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Surface vibrational sum frequency generation technique has been employed to investigate CO and ethylene coadsorption on the Pt(1 1 1) surfaces at CO or ethylene pressures in the 10 −7 to 400 Torr range. Ethylidyne, which was formed when ethylene adsorbed on the Pt(1 1 1) surfaces at 300 K, was partially displaced by CO and underwent orientational change losing its vibrational resonance features in the presence of high-pressure (>1 Torr) CO at 300 K. In contrast, a monolayer of CO was found to be stable on the Pt(1 1 1) surface in the presence of high-pressure ethylene (up to 35 Torr). In the presence of 3 Torr of CO, catalytic ethylene hydrogenation was poisoned due to the site blocking effect of adsorbed CO. No catalytic reactivity was observed at temperatures lower than the CO desorption temperature (400 K). Above the CO desorption temperature, the reaction turnover rate was observed to be three orders of magnitude less than the same reaction without the presence of high-pressure CO. The activation energy of the CO poisoned ethylene hydrogenation reaction is about 20.5 kcal/mol while the activation energy is 10 kcal/mol in the absence of CO. |
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