Dresden 2003 – wissenschaftliches Programm

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O: Oberflächenphysik

O 23: Oberfl
ächenreaktionen II

O 23.6: Vortrag

Dienstag, 25. März 2003, 16:15–16:30, M\"UL/ELCH

CO oxidation reaction on Ir(111) surfaces: bistability and transient behavior — •Stefan Wehner¹, Frank Baumann¹, and Jürgen Küppers^1,2 — ¹Experimentalphysik III, Universität Bayreuth, D-95440 Bayreuth — ²Max-Planck-Institut für Plasmaphysik (EURATOM Association), D-85748 Garching

The oxidation kinetics of CO was investigated under UHV conditions on Iridium(111) surfaces by CO₂ rate measurements utilizing mass spectroscopy. CO₂ rates were measured at constant CO+O₂ gas flux and variable gas composition using a mass flow controller setup. The rate dependence on substrate temperature T and CO fraction in the feed gas Y was studied.
For Y smaller than a specific value Y^∗(T) bistability was observed in a temperature dependent ΔY range, apparent from kinetic measurements performed while ramping Y up or down. This kinetic hysteresis exists in the temperature range 380 K to 520 K. Since the approach to steady state in the bistability region is extremely slow (up to many hours), the kinetic hysteresis was confirmed by cycling through the bistability region as slow as one cycle per week. The center of the bistability region shifts to higher Y with increasing temperature, and ΔY decreases, until at 530 K bistability disappears. Transient gas pulsing experiments with O(CO) rich feed gas enabled to switch from the upper to the lower branch of the hysteresis, and vice versa. All observed features could be explained by a simple kinetic model.