Structural Design And Catalytic Research Of Two-dimensional Transition Metal Chalcogenides High-entropy Alloys

The massive consumption of fossil fuels such as oil and coal has led to excessive emissions of greenhouse gases,triggering a series of environmental issues.Under the promotion of carbon peak,converting CO₂ into valuable carbon-containing compounds is a critical way to achieve the low carbon target.Developing low-cost and efficient CO₂ reduction catalysts is the key to achieving this technology.In recent years,high-entropy alloys（HEAs）have received extensive attention due to their excellent mechanical properties,high catalytic activity,and stability.The catalytic performance of high-entropy two-dimensional transition metal chalcogenides will be particularly significant because they gather the advantages of two-dimensional materials,transition metal chalcogenides,and HEAs.Firstly,two-dimensional materials have a large surface area,more active sites,and high electron transfer efficiency,with very high catalytic performance.Secondly,transition metal chalcogenides have unique electronic structures,low cost,and ease of preparation.They are also widely used in various catalytic reactions.Finally,the HEAs have multiple metal sites,and significant synergistic effects can occur from these metal sites.Optimizing the interaction between adsorbed molecules and active sites can significantly improve catalytic performance.At the same time,the high-entropy effect will significantly improve the stability of the system.Therefore,it can be inferred that the high-entropy two-dimensional transition metal chalcogenides will be a highly efficient electrocatalyst with stable structures and excellent performance.This work is based on density functional theory and takes high-entropy two-dimensional transition metal chalcogenides as the research object.Firstly,structural design work is carried out to study their thermodynamic stability,Then explore the catalytic performance of these stable structures in CO₂ reduction reactions.The main contents are as follows:1.Structural design of high-entropy two-dimensional transition metal chalcogenides:The change of Gibbs free energy can judge the stability of HEAs.However,the number of HEAs will increase sharply with the increase of metal elements,which poses a significant challenge in calculating thermodynamic stability.The enthalpy criterion model can predict the stable high-entropy two-dimensional transition metal chalcogenides solid solution with low calculation costs.We selected 27experimentally existing 1T phase two-dimensional transition metal chalcogenides（MX₂,M is transition metal elements,X=S,Se,and Te）as structural prototypes and generated some binary alloys（M¹M²X⁴）of equal molar ratio based on the special-quasi-random（SQS）method.We calculate the formation enthalpy of binary alloys and determine whether the results meet the criteria.Summarize the transition metal elements that meet the criteria,and use the SQS method to generate a series of five-element disordered alloy structures with equal metal atomic ratios（M¹M²M³M⁴M⁵）_0.2X₂.These structures’formation enthalpy and configuration entropy were calculated to obtain the Gibbs free energy at room temperature.We finally get539 stable five-component high entropy alloys,of which 56,21,and 462 stable structures exist when the X position is S,Se,and Te,respectively.The results show that the structures generated by the enthalpy criterion maintain good thermodynamic stability,and the calculated high-entropy two-dimensional transition metal chalcogenides also provide a reference for related theoretical and experimental work.2.Study on the catalytic mechanism of high-entropy two-dimensional transition metal chalcogenides:Reducing CO₂ by electrocatalytic methods has problems such as complex reaction paths,low reaction rates,and low selectivity.Therefore,stable and efficient electrocatalysts are needed to reduce the reaction’s energy barrier and improve the product’s selectivity.When generating valuable carbon-containing products such as hydrocarbons,CO₂ is first converted to CO,and then CO is converted to hydrocarbons.In this work,we explore the catalytic reaction process of converting CO₂ to CO and search for efficient catalysts.The candidate structure of the catalyst is the stable high-entropy two-dimensional transition metal chalcogenides found in the above study.Considering the chemical inertness of the high-entropy two-dimensional transition metal chalcogenides surface with perfect crystal structure to adsorbed molecules,we used the method of generating sulfur group element vacancies to study the catalytic performance of the local environment（including three transition metal atoms）at each vacancy.We put the intermediate molecules COOH and CO into the vacancy of the optimized structure,construct an adsorption model,and calculate the adsorption energy of small molecules at this site with the high-throughput method and the free energy of each step of the reaction.From the calculated results of more than2000 adsorption and free energies,three optimal catalytic structures were obtained when the X position of the alloy structure was S,Se,and Te,respectively.These three structures require the smallest overpotential during the conversion of CO₂ to CO,and the optimal structure only requires an overpotential of 84 me V.Based on the calculated results of differential charge density and electronic density of states for the intermediate adsorption structure and catalyst structure,we analyzed the reasons for the high catalytic efficiency of these three structures.We analyzed the contribution of transition metal elements in the vacancy-localized environment to the catalytic process.It provides a new guiding idea for researching efficient CO₂ reduction catalysts.