| Abstract
| - The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequencygeneration (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects,surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generationvibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexenehydrogenation/dehydrogenation in the presence and absence of H2 and over the temperature range 300−500K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibitedby the surface species present during reaction. On Pt(100), π-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, π-allyl c-C6H9, 1,4-cyclohexadiene,and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressuresand temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenationof cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered,while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 Kwas a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominatedmobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detectedthe continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During theplatinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hoursusing a metal−semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used asthe catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electronsthrough the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energygenerated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal−semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy ofexothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. Wefound the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured bygas chromatography.
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