Low temperature adsorption of CO on Pt(111): disequilibrium and the occupation of three-fold hollow sites

The adsorption of CO on a Pt(111) surface held at 20 K under ultrahigh vacuum was investigated using vibrational and thermal desorption spectroscopies. In contrast to adsorption at higher temperatures, surface diffusion of chemisorbed CO across the surface was suppressed and vibrational spectroscopy showed non-equilibrium occupation of top, bridge and three-fold hollow sites. The saturation coverage of chemisorbed CO at 20 K was 0.5 ML. The infrared absorption band attributed to three-fold hollow site CO was centered at 1736 cm⁻¹. The integrated absorbance of the 1736 cm⁻¹ band grew in and saturated as the exposure was increased from 0.35 to 1 ML.