HYDROGEN CARBON-MONOXIDE INTERACTIONS ON METAL-SURFACES AT HIGH HYDROGEN COVERAGES

Carbon monoxide displacement has been observed for hydrogen pressures above 0.01 Torr on both the Pt(111) and Ni(100) surfaces. Displacement occurs well below the usual thermal desorption temperatures. ''Crowding'' or repulsive interactions between large coverages of coadsorbed atomic hydrogen and carbon monoxide are consistent with the displacement energetics and kinetics observed. Fluorescence yield near edge spectroscopy (FYNES), a new ultrasoft X-ray method, has been used as an in situ probe of CO surface coverages. Both isothermal and temperature programmed FYNES experiments have been used to characterize hydrogen-induced displacement of CO. On the Pt(111) surface in situ isothermal displacement experiments in the temperature range 318-348 K clearly indicate that CO displacement is a first-order process with an activation energy of about 11 kcal/mol in the presence 0.2 Torr of hydrogen. Activation energies for CO desorption decrease with increasing CO coverage from the 30 kcal/mol value measured in the limit of zero coverage; thus isothermal desorption in this name temperature range cannot be described by a simple first-order exponential process. Temperature-programmed FYNES experiments on the Pt-(111) surface indicate that at a temperature of 168 K even for hydrogen pressures below 0.2 Torr where CO displacement by hydrogen is not complete, hydrogen removes the (3(1/2) x 3(1/2)) compression structure observed for the saturated monolayer (0.67 ML) and induces the formation of the well-known (2 x 2) structure (0.5 ML) with reduced CO-CO repulsive interactions. On the Ni(100) surface in situ isothermal displacement experiments in the 278-330 K temperature range indicate that CO is rapidly displaced by hydrogen pressures above 1.0 x 10(-3) Torr. The displacement rate can be fit by two sequential first-order processes with activation energies of 7 kcal/mol for high CO coverages and 10 kcal/mol for lower CO coverages. These in situ soft X-ray displacement experiments have been independently confirmed by a series of mass spectrometric ex situ isothermal displacement experiments for both the Pt(111) and Ni(100) surfaces. Taken together these displacement experiments suggest that hydrogen-induced displacement processes may be general and clearly may play an important role in the transient behavior of catalytic processes run at ''high'' pressure. These displacement experiments also highlight the importance of understanding the chemistry of coadsorbed overlayers at extremely high coverages. In situ characterization of coadsorbed and surface reaction systems clearly offers an important general approach for discovering new surface phenomena which occur at high coverage and characterizing and establishing the primary mechanisms which govern these new phenomena.