-aminophenyl)adamantine. These two 3D COFs are made up of interpenetrated pts networks according to powder X-ray diffraction and gas adsorption analyses. CoPc-PI-COF-3 doped with carbon black has been employed to fabricate the electrocatalytic cathode towards CO2 reduction reaction within KHCO3 aqueous solution, displaying the Faradaic efficiency of 88-96 % for the CO2 -to-CO conversion at the voltage range of ca. -0.60 to -1.00 V (vs. RHE). In particular, the 3D porous structure of CoPc-PI-COF-3 enables the active electrocatalytic centers occupying 32.7 % of total cobalt-phthalocyanine subunits, thus giving a large current density (jCO ) of -31.7 mA cm-2 at -0.90 V. These two parameters are significantly improved than the excellent 2D COF analogue (CoPc-PI-COF-1, 5.1 % and -21.2 mA cm-2 ).

Maximizing Electroactive Sites in a Three-Dimensional Covalent Organic Framework for Significantly Improved Carbon Dioxide Reduction Electrocatalysis.

The electrocatalytic CO2 reduction reaction (ECO2RR) is considered one of the approaches with the most potential to achieve lower carbon emissions in the future, but a huge gap still exists between the current ECO2RR technology and industrial applications. Therefore, the design and preparation of catalysts with satisfactory activity, selectivity and stability for the ECO2RR have attracted extensive attention. As a classic type of functional porous framework, crystalline porous materials (e.g., metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) and derived porous materials (e.g., MOF/COF composites and pyrolysates) have been regarded as superior catalysts for the ECO2RR due to their advantages such as designable porosity, modifiable skeleton, flexible active site structure, regulable charge transfer pathway and controllable morphology. Meanwhile, with the rapid development of nano-characterization and theoretical calculation technologies, the structure-activity relationships of functional porous frameworks have been comprehensively considered, i.e., metallic element type, local coordination environment, and microstructure, corresponding to selectivity, activity and mass transfer efficiency for the ECO2RR, respectively. In this review, the rational design strategy for functional porous frameworks is briefly but precisely generalized based on three key factors including metallic element type, local coordination environment, and microstructure. Then, details about the structure-activity relationships for functional porous frameworks are illustrated in the order of MOFs, COFs, composites and pyrolysates to analyze the effect of the above-mentioned three factors on their ECO2RR performance. Finally, the challenges and perspectives of functional porous frameworks for the further development of the ECO2RR are reasonably proposed, aiming to offer insights for future studies in this intriguing and significant research field.

A rational design of functional porous frameworks for electrocatalytic CO2 reduction reaction.

Porous crystalline metal-organic frameworks (MOFs) are one class of promising electrode materials for CO2 electroreduction reaction (CO2RR) by virtue of their large CO2 adsorption capacities and abundant tunable active sites, but their insulating nature usually leads to low current density. Herein, a two-dimensional (2D) Ni-phthalocyanine-based MOF (NiPc-Ni(NH)4) constructed by 2,3,9,10,16,17,23,24-octaaminophthalocyaninato nickel(II) (NiPc-(NH2)8) and nickel(II) ions attained high electrical conductivity due to the high overlap of d-π conjugation orbitals between the nickel node and the Ni-phthalocyanine-substituted o-phenylenediamine. During CO2RR, the NiPc-Ni(NH)4 nanosheets achieved a high CO Faradaic efficiency of 96.4% at −0.7 V and a large CO partial current density of 24.8 mA cm−2 at −1.1 V, which surpassed all the reported two-dimensional MOF electrocatalysts evaluated in an H-cell. The control experiments and density functional theory (DFT) calculations suggested that the Ni-N4 units of the phthalocyanine ring are the catalytic active sites. This work provides a new route to the design of highly efficient porous framework materials for the enhanced electrocatalysis via improving electrical conductivity.

Conductive phthalocyanine-based metal-organic framework as a highly efficient electrocatalyst for carbon dioxide reduction reaction

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Conductive phthalocyanine-based metal-organic framework as a highly efficient electrocatalyst for carbon dioxide reduction reaction