CHEMISTRY OF THE DIRECT SYNTHESIS OF METHYLCHLOROSILANES FROM METHYL PLUS CHLORINE MONOLAYERS ON A CU3SI SURFACE

The direct process for synthesizing methylchlorosilanes from methyl ch loride + silicon in the presence of catalytic amounts of copper has be en studied in vacuum using a sample of Cu3Si alloy, the bulk phase tha t is present in active regions of the catalytic direct process. From t he melt containing 23% of excess silicon, a two-phase Cu-3 Si + Si Sam ple was prepared. Free silicon phase served both to replenish silicon reacted from Cu3Si and to provide the grain boundaries found in an ind ustrial process. Atomically clean surfaces of this material with varyi ng Cu/Si atomic ratios were prepared by ion bombardment over a range o f temperatures. While the dissociative adsorption of CH3Cl was observe d to be immeasurably slow, on these surfaces under ultrahigh vacuum co nditions, methyl + chlorine monolayers generated by the coadsorption o f methyl radicals and Cl-2 led to selective formation of dimethyldichl orosilane. Adsorption of methyl groups alone produced trimethylsilane from two different active sites with very different kinetics. Adsorpti on of chlorine alone produced SiCl4.