The Adsorption And Catalytic Properties Of Copper Clusters By First Principles Calculation

Due to the distinctive properties,the nano-cluster materials have attracted great interesting in the science field.Most of the atoms in cluster standing on surfaces and possessing lower coordination numbers,result in a large number of dangling bonds existing.So,if compared with corresponding bulk materials,they have large surface/volume ratio,higher surface energies,special chemical activity,superior catalytic properties and reaction selectivities.All these properties have important applications in the related physical or chemical processes,such as phase transition,adsorption,catalysis and diffusion,etc.The interaction between cluster and reactants or intermediates is an important parameter to describe catalytic activity.It is well known,this interaction is directly related to the adsorbed sites,since their coordination numbers are different,which means the adsorption or catalytic behaviors at different sites are also influenced by site.In addition,the effects of both size and shape are also very inevitable.Therefore,site,size and even shape of clusters are required clarifying,especially in identifying the optimum active sites of clusters in the catalytic process.In this thesis,based on density function theory?DFT?,we investigated the adsorption behavior of some molecules on Cu clusters and also induce these clusters'catalytic activity in some important reactions.The main contents are summarized as following:1.The adsorption behavior of two-?2D?and three-dimension?3D?Cu clusters was studied.The stable structures of 2D and 3D Cu clusters and the adsorption of CH₄ and CH₃ on these clusters with atom number ranging from 6 to 20,are calculated by DFT method.The calculation results show that the binding energies of these clusters decrease with size increasing,while their second-order difference in energy??₂E?possess an even-odd oscillation with size.?₂E indicates the difficulty of adding or removing a Cu atom on Cu_n cluster.However,both 2D and 3D Cu clusters have a weak adsorption for CH₄,and the even-odd oscillation is not evident with size.Moreover,the similar even-odd oscillation is yet not found in 2D Cu clusters when they adsorb CH₃,while it is existing in3D clusters.Considering methane dehydrogenation reaction,the stronger adsorption for CH₄ and the weaker adsorption for CH₃ are required,so the clusters which will possess potential catalytic ability for methane dehydrogenation are presented,that is,3D Cu_n clusters with n=10 or 12,and the 2D cluster with n=7,respectively.2.A systematic study of the binding and adsorption energies of CO on different surface sites?vertex,edge and?111?facet?of Cu_n?n=10,20,35,56,84 and 120?clusters with tetrahedral shape is performed using DFT.It is found that binding energy of Cu atoms always follows the sequence:E_c-v>E_c-e>E_c-f respectively at vertex,edge and?111?facet sites,indicating the binding energy at vertex is highest and binding energy at?111?facet is most stable.Correspondingly,the adsorption energy of these Cu atoms for CO has similar trend,and so vertex atom has the strongest adsorption and the weakest adsorption occurs on?111?facet atom.However,not all clusters have such a trend,for example,Cu₃₅and Cu₅₆ at edge have strongest adsorption of CO.On other hand,even though there is no unified change trend for the size dependence of binding energy or adsorption energy,Cu₂₀ has the lowest binding energy values at three sites,and so the relatively weaker adsorption for CO.3.Due to tetrahedral Cu₂₀ surface atoms possess strong stability and weak adsorption behavior,Cu₂₀ as catalyst for CO₂ reduction?CO₂RR?is studied in this section.To some extent,the CO₂RR efficiency is limited due to the competed hydrogen evolution reactions?HER?.To address this issue,the interaction between Cu₂₀ and some intermediates for HER and CO₂RR is studied by DFT calculations.H,COOH and HCOO are three key intermediates respectively for HER and CO₂RR.The calculation results show Cu₂₀ have weaker adsorption for H,with adsorption energies being 0.26?-0.02 and 0.02 eV at vertex edge and?111?facet site,while it will have stronger adsorption energy for HCOO.One can find,the adsorption energies are-0.85,-1.10 and-0.79 eV for HCOO.In addition,according to the Gibbs free energy calculation,HER has higher Gibbs free energy change??G?than that of CO₂RR.Furthermore,the lower?G is found for CO₂ reduction to HCOOH than to CO.So,Cu₂₀ shows its potential catalytic properties in CO₂RR.