Physisorbed, chemisorbed and dissociated O2 on Pt(111) studied by
different core level spectroscopy methods
Surface Science, 342 (1-3):
119 - 133 (1995)DOI: DOI: 10.1016/0039-6028(95)00798-9
Abstract
For O2/Pt(111) we have found four different adsorption phases which
are formed at different substrate temperatures. At about 25 K the
oxygen molecules physisorb on the surface. Two chemisorbed phases
are observed at 90 and 135 K, respectively. An atomic phase, characterized
by a sharp (2 � 2) LEED pattern, exists at a temperature above 150
K. Different spectroscopic techniques have been used to characterize
the four different adsorption states: XPS studies of adsorbate and
surface core level shifts, UPS, NEXAFS, autoionization and Auger
spectroscopy. We conclude that oxygen adsorbs in two different molecular
chemisorbed states which can be considered to be precursors for the
thermally activated atomization process. The first of these molecular
states is weakly chemisorbed at 90 K. It is adsorbed in a hollow
site with a saturation coverage of 0.23 (molecules per Pt surface
atom). We have identified this phase as a superoxo-like configuration.
The second phase is more strongly bonded to the Pt substrate. It
is characterized by a longer and weaker molecular sigma bonding
due to more charge transfer from the metallic substrate to the antibonding
molecular 1pig orbitals than for the first chemisorbed phase. With
a coverage of 0.15 the oxygen molecules seem to be adsorbed in hollow
or hollow-bridge sites. We have characterized this phase as a peroxo-like
configuration of the oxygen molecule. For atomic oxygen on platinum
we have found a coverage of 0.25 (oxygen atoms per Pt surface atom)
and a threefold adsorption site, in agreement with previous studies.
We discuss the XAS results according to a model for the density of
states induced by the hybridization of the 2p atomic orbitals with
the 6sp states and 5d band of the metal.