Adsorption of pentamethylcyclopentasiloxane on the Pd(100) surface studied by high resolution X-ray photoemission spectroscopy

Adsorption and reaction of pentamethylcyclopentasiloxane (PMPS) on a clean Pd(100) surface were investigated with the goal to determine whether anchoring of siloxane compounds on this metal occurs via strong dissociative chemisorption and whether siloxane monolayers can be formed. The room temperature exposure resulted in disordered, partially dissociated PMPS while retaining the Si-O backbone bonds, as indicated by high resolution X-ray photoemission spectroscopy (HRXPS). The quantitative HRXPS analysis showed that the surface Si coverage corresponded to 0.74 ML and that the PMPS adsorbed "flat" with partially broken Si-CH3 bonds serving as anchoring points. Subsequent exposure of the PMPS-covered Pd surface to CO and O-2 showed no adsorption of these molecules, demonstrating that the adsorbed PMPS completely blocked the metal surface. However, exposure to Cl-2 gave rise to two distinct Cl2p peaks in the HRXPS spectrum: a main component at 199.73 eV and a much smaller peak shifted from the main peak by Delta BE = -2.11 eV, corresponding to Cl atoms covalently bonded to PMPS and atomic Cl bonded to the Pd metal respectively. Thus, Cl-2 reacts with PMPS and the Cl atoms gain access to the Pd surface by rearranging the Cl/PMPS overlayer. (C) 1998 Published by Elsevier Science B.V. All rights reserved.