Green Chemistry Upgrades Small Molecules To High-value Chemicals By Electrocatalysis

Under the dual pressures of the exhaustion of fossil resources and the deterioration of the environment,in recent years,the production of high value-added chemicals without relying on fossil resources(e.g.petroleum)has become one of the focuses of cutting-edge research.To date,the distributed energy and basic chemical production heavily rely on fossil resources,and the gradual depletion of fossil resources posed a serious challenge.Hydrogen energy is most likely to replace fossil resources as a distributed energy source,but the hydrogen energy system cannot provide the necessary raw materials for the production of basic chemicals.Therefore,this thesis proposes the use of renewable energy to upgrade small molecules such as CO₂,CO(derived from CO₂),NO₃^-,and H₂O through electrocatalytic chemical upgrading to produce high value-added chemical products(such as ethylene,ethanol,acetic acid,ammonia,etc.)as an alternative approach.However,the lack of model catalysts with a well-defined structure leads to the inability to obtain a clear structure-activity relationship to guide the development of high-performance catalysts for this type of reaction.Therefore,the purpose of this thesis is to design a model catalyst with a well-defined structure for the electrocatalytic green conversion of small molecules to high value-added chemical products.An in-depth understanding of the catalytic process and mechanism guides the design,development,and optimization of high-performance catalysts.In this dissertation,a variety of synthetic methods such as regulating coordination number method,supramolecular self-assembly pyrolysis method,and interface confinement reduction method have been developed to prepare model catalyst materials on a large scale.Employing partial pressure experiments,isotope labeling experiments,in-situ spectroscopy,and theoretical calculations,the catalytic process and reaction mechanism are explored,and the structure-activity relationship of the catalyst is revealed on a microscopic scale.The main research contents of this thesis are as follows:1.To date,copper-based catalysts have attracted much attention because of their unique ability to upgrade CO/CO₂(single carbon)to high value-added multi-carbon(C₂₊)products.However,the structure-activity relationship of copper-based catalysts in CO/CO₂ reduction has not been fully established,mainly because of the lack of high-quality Cu(111)model catalyst materials at the nanometer scale.In response to this problem,the author developed the regulating coordination number method to prepare the thickness of 5 nm copper nanosheet with selective exposure of{111}crystal planes.The Cu nanosheets exhibit an acetate Faradaic efficiency of 48%with an acetate partial current density up to 131?mA?cm^-2 in electrochemical CO reduction.This research not only brings the highly selective and efficient electroreduction of CO into valuable chemicals to a new level but also serves as a model catalyst to provide a deeper understanding of the mechanism of electroreduction CO to multi-carbon products.2.To explore whether a single-site copper catalyst can convert CO into multi-carbon products via electrocatalysis,the author developed a supramolecular self-assembly pyrolysis method to prepare the single-site copper catalyst(Cu-N-C).The Cu-N-C achieved nearly 100%of selectivity for the multi-carbon products,a maximum acetate Faradaic efficiency of 30%in electrochemical CO reduction.Different from the C-C coupling mechanism on metallic copper,we propose a CO insertion mechanism for the acetate production on the single site copper catalyst.3.The copper catalyst derived from copper oxide(OD-Cu)can produce more grain boundaries,which is conducive to enhancing CO adsorption and improving catalytic performance.Based on this,regular Cu₃N(100)nanocubes were synthesized to explore whether the copper catalyst derived from copper nitride(ND-Cu)has catalytic specificity.In-situ STEM electrochemical reduction experiments showed that Cu₃N-derived Cu selectively exposed more(100)crystal planes and produced surface defects,which explained its 36%acetic acid selectivity in electroreduction of CO.4.To prepare the Bi model catalyst on a large scale and overcome the problems of other methods,such as uncontrollable preparation,incomplete reduction,and difficult large scale preparation,the author proposed the interface confinement reduction method for the first time to realize the Bi(001)model catalyst.With the selectively exposed(001)crystal plane and a large number of small-angle grain boundaries in Bi porous nanosheets,95%formic acid selectivity was achieved in CO₂ electroreduction,and the current density of formic acid can reach 72 mA/cm².5.For the green and sustainable electrochemical synthesis of ammonia,the author proposes a route for electroreduction of nitrate to synthesize ammonia.The Cu nanosheets and nanocube as model catalysts are used for electroreduction of nitrate.It is found that the reaction is a structure-sensitive reaction in alkaline electrolytes.The ammonia selectivity on Cu nanosheets is close to 100%,and the ammonia production rate is as high as 390.1?g mg_Cu^-1 h^-1.This research provides a new way for the closure of the nitrogen cycle and electrochemical synthesis of ammonia.