Microcalorimetric and infrared spectroscopic studies of γ-Al2O3 modified by zinc oxide

The number and strength of surface acid and base sites on the ZnO/gamma-Al2O3 catalysts were quantitatively measured by the technique of microcalorimetric adsorption (MA) while the nature of the acid and base sites were identified by Fourier transform infrared spectroscopy (FTIR), by using ammonia and carbon dioxide as the probe molecules. Specifically, the effects of ZnO loading and calcination temperature on the structure and surface acid/base properties were studied. It was found that the acidity was not significantly affected by the ZnO loading for the ZnO/gamma-Al2O3 samples calcined at 673 K. However, the basicity as measured by the coverage of CO2 increased substantially with the ZnO loading although the initial heats were almost not changed. X-ray diffraction (XRD) showed that the 10% ZnO/gamma-Al2O3 sample calcined at 673 K exhibited only the phase of gamma-Al2O3, indicating the well dispersion of ZnO on the surface. ZnO/gamma-Al2O3 samples with higher loadings showed the phase of ZnO when they were calcined at 673 and 873 K, respectively. The spinel structure ZnAl2O4, was formed when the 42% ZnO/gamma-Al2O3 sample was calcined at 1073 K. Accordingly, the surface acidity and basicity were significantly changed. The surface of the 42% ZnO/gamma-Al2O3 sample calcined at 673 K was mainly covered by Zn2+ cations, as evidenced by the FTIR results which showed that this sample exhibited mainly Lewis acid sites associated with Zn2+ and Lewis base sites associated with O2- anions adjacent to Zn2+ cations. When the spinel structure ZnAl2O4 was formed upon the calcination at 1073 K, the surface was mainly covered by Al3+ again, exhibiting acidity and basicity with both Lewis and Bronsted features associated with Al3+ cations. In addition, the number and strength of both acid and base sites on the surface of the spinel were significantly decreased. (C) 2000 Elsevier Science B.V. All rights reserved.