Theoretical Study On The Mechanism Of CO₂ Reduction Reaction Catalyzed By UiO-66 Supported Metal Single Atom Catalys

With the rapid development of modern industry,the massive emissions of greenhouse gases such as carbon dioxide have led to global ecological degradation caused by the greenhouse effect.Therefore,effective emission reduction and comprehensive utilization of carbon dioxide have important strategic and practical significance.Converting CO₂into fuel or other clean energy through electrocatalytic reduction can achieve a closed-loop cycle of carbon,which is considered as one of the cleanest and most effective methods.In this paper,based on density functional theory（DFT）and ab initio molecular dynamics（AIMD）simulations,we studied the electrocatalytic mechanism of three-dimensional metal organic framework（MOF）material Ui O-66 supported metal single-atom catalysts（SACs）for CO₂reduction reaction（CO₂RR）,and predicted two kinds of SACs Ni₁/Ui O-66-NH₂and Pt₁/Ui O-66-H with high catalytic activity and/or selectivity for CO₂RR.The research contents are as follows:（1）In this study,based on the most stable structure of the defect Ui O-66-NH₂,we evaluate the stability of a series of metal single atoms（SAs）anchored on defect Ui O-66-NH₂using DFT calculations and ab initio molecular dynamics（AIMD）simulations,and found that Ni₁/Ui O-66-NH₂is the most stable single atom catalyst（SAC）.Furthermore,the calculations indicate that Ni₁/Ui O-66-NH₂has high activity and selectivity for the electrocatalytic reduction of CO₂to CH₄comparing to the other different products such as HCOOH,CO,CH₃OH.In the process of electrocatalytic CO₂RR to CH₄,the potential determining step（PDS）is*HCOO→*HCOOH with the limiting potential of-0.24 V.In addition,further comparative studies were conducted on the activity and selectivity of Ni₁/Ui O-66-NH₂and Pt₁/Ui O-66-NH₂for CO₂RR.It is found that the electrocatalytic activity and selectivity of Ni₁/Ui O-66-NH₂is higher than that of Pt₁/Ui O-66-NH₂SAC for CO₂RR to CH₄,and the changes in the valence state of Ni SA played an important role in the reaction process of CO₂RR to CH₄.This work predicts a novel non-noble metal SAC Ni₁/Ui O-66-NH₂with high activity and selectivity for CO₂RR to CH₄.（2）This work focuses on studying the electrocatalytic performance of Ui O-66without substituents supported metal SACs M₁/Ui O-66-H（M=Ni,Pt）for CO₂RR.DFT calculations show that both M₁/Ui O-66-H（M=Ni,Pt）SACs can generate the intermediates of*HCOO in the optimal reaction pathways.On Ni₁/Ui O-66-H,the limiting potential for CO₂RR to CH₄is 0.27 V,but the selectivity of hydrogen evolution reaction（HER）is better than that of CO₂RR,indicating that Ni₁/Ui O-66-H is more favorable for HER.However,The PDS of CO₂RR to CH₄on Pt₁/Ui O-66-NH₂is*HCOO→*HCOOH,with a U_Lof 0.65 V.Although the limit potential of this reaction is relatively high,the selectivity of Pt₁/Ui O-66-H catalyst for CO₂RR is still better than that for HER.This study predicted a noble metal SAC Pt₁/Ui O-66-H with high selectivity for CO₂RR and a SAC Ni₁/Ui O-66-H with high activity and selectivity for HER.（3）Direct selective oxidation of methane（DSOM）to high value-added oxygenates under mild conditions is regarded as an atom-economic reaction.Our previous studies show that the Zr_oxo-^·OH active center of the three-dimensional porous MOF material Ui O-66-H containing ligand defects exhibited high reaction activity and 100%selectivity for the conversion of CH₄to methanol under the presence of H₂O₂oxidant.However,it is unclear that the structure-activity relationship between the electronic structure evolution of Zr_oxo-^·OH active center induced by H₂O₂and its catalytic performance for CH₄conversion to methanol.By using DFT calculations and AIMD simulations,we preliminarily explore the relationship between the evolution of Zr-oxo node and the catalytic activity of Ui O-66-X（X=H,NH₂）for DSOM reaction by the regulating of the^·OH radicals generated from different H₂O₂concentrations.The results indicate that there is a typical volcanic curve relationship between the electronic structure of Zr-oxo nodes and the catalytic activity of Ui O-66-H.At low concentration of^·OH radical,Zr_oxo-^·OH active site is formed when the defects of Zr-oxo node are completely covered by^·OH radicals.With the increase of^·OH concentration,the self-oxidation reaction of^·OH radicals can occur and the different species of O₂,H₂O and^·OOH also can be generated at Zr-oxo node,which leads to the decline of the catalytic activity of Ui O-66-H.On the contrary,the change of H₂O₂concentration can’t significantly affect the catalytic performance of Ui O-66-NH₂.This work theoretically revealed the structure-activity relationship between the evolution of the electronic structure of the Zr-oxo active center regulated by the^·OH radical concentration and the catalytic performance of Ui O-66-X for CH₄conversion,providing a reliable theoretical reference for the design and research of Ui O-66 based catalysts for CH₄conversion.