Chemisorption and Decomposition of Tetramethylsilane over Tungsten and Iron Surfaces

Abstract

The adsorption and subsequent decomposition of tetramcthylsilane Si(CH3)4 on evaporated films of tungsten and iron have been investigated. The results are compared with results previously obtained with neopentane C(CH3)4. Si(CH3)4 is rapidly and irreversibly adsorbed on tungsten at 293 K, and gaseous hydrogen and methane arc formed ; further H2 and CH4 arc desorbed on heating the surface to temperatures up to 420 K. Additional information regarding the adsorbed phase has been obtained from deuterium exchange experiments. Similar experiments carried out with iron show that much less dissociation of the Si(CH3)4 occurs on adsorption and on heating the adsorbed layer; no gaseous products arc observed below 320 K. Comparison of both sets of results with those for C(CH3)4 indicates that Si(CH3)4 is more extensively dissociated under all conditions; this is attributed mainly to the weaker C—Si bond. The results suggest that the rate determining step in hydrocracking reactions of hydrocarbons is likely to be hydrocarbon breakdown rather than desorption of products.