X-ray absorption spectroscopy of transition aluminas

The X-ray absorption spectroscopy of a series of transition aluminas derive d from pseudoboehmite (alpha-alumina monohydrate) by calcination at 350, 40 0, 538, 750, 1000 and 1200 degrees C were examined at the Al edge. The tran sitions from ordered to disordered structure and eventually back to the ord ered corundum structure are easily observed in both the XANES spectrum and EXAFS. The near-edge spectrum consists of 5 peaks, three of which are reaso nably constant in intensity throughout the transitions. The first two peaks are attributed to the white lines of tetrahedral and octahedral Al. The te trahedral Al white line coincides with an additional pre-edge peak, which m akes accurate estimation of the tetrahedral content difficult; NMR is more accurate. Other peaks are ascribable to multiple scattering among the Al me tal lattice and other structures. The EXAFS show that, with increasing calc ination temperature, the Al coordination number decreases and disorder incr eases, as expected from the known structure changes. With the formation of a-alumina at 1000 and 1200 degrees C, the coordination number abruptly incr eases, as expected. This data will be of value in examining alumina structu res in supported catalysts.