M. Feist et al. / Thermochimica Acta 524 (2011) 170–178
177
however, correctly reflect the differences between physisorption
and real chemisorption (exothermicity ca. 100 times stronger, cf.
Table 1).
basicity-creating effect of Mg sites can be overcompensated by Cr
doping.
The behaviour of Ba(OH)F-SG was completely different com-
pared with all other phases investigated. 4.3 mol% CO2 uptake
with a qualitatively different curve shape possibly indicate another
3.6. Discussion
chemical situation compared to those of all other MF -SG. The
2
bonding situation of the OH groups must be different from those
induced by the water injection pulses and might be of the mon-
odentate M–OH (type 1) according to the classification proposed in
The compounds investigated were supposed to exhibit rather
similar chemical and adsorption properties as they all represent
sol gel-prepared phases with a more or less large surface. Never-
theless, the experimental findings have to be interpreted in a more
diversified way.
[
26].
Three methodological considerations and a more chemical
aspect can be deduced from a comparison of the experimental
values listed in Table 1:
4. Conclusions
The results presented here demonstrate that sol gel-prepared
alkaline earth fluorides have not only acid, but basic sites as well
which can contribute to the interpretation of the promising cat-
alytic activity of some of them. In particular, they confirm the main
conclusions reported on MgF -SG and its weak basicity simultane-
ous with a strong Lewis acidity [4].
(
i) It was rather surprising to establish to which extent the DTA
traces show fine details of the sorption process. This is espe-
cially true for the change from the exothermal adsorption
range to the immediately following endothermal desorption
range in the case of physisorption thus allowing a precise dis-
tinction of various adsorption features.
2
For the first time, these basic sites have been characterized by
the exothermicity of the first interaction step with pulsed CO . If
2
the exothermal interaction was weak or absent, the injection of
gaseous water led to the deposition of OH groups predominantly
acting as basic sites on the surface of the fluorides, but in few cases
as acid sites as well. This enabled us to establish that the impact of
OH groups onto different fluorides can exhibit a different influence
on the sorption properties of the solids: the adsorption ability vs.
CO2 can be promoted, suppressed or remain unaffected. Hindering
(
ii) The interruption of the CO2 injections by one or more water
pulses allowed us to generate basic sites on the fluoride sur-
face which indeed represented OH groups rather than simply
adsorbed water molecules. The latter show another desorption
behaviour than do deposited OH groups. In certain cases, how-
ever, the generated OH groups showed acid rather than basic
behaviour.
of the CO adsorption can be a hint to acid rather than to basic char-
(
iii) A limiting factor for comparing spectroscopical information on
the adsorption behaviour such as given in [4] with the results
of PTA measurements is the different time scale of the physic-
ochemical processes. The spectroscopic investigations of the
CO2 adsorption are performed at near-to-equilibrium condi-
tions, sometimes under reduced pressure, whereas the PTA
measurements proceed under flow conditions at normal pres-
sure. That is why the first step of interaction revealed to be so
characteristic and yielded the principal information. If, on the
other hand, the interaction is strong enough – as it is the case
for chemisorption – the PTA information is unequivocal and
convincing.
2
acter of the deposited OH groups. This aspect will be investigated in
more detail in a following study. Again the great potential of Pulse
Thermal Analysis for the investigation of solids is demonstrated by
the results presented here.
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(
iv) Chemical conclusions cannot be drawn for the substance group
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Mg to Ba, on the other hand, correctly reflects the increasing
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2
[
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2
[
of the first CO2 adsorption stage for BaF -SG + H O approximates
2
2
that for BaF -SG wet (Table 1).
2
The influence of doping by a second metal is somewhat different
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3
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3
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