Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

increasing propanol coverage, the TiO2-related TPD peaks were occupied before the appearance
of any Au(111)-related peaks, which indicate sufficient mobility of 2-propanol
molecules on both Au(111) and nano-TiO2 surfaces below 270 K. Our calculations and
experimental measurements showed that in comparison with rutile(110), the nano-
TiO2/Au(111) surface shows higher reactivity both in terms of the fraction of reactive
sites and in terms of lower reaction barrier. Also the desorption temperatures of products
from supported TiO2 nanocrystals were shown to depend strongly on the morphology of
the nanocrystals.
The experimental observations made as a result of our work are as follows: (1)
propanol completely fills adsorption sites on TiO2 nanocrystals before filling Au(111)
sites; the local adsorption site density of the nanocrystals is 0.4 ML; (2) almost ½ of propanol
molecules adsorbed on TiO2 nanocrystals desorb at ~310 K, exhibiting behavior,
which is identical to propanol adsorbed on the Ti (5) atoms of rutile(110) surface; (3) more
than ¼ of propanol molecules undergo chemical reactions either of dehydration to form
propene or, in smaller part, dehydrogenation to form acetone. Some sites on TiO2
nanocrystals show almost 40 kJ/mol reduction in the reaction barrier for propene formation
compared to BBOv on rutile(110), while acetone is not produced on rutile(110).
Thus, in comparison with rutile(110), the nano-TiO2/Au(111) surface shows higher reactivity
both in terms of the fraction of reactive sites and of reaction barrier.
STM Tip-Induced Dissociation Dynamics on Surfaces.
Scanning tunneling microscopy offers a unique ap-
proach to exploration of molecular dissociation dynamics
at an atomic scale. Namely, as shown by the
W. Ho Group, an STM allows injection of an electron
or electrons with a specified energy into a selected
molecule in a specific adsorption configuration. Initial
studies of this approach have been carried out in
the Palmer and Polanyi Groups for reactions on semiconductor
surfaces and shown to yield dynamical information
on the chargetransfer
threshold electron
energy for a specific reaction,
the identity of
adsorbate states suitable for
dissociation, fragment trajectories
across the surface
following bond scission,
Fig. 4: Cartoon showing
the change in 2chloroanthracenefollowing
tip-induced e reaction.
The green atom is
the remaining Cl.
nano-TiO 2 /Au(111)
propanol
propene
acetone
100 200 300 400 500 600 700 800
Temperature, K
and the nature and sensitivity to the adsorption geometry of the
products. Note that for photocatalysis, the electron-induced reaction
similarly involves charge transfer from the bulk of a
substrate to an adsorbed molecule followed by bond cleavage.
Such a study thus tries to answer the same fundamental chemical
physics questions as in gas phase reaction dynamics except
that on a surface determine the role of the surface perturbation
on the half-collision event and fragments.
141
M .S. s ignal, a.u.
ML
0.3
0.2
0.1
0.0
2-propanol
(tail)
Fig. 3. TPD spectrum from a nano-
TiO 2/Au(111) surface having 0.1
ML of 2-propanol coverage. The
inserts show the calculated distribution
of the desorption products.
p r op e ne
a c et on e