ACTIVE-SITES ON SIO2 - ROLE IN CH3OH DECOMPOSITION

The dehydroxylation of SiO2 and adsorption and thermal decomposition o f CH3OH on dehydroxylated SiO2 prepared at extreme temperatures up to 1475 K has been studied using transmission infrared spectroscopy. Form ation of Si-O-Si linkages (as evidenced by 888 and 908 cm(-1) bands) i s directly related to the loss of the isolated SiO-H band at 3748 cm(- 1) during SiO2 dehydroxylation to produce H2O. High-resolution (0.75 c m(-1)) studies of the isolated SiO-H band show sharpening and a slight 4 cm(-1) shift to higher frequency with decreasing -OH coverage and m ay be explained by site inhomogeneity effects. Adsorption of CH3OH on dehydroxylated SiO2 occurs first by reaction with SiOSi sites, followe d by exchange with SiOH groups at elevated temperatures leading to SiO CH3 surfaces species. (CH3OH)-O-18 adsorption studies demonstrated tha t adsorption on SiOSi sites occurs by cleavage of the methanol O-H bon d forming SiOH and (SiOCH3)-O-18. Thermal decomposition of SiOCH3 grou ps creates SiHx(x = 1,2) species and prevents formation of the SiOSi s ites indicative of the production of free radical sites at the surface . At temperatures above 1300 K, SiHx decomposition leads to SiOSi site formation.