Artikel,
Adsorption of CO on Pt(111) studied by photoemission, thermal desorption spectroscopy and high resolution dynamic measurements of work function
Surf. Sci., 83 (1): 189 - 227 (1979)
DOI: DOI: 10.1016/0039-6028(79)90488-6
Zusammenfassung
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 greater, approximate0.1 monolayers two states are populated,
the ratio of which is controlled by kinetic considerations at T less,
approximate 320 K and thermodynamics at T greater, approximate
400 K. The state with the lowest heat of adsorption is bridge bonded
CO and has a dipole moment less, approximate 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 theta = 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 Tgreater, approximate
400 K. XPS and UPS measurements indicate that the sequence of CO
levels in the adsorbed state is 1pi, 5sigma, 4sigma in order
of increasing binding energy and that the 1sigma-4sigma 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.
