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Identification of the "Active Sites" of a Surface-Catalyzed Reaction

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Science  20 Sep 1996:
Vol. 273, Issue 5282, pp. 1688-1690
DOI: 10.1126/science.273.5282.1688

Figures

  • Fig. 1.

    STM images of a Ru(0001) surface after the dissociative adsorption of 0.3 L (1 L = 1 langmuir = 1.3 × 10−6 mbar·s) of NO at 315 K. The gray shading was done for the two terraces individually with brightness scaled according to height. Tunneling voltage (Vsample) = −0.6 V; tunneling current (Itunnel) = 1 nA; T = 300 K. (A) Image (380 Å by 330 Å) showing two terraces (upper one to the left, lower one to the right) separated by a monatomic step (black stripe). (B) Close-up (60 Å by 40 Å) of the area marked in (A), showing an O island and individual N atoms. Individual O atoms are imaged as dashes (see arrow) parallel to the scan lines because most of them jump between subsequent scan lines.

  • Fig. 2.

    (A) Side view of the surface, illustrating dissociation at a type II step and reflection of NO from an O-covered type I step. (B) Top view, indicating the structure differences between type I and type II steps: A metal atom at the type I step has one neighbor atom at the bottom of the step, whereas at the type II step it has two. For O atoms (small dark spheres) at type II steps there is a Ru atom between the atom and the step edge, whereas this is not so at type I steps.

  • Fig. 3.

    STM image of a Ru(0001) surface after dissociative adsorption of 0.1 L of NO at 300 K; steps are descending from left to right (Vsample = −0.6 V; Itunnel = 1 nA; area, 240 Å by 190 Å; 300 K). Gray spots are N atoms, rows of black spots with twofold lattice spacings directly at the steps are trapped O atoms (arrows), and dashes represent mobile O atoms on the terraces. For the narrow terrace between the type II and type I steps at the right side, it cannot be decided from which step the N atoms originate.

  • Fig. 4.

    Density of product N atoms near steps after exposure to 0.1 L of NO at 300 K versus the width of the adjoining terraces. Top: type I steps; bottom: type II steps. Open circles correspond to the adjoining top, filled circles to the bottom terraces. The broken line is a linear fit to the data points of the type II steps. Data are from 1450 N atoms from nine 400 Å by 400 Å STM images. Only terraces wide enough to allow clear attribution of the N atoms to the respective left or right step of a terrace were analyzed. The slow diffusion of the N atoms away from the steps limited the measurements to periods of the order of 1 hour after adsorption (depending on the terrace width).

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