TY - JOUR

T1 - Nucleation of chemical waves at defects

T2 - A mirror electron microscopy study of catalytic CO oxidation on Pt(110)

AU - Wei, Han

AU - Lilienkamp, G.

AU - Imbihl, R.

N1 - Funding Information: Financial support from the German-Israeli Science Foundation (GIF) and by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged. FIG. 1. Nucleation of a CO front at surface defect. Experimental conditions: T = 402 K , p O 2 = 2.0 × 10 − 5 mbar , and p C O ≈ 7.0 × 10 − 6 mbar . (a) MEM images showing the development of the CO fronts. Two defects, a larger and a smaller one, are imaged as dark circular spots. We only consider the large defect. The bright halo asymmetrically surrounding the defect is due to electrical charging. Time = 0 s corresponds to the first visible sign of nucleation. Field of view ( FOV ) = 64 μ m and E ≈ 0 eV . The [ 1 1 ¯ 0 ] direction is given as in all other images by the long axis of the CO front ( 8 s ) . (b) Front velocity along the [ 1 1 ¯ 0 ] direction for the upper and the lower CO front. FIG. 2. Nucleation of an oxygen front inside the CO covered area shortly after the CO front has developed (Fig. 1 ). Experimental conditions as in Fig. 1 . (a) MEM image taken shortly after nucleation of the oxygen front ( 0 s ) . A and B mark the nucleation sites and “ 0 s ” marks the current position of the oxygen front. (b) Front velocity along the [ 1 1 ¯ 0 ] direction for the upper oxygen front. FIG. 3. Nucleation of a CO front from three CO pulses bound at the interface defect/surroundings. Experimental conditions as in Fig. 1 . FOV = 64 μ m and E ≈ 0 eV . FIG. 4. Anisotropy in pulse propagation around the defect. The data were taken from a spiral wave (see Fig. 6 ) pinned to the defect (rotational period = 29.5 s ), i.e., the end points with which the spiral wave is pinned to the defect were followed over one rotational period. Experimental conditions: T = 403 K , p O 2 = 2.0 × 10 − 5 mbar , and p C O = 7.0 × 10 − 6 mbar . FIG. 5. Symmetric structure formed during subcritical nucleation of a CO pulse at defect. Experimental conditions: T = 403 K , p O 2 = 2.0 × 10 − 5 mbar , p C O = 7.0 × 10 − 6 mbar , FOV = 64 μ m , and E ≈ 0 eV . FIG. 6. Supercritical nucleation of a CO pulse at defect. Experimental conditions: T = 402 K , p O 2 = 2.0 × 10 − 5 mbar , p C O = 7.0 × 10 − 6 mbar , FOV = 64 μ m , and E ≈ 0 eV . A and B mark the nucleation sites for the CO fronts, C is the nucleation site for an O front in the bottom wave, in D the upper left CO front detaches from the defect. FIG. 7. Steering effect of the defect demonstrated with the collision of two oxygen fronts at the defect. Experimental conditions: T = 403 K , p O 2 = 2.0 × 10 − 5 mbar , and p C O = 6 × 10 − 6 mbar . (a) MEM images showing different stages in the collision. FOV = 64 μ m and E ≈ 0 eV . (b) Development of the front velocities during collision. The velocities were determined along the directions indicated by the arrows C1 and C2 marked in (a). The oscillatory variation in the velocity of front C2 between 0 and 1 s is due to the interaction of the front with a smaller defect visible about halfway between the lower right corner of the images and the large defect. FIG. 8. Collision of a train of CO pulse with the defect leading to the nucleation of two secondary CO fronts ( 9.25 s ) . Their nucleation sites are marked as A and B. Experimental conditions: T = 403 K , p O 2 = 2.0 × 10 − 5 mbar , p C O = 7.0 × 10 − 6 mbar , FOV = 64 μ m , and E ≈ 0 eV . FIG. 9. Influence of the surface topography on the nucleation of a CO front. Experimental conditions: T = 402 K , p O 2 = 2.0 × 10 − 5 mbar , p C O = 10 − 6 mbar range, FOV = 45 μ m , and E ≈ 0 eV .

PY - 2007/7/12

Y1 - 2007/7/12

N2 - Using mirror electron microscopy (MEM) as spatially resolving method the nucleation of chemical waves in catalytic CO oxidation on a Pt(110) surface was investigated in the 10-5 mbar range. The waves nucleated at an electrically insulating impurity of approximately 15 μm diameter (the "defect") which most likely represents a diamond particle left over from the polishing process. Nucleation events are initiated by a dynamic process in a boundary layer of approximately 1 μm width between the defect and the surrounding Pt(110) surface. Depending on the parameter choice the fronts/pulses do not escape from the vicinity of the defect and later on die out or, in a supercritical nucleation, propagate across the surface. Asymmetric nucleation leads to spiral waves which remain pinned to the defect. The defect has a kind of steering effect causing chemical waves to collide exactly at the defect. This steering effect is evidently due to a distortion of the substrate lattice in the vicinity of the defect.

AB - Using mirror electron microscopy (MEM) as spatially resolving method the nucleation of chemical waves in catalytic CO oxidation on a Pt(110) surface was investigated in the 10-5 mbar range. The waves nucleated at an electrically insulating impurity of approximately 15 μm diameter (the "defect") which most likely represents a diamond particle left over from the polishing process. Nucleation events are initiated by a dynamic process in a boundary layer of approximately 1 μm width between the defect and the surrounding Pt(110) surface. Depending on the parameter choice the fronts/pulses do not escape from the vicinity of the defect and later on die out or, in a supercritical nucleation, propagate across the surface. Asymmetric nucleation leads to spiral waves which remain pinned to the defect. The defect has a kind of steering effect causing chemical waves to collide exactly at the defect. This steering effect is evidently due to a distortion of the substrate lattice in the vicinity of the defect.

UR - http://www.scopus.com/inward/record.url?scp=34547260210&partnerID=8YFLogxK

U2 - 10.1063/1.2751151

DO - 10.1063/1.2751151

M3 - Article

AN - SCOPUS:34547260210

VL - 127

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 2

M1 - 024703

ER -