In-situ SHINERS Study Of Oxygen Reduction And CO Electrooxidation Reactions At Pt(hkl) Surfaces

As a very important basic cathode reaction of proton-exchange membrane fuel cell(PEMFC),the research of oxygen reduction reaction(ORR)mechanism has been obtained people's attention for a long time.However,the relationship between the ORR mechanisms with the electrocatalytic surface structure remains a daunting challenge,even on primary low-index Pt(hkl)surfaces,which are known as the best ORR catalytic-active among the pure metals,and was yet with many different views.Meanwhile,the mechanism of the oxidation of CO in the process of small molecule electrooxidation in the fuel cell is also an important problem remains to be solved.As with the atomic level reactivity models and well-defined surface structure,light and electric field,the single crystal surface systems play key roles to probe the reaction process and unravel the reaction mechanism at the solid-liquid interfaces in surface science.In this thesis,we employed in-situ electrochemical Shell-isolated nanoparticle-enhanced Raman spectroscopic(SHINERS)method to investigate the reaction mechanism of both ORR and CO electrooxidation processes at Pt(hkl)electrode surfaces by obtaining direct spectral evidences in an atomic level.In-situ SHINERS method to study the oxygen reduction reaction at Pt(hkl)surfaces.1.In acidic condition,during the ORR process,we found the adsorbed HO2*as an intermediate at the Pt(111)surface,but an adsorbed OH*was found on both Pt(110)and Pt(100)surfaces.The steps to form HO2*and OH*species will directly affect the ORR activity of different single crystal surfaces.We therefore conclude that the protonation process of O2*significantly affects the ORR activity and mechanism.Combining the theoretical calculation results with the available literature,we further explained the ORR mechanism in a greater detail:After absorbing the O2-species it converts into HO2*via a proton coupled an electron transfer,which then quickly dissociated to form a pair of OH*and O*on to the neighboring Pt atoms.The OH*species further combined with "H" to generate H2O.The observed differences between the Pt(111)and the other two facets(Pt(110)and Pt(100))arises because of the different Gibbs free energy and dissociation barrier of the same intermediates on different crystallographic planes.2.In the alkaline condition,O2-species was only found on all the three single crystal surfaces,which was also confirmed by D2O and 18-O2 experiments.We then employed DFT calculation techniques to get better insights of the detailed adsorbed structure of O2-at single crystal Pt{hkl)electrode surfaces.Based on our experiments and DFT results,we propose the pathway of ORR at Pt(hko surface in alkaline condition occurs mainly through the dissociation of O2-species.3.We then employed electrochemical SHINERS method to study the effect of crystal structure,pH,anion and solvents for ORR process.Significantly,our results demonstrate that in-situ electrochemical SHINERS technique offers an effective and reliable way for real-time investigation of ORR processes at a new level.In-situ SHINERS method to study CO electrooxidation reaction at Pt(hkl)surfaces.1.We employed in-situ SHINERS method to study CO electrooxidation at three low-index Pt(hkl)surfaces under acidic conditions.We obtained direct spectral evidence of some important species which should relate with "O" and "H" during the CO electrooxidation process,and mostly related with OH and COOH species.And through a series of controlled experiments and deuterium isotope experiments,we further confirmed that these species indeed appear from the CO electrooxidation.We also studied the corresponding crystal structure and anion effect for the CO oxidation.2.We employed in-situ SHINERS method to study CO electrooxidation at three low-index Pt {hkl)surfaces in alkaline conditions.We obtained direct spectral evidence of some important species which should relate with COOH and OH during the CO electrooxidation process.And through a series of controlled experiments and deuterium experiments,we further confirmed that these species indeed came from the process of CO electrooxidation.We also studied the corresponding crystal structure and anion effect for the CO oxidation.Our results of in-situ SHINERS provides a comprehensive understanding of the CO electrooxidation reaction on the Pt(hkl)electrode surface.