Surface , reconstruction chemisorption

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

Jensen F, Besenbacher F, Laesgaard E and Stensgaard I 1990 Surface reconstruction of Cu (110) induced by oxygen chemisorption Phys. Rev. B 41 10 233... 

Since the pioneering work of Rohrer and Binning,77 scanning tunelling microscopy (STM) has been used to image atomic-scale features of electrically conductive surfaces under ultra-high-vacuum but also at atmospheric pressure and in aqueous electrochemical environments. The ability of STM to image chemisorption and surface reconstruction is well... 

Boundary Conditions. The implicit boundary condition of small surface area can affect surface reconstructions and chemisorption. Surface steps, for example, are important for reconstructions (e.g, 51), and can determine the particular domain that occurs (e.g. 52). 

To probe the early stage of oxygen chemisorption, that is, prior to the onset of surface reconstruction and oxide formation and relevant to our coadsorption reactivity studies, there were obvious advantages for STM observations to be made at cryogenic temperatures. 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Cases of chemisorption are known in which at high coverages the net (two-dimensional lattice) of the adsorbate is not in registry with the lattice of the adsorbent. In such situations, the concept of sites of precise location and fixed number may not be applicable. Similar difficulties about the definition of sites will occur if surface reconstruction takes place upon interaction of adsorbate and adsorbent. 

For reactions of H with H-covered graphite surfaces, the energetics and dynamics are quite different. The H-graphite chemisorption is comparatively weak, and requires significant surface reconstruction. The attraction of the incident H to the chemisorbed H is very strong, leading to steering of the incident... 

Adsorbate-induced surface reconstruction and dissociative chemisorption are merely natural extremes of this delicate balance. In each case, strong surface-adsorbate interactions direct the course of the transformation, either breaking up bonding in the surface, so that it reconstructs, or disrupting the adsorbed molecule.59 An incisive discussion of the latter situation for the case of acetylene on iron and vanadium surfaces was provided by A. B. Anderson.60... 

In general, as I already outlined in a previous section, nondissociative chemisorption is a delicate balance of the very same interactions, which weaken bonds in the adsorbed molecule and in the surface. Dissociative chemisorption and surface reconstruction are just two extremes of the same phenomenon. 

Chemisorption-induced metal atom reorganization at low temperatures had been discovered in the 1960s by field ion microscopy in chemisorption studies of nitrogen and carbon monoxide on a tungsten single-crystal tip (55-58). Likewise, surfece reconstruction was concluded from research of chemisorption on metal and bimetal films (59-61). This surface reconstruction was observed particularly on open crystal faces the more closely packed faces are often corroded from their edges with the former faces. Surface reconstruction is of particular relevance to alloy surfaces, where it leads to chemisorption-induced segregation of one alloy component to the... 

The catalytic activity of aluminosilicate zeolites and aluminas appears to be directly related to the concentration of 30 ppm sites present, leading to the development of super-five materials displaying large NMR signals at this position (Wood et al. 1990). Since catalysis depends on the chemical nature of the Al at the surface, which may not be the same as in the bulk, cross-polarisation experiments between H and Al have been used to distinguish between the surface and bulk species (Coster et al. 1994). Since chemisorption of water provokes extensive surface reconstruction, a more suitable proton source for the CP experiments was found to be ammonia adsorbed on the surface. The results showed the presence of two kinds of surface Lewis sites associated with the non-framework Al (a tetrahedral site at ca.58 ppm with a xq of about 6 MHz, and an Af site at ca. 40 ppm with a slightly smaller xq)- Lewis sites either... 

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