Coadsorption Characteristics

In order to get a deeper insight into the coadsorption of hydrogen and carbon monoxide (CO) on gadolinium it is necessary to understand what happens when exposing the Gd surface to each single type of molecules. The behavior of hydrogen was already discussed in Sect. 4.1 the adsorption of CO will first be described in the following. 

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In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

Thus, it may again be concluded, as in the case of alkali coadsorption with H, that chemisorbed H exhibits amphoteric, i.e. both electron donor and electron acceptor characteristics. 

Coadsorption, in which two different kinds of particles are chemisorbed on the solid surface, may be classified into cooperative adsorption and competitive adsorption. Cooperative adsorption takes place with two different adsorbate particles of opposite characteristics, such as electron-donating particles and electron-accepting particles (e.g. Na and S), and the two adsorbate particles are adsorbed uniformly on the solid surface. On the other hand, competitive adsorption involves two different particles of similar characteristics, i.e. both being electron-donating or electron-accepting particles (e.g. O and S), which are adsorbed separately on the solid surface. 

Primet et al. (250, 251) also showed that aromatic rCH absorptions characteristic of benzene-derived species on Pt/Si02 caused a marked lowering (from 2065 to 2030 cm"1) of the strong bands characteristic of coadsorbed CO. The authors interpreted the vCO shift as arising from donation of electrons from the benzene 77-orbitals to the metal surface. Palazov (245) drew similar conclusions concerning benzene and CO coadsorption on Ni/Si02. 

CO stretching frequency is in many cases characteristic of the binding site, allowing one to differentiate between adsorption on three and fourfold hollow sites, bridge sites, on-top sites, steps, etc. (17-19). One should keep in mind, however, that this differentiation may not always be possible, in particular, when strong adsorbate-adsorbate interactions occur or when coadsorption of multiple species occurs (199,200). 

The chemistry of a surface determines the surface electronic properties. The following chapter was therefore focused on the influence of adsorbates. It was found that hydrogen exhibit unusual adsorption characteristics demonstrating a lagoon-like appearance. The incorporation of hydrogen in Gd thin films leads to a plastic deformation resulting in surface modihcations which were identified by STM. The combination of ultraviolet photoelectron spectroscopy (UPS) and STM allowed to determine the reaction scheme of coadsorption processes, exemplarily presented for hydrogen covered Gd surfaces exposed to CO. 

Structural and Energetic Characteristics of All Bound Water and the SAW Portion (Shown in Brackets) on Coadsorption with Methane on A-300 (Sbet=337 mVg, Vp=0.714 cmVg)... 

Stimming and his coworkers found by in situ STM that adsorbed sulfate ions on Pt(lll) form the same adlayer structure as that found on Au(lll) [34]. Ordered domains with X V7) symmetry appeared in the potential range between 0.5 and 0.7 V versus a reversible hydrogen electrode (RHE) in 0.05 M H2SO4. As shown in Fig. 3, only the (111) surface shows the characteristic butterfly peaks at potentials slightly negative than 0.5 V. Their STM observations confirmed that the butterfly peaks are formed because of the adsorption and desorption of sulfate ions as indicated by the CO replacement technique used by Clavilier as described above [14]. STM images obtained on Pt(lll) were interpreted in terms of the coadsorption of sulfate anions and water. 

In 1956, Fuerstenau coined the term hemimicelle to describe the two-dimensional aggregates that he proposed were responsible for the rise through several orders of magnitude in the observed absorption of surfactant molecules on certain mineral oxide surfaces from aqueous solutions. The study on coadsorption of pinacyanol chloride (cyanine dye) with sodium p-(l-propylnonyl)-benzenesulfonate (dissolved in water) on AI2O3 surface reveals that surfactant adsorption is independent of the concentration of pinacyanol chloride, and the surfactant solution remains clear at total surfactant concentration ([SurfJx) below CMC, whereas the solid surface becomes dyed blue, the color characteristic of... 

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