Adsorption and coadsorption of CO and H 2 on Fe(111) probed by TEAS

Abstract

Thermal energy atom scattering (TEAS), as developed by Comsa and coworkers, has been widely used to study a variety of processes on smooth, close packed metal surfaces such as Pt(111). The He beam specular reflectivity of these surfaces is very high, and absorbates or defects usually have a relatively large scattering cross section. For TEAS to have a wider applicability, studies on more corrugated surfaces should be undertaken. We report here studies of CO and H 2 adsorption and coadsorption on the open Fe(111) surface. The He specular intensity drops rapidly and smoothly with CO exposure on the Fe(111) surface. In contrast to CO/Pt(111), the diffuse scattering of He from CO on Fe(111) is not a negligible fraction of the intensity. The surface exhibits a finite reflectivity even at high CO coverages. An effective scattering cross section of 56Å 2 was derived from a fit of the reflectivity data to a model taking this diffuse scattering into account. For H 2 exposure to the Fe(111) surface, the He specular reflectivity decreases less abruptly, with distinct slope changes as coverage increases. Angular and temperature dependent measurements suggest distinct adsorption sites for the dissociatively adsorbed hydrogen. When CO and H 2 are coadsorbed, the order of adsorption affects the final composition and coverage in the overlayer. If the surface is first saturated with CO, H 2 will not adsorb, as evidenced by constant TEAS signal. When H 2 is adsorbed first, even to saturation coverage, CO will adsorb, displacing H 2 from the surface. Evidence is obtained for CO and H 2 segregation on the surface at intermediate coverages.