Ga2O3-promoted sulfated zirconia systems: morphological, structural and redox properties

Ga2O3-promotion of ZrO2/SO4 (SZ) systems has been shown to influence the catalytic activity of n-butane isomerization. This contribution reports a study on the influence of Ga2O3 addition and of calcination temperature on ZrO2 structure, morphology and activity. Small additions of Ga2O3 (1–5 mol%) bring about an increase of specific surface area (SSA). By N2 physisorption, X-ray diffraction, Raman spectroscopy and TEM microscopy, the increase of an amorphous phase and the decrease of SSA were observed when the Ga2O3 content raised above 5 mol%. The catalytic activity went through a maximum with Ga loading of 3–5 mol%. In all cases, the typical spectral features of surface sulfates were observed, but they became more and more confused as the Ga2O3 content increased. DTG/DSC measurements showed that Ga2O3 addition tends to slow down both the formation of ZrO2 crystallites and the thermal elimination of surface sulfates. After calcination at temperatures as high as 720 C, the amount of amorphous phase is drastically reduced and an appreciable fraction of surface sulfates is lost. In particular, 15 mol% Ga2O3-doped SZ sample, which is amorphous and inactive in n-butane isomerization after a standard calcination at 650 C, exhibits tetragonal crystalline features and a dramatic improvement of catalytic activity after a thermal treatment at 720 C. TPR-MS analyses showed that Ga2O3-promoted SZ catalysts possess enhanced redox properties, with respect to plain SZ systems, and this can probably account for the higher catalytic activity.