The titration of oxygen adsorbed on a polycrystalline iron surface by hydrogen permeation

Abstract

The interaction between oxygen and hydrogen on a clean polycrystalline iron surface was studied by Auger electron spectroscopy and mass spectrometry. To avoid having the reaction kinetics limited by the slow adsorption of hydrogen on the oxidized surface, hydrogen was adsorbed by permeation through a thin iron specimen. For oxygen coverages of more than one monolayer this procedure resulted in the rapid titration of oxygen by the formation and desorption of water. In the temperature range of 475–625 K, the reaction path consists of the initial formation of OH from coadsorbed oxygen and hydrogen, followed by the disproportionation of two hydroxyl species to form H2O. An overall activation energy of 7–8 kcal/mol was measured for the above reaction sequence. Oxygen in coverages of less than one monolayer could not be removed by hydrogenation, in agreement with a previous study on an Fe(100) single-crystal substrate. The present study shows that this apparent stability of the chemisorbed oxygen is not due to the kinetics of hydrogen adsorption.