Synergetic Effects Of Strain Engineering And Substrate Defecton Highly Efficient Single-Atom Catalysts For CO Oxidation: First-principles Calculations

Developing highly efficient single-atom catalysts?SACs?containing isolated metal atom monomers dispersed on appropriate substrates has surged to the forefront of heterogeneous catalysis in recent years,driven by both specificity of the unique active site and cost-effectiveness of the approach.Nevertheless,the instability of the SACs,i.e.,preferential sintering during the chemical reactions,dramatically hinders its development and applications.In this dissertation,by means of first-principles calculations,taking electronically close-shelled Au₂ and open-shelled Pd₂?PdAu?on single-layer WTe₂ as prototypical examples,we investigate the strengthening effect of the electronic metal-substrate interactions?EMSI?via a synergetic effect of strain engineering and substrate defect to prevent clustering at the initial stage of the SACs.It is noted that on the perfect WTe₂?P-WTe₂?,both Au and Pd adatoms prefer dimerization to separation.However,when a defect exists on the same WTe₂ substrate?D-WTe₂?,the situation changes considerably,and substrate defect have a tendency to stabilize the monoatomic phase.Under tension,relative to the electronically close-shelled Au₂ dimer,an electronically open-shelled Au monomer at the Te vacancy site(V_Te)obtains more charge from the WTe₂ substrate,leading to stronger EMSI.However,when an electronically open-shelled PdAu?Pd₂?dimer is located on the compressive strained D-WTe₂,more charge can be transferred to both of the atoms with decreased distances,therefore the increased Coulomb repulsive interactions?CRI?separates them to be stable SACs.Importantly,we find that strain can effectively strengthen the pinning effect of single atom on defect site.So the stabilities of these noble metal single atoms on the defect substrate are significantly enhanced which make them difficult diffusion and reduce the possibility of nucleation.Moreover,strain can further regulate activation of energy barriers?E_a?of SACs for CO oxidation.Our results demonstrate the importance of substrate engineering in stabilizing the SACs and offer a valid approach in fabricating SACs systems.Chapter 1,introduction.We briefly describe the background knowledge of catalysts.The differences in catalytic performance between nano/sub-nano cluster catalysts and SACs,the unique features of SACs and how to characterize and prepare SACs are discussed.At the same time,to solve the bottleneck problem of SACs,we offer a valid approach to modulate of the efficiency in fabricating SACs systems.Chapter 2,theory and methods.The density functional theory in computational physics,first-principles calculation method used in computational simulation,reasonable construction model and the calculation of the binding energy?formation energy?are presented.Chapter 3,we systematically study the adsorption and diffusion behaviors of monoatomic and diatomic systems on perfect and defect substrates,respectively.For the diatomic system,taking electronically close-shelled Au₂ and open-shelled Pd₂?PdAu?on single-layer WTe₂ as prototypes,we try to enhance the EMSI by strain engineering and substrate defect to modify the properties of substrate,which regulates the isolate active sites of catalysts.In our studies,we find that the defect significantly enhanced the binding between the Pd?Au?atom and the substrate.We find that with the external strain changes from compressive strain such as-5%and-2%,through 0%,to tensile strain of+2%,the local electronic density of states?DOS?by the Fermi level significantly increases.These engineering strategies promote the production of isolated active sites,which will help us to synthesize the SACs.Chapter 4,combining with the SACs regulated in Chapter 3,considering the key step of O₂ activation for CO oxidation,we inspect the adsorption configurations of a ground state?spin-triplet?O₂ molecule on Pd--Au@D-WTe₂ and Au--Au@D-WTe₂.Therefore,based on the structures of O₂-Pd--Au@D-WTe₂ and O₂-Au--Au@D-WTe₂we further extensively investigate the CO oxidation processes by examining both Langmuir–Hinshelwood?L–H?and Eley–Rideal?E–R?reaction mechanisms.Further,we examine the synergy of the strain and defect effects on the catalysis of the Pd?Au?active sites in SACs.It is shown that the strain effect and zero-point energy?ZPE?correction can further reduce the values of the activation energy barrier in the rate-limiting steps.Chapter 5,conclusions and outlook.