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 gi ving an exhaustive picture of the distribution of acid surface sites a re stressed. The influence of the chemical nature (TiO2, ZrO2, HfO2 an d Al2O3) and of the structure (monoclinic/tetragonal for ZrO2 and gamm a-/delta theta-phase for Al2O3) Of the metal oxide, as well as the deh ydration degree of the surface and the presence of anionic (sulfates f or zirconia) or cationic (ceria for alumina) surface dopants were cons idered. The blue shift of the stretching frequency of adsorbed CO and the adsorption enthalpy were measured. In the case of the group IV met al oxides the two parameters were found to be correlated, whereas in t he case of pure Al2O3 no correlation was found. The peculiar behavior of alumina was interpreted on the basis of different processes occurri ng at the CO/Al2O3 interface. An endothermic reversible reconstruction of the surface was supposed to occur upon adsorption, leading to exce ptionally low heat values. The presence of small amounts of cations, o ther than Al3+, in the alumina matrix seemed to inhibit this effect. ( C) 1998 Elsevier Science B.V.