Excitation of chemical waves in a surface reaction by laser-induced thermal desorption: CO oxidation on Pt(100)

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If a Pt(100) sample is kept at steady-state conditions of O₂ and CO partial pressures and temperature which are similar to those for the occurrence of autonomous temporal oscillations in the rate of CO₂ formation, then the surface will be largely covered by adsorbed CO which inhibits oxygen adsorption and keeps the catalytic rate low. Irradiation of a small spot with a high power laser pulse causes momentarily local thermal desorption of CO and creation of a reaction front which propagates as a chemical wave across the surface area, as was monitored by the excursion of the integral reaction rate and locally by means of a work function (=oxygen coverage) probe placed at several mm distance from the spot of irradiation. The velocity for wave propagation rises from about 2 mm/min at 480 K to 4 mm/min at 507 K and is not noticeably dependent on the partial pressures. The mechanism is closely related to that for self-sustained kinetic oscillations of this system and exhibits the typical features of trigger waves: Coupling between autocatalytic reaction and diffusion, as well as the occurrence of a refractory period during which the system is ``dead,'' and of a threshold for the intensity of the excitation. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.