Reaction pathways linking chemisorption to desorption of methylchlorosilanes on copper(001)

The interactions of silane molecules with metal surfaces are pivotal in many commercial processes. Of particular interest is the commercial Direct Process," in which methyl chloride (CH3Cl) is exposed to silicon, in the presence of a copper catalyst and other promoters, producing dimethyldichlorosilane as the predominant product. Previous UHV studies have investigated the interaction of pre-dissociated" methyl (CH3) and chlorine (Cl) with copper and copper-silicide surfaces. This thesis investigates the reac- tion mechanisms of pre-associated" methylchlorosilanes ((CH3)xClySiHz, x+y+z=4) adsorbed on a copper(001) surface. The goal was to develop an understanding of the intermediates and transfer processes involved for dissociative adsorption and subse- quent desorption of these molecules. Only molecules containing at least one Si-H bond were observed to undergo chemisorption. It was found that dimethylsilane (CH3)2SiH2 and methylsilane (CH3)SiH3 exhibit ligand transfer on the copper surface, leading to the desorption of trimethylsilane (CH3)3SiH, in both cases. The suggested intermedi- ates present after adsorption of methylsilane were methylsilyl CH3SiH2, methysilylene ii CH3SiH and methylsilylidyne CH3Si. The proposed mechanisms leading to trimethylsi- lane desorption involve rate liming methyl transfer between silicon centers. Chlorinated silane species also exhibited methyl transfer to form desorbing silane species. While methyl transfer appears facile, chlorine transfer among silicon centers was not observed. Investigations were also conducted on absorbed methyl groups alone on the copper sur- face. Under certain conditions of azomethane pyrolysis the adsorption of both CH3 groups and H atoms on the copper surface was observed. The co-adsorption of methyl and atomic hydrogen leads to the simultaneous desorption of methane and molecular hydrogen at 300K. Any remaining methyl groups decompose at 420K, leading to a resumption of the simultaneous methane and H2 desorption. The relative intensities and peak desorption temperatures of the CH4 and H2 desorption were used to study the kinetics of the associative desorption reaction.Ph. D.Includes bibliographical referencesby James Lall