as determined by AES peak intensities [15] and by X-ray photoemission (XPS), using the intensity attenuation of the Mo 3d core level with increasing film coverage
[16]. Both films have also been studied by electron energy loss spectroscopy [16].
In contrast to defect-poor films, which are characterized by a loss at about 6 eV
in the EEL spectra (Figure 17.1a, in good agreement with previous studies on
MgO(100) single crystals [17]), the EEL spectra of defect-rich films exhibit characteristic losses between 1 and 4 eV, lying within the MgO band gap (Figure 17.1a).
Similar loss structures have been observed before [18], and according to firstprinciple calculations using large cluster models, they have been attributed to
transitions characteristic for neutral surface F centers in various coordinations on
flat terraces, at steps and at kinks [19]. The density of these oxygen vacancies is
estimated to be larger than 5 Â 10
13 cm
À2 .
The interaction of methanol with the defect-poor and defect-rich films was
studied using thermal desorption spectroscopy (TDS) (Figure 17.1c). For both
films the desorption of physisorbed methanol at around 180 K is most dominant.
On the defect-poor films small amounts of chemisorbed methanol desorb up to
around 350 K. On defect-rich films the desorption of chemisorbed methanol
evolves in three distinct peaks at 200 K, 260 K, and 340 K. A small reproducible
feature is observed at around 500 K. Most importantly, H 2 desorbs at 580 K only on
defect-rich films. The corresponding infrared spectra taken at 90 K (insets of
Figure 17.1c) confirm the presence of mainly physisorbed CH 3 OH with the typical
vibrational band for the OH group at 3285 cm
À1 , bands of the symmetric CaH
stretching (2930 cm
À1 /2828 cm
À1 ) and the CaH bending (1475 cm
À1 ) modes of
the CH 3 group, as well as the CaO vibrational mode at 1080 cm
À1 /1050 cm
À1 . For
defect-poor films most of the intensity of the vibrational bands disappears between
180 K and 200 K, consistent with the desorption observed at 180 K in the TDS. Up
to temperatures of about 400 K small bands are observed for the CH 3 a and CaO
bands. Their intensity decreases with temperature. These bands are attributed to
chemisorbed methanol. As for the defect-rich films, the evolution of the IR spectra
with temperature is consistent with the corresponding TDS. Physisorbed methanol
desorbs before 200 K (disappearance of the OH band), and the more intense vibrational features of chemisorbed methanol are unambiguously detected up to
360 K. At higher temperatures a clear peak is observed at 1070–1085 cm
À1 [13]. It
is interesting to note that the disappearance of this band correlates with the desorption of H 2 from the surface at around 580 K and therefore this peak is attributed, in accordance with the theoretical studies, to a proton trapped in the cavity of
an F center. The calculations show that F centers easily dissociate methanol giving
a CH 3 O group and an H
þ ion adsorbed into the F center. This adsorbed proton is
strongly bound as the dissociation of a neutral H atom, Fs/H
þ ! Fs
þ þ H . , costs
4.1 eV. This represents a crude estimate of the barrier to be overcome in order to
observe H 2 desorption from the surface. Once the H atom is detached from the F
center it will rapidly diffuse on the surface. In fact, the binding of an H atom to an
O 5c ion on a terrace is about 0.5 eV. Diffusion will eventually lead to recombination
with a second H to form H 2 ; H 2 is weakly bound to the MgO terrace sites and
at 580 K will immediately desorb. The adsorbed hydrogen gives rise to vibrations
17.2 Chemical Reactions on Point Defects of Oxide Surfaces 553
[16]. Both films have also been studied by electron energy loss spectroscopy [16].
In contrast to defect-poor films, which are characterized by a loss at about 6 eV
in the EEL spectra (Figure 17.1a, in good agreement with previous studies on
MgO(100) single crystals [17]), the EEL spectra of defect-rich films exhibit characteristic losses between 1 and 4 eV, lying within the MgO band gap (Figure 17.1a).
Similar loss structures have been observed before [18], and according to firstprinciple calculations using large cluster models, they have been attributed to
transitions characteristic for neutral surface F centers in various coordinations on
flat terraces, at steps and at kinks [19]. The density of these oxygen vacancies is
estimated to be larger than 5 Â 10
13 cm
À2 .
The interaction of methanol with the defect-poor and defect-rich films was
studied using thermal desorption spectroscopy (TDS) (Figure 17.1c). For both
films the desorption of physisorbed methanol at around 180 K is most dominant.
On the defect-poor films small amounts of chemisorbed methanol desorb up to
around 350 K. On defect-rich films the desorption of chemisorbed methanol
evolves in three distinct peaks at 200 K, 260 K, and 340 K. A small reproducible
feature is observed at around 500 K. Most importantly, H 2 desorbs at 580 K only on
defect-rich films. The corresponding infrared spectra taken at 90 K (insets of
Figure 17.1c) confirm the presence of mainly physisorbed CH 3 OH with the typical
vibrational band for the OH group at 3285 cm
À1 , bands of the symmetric CaH
stretching (2930 cm
À1 /2828 cm
À1 ) and the CaH bending (1475 cm
À1 ) modes of
the CH 3 group, as well as the CaO vibrational mode at 1080 cm
À1 /1050 cm
À1 . For
defect-poor films most of the intensity of the vibrational bands disappears between
180 K and 200 K, consistent with the desorption observed at 180 K in the TDS. Up
to temperatures of about 400 K small bands are observed for the CH 3 a and CaO
bands. Their intensity decreases with temperature. These bands are attributed to
chemisorbed methanol. As for the defect-rich films, the evolution of the IR spectra
with temperature is consistent with the corresponding TDS. Physisorbed methanol
desorbs before 200 K (disappearance of the OH band), and the more intense vibrational features of chemisorbed methanol are unambiguously detected up to
360 K. At higher temperatures a clear peak is observed at 1070–1085 cm
À1 [13]. It
is interesting to note that the disappearance of this band correlates with the desorption of H 2 from the surface at around 580 K and therefore this peak is attributed, in accordance with the theoretical studies, to a proton trapped in the cavity of
an F center. The calculations show that F centers easily dissociate methanol giving
a CH 3 O group and an H
þ ion adsorbed into the F center. This adsorbed proton is
strongly bound as the dissociation of a neutral H atom, Fs/H
þ ! Fs
þ þ H . , costs
4.1 eV. This represents a crude estimate of the barrier to be overcome in order to
observe H 2 desorption from the surface. Once the H atom is detached from the F
center it will rapidly diffuse on the surface. In fact, the binding of an H atom to an
O 5c ion on a terrace is about 0.5 eV. Diffusion will eventually lead to recombination
with a second H to form H 2 ; H 2 is weakly bound to the MgO terrace sites and
at 580 K will immediately desorb. The adsorbed hydrogen gives rise to vibrations
17.2 Chemical Reactions on Point Defects of Oxide Surfaces 553