In-situ SHINERS Study Of Electronic Structure And Electrocatalytic Reaction Mechanism At Bimetallic Surfaces

The surface electronic structure of metal catalysts plays a key role during electrocatalytic reactions,which directly affects the interaction between reactants,intermediates species and products and catalysts,determining the activity,stability and selectivity of catalysts.Therefore,analysis of the electronic structure of interface and in situ monitoring its influence on electrocatalytic process by advanced spectroscopic techniques is of great significance for understanding the structure-activity relationship and reaction mechanism of catalytic reaction,however,remains a big challenge.In this thesis,we first precisely controlled construct heterojunction interfaces on single crystals,which has well-defined structure and surface states.Than we use Shell-isolated nanoparticles enhanced Raman spectroscopy(SHINERS)that has various advantages such as low-wave-number detection,high sensitivity and wide applicability,to detect their interface electronic structures and monitor the process of electrocatalytic reaction at these interfaces.In this way,the adsorbed species,their configurations and metal-molecular interaction,reaction mechanism and structure-activity relationship related to the electronic structure of interfaces are revealed at molecular level.Furthermore,we develop a satellite structure that nanocatlysts assemble on SHINs to extend SHINERS from model single crystal systems to practical Pt and Pd-based nanocatlysts.Thus,the process of electrocatalytic reaction,intermediates and products on the surface of nanocatalysts are in-situ tracked by SHINERS.Combined with theoretical calculation,the reaction pathway and mechanism were studied,which is useful for the design of catalysts with high performance and stability.The main results of this thesis are outlined as follows:1.We precisely constructed two types of bimetallic model surfaces namely Pd/Au(hkl)and Pt/Au(hkl)though a novel method of Cu UPD adlayer-controlled growth.Then SHINERS is applied to investigate the adsorbed PIC molecules on these model surfaces.Correlated with XPS results,the electronic structures of these heterojunctions on Au single crystals were analyzed based on the frequency of the C?N stretching.In addition,the effects of crystallographic orientation are also systematically investigated.We proved that SHINERS could become a novel approach to probe electronic structures of heterogeneous metal interfaces under atmosphere environment.Based on the analysis of surface electronic structure o by SHINERS,we found that the bimetallic 1ML Pt/Au(111)and trimetallic 1ML Pt/1ML Pd/Au(111)model interfaces show excellent catalytic activity toward the electrooxidation of small organic molecules.Furthermore,in situ electrochemical SHINERS studies showed that the electronic effects weaken Pt-C/Pd-C bonds,leading to improved activity towards CO electrooxidation2.We successfully developed a satellite structure that the nanocatlysts assembled on SHINs,which made SHINERS could effectively investigate the catalytic process on the surface of practice nanoctalysts.Thue CO electrooxidation was studied on these nanostructures by in-situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy(EC-SHINERS).The in-situ spectroscopic results correlate well with the results of cyclic voltammetries,and show that PtFe bimetallic nanocatalysts have improved CO electrooxidation activity.And such improvement is the results of weaker CO adsorption on PtFe compared to Pt,as revealed by the redshift of the Raman band of Pt-C stretching vibration on PtFe.We used shell-isolated nanoparticle-enhanced Raman spectroscopy(SHINERS)to investigate the ORR process on PtCo nanocatlysts surfaces with the novel satellite structure.The direct spectral proofs of*OOH suggests that ORR undergoes an associative mechanism on PtCo in both of acid and base environments.Combined with density functional theory(DFT)calculation,the weaker*O adsorption arisen from electronic effect on PtCo surface accounts for the about five-fold enhanced activity of ORR3.With the help of SHINERS technique based on satellite structure,the reduction process of benzyl chloride and CO2 on Pd nanocatalyst was explored.We found the spectra evidence of Benzyl radical and benzyl radical anions during benzyl chloride reduction.Besides,The Raman signals of CO and Pd-C were observed during CO2 reduction,and the spectroscopic evidence of OCCOH indicated that CO could be further reduced at the very negative potential.