HYDROGEN-INDUCED CO DISPLACEMENT FROM THE PT(111) SURFACE - AN ISOTHERMAL KINETIC-STUDY

Chemisorbed CO can be completely removed from the Pt(111) surface in the temperature range 318 to 348 K for hydrogen pressures above 2 x 10(-2) Torr. Thermal desorption of CO in this temperature range in the absence of hydrogen removes only a fraction of the adsorbed CO. A series of in situ isothermal kinetic experiments are presented in this paper which show that CO displacement in the presence of 0.2 Torr of hydrogen is a first-order process in CO coverage with an activation energy of 10.9 kcal/mol. We propose that the origin of this effect is that repulsive intereactions between coadsorbed atomic hydrogen and carbon monoxide induce high desorption rates of CO characteristic of high CO coverages, presumably due to lower values of the desorption activation energy. The importance of these results is to show that high coverages of coadsorbed hydrogen resulting from substantial overpressures of H-2 may substantially modify desorption activation energies, and thus the coverages and kinetic pathways available, even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.