High Pressure Adsorbate Structures Studied by Scanning Tunneling Microscopy: CO on Pt(111) in Equilibrium with the Gas Phase

Scanning tunneling microscopy was applied to studies of adsorbates on model catalysts at elevated pressures where dense overlayers in equilibrium with the gas phase can be formed. A structure due an incommensurate hexagonal overlayer is observed for CO on the Pt(111) surface in the 200--750 Torr pressure range. This structure has not been observed before at similar coverage under low pressure and temperature conditions. The results illustrate the inequivalence in this case between the study of surfaces in catalytic conditions of high pressure and the surface science studies carried out in high vacuum.