Adsorption of CO on a Tungsten (100) Surface

Abstract

The room‐temperature adsorption of carbon monoxide on the (100) surface of tungsten has been studied using the techniques of low‐energy electron diffraction, flash desorption, and work‐function measurement. CO is found to exist in two binding states whch have been labeled β_L and β_H and is desorbed from these two states at ∼1150° and ∼1380°K, respectively. At room temperature, CO appears to adsorb into the β_L state, and is associated with a work‐function increase Δφ which is approximately linear with coverage and amounts to +0.43 eV at saturation—a coverage of about 1.0×10¹⁵ molecules/cm². CO in the β_L states can be converted into the β_H state by heating to 900°—1000°K. Associated with the latter is an ordered c(2×2) structure which, at maximum coverage—about 5.0×10¹⁴ molecules/cm²—exhibits a Δφ of −0.16 eV with respect to the clean surface. Adsorption of CO to saturation on the c(2×2) structure appears to return the surface to a condition indistinguishable from that due to adsorption on the clean surface. A possible model for explaining this behavior is proposed.