I.r. spectroscopic evidence for surface vibrational modes formed upon dehydroxylation of alumina

Jean-Claude Lavalley, Mohammed Benaissa
1984 Journal of the Chemical Society Chemical Communications  
Dehydroxylation of alumina at an activation temperature (Tat) >673 K gives rise to a band in the 1020-1050 cm-1 frequency range whose wavenumber and intensity increase with increasing TaC and which is sensitive to l 6 0 + l80 exchange and disappears on adsorption of water or bases; it can be assigned to surface vibrational modes of alumina. Dehydroxylation of metallic oxides produces water by the condensation of hydroxy groups while co-ordinatively unsaturated (c.u.s.) surface oxygen atoms and
more » ... e oxygen atoms and anion vacancies, exposing C.U.S. metal ions, are created. This may produce considerable strain on the surface, which can lead to new surface vibrational states. On silica, the corresponding vibrational modes have been demonstrated by i.r. spectroscopy: they are characterized by two bands at 908 and 888 cm-l,l showing that the new surface and subsurface siloxane groups are spectroscopically distinguishable from those characteristic of the bulk.1-2 On zinc oxide, Zecchina et aZ.3 showed that the force constants of the bonds present at the surface were appreciably increased with respect to those of the bulk. Morterra4 noted that pyridine adsorption on highly dehydrated transitional aluminas increased the transparency on the high-frequency side of the alumina cut-off, suggesting that the vibrations of the surface A1-0 modes are situated in this region. The same result was obtained when alcohols and thiols were adsorbed.5 Using a Fourier transform (F.t.)-i.r. spectrometer we have investigated these vibrational modes of alumina, by comparison of the spectra of alumina samples heated at different activation temperatures, Tat.
doi:10.1039/c39840000908 fatcat:dutpygn25zbb3lzhjwns7gwore