Surface acidity of metal oxides. Combined microcalorimetric and IR-spectroscopic studies of variously dehydrated systems

The systematic use of CO adsorption at room temperature as a probe for (strong) Lewis acidity at the surface of a number of non-d/d(0) metal oxides of interest in catalysis is illustrated. The advantages of the combined use of adsorption microcalorimetry and IR spectroscopy in giving an exhaustive picture of the distribution of acid surface sites are stressed. The influence of the chemical nature (TiO2, ZrO2, HfO2 and Al2O3) and of the structure (monoclinic/tetragonal for ZrO2 and gamma-/delta theta-phase for Al2O3) Of the metal oxide, as well as the dehydration degree of the surface and the presence of anionic (sulfates for zirconia) or cationic (ceria for alumina) surface dopants were considered. The blue shift of the stretching frequency of adsorbed CO and the adsorption enthalpy were measured. In the case of the group IV metal oxides the two parameters were found to be correlated, whereas in the case of pure Al2O3 no correlation was found. The peculiar behavior of alumina was interpreted on the basis of different processes occurring at the CO/Al2O3 interface. An endothermic reversible reconstruction of the surface was supposed to occur upon adsorption, leading to exceptionally low heat values. The presence of small amounts of cations, other than Al3+, in the alumina matrix seemed to inhibit this effect. (C) 1998 Elsevier Science B.V.