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

Infrared spectroscopy and Si-29 MAS NMR are used to probe the structur e Of SiO2 on Al2O3 catalysts prepared by chemical vapour deposition of Si(OCH3)4. SiO2 loadings from about 0.2-2.5 statistical monolayers ar e studied. Infrared shows a gradual disappearance of AlOH vibrations a nd their replacement by SiOH vibrations as the SiO2 loading is increas ed. A population of inaccessible AlOH is also observed. The Si-O-X asy mmetric stretching frequency shows a continuous variation with SiO2 co ntent, consistent with X varying from Al at low levels to Si at high l evels of silica. Si-29 NMR shows composite peaks at all SiO2 levels. A t the lowest level most intensity is concentrated in the - 80 to - 85 ppm range, corresponding to Si with attached -OAl groups. At the highe st 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 m ixtures of Si(OSi)2(OH)2 with -OAl-containing species. A model is prop osed for build up of the silica layer on these catalysts. Random depos ition of silica on any available surface is shown to give a semiquanti tative account of the experimental results. (C) 1994 Academic Press. I nc.