Molecular beam and infrared spectroscopic studies of the thermodynamics of CO on clean and vinylidene-covered Pd„111..
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چکیده
A differentially pumped, capillary array molecular beam source is used to study the reversible adsorption of CO on CO/Pd~111! @Q(CO)50.55# and vinylidene/Pd~111! @Q(vinylidene)51.0# at 300 K. Differentially pumping allows the beam to equilibrate rapidly ~in ;2 s! while maintaining good beam uniformity. The isosteric heat of adsorption of reversibly held CO on a surface precovered with 0.55 monolayers of chemisorbed CO is 5.660.2 kcal/mol at low excess coverages but decreases linearly with coverage so that an additional 2% of a monolayer of CO reduces the isosteric heat of adsorption to 3.060.2 kcal/mol. CO adsorbs reversibly on vinylidene-saturated Pd~111! with an isosteric heat of adsorption of 1.060.5 kcal/mol for coverages up to ;1% of a monolayer of CO. Infrared spectra of CO on vinylidene-covered Pd~111! at higher pressures ~several Torrs! reveal that CO adsorbs on the metal surface. Assuming that the heat of adsorption of CO on vinylidene-covered Pd~111! decreases with CO coverage at higher coverages shows that the low-coverage data measured using molecular beam methods are consistent with the isotherm measured using infrared spectroscopy at higher pressures. © 2001 American Institute of Physics. @DOI: 10.1063/1.1386808#
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