RESUMO
The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (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 generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of 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 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated 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 of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.
RESUMO
Sum frequency generation (SFG) vibrational spectroscopy and high-pressure scanning tunneling microscopy (HP-STM) have been used in combination for the first time to study a catalytic reaction. These techniques have been able to identify surface intermediates in situ during benzene hydrogenation on a Pt(111) single-crystal surface at Torr pressures. In a background of 10 Torr of benzene, STM is able to image small ordered regions corresponding to the c(2 radical3 x 3)rect structure in which each molecule is chemisorbed at a bridge site. In addition, individual benzene molecules are also observed between the ordered regions. These individual molecules are assumed to be physisorbed benzene on the basis of the SFG results showing both chemisorbed and physisorbed molecules. The surface becomes too mobile to image upon addition of hydrogen but is determined to have physisorbed and chemisorbed benzene present by SFG. It was spectroscopically determined that heating the platinum surface after poisoning with CO displaces benzene molecules. The high-coverage pure CO structure of (radical19 x radical19)R23.4 degrees imaged with STM is a verification of spectroscopic measurements.