The effect of surface structure on catalytic reactions: A sum frequency generation surface vibrational spectroscopy study.

摘要

Using Sum Frequency Generation (SFG) and gas chromatography (GC), molecular level investigations of catalytic reactions were performed on platinum single crystal surfaces. The SFG spectra and GC data was correlated to elucidate the nature of active species present on the surface under high-pressure catalytic reactions.; The effect of structure sensitivity and insensitivity of several catalytic reactions were investigated. Ethylene hydrogenation, a structure insensitive reaction, was performed over both Pt(111) and Pt(100). Both ethylidyne and di-σ bonded ethylene, strongly adsorbed species, were present on the surface under reaction conditions. These species were not responsible for catalytic turnover.; Cyclohexene hydrogenation and dehydrogenation were also performed on Pt(100) and Pt(111). From SFG results, it was concluded that dehydrogenation can proceed through both 1,3- and 1,4-cyclohexadiene intermediates, although, the rate proceeds faster through the 1,3-intermediate. The rate dehydrogenation on Pt(100) was higher because there was a higher concentration of 1,3-cyclohexadiene on the Pt(100) surface as compared to the (111) surface.; By exposing Pt(111), Pt(100), and Pt(557) single crystal surfaces to high pressures of CO, it was found that Pt-carbonyls could be produced leading to the dissociation of CO. In addition, it was found that CO dissociation was structure sensitive when the crystals were exposed to 40 Torr of CO. The Pt(100) surface was the most active and showed dissociation at 500 K, while the Pt(111) surface was the least active with a dissociation temperature of 673 K. Pt(557) exhibited a dissociation temperature at 548 K, between the two other surfaces. Surface roughness was found to affect the temperature of dissociation and Pt-carbonyls were responsible for roughening of the surface. Pt(100) was the most active because the surface reconstructs and roughens at a lower temperature than the other two surfaces. In addition, CO oxidation experiments were performed on all three surfaces and the ignition temperature followed the same trend observed for CO dissociation. This indicates that CO dissociation is important for the onset of ignition.; An in depth study of CO oxidation on Pt(557) was performed on both clean and carbon-covered prepared surfaces. (Abstract shortened by UMI.)