Surface characterization of modified aluminas. I. The Lewis acidity of Sm-doped Al2O3

The surface behavior of an alumina catalyst (support) containing ≈3 wt% Sm2O3 and fired at 773, 1273, and 1473 K has been investigated by IR spectroscopy and compared with the surface behavior of the corresponding undoped aluminas. It was observed that the addition of Sm, which at high temperatures stabilizes the transition (spinel) alumina phases, does not modify appreciably the spectral features of the surface hydrated layer (free surface OH groups), but modifies the Lewis surface acidity of the material. The ambient temperature adsorption of CO, pyridine, and CO2 was investigated in some detail, and it was revealed that coordinatively unsaturated Sm centers progressively substitute, with increasing firing temperature, (all) the tetrahedrally coordinated Al centers in the surface layer. Spectral data indicate that the Lewis acidity of surface Sm3+ sites is intermediate between that of tetrahedrally and octahedrally coordinated surface Al sites. Samples activated under nonoxidizing conditions tend to present at the surface (relatively stable) reduced Sm centers.