Methyl chloride and trichlorosilane adsorption on Cu(110)

Abstract

The adsorption of methyl chloride (CH 3Cl) and trichlorosilane (HSiCl 3) on Cu(110) was studied by temperature-programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). Methyl chloride interacts weakly with Cu(110) at 100 K, and we estimate from TPD that the adsorption energy is only 8–10 kcal/mol. It adsorbs nondissociatively and reversibly on Cu(110) under these conditions; no evidence of thermal decomposition was observed. HREELS indicates that chemisorbed CH 3C1 adopts a bonding geometry with a symmetry lower than C 3v, presumably bonded to the surface through the Cl atom with the methyl group tipped over with respect to the surface normal. In contrast, HSiCl 3 interacts strongly with Cu(110) at 100 K. Trichlorosilane is initially dissociatively adsorbed, but most of the HSiCl 3 in the chemisorbed monolayer adsorbs molecularly and reversibly due to the strong poisoning of the Cu(110) surface by Cl adatoms. Thermal decomposition, which leaves some Si-Cl and Si-H bonds intact, occurs readily at 140–300 K. Silicon deposited on the Cu(110) surface during HSiCl 3 decomposition is in a metallic bonding environment, directly bonded to the Cu. Low coverages of HSiCl 3 form an H 2 desorption state near 300 K that is attributed to the decomposition of an SiH surface species with an activation energy for decomposition of 18 kcal/mol. This H 2 desorption state is not observed for the thermal decomposition of large coverages of HSiCl 3 on Cu(110) nor for pre-chlorinated Cu(110) surfaces. On pre-chlorinated Cu(110), HSiCl 3 is only weakly adsorbed with an adsorption energy of 8 kcal/mol and is adsorbed nondissociatively with complete reversibility. The importance of these results for understanding the Cu-catalyzed direct synthesis reaction between CH 3Cl and Si to form methylchlorosilanes is discussed.