| The kinetics and mechanism of the chemical reactions on the catalyst surface play a crucial role in the study of materials science,photoelectrochemistry,surface science,etc.In the research of electrochemical catalysis,the structure and function of adsorbed species in the electric double layer(EDL)region are the key to the efficient and stable operation of electrocatalytic reactions and the electrochemical energy storage and conversion devices.In this thesis,cyclic voltammetry(CV),linear sweep voltammetry(LSV),and chronoamperometry(CA)were combined with online high-performance liquid chromatography(HPLC)and in situ surface-enhanced Raman spectroscopy(SERS)to study the mediation of oxygen species(OHads,Oads,etc.)produced from the dissociation of adsorbed water molecules on the platinum electrode during the electrooxidation of ethanol,and the effect of adsorbed SO42anions on the ethanol electrooxidation.Combing with the adsorption energy and dynamical pathways calculated by the density functional theory(DFT),the mechanism of surface-adsorbed active species acting on the reaction pathway and product selection in the electrooxidation of ethanol were revealed.Firstly,the role of oxygen species from the dissociation of adsorbed water molecules at the platinum surface on the ethanol electrooxidation was studied in the perchloric acid solution at the potential range of 0.05-1.50 V(RHE).Voltammetry and CA-SERS were used to distinguish the platinum oxides(Pt-OHads,Pt-Oads,and PtO2)formed in different potential regions.The voltammetry and CA-HPLC were used to study the correlation between electrooxidation activity of ethanol and the platinum oxides in different potential regions,which illustrated that Pt-OHads andα-PtO2 act as the catalytic active centers of the ethanol electrooxidation,but Pt-Oads does not show the catalytic role on the ethanol electrooxidation.The universality of Pt oxides-mediated processes was further demonstrated on Pt(111),Pt(110),and Pt(100)single crystal electrodes.A Pt oxides-mediated reaction mechanism for ethanol oxidation on Pt catalyst was developed with the help of DFT computation,which explained the dependence of reactivity and product on the pH of solution and applied potential.The influence of specifically adsorbed sulfate at the surface of Pt electrode(SO4 ads 2-)on ethanol electrooxidation has been investigated using voltammetry,CA-SERS and CA-HPLC methods.The peak around 0.55 V(RHE)in CV curve of poly crystalline platinum electrode in sulfuric acid solution was ascribed to the adsorption of SO42-ion.In addition to the well-known inhibition effect,low concentration of SO42-ion can improve the electrooxidation activity of ethanol under certain conditions.In the presence of SO42-,the initial oxidation potential of ethanol shifts negatively and a new current peak appears within the potential range of SO42adsorption(～0.55V)and the change of product concentration can be detected,indicating that electron transfer occurs at this current peak.Combined with the adsorption energy and structural parameters(bond length and bond angle)calculated by DFT,the mechanism of enhancing effect of SO42-ion on ethanol electrooxidaiton in its adsorption region was explained,that is,SO4ads 2-promotes the reaction by inhibiting the formation of CO poisoning species and promoting the adsorption of ethanol on the electrode surface.The potential-dependent distribution of the oxidation product of ethanol in ClO4and SO42-solutions at different pH values was studied by voltammetry and CA-HPLC,which elucidate that the interaction of SO4 ads 2-with active oxides and reaction intermediate on Pt surface significantly affected the selectivity of the ethanol electrooxidation.Specifically adsorbed sulfate inhibits ethanol electrooxidation due to its competitive adsorption with ethanol in 0.5 M sulfate solution,especially strong inhibition on the second anodic peak due to the H-bond interaction in Pt-OHads… SO42-that increase the difficulty of PtOx formation.However,compared with the product concentrations in perchlorate solution,the specifically adsorbed sulfate reverses the product distribution at the first peak catalyzed by Pt-OHads,i.e.,it considerably strengthens the formation of acetic acid.Additionally,the selectivity of acetic acid/acetate increases with the increasing pH of sulfate solutions at upper potential,while the product selectivity is almost insensitive to the pH of the perchlorate solution.Combined with density functional theory calculations,these results reveal that SO4ads 2-strengthens the adsorption of the key intermediate,1,1-ethylene glycol,on the Pt surface and then co-adsorption of SO4 2-and active Pt oxides increase the selectivity of acetic acid/acetate.This thesis elucidated the key role of the water molecules and adsorbed anion at the electrode-electrolyte interface during the electrooxidation of ethanol;indicated the law and theory of adsorbed species in double electric layer act on the electrooxidation reactions is a research field that needs to be broken through;and provided theory and idea for the selection of electrolyte solutions in electrochemical devices such as proton membrane fuel cells(PEMFCs)and direct alcohol fuel cells(DAFCs),also extended to other specifically adsorbed ions or other electrocatalytic reactions.This dissertation contains 100 graphs,11 tables,and 239 references. |
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