Research On Catalyst Design And Reaction Mechanism Of Small Molecules Conversion Reactions

Nitrogen(N2),carbon dioxide(CO2),and water(H2O)are the most common small molecules on the earth,and they are also indispensable components on the earth.The vast majority of life on Earth contains four elements:carbon(C),hydrogen(H),oxygen(O),and nitrogen(N).They are extremely stable small molecules under ambient conditions.The corresponding products obtained from their catalytic reactions,such as ammonia,carbon monoxide,methane,hydrogen,etc.,are important chemical products and important daily energy sources that are indispensable in human life.Therefore,catalytic small molecule conversion reactions have become the core issue of the current catalytic research.Due to the chemical inertness of these small molecules,high temperature and high pressure are usually used in the industry process to achieve the efficient conversion,which consumes numerous energies and causes serious pollution and damage to the environment.By applied electrocatalytic processes using renewable energy,many small molecules can be converted at room temperature.At present,electroreduction of N2 to synthesize ammonia(NH3)has attracted wide attention due to its advantages such as its ability to react at room temperature and pressure,the reaction rate can be adjusted through voltage,and the use of clean water as a hydrogen source.However,the highly stable N2 molecules and the fierce competition for hydrogen production reactions make the current NH3 production rate and Faraday efficiency very limited.Therefore,efficient activation of N2 is a difficult point in the field of electroreduction of N2 for ammonia synthesis.In addition to the optimization of the catalytic process,the research on the mechanism of small molecule conversion reactions is also a frontier direction in recent years.By using the development of advanced technologies such as synchrotron radiation,researchers have developed dynamic characterization techniques under in situ/operando conditions.These technologies can break through the ultra-high vacuum requirements of traditional characterization techniques,and realize direct testing of the solid/gas interface and solid/liquid interface where the catalytic reaction occurs in the presence of gas and liquid reactants,providing direct experimental evidence for the exploration of the catalytic reaction mechanism.This paper reports the mechanism of Fe single atom supported on MoS2 nanosheets(Fe-MoS2)for high-efficiency N2 electroreduction at room temperature and in situ near-atmospheric pressure X-ray photoelectron spectroscopy(APXPS)for carbon dioxide hydrogenation and water electrolysis reactions.The Fe-MoS2 catalyst reported in the article showed excellent catalytic performance for the electroreduction of N2 reaction,and we also studied its catalytic mechanism for the electroreduction of N2 reaction.In the study of the mechanism of the conversion reaction of CO2 and H2O,we monitored the species on the surface of the catalyst during the reaction by in situ techniques and proposed a reliable reaction mechanism.The main content of this paper includes the following aspects:1.This article first reports an Fe-MoS2 catalyst for by loading Fe single atoms on MoS2 nanosheets.The experimental results show that under an overpotential of-0.3 V,the Faraday efficiency and ammonia production rate of the Fe-MoS2 catalyst can reach 18.8%and 8.63 μgNH3 mg-1cat.h-1,respectively,which are much higher than thoseof the MoS2 catalyst(2.23%and 1.11 μgNH3 mg-1cat.h-1).Mechanism studies have shown that the addition of atomically dispersed Fe in Fe-MoS2 reduces the energy barrier for N2 dissociation,and greatly inhibits the hydrogen evolution process of the competitive reaction,thereby improving its catalytic performance for electroreduction of N2 to ammonia.2.We conducted APXPS studies on CO2 hydrogenation reaction.Studies have shown that CO2 dissociates directly on the surface of Pt(111)to form surface CO and O atom adsorbates at room temperature.The CO adsorbate on the surface can further form surface graphitic carbon species through Boudouard reaction.At temperatures above 150℃,graphitic carbon species can be oxidized by CO2 and disappear from the surface.When CO2 and H2 are dosed in at the same time(H2:CO2=1:1),surface O atoms can directly react with H2.When the temperature increased to 300℃,we observed the occurrence of the reverse water vapor shift(RWGS)reaction in the mass spectrum.When we further increase the H2 ratio(H2:CO2=10:1),the starting temperature of RWGS can be as low as 200℃.3.At the end of this paper,APXPS was used to monitor the surface species changes of Ni electrodes under different potentials in alkaline conditions to reveal the species changes on the Ni electrode surface during the water electrolysis process.Under the potential for hydrogen evolution,Ni surface is mainly in the metallic state;whereas under the potential of oxygen evolution,we found that the intensity of the oxidized Ni species becomes higher.It is worth mentioning that we discovered the signal of metal cations in the solution for the first time using APXPS.The addition of halide ions(Br-and I-)can promote the surface oxidation reaction of the Ni electrode at the anodic oxidation potential,and finally form the surface Ni(OH)2 species.