Catalysis : an Integrated Approach to Homogeneous ...

380 10 - CATALYST CHARACTERIZATION WITH SPECTROSCOPIC TECHNIQUES
region of the sample by a collisional cascade. Some of the energy will return to
the surface and stimulate the ejection (desorption) of atoms, ions, and multi
atomic clusters. In SIMS, positive or negative secondary ions are detected directly
with a quadrupole mass spectrometer. In the more recently developed SNMS, the
secondary neutrals are ionized, for example by electron impact ionisation, before
they enter the mass spectrometer.
Figure 10.11 shows the positive SIMS spectrum of a silica-supported zirco-
nium oxide catalyst precursor, freshly prepared by a condensation reaction
between zirconium ethoxide and the hydroxyl groups of the support [21]. Note
the simultaneous occurrence of single ions (H', Sit, Zr') and molecular ions (SiO',
SiOH', ZrO', ZrOH', Zr02+). Also the isotope pattern of zirconium is clearly
visible. Isotopes are important in the identification of peaks, because all peak
intensity ratios must agree with the natural abundances. In addition to the peaks
expected from zirconia on silica mounted on an indium foil, the spectrum of
Figure 10.11a also contains peaks from Na', K', and Ca'. This is typical for SIMS:
sensitivities vary over several orders of magnitude and elements such as the
alkalis are already detected when present in trace amounts.
SIMS is strictly speaking a destructive technique. In the dynamic mode, used
for making concentration depth profiles, several tens of monolayers are removed
per minute. In static SIMS, however, the rate of removal corresponds to one
? -
m -
\
n
I
Si
t SIHS of zirconium ethoxide on Si02
I
\
J - . --.
a.m.u. +
SiO SiOH Zr ZrO
driad
at 40 C
c) -
? -
m . -.
ZrO
15.0 45.0 75.0 105.
i35.
Zr ZrO
1
I In
calcined
at 400 C
. . . . --
0 100. i10. 120. 130. 90.0 . 100. 110. 120. 130.
0.m.u. + 8.m.u. +
Fig. 10.11. Positive SIMS spectra of a ZrO2/SiOz catalyst, (A,B) after preparation from Zr(OCzH5)
and (C) after calcination in air at 775 K (from Meijers et al. [21]).