Metal excess surface species

The surface tension was stated (Section 6.4.5), on general grounds, to be related to the surface excess of species in the interphase. The surface excess in turn represents in some way the structure of the interface. It follows therefore that electrocapillaiy curves must contain many interesting messages about the double layer at the electrode/ electrolyte interface. To understand such messages, one must learn to decode the electrocapillary data. It is necessary to derive quantitative relations among surface tension, excess charge on the metal, cell potential, surface excess, and solution composition. 

CRi reacts both on the oxidised surface of the support giving CO and on the reduced metal producing CHx specie The conversion is lower than CO2 when co-feeded and, on the contrary, higher than CO2 when it is feeded alone Really other reactions have to be considered La203 easily forms carbonate specie or, at reduction conditions, cover the metal surface thus, due to the stability of carbonate groups or the excessive metal decoration, inhibiting the CH4 reaction. 

In a simple example where a small metal particle M and a species X react to a compound MX, the excess surface energy of both reactants and products must be taken into account. Thus, the molar free energy of formation of a particle MX may be described in a simplified manner on the basis of the Gibbs concept in which excess quantities are addressed ... 

Lead(O) is probably most well known for its application as a catalyst poison, especially for transition metals such as palladium, and is most widely used in Lindlar s catalyst, which permits selective reductions of alkynes. In a detailed study of the selective reduction of a triple to a double carbon-carbon bond, it has been shown that two types of surface species, adsorbed lead and bulk lead, are present, and that the preparation of effective catalysts required precise experimental control. Optimisation of the preparation protocol found that the use of surface-oxidised palladium, as opposed to freshly prepared surface-reduced palladium, for the preparation of lead acetate-poisoned catalysts, as well as the removal of excess lead acetate before hydrogenation, is of crucial importance for preparing effective and selective Lindlar catalysts. 

In Chapter 9 we have shown the thermodynamic relationships among the surface tension, y, (which is the same as the excess surface Gibbs energy), the excess charge density on the metal, qM, the double layer capacitance, Cai, and the surface excess of a particular species, F,- (cf. Eqs. (9.8) to (9.14)). 

In the first linear free energy approach (Model 2a. 1), the energy of the electron(s) transferred on anion adsorption is the only potential dependent term in the adsorption free energy. This approach assumes that the electric field caused by the applied potential (or excess surface charge) has no interaction with the adsorbate or influence on the adsorbate-metal interaction. Two methods to approximate the impact of the electric field effect on elementary surface processes that do not require explicitly charging the metal surface (Models 2a.2 and 2a.3) will be discussed. The dependence of a species energy within an electric field F) is... 

More uniform results may be expected if a substantial layer of metal is removed from the specimens to ehminate variations in condition of the original metaUic surface. This can be done by chemical treatment (pickling), electrolytic removal, or grinding with a coarse abrasive paper or cloth, such as No. 50, using care not to work-harden the surface. At least 2.5 X 10 mm (0.0001 in) or 1.5 to 2.3 mg/cm (10 to 15 mg/iu") should be removed. If clad alloy specimens are to be used, specif attention must be given to ensure that excessive metal is not removed. After final preparation of the specimen surface, the speci-... 

Thus the potential difference at the interface between a metal and electrolyte solution is due to both the charges at the interface (electrostatic potential difference) and the surface dipole layers the latter is referred to as the surface or adsorption potential difference. On the basis of the above considerations it might appear that adsorption at a metal surface with an excess charge is solely due to electrostatic interaction with charged species in the solution, i.e. if the metal surface has an excess negative charge the cations... 

When the Gibbs equation is used for an electrode-electrolyte interface, the charged species (electrons, ions) are characterized by their electrochemical potentials, while the interface is regarded as electroneutral that is, the surface density, 2, of excess charges in the metal caused by positive or negative adsorption of electrons ... 

In the enantioselective hydrogenation of isophorone in the presence of (-)-DHVIN modifier the best optical purity was afforded by small dispersion (<0,05) Pd black catalyst (up to 55%) (7). The influence of the preparation method of Pd black on the optical yield was reported (8). A correlation was found between the oxidation state of the metal surface and the enantioselectivity, the catalyst having more oxidised species on its surface giving higher enantiomeric excess, while the Pd black with lower surface area was more enantioselective. 

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