X-ray absorption spectroscopy of transition aluminas

The X-ray absorption spectroscopy of a series of transition aluminas derived from pseudoboehmite (alpha-alumina monohydrate) by calcination at 350, 400, 538, 750, 1000 and 1200 degrees C were examined at the Al edge. The transitions from ordered to disordered structure and eventually back to the ordered corundum structure are easily observed in both the XANES spectrum and EXAFS. The near-edge spectrum consists of 5 peaks, three of which are reasonably constant in intensity throughout the transitions. The first two peaks are attributed to the white lines of tetrahedral and octahedral Al. The tetrahedral Al white line coincides with an additional pre-edge peak, which makes accurate estimation of the tetrahedral content difficult; NMR is more accurate. Other peaks are ascribable to multiple scattering among the Al metal lattice and other structures. The EXAFS show that, with increasing calcination temperature, the Al coordination number decreases and disorder increases, as expected from the known structure changes. With the formation of a-alumina at 1000 and 1200 degrees C, the coordination number abruptly increases, as expected. This data will be of value in examining alumina structures in supported catalysts.