Chemisorption and decomposition of tetramethylsilane over tungsten and iron surfaces

Abstract

The adsorption and subsequent decomposition of tetramethylsilane 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 are formed ; further H2 and CH4 are desorbed on heating the surface to temperatures up to 42OK. 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 are observed blow 320K. Comparison of both sets of mults with those for C(CH3), indicates that Si(CH& is more extensively dissociated under all conditions ; this is attributed rnainly to the weaker C -S i 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.

Surface organometallic chemistry on metals: controlled hydrogenolysis of Me4Sn, Me3SnR, Me2SnR2, MeSnBu3 and SnBu4 (R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto metallic rhodium supported on silica

“…This regioselectivity of the hydrogenolysis is probably governed by the difference in the Sn-C and C-C bond energies. 45,46 (iii) The stoichiometry of the surface species after 1 h shows the presence of more alkyl groups than after 80 h of hydrogenolysis, which supports a step-wise hydrogenolysis of the Sn-C bond.…”

“…This regioselectivity of the hydrogenolysis is probably governed by the difference in the Sn-C and C-C bond energies. 45,46 Accordingly, the introduced tin compound reacts exclusively by tin activation and not through C-H activation at the alkyl moiety. This is because a surface Rh-alkyl species would be expected to decompose either by b-H elimination or by hydrogenolysis of C-C bonds.…”

Formation of Methyl Radicals from Decomposition of Methyl-Substituted Silanes over Tungsten and Tantalum Filament Surfaces

“…These activation energies are higher than the ones determined from this work for the formation of the • CH 3 radical on W. This is likely due to the fact that more chemical bonds need to be broken to form Si radicals. Extensive studies have been dedicated to understand the adsorption and decomposition of methyl-substituted silanes, among which MMS has been studied the most on various metal surfaces, including W. ,− Despite the fact that the bond strength of Si–H is ∼8 kcal·mol –1 (i.e., 33 kJ·mol –1 ) higher than that of Si–C in methyl-substituted silanes, , temperature-programmed desorption and spectroscopic studies have unambiguously demonstrated that MMS adsorbs dissociatively by Si–H bond cleavage. Similar results were obtained when studying the bond scission in ethane hydrogenolysis on various metal surfaces, which showed that C–H bonds are more reactive than C–C bonds, although the latter are weaker by ∼10 kcal·mol –1 (i.e., 42 kJ·mol –1 ).…”