Investigation of non-linear dynamics in catalytic surface reactions

A number of heterogeneous catalytic surface reactions are known to exhibit non-linear behavior such as reaction rate oscillations and adsorbate pattern formation over a wide range of reaction conditions. The current focus of our work is to investigate the dynamics of the CO oxidation reaction over Pt(100) when it is subjected to external forcing. Forcing is achieved by locally introducing reactants to the catalytic surface by way of a capillary located near the surface. The surface is continuously monitored using ellipsomicroscopy for surface imaging (EMSI), an optical technique sensitive to the presence of different adsorbates. The experimental setup is diagrammed below in Fig. 1.

Figure 1.

With this technique, surface reactions may be perturbed in a spatio-temporal selective manner to initiate unique patterning which does not take place in the unforced system under similar global conditions. For example, CO can be locally dosed onto an oxygen covered surface through a capillary having a square cross-section (Fig. 2) to produce square shaped islands on the surface. Shortly after the formation of the square pattern, the planar reaction front along the right boundary of the pattern undergoes a fingering bifurcation, with CO fingers extending out into the O ad-layer as shown in Fig. 3.

Figure 2.

Figure 3. T = 130 °C, p_O2 = 4x10^-4 Torr.

The cause of the fingering instability lies in the reaction-diffusion properties of the CO oxidation reaction. Oxygen is more tightly bound to the Pt surface and therefore exhibits a diffusion coefficient several orders of magnitude less than that of adsorbed CO. Thus, CO diffusion at the planar front acts to destabilize the front resulting in the fingering instability where the most unstable, and therefore dominant, wavelength was found to be approximately 40 µm.

External forcing by local gas dosing is an effective means of perturbing surface reactions on a local scale. The fingering instability of planar CO fronts is just on example of non-linear behavior unique to the externally forced reaction system. Other types of localized patterns created with the local dosing technique include spatially restricted oscillations as well as excitable systems bounded between doser-induced islands.

[1] R. Imbihl, and G. Ertl, Chem. Rev. 95 (1995).

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