Chemistry Reference
In-Depth Information
whereas ring vibrations produce bands in the 1650-1400 cm
1
range. For the sake of
clarity, the difference between spectra recorded after pyridine adsorption and after activa-
tion is given in spectrum d.
The bottom part of Figure 4.3 represents a zoom of the image presented above, in the
most interesting spectral region, i.e. in the range 1700-1400 cm
1
. The main vibrational
modes of the pyridine ring are apparent. From bands
8a
it is possible to get information on
the strength of the acid sites (the higher the wavenumber, the stronger the acidity), whereas
the integrated intensity of the
19b
furnishes a quantitative estimation of the acid sites
concentration. Further details are reported in references 10 and 11.
When H-bonded species are concerned, the corresponding bands for pyridine are less
sensitive and closer to the frequencies observed in the liquid phase. Such a small frequency
shift can make assignments ambiguous when both weak Lewis and Brønsted acid sites are
expected on the surface. In such a case, the only way to prove the formation of H-bonded
species is the concomitant observation of a broad
(OH) band, downward shifted by
several hundreds of wavenumbers from the frequency of the free OH groups. This
observation is not always straightforward because this band can become very broad and
overlap with
(CH) bands. Moreover, for weak interactions, the
8a
band is in a Fermi
resonance with the
1
þ
6a
combination mode, so introducing an additional difficulty to
assess the acid strength of the acid centres. To circumvent these problems it was found that
an accurate analysis of the whole spectrum of pyridine, including the
(CH) range, may
bring further insights to the acidic properties of metal oxides. Using d
5
-pyridine, in
particular, allowed simpler
(CD) spectra to be obtained, showing that both frequencies
and intensities are sensitive to the adsorption mode and making it possible to distinguish
among H-bonded, protonated and coordinated species.
10
Particular attention was paid to
the case of coordinated species, since the strength of coordination increases
(CH/D)
frequencies and decreases their intensities. Such variations were investigated by DFT
calculations and explained by the polarization of electron density towards nitrogen to the
detriment of hydrogen atoms, leading to (i) an increase of C-H bond strength and (ii), a
decrease of the dipole moment derivatives for C-H stretches. These trends were correlated
with the
8a
and
19b
frequency shifts (Figure 4.4). It appeared that the most intense
(CH)
band is more sensitive to small acidity differences than those in the
(C
¼
C) range,
allowing Lewis acid strength to be discriminated even for solids having a similar acidic
behaviour, such as CaO, MgO and CeO
2
.
In many circumstances, the use of a single probe molecule is not sufficient to obtain a
satisfactory view of the solid surface. In such a case the combination of different probe
molecules is recommended. For example, the study of surface properties of
-Ga
2
O
3
,an
oxide structurally related to
-Al
2
O
3
, was undertaken using pyridine, dimethyl pyridine
(lutidine, DMP) (for acidity), CD
3
CN and carbon dioxide (for basicity), and compared
with the vibrations of OH groups, which can be considered as a probe molecule intrinsic
to the solid itself.
11
Among the different properties observed it is noteworthy that,
similar to what is known for
-Al
2
O
3
, the Brønsted acidity of (partially hydroxylated)
-Ga
2
O
3
was found to be very low, albeit not negligible. DMP adsorbed on
-Ga
2
O
3
and
-Al
2
O
3
showed, in both cases, a weak IR absorption band at 1649-1652 cm
1
that
identified the protonated DMPH
þ
species. However, no traces of the pyridinium ion
were found when pyridine was adsorbed on either
-Ga
2
O
3
or
-Al
2
O
3
.Moreover,the
surface Lewis acid strength was found to be slightly smaller in
-Ga
2
O
3
than in
