First-principles Study On The Electrocatalytic Reduction Of CO₂ By Two-dimensional Single-atom Catalysts

With the rapid development of the global economy and the massive consumption of fossil fuels,the energy crisis and environmental problems have become increasingly prominent and have received more and more attention worldwide.Reducing CO₂ to useful fuels and chemicals is an effective way to address energy crises and weaken the greenhouse effect.Among many CO₂ reduction methods,the direct electrochemical reduction method has been favored by researchers because of its mild reaction conditions and simple operation.Although the research on the catalyst for CO₂ reduction reaction?CRR?has made some progress,the catalyst generally has the disadvantages of high overpotential,stability and poor selectivity.Therefore,the design and synthesis that have excellent catalytic activity,good stability and selection,cheap and readily available catalysts will remain a huge challenge for us.Compared with traditional catalysts,single-atom catalysts?SACs?based on two-dimensional materials have attracted extensive attention due to their large specific surface area,high atomic utilization,bare active sites,stable structure and excellent catalytic activity.Based on this that the density functional theory?DFT?is used in this paper to predict the electrocatalytic CO₂ reduction performance of several single-atom catalysts.The details are as follows:?1?The unique macrocyclic structure of the phthalocyanine provides an effective space for the inlay of transition metals,thus providing the possibility of preparing a monolayer SACs with TM-N₄ coordination.In this paper,the spin-polarization DFT calculation method was used to systematically study the catalytic performance of transition metal phthalocyanine?TM-Pc,TM=Sc-Zn?monolayers as SACs for electroreduction of CO₂.The results show that the monodisperse metal atom has a strong binding ability to Pc,which makes the two-dimensional TM-Pc monolayer structure have excellent stability.Through investigation of the hydrogen evolution reaction,it was found that TM-Pc has a good hydrogen suppression effect.The CRR reduction products of the Sc-Pc,Ti-Pc,V-Pc and Fe-Pc monolayers is CH₄,HCOOH dominates for Cr-Pc,Mn-Pc,Ni-Pc,Cu-Pc and Zn-Pc monolayers,while HCHO is the main product for Co-Pc.Except for Sc-Pc,Ti-Pc and V-Pc?overpotential>1 V?,the overpotential of other TM-Pc monolayer catalysts are in the range of 0.017⁰.857 V,especially the Mn-Pc has the lowest overpotential?0.017V?.Therefore,our research opens up new avenues for the development of efficient CRR catalytic materials.?2?The transition metal 7,7,8,8-tetracyanoquinodimethane?TM-TCNQ,TM=Sc-Zn?monolayer was used as SAC for electrocatalytic CO₂ reduction.Density functional calculations show that the TM-TCNQ monolayers have excellent stability and CRR selectivity.The main reduction product of Sc-TCNQ and Ti-TCNQ is CH₄,but requires a higher overpotential?>2V?.The main catalytic products of V-TCNQ,Cr-TCNQ,Mn-TCNQ,Ni-TCNQ and Cu-TCNQ are HCOOH.For Fe-TCNQ and Co-TCNQ,HCHO dominates,while CO is mainly the reduction product for Zn-TCNQ.It is worth noting that the overpotentials of these eight materials are sufficiently small?0.12 to 0.45 V?,which is comparable to the overpotential??=0.465 V?of the experimentally prepared molecular catalyst?Fe-TCNQ molecular compound?,even lower.This makes the TM-TCNQ monolayer expected to be a competitive electrocatalyst.?3?The unique porous structure of the porphyrin?PP?momolayer enables it to accept transition metal atoms to prepare TM-PP monolayer catalyst with SAC structure.The catalytic properties of the first transition metal series TM-PP monolayers on electrochemical reduction of CO₂ were studied by density functional theory.The TM-PP monolayer structure has excellent catalytic stability and CRR selectivity.The reduction products for Sc-PP and Ni-PP are CO,the reduction products for Ti-PP and V-PP are CH₄,and the other six monolayer CRR reduction products are HCOOH.Except for the overpotential of Ti-PP greater than 1 V,the overpotential of other single layers is less than0.82 V,especially the overpotential of Co-PP is as low as 0.127 V.Therefore,the research in this paper not only provides theoretical support for the reduction mechanism of TM-PP monolayer catalytic CO₂ reduction,but its excellent catalytic performance also makes the TM-PP monolayers expected to be a promising electrochemical catalysts for CRR.?4?Organometallic nanosheets are widely used in the field of electrocatalysis because of their high surface area and uniform dispersion of metal active sites.Here,the electrocatalytic performance of the first transition metal series TM₃-C₁₂S₁₂2 monolayers on CO₂ was studied by density functional theory.The calculation results show that TM₃-C₁₂S₁₂2 exhibits excellent catalytic activity and CRR selectivity in the catalytic reduction of CO₂.In addition,the reduction products for Sc₃-C₁₂S₁₂,Ti₃-C₁₂S₁₂2 and Cr₃-C₁₂S₁₂2 is CH₄,V₃-C₁₂S₁₂,Mn₃-C₁₂S₁₂,Fe₃-C₁₂S₁₂2 and Zn₃-C₁₂S₁₂2 is HCOOH,and other monolayers is CO.Except for the overpotential of Cr₃-C₁₂S₁₂2 greater than 1 V,the overpotential of other TM₃-C₁₂S₁₂2 monolayers are comparable to Cu?211?,especially the overpotential of V₃-C₁₂S₁₂,Fe₃-C₁₂S₁₂2 and Zn₃-C₁₂S₁₂2 are as low as about 0.27 V.Therefore,our results indicate that the TM₃-C₁₂S₁₂2 monolayer is expected to be a promisingbe and efficient CRR catalyst.?5?Borene monolayer,as a new two-dimensional material,has many properties such as superconductivity,Dirac semi-metal and so on.Borene with hexagonal voids is considered to be stable.Hexagonal voids also provide space for metal atoms,thus providing the possibility preparing single atom catalyst TM-B_?12.The CRR catalytic performance of TM-B_?1212 monolayer was studied by density functional theory.It was found that TM-B_?1212 monolayer had excellent catalytic stability and CRR selectivity.The reductive product of Sc-B_?1212 is CO and its overpotential is 0.446 V.while other metals?Ti-Zn?is CH₄,and Fe-B_?1212 has the lowest overpotential?0.45 V?,which is much lower than that of copper electrode?about 1 V?.Therefore,our predictions offer the possibility of preparing a novel CRR catalyst(TM-B_?12)that makes a significant contribution to the conversion of CO₂.