Controlled Synthesis Of Single-atom Catalysts Toward Efficient Electro-catalysis

Catalysis science is not only a very important part of the scientific field,but also plays an indispensable role in all fields of human existence.According to statistics,about 30%of global production is closely related to catalysis,and about 85%to 90%of chemical manufacturing involves at least one catalytic process.It is precisely because of the pivotal position of catalysis science in human society that its development is also related to whether human beings can overcome the two major problems we face today-energy problems and environmental problems.Since the Industrial Revolution,both the total population and the energy consumption per capita have increased dramatically,which has undoubtedly caused the gradual depletion of traditional fossil energy sources and the accompanying environmental pollution.In the energy field,people have begun to urgently demand more efficient catalytic technology to meet our requirements for low cost,high output and reliability.The environmental protection field also puts forward higher requirements on catalysis.We need more advanced catalytic technology to quickly treat the increasing amount of industrial waste and reduce pollution to the atmosphere,water and soil.We also need more sensitive detection methods to more accurately monitor the air and water we depend on for survival.The vigorous development of single-atom catalysis in recent years has opened a new chapter in catalysis science.Compared with conventional nano heterogeneous catalysts based on metal nanostructures,single-atom catalysts have 100%metal atom utilization and unique catalytic behavior,which provides the possibility to solve hot and difficult problems in catalysis science.In this paper,we try to apply single-atom catalysts to catalytic reactions that are closely related to energy and the environment.1.Gold single-atom catalyst is used for electrocatalytic reduction of nitrogen to produce ammonia.As one of the most important industrial chemicals,ammonia(NH3)is currently produced at a scale of 150 million tons per year through the Haber-Bosch reaction.This process requires 200-300 atmospheres of pressure and 300-500?temperature.To date,this energy-and capital-intensive process accounts for 1.4%of annual energy consumption and approximately 3%of global carbon dioxide emissions.Electrocatalytic methods,especially those driven by renewable energy sources,are generally considered to be a highly energy-efficient and sustainable process for the synthesis of NH3 at room temperature and pressure.Despite great efforts,few people have achieved energy efficiency comparable to the Haber-Bosch process in experiments.The hindrance is mainly due to the high overpotential caused by the inertness of the reactant nitrogen(N2),and the inefficient Faraday efficiency caused by the unfavorable hydrogen evolution reaction(HER)in the aqueous solution.Therefore,in order to achieve high-efficiency and energy-saving electrochemical synthesis of NH3,the catalyst is required to have both high activity and selectivity.The emerging atom-dispersed metal catalyst has its specific structure as the active center,which provides the possibility to overcome the above problems.Since the size of the metal is reduced to a single atom,the atom-dispersed metal catalyst usually exhibits significant catalytic activity.At the same time,the uniform active center sites in the atom-dispersed metal catalyst ensure high selectivity.In this chapter,we have prepared a catalyst with Au atomically dispersed on carbon nitride(Au1/C3N4),and studied its catalysis for reducing N2 to ammonium ion(NH4+)under acidic conditions(sulfuric acid solution)performance.Compared with Au nanoparticles(Au-NPs/C3N4)supported on C3N4,Au1/C3N4 exhibits excellent Faraday efficiency for NH4+ generation,at a voltage of-0.10 V(vs.RHE,compared to reversible hydrogen electrode).As high as 11.1%,it is better than most reported catalysts.In addition,we also assembled a complete electrolytic cell,using Au1/C3N4 as the cathode to reduce N2 to NH4+;using platinum foil(Pt)as the anode to cause the hydrogen oxidation(HOR)reaction.The experimental results show that nitrogen and hydrogen can be directly electrochemically converted into ammonia,and the energy utilization rate can reach 4.01 mol kJ-1.This research demonstrates the possibility of electrochemical synthesis of ammonia to replace the Haber process.2.Coordination environment regulation is used to prepare high-efficiency carbon dioxide electroreduction single-atom catalyst.In recent years,the concentration of carbon dioxide(CO2)in the atmosphere has been increasing,which has caused a series of environmental problems.The electricity reduction of CO2 into value-added chemicals is a promising way to suppress man-made CO2 emissions and alleviate the energy crisis.In order to obtain practical electrocatalysts with low overpotential and high selectivity,scholars have researched and designed some metal-based nanostructured catalysts,such as functionalized metals,metal oxides and metal disulfides.However,the internal mechanism of the activation of CO2 to CO2·-during the reaction and how the microstructure of metals and natural species affect the electrocatalytic activity remains unclear.In recent years,the emerging atom-dispersed metal catalysts with a defined structure as the active center provide the possibility to explore the structure-activity relationship.In order to enhance the understanding of reaction intermediates and reaction sites at the molecular level,we prepared a series of atom-dispersed cobalt catalysts with different N coordination numbers and studied their catalytic performance for CO2 reduction.The adjustment of the N number around the central Co site is based on the generation of volatile C-N fragments at different pyrolysis temperatures.The reduction of the N coordination number leads to more unoccupied 3d orbitals of Co atoms,which facilitates the adsorption of CO2·-and increases the reduction rate of CO2.It was found that the Co-N2 site-based catalyst has higher activity and selectivity than the Co-N4 catalyst.The current density is about 18.1 mA cm-2,and the CO Faraday efficiency is 94%at an overpotential of only 520 mV.In addition,the conversion frequency(TOF)of CO generation catalyzed by Co-N2 has reached a breakthrough value of 18,200 h-1,which exceeds most reported metal-based catalysts under the same conditions.Since activity,selectivity and stability are highly sensitive to the local coordination environment of single-atom catalysts,our findings emphasize the importance of coordination adjustment at the reaction site for triggering high-efficiency CO2 electroreduction and other catalytic reactions.