Desorption from crystal faces

The R of electropolished Zn single-crystal face electrodes has been obtained from the shape of the adsorption-desorption peak of cyclohex-anol at various Zn and Hg surfaces.154 The roughness factor of Zn electrodes has been found to increase in the order Zn(0001) < Zn(lOlO) < Zn(llZO) with values in the range 1.1 to 1.25. 

It was quickly seen from studies on platinum single crystals that voltammograms for hydrogen adsorption and desorption differ somewhat among the different faces and between the single-crystal faces and polycrystalline platinum. Despite these differences, though, they have common traits as weU. The areas under these curves,... 

The difference between the Ir(l 11) and Pt(l 11) surfaces in their reactivity to C H bond breaking as indicated by flash desorption spectra is striking. From the Pt(lll) crystal face, ethylene, acetylene, and benzene can all be desorbed in large quantities upon heating. On the Ir(l 11) surface, however, benzene is the only adsorbate that can be desorbed upon flash desorption. Ethylene remains largely on the surface, with only a few percent removed by heating, and acetylene cannot be desorbed at all. Only hydrogen evolution is observed under conditions of flash desorption. 

Another potentially interesting zeolite characteristic is the nature of gas diffiision in the intracrystalline pores. It has been suggested in the literature that certain adsorbed gas molecules close in size to the zeolite pores float within non polar zeolite crystals, instead of the standard adsorption-desorption mechanism. This concept opens the possibility that under certain circumstances, the emission of desorbed gas molecules may be directionally coherent as it emerges fi om each zeolite crystal face. Such a coherent gas emission - "a molecular laser" - may find applications in catalytic combustion or in other applications benefitting from "non thermalized" gas emissions. 

The reactivity of surface material depends, to some extent, on its crystallographic environment. The mobility of a species may vary from one set of crystal faces to another. Individual crystal constituents tend to coordinate with a maximum number of nearest neighbours. Coordination on surfaces is necessarily unsymmetrical and isolated constituents tend to migrate to sites such as step edges where stability can be enhanced by increased coordination. Surface mobility is thus often appreciable at temperatures well below the melting point. Such behaviour is intermediate between solid and liquid states and is important both in sintering and as a transfer mechanism in solid-solid reactions. Migration across surfaces may be complicated by concurrent adsorption and desorption processes. The surface is thus a zone of... 

From a physical viewpoint, since the Elovich equation has been found to be obeyed for adsorption rates on finely divided powders and on metal films, one might consider the total surface to comprise a small number of different crystal faces for which the activation energy for adsorption (and desorption) and their area would differ. Owing to the imperfect nature of real crystal planes, these will not have a set of completely uniform sites but such imperfections will not affect the following considerations about the minimum number of different sets of sites required so that the Elovich equation describes the adsorption rate... 

Figure 7.40. Carbon-14-labeled ethylene (or other alkenes) was chemisorbed as a function of temperature on the flat Pt(l 11) crystal face. The (H/C) ratio of the adsorbed species was determined from hydrogen thermal desorption. The amount of preadsorbed alkene that could not be removed by subsequent treatment in 1 atm of hydrogen represents the irreversibly adsorbed fraction. The adsorption reversibility decreases markedly with increasing adsorption temperature as the surface species become more hydrogen-deficient. The irreversibly adsorbed species have long residence times, on the order of days [195].

Equation 6 together with Eqs. 10 and 11 describe a process of onedimensional diffusion, initiated by a change in the surrounding atmosphere so that the corresponding equilibrium concentration varies from Co to Coo-Equation 10 requires that immediately after the pressure step, the concentration at the boundary (namely for y = 0) assumes the new equilibrium value. This means that the existence of additional transport resistances at the surface of the system is excluded. The second term in Eq. 11 indicates that the process has to proceed as in a semi-infinite medium. This means in particular that the transient adsorption or desorption profiles originating from different crystal faces must not yet have met each other. 

Cu crystallizes in the fee system. The experimental data for single-crystal CUIH2O and for PC Cu are controversial [2-5,7,44,45[. The first studies with Cu(lll), Cu(lOO), Cu(llO), and PC Cu in surface-inactive electrolyte solutions show a capacitance minimum at E less negative than the positive limit of ideal polarizability of Cu electrodes. More reliable values of cr=o for Cu single-crystal faces have been obtained by Lecoeur and Bellier [44] with EP Cu(l 11) and Cu(lOO) (Table 1). Eoresti etal. [46] have found =o = -0.93 0.01 V (SCE) for the Cu(l 10)-aqueous solution interface, and the validity of the GCSG model has been verified. As for Zn single-crystal electrodes, reliable values of cr=o have been obtained indirectly from the dependence of the adsorption-desorption peak... 

FIGURE 25 Typical thermal desorption spectra of CO from a Pt(553) stepped crystal face as a function of coverage. The two peaks are indicative of CO bonding at step and terrace sites. The higher temperature peak corresponds to CO bound at step sites. 

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