IR SPECTROSCOPIC AND MICROCALORIMETRIC CHARACTERIZATION OF LEWIS-ACIDSITES ON (TRANSITION PHASE) AL2O3 USING ADSORBED CO

On gamma-Al2O3 and delta,theta-Al2O3, the adsorption of CO at approxim ately 300 K (and occasionally at approximately 77 K) has been investig ated by IR spectroscopy and adsorption microcalorimetry. Up to three t ypes of sigma-coordinated CO adspecies form, two of which are ascribed to sites (most likely tetrahedrally coordinated Al ions) in crystallo graphically defective configurations, and one to sites (still most lik ely tetrahedrally coordinated Al ions) located on regular crystal plan es; the relative amounts of the three CO adspecies depend on the alumi na crystal phase, and on the temperature of vacuum activation. The ove rall amounts of CO adsorbed are quite small, as compared to the extent of surface dehydration: surface reconstruction and ion shielding effe cts are supposed to be involved in the activation process. The heat of adsorption of the three adspecies, extrapolated to zero coverage, cor relates nicely with the corresponding CO stretching frequencies, but a t higher CO coverages the observed heats fall to very low values, inco mpatible with the sigma-coordination mechanism proposed and with the o bserved frequencies. The occurrence of reversed surface mobility proce sses, brought about by the adsorption phenomenon and involving an endo thermic contribution to the net heat release observed, is postulated.