A REEXAMINATION OF THE CHEMISORPTION OF TRIMETHYLALUMINUM ON SILICA

Fourier transform infrared spectroscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy have been used to study the chemisorption and decomposition of trimethylaluminum (TMA) on silica under high vacuum. By annealing a series of silica substrates from 425 to 1573 K prior to TMA exposures at 300 K, we have examined the distributions of chemisorption products as a function of the relative concentrations of isolated hydroxyls (OH(i)), hydrogen-bonded hydroxyls (OH(H)), and siloxane bridges. The observed variation in the Si-methyl to Al-methyl population ratios supports a new chemisorption model in which a monomethylaluminum surface complex and methyl groups bonded to silicon are proposed as the majority species on the surface at 300 K. Although the initial reactive sticking probability for TMA on the silica substrates is < 0.01 at 300 K, TMA chemisorption affects OH(i), OH(H), and siloxane bridges on the surface with equivalent probability. The common reaction probability (equivalent rate constants) implies that similar requirements may be involved in the reactive adsorption that consumes each site. Decomposition of the monomethylaluminum adsorbate begins above 373 K and increases the population of methyl groups bonded to silicon on the surface. The methyl groups react to form methane(g) and ethane(g) and adsorbed hydrocarbon fragments. In addition, the methyl groups also react further with the surface to form tetramethylsilane(g). At coverages of one-third saturation and greater, the monomethylaluminum surface complex can also react with methyl groups to yield TMA(g) above 500 K.