ADSORPTION OF PENTAMETHYLCYCLOPENTASILOXANE ON THE PD(100) SURFACE STUDIED BY HIGH-RESOLUTION X-RAY PHOTOEMISSION SPECTROSCOPY

Adsorption and reaction of pentamethylcyclopentasiloxane (PMPS) on a c lean Pd(100) surface were investigated with the goal to determine whet her anchoring of siloxane compounds on this metal occurs via strong di ssociative chemisorption and whether siloxane monolayers can be formed . The room temperature exposure resulted in disordered, partially diss ociated PMPS while retaining the Si-O backbone bonds, as indicated by high resolution X-ray photoemission spectroscopy (HRXPS). The quantita tive HRXPS analysis showed that the surface Si coverage corresponded t o 0.74 ML and that the PMPS adsorbed ''flat'' with partially broken Si -CH3 bonds serving as anchoring points. Subsequent exposure of the PMP S-covered Pd surface to CO and O-2 showed no adsorption of these molec ules, demonstrating that the adsorbed PMPS completely blocked the meta l surface. However, exposure to Cl-2 gave rise to two distinct Cl2p pe aks in the HRXPS spectrum: a main component at 199.73 eV and a much sm aller peak shifted from the main peak by Delta BE = -2.11 eV, correspo nding to Cl atoms covalently bonded to PMPS and atomic Cl bonded to th e 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.