Expanded Porphyrin Metal-organic Molecules For Electrocatalytic Reduction Of Carbon Dioxide

CO2 may become the renewable feedstock for making fuels and commodity and pharmaceutical chemicals we need to sustain our societies.If we manage to solve the challenges in reaching this goal,we would achieve a circular economy based on the use of renewable energies and CO2.A primary task is to develop selective,fast and efficient reduction processes of CO2 into valuable products.The clear chemical structure and unique spatial configuration of expanded porphyrin compounds contribute to the understanding of catalytic reaction.In this paper,the metal-based porphyrins(involved cobalt,iron and nickel)and doubly N-Confused Hexaphyrins were synthesized through the improved and unique method.The activity of transition metal was identified by analyzing the structure-activity relationship between the structure and electrocatalytic reduction of carbon dioxide.Copper-based Octaphyrins,Hexaphyrins and doubly N-Confused Hexaphyrins were used to control the reduction process of Cu2+,and a unique catalytic environment of metal-metal-organic coordination is constructed.The selective two-carbon products can be obtained in the electrocatalytic reduction of carbon dioxide with an efficiency of more than 70%.The specific work of this paper is as follows:First,by using the additional two carbonyl oxygens in the doubly N-Confused Hexaphyrins,a coordination environment sufficient to accommodate bimetallic ions can be formed within a single molecule by using the additional two carbonyl oxygens in the doubly N-Confused Hexaphyrins.After metallization,the metal-organic complex of cobalt/iron/nickel coordinated with three nitrogen atoms and one oxygen atom were obtained.These three molecules were further tested for electrocatalytic CO2 reduction in a conventional H-cell,and only the nickel-based molecules achieved high CO selectivity.The catalytic performance and electrochemical cyclic voltammetry of the porphyrins were compared with that of iron,cobalt and nickel based mononuclear porphyrins.The experimental results indicated that cobalt and iron metal have catalytic activity when symmetrically coordinated with four nitrogen,but lose catalytic ability when symmetrically coordinated with three nitrogen and one oxygen.The nickel complex showed completely opposite characteristics.For copper ions,the relaxation and coordination environment caused by octant extended porphyrins can also promote the formation of copper in situ,making it a good catalyst for the conversion of carbon dioxide into hydrocarbon products.Secondly,Hexaphyrins and doubly N-Confused Hexaphyrins can be metallized with copper ions.Thus,binuclear copper complexes in different metal coordination environments can be obtained:Hex-2Cu-O,in which two Cu ions share the same oxygen atom,and Hex-2Cu-2O,in which two Cu ions coordinate with three independent nitrogen atoms and one carbonyl oxygen.Detailed detection of catalytic products and characterization of synchrotron radiation X-ray absorption spectra revealed that Hex-2Cu-O partially exfoliated to form inorganic/organic hybrids consisting of low coordination Cu clusters and partially reduced Hex-2Cu-O.This mixed structure,in turn,produced synergies that greatly promote polycarbon products,especially for alcohols,in the CO2RR process.In contrast,Hex-2Cu-2O with independent Cu-N3O motif has better electrochemical stability,but CO2RR showed poor performance and mainly generated H2 and HCOOH.In this paper,the metal-organic compounds of expanded porphyrins were synthesized and applied to the mechanism analysis of CO2 reduction.Through a series of electrochemical methods and in-situ characterization,the specific active sites of transition metal machine molecules during the CO2 reduction were revealed,and some ideas were also provided for the development of metal-organic-inorganic hybrid catalysts.