STRUCTURE OF SIO(2) AND AL(2)O(3) MONOLAYER CATALYSTS - INVESTIGATION BY INFRARED-SPECTROSCOPY AND SI-29 MAS NMR

Infrared spectroscopy and Si-29 MAS NMR are used to probe the structure Of SiO2 on Al2O3 catalysts prepared by chemical vapour deposition of Si(OCH3)4. SiO2 loadings from about 0.2-2.5 statistical monolayers are studied. Infrared shows a gradual disappearance of AlOH vibrations and their replacement by SiOH vibrations as the SiO2 loading is increased. A population of inaccessible AlOH is also observed. The Si-O-X asymmetric stretching frequency shows a continuous variation with SiO2 content, consistent with X varying from Al at low levels to Si at high levels of silica. Si-29 NMR shows composite peaks at all SiO2 levels. At the lowest level most intensity is concentrated in the - 80 to - 85 ppm range, corresponding to Si with attached -OAl groups. At the highest silica levels, the spectrum can be deconvoluted into peaks at - 108, - 100, and - 92 ppm. These correspond to Si(OSi)4, Si(OSi)3OH, and mixtures of Si(OSi)2(OH)2 with -OAl-containing species. A model is proposed for build up of the silica layer on these catalysts. Random deposition of silica on any available surface is shown to give a semiquantitative account of the experimental results. (C) 1994 Academic Press. Inc.