Influence of lanthana on the nature of surface chromium species in La2O3-modified CrOx/ZrO2 catalysts

Infrared spectroscopy of the adsorbed probe molecules CO, NO, and NH3, photoelectron spectroscopy, temperature-programmed desorption of NH3, X-ray diffraction, and BET surface analysis have been used to study surface and bulk characteristics of lanthana-modified CrOx/ZrO2 catalysts. La2O3 addition to CrOx/ ZrO2 strongly modifies the structure of supported chromia species and stabilizes coexisting isolated Cr3+ and highly dispersed Cr2O3 particles. Unlike that, chromia in La2O3-free CrOx/ZrO2 undergoes considerable sintering at 823 K, which is the necessary reduction and reaction temperature for n-octane aromatization, applied as a test reaction. The strong interaction between chromia and La2O3ZrO2 could be due to an increased number of nonacid hydroxyl groups, which are possibly responsible for a more efficient anchoring of chromia species on the La2O3-containing support rather than on pure zirconia. The performances of La2O3-promoted and La(2)O(3)free CrOx/ZrO2 catalysts in n-octane aromatization do not differ so dramatically as would be expected from the large differences in the structure of surface chromia species. (C) 2000 Academic Press.