Chemisorption surface reconstruction model

Nonsteady behavior of electrochemical systems was observed by Fechner as early as 1828 [ii]. Periodic or chaotic changes of electrode potential under gal-vanostatic or open-circuit conditions and similar variation of current under potentiostatic conditions have been the subject of numerous studies [iii,iv]. The electrochemical systems, for which interesting dynamic behavior has been reported include anodic or open-circuit dissolution of metals [v-vii], electrooxidation of small organic molecules [viii-xiv] or hydrogen, reduction of anions [xv, xvi] etc. [ii]. Much effort regarding the theoretical description and mathematical modeling of these complex phenomena has been made [xvii-xix]. Especially studies that used combined techniques, such as radiotracer (-> tracer methods) ig. 1) [x], electrochemical quartz crystal microbalance (Fig. 2) [vii,xi], probe beam deflection [xiii], surface plasmon resonance [xvi] surface stress [xiv] etc. have contributed considerably to the elucidation of the role of chemisorbed species ( chemisorption), surface reconstruction as well as transport phenomena in the mechanism of oscillations. 

Though both chemisorption and desorption kinetics can be explained by a model in which the surface is characterized by a fixed heterogeneity with a uniform energy distribution function, in chemisorption other explanations are possible, like induced heterogeneity or surface reconstruction. 

The first STM evidence for the facile transport of metal atoms during chemisorption was for oxygen chemisorption at a Cu(110) surface at room temperature 10 the conventional Langmuir model is that the surface substrate atoms are immobile. The reconstruction involved the removal of copper atoms from steps [eqn (1)], resulting in an added row structure and the development of a (2 x 1)0 overlayer [eqn (2)]. The steps present at the Cu(llO) surface are... 

Figure 4.5 STM images (6.2 x 6.5 nm) observed in the chemisorption of oxygen at Ni(110) at room temperature (a) the (3 x 1)0 state at 0 = 0.33 (b) the (2 x 1)0 state at 0 = 0.5 (c) the (3 x 1)0 state at 0 = 0.66. Corresponding ball models of these are shown in (d), (e) and (f) and are typical of oxygen-induced reconstructions at metal surfaces. The small black balls represent the O adatoms. (Reproduced from Ref. 12).

Of crucial significance in deciding between various models have been estimates of the number of copper atoms required to transform the surface into a (2 x 3)N phase. This was the approach adopted by Takehiro et al 2 in their study of NO dissociation at Cu(110). They concluded that by determining the stoichiometry of the (2 x 3)N phase that there is good evidence for a pseudo-(100) model, where a Cu(ll0) row penetrates into the surface layer per three [ll0]Cu surface rows. It is the formation of the five-coordinated N atoms that drives the reconstruction. The authors are of the view that their observations are inconsistent with the added-row model. The structure of the (2 x 3)N phase produced by implantation of nitrogen atoms appears to be identical with that formed by the dissociative chemisorption of nitric oxide. 

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. 

It is therefore not unreasonable to ascribe the wide observation of the Elovich behaviour in chemisorption to the occurrence of one or the other of the considered models (surface heterogeneity, induced heterogeneity, or surfaee reconstruction), and to ascribe the smaller set of systems following the Elovich behaviour in desorption to a unique model — fixed surface heterogeneity. 

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