Chemisorbed adlayer

Roasting a bronze artifact in dry NH3 imparts to it a deep-brown appearance, caused by a chemisorbed adlayer of ammonia-bronze complex, which persists long after removing the bronze from the furnace. 

Fig. 16 High-resolution in situ STM images of the chemisorbed adlayers of type IV, formed by substrate-adsorbate coordination, of BA (A), lA (B) and TA (C) on Au(lll)-(1 X 1) ix = 5-50 pA and E = 0.8 V. The corresponding molecular models are shown in the lower panels. Parameters of the indicated unit cells are summarized in Table 3...

Tables Characteristic dimensions of the experimental unit cell parameters of the chemisorbed adlayers IV of aromatic carboxylic acids...

The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

Chronoamperometric transients for CO stripping on polycrystalline platinum were measured by McCallum and Fletcher [1977], Love and Lipkowski [1988] were the hrst to present chronoamperometric data for CO stripping on single-crystalline platinum. However, they interpreted their data on the basis of a different model than the one discussed above. Love and Lipkowski considered that the oxidation of the CO adlayer starts at holes or defects in the CO adlayer, where OH adsorbs. These holes act as nucleation centers for the oxidation reaction, and the holes grow as the CO at the perimeter of these holes is oxidized away by OHads- This nucleation and growth (N G) mechanism is fundamentally different from the mean held model presented above, because it does not presume any kind of mixing of CO and OH [Koper et ah, 1998]. Basically, it assumes complete surface immobility of the chemisorbed CO. 

That chemisorbed oxygen was active in hydrogen abstraction, resulting in water desorption and the formation of chemisorbed sulfur, was first established by XPS at copper and lead surfaces.42 An STM study of the structural changes when a Cu(110)-O adlayer is exposed (30 L) to hydrogen sulfide at 290 K indicates the formation of c(2 x 2)S strings. 

Clearly the molecular events with iron were complex even at 80 K and low NO pressure, and in order to unravel details we chose to study NO adsorption on copper (42), a metal known to be considerably less reactive in chemisorption than iron. It was anticipated, by analogy with carbon monoxide, that nitric oxide would be molecularly adsorbed on copper at 80 K. This, however, was shown to be incorrect (43), and by contrast it was established that the molecule not only dissociated at 80 K, but NjO was generated catalytically within the adlayer. On warming the adlayer formed at 80 K to 295 K, the surface consisted entirely of chemisorbed oxygen with no evidence for nitrogen adatoms. It was the absence of nitrogen adatoms [with their characteristic N(ls) value] at both 80 and 295 K that misled us (43) initially to suggest that adsorption was entirely molecular at 80 K. 

Although no high resolution studies have been reported for ammonia interaction with iron surfaces, two main states of adsorption were recognized. At 80 K adsorption is entirely molecular with a characteristic N(ls) binding energy of 400 eV, but on warming the adlayer to 290 K the N(ls) intensity is mainly at 397 eV, typical of chemisorbed nitrogen adatoms with only a small contribution at 400 eV. 

S oriaga and coworkers [111-113] discovered that the anodic dissolution Pd(lll) and Pd(lOO) single-crystal electrodes in pure H2SO4 solutions was catalyzed by the presence of monolayers of iodine chemisorbed on these surfaces. Large anodic peaks were found for the dissolution even in noncorrosive electrolyte solutions only when the surfaces of Pd were modified by the iodine adlayer. 

Ordered Adlayers. Lateral interactions for chemisorbed adsorbates are generally repulsive, although there have been reports of attractive interactions for adsorbates at distances of about twice the distance between neighboring sites.At low coverage repulsive interactions simply keep the adsorbates apart. The adlayer is disordered. At intermediate and high coverages a rich field of possible adlayer structures can be observed even when there is only one type of adsorbate. 

Unfortnnately, no distinct LEED patterns could be generated from the adlayer of benzene chemisorbed on a Pd(lll) single-crystal electrode hence, meaningful results were obtained only from the HREELS experiments. Figure 10(A) shows the HREEL spectmm of benzene on Pd(lll) formed and emersed at 0.5 V based upon the above EC-STM results, a Pd(lll)-c(2V3x3)-rect-CeHe adlayer ( CgUg = 0.17) was assumed to be present on the surface. Except for the peak at 1717 cm, which is due to adventitious CO, all the peaks, when compared to published vibrational spectra of unadsorbed and adsorbed " benzene, are attributable to chemisorbed starting material. Unique to the surface-immobilized aromatic are the peaks labeled (a), 265 cm, and (b), 515 cm", which arise from direct metal-adsorbate (Pd-C) chemical bonds. Peaks (c) and (d) are out-of-plane C-H bends, y(C-H) peaks (e) and (i) are in-plane stretches, v(C-H), whereas peaks (f) and (g) are both inplane bends, 5(C-H). 

Table 11.1 The reconstruction of chemisorbed oxygen on selected catalytic metal surfaces, and the corresponding coverages of oxygen and metal adatoms in the oxygen adlayer...

A significant research effort has been devoted to the characterization of Pd surface oxides [17, 63]. In situ studies under non-UHV conditions are most relevant, because surface oxides may only be present under reaction conditions (and not accessible by post-reaction (ex situ) characterization). A stepwise oxidation of Pd(lll) was reported [64-66], starting from the well-known (2x2) chemisorbed oxygen adlayer, followed by the formation of a two-dimensional Pd O surface oxide, which eventually transforms to a PdO bulk oxide. At high temperature, PdO decomposes with the concurrent dissolution of oxygen atoms into the bulk. In situ synchrotron HP-XPS [64, 65] indicated that a two-dimensional Pd O surface oxide was formed on Pd(lll) at 0.4 mbar O and >470 K, which transformed to PdO at temperatures >660 K. PdO was found to decompose at temperatures >720 K. 

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