The hydrogen-oxygen reaction over the Pt(111) surface: Transient titration of adsorbed oxygen with hydrogen

Abstract

The kinetics of the hydrogen-oxygen reaction have been characterized on a Pt(111) surface over the 300–450 K temperature range by titration of adsorbed atomic oxygen with hydrogen in the 1.3 × 10 −6 to 1.3 × 10 −7 Pa (10 −8 to 10 −9 Torr) pressure range. These experiments were performed in an apparatus equipped with Auger electron spectroscopy, low energy electron diffraction, and a multiplexed mass spectrometer used for titration and thermal desorption measurements. The hydrogen-oxygen reaction has been studied by monitoring the water formation rate as a function of time for various temperatures, hydrogen pressures, and initial adsorbed oxygen concentrations. Adsorbed atomic oxygen forms islands of ordered atomic oxygen with a (2 × 2) structure under the conditions used during these experiments. Hydrogen reacts rapidly with adsorbed atomic oxygen to form water above 300 K. Typically reaction probabilities per incident hydrogen molecule were as high as 0.5 over the temperature range studied. Isotope exchange experiments indicate statistical amounts of HDO are formed during titration of adsorbed oxygen with H 2-D 2 mixtures. This isotope result coupled with low temperature spectroscopic studies (1) suggests that water formation proceeds via sequential addition of hydrogen first to adsorbed atomic oxygen, then to adsorbed hydroxyl to form the product water. Neither the concerted atomic hydrogen addition mechanism nor the H 2(a) + O(a) → OH(a) + H(a) mechanism can be rigorously excluded, however, several observations suggest they are not major pathways. The titration data indicate that the reaction is basically first order in incident hydrogen over the full range of adsorbed oxygen concentrations. The water formation rate is not a unique function of oxygen coverage, but also depends on the initial surface oxygen concentration (the largest oxygen coverage attained before the reaction begins). This result demonstrates that all of the adsorbed atomic oxygen is not available for reaction. A simple reaction model is proposed based on the assumption that the island structure of the adsorbed atomic oxygen limits the availability of oxygen for reaction; this simple model rationalizes the qualitative features of the titration data obtained. The model suggests that the key parameters affecting the behavior of the water formation reaction are the size and shape of the oxygen islands and the availability of adsorbed atomic hydrogen in the reaction zone.