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Conversely, an unreconstructed surface has a surface structure and a so-called (1 x 1) diffraction pattern that is expected from the projection of the X-ray unit cell for that particular surface. Such a definition of surface reconstruction does not tell us anything about possible changes in the interlayer distances between the first and the second layers of atoms at the surface. Contraction or expansion in the direction perpendicular to the surface can take place without changing the (1 x 1) two-dimensional surface unit cell size or orientation.

Indeed, it was found by experiments that the desorbing HD beam exhibits cosine angular distribution, as seen in Fig. 19. The pressure dependence of the exchange reaction indicates that an atommolecule reaction or possibly an atom-atom reaction on the surface is the ACTIVE SITES IN HETEROGENEOUS CATALYSIS 41 rate-limiting step. The absence of beam kinetic energy dependence of the rate indicates that the adsorption of hydrogen does not require activation energy. The surface is able to store a sufficiently large concentration of atoms, which react with the molecules by a two-branch mechanism.

1 1 , Comparisons of theory and experiment for I - - V profiles from Pt(l11) at room temperature for (a) the (00)beam and (b) the (TO) beam at three angles of incidence. The vertical scales are of relative intensity in arbitrary units and are not necessarily compatible from one curve to the next. The theoretical results were calculaied on the assumption of the bulk interplanar spacing for all atomic layers parallel to the surface. ACTIVE SITES IN HETEROGENEOUS CATALYSIS 21 There are two major features of the electron-solid interaction evidenced in the I-I/ profiles and in other scattering data in LEED electron diffraction that the theory must provide for: (1) In contrast to the case of X-ray scattering, cross sections for lowenergy electrons from atoms are large (on the order of 10 A2/atom).

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Encyclopedia of Nanoscience and Nanotechnology. Catalysis by Supported Gold Nanoclusters


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