Chemistry Reference
In-Depth Information
Fig. 25 TPD spectra of CO exposed to Au/alumina film/NiAl(110) surface at 90 K as a function
of Au coverage. Note that CO does not adsorb on the alumina film above 60 K. Typical
morphology of similarly prepared surfaces is shown in the respected STM images taken in another
experimental setup (see text)
Au coverage-dependent IRA- spectra taken after CO saturation (adsorption and
deposition at 90 K) are shown in Fig.
26
(left panel). At the lowest coverage studied
here, a feature at 2165 cm
1
and a second at 2131 cm
1
are observed with
approximately equal intensity. With increasing coverage, the low-frequency peak
gains intensity and shifts from 2131 to 2108 cm
1
, while the high-frequency peak
gradually attenuates. As shown in the right panel of Fig.
25
, annealing to 500 K in
UHV results in a single state at 2108 cm
1
independent of the Au coverage. Bearing
in mind the sintering effects of annealing, the peak at 2108 cm
1
can be straight-
forwardly assigned to CO adsorbing on metallic gold. The species giving rise to the
peak at ~2165 cm
1
is CO adsorbed on very small clusters, which - as shown above
- undergo charge transfer. In this case a specific interaction with the support leads
to positively charged Au species [
46
,
109
].
Interestingly, even monolayer islands formed on a FeO(111) film surface upon
annealing display the same CO adsorption behavior as large, three-dimensional
particles (see the STM images on the right side of Fig.
26
) or gold single crystals
[
108
]. We thus concluded that the observed dependences are due to a higher
percentage of highly uncoordinated gold atoms found for smaller particles, formed
at low temperatures, which favor CO adsorption. Indeed, the single IRAS peak
observed for the annealed samples did not shift and simply grew monotonically at
increasing Au coverage. This supports the TPD results indicating no apparent
change in the possible adsorption states for CO on monolayer islands as compared
