Some reactions of methylsilane catalysed by metals
Bradshaw, David Ian
Thesis or dissertation
- © 1975 David Ian Bradshaw. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
The chemisorption of methylsilane at 273 K, and the catalysed hydrogen isotope exchange in methylsilane in the range 195 - 300 K have been studied using evaporated metal films of molybdenum, tungsten, rhodium, nickel, and gold.
Chemisorption is strong, and is accompanied by dissociation of hydrogen bonded to silicon, dissociation of carbon-silicon bonds, and dissociation of hydrogen bonded to carbon, the processes occurring in that order. The empirical composition of the adsorbed layer at complete coverage by methylsilane is reported for each metal.
Isotope exchange in the silyl group is rapid, whereas exchange in the methyl group does not occur. Three exchange reactions have been investigated: self exchange in which CH₃SiD₃ was sole reactant, mutual exchange in which the reactants were CH₃SiH₃ and CH₃SiD₃, and exchange with molecular deuterium which involved CH₃SiH₃ and D₂. None of the metals used catalyse self exchange. Mo, W, Rh, and Ni catalyse each of the other two exchange reactions, but Au only catalyses mutual exchange because of its inability to chemisorb molecular deuterium. Specific rates for the two latter exchange reactions respond differently to the presence of surface poisons, which indicates that these reactions occur at different regions of the surface and involve different intermediates. Mechanisms of exchange are proposed.
Means of providing quantitative mass spectrometric analyses of deuteriated methylsilanes were required, and various methods have been developed to achieve this.
Exchange in methylsilane is compared with that in methane and in ethane.
The origin of the activity of gold lies in the ability of this element to chemisorb methylsilane, apparently by electron donation from gold to silicon. A novel associatively adsorbed state may be involved. Gold has a higher specific activity for mutual exchange than the earlier transition metals studied. This
reaction has importance for the general theory of catalysis since gold has negligible catalytic activity for reactions-of carbon compounds, which has been ascribed to the presence of the nominally filled d-band in gold.
- Department of Chemistry, The University of Hull
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