Partially blocked surface

C. Amatore, J.M. Saveant, and D. Tessier, Charge transfer at partially blocked surfaces. A model for the case of microscopic active and inactive sites. J. Electroanal. Chem. 147, 39-51 (1983). 

Figure 9.3 Equivalent electrical circuit for a partially blocked surface.

Example 15.1 2-D and 3-D Blocking Apply the concepts of electrohydrody-namic impedance to scale deposit in seawater. This system shows behavior associated with both partially blocked surfaces and dijfusion through porous layers. 

The time constant with calcareous deposits was found to be the same as for a bare surface in the low-frequency range with a slight shift cf diagrams to lower reduced frequencies when the deposition time increases. At high frequency (above p — 1), a second time constant appeared. Both these features characterized the presence cf a partially blocked surface with a moderate value of the active fraction cf the electrode. The slight separation of the EHD diagrams observed in the low-frequency region, when the deposition time is... 

C. R. S. Silva, O. E. Barcia, O. R. Mattos, and C. Deslouis, "Partially Blocked Surface Studied by the Electrohydrodynamic Impedance," Journal of Electro-analytical Chemistry, 365 (1994) 133-138. 

A distinctive feature of NEEs is that electron transfer kinetics appear slower than those at conventional electrodes (5). In fact, NEEs behave as electrodes with a partially blocked surface (PBE). According to the model elaborated by Amatore et al. (95), the current response at a PBE is identical to that at a bare electrode of the same overall geometric area but with a smaller apparent standard rate constant for the electron transfer which decreases as the coverage with the blocking agent inaeases. Such an apparent rate constant ( tpp) is related to the true standard charge transfer rate constant k°) and the fraction of blocked surface ( 7) by the following relationship (95) ... 

Amatore, C., Oleinick, A. I., Svir, 1. 2010. Diffusion from within a spherical body with partially blocked surface Diffusion through a constant surface area. ChemPhysChem 11 149-158. 

An important feature characterizing NEEs and NEAs is that their responses are very sensitive to the electron transfer kinetics." According to model proposed by Amatore et al., as well as to more recent theoretical studies, NEEs behave as partially blocked surface electrodes (PBEs), whose current responses are identical to those of naked electrodes with the same overall geometric area, but with a smaller apparent rate constant (k°app) for the electron transfer which decreases as the coverage of the surface increases. According to this model, the nanodisks electrodes are the unblocked surface and the template membrane is the blocking material. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Kinetic and equilibrium studies of the sorption of methanol on various coals and on partially acety-lated samples of these coals have been used to elucidate a mechanism for this process. The data are interpreted in terms of partial acetylation blocking surface sites and perhaps interfering with intermolecular hydrogen bonding. It is proposed that the rate-determining step is a set of parallel, competing, second-order reactions involving transfer of methanol from the surface to the interior of the coal. All types of surface sites appear to participate, and the pressure-independent rate constant is considered to be the sum of the rate constants for each type of surface site. The dependence of the experimental rate constant on methanol pressure is a characteristic of the coal rank. 

For the system shown, the view between surface 1 and surface 2 is partially blocked by an intervening circle. Determine the view-factor F1.2. 

The effect of a surface film which partially blocks an electrode has been considered above (Section 10.5.2.1). This section considers the effect of a film which is permeable to reagents but which covers entirely the electrode... 

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