Nucleation of spiral wave patterns at surface defects

The nucleation of spiral waves at a surface defect during catalytic CO oxidation on Pt(110) has been studied with a low energy electron microscope system. It is found that reaction fronts originate from a boundary layer between the defect and the surrounding Pt(110) area. The findings are corroborated by numerical simulations within a realistic reaction-diffusion model of the surface reaction.

Figure 1

Nucleation of a spiral wave at a surface defect. Experimental conditions: $T=402\mathrm{K}$, $p_{\mathrm{C}\mathrm{O}}>7\times {10}^{-6}\mathrm{mbar}$, $p_{{\mathrm{O}}_2}=2\times {10}^{-5}\mathrm{mbar}$, $E=0\mathrm{eV}$. Field of view: $64\times 64\mu {\mathrm{m}}^2$. (a) Two surface defects (dark) under conditions close to nucleation of a CO front. The inset shows the bound CO pulses on an enlarged scale. (b) Nucleation of a CO front at the defect in (a) $(t=0\mathrm{s})$. (c) and (d) Two stages in the rotation of a stable pinned spiral wave with a rotational period of $29.5\mathrm{s}$.

Figure 2

Supercritical nucleation of a CO pulse at defect. The initial situation at $t=0\mathrm{s}$ is given by Fig. 1b, the final state by Figs. 1c, 1d. At point $A$ the CO front breaks and loses contact with the defect. Experimental conditions: $T=402\mathrm{K}$, $p_{\mathrm{C}\mathrm{O}}>7\times {10}^{-6}\mathrm{mbar}$, $p_{{\mathrm{O}}_2}=2\times {10}^{-5}\mathrm{mbar}$, $E=0\mathrm{eV}$. Field of view: $64\times 64\mu {\mathrm{m}}^2$.

Figure 3

Subcritical nucleation of a CO pulse at a defect. The point where the bound CO pulse and the CO front collided has been marked as $A$. Experimental conditions: $T=402\mathrm{K}$, $p_{\mathrm{C}\mathrm{O}}=7\times {10}^{-6}\mathrm{mbar}$, $p_{{\mathrm{O}}_2}=2\times {10}^{-5}\mathrm{mbar}$, $E=0\mathrm{eV}$.

Figure 4

(Color online) Simulation of the supercritical regime with boundary layer from an initial condition similar to the experimental situation in Fig. 3 at $t=3\mathrm{s}$. Red (blue) corresponds to $u=0$ $(u=1)$ and a CO coverage of 0.95 (0.2) monolayers. The first three pictures show the initial dynamics of a CO pulse and CO front growing out of the boundary layer. Time between snapshots is $12\mathrm{s}$. The last picture shows the system at a later state. Parameters are $D_{\mathrm{y}}=0.25$, $b_{\text{bulk}}=0.425$.

Figure 5

(Color online) Simulation of the transition from super-to subcritical regime by reducing $p_{\mathrm{C}\mathrm{O}}$ in the absence of a boundary layer. This leads to a detachment of the wave front from the defect. Color coding as in Fig. 4. Time between snapshots is $9\mathrm{s}$ and $D_{\mathrm{y}}=1$, $b^{\text{new}}=0.406$, $b^{\text{old}}=0.407$.

Figure 6

(Color online) Simulation of the transition from sub-to supercritical regime by increasing $p_{\mathrm{C}\mathrm{O}}$ with bound CO pulse (arrow) in the boundary layer. Color coding as in Fig. 4. Time between snapshots is $24\mathrm{s}$ and $D_{\mathrm{y}}=0.25$, $b_{\text{bulk}}^{\text{new}}=0.425$, $b_{\text{bulk}}^{\text{old}}=0.38$.