Subsurface oxygen and its influence on the chemisorption behaviour of CO on Rh(210)

Abstract

The chemisorption of CO on either clean or oxygen containing Rh(210) single crystal surfaces has been investigated by means of temperature programmed static secondary ion mass spectrometry (TPSSIMS), temperature programmed desorption (TPD), ion scattering spectroscopy (ISS), and Auger electron spectroscopy (AES). Part of the CO, molecularly adsorbed on clean Rh(210), undergoes dissociation during heating. This is evidenced by the occurrence of RhnC+ and RhnO+ cluster ions (n = 1, 2) in TPSSIMS. Recombination of Cad and Oad at elevated temperatures is associated with a β-CO peak in TPD and the disappearance of respective features in TPSSIMS. Small amounts of subsurface oxygen obtained by adsorption of 10 L O2 (PL = 1.3 × 10−4Paṡs) at 1073 K, are not detectable by either ISS or SSIMS but show up in AES in form of KLL transitions shifted towards lower kinetic energies as compared to the respective transitions of chemisorbed oxygen prepared by exposure to 0.5 L O2 at 300 K. Subsurface oxygen is found to prevent dissociation of adsorbed CO. Neither significant amounts of RhnC+ and RhnO+ cluster ions (n = 1, 2) are present in SSIMS, nor does β-CO desorption take place in TPD under these conditions. By contrast, both methods indicate a change in the kinetics of thermal desorption of molecularly adsorbed CO.