Adsorption of co on Pt(111) studied by photoemission, thermal desorption spectroscopy and high resolution dynamic measurements of work function

Abstract

The adsorption of CO on Pt(111) has been studied by XPS, UPS, thermal desorption mass spectroscopy and by dynamic and static work function measurements at 95, 298, 377,403 and 453 K. At all temperatures and coverages ≳0.1 monolayers two states are populated, the ratio of which is controlled by kinetic considerations at T ≲ 320 K and thermodynamics at T ≳ 400 K. The state with the lowest heat of adsorption is bridge bonded CO and has a dipole moment ≲ 0.004 D with the negative end of the dipole pointing outwards. The more strongly bound state is linearly bonded CO which has a dipole moment of ∼0.04 D, positive end outwards. The difference in the heat of adsorption is ∼3800 J mol −1, half the value derived from an analysis of the thermal desorption spectra. It is suggested that the frequency factor for desorption of the bridge species is ∼5 times that of the linearly bonded CO, perhaps because of the greater entropy change in desorption of the former. The change in work function at θ = 0.33 between 80 and 300 K cannot be explained solely by a change in the ratio of bridge to linear bonded CO but must reflect changes in the order of the adsorbed layer which then effects the dipole moment by through-metal interactions. The ratio of bridge to linear states determined from thermal desorption measurements is different from that measured by XPS at 298 or 100 K, because of interconversion at T≳ 400 K. XPS and UPS measurements indicate that the sequence of CO levels in the adsorbed state is 1π, 5σ, 4σ in order of increasing binding energy and that the 1σ-4σ splitting is smaller (by ∼0.5 eV) in bridge bonded than in linearly bonded CO. A very weakly chemisorbed state is detected at 77 K which desorbs below 100 K. The intensity ratio of the observed UPS bands are compared to those strongly in chemisorbed CO, and it is suggested that either (a) the bands result from a different weakly adsorbed state than can tumble on the surface or (b) that the extra peaks are due to shake-up processes which are enhanced because of the weakness of the interaction between CO and Pt(111) at high coverages.