Adsorption site blocker

As a conclusion, the promoting effects of adsorption site blockers on H entry can be understood not by considering the effective diminution of 0 the overall coverage of OPD H (determined by integration of adsorption pseudocapacitance vs. potential curves [117-119]), but by considering the variation of x the local OPD H coverage in the sites not blocked [obtained by normahzation of 0 or from the Tafel and permeation slopes (see Tables 1 and 2)]. If the surfece-bulk transfer step is in equilibrium, a significant increase of the bulk H eoncentration can be induced by ASB surface effects only for the HER mechanism where the steps of electroadsorption and chemical combination are coupled. This analysis provides a quantitative explanation of the effects of promotion of H absorption into iron and ferrous alloys. 

Figure 12 Theoretical variations, for a coupled electroadsorption-chemical combination HER mechanism, of the overall H coverage 0jj (fiill line), the local H coverage Xjj (dotted line), the HER current (dashed line), and the bulk H fraction for charging (dashed line) with the coverage of adsorption site blocker (ASB). The represented case is for Langmuir type H adsorption and a geometric blocking effect, where one ASB atom blocks one H site, with random ASB adsorption 6h asb)% her cc asb) h Above a certain value of 0 gg...

Another explanation for the promoter effects could involve the subsurface sites. This explanation is that whereas surface adsorption is inhibited by adsorption site blockers, subsurface adsorption would still occur and could even be promoted [120a], but no direct experimental evidence exists. 

A.2 Predictions of the Effects of Adsorption Site Blockers on H Entry.141... 

The effects of adsorption site blockers on the rates of the surface reactions involving adsorbed hydrogen may be modeled simply by taking into account simultaneously but separately the blocking effect that reduces the number of sites for H adsorption and the effect on the bond in the sites not blocked, without any a priori assumption on the... 

On a surface irreversibly covered by an adsorption site blocker (ASB) on a significant range of potential, the H coverage at a given potential E/ 0H/ defined as the ratio of the density of H sites occupied by adsorbed H atoms to the total density of H sites existing on the clean surface (rj, can be expressed as... 

An alternative explanation for the promoter effects exists for nonnoble metals, such as Fe or Ni. The HER rarely proceeds on a bare surface of these metals because, except at high overpotentials, they are covered by corrosion products or passive oxide films [55,102]. Adsorption site blockers such as sulfur block sites for O or OH adsorption and thus impede or inhibit surface oxidation. In the gas phase, it has been shown that at low oxygen pressure and room temperature, Ni oxidation is inhibited when the surface is covered by a complete... 

The metal surfaces are always covered with a monolayer of CO upon evacuation of the reactor and transfer to the UHV system. On both Pd and Ir the CO, which desorbs as CO2 when reacted with the oxide species, desorbs at a much higher temperature than CO from the clean surface. This result implies that the oxide species forms an inactive complex with CO upon adsorption of CO under reaction conditions. While the presence of the oxide species reduces the overall rate of reaction, the activation energy is unchanged, suggesting that oxygen serves as a simple site blocker on the surface. 

The adsorption isotherms of three basic drngs, the anxiolytic bnspirone hydrochloride, the antidepressant doxepin hydrochloride, and the Ca + blocker diltiazem hydrochloride, were fitted to the Bilangmnir model, as shown in Figure 10.12 [27]. This finding snggested that the adsorption took place on two different types of sites and that there were two different adsorption mechanisms. 

Similar adsorption data have also been reported for the adsorption isotherms of many compounds in various systems. For example, the adsorption data of several jS-blockers, particularly those of propranolol acquired in a 1 to 7000 relative concentration range, on an immobilized cellulase. Cel 7A, fit very well to the bi-Langmuir model, as illustrated in Figure 3.13 [47,55]). The enantioselective site was identified as a pair of amino acid residues in the tunnel formed by the main chain of the protein. The parameters of the isotherm depend on the pH as illustrated in Figure 3.14. A bi-Langmuir model was also found to accoimt well for the separation of pairs of enantiomers on polymers molecularly imprinted with one of the enantiomers [56]. Note, however, that there are also many systems in which the adsorption isotherms of enantiomers are not accounted for by a bi-Langmuir model showing that enantioselectivity is often achieved by a complex... 

There is still a lack of experiments on the cathodic reaction with well-controlled surfece coverages by hydrogen, oxygen, and adsorption blockers. Such studies, as well as studies of the influence of the blockers on the occupancy of the subsurface sites, are necessary to understand fully the mechanisms of the hydrogen surface reactions involved in eorrosion phenomena. 

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