IR spectroscopic and microcalorimetric characterization of Lewis acid sites on (transition phase) Al2O3 using adsorbed CO
Articolo
Data di Pubblicazione:
1994
Abstract:
On gamma-Al2O3 and delta-theta-Al203, the adsorption of CO at -300 K (and occasionally at 77 K) has been
investigated by IR spectroscopy and adsorption microcalorimetry. Up to three types of sigma-coordinated CO
adspecies form, two of which are ascribed to sites (most likely tetrahedrally coordinated Al ions) in
crystallographically defective configurations, and one to sites (still most likely tetrahedrally coordinated
Al ions) located on regular crystal planes; the relative amounts of the three CO adspecies depend on the
alumina crystal phase, and on the temperature of vacuum activatiort. The overall amounts of CO adsorbed
are quite small, as compared to the extent of surface dehydration: surface reconstruction and ion shielding
effects are supposed to be involved in the activation process. The heat of adsorption of the three adspecies,
extrapolated to zero coverage, correlates nicely with the correspondhg CO stretching frequencies, but at
higher CO coverages the observed heats fall to very low values, iricompatible with the a-coordination
mechanism proposed and with the observed frequencies. The occurrence of reversed surface mobility
processes, brought about by the adsorption phenomenon and invoiiing an endothermic contribution to
the net heat release observed, is postulated.
Tipologia CRIS:
03A-Articolo su Rivista
Keywords:
combined IR spectroscopy and adsorption microcalorimetry; CO adsorption; transition aluminas; Lewis acidity
Elenco autori:
MORTERRA C; BOLIS V; G. MAGNACCA
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